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10.18331/BRJ2024.11.3.1

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Research Paper

Cascading valorization of defatted rice bran for lactic acid fermentation and biogas production.

Pages 2146-2167

10.18331/BRJ2024.11.3.2

Christiane Herrmann; Raj Shekhar Bose; Anna-Katrin Neu; Roland Schneider; Maria Alexandri

Cascading valorization of defatted rice bran for lactic acid fermentation and biogas production

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Sustainable catalytic pathways for biofuel precursors: quantitative conversion of glucose to gluconic acid using Pt-Zn biochar catalyst

Pages 2168-2180

10.18331/BRJ2024.11.3.3

Hengyu Hao; Haixin Guo; Bingkun Chen; Richard Lee Smith Jr; Shen Feng

Sustainable catalytic pathways for biofuel precursors: quantitative conversion of glucose to gluconic acid using Pt-Zn biochar catalyst

Beyond tradition: charting a greener future for cassava starch industry using multi-criteria decision-making

Pages 2181-2193

10.18331/BRJ2024.11.3.4

Varshini Ravichandran; Deepak Kumar; Sivakumar Mani; Karthik Rajendran

Beyond tradition: charting a greener future for cassava starch industry using multi-criteria decision-making

Advanced computational modelling for biofuel catalyst optimization: enhancing beta zeolite acidity for oleic acid upgrading

Pages 2194-2210

10.18331/BRJ2024.11.3.5

Seba AlAreeqi; Daniel Bahamon; Ismail I.I. Alkhatib; Kyriaki Polychronopoulou; Lourdes F. Vega

Advanced computational modelling for biofuel catalyst optimization: enhancing beta zeolite acidity for oleic acid upgrading

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Biofuels: present and future

S.I. : Biofuels
Published: 08 May 2024

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Richard Vincent Asase ORCID: orcid.org/0009-0002-7712-5955 1 ,
Queency N. Okechukwu 1 &
Maria N. Ivantsova 1

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Biofuels represent a promising departure from conventional fossil fuels, presenting viable remedies for both energy security and environmental apprehensions. This review intricately examines the various realms of biofuels, encompassing their historical progression, present status, obstacles, and outlook. Commencing with an in-depth exploration of their historical antecedents and developmental milestones, this paper navigates through the spectrum of biofuel variants, encompassing first, second, and third-generation iterations. It meticulously scrutinizes the methodologies of production, advantages, limitations, and ecological implications associated with each variant, providing a nuanced comprehension seldom found in singular sources. A pivotal emphasis is placed on technological innovations propelling the biofuel industry’s advancement, shedding light on recent breakthroughs such as nanotechnologies and mathematical models in biofuel production. Moreover, the paper thoroughly assesses the current global landscape of biofuel production, dissecting the health, environmental, and socioeconomic ramifications of their utilization. The findings reveal that the field of biofuels is still developing and possesses a lot of opportunities toward environmental sustainability. Only a small part of the world is experiencing the production and utilization of biofuels, hence the need for its campaign to engulf the entire world. Also, most of the modern technologies identified in this work are on a laboratory basis and require the needed scalability skills for industrial production. Finally, this work of varied viewpoints seeks to serve as a significant scholarly resource for stakeholders, policymakers, academics, and professionals within the industry, offering insights to grasp and influence the trajectory of biofuels.

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Global Conclusions and Future Perspectives

Biofuels: An Overview

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The potential of biofuels from first to fourth generation

Contributed equally to this work with: Philipp Cavelius, Selina Engelhart-Straub

Roles Conceptualization, Data curation, Investigation, Validation, Visualization, Writing – original draft, Writing – review & editing

Affiliation Werner Siemens-Chair of Synthetic Biotechnology, TUM School of Natural Sciences, Technical University of Munich (TUM), Garching, Germany

Roles Conceptualization, Data curation, Funding acquisition, Project administration, Supervision, Writing – original draft, Writing – review & editing

Roles Conceptualization, Data curation, Supervision, Writing – review & editing

Affiliation Chair of Technical Chemistry II, TUM School of Natural Sciences, Technical University of Munich (TUM), Garching, Germany

Roles Conceptualization, Data curation, Supervision, Validation, Visualization, Writing – original draft, Writing – review & editing

* E-mail: [email protected] (DA); [email protected] (TB)

Philipp Cavelius,
Selina Engelhart-Straub,
Norbert Mehlmer,
Johannes Lercher,
Dania Awad,
Thomas Brück

Published: March 30, 2023

https://doi.org/10.1371/journal.pbio.3002063
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The steady increase in human population and a rising standard of living heighten global demand for energy. Fossil fuels account for more than three-quarters of energy production, releasing enormous amounts of carbon dioxide (CO 2 ) that drive climate change effects as well as contributing to severe air pollution in many countries. Hence, drastic reduction of CO 2 emissions, especially from fossil fuels, is essential to tackle anthropogenic climate change. To reduce CO 2 emissions and to cope with the ever-growing demand for energy, it is essential to develop renewable energy sources, of which biofuels will form an important contribution. In this Essay, liquid biofuels from first to fourth generation are discussed in detail alongside their industrial development and policy implications, with a focus on the transport sector as a complementary solution to other environmentally friendly technologies, such as electric cars.

Citation: Cavelius P, Engelhart-Straub S, Mehlmer N, Lercher J, Awad D, Brück T (2023) The potential of biofuels from first to fourth generation. PLoS Biol 21(3): e3002063. https://doi.org/10.1371/journal.pbio.3002063

Copyright: © 2023 Cavelius et al. This is an open access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Funding: This work was supported by the German Federal Ministry of Education and Research (BMBF) (031B0853A to NM). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Competing interests: The authors have declared that no competing interests exist.

Abbreviations: EEA, European Environment Agency; EIC, European Innovation Council; GHG, greenhouse gas; GMO, genetically modified organism; ILUC, indirect land use change; IPCC, Intergovernmental Panel on Climate Change; IRENA, International Renewable Energy Agency; RED, Renewable Energy Directive

Introduction

For decades, global energy demand is on the rise due to economic growth and a rapidly growing world population. Additionally, the standard of living is increasing worldwide, in most cases correlating with increased energy consumption, as energy is needed in almost every aspect of our lives, including land, water, and air transport as well as in agriculture, commercial, industrial, and domestic sectors [ 1 ]. To date, fossil fuels account for around 80% of the world’s energy demand [ 2 ], despite being a major instigator for global warming, representing roughly 89% of total greenhouse gas (GHG) emissions in 2020 [ 3 ]. Additionally, fossil fuels are predicted to deplete with the steadily increasing energy demands. As petroleum demand is constantly on the rise, estimations predict a shortage by 2070 to 2080 [ 4 ]. To that end, distinct biofuel types such as liquid and biogas should be methodologically and strategically developed as a preventive measure against predicted energy shortages, all while reducing the anthropogenic climate impact and preserving the environment.

Currently, biofuels are categorized as first to fourth generation, depending on feedstock and/or biosynthetic platform (i.e., genetic engineering). In this Essay, we present comparative advantages and disadvantages among these categories, as well as fossil sources. Furthermore, the development of biofuel technologies hinges on the socioeconomic and political landscape, which can greatly benefit from policy recommendations by respective regulatory bodies. At present, the European Union has the most stringent biofuel legislation and the most ambitious climate impact goals. Hence, we focus on EU-centered development with respect to current biofuel technology platforms at various stages of industrial deployment, the legislative framework implemented in the EU, as well as policy recommendations that would accelerate academic breakthroughs toward industrial implementation. Although, our recommendations are EU-centric, many are also applicable on a global level.

The four generations of biofuels

One alternative to fossil fuels are biofuels, which originate from organic matter and therefore can be regrown and are termed renewable. Biofuels emit less GHGs and are in general more eco-friendly (non-toxic, sulfur-free, biodegradable) than their fossil fuel predecessors [ 5 ]. Biofuels contribute to the achievement of Sustainable Development Goals 7 (affordable and clean energy) and 13 (climate action) of the United Nations [ 6 ]. Global demand for biofuels is set to grow by 41 to 53 billion liters, or 28%, over 2021 to 2026 [ 7 ]. Typically, one can find four main types of biofuel discussed in the context of fermentation: biogas, bioethanol, biobutanol, and biodiesel. The physiochemical properties of these biofuels are compared to fossil-based fuels in Table 1 .

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https://doi.org/10.1371/journal.pbio.3002063.t001

Biogas formation is a fairly simple process that has been utilized for several decades. It includes four stages: hydrolysis, acidogenesis, acetogenesis, and methanogenesis. Mixed microorganisms consortia and waste streams are combined in a sealed fermentation system in the absence of oxygen. During the biogas production process, microorganisms hydrolyze waste materials into sugars, peptides and amino acids, fatty acids, and to some part into acetate and hydrogen. Afterwards, acidogenic bacteria convert those intermediate products into organic acids, mainly constituting acetic acid. In addition, they produce carbon dioxide and hydrogen. In the third step, acetogenesis, acetate is formed from hydrogen and carbon dioxide produced in the previous stage. Lastly, methanogenesis follows, creating methane from the products of acetogenesis and acidogenesis [ 8 ]. These gases can then be transformed into hydrogen and/or electricity, or can be stored as biomethane in existing geological reservoirs [ 9 ]. Since the Ukraine crisis began, the resulting lack of fossil fuel availability in the EU has led to biogas being politically pushed as a substitute to natural gas [ 10 ].

Compared to gas (biogas/hydrogen), liquid fuels offer higher energy density and simplified transport and storage. This renders them more compatible with current engine and turbine technologies [ 11 ]. Most engines and turbines are designed and built for the use of liquid fuels, which makes liquid biofuels an easy drop-in solution without the need for modifying present engine technologies or infrastructure [ 5 , 12 ]. These gaseous fuels pose a significant safety hazard as they ignite at lower energies and are flammable over a range of concentrations, hydrogen to higher extent, requiring high level of safety procedures [ 13 ]. The low boiling point and high octane number of bioethanol allow blending with gasoline to a certain extent. The added benefits include a more complete combustion and reduced tailpipe emissions, boosting the engine performance and reducing CO 2 emissions. It is, however, inapt for blending with diesel. Diesel engines require hydrocarbons of higher chain length and low autoignition temperature. However, biodiesel, being of similar chemical constitution, can be blended with fossil-based diesel and hence constitutes a major energy-dense liquid biofuel. A third increasingly attractive biofuel is biobutanol, which holds high promise as it displays superior properties to bioethanol such as higher energy density (25% more energy than ethanol) and usually lower water content due to increased hydrophobicity. Biobutanol is less volatile and possesses less corrosive properties, making it easier and safer to use and store [ 11 , 14 – 19 ]. More importantly, it can be blended with both gasoline, fossil-based or biodiesel at any ratio without the need of new engine technologies and might even allow complete substitution of gasoline, while the use of ethanol is only possible as additive [ 11 , 18 ].

While the classification of biofuel technologies somewhat varies in the literature, products can generally be classed as first to fourth generation, depending on the type of feedstock and conversion process that was applied ( Fig 1 ) [ 5 ].

https://doi.org/10.1371/journal.pbio.3002063.g001

First-generation biofuels

Biofuels of the first generation are mainly divided into bioethanol and biodiesel. Bioethanol production of the first generation is based on microbial fermentation of edible feedstocks, rich in starch and sucrose, such as wheat, corn, and sugarcane in Europe, North America, and South America, respectively. Commercial strains include but are not limited to Saccharomyces cerevisiae , S . stipites , and S . pombe . Bioethanol production is not limited to first-generation biofuels; depending on the feedstock and production strain, bioethanol can also be categorized as second and third generation [ 32 – 35 ]. Biodiesel is mainly obtained from food-grade rapeseed, soy, or palm oil sourced from Europe, South America, and Asia, respectively. In contrast to bioethanol, it is only partially biosynthesized as its production includes chemically catalyzed steps such as transesterification of the lipids with alcohols. Enzymatic catalysis currently only exists on a lab scale [ 36 , 37 ]. Although biobutanol production is also possible by sugar fermentation from sugar cane, corn, wheat, and other food crops, it is limited by lower productivity and yields, product inhibition, and high costs [ 11 , 16 , 18 , 38 ].

During the global food demand crisis in 2007/2008, crops used for biofuel became more important to be used as food, giving rise to the “food versus fuel” debate that persists to date. Additionally, an increased demand for crops (e.g., corn) for fuel production yielded an increased market price for those foods [ 5 ]. Models predict that massive agricultural areas would be needed for fuel production and still could supply only limited amounts of fuel compared to the overall demand. It is estimated that more than two times the globally available area of arable land would needed to meet the global market demand for biodiesel when produced from rapeseed oil [ 39 ]. Furthermore, increased market values of palm oil and other biofuel cultures prompted extended deforestation of tropical rainforests for biofuel crop plantations, which releases more CO 2 than the emission saved by those biofuels. In 2008, Fargione and colleagues estimated that it would take 319 years to repay the biofuel carbon debt resulting from clearing of tropical rainforest in Brazil and subsequent conversion to soybean plantations [ 40 ].

Second-generation biofuels

As a result of the issues of the first generation, second-generation biofuels were developed, utilizing lignocellulosic biomass from agricultural and woodland residues as well as other waste streams (for example, from food industry like wheat bran, animal fats, or wastes of cooking and frying oil). Other non-food plants like the drought-resistant shrub or tree Jatropha curcas , which can also be grown in wastelands, might yet be a different promising source for second-generation biofuels [ 41 ]. Hence, second-generation biofuels circumvent the need for agricultural land use change and do not compete with food resources. However, often second-generation waste streams represent more complex feedstocks than sugarcane or palm oil, potentially containing compounds able to reduce fermentation efficiency, such as lignin. Therefore, application of additional pretreatment steps are common, increasing process time and costs [ 5 , 42 , 43 ].

For the most part, biofuels of the first and in the vast majority of the second generation are commercially produced, around 4% and 96% in 2019, respectively [ 44 ]. One example is the commercially available sunliquid from Clariant, which is a cellulosic ethanol from currently underutilized agricultural residues, such as straw. The first commercial ethanol plant in Romania started production in 2022, with plans to convert 250,000 tons of locally sourced agricultural residues to 50,000 tons of ethanol per year. After enzyme production, which hydrolyses cellulose and hemicellulose to sugar monomers, optimized microorganisms are used in fermentation to produce ethanol. These microorganisms can utilize various carbon sources like glucose and xylose, ensuring higher yields and enabling high efficiency and flexibility in waste valorization as more building blocks of waste streams can be converted to product [ 45 ]. Alongside ethanol producers, the production of second-generation biodiesel is possible from microbial lipids produced by organisms, such as Cutaneotrichosporon oleaginosus , a yeast capable of producing up to 90% (w/w) lipids per biomass in a fermentation process, which can be grown on residue streams (e.g., wheat bran hydrolysate medium) [ 46 – 49 ]. Second-generation biodiesel can also be sourced from waste oils via catalytic cracking and hydrogenation. Drawbacks of this process include incomplete conversion and coke formation, which leads to the deactivation of the catalyst. [ 50 , 51 ]. Biobutanol production on lignocellulose biomass and other waste streams is most commonly based on Clostridia fermentation, as it is one of the oldest and best-established fermentative processes for butanol production. Many Clostridia are natural butanol producers and possess the ability to metabolize a variety of different substrates. However, similar to its first-generation predecessor, the process is limited by low butanol titers and product inhibition [ 11 , 16 , 18 , 38 ]. Typically, butanol is produced via ABE fermentation, which results in solvents in ratio of 3 parts acetone, 6 parts butanol, and 1 part ethanol, and butanol refinement is not an energetically favorable solution. Other drawbacks also include cell toxicity at low concentration [ 52 , 53 ]. To that end, cell-free isobutanol biosynthesis using a designed artificial metabolic pathway has been developed [ 54 ]. At present, this approach remains costly for commercialization.

Various carbonaceous compounds can be transformed to syngas by gasification. Commonly, it is a gaseous waste stream from industrial processes such as steel manufacture, in which fossil fuels are burned in the process. Syngas is a mixture mainly consisting of carbon monoxide (CO), CO 2 , and hydrogen. It can be derived from biomass, including lignocellulosic compounds, coal, animal or municipal solid waste, and industrial CO-rich gases. This gas can be metabolized by strictly anaerobic, methanogenic archaea as well as by acetogenic bacterial genera such as Acetobacterium or Clostridium , often used in syntrophic fermentations. The process is mostly focused on biosynthesis of organic acids and alcohol compounds such as acetate, ethanol, and butanol [ 55 – 57 ]. Advantages of syngas fermentation compared to other second-generation approaches are high feedstock flexibility as well as high rates of energy and carbon capture. Complicated pretreatments of second-generation feedstocks can be replaced with gasification, using all components of the biomass, including lignin and other recalcitrant compounds [ 58 ]. LanzaTech developed a process converting feedstocks including industrial waste streams to fuel and chemicals utilizing bacteria. They estimate a total product capacity of 600,000 metric tons as well as 1,000,000 metric tons of captured carbon per year, for all their plants combined [ 59 ]. Since 2022, a demonstration plant in Japan has turned municipal solid waste to ethanol, with a production target of 20 tons of ethanol per day [ 60 ].

More than half of the biologically stored carbon is bound in marine biomass, especially macroalgae and seagrass. Detached seagrass material is seasonally washed on beaches and shore lines; due to low biological degradation and herbivore consumption, an excess of it accumulates as waste. Estimations of up to 40 million tons of dry seagrass biomass, which can be used for biofuel production, are given. Through enzymatic hydrolysis, the carbohydrate content of the seagrass can be used in a fermentation medium for microorganisms, additionally offering low nitrogen and phosphorus content, which is typically required for lipid production [ 61 ].

Despite the highly favorable ability to valorize waste streams, second-generation biofuels by themselves will not be sufficient to supply energy for the current worldwide demand. As is the case for food crops with first-generation biofuels, biomass used in these processes is available in limited amounts. Therefore, second-generation biofuels must be combined with other technologies to ensure sufficient provision of fuels. This prompted research on third-generation biofuels. However, scientific estimations predict second-generation biofuels could supply up to 30% of the world’s transportation energy [ 5 ].

Third-generation biofuels

Third-generation biofuels are mainly derived from microalgae and cyanobacteria biomass, which can be used to naturally generate alcohols and lipids to transform into biodiesel or any other high energy fuel product. Algae exhibit 2- to 4-fold higher photosynthesis rates than terrestrial plants, resulting in faster biomass formation [ 62 ]. Algae do not require arable land or fresh water for cultivation. Many cultures can be grown using waste water, brackish or salt water, which is cost efficient and circumvents competition with agricultural activity [ 63 , 64 ]. Most importantly, efficient algae cultivation requires a direct CO 2 supply, which can be derived from industrial emitters or by atmospheric carbon capture. In conventional cultivation systems, around 70% of supplied CO 2 is used for photosynthesis and therefore biomass production [ 65 ]. Hence, algae biofuels potentially could have a negative carbon footprint as they directly bind the GHG in their biomass. One of the most prominent third-generation processes is the production of biodiesel or other energy density biofuels, such as biokerosene, using oleaginous microalgae [ 66 , 67 ].

One of the most economically critical and versatile operations in algal biofuel production is algae cultivation. Algal bioreactors ( Fig 2 ) are independent of location and climate, therefore can be operated almost irrespective of these factors. For low price, high volume products, such as biofuels, algae are commonly cultivated in open ponds. Open pond reactors are significantly cheaper in their construction and operation but have drawbacks like high loss of water through evaporation and lack of temperature control, which lowers biomass productivity. The alternative, preferred for high price, low volume products, such as cosmetic ingredients, is a closed photobioreactor, where process parameters can be precisely controlled, which often leads to higher productivity [ 63 , 68 ]. These bioreactors also enable a three-dimensional mode of cultivation, significantly increasing the productivity per area. In contrast to second-generation biofuels, the third-generation processes completely decouple biofuel production from the need for agricultural land. Additionally, algal-based oil production is likely greater than that in higher plants, as lipids mainly accumulate in specific parts of the plant (e.g., in rape seeds), while in algae, each cell can contain high amount of lipids, making the process more mass efficient. One bottleneck in production is harvesting, as the low size and density of the microalgal cells combined with the sensitivity of the cells to changes in pH render it challenging. [ 66 ]. Furthermore, downstream processing for algal biofuels is commonly more energy intensive than other biofuel productions [ 63 , 69 ]. Araújo and colleagues mapped 447 algae and cyanobacteria Spirulina production units in 2021 in the EU [ 70 ]. Most of these companies directed their biomass to the production of food, feed, and related uses; commercial application of biofuels only had a very small share. Further technological developments in upscaling and reduction of production costs are necessary for commercialization.

This image showcases the open algae cultivation systems located at Technical University of Munich, Ottobrunn.

https://doi.org/10.1371/journal.pbio.3002063.g002

Fourth-generation biofuels

The latest biofuel generation, termed fourth-generation biofuels, encompasses the use of genetic engineering to increase desired traits of organisms used in biofuel production. This applies to a variety of traits from utilizing multiple types of sugars (e.g., pentoses and hexoses), to higher lipid synthesis or increased photosynthesis and carbon fixation. For model organisms, such as Escherichia coli and Saccharomyces cerevisiae , a wide variety of tools for genetically engineering the regulation of endogenous pathways or inserting new pathways are reported. Unfortunately, for most native producers of biofuels, the genetic engineering toolbox is far more limited.

Currently, two different approaches have been adopted: engineering of pathways in native producers (optimizing growth rates, utilization of different carbon sources, directing the metabolic flux toward biofuel production and increased production titers) and reconstruction of pathways identified in natural producers in more genetically accessible model organisms. A wide variety of microorganisms can be used as heterologous hosts for the production of biofuels, including bacteria, yeast, and algae. Their metabolic versatility enables the use of various substrates to produce a wide range of biofuels. For example, butanol pathway genes from Clostridia were introduced into E . coli , Pseudomonas putida , and Bacillus subtilis strains [ 14 , 16 , 19 ]. While the introduction of heterologous genes is well established, a major challenge is the disruption of competing metabolic fluxes. Another obstacle for high product titers can be toxicity of large amounts of product on the cell. To enable increased accumulation of biofuels, the cellular stress response can be modified through genetic engineering, for example, with cell membrane modifications. Through the overexpression of certain membrane transporters, biofuel molecules can be secreted into the medium thereby circumventing accumulation as well as toxicity while simultaneously simplifying product recovery. In E . coli , membrane transporters have been used successfully to excrete n-alkanes, such as n-octane [ 71 , 72 ]. However, the overexpression of transporters is challenging as it modifies the membrane composition, creating a metabolic burden as well as potentially overloading the cellular import and export, thereby disabling the cells ability to regulate its internal environment/homeostasis [ 71 ].

Genetically modified algae can offer higher product yields and a variety of other improvements compared to wild-type algae. In order to enhance photosynthetic efficiency, the antennae systems of algae capable of absorbing a broader range of the light spectrum could be transferred to more suitable production organisms [ 44 , 73 ]. With respect to genetic engineering, CRIPSR/Cas9 is a frequently used tool, as it offers a simple design with efficient transfection and targeted gene disruption [ 74 ].

In fourth-generation biofuel processes that focus on genetically optimized cyanobacteria, the production of ethanol, as well as other fuel products such as butanol, isobutanol, and modified fatty acids have been realized successfully [ 75 , 76 ]. While 1-butanol production reached titers of 300 mg/L, bioethanol titers of up to 5.5 g/L were reported [ 77 – 79 ].

For the efficient optimization of native producers, systems biology can offer many insights. The availability of whole-genome sequences is essential, as this information allows for the annotation of genes to their respective function and reconstruction of the innate metabolic pathways, which can subsequently be modified. Recent advances have been made in the field of genome sequencing allowing for a more rapid and cost-efficient collection of data [ 19 ], while the gene expression patterns in different growth environments can be analyzed by transcriptomics and protein products identified by proteomics.

With genetic engineering tools, the quantity and quality of biofuels can be controlled and increased but will need political acceptance and support to be widely adopted [ 5 ]. There is a controversial debate around genetic engineering in agriculture and medicine, especially in Europe; therefore, similar concerns can be anticipated surrounding the use in biofuel production. A European-based study came to the conclusion that genetically engineered algae for biofuel production would be accepted by the majority of consumers, when the safety of the systems can be guaranteed [ 80 ]. However, with proper containment methods and carefully selected locations, such risks could be drastically minimized. Therefore, closed production systems with high security standards are expected to be built [ 80 ]. Additional biocontainment methods can be directly based on genetic changes inside the production cells such as auxotrophies or kill switches, significantly decreasing the risk of genetically modified organism (GMO) escape [ 44 , 81 ].

One alternative to targeted genetic engineering is random mutagenesis, which can be described as accelerated evolution. Microorganisms and products generated by this approach are not subjected to GMO regulations. Furthermore, this technique can be performed with little knowledge about the production organism and production pathway. Random mutagenesis can be achieved by a variety of methods such as UV light, chemical agents, or fast neutron irradiation. For the first time, the latter was applied on C . oleaginosus , resulting in mutants with elevated lipid titers suitable for biodiesel applications. It is noteworthy that biodiesel from prominent oleaginous yeast platforms, such as Yarrowia lipolytica , C . oleaginosus , Rhodosporidium toruloides , and Lipomyces starkeyi , are compliant with international biodiesel standards, including US ASTM D6751 and EU standard EN 14214 [ 82 , 83 ].

A new, more experimental approach to fourth-generation biofuels is the production of electrobiofuels. These are based on the approach to establish new-to-nature hybrid systems, which are able to use renewable electricity and carbon sources directly for the production of commodity chemicals and biofuels, thereby enabling the conversion of solar energy into storable liquid fuel. Such a process could combine the higher photon efficiency of modern photovoltaic systems (compared to photosynthesis) with the sustainability of biofuel production, increasing overall process effectiveness [ 84 ].

Economics of biofuels in transportation

Apart from reducing GHG emissions and air pollution, biofuel industries can contribute to energy security on a local and national scale, as it is not reliant on local reservoirs of fossil oil. Additionally, the creation of new employment and economic growth, especially in rural locations, should positively impact the social environment as well. However, to fully exploit all the positive traits of biofuels, further research and investments are necessary, as the production of biofuels requires more processing steps compared with the conventional methods of drilling into the ground to obtain crude oil, followed by refining. Therefore, at present, biofuels commonly exceed fossil fuel production costs. Furthermore, raw materials for biofuel production do not compare to crude oil in energy density, requiring far greater amounts of biomass for the same energy output compared to fossil sources. The infrastructure required for the sector of biofuel production has to be extensively developed as well. One example is the primary energy needed to run the process, which should be obtained through sustainable operations. Candidates for that include solar and wind energy among others. Thus, by reducing the overall production cost and increasing process efficiency, biofuels could become more competitive to fossil fuels. Furthermore, by-products of biofuel production should be efficiently utilized in a circular economy, which could increase cost efficiency of such processes.

Transportation is one of the most socioeconomically sensitive sectors for the use of liquid biofuels ( Fig 3 ). It contributes about 17% of global CO 2 emissions [ 85 , 86 ], and so far, sustainable solutions are not fully developed. Due to their limitations, current technologies for biofuels are not likely to completely replace fossil fuels in their entirety but can offer new routes for waste stream valorization in a circular economy and contribute significantly to minimize our dependency on fossil fuels one step at a time. A complementary approach to this goal is electric cars, which have zero tailpipe emissions, although CO 2 emissions are associated with the production of the car and the source of the electricity. Essential in electric vehicle batteries are metals like lithium, cobalt, nickel, and manganese. The demand for these metals is surging, while at the same time toxic waste electronics are accumulating all over the world. Traditional recycling/extraction methods require high temperatures and strong acids. This is a high energy process involving toxic chemicals. One alternative is bioleaching or biomining, which employs microbes such as Acidithiobacillus ferrooxidans that can bind and recover metals, bypassing the need for high temperatures and toxic chemicals [ 87 – 90 ]. This emerging technology offers an eco-friendly approach to recycling but still requires extensive research and development. Additionally, a new infrastructure must be put into place, supporting millions of electric cars at the same time. To that point, a combination of synthetic and biofuels in synergy with electric cars might be an optimal solution for the years to come, partially substituting fossil fuels, thereby drastically reducing CO 2 output of transportation.

The transport sector, specifically, results in 17% of emissions. Adapted from Ritchie and colleagues (2020), Carbon Leadership Forum 2020 [ 85 , 86 ].

https://doi.org/10.1371/journal.pbio.3002063.g003

EU policy recommendations

In order to promote the use of clean and sustainable energy at the industrial, retail, and consumer level, a cohesive framework of policies is imperative. The European Commission and European Environment Agency (EEA) have cooperated with the International Renewable Energy Agency (IRENA) and the Intergovernmental Panel on Climate Change (IPCC) in leading the efforts for clean energy transition through a number of directives and legislations since the 1990s [ 91 – 94 ]. These efforts manifest as a commitment by EU countries to lower GHG emissions and increase the use of renewable energy. Most notable is the Renewable Energy Directive (RED), which came into force in 2009. Through this directive, EU countries set targets for renewable energy consumption, including a subtarget mandating 10% of energy used in transport to be produced from renewables. It is noteworthy that the deployment of renewable energy has continuously grown, exceeding 22% in 2020 [ 92 ]. The legislation also mandates GHG reduction targets for fuel suppliers, requiring a reduction in GHG intensity of the fuel mix by 6% in 2020 [ 92 ]. In 2018, the commission revised the legislative proposal and the European Parliament and the EU Council proposed amendments as RED II. Therewith, the EU aims to increase the share of renewable energy to 32% and in transport to at least 14%, including a minimum share of 3.5% of advanced biofuels (second- and third-generation biofuels). The latter streamlines waste residues, such as agricultural waste (e.g., straw), and also encompasses renewable electricity in road and rail transport [ 95 ].

At present, the industrial biofuel production is dominated by first- and second-generation processes, respectively. Nevertheless, RED II and indirect land use change (ILUC) proposals have initiated the gradual shift toward second- and third-generation processes, which are associated with significant changes in feedstock supply and logistics, as well as technology deployment (e.g., market penetration of advanced biofuels). ILUC qualifies first-generation biofuels based on the unintended consequences of releasing carbon emissions as a result of land use changes [ 96 , 97 ]. While technical process development for third- and fourth-generation biofuels is advancing rapidly in academic and start-up settings, large-scale industrial implementation remains lagging. This indicates a profound gap in transferring technologies from a pilot scale (TRL 5) to an industrial scale (TRL 8). To that end, clear and implementable criteria remain to be addressed by legislators for industrial technology transition toward advanced biofuels with a notable climate impact. Table 2 summarizes our policy recommendations aimed at advancing biofuels implementation as well as their respective expected results and acting entity.

https://doi.org/10.1371/journal.pbio.3002063.t002

First and foremost, legislators need to create stable policies and regulatory frameworks based on measurable cradle-to-cradle sustainability performance indicators. In the past, one of the greatest barriers for industry to adopt new biofuel technologies, at least in the EU, was the constantly changing regulatory and provisions framework, which ultimately led to waves of market and company consolidation for first-generation fuels such as crop-based biodiesel, corn and sugar beet-based bioethanol, and, more recently, corn-based biogas products. Therefore, it is of the utmost importance that policy makers provide clearly formulated, long-term stable policies, provisions, and regulatory frameworks to allow industrial transition to advanced biofuel technologies with clear climate impact.

With respect to sustainability, measurable criteria can be categorized as agriculture biomass, forest biomass with respect to biodiversity, and carbon stocks and emissions. Biofuel ILUC factors could be included in the biannual reports of fuel suppliers and EU countries. Accordingly, biofuel produced from palm oil and soy should carry a high ILUC factor and phasing out these feedstocks could be achieved by encouraging the diversification of feedstock. Reports estimate that 130,000 to 210,000 hectares of deforestation, which has detrimental effect on biodiversity and soil quality, could be avoided by limiting the demand of EU countries for palm oil biofuels [ 98 ]. Land requirement and fresh water use, carbon trading, and carbon offsets should also be factored in upcoming legislations. The criteria should also include GHG emissions that take the levels of methane, nitric oxides, and sulfur oxides into account in addition to levels of CO 2 . Legislation criteria should also take into consideration end-use performance, whereby industry sector, energy efficiency, and socioeconomic impact could represent qualifying measures. Risk determination and possible exceptions could be evaluated for specific industries, such as security and electricity. With respect to energy efficiency, it should be considered that distinct biofuels differ in their output. For example, ethanol yields 25% more energy than that invested in its production, while biodiesel yields 93% more [ 99 ]. To that end, performance-based renewable energy policies are needed. Finally, a reliable system that verifies compliance and reporting is eminent to putting these proposals into practice. In that respect, a mass balance system that observes the global carbon inventory and defines optimal distribution of energy profiles (first to fourth generation) and mixtures (e.g., E10 petrol/ethanol) to ensure minimal climate impact is in order. This system could integrate a range of parameters, including flexible distribution channels, demand management, storage, and price signals in real time [ 97 , 100 ]. Independent auditing services could further ensure compliance, which could also be extended to trading partners of the EU countries at a later stage.

As the implementation of industrial biofuel production sites are associated with immense capital investments, it is crucial to shed light on the financial aspect linked to these policies, primarily, multilevel incentives schemes, investment risk reduction, and infrastructure and logistics. On an EU level, specific funding mechanisms such as European Innovation Council (EIC) pathfinder, EIC Transition, and EIC Accelerator that aim to enable and accelerate the scaling trajectory of new technologies toward market entry already exist. While this is an initial step toward implementing new biofuel technologies, these measures do not translate into national actions and legislation on a member state level, which impedes the regional mobilization of capital, leading to a slow uptake and implementation of new technologies. Hence, a significant step toward rapid technology adoption and implementation would be the regional implementation of funding and capital mobilization as already practiced on the EU level.

An integral element in promoting advanced biofuels could be incentivizing biofuel processes that show favorable sets of sustainability parameters and end-use performance by a higher cost of CO 2 certificates, which realistically should be in the order of 500 to 1,000 Euros/ton CO 2 . Consolidated long-term measures would also provide companies and investors with valuable tools to calculate return of investment and hence de-risk decision-making for iterative technology transition. To enable more efficient technology transfer from academia toward industrial technology deployment, additional factors need to be considered. To that end, academic projects should receive sequential, stage-gated extended funding periods of 4 to 8 years that commonly go beyond a single governmental administration period. This would allow ideas to be developed toward a proof of concept stage, where they can be translated to spin-outs or industry partners. Governments should incentivize start-up formation derived from academic units using focused funding measures, such as the EXIST funding program in Germany [ 101 ]. As technology development from proof of concept (TRL 2 to 4) in academic settings to pilot plant level often requires time periods exceeding 5 to 7 years, synergistic midterm private funding resources also have to be mobilized. To that end, technology familiarity, better understanding of time frames for solid technology development, and proper risk assessment are essential for private capital investors. In order to motivate private capital in the EU to accept development risks and extended time frames for return of investment in biofuel start-up companies, governments could implement tax write-offs for spent risk capital. This legislatively guided de-risking of capital investment into new technologies is already implemented in the United States of America and the United Kingdom, as well as in other, less compliance-driven, financial markets. Hence, the EU has to rapidly implement such legislative tax reliefs to secure innovation on the biofuels and other innovation and sustainability-driven sectors for added economic value and a vibrant job sector.

Capital is also short at the infrastructure and logistics level. Investments are required to construct dedicated pilot plants that allow industrial scale validation and optimization of new technologies, independent of any large-scale industrial partner. In that respect, multiple regionally decentralized pilot plants could provide dedicated instrumental parks that house state of the art fermentation and downstream processing equipment. In the case of gas fermentation, these parks could be associated with significant security measures and demand special regulatory approval and regular inspection. Accordingly, construction and operation by large national research organizations, such as Fraunhofer institutions in Germany, or private–public partnerships is recommended. Governmentally driven funding actions that enable access and use of these pilot plant facilities by innovators in the biofuels sector could further accelerate industrial deployment and market entry. In parallel to technology market readiness, the implementation of biofuels in industrial processes requires a secured feedstock supply.

Contrary to Nordic countries that are the forefront of advanced biofuel processes development, most industrialized countries in the EU with a high population density do not have sufficient land or biomass availability for large-scale biofuel production [ 100 ]. Hence, the location and feedstock supply require strategic positioning. Two routes for biofuels production are viable in the EU: a large production plant located in a region with abundant, long-term feedstock/biomass supply or secured trade routes; or a network of smaller, decentralized production facilities. In the latter case, a farm-integrated production facility with secured access to local residue streams can be envisioned. To optimize the economics of the production facilities, its location should be leveraged with maximal carbon credits in order to meet fuel market prices. To make an informed decision on the location and mode of production, a global carbon inventory map would be extremely beneficial. While we have a good overview of regional carbon emissions, there is little information on correlative carbon storage, which is mostly limited to terrestrial biomass. To that end, other carbon storage mechanisms should be considered, such as existing geological carbon (CO 2 ) capture activities and marine biomass. Considering that 68% of the world population is projected to live in urban areas by 2050, it is sensible to consider urban waste streams, such as sewage sludge and food waste, as yet underutilized biomass feedstocks for biofuel production processes [ 102 ]. More generally, a map of the carbon flux resolved on a country-specific level would enable a more informed decision on the selection of process feedstock (biomass residues/CO 2 ) and trading partners that could secure operation of large-scale production facilities for third- and fourth-generation biofuels. Currently, the major trading partners of the EU are Argentina, Brazil, USA, Indonesia, and Malaysia [ 97 ]. These trading practices do not ensure level field sustainability over the long term. To that end, future trading legislation should consider balanced trade between the global North and global South to ensure long-term beneficial socioeconomic impact on the stability and sustainability of feedstock and biofuel production.

Conclusions

In this Essay, we laid out the reasoning for biofuel production as immediate and long-term measures to limit and eliminate energy and mobility-related GHG emissions. In that regard, biofuels will not be the only solution but an essential building block in a network with other physical (i.e., wind power, photovoltaic systems [ 103 – 105 ]) and chemical technologies (i.e., Sabatier process, Power to X [ 106 , 107 ]) that together can provide carbon neutral or even carbon negative energy and mobility solutions. In regard to transportation, biofuels should act in synergy with other technologies, such as electrified vehicles. In addition to biofuel manufacturing, similar processes could also be implemented in other applications. Here, algal and yeast oil can be transformed into building materials such as carbon fibers and cement additives. Via these routes, atmospheric CO 2 can be absorbed from the environment and stored for very long periods of time. Such technologies could complement materials derived from fossil fuels or that generate large amounts of CO 2 during the manufacturing process (e.g., steel, aluminum and concrete) [ 108 ].

We are convinced that, in the last decades, mankind has been generally too hesitant to adopt climate-centered technologies, which has put the world on a perilous pathway toward catastrophic climate change [ 109 – 111 ]. The destructive outcomes of this scenario have been documented in the scientific literature and are subject to numerous high level reports [ 112 – 117 ].

As time for action is already overdue, it is essential to act now by implementing the tools and technologies we have at hand at the present time. It is our opinion, that the only path to enable climate effective energy security and mobility is to deploy available technologies at a global scale right now. The global implementation of large-scale production infrastructure for sustainable (bio)technologies to kick-start production of renewable energy carriers and sustainable commodities is imperative in this timely development scenario. Once production with a base process has commenced, these processes can be iteratively refined or modulated at scale to evolve toward the next technology generation. This approach demands close, long-term academic and industry partnerships.

This fundamental transition toward sustainable bio-based technologies will require long-sighted, fact-driven legislative guidance and immense capital investments across the private and governmental sectors. However, it will be the only route to limit climate change effects and provide a livelihood for future societies.

With respect to governments, this means that neither ideology nor demagogically driven decision-making will protect any society from the effects of climate change. There are just no simple answers to complex, global problems. What is needed are global governmental alliances that make technocratically oriented long-sighted decisions, aiming for definitively set climate-centered outcomes even if the communication of the measures that have to be taken may not be popular on first sight.

Even outside the scientific communities, people are ready to accept change of the status quo in order to curb climate change effects and transition to a sustainable society. The question remains if the global political elites are ready to communicate and implement this change. Time is running out to maintain the global ecosystems as we know it.

Acknowledgments

The authors dedicate this manuscript to Dr. Christian Patermann (former EU Program Director Biotechnology, Agriculture, and Food) and Dr. Günther von Au (Chairman of the Board of Directors of Clariant AG), each being outstanding political and industrial visionaries, influencers, and decision-makers in the field of sustainable (bio)technologies and the bioeconomy, respectively.

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Inexpensive, carbon-neutral biofuels are finally possible

When it comes to making fuel from plants, the first step has always been the hardest — breaking down the plant matter. A new study finds that introducing a simple, renewable chemical to the pretreatment step can finally make next-generation biofuel production both cost-effective and carbon neutral.

A young Asian researcher in a lab in a blue lab coat, arms crossed

For biofuels to compete with petroleum, biorefinery operations must be designed to better utilize lignin.

Lignin is one of the main components of plant cell walls. It provides plants with greater structural integrity and resiliency from microbial attacks. However, these natural properties of lignin also make it difficult to extract and utilize from the plant matter, also known as biomass.

“Lignin utilization is the gateway to making what you want out of biomass in the most economical and environmentally friendly way possible,” said UC Riverside associate research professor Charles Cai. “Designing a process that can better utilize both the lignin and sugars found in biomass is one of the most exciting technical challenges in this field.”

To overcome the lignin hurdle, Cai invented CELF, which stands for co-solvent enhanced lignocellulosic fractionation. It is an innovative biomass pretreatment technology.

“CELF uses tetrahydrofuran or THF to supplement water and dilute acid during biomass pretreatment. It improves overall efficiency and adds lignin extraction capabilities,” Cai said. “Best of all, THF itself can be made from biomass sugars.”

A landmark Energy & Environmental Science paper details the degree to which a CELF biorefinery offers economic and environmental benefits over both petroleum-based fuels and earlier biofuel production methods.

Rows of mostly bare poplar trees against a blue sky

The paper is a collaboration between Cai’s research team at UC Riverside, the Center for Bioenergy Innovation managed by Oak Ridge National Laboratories, and the National Renewable Energy Laboratory, with funding provided by the U.S. Department of Energy’s Office of Science. In it, the researchers consider two main variables: what kind of biomass is most ideal and what to do with the lignin once it’s been extracted.

First-generation biofuel operations use food crops like corn, soy, and sugarcane as raw materials, or feedstocks. Because these feedstocks divert land and water away from food production, using them for biofuels is not ideal.

Second-generation operations use non-edible plant biomass as feedstocks. An example of biomass feedstocks includes wood residues from milling operations, sugarcane bagasse, or corn stover, all of which are abundant low-cost byproducts of forestry and agricultural operations.

According to the Department of Energy , up to a billion tons per year of biomass could be made available for the manufacture of biofuels and bioproducts in the US alone, capable of displacing 30 percent of our petroleum consumption while also creating new domestic jobs.

Because a CELF biorefinery can more fully utilize plant matter than earlier second-generation methods, the researchers found that a heavier, denser feedstock like hardwood poplar is preferable over less carbon-dense corn stover for yielding greater economic and environmental benefits.

Using poplar in a CELF biorefinery, the researchers demonstrate that sustainable aviation fuel could be made at a break-even price as low as $3.15 per gallon of gasoline equivalent. The current average cost for a gallon of jet fuel in the U.S. is $5.96.

A young Asian male researcher in a blue lab jacket next to a 20-gallon CELF reactor indoors

The U.S. government issues credits for biofuel production in the form of renewable identification number credits, a subsidy meant to bolster domestic biofuel production. The tier of these credits issued for second-generation biofuels, the D3 tier, is typically traded at $1 per gallon or higher. At this price per credit, the paper demonstrates that one can expect a rate of return of over 20 percent from the operation.

“Spending a little more for a more carbon-rich feedstock like poplar still yields more economic benefits than a cheaper feedstock like corn stover, because you can make more fuel and chemicals from it,” Cai said.

The paper also illustrates how lignin utilization can positively contribute to overall biorefinery economics while keeping the carbon footprint as low as possible. In older biorefinery models, where biomass is cooked in water and acid, the lignin is mostly unusable for more than its heating value.

“The older models would elect to burn the lignin to supplement heat and energy for these biorefineries because they could mostly only leverage the sugars in the biomass - a costly proposition that leaves a lot of value off the table,” said Cai.

In addition to better lignin utilization, the CELF biorefinery model also proposes to produce renewable chemicals. These chemicals could be used as building blocks for bioplastics and food and drink flavoring compounds. These chemicals take up some of the carbon in the plant biomass that would not get released back into the atmosphere as CO2.

“Adding THF helps reduce the energy cost of pretreatment and helps isolate lignin, so you wouldn’t have to burn it anymore. On top of that, we can make renewable chemicals that help us achieve a near-zero global warming potential,” Cai said. “I think this moves the needle from Gen 2 biofuels to Gen 2+.”

In light of the team’s recent successes, the Department of Energy’s Bioenergy Technology Office has awarded the researchers a $2 million grant to build a small-scale CELF pilot plant at UC Riverside. Cai hopes that demonstrating the pilot plant will lead to larger-scale investment in the technology, as harnessing energy from fossil fuels adds to global warming and hurts the planet.

“I began this work more than a decade ago because I wanted to make an impact. I wanted to find a viable alternative to fossil fuels and my colleagues and I have done that,” Cai said. “Using CELF, we have shown it is possible to create cost-effective fuels from biomass and lignin and help curb our contribution of carbon emissions into the atmosphere.”

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Review Article
Published: 25 June 2021

Microbial production of advanced biofuels

Jay Keasling 1 , 2 , 3 , 4 , 5 , 6 ,
Hector Garcia Martin ORCID: orcid.org/0000-0002-4556-9685 1 , 4 , 7 , 8 , 9 ,
Taek Soon Lee 1 , 4 ,
Aindrila Mukhopadhyay ORCID: orcid.org/0000-0002-6513-7425 1 , 4 , 9 ,
Steven W. Singer 1 , 4 &
Eric Sundstrom 4 , 10

Nature Reviews Microbiology volume 19 , pages 701–715 ( 2021 ) Cite this article

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Applied microbiology
Metabolic engineering

Concerns over climate change have necessitated a rethinking of our transportation infrastructure. One possible alternative to carbon-polluting fossil fuels is biofuels produced by engineered microorganisms that use a renewable carbon source. Two biofuels, ethanol and biodiesel, have made inroads in displacing petroleum-based fuels, but their uptake has been limited by the amounts that can be used in conventional engines and by their cost. Advanced biofuels that mimic petroleum-based fuels are not limited by the amounts that can be used in existing transportation infrastructure but have had limited uptake due to costs. In this Review, we discuss engineering metabolic pathways to produce advanced biofuels, challenges with substrate and product toxicity with regard to host microorganisms and methods to engineer tolerance, and the use of functional genomics and machine learning approaches to produce advanced biofuels and prospects for reducing their costs.

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Acknowledgements

The authors thank C. Scown (Lawrence Berkeley National Laboratory) for helpful discussions on life cycle and technoeconomic analyses of biofuel production. This work was performed as part of the US Department of Energy (DOE) Joint BioEnergy Institute ( https://www.jbei.org ) supported by the DOE, Office of Science, Office of Biological and Environmental Research, and by the DOE, Energy Efficiency and Renewable Energy, Bioenergy Technologies Office, and as part of the Co-Optimization of Fuels & Engines project sponsored by the DOE, Office of Energy Efficiency and Renewable Energy, Bioenergy Technologies Office and Vehicle Technologies Office, under contract DEAC02-05CH11231 between the DOE and Lawrence Berkeley National Laboratory. The views and opinions of the authors expressed herein do not necessarily state or reflect those of the US Government or any agency thereof. Neither the US Government nor any agency thereof, nor any of their employees, makes any warranty, expressed or implied, or assumes any legal liability or responsibility for the accuracy, completeness or usefulness of any information, apparatus, product or process disclosed, or represents that its use would not infringe privately owned rights. The US Government retains and the publisher, by accepting the article for publication, acknowledges that the US Government retains a non-exclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of the manuscript, or allow others to do so, for US Government purposes. The DOE will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan ( http://energy.gov/downloads/doe-public-access-plan ).

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A standard measure of an engine or aviation fuel capability against compression.

An indicator of the combustion speed of diesel fuel and compression needed for ignition.

(ILs). A highly efficient set of reagents for the depolymerization and deconstruction of a range of feedstocks.

One-carbon microbial substrates, including CO 2 , CH 4 , CO, HCO 2 − and CH 3 OH.

A mixture of CO, CO 2 and H 2 .

Single fuel components that are blended with additional components to produce a finished fuel.

Modelling approach that uses a polynomial of up to grade 2 to predict the response.

Applied statistics techniques that deal with planning, conducting, analysing and interpreting controlled tests to evaluate the factors that control the experimental output under study.

Modelling approach that takes the input of various different models and has them ‘vote’ for a particular prediction.

The exhaust gas stream exiting a bioreactor.

Development of the genetic tools necessary to allow metabolic engineering of a previously unengineered microorganism.

The degree to which a fuel mixture generates black carbon soot when combusted.

Petrol containing 10% ethanol by volume.

Fuel viscosity at low temperature; poor cold flow can lead to gelling and compromise engine operability in cold weather conditions.

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Keasling, J., Garcia Martin, H., Lee, T.S. et al. Microbial production of advanced biofuels. Nat Rev Microbiol 19 , 701–715 (2021). https://doi.org/10.1038/s41579-021-00577-w

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Biofuels production: a review on sustainable alternatives to traditional fuels and energy sources.

1. Introduction

2. biomass energy.

Energy crop-based: Biomass from different energy crops such as herbaceous crops, woody, industrial crops, agricultural and aquatic crops are used for energy purposes.
Agricultural residues and waste: Agriwaste (rice and wheat straw) and animal farm waste.
Forestry waste and residues: Various mill wood waste, logging residues, trees and shrub residues etc.
Industrial and municipal wastes: Municipal solid waste (MSW), sewage sludge and different industrial waste.

3. Types of Biofuels

3.1. conventional/traditional biofuels, 3.2. advanced biofuels.

Enzymatic dry milling
Dry fractionation
Ammoniation process in the wet mill
Continuous membrane reactor for starch hydrolysis
High-gravity fermentation
Improved and efficient yeast (recombinant DNA techniques)
Conversion of pentose and hexoses sugars to ethanol
New enzymes for liquefaction and saccharification
Optimized systems/conditions

4. Biofuels Generations/Classifications

4.1. first-generation biofuels, 4.2. second-generation (2g) biofuels, 4.3. third generations of biofuels, 4.4. fourth generations of biofuels, 5. biomass conversion technologies for biofuel production.

Briquetting
Pelletization
Agrochemical route/processes
Combustion in excess air
Carbonization
Very little air/no oxygen added
750 °F to 1500 °F
Some air/oxygen used but less than for incineration
Begins at 1300 °F
Bacteria breaks down feedstock
Anaerobic process
Microbes used to produce ethanol

6. Comparison between Biofuels and Fossil Fuels

7. future prospective for biofuels production, 8. national policy on biofuels.

To meet the energy needs of India’s rural population and create employment opportunities;
To address global concerns by tightening automotive vehicle emission standards to curb air pollution;
To reduce the dependence on the import of fossil fuels, provide a higher degree of national energy security;
To derive biofuels from non-edible feedstock on degraded soils or wastelands unsuited to agriculture, avoiding a possible conflict between food and fuel.

9. Major Challenges in Biofuels Production

10. conclusions, author contributions, data availability statement, acknowledgments, conflicts of interest.

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Click here to enlarge figure

Classifications	Product	Feedstock
First generation (food feedstocks)	Ethanol	Corn, sugar beet, sugarcane, cereals, sorghum, grains etc.
	Biodiesel	Rapeseed, palm oil, soybean, waste oils, animal fats
Second generation (biomass)	Cellulosic ethanol	Switch grass, wheat straw, jatropha, miscanthus, corn Stover, stalks, stubbles, leaves, seed pods, rice and wheat straw, sugarcane bagasse, fruit and vegetable wastes
Third generation	Bioethanol/ethanol	Algae
Fourth generation	Biodiesel/bioethanol	“Drop in” biofuels, genetically modified crops for biofuels, renewable solar fuel

Type of Biofuel	Product	Feedstock	Conversion Process/Technologies
Biodiesel	Biodiesel (methyl and ethyl esters of fatty acids) derived from energy crops	Oil crops (soybean, sunflower, rapeseed, palm, etc.)	Cold and warm pressing, drying, extraction, transesterification, purification,
	Biodiesel from organic waste materials	Waste oil, cooking/frying oil	Hydrogenation
Bioethanol	Conventional ethanol	Sugar beet, sugarcane	Direct fermentation of juice
	Starchy ethanol	Corn, wheat and other grains	Enzymatic hydrolysis, fermentation

Substrate(s)	Fermentation Mode
Bioethanol
Corn, potato, cassava, sorghum, grains, fruit and vegetables waste, sweet potato, sugar cane, sugar beets, etc.	Simultaneous saccharification and fermentation (SSF), separated hydrolysis and fermentation (SHF)
Lignocellulose materials, such as wheat and rice straw, corn stover, corn cobs, switchgrass, hardwood, sugarcane bagasse, etc.	Simultaneous saccharification and fermentation (SSF), simultaneous saccharification and fermentation with prehydrolysis time (PTSSF)
Alkaline-treated sugarcane bagasse	Consolidated bioprocessing (CBP)
Biohydrogen
Agriculture residue (sugarcane bagasse, rice straw, leaves, etc.)	Dark and photofermentation
Fruits and vegetables waste	Photofermentation
Algae (macro and micro), cyanobacteria	Biophotolysis

Types of Biofuels	Biomass/Substrate	Conversion Process/ Technologies	Applications
Bioethanol	Corn, potatoes, sugarcane, sugar beet crops, Wheat, barley sorghum grains, Poplar and wood chips, agricultural waste (corn, sorghum, oat, barley, wheat, soybean, cotton, bagasse, and rice straws) and energy crops (hybrid sorghum, energy cane, miscanthus, switchgrass, eucalyptus, and pine)	Pre-treatment, Liquefaction, Enzymatic hydrolysis, saccharification and fermentation	Drop in fuel, blending for gasoline engine, alcohol to jet for aviation
Biodiesel	Rapeseed, sunflower, palm, soybean, canola, jatropha oil, algae, etc.	Pressing, Cultivation, Harvesting, extraction, purification and trans-esterification	Drop in fuel, blending for diesel engine
Biogas	Dung, agriwaste, sewage water and sludge, municipal solid waste, organic waste, industrial waste, etc.	Anaerobic digestion (AD)	Blended with natural gas for transportation, cooking
Biohydrogen	Lignocellulosic materials, bio-waste, macro and micro algae, industrial waste, etc.	Dark and photo fermentation, biophotolysis	Internal combustion engines

Characteristics	Biofuels	Fossil Fuels
Type	Renewable	Non-renewable
Impact on health	Nontoxic	Toxic ingredients, chemicals and by-products
State of industry	Growing	declining
Energy production	Provides a low amount of energy per unit biomass	Provides a high amount of energy per unit mass
Environmentally friendly	yes	no
production methods	Safer	unsafe
CO neutral process	Release net zero carbon dioxide	Increase the concentration of carbon dioxide
Sustainability	yes	no
Examples	Bioethanol, biodiesel, methanol, biobutanol, biogas, biohydrogen	Gasoline, ethane, diesel, methane, butane

The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

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Malik, K.; Capareda, S.C.; Kamboj, B.R.; Malik, S.; Singh, K.; Arya, S.; Bishnoi, D.K. Biofuels Production: A Review on Sustainable Alternatives to Traditional Fuels and Energy Sources. Fuels 2024 , 5 , 157-175. https://doi.org/10.3390/fuels5020010

Malik K, Capareda SC, Kamboj BR, Malik S, Singh K, Arya S, Bishnoi DK. Biofuels Production: A Review on Sustainable Alternatives to Traditional Fuels and Energy Sources. Fuels . 2024; 5(2):157-175. https://doi.org/10.3390/fuels5020010

Malik, Kamla, Sergio C. Capareda, Baldev Raj Kamboj, Shweta Malik, Karmal Singh, Sandeep Arya, and Dalip Kumar Bishnoi. 2024. "Biofuels Production: A Review on Sustainable Alternatives to Traditional Fuels and Energy Sources" Fuels 5, no. 2: 157-175. https://doi.org/10.3390/fuels5020010

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Argonne National Laboratory

Biofuel on the road to energy, cost savings.

Semi-truck on highway, mountains, water, sun in background. (Image by Shutterstock/Vitpho.)

Biofuel is closer to becoming a cost-competitive, climate-friendly solution for slashing carbon emissions in cars and trucks, according to two new studies.

The U.S. Department of Energy’s ( DOE ) Argonne National Laboratory collaborated with the DOE ’s National Renewable Energy Laboratory ( NREL ), Pacific Northwest National Laboratory ( PNNL ) and Idaho National Laboratory ( INL ) on the research. Results showed that biofuel combined with advanced engine design can reduce greenhouse gas ( GHG ) emissions by roughly 60% while improving fuel efficiency or reducing tailpipe emissions.

Argonne energy system analyst Pahola Thathiana Benavides, NREL process engineer Andrew W. Bartling and PNNL engineer Steven Phillips were lead analysts for the two studies published in ACS Sustainable Chemistry & Engineering.

“ The idea is to develop new biofuels blended with conventional fuels to improve engine performance. This means a gasoline car or truck could go further on the same amount of fuel or a diesel vehicle could meet more stringent emissions standards.” — Troy Hawkins, Argonne’s group manager, Fuels and Products Group

Biofuel has significant advantages over petroleum gasoline. But the engines themselves are also critical to energy efficiency. Designing low-carbon fuels and engines to work together can maximize energy use and vehicle performance.

“ We are at the intersection of new innovations in both engines and biofuel,” said Troy Hawkins, Argonne’s group manager, fuels and products group, an author on both ACS Sustainable Chemistry & Engineeringstudies.“Our goal was to develop new biofuels blended with conventional fuels to improve engine performance. This means a gasoline-powered car or truck could go further on the same amount of fuel. Or a diesel vehicle could meet more stringent emissions standards.”

In both studies, Argonne scientists worked with other national labs to identify promising fuels for different engine types. Researchers considered cost, environmental impact and potential for expanding to commercial markets.

The research is supported by the Co-Optimization of Fuels & Engines (Co-Optima) initiative jointly led by DOE ’s Office of Energy Efficiency and Renewable Energy, Bioenergy Technologies Office and Vehicle Technologies Office.

Argonne is part of Co-Optima’s consortium of nine national laboratories and over 20 university and industry partners. The consortium studies how simultaneous innovations in fuel and engines can boost fuel economy and vehicle performance while reducing emissions.

Scientists and experts at every DOE laboratory played an important role in each phase of the research, Hawkins said.

“ This research is a really good example of how laboratories can work together to help the DOE accomplish its mission,” Hawkins said.

Finding biofuel pathways

Co-Optima’s research builds on the goal to identify and understand bioblendstocks, or biofuel. Biofuel is produced from biomass — organic materials including plants, agricultural waste and wet waste. Biofuel can be blended with conventional fuel to reduce emissions and improve fuel and engine performance.

Collaborating with Co-Optima fuel experts, researchers used a screening process to develop a list of biofuels for their research, Benavides said.

Argonne scientists developed the list of biofuels working with experts including PNNL technical team manager and Co-Optima Leadership Team member Daniel Gaspar, NREL senior scientist Gina Fioroni, NREL senior research fellow Robert McCormick, and Anthe George, senior manager at DOE ’s Sandia National Laboratories ( SNL ).

“ We worked with other experts to use specific criteria to narrow many biofuel candidates down to a short list for our research. This list was developed based on the required properties and the engine’s combustion mode,” Benavides said.

Converting biomass to biofuel is a complex process involving variables in feedstock, conversion technologies and fuel types. It is especially challenging finding biofuel pathways that also meet economic, technology and energy goals.

One study was co-first-authored by Benavides. The team assessed 12 biofuel production pathways for optimizing multimode internal combustion engines. Multimode engines can deliver greater efficiency and cost savings by using different methods of ignition, combustion and/or fuel-preparation, depending on driving demands.

Researchers used renewable biomass feedstock found in forestry byproducts such as wood waste and agricultural byproducts such as corn stover. They used conversion technologies including either fermentation, catalysis under high heat and pressure, or a combination of both.

“ We found that not only can seven biofuels be produced cost-competitively, but that these seven are varied in terms of feedstock used and conversion technology,” Bartling said. “ This means that biorefineries can be more flexible in choosing where and how to build their facilities.”

NREL and PNNL researchers did a techno-economic assessment of the biofuel production pathways, analyzing cost and technology performance.

“ Our findings showed that many of the biofuels are competitive with the current cost of petroleum fuel,” Phillips said.

Researchers also analyzed environmental impact. A life cycle analysis of the pathways using Argonne’s GREET ® (Greenhouse Gases, Regulated Emissions, and Energy used in Technologies) model showed impressive results. Ten biofuels have the potential to reduce GHG emissions by 60% compared to petroleum gasoline. The list includes alcohols, furan mixtures and olefins.

Biofuel promising for diesel engines

The second study was co-first-authored by Bartling. Researchers analyzed 25 ways of producing biofuel optimized for a type of engine known as mixing-controlled compression ignition. This diesel engine is mainly used in freight transportation.

To develop biofuel production pathways, researchers used feedstocks ranging from plant materials such as wood chips or corn stover, to oils from soybean and cuphea, to wet wastes and recycled grease. They used conversion technologies including fermentation, gasification, and hydrothermal liquefaction.

“ The diverse set of biomass resources available in the U.S. has great potential to replace a portion of fuels and chemicals that now come from petroleum,” said Damon Hartley, INL ’s Operations Research and Analysis Group lead. “ However, one of the largest barriers is the wide variability in quality in the raw materials. This can have a large impact on how the material performs in conversion.”

As with the first study, most of the technologies performed well. Most of the biofuels were cost-competitive with current gas prices.

In terms of environmental impact, GHG emissions were reduced more than 60% in 12 of the 25 pathways, according to the GREET life cycle analysis.

“ We evaluated the life cycle GHG emissions for each mixing-controlled compression ignition engine pathway. This included not only the tailpipe emissions but also upstream emissions resulting from biomass cultivation, feedstock transportation, biofuel production and biofuel distribution,” Hawkins said.

Creating a biofuel playbook

Researchers did not intend to produce a definitive list of biofuels, Benavides said. Instead, the studies offer a guide for stakeholders on selecting biofuel pathways that best meet their needs.

“ We provide researchers and industry guidance on assessing biofuels based on a number of complex variables,” Benavides said. “ The life cycle and techno-economic analysis is important in guiding stakeholders as early as possible. We can’t tell stakeholders what choices to make but these tools can point them in the right direction from the beginning.”

While many of these biofuel pathways could potentially be cost-competitive, it is too soon to lock in prices in a constantly fluctuating gas market. “ The challenge is providing cost-competitive prices in the long term,” Hawkins said.

While these biofuel production pathways target cars and diesel trucks, Argonne researchers are also studying the potential for using these pathways in hard-to-electrify sectors like aviation and maritime industries. The goal is to bring biofuel to market across a range of industries as quickly as possible.

“ DOE is constantly working on sustainable solutions for decarbonizing the transportation sector. Biofuel is a big piece of that,” Hawkins said. “ We will continue to expand on Co-Optima’s important work.”

Along with Argonne, ORNL , NREL , PNNL , INL , and SNL , other U.S DOE national labs in the Co-Optima Initiative are Los Alamos, Lawrence Berkeley, and Lawrence Livermore national laboratories.

ACS Sustainable Chemistry & Engineering research authors:

“ Identification of key drivers of cost and environmental impact for biomass-derived fuel for advanced multimode engines based on techno-economic and life cycle analysis”: Pahola Thathiana Benavides, Argonne, Andrew W. Bartling, NREL , Steven D. Phillips, PNNL , Troy R. Hawkins, Argonne, Avantika Singh, NREL , George G. Zaimes, Argonne, Matthew Wiatrowski, NREL , Kylee Harris, NREL , Pralhad H. Burli, INL , Damon Hartley, INL , Teresa Alleman, NREL , Gina Fioroni, NREL , Daniel Gaspar, PNL .

“ Environmental, economic, and scalability considerations of selected bio-derived blendstocks for mixing-controlled compression ignition engines”: Andrew W. Bartling, NREL , Pahola Thathiana Benavides, Argonne, Steven D. Phillips, PNNL , Troy Hawkins, Argonne, Avantika Singh, NREL , Matthew Wiatrowski, NREL , Eric C. D. Tan, NREL , Christopher Kinchin, NREL , Longwen Ou, Argonne, Hao Cai, Argonne, Mary Biddy, NREL , Ling Tao, NREL , Andrew Young, NREL , Kathleen Brown, NREL , Shuyun Li, PNL , Yunhua Zhu, PNL , Lesley J. Snowden-Swan, PNL , Chirag R. Mevawala, PNL , Daniel J. Gaspar, PNL .

Argonne National Laboratory seeks solutions to pressing national problems in science and technology by conducting leading-edge basic and applied research in virtually every scientific discipline. Argonne is managed by UChicago Argonne, LLC for the U.S. Department of Energy’s Office of Science.

The U.S. Department of Energy’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, visit https://energy.gov/science .

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Published: 18 May 2017

Biofuel research: perceptions of power and transition

Lena Partzsch 1

Energy, Sustainability and Society volume 7 , Article number: 14 ( 2017 ) Cite this article

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Whether biofuels represent a sustainable innovation, a creative alternative, or a gold rush, very much depends on our perception of power and change with regard to sustainability. This article provides an overview of existing understandings of power in the research on biofuels, including positive perceptions that often lead to more optimistic evaluations of biofuels. It exposes the diversity with which one can understand power through three ideal type concepts: “power with,” “power to,” and “power over”. Integrating these concepts in one power framework allows for examining how the three dimensions interrelate with each other and developing the contours of a power lens on biofuel governance and research. With the 2007–2008 food price crisis, critics re-politicized the governance of biofuels. Several farmer associations have completely turned against biofuels. The article argues that this rejection of biofuels is due to a limited perception of power as a coercion and manipulation (power over). While the current governance of biofuels basically reproduces systems and positions, we should start to more seriously and intensively ask questions of where, when, and how the governance of biofuels may also allow for “green” resistance (power to) and collective empowerment (power with).

Introduction

Whether biofuels represent a sustainable innovation, a creative alternative or a gold rush [ 1 ], very much depends on our perception of power and change with regard to sustainability. This leads to the challenge of how to conceptualize these understandings. I gather diverse perceptions of power and illustrate them for biofuel research. The aim is to initiate a broader, more comprehensive debate across ontological and epistemological differences in this field of research. To begin the discussion, I introduce key components of the debate by identifying different perceptions of power that are common to research on biofuels along three ideal type conceptions:

Power with means collective empowerment through convincing and learning with and from each other. It refers to processes of developing shared values, finding common ground, and generating collective strengths [ 2 ]. Based on this understanding of power, biofuels can potentially be a sustainable innovation that serves the common good (climate protection, energy security, regional development, etc.) (e.g., [ 3 , 4 ]).

Power to corresponds to the ability of agents “to get things done” [ 5 ]. While Pitkin [ 6 ] defines power to as non-relational, Barnett and Duvall [ 7 ] define power to as tied to social relations of constitution that define who the actors are, along with their capacities and practices. Footnote 1 Scholars, who take a perspective of power to, may highlight the agency of producing biofuels as a creative alternative in hitherto fossil fuel-dependent societies (e.g., [ 8 , 9 ]).

Power over describes the direct and indirect ability of powerful actors, structures, and discourses to influence the actions and even the thoughts of others. It is based on power concepts by Dahl [ 10 ], Bachrach and Baratz [ 11 ], and Lukes [ 12 ], among others. I also discuss concepts of discursive power under this category (e.g., [ 13 , 14 ]), while I am aware that these concepts partly fall under the category of power to [ 7 ]. From a perspective of power over, biofuels can be seen as a gold rush: While everybody expected sudden wealth in this new field, there are very few winners and many losers (e.g. [ 15 , 16 ]).

I chose this tripartite approach as a framework for my article, because it is most comprehensive and makes an extension of the power discussion on biofuels possible. At the same time, the framework allows for the discussion of the well-known grouping of the four “faces of power” under the category of power over [ 17 , 18 ]. I will argue that in the research on biofuels, the understandings of power as power with and power to tend to prevail, even when they are not made explicit. This means that scholars have overemphasized the potential of biofuels as a creative alternative to fossil fuels and sustainable innovation for rural development. Concepts of power over have only more recently been applied, specifically since research has started to explicitly issue power. This has, in particular, been used to explain why any process of governing biofuels (biofuel governance) did not lead to urgent sustainability transitions, and why the biofuel boom should rather be seen as a gold rush. Scholars have demonstrated that the development of biofuels markets benefitted large companies and conglomerates [ 19 ]. Critical and post-structuralist perspectives have helped to understand this development by exploring structures and discourses favoring them [ 20 ]. Scholars have used Foucault’s concepts to outline how scientific knowledge practices render the very essence of problems (and solutions) raised on the biofuel agenda [ 21 , 22 ].

This article involves first of all implicit and explicit understandings of power (how do biofuel researchers think and talk about power?). These understandings are expressed in empirical research, as I will demonstrate below, and they hence also allow for an illustration of the practice of biofuel governance (how is power exercised in and through biofuel governance?). This makes the article also relevant for political practice. We should understand, not only in theoretical but also in practical terms, how we effectuate or prevent changes towards a more sustainable supply of energy and transport fuel. As in analytical heuristics, it is not possible to offhand separate power with , power to , and power over in empirical research. These categories shine multiple lights on different aspects of the same empirical phenomena. In practice, these forms of power exercise are mostly interrelated. My less concern is to weigh and compare the pros and cons of each perspective, but rather to outline an agenda for a multidimensional analysis of all three mechanisms of power and their interrelations.

In order to get the full picture of how change happens, we should understand how different perspectives add on to each other (besides overlaps and contradictions). To do this, I will begin by describing each perspective in itself. Based on a survey on biofuel research, I will give references for each perspective. These references are only illustrative. Then, I will exemplify the interrelations between each of these perspectives with respect to biofuel research. I explain how power imbalances can affect processes of power with and power to . Again, scholars have demonstrated how large conglomerates have manipulated biofuel governance in their favor, and why therefore the biofuels boom should be considered as a gold rush. However, I argue that interrelations may also work the other way around, and this is particularly relevant to the main argument of this article. Biofuels as a creative alternative and a sustainable innovation may also provoke changes in existing relations of power over and contribute to address asymmetries and inequalities in agrifood and transport systems. We need a multidimensional power approach to explore these interrelations.

Biofuel: sustainable innovation (power with)

Research on biofuel governance and other studies in the field of sustainability are most often based on a positive perception of power in the sense of power with . Power with is a term that refers to processes of developing shared values, finding common ground, and generating collective strengths [ 2 ]. This conception does not necessarily refer to the diffusion of already existing (predefined) norms. Rather, power with implies learning processes that allow actors to question self-perceptions and to actively build up a new awareness of individuals or groups [ 23 , 24 ]. In this vein, with regard to biofuels, scholars have assumed that collective empowerment and solidarity are possible and that biofuel technologies as a “sustainable innovation” can pave the way to post-carbon societies [ 25 , 26 ].

Power with is often linked to Arendt’s definition of power [ 27 ]. Footnote 2 According to Arendt, power always refers to a group or to a collective of individuals:

Power corresponds to the human ability not just to act but to act in concert. Power is never the property of an individual; it belongs to a group and remains in existence only so long as the group keeps together. When we say of somebody that he is ‘in power’ we actually refer to his being empowered by a certain number of people to act in their name ([ 28 ]: 44). Footnote 3

Research on environmental leadership (e.g., [ 29 ]) in pioneer countries, such as Germany and France in the biofuel sector [ 3 , 30 ], most obviously reflects such an understanding of power. Leaders or pioneers are empowered to act in the name of others from this perspective (while they dominate others from a perspective of power over , see below). In this sense, (Young [ 31 ]: 285) defines leadership in the interest of common welfare:

Leadership (…) refers to the actions of individuals who endeavor to solve or circumvent the collective action problems that plague the efforts of parties seeking to reap joint gains in processes of institutional bargaining.

Leaders and pioneers do not enforce their own interests against or over others; rather they seek “to reap joint gains” of environmentalism. Environmental leadership studies, based on such an understanding of power, usually follow the discourse of Ecological Modernization that highlights flexible and cost-efficient problem solving. Ecological modernization outlines a win-win storyline of environmental protection that benefits green (biofuel) business as much as the environment [ 32 , 33 ]. From this perspective, those who are neither leaders nor pioneers are considered free-riders or laggards , rather than subordinates. Non-leaders also benefit, at least in the long run, from power (with), since biofuels are expected to tackle common problems, such as climate change, enhance energy security, and to contribute to regional development [ 3 , 34 ]. Policies promoting biofuels are hence per se seen to be desirable since, from this perspective, they serve everybody’s interest.

Scholars have extensively analyzed the emergence, diffusion, efficiency, and effectiveness of policies promoting biofuels, with the (at least implicit) aim to foster their adoption and implementation [ 30 , 35 ]. In this context, policy learning and experiments have been gaining momentum [ 9 , 26 ]. Deliberative processes, including third-party certification schemes, were initiated and observed with the aim to introduce sustainable biofuel production schemes that would integrate those formerly excluded stakeholders with new technology; in everyday practice, every actor in the field would then become a winner [ 4 , 36 ].

Scholars who share this perspective of power as power with do not think in dichotomies such as winners - losers or good-bad . Instead, they understand power (or similar concepts, such as leadership) as serving the common good (climate protection, energy security, and sustainability). As there are no subordinates from this power perspective, no imperative follows to empower or to resist. The empowerment of non-leaders is not an issue because scholars assume that, in principle, they are also interested in developing sustainable innovations and that they likewise benefit from respective leadership efforts.

Biofuel: creative alternative and “green” resistance (power to)

While power with pertains to collective empowerment and solidarity, power to refers to single actors and separate groups, such as farmers, co-operatives, and individual processors who were initially key players in pioneering biofuel regions [ 19 ]. Accordingly, biofuels are often seen as an opportunity to empower green ideas and values. Pitkin [ 6 ] emphasizes how power can be non-relational, since an actor may have the power to accomplish something all by him- or herself. This understanding of power is related to the development of an individual identity; self-confidence and consciousness raising [ 23 ]. It is here where Nussbaum’s and Sen’s [ 37 ] capability approach comes in, which defines power as “a capability to act upon one’s environment” [ 38 ]. For example, an individual farmer can simply start to produce and use biomass-based fuels without any permission or interference from another actor, such as the petrol industry. However, constructivist research has demonstrated how every actor or group is defined through socially constituted relations that, at least indirectly, shape the actions of individuals [ 7 ]: only a farmer who receives knowledge about alternative technologies may effectively implement them.

Power to can be linked to Parsons’ definition of power as the ability “to get things done” [ 5 ]. It highlights a productive agency, especially in the cases where actors’ goals are opposed or resisted. Biofuel research by small farmers and rural communities is often based on this perception of power [ 9 , 39 ]. Scholars highlight the potential of biofuels for rural development by providing new markets for agricultural production. They assume that through the introduction of radically new technologies in niches, farmers are able to empower themselves in an attempt of an “agro-ecological revolution” [ 8 ]. They highlight the self-empowering agency of hitherto marginalized people to become “energy sheiks” [ 40 ], based on biomass production.

Scholars, who take a perspective of power to , focus on the productive agency of the biofuel sector. They are interested in the empowerment of alternative ideas and values which, in the case of biofuels, allow for transforming fossil fuel-dependent societies. These alternative agents criticize the practices or the authority of the dominant, carbon-intense system and refuse to reproduce their own positions in this system. Their non-conformism is perceived to serve the common good as they develop alternative technologies required by everyone in a world beyond petrol. From a perspective of power to and in difference to a perspective of power with , there are only a limited number of transformational agents: not everybody in the field is assumed to be a “winner” in the first place; there are only a few “energy sheiks”. However, scholars see an imperative to act based on normatively prior “green” values, for example, climate protection and sustainability (and everybody benefits from the realization of these values).

Biofuel: gold rush (power over)

Scholars who explicitly issue power in the context of biofuels usually perceive power as asymmetric. Biofuel governance is seen as a zero-sum game which produces winners and losers. From this perspective, powerful actors, structures, and discourses in the field of biofuel governance influence the actions and even the thoughts of others. In the following, I will illustrate this perspective, further differentiating the “four faces” of power over (see Table 1 ): visible , hidden , invisible , and unconscious power [ 2 , 41 ]. (the fourth dimension does not understand power as a zero-sum game and can also be added to power to , see the first footnote.)

In the first dimension, agents exercise visible power when they directly influence political decision-makers based on their material and ideational resources [ 42 ]. What is visible is not the power as such, but rather its physical means such as lobbying activities, party financing, and armed force. (Dahl [ 10 ]: 201) defines: “A has power over B to the extent that he can get B to do something that B would not otherwise do” (emphasis added). Any kind of state force implementing objectives of sustainability by top downregulation means exercising direct power. Non-state actors may also play a role in this game. Coase [ 43 ] explains this for business firms. Also when Pilgrim and Harvey [ 44 ] demonstrate how NGO lobbying significantly affected biofuel policy changes and sustainability regulation in the UK and in Europe, they assume that NGOs enforce their ideas against others in an arena of obviously competing demands.

The second dimension of hidden power refers to power not obviously opposed by anyone. Bachrach and Baratz [ 11 ] speak of “two faces of power” emphasizing that some issues never even make it onto the political agenda and are dismissed before observable negotiations start. For a long time, the EU issued biofuels only in the context of climate change, completely neglecting aspects of competing food demands and land use change in the Global South [ 45 , 46 ]. Scholars demonstrating such hidden aspects apply this second dimension of power over to analyze biofuel governance.

The traditional conception of structural (hidden) power in international relations aims to address the coercion resulting from the capital mobility of transnational corporations. Threats to shift investments abroad do not even need to be voiced in order to influence policies in their favor [ 42 , 47 ]. More recent studies point to the fact that businesses also exercise structural power by self-regulation and public-private partnerships; these types of governance allow business actors to actively set rules, for example, for the “sustainable” production of biofuels at the expense of state actors [ 42 , 48 ]. In addition, as public authorities have faced challenges in facilitating the implementation of their sustainability criteria outside their jurisdictions, the EU has started to use these private schemes to verify compliance with sustainability criteria in biofuel production outside its own territory [ 49 , 50 ]. As a result, following this perspective, power in the global political economy has been diffused, leaving biofuel conglomerates with considerable power over others [ 51 ].

Further, scholars are increasingly focusing on power relations linked to latent conflicts of interest. In the third dimension, invisible power comes to play as a result of norms and ideas [ 41 ]. Research analyzes discourses, communication practices, cultural values and institutions, which all work to shape relevant thoughts and actions [ 12 ]. With regard to biofuels, Munro [ 22 ] has shown how, in the United States, a powerful coalition of agricultural interests manipulated the governance of biofuels by linking it to public concerns about climate change and energy security. In consequence, corn biofuel received political support, tax reductions, and subsidies. Likewise, Puttkammer and Grethe [ 52 ] have found a coalition of biofuel advocates to dominate the public discourse in Germany, while scientists who doubted the efficiency of biofuels could not make their voice heard. The discourse only shifted with the 2007–2008 food price crisis when scholars demystified the “ethanol bubble” [ 53 ] and outlined potentially devastating implications for global poverty and food security. Experts, NGOs, and business actors who have challenged the sustainability of biofuels on many fronts began to be heard [ 20 , 22 ].

For the most part, these discourse scholars blame other scholars who apply a perspective of power with for neglecting and postponing important questions of social justice linked to biofuel production [ 21 , 54 ]. Win-win rhetoric is demonstrated to manifest global power asymmetries rather than to contribute to more ecology and fairness [ 22 , 53 ]. From this perspective, pioneers and leaders, whose role Young [ 31 ] and Bernard and Prieur [ 30 ], among others, consider to be positive, only serve dominant interests and prevent a more fundamental social transformation to sustainability. With reference to the International Political Economy, most scholars deny a simple confrontation of biofuel proponents (or pioneers) and opponents (or laggards). In this vein, Levidow [ 55 ] outlines how the EU can continue “its global plunder of resources” because it pursues global leadership for sustainable biofuels. Silva-Castaneda [ 56 ] demonstrates how, in Indonesia, some NGOs decided to participate in the Roundtable on Sustainable Palm Oil (RSPO), a certification process initiated by the WWF, among others. The local NGOs managed to include important clauses regarding indigenous and land rights in the RSPO standard. In practice, however, auditors rarely recognize as valid evidence the forms of proof put forward by local communities, and global conglomerates could even use the standards to increase their primacy vis-à-vis local farmers [ 56 ]. These examples reveal power over within multi-stakeholder processes.

Studies demonstrate that the expansion of biofuels in countries of the Global South was only possible through the partial neglect (simplification) of their cultural and ecological diversity [ 57 ]. Nygren [ 58 ] illustrates how leading retailers, in negotiation with environmental organizations, have guided consumers’ expectations of certified Southern forest products by building images of Southern community forest producers as authentic and exotic others . She concludes that certification as a market-based form of governance has only had a limited impact on altering the unequal relationship characteristic of global networks of production and consumption.

With reference to Foucault [ 13 ] and Bourdieu [ 59 ], we can capture links between knowledge, power, and politics in a fourth dimension of power over [ 17 ]. Critical and (post-) structuralist approaches understand power in a way that everything is socially constructed. Scholars analyze the normative impact on (supposed) losers, such as farmers in the Global South, as well as on (supposed) winners, such as major agribusiness actors. All actors work to mainly reproduce systems and positions [ 60 ]. With regard to biofuels, several studies have highlighted the central role of knowledge and framing [ 15 , 16 , 21 ]. Drawing on Foucault, Kuchler and Linnér [ 21 ] have analyzed the discursive practices of the three major international organizations focused on food and agriculture, energy, and climate with regard to biofuels over the last 20 years: the UN Food and Agriculture Organization (FAO), the International Energy Agency (IEA), and the Intergovernmental Panel on Climate Change (IPCC). They found that, in contrast to pro and contra accounts, the arguments of all three organizations reflected a policy consensus based on the mainstream notion of industrial agricultural production, promoting the intensification and expansion of rural production. The biofuel discourse has further constituted a concatenation of the three issues of agricultural production, energy security, and climate change mitigation. When the discourse shifted with the 2007–2008 food price crisis, all the three major organizations adapted to this shift [ 21 ]. Instead of exercising power over by manipulating discourses on biofuels according to specific pro or contra interests, the organizations were found to rather reproduce hegemonic discourses and their own positions.

The gold rush metaphor is used a lot to describe the situation of biofuels from a power over perspective [ 1 ]. Biofuel production, like gold mining, is unprofitable for most farmers, just like it was for diggers and mine owners. Both biofuel production and gold mining can in addition have very negative environmental effects. While, however, people are made to believe that everyone can become abundantly wealthy (“energy sheiks”), only some few investors make large fortunes. Applying discursive approaches of power over , we can argue that even such investors and major businesses are subject to and not only conscious manipulators of discourses of agricultural intensification and economic growth. The analysis of power over helps to understand why change to more sustainable transport and agricultural systems does not happen. However, as I argue in this article, it falls short on explaining when and why there also sometimes is disruptive change and empowerment.

Power to change: interrelations between power with, power to, and power over

While the perspectives of power with and power to (over-) emphasize the potential for change with regard to biofuels, scholars with understandings of power over often exaggerate their negative impacts. The tripartite framework allows for the combining of different analytical perspectives and to examine their interrelations. While the three categories are first of all analytical heuristics, they also stand for different mechanisms of the exercise of power (see Fig. 1 ). Power over affects what is considered a “sustainable innovation” and “creative alternative”. Research has demonstrated this. However, I argue that it is also possible the other way round: there are situations in which power with and power to can address power imbalances and prevent a situation in which there are only a few winners and many losers as a result of biofuel governance.

Agent-based power

As shown in Fig. 1 , besides considering material and ideational sources of power, we also need to consider different mechanisms of power (over/to/with), since they lead to different results of power (leading to a new distribution of sources in a circular process, see the arrow at the bottom of Fig. 1 ). Biofuels per se are neither a sustainable innovation, a creative alternative nor a gold rush. The three metaphors exemplify three different results of power: the exercise of power over leads to a gold rush situation. So, if scholars only ask for power over , they will always find winners and losers. By contrast, if we ask for the exercise of power to , we may find that biofuels are creative alternative. Finally, the exercise of power with can be exemplified by a case of finding an agreement on sustainability criteria of biofuel production. To demonstrate overlaps, especially, in terms of the results of power, I used dashed lines in Fig. 1 .

When, in the field of biofuels, scholars explicitly issue power, they generally use concepts of power over to explain why governance and research in this field have a blind spot for power asymmetries [ 49 , 53 ]. Biofuel opponents may have accomplished a shift in the biofuel discourse after the 2007–2008 food price crisis [ 20 , 22 ]. However, overriding power asymmetries have prevented a structural change in both the energy/transport and the agricultural sectors. The trend is now definitely towards large companies and conglomerates [ 49 , 50 ].

However, the fact that biofuels have caused no structural change and have disadvantaged rather than empowered small farmers in the Global South, does not mean that a structural change is impossible. What I want to argue in this article is that exercising consensual forms of power (power with) as well as self-empowerment and resistance (power to) can also eclipse and overcome power asymmetries (power over). Empirical research on deliberative processes suggests that communication and common action never happen among equals and that they are never free from any form of power over [ 36 , 61 ]. Hence, we need to understand power with as a form of exercising power, which is strategic (bargaining) as well as communicative (arguing). A crucial part of this process is the orientation of agents involved in processes of biofuel governance. If actors are open to changing their positions and developing shared understandings, transitions to sustainability can follow from dialogues [ 61 , 62 ].

Following this perspective, even if small farmers in the Global South have fewer capabilities compared to conglomerates from the EU and the United States, this does not mean that they have no possibility to act independently from them. For example, sugar is costly to establish, and thus is economically most efficient at large plantation scales. However, Jatropha can more readily be produced through outgrower schemes as it is less capital intensive [ 9 , 49 ]. While currently almost all bio-ethanol is produced from grain or sugarcane and therefore competes with food purposes, other efficient and economically viable technologies for ethanol production are available [ 63 ]. The production of perennial energy crops, such as grasses and trees, and crop residues, such as straw, are seen to require fewer inputs and less prime land [ 64 ].

Under specific conditions, empowerment is possible; processes of power with and power to can have a (positive) impact on unwanted relations of power over . For example, processes of stakeholder dialogue and certification demonstrate that an agreement beyond the lowest common denominator is possible. In addition, they can weaken the perceived legitimacy of powerful actors that are producing biofuels unsustainably. The critical discourse on biomass certification has issued consumers’ accountability for harmful social and environmental effects in countries of production [ 55 , 65 ]. When the legitimacy of unconditional import as well as of private certification schemes was put into question [ 50 ], transnational conglomerates lost ideational and material resources on which their power over others was based. In the agrifood sector, we can clearly see that certification has become a new normative obligation [ 66 ].

We can observe various kinds of empowerment and resistance related to biofuels. While Nygren [ 58 ] argues that certification schemes reproduce (inferior) positions of southern producers as authentic and exotic others, she does not completely deny that certification had a positive impact on altering asymmetries in global networks of production and consumption. Silva-Castaneda’s [ 56 ] study discloses new ways in which local communities can legally prove their land rights, for instance, by video documentation to replace missing formal documents or destructed land marks.

Scholars have described movements, such as Via Campesina, in terms of exercising power over and opposing transnational agriculture corporations [ 67 ]. In terms of reducing and overcoming power asymmetries, however, what is most striking is the fact that small farmers within this movement exercise power to by doing healthy and sustainable agriculture independently of the major agribusinesses to which, from a power over perspective, they would only be subordinated. At the same time, when producing organically, small farmers do not reproduce the system of industrial agricultural production (and their inferior positions within that system). So, their way of farming can be considered as a creative alternative and as a way of resistance. Moreover, within this movement of Via Campesina, despite widely different internal cultures, farmers also exercise power with by (re-) constituting a new shared peasant identity. From a perspective of power with, we can argue that, in the long run, everybody, even from outside this movement, may benefit and share norms and values developed here such as sustainability in farming. The movement delegitimizes the acquisition of land by established conglomerates (“land grabbing”), whose ideational sources of power shrink in consequence. The visible result is a new, more equal, and just distribution of (power) resources through land reforms.

Conclusions

This article should not only encourage a debate on power issues with regard to biofuels, but moreover, develop the debate more comprehensively. When political power has been analyzed in the context of biofuels, this has happened so far through using confrontational or structuralist and discursive approaches that are based on an understanding of power over . Respective scholars have accused other researchers of neglecting “real power concentrations” in the biofuels industries. Often quite rightly: biofuel research has neglected the limits of win-win for a very long time. Scholars have taken sides and normatively inflated their own pro biofuel position, while they have dispatched their adversaries as laggards with regard to the future of transport and agriculture. Of course, not every (supposedly) sustainable innovation is necessarily good in the sense that it is completely uncontroversial (even if there is no visible opposition as in the case of biofuels for a long time). In this context, the question of power essentially addresses the re-politicization of decisions perceived to be urgent and without alternative. With the 2007–2008’s shift in discourse, critics re-politicized the governance of biofuels. Several farmer associations have completely turned against biofuels. I argue that this rejection of biofuels is due to a limited perception of power as power over .

Why does it make sense to complement such a perception of power over ? Why does a multidimensional power framework make more sense? Naming different perspectives, as done here, with one and the same term—“power”—means, first, to put them on one normative level. Gold rush (power over) is a term with strongly negative connotations, on the one hand, and leads to normatively inflating sustainable innovations (power with) and creative resistance (power to), on the other. This is often unjustified because the exercise of power with and power to are not per se more legitimate forms of achieving social change. For example, preventing greenhouse gas emissions “from above” can be quite legitimate.

Secondly, as illustrated in this article, all three conceptions of power are already used in research on biofuels (although sometimes only implicitly; this should change). My hope is that this article addresses diverse communities and overcomes boundaries between them with this multidimensional power approach (in particular, between those who still celebrate biofuels as a “sustainable innovation” and those scholars who completely condemn them because of related power asymmetries). Especially those whose research is (implicitly) based on understandings of power as power with and power to could take stronger reference to researchers taking a critical viewpoint on their studies (power over)—in particular, through showing how consensual forms of power exercise (power with) and resistance and empowerment (power to) not only reproduce power asymmetries but also help overcome them. If we look at the gold rush metaphor from a perspective of power to , we may see that there is a lot of entrepreneurship involved in the discovery of gold deposits. From the perspective of power with , we may also see that people in the field of gold mining as well as of biofuel production find common ground among diverse interests and organize with each other.

Third, convincing and learning (power with) as well as creative ability (power to) and coercion and manipulation (power over) do not completely capture concrete change processes. The analytical categories applied in this paper help to cluster the various understandings of power in biofuel research, but they also reflect different mechanisms of power in reality. Power with perspectives focus on the benefits of biofuels (sustainable innovation); power to focuses on how new actors develop alternatives to fossil (and nuclear)-based economies; power over points to the limits of change because of the dominance of specific actors, structures, and discourses. The common terminology allows that the three perspectives on power are not considered as mutually exclusive (different interpretations of the same phenomenon), but as supplementary (different aspects of a change process). It becomes possible to examine their interrelations and their supplementary potential. With this article, I hope to have given an impetus for further research in this direction. A comprehensive analysis of power in diverse parts of biofuel research and governance is definitely a prerequisite for more seriously and intensively exploring questions of where, when, and how the governance of biofuels may also allow for “green” resistance and collective empowerment.

If actors create (reproduce) discourses and structures, I call this power to . Most constructivist studies however deal with identifying dominant (hegemonic) structures and discourses over others that are unconsciously reproduced, i.e., power over .

Power with is not identical to Arendt’s understanding of power or its empirical operationalization hardly accomplishes Arendt’s demands. So deliberative theories of democracy build upon her understanding of power without finding it comprehensively implemented in reality [ 61 , 68 , 69 ]. In difference to deliberative processes, power with encompasses communicative as well as common action.

An example, to which Arendt refers in a footnote to her definition of power, is the student protests at Berkeley and elsewhere at the end of the 1960s. She contrasts the power of the students—“obviously the strongest power on every campus simply because of the students’ superior number” ([ 28 ]: 44)—to the violence of the university authorities. An individual student leader ‘in power’ would speak on behalf of the movement.

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LUNAPHORE PARTNERS WITH DISCOVERY LIFE SCIENCES TO BRING INNOVATIVE SPATIAL BIOLOGY SOLUTIONS TO CLINICAL RESEARCH

MINNEAPOLIS , Sept. 26, 2024 /PRNewswire/ -- Bio-Techne Corporation (NASDAQ: TECH) today announced that one of its spatial biology brands, Lunaphore , is partnering with Discovery Life Sciences (Discovery) to add Lunaphore COMET™ , a best-in-class spatial biology platform, to Discovery's global suite of biospecimen products and specialty lab services supporting customers with clinical research.

The strategic partnership leverages Lunaphore's COMET technology, the only fully-automated, high-throughput, hyperplex platform with superior tissue profiling capabilities, with Discovery's scientific expertise to advance immuno-oncology, immunology, neuroscience, and infectious disease research across all development stages at a new level of precision and speed. Discovery's Lunaphore COMET Hyperplex Immunofluorescence Services will facilitate the development of next-generation assays for clinical trials and provide multidimensional spatial biology insights to advance biomarker discovery, patient stratification, and translational medicine.

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Environmental sustainability of biofuels: a review

Harish k. jeswani.

1 Department of Chemical Engineering and Analytical Science, The University of Manchester, Manchester M13 9PL, UK

Andrew Chilvers

2 Royal Academy of Engineering, 3 Carlton House Terrace, London SW1Y 5DG, UK

Adisa Azapagic

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Biofuels are being promoted as a low-carbon alternative to fossil fuels as they could help to reduce greenhouse gas (GHG) emissions and the related climate change impact from transport. However, there are also concerns that their wider deployment could lead to unintended environmental consequences. Numerous life cycle assessment (LCA) studies have considered the climate change and other environmental impacts of biofuels. However, their findings are often conflicting, with a wide variation in the estimates. Thus, the aim of this paper is to review and analyse the latest available evidence to provide a greater clarity and understanding of the environmental impacts of different liquid biofuels. It is evident from the review that the outcomes of LCA studies are highly situational and dependent on many factors, including the type of feedstock, production routes, data variations and methodological choices. Despite this, the existing evidence suggests that, if no land-use change (LUC) is involved, first-generation biofuels can—on average—have lower GHG emissions than fossil fuels, but the reductions for most feedstocks are insufficient to meet the GHG savings required by the EU Renewable Energy Directive (RED). However, second-generation biofuels have, in general, a greater potential to reduce the emissions, provided there is no LUC. Third-generation biofuels do not represent a feasible option at present state of development as their GHG emissions are higher than those from fossil fuels. As also discussed in the paper, several studies show that reductions in GHG emissions from biofuels are achieved at the expense of other impacts, such as acidification, eutrophication, water footprint and biodiversity loss. The paper also investigates the key methodological aspects and sources of uncertainty in the LCA of biofuels and provides recommendations to address these issues.

1. Introduction

Greenhouse gas (GHG) emissions from transport have been increasing at a faster rate than from any other sector [ 1 ]. The sector relies heavily on fossil fuels, which accounted for 96.3% of all transportation fuels in 2018 [ 2 ]. Transport is also responsible for 15% of the world's GHG emissions and 23% of total energy-related CO 2 emissions [ 1 ]. To reduce dependence on petroleum-based fuels, as well as to mitigate climate change, biofuels are viewed widely as promising alternative transportation fuels.

Biofuels have been used since the early days of the automotive industry. For instance, Rudolph Diesel tested his first engine on peanut oil [ 3 ] after pulverized coal was found to be unsuitable. Until the 1940s, biofuels were seen as viable transport fuels and bioethanol blends, such as Agrol, Discol and Monopolin, were commonly used in the USA, Europe and other regions [ 3 ]. Further development of bioethanol ceased after the Second World War as petroleum-derived fuel became cheaper. During the oil crisis in the 1970s, many countries showed renewed interest in production of commercial biofuels; however, only Brazil started to produce ethanol at a large scale as part of the National Ethanol Programme ‘Proálcool’ [ 4 ]. During the late 1990s, with the rise in crude oil prices and concerns over energy security, the USA and many nations in Europe developed policies in support of domestic biofuel industries [ 5 ]. The interest in biofuels further increased in the past decade with the development of policies on climate change mitigation and strategies to reduce GHG emissions from the transport sector. More than 60 countries have since launched biofuel programmes and set targets for blending biofuels into their fuel pools [ 6 ]. The most notable are Renewable Fuel Standard (RFS) [ 7 ] in the USA and the Renewable Energy Directive (RED) in Europe [ 8 ].

Owing to these policies, world bioethanol production has increased by 67%, from 67 to 110.4 billion litres, over the decade of 2008–2018 [ 2 ]. During the same period, biodiesel production increased more than threefold, from 12 to 41 billion litres. Currently, biofuels account for about 3.4% of total transportation fuels worldwide [ 2 ]. The global production of biofuels is dominated by the USA and Brazil—producing 69% of all biofuels in 2018—followed by Europe (EU-28) with 9% [ 9 ]. Production of bioethanol in the USA is almost exclusively from corn, whereas in Brazil, it is from sugarcane. In Europe, the main feedstocks are corn, wheat and sugar beet for bioethanol, while rapeseed and used cooking oil (UCO) are used for biodiesel production [ 10 ]. Argentina, Brazil and the USA also produce significant quantities of biodiesel, predominantly from soya bean, while Malaysia and Indonesia produce biodiesel from palm oil. Several international and national organizations have made mid- and long-term projections for global production of biofuels. These projections provide wide-ranging estimates of potential future increases in liquid biofuels for transport globally. The International Energy Agency (IEA) estimates that as much as one-third of all transportation fuel could come from biofuels by 2050 [ 11 ], while organizations, such as the OECD and BP, project approximately a 7% share of biofuels by 2030 [ 12 ]. A recent assessment [ 13 ] also suggests that the IEA projections could be impossible to achieve, estimating the maximum potential of transport biofuels by 2050 to be at least 30% lower than those projected by the IEA.

Biofuels can be differentiated according to a number of key characteristics, including feedstock type, conversion process, technical specification of the fuel and its use. Owing to this multitude of possible distinctions, various definitions are in use for biofuel types. Two commonly used typologies are ‘first, second and third generation’ and ‘conventional and advanced’ biofuels. Biofuels produced from food or animal feed crops are referred to as first-generation biofuels. Since first-generation biofuels are produced through well-established technologies and processes, such as fermentation, distillation and transesterification, they are also commonly referred to as ‘conventional biofuels'. A key characteristic for second-generation biofuels is that they are derived from non-food feedstocks, such as dedicated energy crops (e.g. Miscanthus , switchgrass, short rotation coppice (SRC) and other lignocellulosic plants), agricultural residues, forest residues and other waste materials (e.g. UCO and municipal solid waste). Biodiesel produced from microalgae through conventional transesterification or hydro-treatment of algal oil is commonly known as third-generation biofuel. Second- and third-generation biofuels are often referred to as ‘advanced biofuels’ as their production techniques or pathways are still in the research and development, pilot or demonstration phase. In this paper, the terminology ‘first, second and third generation’ has been selected and followed throughout. An overview of different biofuel types, their feedstocks and conversion routes can be seen in figure 1 .

An external file that holds a picture, illustration, etc.
Object name is rspa20200351-g1.jpg

An overview of feedstocks and production processes for different biofuels, also showing the life cycle of fuels from cradle to gate (well to tank) and cradle to grave (well to wheel). Adapted from [ 14 ]. The figure has been simplified and other feedstocks, production routes, products/by-products and uses are possible. The italic font denotes the focus of this review, i.e. bioethanol and biodiesel used for transportation. DDGS, dark distillers grain with solids. (Online version in colour.)

Biofuels offer both advantages and disadvantages in terms of environmental, economic and social sustainability [ 14 ]. On the one hand, reduction in GHG emissions, energy security and rural development are the most important drivers for biofuels globally. On the other hand, there are concerns related to increasing the production of biofuels, such as upward pressure on food prices, the risk of increase in GHG emissions through direct and indirect land-use change (LUC) from production of biofuel feedstocks, as well as the risks of degradation of land, forests, water resources and ecosystems [ 15 ]. The use of first-generation feedstocks, such as corn, has become a particularly contentious issue, largely owing to competition with food production and concerns over diverting agricultural land into fuel production. A growing demand for agricultural produce risks an increase in deforestation and use of land with a high biodiversity value to meet this demand, as well as associated usage of freshwater, fertilizers and pesticides, with negative consequences on the environment. Some of these issues could be addressed by using second-generation feedstocks; however, the economic viability of some second generation of biofuels remains doubtful in the current economic context, largely because of the low oil prices [ 16 – 18 ]. Third-generation (algal) biofuels could also avoid the issue of food competition and land use because microalgae can be grown on non-arable land and in wastewater, saline or brackish water and they grow extremely rapidly. However, the production of biofuels from microalgae is energy-intensive and at present economically unviable [ 19 ].

To encourage sustainable development of biofuels, regulatory policies, such as the RED and RFS, stipulate various sustainability criteria for biofuels. One of the main criteria is related to life cycle GHG emissions. The RED stipulates that biofuels should have at least 50% lower emissions than their fossil fuel alternatives for installations in operation before October 2015 and 60% for installations starting after this date, rising to 65% lower for biofuel plants commencing operation after 1 January 2021 [ 8 ]. RFS requires producers of advanced biofuels to reduce GHG emissions by at least 50%, while standard biofuels have to achieve a 20% reduction in GHG emissions [ 7 ]. The climate change impact related to GHG emissions and other sustainability aspects of biofuels should be evaluated on a life cycle basis via life cycle assessment (LCA) to avoid shifting burdens from one part of the life cycle or supply chain to another.

Numerous LCA studies have considered the potential of biofuels to achieve reductions in life cycle GHG emissions by estimating their potential impact on climate change. However, their findings are often conflicting, with a wide variation in the estimates. A number of review papers have also discussed LCA of biofuels, but these focused on a particular aspect, such as a region, feedstock or type of biofuel. For example, Shonnard et al . [ 20 ] reviewed LCA studies of biofuels in the Pan American region. Morales et al . [ 21 ] and Roy et al . [ 22 ] concentrated on LCA studies of lignocellulosic bioethanol, while Menten et al . [ 23 ] focused on advanced biofuels. Sieverding et al . [ 24 ] conducted a review of soya bean-based biodiesel and van Eijck et al . [ 25 ] reviewed issues pertaining to biodiesel from Jatropha .

However, a comprehensive review of all types of biofuels is not available in the literature. Besides, many more LCA studies on biofuels have been published since the publication of the above-mentioned reviews. Thus, this review paper aims to close this gap by analysing and synthesizing the latest information concerning LCA of biofuels. The main objective is to provide a greater clarity and understanding of the environmental sustainability of different liquid biofuels with the aim of informing future policy. A further objective is to examine the state-of-the-art knowledge on environmental issues associated with the production and consumption of biofuels. The next section provides an overview of the reviewed LCA studies in terms of their coverage with regard to biofuel type, geographical location and their approaches to handling critical methodological aspects in LCA. Section 3 presents results for the climate change impact, energy and water use and other environmental issues associated with different types of biofuels. The key methodological aspects and sources of uncertainty in assessing the environmental impacts of biofuels are investigated and discussed in detail in §4. The paper ends with conclusions and recommendations on addressing the key issues related to sustainability of biofuels.

2. Study methodology and coverage

A systematic literature search was performed in different databases (Science Direct, Web of Science, Scopus and relevant academic journals) to identify academic, peer-reviewed studies on the environmental sustainability of biofuels. To avoid outdated information, the review of the literature predominantly focused on the articles published in the period from 2009 to 2020. Some important earlier publications cited frequently in the literature were also taken into account. In total, 179 articles were primary (original) LCA studies, combining between them 613 assessments of different types of biofuels, all of which are included in this review. In addition, further publications focusing on environmental issues not usually included in LCA studies, such as water footprint, biodiversity and LUC, as well as discussing various methodological aspects were also reviewed. These studies covered a wide spectrum of first-, second- and third-generation biofuels produced from more than 20 types of feedstock. Table 1 provides an overview of the LCA studies related to the types of fuel, feedstock and geographical coverage. Among them, 52% assessed first generation, 38% considered second generation and the remaining 10% assessed third-generation biofuels. Regarding the type of biofuel, 56% of studies were for bioethanol and the rest for biodiesel. Geographically, 36% of studies were based in Europe, 26% in North America, 20% in Asia, 12% in South America, 4% in Africa and 1% in Australia. An overview of how different studies approached some critical LCA methodological aspects, including type of LCA, goal and scope of the study, definition of the functional unit, allocation methods and estimation of the impacts, is provided below.

Table 1.

An overview of the number of LCA studies by biofuel type, feedstock, location and land-use change.

	location						land-use change
fuel type/feedstock	Europe	North America	South America	Asia	Africa	Australia	without	with	total

corn	6	23	0	1	0	0	16	14	30
molasses	4	12	0	25	3	4	30	18	48
sugar beet	19	1	0	0	1	0	14	7	21
sugarcane	0	4	32	1	1	0	28	10	38
wheat	39	0	0	0	0	0	28	11	39

bagasse	1	1	3	1	0	0	6	0	6
forest residue	16	7	0	0	0	0	23	0	23
	14	9	0	0	0	0	16	7	23
short rotation coppice	29	2	0	0	0	0	17	14	31
stover	12	18	0	0	0	0	27	3	30
straw/husk	27	1	0	9	0	0	32	5	37
switchgrass	2	17	1	0	0	0	18	2	20

palm oil	0	0	3	56	0	0	32	27	59
rapeseed	19	13	2	0	4	0	24	14	38
soya bean	3	10	18	5	3	0	29	10	39
sunflower	1	0	2	0	5	0	5	3	8

	1	13	0	0	0	0	14	0	14
	0	0	7	8	7	0	18	4	22
used cooking oil/tallow	17	1	3	5	1	0	27	0	27

algae	13	28	4	13	0	2	60	0	60
total	223	160	75	124	25	6	464	149	613

a The total number of studies or analyses, rather than the number of papers published, as some papers included several studies or analyses.

(a) Type of life cycle assessment: attributional versus consequential

Two general types of LCA studies are distinguished: attributional (ALCA) and consequential (CLCA). They address different questions and follow different methodologies, and will normally have very different results. ALCA accounts for impacts directly related to the system of interest, attributing them to the activities within the system; hence, the term ‘attributional’. For biofuels, it is used mainly as an ‘accounting’ tool for estimating environmental impacts of various activities in the supply chain, comparisons of alternative systems and identification of environmental hotspots that can be targeted for improvements. CLCA, in addition to direct, also examines potential indirect consequences of the system under study by considering various ‘what if’ scenarios that could arise owing to this system; examples include changes in demand for the product of interest or technological improvements. For instance, CLCA can consider potential impacts of biofuel feedstock cultivation on other land-using sectors and the effect this might have on the food production system and LUC elsewhere in the global economy [ 26 , 27 ]. As such, CLCA is more suited for policy applications.

CLCA is still under development and, consequently, most of the LCA studies on biofuels found in the literature are attributional. Nevertheless, both the ALCA and CLCA are considered in this review.

(b) Goal and scope of studies

Goal and scope definition is an important initial step in LCA studies as the specific methodological approaches depend strongly on the specific goal, scope and question being addressed. The goal and scope of the study influence the definition of the system boundary and determine what activities and life cycle stages will be considered [ 28 ]. LCA studies of biofuels have addressed a wide range of goals and research questions, including:

— What are the environmental impacts of the biofuel system under examination?
— How do biofuels compare with fossil fuels?
— What are the environmental hotspots in the life cycle of particular biofuel systems under study?
— What are the improvement options to optimize the supply chain under study?
— What are the environmental implications of biofuel policies?

Although the ISO 14040 LCA standard [ 28 ] requires clear definition of the goal and scope, a lack of or unclear definition of goal and scope is a common problem in LCA studies of biofuels. This can also mean that the study method and rationale can be unclear, making comparability of results difficult [ 29 ].

Two types of system boundaries were used in the reviewed LCA studies of biofuels: ‘cradle to gate’ (or ‘well to tank’) and ‘cradle to grave’ (or ‘well to wheel’); figure 1 . However, the latter system boundary is more appropriate as it is important to include the use of fuels to enable comparisons of biofuels with their fossil substitutes, since the combustion performance and associated emissions of biofuels can significantly differ from their fossil substitutes for the same type of vehicle [ 30 , 31 ]. Around half (48%) of the LCA studies reviewed considered a cradle to grave system boundary to compare environmental impacts of biofuels with fossil fuels, while the rest were from cradle to gate. Other inconsistencies include the omission in some studies of various inputs (such as enzymes, pesticides, fertilizers, etc.) and co-products. These differences are often important enough to influence the results significantly.

(c) Functional unit

In LCA, the term ‘functional unit’ describes the function of the system under study and represents the unit of analysis on which the study is based. The choice of the functional unit is driven by the goal of the study and must be representative of the system(s) studied and their main purpose (function). Biofuels regulations, such as the RED [ 8 ] and RFS [ 7 ], use the energy content of biofuels (MJ) as the functional unit. While this functional unit was often used in the reviewed literature, others include the distance travelled by a vehicle (vehicle . km) [ 21 , 32 ], volume (litre) [ 33 , 34 ] and mass (kilogram or tonne) [ 17 , 35 ] of biofuels. Some studies also used the mass of biofuel feedstock [ 36 , 37 ], agricultural land area [ 38 , 39 ] and annual operation of refinery [ 40 ]. The use of such a wide array of functional units makes comparisons of LCA studies challenging.

(d) Allocation methods

Biofuel production processes often produce several co-products, such as animal feed, heat, electricity and biochemicals. Therefore, to determine the impacts from the biofuel of interest, it is necessary to allocate the impacts between the biofuel and its co-products. The ISO 14040 and 14044 standards recommend that, if possible, allocation should be avoided through subdivision of processes, or by system expansion. The latter involves expanding the system boundary to include alternative ways of producing the co-products. The production system is then credited for displacing production of the co-products in the alternative systems by subtracting their impacts from the impacts of the biofuel production system. Hence, this method is also known as ‘substitution’ or the ‘avoided burden’ approach. If allocation cannot be avoided, the impacts can be apportioned between the biofuel and the co-products using allocation factors based on physical or economic relationships. Mass and energy content of biofuels and co-products are often used to derive allocation factors based on physical relationships. Economic allocation is based on the assumptions that the market prices are the driver for the production process and the impacts are apportioned in proportion to the economic value (cost or price) of the biofuel and the co-products. In LCA of biofuels, the most common approaches used to allocate the impacts are system expansion and allocation by the energy content. This perhaps reflects the regulatory requirements in the USA and Europe: RFS [ 7 ] prefers system expansion, while the RED [ 8 ] favours allocation based on the energy content of biofuels.

(e) Land-use change

LUC is an important source of GHG emissions that contributed 660 ± 290 Gt CO 2 to the atmospheric CO 2 in the period from 1750 and 2011 [ 41 ]. The majority of LUC is driven by demand for food, fibre and fuel [ 42 ]. Converting natural vegetation or forest to cultivate biofuel feedstocks releases a significant amount of carbon from soil and plant biomass, creating a ‘carbon debt’ that can take years to repay [ 43 , 44 ]. Furthermore, cultivation of biofuel feedstocks on land that has high soil carbon content, such as peat land, leads to a considerable increase in GHG emissions [ 45 ]. Besides increasing GHG emissions, changes in land use can have other environmental consequences, such as soil erosion, nutrient depletion, water consumption and loss of biodiversity [ 46 ]. LUC related to biofuels can occur in two ways: direct (DLUC) or indirect (ILUC). DLUC refers to the direct transformation of previously uncultivated areas (such as grasslands and forests) into croplands for biofuel feedstock production. ILUC occurs when additional demand for biofuel feedstock induces displacement of food and feed crop production to new land areas previously not used for cultivation. Only 25% of the reviewed LCA studies took LUC into account.

(f) Environmental impacts

GHG emissions and savings in comparison to fossil fuels are the centre of attention in most LCA studies on biofuels. Other environmental impact categories considered in biofuel LCA studies include acidification, eutrophication, photochemical smog, human toxicity and eco-toxicity. However, the number of studies that have assessed a wider set of impact categories is still limited: of the 179 (primary) LCA studies reviewed, only 40% of such studies were found in the literature. These are discussed in the next section, starting with the climate change impact, or global warming potential (GWP) as often referred to in LCA, and following on to discuss energy and water use, biodiversity and other impacts.

3. Environmental impacts of biofuels

(a) global warming potential.

For this impact, the LCA studies present contradictory results, ranging from favourable to unfavourable, even for the same type of feedstock. This is a result of the differences in the assumptions, data sources, allocation methods and LUC. The influence of these aspects is discussed in detail in §4. The GWP of biofuels reported in the reviewed LCA studies is summarized in figures 2 – 6 and discussed below for different types of biofuel. For further details, see electronic supplementary material, figures S1–S7.

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GWP of first-generation biofuels without land-use change. Based on data from [ 24 , 32 , 34 , 47 – 118 ]. For the box plot legend, see electronic supplementary material, figure S1 and for the data used to plot this graph, see electronic supplementary material, figure S2. ‘Fossil fuel (reference)’ is the average carbon intensity of petrol and diesel supplied in the EU (94 g CO 2 eq. MJ −1 ) as specified in the RED [ 8 ]. ‘ A ’ refers to the number of LCA articles found in the literature and ‘ n ’ denotes the total number of analyses. (Online version in colour.)

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GWP of microalgae biodiesel. Based on data from [ 19 , 85 , 97 , 113 , 141 , 167 , 190 – 209 ]. The negative values are due to the credits for co-products and avoided processes, such as wastewater treatment. For the box plot legend, see electronic supplementary material, figure S1 and for the data used to plot this graph, see electronic supplementary material, figure S6. ‘Fossil fuel (reference)’ is the average carbon intensity of petrol and diesel supplied in the EU (94 g CO 2 eq. MJ −1 ) as specified in the RED [ 8 ]. ‘ A ’ refers to the number of LCA articles found in the literature and ‘ n ’ denotes the total number of analyses. (Online version in colour.)

(i) First-generation biofuels

As first-generation biofuels may involve LUC, which in turn can affect significantly the total GWP, the results reported in the literature are discussed first for the cases without and then with LUC.

GWP without land-use change . As can be observed in figure 2 , the GWP of first-generation bioethanol from different food crops vary considerably, ranging from 3 to 162 g CO 2 eq. MJ −1 (see electronic supplementary material, figure S2). Figure 2 also shows that the average GWP of bioethanol is lower than that of petrol for all the feedstocks (23–59 versus 94 g CO 2 eq. MJ −1 ). However, only bioethanol from sugarcane can meet the RED requirement of 60% reduction in GHG emissions relative to petrol. The average reductions in emissions from the other four feedstocks—corn, wheat, molasses and sugar beet—are not sufficient to meet this requirement. The main reasons that bioethanol from sugarcane can meet the 60% reduction requirement are relatively lower inputs of agro-chemicals and higher yields of sugarcane crops as well as the credits for electricity produced as a co-product in a biorefinery.

The GHG emissions for first-generation biodiesel also show a large variation across the LCA studies, with the GWP ranging between 3 and 111 g CO 2 eq. MJ −1 (electronic supplementary material, figure S2). However, as shown in figure 2 , the average GWP of biodiesel from all the feedstocks considered is lower than that of fossil diesel. Nevertheless, only biodiesel from palm oil meets the RED requirement for 60% reduction of the GWP relative to diesel (average value). Rapeseed and soya bean also come close to fulfilling this requirement, but sunflower biodiesel cannot meet even the 35% reduction.

The large variability in the GWP of first-generation biofuels shown in figure 2 is due to several reasons. For example, the LCA study on corn ethanol and soya bean biodiesel production in China found that the GWP of corn ethanol and soya bean biodiesel were 40 and 20% higher than petrol and diesel, respectively, owing to the relatively higher use of fertilizers, higher process energy consumption and the coal-dominated energy mix of China [ 47 ]. Low or no GHG savings (0–20%) compared to the fossil fuels were reported for South African sugar beet bioethanol as well as rapeseed, soya bean and sunflower biodiesel due to water scarcity affecting crop yields [ 48 , 49 ]. On the other hand, owing to higher yield and lower farming inputs, soya bean biodiesel produced in Brazil, the USA and Argentina achieve more than 60% GHG savings relative to the fossil fuels [ 50 ]. Studies on palm [ 51 , 52 ], rapeseed [ 53 , 54 ] and sunflower [ 55 ] biodiesel have also found the significant effect on GHG emissions of different locations, farming practices and usage of fertilizers. In the case of palm oil, the emissions vary with the options for handling of methane emissions from the treatment of palm oil milling effluent [ 56 ]. The influence of the fuel used in the biorefinery was noted in the case of wheat ethanol, with the GHG savings varying from 4 to 85% depending on whether straw or distillers' dried grains are used as a fuel [ 57 ]. Similarly, another study on molasses-based ethanol found that the use of bagasse instead of fuel oil would reduce GHG emissions of bioethanol from 112 to 51 g CO 2 eq. MJ −1 [ 58 ]. A number of studies on biofuels from different feedstocks have also found that the emissions are highly influenced by the utilization of by-products and the allocation method [ 52 , 59 – 62 ]. The effects of allocation methods and other factors are discussed in more detail in §4.

GWP with land-use change . As shown in figure 3 , if LUC is involved and considering the average GWP values, bioethanol cannot meet the 60% GHG reduction requirement regardless of the type of feedstock [ 62 , 63 , 119 , 120 ]. The increasing demand for bioethanol from sugarcane in Brazil has led to a continuous expansion of land used for sugarcane cultivation [ 44 , 210 ]. If this involves deforestation of tropical rainforest, the GWP of bioethanol from sugarcane can be up to 60% higher than that of petrol [ 58 ]. Similarly, expansion of soya bean cultivation in Central and South America is driving both direct and indirect LUC [ 121 , 211 ]. Furthermore, palm cultivation in Malaysia and Indonesia is associated with deforestation and drainage of peat lands. As a consequence, biodiesel from palm oil on peat and forest lands can have 3–40 times higher GHG emissions than diesel [ 64 ]. A recent study assessing the LUC impact of biofuels consumed in Europe [ 212 ] also found that the GWP of palm oil and soya bean diesel are almost two times higher than that of diesel. The same study also estimated the GWP of biodiesel from rapeseed and sunflower to be 20 to 40% higher than from conventional diesel ( figure 3 ).

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GWP of first-generation biofuels with land-use change. Based on data from [ 32 , 35 , 48 , 51 , 56 , 58 , 62 – 64 , 66 – 72 , 79 , 86 , 87 , 91 , 108 , 116 , 119 – 127 ]. For the box plot legend, see electronic supplementary material, figure S1 and for the data used to plot this graph, see electronic supplementary material, figure S3. ‘Fossil fuel (reference)’ is the average carbon intensity of petrol and diesel supplied in the EU (94 g CO 2 eq. MJ −1 ) as specified in the RED [ 8 ]. ‘ A ’ refers to the number of LCA articles found in the literature and ‘ n ’ denotes the total number of analyses. (Online version in colour.)

The significant variability shown in figure 3 for the GWP related to LUC is due to several reasons. For instance, some studies considered only ILUC [ 62 , 63 , 119 , 120 ] or DLUC [ 58 , 65 – 67 ], while others included both [ 68 , 122 ]. Furthermore, some studies applied partial equilibrium models and counterfactual (what if) scenarios to estimate ILUC emissions [ 62 , 119 ], whereas others used ILUC factors recommended by the US EPA [ 120 ]. The former tended to obtain higher ILUC emissions (34–155 g CO 2 eq. MJ −1 ) [ 62 , 119 , 123 ] than the latter (5–16 g CO 2 eq. MJ −1 ) [ 63 , 68 , 122 ]. For DLUC, several studies focused only on soil organic carbon (SOC) changes [ 65 , 122 ], but others also considered changes in the carbon stock related to the removal of biomass, both above and below the ground [ 58 , 67 , 68 ]. DLUC emissions also depend on the type of converted land and its previous use. For example, studies assuming palm oil cultivation on tropical forest and/or peat land in Malaysia and Indonesia estimated DLUC emissions in the range of 150–530 g CO 2 eq. MJ −1 [ 64 , 69 , 70 , 124 ]. On the other hand, studies on palm oil in Colombia and Thailand considered increase in the carbon stock due to LUC, assuming that the expansion of oil palm cultivation would occur in shrublands, savannahs, paddy fields and other agricultural lands [ 52 , 56 , 71 ]. In the case of sugarcane, molasses and soy, LUC emission reported in the literature varied from 30 to 200 g CO 2 eq. MJ −1 depending on the previous land use [ 35 , 58 , 69 , 72 , 121 ].

(ii) Second-generation biofuels

Figures 4 and and5 5 indicate that in most of the studies, the GWP of second-generation biofuels is considerably lower than that of fossil fuels. However, there is a large variation among different studies and feedstocks, with the values ranging from −115 to 173 g CO 2 eq. MJ −1 for bioethanol and −88 to 150 g CO 2 eq. MJ −1 for biodiesel (see electronic supplementary material, figures S4 and S5). These variations reflect the diversity of feedstocks and production routes, technology assumptions and methodological differences. Furthermore, some studies also considered emissions from ILUC [ 119 ] and SOC sequestration [ 68 , 128 ] associated with the production of SRC and perennial grasses as well as the reductions in SOC with removal of agricultural residues used as biofuel feedstocks [ 68 , 129 ]; for further discussion on SOC, see §4f. It should also be noted that the uncertainties related to technologies plays a particularly important role in the assessment of advanced biofuels as these are yet to be fully commercialized. Therefore, the quality of the available data is not as robust as in the case of the well-established first-generation biofuels.

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GWP of second-generation bioethanol. Based on data from [ 17 , 18 , 33 , 34 , 40 , 66 , 68 , 119 , 120 , 102 , 104 , 105 , 115 , 117 , 128 – 163 ]. The negative values are due to the credits for co-products, such as heat and chemicals. For the box plot legend, see electronic supplementary material, figure S1 and for the data used to plot this graph, see electronic supplementary material, figure S4. ‘Fossil fuel (reference)’ is the average carbon intensity of petrol and diesel supplied in the EU (94 g CO 2 eq. MJ −1 ) as specified in the RED [ 8 ]. ‘ A ’ refers to the number of LCA articles found in the literature and ‘ n ’ denotes the total number of analyses. (Online version in colour.)

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GWP of second-generation biodiesel. Based on data from [ 20 , 47 , 89 , 90 , 97 , 108 , 164 – 189 ]. The negative values are due to the credits for co-products. For the box plot legend, see electronic supplementary material, figure S1 and for the data used to plot this graph, see electronic supplementary material, figure S5. ‘Fossil fuel (reference)’ is the average carbon intensity of petrol and diesel supplied in the EU (94 g CO 2 eq. MJ −1 ) as specified in the RED [ 8 ]. ‘ A ’ refers to the number of LCA articles found in the literature and ‘ n ’ denotes the total number of analyses. (Online version in colour.)

In general, lignocellulosic bioethanol from agricultural and forest residues has a lower GWP than bioethanol from energy crops ( figure 4 ). This is largely due to N 2 O emitted during the cultivation of energy crops, related to the use of fertilizers. The latter are avoided in the case of residues as they are assumed to have no environmental burdens, which are all allocated to the original crop from which the waste is derived. In lignocellulosic bioethanol studies, the residual lignin is assumed to co-generate heat and power to meet the energy needs of the process, with surplus electricity exported to the grid. The biofuel production system is thus credited for avoiding the GHG emissions from the equivalent amount of grid electricity. For some feedstocks (SRC, forest residue, straw and corn stover), the credits for electricity generation and other co-products are higher than the total emissions from the biofuel production. Consequently, these studies report negative GWP, indicating the avoidance (saving) of GHG emissions. Some studies on energy crops also considered the increase in the carbon stock on the land that was converted to produce these crops, which led to the total net-negative GHG emissions [ 68 , 119 ]. On the other hand, harvesting of agricultural and forest residues can result in reduction of the land carbon stock [ 213 – 215 ], thus increasing GHG emissions [ 214 , 215 ]; however, most of the studies did not account for these changes. In the case of bioethanol from agricultural residues, other factors, such as the consideration of agricultural emissions [ 115 ], pre-treatment methods [ 130 ] and source of energy for the biorefinery [ 33 , 131 ], have a significant influence on GHG emissions.

While the LCA literature on second-generation bioethanol covers a wide range of feedstocks, the studies of biodiesel are more limited, focusing largely on three feedstocks: Jatropha, Camelina and UCO/tallow. As can be seen in figure 5 , the GWP of Jatropha and Camelina varies widely because of variations in the yield in different regions and differences in processes and assumptions, especially with respect to co-product allocation. For example, the yield of Jatropha oil seeds varies in different studies by a factor of 30, from 0.4 to 12 t ha −1 yr −1 [ 25 ]. The influence of allocation is also significant: using system expansion according to the US EPA methodology results in the GWP of Jatropha biodiesel of −88 g CO 2 eq. MJ −1 , while energy allocation as per the RED approach leads to GHG emissions of 15–20 g CO 2 eq. MJ −1 [ 20 ].

Although a majority of the studies of biodiesel from UCO report that GHG savings are greater than the RED reduction target of 60%, some studies also estimate that the GHG savings from this type of biodiesel are not sufficient to meet the target ( figure 5 ). This is due to some specific assumptions. For example, Intarapong et al . [ 90 ] considered pyrolysis for conversion of UCO to biodiesel, which is more energy-intensive than transesterification. Similarly, another study [ 47 ] assumed only a 5% biodiesel production yield, which is very low compared to more than 90% considered in other studies. Furthermore, Escobar et al . [ 171 ], who used consequential LCA methodology, considered indirect impacts, such as changes in the production of palm oil, soya bean and animal feed, found that the GWP of UCO biodiesel would be only 25% lower than that of diesel if ILUC and other indirect market impacts are considered.

(iii) Third-generation biofuels

In total, 27 LCA studies have estimated the GWP of third-generation algal biodiesel. However, they have all used very different approaches, process designs, system boundaries, methodologies and assumptions for feedstocks, nutrients and co-product management. As a result of the variation in these choices, the GWP differs widely between the studies, ranging from −2400 to 2880 g CO 2 eq. MJ −1 ( figure 6 ; electronic supplementary material, figure S6). These results would suggest that microalgae diesel can either reduce or increase GHG emissions significantly, relative to diesel, depending on the assumptions. However, a majority of the studies conclude that, at present state of development, algal biodiesel has higher life cycle GHG emissions than that of fossil diesel. The main reasons for higher emissions include lower algal yield [ 19 , 190 ] and high energy use in the cultivation, harvesting and drying stages [ 191 – 193 ].

Some studies which reported the high savings of GHG in comparison to diesel are based on the best-case assumptions that may not be feasible for large-scale implementation. These include the use of CO 2 from cement plants as a feedstock [ 216 ], cane sugar as a nutrient/feedstock [ 217 ] and recycling of nutrients from anaerobic digestion plants [ 194 ] or wastewater [ 190 ].

(b) Energy use

Various indicators have been used in LCA studies to quantify energy use in the life cycle of biofuels, including fossil energy consumption, primary, secondary or cumulative energy demand and net energy ratio [ 218 ]. However, many focused on fossil energy consumption, given that energy security and reducing dependence on fossil fuels are key objectives of national policies on biofuels, in addition to climate change mitigation.

As indicated in figure 7 , most of the studies estimate that the fossil energy consumption for first- and second-generation biofuels is below 0.5 MJ MJ −1 . However, there is a wide variation across different types of biofuel, ranging from 0.04 to 0.86 MJ MJ −1 for first generation and from −0.57 to 0.87 MJ MJ −1 for second-generation biofuels (see electronic supplementary material, figure S7), where negative values are due to energy credits for co-products, such as electricity and heat. These variations are due to several factors, including differences in feedstock productivity, agricultural practices, conversion technologies and allocation methods. The results are also affected by the assumption on the type of energy (biomass or fossil) used in the conversion process.

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Fossil energy use in the life cycle of biofuels. Based on data from [ 17 – 19 , 40 , 47 , 48 , 54 , 55 , 57 , 58 , 60 , 68 , 76 , 77 , 80 , 85 , 89 , 92 , 93 , 95 , 96 , 98 , 108 , 113 – 115 , 120 , 121 , 129 , 132 , 133 , 136 , 138 , 139 , 143 , 144 , 147 , 152 – 154 , 156 , 159 , 161 , 162 , 164 – 167 , 169 – 171 , 175 , 176 , 179 – 181 , 187 , 190 – 192 , 195 – 199 , 204 , 206 , 208 , 219 – 223 ]. For the box plot legend, see electronic supplementary material, figure S1 and for the data used to plot this graph, see electronic supplementary material, figure S7. The value for third-generation biodiesel should be multiplied by 10 to obtain the actual value. ‘ A ’ refers to the number of LCA articles found in the literature and ‘ n ’ denotes the total number of analyses. (Online version in colour.)

The range of estimates for fossil fuel demand in the life cycle of algal biodiesel is even wider, ranging from 0.15 to 40.5 MJ MJ −1 ( figure 7 ; electronic supplementary material, figure S7). Like the GWP, the reasons for these variations are technological uncertainties and the diversity of potential feedstocks and production systems. However, most studies agree that algal biofuels are not energetically viable because of high energy requirements for pumping, dewatering, lipid extraction and thermal drying [ 141 , 191 , 224 – 226 ]. In general, algae cultivation in raceway ponds has lower energy demand than photo-bioreactors, with some studies suggesting that the former can have energy demand below 1 MJ MJ −1 [ 225 ].

(c) Water use

Water use in the production of feedstocks can be high, particularly for first-generation biofuels [ 5 , 227 , 228 ]. This is of concern where requirements for irrigation water for certain feedstocks might compete with water used for other purposes, such as food production. With increased agricultural biomass production for biofuels, the total global water consumption could increase significantly by 2050 [ 229 ] and, in areas that are already water stressed, additional water demand has a potential to substantially increase the overall environmental impacts of biofuels.

Water use is usually not included in LCA studies of biofuels, but there are numerous studies that have specifically focused on this aspect of biofuels production. Most of these provide a volumetric usage of water, such as the amount of green (soil moisture) and blue (surface) water. This is not sufficient to assess local environmental impacts of water consumption as these are highly dependent on the level of water availability in the local area and the specific characteristics of the hydrological cycle, even if the quantity used is the same for a particular product [ 230 ]. Furthermore, consideration of green water results in very large total water use for most agricultural crops. Since the local hydrological cycle may in reality be affected little by the use of green water in agriculture, inclusion of green water could overestimate the actual impact of water use for biofuels [ 231 ].

A more recent study [ 232 ] that assessed the water footprint of first-generation biofuels consumed in Europe suggests that blue water consumption of biofuels is very diverse, depending on the underlying crop and country ( figure 8 ). Bioethanol from sugar beet and wheat has lower water consumption because many countries produce crops using little or no irrigation. By contrast, the production of bioethanol from corn in Portugal consumes 86 m 3 GJ −1 . Furthermore, while no irrigation is needed to cultivate crops for biodiesel in the UK, Poland and Germany, in Spain, on average 90 m 3 of irrigation water is consumed to produce 1 GJ of crop-based biodiesel [ 232 ].

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Blue water consumption for biofuels consumed in Europe. Based on data from [ 232 ]. Data labels represent the average values. (Online version in colour.)

As indicated in figure 8 , the average blue water consumption of bioethanol and biodiesel consumed in Europe is 3.3 m 3 GJ −1 and 1.9 m 3 GJ −1 , respectively—this is 40 and 60 times higher compared to their respective fossil alternatives. If regional water stress is taken into account, as opposed to just the volume of water consumed, biofuels have water footprints a factor of 55–246 higher than fossil fuels [ 232 ]. This is a result of a large share of water consumption in the production of biofuels occurring in relatively water-stressed countries.

The blue water consumption of algae-based biofuels depends on the geographical location, production systems and conversion routes [ 233 ]. For example, the blue water consumption for biofuels produced in a closed photo-reactor in the Netherlands is estimated at 8 m 3 GJ −1 , while it can be as high as 193 m 3 GJ −1 if algae are cultivated in open pond systems in Hawaii [ 233 ]. There is also a difference between dry and wet conversion, with the blue water consumption being higher for the latter.

(d) Biodiversity

Biofuels have the potential to contribute to loss of biodiversity through habitat loss and degradation, excessive nutrient load and other forms of pollution, over-exploitation and unsustainable use of land, as well as the cultivation of invasive alien species used as feedstocks [ 234 ]. The impact of biofuel production on biodiversity depends on the feedstock used and scale of production, management practices and LUC [ 235 ].

Intensive cultivation and use of agro-chemicals in the feedstock production for first-generation biofuels can create direct threats for local biodiversity [ 236 , 237 ]. LUC resulting from increased biofuel production exacerbates the risk of losing biodiversity through the direct loss of wildlife habitats, such as tropical rainforests [ 5 , 15 ].

Compared to first generation, second-generation biofuels are considered to have fewer negative impacts on biodiversity and could even have a positive effect [ 238 ]. For plant-based lignocellulosic feedstocks, this is because of their long growth cycle, low requirement for fertilizers and pesticides and less human intervention needed during the growth period. For example, large-scale SRC willow can provide benefits for some bird species, butterflies and flowering plants [ 239 ]. Furthermore, if degraded land is used for cultivation of feedstocks, the diversity of species might be enhanced. Similarly, perennial grasslands used for biomass production may enhance avian diversity, including migratory species. However, large energy crop monocultures can be detrimental to local biodiversity, particularly through habitat loss and the expansion of invasive species [ 5 ]. Eucalyptus, switchgrass and some Miscanthus species exhibit some traits of invasiveness [ 240 ].

The use of forest and agricultural residues as biofuel feedstock is expected to have a lower negative impact on biodiversity than dedicated energy crops [ 238 ]. Some of the impact on biodiversity associated with the use of forestry residues includes reduction in the amount of decaying wood—a niche habitat—and disturbance of wildlife caused by increased forest access. Excessive removal of agricultural residue from fields would also be a concern as it may increase weed growth, which could lead to the increased use of herbicides and thus affect local biodiversity.

For algal biofuels, the impact on biodiversity is uncertain. The large-scale cultivation of algae can bring significant risk to coastal biodiversity through invasion by algal species of coastal shallow ecosystems, such as mud flats, salt marshes, mangroves, sea grass bed and coral reefs [ 236 ].

Although the biodiversity loss is identified as one of the current key environmental concerns, it is only seldom included as an impact category in LCA studies of bioenergy systems [ 241 ]. Preserving biodiversity or avoiding biodiversity loss from biofuels is one of the criteria in sustainability certification schemes. However, biodiversity loss is difficult to measure and there are no standard ways of identifying and measuring systems that promote biodiversity.

(e) Other environmental impacts

The LCA studies on biofuels have used different impact assessment methods to estimate the other environmental impacts. Therefore, it is difficult to compare them and provide a meaningful range of impacts for different biofuels. Furthermore, the studies differ in scope, with some considering the cradle-to-gate and others cradle-to-grave system boundary. Results of the latter studies also depend on assumptions regarding the type of vehicle in which biofuels are used. Nonetheless, several studies suggest that reduction in GHG emissions from biofuels compared to fossil fuels is carried out at the expense of other impacts, such as acidification and eutrophication [ 32 , 54 , 76 , 81 , 83 , 88 , 121 , 129 , 139 , 148 , 216 , 242 – 244 ].

These two impacts are compared in table 2 for different feedstocks relative to fossil fuels. As can be seen, first-generation bioethanol has up to three times higher acidification and 3–20 times higher eutrophication. Similarly, first-generation biodiesel has 30–70% higher acidification and 3–14 times greater eutrophication than the fossil alternative. These impacts are largely due to the use of fertilizers and associated emissions of acid gases and nutrients to air and water.

Table 2.

Acidification and eutrophication of biofuels relative to fossil fuels.

biofuel type	feedstock	acidification	eutrophication	source

bioethanol	corn	1.4–3	4.4–20	[ ]
	wheat	3	5	[ ]
	sugar beet	1.4–1.8	6–15	[ ]
	sugarcane	2	2.8	[ ]
biodiesel	rape seed	1.3–1.7	3.1–5	[ , ]
	soya bean	1.3–1.7	4–5	[ ]
	palm oil	1.3	14	[ ]

bioethanol	short rotation coppice	0.45	1.2	[ ]
	switchgrass	1.1	3.2	[ ]
	straw	1.6–3	2–3.6	[ ]
biodiesel	used cooking oil	0.2	0.63	[ ]
		1	1	[ ]

biodiesel	algae	2.6–3	2.1–3.2	[ ]

a The values represent the ratio of impacts from biofuels over fossil fuels and are dimensionless.

Lignocellulosic bioethanol from SCR performs better for acidification, but bioethanol from switchgrass and straw is worse than petrol for both impact categories. However, biodiesel from UCO has lower acidification and eutrophication than fossil diesel. These two impacts are also higher for algal biodiesel than for the fossil equivalent [ 216 ]. However, as mentioned earlier, the absence of full-scale plant data, large variability in production parameters and various assumptions lead to high uncertainty in the LCA estimates for algal biofuels [ 197 ].

4. Discussion

While many studies on biofuels have examined multiple scenarios and conducted sensitivity analyses, only a few have conducted comprehensive uncertainty analyses [ 63 , 197 , 245 ], demonstrating that the variability in results can be large. It is also clear from the findings discussed above that the outcomes of LCA studies are highly situational and dependent on many factors, including assumptions, data variation and gaps, models, methodology and software tools used. The outcomes of the study are also affected by the choice of allocation method, system boundaries and the cut-off criteria for auxiliary inputs. Especially in relation to GWP, there are significant uncertainties in models for estimating soil N 2 O emissions, direct and indirect LUC and the extent and duration of changes in soil and vegetation carbon stocks. The effects of these aspects on LCA results are discussed below.

The problems related to data availability and quality are inherent to LCA. It is always preferable to use site-specific inventory data for developing LCA models of biofuels. However, data availability is often limited, particularly for second- and third-generation biofuels that, along with the associated process technologies, are still under development. As such, the use of unrepresentative data or assumptions to fill data gaps becomes a source of uncertainty [ 29 ]. There is also a great deal of technical, spatial and temporal variability associated with agronomic practices, such as fertilizer inputs, cultivation intensities and yields, as well as with biofuel conversion processes. LCA results are highly sensitive to variations in crop yields, use of nitrogen fertilizer and energy sources for biofuel conversion processes. Original and measured field data are still scarce and many studies rely on secondary data. There is also a room for improvement in existing LCA databases and a need to develop better, open access databases with common assumptions. Many data in common usage are reportedly out-of-date and finding new data is often difficult and time-consuming.

(b) Methodological approaches

As mentioned earlier, ALCA and CLCA are different techniques that follow different methodologies and will normally have very different results that must be interpreted carefully based on the goal and scope of the study. For example, Searchinger et al . [ 246 ] found that using ALCA resulted in a 20% saving in GHG emissions from US corn ethanol compared to petrol. However, following a CLCA approach and considering the increase in output required by the US Energy Independence and Security Act lead to a 47% increase in emissions relative to petrol. This increase was related to LUC induced by higher prices of corn, soya bean and other grains as a consequence of the additional demand for corn for ethanol production.

As also mentioned earlier, CLCA is more suited for policy applications. However, the use of CLCA for policy is still in infancy and its application to biofuels is controversial and subject to criticism [ 29 , 247 ]. One of the main reasons is that consequential analysis is highly complex, being dependent on future projections, formulation of possible ‘what if’ scenarios and counterfactual circumstances, economic models of relationships between demand for inputs, price elasticities, supply and market effects of co-products, all of which can be highly uncertain [ 26 , 248 , 249 ]. There is also a real challenge in defining meaningful scenarios for how the world would develop with a biofuels policy or production in place. This is true for individual feedstocks all the way up to the economic and energy system models incorporated into CLCA studies. Therefore, caution should be exercised with the interpretation of CLCA results [ 249 ]. Furthermore, unlike ALCA, there is still no internationally agreed methodology for CLCA, making it difficult to carry out and compare different studies.

(c) Allocation methods

Allocation is one of the most controversial issues in LCA. Both system expansion and allocation are subject to shortcomings: for system expansion, the difficulty is to estimate various substitution effects (similar to the related consequential issues in CLCA), while different allocation methods produce very different results. For instance, allocation by mass could result in the majority of impacts being allocated to the co-products rather than the biofuel which is the main (economic) product, while allocation by product cost/price leads to changes in the estimates of environmental impacts over time with variations in costs/prices without any other changes in the system. Several studies considered more than one allocation approach and found that the results were highly affected. For instance, some authors [ 17 , 132 ] showed that biofuels had significantly lower environmental impacts when using system expansion instead of allocation. In some cases, system expansion can lead to the negative values, suggesting net savings in environmental impacts, including in GHG emissions. However, studies assessing uncertainty in LCA of biofuels showed that system expansion also results in higher uncertainties [ 63 , 65 ]. Other authors found that environmental impacts were higher if economic allocation was used instead of mass and energy allocation [ 250 ]. For some biofuels, the co-products are sufficiently substantial that choice of allocation procedure can tip the balance between net benefit and net impact.

(d) Emissions of soil nitrous oxide

Emissions of N 2 O arise from application of nitrogen fertilizer and decomposition of organic matter in soil. N 2 O is a potent GHG with a GWP 265 times higher than CO 2 [ 41 ]; hence, its emission can have a significant effect on the GHG balance of biofuels. The N 2 O emissions are particularly significant for first-generation biofuel crops since fertilization rates are larger for these than for second-generation biofuels from perennial energy crops, which are usually grown without fertilizers, except during the initial establishment of the crop [ 251 , 252 ].

LCA studies often use the ‘Tier 1’ methodology developed by the Intergovernmental Panel for Climate Change (IPPC) to estimate N 2 O emissions from fertilizers [ 253 ]. According to this method, 1–1.5% of nitrogen in synthetic fertilizer applied to crops is emitted as N 2 O [ 253 ]. Since in reality, the occurrence and level of N 2 O emissions depend on many factors, including soil characteristics and local weather following fertilizer application on the soil, the default IPCC emission factors represent an uncertain estimate [ 23 ]. For example, a study by Crutzen et al . [ 254 ] suggested that N 2 O emissions in feedstock production can be three to five times higher than those estimated based on the IPCC methodology. Inclusion of these variable N 2 O rates leads to dramatically different estimates of GHG emissions in the life cycles of biofuels. For instance, for corn ethanol, the nitrogen conversion of 5% instead of 1.5% could change its GHG savings relative to petrol from around 40% to zero [ 255 ].

Conversely, a recent study in the UK concluded that N 2 O emissions averaged across arable land in the UK are below those determined by following the IPCC guidelines [ 256 ]. Compared to the default IPCC emissions factor of 1% (of the amount of nitrogen applied), direct N 2 O emissions from soil related to the use of fertilizers on crops for first-generation biofuels were estimated to be, on average, 0.46% of the nitrogen applied. However, the study noted that any one instance of fertilizer application is subject to interacting effects of rainfall and soil type, such that fertilizer-induced emissions could also be larger than the default IPCC emission factors in the wetter regions of the UK. Thus, in summary, the estimates of N 2 O emissions are highly variable and uncertain and should be treated with caution when interpreting the results.

An increasing global demand for biofuels highlighted the potential for the competition for land between cropland and natural ecosystems. Early LCA studies on biofuels, which excluded LUC, concluded that first-generation biofuels, such as corn ethanol, had lower GWP than petrol [ 257 ]. However, when attempts were made to account for the LUC effects of the expansion of first-generation biofuels, these findings came under question [ 43 , 246 ]. Since then, several other studies have cast doubt on the ability of first-generation biofuels to meet mandatory GHG savings targets if LUC is involved [ 119 , 258 ].

From an LCA perspective, DLUC is relatively straightforward and easy to include in the assessment, although the uncertainty remains high. However, estimating ILUC related to biofuels remains difficult, complex and highly uncertain [ 259 , 260 ]. The latter is exemplified by that fact that estimates of GHG emissions from ILUC range widely, from very small to very large [ 261 ]. For instance, a study on the ILUC associated with US corn ethanol found that the ILUC emissions varied from 10 to 340 g CO 2 eq. MJ −1 [ 262 ]. For these reasons, the effects of ILUC and how to account for them in assessing the sustainability of biofuels are key areas requiring further research and consensus building [ 42 , 260 ]. Part of the challenge is constructing and analysing credible counterfactual scenarios. Another challenge is the economic (equilibrium) models used for consequential modelling [ 247 , 263 ] and the assumed yield-price elasticities for crops [ 26 ]. ILUC models make various assumption to estimate how much indirect change might be induced up to 20 years into the future under prescribed scenarios. Therefore, such estimates would only apply for the assumed conditions and must be interpreted with caution [ 264 ]. The lack of transparency in ILUC models, many of which are proprietary, is also problematic.

There is an ongoing question about how policymakers should respond to the growing evidence on ILUC from biofuel production. The blanket application of ‘ILUC factors’ according to feedstock type is unpopular as it offers producers no opportunity to improve the performance of their individual supply chains [ 265 ]. Moreover, there are many other drivers of LUC besides biofuels, such as demand for food and timber, urban development and infrastructure, leading some to argue that it is unfair to consider ILUC only for biofuels [ 247 , 266 ].

(f) Soil organic carbon

SOC is one of the largest carbon pools in the terrestrial ecosystem [ 267 ]. Its balance is affected because of agricultural activities and LUC. Depending on various soil characteristics and agricultural practices, soil can act as either a sink or a source of carbon emissions. Soils may lose SOC by mineralization through cultivation, emitting CO 2 to the atmosphere. Alternatively, SOC may increase through cropping or from repeated addition of crop residues or organic manures [ 268 ].

When biomass is left to decay in the soil, a part of the carbon in the biomass is sequestered into soil. Therefore, assuming biomass would have otherwise been left to decay in the soil, harvesting it decreases SOC and this may affect significantly the GHG balance of a biofuel [ 269 , 270 ]. For example, a study that included the effects of the removal of corn residue across the US corn belt concluded that the GWP of corn stover ethanol may exceed that of conventional petrol [ 271 ]. Another study on wheat-straw ethanol suggested that there is only a 30% probability that its GHG emissions will be 35% lower than that of petrol if SOC changes are included in the analysis [ 272 ]. A study on sugarcane ethanol claimed that the GHG balance of sugarcane ethanol could be significantly higher if the impacts on SOC from pre-harvest burning were considered [ 210 ]. The burning of biomass in the field, which is often carried out prior to a sugarcane harvest to help manual harvest, means that far less crop residues are left on the land to be incorporated into the soil.

Changes in SOC can also have a major influence on GHG emissions from LUC associated with biofuel feedstock production [ 267 , 273 ]. For example, reversal of grassland, woodland and perennial crops back to arable lands could reduce soil carbon by 0.6–1.7 t C ha −1 yr −1 , which would be emitted to the atmosphere as CO 2 (2.2–6.2 t ha −1 yr −1 ). On the other hand, cultivation of perennial energy crops, such as SRC and Miscanthus , could sequester CO 2 from the atmosphere into the soil at the rate of 2.2 t CO 2 ha −1 yr −1 [ 267 ]. However, the sequestration potential is very site-specific and highly dependent on former and current agronomic practices, previous land use, as well as climate and soil characteristics [ 17 , 40 , 267 , 274 – 276 ]. Therefore, quantifying changes in SOC storage is an important factor in estimating GHG emissions of biofuels [ 277 ]. However, most LCA studies do not account for potential SOC changes from biomass cropping systems. This is probably due to inherent complexity of soil science, the high degree of intra- and inter-site variability, substantial data uncertainties and the challenges of linking biomass feedstock supply to specific soils [ 46 ]. Furthermore, there is no consensus in LCA on how to account for SOC change of agricultural activities and delayed GHG emissions [ 278 ]. However, the work on developing models to estimate SOC emissions related to biofuels is ongoing [ 273 – 275 ].

(g) Biogenic carbon

In the context of biofuels, the term biogenic carbon refers to CO 2 that is sequestered from the atmosphere during the growth of feedstocks and subsequently released during the combustion of the biofuel. ‘Carbon neutrality’ is achieved when CO 2 sequestered and subsequently released are in balance. However, carbon neutrality cannot be claimed if there is a potential imbalance or a time delay between the amount of CO 2 taken up during feedstock growth and the amount released through biofuel production and use. Since many bioenergy products—including annual crops and perennial grasses—have relatively short lifespans, carbon neutrality is commonly assumed in LCA standards and regulations. Hence, most LCA studies of biofuels assume that biogenic CO 2 emissions, both from end-use combustion and the burning biomass to produce energy for conversion processes, are fully balanced by CO 2 uptake during feedstock growth. While this assumption is reasonable for fuels from annual crops and perennial grass feedstocks, it is open to challenge in relation to biofuel production from feedstocks with harvest cycles of more than a few years—such as longer-lived lignocellulosic feedstocks from forestry [ 26 , 279 ]. For such feedstocks, it is important to consider the balance of carbon sequestered during feedstock growth versus that which is emitted during biofuel production and use, together with the overall time profile of biogenic carbon storage, emission and re-sequestration [ 279 ].

Different approaches to account for the temporal impact of carbon emissions are suggested in the literature; for example, carbon payback period, carbon discounting and time-integrated accounting of biogenic carbon [ 279 , 280 ]. Where accounting for the carbon storage in other, more long-lived bio-based products is required, there are various standards and methods [ 46 ] and these contain significant procedural differences. For example, GHG Protocol [ 281 ], PAS 2050 [ 282 ] and ISO 14067 [ 283 ] require reporting of emissions and removal of GHG emissions from biogenic carbon sources, while regulations the RED [ 8 ] and RFS [ 7 ] do not require such reporting. Furthermore, the time between the production of the product (storage of biogenic carbon) and its end of life (release of biogenic carbon), referred to as ‘delayed emissions’, varies among the standards. For instance, in PAS 2050 [ 282 ], all emissions that occur within a 100-year period are quantified and treated as if they occurred at the beginning of the time period. By contrast, ISO 14067 [ 283 ] makes a distinction between emissions released within and after the first 10 years.

(h) Other environmental impacts

Production and use of biofuel generate emissions of various air pollutants, including particulate matter (PM), carbon monoxide (CO), nitrogen oxides (NO x ), hydrocarbons and volatile organic compounds (VOCs). Unburned hydrocarbons, VOCs and NO x are precursors for the formation of summer smog and ground-level ozone. These pollutants are associated with increased morbidity and mortality from cardiovascular and respiratory diseases and certain cancers [ 284 , 285 ]. Air quality modelling studies show that life cycle emissions of some pollutants may be higher for biofuels when compared with fossil fuels, largely resulting from the emissions associated with feedstock production and biofuel processing [ 284 , 286 ]. For example, in the case of sugarcane ethanol in Brazil, burning of straw in fields is the common practice in certain areas and is the predominant source of PM [ 284 , 286 ]. Studies on health impacts of sugarcane ethanol in Brazil suggest that there is strong evidence that burning straw in sugarcane fields causes substantial respiratory diseases, such as asthma and pneumonia, in sugarcane fieldworkers and local populations [ 284 , 286 – 289 ]. These effects are often ignored in LCA studies.

In cradle-to-grave LCA studies, assessing impacts of vehicular exhaust emissions is another challenge as they are affected by many different parameters, including the type of engine and how it is run (the operational drive cycle), vehicle age and maintenance, the quality of the base fuel and exhaust after treatment [ 290 ]. Vehicular exhaust emissions of bioethanol blends vary with blend strength. However, in general, lower bioethanol blends (E5–E15) have lower CO and PM emissions compared to petrol [ 290 , 291 ]. Beer et al . [ 291 ] suggest that lower PM emissions from low-ethanol blends used in spark-ignition vehicles have slight health benefits over petrol. However, they lead to significantly higher emissions of acetaldehyde, which is one of the precursor VOCs involved in ground-level ozone formation. Similarly, higher ethanol blends (E85) lead to comparable, or slightly lower, levels of PM, NO x and CO emissions than petrol, but 5–10 times higher acetaldehyde emissions [ 290 , 292 , 293 ].

Compared to fossil diesel, biodiesel has generally lower exhaust emissions of PM, CO, hydrocarbons and VOCs, but higher NO x emissions [ 294 , 295 ]. These differences are small for 5–20% biodiesel blends and would lead to negligible or non-measurable impacts on air quality [ 294 ], but increase with higher blends [ 290 ]. On the other hand, Larcombe et al . [ 296 ] argue that, despite having lower PM emissions, biodiesel exhaust emissions could potentially be more harmful to human health because of higher proportion of ultra-fine particles (less than 100 nm diameter) compared to diesel exhaust. This is due to the fact that smaller particles remain suspended in the air for longer, are more easily inhaled and are able to penetrate more deeply into the lungs. However, other assessments on the potential human health implications of biodiesel suggest that the use of biodiesel fuel blends compared to fossil diesel results in minimal changes in health impacts [ 294 , 295 ]. Thus, the topic of human health impacts from biofuels remains open to debate, requiring further research and evidence.

Besides air pollution, production of liquid biofuels could affect human health directly through water and soil pollution and occupational hazards [ 284 ]. However, these effects are scarcely discussed in the literature and should be explored further to understand whether there are risks that need to be addressed.

5. Conclusion and recommendations

LCA is widely used as a tool to estimate GWP and other environmental impacts of biofuels. However, as evident from this review, the estimates vary widely among the studies owing to a wide range of methodological choices in LCA and various uncertainties. Despite this, the existing evidence base is instructive. Firstly, it shows that, if no LUC is involved, first-generation biofuels can—on average—have lower GHG emissions than fossil fuels, but GHG savings for most of the feedstocks are not sufficient to meet those required by the RED. Secondly, in general, second-generation biofuels have a greater potential than first generation to reduce GHG emissions, again provided there is no LUC. However, the development of second-generation biofuels will take time and is likely to depend on the continued support of first-generation fuels to give the industry the confidence to invest. Thirdly, it is also clear that, at present state of development, third-generation biofuels from algae are unlikely to make a contribution to the transport sector as their GHG emissions are higher than those from fossil fuels. Moreover, they are unproven and expensive to produce and, as such, the algal feedstock will continue to be restricted to high-value markets, such as cosmetics and dietary supplements.

LCA is a complex tool that lies at the interface between science, engineering and policy. Despite this complexity, it is often perceived as a tool that can give a definitive answer to multifaceted questions. As the findings in this review demonstrate clearly, there are no definitive answers. Even focusing only on the GWP of biofuels—one of the main drivers for their development—brings with it a host of uncertainties. Moreover, almost every aspect related to biofuels is dynamic in nature across different scales, which adds to the complexity. Examples include changes in soil carbon content over time (micro-scale); time needed to replace vegetation used as feedstock for biofuels (meso-scale); and development of global biofuel supply chains (macro-scale). Considering these dynamic aspects and their interconnections presents a considerable challenge. There are also significant uncertainties in the models for estimating direct and indirect LUC, changes in SOC stocks and N 2 O emissions. It is important to recognize these limitations and interpret the results accordingly.

In addition to the environmental impacts, there are many other sustainability issues that must be considered when assessing the sustainability of biofuels. These include: costs of production and competitiveness with fossil fuels; food, energy and water security; employment provision; rural development; and human health impacts. It is essential that the sustainability aspects of biofuels be evaluated on a life cycle basis across full supply chains to avoid shifting the burdens from one part of the life cycle or supply chain to another. It is also important to note that LCA and wider sustainability assessments are of little use if the results cannot be trusted. Therefore, strong auditing of biofuel supply chains is vital to prevent negative socio-economic effects as well as to ensure traceability of the fuels and to mitigate the risk of fraud. Moreover, improving transparency, data availability and sharing are key if LCA is to be trusted and useful for policy. This could be achieved through development of open national and global databases, in a similar way that national inventories have been developed for GHG reporting under the Kyoto Protocol. It is also important to ensure that the data and models from different disciplines that are used in LCA preserve reasonable levels of transparency, rigour and robustness to avoid misuse and misinterpretation.

ALCA studies, which account for the direct impacts, should follow the ISO 14040 and 14044 standards more rigorously. For CLCA, both methodological and practical aspects need improvements. For the former, further work is required towards the standardization of CLCA methodology. As part of that, there is a need to improve development of counterfactual (what if) scenarios and ILUC models. Involvement of multiple stakeholders can help to build consensus on the definition of the scenarios and to improve the transparency of ILUC models, their assumptions and the associated uncertainty. In addition to improving the CLCA methodology, much work is required in its application in practice. Specifically, there is a need to validate ILUC models with empirical evidence; empirical methods to test alternative hypotheses also require attention. Further work is also needed on the development of models and empirical evidence of changes in soil and plant carbon stocks as well as emissions of nitrous oxide related to the application of fertilizers. Research is also needed on estimations of biogenic carbon, particularly changes in the forest carbon stock that may be affected by an increase in biofuels demand.

It is also important to take into account that biofuels do not exist in isolation but are part of much wider systems, including energy, agriculture and forestry. Like other production systems with which they interact, biofuels impact on various ecosystem services, such as land, water and food. It is, therefore, essential to take an integrated, systems view to developing future policy to ensure that biofuels are not disadvantaged relative to other sectors or that progress made in this sector is not undone by unsustainable practices in others. Analysis and, ultimately, policies based on ecosystem services and natural capital at a landscape level are needed to make the best overall use of land. This would, in turn, optimize ecosystem services, such as carbon storage, biodiversity, reductions of agricultural run-off and increases in water quality and flood risk management. Complete value chains rather than single bioenergy products should be analysed together to understand the interactions across sectors and land uses with the goal of identifying opportunities where collective benefits can be realized.

Supplementary Material

Acknowledgements.

This review was originally carried out as part of the Royal Academy of Engineering study on Sustainability of liquid biofuels and subsequently fully updated as part of a Research Councils UK project (EP/K011820/1). The suggestions and comments by members of the Academy's expert working group, the Academy's Engineering Policy Committee as well as external stakeholders are gratefully acknowledged.

Data accessibility

Authors' contributions.

H.K.J.: literature review, analysis and presentation of results, paper writing. A.C.: literature reivew, paper drafting. A.A.: conceptualization, supervision, paper writing.

Competing interests

We declare we have no competing interests.

The study was funded by Department for Business, Energy and Industrial Strategy (BEIS), Department for Transport and Research Councils UK.

Aboriginal Heart Health Grants

Fostering Aboriginal innovation and building capacity in cardiovascular health research.

The Aboriginal Heart Health Grants fund high impact Aboriginal-led research. The $5 million in funding is part of the Cardiovascular Research Capacity Program, the NSW Government’s $150 million investment over 10 years into cardiovascular research.

The Aboriginal Heart Health Grants aim to:

improve cardiovascular health outcomes for Aboriginal peoples
increase the number of targeted Aboriginal cardiovascular research projects being undertaken in NSW, and
build the capacity of Aboriginal communities and researchers in cardiovascular focused research.

Aboriginal Heart Health Grants 2024 – Expressions of Interest now open

EOI Applications close 13 December 2024 at 5pm AEDT.

Information webinar: 16 October 2024, 12pm – 1pm – Register here

Aboriginal Heart Health Grants have a three-year duration.

There are two funding streams:

Stream 1: Maximum grant of $750,000 to conduct a multidisciplinary project, which has undertaken a consultation process with Aboriginal communities to identify a need/priority and solution/ innovation/strategy to be tested in the cardiovascular space. Feasibility studies can be included in this stream. Research in this stream can use quantitative and/or qualitative methods.

Stream 2: Maximum grant of $250,000 for early-stage developmental/exploratory research project, where a consultation process is required with Aboriginal communities to determine an identified priority/need, better understand its causes, and suggest solution(s)/innovation(s)/strategy(ies) to address the need in the cardiovascular space. This type of research should provide the groundwork for a feasibility study. Research in this stream can use quantitative and/or qualitative methods e.g. yarning interviews or circles with community or research participants.

The grant requested should be appropriate for the type, stage and scale of research proposed.

The artwork depicts a heart, surrounded by veins, arteries, blood cells and icons relating to Aboriginal and Torres Strait Islander people.

‘A Heart for Health’ by Carissa Paglino. More details in the artist bio below. “The interwoven lines in the artwork show the connection between heart health and Aboriginal and Torres Strait Islander people as well as cardiovascular research. Various aspects of cardiology and the heart anatomy, such as veins, arteries and blood cells, are represented in the piece. The artwork calls for healthier communities, better heart health research and improved health outcomes. The colours are reflective of the circulatory system.”

Who can apply

All projects must be led by a Lead Chief Investigator who identifies as Aboriginal or Torres Strait Islander.

Applications are encouraged from Aboriginal Community Controlled Organisations, Aboriginal Medical Services, Aboriginal peak bodies, local health districts, specialty health networks and other public health organisations.

Researchers from universities, medical research institutes and not-for-profit organisations are also eligible to apply.

Support for applicants

Information webinar: 16 October 2024, 12pm – 1pm – Register h ere

The Office for Health and Medical Research will host an information webinar to help applicants understand NSW Health requirements and gain advice on how to prepare a high quality EOI. There will also be an opportunity for applicants to ask any questions about the grant round.

Registrations for Sax Support: 18 October 2024 – Register here

The Sax Institute is offering two optional services for Aboriginal Heart Health Grant applicants:

a concierge service that links applicants to academic partners or researchers with appropriate methods expertise for the project
support with the development of applications.

Further information about the Sax Institute support service is in the Grant Guidelines on page 12.


Call for Expressions of Interest opens	September 2024
Information webinar for Expressions of Interest	16 October 2024
Registration for Sax Support close	18 October 2024
Expressions of Interest close	13 December 2024 (5pm AEDT)
Applicants notified of Expressions of Interest outcome Full Applications open	23 May 2025
Information webinar for Full Applications	29 August 2025 (5pm AEST)
Full Applications close	11 June 2025
Applicants notified of Full Application outcome	By 19 December 2025

Depending on the number of expressions of interest received, this timeline may be shortened.

Application and submission process

Aboriginal heart health grant guidelines 2024, stream 1 expression of interest form.

DOCX - 2 MB

Stream 2 Expression of Interest Form

NSW Health invites eligible researchers to submit an Expression of Interest for Aboriginal Heart Health Grants.

The Application process includes two stages:

Stage 1: Expression of Interest

All EOIs should be submitted by email to [email protected] by 5pm (AEDT) on 13 December 2024 .
Applicants must use the Expression of Interest Form above and attach any supporting evidence. Please submit both a word and PDF version of the application.
All email applications will receive an email to acknowledge receipt within 72 business hours. It is the applicant’s responsibility to follow up if no acknowledgement email is received.
The maximum email size is 20MB . Larger emails will be rejected by the NSW Health server and you may not be notified that the email has been rejected.

Any queries regarding Aboriginal Heart Health Grants may be directed by email to: [email protected] .

Stage 2: Full application

Detailed instructions will be provided to applicants successful at EOI stage.

Further information around the application submission process is in the Grant Guidelines on page 14.

Additional resources

Researchers may find the following resources useful in developing their application.

Aboriginal Health Ethics Guidelines Key Principles – AHMRC

AIATSIS Code of Ethics for Aboriginal and Torres Strait Islander Research

AIATSIS Engaging with traditional owners

AIATSIS Principles for engagement in projects concerning Aboriginal and Torres Strait Islander peoples

Ethical conduct in research with Aboriginal and Torres Strait Islander Peoples and communities | NHMRC

NSW Aboriginal Health Plan 2024‑2034 – Aboriginal health

NSW Ministry of Health: Quick-Guide-on-Undertaking-Appropriate-Aboriginal-Health-Research

Pathways into Research Toolkit – Lowitja Institute

Carissa Paglino was born and raised in the city of Newcastle. She is a proud descendant of the Wanaruah nation. She has always had a great passion for art and design, with her art being featured in several exhibitions and winning various awards. From 2004 to 2015, she worked as a Graphic Designer for Miromaa Aboriginal Language & Technology Center. She has pursued a full-time career as a Freelance Graphic Designer since 2015. Although she practices many forms of art, her professional forte is contemporary Aboriginal Graphic Design. She designs symbols and motifs that are influenced by traditional Aboriginal art and present them in bold, new ways, creating her own unique style.

Updated 2 days ago

Projects Align with Administration’s New Goal of Decarbonizing the Aviation Sector by 2050

WASHINGTON, D.C. – As part of a White House roundtable to launch the Sustainable Aviation Fuels (SAF) Grand Challenge to decarbonize the aviation sector by 2050, the U.S. Department of Energy (DOE) today announced $64.7 million in funding for projects focused on producing cost-effective, low-carbon biofuels. These investments will advance technologies to create replacements for petroleum fuels used in heavy-duty forms of transportation, like airplanes and ships, and accelerate America’s path to a net-zero emissions economy by 2050.

“Decarbonizing transportation – particularly planes and ships that are difficult to electrify – is an essential part of the path to a net-zero carbon future,” said Secretary of Energy Jennifer M. Granholm. “These investments mobilize industries to join this effort, which will create new, good-paying jobs across the biofuels, chemical, and agricultural supply chains and boost economic activity in rural economies.”

As part of the SAF Grand Challenge, DOE also signed a memorandum of understanding with the U.S. Department of Transportation and U.S. Department of Agriculture to collaborate on the needed research, development, and demonstration (RD&D) to reach the goals of supplying at least 3 billion gallons of SAF per year by 2030 and sufficient SAF to meet 100% of aviation fuel demand – currently 35 billion gallons per year – by 2050.

These efforts seek to cut carbon emissions from the aviation and shipping industries, which – because of their size – are more challenging to electrify. Biofuels, which are produced by converting the renewable carbon from recently living organic materials like crop waste, food waste, and algae into a liquid fuel, can serve as a low-carbon equivalent to fossil-based fuels such as gasoline, jet, and diesel fuel.

“I’m thrilled that the Department of Energy has awarded a grant to Lignolix, a company with roots in Professor Thomas Epps’ lab at the University of Delaware, to support their work to transform plant waste into clean fuels and sustainable materials – another success story out of Delaware's entrepreneurship community,” said U.S. Senator Chris Coons (D-DE).

“We must cut emissions in aviation and commercial shipping if we’re going to meet our net-zero goals,” said U.S. Senator John Hickenlooper (CO). “This DOE research grant will put Colorado’s National Renewable Energy Research Lab at the center of those efforts and at the forefront of biofuel research.”

“I am proud that the National Renewable Energy Laboratory (NREL), located in my district, is helping lead the way to reduce emissions from the aviation industry. NREL’s work will help lower the price of cellulosic sugars, which can be used to make sustainable aviation fuel or other biofuels and bioproducts,” said U.S. Representative Ed Perlmutter (CO-07). “By reducing the price of biofuels, we can incentivize the use of more sustainable options in the aviation industry and help accelerate our path to a cleaner future.”

“Biofuels are a critical component of our nation’s energy portfolio and our agriculture economy,” said U.S. Representative Rodney Davis (IL-13). “They help reduce emissions while promoting American energy independence. This DOE grant will assist ADM and others in developing new innovations in the biofuels sector. I look forward to watching ADM utilize this grant and continuing to partner with industry stakeholders to further promote the production and use of biofuels.”

The 22 selected projects target high-impact bioenergy technology RD&D to bolster foundational knowledge and scale up systems to produce low-carbon biofuels at lower costs. Among the projects are:

National Renewable Energy Laboratory (Golden, CO) will lower the cost and carbon intensity of producing a highly fermentable sugar from corn stover. (Award amount: $2,800,000)
Archer Daniels Midland (Decatur, IL) will couple isobutanol (a precursor for sustainable aviation fuel) fermentation with a membrane separations system – reducing energy used in the separation process by 50%. (Award amount: $3,466,844)
Alder Energy (Charleston, SC) will convert miscanthus, a highly promising biomass crop, to SAF through their advanced pyrolysis oil technology, a process that utilizes heat, pressure, and solvents to deconstruct the miscanthus into oils for conversion to SAF. (Award amount: $3,000,000)
D3MAX (Grand Forks, ND) will design a pilot plant to validate technology for first generation ethanol plants to produce ethanol from corn stover, which is then converted to SAF. (Award amount: $499,988)
T2C Energy (Pinellas Park, FL) was selected to design a demonstration scale plant that converts waste landfill gas to SAF or renewable diesel. (Award amount: $533,619)
AVAPCO (Thomaston, GA) will demonstrate a production process for clean, affordable cellulosic sugars, which are derived from agricultural or woody waste residues and can help reduce greenhouse gas emissions for a variety of products including SAF, bioplastics, and biopolymers. (Award amount: $2,800,000)
Quasar Energy Group (Independence, OH) will use an anaerobic digestor to convert food waste to SAF precursors. The microbes that digest the food waste produce a type of chemical called a volatile fatty acid, which can be converted to SAF. (Award amount: $3,500,000)

These investments are administered by DOE’s Bioenergy Technologies Office (BETO), which is focused on developing technologies that convert domestic biomass and other waste resources into low-carbon biofuels and bioproducts. BETO is increasing its emphasis on partnering with industry to demonstrate technologies at large scale in recognition of the urgent need to reduce risks and scale-up SAF production.

For more information about the selected projects, visit BETO’s website .

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DOE Office of Science: Contributions to Biofuel Research. The Department of Energy Office of Science, Biological and Environmental Research program provides support for research on advanced biofuels and bioproducts developed from non-food lignocellulosic plant biomass. These efforts take place in the four DOE Bioenergy Research Centers.
Recent advances and viability in biofuel production
However, scientific research has shown that various biofuels differ massively in the greenhouse gas balance when compared with petrol despite the potential advantages. Based on the techniques used for processing the fuel and production of the feedstock, certain crops may also emit more greenhouse gases than fossil fuels do [52], [53].
Biofuel Research Journal
Cover art by BiofuelResJ. ©2024. Biofuel Research Journal (BRJ) is a leading, peer-reviewed academic journal dedicated to publishing high-quality research on biofuels, bioproducts, and related biomass-derived materials and technologies. BRJ is an open-access online journal and completely free-of-charge, aiming to advance knowledge and ...
Biofuels and their sources of production: A review on cleaner
The third and fourth generation also provides a better option for generating biofuels, but the research expedition is time-consuming and requires expert handling. One of the significant limitations of biofuel production is its implication on energy, water and food nexus. The growing demand and utilization of biofuels has developed a direct ...
Biofuels: present and future
Biofuels represent a promising departure from conventional fossil fuels, presenting viable remedies for both energy security and environmental apprehensions. This review intricately examines the various realms of biofuels, encompassing their historical progression, present status, obstacles, and outlook. Commencing with an in-depth exploration of their historical antecedents and developmental ...
Biofuels an alternative to traditional fossil fuels: A comprehensive
Biofuels are fossil fuel alternatives produced from agricultural biomass or other organic matter; considered sustainable, eco-friendly, and bioeconomic biofuels have come up as a topic of discussion for over a decade. Their practical use depends on the production methods, low cost-technology implementation, and substrate used.
The potential of biofuels from first to fourth generation
The four generations of biofuels. One alternative to fossil fuels are biofuels, which originate from organic matter and therefore can be regrown and are termed renewable. Biofuels emit less GHGs and are in general more eco-friendly (non-toxic, sulfur-free, biodegradable) than their fossil fuel predecessors [5].
Inexpensive, carbon-neutral biofuels are finally possible
Inexpensive, carbon-neutral biofuels are finally possible. February 15, 2024. Jules Bernstein, UC Riverside. Credit: iStock/Dudaeva. When it comes to making fuel from plants, the first step has always been the hardest — breaking down the plant matter. A new study finds that introducing a simple, renewable chemical to the pretreatment step can ...
Sustainability of the four generations of biofuels
Biofuel has emerged as an alternative source of energy to reduce the emissions of greenhouse gases in the atmosphere and combat global warming. Biofuels are classified into first, second, third and fourth generations. Each of the biofuel generations aims to meet the global energy demand while minimizing environmental impacts.
Environmental sustainability of biofuels: a review
Research is also needed on estimations of biogenic carbon, particularly changes in the forest carbon stock that may be affected by an increase in biofuels demand. It is also important to take into account that biofuels do not exist in isolation but are part of much wider systems, including energy, agriculture and forestry.
Biofuel Innovation: Clean Energy Solutions, Ready Today Fact Sheet
Innovations in biofuels research are leveraged today in transportation technologies and infrastructure. The clean energy future is enabled by the U.S. Department of Energy (DOE) Bioenergy Technologies Office (BETO) investment of $255 million enacted in Fiscal Year 2021 for biofuels research and development (R&D). With over 300 active R&D ...
Microbial production of advanced biofuels
Carbon-efficient biofuel production. An important aspect of microbial production of biofuels is the conservation of carbon that is converted from biomass substrates to fuel products. A challenge ...
Biofuels Production: A Review on Sustainable Alternatives to
Nowadays, biofuel is a tremendous research area in which various advanced technologies used for biofuel production have also been developed, but fossil fuels cannot be completely replaced, and numerous integrated approaches in biological and genetic engineering are still important to standardize biofuel production at a marketable scale ...
Biofuel Basics
ETHANOL. Ethanol (CH3CH2OH) is a renewable fuel that can be made from various plant materials, collectively known as " biomass.". Ethanol is an alcohol used as a blending agent with gasoline to increase octane and cut down carbon monoxide and other smog-causing emissions. The most common blend of ethanol is E10 (10% ethanol, 90% gasoline ...
(PDF) Biofuel production: Challenges and opportunities
Predominantly, biofuels are produced from photosynthetic organisms. such as photosynthetic bacteria, micro- and macro-algae and vascular land plants. The. primary products of biofuel may be in a ...
Biofuels for a sustainable future
Biofuel technology has evolved through several generations of significant advancements. The predominant problem with first-generation biofuels is that they are derived from food crops (e.g., corn and sugar cane), which require fertilization, water, and soil, and thus directly compete with food production. ... This is a novel research area that ...
Biofuel on the road to energy, cost savings
Finding biofuel pathways . Co-Optima's research builds on the goal to identify and understand bioblendstocks, or biofuel. Biofuel is produced from biomass — organic materials including plants, agricultural waste and wet waste. Biofuel can be blended with conventional fuel to reduce emissions and improve fuel and engine performance.
Biofuel research: perceptions of power and transition
Whether biofuels represent a sustainable innovation, a creative alternative, or a gold rush, very much depends on our perception of power and change with regard to sustainability. This article provides an overview of existing understandings of power in the research on biofuels, including positive perceptions that often lead to more optimistic evaluations of biofuels. It exposes the diversity ...
Biofuel
Biofuel is a fuel that is produced over a short time span from biomass, rather than by the very slow natural processes involved in the formation of fossil fuels such as oil. Biofuel can be produced from plants or from agricultural, domestic or industrial biowaste. [1] [2] Biofuels are mostly used for transportation, but can also be used for heating and electricity.
6 New Things Happening with Biofuels
While fuels for transportation are the primary targets of biofuels research, many products today -- like plastics and paints -- are also made from petroleum, which can be replaced with new forms of biofuel. Photo courtesy of the California Center for Algae Biotechnology. 4. Robots are going to improve the plants used to make fuel.
Press Releases
About Bio-Techne. Bio-Techne Corporation (NASDAQ: TECH) is a global life sciences company providing innovative tools and bioactive reagents for the research and clinical diagnostic communities. Bio-Techne products assist scientific investigations into biological processes and the nature and progress of specific diseases.
Environmental sustainability of biofuels: a review
Research is also needed on estimations of biogenic carbon, particularly changes in the forest carbon stock that may be affected by an increase in biofuels demand. ... Environmental life cycle assessment of bio-fuel production via fast pyrolysis of corn stover and hydroprocessing. Fuel 131, 36-42. ( 10.1016/j.fuel.2014.04.029) [Google Scholar ...
Stifel Nicolaus Remains a Buy on Cabaletta Bio (CABA)
Stifel Nicolaus analyst Benjamin Burnett maintained a Buy rating on Cabaletta Bio (CABA - Research Report) today and set a price target of $26.00.The company's shares closed yesterday at $3.95
Biofuels at Sea: Marine Research Engine Plots ...
A marine research engine at the U.S. Department of Energy's (DOE) Oak Ridge National Laboratory (ORNL) is providing scientists with valuable insights into biofuel design for large ocean-going vessels (OGVs) in a multi-lab project focused on reducing total life-cycle carbon emissions from this vital transportation sector. When the International Maritime Organization (IMO) announced in 2023 ...
NSW Health & Medical Research
Research in this stream can use quantitative and/or qualitative methods e.g. yarning interviews or circles with community or research participants. The grant requested should be appropriate for the type, stage and scale of research proposed. 'A Heart for Health' by Carissa Paglino. More details in the artist bio below.
DOE Announces Nearly $65 Million for Biofuels Research to Reduce
WASHINGTON, D.C. - As part of a White House roundtable to launch the Sustainable Aviation Fuels (SAF) Grand Challenge to decarbonize the aviation sector by 2050, the U.S. Department of Energy (DOE) today announced $64.7 million in funding for projects focused on producing cost-effective, low-carbon biofuels. These investments will advance technologies to create replacements for petroleum ...

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Biofuels: present and future

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The potential of biofuels from first to fourth generation

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Second-generation biofuels

Third-generation biofuels

Fourth-generation biofuels

Economics of biofuels in transportation

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Inexpensive, carbon-neutral biofuels are finally possible

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