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Biodiesel sustainability: review of progress and challenges of biodiesel as sustainable biofuel.

1. Introduction

Click here to enlarge figure

2. Biodiesel Sustainability

2.1. sustainability paradigm, 2.2. key aspects of sustainability, 2.3. sustainability assessment, 3. application of biodiesel in different countries.

3.1. Biodiesel in the USA

3.2. biodiesel in the european union (eu), 3.3. biodiesel in asia, 3.3.1. china, 3.3.2. india, 3.4. biodiesel in asean, 3.4.1. indonesia, 3.4.2. thailand, 3.4.3. malaysia, 4. conclusions, author contributions, institutional review board statement, informed consent statement, data availability statement, conflicts of interest.

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Share and Cite

Suhara, A.; Karyadi; Herawan, S.G.; Tirta, A.; Idris, M.; Roslan, M.F.; Putra, N.R.; Hananto, A.L.; Veza, I. Biodiesel Sustainability: Review of Progress and Challenges of Biodiesel as Sustainable Biofuel. Clean Technol. 2024 , 6 , 886-906. https://doi.org/10.3390/cleantechnol6030045

Suhara A, Karyadi, Herawan SG, Tirta A, Idris M, Roslan MF, Putra NR, Hananto AL, Veza I. Biodiesel Sustainability: Review of Progress and Challenges of Biodiesel as Sustainable Biofuel. Clean Technologies . 2024; 6(3):886-906. https://doi.org/10.3390/cleantechnol6030045

Suhara, Ade, Karyadi, Safarudin Gazali Herawan, Andy Tirta, Muhammad Idris, Muhammad Faizullizam Roslan, Nicky Rahmana Putra, April Lia Hananto, and Ibham Veza. 2024. "Biodiesel Sustainability: Review of Progress and Challenges of Biodiesel as Sustainable Biofuel" Clean Technologies 6, no. 3: 886-906. https://doi.org/10.3390/cleantechnol6030045

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• Published: 31 August 2024

Exploring alternative fuel solutions: lemon grass oil biodiesel blend with dibutyl ether additive for VCR diesel engines - an experimental analysis

• Prabhu Paramasivam 1 , 2 ,
• Arun Balasubramanian 3 ,
• Adekunle Akanni Adeleke 4 ,
• Peter Pelumi Ikubanni 5 ,
• Sandeep Kumar 6 ,
• Chander Prakash 7 &
• Rahul Kumar 6 , 8  

Scientific Reports volume  14 , Article number:  20272 ( 2024 ) Cite this article

Metrics details

• Environmental impact
• Mechanical engineering

There has been an intense surge in interest in the search for alternative sources of petroleum fuels in the modern world as a result of the inflation of fuel prices and the historic supply gap. When compared to petroleum fuels, biodiesel is becoming an increasingly valuable option due to the fact that it produces less emissions and provides the almost same amount of energy. In point of fact, the prime aim of this work is to explore the possibility of utilizing biodiesel derived from lemongrass oil and including dibutyl ether as an additive for the test diesel engine operating on varied compression ratios. The findings showed that the best operating settings are a 17.5 compression ratio with a blend of 30% biodiesel and 70% diesel fuel. At greater loads, brake thermal efficiency is lower than that of diesel engines. Lower loads result in lower specific fuel usage. Mechanical efficiency at higher loads is highest in the B30 blend, but emission metrics such as CO, CO 2 , HC, and NOx were reduced with the inclusion of an additive, though HC rose with higher loads of lemongrass oil biodiesel blends. When compared to the B30 biodiesel blend with various composition additives, the B30 + 4% additive has the highest efficiency at the fourth load in terms of both brake power and mechanical efficiency.

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Introduction.

The usage of biodiesel has the potential to significantly contribute to sustainable development goal 7 (SDG 7), which seeks to provide all people with economical, sustainable, and modern energy 1 . Biodiesel, being an ecologically beneficial fuel, has the potential to improve energy availability in isolated and rural regions, enhance energy independence, and minimize reliance on imported fossil fuels 2 . Biodiesel, derived from renewable feedstocks, decreases reliance on finite fossil fuel supplies, leading to a more environmentally friendly and sustainable energy mix 3 . Because of its lower greenhouse gas emissions, it helps to combat climate change and supports SDG 13 on climate action. Biodiesel improves air quality by generating lower amounts of pollutants, thereby helping achieve SDG 3 on good health and well-being 4 . In addition, biodiesel production and utilization are in line with SDG 12 on responsible consumption and production. Furthermore, biodiesel operations have the potential to boost local economies, generate job opportunities, and contribute to SDG 8 on decent work and economic growth 5 . SDG 9 on innovation, industry, and infrastructure may be promoted through fostering the transfer of knowledge, building capacity, and research in the biodiesel area. Overall, the use of biodiesel promotes sustainable energy practices, decreases emissions, improves air quality, and contributes to sustainable development, making it an important instrument in reaching SDG 7 and progressing towards a more sustainable future for all 6 .

The diesel engine has become well-known for its contributions to society. Its main draws are its tough structure, ease of support, and simplicity of use. Because of the higher brake thermal efficiency (BTE) and lower fuel consumption, it has become an established machine in the transportation and agriculture sectors 7 . Excellent compression ratios, leaner fuel–air mixes, and little pumping losses as a result of throttle absence all contribute to high thermal efficiency 8 . However due to the rapid depletion of fossil fuel, rising costs, erratic supplies, rising petroleum demand, and, most crucially, strict emission regulations, experts are now seriously looking for alternatives to diesel fuel 9 . Therefore, it is crucial to create a long-term plan for the development of alternative energy sources that are balanced and makes the best use of the resources available in terms of land and labor. It is crucial to look into the possibilities of replacing diesel with an alternative fuel that can be generated on a large scale in the country for commercial use 10 . As a result, efforts are being made globally to identify suitable replacement fuel for diesel engines. Due to their greater heat efficiency, lower carbon monoxide emissions (CO), higher load capacity, and unburned hydrocarbons (UHC), diesel engines are significantly more cost-effective 11 . Diesel fuel consumption has increased exponentially as a result, and demand has recently grown. Rapid exhaustion of fossil fuel reserves owing to increased diesel fuel utilization increases demand and diesel market prices 12 . In-depth research is being done in the study community to develop an alternative fuel that can take the place of diesel fuel to meet supply and demand 13 . Soot emissions and nitrogen oxide (NOx) emissions from diesel engines are also very concerning. Lowering NOx and soot simultaneously is a challenging task for academics since they both have trade-off attributes 14 .

Suresh et al. 15 studied the application of green synthesized carbon nanotubes as nano additives in ternary fuel mixes for a CRDi diesel engine (diesel (60%), ethanol (20%), and algal biodiesel (20%). When the carbon nanotube concentration in the fuel blend was increased from 25 to 100 ppm, engine performance improved, with a 9% increase in BTE, an 11.76% reduction in BSFC, and a 12.79% decrease in BSEC compared to diesel. In contrast to diesel and other blends, nanotubes blended biodiesel blend helped in emission reduction of HC by half, CO almost 14%, a reduction of 13.42% in NOx. Joshi et al. 16 tested algae biofuel combined with diethyl ether (DEE) in a mono-cylinder, direct-injection CI engine. When compared to diesel, the inclusion of DEE resulted in a 7.2% gain in maximum BTE and a 6.3% reduction in minimum BSFC, with reductions in HC (12%), CO (19%), and a little rise in NOx (3%) emissions. Balasubramanian et al. 17 explored the influences of CR on a 5.5-kW diesel engine running on B100 biodiesel. The engine's thermal efficiency improved as the CR was increased, with a 9.5% and 4.63% gain in brake thermal efficiency at full load for CRs of 21:1 compared to the original CR of 19:1. However, at higher CRs, NOx emissions showed elevated results. However, higher CRs increased NOx emissions, which might be mitigated by delaying injection timing. Overall, these studies show that using new fuel blends and engine changes can improve performance and reduce emissions in diesel engines. Yildirim et al. 18 set out to assess the vibroacoustic characteristics of biodiesel made from used cooking oils with petroleum-based diesel fuel. The tests were carried out on a diesel engine with a 6 L cylinder volume, with an emphasis on altering engine speed & load. At high engine speeds, a vibration study using root mean square and coherence methods found that the vibration amplitude of B100 (100% biodiesel) was slightly greater than pure petroleum-based diesel fuel (PBDF). At 1500 rpm, the maximum vibration amplitude of B100 was found to be 8.5% more than that of PBDF. Coherence investigation revealed that engine sound increased with engine speed, reaching a maximum noise level of 94.9 dB with B100 at 2000 rpm. Another investigation by Rao et al. 19 examined the effect of carbon nanotubes (CNTs) on the efficiency of a diesel engine utilizing a biodiesel-diesel blend (Y20). The addition of optimal quantities of CNT nano additions resulted in considerable increases in a variety of performance and emission metrics, including BSFC, BTE, CO 2 , CO, HC, and NOx. CNTs also increased cylinder pressure, heat release rate, pressure rise rate, ignition delay, and combustion duration. Overall, the inclusion of CNTs was found to improve performance and lower emissions in the Y20 blend, demonstrating CNTs' potential as useful fuel additives.

For the literature analysis, it could be seen that there has been not much research into the emissions and performance features of fuel mixes containing diesel, lemongrass biodiesel, and dibutyl ether. Moreover, the impact of different engine loads and compression ratios on the combustion and emission characteristics associated with these fuel blends have not been thoroughly investigated. In addition, the synergistic effects of mixing lemongrass biodiesel with dibutyl ether as diesel fuel additives have not been properly studied. Thus, the prime objective is to explore the emissions and performance features of test blends comprising diesel, lemongrass biodiesel, and dibutyl ether under various engine load circumstances and compression ratios. The study aims to fill current gaps in the literature by:

Evaluating the impact of different blend compositions on the performance of engine and efficiency.

Examining the effect of various blend ratios on exhaust emissions like CO, HC, and NOx.

The effects of combining lemongrass biodiesel with dibutyl ether additives on combustion effectiveness, ignition characteristics, and engine sound are being studied.

The effects of combining lemongrass biodiesel with dibutyl ether additives on combustion effectiveness, and ignition characteristics are being studied.

Offering insights into ideal blend compositions that could enhance engine performance while lowering emissions, hence contributing to the development of environmentally friendly and sustainable fuel solutions.

By addressing these objectives, the research aims to add to existing knowledge on the use of lemongrass biodiesel and dibutyl ether as additives in diesel fuel, providing valuable insights for engine optimization and the development of more efficient and cleaner fuel blends.

Material and methods

Lemongrass biodiesel.

Lemongrass biodiesel is produced from citronella grass ( Cymbopogon Nardus ). Lemongrass is a special plant with fragrant and therapeutic characteristics. This Poaceae family perennial grass is native to tropical climates. Citronella grass grows to a height of one to two meters owing to its tall and thin stems 20 . The leaves are thin and long, and when crushed, they emit a strong lemon-like aroma, hence the name. Lemongrass oil was extracted by using steam distillation 21 . The process involves heating lemongrass in a chamber with steam from an external boiler. The high temperature causes the oil to be removed from the lemongrass. The biodiesel was prepared using well well-established transesterification process. The main properties of the test fuel are depicted in Table 1 .

Dibutyl ether

Dibutyl ether is an additive that contains oxygen. It is a flammable, colorless liquid with the chemical formula C 8 H 18 O. Because there is oxygen present, the fuel burns more efficiently, resulting in good combustion. Dibutyl ether, abbreviated as DBE, is a multifunctional chemical molecule that plays a significant part as a diesel fuel additive. It is an ether that is formed up of two butyl groups connected to an oxygen atom. DBE is a colorless, transparent liquid with a distinct odor. DBE has multiple significant benefits as a diesel additive 22 . For example, it improves the combustion qualities of diesel fuel, resulting in improved fuel economy and lower pollutants. DBE enhances the brake thermal efficiency (BTE) of diesel engines by helping in the improvement of combustion, resulting in greater energy efficiency and lower specific fuel consumption. DBE helps to maintain optimal engine efficiency, avoid injector fouling, and reduce fuel filter blockages by keeping the fuel system clean, therefore increasing the engine's lifespan 22 . Furthermore, DBE improves the ignition quality of diesel fuel by acting as a cetane number improver. A greater cetane number enhances combustion, resulting in smoother engine running, less ignition delay, and lower amounts of unburned hydrocarbons and particulate matter in the exhaust. This significantly reduces hazardous pollutants like CO, NOx, and HC, assisting in satisfying severe environmental standards and improving air quality. DBE has high stability, compatibility, and miscibility with diesel fuel, in addition to its combustion-enhancing qualities 23 . It easily mixes with diesel, guaranteeing a uniform mixture without requiring sophisticated or costly adjustments to the current fuel infrastructure. As a result, DBE is a viable and affordable diesel additive for both traditional and modern diesel engines 24 . The properties of DEE are listed in Table 2 .

Transesterification process

Biodiesel shares many of the same characteristics as diesel fuel. The esterification process improves the vegetable oil's density, viscosity, cetane number, calorific value, atomization and vaporization rate, molecular weight, and fuel spray penetration range 25 . In the biodiesel production process, a 500 mL conical flask was filled with Lemongrass oil and heated for 15 min (55 °C). A solution was made by combining a pellet of sodium hydroxide weighing 4 g with 60 ml of ethanol in a beaker that held 250 ml. Following a sustained period of intense agitation of the solution, the sodium hydroxide pellet was completely dissolved. With the addition of the sodium ethoxide solution, the lemongrass oil was prepared. To get the temperature of the blended solution up to sixty degrees Celsius, it was heated in a water bath for sixty to ninety minutes. After that, the mixture was allowed to settle in a burette for twenty-four hours. The two components that make up the bottom layer are glycerol and soap, while the top layer is composed of crude biodiesel. The next step is to separate the crude biodiesel and glycerol into their respective containers. In the subsequent step, water was utilized to remove any traces of soap and glycerol that were still present in the crude biodiesel. Up to the point where the biodiesel could be seen beneath the clear water in the burette/separating funnel, this process was carried out. After washing the sample, it was placed on a hot plate to dry, and any water that was still present in the biodiesel was extracted to eliminate it. After the collection was complete, the volume of pure biodiesel was meticulously measured and documented.

Ethics approval and consent to participate

Plant guidelines.

The Lemongrass plants used in our research were cultivated according to locally established agricultural practices. We confirm that no wild plant materials were utilized in our study. This study including plant material complied with relevant institutional, national, and international guidelines and legislation.

Permission to collect the plants/plant parts

The Lemongrass plants utilized in our study were sourced from a local nursery that specializes in herb cultivation. The established parameters were followed when gathering the cultivated lemongrass plants.

Source of the plant used in your study

The Lemongrass plants utilized in our research were sourced from a local reputable nursery specializing in herb cultivation. We ensured that the nursery followed proper agricultural practices and adhered to any relevant regulations governing plant cultivation and distribution. Additionally, we maintained records of the specific nursery from which the Lemongrass plants were obtained for traceability purposes.

Experimental setup

Figure  1 a presents a schematic overview of the experimental setup, and Table 3 provides information regarding the parameters of the Kirloskar make diesel engine. The engine was allowed to run at a low load for twenty minutes in order to warm up. It was done so that temperature stabilization of engine oil and cooling takes place. The engine load was increased gradually while keeping the engine speed of 1500 revolutions per minute. At this fixed engine speed, other parameters were varied like compression ratio (CR), load, and blending ratio. The load on the engine was varied using an eddy current-based dynamometer. The engine has an in-built provision for varying the CR. The different blending ratios of test fuel samples were prepared using the ultra-sonication-based mixer. The data for the study was gathered through the utilization of an electronic acquisition system supplied by Apex Innovation, India. The engine is outfitted with a Kistler make pressure sensor for measuring the cylinder pressure. There are five thermocouples distributed over five separate sites on the engine. The data-collecting device is linked to all of the sensors and displays real-time data on the personal computer. The emission analyzer is Testo make and was used to measure different emissions such as CO, unburned HC, and NOx. The emission analyzer is shown in Fig.  1 b. Further details on the engine and test setup are listed in Table 3 .

Test set up ( a ) engine ( b ) emission analyzer.

In experimental studies, it is important to minimize the error and uncertainties 26 . Each set of engine tests was conducted thrice to reduce the uncertainty and errors. By choosing the appropriate instruments, their condition, calibration, observation process, test procedure, and planning, it is possible to decrease the number of errors and uncertainties that occur during the performance of experiments. However, it is impossible to eliminate them 27 . The uncertainties and errors may creep in due to ambient conditions, manufacturing defects, calibration errors, etc. The uncertainty can be estimated as 28 :

The partial derivative is given as 29 :

Herein, R denotes an independent variable, \(\frac{\partial R}{{\partial Z}_{1}}\) is the estimated sensitivity of individual variables. The accuracy, range, and uncertainties are listed in Table 4 .

Results and discussion

The experimental results that were acquired from the studies are given an in-depth analysis in this part. In the course of the trials, the performance and emissions characteristics of fuel blends that contained diesel, biodiesel (varying from 0 to 30%), and dibutyl ether were analyzed. The engine load and compression ratio settings were changed during the course of the studies. There is a greater knowledge of the effect that certain fuel blend compositions have on engine performance, including characteristics such as SFC, BTE, mechanical efficiency, and other emission metrics, thanks to this section's in-depth examination and interpretation of the data that was acquired. During the course of the conversation, we looked into the patterns, correlations, and amazing outcomes that were discovered, providing an understanding of the potential benefits and drawbacks associated with the different fuel mix compositions. The data was additionally compared to the base diesel fuel to see whether there were any benefits or drawbacks in terms of engine performance and emissions caused by the various blending levels of test samples.

Effect of compression ratios

When it comes to evaluating an engine's performance and fuel consumption, the connection between compression ratios and the thermal efficiency of the engine is of the utmost importance. Under pure diesel circumstances, Fig.  2 illustrates the load vs brake thermal efficiency of four different compression ratios (CR16, CR17.5, and CR18) at five distinct load points: zero, twenty percent, fifty percent, eighty percent, and one hundred percent. According to the findings of the tests, higher compression ratios are associated with improved thermal efficiency of the brakes, which ultimately leads to lower consumption of test fuel 30 . The findings of this investigation are in agreement with the fundamental laws of thermodynamics, which suggest that higher compression ratios result in improved combustion efficiency and energy conversion within the engine 31 .

Load vs BTE at the different compression ratios.

The study also highlighted the effect of varying the CRs on BTE. The analysis discovered that increasing the cut-off ratio reduced brake thermal efficiency. This fact implies that at greater cut-off ratios, the engine's expansion mechanism became less successful, resulting in a loss in total thermal efficiency 32 . When the performance of the compression ratios CR16, CR17.5, and CR18 were examined, it was revealed that CR17.5 gave the best combination. This means that a compression ratio of CR17.5 achieves a reasonable balance of boosting brake heat efficiency while reducing the drawbacks associated with exceptionally high compression ratios 33 . These findings have significant implications for engine design and optimization because they highlight the need to select an appropriate compression ratio for best performance in terms of BTE as well as BSFC. Similar results are reported in the case of biodiesel used to power the diesel engine 34 .

Figure  3 demonstrates the relationship between load and brake-specific fuel consumption (BSFC) using pure diesel fuel at five load levels (0, 20, 50, 80, and 100%) for different compression ratios (CR16, CR17.5, and CR18). The testing findings reveal that BSFC reduces at higher engine load across all tested fuels. This is because enhanced Brake Thermal Efficiency (BTE) at higher loads exceeds the equal rise in fuel consumption 35 . When the performance of the three compression ratios, CR16, CR17.5, and CR18, are examined, it is obvious that the CR17.5 mix beats the others at both low and high loads.

Load vs BSFC of different compression ratio.

This demonstrates that the CR17.5 has the best BSFC performance across a wide range of loads. When compared to other compression ratios, CR17.5 has lower BSFC ratings, indicating its capacity to achieve higher fuel efficiency and energy conversion. These findings highlight the significance of compression ratio selection in achieving efficient combustion and peak engine performance 36 . The preference for CR17 over CR16 and CR18 demonstrates that it strikes a favorable balance between compression efficiency and load requirements. This information contributes to the scientific study of engine performance characteristics and can aid in the development of strategies to increase fuel efficiency in compression ignition engines 9 .

Figure  4 demonstrates the relationship between load and mechanical efficiency while utilizing pure diesel fuel for various compression ratios (CR16, CR17.5, and CR18) at five distinct load levels (0, 20, 50, 80, and 100%). Mechanical efficiency is an important metric that quantifies the ratio of brake power generated at the crankshaft to the indicated work occurring within the combustion chamber of an engine. Analyzing the data reveals that mechanical efficiency levels vary depending on compression ratio and load conditions. Notably, when the compression ratios CR16, CR17.5, and CR18 are examined, CR17.5 consistently has the highest mechanical efficiency rating across all load levels. The importance of choosing the appropriate compression ratio in achieving the best engine performance and fuel efficiency cannot be overstated. According to data, CR17.5 offers the finest conditions for optimizing mechanical efficiency over the whole range of loads assessed. This conclusion highlights the importance of selecting an optimal compression ratio, as it directly affects the engine's overall performance and energy conversion efficiency 37 .

Load vs mechanical efficiency at different compression ratios.

According to the data presented in Fig.  4 , CR17.5 is the best combination of the compression ratios investigated (CR16, CR17.5, and CR18) in terms of mechanical efficiency, as it consistently produces the highest values across all tested loads. This information is useful for engine designers and researchers who wish to increase the efficiency and effectiveness of internal combustion engines by selecting the appropriate compression ratio. Similar trends are reported by other researchers 38 .

Effect of blending ratios of biodiesel

An engine's brake thermal efficiency (BTE) is a vital performance indicator that measures how efficiently fuel energy is transformed into usable effort. It denotes the link between the engine's output and the amount of fuel energy delivered. Figure  5 demonstrates the relationship between engine load and BTE for a specific Lemongrass oil biodiesel mix.

Load vs brake thermal efficiency at different biodiesel blend.

BTE is synonymous with fuel conversion efficiency and provides information about the engine's ability to convert fuel into mechanical work. In this study, the researchers investigated the BTE of the test engine employing a lemongrass oil biodiesel blend as the fuel source. The maximum and minimum BTE values for diesel were discovered to be 23.76 and 22.74%, respectively. Higher values indicate more efficient fuel usage. These values represent the engine's ability to convert the energy in the fuel into usable work 39 . The findings in Fig.  5 and associated BTE values illustrate the need to maximize fuel economy in internal combustion engines. Increasing BTE values is crucial for enhancing energy sustainability and lowering fuel use. This experiment provided valuable information about the performance parameters of the lemongrass oil biodiesel blend and its impact on engine efficiency 40 . These findings contribute to the body of knowledge in the field of alternative fuels and engine optimization, paving the way for the creation of more energy-efficient and environmentally friendly technology.

The concept of brake-specific energy consumption (BSEC) is an important criterion in evaluating the efficiency and performance of diesel engines. It is computed by multiplying the brake-specific fuel consumption (BSFC) by the fuel's calorific value. In Fig.  6 from the study "Load vs. Brake Specific Fuel Consumption of lemongrass oil biodiesel Blends," the relationship between load and BSFC for numerous lemongrass oil biodiesel blends is investigated.

Load vs brake-specific fuel consumption at different biodiesel blends.

The amount of energy needed to generate one unit of braking power is measured by BSEC, which provides information on the engine's overall efficiency. When employing BSEC as a metric, the evaluation of diesel engine capabilities becomes more precise because a variety of fuel blends with varying densities and calorific values are taken into account. Notably, the study discovers that the B30 blend has a higher BSFC than other mixes. This means that the engine consumes more fuel per unit of power produced in the B30 mix. The BSFC drops as the load increases, leading the BSEC to fall as well. This research reveals the engine's improved efficiency under high-load conditions 41 .

The use of biodiesel blends in the study underscores the importance of understanding the impact of fuel composition on engine performance. The data demonstrates the potential advantages of biodiesel blends in terms of reduced fuel consumption and increased energy efficiency. It was also reported by other researchers 42 . The load vs. mechanical efficiency of the biodiesel blends of lemongrass oil is shown in Fig.  7 , which demonstrates how mechanical efficiency rises with brake power. The B40 and B30 blends are more efficient than the remaining, almost identical diesel mixtures at higher loads. B10, B20, B30, and B40 are virtually as efficient as diesel at lighter loads. The percentages of diesel in B10 to B40 lemongrass oil biodiesel blends were 46.11%, 47.96%, 48.43%, and 49.09%, respectively.

Load vs mechanical efficiency at different biodiesel blends.

Figure  8 depicts the behavior of carbon monoxide (CO) emissions for several diesel blends (B10, B20, B30, and B40) under various engine loads. It has been noted that when the load increases, the CO emissions for B10 and B20 mixes stay high. This is due to inefficient fuel combustion, particularly when the engine operates at low loads and under air control, with the throttle just partially open 43 .

Load vs carbon monoxide at different biodiesel blends.

In contrast, both B30 and B40 blend significantly reduce CO emissions. This increase in combustion efficiency can be ascribed to an increase in engine load and a wider throttle. Increased engine load improves the combustion process for all mixes, resulting in less CO in the exhaust emissions. Surprisingly, for low-load mixes with low combustion efficiency, wide-open throttle has a large favorable effect on the fuel–air combination 44 . This design dramatically reduces CO emissions, indicating a more complete and efficient combustion process. These findings highlight the need to manage engine operating conditions such as load and throttle position to improve combustion efficiency and reduce CO emissions 45 . This knowledge can assist guide the development of ways for reducing emissions in diesel engines, ultimately contributing to the expansion of environmentally friendly and sustainable transportation systems. The results corroborate similar studies in this domain 46 .

Figure  9 depicts the connection between loading and hydrocarbon (HC) emissions in lemongrass oil biodiesel mixes. According to the findings, when the load grows, lemongrass oil biodiesel blends' HC emissions exceed those of diesel. However, when the load increases, the HC emissions from B10 to B40 approach diesel emissions levels. At lower loads, lemongrass oil biodiesel blends perform nearly as well as diesel while producing fewer tailpipe pollutants. This problem is caused by an oxygen deficit in the engine cylinder caused by high loads 47 . Unburned hydrocarbons persist in the exhaust gases under these conditions, resulting in increased HC emissions. The behavior of all blends in terms of HC emissions closely resembles that of carbon monoxide (CO).

Load vs hydrocarbon emission at different biodiesel blends.

Both HC and CO are byproducts of combustion that is incomplete, that occur when there is more fuel in the combustion process than available oxygen. These results demonstrate the complex interplay of load, oxygen availability, and fuel concentration in influencing HC emissions in lemongrass oil biodiesel blends. Understanding these relationships is crucial for optimizing combustion processes, minimizing emissions, and encouraging the adoption of cleaner, more sustainable energy sources.

The load vs. CO 2 emissions for lemongrass oil biodiesel blends are depicted in Fig.  10 . At higher loads, B10 to B40 CO 2 emissions are similar to CO 2 emissions from pure diesel. At lower loads, all lemongrass oil biodiesel blends generate almost the same amount of CO 2 . Due to a poor combustion process brought on by a lack of oxygen in the air, CO is predicted to decrease with altitude rather than CO 2 . The results show that increasing CO 2 emissions cause an oxygen shortage in the atmosphere 48 .

Load vs carbon dioxide at different biodiesel blends.

Figure  11 demonstrates the relationship between load and NOx emissions for several lemongrass oil biodiesel blends. Surprisingly, when the load increases, all blends create NOx emissions that are roughly comparable to diesel. At higher loads, however, B10 and B20 mixes generate less NOx than other lemongrass oil biodiesel blends. This finding implies that adopting these specific blends can effectively reduce NOx emissions.

Load vs NOx at different biodiesel blends.

NOx emissions levels are influenced by a number of parameters, including exhaust gas temperatures and the presence of oxygen in the combustion chamber. Blends with increased brake power may be more prone to producing excessive NOx levels as temperatures rise 49 . Higher temperatures promote more powerful reactions, which increase NOx production. However, combustion efficiency and the availability of air for reactions can counteract this trend. NOx creation can be minimized when combustion is efficient and there is enough air present, resulting in lower emissions. This emphasizes the need to improve combustion conditions and ensure adequate oxygen supply to reduce NOx formation 50 . From the above analysis, it could be found that B30 is the best selection to get high combustion efficiency, showing lower emission and higher performance than B10, B20, and B40. Thus, B30 is selected for the next experiments, in which B30 is blended with additives at a 2%, 4%, and 6% rate.

Effect of blending ratios of additives

A comparison of the thermal efficiency of lemongrass oil biodiesel blends with regard to load and brakes is depicted in Fig.  12 . Dibutyl ether is a component that is added to a biodiesel mixture at the additive level. In the process of combustion, the quantity of energy that is produced by a fuel and then converted into work that can be utilized is referred to as thermal efficiency. Because of the increased heat transfer to the cylinder wall, the BTE decreases in case of low engine speed. This is because of the temperature rise. When compared to B30 and its additions of 1%, 3%, and 5% dibutyl ether, the graph indicates that the biodiesel blend with the highest braking power efficiency is B30 + 3% DBE.

Load vs BTE at the different additive rate.

The load vs. brake-specific fuel consumption of a lemongrass oil biodiesel blend with dibutyl ether is shown in Fig.  13 . The effect of engine speeds at full load on the BSFC for lemongrass oil biodiesel mixes. Low engine speed leads to inefficient combustion and a strong quenching effect, which increases the BSFC. BSFC decreases as engine speed rises, combustion improves, and overall energy consumption increases. The density, viscosity, and heating valves create the BSFC. According to the graph, B30 + 4% DBE is the biodiesel mix with the best brake power efficiency among B30 and its additives of 2%, 4%, and 6% dibutyl ether 51 .

Load vs BSFC at the different additive rates.

Figure  14 shows the relationship between load and mechanical efficiency for a B30 lemongrass oil biodiesel mix with various additives at 2%, 4%, and 6% dibutyl ether. The evaluation of a mechanical system's performance efficacy. It is typically the ratio of the mechanical system's output to input power, and because of friction, this efficiency is less than one.

Load vs mechanical efficiency at the different additive rate.

This research examines the efficiency and emissions of a diesel engine that runs on lemongrass oil biodiesel-diesel blends with dibutyl ether added at 1%, 3%, and 5% rates. The results of the lemongrass oil biodiesel blend are compared to those of diesel. The following are the main observations:

Blends of biodiesel have the same physical characteristics as diesel. A diesel engine that has not been modified in any manner is used to evaluate the prepared fuel samples.

The CR17.5 is the best of the other two diesel compression ratios, which are all different. Because performance gains are greatest and emissions are least with higher loads

Diesel engines can be powered with blends of lemongrass oil biodiesel without modification. Citronella oil gives the engine a smooth, diesel-like performance.

In comparison to diesel, brake thermal efficiency is worse at higher loads. Lower loads lead to lower specific fuel consumption. In the B30 blend, mechanical efficiency at higher loads is at its highest, while emission metrics including CO, CO 2 , HC, and NOx were lowered with the introduction of an additive, although HC is increased with greater loads of lemongrass oil biodiesel blends.

The B30 + 4% additive achieves maximum efficiency at the fourth load in terms of both brake power and mechanical efficiency when compared to the B30 biodiesel blend with various composition additives.

Future studies should focus on optimizing additive concentrations, exploring long-term engine durability, and assessing the economic and environmental impacts of large-scale lemongrass oil biodiesel production and use.

Data availability

The data is available within the manuscript.

Abbreviations

90% diesel + 10% biodiesel

80% diesel + 20% biodiesel

70% diesel + 30% biodiesel

60% diesel + 40% biodiesel

Compression ratio

Common rail diesel injection

Variable compression engine

Brake specific fuel consumption

Carbon nanotube

Petroleum-based diesel fuel

Specific fuel consumption

Unburnt hydrocarbon

Brake specific energy consumption

Brake thermal efficiency

Oxides of nitrogen

Carbon mono oxide

Carbon dioxide

Equivalence ratio

Mean absolute percentage error

Mean squared error

Grey wolf optimizer

Sustainable development goal

Hydrocarbon

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Paramasivam, P., Balasubramanian, A., Adeleke, A.A. et al. Exploring alternative fuel solutions: lemon grass oil biodiesel blend with dibutyl ether additive for VCR diesel engines - an experimental analysis. Sci Rep 14 , 20272 (2024). https://doi.org/10.1038/s41598-024-70491-7

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Biofuels from Renewable Sources, a Potential Option for Biodiesel Production

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Not applicable. There is no primary data used in this study. However, secondary data that supports this study are available from the corresponding author (D.N.) upon reasonable request.

Ever-increasing population growth that demands more energy produces tremendous pressure on natural energy reserves such as coal and petroleum, causing their depletion. Climate prediction models predict that drought events will be more intense during the 21st century affecting agricultural productivity. The renewable energy needs in the global energy supply must stabilize surface temperature rise to 1.5 °C compared to pre-industrial values. To address the global climate issue and higher energy demand without depleting fossil reserves, growing bioenergy feedstock as the potential resource for biodiesel production could be a viable alternative. The interest in growing biofuels for biodiesel production has increased due to its potential benefits over fossil fuels and the flexibility of feedstocks. Therefore, this review article focuses on different biofuels and biomass resources for biodiesel production, their properties, procedure, factors affecting biodiesel production, different catalysts used, and greenhouse gas emissions from biodiesel production.

1. Introduction

The rising world population is predicted to reach over 9 billion by 2050 [ 1 ]. Increasing global prices and higher energy demand have put tremendous pressure on natural energy reserves, causing their depletion [ 2 , 3 , 4 ]. The burning of fossil fuels has several environmental implications, including an increase in greenhouse gas (GHG) emissions, particularly carbon dioxide (CO 2 ) [ 5 , 6 ]. Over the last few decades, global primary energy consumption has increased dramatically due to rapid industrialization and higher living standards [ 2 , 7 ]. Developing countries such as Brazil, the South Asian region, and South Africa require 12–24 gigajoules (GJ)/cap of energy annually to have a decent standard of living [ 8 ]. Currently, over 80% of the world’s energy comes from fossil fuels, including natural gas, oil, and coal, and about 98% of it is generated via carbon emissions from fossil fuels [ 8 , 9 ]. The duration and intensity of drought are expected to become more severe, thus reducing water reserves by five-fold throughout the 21st century [ 1 ].

An increased share of renewable energy in the global energy supply will help to stabilize surface temperature rise to 1.5 °C compared to pre-industrial levels [ 10 ]. The temperature increase could be as much as 3–5 °C depending on certain regions [ 11 ]. Further, a shift in rainfall was found, ranging from 19.2 to 37.2 mm over different growing seasons [ 12 ]. With the inadequate pool of sources, particularly water, and an ever-increasing need for global energy, alternative fuels are the most practical way to meet the rising demand [ 13 ]. Researchers have already figured out alternatives to address this demand [ 14 ]. Further, the potential options to mitigate the effect of climate change and reduce dependence on fossil fuels are urgently needed and are already in development. There is an increasing interest in growing biofuels at a global and national level as a low-carbon alternative to fossil fuels due to their potential to reduce GHG emissions and the associated climate change impact from transport [ 15 ]. The use of bioenergy/biofuels is one of the promising renewable energy alternatives [ 16 ] because these are cheaper in synthesis [ 4 ]. Biofuels, generally biodiesel, have attracted researchers’ attention due to their potential benefits over fossil fuels and the flexibility of feedstocks. For example, sulfur-free, adequate oxygen content, an easy manufacturing process, and reduced GHG emissions are critical advantages of biodiesel [ 17 ]. Biodiesel as a diesel fuel is bio-degradable [ 18 ], non-toxic [ 19 ], portable, environmentally sustainable [ 20 ], efficient, and has low sulfur as well as aromatic content [ 21 ]. Additionally, due to the higher flask point of biodiesel, transportation and storage of biodiesel are safer than diesel fuels. However, it has some disadvantages; biodiesel is more expensive and emits more NO gas than diesel [ 9 ].

Due to its crucial characteristics and usage of versatile feedstock, for example, from waste frying oil to cheap non-edible resources, biodiesel has tremendous potential to use as an alternative fuel [ 22 ]. It is a promising and economical alternative to diesel that can reduce the global reliance on imported petroleum fuels. This article provides a comprehensive review of the types and generation of biofuels, biomass sources, properties, and factors affecting biodiesel production. This article also highlights various catalysts in biodiesel production, greenhouse gas emissions from several literatures, and finally, the conclusion and future perspective.

2. Types and Generation of Biofuels

Biofuels are classified into four generations, namely first, second, third, and fourth based on their sources and production of various biomaterials. A brief description of each of the generations is highlighted below.

2.1. First-Generation Biofuels

First-generation biofuels are conventional biofuels, mainly generated from two types of edible feedstock, namely starch-based (e.g., potato, corn, barley, and wheat) and sugar-based (e.g., sugarcane and sugar beet) feedstocks [ 23 , 24 ]. The main advantages of first-generation raw materials are the availability of crops and comparative simple conversion processes. However, using edible food crops for biodiesel production, there is a reduced food supply, thus potentially increasing food prices [ 25 ]. Another concern is the diverting of agricultural land to fuel production. Using a significantly large amount of fertilizer and pesticides for agricultural production could negatively impact the environment [ 15 ]. There are several types of conventional biofuels based on the technological approach they use to generate ( Figure 1 ).

Types and generations of Biofuel. Adopted from [ 23 ].

2.1.1. Bio Alcohols

Bio alcohols are extracted with the help of enzymes and microorganisms by alcohol fermentation of cellulose, glucose, starches, carbohydrates, and other sugars. Bio alcohols are further categorized into bioethanol, biopropanol, and biobutanol [ 26 ].

2.1.2. Biodiesels

Biodiesels are the forms of diesel extracted from renewable feedstocks, including lignocellulosic biomass, which consists of long-chain fatty acid esters. Biodiesels are produced chemically by reacting lipids, such as animal fat (tallow), soybean oil, or other vegetable oils with alcohol and produce methyl, ethyl, or propyl ester [ 27 ]. The commonly used catalyst used during biodiesel production includes NaOH or KOH [ 28 ].

2.1.3. Vegetable Oil

Vegetable oils are produced from fat, olive oil, castor oil, and sunflower oil. The fuels produced from vegetable oil are economical and environmentally friendly. Recent studies reported that waste cooking and vegetable oils are considered alternative fuels for diesel engines in some precise applications [ 29 ].

2.1.4. Green Diesel

The hydrotreating of triglycerides produces green diesel in vegetable oils with hydrogen. Three main reactions during the process are hydrodeoxygenation (HDO), decarbonylation (DCO), and decarboxylation (DCO 2 ) [ 30 ].

2.1.5. Biogas

Biogas is produced by anaerobic digestion with the help of microbial consortium without oxygen, and digestate as a nutrient-rich byproduct is also produced [ 31 , 32 ]. Biogas produced during the process contains about 60% CH 4 , 35%CO 2 , and 5% a mixture of H 2 , N 2 , CO, NH 3 , O 2 , and volatile amines [ 33 ]. Biogas can be used for industrial energy, cooking in rural areas [ 33 , 34 ], and combined heat and power production [ 10 ].

2.1.6. Solid Biofuels

Raw materials, including wood, wood chips, leaves, sawdust, charcoal, and animal dung, are commonly used as solid biofuels. The use of solid biofuels in the energy sector is limited to particular markets [ 23 ]. For example, firewood is the most common strategy to generate bioenergy, which can be used for cooking food [ 28 ].

2.2. Second-Generation Biofuels

The controversy of using first-generation biofuel feedstock due to the food vs. energy debate has forced us to move to second-generation biofuels, such as lignocellulosic or carbohydrate biomass, as the potential alternative source for biofuels and chemical production [ 24 ]. These feedstocks do not rely on edible plants and do not require agricultural land [ 35 ]. Cellulosic biomass comprises various chemical compositions such as cellulose, lignin, and polyose. Lignocellulosic biomass is composed of cellulose (35–50%), lignin (15–20%), hemicellulose (20–35%), and other components (15–20%). The lignocellulosic-based biofuel production process has the potential to lower GHG emissions, boost the economy, and aid energy security. The biotechnological approach in the United States has been estimated to produce 1.3 billion tons of dry biomass annually without compromising food security [ 36 ]. Second-generation biofuels are advanced biofuels obtained from several trees, grass, bushes, and agricultural residues [ 23 ]. Based on the technologies used to produce them, second-generation biofuels include the following ( Figure 1 ).

2.2.1. Cellulosic Ethanol

The fermented sugars obtained from polyose and cellulose compounds of lignocellulose are used for making cellulosic ethanol [ 23 ]. Cellulosic biofuels can contribute to rural economic development and enhance the sustainability of agricultural landscapes [ 37 , 38 ].

2.2.2. Algae-Based Biofuels

Algae is the fastest-growing raw material for biofuel production and an essential substitute for biofuel extraction. Techniques of extraction and concentration of biomass from algae include processes such as centrifugation, aggregation, floatation, purification, and flocculation [ 39 , 40 ]. Biofuels such as biodiesel, biogas, and hydrogen can be produced from algae using the advanced feature [ 41 ].

2.2.3. Alcohol

Alcohol is obtained from syngas by fermenting biomass with the help of specific microorganisms [ 42 ].

2.2.4. Dimethylfuran

Dimethylfuran is an oxygenated hydrocarbon with an oxygen content of 17%. It is an additive in diesel fuels. This is highly competitive in reducing emissions from engines [ 43 ].

2.2.5. Biosynthetic Natural Gas (Bio-SNG)

Biogas can be produced from anaerobic digestion with the help of microbes. Bio-SNG is used in the form of CNG and LNG in vehicles and for refilling a natural gas cylinder [ 44 ].

2.3. Third-Generation Biofuels

Third-generation biofuels are produced from algal biomass and waste oil. The advantages of using third-generation biofuels include higher growth and productivity, no agricultural land required, higher oil content, and less impact on food supply. Microalgae, fish oil, animal fat, and waste cooking oil are the primary sources of third-generation biodiesel feedstocks [ 45 ]. Because of the cost involved during harvesting, drying, and extraction processes, using algal biomass as biodiesel feedstock is expensive. However, it produces about 10–100 times more biofuel or oil per unit area. Seaweed or macro-algae is third-generation biomass that can be used in bio-energy production and has many advantages such as short cultivation time, high carbohydrate, proteins, and lipids content, and low or no lignin content [ 46 ]. Algal-based biofuel includes bioethanol, biodiesel, and biohydrogen (by the process of bio photolysis, photo fermentation, and dark fermentation) [ 47 ]. A study showed that the lipid in algae could be converted to biodiesel by the conventional approach, such as the conversion method used for vegetable oil. The conversion process of algal biodiesel production involves transesterifications, enzymatic, wet extraction, alcoholysis and acidolysis, and finally, biodiesel [ 48 ]. Algal oil blended with diesel fuel in a 20% ratio reduced hydrocarbon exhaust and better emission characteristics [ 49 , 50 ]; however, the complete combustion of algae releases a higher % of NO x into the atmosphere due to the significant presence of nitrogen in algae (5–8%) [ 51 ].

In the case of waste oil or waste cooking oil, the variation in using different feedstocks and their chemical composition, and impurities, limit their productivity at large scale [ 52 ]. Waste coffee ground oils and bardawil lagoon are an example of third-generation feedstock used in recent years [ 53 ].

2.4. Fourth-Generation Biofuels

With the application of molecular biology, genetic engineering, and interdisciplinary physicochemical approaches, which include the use of CRISPR/Cas9 with guided RNA for genetic modification in algae [ 54 ] to optimize and enhance the yield of biofuel production, the biofuel generated by such process is considered a fourth-generation biofuel. The fourth-generation biofuel production employs genetically modified algae that accumulate high lipid and carbohydrate content to improve biofuel yield [ 55 ]. The raw materials used for biofuel production are microalgae, macroalgae, and cyno-bacteria. Cyno-bacteria are non-photosynthetic prokaryotes, and micro and macro algae are eukaryotes [ 56 ]. The inactivation of ADP-glucose phosphorylase in a Chlamydomonas starchless mutant led to a 10-fold increase in TAG [ 57 ]. Similarly, a modification in the CoA-dependent 1-butanol production pathway into a cyanobacterium, Synechococcus elongatus , can produce butanol from CO 2 directly [ 58 ].

3. Biomass Sources for Biodiesel Production

Biodiesel or fatty acid methyl ester (FAME) is a processed diesel fuel from different biological sources, including edible, non-edible, animal fats, and waste cooking oils. FAME combines long-chain fatty acid monoalkyl esters of fatty acids [ 2 ]. It is a green biological ester-based oxygenated oil that comprises organic fats and oils [ 3 ]. The world’s biodiesel production is projected to reach 10.3 billion gallons by 2024, which reached 8.5 billion gallons in 2016 [ 59 ]. It is also estimated that food-based feedstocks (first-generation biofuel) will dominate the world’s market [ 60 ].

Biodiesel is intended to be used in standard diesel engines as a standalone fuel or blended with petroleum. In 2021, the total volume of biodiesel production in the United States amounted to over 1.6 billion gallons, compared to 9 million gallons in 2001 and 991 million in 2012. After 2012, there were fluctuations in biodiesel production volume in different years, with the highest quantity attained in 2018 ( Figure 2 a). Similarly, total biomass production in the United States was 1375.56 billion kW hours in 2021, which is expected to increase gradually in the coming decades. It is estimated to reach 1630.73 billion kW hours by 2050 ( Figure 2 b).

( a ) US biodiesel production change for the past 20 years since 2001 [ 61 ] (Source: U.S. Energy Information Administration (EIA), Monthly Energy Review, Table, 10.4. Release date: April 2022. Available at: https://www.eia.gov/totalenergy/data/monthly/pdf/mer.pdf ), and ( b ) US biomass energy production forecast from 2021 to 2050 [ 62 ] (Source: EIA, Annual Energy Outlook 2022, Table 1 . Available online: https://www.statista.com/statistics/264029/us-biomass-energy-production/ . Accessed on 21 June 2022).

Various feedstock sources can be used for biodiesel production, including various vegetable oils, animal fats, microbial oil, algal oils, and waste oils [ 63 , 64 ]. Palm oil, stearic oil, lauric oil, oleic oil, soybean oil, sunflower oil, palmitic oil, rapeseed oil, canola oil, and vegetable derivates are included under vegetable oils. With the use of catalyst, animal fats or vegetable with alcohols also produces biodiesel and glycerin [ 3 ]. Feedstock selection is a crucial step in biodiesel production, which is impacted by different factors, such as yield, cost, composition, and purity of the produced biodiesel. Another significant factor affecting biodiesel production is availability and the types of sources (non-edible, edible, or waste) [ 65 ].

Further, the choice of materials used for its production depends on the geographical regions; for example, soybean is the primary source of biodiesel in the United States, whereas, in Europe and the tropical parts of the world, rapeseed (canola) and palm oil serve as the primary sources [ 66 , 67 , 68 ]. Different feedstocks produce biodiesel with distinct qualities that must be considered when blending biodiesel with petroleum diesel for their use in transportation. Biodiesel is blended with petroleum diesel from 5% to 20% biodiesel, or B5-B20. However, the Renewable Fuel Standard (RFS), a federal program that mandates the blending of biofuels into the nation’s fuel supply, has suggested including higher biodiesel blends. Soybean and canola oil are the most common biodiesel in the United States. Soybean accounted for about 50% of biodiesel feedstock input between 2014 and 2017. The soybean oil used for biodiesel production increased by 30% in 2017 compared to 2014 ( https://www.eia.gov/todayinenergy/detail.php?id=36052 . Accessed on 3 May 2018).

In 2020, approximately 71.7 % of the biodiesel feedstock came from soybean, while other small amounts of vegetable oils and animal fats (AF) such as canola oil (10.7 %), corn oil (13.0%), tallow beef fat (3.1%), poultry fat (1.5%), and other (0.1%) were used ( Table 1 ). Based on 2012–2019 data ( Table 2 ), rapeseed oil is still the dominant biodiesel feedstock in Europe and worldwide. In 2016, rapeseed (canola) input to global contribution for biodiesel production was 68%, followed by soybean (15%), animal fat and yellow grease (5% each), palm oil (6%), and sunflower (1%) [ 68 ]. However, rapeseed share in the feedstock mix in Europe has significantly decreased; for example, its share was 62.3% in 2012 compared to only 37.9% in 2019 ( Table 2 ). This decrease in the share of rapeseed oil in Europe is primarily because of recycled vegetable oil/used cooking oil (UCO) and palm oil. UCO, or yellow grease, has become the second-most important feedstock for Europe since 2015. In the USA, biodiesel production from yellow grease (13%) dominated both rapeseed-based biodiesel (10%), corn-based biodiesel (12%), and animal fats-based biodiesel (10%) (based on 2016 data reported by Kim et al., 2018 [ 68 ]).

US inputs to biodiesel production (million kilograms).

Table with -, W, S indicates no data, withheld to avoid disclosure of individual company data, and value is less than 0.5 of the table metrics. However, the value is included in any associated total. Source: U.S. Energy Information Administration (EIA), Form EIA-22M “Monthly Biodiesel Production Survey.” U.S. EIA| Monthly Biodiesel Production Report (2020).

The feedstock was used for biodiesel + renewable diesel (HVO; hydrotreated vegetable oil) in Europe from 2012 to 2019.

The original data were collected in a metric ton (MT) and then converted to kilogram (kg) using a conversion rate of 1 MT = 1000 kg (Source: EU-28. Available online: https://apps.fas.usda.gov/newgainapi/api/report/downloadreportbyfilename?filename=Biofuels%20Annual_The%20Hague_EU-28_7-15-2019.pdf . Accessed on 15 July 2019).

The types of raw materials/feedstocks for biodiesel production rapidly diversified for economic and environmental reasons [ 69 ]. A market survey reported that biodiesel’s feedstock market is transitioning from first-generation feedstock such as soybean, rapeseed, and palm oil to non-food and lower-cost feedstock such as jatropha, castor, UCO, and AF [ 70 ]. In countries such as Brazil, effective programs are underway to promote jatropha and castor production for biodiesel production. Similarly, another emerging feedstock for Biodiesel is HVO, which is produced through hydrotreating [ 69 ]. Production and use of biofuel generate emissions such as particulate matter (PM), carbon monoxide (CO), carbon dioxide (CO 2 ), nitrogen oxides (NOx), hydrocarbons, and volatile organic compounds (VOCs). The VOCs, unburnt hydrocarbon (UBHC), and NOx are the precursors for forming smog and ground-level ozone, which are associated with increased morbidity and mortality from cardiovascular and respiratory diseases and certain cancers [ 15 ]. Compared to fossil diesel, biodiesel produces lower PM, CO, VOCs, and NO X emissions [ 71 ]. Among NOx, nitrous oxide (N 2 O) is only the greenhouse gas of great environmental concern. It is a substantial anthropogenic greenhouse gas, and agriculture represents its most significant source. The global warming potential of N 2 O is 298 times that of CO 2 [ 72 ]. Previous studies on biofuel production systems revealed that emissions of N 2 O may counterbalance a substantial part of the global warming reduction by fossil fuel displacement [ 73 ]. Using optimized crop management, which involves state-of-the-art agricultural technologies coupled with an optimized fertilization regime, and nitrification inhibitors, N 2 O emissions can significantly be reduced by −135% points (pp) compared to conventional management. However, uncertainties in using statistical N 2 O emission models and data on non-land use GHG emissions due to biofuel production are significant, which can change the GHG emission reduction by between −152 and 87 pp [ 74 ].

While selecting the raw materials for biodiesel production, various parameters are considered, including oil content, suitability, chemical composition, and physical properties [ 75 ] ( Table 3 ).

Different sources of feedstocks/raw materials are used for the production of biodiesel [ 75 ].

1 represents feedstocks for biodiesel production reported by [ 86 , 87 , 88 ]; Ambat et al., 2018 [ 75 ] gathered information on sources of biodiesel feedstocks from different studies and reported them in their review paper.

A brief description of various feedstocks used for biodiesel production with their oil content is summarized in Table 3 .

Different studies were conducted to investigate the suitability of various feedstocks, for example, edible, non-edible oils, animal fats, and algal oils, for biodiesel production. The transformation of edible oil is biodiesel was considered the most feasible approach. As reported in Table 3 , the biodiesel feedstocks such as olive oil and microalgae oil have the highest oil content, up to 70%, followed by rubber seed oil (up to 68.4%) and coconut oil (up to 65%). The lowest oil content was reported for soybean oil (15–20%). Edible oils such as sunflower, soybean, and rapeseed ( Table 3 ) served as important substrates for biodiesel production. However, a vast disparity in food use affects the use of these first-generation feedstocks as fuel [ 76 ]. This will create a significant conflict with food vs. fuel, and competition with the food market can also adversely affect the price of biodiesel. The shift for non-edible oil such as castor oil, jatropha oil, and rubber seed oil was associated with the higher price of biofuel from edible oils because of their higher demand for food. Using raw materials from non-edible oils, animal fats, and waste oils has several advantages, including reducing the price of raw materials and avoiding competition with the food market [ 25 , 64 ].

In recent years, there has been significant interest in renewable and sustainable oils, and the life cycle assessment of raw materials plays a vital role in biodiesel production, it is essential to consider the oil content (%) and oil yield to determine the quality of biodiesel [ 65 ]. Additionally, microalgae are a great source of biodiesel production. These organisms can produce well-graded bioactive compounds by converting carbon dioxide (CO 2 ) with the help of sunlight [ 77 , 78 ]. With the increase in the price of petroleum and the concern with greenhouse gas emissions, microalgae have become an environmentally friendly alternative for biodiesel production. Though it is challenging for commercial-scale production, several companies have already started algal-based fuel production [ 77 , 78 ].

Similarly, animal fats, the byproducts of meat processing and cooking, are also important sources for biodiesel production. These include mutton or beef tallow, yellow grease, and lard, the residues after producing omega-3 fatty acids from fish oil [ 78 ]. Commercial-scale biodiesel production has been attained from animal fat-based feedstocks such as tallow, lard, and chicken fats. Unlike edible oils, animal fats-based biodiesel feedstocks have economic, environmental, and food security advantages. However, higher amounts of saturated fatty acids and free fatty animal fats demand complex production techniques. On the other hand, animal waste fats with lower saturated fatty acids have good oxidative stability, elevated calorific value, and shorter ignition [ 78 , 79 ]. Another important source of biodiesel feedstock is waste cooking oil. The waste cooking or frying oils include yellow and brown grease that does not directly conflict with food security. Yellow grease has < 15% fatty acid and can be used as a potential low-cost raw material for biodiesel production compared to brown grease (>15% fatty acid), which has an adverse effect on biodiesel production [ 79 ].

Feedstocks’ chemical composition and physical properties are essential when selecting raw materials for biodiesel production. The chemical composition of different fatty acids from different sources is highlighted in Table 4 . The differences in the degree of saturation and the carbon chain length are mainly due to the fatty acids of different architecture in the oil [ 66 ]. The degree of saturation from different sources is 14.7 % (soybean oil), 49.6% (palm oil, 6.1% (rapeseed oil), 21.6% (jatropha oil), 28.7% (used cooking oil), 46.9 % (animal fats), and 36.1% (algal oil) [ 68 ]. The percentage of carbon found at higher concentrations with C ≥ 18 in most of the feedstock oils except for algal oil, which has only 33.1% compared to 85% (soybean oil), 55% (palm oil), 87.4% (rapeseed oil), 85.7% (jatropha oil), 73.1% (used cooking oil), and 68.9% (animal fats) [ 68 ]. This study compiled the fatty acid profile of different fatty acids from various sources, including edible and non-edible oil, animal fats, and other sources. The predominant fatty acids were monosaturated fatty acids, saturated fatty acids and polyunsaturated fatty acids, oleic acid (C18:1; 2.9–72.2), palmitic acid (C16:0; 1.3–48), and linoleic acid (C18:2; 1–70) ( Table 4 ).

Fatty acid composition (%) of different biodiesel feedstocks.

The values of different fatty acids reported by different studies are represented by superscripts a [ 88 ], b [ 89 ], c [ 68 , 90 , 91 , 92 , 93 , 94 ]; microalgae species ( Nannochlopsis oculate ) d [ 95 ], e [ 66 ], f [ 96 ], g [ 75 ] and h [ 97 ].

4. Biodiesel and Its Properties

Biodiesel, also known as FAME, is produced by mixing methanol with vegetable oil, animal fat, or other triacylglycerol-carrying material. Differences in feedstocks significantly fluctuate the value of characteristics of FAME, including cloud point, Cetane number (CN), oxidative stability, saponification value, iodine value, and acid value [ 88 ]. The main physicochemical properties of biodiesel obtained from various feedstock/raw materials are discussed below ( Table 5 ).

Physicochemical properties of different biofuel feedstocks.

Cloud point (CP), cetane number (CN), oxidative stability (OS), saponification value (SN), iodine number (IN), and Acid value (AV). Values shaded with green are adopted from [ 88 ], blue from a study by [ 75 ], and not highlighted text black are from the U.S. Energy Information Administration (EIA), compiled from the U.S. Department of Energy, National Renewable Energy Laboratory, and Renewable Energy Group.

4.1. Cloud Point

The cloud point (CP) is the minimum temperature below which wax begins to form crystals in fuels, resulting in a cloudy appearance [ 98 ]. Solidified waxes can clog engine fuel filters and injectors. Biodiesel has higher CP due to the high melting points of saturated fatty acids compared to unsaturated fatty acids [ 88 ]. Biodiesel produced from feedstocks such as inedible tallow and waste frying oil may require additives or blend at higher levels with lower cloud point ULSD to mitigate cold weather concerns.

4.2. Cetane Number

The cetane number (CN) represents the ignition behavior and quality of the fuel. Higher cetane is often associated with improved performance and a cleaner burning fuel [ 99 ]. Most biodiesel feedstocks have slightly higher cetane numbers than ultra-low sulfur diesel (ULSD), which usually has a minimum allowable cetane value of 40 ( https://www.eia.gov/todayinenergy/detail.php?id=36052 . Accessed on 3 May 2018). The CN value of biodiesel increases with the length of the fatty-acid chain and the degree of saturation; hence, a higher CN means a higher oxygen concentration in the biodiesel and a better combustion efficiency [ 98 ]. Studies reported the highest CN value of 70 for Spirulina platensis [ 100 ] vs. the lowest CN value of 34.6 for biodiesel obtained from linseed oil [ 101 , 102 ]. The raw materials and feedstocks reported in this study have a CN value range between 47 for soybean oil and 64 for choice white grease ( Table 5 ).

4.3. Oxidative Stability

Oxidative stability is the ability of the fuel to resist oxidation during storage and use. This essential factor significantly influences the storage duration and condition [ 103 ]. Fuels with lower oxidative stability are more likely to form peroxides, acids, and deposits that adversely affect the engine performance. Because it generally has lower oxidative stability, petroleum diesel can be stored longer than biodiesel feedstocks such as white grease and tallow. Biodiesel producers may use additives to extend the storage and usage timelines of Biodiesel (Source: EIA). Biodiesel with high oxidative stability is highly susceptible to oxidation deterioration. Oxidative stability varies according to fatty acid composition [ 104 ]. The fuel’s oxidative stability is greatly affected by polyunsaturated FAME. For example, Camelina-oil-based Biodiesel has low oxidative stability because it has approximately 35% polyunsaturated FAME occurrence (i.e., α -linolenic [C18:3]) [ 105 ] compared to coconut-oil-based biodiesel, which has better oxidative stability due to 2% polyunsaturated FAME in its oil [ 88 ]. Oxidative stability reported in this study ranges from 2.3 mg/100 mL (yellow grease) to 44.9 mg/100 mL (Canola oil) ( Table 5 ).

4.4. Saponification Value

Saponification value (SV) is an index of the molecular weights of triglycerides in the oil. It is inversely proportional to the average molecular weight or the chain length of the fatty acids [ 106 ]. Thus, the shorter the chain length, the higher the SV of the oil. The expected SV should range between 195 and 205 mg/KOH/g of oil [ 107 , 108 ]. Any value below that value needs refining to meet the required standard and would be better fitted for an industrial purpose [ 109 ]. The SV reported in this study is comparable and lies close to the required range of 195–205 ( Table 5 ).

4.5. Iodine Number

Iodine number (IN) represents the amount of iodine absorbed by double bonds of the FAME molecules in 100 g of the fuel sample. A higher iodine value indicates higher fats and oils [ 110 , 111 ]. In the case of biodiesel fuels, linseed methyl ester showed the highest IN of 178 compared to the lowest IN of 37.59 reported for Kusum-oil-based Biodiesel [ 102 , 112 ]. This study reported the lowest IN for Palm oil (35–61) vs. the highest value of IN for Camelina oil (146.5) ( Table 5 ).

4.6. Acid Value

The acid value represents the fuel sample’s quantity of free fatty acids. A high acid number causes corrosion problems in the engine’s fuel delivery system [ 112 ]. A high acid value of 6.9–50.8 mg KOH/mg of oil is reported for biodiesel from palm oil compared to the lowest acid value of 0.1–0.2 mg KOH/mg of oil from soybean oil ( Table 5 ). Further, descriptions of additional fuel properties of biodiesel from different generation oil feedstocks are reported in our previous study [ 88 ].

5. Procedures for Biodiesel Production

Different physicochemical processes could produce biodiesel, and the primary methods include pyrolysis, micro-emulsion, and transesterification [ 113 ]. Each method has its merits and demerits. For example, micro-emulsion is a simple and environmentally safer method that generates fewer pollutants. Biodiesel synthesized using this method has a good cetane number (CN). Similarly, alcohol in the micro-emulsion process improves the CN of Biodiesel [ 114 ]. Microemulsion-based fuel systems reduce the combustion temperature, which leads to lower emissions of thermal NO x , CO, black smoke, and particulate matter. However, one major problem of using ethanol to formulate a microemulsion system is its lower miscibility with diesel. The immiscibility can be visualized for a wide range of temperatures, particularly at lower temperatures [ 115 , 116 ]. Furthermore, environmentally benign bio-based non-ionic surfactants and cosurfactant without N and S are of environmental concern [ 117 , 118 ]. Biodiesel production from the pyrolysis method (also known as thermal cracking) has low CN, volatility, and high viscosity [ 21 ]. By comparing these methods, the transesterification method is reliable and effective because the transesterification method demands low temperature, low pressure, and less processing time. The transesterification method is simple and highly efficient [ 119 ]. A description of various procedures to generate biodiesel is highlighted below.

5.1. Micro-Emulsion

This method uses isotropic fluid to form a colloidal dispersion of dimensions ranging from 1 to 150 nm. A study using soybean oil has already demonstrated that by using this method, maximum viscosity was achieved that involves both ionic and non-ionic aqueous solutions [ 120 , 121 ]. A study revealed that using a ternary phase system (a clear and thermodynamically stable, isotropic liquid mixture of oil, water, and surfactant) counters the viscosity problems of vegetable oils by forming micro-emulsions with different solvents (ethanol, methanol, propanol, n-butanol, and hexanol). These alcohols act as emulsifying agents, dispersing the oil into tiny droplets, usually with diameters ranging from 100 to 1000 Å [ 122 ].

5.2. Pyrolysis

Thermal cracking or pyrolysis converts organic materials to fuels without oxygen using thermal decomposition (temperature: 300–1300 °C) [ 122 ]. Chemically, pyrolysis reaction cleaves the bonds in a substance, converting it into many smaller compounds. The process is similar to the process used to synthesize petroleum-diesel; therefore, it yields a product with similar combustion characteristics and results in less waste formation and no pollution [ 123 , 124 ].

The substrate used for pyrolysis includes vegetable oils, animal fats, natural fatty acids, or methyl esters of fatty acids. It sometimes produces a higher yield than the transesterification reaction, which is the most widely used [ 122 ]. The pyrolysis of organic feedstock for the manufacture of synthetic diesel has yet to be viable on an economic scale [ 124 ]. Based on operating parameters, pyrolysis can be divided into three types, namely conventional pyrolysis (550–900 K), fast pyrolysis (850–1250 K), and flash pyrolysis (1050–1300 K) [ 124 ]. The pyrolysis of biomass for bio-oil generation can be performed using both conventional and flash pyrolysis. In conventional pyrolysis, the vapor residence time ranges from 5 to 30 min, and thus this contributes to overall reaction time. Depending upon residence time, the vapors can be removed continuously. Whereas in flash pyrolysis, the heating rate is predominantly high. Some of the prerequisites for flash pyrolysis include a high heat transfer rate, finely grounded materials, and short vapor residence times (<2 s) [ 125 ]. The product obtained from pyrolysis has desired characteristics of biodiesel, such as low viscosity, less amount of sulfur and water, and high cetene number; however, it has less ash and residual carbon content than the desirable amount [ 123 , 124 , 126 , 127 ].

5.3. Transesterification

Transesterification is a standard and widely used procedure for high-quality biodiesel production [ 128 ]. This procedure involves the transformation of fats or oils using alcohol, particularly methanol or ethanol, with the help of catalysts (e.g., heterogeneous, homogeneous, or enzyme) [ 129 , 130 ]. Compared to the transesterification process facilitated by enzymes, the process is energy-consuming because of the presence of soap byproducts, and separation and purification of the chemically produced biodiesel require more complex steps than enzymatically produced biodiesel [ 131 ]. Ethanol is cost-effective and abundant commodity obtained from the fermentation of sucrose from sugarcane. Propanol or butanol could be a better option because these two alcohols promote better miscibility between the alcohol and the oil phases [ 132 ]. Transesterification can be combined with ultrasound-assisted member technology [ 25 , 66 , 120 ]. There are merits and demerits of using various biodiesel production technologies based on several studies ( Table 6 ). However, these production technologies were centered on reducing problems during biodiesel production, such as oil’s high viscosity, acid value, and fatty acid content [ 97 , 133 ]. Among those technologies, transesterification using a homogeneous catalyst was the most typical and commercially used technology [ 134 ]. From an environmental point of view, enzyme catalysts and heterogeneous catalysts are suitable options for the future [ 75 ].

Merits and demerits of using various biodiesel production technologies [ 27 , 97 , 133 , 135 ].

6. Factors Affecting Biodiesel Production

Biodiesel production using biomass feedstock is influenced by several factors described below.

6.1. Free Fatty Acids

Free fatty acids affect biodiesel production. The higher amount of free fatty acid leads to soap and water formation [ 146 ]. The slow rate of acid-catalyzed reaction requires low-temperature conditions [ 147 ]. Base-catalyzed transesterification reactions demand raw materials with low acid value (<1) and free from water [ 148 ]. With 3% free fatty acids, there is no need to use a homogeneous base catalyst during the transesterification reaction [ 149 ].

6.2. Water Content

The amount of water content in the feedstock accelerates the hydrolysis and lowers the formation of ester [ 150 ]. A study has revealed that for a 90% biodiesel yield for an acid-catalyzed reaction, the water content should be less than 0.5% [ 151 ]. Additionally, water obtained as a byproduct inhibits the reaction and decreases engine performance. However, water in oil can be removed by preheating it up to 120 °C or by using anhydrous sodium sulfate or anhydrous magnesium sulfate [ 152 ].

6.3. Types of Alcohol

Methanol is used for biodiesel production for a higher conversion rate from waste cooking oil with lower viscosity and is cheaper than other alcohol-based biofuels [ 153 ]. However, it is more toxic [ 154 ] and causes enzyme deactivation, denaturation, or inhibition at higher concentrations [ 155 ]. In order to address these issues, ethanol is used in most enzymatic reactions [ 153 ].

6.4. Alcohol to Oil Ratio

In order to obtain one mole of alkyl ester, 3 mol of alcohol and 1 mol of triglyceride are needed [ 156 ]. The rate of biodiesel production increases with higher alcohol concentration, i.e., increasing the alcohol-to-oil ratio [ 157 ]. The maximum conversion with 99% biodiesel production was achieved from waste sunflower oil transesterification using methanol and NaOH as the catalyst, with an alcohol-to-oil ratio of 6:1 [ 158 , 159 ], compared to 49.5% yield in waste canola petroleum using 1:1 methanol to oil [ 158 ].

6.5. Reaction Time

Reaction time plays a significant role in product conversion. Suppose more time is needed to give to the reaction. In that case, some parts of the oil may remain unreacted and ultimately reduce ester yield and exceed reaction time than usual, affecting the end product and leading to soap formation [ 160 ]. The reaction time for lipase-catalyzed reactions differs from 7 to 48 h [ 161 ]. Studies also suggested that reaction time also controls production costs. A study found no significant change in the conversion of biodiesel with the reaction time of 1 h (96.10%) versus 3 h (96.35%) [ 162 ]. However, a longer reaction time may lead to the reduction in biodiesel due to reversible transesterification reaction resulting in loss of esters and soil formation. Thus, reaction time needs to be optimized to bring the production cost down to a minimum. Maximum ester conversion can be achieved within <90 min.

6.6. Reaction Temperature

High temperatures lead to lower oil viscosity, resulting in a high reaction rate and reduced reaction time. However, if the temperature increases beyond the desirable range, the biodiesel yield is lowered due to the saponification of triglycerides accelerated by high temperature [ 163 ]. Biodiesel viscosity improves as the reaction temperature falls below 50 °C. For waste cooking oil, it is necessary to pre-heat up to 120 °C and cool down to 60 °C [ 164 ]. Higher reaction temperature increased the reaction rate and shortened the reaction time due to the reduction in the viscosity of oils. For the esterification reaction, the temperature should be below the boiling point of alcohol to prevent alcohol evaporation [ 165 , 166 ]. The highest conversion was achieved for cottonseed oil at 50 °C and Jatropha oil at 55 °C using lipase as a catalyst [ 167 ]. Further, the maximum yield of biodiesel was reported at 65 °C for domestic and commercial (waste and fresh) oils using KOH as a catalyst [ 162 ].

Though pH is not crucial for acid/base catalysts, for lipase catalysts, pH plays an important role; for example, the enzyme may decompose at higher or lower pH. For example, a study found that a pH of 7 is optimal for biodiesel production using Jatropha oil-immobilized Pseudomonas fluorescence [ 168 ].

6.8. Catalyst Concentration

The most commonly used catalyst for biodiesel production is sodium hydroxide (NaOH) or potassium hydroxide (KOH) [ 165 ], and other catalysts used are sodium methoxy and potassium methoxide [ 169 ]. Increasing the catalyst concentration with oil samples also increases the conversion of triglycerides into biodiesel. However, it also increased soap formation. Lowering the amount of catalyst leads to incomplete conversion into fatty acid ester, resulting in lower methyl esters yield [ 166 ]. Optimum biodiesel production is achieved when the concentration of NaOH reaches 1.5% weight [ 93 ]. Again, using an excess amount of catalyst can have a negative impact on biodiesel yield [ 93 , 170 ]. For soybean oil biodiesel, a 1.5 % copper vanadium phosphate (CuVOP) concentration was found to be the most effective [ 171 ].

6.9. Agitation Speed

Agitation is mandatory for the reaction, and its speed is essential for product formation. Lower agitation speed cause less product formation. Lower agitation speed cause less product formation. However, higher agitation speed favors soap formation [ 166 ]. There should be an optimum stirrer speed, which varies with our feedstocks. A study revealed a stirrer speed of 200 mm found to be optimum for biodiesel production using enzymatic reactions [ 172 ]. However, another study reported that at 400 rpm, there was a higher conversion of end product compared to 200, 600, and 800 rpm for an hour [ 165 ].

7. Catalyst Use for Biodiesel Production

Biodiesel is fatty acid methyl esters (FAME) with lower alkyl esters and long-chain fatty acids. It is synthesized by two procedures: esterification of fatty acids and transesterification with lower alcohol. Even without a catalyst, transesterification reactions can happen. However, they demand high temperatures, pressure, and reaction time. It also increases the overall cost of the reaction process [ 173 ]. The biodiesel thus produced has high purity of ester and glycerol (soap-free); however, from a commercial scale standpoint, it is imperative to use catalysts. Hence, there are three different catalysts: acidic, alkaline, and enzyme [ 174 ].

7.1. Acidic Catalysts

Acidic catalysts support higher efficiency for the esterification of FFAs over alkaline catalysts, with up to 90% conversion [ 175 ]. These catalysts favor feed oil with high acid value (including edible waste oil) and have good potential for transesterifying low-quality feeds [ 3 ]. Transesterification is performed at high temperatures (100 °C), pressure (~5 bar), and a high amount of alcohol. However, the process is slower compared to alkaline catalysts [ 3 ]. The most commonly used acid catalysts are sulfuric acid, hydrochloric acid, organic sulfonic acid, sulfonic acid, and ferric sulfate [ 75 ].

7.2. Alkaline Catalysts

Alkaline catalysts for biodiesel production include NaOH, KOH, alkaline metal carbonate, sodium and potassium carbonates, sodium methoxide, and sodium ethoxide. These catalysts are appropriate for oil with low FFAs due to the sensitivity as oils with higher FFAs, are converted to soap rather than biodiesel. This process restricts the separation of glycerin, biodiesel, and water. In order to cope with the issue, a deacidification step is necessary before the transesterification of vegetable oil [ 3 ].

7.3. Enzyme Catalysts

Enzymes such as lipases from microorganisms act as catalysts during transesterification reactions [ 176 ]. Lipase enzymes are abundant in nature and are synthesized by microorganisms (fungi, bacteria, and yeast), plants (rapeseed, oat, papaya, latex, and caster seeds), and animals (cattle, pigs, hogs, and pancreases of sheep) [ 177 ]. During biodiesel production, no or little residual or soap is formed at the end, resulting in high-quality glycerol production. This is also useful for feedstocks with high acidic values. Some limitations of using enzyme catalysts are high concentration and long reaction time. Separating the final product from the reaction results in a high cost of biodiesel production [ 3 ]. Further, applying metagenomics in enzyme technology opens the door for developing stable and solvent-tolerant biocatalysts for biodiesel production [ 178 ].

7.4. Homogeneous Catalysts

Homogeneous catalysis involves a series of reactions involving a catalyst from the same phase as the reactants, whether in the liquid or gaseous state. A homogeneous catalyst is dissolved or co-dissolved in the solvent with all the reactants [ 166 ]. Sodium hydroxide (NaOH) or potassium hydroxide (KOH) is the most popular homogeneous catalyst for biodiesel production [ 179 ]. Homogeneous catalysts are acidic and basic and widely used for biodiesel production. Acid catalysts are less active than base catalysts (i.e., lower reaction time). Therefore, a base catalyst involves high temperature and pressure. When FFAs exceed 1% in the oil, acid catalysts become effective. Acid catalysts prevent soap from forming. These catalysts catalyze the esterification of FFAs to form FAME and thus enhance biodiesel production [ 75 , 180 ]. The deep eutectic solvents (DESs) with acidic nature were evaluated for biodiesel production and found to have over 90% conversion efficiency [ 3 ]. Alkaline catalysts react with alcohol to form alkoxide and protonated catalysts. The carbonyl atom of the triglyceride molecule is attacked by nucleophilic alkoxide to form a tetrahedral intermediate, which reacts with alcohol to revive the anion. Further, the tetrahedral structure undergoes structural reorganization to form a fatty acid ester and diglyceride [ 66 , 181 ]. The higher conversion rate is obtained at low temperatures and pressure, resulting in lower production costs of biodiesel [ 3 ]. The alkaline catalysts are less efficient than acidic catalysts for converting oils containing high FFAs, producing soap, and inhibiting the separation of ester and glycerin. Thus, acid catalysts are recommended for biodiesel production [ 75 ].

7.5. Heterogeneous Catalysts

Catalysts with a state or phase different from reactants are heterogeneous catalysts. Most of the heterogeneous catalysts are solid. However, reactants are either in liquid or gaseous forms [ 166 ]. The separation process in heterogenous catalysts is easy and aids faster recycling and reuse than homogeneous catalysts. Therefore, it resolves problems related to homogeneous catalysis while lowering the material and processing costs [ 25 , 120 , 182 , 183 , 184 , 185 ]. Heterogenous catalysts can also tolerate high FFA and moisture content [ 186 ]. These catalysts, even at severe reactions conditions, can recover from a reaction mixture, stand up to aqueous treatment steps, and can be easily modified to achieve a high level of activity, selectivity, and long lifetime. Solid base heterogeneous catalysts include hydrotalcite, metal oxides (CaO, MgO, or SrO), oxides of mixed metals (Ca/Mg, Ca/Zn), alkali metal oxides (Na/NaOH/y-Al 2 O 3 , K 2 CO 3 /Al 2 O 3 , magnetic composites, and alkali-doped metal oxides (MgO/Al 2 O 3 , CaO/Al 2 O 3 , Li/CaO) [ 187 ]. However, some limitations of using heterogenous catalysts include diffusion due to phase separation between alcohol and oil, low surface area, and leaching. Strategies to resolve these issues include using n-hexane and tetrahydrofuran as co-solvents, increasing the area of specific activities, and providing more pores for reactive components. This can be possible with the help of supporters for catalysts as well as promoters for its structure [ 25 , 120 ]. The study also suggested that through immobilization or in the liquid phase, higher biodiesel yield can be obtained with robust lipase enzymes (how lipase technology contributes to the evolution of biodiesel production using multiple feedstocks).

A clear distinction between acid versus alkali and homogeneous versus heterogeneous catalyzed transesterification reactions is shown in Table 7 and Table 8 .

Comparison of acid versus alkali-catalyzed transesterification process of biodiesel production, reported by [ 27 ].

Comparison of homogeneous versus heterogeneous-catalyzed transesterification process of biodiesel production, reported by [ 27 ].

Overall, the reusability and recyclability are complex in homogeneous catalysts, whereas heterogeneous catalysts offer efficient, yielding results and can be reused again. Nanocatalysts that come under the heterogeneous catalyst group reveal better yield due to the large surface area at the Nanoscale and are preferable for biofuel reproduction with the help of transesterification [ 4 ]. Homogeneous catalysts are also considered fuel performance catalysts due to their ability to improve fuel efficiency and reduce smoke emissions, unburned hydrocarbons, and carbon monoxide.

8. Evaluation of Greenhouse Gas Emissions from Biodiesel Production

Overall, biodiesel reduces GHG emissions of carbon monoxide (CO), carbon dioxide (CO 2 ), unburnt hydrocarbon (UBHC), and particulate matter (PM) to a significant extent, except nitrogen oxide (NO X ), compared to diesel. These emission gases are the primary causes of atmospheric pollution and human health [ 71 ].

Carbon monoxide reduction from different biodiesel feedstock range from 9.4% (microalgae) to 63% (palm oil) compared to CO emissions from diesel [ 188 ]. Several studies have shown the different proportions of CO emission reduction as engine speed increases. For example, CO emission reduction for soybean biodiesel was reported at 14% at 1400 rpm, 27% at 2000 rpm [ 189 ], and 37% at 3600 rpm engine speed [ 90 ]. Carbon monoxide reduction using rapeseed oil was 29.7% [ 91 ] and 26% [ 90 ]. Similarly, CO emission from jatropha oil ranged from 14 to 30% [ 190 , 191 , 192 ], waste cooking oil ranged from 17.8 to 20% [ 91 , 193 ], animal fats from 26% [ 90 ], and microalgae ranged from 9.4 to 32% [ 194 , 195 , 196 ].

Carbon dioxide emissions in some biodiesels are almost the same or even higher than the regular diesel [ 68 ]. For example, CO 2 emissions from soybean oil (SO) and used/waste cooking oil (UCO)-based biodiesel were 60% and 33% more compared to regular diesel [ 90 ]. However, another study reported an increase in CO 2 emission from SO and UCO by 1.8% and 1.2%, respectively. Similarly, palm, rapeseed oil [ 90 ], and jatropha oil [ 191 ] generated 41%, 32%, and 3% more CO 2 emissions than regular diesel. Animal fats and microalgae biodiesels emitted 3% and 2.6% more CO 2 emissions than regular diesel [ 197 ].

Nitrogen oxide emissions from biodiesels are more than diesel, except for palm oil and microalgae-based biodiesel. Studies have shown that biodiesel with long-chain fatty acids produced fewer NOx emissions than short-chain fatty acids. On the contrary, NOx emissions increased as the number of double bonds, i.e., the degree of saturation of fatty acids, increased [ 68 ]. NOx emissions from soybean biodiesel increased due to its highest degree of saturation (14.7%) and 85.3% of the chain. NOx emission was lowered for palm-based biodiesel due to more short chains and a high degree of saturation than other biodiesels [ 68 ].

PM emissions from biodiesel are lowered compared to diesel. PM emissions from SO biodiesel decreased from 56% [ 90 ] to 69% [ 198 ], palm oil by 50% [ 91 ], rapeseed oil by 36% [ 90 ] to 70% [ 198 ], jatropha oil by 11% [ 191 ] to 15% [ 192 ], and used/waste cooking oil by 17% [ 90 ]. Similarly, PM emissions from animal fats-based biodiesel decreased by 61% [ 99 ] to 77% [ 198 ] and microalgae by 31% [ 196 ].

Overall, soybean and animal fats-based biodiesel produced the lowest PM, jatropha oil-, animal fats-, and microalgae-based biodiesel produced the lowest CO 2 emissions. Palm oil-based biodiesel produced the lowest CO and NOx emissions [ 68 ].

9. Conclusions and Recommendations

The demands for fossil fuels are gradually increasing due to the improvement in technology (for example, urbanization and improved life standard), which also requires more fuels. This increasing use of energy reserves will decrease fossil fuels in the future. A rapid increase in population and associated energy demand cannot be fulfilled by using fossil fuels alone. Using first-generation crops such as soybean and corn as bioenergy creates conflict in the food versus energy debate. Likewise, second-generation crops, particularly grasses, are unsuitable for biodiesel production. One of the significant problems in using second-generation vegetable oil is that it lessens engine life if the oil is not refined correctly. These issues of using first- and second-generation biofuels, such as economic, social, and food insecurity [ 48 ], can be resolved using third and fourth-generation biofuels. Third and fourth-generation biofuels are generated from various types of algae, which is highly efficient, and algal-based biofuels have great potential and no competition for food or land. In recent times, fourth-generation biofuels have great promise to overcome the inherent flaws and meet the world’s growing energy demands. Though algal cultivation is simple, feedstock production is complex due to high lipid content, and harvesting needs should be addressed. Detailed work on the parameters for fuel compatibility is required. Many things need to be worked out to make an algal biofuel a commercially viable option to fossil fuel, as the production of biofuels from microalgae is an energy-intensive process [ 199 ]. Further, greenhouse gas emissions are much lower; mainly, there is no emission of CO or CO 2 using this generation of biofuels. Thus, these fuels could be potential options to replace fossil fuels. It is also recommended to consider the potential benefits of using other resources for energy sources that are more cost-effective, climate resilient, and sustainable. This could reduce the burden on fossil fuels in the future.

Acknowledgments

The author would like to acknowledge Eric Olson, an Associate Professor of the Department of Mathematics and Statistics at the University of Nevada, Reno, for critical reading of the manuscript. The author would also like to acknowledge all individuals, including anonymous reviewers and editors, for their valuable comments and suggestions to improve the quality of this article.

Abbreviations

GJ: gigajoules; GHGs: Greenhouse gases; CO 2 : carbon dioxide; NO X : nitrogen oxides; CH 4 : methane; FAME: fatty acid methyl ester; EIA: energy information administration; AF: animal fats; UCO: used cooking oil; UBHC: unburnt hydrocarbon; FFA: free fatty acid.

Funding Statement

There is no external funding (except the reviewers’ vouchers) for this publication.

Author Contributions

Conceptualization and writing, D.N., writing—review and editing, compilation, and supervision, D.N. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Data availability statement, conflicts of interest.

The authors declare no conflict of interest. The funders had no roles in the design of the study; in the collection, analysis, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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Biodiesel Benefits and Considerations

Biodiesel is a domestically produced, clean-burning, renewable substitute for petroleum diesel. Using biodiesel as a vehicle fuel improves public health and the environment, provides safety benefits, and contributes to a resilient transportation system.

Public Health and the Environment

The transportation sector is the largest source of greenhouse gas emissions in the United States. A successful transition to clean transportation will require various vehicle and fuel solutions and must consider life cycle emissions. Engines manufactured in 2010 and later must meet the same emissions standards, whether running on biodiesel, diesel, or any alternative fuel. Selective catalytic reduction (SCR) technology in diesel vehicles, which reduces nitrogen oxide (NOx) emissions to near-zero levels, makes this possible. The criteria air pollutant emissions from engines using diesel fuel are comparable to those from biodiesel blends.

Using biodiesel reduces life cycle emissions because carbon dioxide released from biodiesel combustion is offset by the carbon dioxide absorbed from growing soybeans or other feedstocks used to produce the fuel. Life cycle analysis completed by Argonne National Laboratory (PDF) found that B100 use reduces carbon dioxide emissions by 74% compared with petroleum diesel. The California Air Resources Board (CARB) reported similar values from various sources for its life cycle analysis of biodiesel.

Air quality benefits of biodiesel are roughly commensurate with the amount of biodiesel in the blend. Learn more about biodiesel emissions .

Engine Operation

Biodiesel improves fuel lubricity and raises the cetane number of the fuel. Diesel engines depend on the lubricity of the fuel to keep moving parts from wearing prematurely. One unintended side effect of the federal regulations, which have reduced allowable fuel sulfur to only 15 ppm and lowered aromatics content, has been to reduce the lubricity of petroleum diesel. To address this, the ASTM D975 diesel fuel specification was modified to add a lubricity requirement (a maximum wear scar diameter on the high-frequency reciprocating rig [HFRR] test of 520 microns). Biodiesel can improve the lubricity of diesel fuel, even at very low levels. The amount of biodiesel required depends on the specific properties of the diesel fuel, but 2% biodiesel is almost always sufficient for adequate lubricity.

Before using biodiesel, check your engine original equipment manufacturer (OEM) recommendations to determine the allowable blend for your vehicle (see the Engine Technology Forum’s list of diesel vehicles available in the United States for light-duty vehicles and a fact sheet from Clean Fuels Alliance America for heavy-duty vehicles that are compatible with biodiesel .)

Biodiesel in its pure, unblended form causes far less damage than petroleum diesel if spilled or released to the environment. It is safer than petroleum diesel because it is less combustible. The flashpoint for biodiesel is higher than 130°C, compared with about 52°C for petroleum diesel. Biodiesel is safe to handle, store, and transport. For additional guidance on handling, storing, and transporting biodiesel, reference the Biodiesel Handling and Use Guide (Sixth Edition) .

Energy Resilience and Balance

The transportation sector accounts for approximately 30% of total U.S. energy needs and 70% of U.S. petroleum consumption. Using biodiesel and other alternative fuels and advanced technologies to provide diverse clean transportation options strengthens national energy security by increasing resilience to natural disasters and fuel supply disruptions.

Biodiesel is produced in the United States and used in conventional diesel engines, directly substituting for or extending supplies of traditional petroleum diesel. Soybean biodiesel has a positive energy balance, meaning that soybean biodiesel yields 4.56 units of energy for every unit of fossil energy consumed over its life cycle. (See USDA study for more details.)

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Article Contents

1 introduction, 2 theoretical consideration, 3 experimental consideration, 4 results and discussion, 5 conclusions.

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Comparison of the performance and emissions of different biodiesel blends against petroleum diesel

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P. McCarthy, M.G. Rasul, S. Moazzem, Comparison of the performance and emissions of different biodiesel blends against petroleum diesel, International Journal of Low-Carbon Technologies , Volume 6, Issue 4, December 2011, Pages 255–260, https://doi.org/10.1093/ijlct/ctr012

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Biodiesel, an alternative fuel of petroleum diesel, is mainly used to reduce the environmental impact of emissions without modifying engines. This study compares the performance and emissions characteristics of different biodiesel blends with petroleum diesel using an internal combustion engine (Kubota V3300) and following ISO 8178 standards. Two types of biodiesel, type A (80% tallow and 20% canola oil methyl ester) and type B (70% chicken tallow and 30% waste cooking oil methyl ester), were tested in this study. It was found that the performance (mainly torque and brake power) of both biodiesel fuels reduces with increasing blend ratio which can be attributed to lower energy content of biodiesel. Specific fuel consumption increases for both biodiesels compared with diesel fuel, as expected. Some of the greenhouse gas emissions were found to be higher than petroleum diesel, whereas some were lower. Overall, Biodiesel A was found to produce lower emissions across the board compared with diesel and Biodiesel B.

It is well known that petroleum diesels are the major source of air pollutions that create an adverse impact on human health and overall greenhouse gases. Biodiesel has some great benefits over petroleum diesel, such as it produces 4.5 units of energy against every unit of fossil energy [ 1 , 2 ] and also it has some environment-friendly properties such as it is non-toxic, biodegradable and safer to breathe [ 3 ]. Biodiesel is also a clean-burning and stable fuel [ 3 ]. Properties of biodiesel such as oxygen content, cetane number, viscosity, density and heat value are greatly dependent on the sources (soybean, rapeseed or animal fats) of biodiesel [ 4 , 5 ]. Engine performance and emissions depend on the properties of biodiesels. Biodiesel is a highly oxygenated fuel that can improve combustion efficiency and can reduce unburnt hydrocarbons (HCs), carbon dioxide (CO 2 ), carbon monoxide (CO), sulphur dioxides (SO 2 ), nitric oxide (NO x ) and polycyclic aromatic HC emissions. However, brake-specific fuel consumption slightly increases [ 6 ].

Popularity of biodiesel as renewable sources of alternative fuel of petroleum diesel is growing quickly due to increased environmental awareness and the rising price of diesel. It is an earth-friendly choice of consumers that already occupies a great volume of the world's fuel sector due to its clean emission characteristics.

Developments of biodiesel fuels in many countries are driven by the necessity to reduce the greenhouse gas emissions which is the major issue for today's world, and the scarcity of the source of petroleum diesel also enhances the development and production of biodiesel fuel around the world. Biodiesel is generally produced from vegetable oils or animal fats through a chemical process known as transesterification process.

Vegetable oil was first used to run an engine by Rudolf Diesel (1858–1913) who developed the first engine. But sometimes, vegetable oils create adverse effects on engine components which may be due to their different volatility and molecular structure from diesel fuel as well as high viscosity compared with diesel fuel [ 4 , 5 , 7 ]. Currently, this problem is being eliminated by applying different chemical processes such as transesterification, supercritical, catalyst-free process etc., on vegetable oils to convert into biodiesel.

This paper aims to investigate the engine performances (power, torque, fuel consumption) and emissions (unburnt HCs, carbon dioxide, carbon monoxide and nitric oxide) of a diesel engine using two different biodiesels. Two different sources of biodiesel, type A [80% tallow (beef, pork and sheep) and 20% canola oil methyl ester] and type B (70% chicken tallow and 30% waste cooking oil methyl ester), were used for the experimentation in this study. Fuel types such as B5, B10, B20, B50 and B100 are analysed and discussed.

Specifications of Kubota V3300 [ 20 ].

Experimental set-up of Kubota V3300 Indirect Injection, four cylinders naturally aspirated CI engine.

The biodiesel–diesel blends that referred to as B5, B20, B50 and B100 were used in this study, where the percentage ratios of biodiesels are 5%, 20%, 50% and 100%, respectively. Two types of biodiesels were used in this study to blend with petroleum diesel, type A [80% tallow (beef, pork and sheep) and 20% canola oil methyl ester] and type B (70% chicken tallow and 30% waste).

ISO 8178 test procedure was used in this study which is an eight-mode steady-state test procedure that comprises three engine speeds, rated speed, intermediate speed and low idle for testing. The minimum test mode length of each mode is 10 min and emissions are measured in the last 3min of each mode. The engine is preconditioned by warming up the engine at its rated power for 40 min before each test cycle and minimum 50 data were taken for each mode in each test cycle, and three cycles are run per test fuel and then average. Experiments were done at both 2600 and 1400 rpm.

Fuel consumption of biodiesel is expected to be slightly higher than petroleum as density of the biodiesels is higher than petroleum diesel [ 11 ]. Sources of biodiesel greatly influence the engine performance, e.g. the engine fuelled with palm oil biodiesel is more efficient than biodiesel produced from tallow and canola oil [ 12 ]. Biodiesel is likely to produce less power with high fuel consumption than diesel as the gross calorific value (energy content) of biodiesel is lower than petroleum diesel. Blends of biodiesel with petroleum fuel are widely used in the diesel engine [ 13 ]. High viscosity of the fuels causes fuel flow and ignition problems in unmodified CI engines and also decreases the power output [ 11 , 14 ]. The lubricity and oxidative stability of the animal fat-based biodiesels are better than soy-based biodiesel [ 15 , 16 ]. The composition of animal fatty acid methyl esters is different from vegetable fatty acid methyl (ethyl) esters.

The results of performances and emissions of biodiesels tested in this study (Biodiesels A and B) compared with diesel are presented and discussed below.

4.1 Torque and power

Figure  2 shows the torque as a function of diesel and biodiesel blends for both Biodiesels A and B using modes 1 and 5 of the ISO 8178 test procedure. Mode 1 corresponds to the rated speed of the engine (2600 rpm) at 100% throttle, and mode 5 corresponds to the intermediate speed of the engine (1560 rpm) at 100% throttle. These two modes are the only ones that give a good indication of the differences in torque when using biodiesel, as the other modes require the torque to be set to a value (therefore reducing the throttle from 100%) which is the same for all test fuels. It can be seen from Figure  2 that the output torque decreases with increasing blend ratio for both biodiesels. The percentage decrease for both biodiesels at these modes is in the range of 4–5%. A decrease in this magnitude is to be expected, due to the lower energy content of biodiesel. The decrease in output torque at these two modes also affects the power output of the engine, since torque and power are directly proportional when the engine speed is fixed. As a result, the power output will also decrease by 4–5%. A decrease in both power and torque is due to their lower energy content of biodiesel.

Torque comparison for different biodiesel blends [B5(5% biodiesel 95% diesel), B20 (20% biodiesel 80% diesel), B50 (50% biodiesel 50% diesel) and B100 (100% biodiesel)] using Biodiesel A (80% beef, pork and sheep tallow and 20% waste cooking oil methyl ester) and Biodiesel B (70% chicken tallow and 30% waste cooking oil methyl ester).

4.2 Specific fuel consumption

Figure  3 compares the specific fuel consumption for the two biodiesels over the ISO 8178 test procedure. Even though this test procedure is designed to evaluate exhaust emissions, it can also be used in the same way to measure fuel consumption. During testing, the fuel flow rate at each mode was measured, and by using the weighting factors designated in the test procedure, a value for fuel consumption over the duration of the test was found, and averaged over the three tests for each fuel. Since the test procedure requires set values of torque and rpm, fuel consumption should be higher for a fuel with lower energy content.

Fuel consumption comparisons.

From Figure  3 , it can be seen that fuel consumption increases with blend ratio for both Biodiesels A and B. For Biodiesel A, the fuel consumption is 7% higher than diesel and for Biodiesel B, it is +10% higher which indicates that Biodiesel B has lower energy content than Biodiesel A and both biodiesels have lower energy content than diesel.

4.3 Exhaust emissions

Figure  4 compares the NO x emissions for Biodiesel A, Biodiesel B and diesel. Biodiesel A nitric oxide emissions show a decreasing trend with increasing blend ratio, whereas Biodiesel B emissions increase with the blend ratio. NO x emissions can increase or decrease depending on a number of factors such as biodiesel type, engine type and test procedure used. The US EPA reports a 10% increase in NO x emissions for B100 when compared with diesel.

Comparison of NO x emissions.

Figure  5 shows the carbon monoxide emissions for Biodiesels A and B over the ISO 8178 test procedure. Both biodiesels displayed a significant decrease in CO emissions with increasing blend ratio. For Biodiesel A, the decrease is ∼55% and for Biodiesel B, the decrease is ∼30%. This decrease fairly agrees with US EPA [ 4 ] who reported 51% decrease in CO emissions for biodiesel. This decrease could be attributed to the biodiesels having higher oxygen content than diesel which can result in a more complete combustion, leading to less CO in the exhaust stream.

Comparison of carbon monoxide emissions.

The HC emission results for the biodiesels are shown in Figure  6 . It can be seen that both Biodiesels A and B show an increase in HC emissions with increasing blend ratio. Conversely, the US EPA reports that HC should decrease with increasing blend ratio. It should be noted that the HCs measured during testing were very low (<0.002%). This brings into question the validity of these results, since other studies have found significantly higher levels of HCs in diesel exhaust emissions. These low readings could be attributed to a number of factors, one being that the EGA is optimized for measuring petrol engine exhaust emissions, not diesel. Petrol engine emissions contain different HCs to diesel engines, and higher concentrations of HC. If diesel/biodiesel HCs were to be measured accurately, a flame ionization detector would need to be used instead of the infrared sensor that was used for this testing, but this equipment is extremely expensive. Another explanation for inaccurate HC readings is that HC drift was occurring. Drift occurs when the emissions sample point is too far down the exhaust stream, which gives the HCs a chance to break down into other compounds such as carbon dioxide and water vapour. Since the sample point is ∼3m down the exhaust stream on the test rig, it is possible that this is sufficient distance for some of the HCs to break down; therefore, a reduced amount is actually being measured.

Comparison of HC emissions.

Figure  7 shows the carbon dioxide emissions for the biodiesels over the ISO 8178 test procedure. It can be seen that both biodiesels display an increase in CO 2 emissions with increasing blend ratios, although a decrease in CO 2 emissions was expected as CO emissions presented in Figure  5 . For Biodiesel A, the increase is ∼6% and for Biodiesel B, the increase is ∼18% compared with diesel. It is to be noted that CO 2 is a non-regulated emission (i.e. not limited), but is frequently measured when analysing exhaust gas emissions as it gives valuable clues on fuel consumption in dynamometer tests [ 17 ]. Studies have shown that biodiesel can decrease CO emissions up to 51%, whereas it can increase or decrease CO 2 emissions, with the percentage change ranging from −7% to +7% depending on the type of biodiesels [ 18 , 19 ]. In the current study, Biodiesel A clearly agrees with the literature findings both in terms of CO and CO 2 emissions; however, a higher increase in CO 2 emissions was found for Biodiesel B compared with literature findings. This difference can be considered as not very significant, as CO 2 emissions are not regulated. However, the specific reason for increase in CO 2 emission for both the biodiesels studied in this study (i.e. Biodiesels A and B) needs further investigation.

Comparison of carbon dioxide emissions.

4.4 Summary of discussion

The summary of discussion based on the experimental findings is outlined below.

Lower energy content of biodiesel results in the lower performance (torque and power). It shows a decrease in both power and torque for biodiesel fuels.

Emissions of HC and CO 2 from both biodiesels increase with increasing the amount of biodiesel in their blend, whereas CO emission decreases with increasing amount of biodiesel in the blend.

Fuel consumption for Biodiesel B is higher than Biodiesel A, and Biodiesel B has lower energy content than Biodiesel A. This indicates that fuel consumption is higher for fuel with lower energy content.

Biodiesel A has lower exhaust emissions and better performance compared with Biodiesel B.

NO x emission depends on a number of factors such as biodiesel type, engine type and test procedure used. In this experiment, Biodiesel A shows a decreasing trend with increasing blend ratio whereas Biodiesel B shows increasing trend with increasing blend ratio for NO x emission.

Biodiesels having higher oxygen content can lead to less CO emissions with increasing blend ratio due to complete combustion in the diesel engine.

A diesel engine fuelled with biodiesel can make complete combustion due to the presence of oxygen content in the molecule of biodiesel.

Fuel consumption of biodiesel is expected to be higher when engine fuelled with higher density biodiesel.

An engine fuelled with biodiesel containing higher cetane number and higher lubricity is more efficient.

Biodiesel with higher gross calorific value (energy content) produces higher power.

High viscosity of the biodiesel causes fuel flow and ignition problems in engines and decreases in power output.

The results of this study indicated that biodiesel is a more environmental-friendly option than petroleum diesel based on the reductions in CO and NO x in the tailpipe emissions. This comes at the cost of performance, though biodiesel has lower energy content than petroleum diesel. Biodiesel A (the 80% beef, pork and sheep tallow and 20% waste cooking oil methyl ester) was found to have lower exhaust emissions across the board compared with Biodiesel B (70% chicken tallow and 30% waste cooking oil methyl ester). Without knowing more about the exact fuel properties of these two fuels, such as ultimate analysis, it was difficult to draw any definitive conclusions about why emissions were higher for biodiesels. It is recommended that a follow-up study should be completed to further investigate the fuel properties of Biodiesels A and B in order to determine how the differences in chemical properties affect performance and emissions. Once these fuel properties data are obtained, it could be inputted into an appropriate engine simulation programme to analyse theoretical emissions data. If the model was found to be accurate enough, these theoretical data could be compared against the practical data found in this study, which would provide more insight into the performance and emissions of biodiesel fuels.

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Biofuels are liquid fuels produced from renewable biological sources, including plants and algae. Biofuels offer a solution to one of the challenges of solar, wind, and other alternative energy sources. These energy sources have incredible potential to reduce our dependence on fossil fuels and yield environmental and economic benefits. But many of these sources have a limitation: they can’t replace liquid fuels such as jet fuel, gasoline, and diesel fuel that are critical to our transportation needs. That’s where biofuels could help.

Now, bioenergy researchers are developing a future generation of advanced biofuels and bioproducts. These efforts, spread across many institutions and many scientists taking incremental steps to advance biofuels science, share several important steps. Scientists must develop sources of biofuel raw material (called feedstocks) that are sustainable and environmentally friendly. For example, switchgrass and poplar are fast-growing, non-food species that may be ideal feedstocks. Scientists may improve genes in plants to make those plants easy to break down for processing into biofuels and other bioproducts. They must design enzymes and microbes (such as yeast) tailored to breaking down plant material into sugars and the substances that give plants their structure. And they must design microbes that excel at converting these materials into chemicals used for fuel and other products. Finally, scientists must work with engineers and technicians to design processes that produce feedstock in ways that are cost effective and that minimize their need for water, electricity, and other resources.

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. The Center for Advanced Bioenergy and Bioproducts Innovation—a collaboration between the University of Illinois at Urbana-Champaign’s Institute for Sustainability, Energy, and Environment and the Carl R. Woese Institute for Genomic Biology—focuses on a “plants as factories” approach to develop fuels and chemicals. The Center for Bioenergy Innovation, led by Oak Ridge National Laboratory, works on creating new bioenergy-relevant plants and microbes. At the Great Lakes Bioenergy Research Center, led by the University of Wisconsin—Madison in partnership with Michigan State University, researchers are developing the science and technology to sustain biofuels production processes. Finally, at the Joint BioEnergy Institute led by Lawrence Berkeley National Laboratory, scientists are deploying tools in molecular biology, chemical engineering, and computational and robotics technologies to address challenges in biofuels and bioproducts production.

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Boosting glycerol's value: A new process makes biodiesel more profitable

by Tokyo Institute of Technology

Biodiesel, a green alternative to conventional diesel, has been shown to reduce carbon dioxide emissions by up to 74%. Biodiesel is produced through transesterification, converting triglycerides into biodiesel and producing glycerol as a low-value byproduct.

Since glycerol makes up about 10% of the output, efforts have focused on boosting its value. One method involves electrochemical oxidation, turning glycerol into high-value three-carbon compounds like dihydroxyacetone (DHA) and glyceraldehyde (GLYD), though past approaches often yielded unstable or low-value products under strong alkaline conditions.

In a study published in the Journal of Catalysis on 15 August 2024, researchers led by Associate Professor Tomohiro Hayashi from Tokyo Institute of Technology (Tokyo Tech) and Professor Chia-Ying Chiang from National Taiwan University of Science and Technology, Taiwan, have developed a highly selective and efficient glycerol electrooxidation (GEOR) process that can lead to the production of valuable 3-carbon (3C) products.

"Establishing an electrochemical route for a highly selective and efficient glycerol electrooxidation process to desirable 3C products is essential for biodiesel production," say Hayashi and Chiang.

Selective oxidation of glycerol is challenging due to its structure. Glycerol has three –OH groups: two on primary carbon atoms and one on a secondary carbon atom. This arrangement creates steric hindrance, making it hard for reactants to target specific –OH groups for oxidation. In alkaline conditions, the –OH groups also cause unwanted side reactions that break carbon-carbon bonds, resulting in two-carbon or one-carbon compounds instead of the desired three-carbon products.

To address this, the researchers conducted GEOR using sodium borate and bicarbonate buffer as a mild alkaline electrolyte and a nickel-oxide (NiO x ) catalyst. The sodium borate helps protect a certain –OH group, improving the selectivity of the reaction, while the NiO x catalyst enhances the efficiency of the electrooxidation process. Sodium borate forms coordination complexes with glycerol's primary and secondary alcohol groups to form GLYD and DHA respectively.

However, the final product depends on the ratio of borate to glycerol. To understand how different concentrations of glycerol and borate affect the electrooxidation process, a fixed concentration of 0.1 M borate buffer was reacted with varying concentrations of glycerol (0.01, 1, 2.0 M) and a fixed concentration of 0.1 M glycerol with varying concentrations of borate buffer (0.01, 0.05, 0.10, and 0.15 M). while maintaining a pH of 9.2.

Higher borate concentrations were found to increase the selectivity for 3C products, particularly DHA, with the highest selectivity of up to 80% observed at a borate concentration of 0.15 M. This improvement is attributed to the increased buffer capacity provided by the borate solution, which helps maintain a stable pH during the reaction and stabilizes the borate-glycerol complex for further oxidation into 3C compounds.

Conversely, increasing the glycerol concentration reduced both the yield and selectivity of 3C products. At a glycerol concentration of 1 M, GLYD was the main product, with a selectivity of 51%.

The difference in the type of 3C product was found to be related to the formation of different glycerol-borate complexes. Using Raman spectroscopy, the researchers found higher borate concentrations favor six-membered ring complexes, promoting secondary –OH oxidation and DHA production. Conversely, higher glycerol concentrations favor five-membered ring complexes, leading to primary –OH oxidation and GLYD formation.

"Five-membered ring complexes were more likely to form in the electrolyte with a borate-to-glycerol ratio of 0.1, whereas six-membered ring complexes became more prominent in the electrolyte with a borate-to-glycerol ratio of 1.5," say Hayashi and Chiang.

These findings present a promising strategy for transforming glycerol into valuable products, boosting the sustainability and profitability of biodiesel production.

Provided by Tokyo Institute of Technology

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Spanish energy giant turns trash into jet fuel

CARTAGENA, Spain – In a narrow valley with steep sides near the ancient city of Cartagena in Spain, a team of 150 engineers has just finished building a plant that could be a game changer for Spanish energy company Repsol and a bellwether for the transport industry.

Mr Emilio Mayoral, who manages the unit, said his colleagues were in the early days of brewing fuels for trucks and airplanes from what was formerly garbage. “It’s quite flexible,” he said. “We are currently using used cooking oil, but we can use other waste.”

Repsol says these alternative fuels will cut emissions by up to 90 per cent compared with the petroleum-based products they will replace. The new fuels emit some carbon dioxide (CO2) when used, but they are produced from plants and other organisms that absorbed CO2 during their lifetimes, which is factored into the emissions calculation.

As an added benefit, these new biofuel products perform as well as their fossil fuel counterparts, even in cold northern European weather that creates problems for some fuels, Mr Mayoral said.

Madrid-based Repsol is one of Europe’s largest energy companies, with 26,000 employees and more than 4,500 service stations as well as investments in renewable energy such as wind and solar power. Repsol reported income of €1.6 billion (S$2.3 billion) for the first half of 2024.

Energy companies like Repsol are betting that advanced biofuels like the ones being made at the Cartagena plant will play an important role in transportation well into the future. They figure that airplanes and heavy trucks as well as a significant portion of the passenger car fleet will continue to be powered by liquid fuels like diesel and jet fuel, despite growth in the market for electric vehicles.

Tightening regulations on emissions, they calculate, will force greater use of fuels that emit less CO2. Both energy companies and their customers consider biofuels – which can make use of large parts of existing infrastructure like petrol station pumps and storage tanks – to be a practical and relatively inexpensive solution for navigating this technological and regulatory gauntlet.

Until a cheaper, more convenient clean energy source for transportation emerges, “legislators don’t appear to have much choice but to continue to lean on biofuels”, said Mr Charles Jans, vice-president for consulting at Argus Media, a commodities research firm.

The International Energy Agency, a Paris-based policy research group, has forecast that consumption of these fuels will increase 20 per cent globally by 2030.

Repsol anticipates that its home market, Spain’s Iberian Peninsula, will prove to be a kind of paradise for biofuels. Some of its rivals have closed traditional oil refineries, but Repsol plans to gradually retrofit its facilities to produce greener fuels from various forms of waste and, eventually, so-called e-fuels from gases like hydrogen and CO2.

In this way, the firm is betting that it can continue to make profits from trading and processing oils, taking advantage of reduced competition and the higher prices that lower-carbon fuels may bring.

“We are pushing very much not to close our refineries but to transform them,” said Dr Luis Cabra, the company’s deputy chief executive, who is a point person for Repsol’s energy transition.

“We still believe in the internal combustion engine,” he added.

The new plant in Cartagena, a port city built around the ruins of a Roman-era theatre, is the first big step in this makeover, which has been aided by research at Repsol’s laboratories outside Madrid.

Repsol spent €250 million grafting the new plant onto what had been a conventional refinery. Making use of existing facilities reduced the costs involved in the switch, Dr Cabra said.

The overhaul at Repsol and other companies was prompted by European Union regulations intended to tackle climate change. For instance, energy companies like Repsol are required to supply airports with jet fuel that includes a growing proportion of what is known as sustainable aviation fuel – ingredients that did not come from fossils – starting with 2 per cent in 2025 and rising to 70 per cent by 2050.

A similar set of requirements exists for motor vehicle fuels, and the rules are complicated by other priorities, like protecting the food supply and tropical forests.

As a result, the path to lower emissions faces dizzying complexity in the coming years. “It gives people a headache from the long-term planning and business management perspective,” said Argus’ Mr Jans.

The alternative fuels that companies like Repsol are proposing are a relatively easy way to meet the EU requirements because they give customers who purchase them green credentials with little effort.

Drivers, for instance, can buy what is described as 100 per cent renewable diesel at Repsol petrol stations in Spain. The EU pegs the biodiesel market in the trading bloc at €31 billion.

That solution is particularly welcome in areas of transportation like aviation that, analysts say, will face decades of difficulty in their conversion to new energy sources such as electricity or hydrogen.

For airlines, the so-called sustainable aviation fuel that Repsol will be brewing from waste in Cartagena looks like a much easier way to comply with tightening European standards.

“It’s like magic,” said Dr Teresa Parejo Navajas, head of sustainability at the Spanish airline Iberia. “You can use it in the same aircraft with the same engines and the same airport infrastructure.”

But questions about the availability and sourcing of the waste, like used cooking oil, are potential stumbling blocks for the industry.

Some environmentalists are sceptical about how green the new fuels will be, saying, for instance, that the ingredients will probably need to be transported from around the world, which will create emissions in the process.

“It’s going to be very difficult to get enough oil,” said Mr Javier Andaluz, coordinator for climate and energy at Ecologistas en Accion. “Even if they took used oil from all the restaurants in Spain, it wouldn’t be enough.”

At its refinery in Cartagena, Repsol gives preference to the Spanish market for waste collection, Mr Mayoral said, but he conceded that some of it could come from elsewhere, including Asia.

With energy companies scouring the globe for material to feed refineries, cooking oil and other forms of waste have become valuable and even scarce commodities.

“In essence, there is a limit on the amount of feedstock,” said Mr Alan Gelder, an analyst at consulting firm Wood Mackenzie, though he added that supply could be expanded by loosening rules on what could go into the mix.

The scarcity has created a search for alternatives. Oleofat, a Spanish company that supplies ingredients for refining into biofuel, says it is working with dozens of sources of waste, including residues from water treatment plants.

Compounding the problem, analysts say, prices in Europe are being depressed by imports of cheap biodiesel from China and worries about oversupply. The price of sustainable aviation fuel has fallen more than 40 per cent over the past year to about US$1,769 (S$2,300) a tonne, according to Argus, although it is still much higher than the price of ordinary jet fuel, which sells for about US$750 a tonne.

The challenges have led some energy companies to pull back on biofuels. In July, Shell, Europe’s largest energy company, said it would temporarily suspend the construction of a large biofuel plant in the Dutch city of Rotterdam, partly because of market conditions. Shell said it would take a write-down of nearly US$800 million.

In December, the EU began an investigation into the alleged dumping of Chinese biodiesel, and said in July that it would provisionally impose tariffs of up to 36.4 per cent on imports from China.

Dr Cabra of Repsol said he expected more “scrutiny” of Chinese imports because of concern that they might not be produced “under the same sustainability criteria” as in Europe.

Still, analysts say that the type of fuel Repsol is producing is likely to have a future and that the company is well placed because it started early, likely locking in suppliers. In July, the company said it expected the Cartagena plant to make a profit of as much as €140 million in 2024.

Dr Cabra said there were still reasons for optimism despite falling prices. “Definitely, there will be more demand of this product in Europe. You are obliged every year to increase the volume that you sell.” NYTIMES

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Exploring the performance and emission characteristics of a dual fuel CI engine using microalgae biodiesel and diesel blend: a machine learning approach using ANN and response surface methodology

• Published: 03 September 2024

Cite this article

• Chandrabhushan Tiwari 1 ,
• Gaurav Dwivedi 2 &
• Tikendra Nath Verma 1  

Alternative fuels in internal combustion engines have gained significant attention to environmental sustainability and energy security. The study employs a machine-learning (ML) approach, integrating artificial neural networks (ANN) and response surface method (RSM), to analyze the engine characteristics. The experimental data used to train the ANN and RSM model was obtained by employing different combinations of input parameters obtained by the Design of the experiment tool. Four input parameters load 25–100% ((1.3, 2.6, 3.9, and 5.2 kW) loading condition, speed (1200, 1500, and 1800 RPM), compression ratio (17.5 and 18.5), and biodiesel–diesel blends (Diesel, SM 20 , SM 40 , SM 60 , SM 80 and SM 100 ) were used. The results show predictability for ANN with training and test regression coefficients (R 2 ) of 0.975 and 0.948 whereas RSM with R 2 of 0.992. Optimized results for RSM and ANN, BTE (29.4% and 29.1%), BSFC (0.0.3201 and 0.334 kg/kWh), IMEP (2.83 and 2.69 bar), and CO 2 (922.72 and 940.87 g/kwh), NOx (964 and 937 ppm). When compared with experimental data, the error was about 5%. It can be inferred that RSM and ANN may be used to model processes with high predictability and that optimization can be carried out using various techniques depending on the process or problem.

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• Artificial Intelligence

Data availability

The datasets used and/or analysed during the current study are available from the corresponding author upon reasonable request.

Abbreviations

Artificial intelligence

Artificial neural network

Box Behnken design

Brake thermal efficiency

Brake specific fuel consumption

Compression ignition

Compression ratio

Diesel fuel

Root mean square error

Response surface method

Spirulina microalgae

Particulate matter

Brake mean effective pressure

Common rail direct ignition

Electronic circuit unit

Rectified linear unit

Data acquisition system

Exhaust gas temperature

Free fatty acid

Greenhouse gases

Hydrocarbon

Indicated mean effective pressure

Multi-layer perceptron

Machine learning

Mean root error

Support vector regression

Support vector machine

Regression coefficient

Brake power

Internal combustion engine

Revolution per minute

Design of experiments

Diesel biodiesel blend (20% biodiesel and 80% diesel)

Diesel biodiesel blend (40% biodiesel and 60% diesel)

Diesel biodiesel blend (60% biodiesel and 40% diesel)

Diesel biodiesel blend (80% biodiesel and 20% diesel)

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Tiwari, C., Dwivedi, G. & Verma, T.N. Exploring the performance and emission characteristics of a dual fuel CI engine using microalgae biodiesel and diesel blend: a machine learning approach using ANN and response surface methodology. Environ Dev Sustain (2024). https://doi.org/10.1007/s10668-024-05362-2

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Received : 24 May 2023

Accepted : 25 August 2024

Published : 03 September 2024

DOI : https://doi.org/10.1007/s10668-024-05362-2

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