However, these feedstocks pose several challenges that harm scalability, including sustainability, land exploitation, and food production competition.
For example, in the US Midwest, vast acres of farmland are used to produce solely corn and soybeans, with a large portion used as biofuel feedstock.
The machines and land use during the agricultural process release emissions, displace land, and cause water and air pollution that negatively impact human health [2].
As a result of these issues, researchers are exploring alternative raw materials that are more sustainable, cost-effective, and eco-friendly.
This paper synthesises numerous recent advancements in the development and optimization of raw materials, specifically exploring lignocellulosic biomass and other waste products, like cooking oil and plastic solid waste, as emerging or re-evaluated feedstocks.
Advancement in Biofuel Raw Materials Literature Review
Lignocellulosic biomass, composed of cellulose, hemicellulose, and lignin, has emerged as a potential feedstock due to its abundance, low cost, and sustainability.
The ratio of cellulose (40-60%), hemicellulose (20-40%), and lignin (10-25%) varies among different lignocellulose biomasses depending on the composition, and it dictates the pretreatment method that will be most effective [3].
Figure 1 illustrates lignocellulose feedstock composed of raw material sourced from forests and crops, agricultural waste, and grasslands.
The pie chart shows the average percentage of different materials as feedstock, which affects the composition of cellulose, hemicellulose, and lignin with varying percentages.
In recent years, progress has been made in the pretreatment process and catalytic conversion process, emphasizing more efficient biofuel production and higher yields.
Since lignocellulose composition varies based on the feedstock, the percentages show the average annual yield in each section [3].
Pretreatment is an important process in lignocellulose refinement, as it disrupts the compound’s rigid structure of its composition to enhance the activity between cellulose and hemicellulose with enzymes or catalytic conversion.
According to Deng et al. [3], cellulose is a linear polymer of glucose units that form crystalline and amorphous regions that have high mechanical strength and are chemically stable, serving as structural support, with the amorphous regions degrading 3-30 times faster than the crystalline regions. Hemicellulose, the second most abundant material, is completely amorphous with no crystalline structures, which facilitates easy hydrolysis and solubility in alkali metals, enabling binding to cellulose and lignin. Lignin is a crosslinked aromatic polymer connected to cellulose with ether bonds and to hemicellulose with ester bonds.
The purpose of pretreatment is to break the lignin and reduce the crystalline structure of the cellulose to expose the cellulose and hemicellulose to “increase the accessibility of various reagents, solubilize or separate certain components, and/or enhance the reactivity of polymers” [3].
Traditional pretreatment [1] methods fall under three categories: physical, chemical, and biological, as seen in Figure 2 [4].
Physical treatment of the lignocellulose biomass is the mechanical process of milling, grinding, and crushing to increase the surface area of reaction while reducing the size, as well as irradiation using gamma rays to porosify the biomass and increase the contact with cellulase enzymes.
Recent advances in physical pretreatment involve using microwaves to generate internal heat through dielectric polarization, polar molecules aligning with the electric field and generating heat, to disrupt the bonds between lignin and cellulose. When combined with enzymes and diluted acids, a high glucose concentration can be produced in 15 minutes.
In addition to being time-efficient, microwaves are eco-friendly, treating sludge and other waste products, cost-effective, costing $0.093/kg sugar, and have low energy consumption, only 0.46 kWh [5]. Chemical treatment uses solvents to break bonds.
Acid pretreatment dissolves and removes hemicellulose for more surface area for enzymes but generates fermentation inhibitors and requires neutralisation since it uses strong organic acids. Alkaline pretreatment, on the other hand, saponifies ester bonds and disrupts lignin structure, causing the biomass to swell and therefore expose more cellulose.
This method is slower than using acid and has a high chemical consumption rate. Another form of chemical treatment is organosolv treatment, which uses pure organic solvents to produce porous cellulose-rich structures.
It works with both polar protic and aprotic solvents, producing a high-purity cellulose. Recent approaches to organosolv include using a two-step anhydrous process, modifying soluble components like the production of organosolv sugars and organosolv-modified lignin.
The two-step method improves lignin recovery while also reducing water consumption. However, organic solvents are more expensive, more hazardous, and energy-intensive [5]. Finally, biological treatments use microbes to decompose lignin; white-rot and brown-rot are most used, with white-rot being more desirable due to its natural ability to decompose lignin.
Although biological pretreatment requires less energy, it is less effective as it needs more time, and fungi also break down the desired cellulose and hemicellulose [3].
After pretreatment, catalysts are used to “achieve high selectivity towards the desired products” as they effectively remove the oxygen content in the biomass after selectively breaking bonds [2]. For autohydrolysis, cellulose and hemicellulose degrade into oligosaccharides (glucose) or monosaccharides (xylose) using hot water under pressure.
This is a slow process, using green solvent with no involved corrosion. NaCl acts as an ionic facilitator, where Cl- ions disrupt the hydrogen bonds between xylo-oligosaccharides, to shorten the degradation time for hemicellulose by accelerating acetyl group removal.
For the cellulose, NaCl and microwave treatment compresses and concentrates the H+, further breaking bonds to form carboxylic acids.
However, prolonged reaction time with NaCl can produce furfural, a fermentation inhibitor and since the desired sugar products degrades to furfural, more furfural means reduced yield [3].
Other catalysts can be used to further accelerate the process.
For homogeneous catalysts, diluted mineral acids hydrolyze cellulose and hemicellulose into monosaccharides by adding H+ ions and breaking bonds. Hemicellulose would be the first to degrade, while cellulose requires extreme pressures and temperatures.
This method is fast, but it is also corrosive and can degrade the xylose to furfural, an inhibitory compound. Heteropoly acids (HPAs) are even stronger than mineral acids and can effectively hydrolyse cellulose, primarily yielding glucose.
Organic acids are less corrosive than mineral acids, avoiding the degradation to furfural, and they have high efficiency in hydrolysis. Lastly, Lewis acids are primarily used to produce oligosaccharides for hemicellulose degradation [3].
Heterogeneous catalysts like metal oxides can also hydrolyze cellulose and hemicellulose while preventing recrystallization from happening.
Additionally, the hydrolysis of the cellulose is further accelerated due to the abundant Lewis acid sites on the metal.
H-form zeolites similarly have acid sites but also have porous surfaces that directly increase the effectiveness relative to the size of the pores. Acidic resin allows hydrolysis in ionic liquids at mild temperatures.
The macroporous surface allows for reaction to accelerate due to larger surface contact. Finally, acidic carbon catalysts use sulfonated carbons to yield glucose with a high surface area, though they are corrosive.
Waste Cooking Oils
While lignocellulosic biomasses remain a large focus of research and development, waste materials like used cooking oil have gained prominence as an alternative feedstock. Rahman et al. [6] researched the feasibility of using waste cooking oil as a raw material for biofuel production in Indonesia and analyzed it through a Life Cycle Sustainability Assessment.
This assessment examines three factors: life cycle assessment (LCA), social implications (SLCA), and economic viability (LCCA).
Waste cooking oil (WCO) typically accumulates in urban areas like neighborhoods, hotels, restaurants, cafes, and other businesses. Since cooking oil is typically made from oilseed plants, it has a high biofuel potential while not competing with food production, as it is a waste product which is unlike some other feedstocks that are food crops.
Although Indonesia is a large consumer of palm cooking oil, going up to 20.9 million tons during 2020, only around 18.5% has been collected, meaning the rest are improperly disposed of.
Firstly, the LCA assessment analyses environmental impact, human health effects, and resource usages following the International Standards Organization (ISO) guidelines and specifically ISO 14040, which describes what is included in the LCA, and ISO 14044, which shows the steps on how to do the LCA. These guidelines allow a thorough comparison between the sustainability factor of biodiesel compared to using fossil fuels, with variations in results due to the differences in system boundaries, tools and units used, methodology, and databases.
Researchers employed LCA to identify environmental hotspots and found WCO to have lower GWP during production compared to other feedstocks like crop-based oils, confirming that WCO biodiesel is a greener fuel, but requires optimisation, as pretreatment and transesterification impact the environment more than other production processes.
The second part of the LCA is the life cycle inventory (LCI) assessment. This test quantifies the inputs, like raw material and energy required, and outputs, like products, pollution, and waste, in the biodiesel’s life, involving extensive data and time-intensive calculations across a large scope.
The third stage of the LCA is the life cycle impact assessment (LCIA). The LCIA evaluates environmental impacts using midpoint and endpoint categories.
The midpoints identify areas of improvement, for example, using the global warming potential to assess CO2 emissions, human toxicity, acidification potential, etc.
Endpoints then show the consequences and the damage done from the midpoints, like human health, ecosystem quality, and resource depletion. WCO biodiesel has been proven to be greener than fossil fuel and most other crop-based biodiesels, depending on the process methods and energy sources. According to Rahman's findings, bio-jet fuel WCO has a carbon intensity 63.7 % lower than conventional jet fuel, and hydrogen from WCO accounts for 18.7 % of GHG emissions.
However, for transesterification in GWP values, WCO has a value of 799-1382 kg CO2 eq/ton, higher than mazut, which is 553 kg CO2 eq/ton.
Regarding freshwater ecotoxicity, since WCO used organic solvents, WCO has a value of 19.8 kg 1,4-DB eq, which is the highest compared to the 7.32 kg 1,4-DB eq of other biofuels. Further research needs to be done to optimize the transesterification of WCO since it has a higher environmental impact in GWP and freshwater ecotoxicity values [6].
The social implications (SLCA) stage evaluated the social and socioeconomic impacts of the WCO biodiesel’s lifespan.
Several positive impacts emerged, like increasing employment in the local community, heightening engagement as people actively contribute to the production of biodiesel.
However, the negative impacts also followed, including illegal markets for collecting WCO to be reused as food or feed, hazardous WCO handling, and differences in policies across regions, based on a study done in Colombia.
To mitigate these risks, Rahman et al. suggest standardised WCO handling methods to ensure industrial safety for all parties involved.
The economic viability (LCCA) stage adopts a circular economy perspective as it develops a method to recycle waste. WCO aligns with the global sustainable development goals by reducing waste and decreasing carbon footprints. However, the cost of pretreatment is also high, as WCO requires purification, and offsets sustainability if transesterification chemicals are derived from fossil fuels.
The production of a ton of WCO biodiesel ranges from 224-855 USD, depending on regions, which is cheaper than most plant-based oils but costs slightly more than fossil diesels (395 USD). Ultimately, this study demonstrates that WCO production is economically feasible due to lower production and material costs compared to other feedstocks [6].
Despite WCO biodiesel being theoretically viable, the complex production process and low recovery rate increase the demand for WCO. In addition, if the market for WCO opens with more external businesses buying the raw material, production prices will increase, which lowers the cost advantage over conventional diesel.
The collection process of WCO is also not yet developed, and without a stable collection network, profit motives are unstable and risky for biodiesel businesses. Social issues are also prevalent, as a lack of awareness leads to WCO being handled improperly and disposed of instead of being recycled as feedstock, worsening environmental damage.
Finally, government intervention is required to instate policies and regulations for the production and trade of WCO biodiesel. While WCO biodiesels have environmental and economic advantages compared to fossil fuels and food crop feedstock biodiesels, optimization still needs to be made in the transesterification, collection, and regulation [6].
Plastic Solid Waste
Another viable form of waste material that is explored is plastic solid waste. Plastics have been a large environmental concern, as 4-12 million tons of plastic enter our oceans alone.
Since they have highly volatile content (60-80%), plastics can be efficiently converted to biofuel [8]. One way plastic is converted into biofuel is through pyrolysis, heating organic materials at high temperatures with no oxygen, to produce three main products: biochar, bio-oil, and synthesis gas. Several varieties of pyrolysis produce a wide range of products. Slow pyrolysis is used for producing biochar, which can be burnt for energy, and smaller amount of tar byproducts at low temperatures and over a long period of time. Fast pyrolysis, also known as flash pyrolysis, primarily yields liquid fuels like bio-oil, a potential replacement for conventional crude oil, as it undergoes rapid heating and high temperatures [7]. Figure 3 outlines the process of pyrolysis as it goes from a feedstock into the different pyrolysis methods.
One way of examining pyrolysis is through thermogravimetric analysis (TGA) by measuring weight loss with temperature.
Through this analysis, plastics like LDPE, HDPE, PET, PP, and PS degrade in one step, making them ideal for pyrolysis, while PVC is problematic as it releases HCl and requires two-stage pyrolysis or pretreatment. When examining the activation energy, similar results are found with PP and PS, which require lower energy thresholds than PVC. These findings suggest that when using plastics as feedstock, PVC is not an ideal candidate since it takes more energy and causes HCl emissions [7].
In addition to pyrolysis, hydrothermal liquefaction (HTL) converts the plastic solid wastes to biofuels by using water at high temperatures and high pressure. Plastics like PE, PP, PS, and PET contain high hydrogen and volatile content while being low in oxygen content, decreasing unwanted compounds in the bio-oil. At nearly supercritical points, 374°C and 22.1 MPa, water acts as a solvent to accelerate depolymerization for PE and PP. This process yields 80-92% oil yield while also eliminating the need for organic solvents. Fast HTL, inspired by fast pyrolysis, reduces the reaction time from hours to minutes by using higher temperatures, 500-600°C, reducing the energy input [8].
Conclusion
Biofuel remains a promising fuel, with three feedstocks uniquely suited for biofuel production: lignocellulosic biomass, waste cooking oil (WCO), and plastic solid waste. Each material has its own set of advantages and issues.
The structural and compositional properties of lignocellulosic biomass make it heavily reliant on pretreatment and catalytic processes to increase yield. WCO provides a greener alternative but faces logistical and social challenges with the collection process. Meanwhile, pyrolysis and hydrothermal liquefaction offer a sustainable and efficient method to convert plastic waste into biofuel. Continued research is still needed to optimize the efficiency and sustainability of converting these waste materials into biofuels. As the world moves forward into using renewable energy, optimizing biofuel feedstock plays an important role in lowering carbon emissions as it transforms waste into energy and mitigates environmental harm.
About the Authors
Dr. Raj Shah is a Director at Koehler Instrument Company in New York, where he has worked for the last 25 plus years. He is an elected Fellow by his peers at ASTM, IChemE, ASTM,AOCS, CMI, STLE, AIC, NLGI, INSTMC, Institute of Physics, The Energy Institute and The Royal Society of Chemistry. An ASTM Eagle award recipient, Dr. Shah recently coedited the bestseller, “Fuels and Lubricants handbook”, details of which are available at ASTM’s Long-awaited Fuels and Lubricants Handbook https://bit.ly/3u2e6GY.
He earned his doctorate in Chemical Engineering from The Pennsylvania State University and is a Fellow from The Chartered Management Institute, London. Dr. Shah is also a Chartered Scientist with the Science Council, a Chartered Petroleum Engineer with the Energy Institute and a Chartered Engineer with the Engineering council, UK. Dr. Shah was recently granted the honorific of “Eminent engineer” with Tau beta Pi, the largest engineering society in the USA. He is on the Advisory board of directors at Farmingdale university (Mechanical Technology), Auburn Univ (Tribology), SUNY, Farmingdale, (Engineering Management) and State university of NY, Stony Brook (Chemical engineering/ Material Science and engineering).
Professor at the State University of New York, Stony Brook, in the Department of Material Science and Chemical Engineering, Raj also has over 700 publications and has been active in the energy industry for over 3 decades. More information on Raj can be found at https://shorturl.at/JDPZN
Contact: rshah@koehlerinstrument.com
Ms. Ivy Li is part of a sought after alternative fuel internship program at Koehler Instrument company in Holtsville, NY.
References
[1] US EPA. “Biofuels and the Environment.” Www.epa.gov, 14 Jan. 2021, www.epa.gov/risk/biofuels-and-environment.
[2] Scafidi, Angela, and Haley Leslie-Bole. “Increased Biofuel Production in the US Midwest
May Harm Farmers and the Climate.” Www.wri.org, Feb. 2024,
www.wri.org/insights/increased-biofuel-production-impacts-climate-change-farmers.
[3] Deng, Weiping, et al. “Catalytic Conversion of Lignocellulosic Biomass into Chemicals and Fuels.” Green Energy & Environment, vol. 8, no. 1, July 2022, https://doi.org/10.1016/j.gee.2022.07.003.
[4] Zhao, Lei, et al. “Advances in Pretreatment of Lignocellulosic Biomass for Bioenergy Production: Challenges and Perspectives.” Bioresource Technology, vol. 343, Jan. 2022, p. 126123, https://doi.org/10.1016/j.biortech.2021.126123. Accessed 31 Jan. 2022.
[5] Chakraborty, Prasenjit, et al. “Technological Advancements in the Pretreatment of
Lignocellulosic Biomass for Effective Valorization: A Review of Challenges and
Prospects.” Journal of Industrial and Engineering Chemistry, vol. 137, Elsevier BV, Sept. 2024, pp. 29–60, https://doi.org/10.1016/j.jiec.2024.03.025. Accessed 28 Aug. 2024.
[6] Rahman, Arif, et al. “Current Scenario and Potential of Waste Cooking Oil as a Feedstock for Biodiesel Production in Indonesia: Life Cycle Sustainability Assessment (LCSA) Review.” Case Studies in Chemical and Environmental Engineering, vol. 11, Elsevier BV, June 2025, p. 101067, https://doi.org/10.1016/j.cscee.2024.101067. Accessed 21 Feb. 2025.
[7] Escalante, Jamin, et al. “Pyrolysis of Lignocellulosic, Algal, Plastic, and Other Biomass Wastes for Biofuel Production and Circular Bioeconomy: A Review of Thermogravimetric Analysis (TGA) Approach.” Renewable and Sustainable Energy Reviews, vol. 169, Nov. 2022, p. 112914, https://doi.org/10.1016/j.rser.2022.112914. Accessed 12 Sept. 2022.
[8] Zaman, Muhammad Gul, et al. “Biofuel Production Boosted by Plastic Waste: Co-Hydrothermal Liquefaction of Plastic and Biomass toward Sustainable Energy.” Industrial & Engineering Chemistry Research, American Chemical Society, Jan. 2025, https://doi.org/10.1021/acs.iecr.4c03663. Accessed 1 May 2025.
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