Recent Innovations in Turning Agricultural Waste into Biofuels

August 19, 2021

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by Raj Shah and Amanda Loo (altenergymag.com/Koehler Instrument Company) Wastes from fields and wastes from processing are the two kinds of agricultural wastes. Field wastes are present after harvesting crops and include stems, leaves, and stalks, and waste after processing crops includes seeds, peels, husks, etc.

Lignocellulosic biomass is composed of four different categories that include agricultural wastes, hardwood, softwood, and grass [2]. These biomasses normally do not receive treatment and are disposed of, which can contaminate the environment and cause environmental load. Wastes from fields and wastes from processing are the two kinds of agricultural wastes. Field wastes are present after harvesting crops and include stems, leaves, and stalks, and waste after processing crops includes seeds, peels, husks, etc. [3]. The lignocellulosic biomass consisting of hemicellulose, cellulose, and lignin from agricultural waste can be converted into biofuels [4].

Pretreatment processes are typically required before converting agricultural waste into biofuels as these processes break down the waste into smaller components. Microwave pyrolysis is a very popular thermochemical method to convert biomass into biofuels, specifically biochar, bio-oil, and syngas. Many pathways have also been studied to convert biomass feedstock such as oil, syngas, alcohols, and sugars into aviation fuels.

Pretreatment Processes

From lignocellulose wastes, substances such as methane, carbohydrates, enzymes, amino acids, etc. can be obtained [5]. However, lignocellulose wastes are often disposed of, limiting their use potential. As previously mentioned, lignocellulosic biomass can be converted into biofuels as well demonstrating great advantages. Furthermore, lignocellulose materials are CO2 neutral as the CO2 from the biomass does not add any additional carbon to the carbon cycle [6].

Breaking down the cell wall of the biomass is complicated and requires pretreatment before converting it into biofuels. Physical, chemical, and biological approaches are common in the pretreatment process of lignocellulosic biomass. The major goals of pretreatment are to form sugars with hydrolysis, to use energy efficiently, and to reduce biofuel production costs [7]. Physical pretreatment is relatively inexpensive and convenient and includes radiation, milling, and grinding. Wheat straw has been previously studied, and it was determined that there was a 53% increase in biofuel yield with pretreatment at 180℃ than without pretreatment [8].

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More green pretreatment methods have been studied in recent years, which include ultrasound, electron beam, and pulsed-electric field technologies. A significant shortcoming of these green technology approaches is that they demonstrate poor efficiency and limited industrial scaling ability. As Figure 1 shows, pretreatment breaks down the biomass into its hemicellulose, cellulose, and lignin components, making it easier for conversion into biofuel.

An increasingly popular chemical pretreatment method is using dilute acid.

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Ammonia fiber explosion/expansion (AFEX) is a physico-chemical pretreatment that branched off the ammonia-based chemical pretreatment [7], with the main difference being that AFEX instead uses anhydrous liquid ammonia.

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Microwave Pyrolysis

The process of biochemical conversion requires pretreatment processes for the lignocellulosic components and is not as efficient compared to thermochemical conversion. Biomass recalcitrance, where the cell wall of crops and plants resists enzymatic degradation to process the biomass, limits the efficiency of biochemical conversion [9]. Instead, thermochemical conversion methods are used to convert agricultural waste to biofuels, and the main five processes are combustion, pyrolysis, gasification, combustion, and hydrothermal liquefaction [4].

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Another study using solar-powered microwave pyrolysis was done, but corn stover was studied instead of rice straw. The goal was to maximize wood vinegar and biochar products from corn stover as biomass feedstock that is plentiful in India.

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A general trend of an increase in bio-oil production is seen with the presence of the bio-char additive.

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Aviation Fuels

Commercial airline aviation fuel consumption was 260 million m3/year in 2005, which has increased to more than 340 million m3/year in 2018 and is expected to continue to grow [13]. 669 million tons of CO2 are released by the aviation sector annually, with an expected six times increase by 2050 [14]. Road, rail, and maritime sectors all use biofuels, but the most promising changes are expected to be seen in the commercial aviation sector in reducing greenhouse gas emissions and reducing oil dependency [15]. However, the ASTM (American Society for Testing Materials) and EU standards of bio-aviation fuels are that they need to be hydrocarbons without oxygen, making it difficult to produce from biomass as biomass has a high wt% of oxygen [13]. The ASTM D1655-09 lists the specifications of jet fuels, such as density, viscosity, energy density, etc. that must be met by the aviation industry [14].

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In oil-to-aviation fuel conversion, catalytic hydrotreating is performed at temperatures of 250-400℃ with high pressure, resulting in a product referred to as hydroprocessed renewable jet fuel. Noble metal-supported and alumina-supported metals are used as catalysts. The unsaturations in the vegetable oil feedstocks transform into branched hydrocarbons during the hydrotreating process. One shortcoming of the oil-to-aviation process is that hydrotreating does not produce cycloalkanes or aromatics. The process of gas-to-jet includes the gasification of biomass to syngas, then undergoing Fischer-Tropsch synthesis of syngas to hydrocarbons of the C1-C50 range.

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The alcohol-to-jet process can convert oxygenated fuel to hydrocarbons that are compatible with today’s transportation fuels. Finally, the sugar conversion pathway of sugars to hydrocarbons is complex as there are only six carbon atoms in sugars and their derivatives, while jet fuels require longer carbon chains. Removing oxygen from the products with processes such as hydrogenation and dehydration must then be done [14].

In a 2020 experiment using waste cooking oil from Egyptian fast-food restaurants to create jet fuel, 2.5% w/v zinc aluminate (ZnAl2O4) nanoparticles were used as catalysts because they can decrease the H/C ratio in thermal cracking [17].

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Conclusion

The pretreatment processes needed for biochemical conversion methods break down the lignocellulose biomass feedstock into smaller components, making it easier to convert the input biomass into biofuels. READ MORE

References

[1] Duque-Acevedo, M., Belmonte-Urena, L. J., Cortés-García, F. J., & Camacho-Ferre, F. (2020). Agricultural waste: Review of the evolution, approaches and perspectives on alternative uses. Global Ecology and Conservation, 22, e00902.

[2] Hassan, S. S., Williams, G. A., & Jaiswal, A. K. (2018). Emerging technologies for the pretreatment of lignocellulosic biomass. Bioresource Technology, 262, 310-318.

[3] Rao, P., & Rathod, V. (2019). Valorization of food and agricultural waste: a step towards greener future. The Chemical Record, 19(9), 1858-1871.

[4] Ge, S., et al. (2021). Progress in microwave pyrolysis conversion of agricultural waste to value-added biofuels: A batch to continuous approach. Renewable and Sustainable Energy Reviews, 135, 110148.

[5] Aftab, M. N., Iqbal, I., Riaz, F., Karadag, A., & Tabatabaei, M. (2019). Different pretreatment methods of lignocellulosic biomass for use in biofuel production. Biomass for Bioenergy-Recent Trends and Future Challenges.

[6] Sasmal, S., & Mohanty, K. (2018). Pretreatment of lignocellulosic biomass toward biofuel production. In Biorefining of Biomass to Biofuels (pp. 203-221). Springer, Cham.

[7] Kumari, D., & Singh, R. (2018). Pretreatment of lignocellulosic wastes for biofuel production: a critical review. Renewable and Sustainable Energy Reviews, 90, 877-891.

[8] Mahmood, H., Moniruzzaman, M., Iqbal, T., & Khan, M. J. (2019). Recent advances in the pretreatment of lignocellulosic biomass for biofuels and value-added products. Current Opinion in Green and Sustainable Chemistry, 20, 18-24.

[9] Fodah, A. E. M., Ghosal, M. K., & Behera, D. (2021). Studies on microwave-assisted pyrolysis of rice straw using solar photovoltaic power. BioEnergy Research, 14(1), 190-208.

[10] Wang, Y., et al. (2018). Production of bio-oil from agricultural waste by using a continuous fast microwave pyrolysis system. Bioresource technology, 269, 162-168.

[11] Wu, C., Budarin, V. L., Gronnow, M. J., De Bruyn, M., Onwudili, J. A., Clark, J. H., & Williams, P. T. (2014). Conventional and microwave-assisted pyrolysis of biomass under different heating rates. Journal of Analytical and Applied Pyrolysis, 107, 276-283.

[12] Fodah, A. E. M., Ghosal, M. K., & Behera, D. (2021). Solar‐powered microwave pyrolysis of corn stover for value‐added products and process techno‐economic assessment. International Journal of Energy Research, 45(4), 5679-5694.

[13] Wang, M., Dewil, R., Maniatis, K., Wheeldon, J., Tan, T., Baeyens, J., & Fang, Y. (2019). Biomass-derived aviation fuels: Challenges and perspective. Progress in Energy and Combustion Science, 74, 31-49.

[14] Díaz-Pérez, M. A., & Serrano-Ruiz, J. C. (2020). Catalytic production of jet fuels from biomass. Molecules, 25(4), 802.

[15] Kousoulidou, M., & Lonza, L. (2016). Biofuels in aviation: Fuel demand and CO2 emissions evolution in Europe toward 2030. Transportation Research Part D: Transport and Environment, 46, 166-181.

[16] Wang, H., Yang, B., Zhang, Q., & Zhu, W. (2020). Catalytic routes for the conversion of lignocellulosic biomass to aviation fuel range hydrocarbons. Renewable and Sustainable Energy Reviews, 120, 109612.

[17] El-Araby, R., Abdelkader, E., El Diwani, G., & Hawash, S. I. (2020). Bio-aviation fuel via catalytic hydrocracking of waste cooking oils. Bulletin of the National Research Centre, 44(1), 1-9.

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Tags

biomass recalcitrance Jetfuel (Sustainable Aviation Fuel (SAF)) waste-to-fuel Agricultural waste/residue food processing waste hemicellulose cellulose lignin Lignocellulosic Biofuel Pretreatment pyrolysis thermochemical conversion conversion technology biochar bio-oil/pyrolysis oil syngas (synthesis gas) Fischer-Tropsch/FT alcohol fuels alcohol-to-jet (ATJ)/ethanol-to-jet (ETJ) sugars lignocellulosic sugars sugar-to-jetfuel Aviation Fuel/Sustainable Aviation Fuel (SAF) methane/biomethane enzymes plant cell walls biomass hydrolysis wheat straw dilute acid hydrolysis pretreatment ammonia fiber explosion (AFEX) corn stover rice straw/paddy straw ASTM European Union (EU) hydrotreating Egypt BioChemicals/Renewable Chemicals BioRefineries/Renewable Fuel Production palm fiber rice husks palm kernel shell Coconut New York

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