The increase in global energy demands, diminution of fossil fuels, and its harmful effects on the environment have attracted the use of lignocellulosic biomass as an alternative sustainable source for chemicals, materials, and liquid fuels. Leveraging biomass presents a potential avenue to contribute to the global objective of achieving a carbon-neutral human society. The Energy Information Administration (EIA) notes that global energy consumption, which was 282.817 quadrillion Btu in 1980, exceeded 500 quadrillion Btu in the 2010s. Projections suggest it will hit 815 quadrillion Btu by 2040, a 48% increase. Fossil fuels currently meet this majority demand, raising concerns due to potential CO2 (a major greenhouse gas) doubling compared to preindustrial levels (approximately 285 ppm for about 400 000 years before the industrial revolution and reached 376 ppm as of 2005 and is still increasing). Halting fossil fuel use by 2050 is crucial to avoid exceeding the critical 2°C temperature increase threshold. Lignocellulosic biomass, a renewable and abundant resource, holds great promise for sustainable production of valuable chemicals and fuels, currently derived mainly from oil. Currently, biomass contributes 10% (5 x 10-19 kJ) to global energy consumption. Predictions estimate this value to reach 150 x 10-19 kJ by 2050, considering the diverse range of biomass resources available.Biomass, deriving its carbon from atmospheric CO2, is deemed a carbon-neutral fuel (Figure 1). By cycling carbon through energy generation systems as biomass, it offers a means to address climate change. While solar and wind provide carbon-free electricity or H2 through water splitting, they lack the ability to directly produce carbon-based fuels and chemicals without additional carbon sources. Given the current imperative, the primary focus is on biomass-based fuels and chemicals to enable a sustainable future free from carbon emissions.
Figure 1. Carbon dioxide cycle of biomass-based fuels.
The term ‘biofuels’ encompasses solid, liquid, and gaseous fuels primarily derived from biomass, including animal and plant wastes and residues. Bioethanol, biodiesel, biomethanol, biogas, and syngas constitute the primary categories of biofuels. Lignocellulosic feedstocks, potentially abundant, consist of three main components: cellulose (~28-55%), hemicellulose (~17-35%), and lignin (~17-35%). Extracted from trees, grasses, and other biomass, cellulose finds applications in products like paper and ethanol. However, lignin, a complex material providing plant strength, remains largely unused due to challenges in breaking it down into low-viscosity oils essential for producing kerosene or diesel fuel. In biorefineries, lignin is used for producing heat to run the turbines which substantially affects the economy. (Prof. Yulin Deng, https://news.gatech.edu/news/2020/09/09/new-process-boosts-lignin-bio-oil-next-generation-fuel)
The primary biofuels in use today, ethanol and biodiesel, belong to mostly the first generation of biofuel technology. Ethanol, a renewable fuel (CH3CH2OH), is commonly blended with gasoline to boost octane levels and reduce carbon monoxide and other emissions contributing to smog. Ethanol blending is feasible in various ratios, including E10 (10% ethanol, 90% gasoline), E15 (15% ethanol, 85% gasoline), and E85 (flex fuel), depending on the approved vehicle model. Biodiesel, derived from renewable sources like vegetable oils and animal fats, serves as a cleaner-burning alternative to petroleum-based diesel fuel. It’s both nontoxic and biodegradable, produced by combining alcohol with vegetable oil, animal fat, or recycled cooking grease.
Biomass can be converted into fuels through several steps. The initial step involves the deconstruction and fractionation of biomass into intermediates, such as sugars, intermediate chemical building blocks, bio-oils, and gaseous mixtures. These intermediates are then further converted into fuels, chemicals, and power through synthesis or upgrading processes. (https://www.energy.gov/eere/bioenergy/conversion-technologies) Present biomass conversion technologies can typically be classified into low-temperature (mostly chemical or biochemical) and high-temperature processes (thermochemical or thermal).
In low-temperature processes, biological catalysts known as enzymes or chemicals are used to break down biomass into intermediates. High-temperature conventional thermochemical conversion approaches, including combustion, gasification, and pyrolysis, originally developed for fossil fuels, can now be applied to biomass feedstocks. The intermediates obtained through both methods are then upgraded to finished products. (DOI: 10.1039/c6ee03718f)
Recently, the Editorial in Sustainable Energy & Fuels highlighted biorefining as a promising solution to enhance economic viability in biobased processes and address climate change challenges. (DOI: 10.1039/d3se90047a) Biorefining involves transforming biomass into valuable products like renewable fuels and platform chemicals. Liquid hydrocarbon fuels have almost 100 times higher energy density than batteries, making them the practical energy source for vehicles that require a high-power requirement like aviation, heavy duty vehicles and shipping.
Developing economically viable, scalable, and sustainable technologies to convert lignocellulosic polysaccharides into liquid fuels is crucial for the global bioeconomy and integral to achieving carbon neutrality. Among the various considerations for biomass conversion, the following factors must be addressed to ensure the sustainability of a biorefinery and achieve its objectives:
Feed-stock flexibility
The concept of feedstock flexibility is essential in diversifying biomass sources while minimizing competition with food production. Bioethanol, can be derived from various sources such as sugar, corn, wheat, agricultural wastes, molasses, macroalgae, microalgae, and seaweed. While edible resources like sugar, corn, and potato are not recommended due to the food versus fuel debate, utilizing agricultural wastes and other non-food sources offers a sustainable solution. Additionally, employing waste materials as feedstock presents another promising approach to ensure long-term viability and environmental benefits.
Deconstruction of biomass
The conversion process of lignocellulosic biomass to ethanol typically consists of three steps: (1) pretreatment; (2) hydrolysis of cellulose and hemicellulose into fermentable sugars; and (3) fermentation of the sugars into liquid fuels (ethanol) and other commodity chemicals. Efficient conversion of lignocelluosic biomass to fermentable sugar depends largely upon the physical and chemical properties of biomass, pretreatment methods, effective microorganisms, and optimization of processing conditions. The ideal pretreatment should break the lignocellulosic complex, increase the active surface area and decrease the cellulose crystallinity, while limiting the generation of inhibitory byproducts and minimizing hazardous wastes and wastewater. The schematic of biomass to ethanol production is shown in Figure 2.
Figure 2. Schematic of biomass to bio-oil conversion process.
Lignin valorization
The valorization of lignocellulosic biomass, especially underutilized lignin, is essential for enhancing the sustainability of downstream biomass processing in a biorefinery. Developing selective lignin depolymerization methods to produce valuable chemicals contributes significantly to this goal. However, lignin depolymerization is challenging due to its complex structural pattern and changes during the process.
The process yields chemicals such as vanillin, organic acids, and aldehydes etc. ( https://doi.org/10.1016/j.fuel.2020.118799) Pyrolysis of lignin produces bio-oil, considered the next-generation fuel for the transportation sector, including sustainable aviation fuel. The schematic of bio-oil production is shown in Figure 3. Bio-oils contain more than 300 small molecules such as phenols, aldehydes, ketones, and carboxylic acids, etc. The oxygen content in bio-oil is high, which results in low heat value, high acidity and corrosivity.
Therefore, upgrading is necessary for its removal through catalytic hydrodeoxygenation (HDO) before conversion to hydrocarbon fuel or chemicals (benzene, toluene, and xylene, etc.). Bio-oil HDO processes are typically energy-intensive and take place at high hydrogen gas pressure (2-200 bar) and temperature (200-500 0C) using a solid-phase catalyst such as metals, metal oxides, and bifunctional catalysts, etc.
The precious noble metals (Pd, Pt, and Ru, etc.) or transition metals (Ni, Cu, Mo, Cu, and Fe, etc.) and their derivatives have shown suitable activities towards HDO reactions. (https://doi.org/10.1038/s41560-020-00680-x) However, the HDO reactions are restricted by the high temperature, which causes catalyst deactivation and tar formations. Moreover, the cost of the catalyst, specially noble metals, hindered its acceptability. Further, extensive research needs to be done in this direction to develop various economically viable methods to meet the industry’s needs.
Figure 3. Schematic of biomass to bio-oil conversion process.
The preceding discussions offer insights into strategies for converting biomass into fuel within the framework of biorefining. Looking ahead, it is imperative that scientists, engineers, and industry professionals collaborate and commit to research efforts to develop sustainable technologies, ultimately realizing a greener and more efficient future.
(Disclaimer: Dr. Parikshit Gogoi is a visiting research scholar at the Illinois Sustainable Technology Center (ISTC), University of Illinois at Urbana-Champaign, USA under the Fulbright-Nehru Academic and Professional Excellence fellowship. Dr. Gogoi is also Asst. Professor of Chemistry at Nowgong College in Assam, India. His research focuses on biomass-based chemicals, fuels, and materials. Views are personal)
