Research review paperBiomass pretreatment: Fundamentals toward application
Introduction
Biomass has been defined to be “any material, excluding fossil fuel, which was a living organism that can be used as a fuel either directly or after a conversion process” (ASTM 2002). The use of biomass as source of fuel dates back to Peking Man, 460,000 to 230,000 years ago (Binford and Ho 1985), and the use of plant biomass as a fuel for heat, light, and food preparation has been central to the evolution of our species (Gowlett 2006). Although such empirical forms of biomass-derived fuels are still in use today, the bioprocessing options (aerobic and anaerobic fermentation) to produce liquid transportation fuels is of paramount importance to any sustainable energy development scheme. The global rise in energy consumption, predicted increase in energy demands in the near future, the depletion of low extraction cost fossil fuel reserves, and climate change have converged to create an urgent need to develop more sustainable energy systems based on renewable lignocellulosic biomass.
Traditional production of bioethanol involves batch fermentation of sugars derived from sugarcane (in Brazil) or from starch from grain (predominantly corn in the US and eastern Canada, and wheat in the prairie provinces of Canada), followed by ethanol recovery from the fermentation broths using distillation (Khanal 2008). Biofuels produced from these processes are referred to as “first generation biofuels”.
The long-term viability of grain-based ethanol production, however, is in question (Groom et al., 2008, Searchinger et al., 2008, Simpson et al., 2008). Issues concerning the impact of expanded corn production for fuel ethanol in the US, for example, include poor energy balance (energy output of the biofuel/energy inputs required during production) and negative impacts on regional water resources, biodiversity, and soil quality (Groom et al., 2008, Simpson et al., 2008). Estimates suggest that in the USA, starch based ethanol output will reach a maximum between 12 and 15 billion gallons per year, one tenth of the projected 140 billion gallons per year, required to impact use of petroleum in the states (Jessen 2006). Moreover, corn-based ethanol production may actually result in increased greenhouse gas emissions (Searchinger et al. 2008). Grain-based fuel ethanol production also creates a moral dilemma over the use of agricultural crops and/or land for fuel vs. food (Brown, 2006, Sun and Cheng, 2002). Based on protein weights it takes anywhere from 40–100 times more enzymes to breakdown cellulose than starch, yet the cost of enzyme production is not substantially different (Merino and Cherry 2007).
Cellulosic materials are particularly attractive as feedstocks for biofuel production because of their relatively low cost, great abundance, sustainable supply (Lynd et al. 2002). Cellulose is the most abundant biopolymer on earth (O'Sullivan, 1996, Zhang and Lynd, 2006) and biofuel production from cellulosic biomass has become the major focus of intensive research and development (Lynd et al. 1999). Raw biomass is composed mostly of cellulose, hemicelluloses, lignin, and proteins. Lignocellulosic material often requires pretreatment to liberate the sugars contained within cellulose fibres embedded in the hetero-matrix of plant cell walls.
The process that converts any source of lignocellulosic biomass from its native form, which is recalcitrant to hydrolysis with cellulase enzyme systems, into a form which enzymatic hydrolysis is effective is referred to as ‘pretreatment’ in bioprocess engineering (Lynd et al. 2002). An ideal pretreatment step by this definition should render lignocellulosics completely susceptible to the action of cellulases. Various types of pretreatment have been developed, but key bottleneck in biofuel production is the initial conversion of biomass to sugars. New biotechnological solutions for the decomposition of lignocellulosic biomass are required to improve the production efficiencies and reduce the costs of cellulosic biofuel production (Lynd et al. 2008). This paper reviews developments in biomass pretreatment technologies with emphasis on concepts, strategies, and practicality for industrial applications.
Section snippets
Bioprocessing of lignocelluloses and fibre analysis
Lignocellulosic materials consist mainly of three polymers: cellulose, hemicellulose and lignin. These polymers are associated with each other in a hetero-matrix to different degrees and varying relative composition depending on the type, species and even source of the biomass (Carere et al., 2008, Chandra et al., 2007, Fengel and Wegener, 1984). The relative abundance of cellulose, hemicellulose, and lignin are inter alia, key factors in determining the optimum energy conversion route for each
Biomass types, properties, and recalcitrance
Different types of biomass, such as woody plants, herbaceous plants, grasses, aquatic plants, agricultural crops and residues, municipal solid waste and manures, contain different amounts of cellulose, hemicellulose, lignin, and extractives (Chandra et al. 2007). Generally plant biomass contain 40–50% cellulose (with exception to a few plants, such as cotton and hemp bast-fibre that are made up of ≈ 80% cellulose), 20–40% hemicellulose, 20–30% lignin by weight (Chandra et al., 2007, Mckendry,
Pretreatment methods
A rough classification of pretreatment based on pH divides it into acidic, alkaline and neutral pretreatments (Galbe and Zacchi 2007). This classification is focused on chemical pretreatment only. It does not encompass other pretreatment methods such as physical or biological pretreatments. A generalized classification of pretreatment methods groups them into; physical, chemical, biological and multiple or combinatorial pretreatment. In combinatorial pretreatment methods, physical parameters
Conclusion
Biomass pretreatment remains a key bottleneck in the bioprocessing of lignocellulosics for biofuels and other bioproducts. Although some pretreatment methods show apparent advantages, it is unlikely that one method will become the method of choice for all biomass, at least, not for all feedstocks. Co-digestion of non-food cellulosic biomass feedstocks from different pretreatment technologies as a substrate quality optimization attempt is worth investigating. Bioprocessing of pretreated
Competing interests
The authors declare that they have no competing interests.
Acknowledgements
This is supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) through a Strategic Network grant (Hydrogen Canada), and by Husky Energy through an NSERC Collaborative Research and Development (CRD) grant fund.
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