Lignocellulosic feedstocks

Second-generation biofuel technologies make use of a much wider range of biomass feedstock (e.g., forest residues, biomass waste, wood, woodchips, grasses and short rotation crops, etc.) for the production of ethanol biofuels based on the fermentation of lignocellulosic material, while other routes include thermo-chemical processes such as biomass gasification followed by a transformation from gas to liquid (e.g., synthesis) to obtain synthetic fuels similar to diesel. The conversion processes for these routes have been available for decades, but none of them have yet reached a high scale commercial level. 

Various processes have been developed for hydrolyzing lignocellulose to its major constituents, i.e., to sugars and (partly) depolymerized lignin. The lignin is usually precipitated from the aqueous solution and either used as chemical feedstock or burned as process fuel. The aqueous sugar solution is then applied for fermentation to ethanol after neutralization and purification. 

Figure 2.13 did not include all the biomass conversion processes discussed above. It only considered those that produce transportation fuels. The processes that convert bio-feedstock into biocrude or electricity could not be included because their products have a different value than the transportation fuels. Such a comparison can be attempted by displaying the total manufacturing cost of biobased products in a graph that shows typical relationships between the price of crude and that its derivatives, i.e., of fuel oil, transportation fuel and electricity. This has been done in Fig. 2.14 for the lignocellulose conversion processes. 

Biomass conversion processes are still expensive today, being competitive at crude oil prices between 50 and 100 bbl-1. Lignocellulose might be a fairly cheap feedstock, cheaper than crude oil. However, its conversion requires large... 

The present chapter discusses aspects, known by the authors, of (a) biomass as feedstock, (b) the concept of bio-refinery, (c) thermochemical routes from lignocellulosic biomass to fuels, and (d) the contribution of catalytic technology. The main focus will be on the catalytic conversion of fast pyrolysis oil into fuels with regard to problems encountered currently and the challenges for future research and development. 

Both in the USA and the EU, the introduction of renewable fuels standards is likely to increase considerably the consumption of bioethanol. Lignocelluloses from agricultural and forest industry residues and/or the carbohydrate fraction of municipal solid waste (MSW) will be the future source of biomass, but starch-rich sources such as corn grain (the major raw material for ethanol in USA) and sugar cane (in Brazil) are currently used. Although land devoted to fuel could reduce land available for food production, this is at present not a serious problem, but could become progressively more important with increasing use of bioethanol. For this reason, it is important to utilize other crops that could be cultivated in unused land (an important social factor to preserve rural populations) and, especially, start to use cellulose-based feedstocks and waste materials as raw material. 

It may be estimated that ethanol yields from lignocellulosics will range between 0.12 and 0.32 L kg-1 undried feedstock, depending upon the efficiency of five-carbon sugar conversion [26]. Other types of fermentation, including bacterial fermentation under aerobic and anaerobic conditions, can produce various other products from the sugar stream, including lactic acid. 

The next generation of biofuel processes should differ from the first in (a) utilizing the whole plant as a feedstock and (b) the use of non-food perennial crops (woody biomass and tall grasses) and lignocellulosic residues and wastes (woodchips from forest thinning and harvest residues, surplus straw from agriculture). 

Lignocellulosic perennial crops (e.g., short-rotation coppices and grasses) are a promising feedstock becanse of high yields, low costs, good snitability for low-quality land (which is more easily available for energy crops), and the low environmental impacts. 

The list of plants, by-products and waste materials that can potentially be used as feedstock is almost endless. Major resources in biomass include agricultural crops and their waste by-products, lignocellulosic products such as wood and wood waste, waste from food processing and aquatic plants and algae and effluents produced in the human habitat. Moderately dried wastes such as wood residue, wood scrap and urban garbage can be directly burned as fuel. Energy from water-containing biomass... 

Lignocellulosic biomass is a valuable and plentiful feedstock commodity and its high cellulose and hemicellulose content (about 80% of total) provides considerable potential for inexpensive sugars production. However, enzymatic deconstruction of these polysaccharides remains a costly prospect. Strides in cellulase cost reduction have been made, yet further improvements are needed to reach the goal of 0.10/gal of EtOH expected to enable this new industry. Strategies to reach this goal will combine reduction in the cost to produce the needed enzymes as well as efforts to increase enzyme efficiency (specific activity). As this work proceeds, the more easily attained achievements will be made first, and thus the overall difficulty increases with time. 

Recent studies have proven ethanol to be an ideal liquid fuel for transportation and renewable lignocellulosic biomass to be an attractive feedstock for ethanol fuel production by fermentation (1,2). The major fermentable sugars from hydrolysis of lignocellulosic biomass, such as rice and wheat straw, sugarcane bagasse, corn stover, corn fiber, softwood, hardwood, and grasses, are D-glucose and D-xylose except that softwood... 

Many reseachers have previously studied the use of ultrafiltration to recycle cellulase during saccharification (5-22). However, much of this previous work involved either purified cellulosic substrates, such as Solka-Floc, which are not representative of feedstocks in large-scale operations, or lignocellulosic substrates at concentrations that would be much too low to be economical in a full-scale process. 

Industrialbiobased products have enormous potential in the chemical and material industries. The diversity of biomass feedstocks (sugars, oils, protein, lignocellulosics), combined with the numerous biochemical and thermochemical conversion technologies, can provide a wealth of products that can be used in many applications. Targeted markets include the polymer, lubricant, solvent, adhesive, herbicide, and pharmaceutical markets. Industrial bioproducts have already penetrated some of these markets, but improved technologies promise new products that can compete with fossil-based products in both cost and performance. 

Lignocellulosics are the most abundant renewable organic materials in the biosphere. They account for approx 50% of the total biomass in the world, with an estimated annual production of 1-50 x 1091 (4). Lignocellu-losic materials, particularly the residues obtained from wood processing, are usually much cheaper than sugar- and/or starch-derived feedstock, such as sugarcane and corn. They also have no competitive use as human or animal foodstuffs. 

Figure 1.5 Simplified schematic diagram of a lignocellulosic feedstock biorefinery...

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