In the last century, innovation in various disciplines, such as plant breeding, crop protec­tion, soil fertility, and plant nutrition have supported an enormous increase in agricultural productivity (Sambamurty 2002; Tilman et al. 2002; Jauhar 2006; Wenzel 2006). As a result, the United States has large amounts of (1) crop residues, (2) agricultural processing residues, (3) animal manures and other wastes, and (4) grasses. Feedstock compositions directly affect product yield and tech-economical feasibility of ethanol conversion process and vary significantly among different kinds of agricultural residues (Table 9.1) (Wyman 2007). As shown in the second through eighth columns of Table 9.1, the high carbohydrate content (i. e., cellulose and hemicellulose) of many crop residues, such as corn stover, result in high theoretical ethanol yields, making them attractive candidates for fuel ethanol production. Among these, corn production residues, such as corncobs and corn stover, cotton processing residues, and sugarcane bagasse contain relatively high fractions of carbohydrates and relatively low lignin, making them particularly amenable for making fermentable sugars. On the other hand, nutshells are not promising feedstocks for bio­conversion to fuel ethanol due to their high lignin content and resulting low amounts of carbohydrates.

Competition for feedstocks and harvesting and transport costs are critical, particu­larly for initial commercial ventures. For example, because the stalks left after extrac­tion of sugar from sugarcane are already at a central location, no additional costs are incurred for collection and transport. However, these materials have value as a fuel for generating process heat and possibly electricity, which still must be taken into consid­eration. Corn fiber is also attractive because of its availability at a processing facility, but it has value as a binder and source of protein for cattle feed. Because residues such as corn stover or just the stalks are left on the field after harvesting the kernels, additional costs are incurred to gather and transport these materials compared to bagasse or corn fiber, and such residues frequently have value as a soil stabilizer and nutrient source, with the result that some must be left in the field (Karlen et al. 1984, Randall et al. 2006; Hoskinson et al. 2007). Other feedstocks such as rice straw are of interest because they are burned following rice harvest to prevent spread of plant diseases, making them potentially available. However, additional costs are incurred to harvest and transport such materials to a central processing site, and the high amounts of silica have a large impact on sugar and resulting ethanol yields and complicate processing to ethanol.

Table 9.2 summarizes the production of various categories of agricultural residues potentially available for conversion to ethanol and other products by 2030. Figure 9.1 breaks these totals down to show the current and predicted availability of those feedstocks with greatest potential impact in the United States (Perlack et al. 2005) . The amount of available feedstock is the residue that can be sustainably removed from the field, which is less than the total produced. The sustainably removable amounts depend on various factors, such as the annual crop residue collection technology, equipment used, soil type, climate, and crop tillage practices (Blanco-Canqui et al. 2006; Hoskinson et al. 2007). The predicted feedstock availabilities listed are based on two different scenarios: a rela­tively conservative assumption of moderate crop yield increases without land use changes to accommodate perennial crops (energy crops) and a high-end assumption that crop yields increase significantly with land use change to accommodate energy crops. As the data show, corn stover ranks first by a large margin in terms of availability in all sce­narios. Yet various crop residues can play an important role for fuel ethanol production, particularly when they are combined with others. As crop yields increase with land use change, the availability of some feedstocks, such as soybean straw and sorghum, could increase significantly.

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Table 9.1. Continued. Feedstock Arabinan Xylan Mannan Galactan Glucan Cellulose Hemicellulose Extractives Lignin Ash Reference Processing residues Corncobs 45 35 5 15 Koch 2006 Corn fiber 10.0 19.0 4.0 30.0 35.3 8 Allen et al. 2001a Corn husk 43.2 21.8 10.9 Kurakake et al. 2001 Cotton linter, 85-90 1-3 0.7-1.6 0.8-2 Han 1998 seed hull Sugarcane 1.6 23.11 0.3 0.45 43.39 43.39 25.46 1.52 24.09 2.84 bagasse Rice hull 1.9 14.4 0 1.1 31.2 30.9 16.8 35.9 Laser et al. 2002 Oat hull 1.2 15.5 0 0.0 48.9 48.4 16.1 16.2 Antal et al. 2000 Almond shells 0 26.1 0 1.8 25.0 24.7 27 27.2 Antal et al. 2000 Walnut shell 0 17.2 0 2.2 21.2 21.0 18.8 32.7 Antal et al. 2000 Pecan shell 0 2.8 0 1.1 5.6 5.6 3.8 70 Antal et al. 2000 Carrot peelings 39.49 9.71 Li et al. 2007 Coconut fiber 0.1 17 0.1 0.4 34.9 34.9 17.5 33.5 Han 1998 Coconut shell 0 25.5 0 0.0 24.5 24.2 24.7 34.9 Antal et al. 2000 Oil-palm empty 35.71 5.67 21.97 6.82 Minowa et al. 1998 fruit bunch Olive husk 24 23.6 9.4 48.4 Demirbas 1997 Orange peel 9.91 8.7 3.45 Grohmann et al. 1995 Manure Cow manure 32.6 23.8 26.8 43.7 Stiller et al. 1996 Horse manure 37.8 32.4 19.6 11.2 Stiller et al. 1996 Broiler litter 18.1 18.05 2.82 Rasool et al. 1996