Starch is converted using two main enzymes alpha

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and the mixing power requirements are higher due to a higher solids content. Starch is converted using two main enzymes, alpha-amylase and gluco-amylase. These enzymes have improved over the years, and now convert essentially 100% of the starch to glucose, provided that the corn is finely ground and properly cooked. The residual solids from each process have value as a by-product. The DDG is high in protein and is sold for animal feed. The lignocellulosic residue has no food value but has a high energy value and can be used for fuel. Table 2 shows the composition of the DDG and lignocellulosic residue and their relative amounts for a 25 million annual gallon fuel ethanol plant. The lignocellulosic residue composition is determined in the process model. It should be noted that ethanol and possibly electricity are the only products of the lignocellulose plant considered here. Certainly, smaller-volume niche products will emerge - products that can also be produced from the lignocellulose-derived sugars and that will have a significantly higher profit margin. This is also true for the starch process; higher value co-products such as zein proteins and corn fiber-based products are under study by the USDA. When these other products and their selling prices are figured into the analysis, the cost of fuel ethanol will decrease, just as the cost of gasoline is lowered by the sale of other petroleum products of crude oil. Table 2. DDG and Lignocellulosic Residue Composition and Production DDG 17 % As-is Basis Lignocellulosic Residue % As-is Basis Neutral Detergent Fiber 44.0 Cellulose 4.6 Protein 27.0 Hemicellulose 3.6 Fat 9.0 Lignin 12.3 Ash 5.0 Protein 1.7 Other (glycerol, other organics) 6.0 Other Organics 14.7 Moisture 9.0 Ash 4.5 Total 100.0 Moisture 58.6 Total 100.0 Tons per day at 9% moisture 243.6 Tons per day at 58% moisture 1481 Pounds per gallon fuel ethanol 6.4 Pounds per gallon fuel ethanol 39.1 IV Normalization of Design and Economic Models A large part of this joint effort was to put the two models, developed separately, on common design and costing bases. While not a trivial effort, it was encouraging to find that much of the design assumptions and costing methodologies were, though not identical, definitely comparable. In 1999, NREL completed a comprehensive review of its design and costing with Delta-T Corporation, which designs, constructs, and operates corn ethanol plants. 15 The majority of the costs used in the USDA process model were also from Delta T. USDA and NREL staff evaluated the physical properties, equipment specifications and costs, and operating costs. When necessary, modifications to one or
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11 both models were discussed and agreed upon. It was agreed that some differences would remain, particularly in modeling the utilities, to aid in combining the two models later. Both the USDA and NREL use ASPEN Plus™, 18 a chemical engineering simulation software package to model the mass and energy balances for both of the ethanol processes, and Microsoft Excel™ for creating costing and economic analysis models. In order to make the comparison, both portions of the models had to be aligned. This
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