PEP Review 2000-7
Ethanol From Corn Stover
Published: November 2003
Ethanol's biggest problem as a fuel is its high cost. The U.S. wholesale price in 2002 was around $1.10 per gallon. At this price it is not competitive with gasoline. However, tax breaks provide a stimulus for using ethanol as a fuel in the United States. A strong factor in the U.S. ethanol market is the expectation that it will replace MTBE as an oxygenate in reformulated gasoline or become the main renewable additive in gasoline, should the federal government adopt a renewable fuels requirement. As a result, U.S. ethanol capacity is growing significantly.
For over 20 years the U.S. Department of Energy (DOE) has funded research on the development of renewable, domestically produced fuels for transportation. The Office of the Biomass Program manages biomass conversion technology for DOE. The Biomass Program includes research in feedstock development, bioconversion, thermochemical conversion, and integrated biorefineries. Much of the bioconversion research takes place at the National Renewable Energy Laboratory (NREL). NREL has modeled many potential ethanol process designs during the last 20 years. The most recent NREL design for producing ethanol from corn stover, published in 2002, serves as the basis for this PEP Review. The core technology is co-current dilute acid prehydrolysis of the lignocellulosic biomass with enzymatic saccharification of the remaining cellulose and co-fermentation of the resulting glucose and xylose to ethanol.
Organic wastes are potential low cost fermentation substrates for making ethanol. Much of this waste is crop residues, with corn stover being the dominant material. Stover is the material left in the field after harvesting the kernels and is comprised of stalks, leaves, cobs and husks. It contains considerable quantities of cellulose, a beta-linked glucose polymer, which is difficult to break down into glucose. In addition, it contains hemicellulose, which is a more complex polymer of several sugars including xylose and arabinose. Entwined around the two sugar polymers is lignin, a polymer that does not contain sugar. In NREL's process, cellulose and hemicellulose are converted to ethanol, but lignin is not.
Our economic evaluation, based on PEP's concept of the process, indicates that significant progress must be made in the NREL design to meet the target ethanol selling price of $1.07 per gallon. Capital-related items make up a large share of the overall economics. Yield improvements in all three major steps of the process would enable lower capital requirements. Significant improvement in the process economics could result if revenue were to be generated by sale of the residual lignin.