(25c) Effect of Biomass Feed and Pretreatment Method on the Economic Viability of a Bioethanol Plant Utilizing Biochemical Production Pathways

Petroleum derived energy sources such as gasoline are generally expensive, nonrenewable, and can negatively impact the environment during production and use. Because of this, green energy alternatives have been investigated. At the forefront of these alternatives is the production of ethanol from lignocellulosic biomass. Mainstream production of biologically derived ethanol is done thermochemically via gasification and pyrolysis using a syngas intermediate. An alternative conversion method is done via a biochemical reaction pathway, converting the plant material to simple sugars that can be further processed into ethanol. However, this process must be able to compete with the production levels and efficiency of the thermochemical pathway as well as the current petroleum industry.

Our research focuses on the optimization of bioethanol production utilizing biochemical pathways and sugar intermediates. Our current interest includes the effects of various pretreatment methods, used in the enzymatic saccharification and fermentation of cellulosic biomass, and biomass feed composition on the economic viability of the bioethanol production plant. Pretreatment of the biomass involves the breakdown of lignin and the rigid cell wall, leading to greater accessibility of enzymes to the cellulosic material in further downstream processing. Many pretreatment options have been widely studied, each requiring different considerations in process development. Our research focuses on four of these technologies, ammonia fiber expansion (AFEX™), liquid hot water, sulfur dioxide and dilute acid, as investigated within the Biomass Refining Consortium for Applied Fundamentals and Innovation (CAFI).^[1][2] The effectiveness of the pretreatment process varies based on biomass type, so three different biomass alternatives, corn stover, switchgrass and hybrid poplar, are investigated as possible feed components.

To find the optimal pretreatment method and biomass feed composition, a mixed integer nonlinear program for the plant was developed. This model consisted of mass balances around all of the unit operations in the plant along with cost correlations to determine the resulting capital and operating costs. The biomass feed consisted of any or all of the three different biomass options. The total cost of the plant was then minimized to find the break-even selling price per gallon of the ethanol produced. Biomass supply ranged from 2000 to 3000 US tons per day and it was assumed the plant is operation for 350 days of the year. Loans on the capital costs of the plant were assumed to be paid back in 7 years. It was found that the optimal break even selling price of ethanol was $2.68, using a of 85.43% corn stover, 14.52% switchgrass and 0.05% hybrid poplar and liquid hot water pretreatment. Sensitivity analyses done on the capital cost, enzyme cost and loan period showed that the break-even ethanol price and biomass feed composition was affected most by the capital cost loan period. Capital costs showed an effect on the biomass feed composition but little effect on the ethanol price; the enzyme price showed little effect on either variable.

1. Humbird, D. et al. “Process Design and Economics for Biochemical Conversion of Lignocellulosic Biomass to Ethanol: Dilute-Acid Pretreatment and Enzymatic Hydrolysis of Corn       Stover” NREL Technical Report. May 2011.
2. Tao, Ling et al. “Process and Technoeconomic Analysis of Leading Pretreatment Technologies for Lignocellulosic Ethanol Production Using Switchgrass.” Bioresource Technology 102.24 (2011): 11105–11114
3. Elander, Richard T. et al. “Summary of findings from the Biomass Refining Consortium for Applied Fundamentals and Innovation (CAFI): corn stover pretreatment.” Cellulose 16.4 (2009): 649–659.