Tuesday, May 3, 2011
Liquid fuels can be produced from lignocellulosic biomass and used as a replacement for fuels made from petroleum. One likely near-term transportation fuel is ethanol made by fermenting sugars derived from biomass. Converting biomass requires a pretreatment step, followed by enzymatic cellulose hydrolysis to convert the carbohydrate polymers in biomass to fermentable sugars that are subsequently fermented to ethanol. Dilute sulfuric acid pretreatment is commonly used to convert hemicellulose to sugars while improving the susceptibility of cellulose to enzymatic hydrolysis. There are several ways to configure the separate processes of hydrolysate conditioning, enzymatic hydrolysis, and fermentation; and we need to understand how each step affects the other. In this study, we evaluated two whole slurry processes and a separate hemicellulose/cellulose (C5/C6) conversion process using ammonium hydroxide to condition or adjust pH of the hydrolysate. All work was done with dilute-acid pretreated corn stover produced in a 200 kg/day horizontal pretreatment reactor. In an effort to understand the interactions between solids loadings, conversion yields, and process costs, each process configuration was tested at total solids loadings of 15%, 17.5%, 20%, 22.5%, and 25% (w/w). We used the data to perform an economic analysis to understand the relevant impact of these variables on ethanol production cost. We found that enzymatic cellulose hydrolysis yield decreases significantly above 20% total solids and that all conversion yields are generally better for the separate C5/C6 configuration.