5-02: Impact of solids loading on the economics of a lignocellulosic biomass to ethanol conversion process

Technoeconomic modeling describes the impact of performance tradeoffs on process economics and helps guide research efforts. A rigorous model in Aspen Plus was used to compute material and energy balances for a biomass-to-ethanol conversion process using dilute acid pretreatment of corn stover, separate enzymatic cellulose hydrolysis and fermentation, and ethanol distillation. Subsequent economic analysis determined the minimum ethanol selling price (MESP) for the process, assuming n^th-plant equipment and operating costs. To understand the cost impact of solids loading in the hydrolysis step, a correlation of cellulose conversion as a function of enzyme and solids loading was developed from bench-scale enzymatic hydrolysis experiments on pretreated corn stover slurries. Higher solids processing should show more favorable economics, since stream volumes are reduced and less energy is required to separate the product from water. However, using the correlation it was found that the MESP rises at high solids loading due to a drop in cellulose conversion yields. For assumed enzyme costs of $10-$15/kg protein, the minimum MESP was between 15-20% total solids. For projected costs of $2.50-$5/kg, the minimum MESP occurred at 25% total solids, and was relatively flat from 15-25%. Only in the ideal case where conversion was independent of solids loading did the MESP decrease monotonically from 5 to 30% total solids. These results indicate the economic benefit of processing at higher solids loading, but highlight the need to develop enzymes that maintain conversion yields at high solids.