Biomass Conversion Research Laboratory, Department of Chemical Engineering and Materials Science, Michigan State University, 3900 Collins Road, Lansing, MI 48910-8596, USA. garlock1@msu.edu.
ABSTRACT: BACKGROUND: Corn stover composition changes considerably throughout the growing season and also varies between the various fractions of the plant. These differences can impact optimal pretreatment conditions, enzymatic digestibility and maximum achievable sugar yields in the process of converting lignocellulosics to ethanol. The goal of this project was to determine which combination of corn stover fractions provides the most benefit to the biorefinery in terms of sugar yields and to determine the preferential order in which fractions should be harvested. Ammonia fiber expansion (AFEX) pretreatment, followed by enzymatic hydrolysis, was performed on early and late harvest corn stover fractions (stem, leaf, husk and cob). Sugar yields were used to optimize scenarios for the selective harvest of corn stover assuming 70% or 30% collection of the total available stover. RESULTS: The optimal AFEX conditions for all stover fractions, regardless of harvest period, were: 1.5 (g NH3 g-1 biomass); 60% moisture content (dry-weight basis; dwb), 90 degrees C and 5 min residence time. Enzymatic hydrolysis was conducted using cellulase, beta-glucosidase, and xylanase at 31.3, 41.3, and 3.1 mg g-1 glucan, respectively. The optimal harvest order for selectively harvested corn stover (SHCS) was husk > leaf > stem > cob. This harvest scenario, combined with optimal AFEX pretreatment conditions, gave a theoretical ethanol yield of 2051 L ha-1 and 912 L ha-1 for 70% and 30% corn stover collection, respectively. CONCLUSION: Changing the proportion of stover fractions collected had a smaller impact on theoretical ethanol yields (29 - 141 L ha-1) compared to the effect of altering pretreatment and enzymatic hydrolysis conditions (150 - 462 L ha-1) or harvesting less stover (852 - 1139 L ha-1). Resources may be more effectively spent on improving sustainable harvesting, thereby increasing potential ethanol yields per hectare harvested, and optimizing biomass processing rather than focusing on the selective harvest of specific corn stover fractions.