Optimizing harvest of corn stover fractions based on overall sugar yields following ammonia fiber expansion pretreatment and enzymatic hydrolysis

Biotechnology for Biofuels Optimizing harvest of corn stover fractions based on overall sugar yields following ammonia fiber expansion pretreatment and enzymatic hydrolysis Rebecca J Garlock 0 Shishir PS Chundawat 0 Venkatesh Balan 0 Bruce E Dale 0 0 Address: Biomass Conversion Research Laboratory, Department of Chemical Engineering and Materials Science, Michigan State University , 3900 Collins Road, Lansing, MI 48910-8596 , USA 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), 90C and 5 min residence time. Enzymatic hydrolysis was conducted using cellulase, -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. - Background Corn stover, the aboveground, vegetative portion of maize (Zea mays L.), makes up roughly 80% of all agricultural residues produced in the USA [1]. Data on annual corn stover production in the USA are not readily available, so various sources have independently estimated that anywhere from 200 to 250 million dry tons of corn stover are produced per year [1-4]. Sustainably harvested corn stover could be used as a feedstock for a variety of applications, including lignocellulosic ethanol production. It has been estimated that 38.4 billion liters of ethanol per year could be produced from North American corn stover, assuming that 40% of the stover is collected [5]. It is widely acknowledged that a percentage of the produced corn stover should be retained on the field following harvest in order to prevent soil erosion and maintain soil organic carbon (SOC) levels. The amount that can be sustainably harvested is highly debated and depends heavily on cropping practices, climate, topography and soil type [4,6-8]. Estimates on the amount of corn stover that can be sustainably harvested vary widely because of these factors, anywhere from 20-80% [1,5,6]. Lignocellulosic feedstocks, such as corn stover, derive their name from the three primary components of the plant cell wall: cellulose, hemicellulose and lignin. The complex polysaccharides, cellulose and hemicellulose, must be broken down into monomeric form (primarily glucose and xylose) prior to microbial fermentation into ethanol or other valuable products. High sugar yields require a two-step process: generally a chemical and/or physical pretreatment step followed by enzymatic hydrolysis of the polysaccharides. Previous work has shown that ammonia fiber expansion (AFEX) is a promising pretreatment that can be used in the process of converting corn stover polysaccharides into ethanol as a liquid fuel source [9-12]. AFEX pretreatment uses concentrated ammoniawater mixtures under moderate temperatures (60180C) and high pressures (200-1000 psi) to disrupt the cellular structure of the plant material by decrystallizing the cellulose, partially depolymerizing and solubilizing the hemicellulose and altering the form, location and structure of lignin [9,11]. The structure and composition of the plant cell wall depends on a number of factors including: developmental stage at harvest, geographical origin, type of tissue and other external factors including season of harvest and environmental conditions experienced dur (...truncated)