Abstract
BackgroundCorn 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.ResultsThe 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°C and 5 min residence time. Enzymatic hydrolysis was conducted using cellulase, β-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.ConclusionChanging 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.
Highlights
Corn stover composition changes considerably throughout the growing season and varies between the various fractions of the plant
ammonia fiber expansion (AFEX) followed by enzymatic hydrolysis was performed on four different corn stover fractions from September and November harvests. The objectives of this project were: (1) to determine whether individual stover fractions have different optimal AFEX conditions and whether this is different from previously optimized values for homogeneously milled corn stover [9,11]; (2) to discover which fractions give the highest glucose and xylose yields at optimal pretreatment conditions; and (3) to model optimal harvest scenarios, assuming 30% and 70% collection of total available dry corn stover, based on the maximum monomeric glucose and xylose yields from each fraction
The main reason for this effect, as can be observed from Figure 4, is due to the cob fraction which was much more strongly affected by the addition of xylanase than all of the other fractions, whose responses were fairly similar. This conclusion is supported by the fact that when the data for the late cob was removed from the analysis, the fraction × xylanase interaction became non-significant. These results indicate that of all the corn stover components, the cob reacts more differently during enzymatic hydrolysis
Summary
Corn stover composition changes considerably throughout the growing season and 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. 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]
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