Characterization of wet storage impacts on bioprocessing of corn stover to biofuels
Open Access
- Author:
- Darku, Irene Dzidzor
- Graduate Program:
- Agricultural and Biological Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- September 23, 2013
- Committee Members:
- Thomas Lehman Richard, Dissertation Advisor/Co-Advisor
Thomas Lehman Richard, Committee Chair/Co-Chair
Jude Liu, Committee Member
Paul Heinz Heinemann, Committee Member
Evelyn Ann Thomchick, Committee Member - Keywords:
- Biomass
Wet storage
Ensilage
Biofuel
Logistics
Ethanol
Fiber reactivity
Pretreatment
Corn stover
Biofuel
Delivery cost - Abstract:
- Narrow harvest windows and contamination concerns with field drying suggest that wet storage will likely be the preferred storage method for biofuel feedstocks in humid regions of the U.S. There are, however, at least two major setbacks to the adoption of this storage method. The first relates to the impact of wet storage on biomass quality, biofuel yield, and biorefinery system performance. The second relates to the impact of wet storage system on supply chain management and logistics. These impacts have not been well addressed in prior wet storage studies, and several logistic models have overlooked the wet storage option entirely. Feedstock suppliers and feedstock buyers, therefore, have no prior expectation of how wet storage outcomes compares with conventional dry storage, hence making the adoption of this storage option a high risk venture. One of the ways in which wet storage is anticipated to affect downstream processes, especially biological processes like fermentation, is through the organic acids produced during storage. These organic acids have the potential to alter the feedstock structure and provide partial pretreatment, but can also inhibit subsequent biofuel fermentation. Pretreatment is necessary for lignocellulosic feedstocks since it allows plant cell wall degrading enzymes to have access to structural sugars (cellulose and hemicelluloses) and convert them to glucose and other simple sugars that can be fermented to ethanol. Inhibition results in reduced specific biofuel productivity (the amount of biofuel produced from the feedstock within a given time), when using biological route of conversion. The contradictory functionality of organic acids generated during wet storage made the impact of organic acids a major focus of this study. To address these concerns, a series of experiments were performed to characterize the impact of a wet storage system on biofuel production and elucidate how this storage option compares with conversional dry storage systems. The most familiar wet storage process applicable to biofuel production is ensilage, which has been practiced for centuries in the animal feed industry. The feedstock is stored under anaerobic conditions, typically at moisture contents of 55 - 75% wet basis (w.b.), and these conditions result in the production of organic acids by natural acidogenic microorganisms. In this study, moisture levels outside these conventional ensilage levels were investigated in order to define the full range of suitable levels for the storage of industrial biomass destined for cellulosic biofuels. The feedstock used in this wet storage study was corn stover, which is an agricultural residue and includes the above ground biomass after corn grain is harvested. Corn stover at seven different moisture contents (15-75%) was incubated under anaerobic and aerobic conditions at two temperature levels (23oC and 37oC) for 0, 21, 90 and 220 days. The impact of wet storage was evaluated through its effect on dry matter loss; feedstock composition; reactivity of corn stover fibers, after wet storage, to enzymes; and production of ethanol, which was used as a model fuel. The impact of organic acids was examined specifically through their effect on fiber reactivity; their interaction with subsequent liquid hot water pretreatment process; and interference with ethanol fermentation, when Saccharomyces cerevisiae is used as the fermenting microbe. A process cost model was developed, using results from this study, to explore the cost implications of wet storage and effect on ethanol production. Results from this study show that the most influential factor with respect to change in feedstock composition during anaerobic wet storage is storage duration. Generally, there were no significant differences in feedstock composition and in feedstock response to subsequent downstream processes when corn stover was stored at moisture levels of 35% to 65%. Although maximum dry matter loss observed under anaerobic storage was approximately 9%, the average loss at 220-day storage period was less than 3%. Total organic acid content after wet storage was up to 9.1%. Hemicellulose degradation during wet storage, which is an indication of the pretreatment capability of organic acids, ranged from ~6% to ~30%. Some key findings from this study are: (1) the extent of organic acid pretreatment during storage was not adequate to serve as sole pretreatment, implying post storage pretreatment would still be necessary; (2) the organic acid profile that develops during storage is considerably changed during subsequent pretreatment. The amounts and types of acid after pretreatment depends on whether feedstock was dried after storage, washed before pretreatment, or used “as is”, that is without any processing before pretreatment; (3) acetic acid amounts greater than 6% g/g dry basis can inhibit ethanol fermentation if butyric acid is also present. But these high concentrations are observed only in 75% moisture feedstock; (4) if samples stored at 75% are excluded, the organic acids produced during wet storage had no inhibitory effect on ethanol fermentation and in fact enhanced the yield by a mean factor of ~1.11; (5) modeling output showed that at moisture levels ≤ 35%, the minimum delivery cost of wet storage feedstock was lower than cost of dry storage at 25% moisture. (6) A well preserved dry biomass storage system is likely less expensive than high moisture (≥ 45%) wet storage systems in terms of feedstock delivery cost. Considering findings 5) and 6), the optimum wet storage system for biomass is likely to be approximately 35 - 40% moisture, which is lower than for animal feed. This lower optimum is primarily driven by transportation costs of the additional water at higher moisture levels, and will vary with the distance biomass needs to move to a biorefinery. This study is useful in providing feedstock suppliers and feedstock buyers with a good understanding of wet storage systems and their impact on feedstock composition, feedstock logistics and downstream processing outcomes. Several major concerns about the appropriateness of wet storage systems for the biofuel industry have been addressed. The information and model from this study provides a basis for comparison of wet storage and conventional dry storage systems and will facilitate the cost effective adoption of wet storage for biofuel production where appropriate. In addition, the results from this study can be used in developing quality indices to facilitate fair trade between feedstock suppliers and buyers.