Location: Forage-animal Production Research2013 Annual Report
1a. Objectives (from AD-416):
1. Demonstrate and test a universal bio-energy crop single-pass harvesting system applicable to agricultural residues (corn stover, wheat straw), switchgrass, and miscanthus with bale densities at or above 210 kg/m3 with appropriate best management practices for sustainable biomass harvest. 2. Demonstrate the technical feasibility of on-farm storage and processing of high density bio-energy crops to enhance biomass conversion to value added products using a solid substrate fungal cultivation followed by a percolating anaerobic fermentation with recycle. 3. Develop and validate integrated geographic information system (GIS)-based economic and life cycle analysis models for the proposed on-farm processing system, and use these models to evaluate different landscape-scale management scenarios on food and energy production and the environment. Determine the incentives required to increase carbon sequestration and bioenergy production when they conflict with maximum farm profitability.
1b. Approach (from AD-416):
Two University of Kentucky cooperators are available with highly productive soils and are willing to provide access to miscanthus, switchgrass, and cereal crops. Additional switchgrass and miscanthus sites are located in Northeastern Kentucky where biomass collection on marginal agricultural land can be performed and its impact evaluated. In addition, both the University of Kentucky and North Carolina State University have research farms that allow for accurate quantification of inputs, water quality changes, and soil changes during feedstock collection. Scale up of the bunker silo conversion would also be done at university farms. The on-farm processing is envisioned to occur in bunker silos containing 360 bales of feedstock. The lignin will be depolymerized by inoculating the bales with Phanerochaete (P.) chrysosporium, and forced aeration. Alternatively, a biomimetic route will be explored. The Fenton reaction likely to be a part of this mechanism, and has been shown to be a biomimetic route to degrade lignin. The Fenton reaction requires inexpensive reagents that are amenable to current agricultural practices. When sufficient lignin has been removed to allow access to cellulose, the reactor will be switched to anaerobic solid-state thermophilic conditions, and innoculated with Clostridium (C.) thermocellum, which degrades pretreated lignocellulosic material very efficiently when the culture media is replaced on regular intervals. Water will be degassed and heated to 60°C using waste heat generated from gasifying the residual lignin from a previous bunker. After C. thermocellum is established, the reactor temperature will be dropped back to 37°C, and Clostridium (C.) acetobutylicum will be introduced. C. thermocellum will stop metabolizing at this temperature, thus allowing C. acetobutylicum to convert sugars to butanol generated by the (still active) cellulosomes of C. thermocellum. A modular, synthetic, semi-permeable membrane separation system will be developed to separate product streams, concentrate the products, remove inhibitory products and permit recycling of water and nutrients. Membrane modules can be customized from filtration of cells and debris to small molecule separation (such as butanol from acetone). Membrane separations do not involve a phase change (unlike distillation), and are energy efficient. Our proposed separations may be less stringent because economic benefit is expected from any preconcentration of the product streams prior to shipping or purification of the recycle stream prior to reuse. Residual biomass will be evaluated as a ruminant feed. Distiller’s grains are valuable as a feed additive because of high residual protein concentrations. This process will produce C. thermocellum and C. acetobutylicum cells in the same manner that bio-ethanol production yields yeast cells. The residual biomass will be evaluated as a value-added product.
3. Progress Report:
The topic of this year’s experiments was switchgrass (Panicum virgatum) fermentation by sequential culture of Clostridium (C.) thermocellum and C. beijerinckii. Specifically, an ARS Scientist in Forage-Animal Production Research Unit at Lexington, KY, and a University of Kentucky collaborator quantified the cultures’ ability to produce and sustain anaerobic conditions. The hypothesis was that there would be an inverse relationship between particle size and gas production. The ability of C. thermocellum and C. beijeriinckii to catabolize cellulosic biomass was evaluated through gas production experiments on different particle sizes of switchgrass. Among the gasses produced, carbon dioxide (CO2) is the most universal measure of substrate fermentability. Moreover, because not only carbon dioxide but also hydrogen is produced during the fermentation, the cumulative pressure obtained in order to maintain adequately anaerobic conditions without supplying gas continuously was quantified. The optimum particle size was determined by the maximum gas production of sequential cultures. C. beijerinckii produced approximately 1,000 µmol CO2 from 2 mm or 5 mm particle size switchgrass in 24 h. Gas production was 30% less when the particle size was 15 mm. Sequential culture promoted gas production. Specifically, when C. thermocellum was grown on the substrate (2 or 5 mm) prior to inoculation with C. beijerinckii, 5-times as much CO2 was produced by C. beijerinckii. Even gas production from 15 mm particle switchgrass was more than doubled by sequential culture.