Cotreatment Enhanced Anaerobic Digestion of Lignocellulosic Biomass

Open Access
- Author:
- May, Drew
- Graduate Program:
- Agricultural and Biological Engineering
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- October 29, 2020
- Committee Members:
- Thomas L Richard, Thesis Advisor/Co-Advisor
John M Regan, Committee Member
Juliana Vasco-Correa, Committee Member
Paul Heinz Heinemann, Program Head/Chair - Keywords:
- anaerobic digestion
lignocellulose
bioprocess modeling
cotreatment
bioenergy
switchgrass
biomass milling - Abstract:
- Anaerobic digestion generates storable renewable energy from organic matter. In this process, organic compounds are degraded in the absence of oxygen by a mixed microbial community into carbon dioxide and methane, which can be burned as an energy source. Lignocellulosic feedstocks like grasses and corn stover are cheap and abundant, but without expensive and energy intensive thermochemical pretreatment their recalcitrance to biological decomposition has thus far limited usage in commercial biogas production. A possible solution to this problem is demonstrated by ruminant animals, which degrade lignocellulosic material efficiently by occasionally regurgitating and chewing the plants they consume. The mechanical analog of this strategy is termed cotreatment and involves mechanical milling during fermentation. This study evaluated the potential of intermittent cotreatment through observation of a pilot scale lignocellulosic anaerobic digester, determination of cotreatment energy consumption, and process modeling at industrial scale. Cotreatment had previously been studied using systems that milled the material only on a single occasion in a batch process, while in this study cotreatment was implemented daily in a continuous process using a recirculating loop design. A pilot scale 80-liter fermenter was coupled with a conical wet disk mill where the mill rotation also provided the pumping force for recirculation. The system was able to tolerate daily milling of 50% of the reactor volume, but a small decrease in biogas production was observed compared to performance without cotreatment. This could be due to inadequate recovery time for the microbes between milling events, poor milling due to an improper adjustment or an inadequate mechanism, a substrate whose recalcitrance was not sufficiently limiting in the base case without cotreatment, or some combination of these and perhaps other factors. Nutrient limitations were unlikely, as nutrient requirements were tested during system setup and supplemented with at least 150% of requirements. The nutrient supplement studies suggested that while nitrogen and mineral additions were necessary for a switchgrass feedstock that had senesced in the field and was stored dry, addition of vitamins was not required for continuous anaerobic digestion. The energy necessary to mill switchgrass slurries tended to increase when solids content increased or when the radial gap size decreased. Even the most energy intensive conditions consumed only about 2.3% of the higher heating value of the cellulose and hemicellulose fraction of the switchgrass per pass through the mill. The magnitude of particle size reduction mirrored that of energy consumption. Increased solids loading caused larger magnitude particle size change despite maintaining the same radial gap size, suggesting that interactions between particles play a role in size reduction during milling. Using a model to simulate cotreatment at industrial scale for a lignocellulosic anaerobic digester fed with switchgrass, varying degrees of milling effectiveness were evaluated in combination with recycling of a portion of water from the digestate waste stream. For a scenario of 40% cotreatment conversion increase and 80% water recycling, the model predicted the breakeven subsidy price would decrease by 31% from the base case without these innovations, and the net energy return on energy invested for the system would be as high as 2.06. However, the net present value of the investment was -$5.3 million for a system processing 12 dry tons of switchgrass per day. Additional subsidies or revenue streams are necessary, in combination with technology innovations like cotreatment and water recycling, for lignocellulosic anaerobic digestion to become economically feasible.