Efficient feedstock use for upgrading biogas to renewable natural gas: An economic and environmental assessment
Date
2024-05
Authors
Wild, Katherine Jeanne
Major Professor
Advisor
Mba Wright, Mark
Schulte, Lisa A.
Heindel, Theodore J.
Committee Member
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Abstract
This thesis describes an analysis of the economic and environmental implications of anaerobic digestion (AD) process additions for upgrading biogas to renewable natural gas (RNG). These integrations are designed to increase the efficiency of feedstocks used for AD. The addition of a pretreatment solution and various co-digestion ratios are analyzed to increase feedstock digestibility. Two papers are included: “Techno-economic and environmental assessment of converting mixed prairie to renewable natural gas with co-product hydroxycinnamic acid” and “Integration of biogas model (ADM1) for economic and environmental analysis of RNG production”. Results from the pretreatment addition of the first paper show that increasing the efficiency of the feedstock decreases the minimum fuel selling price (MFSP) and increases the net present value (NPV) of the system due to higher methane (CH4) yields. The MFSP of the scenarios involving pretreatment decreased by 12% to 34% due to the biomass digestibility increasing and collection of hydroxycinnamic acid (HCA), a co-product from the pretreatment process. These system updates led to environmental impact changes due to the integration of pretreatment chemicals. All analyzed scenarios producing RNG emit less greenhouse gas (GHG) emissions when compared to the production of fossil fuel natural gas. Results from the second paper, involving the co-digestion of biomass and manure with no pretreatment, showed the addition of manure decreased the MFSP by 7% to 19% when added to prairie biomass. While our biogas model did not fully capture the co-digestion relationship seen within experimental literature for these feedstocks, two scenarios of co-digestion had a lower MFSP than the mono-digestion of cattle manure. Additionally, the feedstock source greatly impacts the environmental results. Impact categories analyzed include the potential for global warming, acidification, ecotoxicity, eutrophication, ozone depletion, photochemical oxidation, carcinogenics, non-carcinogenics, and respiratory effects. Avoidance of manure alleviates burdens while the cultivation of prairie biomass can require additional resources and therefore impact system burdens. Prairie biomass cultivation alleviated burdens within the ecotoxicity category due to the beneficial impacts seen on the environment. Farm AD is a viable method for alternative energy sources and additional revenue streams. Prairie biomass was determined to be an ideal candidate for co-digestion due to the positive impacts seen from cultivation. This paper exemplifies benefits of prairie biomass use for co-digestion, including synergistic properties and additional co-products from prairie biomass pretreatment.
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Mechanical Engineering
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article