(237d) Techno-Economic Analysis of Grass2Gas Systems and CO2 Utilization on Swine Farms.

Techno-Economic Analysis of Grass2Gas Systems and CO2 Utilization on Swine Farms

Authors: Jesus de Leon Hernandez, Elmin Rahic, Mark Mba-Wright, Lisa Schulte Moore

Abstract

In recent years, anaerobic digestion (AD) has become a growing topic in the research community. AD involves the biological decomposition of organic feedstocks (manure, crop residues, food wastes, etc.) into methane gas that can be upgraded into electricity or renewable natural gas. Most farms use manure and food waste as feedstocks for AD, which provides benefits to reducing greenhouse gas emissions, but not to other environmental considerations, like water quality or biodiversity. As such, this study explores whether AD can be used to promote conservation practices by using perennial and winter grasses as bioenergy feedstocks.

We ran a continuous AD experiment to model the effects of hydraulic retention time and feedstock composition using winter rye, prairie biomass and swine manure as feedstocks. The experiments provided data on feedstock properties, methane yield, and digestate composition.
The grass-to-manure ratio tested ranged from 50% to 90%, while the grass fraction varied from 100% rye to 100% prairie grass. Methane yield ranged from approximately 200 to 320 mL/gVS, increasing with both higher grass content in the mixture and greater rye contribution within the grass. Experimental results indicated that at these longer retention times, feedstock compositions consisting of 70–80% grasses with a higher proportion of rye relative to prairie produced the greatest methane yields. At shorter retention times, with the same range of grass-to-manure ratios and grass fractions, the optimal composition shifted to approximately 50% grasses, again favoring a higher rye contribution. These findings provide guidance for optimizing feedstock mixtures based on on-farm factors such as herd size and the relative availability of prairie and rye biomass.

A techno-economic model was developed to study the economics of Grass2Gas on Iowa swine farms using BioSTEAM. Key economic outputs like minimum fuel selling price (MFSP) and biomass price were modeled over a range of feedstock composition scenarios (grass-to-manure ratio and prairie-to-rye ratio) for system optimization.

Further valorization options are expected to be necessary, so this study also explores ways to utilize the carbon dioxide (CO) produced by AD. Usually, this CO is vented into the atmosphere, contributing to greenhouse gas emissions. Two of the pathways we are exploring are biological methanation, which involves injecting hydrogen gas into AD systems to biologically convert CO into methane, and catalytic methanation, which takes CO and hydrogen through a nickel-based catalyst to yield water and methane. The economics of these upgrading processes will be compared and evaluated in addition to the baseline model.

References

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[RE1]I think you can remove the details of the reactors. Using only two sentences, just mention that you ran continuous AD experiments to model the effects of __, __, and __, using winter rye, prairie biomass, and swine manure as feedstocks.