Experimental and techno-economic assessment of renewable natural gas production from swine slurry, prairie, and cereal rye

Farm-based anaerobic digestion (AD) has been increasingly adopted on dairy farms in the United States, but not yet on swine operations, largely due to scale limitations. While AD is often acknowledged for lowering greenhouse gas emissions, AD's effects on other agriculturally relevant impact categories (e.g., water quality or biodiversity) are less clear. This study attempts to address both limitations by evaluating the feasibility of scaling swine-based AD systems using herbaceous feedstocks that provide environmental co-benefits, such as annual cereal rye ( Secale cereale ) and mixed-species perennial prairie; a concept referred to as “grass-to-gas.” Semi-continuous AD systems were used to model process outcomes under varying feedstock mixtures and solids retention times using response surface methodology, with predicted methane yields ranging from approximately 110 – 365 mL g -1 of VS. The results were integrated within techno-economic models to evaluate and optimize the economic viability of renewable natural gas production under eight scenarios, considering retention times, methane recovery from digestate, and scale. Minimum fuel selling prices ranged from 47.5 to 160 $ GJ −1 depending on feedstock mixture and operating conditions, indicating that grass-to-gas systems are not yet economically competitive under current market conditions, largely due to high capital and feedstock costs. These findings highlight the importance of optimizing both technical and economic parameters when designing AD systems, as operational scenarios that provided the greatest methane yields were not necessarily correlated with the greatest economic outcomes. Lastly, to compensate for high feedstock costs, future research into the grass-to-gas concept should seek to quantify the ecosystem services these feedstocks may provide and identify possible measures to monetize that value.