(304c) Meta-Analysis of Prairie Grass Anaerobic Digestion Life Cycle Assessment

There is growing interest in developing resources that provide clean energy while supporting rural communities. Renewable natural gas (RNG) production has increased significantly in recent years. Several substrates have been developed for RNG production, including prairie grasses. Prairie grass once covered 40% of the US land area but now occupies about 4% due to the expansion of commodity crops and increasing loss of natural habitats. These losses are accompanied by decreased ecological services offered by prairie grasses. Prairie grass anaerobic digestion could reverse this trend by generating additional revenue. Recent work demonstrated the economic and environmental benefits of prairie grass to RNG biorefineries [1,2]. However, those studies identified uncertainties related to lifecycle assessment (LCA) parameters and assumptions. There is a need to understand the inherent and simulated variability in key factors that impact the economic and environmental performance of prairie grass anaerobic digestion to optimize future RNG LCA studies.

This meta-analysis investigates the contributions of key LCA factors to the global warming potential estimates for prairie grass in AD feedstocks, providing a benchmark for researchers to validate their assumptions and findings. Furthermore, this study evaluates the use of large language models for the automated screening and gathering of LCA inventory data. We also apply well-established methods, such as PRISMA for article screening and funnel plots for publication bias assessment. The statistical analysis involves multi-parameter regression, correlation analysis, and random-effects models.

The keywords used for this meta-analysis paper are “grass,” “anaerobic digestion,” and “life cycle assessment.” Following PRISMA guidelines, we identified 255 papers through our search in Scopus, screening 255 studies from 2015 to 2024. The number of retrieved studies is 212. After applying the screening criteria, the number of studies was reduced to 127. The screening criteria excluded studies categorized as theses, book chapters, review papers, and duplicates. It also excluded studies lacking global warming potential (GWP) values or with GWP outside of the expected range of -900 to 300 kg CO₂/MJ.

The percent of studies evaluating prairie grass varied by digester type. Prairie grasses are common substrates for batch digestion studies (~40%) but less than 25% of studies using CSTR, Plug Flow, or other digester designs. This finding suggests that most prairie grass studies are at the lab-scale, and further scale-up testing would be recommended. Statistical analysis found that most herbaceous feedstock have GWPs between -102 and 151 kg CO2/MJ. Other factors explored include the type of digester, pretreatments, and Carbon to Nitrogen ratio.

[1] Wild, Katherine, Elmin Rahic, Lisa Schulte, and Mark Mba Wright. "Techno‐economic and environmental assessment of converting mixed prairie to renewable natural gas with co‐product hydroxycinnamic acid." Biofuels, Bioproducts and Biorefining 19, no. 2 (2025): 288-304.

[2] Olafasakin, O., Audia, E. M., Mba‐Wright, M., Tyndall, J. C., & Schulte, L. A. (2024). Techno‐economic and life cycle analysis of renewable natural gas derived from anaerobic digestion of grassy biomass: A US Corn Belt watershed case study. GCB Bioenergy, 16(6), e13164. https://doi.org/10.1111/gcbb.13164