Economic analysis and optimization of renewable natural gas production from prairie biomass

This study coupled continuous anaerobic digestion (AD) experiments with a techno‑economic model to evaluate renewable natural gas (RNG) production from co‑digesting prairie biomass with beef manure on farm. A Box-Behnken design quantified the effects of solids retention time (SRT), organic load (OL), prairie:manure ratio (P:M), and 50% liquid‑digestate recirculation on methane yield, digestate nutrient value as fertilizer, and residual methane potential of the liquid fraction. Experiment‑based regressions were used to drive process models for continuously stirred AD systems with one‑stage and two‑stage AD configurations, the latter including a covered lagoon as the second stage to recover residual methane from the liquid digestate. Longer SRTs increased methane yield, while recirculation shifted the optimal OL and P:M, while raising the maximum methane yield by 15-35%. Two‑stage AD increased total methane by 12-67% without recirculation and 1-33% with recirculation by capturing residual methane from liquid digestate. Minimum fuel selling price (MFSP) decreased with higher P:M and, depending on recirculation, with lower SRT. Two‑stage AD with 50% recirculation resulted in the lowest MFSP of $36.1/MMBTU. Sensitivity analysis identified biogas yield and capital cost as dominant levers, suggesting benefits from improved feedstock quality (e.g., harvest timing, winter annual co‑feeds) and lower‑cost reactors. Our results quantify process settings that move prairie‑based RNG toward viability and align bioenergy with conservation goals.