Limit of hydrogen addition for enhanced methane concentration and production during syngas biomethanation in thermophilic trickle-bed reactor

Syngas biomethanation is an emerging technology that converts synthesis gas, primarily composed of hydrogen (H 2 ), carbon monoxide (CO), and carbon dioxide (CO 2 ), into methane (CH 4 ) through microbial activity. In this study, the effect of changing syngas composition with increased H 2 shares on CH 4 concentration and production was assessed for 125 days, using a thermophilic trickle-bed reactor (5 L). With the experimental upper limit of 71% H 2 (14% CO, 10% CO 2 , 5% N 2 ) in the syngas, the maximum CH 4 concentration was 65%, maintaining high methane evolution rates (4 L/(L pbv ·d)) and high H 2 and CO conversion rates (>95%). Targeted sulfur supplementation (Na 2 S) did not improve H 2 and CO conversion or CH 4 productivity, indicating that sulfur was no limiting factor under digestate-based operation. Reactor performance was instead constrained by system-level factors, including low gas retention time, gas–liquid mass transfer limitations, and inhibition of CO-converting pathways at elevated H 2 partial pressure. 16S rRNA gene sequencing revealed a highly stable microbial community dominated by the hydrogenotrophic methanogen Methanothermobacter . CO conversion occurred via direct methanogenesis and acetate formation, followed by syntrophic acetate oxidation. Overall, increasing H 2 availability enhanced CH 4 production only up to a system-specific threshold, beyond which microbial and transport limitations dominated. • Addition of H 2 increased CH 4 concentrations in the product gas from 30 to 65%. • High methane evolution rate of 4 L/(L pbv ·d) was achieved. • The experimental maximum share of H 2 in the syngas reached 71%. • S supplementation did not improve syngas biomethanation. • High H 2 partial pressure likely reduced acetogenic CO conversion.