Pressure-regulated mixed-culture methanation: A Power-to-Methane (PtM) system designed to dynamically accommodate fluctuating renewable hydrogen generation

The intermittent nature of renewable power sources poses challenges for the biological power-to-methane (PtM) process, especially in dealing with variable hydrogen feeds. This study showcases that the microbial culture driving the process lacks the speed to adjust to rapid changes in hydrogen feed, leading to product of variable quality. To address the challenge, a strategy is proposed and validated using the controlled pressurisation of the reactor's headspace to enhance gas diffusion and provide the culture with additional time to respond. Results indicate the effectiveness of this mechanism in managing short-term variations (hours) in the gas feed. Specifically, the study shows that a gas product with over 93 % CH4 v/v can be maintained during a 35-min increase in gas feeding rates, equivalent to 5 times the baseload feeding rate. Additionally, a gas product with over 92 % CH4 v/v can be sustained during a 15-min increase in gas feeding rates, equal to 10 times the baseload feeding rate. Accumulation of organic acids in the culture was observed and is attributed to non-methanogenic antagonistic species utilizing a portion of the H2/CO2 for acetate production. Acetate accumulation appears to be linked to excess gas feeding rates and not changes in pressure.