Electromethanogenesis for the conversion of hydrothermal carbonization exhaust gases into methane.

Hydrothermal carbonization (HTC) is a biomass conversion process that generates a CO 2 -rich gaseous phase that is commonly released directly into the atmosphere. Microbial electromethanogeneis (EM) can potentially use this off-gas to convert the residual CO 2 into CH 4, thus avoiding GHG emissions while adding extra value to the overall bioprocess. In the present work, the HTC gas phase was fed to two mixed-culture biocathodes (replicates) polarized at −1.0V vs. Ag/AgCl. Compared to pure CO 2 , HTC gas had a marked negative effect on the process, decreasing current density by 61%, while maximum CH₄ yield contracted up to 50%. HTC also had an unequal impact on the cathodic microbial communities, with the methanogenic hydrogenotrophic archaea Methanobacteriaceae experiencing the largest decline. Despite that, the present study demonstrates that HTC can be used in EM as a raw material to produce a biogas with a methane content of up to 70%. • Electromethanogenesis can convert hydrothermal carbonization off-gas into methane. • HTC off-gas had a marked negative effect on the process in comparison to pure CO 2. • qPCR analyses revealed a larger impact on methanogenic communities. • The presence of CO in the HTC off-gas would explain the poorer performance. [ABSTRACT FROM AUTHOR]