Biological Syngas Methanation via Immobilized Methanogenic Archaea on Biochar

Abstract

Abstract Syngas containing H 2 , CO, CO 2 and CH 4 produced by thermal processes such as gasification or pyrolysis is typically converted to methane via thermochemical methanation. This process is characterized by a high heat demand utilizing a sensitive chemical catalyst at increased pressure conditions. Alternatively, methanogenic archaea could be exploited as a natural catalyst in a biological methanation process with a lower energy demand. However, the mass transfer between the gas phase and the microbial cell is a major challenge for efficient conversion of the syngas components. Therefore, in this work methanogenic archaea from anaerobic digestion residues were successfully immobilized on biochar particles obtained from green waste pyrolysis with two distinct particle sizes (0.25-1 mm and 1-2 mm). After incubation of the inoculated particles with an artificial syngas mixture CH 4 was formed within the first 24 hours, while H 2 , CO 2 and CO simultaneously declined. However, the particle size had no influence on the CH 4 yield, content and conversion efficiency. According to the maximum theoretical conversion rate of H 2 with CO 2 and CO to CH 4 only about 50% of the syngas components were converted to methane. These results suggest that CO was rather utilized by the methanogens involved for acetate/formate formation than for methanogenesis due to slight inhibition of the latter process by CO present in the syngas. The impact of CO inhibition during biological syngas methanation needs to be further evaluated for a continuous application of the process. However, a proof of concept for this process using inoculated biochar particles could be shown within the study presented here.