Producing methane from dry municipal solid wastes: A complete roadmap and the influence of char catalyst

The lack of standardized products is a key hindrance to the widespread commercialization of municipal solid wastes (MSW) pyrolysis technology. In this paper, a cost-effective roadmap employing pyro-gasification and methanation processes is developed, with CH4-rich gas as the target product. In this roadmap, a two-stage catalytic conversion of “MSW → syngas → CH4-rich gas” was realized using MSW char, a by-product of the pyrolysis process as the catalyst/catalyst carrier. The results showed that, in the pyro-gasification stage, a syngas yield of 1.3 L/gMSW could be obtained at 800 °C and a steam-to-MSW mass ratio of 0.4, with a H2 fraction reaching 56.6 vol%. The effective conversion of MSW to syngas was attributed to the superior catalytic activity of the MSW char. By comparing the catalytic activity and structural characteristics of the chars produced from various waste components, it could be determined that the high catalytic activity of the char required both active minerals derived from contaminated plastics and residue, and an appropriate proportion of carbon matrix from the biomass components in MSW. The gasified MSW char (G-MSWC), co-produced with the syngas, served as a carrier for the subsequent methanation catalyst. Due to the activation effect of steam during the syngas production stage, the pore structure of G-MSWC was improved and its defect density also increased. Consequently, G-MSWC demonstrated enhanced Ni–C interaction, facilitating the methanation activity. The final product from the methanation stage achieved a CH4 concentration of 52.9 vol%, and a CH4 yield of 11.25 mol/kgMSW. The above findings demonstrate the feasibility of the complete roadmap from MSW to CH4 and provide valuable guidance for decentralized MSW management based on pyrolysis technology.