Waste-derived chars as methanation catalyst support: Role of inorganics in the char and its guide to catalyst design

Abstract

The municipal solid waste-derived biochar (MSWC) can be applied as a catalyst support for low-cost syngas methanation. However, the mechanism of the char support as a catalyst in methanation process remains unclear. Based on the similarity between the intrinsic inorganics present in the chars and the promoters used for methanation catalyst modification, the impacts of the inorganics in different waste-derived chars on the methanation reaction were investigated. The results showed that the acid-soluble inorganics in the MSWC significantly improved the methanation activity of the MSWC-supported Ni catalysts. The acid-soluble inorganics were proved to have the role of promoting the formation of suitable Ni/support interactions, improving the dispersion of the Ni species, and increasing the concentration of the basic sites on the catalyst’s surface. Compared with the MSWC, the biochar contained much more water-soluble alkali metal salts (up to 15–57% of ash), which may be detrimental to the methanation activity of the biochar-supported Ni catalysts by leading to increases in Ni particle size and the concentration of the strong basic sites on the catalyst’s surface, especially when existing alone. A new alkali–carbon index was created to evaluate the effect of the inorganics in the waste-derived char on the methanation activity of the char-supported catalyst. A higher alkali–carbon index reflects improved methanation activity. Thus, the findings of this study can provide quantitative guidance for designing suitable waste-derived char as methanation catalyst supports.