Comparison of chars from municipal solid waste and wheat straw for understanding the role of inorganics in char-based catalysts during volatile reforming process

Highlights: • Role of inorganics in different speciation in char-based catalysts explored. • MSW char with more inorganics showed better catalytic performance than porous WS char. • Acid-soluble inorganics hinder microcrystallization and enrich functional groups on the surface. • Migration of organically bound and water-soluble AAEMs led to larger aromatic ring size. Chars from wastes/biomass pyrolysis are widely used as catalysts. But their catalytic mechanism is not clear enough. To explore the role of inorganics in char-based catalysts, in this research, chars from municipal solid wastes (MSW) and wheat straw (WS) were compared for their catalytic performances and structure changes in reforming volatiles from MSW and WS pyrolysis. Results showed that MSW char (ash: 43 wt%) was more active than WS char (ash: 16 wt%) in reforming both volatiles within 500–700 °C, which facilitated higher syngas yield. 74% of the metals existed as non-evaporable acid-soluble inorganics in MSW char, which favored the transformation of microcrystalline carbon into amorphous carbon by carbon gasification yet increased aromaticity of its carbon matrix; while 27% of metals in WS char were organically bound alkali and alkaline-earth metals (AAEMs), and aromaticity of its carbon lattice increased from 0.39 to 0.67 with increase in reforming temperature. Additionally, WS char was characterized with larger aromatic ring size after reforming temperatures. Rich acid-soluble inorganics in MSW char were found to be favorable to its high activity; while migration of AAEMs caused the lower activity of WS char. These findings will help to improve catalytic activity of char-based catalysts.