Clean conversion of pig manure via supercritical water gasification: Hydrogen-enriched syngas generation, mechanisms analysis, and environmental impacts

The energy potential of pig manure (PM) has garnered attention. Supercritical water gasification (SCWG) is proposed to convert PM to H₂-enriched syngas without the drying process for wet materials. This paper studied the effect of diverse parameters on H₂-enriched syngas production. The results revealed that higher temperatures and alkaline catalysts elevated total gas yield and H₂ yield, with the addition of K₂CO₃ at 620 °C, 30 min, and 6 wt% feed concentration generating the highest total gas yield (36.55 mol/kg), H₂ yield (18.89 mol/kg), and mole fraction (51.55%), accompanied by a high gasification efficiency (87.59%). Based on product characterization and thermodynamic analysis, reaction mechanisms were proposed to provide insight into enhancing syngas production, indicating that the suppression of polymerization reactions between cyclic compounds and the promotion of CH₄, C₂H₆, and long-chain alkanes conversion were essential. Further, the environmental impact evaluation showed that heavy metals (Cu, Cr, As, and Zn) were concentrated in solid products by adsorption and passivation. The solid products exhibited negligible environmental hazards. This work showed that PM SCWG realized syngas production as well as heavy metals enrichment and stabilization, which could provide an alternative solution to the clean conversion of other types of organic wastes, especially with high heavy metals content, such as oily sludge and sewage sludge.