Hydrothermal carbonization coupled with steam gasification upcycles biomass-PVC waste into CO₂-suppressed syngas

Thermochemical upcycling is a promising waste-to-energy pathway, but it faces technical challenges, particularly the heteroatom content in plastics and the high ash content in biomass. This study introduces an integrated hydrothermal carbonization-steam gasification (HTC-SG) strategy for the synergistic valorization of cow manure and polyvinyl chloride (PVC). Systematic experiments revealed that HTC treatment effectively dechlorinated PVC and removed ∼40% of ash through pH-dependent mineral dissolution. Subsequent steam gasification of hydrochar generated H₂-rich syngas and significantly suppressed CO₂ (min. 5.78 vol%). The carbonaceous char exhibited enhanced structural properties, including elevated carbon content (∼45%) and well-developed mesoporous architectures, indicating potential as functional carbon materials. Mechanistic analysis elucidated that inherent ash components, especially alkali and alkaline earth metals, regulated both gas composition and byproduct evolution by catalyzing char gasification, water-gas shift reactions, and tar cracking. Overall, the HTC-SG system provides an effective approach for co-processing biomass-plastic mixtures while contributing to sustainable waste management and renewable energy production.