H2-rich syngas production from biowastes: Steam gasification and supercritical water gasification

The growing demand for renewable energy and cost-effective waste management has intensified interest in converting biowaste into fuels and chemicals with added value. Among thermochemical conversion methods, steam gasification and supercritical water gasification (SCWG) stand out as promising technologies for producing hydrogen-rich synthesis gas (syngas) from a wide variety of feedstocks. This review compares these two approaches in detail, emphasizing reaction mechanisms, operating parameters, and the role of feedstock properties. Factors such as temperature, residence time, steam or water ratio, and catalyst choice are examined with respect to their influence on gas composition, hydrogen yield, carbon conversion, and tar reduction. Recent progress in catalyst development is also highlighted. In addition, findings from life cycle assessment (LCA) and techno-economic analysis (TEA) are synthesized to evaluate both environmental and economic feasibility. While steam gasification is particularly competitive for large-scale hydrogen and methanol production, SCWG is more effective for processing high-moisture feedstocks and offers stronger environmental benefits when integrated with carbon capture. The review concludes by identifying current challenges and outlining opportunities for advancing the commercial deployment of biowaste gasification technologies within the context of a circular bioeconomy and low-carbon hydrogen strategies. • Comparative evaluation of steam gasification and SCWG for hydrogen-rich syngas production from biowastes. • Steam gasification is technologically mature for dry biomass but limited by tar formation and energy-intensive drying. • SCWG converts wet biowastes directly with >95 % carbon conversion but faces high-T/P and long-term reactor safety challenges. • TEA and LCA indicate steam gasification is more economical, whereas SCWG offers greater sustainability with CO 2 capture. • Future priorities include catalyst stabilization, corrosion-resistant reactor materials, and hybrid system integration for scale-up.