A comprehensive review on degradation pathways, catalyst screening and machine learning in supercritical water gasification from biomass for sustainable hydrogen production

Supercritical water gasification (SCWG) efficiently converts biomass into hydrogen-rich syngas. This review analyzes biomass degradation pathways, process parameters, reactor configurations, and catalytic advancements to enhance hydrogen selectivity and carbon gasification efficiency. Biomass components (cellulose, hemicellulose, lignin, lipids, proteins) undergo hydrolysis, decarboxylation, dehydration, and reforming to produce H₂, CO, CO₂, and CH₄. Key operational parameters (temperature, pressure, feed concentration, residence time) are evaluated to optimize syngas yield and minimize energy use. Reactor designs (batch, fluidized bed, continuous systems) are compared for scalability and efficiency. Catalytic enhancements involve homogeneous catalysts (KOH, Na₂CO₃) and heterogeneous catalysts (Ni, Co, Fe, Ru, Pt), focusing on synthesis methods to improve catalyst performance. Catalysts influence Water Gas Shift (WGS), methanation, steam reforming, and bond cleavage under supercritical conditions. Na and K promote biomass decomposition via formate intermediates; Ca enhances CO₂ capture and reforming; Ni facilitates H₂-selective bond activation; Ru aids organic breakdown; Fe, Ce, Cu, and Co support gasification. Furthermore, application strategies that affect dispersion, scalability, and regeneration potential. Alongside regeneration techniques like oxidative purging, steam stripping, solvent washing, calcination, and in situ rejuvenation, deactivation mechanisms are covered. Additionally, advancements in machine learning (ML) applied to SCWG include models like artificial neural networks (ANNs), support vector machines (SVMs), Random Forests (RF), and Gradient Boosting Regressors (GBR), successfully predicting hydrogen yield, syngas composition, and optimizing process parameters. Finally, end-product and waste stream management are addressed, covering gaseous, liquid, and solid residues, with effective mitigation strategies outlined for each stream.