Mechanistic insights into sulfate reduction and suppression of SO2 accumulation in supercritical water gasification

Sulfate transformation during supercritical water gasification (SCWG) is important for understanding sulfur migration, gas purification, and corrosion control, but its molecular reaction mechanism remains unclear. In this study, density functional theory calculations and model sulfate transformation experiments, rather than complete organic-feedstock gasification tests, were combined to investigate the reduction pathways of SO 4 2- and HSO 4 - under gas-phase and supercritical-water conditions. H 2 and CH 4 were selected as representative reducing gases. The calculations show that sulfate reduction mainly proceeds through stepwise S–O bond cleavage toward reduced sulfur species such as HS - and H 2 S. In the gas phase, the consumption barrier of SO 3 2- is higher than its formation barrier, indicating the possible accumulation of sulfite-like intermediates. In contrast, the comparison with protonated sulfite-related pathways suggests that sulfite-derived intermediates can be further reduced more readily in the water-rich supercritical environment, thereby suppressing SO 3 2- accumulation and limiting SO 2 accumulation. CH 4 can participate in sulfate activation, but its reduction pathway is restricted by high reaction energy barriers and difficult HSO 3 - conversion, especially under supercritical-water conditions. Model sulfate transformation experiments supported the main mechanistic trends predicted by DFT: Na 2 SO 4 was extensively converted in the H 2 system, reduced sulfur species dominated the products, and no SO 2 was detected. A small amount of SO 3 2- appeared in the gas-phase H 2 experiment, whereas SO 3 2- was absent in baseline supercritical water without alkaline additives. These results provide a mechanistic explanation for the limited accumulation of SO 3 2- and SO 2 under SCW conditions.