Effect of alkali additives on desulfurization of syngas during supercritical water gasification of sewage sludge

Amount of H₂S and SO₂ were generated during the SCWG of sewage sludge. It is essential to reduce the sulfur concentration in syngas for depth utilization of syngas from SCWG of sewage sludge. The syngas desulfurization ability of five additives (KOH, K₂CO₃, NaOH, Na₂CO₃, AC) were tested and the result indicated that K₂CO₃ had the best syngas desulfurization effect while KOH could significantly promote the yield of syngas at 450 °C and 4% loading. Increasing KOH and K₂CO₃ loading to 12% could reduce around 90% of sulfur in syngas comparing to no additives. The XPS analysis results indicated that alkali additives promoted the cyclization and oxidation of unstable sulfur compounds in raw sludge, which can convert it into stable sulfur compounds such as thiophene, sulfone and sulfate. The sulfur in liquid was mainly in the forms of sulfate, and the effect of alkali and AC additive on sulfur in liquid was relatively weak.