Behavior of sulfur under hydrothermal conditions: removal strategies for catalytic processes

Catalytic hydrothermal gasification (cHTG) (T > 374 °C, P > 22.1 MPa) converts wet biomass into renewable methane. As sulfur compounds in biomass can deactivate metal catalysts, desulfurization of the cHTG stream is crucial. Sulfides, sulfates, and organosulfur compounds (OSC) are the sulfur species that contribute to catalyst deactivation via sulfidation. Researchers have optimized cHTG with salt separator and sulfur trap to remove sulfates and organosulfur compounds (OSC), respectively, but some still enter the catalytic reactor causing deactivation. To further improve the desulfurization and to limit costs linked to the use of sulfur trap, this study explores two approaches. The first one is thermochemical sulfate reduction (TSR), aiming at converting sulfate fully into sulfide to remove sulfur by upstream absorption, but it remains unexplored under cHTG conditions. In this work, the extent to which TSR occurs under these conditions was investigated. Results showed that sulfate reduction by glycerol started forming volatile OSC only above 440 °C within 60 min with no H2S produced. Ni- and Mo-based materials were used as potential catalysts, but no catalytic effect was found despite their excellent redox activity and sulfur compatibility. TSR occurs under cHTG conditions slowly under cHTG conditions. The escaped sulfate still poses a threat for the catalyst. However, a reaction pathway for the formation of a key volatile OSC (thiophene) was unraveled which should trigger further research to promote OSC removal. The second approach is oxidative desulfurization (ODS), targeting OSC oxidation to sulfate for salt separation and reducing the demand of sulfur trap. Typical ODS requires milder conditions than cHTG. To assess the viability of ODS in cHTG with H2O2, the effects of temperature and O/S ratio on sulfate yield were examined. Results showed minimal effect of temperature (50 to 400 °C) and O/S ratio (0 to 116). Mo- and W-based carbon materials were used as potential catalysts. Carbon support (CNF) alone tripled the sulfate yield to 2.3 % at 400 °C, due to its oxygen-containing surface functional groups. Surface oxidation by acid treatment further promoted the effect of CNF on sulfate yield (to 7 %). Additionally, Mo (IV) was inferred to be an active phase for ODS, evidenced by the significantly enhanced sulfate yield (to 12 %) over MoO2 while no effect was observed from MoO3 and WO3. However, loading metal oxides on CNF altered the surface properties of the carbon support, which might be the cause for its reduced promoting effect. These results should serve as foundation for further development of Mo (IV) based carbon material, with the aim to minimize the change of carbon surface species. In ODS under cHTG conditions, thiols were the most sensitive to oxidation with the following sulfate yield order at 400 °C and 25 MPa: 1-pentanethiol (29 %) > benzothiophene (14 %) > DMDS (9 %) > thiophene (0.6 %). The low sulfate yield from thiophene is du