Catalytic supercritical water gasification: Interaction of sulfur with ZnO and the ruthenium catalyst

•Good sulfur adsorption performance of a commercial ZnO adsorbent.•The commercial ZnO adsorbent exhibited a high mechanical stability.•Sulfur reacts with ZnO to form ZnS under supercritical water conditions.•The deactivation causes of the catalyst are investigated.

Continuous catalytic supercritical water gasification (CSCWG; 400 °C, 28 MPa) of microalgal biomass (Chlorella vulgaris) was carried out at the microalgae production site of ZHAW in Wädenswil (Switzerland) non-stop over a period of 100 h. Characterization of the spent catalyst showed that mainly sulfur poisoning, and to a lesser extent coking, salt deposits, and some sintering of the Ru nanoparticles were responsible for the deactivation of the catalyst after 55 h of time on-stream. The commercial zinc oxide adsorbent exhibited a high mechanical stability and good sulfur adsorption performance under supercritical water conditions although its specific surface area collapsed. In summary, the use of a zinc oxide adsorbent upstream of the catalyst bed, together with a higher ruthenium loading of the catalyst, improved the long-term performance of the CSCWG process significantly.

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