On the selective desulphurization of biomass derivatives in supercritical water

The nature of organosulphur compounds formed during the reductive supercritical water treatment of sewage sludge, miscanthus and Spirulina was investigated by gas chromatography coupled to a sulphur chemiluminescence detector (GC-SCD). Methanethiol was found to be the dominating species after hydrothermal liquefaction and gasification of all process waters, along with its derivatives dimethyl disulphide and dimethyl trisulphide, likely formed upon quenching and/or after sampling. Various alkylthiophenes, short-chain sulphides and their corresponding disulphides were also found in varying amounts strongly depending on the biomass used. Various commercial sulphur absorbing materials and pure oxide nanoparticles were tested under supercritical water conditions to selectively desulphurize the various mixtures. Cu-based materials were found to be the most active in desulphurizing the organic mixture in hydrothermal conditions as well as absorbing sulphur. ZnO, MoO2, MnO and CeO2 were found to have an intermediate activity in decreasing organosulphur compounds concentration but had a good sulphur absorption capacity. Surprisingly, only Zn and Mn materials led to the formation of crystalline sulphide phases. Iron oxide showed intermediate desulphurization but surprisingly no sulphide absorption capabilities. All of these materials suffered various extents of sintering/coarsening under such harsh conditions, with cerium oxide showing very good structural and crystalline stability. All these results orientate further development of absorbing materials and/or catalysts for selective desulphurization of biomass under hydrothermal conditions.