The nature of carbon deposits and their formation pathways during catalytic supercritical water gasification of glycerol

Supercritical water gasification (SCWG) is a promising technology to convert wet biomass to renewable natural gas. The present study sheds light on the chemical composition and the formation mechanism of coke deposits on a Ru/CNF catalyst during continuous SCWG of glycerol in a fixed-bed reactor. SCWG experiments were performed using pristine carbon nanofibers (CNF) and 5 wt% Ru/CNF catalysts, which suffered from partial deactivation. Transmission electron microscopy (TEM) indicated a 34-42 % dispersion loss independently of the conditions, only partially explaining the activity loss observed during the test at very high space velocity. When operated under thermodynamic conditions, no deposits could be observed. A nanometric layer of deposits on the catalysts was present under intermediate conditions, while in the kinetic regime (conversion of glycerol down to 4 %), significant carbon deposition was observed. The chemical compositions of aqueous phases and organic deposits extracted from the catalysts and support showed that the presence of Ru enhanced the formation of unsaturated compounds from glycerol. In particular, molecules containing aromatic rings were detected. The space velocity significantly affected the composition of the coke deposits. The number of carbons in the molecules increased from 5-25 to 10-45 when moving away from the thermodynamic regime (full conversion), and the O/C ratio significantly decreased from 0.3-0.6 to 0.05-0.3, maintaining a very broad H/C ratio. The extracted organic deposits contained various aromatic compounds but were mostly represented by unsaturated aliphatic compounds. All these results were used to elucidate a reaction pathway for the formation of organic deposits from glycerol.