Influence of process parameters on the hydrothermal decomposition and oxidation of glucose in sub- and supercritical water

Supercritical water oxidation (SCWO) of wet waste biomass for energy recovery could be an advantageous alternative to conventional combustion with preceding drying. Therefore the reactions of glucose as a model substance for cellulosic biomass were investigated in sub- and supercritical water. The results of hydrothermal and oxidative experiments carried out in a continuous high-pressure plant with a feed solution of 0.2–1.2% (g g−1) glucose at 24–34 MPa, 250–480 °C and residence times of 2–35 s are presented. In the presence of a stoichiometric oxygen concentration (for total oxidation to carbon dioxide and water) glucose decomposes already at subcritical temperatures readily to carbon monoxide and low molecular liquid substances, chiefly organic acids like e.g. acetic acid and glycolic acid. In turn these are in general more stable and react only slowly with oxygen. The effect of temperature, residence time, pressure, reactant concentration and addition of zinc sulfate on the conversion and the yields of reaction products was demonstrated. Already at 350 °C (24 MPa and 30 s) 99% of the glucose are converted. With increasing temperature the production of CO2 increases. However, even at 480 °C (34 MPa and 4 s) significant amounts of CO are formed and the reaction of glucose to CO2 and H2O is not complete. Higher temperatures or greatly longer residence times are needed for a total combustion of the glucose.