Oxidation of Carbon Particles in Supercritical Water:  Rate and Mechanism

Oxidation of a carbon particle in supercritical water was studied in a flow-type reaction cell by directly observing the change in the size and shape of the particle through a sapphire window attached to the cell. Three types of carbon were investigated: activated carbon, synthetic graphite, and highly oriented pyrolytic graphite (HOPG). The reaction was observed at a pressure of 23−30 MPa and a temperature of 673−873 K. The spherical particles (3−4 mm in diameter) of both the activated carbon and the synthetic graphite became smaller while maintaining their shape, indicating that the reaction occurred nearly at the surface of the sphere. The reaction was first order with respect to O2 concentration. The rate of the decrease in the radius was in the order of μm s-1 for the activated carbon, and the rate was about an order of magnitude slower for the synthetic graphite. The rate of O2 mass transfer was estimated by computational fluid dynamics (CFD) calculations, showing good agreement with the reaction rate of the activated carbon. Calculated surface temperature of the particle was higher than the fluid temperature by 200 K at its highest due to the heat of the surface reaction, C+O2 → CO2. This temperature difference caused density distribution in the fluid, and led to significant natural convection above the particle. The observed shadowgraph around the particle showed a texture corresponding to the above calculated flow field. This phenomenon was also taken into account in the CFD calculation of O2 mass transfer rate. Thus, the external mass transfer of O2 to the particle surface limited the reaction rate of the activated carbon. On the other hand, the reaction rate of the synthetic graphite was limited by the surface reaction. Oxidation rate of HOPG was so slow that no reduction in size or weight was observed. The activation energy for the oxidation rate of the synthetic graphite under surface-reaction-limited kinetics, 127 ± 10 kJ mol-1, was smaller than the values reported for the graphite combustion rates at ordinary pressures. The discrepancy can be partly due to characteristic reactions in supercritical water oxidation.