Supercritical water oxidation of coal: Investigation of operating parameters' effects, reaction kinetics and mechanism

Supercritical water oxidation (SCWO) of coal was conducted in a continuous tubular reactor under various reaction conditions. Our experimental results show that the removal rate of chemical oxygen demand (COD) had no significant dependence on the temperature variations. Effect of residence time was less significant as exceeded fixed values. Free radical mechanism of SCWO reaction may be a possible explanation for the relative low conversion rate of coal at the range of tested oxygen excess. Compared with other parameters, effect of pressure was less significant. A global power-law rate expression was regressed from experimental data. The reaction orders for coal slurry and oxidant were 1.79 and 0.28 respectively. The reaction activation energy E_a was determined to be 112.3 kJ mol^-1, and the pre-exponential factor k₀ was 412 (mol/L)^-1.07 s^-1. The deviation between the model and experimental data was within ± 9%. Free radical mechanism, oxidation and hydrolysis mechanisms and phenolic hydroxyl oxidation mechanism were considered to be the possible mechanisms for the SCWO process of coal.