Experimental studies on the supercritical water oxidation of municipal sludge and its reaction heat

The experiments of supercritical water oxidation (SCWO) of municipal sludge were conducted in a batch reactor. The experimental results show that the sludge COD reduction can be over 99.98%, and the COD reduction increases with increasing reaction time and temperature, whereas scarcely changes with pressure. As the excess oxidant ratio is higher than certain value, the effect of the oxidant ratio can be neglected. Applying SCWO can rapidly reduce the sludge volume and make it becomes harmless. After 95s SCWO treatment at 450°C, the mass fraction of remainder solid is about 8% of the dehydrated sludge and the COD of reaction liquid is less than 40mg · L^-1. Based on the enthalpy changes of the reaction mediums, the reaction heat was studied. The calculating equations determining the maximal water ratio and the minimal mass fraction of organic matters in the sludge for self-sustaining the energy balance of the SCWO process were derived. The results indicate that the reaction heat is 21319.15 kJ · kg^-1 when the organics are completely oxidized, and under 400°C and 26 MPa, the self-sustaining energy balance of the SCWO reaction can be realized as the mass fraction of organics in municipal sludge is over 3.0%.