Thermal degradation and kinetic analysis of organic constituents in coal-gasification wastewater with a novel treatment

The traditional treatment of coal-gasification wastewater produces high solvent and operation cost, secondary pollution and long processing cycle. The aim of the paper was to attempt an alternative approach of wastewater treatment in coal gasification process. With wastewater being heated and sprayed into the gasifier, water participates in the water–gas shift reaction; meanwhile, organic constituents in wastewater are thermally degraded in specific conditions. In the study, thermal degradation and kinetic analysis of COD and NH3-N from Lurgi coal-gasification wastewater were conducted experimentally. The results showed that COD degradation can be divided into three reaction regions: 200–600, 600–1000 and 1000–1200°C. Also, NH3-N degradation can be divided as 200–400, 400–800 and 800–1200°C. The reaction temperature, oxygen concentration and reaction residence time can improve organic constituents’ degradation rate. The COD and NH3-N degradation rate ranks in the order oxidative > inert > reductive. It is because increasing oxygen concentration indicates more free radical generation and aromatic hydrocarbon polymerization was weakened. In addition, NO conversion with NH3 occurs within a narrow temperature window (800–1000°C). Thus, NO concentration reached the peak 230 mg/m3 at 800°C and then reduced with the increase in reaction temperature. Furthermore, a pseudo-first-order reaction model was implemented to analyse the kinetics of COD and NH3-N degradation rate. The results of the present study indicate that the proposed wastewater treatment is feasible and can be preferable reference for further practical application.