Supercritical water oxidation of semi-coke wastewater: Effects of operating parameters, reaction mechanism and process enhancement

Semi-coke wastewater is a kind of industrial wastewater with complex composition, high concentration of organic pollutants and high chroma, seriously threatening the ecological environment and requiring to be effectively degraded. Supercritical water oxidation (SCWO), as for a promising environmental technology, was applied to treat semi-coke wastewater in this work. The influences of key operating parameters such as reaction temperature (400–600 °C), oxidation coefficient (1.0–4.0) and residence time (0.5–10 min), the reaction mechanism for organics in semi-coke wastewater and the process enhancement methods like catalytic oxidation and segmented oxidation were systematically investigated. Experimental results showed that the removal efficiency of COD and NH₃-N both significantly increased with the increasing of temperature, oxidation coefficient and residence time, the COD removal efficiency and NH₃-N removal efficiency could be 99.02% and 63.94% obtained under the condition of 600 °C, 25 MPa, 1.3 times oxidation coefficient and 10 min. The residual organics in liquid products were mainly phenols, ketones, imidazoles, esters and pyridines, which produced from the cyclization and esterification reaction between intermediate products such as alcohols, aldehydes, acids and NH₃-N, etc. What's more, NH₃-N was proved to have inhibitory effect on the degradation of phenol by generating more stubborn nitrogen-containing compounds with that. Besides, compared with single catalyst, the composite catalyst of MnO₂/CeO₂ exhibited the highest catalytic activity, which could synergistically degrade 98.52% COD and 67.18% NH₃-N under a relatively mild reaction condition (550 °C, 25 MPa, 1.3 times oxidation coefficient, 2 min). Moreover, the segmented oxidation, combining the pre-oxidation in preheater and oxidation in reactor, was firstly observed and analyzed here, could achieve a higher COD removal efficiency with a shorter length of the reactor. The results obtained in this paper proved the technical feasibility and could provide basic data support for the industrialization of semi-coke wastewater treatment by SCWO.