Supercritical water co-oxidation of swine manure with methane and lignite in the flow mode

Abstract

This paper presents the results of a comparative study of supercritical water (SCW) co-oxidation of swine manure (SM) with methane and lignite continuously fed into a flow reactor. The tests were carried out at a temperature gradient along the vertical axis of the reactor (390–600 °C), a pressure of 25 MPa, varying reactant flow rates and oxidation ratio (OR). An analysis of the time dependence of the reactor wall temperature and the power of the resistive heaters was carried out and has shown that, compared to the SCW oxidation of SM, its co-oxidation with methane and lignite led to a 1.5 and 1.8-fold reduction in the power of the heaters, respectively. The gas products were found to be CO₂, N₂, and traces of N₂O. Saturated carboxylic acids C₅–C₁₆, N-bearing aromatics, phenols, chlorophenols, and polycyclic aromatic hydrocarbons (PAHs) were identified in the aqueous effluent by gas chromatography-mass spectrometry. The levels of phenols, chlorophenols, and PAHs in the effluent decreased with increasing OR and with the addition of NaOH. At OR ≥ 1.14, their contents were below the maximum permissible concentrations of these compounds in wastewater. Furthermore, the co-oxidation of SM and lignite increases the content of carboxylic acids, nitrate and sulfate ions in the effluent. The formation of mineral acids during the SCW oxidation of SM intensifies the corrosion of structural materials, resulting in increased contents of chromium, copper, and nickel in the effluent. The paper discusses the transformation mechanisms of organic and mineral components of SM and lignite, as well as ecological aspects of contaminants migration and the efficiency of their removal by SCW oxidation in the SCW/O₂ fluid.