Ignition of hydrothermal flames

Abstract

Supercritical water oxidation is one of the most promising technologies for complete oxidation of complex organic compounds. Flames in supercritical water, often referred to as hydrothermal flames, improve the oxidation rates of reactants in an organic waste stream. The ignition and control of flames in supercritical water could potentially be used to reduce the reaction time (from seconds to milliseconds) and enhance the thermochemical decomposition rates of recalcitrant molecules without the release of any harmful intermediates. This provides a platform to design compact reactors for processing complex organic waste followed by their conversion to valuable compounds. This paper reviews some notable work focused on the ignition and qualitative observations of hydrothermal flames as an environmentally friendly technology. More specifically, the review highlights the classification and characterization of hydrothermal flames with several demonstrations of laboratory scale (e.g., visual flame cell) and pilot scale (e.g., transpiring wall reactor) reactor configurations. The process parameters such as feed concentration, reaction temperature, oxidant temperature, oxidant flow rate, and transpiration flow properties (in the case of transpiring reactors) are comprehensively discussed for their influence on the ignition and stability of hydrothermal flames, and total organic carbon removal. In addition, the impact of these parameters on the performance of various flame reactors is presented. Finally, the paper also outlines some wide-ranging applications and challenges concerning the industrial utilization of hydrothermal flames.