Hydrothermal flames: From phenomenological experimental demonstrations to quantitative understanding

Investigations of hydrothermal flames were reviewed with three primary objectives: (1) to classify and describe experiments performed to date, (2) to assess the state of the art of experimental instruments and measurement techniques, and (3) to characterize current understanding and outline future research needs of hydrothermal flame processes. For this review, hydrothermal flames are defined as rapid oxidation reactions occurring in a supercritical aqueous environment at sufficient temperature (∼1000 °C and higher) and rate to produce a luminous flame. Under these conditions, chemical kinetics are very fast, with reactions going to completion within 10–100 ms. Hydrothermal flame experiments were divided into two categories: laminar inverse diffusion flames in semi-batch reactor systems and turbulent diffusion flames in continuous reactor systems. Laminar diffusion flame studies have demonstrated the range of temperatures, pressures, and fuel and oxidant types and concentrations from which hydrothermal flames can be produced, while turbulent diffusion flame studies have focused on the application of hydrothermal flames to the remediation of aqueous waste streams. Experimental apparatuses, instrumentation, and techniques used in hydrothermal flame investigations are presented in some detail. Models developed to describe hydrothermal flame processes, especially flow fields in turbulent diffusion flame studies, are presented as well.