Supercritical Water Oxidation (SCWO) – Observations of Hydrothermal Flames in a Co-Flow Constant Pressure Reactor

Under supercritical temperatures and pressures, water demonstrates profound changes in transport properties, dielectric properties, thermal and caloric state variables, and solvating properties. These attributes combine to create highly efficient oxidizing environments that allow for complete conversion of a wide range of wet organic waste streams. Supercritcal Water Oxidation (SCWO) technology, recognized as “green technology” because of its ability to extract useful energy while converting environmentally challenging waste streams into benign products, is receiving renewed attention for a variety of applications. Of particular interest to NASA is the fact that SCWO is especially well suited for conversion and resource recovery from wet organic wastes. Although SCWO reactors have traditionally been designed as low temperature systems, hydrothermal flames, if properly controlled, can be used to accelerate conversion rates.

Results reported in this study demonstrate the feasibility of spontaneously igniting and stabilizing hydrothermal flames in a SCWO reactor operating at constant pressure. Hydrothermal flames are observed as highly localized and luminous reaction zones occurring in supercritical water; i.e., water at conditions above its critical point (218 atm and 374 °C). A co-flow injector is used to inject fuel (inner flow), comprising an aqueous solution ranging from 10%‑vol to 50%‑vol ethanol, and air (annular flow) into a reactor filled with supercritical water at approximately 240 atm and 425 °C. Results show hydrothermal flames are auto‑ignited and quickly stabilize as either laminar or turbulent diffusion flames, depending on the injection velocities, buoyant forces and test cell conditions. Two orthogonal camera views are used; one providing a backlit shadowgraphic image of the co-flow jet and the other providing color images of the flame geometry. Depending on the fuel/air flow ratios, varying degrees of sooting are observed and are qualitatively compared using light absorption measurements from the backlit images.