(178d) Hydrothermal Flames in a Novel Supercritical Water Oxidation (Scwo) Reactor

Supercritical Water Oxidation (SCWO) is a high-pressure high-temperature process for the disposal of toxic, hazardous or non-biodegradable aqueous waste streams. The process is carried out at conditions above the critical point of water (Tc=374°C, pc=221bar). As a result of the complete miscibility of organic compounds and gases in supercritical water, high reaction rates and conversions close to unity can be achieved. Furthermore, the lack of interfacial mass transfer resistances in such a single-phase mixture combined with high reaction temperatures, leads to short residence times and small reactor volumes. The problems of corrosion and plugging of the reaction vessels and components due the precipitation of salt particles in supercritical water are the major challenges in the realization of SCWO. Several approaches can be found in literature to overcome these problems. One of the promising reactor concepts involves a flame as internal heat source in the process. Experiments showed that SCWO can be carried out in the regime of a hydrothermal flame. A transpiring-wall reactor with a hydrothermal flame as internal heat source [1] was used for the investigation of degradation of salt containing artificial waste water streams with methanol and sodium sulfate mass fractions up to 6 wt.% and 3 wt.%, respectively. Further experiments focused exclusively on the hydrothermal flame. To gain insight into the complex phenomena a novel reactor with optical access to the full length of the flame was developed. The flame is produced by mixing a fuel (methanol-water mixture with methanol mass fractions up to 20 wt.%) and an oxygen stream in a geometrically simple wall-cooled coaxial burner. The ignition and extinction process and flame temperature was examined for a broad range of operating conditions. Fuel injection temperatures above 410°C typically led to ignition for fuel mixtures with 16 wt.% methanol and more. Maximal flame temperatures reached values of around 1300°C for fuel mixtures with 20 wt.% methanol. Position, size and shape of the flame were determined using optical diagnostics, e.g. laser-induced fluorescence (LIF). Computational fluid dynamics were also involved to gain deeper understanding of the interdependency between thermophysical properties, heat and mass transfer phenomena, and chemical reactions. Numerical results using an adapted, commercially available software with adjusted properties were in a good agreement with the experimental results and will be used for further improvement of SCWO flame reactor design.

References: [1] B. Wellig, K. Lieball, P. Rudolf von Rohr, Operating characteristics of a transpiring-wall SCWO reactor with a hydrothermal flame as internal heat source, J. Supercrit. Fluids 34 (1) (2005) 35-50.