Supercritical water oxidation or SCWO is a process that occurs in water at temperatures and pressures above a mixture's thermodynamic critical point. Under these conditions water becomes a fluid with unique properties that can be used to advantage in the destruction of hazardous wastes such as PCBs. The fluid has a density between that of water vapor and liquid at standard conditions, and exhibits high gas-like diffusion rates along with high liquid-like collision rates. In addition, the behavior of water as a solvent is altered (in comparison to that of subcritical liquid water) - it behaves much less like a polar solvent. As a result, the solubility behavior is "reversed" so that chlorinated hydrocarbons become soluble in the water, allowing single-phase reaction of aqueous waste with a dissolved oxidizer. The reversed solubility also causes salts to precipitate out of solution, meaning they can be treated using conventional methods for solid-waste residuals. Efficient oxidation reactions occur at low temperature (400-650 °C) with reduced NOx production.
SCWO can be classified as green chemistry or as a Clean Technology. The elevated pressures and temperatures required for SCWO are routinely encountered in industrial applications such as petroleum refining and chemical synthesis.
A unique addition to the world of supercritical water (SCW) oxidation is generating high-pressure flames inside the SCW medium. The pioneer works on high-pressure Supercritical Water Flames were carried out by professor EU Franck at the German University of Karlsruhe in the late 80s. The works were mainly aimed at anticipating conditions which would cause spontaneous generation of non-desirable flames in the flameless SCW oxidation process. These flames would cause instabilities to the system and its components. ETH Zurich pursued the investigation of hydrothermal flames in continuously operated reactors. The rising needs for waste treatment and destruction methods motivated a Japanese Group in the Ebara Corporation to explore SCW flames as an environmental tool. Research on hydrothermal flames has also begun at NASA Glenn Research Center in Cleveland, Ohio.
Commercial applications
Several companies in the United States are working to commercialize supercritical reactors to destroy hazardous wastes. Widespread commercial application of SCWO technology requires a reactor design capable of resisting fouling and corrosion under supercritical conditions.
In Japan a number of commercial SCWO applications exist, among them one unit for treatment of halogenated waste built by Organo. In Korea two commercial size units have been built by Hanwha.
In Europe, Chematur Engineering AB of Sweden commercialized the SCWO technology for treatment of spent chemical catalysts to recover the precious metal, the AquaCat process. The unit has been built for Johnson Matthey in the UK. It is the only commercial SCWO unit in Europe and with its capacity of 3000 l/h it is the largest SCWO unit in the world. Chematur's Super Critical Fluids technology was acquired by SCFI Group (Cork, Ireland) who are actively commercializing the Aqua Critox SCWO process for treatment of sludge, e.g. de-inking sludge and sewage sludge. Many long duration trials on these applications have been made and thanks to the high destruction efficiency of 99.9%+ the solid residue after the SCWO process is well suited for recycling – in the case of de-inking sludge as paper filler and in the case of sewage sludge as phosphorus and coagulant. SCFI Group operate a 250 l/h Aqua Critox demonstration plant in Cork, Ireland.
Turbosystems Engineering (California, USA) is actively commercializing their patented transpiring wall SCWO reactor ("TWR") with a focus on renewable energy applications. The TWR has been demonstrated to provide superior resistance to fouling and corrosion. The TWR is capable of operating at reaction temperatures in excess of 800 C, at pressures above or below the critical pressure of water, and has been demonstrated at a feedrate of 20 l/h.
Recently, most supercritical fluid technology has been confined to the laboratory since it is expensive and usable only on a small scale. This technology will enable commercial applications through reduction of the operation pressure, and subsequent drastic reduction in cost of the production equipment.