Supercritical water oxidation (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, 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 a Green or Clean Technology but care should be taken because of high pressures and temperatures.

Commercial applications

The development of supercritical water oxidation is in its infancy. Several companies in the United States are trying to commercialize supercritical reactors to destroy hazardous wastes, but due to resource limitations they are unable to address the fundamental issues involved in the process. Chemical mechanisms and kinetics at high pressure, the effects of halogens, mixing of the fuel and oxidizer, mass transport, heat transfer, and the existence and structure of supercritical flames are only a few of the phenomena that need to be clarified.

Useful links

There are some research groups working in this topic through the world:

• High Pressure Processes Group (Spain) (http://www.iq.uva.es/hpp/linea1/linea1.php)
• Clean Technology Group (UK) (http://www.nottingham.ac.uk/supercritical)