Water enters a special condition, or 'fourth phase', in addition to the familiar solid, liquid and gaseous phases, when its temperature and pressure are above 374°C and 221 bar. In this region, its properties change, density being less than that of the liquid, viscosity the same as the gas and diffusivity about mid-way between the liquid and the gas. Most importantly, the solubility of gases and organic compounds are increased to almost 100 per cent while inorganic compounds become almost insoluble.
These special properties have been known theoretically for a long time and in the 1970s and 1980s work began on exploiting them for practical applications. An oxygen supply was introduced and the process is now know as Supercritical Water Oxidation (SCWO). With organic molecules and the added oxygen fully dissolved, a uniform homogeneous mixture is created and reactions can proceed at the intrinsic rate, i.e., the theoretically maximum rate for chemical reactions. Consequently, residence time in the reactor is only about one minute. Despite this brief residence, the method achieves a 99.99 percent destruction of the organic contaminants.
Incineration is also an oxidation process but suffers from several disadvantages, one of which is incomplete combustion and the consequent need to scrub the stack gases to rid them of environmentally dangerous compounds such as dioxins. SCWO results in complete destruction irrespective of the feed entering the process, even PCBs. Due to the lower reaction temperatures, the harmful oxides of nitrogen are not formed; these are toxic and acidic and can cause eutrophication (algal blooms) in fresh water. Traces of nitrous oxide (laughing gas, N2O) may be formed if the feed contains a nitrogen source. However N2O is easily separated into nitrogen and oxygen if required.
Unlike incineration, SCWO is a totally enclosed process and the reaction products are discharged at very close to atmospheric pressures and temperatures. Furthermore, they are benign, consisting mainly of CO2, water and nitrogen. These substances do not need expensive scrubbing to make them suitable for discharge to the environment. Organic and inorganic halogens are converted to the corresponding acids and organic and inorganic sulphur are converted to sulphuric acid. These are far easier to deal with in liquid form than as gases like sulphur dioxide which causes acid rain. Heavy metals are oxidised to their highest oxidation state and are separated together with any inert materials as a fine, non-leachable ash which can be used much like power station ash for landscaping, aggregates and similar applications.
Both SCWO and incineration are auto-thermal, or self-sustaining, once the process is up to operating temperature. However, they both need sufficient organic material in the feed stock or external heating must be applied; incineration needs well in excess of 25 percent before becoming auto thermal whereas SCWO can be auto thermal with only 3-4 percent of organic material in the feed. The SCWO process can therefore show considerable primary fuel savings when compared with incineration.