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Savage [11] also presented an excellent review of some other organic reactions
in SCW. These reactions include decomposition of complex materials, individual
hydrocarbons, nitrogen-containing compounds, sulfur-containing compounds,
oxygen-containing compounds, compounds with two heteroatoms, and chlorineand fluorine-containing compounds. He noted that the rates and selectivity of these
and other reactions can be manipulated by judicious selection of temperature, pH,
catalyst, and water density; one can thus control the functional group transformation
in SCW.
10.4 OXidatiOn in sCW
Catalytic oxidation that has been used for many wastewater treatment, wet air
oxidation, and photolysis is now being used for oxidation of organic compounds in
SCW medium [12,14,15,29–72]. As mentioned earlier, water in supercritical conditions behaves like many organic solvents, and it is miscible with these solvents. Thus,
SCW provides a homogeneous, benign, and nontoxic environment for many organic
reactions in the presence or absence of a catalyst.
Oxidation in SCW (SCWO) is a rapidly developing technology for the
destruction of organic wastes [34–39]. Hazardous organic pollutants can be
destroyed by SCWO at temperatures around 500°C in less than 1 min [34–47].
The world’s first commercial SCWO facility for treating industrial wastewater
became operational in 1995 [48,49]. In order to increase process capacity and
handle more stubborn refractory compounds and condensation products with
an ease, catalytic oxidation in supercritical conditions has become more important. The SCW allows maximum concentration driving forces for the reaction
because there are no interfacial mass or heat-transfer resistances to hinder the
reaction rate.
In 2000, General Atomics was selected by DOE’s hydrogen program to carry
out SCW partial oxidation (SWPO) of biomass, municipal solid waste (MSW), and
high sulfur coal to generate hydrogen. SWPO carries out oxidative reactions in the
SCW environment akin to high-pressure steam in the presence of substoichiometric
oxygen or air. SWPO forms more hydrogen and less char and tar than the similar
operation in the subcritical conditions. Furthermore, SWPO eliminates the formations of particulates NO x , SO x , and hazardous air pollutants. High-density aqueous
environment is also ideal for reacting and gasifying organics. The high density also
allows utilization of compact equipment that minimizes capital cost and the plant
footprint requirements.
SCW has density one-tenth of the liquid water and solubility behavior that of
high-pressure steam, hydrogen bonding in SCW is totally disrupted, and polarity
and many thermal properties are such that they facilitate mass and heat-transfer
operations along with many different types of chemical reactions. The effectiveness of SCWO has been demonstrated at the laboratory and pilot scale on hundreds
of feedstock, which include sewage sludge; coal slurry; pig manure; various biomass slurries including pulp mill sludge, pulverized wood with ground plastic, rubber, and charcoal; fermentation waste; ground cereal; highly refractory hazardous