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Fuel Production by Supercritical Water
wastes such as hexachloro-benzene; and many more [29]. Maximum gaseous hydrogen yield that can be obtained can vary to as high as 26.1 g H 2 /100 g dry feed for
ethanol and 42.9 g H 2 /100 g dry feed for polyethylene to as low as 13.7 g H 2 /100 g
dry feed for cornstarch. Some of the practical results obtained in SCW conditions
are described by Johanson et al. [30] and Hong and Spritzer [29]. One of the earliest
patents on processing methods for the oxidation of organics in SCW was published
by Modell [14].
In recent years, more efforts have been made to find (1) suitable catalysts to carry
out SCWO most efficiently, (2) novel reactor designs to obtain clean syngas through
oxidation, and (3) novel approaches to convert methane to methanol in economically
viable way under supercritical conditions. Numerous compounds such as alcohols, acetic acid, ammonia, benzene, benzoic acid, phenol, pyridine, quinolone, MEK (methylethyl ketone), and dichlorobenzene have been catalytically oxidized in SCW [15].
Special applications have been targeted to various aromatic and aliphatic organic
compounds, inorganic compounds, and various wastewaters and sludges. The most
notable catalysts used for these purposes are oxides of copper, zinc, vanadium, manganese, as well as noble metal such as platinum. Additional data are reported by Savage
et al. [26,53,54], Savage [11], Subramaniam and McHugh [9], Thomason et al. [59],
and Tester et al. [48]. Various mechanisms for oxidation reactions are outlined by Ding
et al. [15] and Savage [11].
A two-stage approach to SCWO has also been investigated. In the first stage,
contaminated waste is exposed to hydrothermal carbonization or liquefaction to
extract harmful substances (such as chlorinated and other toxic components) from
waste. Biocoal, biocrude, or biochar produced from this first stage then undergo
oxidation and reforming in SCW to decompose organic compounds and generate
syngas containing hydrogen, carbon dioxide, carbon monoxide, and may be some
lower hydrocarbons depending on the temperature of the gasification and the nature
of the catalyst. Some practical examples of multistage operations are outlined by
Brunner [10].
The most extensive and critical review of oxidation of methanol in SCW was
carried out by Vogel et al. [31]. This study is very important for treating aqueous effluents containing methanol by SCWO (an exothermic reaction) and for performing hydrothermal reforming under autothermal (i.e., in the presence of partial
oxidation) conditions. They critically evaluated all existing literature data and concluded that there are important differences in the reported kinetics of methanol
oxidation. The factors responsible for these differences are (1) the methanol feed
concentration, (2) insufficient reaction heat removal from tubular or coiled flow
reactors, and (3) inherent difference in apparent kinetics of autocatalytic reactions
in continuous stirred-tank reactor (CSTR) and in plug flow reactor (PFR) due to
recirculation of radicals (i.e., mixing effect) in a CSTR. The study indicated that
the best kinetic data for methanol SCWO cannot be recommended because of lack
of information on (1) induction time and (2) influence of wall catalysis on the
apparent reaction rate.
Watanabe et al. [41,42] showed that NaOH and ZrO 2 have catalytic effects for partial oxidation of n-hexadecane and lignin in SCW. The experiments were carried out