269
Fuel Production by Supercritical Water
the liquid productions are coal, polymeric materials, rubber tires, cellulose among
others, or mixtures of them.
The extraction of coals with SCW is a promising route for the production of liquid
fuels and chemical feedstock from coal. Deshpande et al. [73] obtained high conversion for extraction of a German Brown coal and a Bruceton bituminous coal by SCW
at 375°C and 23 MPa. They reported conversions of 70%–79% for the brown coal
and about 58% for the bituminous coal. Pauliatis et al. [74] reported 35% conversion
and only 10% liquid yield for North Dakota lignite at 400°C and 28 MPa pressure.
Deshpande et al. [73] also obtained low liquid yield with lignite coal with high sodium
content. Other studies report low conversion for bituminous coal, particularly when
solvent density is low [75–80,82–84] (Swanson et al., 2012, pers. comm.). Kershaw
and Bagnell [78] showed that at 380°C and 22 MPa, the conversions of Australian
brown coals were considerably higher for supercritical extraction of water than with
toluene. The reverse was, however, true for black coals. In general, they found SCW
extraction was more effective for low-rank coals than high-rank bituminous coals.
The extraction by water was also more dependent on pressure presumably due to solvent density effect. The hydroxyl concentration of liquid yield by SCW extraction was
higher than that obtained in the liquid produced by toluene extraction.
Swanson et al. (2012, pers. comm.) showed that for low-rank coals, the conversion
and extract yields increased with increasing temperature and pressure. The conversion also decreased with increasing coal rank and correlated well with the percent
volatile matter in the coals. The study also indicated that SCW extracts the volatile hydrogen-rich fraction of the coal. The extract was found to be highly polar in
nature, with significant quantities of phenols and long-chain aliphatic fatty acids.
Numerous other studies have also addressed the behavior of coal, shale oil, biomass and mixtures of coal and biomass, polymers, rubber, algal oil, lignin, residual oil, and so on under SCW conditions [77–115] (Kim and Mitchell, 2012, pers.
comm.; Swanson et al., 2012, pers. comm.). Three typical studies illustrating the
coal decomposition in SCW are reported by Nonaka [89], Nonaka et al. [97], Li and
Eglebor [106], Vostrikov et al. [84], and Cheng et al. [83]. These studies showed that
as the temperature of SCW increases, more gas and less liquid are produced.
SCW has also been explored as a medium for the degradation of waste synthetic
polymers [107–116]. Rubber tires were converted to a 44% oil yield by reaction in
SCW at 400°C. When polystyrene-based ion exchange resins were subjected to SCW
at 380°C for 1  h [107–116], less than 5% polymer decomposed and the products
included styrene and several oxygenated arenes such as acetophenone and benzaldehyde. SCW is also used to extract oil and oil precursors from oil shale [98,100–105].
The process involved C–C bond cleavages, and in the presence of CO, higher hydrocarbon yields were obtained than those obtained in conventional pyrolytic treatment.
Holliday et al. [115] showed that water near its critical point is a good medium for
the hydrolysis of triglyceride-based vegetable oils into their fatty acid constituents.
A number of studies examined the decomposition of mixed feedstock under SCW
conditions [89,95–99]. Veski et al. [98] examined the decomposition of a mixture of
kukersite oil shale and pinewood and showed improved liquid and gas yields at 380°C
temperature. The mixture indicated a synergistic effect and showed the product to be
1.5–2.0 times better than what would be predicted based on simple additive yields.