As reported in the previous chapter, supercritical extraction of coal, biomass, and other organic wastes by CO2, water, and other organic solvents have been extensively reported [146-160]. These studies have clearly indicated the value of this process. Outstanding reviews of supercritical extraction by
water both at high and low temperatures are given by Guo, Cao, and Liu [161] and Peterson et al. [124]. Only limited studies [161-163] for supercritical extraction of a mixed feedstock by water have been reported. Once again, there seems be no synergy for the high-temperature supercritical extrac­tion of coal and biomass in the mixture environment by water. The work of Veski, Palu, and Kruusement [163], however, shows an interesting interaction between liquefaction reactions of oil shale and wood. As shown in Chapter 6 and the previous section, syngas and hydrogen productions by supercriti­cal water gasification of pure and mixed feedstock have been the subjects of significant recent interest [66, 124, 161].