Water, particularly at high temperature and pressure is a good solvent for both biomass and coal [124-131]. For biomass, as described in the previous chapter, between temperatures of about 180-280°C, hydrothermal carbonization occurs which produces a heavy biocrude. For temperatures close to the critical tem­perature, hydrothermal liquefaction occurs producing a higher quality crude and this can be further upgraded with the use of hydrogen by a hydrothermal upgrading process. These processes are described in Chapter 6 and are not repeated here. Similarly finely pulverized coal and high-temperature water can also depolymerize coal producing asphaltene and preasphaltene types of materials. Initial tests done at Auburn University indicate that liquefaction of a mixed feedstock in water at high temperatures results in a product similar to biocrude, bio-oil, and asphaltene types of materials. There was no syner­gistic effect between liquefaction of biomass and coal. The tests were done for different types of coal and biomass. Because water is not a hydrogen donor, further upgrading of these crudes (or heavy materials) will require the use of hydrogen. Hydroliquefaction is a good method to produce heavy crude from coal and biomass mixture which may have some practical applications in the construction (e. g., cement) or fertilizer industries.

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