The thermochemical direct liquefaction process involves converting biomass to an oily liquid by contacting the biomass with water at elevated temperatures (250-350°C) with sufficient pressure to maintain the water primarily in the liquid phase (12-20 MPa) for resi­dence times up to 30 minutes (Figure 2.3). It mimics the natural geological processes thought to be involved in the production of fossil fuels. Alkali may be added to promote organic conversion. In the liquefaction process, the carbonaceous materials are converted to lique­fied products through a complex sequence of changes in physical structure and chemical bonds [58]. The primary product is an organic liquid with reduced oxygen content (about 10%) and the primary by-product is water containing soluble organic compounds. The resulting intermediates can be converted to hydrocarbon fuels and commodity chemicals for products similar to those produced from petroleum [16]. Work done on the determina­tion of the reaction mechanisms of liquefaction, mainly with pure cellulose, suggests that the technique offers a potential alternative synthetic route to phenolics, furans and other chemicals [59]. Liquefaction is suitable for high moisture content biomass, such as aquatic biomass, garbage, organic sludge and so on.

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