The process of liquefaction has been widely used for fossil energy such as direct liquefaction of coal, shale, bitumen, heavy oil, and the like. The same concept can be easily applied to biomass which produces a water-insoluble bio-oil at high pressure (50-200 atm) and low temperature (250-450°C). These process conditions are less severe than the ones normally used for coal. The liquefaction involves all kinds of processes such as solvolysis, depolymer­ization, decarboxylation, hydrogenolysis, and hydrogenation (which can be accompanied by mild hydrocracking). The overall objective of biomass (and waste) liquefaction is to control the reaction rate and reaction mechanisms using pressure, gases, and catalysts to produce a high-quality liquid oil. The reactor feed generally consists of solid biomass feed (or a suitable waste), sol­vent, reducing gas such as H2 or CO, or a catalyst. The bio-oil produced via liquefaction has a lower oxygen content, lower viscosity, and higher energy density than pyrolysis-derived oil. Fundamentally, the nature of the lique­faction process can be broken down into several categories depending on the nature of the solvent and gas. For example, hydrothermal liquefaction uses water as the solvent, hydropyrolysis uses H2 or a reducing gas but no solvent, and solvolysis uses a reacting or hydrogen donor solvent. A review of biomass liquefaction research done between 1920 and 1980 is presented by Moffatt and Overend [77].

The nature of solvent, gas, and catalyst dictate the operating conditions and the quality of the liquefaction product. A number of solvents such as water, creosote oil, ethylene glycol, methanol, and recycled oil have been tested. Water is the most attractive because it is cheaper, and it does not require drying of waste or biomass. Hydrothermal liquefaction is sepa­rately described in this section. The recycled oil increases the selectiv­ity, and it also makes the process self-sufficient with no need to add new solvent in the process. Creosote oil, ethylene glycol, tetralin, phenathrene, alcohols, and phenols among other hydrogen donor solvents are used in the solvolysis process. They react with the solid materials during the liq­uefaction process. Hydrogen, carbon monoxide, and even methane act as reducing agents during hydropyrolysis in the presence of a catalyst and give a higher quality bio-oil. A number of catalysts have been used for liquefaction including alkali (from the alkaline ash components in the wood, alkaline oxides, carbonates, and bicarbonates) and metals such as zinc, copper, nickel, formate, iodine, cobalt sulfide, zinc chloride, and fer­ric hydroxide, as well as Ni, Mo, Ru, Co (which aid in hydrogenation/ hydrocracking).

Akhtar and Amin [80] studied the effects of various reaction variables on the hydroliquefaction process. They concluded that the major parameters influencing the yield and composition of bio-oil are temperature, proper­ties of solvent, solvent density, and type of biomass or waste. Temperature is the most important parameter, and they recommended a temperature range of 300-374°C depending upon the biomass type and specifications for the composition of bio-oil. At temperatures higher than 350°C, too much gas is formed and for temperature less than 280°C, conversion to oil is low. They recommended the use of water, methanol, ethanol, acetone, tetralin, and benzene, among others as suitable solvents. Residence time, heating rates, pressure, biomass particle size, presence of reducing gas, or hydrogen donor species are found to be of secondary importance in the hydrolique­faction process. The nature of the biomass waste also influences the yield of liquid production.