Currently, more research is being done on direct and indirect thermal liquefaction methods for biomass and wastes than on the other methods. Direct liquefaction is either reaction of bio­mass components with smaller molecules such as H2 and CO (e. g., Pittsburg Energy Research Centre (PERC) and Lawrence Berkeley Laboratory, Berkeley, USA (LBL) processes) or short­term pyrolytic treatment, sometimes in the presence of gases such as H2. Indirect liquefaction involves successive production of an intermediate, such as synthesis gas or ethylene, and its chemical conversion to liquid fuels, In 1983, after several years of laboratory and pilot-plant work on the PERC and LBL processes, which involve reaction of product oil or water slurries of wood particles with H2 and CO at temperatures up to about 370 °C and pressures up to 272 atm in the presence of sodium carbonate catalyst, researchers concluded that neither process can be commercialized for liquid fuel production without substantial improvement. The most attractive approach to such improvement is believed to be a combination of solvolysis with a pyrolysis or reduction step. However, the oxygen content of the resulting complex liquid mix­ture is still high (6-10 wt%), and considerable processing is necessary to upgrade this material (http://journeytoforever. org/biofuel_library/ liquefaction. html).

Direct liquefaction has some similarity with pyrolysis in terms of the target products (liquid products). However, they are different in terms of operational conditions. Specifi­cally, direct liquefaction requires lower reaction temperatures but higher pressures than pyrolysis (0.5-2 atm for liquefaction vs. 0.01-0.05 atm for pyrolysis). In addition, drying of the feedstock is not a necessary step for direct liquefaction, but it is crucial for pyrolysis. Moreover, catalysts are always essential for liquefaction, whereas they are not as critical for pyrolysis. At the beginning of the liquefaction process, biomass undergoes depolymeriza­tion and is decomposed into monomer units. These monomer units, however, may be repolymerized or condensed into solid chars, which are undesirable (Zhang et al., 2010).