Hydrocarbons of algal cells have been separated by extraction with organic solvent after freeze-drying and sonicating the algal cells. However, these procedures are not suitable for separation on a large scale because they are costly. Therefore, an effec­tive method is liquefaction for separating hydrocarbons as liquid fuel from harvested algal cells with high moisture content. The direct thermochemical liquefaction can convert wet biomass such as wood and sewage sludge into liquid fuel at around 575 K and 10 MPa using a catalyst such as sodium carbonate (Demirbas 2007). At the same time, the liquid oil can be easily separated (Ogi et al. 1990).

Processes relating to liquefaction of biomass are based on the early research of Appell et al. (1971). These workers reported that a variety of biomass such as agri­cultural and municipal wastes could be converted, partially, into a heavy oil-like product by reaction with water and carbon monoxide/hydrogen in the presence of sodium carbonate. The heavy oil obtained from the liquefaction process is a viscous tarry lump, which sometimes causes troubles in handling. For this purpose, some organic solvents can be added to the reaction system. These processes require high temperature and pressure.

In the liquefaction process, biomass is converted into liquefied products through a complex sequence of physical structure and chemical changes. The feedstock of liquefaction is usually a wet matter. In liquefaction, biomass is decomposed into small molecules. These small molecules are unstable and reactive and can repoly­merize into oily compounds with a wide range of molecular weight distributions (Demirbas 2000).

Liquefaction can be accomplished directly or indirectly. Direct liquefaction in­volves rapid pyrolysis to produce liquid tars and oils or condensable organic va­pors. Indirect liquefaction involves the use of catalysts to convert noncondensable, gaseous products of pyrolysis or gasification into liquid products. Alkali salts, such as sodium carbonate and potassium carbonate, can induce the hydrolysis of cellulose and hemicellulose into smaller fragments. The degradation of biomass into smaller products mainly proceeds by depolymerization and deoxygenation. In the liquefac­tion process, the amount of solid residue increases in proportion to the lignin con­tent. Lignin is a macromolecule, which consists of alkylphenols and has a complex three-dimensional structure. It is generally accepted that free phenoxyl radicals are formed by thermal decomposition of lignin above 500 K and that the radicals have a random tendency to form a solid residue through condensation or repolymerization (Demirbas 2000).

The changes during liquefaction process involve all kinds of processes such as solvolysis, depolymerization, decarboxylation, hydrogenolysis, and hydrogenation. Solvolysis results in micellarlike substructures of the biomass. The depolymeriza­tion of biomass leads to smaller molecules. It also leads to new molecular rear­rangements through dehydration and decarboxylation. When hydrogen is present, hydrogenolysis and hydrogenation of functional groups, such as hydroxyl groups, carboxyl groups, and keto groups, also occur.

Direct hydrothermal liquefaction in subcritical water conditions is a technology that can be employed to convert wet biomass material into liquid fuel. A number of technical terminologies have been used in the literature to refer to this technology, but it essentially utilizes the high activity of water in subcritical conditions in order to decompose biomass materials down into shorter and smaller molecular materials with a higher energy density or more valuable chemicals.

Past research in the use of hydrothermal technology for direct liquefaction of al­gal biomass was very active. Minowa et al. (1995) reported an oil yield of about 37% (organic basis) by direct hydrothermal liquefaction at around 300 °C and 10 MPa fromDunaliella tertiolecta with a moisture content of 78.4%wt. The oil obtained at a reaction temperature of 340 °C and holding time of 60 min had a viscosity of 150 to 330 mPas and a calorific value of 36 kJ/g, comparable to those of fuel oil. It was concluded that the liquefaction technique was a net energy producer from the energy balance. In a similar study on oil recovery from Botryococcus braunii, a maximum yield 64% dry wt. basis of oil was obtained by liquefaction at 300 °C catalyzed by sodium carbonate (Sawayama et al. 1995). Also, Aresta et al. (2005) have compared different conversion techniques, viz., supercritical CO2, organic solvent extraction, pyrolysis, and hydrothermal technology, for the production of microalgal biodiesel. The hydrothermal liquefaction technique was more effective for extracting microal­gal biodiesel than supercritical CO2. From these two studies, it is reasonable to believe that, among the selected techniques, hydrothermal liquefaction is the most effective technological option for the production of biodiesel from algae. Never­theless, due to the level of limited information in the hydrothermal liquefaction of algae, more research in this area is needed.

Liquefaction of B. braunii, a colony-forming microalga, with high moisture con­tent was performed with or without sodium carbonate as a catalyst for conversion into liquid fuel and recovery of hydrocarbons. A greater amount of oil than the con­tent of hydrocarbons in B. braunii (50 wt% db) was obtained, in a yield of 57 to 64wt% at 575 K. The oil was equivalent in quality to petroleum oil. The recovery of hydrocarbons was maximized (>95%) at 575 K (Banerjee et al. 2002).