Many petroleum-derived chemical compounds have been developed and used to enrich human life during the 20th century. Human and industrial use of petroleum bring about a rapid increase in the amount of consumption and will eventually lead to the depletion of petroleum. Many attempts have been made to find a substitute for petroleum as energy sources. Coal is more promising resource than other various ones such as solar energy, nuclear energy, hydraulic power and wind power in that it can be both carbon and energy sources. Especially, low-rank coal is an abundant resource. The advantages of coal liquefaction lie in easy transport, reduction of dust pollution, and easier removal of nitrogen and sulfur compared to solid coal. Conventional processes, such as solvent extraction, catalytic hydrogenation, pyrolysis, indirect liquefaction and hydrogasification, usually are carried out under extreme conditions : high temperature and high pressure. Therefore, they are energy consumption and complex process.
During the last decades, an alternative way of coal liquefaction, microbial solubilization of coal, has been introduced and attracts considerable interests among researchers. Biosolubilization of coal is performed under mild conditions, i.e., room temperature and ambient pressure. The process is energy efficient, comparatively simple and economically and technically advantageous over conventional ones. In most microbial liquefaction of coal into liquefied fuel, lignite would be the best candidate because it contains much oxygen and can be more easily accessible to microorganisms than other coals. Because oxygen content is the most important factor in microbial liquefaction, pretreatment of coal is required before microbial attack : chemical oxidation method. Microbial coal liquefaction can provide raw materials for chemical industries and additive of coal-water mixture as well as fuel liquid as an energy substitute. Although coal liquefaction processes are not economically feasible at present, it is anticipated that it will be commercialized within one or two decade through technical development of screening and mutation of microorganisms. Effective pretreatment methods and design of optimized bioreactor also could be important factors for the industrialization. The purpose of this research is to analyze the coal solubilization process, to establish a method of pretreatment, and to increase the solubilization efficiency and solubilization rate.