Coal is a physically and chemically complex substance that has been defined in different ways over the years. Currently, the most widely accepted definition is that adopted by the American Society for Testing & Materials (ASTM) which is as follows: “Coal is a readily combustible rock containing more than 50 percent by weight and more than 70 percent by volume of carbonaceous material including inherent moisture, formed from compaction and induration of variously altered plant remains similar to those in peat. Differences in the kinds of plant materials (type), in degree of metamorphism (rank), and in the range of impurity (grade) are characteristic of coal and are used in classification (ASTM, 1970, p.70).”