Coal is a kind of solid combustible mineral which is formed gradually by the complex biochemical and physical chemical changes of ancient plants buried underground. It is a kind of solid combustible organic rock, which is mainly formed by the biochemical process of plant remains, buried and then transformed by geological process, commonly known as coal. According to the different primitive materials and conditions of coal formation, natural coal can be divided into three categories, namely humic coal, residual coal and sapropelic coal. Coal is the most widely distributed fossil energy resource in the world. It is mainly divided into bituminous coal and anthracite, sub-bituminous coal and lignite. Sixty percent of the world's recoverable coal reserves are concentrated in the United States (25 percent), the former Soviet Union (23 percent) and China (12 percent). In addition, Australia, India, Germany and South Africa account for 29 percent of the world's total coal production. The above-mentioned seven countries or regions account for 80 percent of the world's total coal production. The proven coal reserves are more than 63 times the oil reserves. Abundant countries are also major producers of coal.