The specific gravity of coal, also known as the density of coal, is the ratio of the weight of a certain volume of coal after removing pores to the weight of water at the same temperature and volume. The bulk density of coal is also called the weight or false specific gravity of coal, which is the ratio of the weight of a certain volume of coal including pores to the weight of water at the same temperature and volume.

The bulk density of coal is an important index for calculating coal seam reserves. The bulk density of lignite is generally 1.05 ~ 1.2, that of bituminous coal is 1.2 ~ 1.4, and that of anthracite varies greatly between 1.35 ~ 1.8. The composition of coal and rock, the degree of coalification and the composition and content of minerals in coal are the main factors affecting the specific gravity and bulk density.

With the same mineral content, the proportion of coal increases with the deepening of coalification. Domestic anthracite, about 1.35 ~ 1.8 tons per cubic meter.

Coal is mainly composed of carbon, hydrogen, oxygen, nitrogen, sulfur and phosphorus. The sum of carbon, hydrogen and oxygen accounts for more than 95% of organic matter. It is a very important energy source and an important raw material for metallurgical and chemical industries. There are lignite, bituminous coal, anthracite and semi-anthracite Zhu Ziqing, a famous writer, once wrote a poem on coal, giving it a unique symbolic meaning.

As of 20 1 1, China is the country with the largest coal output in the world, with a coal output of 3.24 billion tons, equivalent to180.04 million tons of oil equivalent, accounting for 48.3% of the world. Followed by the United States, accounting for14.8% of world output; Ranked third is Australia, accounting for 6.3% of the world's output; India and Indonesia rank fourth and fifth respectively, accounting for 5.8% and 5.0% of the world output respectively.