Geologists believe that underground coal mines were formed in the Carboniferous geological age 290-360 million years ago, when most of the surface was covered by muddy land, swamps, and dense forests (Moore 1922, Jeffry 1924). As the flowers and trees grow and wither, plant residues are deposited under the wet ground surface, and the hypoxic environment below the surface slows down the rate of corruption, so the plant residues are turned into peat through a biochemical process. After millions of years of changes, the physical and chemical properties of the peat layer have changed. This process is called “coalification”. Because the newborn oceans and continents cover the surface of peat, the peat forms tightly compacted sediments under the combined action of heat (heat comes from the earth’s interior or nearby volcanic sources) and pressure. After repeated accumulation, sediments undergo geochemical processes (ie, biological and geological reactions). In other words, under the effects of decay and compaction, heat and time, the properties of peat have changed. The effects of different degrees of temperature, time and external pressure determine the formation of different types of coal (peat, lignite, sub-bituminous coal, bituminous coal and anthracite). Figure 1 shows the sequence of coal formation (Kentucky Geological Survey: How is coal formed?). It is estimated that it takes 3~7t9 plant residues per foot of coal seam. Some modern coals were formed 15 to 100 million years ago, while the newest coal was formed only about 1 million years ago. The different vegetation and climatic conditions at the time of coal formation and subsequent geophysical reactions have led to huge differences between different coals. The biological and chemical properties of plants in different geological periods are different. Coal seam depth, temperature, acidity, and natural water flow in wetlands are all factors that affect the process of coal formation.
Coal, like oil and natural gas, comes from ancient organisms, so coal is called fossil fuel. Coalification consists of three chemical processes: microorganisms degrade cellulose in plants, lignin in plants is converted into humus, and humus is reduced into larger coal particles.
Coal rank is a quantity used to describe the degree of coalification during coal formation. Both physical and chemical properties of coal will affect coal rank. With the deepening of coalification, coal evolved from the original peat, in the order of lignite, sub-bituminous coal, bituminous coal, and anthracite. Figure 2 shows the various stages of coal formation and the corresponding chemical reactions. Peat has a high water content and low calorific value; the organic matter maturity and calorific value of the lignite formed at the beginning is low, and it is a soft, brown-black substance. The reserves of lignite rank first among all kinds of coal reserves in the world.
After millions of years of evolution, lignite has further changed under the continuous action of temperature and pressure, and its organic matter maturity has gradually increased, thereby evolving into sub-bituminous coal, which is pitch black. As further physical and chemical changes proceed, sub-bituminous coal becomes harder and darker, forming bituminous coal. With the gradual increase in the maturity of organic matter, anthracite is finally formed. The first two types of coals are soft coals, which are the main coal for power plants; the last two types of coals, bituminous coal and anthracite, are “black coal” or “hard coal”.
Coal usually appears in the seams or veins of sedimentary rocks; coal seams have different thicknesses, some underground coal mines are 0.7~2.4m (2.5~8ft) thick, and some open-pit coal mines are 30m (100t) thick, such as open-pit mines in the western United States. Coal mine.