coal is the plant body through biological chemistry and physical chemistry and transformed into sedimentary organic minerals, is composed of a variety of polymer compounds and mixtures.
Coal was slowly formed by plants buried in the ground hundreds of millions of years ago.
So?
Coal is a product of the earth’s crust.
As far back as 300 million years ago in the Paleozoic and more than 100 million years ago in the Mesozoic and Cenozoic period of tens of millions of years ago, a large number of plant debris after a complex biological chemistry, geochemistry and physical chemistry into coal, after death, accumulation, burial to shift from plants into coal through a series of evolution process, this process is known as coal.
It is generally believed that the coal formation process can be divided into two stages: peatification stage and coalification stage.
The former is mainly a biochemical process, while the latter is a physicochemical process.
Peatification Stage The first stage of peatification is the continuous reproduction of plants in peat bogs, lakes or shallow seas, and their remains are continuously decomposed, combined and accumulated with the participation of microorganisms. Biogeochemistry plays a leading role in this stage.
Lower plants form sapropelation through biogeochemistry, while higher plants form peat, so the first stage of coal formation can be called sapropelation stage or peatization stage.
Coalification stage The coalification stage consists of two continuous processes: the first process, under the action of geothermal heat and pressure, the peat layer occurs compaction, water loss, limb aging, induration and other changes to become lignite.
Lignite, which is denser than peat, also has a significant change in composition, with a relative increase in carbon content, a decrease in humic acid content and a decrease in oxygen content.
Because coal is an organic rock, this process is also called diagenesis.
The second process is the transformation of lignite into bituminous coal.
In this process the properties of coal change, so this process is also called metamorphism.
As the earth’s crust continues to sink, the lignite overburden thickens.
Under the action of geothermal and static pressure, lignite continues to undergo physical and chemical changes and is compacted and loses water.
Its internal composition, structure and nature are further changed.
This process is the metamorphism of lignite to bituminous coal.
Compared with lignite, bituminous coal has higher carbon content and lower oxygen content. Humic acid is no longer present in bituminous coal.
Bituminous coal continues to undergo metamorphism.
Change from low metamorphic degree to high metamorphic degree.
Thus appear low metamorphic degree of long flame coal, gas coal, medium metamorphic degree of fat coal, coke coal and high metamorphic degree of lean coal, lean coal.
The carbon content between them also increases with the deepening of metamorphism.