Coal formation requires certain conditions and only occur in certain eras in the history of geology. Carboniferous Period, about 340 million years ago (JTL), is the formation of the most productive coal where almost all deposits of coal (black coal) that is economical in the northern hemisphere is formed.
In the Permian Period, about 270 JTL, also formed coal deposits that are economical in the southern hemisphere, like Australia, and continue until the Tertiary Period (70-13 JTL) in various other hemisphere.
Coal-forming material
Almost all the coal-forming derived from plants. The types of coal-forming plants and age according to Diessel (1981) are as follows:
- Algae, from the Age of Pre-Cambrian to Ordovician and single-celled. Very few coal deposits of this period.
- Silofita, from the Silurian to Devonian Middle Ages, is derived from algae. A little coal deposits of this period.
- Pteridofita, age of Upper Devonian to Upper Carboniferous. The main coal-forming material Carboniferous age in Europe and North America. Plants without flowers and seeds, and reproduce by spores grow in warm climates.
- Gimnospermae, a period ranging from Permian to Cretaceous Middle Ages. Heterosexual plants, seeds encased in fruit, such as pine, contain levels of sap (resin) high. Type Pteridospermae like gangamopteris and Glossopteris is the main constituent such as the Permian coal in Australia, India and Africa.
- Angiosperms, from the Upper Cretaceous period until now. Modern plants, fruit that covers the seed, the male and female in one flower, less gummy than gimnospermae so that, in general, less can be preserved.
Coal mining is the mining of coal from the earth. Coal is used as fuel. Coal can also be used to make coke for steel making. [1]
Classes and types of coal
Based on the formation process is controlled by pressure, heat and time, coal is generally divided into five classes: anthracite, bituminous, sub-bituminous, lignite and peat.
- Anthracite is the highest grade of coal, with black sparkling (luster) metallic, containing between 86% - 98% elemental carbon (C) with water content less than 8%.
- Bituminous coal contains 68-86% elemental carbon (C) and yield of 8-10% water by weight. Class coal most widely mined in Australia.
- Sub-bituminous coal contains less carbon and more water, and therefore a source of heat is less efficient than the bituminous.
- Lignite or brown coal is very soft coal containing 35-75% water by weight.
- Peat, porous and has a moisture content above 75% and the lowest calorific value.
The process of change in plant debris into the peat to coal is termed pembatu baraan (coalification). In summary there are two stage process that takes place, namely:
- Phase Diagenetik or Biochemistry, began at the plant material deposited to form lignite. The main agents involved in this change process is the water content, degree of oxidation and biological disturbance that could cause the process of decay (decomposition) of organic material and compacting and forming peat.
- Metamorphic stages or Geochemistry, covers the process of change from lignite to bituminous and finally anthracite.
