The Occurence of Coal
There are 2 theories that describe the occurence of coal :
In-situ Theory : Coal is shaped from plants or trees that come from forests where that coal was shaped. Coals that are shaped according to the in-situ theory usually occur in wet forests and swamps, causing trees in that forest when die or tumble down, to directly sink into the swamp and the rest of the plants will not go through putrefaction perfectly, which in the end becomes a plant fossil that forms an organic sediment.
Drift Theory : Coals formed from plants or trees that are not from forests where the coals were originally formed. Coals that are formed according to the drify theory usually happen in deltas, having the characteristics of a thin layer of coals, splitting, multiple seam and has ash contents that are relatively high. The coal forming has two phases which are biochemically (peatification) and geochemically (coalification).
Peatification is a phase where the accumulated plant leftovers are stored in an oxygen free (anaerobic) condition in swamp areas with a bad drying system and is always stagnant with water in the 0.5 – 10 meters depth. These rotting Plant materials release H, N, O and C elements in a compound shaping CO2, H2O, and NH3 to become humus. Which later, by anaerobic bacterial and fungis will change into peat moss. (Stach, 1982, op cit Susilawati 1992).
Coalification is a colaboration process of biology, chemical and physics that happens because of loading effects from sediments that are covering it, temperature, pressure and time toward organic component from peat moss (Stach, 1982 op Susilawati 1992). During this phase, the carbon level percentage will increase, while the hydrogen and oxygen level percentage will decrease (Fischer, 1927, op cit Susilawati 1992). This process will produce Coal in a different maturity levels in its organic materials starting from lignite, sub bituminous, bituminous, semi-anthracite, anthracite, to meta-anthracite.
There are three factors that affects the coal forming process which are: Age, Temperature and Pressure.
The sediment quality is also affected by temperature, pressure and time length for forming, which is called organic maturity.
Coal forming starts since the period of Carbon Forming (Carboniferous Period) known as the first coal era that happened between 360 to 290 million years ago. In the beginning, the plant sediments changed into peat (C60H6O34) which supposed to turn into lignite or also called brown coal. Lignite is a type of coal with low organic maturity.
After receiving the affect of temperature and pressure continuously for million of years, lignites will experience changes periodically adding to its organic maturity and change the lignite into sub-bituminous coals. Chemical and Physics changes will continuously happen to the point where the coal will be much harder and will be more black in color to form bituminous or anthracite. In the right conditions, the increasing organic maturity will form anthracite.
In the Coal forming process, the real organic maturiy pictures the change of concentration from every main elements that form coals. Here shows the coal forming process:
There's a process of coal :
Aside of it, the higher the coal rank, the higher the carbon level while hydrogen and oxygen levels will decrease. Because coalification is generally associated with quality or coal quaity, hence coals with low coalification is also called a low quality coal such as lignite and sub-bituminous are usually softer with fragile material and dull color like soil, has high moisture level and low carbon level, hence the energy content is also low. The higher quality the coal is, generally it would be much harder and cohesive with its black glossy color. Aside of that, the moisture level will also decrease and carbon level increases, hence the energy content is bigger.