Coalification is the process of coal formation from peat via various intermediate phases. This is a short introduction to the process of coal formation.
Peat-Lignite-Sub Bitunimous- Bitunimous- Semi Anthracite – Anthracite – Meta anthracite.
The “rank” of coal refers to the degree of coalification.
The coalification process has two main distinct phases
- Initial Biochemical Phase – Processes that operates during the diagenesis of swamp deposits. The process stops with the formation of “Hard Brown Coal”. Biochemical phase of coalification is primarily controlled by bacteria. Other factors such as pH and depth to the groundwater can also affect the coalification process during this stage. Reduction of moisture content is the most obvious end result of the biochemical phase.
- Geochemical Phase / Metamorphic phase: Geochemical phase could be intense with rapid change in carbon content of the initial peat deposit due to change in pressure and/or temperature. With increase in rank of the coal, the % of carbon increases with the reduction of % hydrogen and % Oxygen. Methane and CO2 are the main by-products of this phase.
- Change in Temperature: This can be either direct contact with minor intrusion producing high heat or just increase in temperature with burial.
- Time – if the rate of burial is too high, sediments will not have enough “cooking time” to produce high rank coal from peat.
- Pressure: Most pronounced requirement for the compaction phase.
- Radioactivity: It is rarely observed to achieve higher rank due to readioactive material within the rock.
Hilt’s Law: In a vertical sequence, at any one locality in a coalfield, the rank of coal seams rises with increasing depth