As long ago as 600 million years, lush growing plants deposited thick layers of dead material in shallow swamps. The initial transformation was due to biochemical decomposition to form peat bogs.
Due to local movements of the earth’s crust the peat bogs sank, increasing the depth of water over the deposit and allowing mineral sedimentation to cover the layer, which became the beginning of a coal seam. These processes of plant deposition, further sinking and covering with sediment may be repeated several times, forming a multi-seam coal deposit.
Continuing movements of the earth’s crust with deposition of mineral matter caused the new coal seams to sink to depths of several kilometres, where elevated temperatures and pressures progressively changed the chemical and physical properties of the coal. High temperatures from volcanic activity sometimes played a part also. The most pronounced change was an increase in the carbon content and this process is known as coalification. Due to coalification the coal progresses from peat, to lignite, to coal, and finally to anthracite.
The coalification path may be followed relatively quickly or slowly depending on the severity of the conditions to which the coal is exposed. The path is not identical for all coals, as there may be differences in:
· The raw materials, that is types of plant matter – species, foliage, branches, spores,algae,
· The chemical environment for the initial plant decomposition – availability of oxygen,
· The temperature and pressure history.
Though the paths differ, coal geologists like to define the progress a coal has made on its journey from peat to anthracite. This leads to definitions of coal rank. Coal rank is the term used to describe the extent of the coalification process, going from low rank to high rank.
Any discussion of coal formation should recognise that dirt was also deposited in the swamps simultaneously with the plant matter, so that all coals contain intimately mixed mineral matter in varying amounts. Many utilisation properties of coal tend to change with coal rank, making it a useful concept.
However, it must be reiterated that different coals of the same rank may differ markedly in the chemical composition and heterogeneity of the organic matter, as well as of the mineral matter.
Definition of Coal Rank
The simplest definitions of coal rank are based on a single coal quality parameter that changes progressively during coalification. There are two main examples:
Carbon Content: It is necessary to exclude the mineral matter in coal so as to characterise the organic component, consequently organic carbon content on a dry mineral matter free basis is the most relevant. This differs slightly from the organic carbon on a dry ash free basis, but the latter is more easily measured and is therefore often used. Carbon (daf) varies from approximately 65% for lignite to 95% for anthracite.
Reflectance of Vitrinite: The metallurgical coal trade makes extensive use of the Reflectance of the Vitrinite maceral. Reflectance of vitrinite generally increases as coalification proceeds1, typical values being 0.3% for lignite, against 3% for anthracite.
Descriptive ranking makes use of traditional terms used for coal types. The ASTM Ranking System2 is defined in Table 1 and classifies coals according to their Volatile Matter content
1. Coal is composed of microscopically identifiable components termed the maceral groups vitrinite, inertinite and liptinite. The relative proportions of these in a coal relate principally to the types of plant matter that were present and the biochemical conditions that applied during the initial transformations, and not so much to the degree of coalification. Both the reflectance and the carbon content differ significantly between the maceral groups within a single sample, therefore the measurement of a property (such as reflectance) of a single maceral group gives technically a more precise measure of the degree of coalification than does a value averaged over the three maceral groups.
2. ASTM D388-95 (1997), Coal by Rank and/or Calorific Value. The system is widely used but the definitions depend on unusual bases: dry mineral matter free (dmmf) basis requires either a measurement or estimate of the mineral matter content of a coal, while moist mineral matter free (mmmf) basis requires a measurement or estimate of the Equilibrium Moisture content.
Table 1: The ASTM Ranking of Coals
* Dry mineral matter free basis
# Moist mineral matter free basis. This refers to the moisture content determined by the ASTM Moisture Holding Capacity Test