Coal is the altered remains of vegetation which originally accumulated in swamps. Organic matter transforms to coal when it is buried and subjected into increasing temperature and pressure over an extended period of time as the sedimentary pile above the organic matter accumulates. The level of coalification controls the way the coal is used in industrial processes.
The temperature of the earth rises with increasing depth underground. Most bituminous coals are thought to have formed between 50°C and 150°C. The geothermal gradient in sedimentary basins is approximately 25°C to 30°C for every kilometre depth. Thus, the depth of burial for bituminous coals is between 1.7 km and 6 km. As the coalification process occurs at relatively low temperatures, the coal has to remain buried at that depth for several million years. Although this seems an extensive length of time, within the geological timescale this is readily achievable..
The alteration of vegetation to coal is termed the coalification pathway. The degree of alteration of a particular coal is referred to as the coal’s rank.
In the transformation of peat to brown coal, the water saturated wood in the swamp is partly consumed by bacteria. This produces humic acids which further break down the wood material into an acid organic mush. As more material is overlaid on the pile, the force exerted on the organic material squeezes out excess water and the pore space decreases. Ultimately, more complex changes begin to occur with the release of water loving OH-groups (-OH, -COOH, -OCH etc). With increasing temperature, the organic mush is gelified and subsequently dries to a relatively solid form. At this point the organic matter has transformed to bituminous coal
In the bituminous rank range, increasing temperature results in additional water loss and initiation of the release of gases, notably carbon dioxide (CO2). Ongoing coalification into the mid-rank bituminous coals allows continued release of carbon dioxide (CO2) and increasing release of methane (CH4). Consequently, the residual material deoxygenates and dehydrogenates and the structure of the coal becomes carbon-rich. Via this process, by the time the coal transforms to anthracite is contains more than 90% carbon.
It is not only the chemistry of coal that changes with increasing rank. The structure of coal also changes. Low-rank bituminous coals contain groups of aromatic rings, crosslinked into a complex three-dimensional structure comprising carbon (C), oxygen (O), hydrogen (H), nitrogen (N), sulfur (S) and various radicals.
The bonds within the crosslinking structure are mostly strong co-valent bonds. However, a proportion of the C-H bonds are significantly weaker Van der Wall force linkages. The different rates at which CO2 and CH4 is driven from the coal results in an enhanced concentration of C-H Van der Wall bonds in within a sub-set of bituminous coals. It is these coals, with enhanced C-H bonds, that exhibit plastic behavior when heated in the absence of air and it is these coals that are used to make coke. Coal that are lower in rank or higher in rank compared to this subset are typically used as thermal coals in power generation. Thus, the structural changes that occur during coalification determine coal use.
There are various ways to measure coal rank. The most common analyses are:
At all levels of observation, coal is not homogeneous. For example, coal seams exhibit dull and bright bands. Using a microscope, these differences are more obvious in the inorganic components of coal. The organic portion of coal can be divided into three recognizable components which are termed macerals. The term "macerals" is supposed to be analogous to minerals. However, macerals do not have well-defined crystallographic structures found in minerals due to chemical and structural changes that occur in coal because of the coalification process.
There are three main maceral groups:
There are various ways to measure coal type. The most fundamental analysis is Maceral Analysis performed using a reflected light optical microscope. As this is a time-consuming and subjective test, various surrogate tests have been designed to determine the proportion of fusible to infusible components in a coal. Thus the typically tests used to measure coal type are:
There is some correlation between tests that measure coal type. However, the correlation is not as good as that found between tests that measure coal rank. This is primarily because there is also an underlying relationship between surrogate coal type tests and coal rank.