1.5 The physics and chemistry of coal formation
Coal is a type of sediment made up mainly of lithified plant remains. But how does spongy, rotting plant debris become a hard seam of coal? As discussed earlier, plant material growing in mires dies, and then rots under anoxic conditions to form peat (by the process of humification). With time, the mire becomes covered with layers of sediment, the weight of which squeezes water and gas out of the pore spaces and compacts the vegetation. As subsidence allows deposition of further mire-sediment cycles, the process of compaction continues. The vegetation matter interbedded with sand, silt and mud progressively increases in density to become indistinguishable from coal.
The first stage in the chemistry of turning plant material into coal is one of biochemical decomposition. Bacterial breakdown of the more soluble components, principally the cellulose, results in enrichment of the more resistant, waxy leaf coatings, spores, pollen, fruit and algal remains. Decomposition also expels some gases originally contained in the rotting matter — chiefly water, carbon dioxide and methane — leaving organic residues rich in carbon.
The second phase starts when the plant deposits are progressively buried beneath substantial amounts of mud, sand and silt. As depth of burial increases so too does pressure. Because of the Earth's internal heat flow, temperature also increases with depth. Coalification of the deposit involves progressive physical and chemical changes brought about by the increased temperature and pressure. The degrees of change result in distinguishable stages of coal quality, or rank, which reflect the maturity of the coal. The different rank stages are listed in Table 1, together with some of the parameters used to define them. Changes in rank are gradual and so the boundaries of the rank categories are somewhat arbitrary.
Compaction under pressure progressively increases the hardness of the coal. The continuous variation of chemical composition through the rank series is shown in Figure.7. Low-rank coals (b, c) contain more volatiles than do high-rank coals (d, e), reflected by the variation in their oxygen and hydrogen content in Table 1. Anthracites, for example, usually contain less than 10% volatile matter. To form, they require high pressures associated with tectonic deformation or high temperatures near to igneous intrusions. Metamorphism at very high temperatures and pressure transforms coal to graphite deposits, which are not energy resources, although valuable in their own right.
The most important chemical change shown by increasing rank is the increase in the amount of carbon at the expense of oxygen. The proportion of hydrogen present remains relatively constant at 6-9% by weight over much of the rank series until about 90% carbon, where a significant reduction in hydrogen occurs.
Changes in rank involve the expulsion of water, carbon dioxide and methane (CH4). Only small amounts of methane are liberated during the early stages of coalification, but during the transition from bituminous coal to anthracite (particularly over the range 85-92% carbon), expulsion of methane and other hydrocarbons removes hydrogen, whereas the emission of carbon dioxide declines. As the complex organic compounds in buried vegetation are slowly transformed to simpler, more carbon-rich compounds, coal changes colour from brown to black.
Would you expect the density of coal to increase or decrease with increase in rank?
As rank increases, the porosity and water content of the coal decrease, so density increases.
The density of coal is reflected in its structure, hence the arbitrary distinction between 'hard' and 'soft' coals used in Table 1. In fact all coal is soft compared with most rock forming minerals, and the terms refer to how coal breaks: soft coal is crumbly, whereas hard coal breaks in a brittle fashion, and remains in lumps when transported.
The relative proportions of carbon and volatiles in coals affect their physical properties and their uses.
Low-rank coals rich in volatile matter (more than 30%) are easy to ignite and burn freely but with a smoky flame. Low-volatile (high-rank) coals are more difficult to ignite, but they burn with a smoke-free flame; they are natural smokeless fuels.
For industrial use, coal is usually heated to expel remaining volatiles. The carbonized residue that remains after the volatile matter has been driven off in the absence of air is called coke. High-rank bituminous coals become partly fluid on heating and swell up to form a porous coke, especially valuable for the iron and steel industries. Coke makes a useful artificial smokeless fuel because it is free of volatiles.
The calorific value of coal (i.e. the amount of heat liberated under controlled conditions) generally increases with rank. Nevertheless, coals with a high volatile content (>30%) are usually burned in power stations for the ease with which they burn, even though they give out less heat than higher ranked coals.