coal

coal Coal is an important fossil fuel. As with ‘oil’, ‘coal’ is an umbrella term encompassing a wide range of fuels varying considerably in their physical, chemical, and technological properties. Although solid, and thus less convenient to handle and less versatile in usage than oil, coal continues to be a diverse energy source. Although the use of coal for gasification has diminished in many economies, if not disappeared completely, coal remains a major resource to meet growing energy demands, particularly in emerging economies.

Once a land flora was extensively established some 400 million years (Ma) ago, the potential for the formation of substantial coal deposits existed. Coals have formed at all latitudes, but the greatest volume has accumulated at middle latitudes. Major coal-forming events in the Earth's history are episodic, but only one such event has widely affected both hemispheres simultaneously. This took place during the Carboniferous and Permian periods, reaching its acme c. 300 Ma ago. In the Mesozoic era (249 to 65 Ma ago), there were several less extensive periods of coal formation. Throughout the Cenozoic (from 65 Ma ago to the present time), further coal-forming episodes have occurred and still continue, particularly in the Far East.

Older coals are usually more valuable as energy sources than younger coals because of their lengthy maturation in the Earth's crust. Their longer history has the disadvantage that the extensive depositional areas of which they were once part have been disrupted and fragmented by structural movements and, when exposed at surface, they have been weathered and eroded, leaving isolated coalfields.

Formation of coal-forming peats

Most economic humic coal deposits are autochthonous; that is, the peats from which the coals formed largely accumulated in situ from the fallen debris of trees and plants with relatively little movement of the dead vegetation. The allochthonous coals are a smaller group, forming chiefly from finely degraded vegetation, which was transported into lakes and ponds, principally by water, but also by wind. These organic muds form sapropels (cannels, algal cannels, and boghead coals), which are usually thin, of limited extent, and may occur within or on top of normal humic coals. Parts of some humic coal seams may be allochthonous.

An essential requirement for economic coals is that at no time in their history should they be heavily contaminated by mineral matter. A long-established view is that coal-forming peats accumulated in swamps on delta surfaces or in coastal swamps. Some coals may have formed in such environments, but they would not be ‘clean’ coals. Delta surfaces and coasts receive an influx of clastic material at times of flooding and during storms. The mixing of this mineral matter with the organic debris would be more likely to give rise either to high-ash coals or to carbonacous shales or mudstones, but certainly not to coals low in mineral matter.

‘Clean’ autochthonous peat swamps in sedimentary basins are of three types: ‘floating’, which form only thin peats, ‘low-lying’, and ‘raised’; those of the second and third types form thick high-quality peats. The peats may pass into one another, according to the depositional conditions. Active clastic deposition can occur where the swamps develop, but all the peats are low in mineral matter. In the floating swamp, the top of the peat mat is above water level; where the swamp is low-lying, covering pre-existing topography, clastic material and the peat are deposited at different times; and in raised swamps the peats lie above flood levels and are not contaminated by water-borne minerals; they also possess high acidity which may leach out any minerals present.

For peat accumulation to proceed satisfactorily, organic production and decay must roughly balance. Both are controlled by climate, particularly by temperature and humidity. Decay can be halted or retarded only if the vegetation accumulates under oxygen-free, stagnant conditions below the water table, which must be at the surface of the sediment. If the water table rises at an appropriate rate, peat growth will keep pace with this rise. The water table may fluctuate; the peat will then either decay because of exposure to the air or it may temporarily be drowned. Fluctuations of this kind occur throughout peat accumulation, but when subsidence becomes rapid and continues, the drowned peat will then be covered by clastic sediments. Most major coalfields have formed by drowning of the peats after each has had a period of successful growth.

Composition of coals

In the field and under the microscope most coals display heterogeneity governed by their initial compositions and the biochemical histories of the peats from which they formed. While sapropelic coals are massive and fine-grained, humic coals are banded, the bands being composed of differing proportions and sizes of constituents that reflect the changing character and depositional conditions within the peat. Plant components range from the massive parts of trees, shrubs, and plants to small fragments of tissues, seed and spore coats, cuticles, and sometimes algae and highly resistant plant impregnations. After deposition these components offer differing degrees of resistance to influences such as oxidation and microbiological activity, which continue to operate (the latter especially), until the peat is buried so deeply that they are no longer effective. Thus, not only do the initial compositions of the peats vary, but the peats are subsequently modified during what is termed the ‘biochemical stage of coal formation’.

The plant components in peat are transformed into ‘macerals’ (broadly analogous to the minerals of inorganic rocks), which fall into three groups.(1) Vitrinite, the dominant constituent of humic coals and formed from the lignin-cellulose complexes of trunks, branches, roots, and other plant organs along with degraded plant and humic-peat material, some of which forms gels;(2) Liptinite (exinite) formed of the waxy resistant parts of plants, spore and seed coats, resins, waxes, tannins, algae, and degradation tissues;(3) Inertinite, much of which is material oxidized either before or after incorporation into the peat, with a similar origin to those components forming the vitrinite group, and also other material of fungal origin, particles, and granules arising from redeposition of inertinite and degradation of liptinites. Minerals form a fourth group of constituents, of which mineral sulphides and carbonates are the principal components. The differing proportions of these constituent groups govern the properties and technological behaviour of any coal. Figure 4(a)-(c) illustrates typical appearances of a humic coal under the microscope. The highly reflecting material is inertinite, with some mineral matter (pyrite in Fig. 4a); the medium-reflecting constituent is vitrinite, and the dark grey constituents are liptinite, mainly spores.

The petrographic compositions of coals and their technological potential are best assessed by light microscopy. The macerals present, and their proportions, are determined together with that fraction of contaminating mineral matter which is not too finely divided to be detected by light microscopy. From an economic viewpoint, coals are categorized chemically most effectively by proximate and ultimate analyses. Proximate analysis defines the moisture content of a coal, its ash content, and its yield of volatile matter (which is directly related to the gas potential of the coal), and fixed carbon. Ultimate analysis yields the elementary composition of a coal: its carbon, hydrogen, oxygen, nitrogen, and sulphur contents and other properties. The calorific value and the coking potential of the coal are decided by other tests.

The properties of any coal are directly related to its original composition and to its biochemical and geochemical histories. Biochemical activity occurs throughout the peat stage when coal types are differentiated, including a range of humic-coal types and the sapropelic coals: the cannels, algal cannels, and boghead coals. Once all biochemical activity has ceased, the ‘geochemical stage of coal formation’ begins. The coal is subjected only to physical influences: rise of temperature, length of exposure to increased temperature, raised overburden pressure, and sometimes stress. With increasing length of geological time, as the organic mass is exposed to raised temperatures, coals pass through a series of coalification (rank) stages: lignite, sub-bituminous coal up to meta-anthracite. Table 1 illustrates how some important properties of coals vary with rising rank. The coal depicted in Fig. 4 belongs to the high-volatile bituminous group.

Reserves and consumption of coal

Proven reserves of coal are usually taken as those quantities that geological knowledge and current engineering technology indicate can be recovered from known deposits. Table 2 shows the world distribution of major reserves in 1997, dominated by the Asia-Pacific region. Only in this region do the reserves of anthracite and bituminous coal exceed the reserves of sub-bituminous coal and lignite.

All regions show interest in their reserves, but the greatest concentration is focused on the length of time the reserves will last. This is estimated by dividing the known reserves remaining at the end of a year by the production in that year. The calculation gives the ‘Reserves/Production (R/P) ratio’, representing the number of years the reserves will last if production continues at that level.

Table 2 Distribution of major coal reserves in 1997.

Region	A/B	SB/LT	Total	R/P
thousand million tonnes	years
A/B, Anthracite/Bituminous coal
SB/L, Sub-bituminous coal/Lignite
R/P, Reserves/Production ratio
North America	111.9	138.5	250.4	233
Europe	59.1	97.6	156.7	191
Former Soviet Union	104.0	137.0	241.0	>500
Asia/Pacific	178.2	133.3	311.5	146
Total World	519.4	512.3	1031.6	219

Table 3 Variation in world consumption of primary energy (milliontonnes of oil equivalent) over 25 years.

Fuel	1972	1977	1982	1987	1992	1997
Oil	2591	3000	2773	2955	3182	3409
Natural gas	1045	1182	1318	1573	1818	2000
Nuclear energy	24	136	245	455	545	636
Hydroelectric	91	118	136	182	182	218
Coal	1545	1727	1864	2182	2145	2309

Coal production is closely related to consumption: most coal is consumed in the region in which it is produced. The balance between production and consumption is roughly equal in most regions, but North America is a net coal exporter while Europe is a net importer.

The future for coal

Table 3 illustrates the pattern of world energy consumption over the period 1972–97, when consumption increased by over 60 per cent. The largest increase (c. 90 per cent) was in natural gas. Consumption of oil rose by only some 36 per cent, but in total substantially exceeded consumption of any other individual fuel. Usage of coal increased by approximately 44 per cent, and was the second largest fuel consumption over the period.

Consumption of energy varies from region to region. In the Asia/Pacific region the largest usage in 1997 was of coal; in the Former Soviet Union it was of natural gas; but elsewhere oil was dominantly consumed. Table 3 illustrates the implications of these patterns of usage upon the world reserves' position. Even allowing for substantial future discoveries of oil and natural gas, but discounting a dramatic development of new technologies, which would diminish or replace the use of oil, or a reversal of attitudes to large-scale expansion in nuclear-energy programmes, use of coal as an energy source must increase apace, particularly in developing countries with few other fossil-fuel reserves. The greatest demand for coal will be for electricity generation, but coal will continue to feature in conversion processes such as gasification, liquefaction, and pyrolysis, and in the production of metallurgical coke.

Duncan Murchison