Coal, a black, brittle, sedimentary rock that burns, consists of elemental carbon mixed with minor amounts of organic chemicals, quartz, and clay. Typically, the carbon atoms in coal have bonded to form complicated molecules. Note that coal and oil do not have the same composition or origin. In contrast to oil, coal forms from plant material (wood, stems, leaves) that once grew in coal swamps, regions that resembled the wetlands and rain forests of modern tropical to semitropical coastal areas. Like oil and gas, coal is a fossil fuel because it stores solar energy that reached Earth long ago. Signiﬁcant coal deposits could not form until vascular land plants appeared in the late Silurian Period, about 420 million years ago. The most extensive deposits of coal in the world occur in Carboniferous-age strata (deposited between 286 and 354 million years ago). During the Carboniferous, the continents were assembled in Pangaea, and large areas straddled warm tropical regions where vegetation ﬂourished. Also during this time, sea level was so high that large areas of continents were ﬂooded by shallow seas along which vast swamps grew; the plant debris of these swamps, once buried and heated, turned into coal. Not all coal reserves, however, are Carboniferous during the Cretaceous (144 to 65 Ma), large areas of freshwater coal swamps developed in Wyoming and adjacent states. Coal commonly occurs in beds, or seams, that may be centimetres to meters thick and may be traceable over very large regions. Broad, continuous coal seams develop when sea level rises slowly relative to the land surface so the coastline, and therefore the coal swamp, migrates slowly inland. During such a transgression, the submerged portion of the coal swamp eventually gets buried by layers of other sediments, such as sand and mud. When lithiﬁed, the succession turns into beds of sedimentary rock, with the coal occurring as a sedimentary bed, inter-layered with sandstone and shale.
The Formation of Coal
|The formation of coal. Coal forms when plant debris becomes deeply buried.|
How do the remains of plants transform into coal? The vegetation of an ancient swamp must fall and be buried in an oxygen-poor environment, such as stagnant water, so that it can be incorporated in a sedimentary sequence without ﬁrst decaying by reacting with oxygen and/or by being eaten by microbes or larger organisms. Compaction and partial decay of the vegetation transforms it into peat. (Peat, which contains about 50% carbon, itself serves as a fuel in many parts of the world.) To transform peat into coal, the peat must be buried deeply (4–10 km) by overlying sediment. Such deep burial can happen where the surface of the continent gradually sinks, creating a depression, or sedimentary basin, that can collect sediment. Over time, many kilometers of sediment containing numerous peat layers accumulate. At depth in the pile, the weight of overlying sediment compacts the peat and squeezes out any remaining water. Then, because temperature increases with depth in the Earth, deeply buried peat gradually heats up. Heat accelerates chemical reactions that gradually destroy plant ﬁber and release elements such as hydrogen, nitrogen, and sulfur in the form of gas. These gases seep out of the reacting peat layer, leaving behind a residue concentrated with carbon. Once the proportion of carbon in the residue exceeds about 50%, the deposit formally becomes coal. With further burial and higher temperatures, chemical reactions allow additional hydrogen, nitrogen, and sulfur to escape, yielding progressively higher concentrations of carbon.
The Classiﬁcation of Coal
Geologists classify coal according to the concentration of carbon. With increasing burial, peat transforms into a soft, dark-brown coal called lignite. At higher temperatures (about 100n–200nC), lignite, in turn, becomes dull, black bituminous coal. At still higher temperatures (about 200n–300nC), bituminous coal transforms into shiny, black anthracite coal (also called hard coal). The progressive transformation of peat to anthracite coal, which occurs as the coal layer is buried more deeply and becomes warmer, reﬂects the completeness of chemical reactions that remove water, hydrogen, nitrogen, and sulfur from the organic chemicals of the peat and leave behind carbon (Fig. 12.6c). Thus, lignite contains only about 50% carbon, bituminous about 70%, and anthracite about 90%. As the carbon content of coal increases, we say the coal rank increases. Notably, the formation of anthracite coal requires high temperatures that develop only on the borders of mountain belts, where mountain-building processes can push thick sheets of rock up along thrust faults and over the coal-bearing sediment, so the sediment ends up at depths of 8 to 10 km, where temperatures reach 300nC. Hot groundwater ﬂowing through the rock may also provide enough heat to produce anthracite.
Finding and Mining Coal
|The distribution of coal reserves. Vast quantities lie buried in continental sedimentary basins.|
Because the vegetation that eventually becomes coal was initially deposited in a sequence of sediment, coal seams interlayered with other sedimentary rocks. To ﬁnd coal, geologists search for sequences of strata that were deposited in tropical to semitropical, shallow-marine to terrestrial environments the environments in which a swamp could exist. The sedimentary strata of continents contain huge quantities of discovered coal, or coal reserves. The way in which companies mine coal depends on the depth of the coal seam. If the coal seam lies within about 100 m of the ground surface, strip mining proves to be the most economical method. In strip mines, miners use a giant shovel called a dragline to scrape off soil and layers of sedimentary rock above the coal seam. Draglines are so big that the shovel could swallow a two-car garage without a trace. Once the dragline has exposed the seam, miners use smaller power shovels to dig out the coal and dump it into trucks or onto a conveyor belt. Before modern environmental awareness took hold, strip mining left huge scars on the landscape. Without topsoil, the rubble and exposed rock of the mining operation remained barren of vegetation. In many contemporary mines, however, the dragline operator separates out and preserves soil. Then, when the coal has been scraped out, the operator ﬁlls the hole with the rock that had been stripped to expose the coal and covers the rock back up with the saved soil, on which grass or trees may eventually grow. In hilly areas, however, miners may use a practice called mountain top removal, during which they blast off the top of the mountain and dump the debris into adjacent valleys. This practice can disrupt the landscape permanently.
Deep coal can be obtained only by underground mining. To develop an underground mine, miners dig a shaft down to the depth of the coal seam and then create a maze of tunnels, using huge grinding machines that chew their way into the coal. Underground coal mining can be very dangerous, not only because the sedimentary rocks forming the roof of the mine are weak and can collapse but also because methane gas released by chemical reactions in coal can accumulate in the mine, leading to the danger of a small spark triggering a deadly mine explosion. Unless they breathe through ﬁlters, underground miners also risk contracting black lung disease from the inhalation of coal dust.
Gas from Coal
Coalbed Methane As we’ve noted, the natural process by which coal forms underground yields methane, a type of natural gas. Over time, some of the gas escapes to the atmosphere, but vast amounts remain within the coal in pores or bonded to coal molecules. Such coalbed methane, trapped in strata too deep to be reached by mining, is an energy resource that has become a target for exploration in many regions of the world. Obtaining coalbed methane from deep layers of strata involves drilling, rather than mining. Drillers penetrate a coalbed with a hole and then start pumping out groundwater. As a result of pumping out water, the pressure in the vicinity of the drill hole decreases relative to the surrounding bed. Methane bubbles into the hole and then up to the ground surface, where condensers compress it into tanks for storage.
Coal Gasiﬁcation Solid coal can be transformed into various gases, as well as solid by-products, before burning. The process of producing relatively clean-burning gases from solid coal is called coal gasiﬁcation. Coal gasiﬁcation involves the following steps. First, pulverized coal is placed in a large container. Then, a mixture of steam and oxygen passes through the coal at high pressure. As a result, the coal heats up to a high temperature but does not ignite; under these conditions, chemical reactions break down and oxidise the molecules in coal to produce ﬂammable gases as well as water and CO2. Solid ash, as well as sulphur and mercury, concentrate at the bottom of the container and can be removed before the gases are burned.
Underground Coalbed Fires
Coal will burn not only in furnaces but also in surface and subsurface mines, as long as the ﬁre has access to oxygen. Coal mining of the past two centuries has exposed much more coal to the air and has provided many more opportunities for ﬁres to begin; once started, a coalbed ﬁre that progresses underground (sucking in oxygen from joints in overlying rock) may be very difﬁcult or impossible to extinguish. For the past 50 years, a coalbed ﬁre has progressed under the town of Centralia, Pennsylvania, eventually burning coal seams beneath the town itself. The ﬁre produces toxic fumes that rise through the ground and make the overlying landscape uninhabitable, and it also causes the land surface to collapse and sink. And today, over 100 major coalbed ﬁres are burning in northern China. Recent estimates suggest that 200 million tons of coal burn underground in China every year, an amount equal to approximately 20% of the annual national production of coal in China.