Coal seam

Figure 1. Coal seams (darker black bands) in a rock.

A coal seam is a dark brown to black, banded deposit of coal that is visible within layers of rock. Coal seams are located underground and can be mined using either underground mining or strip-mining techniques, depending on their depth below the surface. These seams form through the natural process of coal formation (coalification) and are considered a conventional coal resource. Global coal reserves are immense and are the largest of all of the fossil fuels reserves.

Coal seams can also act as an unconventional source of natural gas. When natural gas is extracted from a coal seam, it is known as coal seam gas or coal bed methane. This gas is adsorbed onto the surface of underground coal seams, which are typically saturated with water. The pressure of the groundwater helps keep the methane adsorbed onto the surface of the coal. The amount of methane adsorbed onto a coal seam depends on factors such as the thickness and depth of the coal seam, its permeability, and the characteristics of the coal itself.

Formation

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Coal seams are formed from the accumulation of dead and decaying plant matter. Generally, this plant matter gathered in ancient peat-formed wetlands, such as swamps and peat bogs, where it was buried in shallow, stagnant water. As this plant material partially decomposed in the oxygen-poor environment, it gradually formed peat. As this peat became buried under layers of sediment, increasing temperature and pressure caused it to undergo coalification. Over millions of years, this heat and compression transformed the peat into lignite, the lowest rank of coal. With continued burial and increasing heat and pressure, brown lignite coal became sub-bituminous coal, then bituminous coal, and finally anthracite coal, the highest rank of coal. Regardless of the size or location of a coal seam, the overall process of coal formation remains the same.

Location

The size, depth, and accessibility of coal seams can vary widely. During coal formation, the combined effects of heat, pressure, and compaction reduce the thickness of peat substantially, so that about 10 metres of peat typically form approximately 1 metre of coal. However, the thickness of coal seams can vary significantly depending on the amount of peat that was initially deposited and the geological conditions during coal formation. Coal seams can range from less than 30 centimetres (approximately a foot) wide to more than 30 meters wide. Additionally, coal seams may be buried either very deeply or fairly close to the surface. Deeply buried coal seams are generally extracted using underground mining techniques, whereas shallow coal seams are generally obtained using surface mining techniques, such as strip mining.

Fires

Figure 2. A portion of a highway destroyed by the Centralia mine fire.

Coal seams can pose a fire hazard and may sometimes begin to smoulder. These fires are often ignited by surface fires or human activities, but they can continue to smoulder underground for many years. Some coal seam fires can burn for decades or even centuries until the underground fuel is used up, or the fire is extinguished. One of the most widely known coal seam fires is the Centralia mine fire, which has been burning since 1962. Although it is important to extinguish these fires because of the hazards they pose, doing so is often costly and difficult to extinguish. Internationally, thousands of underground coal fires are burning on every continent except Antarctica. These fires are hazardous because they release harmful gases and pollutants including carbon dioxide, methane, mercury, and often other toxic compounds, and can spark new surface fires. Global coal fire emissions are estimated to release about 40 tonnes of mercury into the atmosphere annually and account for approximately 3% of the world's yearly carbon dioxide emissions.

For Further Reading

• Strip mining
• Coal bed methane
• Sediment
• Peat
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