Coal is a very effective solid fuel as it burns to easily and produces a great deal of heat. Until relatively recently, coal-fires were the most common means of domestic heating. Coal is also used very extensively to fuel the power stations that produce electricity. However, there are two major concerns over the use of coal: firstly, it is a fossil fuel, so supplies are finite; and secondly, the levels of carbon dioxide emissions that result from the combustion of coal are believed to be damaging to the environment.
The Formation of Coal
Coal is the compressed remains of vegetation that grew hundreds of millions of years ago.
Living vegetation absorbs sunlight and water, converts them into energy and stores them in the form of hydrocarbons. When the vegetation dies, it still contains stored hydrocarbons. Once created, energy cannot be destroyed; therefore the potential energy in the hydrocarbons will remain trapped until it is converted into a form in which it can be released and used. If the vegetation is exposed to air it can oxidise, releasing energy as carbon dioxide. However, if it collects in thick layers or becomes buried beneath water and mud so that it cannot react with the oxygen in the air, it will not biodegrade.
We find layers of coal in places where decaying vegetation was buried and preserved beneath the surface of the earth. Over time, layers of rock formed above them and compressed the vegetal matter until eventually it became the hard, black substance that we mine as coal.
Different Types of Coal
The coalification process begins with the formation of peat. Peat, which can itself be used as a fuel, is found close to the earth’s surface. Commonly called ‘turf’ in Ireland, when peat is first cut and before it is dried it bears more resemblance to decaying vegetation mixed with earth, than to coal. As more and more sediment accumulates over a layer of peat, it is subjected to more and more pressure and heat, and its composition changes. Water is squeezed out of it, the methane and other gases are released, and the ratio of carbon to other elements increases. It successively becomes lignite, then sub-bituminous coal and then bituminous coal.
These three substances are used as industrial fuel, especially in power plants. The next stage is anthracite. Household coal is normally anthracite: it is very black, hard, and quite glossy in appearance, and burns well, giving a lot of heat. Finally, anthracite becomes graphite, which is a pure carbon mineral. Graphite is very hard, and since it does not ignite easily it is not suitable for use as a fuel. Its main use is in pencils, and, in powdered form, as a lubricant.
What Happens When We Burn Coal?
When coal is burned, it reacts with the oxygen in the air. This chemical reaction converts the stored solar energy into thermal energy, which is released as heat. But it also produces carbon dioxide and methane. During the 20th century, scientists found growing evidence to indicate that the large quantities of these ‘greenhouse gases’ being produced by combustion of fossil fuels are causing damage to the environment. Coal that contains large quantities of sulphur also emits significant levels of sulphur dioxide, which causes acid rain.
The Uses of Coal
Coal remains a major source of heat and power. It was the traditional fuel for steam-driven machinery in mills and other industries, and for steam locomotives; in homes, coal fires provided the heat for warming the house, heating water and cooking. We are now moving towards cleaner, more environmentally-friendly domestic heating systems and industrial processes, but as yet no other source of energy has emulated coal in the production electricity, either in conventional power stations or in the new generation of combined cycle power plants. Far from being phased out, coal-fuelled electricity production looks set to continue well into the future.
The Future of Coal
Our reliance on coal is a source of grave concern to environmentalists, and coal reserves are being depleted rapidly. It is impossible to predict when supplies will be exhausted to the point where coal can no longer be mined on an economically-viable scale; some scientists estimate that this may happen within the next 100 – 150 years, while others say reserves will last for 400 years or more.
We cannot be certain how much coal there is underground, nor can we predict future trends in consumption. It is likely that consumption will fall because of environmental concerns. But at present, with coal still responsible for producing more than a third of the UK’s electricity and probably a similar proportion worldwide, it is difficult to see how industrialised society could continue functioning without the energy provided by coal.