Coal is the most widely used fossil fuel for electricity generation; its composition is a complex combination of organic compounds and inorganic, mineral matter.
Coal is fossilized plant material preserved by burial in sediments and changed by geological forces that compact and condense it into a carbon-rich rock.
It has been suggested that coal formation may date back to Precambrian times but most of the coal was originally laid down as organic material during the carboniferous period, 286 million to 360 million years ago, when the earth’s climate was warmer and wetter.
Coal is considered a non-renewable energy source since it takes so long to form [16, 47, 48].
Coal is classified by type based on its stage of formation. This classification consists of five categories: peat, lignite, sub-bituminous, bituminous and anthracite (see Figure 3.2).
Younger coals, such as lignite and sub-bituminous coals, are easier to burn because they contain a larger amount of volatile compounds that evolve as gases when the coal is heated. In contrast, older coals are more difficult to burn as they are made almost entirely of solid carbon.
However, anthracite was preferred in the past instead of bituminous because it burns cleaner, producing less smoke and leaving less ash—and is more efficient in terms of units of heat produced per unit of weight.
Emissions from coal combustion strongly depend on the rank and composition of the coal. Pollutants emitted from coal combustion include greenhouse gases (mainly CO2), particulate matter (including ash and unburned carbon resulting from incomplete combustion), nitrogen oxides and sulfur oxides.
Other emissions from coal power stations are carbon monoxide, unburned hydrocarbon compounds, some carcinogens like dioxins and furans, and trace metals (e.g., lead and mercury).