All living plants store solar energy through a process known as photosynthesis. When plants die, this energy is usually released as the plants decay. Under conditions favorable to coal formation, the decaying process is interrupted, preventing the release of the stored solar energy. The energy is locked into the coal.
Coal formation began during the Carboniferous Period - known as the first coal age - which spanned 360 million to 290 million years ago. The build-up of silt and other sediments, together with movements in the earth's crust - known as tectonic movements - buried swamps and peat bogs, often to great depths. With burial, the plant material was subjected to high temperatures and pressures. This caused physical and chemical changes in the vegetation, transforming it into peat and then into coal.
The degree of change undergone by a coal as it matures from peat to anthracite is known as coalification. Coalification has an important bearing on coal's physical and chemical properties and is referred to as the 'rank' of the coal. Ranking is determined by the degree of transformation of the original plant material to carbon. The ranks of coals, from those with the least carbon to those with the most carbon, are lignite, sub-bituminous, bituminous and anthracite.
The quality of each coal deposit is determined by:
- Types of vegetation from which the coal originated
- Depths of burial
- Temperatures and pressures at those depths
- Length of time the coal has been forming in the deposit
Coal is extracted from the earth through underground mining or surface mining. The choice of mining method is largely determined by the geology of the coal deposit and its distance to the surface.
Coal can be burned for heating or to produce electricity. To convert thermal coal to electricity, it is first milled to a fine powder, which increases the surface area and allows it to burn more quickly. The hot gases and heat energy produced from combustion converts water into steam to run a turbine and generator.
High quality coal can be converted to coke for steel-making. It can also be converted to liquid or synthetic gas by advanced chemical processes, making it a possible, but currently costly, replacement for natural gas or liquid fuels in terms of transportation.
Coal is a highly abundant and cheap energy resource. Coal has powered the industrialization of many nations throughout history and continues to today. It is a big player in today’s energy system, providing the dominant share of the world’s electricity and a vital component of many industries including cement and metal production.
Coal reserves are available in almost every country worldwide, with the biggest reserves being in the China, USA, Russia, and India. Around 70 countries have recoverable coal reserves. Coal is readily available from a wide variety of sources in a well-supplied global market. Over 50 countries mine coal commercially and over 70 countries use it.
The largest coal-producing countries are not confined to one region; the top five hard coal producers are China, USA, India, Australia and Indonesia. Much of global coal production is used in the country in which it was produced.
There are well-developed market pricing systems for coal including cash, futures and options markets for producers and consumers to hedge production and use.
Coal is not polluting of itself however when combusted it the most CO2 intensive fossil fuel because it is composed largely of carbon. Concerns about climate change from greenhouse gas emissions have put a spotlight on coal and have prompted the development of numerous ‘clean coal’ technologies such as carbon capture and storage or sequestration (CCS) and high efficiency low emission (HELE) applications.
As the world's largest producer and consumer of coal and the world's largest emitter of emissions China has invested heavily in CCS and HELE to lower emissions and meet it's COP 21 pledges. The cutting-edge Chinese coal power plants of Zhousan, Sanhe and Waigaoqiao have lowered emissions beyond national and COP21 requirements and below those of Natural Gas.