Coal is a sedimentary black or dark brown rock that varies in composition. Coals of varying composition are used around the world as a combustible fossil fuel for generating electricity and producing steel. According to the World Coal Association, a total of 7229 million tonnes of coal was produced globally in 2010.
People don't exactly "produce" coal; geological processes and decay of organic matter create coal over thousands of years. People mine coal from underground formations, or seams, through underground tunnels or by removing large areas of the earth's surface. The excavated coal must be cleaned, washed, and processed to prepare it for commercial use.
Coal Exporters and Importers
Australia tops the world wide list of coal exporters, having sent 298 million tonnes of coal overseas in 2010. Indonesia and Russia ranked second and third, exporting 162 and 109 million tonnes respectively. The U.S. came in fourth globally, having shipped 74 million tonnes of coal beyond its borders that same year.
Reliance on Coal
South Africa relies most heavily on coal, getting a full 93 percent of its electric power from coal. China and India also rely on coal for substantial amounts of their energy, at 79 and 69 percents, respectively. In recent years, the U.S. has gotten 45 percent of its electricity from coal. That ranks the U.S. 11th on the global list of countries that generate power from coal.
Hard and Soft Coal
Coal falls into two main categories: hard coal and soft coal (which is also known as brown coal or lignite). The country that produces more hard coal than any other, by a factor of about three, is the People's Republic of China. The whopping 3162 million tonnes of coal that China produces dwarfs the output of the second and third ranked country producers -- the U.S. at 932 million tonnes and India at 538 million tonnes.
For soft brown coal, Germany and Indonesia nearly tie for the honor of top honors. In 2010, they dug up 169 million tonnes and 163 million tonnes, respectively.
Coking Coal Versus Steam Coal
Coking coal, also known as metallurgical coal, has low sulfur and phosphorus content. Coking coal is able to withstand high heat. Coking coal is fed into ovens and subjected to oxygen-free pyrolysis, a process in which the coal is heated to approximately 1100 degrees Celsius. This melts the coal and drives off any volatile compounds and impurities to leave pure carbon. The purified, hot, liquified carbon solidifies into lumps called "coke" that can be fed into a blast furnace along with iron ore and limestone to produce steel.
Steam coal, also known as thermal coal, is suitable for electric power production. Steam coal is ground into a fine powder that burns quickly at high heats and is used in power plants to heat water in a boiler that runs steam turbines. It may also be used to provide space heating for homes and businesses.
Energy in Coal
All types of coal contain fixed carbon, which provides stored energy, plus varying amounts of moisture, ash, volatile matter, mercury, and sulfur. Because physical properties of coal vary widely, coal-fired power plants must be engineered to accommodate the specific properties of available feedstock and to reduce emissions of pollutants such as sulfur, mercury, and dioxins.
The stored energy potential within coal is described as the “calorific value,” “heating value,” or “heat content,” and is measured in Btu. The harder the coal, generally the higher its energy value.
Coal that is burned releases thermal energy, or heat, along with carbon and ash. Ash is made up of minerals such as iron, aluminum, limestone, clay, silica, and trace elements such as arsenic and chromium.
Types of Coal Compared and Ranked
The international standards organization ASTM has issued a ranking method for classifying grades of coal formed from biodegraded peat-based humic substances, organic material (vitrinite). The ranking is based on levels of geological metamorphosis, fixed carbon, and calorific value, and known as ASTM D388 - 05 Standard Classification of Coals by Rank.
How do the four types of coal compare? Following is a summary of the comparative ranking of coal, from the most energy dense to the least energy dense.