Coal is primarily used as fuel to generate electric power in the United States. The coal is burned and the heat given off is used to convert water into steam, which drives a turbine. In 2012, about 39 percent of all electricity in the United States was generated by coal-fired power plants, according to the U.S. Energy Information Administration.
Certain types of coal can also be used for metallurgical processes, like forging steel, smelting metals, or even in smelting sands, which are used to cast metal. Finally, coal can be burned to provide heat for individual homes.
Coal is abundant in the U.S., is relatively inexpensive, and is an excellent source of energy and byproduct raw materials. Because of these factors, domestic coal is the primary source of fuel for electric power plants in the U.S., and will continue to be well into the 21st century. In addition, other U.S. industries continue to use coal for fuel and coke production and there is a large overseas market for high-quality American coal.
Because humans have used coal for centuries, much is known about it. The usefulness of coal as a heat source and the myriad of byproducts that can be produced from coal are well understood. The continued and increasingly large-scale use of coal in the United States and in many other industrialized and developing nations has resulted in known and anticipated hazards to environmental quality and human health. As a result, there is still much to be learned about the harmful attributes of coal and how they may be removed, modified, or avoided to make coal use less harmful to humans and nature. These issues of coal quality have not been examined carefully until recently.
Coal is a sedimentary rock made predominantly of carbon that can be burned for fuel. Coal is readily combustible, black or brownish-black, and has a composition that, including inherent moisture, consists of more than 50 percent by weight and more than 70 percent by volume of carbonaceous material. It is formed from plant remains that have been compacted, hardened, chemically altered, and metamorphosed by heat and pressure over geologic time.
Coal is found all over the world including our country, predominantly in places where forests and marshes existed prehistorically, before being buried and compressed over millions of years. Some of the largest deposits, though, are located in areas of the Appalachian basin in the eastern U.S., the Illinois basin in the mid-continent region, and throughout the Rocky Mountain basins in the western U.S.
What are the types of coal?
There are four major types (or “ranks”) of coal. Rank refers to steps in a slow, natural process called “coalification,” during which buried plant matter changes into an ever denser, drier, more carbon rich, and harder material. The four ranks are:
- Anthracite: The highest rank of coal. It is a hard, brittle, and black lustrous coal, often referred to as hard coal, containing a high percentage of fixed carbon and a low percentage of volatile matter.
- Bituminous: Bituminous coal is a middle rank coal between subbituminous and anthracite. Bituminous usually has a high heating (Btu) value and is the most common type of coal used in electricity generation in the United States. Bituminous coal appears shiny and smooth when you first see it, but look closer and you may see it has layers.
- Subbituminous: Subbituminous coal is black in color and dull (not shiny), and has a higher heating value than lignite.
- Lignite: Lignite coal, aka brown coal, is the lowest grade coal with the least concentration of carbon.
Also, there is peat. Peat is not actually coal, but rather the precursor to coal. Peat is a soft organic material consisting of partly decayed plant and, in some cases, deposited mineral matter. When peat is placed under high pressure and heat, it becomes coal.
Different coal types are all minerals and rocks made largely of carbon. This fossil fuel generates ~40% of the world’s electricity and about 25% of the world’s primary energy. However, not all coal used is the same; it comes in different quantity levels of carbon—which dictates the quality of the coal.
Higher quality coal produces less smoke, burns longer, and provides more energy than lower quality coal.
The table below includes the carbon contents, and energy densities of coal. In addition, it states the moisture content before drying, and the amount of volatile content, after it’s dried.
| Coal | Dry, Carbon content (%) | Moisture content before drying (%) | Dry, volatile content (%) | Heat Content (MJ/kg) |
|---|---|---|---|---|
| Anthracite | 86-92 | 7-10 | 3-14 | 32-33 |
| Bituminous coal | 76-86 | 8-18 | 14-46 | 23-33 |
| Sub-Bituminous coal | 70-76 | 18-38 | 42-53 | 18-23 |
| Lignite | 65-70 | 35-55 | 53-63 | 17-18 |
| Peat | <60 | 75 | 63-69 | 15 |
The following is an overview of the different grades of coal, ordered from lowest to highest quality. Please see their main pages to learn more about each type.
What it is used for ?
In 2019, about 539 million short tons (MMst) of coal were consumed in the United States. On an energy content basis, this amount was equal to about 11.3 quadrillion British thermal units (Btu) and to about 11% of total U.S. energy consumption. Although coal use was once common in the industrial, transportation, residential, and commercial sectors, today the main use of coal in the United States is to generate electricity.
The electric power sector accounts for most of U.S. coal consumption.
U.S. coal consumption by consuming sector by amount and percentage share of total in 2019
- Electric power—539.4 MMst—91.8%
- Industrial total—47.1 MMst—8.0%
- Industrial coke plants—17.9 MMst—3.1%
- Industrial combined heat and power—11.2 MMst—1.9%
- Other industrial—17.9 MMst—3.0%
- Commercial—0.9 MMst—less than 1%
- Residential and transportation—each less than 1 MMst—less than 1%
U.S. coal consumption peaked in 2007 and declined in most years since then, mainly because of a decline in the use of coal for electricity generation.
● transportation: 0.00 million short tons
● residential and commercial: 8.24 million short tons
● coke plants: 40.91 million short tons
● other industrial: 64.10 million short tons
● electric power: 593.67 million short tons