Coal is a burnable rock, usually black or brown, that serves as a source for hydrocarbons. Coal is classified as a nonrenewable energy source, because it takes millions of years to form and cannot be replenished with a practical time frame. Coal contains the energy stored by plants from hundreds of millions of years ago. More information about coal formation and coalification can be found here. There are four main types of coal, each with its own unique properties and uses:
- Anthracite – Made of at least 85 percent carbon, anthracite has the highest heating value of all ranks of coal. This makes it valuable for industrial heating applications, particularly industries that deal with metalworking. However, anthracite coal is rare, and accounted for less than 1 percent of the mined coal in the USA in 2015. All of the anthracite mines in the USA are in northeastern Pennsylvania. Anthracite is mainly used by the metals industry.
- Bituminous – Bituminous coal contains at least 45 percent carbon by weight, and is the most available coal in the USA. About 45 percent of US coal production falls under this category. Bituminous coal can be used as thermal coal to generate electricity or as metallurgical coal to make iron and steel. West Virginia, Kentucky, Illinois, Pennsylvania, and Indiana all produce bituminous coal.
- Sub-bituminous – Sub-bituminous coal is considered a lower-rank coal, since it contains at least 35 percent carbon by weight. Some uses include cement production, various industrial uses, and power generation. Most sub-bituminous coal is produced in Wyoming.
- Lignite – The lowest rank of all coals, lignite contains at least 25 percent carbon, since it is the most newly developed. As such, it has the lowest energy content of all coal ranks. Lignite cannot be used in industrial applications, so it is usually used solely for power generation. Lignite accounted for about 8 percent of the total coal production in 2015. Most lignite coal comes from Texas and North Dakota.
In March 2017, coal provided just over a quarter of Texas’ electricity. This number has been trending downwards, as natural gas and renewable resources like wind have been able to provide electricity at much cheaper rates.