Consisting of altered remains of plants, coal is a widely used fossil fuel. Generally, the older the coal, the higher the carbon content and heating value. Anthracite coal ranks highest in carbon content, then bituminous coal, subbituminous coal, and lignite (as determined by the American Society for Testing Materials). Over 80% of the world's vast reserves occur in the former Soviet Union, the United States, and China. Though globally abundant, it is associated with many environmental problems, including acid drainage, degraded land, sulfur oxide emissions, acid rain, and heavy carbon dioxide emissions. However, clean coal-burning technologies, including liquified or gasified forms, are now available.

Anthracite, or "hard" coal, differs from the less altered bituminous coal by having more than 86% carbon and less than 14% volatile matter. It was formerly the fuel of choice for heat purposes because of high Btu (British Thermal Unit) values, minimally 14,500, and low ash content. In...