Coal - What Is Coal?

Coal is a black, combustible, mineral solid that develops over millions of years from the partial decomposition of plant matter in an airless space, under increased temperature and pressure. Coal beds, sometimes called seams, are found in the earth between beds of sandstone, shale, and limestone and range in thickness from less than an inch to more than one hundred feet. Approximately five to ten feet of ancient, composted plant material have been compressed to create each foot of coal.

Coal is used as a fuel and in the production of coke (the solid substance left after coal gas and coal tar have been

FIGURE 4.1

extracted), coal gas, water gas, and many coal-tar compounds. When coal is burned, its fossil energy—sunlight converted and stored by plants over millions of years—is released. One ton of coal produces an average of 22 million Btu, about the same heating value as 22,000 cubic feet of natural gas, 159 gallons of distillate fuel oil, or one cord of seasoned firewood. (A cord is a stack of wood four feet by four feet by eight feet, or 128 cubic feet.)