From Appropedia

Coal is a combustible black or brownish-black sedimentary rock normally occurring in rock strata in layers or veins called coal beds or coal seams. Coal is composed primarily of carbon, hydrogen, sulfur, oxygen and nitrogen. Coal has a wide variety of uses, from fuel to roles in industrial processes. It is used to generate electricity by burning it, using the heat to vaporize water, and then using this steam to turn turbines. The process of burning the goal gives off many polluting agents. Regardless, coal generates 40% of the world's electricity and 42% of the electricity in the United States in 2011. (EIA, 2012)

Coal has an EROI (Energy Return on Investment) ratio of 6:1. (Department of Energy Efficiency Fact Sheet)