Most thermal power plants are steam driven. Typically, water is heated into steam and directed across the blades of a turbine causing it to spin. The turbine is commonly attached to an electrical generator or is used for other work, like turning a ship propeller.
After passing through the turbine the steam is cooled and condensed back into liquid form before being returned to the heat source where it will again be converted to steam. Power plants use a wide range of fuels to heat liquids into steam. Some of the more common fuels are natural gas, coal, uranium (nuclear), diesel, oil and biomass materials.
Most thermal plants are large industrial installations used to generate electricity. Coal is the most common fuel for generating electricity in the U.S. In 2007, nearly half (49 percent) of the nation's 4.1 trillion kilowatt-hours of electricity used coal as its source of energy. Natural gas accounts for 22 percent, and nuclear just under 20 percent.