Single cycle steam turbine power plant
The vast majority of fossil fuel energy generation in the world is based on a steam cycle. Water is heated and turned into steam, driving a steam turbine to deliver either electric energy through a generator, or mechanical work through a drive shaft. Steam cycle power plants are often referred to as thermal power plants.
For solid fuels such as coal, this is the only practical power plant design, unless the solids are transformed to gases or liquids before combustion. Light liquid and gas fuels can also use a gas turbine cycle or a combined gas turbine/steam turbine cycle, but these fuels are also commonly used in single cycle steam turbine setups. Therefore, the single cycle steam turbine power plant is the most common fossil fuel power plant design in the world, by a great margin.
The steam turbine cycle is highly versatile, easily scaled to different size applications, and adaptable to any fuel that provides enough energy by combustion to vaporize water.
Schematic representation of a typical single cycle coal power plant. Source: wikipedia.org
In a coal power plant, finely powdered coal is mixed with pre-heated air and injected into a combuster, where it is ignited. The hot combustion gas rises through the boiler and heats the water that enters the steam generator through an elaborate system of pipes, to capture as much of the thermal energy as possible. The water is only partially vaporized on its first pass through the boiler. It enters the steam drum, where steam is separated from the water, and the remaining water cycles through the boiler again, through pipes lining the furnace walls.
The steam, being only at equilibrium temperature, is passed through another system of pipes, known as the superheater, where the temperatures are increased to well above boiling, increasing the thermal energy of the steam and preventing condensation in the turbine.
The superheated steam, know at a very high pressure, passes through a high pressure turbine, where some of the pressure is converted into rotary motion, running an electrical generator. As steam moves through the system and loses pressure and thermal energy it expands in volume, requiring increasing diameter and longer blades at each succeeding stage to extract the remaining energy.
After passing through the high pressure turbine, the steam piped back to the boiler to be reheated, before entering an intermediate pressure turbine. Finally it passes through a low pressure turbine. The steam passes through a condensator, and is usually cycled back to the steam generator for reuse.
The electric efficiency of a conventional thermal power station, calculated as electric energy produced as a percent of the heating value of the fuel consumed, is typically 33% to 48%. The rest of the energy must leave the plant in the form of heat. If the waste heat is instead utilized for district heating, it is called cogeneration. A power plant that utilizes its excess heat for industrial or residential heating purposes is often referred to as a Combined Heat and Power (CHP) plant.