How Thermal Power is Generated

In a conventional thermal power station, a fuel is used to heat water, which gives off steam at high pressure. This in turn drives turbines to create electricity.

At the heart of a power stations is a generator, a rotating machine that converts mechanical energy into electrical energy by creating relative motion between a magnetic field and a conductor. The energy source harnessed to turn the generator varies widely. It depends chiefly on which fuels are easily available and on the types of technology used.

Thermal power plants are classified by the type of fuel used

• Nuclear power plants use a nuclear reactor’s heat to operate a steam turbine generator
• Fossil fuelled power plants may also use a steam turbine generator or in the case of natural gas fired plants may use a combustion turbine.
• Geothermal power plants use steam extracted from hot underground rocks
• Renewable energy plants may be fuelled by waste from sugar cane, municipal solid waste, landfill methane, or other forms of biomass
• In integrated steel mills, blast furnace exhaust gas is a low-cost, although low-energy-density, fuel
• Waste heat from industrial processes is occasionally concentrated enough to use for power generation, usually in a steam boiler and turbine
• Solar thermal electric plants use sunlight to boil water, which turns the generator

Coal Based Power Plants

When coal is used for electricity generation, it is usually pulverized and then burned in a furnace with a boiler. The furnace heat converts boiler water to steam, which is then used to spin turbines which turn generators and create electricity.

The thermodynamic efficiency of this process has been improved over time. ‘Standard’ steam turbines have topped out with some of the most advanced reaching about 35% thermodynamic efficiency for the entire process, which means 65% of the coal energy is waste heat released into the surrounding environment. Old coal power plants, especially ‘grandfathered’ plants, are significantly less efficient and produce higher levels of waste heat. About 40% of the world's electricity comes from coal.

Byproducts of power generation

Byproducts of power plant operation need to be considered in both the design and operation. Waste heat due to the finite efficiency of the power cycle must be released to the atmosphere, often using a cooling tower, or river or lake water as a cooling medium. The fuel gas from combustion of the fossil fuels is discharged to the air; this contains carbon dioxide and water vapor  as well as other substances such as nitrogen, nitrous oxides, sulfur oxides, and (in the case of coal-fired plants) fly ash and mercury. Solid waste ash from coal-fired boilers is removed. Ash generated can be re-used for building materials.