Fossil-fired power plants are large electricity generation plants powered by fuels such as lignite, coal or gas. Basic principle: The heat released by combustion is absorbed by a tube boiler, where the feedwater is turned to steam. The steam flows through pipes to the steam turbine, where the steam releases some of its energy by expansion. In the condenser beneath the turbine, the steam transfers most of its heat to the cooling water. This causes the steam to condense. A feed pump returns this liquid water to the tube boiler as feedwater. This completes the cycle. Some of the generated steam is used to preheat the feedwater in the economiser and the combustion air in the air preheater. The rotary motion produced in the turbine drives the connected electrical generator.