The fuel used in thermal power plants is:
fossil fuel
In a thermal power station, fuel like coal, naphtha, lignite, etc., are ignited. The heat energy liberated is used to convert water into steam at high pressure and temperature. Hence, thermal power plants use fossil fuels.The energy of the steam is used to rotate the turbine to generate electricity.
The energy changes taking place here is:
Chemical energy -> Heat energy -> Mechanical energy -> Electrical energy.