section_path: History/Human control of fire/Productive use for energy
section_name: Productive use for energy
paragraph_text: Productive use for energy
  
  Burning fuel converts chemical energy into heat energy; wood has been used as fuel since prehistory. The International Energy Agency states that nearly 80% of the world's power has consistently come from fossil fuels such as petroleum, natural gas, and coal in the past decades. The fire in a power station is used to heat water, creating steam that drives turbines. The turbines then spin an electric generator to produce electricity. Fire is also used to provide mechanical work directly by thermal expansion, in both external and internal combustion engines.
  
  The unburnable solid remains of a combustible material left after a fire is called clinker if its melting point is below the flame temperature, so that it fuses and then solidifies as it cools, and ash if its melting point is above the flame temperature.