PURPOSE: To avoid use of high grade heat for vapor generation by heating nitrogen through heat exchange with hot fluid from a chemical process utilizing the oxygen product of air separation and expanding the heated nitrogen flow in a turbine through action of external work.

CONSTITUTION: An air separator 2 supplies separated oxygen to a plant 6 where reaction substances form a fluid product which is taken out from the plant 6 through a pipeline 10. Nitrogen is produced under a pressure of at least 5 atm and passed through a duct 16 penetrating a heat exchanger 14. Nitrogen is heated up to 700°C or above in the heat exchanger 14. The heat is obtained by taking out hot fluid from the plant 6 and passing through the duct 16 while facing nitrogen. Consequently, the fluid is cooled and normally returned back to the plant 6. Subsequently, the fluid enters into a turbine 18 and expanded through action of external work (typically, generation of electricity).