A waste heat boiler is a system that captures excess thermal energy from a combustion plant and uses it to convert water into steam to produce electricity. Walls of pipes in the boiler circulate the captured flue gases, which reach up to 1,000 degrees Centigrade; the water boils, creating steam.
The energy used to heat the water reduces the flue gas temperature to approximately 300 degrees Celsius. A separate system removes and purifies the cooler gas. Refuse incinerators, cement kilns and coal power plants produce heat energy. Waste heat boilers recycle the plants' excess energy to produce steam energy for electricity-producing turbines and generators.