The process of recovering the thermal energy produced when fuels are converted to gases and residues through the combustion process. The thermal energy generally is recovered through the use of heat exchangers that extract the energy from the hot combustion gases. Heat exchangers can be air to air units similar to those used in residential or commercial hot air heating systems or air to water/steam units (boilers) that can be designed to generate either hot water or steam, similar to residential and commercial hot water and steam generation heating systems. Large electric power production facilities, including modern waste-to-energy plants, that supply needed power to our homes, hospitals and factories, maximise thermal energy recovery efficiency through the utilisation of high temperature, high pressure steam generating boilers that recover both the radiant energy from the combustion process inside the furnace as well as the energy in the hot combustion gases. The high heating value of plastics makes them a valuable source of energy that can be readily recovered in modern waste-to-energy plants. (Tchobanoglous, George, Hilary Theisen and Rolf Eliassen, "Solid Wastes, Engineering Principles and Management Issues," McGraw-Hill, Inc., New York, 1977; Gershman, Brickner & Bratton, Inc., "Small-Scale Municipal Solid Waste Energy Recovery Systems," Van Nostrand Reinhold Company, New York, 1986).