Heat from fuel is the main source of energy for CHP plants. This energy is released by burning the fuel with air to produce high-temperature combustion gases. If burned in a confined space, some of the energy released pressurises the exhaust gases, thereby providing the power to drive an engine or gas turbine and generate electricity. The exhaust gases are subsequently released at a lower pressure and temperature: they can then be the main source of heat for on-site use. Packaged CHP units are normally based on engines, although gas turbines can be used. Units are designed to operate on gaseous fuel only, usually using natural gas provided through the national supply system. Some units are capable of operating on other gases such as gaseaous waste fuel, which may be available locally as by products of other plants or processes. Some typical fuel calorific values have been included as a guide.
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