A simple and low-cost cogeneration power plant can be constructed by combining a gas turbine and a heat recovery boiler as shown in the following figure.

 
 

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Figure 5. Gas Turbine Heat Recovery Boiler Power Plant.

 

Heat is generated by the hot exhaust gases from the gas turbine. Heat can be taken out of the heat recovery boiler either in form of district heat or process steam.

The amount of heat that can be recovered depends on the required temperature level of the heat recovery and on the fuel used. Due to sulphur corrosion risk, the exhaust gas temperature must not go below 120... 170 oC with oil firing depending on the sulphur content of the oil. When natural gas is used, the temperature can be reduced to 60-100 oC depending on the returning district heating water temperature and thereby more heat can be recovered from the flue gases.

In some applications the power plant is equipped with a supplementary burner, which utilises the exhaust gases of the gas turbine as combustion air, since the gases still contain about 15 % of oxygen. Naturally, the heat generated with the supplementary burner is not CHP Heat by definition. The same applies to fresh air burners, which are not dependent on the gas turbine, because they have separate combustion air fans.

In some cases, the power plant contains a by-pass stack past the heat recovery boiler, so that the gas turbine can be operated without the boiler, if necessary. In this case, the electricity generated without the heat recovery boiler is not CHP Electricity. The same applies, when the capacity of the heat recovery boiler is much less than what would be economically feasible, in which case only part of electricity generation can be regarded as CHP Electricity.