The exhaust heated supplementary fired CCGT

The exhaust gases from a gas turbine contain substantial amounts of excess air, since the main combustion process has to be diluted to reduce the combustion temperature to well below that which could be obtained in stoichiometric combustion, because of the metallurgical limits on the gas turbine operating temperature. This excess air enables supplementary firing of the exhaust to take place and higher steam temperatures may then be obtained in the HRSG. 

For a mass flow of air (Mg) to the compressor of the gas turbine plant, a mass flow Mf of fuel (of specific enthalpy hfo) is supplied to the two combustion chambers (Mf = (Mf)ff + (Mf)f). The overall efficiency of the combined plant is then 

Then with vl = GlAGl + Gh) and vun = GunAGl + Gh). it follows that 

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