The recuperative STIG plant

Consider next a recuperative STIG plant (Fig. 6.5, again after Lloyd [2]). Heat is again recovered from the gas turbine exhaust but firstly in a recuperator to heat the compressed air, to state 2A before combustion and secondly in an HRSG, to raise steam S for injection into the combustion chamber. 

Again we analyse the open cycle version of this plant, but with a fuel input/ (per unit air flow) at ambient temperature Tq, i.e. a fuel enthalpy flux of/ /rfo- For the combustion chamber we may now write 

But of course these two equations may be combined with Eq. (6.12) to give the steady flow energy equation for the whole plant as 

16) is essentially the same as Eq. (6.8) for the basic STIG plant which, on reflection, is not surprising. If the states 1,2, 3, 4 and 5 and the steam quantity S are all the same then expressions for the work output, the heat input (or fuel energy supply) and the heat rejected are all unchanged. The total amount of heat transferred from the exhaust is also unchanged, but two separate flows, of air and of water/steam, have been raised in enthalpy before entry to the combustion chamber, rather than one (water/steam) in the earlier analysis. 

However in practice, for the same states 1-5 the steam raised S will be less hence there is no advantage in operating a STIG plant in this variation of the basic CBTX recuperative gas turbine plant. Nonetheless, this form of analysis as developed by Lloyd will prove to be useful in the discussion of the chemical recuperation plant in Chapter 8. 

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