Page 145 - Advanced Gas Turbine Cycles

116 Advanced gas turbine cycles

7.4.3. The exhaust heated (supplementary $red) 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.
The T, s diagram for a combined plant with supplementary firing is illustrated in Fig. 7.5
(again the steam entropy has been scaled). Introduction of regenerative feed heating of the
water is of doubtful value, as will be discussed later. Supplementary heating generally
lowers the overall efficiency of the combined plant. Essentially this is because a fraction of
the total heat supplied is utilised to produce work in the lower cycle, of lower efficiency
than that of the higher cycle.
",
M'

, , f
, CON

3

0 Sgr P'ss
Fig. 7.5. Open circuit gas turbindclosed steam cycle combined plant (CCGT). With supplementary firing (after
Ref. [ 11).