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Combined cycle components and waste heat use

Short answer

Basic components: compressor, combustion chamber, gas turbine and generator; HRSG with economiser, evaporator, superheater; steam turbine, generator, condenser, feed pump. Waste heat: 500-600 C exhaust with 15-16% oxygen passes through HRSG, superheating steam, evaporating water, preheating feed water; stack gas leaves at 100-150 C. Steam produces about one-third of output without extra fuel; overall efficiency 50-60%.

Draw the basic component of a gas and steam turbine combined cycle. Explain how the waste heat is utilized in this cycle.

Basic components of a gas and steam turbine combined cycle

      air 1                fuel
      |                     |
      v                     v
   +------+     2     +-----------+   3   +------+
   | Comp |---------->| Combustion|------>|  GT  |===> G1
   +------+           |  chamber  |       +------+
      ||              +-----------+        ||  |
      ++=============== shaft =============++  | 4 hot exhaust
                                               v   (~500-600 C)
   stack <-- 5 --+-------------------------------+
                 |  HRSG (economiser,            |
                 |  evaporator, superheater)     |
                 +-------------------------------+
                     ^ d                  | a steam
                     |                    v
                 +------+             +------+
                 | Pump |             |  ST  |===> G2
                 +------+             +------+
                     ^ c                  | b
                     |   +-----------+    |
                     +---| Condenser |<---+
                         +-----------+
  • Gas turbine plant: compressor (Comp), combustion chamber, gas turbine (GT), generator G1.
  • Heat recovery steam generator (HRSG): economiser, evaporator and superheater in the exhaust duct; optional duct burner.
  • Steam plant: steam turbine (ST), generator G2, condenser, feed pump.

How the waste heat is utilised

  1. The gas turbine exhaust leaves at about 500-600 C and still contains 15-16% oxygen; in a simple gas turbine plant this heat (about 60-65% of fuel energy) would go to the atmosphere.
  2. In the HRSG the exhaust gives up this heat in stages: first superheating the steam, then evaporating water, and finally preheating feed water in the economiser. Gas leaves the stack at about 100-150 C.
  3. The steam produced drives the steam turbine and generates about one-third of the total plant output without any extra fuel.
  4. If more steam is needed, extra fuel is burnt in the exhaust (supplementary firing), using its remaining oxygen.

Thus the heat rejected by the gas turbine cycle becomes the heat input of the steam cycle, and the overall efficiency rises to about 50-60%:

ηcc=ηGT+ηST−ηGTηST\eta_{cc} = \eta_{GT} + \eta_{ST} - \eta_{GT}\eta_{ST}

✓ Answer checked by Claude Opus 5.5 (high effort)

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