Working of combined gas-steam plant

A combined power plant (combined cycle plant) uses two thermodynamic cycles in series, so that the heat rejected by the first (topping) cycle becomes the heat input of the second (bottoming) cycle. The most common is the gas turbine - steam turbine combined cycle: a gas turbine (Brayton cycle, heat added at 1100-1500 C) exhausts at 450-600 C, and this exhaust raises steam for a steam turbine (Rankine cycle, heat rejected at 30-45 C). Overall efficiency of 50-60% is achieved, compared with 30-40% for each plant alone.

Layout

      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 |<---+
                         +-----------+

Components

• Gas turbine unit: axial compressor, combustion chamber and gas turbine on one shaft with generator G1.
• Heat recovery steam generator (HRSG): banks of economiser, evaporator (with drum) and superheater placed in the exhaust duct; may have a duct burner for supplementary firing.
• Steam unit: steam turbine with generator G2, condenser, condensate and feed pumps, deaerator.

Working

1. Air is compressed in the compressor (1-2), burnt with fuel in the combustion chamber (2-3) and expanded in the gas turbine (3-4), which drives the compressor and generator G1.
2. Exhaust gas at about 500-600 C enters the HRSG. Here it heats the feed water in the economiser, evaporates it in the evaporator and superheats it in the superheater (d-a). The cooled gas goes to the stack at about 100-150 C (4-5).
3. Steam expands in the steam turbine (a-b) and drives generator G2.
4. Exhaust steam is condensed (b-c) and pumped back to the HRSG (c-d).
5. Optionally, a duct burner before the HRSG burns extra fuel (supplementary firing) to raise steam output.

Heat added at high temperature in the gas cycle and rejected at low temperature in the steam condenser gives a high overall efficiency.

T-s diagram

  T
  ^              3
  |             /|
  |            / |   Gas cycle 1-2-3-4
  |           /  |   (Brayton, top)
  |          /   |
  |         2    |
  |         |    4
  |    a    |   /    4-5: exhaust heats
  |   /|    |  5     steam (d-a) in HRSG
  |  / |    | /      5-1: gas to stack
  | d  |    |/
  | |  |    1
  | c--b       Steam cycle c-d-a-b
  |            (Rankine, bottom)
  +--------------------------> s
  (gas and steam drawn per kg of
   their own fluid, schematic only)

Process summary:

Efficiency

$$\eta_{cc} = \frac{W_{GT} + W_{ST}}{Q_{fuel}} = \eta_{GT} + \eta_{ST} - \eta_{GT}\eta_{ST}$$

With $\eta_{GT} = 0.35$ and $\eta_{ST} = 0.30$: $\eta_{cc} = 0.35 + 0.30 - 0.105 = 0.545$, about 54.5%. Typical modern plants reach 55-60%.