Three popular combined cycle designs

The three popular ways of combining a gas turbine with a steam cycle are: (1) unfired/supplementary fired heat recovery boiler, (2) exhaust used as combustion air in a fired boiler, and (3) exhaust used for feed water heating.

Design 1: Unfired (or supplementary fired) heat recovery boiler (HRSG)

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

The GT exhaust raises all the steam in a heat recovery boiler. A duct burner may add supplementary fuel. This is the most common modern design (efficiency 50-60%).

Design 2: Exhaust used as combustion air in a fired boiler

  air      fuel
   |1        |
   v         v
  [C]--2-->[CC]--3-->[GT]==> G1
                      | 4 hot exhaust (~15% O2)
                      v   used as combustion air
         fuel --> +-----------------+  a  +----+
                  | Fired steam     |---->| ST |==> G2
       stack <----| boiler          |     +----+
                  +-----------------+       | b
                     ^ d                    v
                     |  +------+  c  +-----------+
                     +--| Pump |<----| Condenser |
                        +------+     +-----------+

The hot, oxygen-rich exhaust replaces the air preheater and forced draft air of a conventional fired boiler. Steam turbine output is large compared to gas turbine output.

Design 3: Exhaust heats the feed water

  air       fuel
   | 1        |
   v          v
  [C]--2-->[ CC ]--3-->[GT]==> G1
   ||                   |  4  exhaust gas
   ++=====shaft=========+  v
          +--------------------------+
 stack <-5| Feed water heater (gas   |
          | to water heat exchanger) |
          +--------------------------+
             ^ d             | e  hot feed water
             |               v
          +------+     +----------+  a  +----+
          | Pump |     |  Steam   |---->| ST |==> G2
          +------+     |  boiler  |     +----+
             ^ c       +----------+       | b
             |              ^ fuel        v
             |           +-----------------+
             +-----------|    Condenser    |
                         +-----------------+

The exhaust heats the condensate/feed water of a conventional steam plant, replacing bled-steam feed heaters; the boiler supplies the rest of the heat.

Explanation of design 1 (heat recovery boiler) with T-s diagram

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
  ^              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)