Combined plant heating feed water with exhaust

In this scheme a gas turbine is added to a conventional steam plant and its exhaust is used to heat the boiler feed water, replacing some or all regenerative (bled-steam) feed water heaters.

Layout

  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    |
                         +-----------------+

Working:

1. Air is compressed (1-2), burnt with fuel in the combustion chamber (2-3) and expands in the gas turbine (3-4), driving the compressor and generator G1.
2. The hot exhaust (about 450-550 C) passes through the feed water heater (an economiser-type gas-to-water heat exchanger) and leaves to the stack at 5.
3. Condensate from the steam plant is pumped (c-d) and heated by the exhaust gas (d-e) nearly up to the boiling temperature. This replaces the bled-steam feed heaters, so steam that would have been extracted now expands fully in the turbine and produces extra power.
4. The hot feed water enters the conventional fired steam boiler, where fuel supplies the latent heat and superheat (e-a).
5. Steam expands in the steam turbine (a-b) driving G2, is condensed (b-c) and the cycle repeats.

The steam boiler is still fully fired, so this scheme gives a modest gain (about 3-5% points) and is often used to repower an existing steam plant by adding a gas turbine.

T-s diagram

  T
  ^          3
  |         /|
  |        / |     gas cycle 1-2-3-4,
  |       2  |     exhaust 4-5 heats
  |       |  4     feed water d-e
  |       | /
  |       |5
  |       1            a
  |       f---------g /|
  |      e             |
  |     /              |
  |    d               |
  |    c---------------b
  +---------------------------> s
  c-d pump, d-e heating by GT exhaust,
  e-f-g-a heating in fired boiler,
  a-b steam turbine, b-c condenser
  (gas and steam drawn per kg of
   their own fluid, schematic only)

The heat given by the gas, $\dot m_g c_p (T_4 - T_5)$, equals the heat taken by the water, $\dot m_w (h_e - h_d)$.