Waste heat methods and feed water heating layout

Methods of utilising waste heat

Common methods of utilising the gas turbine exhaust (waste) heat:

1. Heat recovery steam generator (HRSG), unfired: exhaust passes through an economiser, evaporator and superheater and raises steam for a steam turbine. Simplest and most common; steam turbine gives about one-third of the plant output.
2. Supplementary fired HRSG: extra fuel is burnt in a duct burner in the exhaust (it still contains about 15-16% O2) to raise gas temperature and steam output/temperature; increases output and flexibility.
3. Exhaust-fired (fully fired) boiler: GT exhaust is used as hot combustion air for a conventional fired steam boiler, replacing the air preheater.
4. Feed water heating: exhaust heats the boiler feed water in a gas-to-water heat exchanger (economiser), replacing some or all bled-steam feed heaters of a steam plant.
5. Pressurised (supercharged) boiler: combustion takes place under pressure in the boiler furnace; after raising steam, the gases expand in a gas turbine that drives the compressor.
6. Cogeneration / process heat: exhaust or exhaust-raised steam is used for industrial process heating, district heating or absorption cooling.
7. Regenerator / recuperator: exhaust preheats the compressed air of the gas turbine itself (regenerative gas turbine).

Layout: gas turbine exhaust used to heat 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    |
                         +-----------------+

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)