Combined plant introduction and popular designs

Introduction

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.

Main parts: compressor, combustion chamber, gas turbine, heat recovery boiler or fired boiler, steam turbine, condenser, feed pump and two generators. The arrangement depends on how the exhaust heat is used; the three popular designs are shown below.

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.