Combined cycle merits, demerits and designs

A gas and steam combined cycle uses the exhaust heat of a gas turbine to generate steam for a steam turbine, so overall efficiency becomes 50-60%.

Advantages

• High overall efficiency (50-60%) against 35-40% for steam and 30-38% for gas turbine plants, so lower fuel cost per kWh.
• Lower capital cost per kW than coal or nuclear plants; the gas turbine part is compact and factory built.
• Short construction time (2-3 years); can be built in phases (gas turbine runs first in open cycle).
• Quick start-up and good load-following, so useful for intermediate and peak loads as well as base load.
• Less cooling water needed, since only the steam part (about one-third of output) has a condenser.
• Lower emissions of CO2, SO2 and particulates per kWh (usually natural gas fuel); no ash handling.
• Less space and smaller staff; high reliability and availability.
• Flexible: supplementary firing, cogeneration (process steam), repowering of old steam plants.

Disadvantages

• Needs clean, costly fuel (natural gas or distillate oil); not suited to coal unless gasified.
• Output and efficiency fall with high ambient temperature and altitude (gas turbine takes less mass of air).
• More complex plant and controls; two kinds of machines to maintain.
• Gas turbine hot parts need frequent inspection; high-temperature materials are costly.
• Unit size limited by available gas turbine sizes.
• Part-load efficiency falls when the gas turbines run at reduced load.

Popular designs of combination cycles

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.