Combined plant layouts, performance and economics

What is a combined power plant

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.

Layouts of various types

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.

Performance and economics

Performance of a combined plant is judged by its overall thermal efficiency (or heat rate):

$$\eta_{cc} = \frac{W_{GT} + W_{ST}}{Q_{GT} + Q_{sf}}$$

where $W_{GT}$, $W_{ST}$ are net outputs of gas and steam units, $Q_{GT}$ is fuel heat to the gas turbine and $Q_{sf}$ is any supplementary fuel heat.

For an unfired HRSG with no losses, all heat rejected by the gas turbine, $Q_{GT}(1-\eta_{GT})$, goes to the steam cycle, so

$$\eta_{cc} = \eta_{GT} + \eta_{ST} - \eta_{GT}\,\eta_{ST}$$

Example: $\eta_{GT} = 0.35$, $\eta_{ST} = 0.32$ gives $\eta_{cc} = 0.35 + 0.32 - 0.112 = 0.558$ (55.8%).

If the HRSG (heat recovery) effectiveness is $\eta_{HR}$ (part of exhaust heat lost to stack), $\eta_{cc} = \eta_{GT} + \eta_{HR}\,\eta_{ST}(1 - \eta_{GT})$.

Other performance figures: heat rate = 3600 / $\eta_{cc}$ kJ/kWh (about 6000-7000 kJ/kWh), power ratio $W_{ST}/W_{GT}$ (about 0.5), part-load efficiency and stack temperature.

Economics:

• Capital cost per kW is lower than for coal or nuclear plants (the gas turbine is factory made, the HRSG is simple, and there is no coal or ash handling); it is often about half that of a coal plant, or less.
• Construction time 2-3 years; phased construction earns revenue early.
• Fuel cost is the largest cost item; high efficiency reduces it, but gas/oil price decides the cost of energy.
• Operating and maintenance cost is moderate; fewer staff, but GT hot parts need regular overhaul.
• Cost of electricity is lowest for base and intermediate load where cheap natural gas is available; supplementary firing gives cheap peaking capacity.