Working and performance of combined plant

Working of 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.

      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 |<---+
                         +-----------+

Working:

1. Air is compressed in the compressor (1-2), burnt with fuel in the combustion chamber (2-3) and expanded in the gas turbine (3-4), which drives the compressor and generator G1.
2. Exhaust gas at about 500-600 C enters the HRSG. Here it heats the feed water in the economiser, evaporates it in the evaporator and superheats it in the superheater (d-a). The cooled gas goes to the stack at about 100-150 C (4-5).
3. Steam expands in the steam turbine (a-b) and drives generator G2.
4. Exhaust steam is condensed (b-c) and pumped back to the HRSG (c-d).
5. Optionally, a duct burner before the HRSG burns extra fuel (supplementary firing) to raise steam output.

Heat added at high temperature in the gas cycle and rejected at low temperature in the steam condenser gives a high overall efficiency.

Determining performance

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.