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Feed water heating combined circuit and reasons

Short answer

Circuit: gas turbine with regenerator preheating compressed air; remaining exhaust heats feed water in a gas-to-water heater before the fired steam boiler, replacing bled-steam heaters. Reasons: exhaust carries 60-70% of fuel energy, giving 50-60% overall efficiency, lower fuel cost and emissions, lower capital cost, fast start, less cooling water, and repowering old steam plants.

In a combined gas and steam turbine power plant, exhaust gases of regenerative gas turbine is used to heat feed water for the boiler of a steam power plant. According to this concept draw a complete circuit of a combined power plant showing "Heating feed water with exhaust gases". What are the reasons that inspire you to construct a combined cycle power plant?

Circuit: heating feed water with exhaust gases

The gas turbine is regenerative: its exhaust first preheats the compressed air in a regenerator, and the remaining exhaust heat heats the boiler feed water.

  air 1                fuel
   v                    v
  [C]-2->[ Regen ]-x->[ CC ]-3->[GT]==> G1
           ^   |                  | 4
           +---|------------------+
               v y exhaust
   stack <-- +------------------+
             | Feed water heater|<--d--+
             +------------------+      |
                   | e hot water   +------+
                   v               | Pump |
  fuel --> +--------------+   a    +------+
           | Steam boiler |--+       ^ c
           +--------------+  v       |
                           [ST]==>G2 |
                             | b     |
                             v       |
                        [Condenser]--+
  1. Compressed air (2) is preheated by the turbine exhaust in the regenerator (x), heated in the combustion chamber (3) and expanded in the gas turbine (4).
  2. Exhaust leaving the regenerator (y), still at about 300-400 C, flows through the feed water heater and then to the stack.
  3. Condensate pumped from the condenser (d) is heated by this gas to state e and enters the fired steam boiler, which produces superheated steam (a) for the steam turbine.
  4. Since bled-steam heaters are no longer needed, more steam expands fully in the turbine, increasing steam plant output with the same boiler fuel.

Reasons for building a combined cycle plant

  • Gas turbine exhaust (450-600 C) carries 60-70% of fuel energy; using it gives overall efficiency of 50-60%.
  • Lower fuel cost and CO2 per kWh; lower emissions with natural gas.
  • Lower capital cost and shorter construction time than coal or nuclear plants; phased installation possible.
  • Fast start and load-following, useful for varying demand.
  • Less cooling water and space than a steam plant of the same output.
  • Old steam plants can be repowered by adding a gas turbine.

✓ Answer checked by Claude Opus 5.5 (high effort)

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