Combined Gas Turbine Cycle Power Plants

Combined Gas Turbine Cycle Power Plants

The temperatures of gas turbine exhaust lies between the range of 400° C to 525° C with stygen content ranging from 15 to 18 % depending upon the air-fuel ratios used. Therefore, it spetsible to utilise the large heat content of the turbine exhaust effectively in a heat recovery putem for the following purposes.

1. Combustion of fuel in boiler furnace
2. As process heat
3. Generation of steam for process industry.
4. Generation of steam for power generation

The combined gas turbine cycle improves the thermal efficiency of the system considerably. The effectiveness of the system would depend upon the number of hours of plant running and the cost of additional fuel. In case the exhaust of the gas turbine for power generation using steam as working fluid, the combination of gas turbine and steam turbine cycle is called combined gas and steam turbine cycle. Some of the arrangements are being discussed below :

1. Combined Gas Turbine Cycle with Vapour Power Cycle :

Figure A shows the combination of gas turbine and steam vapour cycles. The exhaust of the gas turbine is used in a heat exchanger called waste heat boiler or heat recovery steam generator (HRSG) for generation of low pressure steam before it is exhausted to the atmosphere. The steam generated is used for power generation in the conventional steam power plant cycle.

Simple gas turbine cycle and the Rankine vapour cycle is shown in (T-S) diagram. To improve further the operational efficiency, we may us either the closed gas turbine cycle using helium instead of air.

Also, the combined cycle performance can be improved upon by using inter-cooling and Tebeat cycles with complexities of the plant. For better heat utilization in HRSG, steam may be generated at two or three different pressure with reheat called reheat steam cycle instead of single pressure arrangement alongwith feed water heating cycle.

Figure A

Figure B

Figure B shows the utilisation of gas turbine exhaust in the furnace of a steam power plant. In the boiler furnace the additional fuel is added for raising the temperature of flue gases and the steam generated is used for power generation. It can improve the pressure of steam generation. Thus the combined cycle improves the thermal efficiency upto 5%.

Figure C

2. Combined Gas Turbine Cycle with Nuclear Power Plant :

The gas turbine cycle can be combined with nuclear power plant having a high temperature gas cooled reactor, the coolant being helium gas. The combined cycle is shown in Figure D.

IMAGE

The system consists of two turbines and two generators, one each for nuclear power plant using helium gas and another for steam circuit. Helium gas after absorbing heat and attaining the temperature in the range of 750 to 800° C in the high temperature gas reactor (HTGR), is expanded in the helium turbine. The exhaust from this turbine is passed through the regenerator where it transfers heat to incoming helium from compressor and then it is passed through a feed water heater. Finally, helium is supplied to the compressor and after its compression it is passed through the regenerator to HTGR.

In the steam circuit, the feed water circulated from feed pump is heated in feed water heater by the helium gas. Heated feed water passes to the boiler and the steam generated in the boiler is expanded in the steam turbine. Finally, the steam exhausted from the turbine is condensed in the condenser.

3. Efficiency of Combined Cycles :

A combined cycle is shown in Figure E Topping cycle is gas turbine cycle having efficiency and bottoming cycle is a steam turbine cycle having efficiency η₁.

Figure D

Let :

η₁ = Efficiency of topping cycle

Q₁ = Heat supplied to topping cycle

Q₂ = Heat rejected by topping cycle fluid to bottoming cycle fluid in heat exchanger i.e. heat added to bottoming cycle

Q₃ = Heat rejected in bottoming cycle

η₂ = Efficiency of bottoming cycle

η = Efficiency of the combined cycle

η₁ = 1 – Q₂ / Q₁ ;    Q₂ = (1 – η₁) Q₁      ….(ii)

And, Q₂ / Q₁ = (1 – η₁)   ….(ii)

η₂ = 1 – Q₃ / Q₂ ;    Q₃ = (1 – η₂) Q₂

Efficiency of the combined cycle,

η  = 1 – Q₃ / Q₁  (1 – η₂) Q₂ / Q₁ = 1 (1 – η₂) (1 – η₁)

∴ (1 – η₁) = (1 – η₁) (1 – η₂)   …(1)

If there are y number of cycles coupled in series then,

(1 – η) = (1 – η₁) (1 – η₂)  …. (1 – η_y)      …(2)

Again, from equation (1) we have

1 – η = 1 – η₂ – η₁ + η₁ – η₂

η = η₁ + η₂ – η_{1 .} η₂     …(3)

_{Therefore the overall efficiency of the combined cycle in series is equal to the sum of efficiencies of individual efficiencies minus their product.}

_{Let : Efficiency of simple gas turbine cycle.} η₁ =  0.27

_{Efficiency of simple steam cycle.} η₂ = 0.30

_{Then the overall efficiency of combined gas and steam power according to above equation will be m = 0.27 +0.3 -0.27×03 = 0.489 OF 1893}

_{Conclusion :}

_{Thus we find that the efficiency of combined cycles considerably increases.}

_{Typical range of efficiencies of fossil fuel power plants is given below}

1. _{Gas Turbine Plants : 25% to 30%}
2. _{Medium size steam power plants : 27% to 33%}
3. _{Nuclear power plants : 30% to 33%}
4. _{Large steam power plants : 36% to 40%}
5. _{Diesel power plants : 40% to 42 % 6}
6. _{Combined cycles : 40% to 50%}