It has been found that a considerable amount of heat energy goes as a waste with the exhaust of the gas turbine. This energy must be utilized. The complete use of the energy available to a system is called the total energy approach. The objective of this approach is to use all of the heat energy in a power system at the different temperature levels at which it becomes available to produce work, or steam, or the heating of air or water, thereby rejecting a minimum of energy waste. The best approach is the use of combined cycles. There may be various combinations of the combined cycles depending upon the place or country requirements. Even nuclear power plant may be used in the combined cycles.
Fig. shows a combination of an open cycle gas turbine and steam turbine. The exhaust of gas turbine which has high oxygen content is used as the inlet gas to the steam generator where the combustion of additional fuel takes place. This combination allows nearer equality between the power outputs of the two units than is obtained with the simple recuperative heat exchanger. For a given total power output the energy input is reduced (i.e., saving in fuel) and the installed cost of gas turbine per unit of power output is about one-fourth of that of steam turbine. In other words, the combination cycles exhibit higher efficiency. The greater disadvantages include the complexity of the plant, different fuel requirements and possible loss of flexibility and reliability. The most recent technology in the field of co-generation developed in USA utilizes the gaseous fuel in the combustion chambers produced by the gasification of low quality of coal. The system is efficient and the cost of power production per kW is less.
The worldwide demand for combined cycle power plant is growing dramatically, with some experts forecasting explosive growth over the next decade . In its basic form, a gas turbine exhausting into a heat recovery steam generator (HRSG) that supplies steam to a steam turbine cycle is the most efficient system of generating electricity today. Thanks to defence and aircraft research and development programs, the gas turbine technology has forged ahead considerably during the last two decades with unit plant efficiencies of 30 to 35%, comparable to the state-of-the-art fossil-fired power stations.
The advantages of combined gas-steam cycles may be summarized as follows:
1. High overall plant efficiency: Efficiencies exceesing 50% can be attained.
2. Low investment costs: Because 2/3 of the output is produced in a GT and only 1/3 in the simple ST, the investment costs required are approximately 30% less than those for a conventional steam power plant.
3. Small amount of water required: The amount of cooling water required is only about 40 to 50% as much as for a steam plant.
4. Great operating flexibility: The simple steam cycle makes it possible to start-up and shut-down the plants quickly, which also affects efficiency in a positive direction (reducing start-up losses).
5. Phased installation: Because the gas turbines can go into operation much sooner than the steam plant, installation in stages is possible. The gas turbine plant can keep on generating power as the steam plant is under construction. This makes it possible to adjust the growth in demand for energy in a grid. Later, a coal gasification unit can be installed if there is too sharp an increase in the price of oil or gas.
6. Simplicity of operation: It is similar to run than a conventional steam power plant. Moreover, because combined cycle plants are generally operated fully automatically, they are especially suitable for use where operating staff is less experienced.
7. Low environmental impact: Gas burning combined cycle (CC) plants in particular are ideally suitable for use in heavily populated regions because of their high efficiency and low emission levels of pollutants. In particular, very low NOx levels of clean CC plants are one of their most attractive features. Furthermore, gas-fired CC plants produce per kWh only 40% of the CO2 produced by a coal-fired plant .
8. Advantages for cogenerations of heat and electricity
The good thermodynamic properties of CC plants are highly suitable for cogenerations of heat electricity. Electrical yields of more than 40% are quite common in heating or industrial power plants with a back pressure turbine.
Large output combined with high cycle efficiency, low emission level and low investment cost are the main attractive features of the CC power generation.
By dividing the expansion process into two temperature ranges, 1100-550 0C in the gas turbine and 550 0C to ambient temperature in the steam turbine, high overall efficiencies exceeding 50% are achieved.
DISADVANTAGES
Major part of the work developed in the turbine is used to derive the compressor. Therefore, network output of the plant is low.
Since the temperature of the products of combustion becomes too high so service conditions become complicated even at moderate pressures.
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