Gas Turbine Power Plants

Q. How does a gas turbine differ from a diesel engine?
Ans. The gas turbine is different from the diesel engine in the respect that the combustion region is external to the prime mover.
Q. Why are the majority of gas turbine power plants employed as peak load plants?
Ans. The majority of such power plants are usually used for driving generators and supplying peak loads in other types of power plants (steam and hydroelectric power plants) because they are started and loaded quickly. Other reasons for it are low initial costs and higher fuel costs.
Q. Why are coal and peat not used as fuel in gas turbine power plants?
Ans. Though coal and peat can be employed as gas turbine fuels but these are not used mainly because of coal handling and if handling problems.
Q. Why is it advantageous to use high-pressure turbine for the driving compressor and low-pressure turbine for driving alternator in gas turbine power plants?
Ans. This arrangement has the advantage that the speed of the power turbine (the turbine driving the alternator) can be kept constant at synchronous speed while the speed of the turbine driving the compressor can be varied according to the output required.
Q. Why is the efficiency of open-cycle gas turbine power plants quite low?
Ans. The overall efficiency of such power plants called the open cycle plants is very low as a greater part (around 65%) of the power developed by the turbine is used in driving the compressor. The fact that the hot gases from the combustion chamber are cooled to a safe temperature before admitting it to the turbine also reduces the efficiency. The overall efficiency of the plant, owing to above reasons seldom exceeds 20%.
Q. Compare the diesel power plants and gas turbine power plants of the same capacity.
Ans. The gas turbine power plants have numerous inherent advantages over diesel power plants such as smaller site area, lower cost of buildings, low initial cost, better heat rate, less water requirement, easier maintenance, reduced attendance charges, improved reliability, fewer vibrations, no cyclic variations etc. There are no problems of fuel injection and cylinder lubrication as with diesel engines. Possibilities of use of lower grades of fuel oil or low-grade waste gases with gas turbines are also there.
The only drawback of a gas turbine power plant in comparison to diesel power plants is higher fuel consumption (0.2 – 0.25 kg per kWh output) which is insignificant against their inherent advantages, mentioned above.
Q. In what respects gas turbine power plants are inferior to steam power plants?
Ans. The drawbacks of gas turbine power plants over steam power plants are:
(a). The operating costs of a gas turbine power plants are much higher than those of a steam power plant.
(b). Gas turbines can be employed only in small sizes (about 50 MW or so)
(c). Like steam turbines, the gas turbine is not readily reversible.
(d). In a case of a gas turbine power plant, the compressor has to be started before starting of the turbine and the former requires power from some external source.

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