Mumbai University > Mechanical Engineering > Sem 7 > Power Plant Engineering

Marks : 5M

Year: Dec 2015

The economics of power generation by gas turbines is proving to be more attractive, due to low capital cost, and high reliability and flexibility in operation. Quick starting and capability of using wide variety of fuels from natural gas to residual oil or powdered coal are other outstanding features of gas turbine power plants. Major progress has been made in three directions namely increase in unit capacities of gas turbine units (50—100 MW), increase in their efficiency and drop in capital cost, (about Rs. 700 per kW installed). Primary application of gas turbine plant is to supply peak load. However gas turbine plants now-a-days is universally used as peak load, base lead as well as standby plants.

1. It is smaller in size and weight as compared to an equivalent steam power plant. For smaller capacities the size of the gas turbine power plant is appreciably greater than a high speed diesel engine plant but for larger capacities it is smaller in size than a comparable diesel engine plant. If size and weight are the main consideration such as in ships, aircraft engines and locomotives, gas turbines are more suitable.

2. The initial cost and operating cost of the plant is lower than an equivalent steam power plant. A thermal plant of 250 MW capacity cost about Rs. 250 crores. Presently whereas gas turbines plant of that same-size cost nearly 70 crores.

3. The plant requires less water as compared to a condensing steam power plant.

4. The plant can be started quickly, and can be put on load in a very short time.

5. There are no standby losses in the gas turbine power plant whereas in steam power plant these losses occur because boiler is kept in operation even when the turbine is not supplying any load.

6. The maintenance of the plant is easier and maintenance cost is low.

7. The lubrication of the plant is easy. In this plant lubrication is needed mainly in compressor, turbine main bearing and bearings of auxiliary equipment.

8. The plant does not require heavy foundations and building.

9. There is great simplification of the plant over a steam plant due to the absence of boilers with their feed water evaporator and condensing system.

Advantages of combined cycle power generation 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.