9 points to differentiate Gas turbine and steam turbine power plant

Hola! power plant professionals, today we will discuss about one of the important topic which is important from interview's perspective. Because while facing interview for professionals who are having experience in both gas turbine operated power plant or combined cycle power plant and steam turbine operated power plant or thermal power plant  first question will be asked is How would you differentiate both from your work experience point of view ? So everyone is having his own observations but I will share by experience of mine,if you find anything to add please share.

Gas turbine or combined cycle power plant :-

1. First of all if you look theoretically gas turbine power plant is working on Brayton Cycle which is gas power cycle i.e. considering gas as a working fluid. 

2. Plant availability factor depends upon unplanned shutdowns and breakdowns in plant and which keeps machine idle for that time .This factor decides availability of machine throughout year and availability factor for Gas turbine power plant is more as compared to thermal power plant i.e. in the range of 80-99%. And it calculated as follows,

Plant availability factor			=	P - S- F
				P

Where,

          P = Total time considered in hours i.e. 8760 hrs for 1 year

          S = Scheduled outage for planned maintenance (hrs)

          F = Forced outage due to breakdown (hrs)

3. Overall cycle efficiency is 40-45% which is better than steam operated thermal power due to operation of duel cycle.

4. Auxiliary power consumption is in the range of 2-5% of total power generation which is comparatively lower than thermal power plant.

5. Heat rate of complete cycle i.e amount of heat required for generation of one unit of power is less as compared to thermal power plant.

6. Start up time of Gas turbine is less as compared to steam cycle power plant.Gas turbine will start within few minutes of time and ready to serve full MW load in short period of time. 

7. Temperature variation of working fluid inside gas turbine is more as combustion of gas takes place inside turbine. 

8. Initial investment for plant setup and time required for completion of erection and commissioning is less .

9. Ability   of  gas turbine is limited to handle multiple fuels, even slight change in fuel specification will cause drop in efficiency, increased maintenance and frequent inspection etc. It handles only gaseous or liquid fuels.

Steam turbine or thermal power plant :-

1. Steam turbine is working on Rankin cycle which is steam power cycle means considering steam as a working fluid.

2. Plant availability factor is less as compared to gas turbine because boiler is always under the threat of problems like tube leakage,clinker formation,fuel handling plant problem in rainy season,etc.And hence it comes to the range of 70-90%.

3. Overall cycle efficiency is less when it is compared with gas turbine.Major factors responsible for lower efficiency are heat loss in steam turbine condenser and wastage of heat from boiler flue gas. Cycle efficiency of steam turbine cycle is 30-36%.

4. Auxiliary power consumption is more in thermal power plants due to operation of power consuming fans,required to create draft inside boiler furnace to handle flue gases.Which is in the range of 9-11% of total power generated.

5. Heat rate of steam turbine is more as compared to gas turbine i.e. heat required per unit of generation is more.

6. Start up time for boiler and steam turbine is more as compared to gas turbine because it is very difficult to increase temperature by considering thermal expansion constraints in boiler and steam turbine.In steam turbine it will cause  high vibrations and uneven differential expansion.

7. Temperature variation in working fluid i.e steam is very less as compared to gas turbine as highly precise temperature controllers or attemperators are provided at boiler steam outlet.

8. Initial investment and time required from erection to operation phase is more as compared to gas turbine because in steam operated power plant boiler and fuel handling plant will take comparatively more time for erection and commissioning.

9. Fuel flexibility is more in utility Boilers because variety of fuels from coal, biomass, wood chips to waste sludge of plant etc could be burnt without any difficulty.

Hope it will help in understanding the power plant cycles from

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