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Gas Turbine Vs Steam Turbine


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#1 Tahir

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Posted 17 April 2006 - 04:58 AM

Dear Members!

I am confused which is the best option steam turbine or gas turbine for power generation in terms of cost and performance.

#2 Adriaan

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Posted 17 April 2006 - 06:00 AM

A gas turbine uses high energy fuel that is burned in the combustion chamber with compressed air (and because of the compression that air gets HOT). The EFFICIENCY of a gas turbine is determined by the combustion temperature first and foremost. In and of itself a gas turbine is simple to use (quick to start etcetera). The exhaust gas of a gas turbine is usually still HOT, so in that respect there is an efficiency problem; you would normally add an exhaust heat boiler to use that remaining heat (and use the steam thus generated .... probably with a steam turbine).

A steam turbine does not use fuel directly which makes a steam system usefull if you do not have high calorific gas available. Starting a steam turbine takes time (slowly preheating the system, otherwise you get mechanical problems). A steam turbine itself is very efficient, the energy losses in a steam powerplant occur elsewhere mostly (in the condensor / cooling tower or electric generator (cos phi)). Once a steam turbine is in operation its load can be varied without influencing the efficiency too much (within limits).

Any turbine is expensive because there is a lot of work needed to MAKE the turbine (all those turbine blades, a different shape for each stage). Gas turbines are rather more expensive to make because the turbine blades (especially in the first stages) need to be made of exotic alloys that can withstand the very high temperatures. Also, because you need some system to use the remaining heat in the gas turbine exhaust gasses, you still usually need a steam system - a gas turbine system itself is simpler than a steam system but you need a steam system for efficiency.

The biggest advantage of gas turbines is their flexibility, giving a lot of performance whenever it is needed. Steam turbines take time to start so you have to run inefficiently in order to provide the same flexibility gas turbines do. Ultimately the choice between a steam or gas turbine is decided by many factors (availability of cooling being one of them).

#3 Tahir

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Posted 17 April 2006 - 07:08 AM

Dear Sir!
I'm still confused as the gas tubine is more efficient in cogeneration setup and operating above 90%.
But what about life cycle analysis? and the ambient air temperature above 40 Celsius.As i know the peak load reduction in gas turbine is 2 MW per 10 degree rise in ambient air temperature. So comparing the two on equal basis how is the question mark.












QUOTE (Adriaan @ Apr 17 2006, 04:00 PM) <{POST_SNAPBACK}>
A gas turbine uses high energy fuel that is burned in the combustion chamber with compressed air (and because of the compression that air gets HOT). The EFFICIENCY of a gas turbine is determined by the combustion temperature first and foremost. In and of itself a gas turbine is simple to use (quick to start etcetera). The exhaust gas of a gas turbine is usually still HOT, so in that respect there is an efficiency problem; you would normally add an exhaust heat boiler to use that remaining heat (and use the steam thus generated .... probably with a steam turbine).

A steam turbine does not use fuel directly which makes a steam system usefull if you do not have high calorific gas available. Starting a steam turbine takes time (slowly preheating the system, otherwise you get mechanical problems). A steam turbine itself is very efficient, the energy losses in a steam powerplant occur elsewhere mostly (in the condensor / cooling tower or electric generator (cos phi)). Once a steam turbine is in operation its load can be varied without influencing the efficiency too much (within limits).

Any turbine is expensive because there is a lot of work needed to MAKE the turbine (all those turbine blades, a different shape for each stage). Gas turbines are rather more expensive to make because the turbine blades (especially in the first stages) need to be made of exotic alloys that can withstand the very high temperatures. Also, because you need some system to use the remaining heat in the gas turbine exhaust gasses, you still usually need a steam system - a gas turbine system itself is simpler than a steam system but you need a steam system for efficiency.

The biggest advantage of gas turbines is their flexibility, giving a lot of performance whenever it is needed. Steam turbines take time to start so you have to run inefficiently in order to provide the same flexibility gas turbines do. Ultimately the choice between a steam or gas turbine is decided by many factors (availability of cooling being one of them).


#4 Guest_rad_*

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Posted 30 April 2006 - 10:33 AM

more than turbine performance, i think most important factor should be your refinery overall steam balance.

if you have a lot of excess HP steam, then steam turbine/turbo-alternator may be best option for power generation (and getting better refinery steam balance/heat recovery). If you do not have sufficient HP steam then, GTG might be your best choice with option of provision for HRSG to provide more HP steam as well. I think its impractical to provide new boiler just for steam turbine power generation if you don't have enough HP steam in the first place. for one, the boiler will just use up the fuel gas you could have used for the GTG but additionally, you lose net HP steam as it will be letdown in the steam turbine.

#5 zartisht

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Posted 02 May 2006 - 12:47 AM

I thik you should use a combination of steam plus gas turbine i-e combine cycle power plant ....!!!
siemens is a best combine cycle provider

#6 Guest_Kantreddi_*

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Posted 02 May 2006 - 06:06 AM

Usually these two are never compared just like that. You can compare thermal efficiency of conventional power palnt with that of combined cycle power plant :

that is Fired Boiler + Steam Turbine vs Gas Turbine + HRSG

Yes, combined cycle power plants offer much higher efficiency up to 55% compared to conventional up to 45%

Kantreddi

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#7 Guest_Guest_Pawan_*_*

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Posted 02 May 2006 - 11:52 PM

I've made many financial and technical comparisons for such options.
The first Important thing is what about ur complex steam balance and power requirement.
If u need standalone system for power only then economics depends on power requirement.
SIZE does matter here. I am ready to help u if u wish for a detailed comparison.

YES, ALSO Remove the myth that cogeneration will give you >90% overall efficiency.

#8 panduru

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Posted 03 May 2006 - 05:48 AM

QUOTE (Guest_Pawan_* @ May 3 2006, 10:22 AM) <{POST_SNAPBACK}>
I've made many financial and technical comparisons for such options.
The first Important thing is what about ur complex steam balance and power requirement.
If u need standalone system for power only then economics depends on power requirement.
SIZE does matter here. I am ready to help u if u wish for a detailed comparison.

YES, ALSO Remove the myth that cogeneration will give you >90% overall efficiency.


@Guest_Pawan_:
I feel you should join the forum as a member so that you can give us the benefit of your professional knowledge and experience.

Thanks n Regards

#9 Guest_Guest_Pawan_*_*

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Posted 05 May 2006 - 11:01 PM

Ya, actually I joined way back (some 3-5 years) but forgot all informations regarding my registration.
Anyway registering again.

#10 hamadi

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Posted 06 May 2006 - 04:37 PM

Hi

What I know about steam turbine is more efficient than gas turbine and could provide more power than gas turbine and its efficiency is double the gas turbine. However it takes time for startup in which it it takes hour in the other hand gas turbine approximately 30 minutes for normal operating speed and also gas turbine is easier to maintain.

#11 Tahir

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Posted 08 May 2006 - 04:37 AM

QUOTE (Guest_Pawan_* @ May 3 2006, 09:52 AM) <{POST_SNAPBACK}>
I've made many financial and technical comparisons for such options.
The first Important thing is what about ur complex steam balance and power requirement.
If u need standalone system for power only then economics depends on power requirement.
SIZE does matter here. I am ready to help u if u wish for a detailed comparison.

YES, ALSO Remove the myth that cogeneration will give you >90% overall efficiency.



Dear Pawan!
Thanks for the response . We have three Borsic boiler with 80 t/h steam generation at 40 bar and three turbo set of 9 MW each. Our normal plant load is around 18 MW and all three turbo sets are in operation shearing the load. The steam consumption per MW is 7.6 tons. The power plant exports about 80 tons per day steam to other plants. As we are operating in an island mode we need to have the standby power which is 9 MW. Due to this the turbo sets are operated at 66% load. One idea is to replace the existing system with GTs and HRSG to meet the electric power and export steam of the complex and it will save us money. Pl advise. Pl also tell what you mean by the myth that GTs >90% overall efficiency.The companies offering the system claim the same.

Tahir

#12 bramantyo

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Posted 28 December 2011 - 09:53 PM

Dear Members,

Proceeding your previous discussion, I have question regarding the selection of steam or gas turbine for FLNG. I read lot of articles saying that gas turbine (aero derivative) is the best option for FLNG as it is compact, light weight, simple in operation & maintenance, and high availability & reliability. But other FLNG project (still in detail engineering) use steam turbine instead of gas turbine for the power generation system. And the reason behind this selection is that steam turbine offers proven high reliability in marine setting, ALSO simple in operation & maintenance, fuel gas composition flexibility, and avoid the use of fired equipment in liquefaction module. Actually, what is the most important point to consider regarding this selection? Which option is the best for FLNG: steam or gas turbine?

Thanks & Regards.




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