Gas Turbine Site Rating to Heat exhaust Heat

[Vivaldi M. Smith]

I am trying to figure out if the site rating of my Gas Turbine-Compressor can be translated into the amount of heat that the exhaust of this turbine will deliver.
Turbine Site rating is 30000 HP. Will it be fair to translate that into KJ/hr and assume that is the correct amount of Heat the exhaust delivers?
When I look at the data it says 12 Kj/kw*hr but thats the heat rate and does not match to the HP value .

 

[LittleInch]

30000 HP eh so 23MW. Not a bad size.

That's usually shaft output power at 20C intake air.

If this is a single open cycle GT, then its about 30-35% efficiency. All the rest goes as heat. So you're dumping something like 40MW up the stack.

I've no idea what that 12KJ/kWhr refers to as it's too vague and kWhr of what? energy in, shaft power out?

Pretty basic question here - what are you trying to do?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Vivaldi M. Smith]

it is a MS50002 D gas turbine that drives a compressor ( double shaft) , I am pretty much trying to figure out if the exhaust outlet heat can be reused for another service.

 

[LittleInch]

Heat rate is basically the amount of heat you put in in the form of fuel to get 1KW hr of energy out.

1KWhr is basically 3,600 kJ
Your heat rate in is 12,400kJ,
I.e. 29% efficiency.

So you have the difference as "waste" heat. The lower the heat rate per KWhr the better efficiency - your turbine is quoted at 28% thermal efficiency.

So yes in theory you have 40MW of heat to extract. Reality is that the heat exchangers to get all that heat out get to be enormous.

But that's how Combined cycle GTs work - they use the heat out of the back of the turbine to create steam which runs a steam turbine.

What do you want your heat for? How much? And more importantly how much money do you have to spend? high temperature high volume gas heat exchangers don't come cheap.

Also you need to talk to the OEM of the turbine as introduction of a second HX will impact the operation of main turbine as it wasn't designed with the back pressure on the exhaust that would be required.

sounds good - Often very complex and costly.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[EdStainless]

I'll wager that there is an HRSG (heat recovery steam generator) design for this turbine that will get you about 20-25MW of recovered energy as steam.
Go back to the OEM and ask.
A caution, HRSGs are not simple to operate. They require high quality feedwater, and care and attention.
There is a high fraction of the combined cycle plants in the world that cannot operate CC, because they have trashed their HRSG and/or condensers.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[georgeverghese]

Also note that the exhaust heat rate is a function of the turbine speed at fixed torque, and will be at a minimum when turbine-compressor speed is lowest. For single shaft turbines, speed does not turndown less than 80% of max, while speed can go down to as low as 60% of max (from memory - some one may verify) for a dual shaft GT. Ask the GT vendor for the applicable exhaust heat rates at min speed at normal operating torque.

You can get out of this dependence on speed by implementing supplementary firing to keep derived heat constant.

Some of the GT waste heat recovery units I've seen were not in service simply because the diverter damper in the WHRU for capacity control had failed. There have been accidents at these GT - WHRUs' in hot oil service in LNG plants in the past - shoddy expander rolled tube - tubesheet joints. Use seal welded joints (with double tube sheets preferably) for flammable heat transfer media.

 

[EdStainless]

George, Yes I have seen much the same.
Damper failures are common as well.
I have only seen one hot oil failure.
And while it looked like tube to tubesheet failure it was actually a very poor design that didn't properly handle differential thermal expansion.
Keep the tubes thin and seal weld. But then this is nearly always good advice.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[georgeverghese]

Also, note that "normal operating torque" will be lower when the turbine is new and unfouled as compared to when the turbine has taken up the entire allocated fouling margin when it has done several thousand hours in operation. Typical fouling margin is 10% of ISO rating.
@Ed, agreed, root cause for these joint weld failures may well be differential thermal expansion.

 

[Vivaldi M. Smith]

Replying to LittleInch . I am just looking at the option of using a WHRU instead of a direct fire heater. The current heater has a design duty of 139MMBUT/hr . Therefore, I was trying to see if the exhaust heat that my Turbine double stage Compressor can be good enough to supplement the heat that my heater supplies.

 

[LittleInch]

I get 139 million BTU/hr to be 40 MW?

So in theory yes, but in practice you will only get 50% of that demand out of the waste heat if you're lucky, and at a significant expense.

Were you just thinking to direct the waste gas through the heater? Not sure if that's possible or not, but note the exhaust temp of the gas out the back of the GT - about 500C. Your fired heater might be more or less than than that - I don't know.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Vivaldi M. Smith]

The plan is to build a WHRU from the exhaust of the Turbine Compressor, that way we can bypass the Direct fire heater. But if we have to do that we must ensure that the 139 MM BTU/hr around 150 MMKJ/Hr, that our heater heat supplies can be compensated with the Turbine exhaust. That said, for a Turbine with a site rating of 36K HP about 99 MMKJ/hr, taking into account most a typical simple-cycle gas turbine will convert 30% to 40% of the fuel input into shaft output. All but 1% to 2% of the remainder is in the form of exhaust heat it does not seem feasible. What are your thoughts?

 

[LittleInch]

Single cycle is more like 30% efficiency.

Given that the best CCGT gets up to no more than 63%, then economically recoverable heat is no more than 99 MM KJ/hr and probably less, So you're going to be a long way short of your heater demand, probably about half.

But doing something is better environmentally than nothing to improve the overall efficiency.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

Recovered heat from the turbine exhaust will depend on the process fluid exit temp at the direct fired heater - you havent said what this is.

 

[crshears]

A caution, HRSGs are not simple to operate. They require high quality feedwater, and care and attention.
There is a high fraction of the combined cycle plants in the world that cannot operate CC, because they have trashed their HRSG and/or condensers.

Interesting! I had not heard this before . . .

So is the problem that the owners of the plants cheap out on training their operators, or won't pay them a decent wage, or won't spend the money on the needed water analysis and/or treatment chemicals, or simply that good, competent and qualified operators with a good work ethic are hard to find, or . . . ?



CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[georgeverghese]

At the outset, it must be stated that this dual shaft GT is less suited for heat recovery application, since it has a wider speed range and fuel consumption range. Hence exhaust gas rate and temp will vary much more with a dual shaft GT than with a single shaft GT as compressor load varies. This trend is only valid when the heat load at the new proposed WHRU (or the existing direct fired heater) remains essentially constant even while compressor load varies.

It looks like this dual shaft GT was selected merely from a fuel consumption perspective at this GT alone, without any consideration for future WHRU.

Hence you may be short on WHRU capacity a lot of of the time when compressor capacity drops and speed decreases with this dual shaft GT. The alternative is to keep this GT, and raise the min speed setting on the compressor speed controller so that it does not turn down on speed as much as it does now as compressor capacity drops. This will force the compressor into recycle much more often, use more fuel gas, but you'd have a more stable operation of the WHRU. Acceptable minor mod on process controls which helps to make the WHRU viable.

 

[pmover]

a regenerator for this application may be the better option. depends upon fuel cost/savings and HP losses and plot space available. typically, there are two regenerators, one on each side of the unit.

 

[georgeverghese]

There are many parameters the affect recoverable heat from this GT, some of which are : (a) current direct fired heater process fluid exit temp (b) max summer time temp (c) design case compression power (d) GT exhaust heat rates and temp at part load at 100% speed from GE.

Approximate trends for (d) for single shaft and dual shaft GT are to be found in Perry Chem Engg Handbook 6th edn in the chapter on process machinery drives.

We dont know any of this at the moment, so only loose estimates of recoverable heat at the WHRU can be made. It is likely, however, that this compressor would have to be operated at fixed speed to maintain WHRU capacity.

 

[pmover]

"It is likely, however, that this compressor would have to be operated at fixed speed to maintain WHRU capacity."

correct. since the unit is a split shaft driving a compressor, a regenerator is the better fit.

 

[georgeverghese]

@pmover,
Perry Chem Engg Handbook 7th edn compares the cycle eff of various GT heat recovery options on page 29-39 in fig 29-40:

At a compression ratio of say 10 in the gas producer section of the GT,
The dual shaft simple cycle eff is about 28%
Dual shaft regenerative cycle (combustion air preheater) eff is about 35%
Dual shaft combined cycle, based on steam, which can also be used for any other process heating WHRU also (provided process fluid exit temp is not very high), eff is about 43% (think this is a conservative estimate), and is the highest of all heat recovery schemes.

At considerable increase in complexity and cost, the intercooled, regenerative and reheat cycle approaches that of the combined cycle.

The process control mod for operating the dual shaft GT on fixed speed is easily done at DCS even while the compressor-GT unit is still running.

 

[pmover]

@georgeverghese

ok, & agree.

my point is that the unit may not continuously operate at full load - likely not since a GT driven compressor. so, this is a matter for the designer/operator to consider. if operated constantly at full-load; heck ya, power generation.

utilizing any type of heat recovery benefits the end user. also, the end user will need to be permitted to sell electricity if not a utility. not a big deal, but these details are unknown at this time.

the GT unit will operate the compressor to meet process demand; hence operation of the unit "may very well" vary.