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Wednesday, May 23, 2018
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Accounting for Water Injection in an LM6000
A question about what exactly happens with the water injection at the combustor in a turbine.

Afternoon,

I am in the process of validating an update to an EPA method for measuring moisture in stack gases. I am trying to account for all the H2O in the system.

Specifically a simple cycle LM6000 turbine with water injection CO catalyst NH3 injection with an SCR.

I'll post the assumptions here and hopefully some one here can help me understand where the H2O from the water injection goes or if I'm misunderstanding what exactly is happening there.

ATM Conditions
RH 43%
AT 70 degrees
BP 29.7

Fuel - Nat Gas assumed per EPA to have no fuel bound H2O
FD 8710
FW 10610

Firing rate
550 MMBtu

Stack Conditions (measured)
Flow Rate 550K DSCFM
O2 14.8
CO2 3.4
Moisture (H2O) 9.8%

Based on these the EPA formulas yield the following
Fuel Contribution to stack moisture = 6.3%
Aqueous NH3 (19%) @ 129#/hr= 0.14
Water Injection @ 40 GPM = 26%

As you can see, and I've done hundreds of stack tests the water injection contribution is something that is never seen in actuality.

I'll expand on how that 26% was derived and perhaps some one here who is familiar with this can educate me or correct any of my assumptions...

40 GPM = 20029 lb/hr
H2O #/mole = 4.6754E-08
20029 / (4.6754E-08 * 550k DSFM *60)
~26% Moisture

Hopefully all that background doesn't confuse the issue and one of you can help me understand what happens to all that water injected.

It's always been my understanding that any water or steam injected into a gas turbine was vapourized and lost and gone forever out the exhaust stack. There were some attempts a few years back to try to recover (by various methods) at least some of the water vapour, but they weren't economically viable on larger scales.

There may be some kind of reaction with the catalyst and/or the ammonia (I'm not a chemist, and it was my worst subject in secondary and university--by far), but other than that I believe any water or steam is vapourized or superheated in the combustion section and exits the stack as water vapour.

Hope this helps.