Co2 in Gas Turbine emissions 1

[1PowerGuy]

What relationship dose Co2 play in the combustion process of a Gas Turbine as it relates to Co emissions?

 

[rmw]

The more CO2 you have, the less CO; that simple. Your question is too general. It needs to be more specific. In a perfect combustion system there will be only CO2 and no CO. The presence of CO is an indication of incomplete combustion.

rmw

 

[1PowerGuy]

Sorry about that, I was afraid it may have been to general of a question. Here is the entire scenario; I am dealing with high quantities of Co2 in my fuel gas stream (18-30%). It would appear that the more Co2 I have the higher my Co stack number climbs. The Co # is somewhat controllable by augmenting combustion temperatures. We deal with constantly radical fuel gas composition, and up to this point (2.5 years on line) I have not noticed any major variations in the profile of my flue gas stream.In this scenario would Co2 have a direct relationship to the amount of Co in the flue gas stream? I should also note that I have not noticed any significant reduction in power of these large base loaded aero derivative units (50+mw).

 

[btrueblood]

Yes, LeChatlier's principle would say that if you have more CO2 in your fuel stream, you would expect more CO in the exhaust, all else being equal. For the most part, though, the CO2 just goes along for the ride as an inert part of the fuel, the controller should be able to adjust for variations in the fuel heating value up to some reasonable limit (but 30%? wow).

 

[1PowerGuy]

The problem being that we are only profitable in today’s market by burning tail-gas from a crude oil Co2 reinjection facility. So we end up trying to perform on whatever they send us. Though for the most part we get excellent performance numbers, 7800-7900HR @43% efficiency, just this odd shift on two units Co levels...

 

[rmw]

OK,

In a perfect world you would have a fixed amount of air produced for combustion by the compressor being heated by the appropriate amount of fuel gas that was all CH4 and you would end up with an exhaust of nothing more than N2 and C02 ignoring excess O2, and other small amounts of impurities in the gas, etc.

In your case, because a high percentage of your fuel gas is CO2 and is inert, the fuel control has to feed even more fuel volume into the combustors than if it were pure methane gas. The inert CO2 in the fuel also has to be heated by fuel to the turbine inlet temperature in order to pass through the hot gas path with the other products of combustion, necessitating the need for even more fuel.

All that fuel in a combustion can that was designed for and would otherwise love to be operating on pure methane (or distillate) displaces some of the air that would have been beneficial to the combustion process, thereby displacing the O2 necessary for complete combustion and upping your levels of CO which is an indicator of incomplete combustion as I stated before.

Your turbine runs on hot gas products of the combustion system. It doesn't care if it is hot N2/CO2 at stoichiometric ratios or other hot gasses (with proper considerations for volume ratio differences between the constituents) so it is not surprising that you get the HR and the efficiency numbers that you do. You could get those numbers if you were burning turpentine if you could get it in in a fashion it could be burnt completely.

You might ask your turbine mfg'r about it as they might have a different combustion liner or can (whatever your airplane engine has in it-whatever those guys call it) for a more "difficult" fuel that would get more air into the combustion area or with better mixing to burn your junk fuel.

That is my theory anyway, and I am sticking to it.

rmw

 

[davefitz]

Other radicals in the CTG exhaust affect CO production. NO and unburned hydrocarbons will borrow O from CO2 , and lead to higher CO.