Gas Turbine Exhaust Composition for auxilarly firing a HRSG 2

[jdaye]

I am currently working as an intern at a company and am trying to complete calculations for a HRSG which auxiliary fires using the exhaust from a gas turbine (the size of which is required I am trying to determine). Many of the studies I have read state that the gas turbine exhaust contains approximately 15 % oxygen by volume. I am having trouble understanding where this value comes from. I was hoping someone could please explain this to me, and how to determine the amount that is in the exhaust.I believe this is where I am having difficulty as my current percentage of oxygen in my turbine exhaust is less than 5%.

 

[GHartmann]

Most combustion processes require excess oxygen for proper combustion to occur and avoid excess NOx formation.

You would need to do a complete material balance to calculate the excess O2. This would require:

Air flow measurement (you may not have this or it may be inaccurate)
Gas flow measurement

You can also find some excel sheets that calculate the amount of O2 required for each component in the natural gas. A complete material balance can then be calculated.

I would suggest a Google search of "combustion calculations"

See my link for some information provided by GE.

 

[rmw]

Gas turbine exhaust, unlike boiler exhaust has more Oxygen in it. In a boiler, the goal is to only add enough air (plus just a little excess) to burn the fuel. The goal is to heat water to make steam to drive a turbine, etc. O2 levels are run as close to stochiometric as possible without creating CO.

In a gas turbine, on the other hand, the working fluid is air and the goal is to heat as much air mass as the machine can "stuff" through it in order for that heated air to drive a turbine or turbines and overcome its own parasitic losses and produce net power.

15% sounds reasonable, but I seem to remember somewhere seeing as low as 2-3% but I can't substantiate that. I do know that some gas turbines are very difficult to supplemental fire due to the lack of O2 in the exhaust.

rmw