Boiler Combustion Efficiency

[ahramos]

I'm looking for some Boiler Combustion Efficiency tables.
I seem to recall one with %Excess Air, %02 and temperature.
Does anyone know where I can find one? I have flue gas analysis data telling me I have 8.3% O2. I believe it should be in the 2-3% range. Oh yeah, the fuel is natural gas. Thanks in advance for any help.

 

[Combustor1]

Ahromas,

Maybe you could provide more details of your system.

You can probably find some tables in the Noth American Combustion Handbook. If you can not find one then leave you email address and I will forward one to you.

As a note - what you are measuring in your stack might be inpart from tramp air coming into your furnace (?) from holes/cracks - so not actually what the burner is seeing. This is especially true if you have an exhaust fan (furnace is negative). Also the hole you place your Analyzer probe into must be sealed properly.

Assuming all is OK then if you do decide to reduce O2 then you might be wise to monitor CO. Generally depending on your furnace temperature if you are over 1400F then you should have 0 ppm CO - but never over 200 ppm - normal <50 ppm. If CO starts rising - then that is the limit of O2 reduction - trim back a little.

Next you require higher amounts of O2 at lower firing rates. A burner can turn down from 5 to 20:1 (depending on the burner and your control system) a fan will generally turn down only 5:1 - so you want to get O2 levels approaching 3% when your firing rate is over 50% (unless you are using ratio contol or similar) - low O2 at low fire can get you into trouble if you ever have fuel problems.

One other thing - your burner might be setup for reduced emmissions by firing excess air. If you reduce your excess air then your NOx will climb - will need to check your permitting.

Give some more details ...

 

[ahramos]

Combuster,

There are two Cleaver Brooks DL-60 boilers serving a hospital. These boilers have been de-rated years ago once the absorption chillers were removed. One boiler has a new Power Flame boiler rated at 20,696 MBH with FGR and a modified version of Parallel Position Controls. The other boiler has a single point jackshaft, no FGR, but has O2 trim controls and a VFD on the draft fan. I believe it is rated at 54,426 MBH. However, I'd take that rating with a grain of salt.

I do not operate these boilers, nor did I do the flue gas analysis. I am an energy engineer making some recommendations to improve controls and combustion efficiency. I am looking for these efficiency tables to help me calculate savings for some projects. I've seen some online, but they were for fuel oil No. 6 I believe.

Thanks

 

[25362]

If you have complete and perfect combustion and no external air leaks a value of 8.3% oxygen in your flues (by Orsat) would mean all the xs air has been mixed with the fuel gas and it is not tramp air.

For a gas composition near methane the xs air (on top of the stoichiometric need) would be 62%, and the CO₂ in the flue gases should amount to about 7.5%. This is to say that if you know the average formula C_xH_y, or better the y/x ratio, of your gas, the amount of CO₂ measured in the flue gases would suffice to determine the xs air used for combustion. The formula would be:
% CO₂ = 21 /[n + y/x(0.25n-0.05)].

Where (n -1)100 is the % xs air. As an example if y/x = 3.75, and the CO₂ in the flue gases is 7.2, the xs air would be 1.6 or 60%.

In addition to what has been said above by Combuster, 60% xs air means a loss of some 300^oC in the adiabatic flame temperature, and a drop of some 170 kcal/Nm³ in flue gas enthalpy, when comparing with values using 20% xs air, very important factors in heat transfer.