Mass Percent of Excess Air In Flue Gas

The mass percentages need to be adjusted according to the molecular weights of the various constituents. I haven't done such calculations in a long time, but maybe later I might attempt an example. Air ≈ 29, N₂ ≈ 28, O₂ ≈ 32, CO₂ ≈ 44, CO ≈ 30, etc.

But how would I be able to get mass of Excess air? I mean i need mass percent of O2 in flue gases:

(Excess air (i.e. O2)/mass of flue gases)*100

Usually one uses these things called oxygen sensors. They are essentially a zirconium oxide cell that generates an emf proportional to oxygen concentration. Get yourself one. You will be a guru.

The problem is the steam generator I am working on has the sensor which measures percentage volume of excess air in flue gases. What I need is percentage mass of excess air. How can I find mass percent without knowing the density of flue gases?

Their densities are proportional to their molecular weights, as already mentioned.

If you already know the volume excess air, and have the temperature and pressure, you can calculate the mass from the ideal gas law. Air (with all the oxygen still in it) has an approximate molecular mass of 29. Air with the oxygen removed (at the same temperature and pressure and constituents), has a molecular mass of very near 28.

One mole of gas at standard temperature of 273K and 1 atmosphere pressure (~ 1Bar), will occupy a volume of 22.414 L.

Er, the objective remains: to reduce <...excess air...> to zero. Oxygen trim remains the technique.

This should help....

Tables and other calculations in link below...

http://www.nrcan.gc.ca/energy/efficiency/industry/technical-info/tools/boilers/5439

This is a huge help. Thank you SolarEagle.

Here is a hypothetical example (underscores inserted for alignment):

Constituent..: N₂_ O₂_ CO₂ H₂O
Molecular wt: 28_ 32_ 44_ 18
Vol. fraction.: .80 .05 .10_ .05 (Arbitrary example)
Weight____:22.4 1.6 4.4_ 0.9 (Mol. wt. x Vol. fr.) (Total 29.3)
Wt. fraction: .76_.05 .15_ .03 (Individual wt./total wt. of 29.3)

How to measure vol. fraction of all these gases?

I thought you already did that and had the information. An Orsat apparatus is the old way; various electronic gas analyzers are the new way.

How about getting a handle on the air fan output? You do have forced draft fans, yes?

...and what is this "additional moisture in flue gas" mysterious term coming from? Boiler leaks?

Moisture comes in with the combustion air, it must be heated and consumes BTUs, plus fuel combustion creates H2O based on the hydrogen content of the fuel. Hence HHV & LHV especially for hydrogen rich fuels like natural gas.

Yes there are forced draft fans.

The additional moisture is mostly due to sootblowers used upstream of the flue gas analyzer path.

Nice job sir!

Nicely done, and clever use of minimal available information. All that remains is to relate the fuel flow volumetric/mass rate to total stack flow, so he can get his mass flow.

I think his instrument already gives him the dry concentration of excess air, so with the fuel type and approximate composition, you should be able to calculate the stoichiometric conditions, and then correct for the excess air as you show, and come up with a probable gas flow in the stack, in mass lb/hr or CFM.

With a fuel type and some actual figures, assuming he does have a fuel flow instrument of some type, an example of how to convert those values to mass, and correct for temperature & pressure might be helpful to all of us. I've done it for theoretical situations, sizing stacks and flue gas ducts, but never tried backing into an actual operating installation. Excel spreadsheets help keep the calculations all straight.

Thanks for the feedback. I do this for air permitting, so it is second nature to me. I should have mentioned that the most important aspect of this calculation is the O2 demand at minimum stoichiometric balance. The N2 brought in with the minimum O2 will be (.78/.22)x minimum O2 (volumetrically), which will be by far the main component of the flue gas flow, with the added N2 and O2 from excess air.

I have seen any number of boiler setups (older boilers, usually) which label their O2 monitors as "Excess Air", which is why I mention that. There was a publication from Combustion Engineering (a large scale boiler manufacturer) simply named "Combustion" that has a treasure trove of information (in just a few pages in a book 2 inches thick) on just this type of calculations. I probably have an old copy of that somewhere, if the termites haven't gotten into my old reference books. The bugs have gotten into my storage shipping container in the country and done a lot of damage. Still waiting for the exterminator to come out and nuke the rascals! That book was the best manufacturer "perk" I ever got in 35 years of utility engineering work!

Sorry about (and for) the bugs. You would think they could at least eat some old issues of Playboy, or something. Cockroaches with bunny ears is an image you can't un-see.

Dear Mr.maamurrehaman13,

In the Thermo Dynamics, as well as Chemical Engg. books, there is a formula to calulate excess air by the formula and it is

((O2 / (21-O2) ) x 100 in Percentage. where O2 is Oxygen in the Flue Gas which can be measured with Orstat apparuts and also latest gadgets are available like O2 analyzer in on-line position.

Other method is to measure the conductivity of the flue gas which will vary in accordance with the excess air and apply correction for temperature.

DHAYANANDHAN.S

I am unfamiliar with flue gas conductivity measurement principle: is that done by bubbling the gas through water?

Is this a direct measurement between grid electrodes?

I trust the O2 monitor method more, BUT you must know where the O2 is measured, and some basics about your combustion system. O2 may be measured in the boiler, near the exit, or up on the stack. If your boiler is positive pressure, it should be effective anywhere (within temperature limitations on your measurement system and mixing characteristics of the flue gas) after the combustion zone(s). If you have induced draft fan, creating a negative pressure in the boiler, you should measure in the stack, to get a hand on fan loading. Especially with heat exhanger or possible leaking seals. A negative pressure boiler's leakage can add a lot of fan demand. Even leakage around the fan seals can impact load.

No mention in there in your references on how to calculate the mass of the flow, other than direct flue gas velocity measurement, or back-calculate from mass of fuel burned, or even room for more error, calculate gas flow based on fan curves, static pressures, kw or hp consumption...?

Volume of gas flow (scfm, or lpm) can be converted to mass flow using ideal gas equation at stack pressure.

PV= (m/M)RT is the statics version of the equation. M is molecular weight (about 29 for air).

thus m=PVM/RT now turn it into a flow equation: m•=dm/dt = V• (PM/RT)

It really is not too hard for a high school student.

all we need now is a way to compute the volume...

See if you can see this (Sorry for the resolution, I can't get it any better:

I can see enough, that's similar to calculations I see in the B&W little green book I have floating around someplace. I use those equations to populate a handy-dandy Excel spreadsheet...

We still need the actual fuel rate in lbs/hr or CFM/SCFH in order to get the mass, it looks like the original poster has either solved the problem or lost interest...

I got it down to dry acfm per gallon in the calcs. To get SCFM, use the reference conditions of the standard as applied (I have found sometimes "Standard" is not always "standard"). It is easy if you have the fuel specs as in lb/cu ft, or lb/gal. to convert. In air permitting and production calcs, I often convert to cfm per MMBtu, since I started with lb mol/100 lb of fuel. The conversions are simple once you develop the stoichiometry. I have used similar calculations for utility boilers that use up to 300 tons of coal per hour down to office heaters of a residential scale on either oil or natural gas. The results, if your instrumentation is good, are usually very close to stack measurements, after the units are started.

The key is to double check your units through the entire calculations to be sure your results are supportable. I consider it to be more book keeping than "real math". For permitting purposes, we must either go back to a reference method of calculations, or show the consistent units.