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Mass Percent of Excess Air In Flue Gas

07/15/2016 3:27 AM

I am calculating dry loss in oil fired steam generator. I am following ASME PTC 4.

To calculate dry gas loss we need to know mass of dry gas, which can be found by; M(dry gas)=M(wet flue gas)-M(moisture in flue gas).

Now to calculate the mass of wet flue gas;

M (wet flue gas)=M(dry air)+M(moisture in air)+M(wet gas from fuel)+M(additional moisture in flue gas)

All of these above parameters can be measured except for Mass of dry air. Mass of dry air is calculated using the following formula:

M=M(theoretical air)*(1+(Mass percent of excess air/100)).

The issue is regarding mass percent of excess air. I am not sure how to measure it. However I have percentage volume of excess air in flue gas. Is there are any way of calculated mass percent of excess air in flue gas?

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#1

Re: Mass percent of Excess air in flue gas

07/15/2016 4:30 AM

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.

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#3

Re: Mass percent of Excess air in flue gas

07/15/2016 7:40 AM

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

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#13

Re: Mass percent of Excess air in flue gas

07/18/2016 4:52 PM

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.

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#2

Re: Mass percent of Excess air in flue gas

07/15/2016 4:39 AM

Yes, there is, however most combustion processes of any size will operate at their most economical point when there is only just over zero excess air. The practice is called oxygen trim, among other names; sensors are fitted in the exhaust stack and the oxygen reading trimmed by reducing the excess air to the furnace. In this way, the minimum amount of heat disappears up the stack as waste, as it is not being carried away in the stream by the excess air.

Trimming can be done manually or automatically, depending on the complexity of the installation and economics of size.

So, <...mass percent of excess air...> relies on oxygen measurement in the flue gas. A Google (other search engines are available) search will reveal more on this topic, including contact details of suppliers that can help.

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#4

Re: Mass percent of Excess air in flue gas

07/15/2016 7:45 AM

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?

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#6

Re: Mass percent of Excess air in flue gas

07/15/2016 5:30 PM

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

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#14

Re: Mass percent of Excess air in flue gas

07/18/2016 4:55 PM

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.

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#5

Re: Mass Percent of Excess Air In Flue Gas

07/15/2016 11:29 AM

This should help....

Tables and other calculations in link below...

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

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#7

Re: Mass Percent of Excess Air In Flue Gas

07/16/2016 2:41 AM

This is a huge help. Thank you SolarEagle.

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#8

Re: Mass Percent of Excess Air In Flue Gas

07/16/2016 9:50 PM

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)

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#9

Re: Mass Percent of Excess Air In Flue Gas

07/17/2016 11:42 PM

How to measure vol. fraction of all these gases?

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#10

Re: Mass Percent of Excess Air In Flue Gas

07/18/2016 1:12 AM

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.

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#11

Re: Mass Percent of Excess Air In Flue Gas

07/18/2016 9:06 AM

If your instrument already reports the excess air component by volume, then you know the molecular weight already, it is usually called 28.97 (dry). If your instrument reports air by volume at standard conditions, (maybe 70F and 14.686 PSIA in English units), then these are the conditions to convert to dry air mass, or pounds mass per cubic foot of volume. This is often reported as 0.075lb/cu ft.

After you convert to the conditions of your volume measurement, hopefully STP Standard Temperature & Pressure, and then check what are the actual STP, because it seems everybody is different, take your volume measure and multiply it by the weight, that is your mass per unit time, to close up your combustion calculations.

i.e. 1500 SCFM dry Excess Air (70F/1atm) = 1500cfm x 0.07492lb/cuft= 112.38lb/min

http://www.engineeringtoolbox.com/molecular-mass-air-d_679.html

See this link above also for metric examples

Here is some information on Standard Temp & Pressure...

http://www.engineeringtoolbox.com/stp-standard-ntp-normal-air-d_772.html

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#17

Re: Mass Percent of Excess Air In Flue Gas

07/19/2016 9:08 AM

I did leave one important thing out, you do need to know the total flow converted to SCFM usually. Your instrument reports in concentration, ppm of the total.

I usually use a pitot to measure total stack flow, it gives you a velocity, you need temperature and the dimensions of the flue at the point of measurement, and a nice straight section so the flow is somewhat uniform. The calculations to convert the pitot velocity pressure readings to flow are less tedious when put into a spreadsheet.

Often the traverse points are required by emission permitting & regulatory testing, so they are visible on the stack, capped pipes maybe 100mm diameter just above a platform.

https://www.dwyer-inst.com/Products/AirVelocityIntroduction.cfm

I have had success correlating a single point thermal anemometer (FCI is good) to pitot traverse, proving the flow doesn't drop below the dew point (moisture raises hell with the sensor). Have not had good results with traversing a thermal anemometer, at least in industrial environments.

www.fluidcomponents.com/products/mass-flow-meters

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#12

Re: Mass Percent of Excess Air In Flue Gas

07/18/2016 4:50 PM

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?

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#15

Re: Mass Percent of Excess Air In Flue Gas

07/18/2016 5:12 PM

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.

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#16

Re: Mass Percent of Excess Air In Flue Gas

07/19/2016 12:23 AM

Yes there are forced draft fans.

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

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#18

Re: Mass Percent of Excess Air In Flue Gas

07/23/2016 10:30 PM

If you have a basic understanding of chemistry, and assuming that you are using a fuel that can be characterized with a simple proximate analysis, given the specific gravity and fuel flow rate, and excess 02 readings, you can calculate a theoretical dry gas flow fairly easily. You must assume complete combustion and calculate the total molar percentage of each component (all gas is measured volumetrically, and it is a safe assumption that under most stack conditions, it will behave ideally). You can boil it down to computing total CO2, Total H2O, and N2 resulting from incoming air, at minimum combustion air demand.

Let's just say it ain't rocket science, HOWEVER, you need to remember that the measured O2 is NOT the same as excess air, but you can calculate the excess air from measured O2 in the flue gas, with some simple algebra. You have to remember that the excess O2 also came with atmospheric nitrogen (assume that your air is 22% O2 and 78% N2), which must be figured into the total dry air mass. Also, your O2 monitor may be reading total flow, or dry basis, so you need to know your system setup.

So remembering that measured O2 (dry basis) was 22% of the total excess air (the total excess air) and taking a shortcut through the algebraic logic, excess air is defined as 100(1-(22%/(22%-Measured O2 %))). So, for example, if your measured O2 is 5%, you actually have an excess air flow of 100x(1-22%/17%), or 29.4%.

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#19

Re: Mass Percent of Excess Air In Flue Gas

07/25/2016 10:48 AM

Nice job sir!

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#20

Re: Mass Percent of Excess Air In Flue Gas

07/25/2016 4:53 PM

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.

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#21

Re: Mass Percent of Excess Air In Flue Gas

07/26/2016 11:36 AM

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!

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#22

Re: Mass Percent of Excess Air In Flue Gas

07/26/2016 12:29 PM

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.

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#23

Re: Mass Percent of Excess Air In Flue Gas

07/26/2016 12:48 PM

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

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#24

Re: Mass Percent of Excess Air In Flue Gas

07/26/2016 1:38 PM

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?

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#25

Re: Mass Percent of Excess Air In Flue Gas

07/26/2016 1:41 PM

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.

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#26

Re: Mass Percent of Excess Air In Flue Gas

07/26/2016 2:38 PM

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...?

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#27

Re: Mass Percent of Excess Air In Flue Gas

07/26/2016 3:35 PM

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.

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#28

Re: Mass Percent of Excess Air In Flue Gas

07/26/2016 4:54 PM

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

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#29

Re: Mass Percent of Excess Air In Flue Gas

07/26/2016 6:38 PM

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

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#30

Re: Mass Percent of Excess Air In Flue Gas

07/27/2016 1:15 PM

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...

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#31

Re: Mass Percent of Excess Air In Flue Gas

07/27/2016 4:41 PM

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.

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