Flue gas question 1

[kbs]

Is there a formula that would give me the volume of flue products leaving a boiler if I know the btu/hr, temp. of flue gases, percent O2, CO2 and excess air. I am trying to size an economizer for a small boiler (4100cf/hr CH4) and use the captured heat to help heat a swimming pool.

In the past, I have measured velocity of flue gases and, using velocity and volume formulas, found the volume of gases that way. Just curious if there is another way.

Thanks for any help.

 

[mbeychok]

kbs:

Using a pitot tube to ascertain the flue gas velocity and then calculating the volumetric flow rate of the flue gas is a good way of doing what you want.

If you could contact a chemical engineer and ask him/her to perform a combustion calculation to obtain the volumetric flow is another way of doing it. Assuming that you are talking about a natural gas fuel, a "ball-park" figure is 11,000 to 12,000 standard cubic feet of wet flue gas per million Btu of fuel gas. (Standard cubic feet are measured or calculated at 60 degrees F and atmospheric pressure).

Milton Beychok
(Contact me at www.air-dispersion.com)
.

 

[kbs]

Thanks for the input. Stoichemetric combustion would be approximately 10 to 1 but how would the excess air percentage be worked out. Would 50% excess air be the equivalent of 5 times the cu.ft of gas? Oh, yes it is natural gas at 1040 btu/cuft.

Thanks again for the help.

 

[mbeychok]

kbs:

Going through the detail of a combustion calculation by posting a message on an online forum is just too difficult and time consuming. Any chemical engineer you contact can and should be able to easily work such a calculation through for your boiler's specific amount of excess combustion air (which is not the same as the amount of oxygen in your flue gas).

I very seriously doubt that your boiler uses 50 percent excess combustion air ... it probably uses 5 to 10 percent excess combustion air or even less.

The number that I gave you in my earlier response (11,000 to 12,000 standard cubic feet of wet flue gas per million Btu of fuel gas) was based on a combustion calculation using a natural gas with a heating value of 1093 Btu/scf and 12 percent excess combustion air.

Milton Beychok
(Contact me at www.air-dispersion.com)
.

 

[kbs]

Hi Milton

As an industrial gasfitter I regularly use combustion analyzers. My current is a KaneMay Equinox. It measures excess air in percentages above stoichemetric. On the smaller (under 400hp) package non-condensing hot water boilers such as Cleaver Brooks, their design rarely allows less than 20% excess air. My example of 50% was used in context.
I can always tell how much natural gas is entering the combustion chamber using manifold pressures, and have always measured flue gas volumes using a pitot tube and with velocity and volume formulas. I was hoping there may be a rule of thumb method I could use from the analyzer printouts to get a flue gas volume instead of climbing and drilling into stacks.
The rule of thumb we use is 10 cuft air to 1 cuft gas for near stoichemetric. Therefore 10% excess would be 1 extra cuft of air than is required. This acheives what you find with your calc.
All in all, I think you are right, measuring the velocity, etc. is probably my best alternative.

Thanks for your input.

kbs

 

[mbeychok]

kbs:

Let's do it quick and dirty by assuming that natural gas is pure methane (actually, most natural gas is 90 to 95% methane). Now let's write the equation for the combustion of methane (CH₄) at exactly stoichiometric conditions (which means no excess combustion air):

CH₄ + 2 0₂ + 8 N₂ ==> CO₂ + 2 H₂O + 8 N₂

In English, the above equations says that 1 mole of methane plus 10 moles of air (i.e., 2 moles of oxygen plus 8 moles of nitrogen) will burn to yield 11 mols of wet flue gas (i.e., 1 mole of carbon dioxide plus 2 moles of water vapor plus 8 moles of nitrogen).

Since moles are directly proportional to standard cubic feet (scf) of any gas, we can say that:

Burning 1 scf of natural gas requires at least 10 scf of air ... which is your rule of thumb

and also:

1 scf of natural gas burns to yield 11 scf of wet flue gas

and since 1 scf of natural is about 1,000 Btu, we can also say that:

1,000 btu of natural gas burns to yield 11 scf of wet flue gas

Therefore:

1,000,000 Btu of natural gas burns at stoichiometric conditions to yield 11,000 scf of wet flue gas which is pretty darn close to the ball-park figure I gave you for burning fuel gas with 10 percent excess combustion air.

Milton Beychok
(Contact me at www.air-dispersion.com)
.

 

[kbs]

Hi Milton, I can work with this. Thanks for your patience and info.