Minimum Oxygen Level For Combustion 3

[XELR8]

I have a dryer that uses off gas heat for evaporating moisture from my product. I want to know what is the required level of oxygen in the air to initiate comustion? The incoming air temperature is 350F and moisture is 0.020 lbs H2O/lbs Air. The exhaust air is 250F and 0.040 lbs H2O/lbs Air. What is the level of O2 necessary for combustion? Are there any graphs for O2 vs Temperature for combustion?

 

[Guidoo]

Basically, what you are looking for is the combustion envelope of a hydrocarbon gas mixture at an elevated temperature. Can you give an idea of the composition of this off-gas?

More on the construction of the flammability envelope can be found in thread283-99596 and in NFPA 69.

The best advice would be to let a laboratory do some tests on samples of your off-gas, but following method could give you some idea:

1) Collect data for Lower Flammable Limit (LFL) and Upper Flammable Limit (UFL), both in air and in pure oxygen for the components in your mixture. These data will be valid at room temperature.

2) Using this data, estimate the LFL and UFL of your mixture in air and pure oxygen at room temperature. For this you use Le Chatelier's law as described in thread798-33615

3) Estimate LFL and UFL of your mixture at the elevated temperature using the modified Burgess-Wheeler law (thread283-99596).

4) Construct the flammability triangle with the flammability envelope as described in above mentioned threads.

Check also thread816-96027.

I can recommend you to get a copy of
M.G. Zabetakis; Flammability Characteristics of combustible gases and vapors. Bulletin 627. Bureau of Mines. Washington, US Government Printing Office, 1965.

 

[Guidoo]

Another interesting document on flammability envelopes and effect of elevated temperatures and pressures can be found on:

http://www.airproducts.com/NR/rdonlyres/TempInfluenceFlammab48b7xahxlvkfrkwuajtp.pdf

In my previous post I assumed that you operated at or near atmospheric pressure. If this is not true you have to make a correction for that as well...

 

[XELR8]

Guidoo,

Thank you for your quick response and many helpful resources.

It will take me a little time to digest all the information you mentioned above but I want to elaborate a little bit more on my exact "theory".

My dryer contains charcoal and charcoal dust. When the product becomes too dry it can combust. Theory " If I increase moisture content in my dryer I can decrease the O2 level and therefore reduce fires". True or False?

Also, does reducing the density of air (by raising the temperature) reduce the O2 content?

Thanks for your help.

 

[Guidoo]

Dracula,

From your second post I understand that I misinterpreted your original question. I thought you were interested in the minimum oxygen concentration required to burn your offgasses...

Now I understand you are interested in the maximum oxygen concentration to prevent combustion of your charcoal dust. This is called the Limiting Oxygen Concentration (LOC). This method is described in NFPA 69. According to table C1(b) in NFPA 69, the LOC for charcoal is 17 volume% when CO2 is used as the diluent (so we are dealing with an atmosphere that is a CO2/Air mixture). This LOC is determined at room temperature and ambient pressure.

Again, to be sure the best thing you can do is to have some laboratory do experiments on your system. This is also stated in NFPA 69 "The extent of oxidant reduction shall be determined by testing where conditions vary significantly from the test conditions under which the data were obtained"

About your questions:
- If you increase the water vapor concentration you decrease the volume% of oxygen. You are diluting the air using water vapor as diluent.

- If you reduce the density of air you do not decrease the oxygen concentration in volume%.

 

[XELR8]

Guidoo,

Please see the below answers to your request:

1) Gas composition of off-gas:
Moisture = 13.8%
O2 = 16.8%
CO2 = 3.7%
Percent Isokinetic = 88%
Total Particulate Concentration = 0.20 gr/dscf
Rate = 82.8 lb/hr

Pressure is atmospheric 14.7 psi.

I hope this helps.

Thanks!

 

[Guidoo]

Dracula,

Just for my understanding:

What is "isokinetic"?

Why do your percentages add up to 122.3% and not 100%?

Is there any nitrogen in the atmosphere?

 

[25362]

Guidoo, I'm thinking aloud.

Isokinetic sampling would mean the analysis was done on a stream flowing at the same velocity as the main stream. Nitrogen would bring the gas composition to 100% in the sample, whether isokinetic or not. Let's hear what Dracula has to say in this respect.

Dracula, pleae confirm whether the solid particle concentration is given in grains per dry standard cubic feet.

 

[XELR8]

Guidoo,

The percent isokinetic is the flow velocity going into a sampling nozzle as comapred to the flow velocity coming from the exhaust port from the stream you are testing. This information was obtained from a Mehtod 5 emission test. So 25362 is correct.

25362, your correct again, gr/dscf is grains per dry standard cubic feet.

Also, the N2 would bring the gas sample to 100% as noted by 25362. My test only measures moisture, O2 and CO2.

Guidoo, many thanks for your LOC information. It was exactly what I wanted.

25362, I have ready many of your threads and they are excellent. Keep contributing.

 

[25362]

Dracula, thanks for your compliment. I like participating in the eng-tips forums. I don't know if my posts are of value, but I can assure you that I learn a lot from this website.

Now, some thoughts on charcoal drying with hot air.

There are factors that may dramatically reduce the activation energy to start a fire. Among them one can mention: the unknown pore surface of the charcoal particles and their size distribution, the amount of still occluded pyrolytic organics that may have low ignition points, the various possible catalysts present, the way the charcoal bed is arranged, the contact time with hot air.

Although evaporating water is bound to cool the gas and the charcoal, it may sometimes act as a catalyst to enhance combustion. Under special circumstances I heard that even CO₂ may oxidize charcoal to CO.

Thus, to be on the safe side, whatever a book might say about charcoal burning with air at various temperatures, I think that Guidoo's advice on trial bench tests is the right approach, and deserves a star for that.

 

[XELR8]

25362,

Thanks for the additional information. I agree with the trial test recommended by Guidoo is the right the thing to do and I will proceed accordingly.

I have measure CO2 in our evaporative exhasut gas but I have not measured CO (it is probably there). I will follow up on your information.

Thanks again for your help and Guidoo's solution to my problem. ( I did give Guidoo 2 stars for the soulition).