Soot distribution in a firetube boiler burning natural gas 2

[skbutler]

Having spent 36 years in the boiler business I've seen a lot of sooted up boilers but I never gave a lot of thought to the soot distribution on the heating surfaces. But now I'm writing a book on packaged boilers and would like a better explanation than.. 'that's just the way it is."

I'm interested in why soot collects in certain areas of 2 and 3 pass firetube boilers firing natural gas, (boilers 100 hp and above.) In 2 pass boilers it is common for soot to collect in the bottom of the turnaround space where the flue gas exits the tubes and enters the stack.

Example 1 shows a standard 2 pass firetube boiler. As you can see, soot has collected in the bottom of the turnaround space. You will also notice that there is no soot on the tube sheet or inside the tubes.

Example 2 shows a 3 pass firetube boiler. 'A' shows the end of the second pass tubes and the beginning of the third pass tubes, 'B' shows the Morrison tube, 'C' shows end of the beginning of the second pass tubes and 'D' shows the end of the third pass tubes. (sorry of the slightly out of order pics)

Since the soot is created in the Morrison tube, why isn't it evenly distributed?

I'm assuming the fact that more soot is found in the areas where the flue gas makes a rapid directional change is significant. Look at the first example again. That soot deposit is pure carbon and light as a feather. Why didn't it stick to the tubes or tube sheet?

Thanks

Steve

 

[rmw]

Do some calculations on the gas velocity in each tube, in the uptake zone after the tube exit as it goes toward the stack and in the stack itself (and in similar zones in 3 and 4 pass boilers) and draw you own conclusions.

rmw

 

[HKoehler2]

I also would recommend to do some gas velocity calculations or better a 3D-CFD-Simulation.
I assume because of the low velocity the soot will stay on the surfaces at the bottom of the turnaround.
It is also dependent of the swirl situation of the flow and the temperature(s) in the combustion chamber itself and on the surfaces.

That in D no soot is should be clear due to the fact that there is a pure free jet (without change of the gas flow direction)

Best regards

Hartmut

 

[rmw]

I went back and read the OP again. My question is; why is soot present when firing natural gas? Maybe you should include a chapter in your book on burner tuning. I have seen lots of nat gas fired boilers because of the region of the world that I live in, and rarely did I see one sooted up unless it had some serious burner problems or tried to run flue gas O2 trim control too tight. That, of course, was a controls issue.

rmw

 

[skbutler]

rmw, you're right, natural gas should burn clean and usually does. That's a tribute to modern gas burners but the combustion of natural gas is a little more complicated than is usually shown in text books. The equation of methane is often used to illustrate the combustion of natural gas as
CH4 + 2O2 ==> CO2 + 2H2O
Actually the oxidation of methane might take a dozen steps, not one as shown above. And each step produces intermediate products. Soot, or pure carbon is produces in large quantities even in clean burning natural gas flames. The free carbon might last only nanoseconds and is quickly oxidized into CO or some higher order aldehyde. All of this is relatively simple compared to the distribution of soot in those cases where soot is produced as a final product of combustion.

Obviously the velocity of the flue gas plays some role... maybe that's the full explanation.

It's great to get different view points... thanks

 

[ash9144]

You may also want to look at cool spots as you travel through your passes. One potential is some of the combustion products are reaching a dew point and condensing allowing soot to gather. This is a common concern in sulfur burners due to corrosion more than soot.