Soot distribution in a firetube boiler burning natural gas
Soot distribution in a firetube boiler burning natural gas
(OP)
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
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





RE: Soot distribution in a firetube boiler burning natural gas
rmw
RE: Soot distribution in a firetube boiler burning natural gas
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
RE: Soot distribution in a firetube boiler burning natural gas
rmw
RE: Soot distribution in a firetube boiler burning natural gas
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
RE: Soot distribution in a firetube boiler burning natural gas