Protective coating for carbon steel

[Technopriest]

Greetings.
We are due to produce a thermal oxidizer treating air with a 3,33g/Nm³ concentration of CH2CL2. After the burner, there will be steamm addition; our customer fears that when the air temperature drops in the range 60-150°C chloridric acid might concentrate.
Our proposed solution is coating with acid resistant bricks, but this leaves a lot of unprotected spots in ducting and near valves nozzles. Is there any other solution?
We have received PTFE coating offers, but the cost exceeds refractory bricks (and requires transportation of the entire structure to the supplier, while the bricks may be placed at our factory or in the field).
Thanks in advance for any tip!

Che Dio ce la mandi buona, o almeno ce la mandi- Massimiliano Eusebi
Check eng-tips rules: faq731-376

 

[gr2vessels]

Dear priest,
I'm curious to understand you burning process a bit better than described above;- do you realy expect to find in the thermal oxidizer temperatures less than 150 deg C? I assume that you have downstream the incinerator a heat recovery system where you could find some "lower" temperatures, but from what I have seen so far, the gas temperature at the outlet of a boiler (for eg.) was well above the acid dew temperature. Obviously, the gas is then sent to a stack and the design of the stack is a different story.
I believe that the steam addition in incinerator is controlling the upper temperature limits, I doubt it is there for helping the acid formation and condensation.
Just remember to keep the gas temperature above the acid dew temperature and you don't have corrosion! Can you please elaborate a bit, how do you treat the air?
Cheers,
gr2vessels

 

[Technopriest]

I apologize for the lack of information.
I'll try to explain a little more.
We have an inlet foul air comprehensive of a 3 mg/Nm³ of dichloromethane. This foul air goes to the burner, where it is mixed with another inlet of concentrated solvents (to date, unknown chemical composition). A steam-powered device pushes these solvents in- the steam enters as a part of this process (As far as this moment, my comprehension of this part of the device is poor, but this does not influence the problem: the total inlet is steam, foul air and concentrated solvents).
I'm not a chemist, but I'm told that the final composition of the air in the burner can condensate in hydrocloric acid (HCl) in unknown concentration if the temperature drops below 130°C or so.
In the combustion chamber the temperature is far above 700°C, so no problem.
After the combustion chamber, the air goes through a ceramic media (C-130 porcelain bricks); this ceramics cool down the air a lot (expected temperature after the ceramic media is about 150°C). The ceramic media is needed; the oxydizer works in cycles of about two minutes, and the media that in the first cycle cool down the air, in the second cycle preheates it (so, there is no way of eliminating the media, as this solution drops the methane consumption a lot).
After the media we have long ducts- in these ducts the temperature may (and, in start-up and shut-down, will) drop below the drew point.
I'm not sure if my english is good enough, but I hope I explained our situation a little better than in my first post.
Our actual solution is Duplex Alloy 2507 for the parts that can't be covered by refractory bricks- but it's still very expensive...


Che Dio ce la mandi buona, o almeno ce la mandi- Massimiliano Eusebi
Check eng-tips rules: faq731-376