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LEL control for a thermal oxidizer

We have a RTO Regenerative Thermal Oxidizer and I was told that our process has over 55 to 70% LEL phenol and other. Our contractor wants to open a damper to the central chamber (chamber temp at 1,550 F Deg) and allow this to the chamber at 40,000 SCFM of air flow. At one point we had a minor explosion. Is this a safe way of doing this and were can I find out about the codes of LEL.

Mike W [last name deleted for privacy by Editor]
- Toledo, Ohio
2003

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Tom,
It depends upon how they're wanting to set up the damper. If they're bringing in fresh air into the chamber then it's a bad idea in many ways.
If they're bleeding off heat from the chamber to contain the combustion chamber temperature, then it's not a bad way to go, however there are much better ways.
If you have a high VOC content in the SLA then the best way to minimize excessive temperature in the chamber is by diluting the air using the fresh air damper (a common practice).
The VER damper, which is what it sounds like the contractor is recommending is not a proven technology. I have seen many problems from these combustion chamber dampers. They have an impact on Destruction Efficiency and can increase utility consumption.
Personally I'd recommend setting up a PID to the Fresh Air Damper (using 4-20 ma control is a standard). That way you mitigate the concentration at the cold face while not affecting your combustion chamber dynamics.
I'm assuming your system has self sustain? If not that's definitely something worth looking into as it will save you hundreds of thousands of dollars a year.

Russ Friend
- Alliance, Ohio
July 1, 2008

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August 2, 2010

Often when the process stream is not sampled regenerative thermal oxidizers are designed to small or without ample protections.

This is what you have here. The only way to solve your problem is to place in the line a dilution damper of proper size that will allow the air to mix with your plant process gases. The best way to control the dilution is through a modulating dilution air valve. This would modulate based on your process streams LEL.

Hot air by-pass systems are ways to reduce the chamber temperature so that the media bed remains unaffected during extreme temperature increases and are good practice on a plant such as yours, but this should be used as protection and should only open during emergency states.

If your regenerative thermal oxidizer has capacity then add your dilution dampers above stream from your LEL monitor and control the valve by means of 4-20 ma based on LEL. I hope this helps.

Scott Manes
- Hulbert, Oklahoma