High/Low O2 in BMS 1

[Sam654]

I recently started at a large chemical company after 20 years at smaller chemical plants as a utility engineer.

Several years ago, this plant retrofitted a boiler with a low NOx burner using only Natural gas as a fuel. During the modification, the manufacturer recommended a boiler shutdown at either 12% and 1.5% O2. They put a 3 minute delay in the shutdown.

We have recently experienced several shutdowns at high O2 with an adverse effect on production. In my 20 years experience with boilers, I have NEVER had a shutdown tied either on high or low O2, only alarms on low. NFPA doesn't require it or O2 analyzers for that matter. I understand there is a reason for the high or low O2 and I am indeed looking into that.

My questions...
Why the shutdown on HIGH O2? Does this have to do anything with a low NOx burner and why?

Does anyone else have boilers without low O2 shutdown? I worked at two other plants that had no O2 shutdown.

I have never experienced any problems running without. If this were a severe problem, I'm sure NFPA would make it a requirement. Sounds like over kill by the manufacturer to me. When you call them, they talk down to you as if they are God's gift to the world of boilers.

 

[danw2]

Low Excess Air (LEA) Firing

http://www.energysolutionscenter.org/boilerburner/Workshop/RCTOverview.htm

quote:
As a safety factor to assure complete combustion, boilers are fired with excess air. One of the factors influencing NOx formation in a boiler is the excess air level. High excess air levels may result in increased NOx formation because the excess nitrogen and oxygen in the combustion air entering the flame will combine to form thermal NOx.

 

[danberry]

High O2 shutdown after 3 minutes shouldn't be a problem. What is going on in the control room during these 3 minutes?

If it can't be helped, you can remove the shutdown feature or extend the grace period. It can't damage the boiler; worst case with too much air is you pull the flame off the burner and trip on a flameout.

 

[rmw]

Is it an emissions problem? Does the boiler have CEMS equipment? Is the site emissions limit exceeded when the NOx limits on this particular boiler are out of limits?

rmw

 

[lownox]

I would be intersted in knowing the boiler input and the type of lownox burner being used. i.e. FGR, IFGR, premix, etc...

I can't imagine the NOx being stable across a range of excess air like that though. My guess would be it is related to flame stability???

 

[davefitz]

Sound like you do not have a reliable flame scanner or burner safety system, and you're trying to get by on O2 only.

If so, 3 minutes is way too long for flame safety reasons. It only takes 10-20 seconds to fill a furnace with enough gas to be in the dangerous range , ifno flame is detected.

If you have a reliable flame scanner and flame safety system, the first control repsonse toa 1.5% O2 signal is to increase combustion air flow. If the fans are unresponsive, then you should runback the fuel heat input continuously until O2 setpoint is reached . The time delay to fuel runback should be not much longer than the response time of the fan dampers + O2 monitor.

High O2 on a single burner boiler with lo nox burners may generate either an unstable flame or a flame that cannot be reliably detectted by the flame scanner. If the fan damper cannot runback the air flow to 25% MCR air flow, then you shuld look into improive the fan dampers ormaybe a VS drive for the fans. Alternatively, a different burner tha will be stable at 10% heat input and 25% air flow should be found.

 

[Sam654]

The reason for going to a low NOx burner was we lost our grandfather exclusion for burning #6 oil when we had to change the burner. For some unknown reason (to me), the decision was made to go with a low NOx system, not that we are in a non-attainment area. There is no CEM on the boiler.

After digging through the old project files, I came across an incident during commissioning that explains why they put the high O2 shutdown in place. As lownox construed above, it was a flame stability issue.

We do have a reliable flame scanner system measuring both IR and UV. There is a burner management system in place built IAW NFPA 85. In fact, we have separate flame scanners for the main flame and the ignitor so we can maintain the ignitor flame during operation IAW NFPA 86 para 5.3.4.1.1.2.

The concern is when the O2 sensor fails, the boiler will shutdown (lost production), not that the operator is unresponsive after an alarm. Our procedures are such that any alarm is investigated.

Its my position that O2 excursions should be alarm points with trained field operators responding to investigate to determine the cause of the alarm and assess the safety of the equipment and correct if possible. The failure of an O2 monitor is not a reason for boiler shutdown at a supervised operation.

Is there any agreement or disagreement with this?

 

[lownox]

There is certainly no code requirement that you monitor O2 in the flue gas and interlock the burner that I am aware of. Maybe a local thing? The problem is that the flame could become unstable while the scanners are still "seeing" the flame. You could get nasty pulsation, noise, and worst case delayed ignition. If you have an operator there all the time, maybe rewire so it is annunciation only and not a lock-out.

Other question is why is he going from normal O2 (which in this case is probably about 6% since it is low NOx) to high O2? Gas pressure dropping? Air linkage slipping or misadjusted? If it is modulating, maybe the unit is not characterized well throughout it's low to high fire transition and you have a really lean spot in the middle. Just thinking out loud.

Who manufactured the burner? I know a lot of those guys... it isn't a huge industry with many engineers going from company to company and back again. LOL.

 

[Sam654]

Thanks again for your response. I appreciate it very much.

We've had a couple of monitor failures (thats another issue with reliability). The burner has been characterized and works reasonbly well. I have it operating in the 3-4% range above ~40% load. We operate through a Delta V DCS with an A-B plc w/ Fireye scanners for burner management.

Because of my company's requirement for 100% compliance, I have to build a strong arguement to overturn the recommendation of the manufacturer's startup technician. The manufacturer won't back down for COA/liability reasons.

Its a NatCom burner from Canada. My experience has been with Coen, Todd, Zink and Peabody. As I said originally, this is my first exposure to low NOx.

 

[danberry]

Ah! A Natcom burner. That explains it. These burners need a lot of air at low and high outputs. In the middle, they can be made to run efficiently. I am sure the O2 trip is a work-around for this unpublished problem.

You should defenitely monitor O2 and control the X-air with it. You need to build a set-point curve that varies with the boiler load.