Rising CO and Unstable Excess O2 on a Packaged Watertube Boiler
You are the shift engineer at a 24/7 processing plant. No. 2 boiler is a packaged watertube unit rated 60,000 lb/h at 250 psig saturated, firing natural gas through a single low-NOx burner. At 06:00 the load is steady at 44,000 lb/h and the O2 trim is in automatic. Over two hours the CEMS shows CO rising from 35 ppm to 190 ppm, O2 swinging between 2.4% and 4.1%, and the trim actuator moving full stroke. Stack temperature is 408°F against a 355°F baseline measured after a recent tube cleaning. Furnace draft is −0.05 in. w.c. but fluctuates ±0.03 in. w.c. The FD fan is at 78% demand, fuel gas pressure is steady at 12 psig, and the flame scanner signal is steady. The plant is behind on steam demand and the boiler cannot be removed from service without curtailing production. Last week the combustion air damper linkage was adjusted during a forced-draft fan bearing change.
1. What is the most likely explanation for the simultaneous CO rise, O2 swing, and stack temperature increase, and what competing explanations should be ruled out before you act?
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The pattern points to unstable combustion air control, not a fuel problem. CO rising while O2 is not low means fuel and air are not mixing properly at the burner. The O2 swings show the air flow itself is moving, and the trim actuator moving full stroke shows the controller is chasing a feedback that will not settle. The stack temperature rise after tube cleaning is consistent with more excess air, not with fouling. The draft fluctuation of ±0.03 in. w.c. reinforces a damper or actuator problem.
Rule out: fuel gas pressure or heating value change (pressure is steady, but check heating value); burner tip fouling or flame impingement (a steady scanner does not prove good flame shape); O2 analyzer drift (it would not move the actuator); air in-leakage (raises O2 without raising CO); and load change (load is steady). The strongest candidate is a slipped or loose linkage that gives the FD fan a different flow curve, so the O2 controller hunts and periodically creates poor mixing.
2. What immediate operating actions would you take while keeping the boiler online?
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Place the O2 trim in manual and hold a fixed firing rate, or reduce load slightly, so the controller stops chasing the swing. Raise excess air until CO is below the site limit, for example below 100 ppm, while watching stack temperature and flame stability; typical O2 would be 4–5% at this load. Do not keep driving air down to chase an O2 setpoint, because that deepens the CO problem.
Verify the linkage, actuator stroke, and damper position feedback, comparing command with actual position. Trend CO and O2 every 5–10 minutes. Tell the control room and production that steam may be curtailed. If CO cannot be held below the limit or the flame scanner becomes unstable, trip the boiler; a boiler that is smoking CO is also at risk of a furnace explosion. Do not allow production pressure to justify overriding combustion limits.
3. If the linkage checks out, what diagnostic steps and criteria determine whether the boiler may stay in service overnight?
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Inspect the burner air register, diffuser, and gas spuds during a brief outage; calibrate the O2 and CO analyzers; check fuel gas heating value; and perform a burner setup with a portable analyzer across the firing range. Look for a partly plugged air passage or a damper that is mechanically loose on its shaft even if the linkage appears tight.
Criteria to remain in service overnight: CO stable below the site limit at O2 of at least 3% with no more than about ±0.5% O2 variation; damper command and feedback within 2%; stable furnace draft; and stack temperature not climbing. If those cannot be met, reduce maximum load or shut down. Document the decision, the readings, and who authorized continued operation.