Low-Load CO Excursion on a Gas-Fired D-Type Boiler
A 150,000 lb/h, 600 psig D-type packaged boiler fires natural gas through a parallel-positioning burner with separate forced-draft and fuel-control valves. The unit carries plant load between 20% and 85%. An in-situ O2 probe is installed in the stack downstream of a common air heater that has 8% air leakage; the probe was calibrated two weeks ago at high fire. At 30% load the O2 trim holds 2.0% indicated O2, but CO rises from 15 ppm to 650 ppm and unburned hydrocarbons rise from 5 ppm to 120 ppm. At 60% load and the same 2.0% O2 setpoint, CO is 20 ppm. The burner management system has no CO trim and no individual burner air measurement. Operations proposes to raise the O2 setpoint to 3.5% and continue running at 30% load until the next outage.
1. Determine the most probable reason the CO excursion appears at low fire but not at high fire, including the role of the air heater leakage and probe location.
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The indicated O2 is measured downstream of an air heater with 8% leakage, so part of the sample is air that never entered the burner. At 30% load the flue-gas mass is roughly one-third of high-fire mass, so the same leakage mass is a much larger share of the sample; true combustion O2 is therefore well below the 2.0% indicated. That, combined with poorer low-fire fuel/air mixing, produces CO and unburned hydrocarbons. At 60% load the dilution error is smaller and mixing is better, so CO remains low. Raising the setpoint to 3.5% treats the diluted reading, not the root cause; it may clear CO temporarily but wastes fuel and leaves the low-fire mixing and leakage problems unresolved. Confirm by measuring O2 and CO at the burner windbox and at the stack with a portable analyzer, and by comparing true O2 before and after the air heater.
2. List immediate operating actions and the follow-up test plan you would require before accepting the 3.5% setpoint as a permanent fix.
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Immediate: as an interim step raise the indicated O2 setpoint by no more than 0.5 to 1.0 point, to about 2.5 to 3.0%, watch CO and unburned hydrocarbons continuously, and reduce load or take the burner out of service if CO exceeds the site limit or hydrocarbons rise further. Do not treat 3.5% as permanent. Follow-up: perform an air-heater leakage test and repair the leakage; recalibrate the O2 probe with certified gas and compare it against a portable analyzer at the same plane; inspect the low-fire fuel-valve position, forced-draft damper, burner diffuser, and linkage for hysteresis; conduct a low-fire combustion tune with a portable analyzer; verify that O2 trim has adequate authority and that its low-fire setpoint curve is not based on a diluted signal.
3. If the air heater leakage is corrected but low-fire CO persists, what design or control changes would you evaluate?
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Evaluate the burner turndown limit, fuel/air characterization at 20% to 30% load, individual burner air registers, diffuser position, and fuel-pressure stability. Add a CO or unburned-hydrocarbon analyzer for trim, revise the O2 setpoint curve with load, and consider flue-gas recirculation or staged air if NOx limits allow. If the burner cannot be tuned to hold CO within limits at the required low load, reduce the operating turndown or replace the burner. Any change must be verified by a combustion test across the full load range, not just at the low-fire point.