High CO After a Natural Gas Burner Tune
A 600 hp natural gas-fired firetube boiler has just been tuned by a contractor. At high fire, the portable analyzer shows 2.0 percent O2, 850 ppm CO, 520°F stack temperature, and furnace draft of -0.04 in. w.c. The gas supply pressure is 7 in. w.c., and the burner is a forced-draft design with a fixed-position air damper linkage. The boiler normally runs at 3 to 4 percent O2 with less than 50 ppm CO and a 380°F stack. The contractor says the low O2 is good for efficiency and suggests opening the air damper slightly to bring CO down. The plant has a CO alarm in the boiler room but no continuous flue gas analyzer. The boiler is on line and supplying a process that cannot be interrupted without a scheduled shutdown.
1. Interpret the O2, CO, stack temperature, and draft readings. What do they indicate?
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At 2.0 percent O2, the burner is running with very little excess air, roughly 10 percent. That is not automatically a CO problem, but 850 ppm CO shows incomplete combustion. The high stack temperature of 520°F, well above the normal 380°F, points to either excessive heat input, poor heat transfer, or flame impingement and afterburning in the passes. The -0.04 in. w.c. furnace draft is within a typical operating range, but it does not prove the burner has enough combustion air; a partly closed air damper, a dirty burner diffuser, or a misadjusted linkage can starve the flame while draft remains normal. The combination of low O2, high CO, and high stack temperature is not an efficiency success. It is a combustion problem that can produce carbon monoxide in the boiler room and unsafe flame conditions.
2. What immediate actions and diagnostic checks should be taken while the process remains on line?
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First protect people: verify the boiler-room CO alarm is working and ventilate if needed. Do not simply open the air damper blindly; a small air increase may reduce CO, but if the root cause is a dirty burner, a bad linkage, or flame impingement, adding air can shift the flame and create a worse condition. Increase excess air in small steps while watching CO and flame stability, and be ready to reduce firing rate or shut down if CO rises or flame becomes unstable. Check the burner diffuser and air passages for dirt, confirm the linkage is actually moving the damper, check gas pressure at high fire, inspect the flame through the sight port, and check for air leakage into the furnace. Confirm the analyzer is calibrated and the sample is taken at the correct location. If CO cannot be brought below a safe limit with stable flame, the boiler should be taken off line for burner service.
3. What final combustion targets and documentation should the contractor provide?
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The final tune should establish stable flame with enough excess air to keep CO low across the firing range, not just at high fire. A reasonable target for this boiler is 3 to 4 percent O2 at high fire with CO well under 50 ppm, but the manufacturer's data and the burner's acceptable range govern. The stack temperature should return near the normal 380°F if the high reading was caused by combustion rather than fouling. The contractor should record O2, CO, CO2, stack temperature, draft, and gas pressure at low, mid, and high fire, and verify that the linkage positions match the commissioning data. The boiler log should note the before-and-after readings, the repairs made, and the date. If heat transfer surfaces are fouled, the stack temperature will remain high even after a good tune, and the tubes should be cleaned.