High CO and Yellow Flame on a Fire-Tube Boiler
A plant operates a 15 MMBtu/h fire-tube boiler on natural gas at 1,000 Btu/ft³ and 0.60 specific gravity. A portable flue-gas analyzer at the stack outlet reads O₂ 3.4%, CO 410 ppm, CO₂ 10.1%, flue temperature 212°C, and ambient 18°C. Stack draft is −0.12 in. w.c. at the outlet, and the burner was last tuned 14 months ago. The boiler cycles between 40% and 70% load. Operators report a faint yellow tip on the main flame and occasional rumble at low fire. The combustion-air louver has 60% free area, and its bird screen is partly blocked with lint. The nameplate specifies 4.0 in. w.c. manifold pressure; a gauge reads 4.0 in. w.c. at high fire. The plant must decide whether to retune, clean, or investigate further before the next production run.
1. Calculate excess air from the measured O₂ and assess whether the CO reading is acceptable. Show the calculation.
Read the worked answer
Excess air from O₂: EA (%) = O₂/(20.9−O₂) × 100 = 3.4/(20.9−3.4) × 100 = 19.4%. That is a reasonable excess-air band for a gas boiler, so the O₂ alone does not explain the CO. Convert CO to air-free: CO_af = 410 × 20.9/(20.9−3.4) = 490 ppm. A 490 ppm air-free CO reading is high for a clean gas boiler and warrants correction before returning to normal service; many commissioning targets are below about 100 ppm, with investigation triggered well before 400 ppm. The combination of acceptable O₂ and high CO points to incomplete mixing or flame impingement, not simply a lack of total air.
2. Identify the most likely causes of the yellow flame tip and low-fire rumble, and explain how the blocked louver screen could produce high CO even though O₂ is only 3.4%.
Read the worked answer
Likely causes: (1) dirty burner or primary-air passages, especially after 14 months; (2) partially blocked combustion-air screen, which raises the pressure drop across the louver and can starve the burner at high fire; (3) low-fire air/fuel mismatch causing rumble; (4) flame impingement on the fire-tube due to misaligned burner or incorrect orifice; (5) draft instability. The blocked screen can produce high CO while O₂ remains moderate because the analyzer samples the whole flue, not the local flame zone-CO forms in fuel-rich pockets or quench zones even when average O₂ is 3.4%. The yellow tip supports insufficient primary air or a dirty burner, not a global air shortage.
3. Recommend a sequence of corrective actions and the final combustion targets you would use to return the boiler to service.
Read the worked answer
Sequence: (1) Lock out and allow cooldown; (2) clear the louver bird screen and verify free area; (3) open the boiler, clean the burner, air passages, and fire-tube; (4) verify gas supply pressure and manifold at high and low fire; (5) set the burner air shutter/primary air per manufacturer; (6) re-tune high fire, low fire, and mid fire; (7) record O₂, CO, CO₂, flue temperature, and draft. Targets: O₂ about 3–5% (or manufacturer's range), CO air-free below 100 ppm (and never above the manufacturer's limit), stable flame without yellow tip or rumble, and draft within nameplate range. If CO remains high after cleaning and tuning, inspect the orifice, gas valve, and fire-tube for scaling or baffle damage before returning to service.