High CO After a Natural-Gas Combustion Tune
A 300 hp natural-gas boiler at 100 psig has just been tuned. At steady 80% load the flue gas reads 2.0% O2 dry and 600 ppm CO; stack temperature is 460°F, furnace draft is -0.02 in. w.c., and the forced-draft fan is at 85% with the burner air damper at 60%. Gas supply pressure is 7 in. w.c. The outside temperature has dropped, and the burner front shows a slightly yellow-tipped flame. The operator wants to open the air damper until CO disappears. The plant has a 30 ppm NOx limit and is sensitive to stack loss.
1. Estimate excess air at 2.0% O2 and interpret the 600 ppm CO reading.
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For natural gas, a quick estimate is excess air ≈ O2/(20.9 - O2) × 100. At 2.0% O2, excess air ≈ 2.0/18.9 × 100 ≈ 10.6%. That is not extremely low, yet 600 ppm CO is high for a gas flame. The CO indicates incomplete combustion, but it does not by itself prove that excess air is too low. Likely causes include poor fuel-air mixing, a dirty or misaligned burner, flame impingement, incorrect gas pressure, or poor draft distribution. The yellow-tipped flame supports a mixing or flame-shape problem rather than simply a shortage of total air.
2. Evaluate the operator's plan to open the air damper until CO disappears. What O2 would result if CO clears only at 4.0% O2, and what is the tradeoff?
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Opening air may reduce CO temporarily, but if CO clears only at 4.0% O2, the estimated excess air becomes 4.0/(20.9 - 4.0) × 100 ≈ 23.7%. That is a large increase from about 10.6%. The extra air lowers flame temperature, increases stack loss, and may increase NOx or fan loading. If a large air increase is needed to control CO, the real problem is likely mixing, draft, burner alignment, or gas pressure, not total air. Using air to mask the symptom wastes fuel and may still leave CO spikes at other firing rates. Short-term, raise air only as needed for safe CO, but do not treat that as the final tune.
3. Outline the diagnostic steps and the final recommendation.
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First confirm the analyzer with a known gas and check that the sample is not contaminated. Then map CO versus O2 at steady load, and check furnace draft at the burner, overfire draft, gas pressure at the burner manifold, burner insertion, air damper linkage, and flame shape. Inspect and clean the burner and air passages. Verify that the forced-draft fan and damper are not fighting each other. If CO falls sharply only after a large air increase, correct mixing or draft rather than accepting a high excess-air setting. The final recommendation should be a repeatable tune with stable flame, CO well below the site limit, O2 at the lowest value that preserves CO margin, and NOx within the 30 ppm limit across the firing range.