Black Smoke and High Stack Temperature After a Fuel Switch
A 400 hp firetube boiler burning No. 2 oil is switched to No. 6 oil because of a supply contract change. The burner was set up for No. 2; the only change made was to open the fuel heater to bring the heavier oil to 140°F at the burner. Within an hour the stack shows heavy black smoke, the stack temperature has risen 90°F above the No. 2 baseline, and the firebox is visibly smoky through the peep sight. The forced-draft fan is at its normal position, atomizing steam pressure is unchanged, and the oil pressure at the burner reads the same as before. The operator wants to open the draft damper and increase atomizing steam to clear the smoke.
1. Diagnose the combustion problem and explain why the two proposed adjustments may not solve it.
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The symptoms - black smoke, rising stack temperature, smoky firebox - indicate incomplete combustion with excess unburned carbon, i.e., the burner is not properly atomizing and mixing the heavier fuel.
No. 6 oil is far more viscous than No. 2 and needs both higher preheat (typically 180–220°F at the burner, not 140°F) and appropriate atomization. At 140°F the oil is still too thick to atomize into fine droplets; large droplets burn slowly and smoke.
Opening the draft damper adds air but does not fix atomization - it may cool the firebox and worsen combustion while raising excess air and stack loss. Increasing atomizing steam can help break up the oil, but if the oil is too cold it will still form coarse droplets and can also quench the flame. Neither adjustment addresses the root cause: fuel viscosity at the nozzle.
2. What is the correct corrective sequence, and how would you verify the fix?
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- Raise the fuel oil preheat to the manufacturer's specified viscosity for No. 6 oil at the burner (commonly targeting roughly 150–300 SSU at the nozzle; check the burner data).
- Verify nozzle size and type are suitable for the heavier oil; a nozzle sized for No. 2 may be undersized or wrong pattern.
- Recheck atomizing steam pressure and temperature for the heavier fuel.
- Set draft and excess air to the burner manufacturer's curve for No. 6 oil - do not simply open the damper.
- Verify with a combustion test: measure O2/CO2, CO, and stack temperature; confirm no visible smoke (Ringelmann/Bacharach) and that stack temperature returns toward baseline.
Document the fuel change and update the burner setup and operating procedure.