Black Smoke and High Stack Temperature After a Burner Change
A shell boiler rated at 5 000 kg/h has been converted from heavy fuel oil to natural gas. After commissioning, the stack temperature reads 240 °C against 190 °C before conversion, and the boiler is producing dense black smoke at high fire. The gas pressure at the burner is 80 mbar against a design of 120 mbar. The combustion-air damper is set at its old oil-fired position. The boiler is also short-cycling: it reaches pressure in about 6 minutes and then shuts down for 4 minutes. The operator proposes to reduce the gas pressure further to stop the smoke and to open the air damper fully.
1. Diagnose the likely causes of the black smoke and high stack temperature, and evaluate the operator's proposal.
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Black smoke on gas firing indicates incomplete combustion, usually from insufficient air, poor fuel-air mixing, or a gas supply that is below the burner's design pressure so the burner cannot entrain enough air at high fire. Here the gas pressure is only 80 mbar against a 120 mbar design, and the air damper is still in the oil-fired position, so the mixture is almost certainly air-starved at high fire. The high stack temperature is consistent with afterburning of unburnt fuel and with short-cycling, which wastes heat in purge and warm-up losses.
Reducing gas pressure further would make the mixture even richer and the smoke worse, and opening the air damper fully without matching the fuel may cool the flame and cause instability. The correct approach is to restore gas pressure to design, set the burner to the manufacturer's gas-fired air/fuel curve, and check the flue-gas oxygen and carbon monoxide with a analyser.
2. What measurements would you take to confirm the diagnosis, and what would you adjust first?
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Take flue-gas oxygen and carbon monoxide or carbon dioxide at low, medium, and high fire; measure gas pressure at the burner inlet with a manometer; measure combustion-air temperature and draught; and record stack temperature and boiler output. Also check the burner diffuser and gas nozzle for the correct gas-sized components.
Adjust first the gas supply pressure to the burner design, then reset the air/fuel linkage to the gas curve, starting at low fire and working up. Confirm that the smoke clears and that oxygen is in the normal band (roughly 3 to 6 percent for gas) without excessive carbon monoxide. Only then should the stack temperature and short-cycling be addressed, for example by checking the burner modulation and the boiler load.
3. How would you deal with the short-cycling and its effect on efficiency?
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Short-cycling at 6 minutes on and 4 minutes off suggests the boiler is oversized for the load, or the burner's high-fire output is too large, or the pressure band is too narrow. Each start brings a purge that carries heat up the stack and cools the boiler, lowering seasonal efficiency.
Options include widening the pressure band within the boiler's design limits, staging or modulating the burner so it matches the load, reducing the burner's high-fire nozzle or gas orifice, or adding thermal storage. The choice should be based on the measured load profile, not guesswork. After adjustment, re-check stack temperature, oxygen, and cycling frequency to confirm the improvement.