Adding flue-gas oxygen trim control
Oxygen trim control continuously measures the oxygen left in a burner's flue gas and automatically adjusts the air-to-fuel ratio to hold the optimum small excess of air. It corrects the drift that pushes manually tuned burners toward either wasteful excess air or dangerous incomplete combustion as conditions change.
What it is
Combustion needs slightly more air than the theoretical minimum to burn fuel completely, but only slightly — every extra unit of air is heated and sent up the stack as waste. Oxygen trim adds a flue-gas oxygen sensor and a control loop that trims the combustion-air damper or fan to keep excess oxygen at the optimum target across the whole firing range, automatically and continuously.
Why it is done
A burner tuned once stays tuned only briefly: fuel quality, air density, temperature and mechanical wear all shift the ratio. Without trim, operators set a generous air margin to stay safe, paying a permanent efficiency penalty. Trim holds the ratio close to optimum despite drift, recovering that margin while avoiding the soot, carbon monoxide and hazard of running short of air.
How it is done
A flue-gas oxygen analyser is installed in a representative location, and the optimum excess-oxygen target is established by combustion testing across the firing range. The trim loop is configured to adjust the air control around the base air-fuel curve, bounded by safe limits so it cannot drive combustion sub-stoichiometric. The loop is tuned for the burner's response, and performance is verified by checking stack oxygen and combustibles hold target as load and conditions vary.
- Install O2 analyser
- Set excess-air target
- Configure trim loop
- Bound safe limits
- Tune loop response
- Verify across load
What to watch for
Trimming toward too little excess air to maximise efficiency risks incomplete combustion, soot and carbon monoxide — the loop must be hard-limited against going sub-stoichiometric. A drifting or sludged oxygen probe feeds the loop bad data and must be maintained and calibrated.
Related practices
Running a compressed-air leak survey programme
Retrofitting waste-heat recovery
Retrofitting variable-speed drives
Related topics
How to Improve Boiler Efficiency: A Practical Guide · Flue Gas Loss · Control Loop · Boiler Efficiency
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