Combustion control

Combustion control involves measuring and adjusting how a burner (gas, oil, biomass) mixes air and fuel to produce a stable, clean, and efficient flame.

Using a flue gas analyzer (O₂, CO, NOx, CO₂, flue gas temperature), excess air (λ) is checked, combustion chamber/flue gas temperature and draft are monitored, and the air/gas ratio or oil burner manifold is calibrated. When properly conducted at minimum, medium, and maximum loads, this control limits pollutants, improves efficiency (especially in condensing boilers thanks to cold return air), and reduces cycling and breakdowns.

Combustion control: principle, measurements and settings

The control relies on a measurement protocol and correlated settings:

- Key measurements : O₂ (or λ), CO (safety/mixture quality), NOx (hot/oxidizing combustion), flue gas temperature, draft (ΔP chimney), ambient temperature and sometimes H₂ content of the gas / LHV.

- Interpretation : too much excess air = ↓ efficiency (hotter fumes, heat carried away); not enough air = ↑ CO, soot, risk of reignition. The best compromise is sought with each load: "low O₂ without CO".

- Settings : fan speed (VFD), air damper, proportional gas valves, nozzles/fuel oil pressure; synchronization of cams/servomotors on mixed burners.

- Steps : safety checks (pressure switches, flame detection), combustion chamber purging, ignition timing (avoid CO spikes), min/mi/max adjustment, acceleration ramp check (avoid stalling), stability validation.

- Advanced tools : O₂-trim (flue gas probe that continuously corrects the combustion point), gas self-adaptation (variable quality), fuel oil/biomass lambda probes, altitude/barometric compensation.

- Hydraulic context : perfect combustion does not compensate for an uncontrolled temperature difference (ΔT). Balancing, heating curve, and cold return temperatures remain essential, especially in condensing boilers.

Advantages, limitations and points to consider

Interests

• Increased efficiency: less excess air, optimized flue gas temperature → kWh saved.
• Safety & reliability: controlled CO, stable ignition, reduced fouling (clean heat exchanger).
• Emissions ↓: NOx/CO/particulates reduced with fine adjustments (and premix burners).
• Comfort: stable flame → reduced cycles and noise, more regular temperatures.

Boundaries

• Fuel variability (gas, biomass moisture) which shifts the settings.
• Unstable chimney/draft (wind, temperatures) can disrupt combustion.
• Equipment: poorly calibrated analyzer = erroneous decisions.

Points to consider

• Documented procedure: measure min/mi/max, record O₂/CO/NOx/T° and organ positions.
• Maintenance: clean burner head, electrodes, heat exchanger, air/fuel filters, check gas circuit tightness.
• Safety: test pressure switches, ionization/UV, emergency cut-offs; monitor ambient CO in boiler room.
• Coupled hydraulics: guarantee minimum generator flow rate, target ΔT, returns < 55 °C for condensation (gas boiler).
• GTB: trace trends (O₂, flue gas temperature, modulation), alerts on drifts; harmonize burner PID and water law (avoid aggressive double loops).
• Regulatory context: comply with local emission limit values ​​and verification frequency.

Anecdote — “The subdued flame of Le Havre”

In Le Havre, a gas-fired boiler room in an office building was experiencing CO spikes at ignition and excessively high flue gas temperatures at half load. The result: excessive air intake "as a precaution," desynchronized valve timing, and fluctuating draft in sea breezes. After cleaning the boiler head, recalibrating the servos, activating a soft O₂ trim, and adjusting the acceleration ramp, measurements showed an O₂ level reduced by 1 point with no CO, a flue gas temperature of -25 K, smoother ignition, and more frequent condensation. The building manager summarized: "We stopped burning kWh... up the chimney."