All Topics
Combustion & Burners
Fuel combustion principles, burner types, air/fuel ratio, and flame safeguards.
Combustion Fundamentals
Combustion is the rapid oxidation of fuel that produces heat. Complete combustion requires the right combination of fuel, air, heat, and mixing.
- Three requirements: fuel + oxygen (air) + heat (ignition)
- Fourth element: proper mixing of fuel and air
- Complete combustion: all fuel is burned — produces CO₂ and H₂O
- Incomplete combustion: produces CO (carbon monoxide) — toxic and wastes fuel
- Flue gas analysis is the best indicator of combustion efficiency
Excess Air
Some excess air above the theoretical requirement is needed for complete combustion, but too much wastes energy.
- Too much air: lowers efficiency — extra air absorbs heat and carries it up the stack
- Too little air: produces CO — dangerous, smoky, and wastes fuel
- Typical excess air: 10–25% above theoretical air
- High O₂ in flue gas = excess air
- High CO in flue gas = insufficient air or poor mixing
Never reduce combustion air to the point where CO is produced — this is dangerous.
Burner Types
Different burner types are used depending on fuel type and boiler application.
- Gas burners: atmospheric (natural draft) or power burners (forced draft)
- Oil burners: pressure-atomizing, rotary cup, or steam/air-atomizing
- Combination burners: can fire gas or oil (dual-fuel)
- Modulating burners: adjust firing rate to match load
- On/off burners: simple — either fully on or fully off
Flame Safeguard System
The flame safeguard system (burner management system) ensures safe startup, operation, and shutdown of the burner.
- Pre-purge: blows air through the furnace to remove combustible gases before ignition
- Pilot flame: small flame that ignites the main burner (proved by flame scanner)
- Main flame: monitored continuously by flame scanner (UV, IR, or flame rod)
- Flame failure: if the flame scanner loses the flame signal, fuel is shut off immediately
- Lockout: after a flame failure, a manual reset is required before restarting
NEVER bypass flame safeguards. NEVER immediately relight without purging.