Fuels, Combustion and Burners
Fuels, Combustion and Burners
This domain tests fuel properties, stoichiometry, excess air, draft, burner operation, and flame safety. Expect questions on air-fuel ratio, excess oxygen, purge requirements, turndown, and emissions rather than detailed fuel chemistry.
Core rules and concepts
- Higher heating value includes latent heat of water vapor in products; lower heating value does not. Use one basis consistently.
- Stoichiometric air is the minimum air for complete combustion. Practical firing uses excess air to avoid CO and unburned fuel, but too much excess air lowers efficiency and can increase NOx.
- Burners must atomize liquid fuel or mix gaseous fuel with air, stabilize ignition, and support turndown. Flame safeguards supervise purge, trial for ignition, main flame, and shutdown.
- A furnace purge before ignition removes combustible gases. Never light a burner without proven purge and airflow.
- Draft systems may be natural, forced, induced, or balanced. Furnace pressure and stack temperature are key operating indicators.
Worked example
Methane burns as CH4 + 2O2 → CO2 + 2H2O. Two volumes of oxygen are needed per volume of methane. Air is about 21% oxygen, so theoretical air is about 2 ÷ 0.21 = 9.5 volumes of air per volume of methane. If flue gas shows 3% O2 dry, approximate excess air is 3 ÷ (20.9 − 3) × 100 ≈ 17%. This formula assumes no CO and is an approximation; the reasoning is that remaining oxygen represents air beyond the theoretical amount.
Traps that cost marks
- Firing at 100% theoretical air and expecting clean, stable combustion.
- Skipping or bypassing the purge before ignition.
- Trusting a flame scanner that sees hot refractory instead of flame.
- Increasing excess air indefinitely to reduce CO without considering efficiency and NOx.