Boiler Operation, Combustion, Fuels and Emissions Control
This domain tests safe and efficient operation from purge to stack. It combines burner management, drum-level control, combustion stoichiometry and emissions-control techniques. The exam expects you to explain why a parameter is controlled, not just recite a setpoint.
Safe operating sequence
- A furnace purge must establish airflow through the furnace and remove combustible gases before ignition. Purge credit, airflow proving and flame-safeguard logic are interlocked; never bypass them for convenience.
- Light-off requires proven ignition, flame detection and stable fuel pressure. Loss of flame must trip fuel within the flame-safeguard response time.
- Drum level is critical. Single-element control uses level alone; two-element adds steam flow; three-element adds feedwater flow to reject disturbances and maintain level during load changes.
- Carryover, foaming and priming are caused by high dissolved solids, high alkalinity, oil contamination or sudden load swings. Blowdown and water chemistry control are the remedies.
Combustion and emissions
- Stoichiometric air is the exact air needed for complete combustion. Excess air ensures complete burning but lowers efficiency by heating nitrogen and unused oxygen.
- O2 in flue gas is the practical excess-air indicator; CO indicates incomplete combustion. Low excess air saves energy but risks CO, smoke and unstable flame.
- NOx control options include low-NOx burners, overfire air, flue-gas recirculation, staged combustion and post-combustion SCR or SNCR. SO2 is controlled by fuel selection, limestone injection or scrubbers; particulates by baghouses or precipitators.
Worked example
A dry flue-gas analysis shows 3.0% O2. Approximate excess air = O2 / (21 - O2) x 100 = 3.0 / 18 x 100 = 16.7%. If O2 falls to 1.5%, excess air is about 7.7%. The lower figure improves efficiency but must be checked against CO and flame stability. Measuring O2 on a wet basis without correcting to dry gas would misstate excess air.
Traps that cost marks
Treating O2 as a direct efficiency percentage; reducing excess air without watching CO; ignoring air in-leakage, which raises O2 and disguises excess-air control; and using a purge timer without proving airflow. Also remember that NOx and efficiency can conflict: overly aggressive low-NOx operation may increase CO or unburned carbon.
Key points: purge must be proven before light-off; three-element control uses level, steam flow and feedwater flow; excess air is estimated from dry O2; CO indicates incomplete combustion; NOx controls must be balanced against efficiency and CO.