Fuels, Combustion, and Combustion Controls
Core concepts
Understand fuel properties: heating value (HHV vs LHV), stoichiometric air, excess air, and the three T's (time, temperature, turbulence). Know how to read flue gas analysis (O2, CO, CO2) and how excess O2 trades off against stack loss. Learn burner types (register, packaged, low-NOx), fuel/air ratio control, and the purpose of the burner management system (BMS). Grasp the difference between the BMS and the combustion control system (CCS).
Traps
Questions often swap HHV and LHV or ignore the latent heat of water vapor. Excess air is not the same as excess O2; be ready to convert. Watch for items that treat CO as an indicator of too much air - high CO usually signals incomplete combustion from insufficient air, poor mixing, or flame impingement. Also distinguish natural draft from forced draft effects on excess air readings.
Worked example
A natural gas fuel with 10% excess air shows about 2% O2 in the flue gas; 5% excess air corresponds to roughly 1% O2. If O2 rises to 6%, excess air is roughly 35–40%, raising stack losses. Rule of thumb: each 1% reduction in excess O2 (within safe limits) can improve efficiency by roughly 0.5–1%, but never reduce air below the burner manufacturer's minimum.