Combustion, Fuels, and Burner Systems
Core concepts
Combustion is rapid oxidation releasing heat. The three T's govern completeness: temperature, turbulence (mixing), and time. Stoichiometric (theoretical) air is the minimum air that supplies exactly enough oxygen for complete combustion; excess air is the extra supplied to ensure complete burning. Know the approximate stoichiometric air for common fuels: about 17.3 lb air per lb of fuel oil, about 17.2 lb per lb of natural gas, and roughly 10-11 lb per lb of coal depending on rank. Products of complete combustion are CO2, water vapor, and, when excess air is present, O2 and N2; incomplete combustion produces CO and unburned hydrocarbons.
Know the burner types: atmospheric (natural draft, gas), power (forced draft), pressure jet (oil atomization), rotary cup, and combination burners. Understand the roles of the fuel oil train: pump, heater, filter, pressure regulator, atomizing nozzle, and the importance of viscosity and atomization for clean burning. Understand draft: natural, forced, induced, and balanced, and how a stack produces natural draft from the density difference between hot flue gas and ambient air.
Traps
Questions confuse theoretical air with excess air, ask for the O2 reading that indicates excess air, and test whether CO rises or falls as excess air increases near the optimum. Another trap: confusing a rich (fuel-heavy) mixture with a lean one when diagnosing smoke or high stack temperature.
Worked example
Rule of thumb: roughly 3 percent O2 in flue gas corresponds to about 15 percent excess air for typical gaseous fuels; if O2 climbs well above that, excess air is wasting heat up the stack.