Fuels, Combustion and Firing Equipment
Fuel properties
Understand the composition of solid, liquid and gaseous fuels: carbon, hydrogen, sulphur, moisture and ash. Know the meaning of calorific value (gross and net), volatile matter, fixed carbon and ash fusion temperature. For liquid fuels, understand viscosity, flash point, pour point and specific gravity, and why preheating is used. For gas, know the difference between natural gas and liquefied petroleum gas and the significance of the Wobbe index.
Combustion fundamentals
Master the three T's: temperature, time and turbulence, plus the correct air-to-fuel ratio. Know stoichiometric air, excess air, and why some excess air is needed. Understand the products of complete and incomplete combustion, and the danger of carbon monoxide from insufficient air or poor mixing. Know why sulphur produces corrosive deposits and why ash causes fouling.
Firing equipment
Recognise stokers (chain grate, spreader), pulverised fuel burners, pressure-jet oil burners, rotary cup burners and gas burners. Understand atomisation, draught systems (natural, forced, induced, balanced) and the role of the furnace draught. Know how flame failure and purge interlocks protect against explosion.
Traps
Questions often confuse gross and net calorific value, or ask for the effect of too much excess air: it lowers efficiency by carrying heat up the stack, but too little raises carbon monoxide and smoke. Do not assume more air always means better combustion. Another trap is the flash point: it is a safety property, not an ignition property.
Worked example
If flue gas shows high carbon monoxide and low oxygen, reason that air is insufficient or mixing is poor. Correct by increasing air or improving turbulence, not by reducing fuel alone.
Rule of thumb
Aim for the least excess air that still gives complete combustion, judged by carbon monoxide and smoke.