Fuels, Combustion and Firing Equipment
The three elements and the three T's
Combustion requires fuel, oxygen and heat. For complete combustion you need the right proportions (air-fuel ratio), temperature high enough to ignite, turbulence to mix air and fuel, and time to finish burning. Incomplete combustion wastes fuel and produces carbon monoxide and smoke.
Fuels
Solid: coal graded by volatile matter and ash; burnt on grates (chain-grate stoker, spreader stoker) or as pulverised fuel. Liquid: fuel oils classified by viscosity and grade; atomised by pressure-jet or rotary-cup burners. Gaseous: natural gas, LPG, blast-furnace gas; burnt in premix or nozzle-mix burners with flame safeguards.
Air and excess air
Theoretical (stoichiometric) air is the minimum for complete combustion. In practice excess air is supplied: too little gives CO and smoke; too much carries heat up the stack. Typical shell-boiler values are around 10–20% excess air for oil and gas, higher for coal. Draught (natural or induced/forced) supplies air and removes products.
Traps
Questions confuse theoretical with actual air, or claim excess air always improves efficiency. Remember: the burner must be purged before ignition to prevent furnace explosion, and flame failure devices must cut off fuel.
Worked rule of thumb
Smoky, CO-rich flue gas with a high stack temperature suggests either insufficient air or poor atomisation/mixing. If CO rises and oxygen falls, increase air or improve atomisation - do not simply add more fuel.