- Combustion Definition: Combustion is defined as a chemical process where oxygen combines with fuel elements to produce heat.
- Stages of Combustion: Combustion involves heating fuel, burning volatile gases, and combusting fixed carbon.
- Boiler Combustion Chamber: he boiler combustion chamber is where volatile hydrocarbons burn, producing heat absorbed by the boiler.
- Grate Function: The grate supports solid fuel in the furnace and allows air to pass through for combustion.
- Air Supply: Adequate air supply is crucial for complete combustion, but too much air can disrupt the process.
What is Combustion?
Combustion is the chemical reaction of a fuel with oxygen that releases heat. In coal, carbon, hydrogen and sulfur can react with oxygen. The fuel composition and the completeness of those reactions affect the heat released.
Coal also contains mineral matter, including compounds of iron and silicon, that can remain as ash after firing. Bottom ash falls into the ash pit of a stoker-fired steam boiler furnace, while finer fly ash may leave with the flue gas. The frozen list below gives a simplified three-stage account of coal combustion.
- The absorption of heat to rise the temperature of the fuel to the point of ignition.
- The distillation and burning of volatile gasses.
- Combustion of fixed carbon.
As coal enters a boiler furnace, heating first drives off moisture and volatile matter. Those gases mix with combustion air and burn above the fuel bed in stoker firing, or around the coal particles in pulverised firing. The remaining char then burns in the steam boiler furnace.
Carbon in the remaining char reacts with oxygen. Complete oxidation forms carbon dioxide, while incomplete combustion can form carbon monoxide and leave unburned carbon in the ash. In grate firing, some non-combustible residue falls into the ash pit below the boiler furnace.
The furnace must supply enough air and mix it with the fuel at a temperature and residence time that allow combustion to finish.
The theoretical air requirement depends on the coal analysis, so 12 pounds of air per pound of coal is only a rough example. Operators add controlled excess air to cover imperfect mixing and load changes. Supplying twice the theoretical amount is not a general rule because excess air carries extra heat up the stack. Forced-draft and overfire-air systems distribute air through the steam boiler furnace.
Too little air produces carbon monoxide, smoke and unburned carbon. Too much air cools the flame, raises flue-gas losses and can entrain fuel or ash. Combustion controls therefore hold excess air within the range required for the fuel, furnace and load.
A boiler furnace is designed for stable combustion with low visible smoke. Smoke matters for the two reasons listed below:
- Smoke causes air pollution
- Indicates incomplete combustion, as unburned gases become visible as smoke.
Complete combustion in a steam boiler furnace requires more than fuel at its ignition temperature and enough total air. The air must reach the combustible gases and char before the flame is cooled or the gases leave the furnace.
Air and combustible gases must mix while the gas remains above its ignition temperature. Fresh bituminous coal releases volatile matter over the fuel bed. Poor mixing or early cooling can carry carbon monoxide, hydrocarbons and smoke into the flue instead of completing combustion.
Stoker furnaces may use overfire-air jets, arches or baffles to mix volatile gases with air and keep the flame away from cool heat-transfer surfaces until combustion has progressed. A refractory-lined chamber can retain heat and provide residence time, but its design must match the fuel and firing system.
Construction of Boiler Furnace
For solid-fuel combustion, a boiler furnace includes the parts in this frozen construction list:
- A grate for supporting fuel (coal).
- Combustion chamber – in which the combustion takes place Means of supplying fresh air.
- An ash pity for collecting and catching the refuses from the fuel during combustion.
Boiler Furnace Grate
A furnace grate supports solid fuel in a stoker-fired steam boiler furnace. Its openings admit undergrate air to the fuel bed and let ash move towards the collection area.
The openings in the grate must retain the fuel while admitting enough air for combustion. Opening size and air distribution depend on the fuel size, grate design and firing rate.
Firebox and Combustion Chamber of Furnace
The Firebox is the furnace space directly above the grate. The combustion chamber extends that space so volatile gases can mix with air and burn. Heat then transfers to the steam boiler surfaces around the combustion chamber. Refractory walls, arches and baffles may control temperature and gas flow.
Ash Pit
The Ash pit below a fixed grate collects bottom ash and can serve as an undergrate-air plenum. Clearance between the pit floor and grate lets air spread before it passes through the fuel bed. A floor sloped towards the access side makes ash removal easier.
