Fuel Burning Furnaces

Fuel is burnt in a confined space called furnace. The furnace provides supports and enclosure for burning equipment. Solid fuels such as coal, coke, wood etc. are burnt by means of stokers whereas burners are used to burn powdered (Pulverized) coal and liquid fuels. Solid fuels require a grate in the furnace to hold the bed of fuel.

Types Of Furnaces

According to the method of firing fuel furnaces are classified into two categories :

(i) Grate fired furnaces

(ii) Chamber fired furnaces. Grate fired furnaces. They are used to burn solid fuels. They may have a stationary or a movable bed of fuel.

These furnaces are classified as under depending upon the method used to fire the fuel and remove ash and slag.

(i) Hand fired

(ii) Semi-mechanized

(iii) Stocker fired.

Hand fired and semi-mechanized furnaces are designed with stationary fire grates and stoker furnaces with traveling grates or stokers.

Chamber fired furnaces.

They are used to burn pulverized fuel, liquid and gaseous fuels. Furnace shape and size depends upon the following factors:

(i) Type of fuel to be burnt.

(ii) Type of firing to be used.

(iii) Amount of heat to be recovered.

(iv) Amount of steam to be produced and its conditions.

(v) Pressure and temperature desired.

(vi) Grate area required.

(vii) Ash fusion temperature.

(viii) Flame length.

(ix) Amount of excess air to be used.

Simply furnace walls consists of an interior face of refractory material such as fireclay, silica, alumina, kaolin and diaspore, an intermediate layer of insulating materials such as magnesia with the exterior casing made up of steel sheet. Insulating materials reduce the heat loss from furnace but raise the refractory temperature. Smaller boilers used solid refractory walls but they are air cooled. In larger units, bigger boilers use water cooled furnaces.

To burn fuels completely, the burning equipment should fulfill the following conditions :

1. The flame temperature in the furnace should be high enough to ignite the incoming fuel and air. Continuous and reliable ignition of fuel is desirable.

2. For complete combustion the fuel and air should be thoroughly mixed by it.

3. The fuel burning equipment should be capable to regulate the rate of fuel feed.

4. To complete the burning process the fuel should remain in the furnace for sufficient time.

5. The fuel and air supply should be regulated to achieve the optimum air fuel ratios.

6. Coal firing equipment should have means to hold and discharge the ash.

Following factors should be considered while selecting a suitable combustion equipment for a particular type of fuel :

(i) Grate area required over which the fuel burns.

(ii) Mixing arrangement for air and fuel.

(iii) Amount of primary and secondary air required.

(iv) Arrangement to counter the effects of caping in fuel or of low ash fusion temperature.

(v) Dependability and easier operation.

(vi) Operating and maintenance cost.