Power Plant Engineering

4.4 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 where as
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.
4.4.1 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.

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STEAM POWER PLANT
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Following factors should be considered while selecting a suitable combustion equipment for a