Power Plant Engineering

Fig. 4.11. Hand Fire Grate Furnace.
Fig. 4.11 shows a hand fire grate furnace with a stationary fuel bed. The grate divides it into the
furnace space in which the fuel is fired and an ash pit through which the necessary air required for com-
bustion is supplied. The grate is arranged horizontally and supports a stationary bed of burning fuel.
The fuel is charged by hand through the fire door. The total space in the grate used for the
passage of air is called its useful section.
In a hand fired furnace the fuel is periodically shovelled on to the fuel bed burning on the grate,
and is heated up by the burning fuel and hot masonry of the furnace. The fuel dries, and then evolves
gaseous matter (volatiles combustibles) which rise into the furnace space and mix with air and burn
forming a flame. The fuel left on the grate gradually transforms into coke and burns-up. Ash remains on
the grate which drops through it into ash pit from which it is removed at regular intervals. Hand fired
furnaces are simple in design and can burn the fuel successfully but they have some disadvantages also
mentioned below:

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(i) The efficiency of a hand fired furnace is low.
(ii) Attending to furnace requires hard manual labour.
(iii) Study process of fuel feed is not maintained.
Cleaning of hand fired furnaces may be mechanized by use of rocking grate bars as shown in
Fig. 4.12. The grate bars loosen the slag and cause some of it to drop together with the ash into the
bunker without disturbing the process of combustion.
Lever
Grate bars
Fig. 4.12. Rocking Grate Bars.
4.5.2 MECHANICAL FIRING (STOKERS)
Mechanical stokers are commonly used to feed solid fuels into the furnace in medium and
large size power plants.
The various advantages of stoker firing are as follows :
(i) Large quantities of fuel can be fed into the furnace. Thus greater combustion capacity is
achieved.
(ii) Poorer grades of fuel can be burnt easily.
(iii) Stoker save labour of handling ash and are self-cleaning.
(iv) By using stokers better furnace conditions can be maintained by feeding coal at a uni-
form rate.
(v) Stokers save coal and increase the efficiency of coal firing. The main disadvantages of
stokers are their more costs of operation and repairing resulting from high furnace
temperatures.