Power Plant Engineering

152 POWER PLANT ENGINEERING

The main characteristic of a grate fired furnaces are the heat liberation per unit of grate area and
per unit of volume. The heat liberation per unit area of fire grate area is calculated as follows:

H = (W × C)/A

where H = Heat liberation per unit of fire grate area

W = Rate of fuel consumption (kg/sec)

C = Lower heating value of fuel (kcal/kg)

A = Fire grate area (m^2 )

The heat liberation per unit of furnace volume is given by the following expression:

H = (W × C)/V

where H = Heat liberation per unit volume

W = Rate of fuel consumption (kg/sec)

C = Lower heating value of fuel (kcal/kg)

V = Volume of furnace (m^3 ).

These two characteristics depend on the following factors :

(i) Grade of fuel

(ii) Design of furnace

(iii) Method of combustion.

Ash pit

Furnace

Space

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: