Power Plant Engineering

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.
4.5 METHOD OF FUEL FIRING
The solid fuels are fired into the furnace by the following methods :
1. Hand firing.
2. Mechanical firing.
4.5.1 HAND FIRING
This is a simple method of firing coal into the furnace. It requires no capital investment. It is used
for smaller plants. This method of fuel firing is discontinuous process, and there is a limit to the size of
furnace which can be efficiently fired by this method. Adjustments are to be made every time for the
supply of air when fresh coal is fed into furnace.
Hand Fired Grates. A hand fired grate is used to support the fuel bed and admit air for combus-
tion. While burning coal the total area of air openings varies from 30 to 50% of the total grate area. The
grate area required for an installation depends upon various factors such as its heating surface, the rating at
which it is to be operated and the type of fuel burnt by it. The width of air openings varies from 3 to 12 mm.
The construction of the grate should be such that it is kept uniformly cool by incoming air. It
should allow ash to pass freely. Hand fired grates are made up of cast iron. The various types of hand
fired grates are shown in Fig. 4.10. In large furnaces vertical shaking grates of circular type are used.
Tuper grate
Common Grate Bar
Sawdust Grate
Fig. 4.10. Various Types of Hand Fired Grates.

---

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