Power Plant Engineering

176 POWER PLANT ENGINEERING

(iv) The size of coal used has pronounced effect on the operation and performance of FBC sys-

tem. The particle size preferred is 6 to 13 mm but even 50 mm size coal can also be used in

this system.

4.13.3 Types of FBC Systems

FBC systems are of following types :
(i) Atmospheric FBC system :
(a) Over feed system
(b) Under feed system.
In this system the pressure inside the bed is atmospheric.
Fig. 4.42 shows commercial circulation FBC system. The solid fuel is made to enter the furnace
from the side of walls. The Low Velocity (LV), Medium Velocity (MV) and High Velocity (HV) air is
supplied at different points along the sloping surface of the distribution ash is collected from the ash
port. The burning is efficient because of high lateral turbulence.

(ii) Pressurised FBC system. In this system pressurised air is used for fluidisation and combus-
tion. This system : the following advantages: (a) High burning rates. (b) Improved desulphurisation
and low NO, emission. (c) Considerable reduction in cost.

Solid

Fuel

Light

Matter

Distributer

Air

LVAir MVAir HVAir

Ash

Start up

Burner

Deflector

Wall

Liquid

Fuel

Ash Port

To B o i l e r

Fig. 4.42

EXERCISES

1.(a) What is meant by ‘over feed’ and ‘under feed’ principles of firing coal?

(b) What are the different methods of firing coal? Discuss the advantages of mechanical

methods of firing coal.

(c) Make neat sketch and explain the working of: (i) Chain grate stoker (ii) Spreader stoker.

(iii) Multi retort stoker.

(d) What is Fluidised Bed Combustion system? Sketch and describe a Fluidised Bed

Combustion (FBC) system. State the advantages of FBC system.