Power Plant Engineering

174 POWER PLANT ENGINEERING

Depending on the type of fuel and the power of bailer the ash collection in industrial boilers and
thermal power stations can be effected by mechanical ash collectors, fly ash scrubbers and electros-
tatic precipitators.
For fly ash scrubbers of large importance is the content of free lime (CaO) in the ash. With a
high concentration of CaO the ash can be cemented and impair the operation of a scrubber.
The efficiency of operation of gas cleaning devices depends largely on the physico-chemical
properties of the collected ash and of the entering waste gases.
Following are the principal characteristics of the fly ash:
(i) Density
(ii) Dispersity (Particle size)
(iii) Electric resistance (For electrostatic precipitators)
(iv) Coalescence of ash particles.
Due to increasing boiler size and low sulphur high ash content coal the problem of collecting fly
ash is becoming increasingly complex. Fly ash can range from very fine to very coarse size depending
on the source. Particles colour varies from light tan to grey to black. Tan colour indicates presence of
ion oxide while dark shades indicate presence of unburnt carbon. Fly ash particles size varies be-
tween 1. micron (l μ) to 300 μ. Fly ash concentration in flue gases depends upon mainly the follow-
ing factors :
(i) Coal composition.
(ii) Boiler design and capacity.
Percentage of ash in coal directly contributes to fly ash emission while boiler design and opera-
tion determine the percentage retained in the furnace as bottom ash and fly ash carried away by flue
gas. Fly ash concentration widely varies around 20-90 g/mm^3 depending on coal and boiler design. Fly
ash particle size distribution depends primarily on the type of boiler such as pulverised coal fired boiler
typically produces coarser particles then cyclone type boilers. Electrostatic precipitator (ESP) is quite
commonly used for removal of fly ash from flue gases.

4.13.1 Fly Ash Scrubber

Fig. 4.40 shows a fly wash centrifugal scrub-
ber. It is similar to a mechanical ash collector but
has a flowing water film on its inner walls. Due to
this film, the collected ash is removed more rapidly
from the apparatus to the bin and there is less pos-
sibility for secondary. Capture of collected dust par-
ticles by the gas flow. The degree of ash collection
in scrubbers varies from 0.82 to 0.90. The dust laden
gas enters through the inlet pipe.

Cinder Catcher. Cinder catcher is used to
remove dust and cinders from the gas. In this catcher
the dust laden gas is made to strike a series of verti-
cal baffles that change its direction and reduce its velocity. The separated dust and cinders fall to the
hopper for removal. Cinder catchers are ordinarily used with stoker firing.

Baffles

Clean Gas

Dust

Ladden

Gas

Dust Hopper

Dust

Fig. 4.40