| Common Pollution
Control System |
The emissions from cupola consist of particulate
matter, sulphur dioxide, oxides of nitrogen
and carbon monoxide. The emission level of
particulate matter depends on a number of
variables such as the size and design of cupola,
size and composition of raw materials (specifically
the ash content of coke), volume and velocity
of the air blast, blast temperature, melting
practice etc. |
| |
| Sulphur dioxide mainly arises out of the
sulphur in the coke. Approximately, 50% of
sulphur in coke is usually absorbed in the
metal and slag, and the rest gets discharged
as sulphur dioxide through the stack along
with the exhaust gases. |
| |
| Nitrogen oxide is emitted due to the intense
combustion conditions prevailing near the
tuyeres. |
| |
There is high presence of carbon monoxide
in the exhaust gases which is to be expected
considering the design of a cupola to ensure
that the melt is not exposed to oxidising
atmosphere which will adversely affect the
melt quality. |
| |
| Depending upon the sizes of the particulates
and their distribution in the exhaust gas
and presence of gaseous pollutants, a controlling
device or combination of device may be selected
for use. The different pollution control options
for cupola furnace are given in the table
below. |
| |
| Common pollution control systems |
| Equipment |
Minimum
particle
size, µm |
Overall
collection
efficiency, % |
| |
|
|
| Centrifugal separators |
|
|
| |
|
|
| Cyclone |
>10 |
<85 |
| Multi cyclone |
>5 |
<95 |
| |
|
|
| Wet Scrubber |
|
|
| |
|
|
| Centrifugal |
>5 |
<90 |
| Impingement |
>5 |
<95 |
| Packed bed |
>5 |
<90 |
| Jet |
0.5 - 5 |
<90 |
| Venturi |
>0.5 |
<99 |
| Fabric Filter |
>0.2 |
<99 |
| Electrostatic precipitators |
>2 |
<99 |
| |
|
|
|
|
Some of the more common pollution control
systems employed in the foundry industry are
discussed below, particularly from the point
of their suitability to cupola melting. |
| |
Cyclone
The major drawback of cyclone is that it is
not effective for controlling the finer size
(<10 µm) particulate matter whereas
overall control efficiency can be achieved
up to maximum 85% which is considered as low.
It may be very difficult to meet the emission
standard of 450 mg/Nm3 (for cupolas capacity
below 3 tph) by cyclone with certainty particularly
when ash content in coke fluctuates widely.
Moreover, emission standard for suphur dioxide
for cupola furnace has also been prescribed
by CPCB which is not possible to control by
dry cyclone.
|
Wet arrester or cap
Wet arrester can capture a large amount of
dust particles and also dissolve a part of
sulphur dioxide emitted. Water is sluiced
back into settling tank and can be recycled
by providing a proper provision for settling.
It’s overall efficiency is of the order
of 90%. However, the device is not very effective
for cleaning fine particulate matter (<
5 µm) contained in the gases.
|
Multi-cyclone
Moderate pressure loss (approx. 150 mm Hg)
occurs across the collector and hence it is
more energy consuming device than a simple
wet arrester. Multi-cyclones are effective
for meeting the emission standard of 450 mg/Nm3
with greater degree of certainly as their
overall efficiency goes up to 95%.
|
Venturi-scrubber
Venturi-scrubbers are highly efficient for
meeting the stringent standard of particulate
matter and sulphur dioxide. A large induced
draft fan is required to overcome the high
pressure drop (as high as 1000 mm Hg). It
is possible to collect sub-micron particulates,
fumes and smoke (upto 0.5 µm) with 99%
efficiency using venturi-scrubbers.
|
Fabric filter
Fabric filter is one of the most efficient
particulate collectors for foundry units.
The device can even remove the finer particulates
upto 0.2 µm with 99% efficiency. However,
it is more energy intensive and the hot gas
needs to be cooled to below 140 oC before
passing it through the bag filter. Fabric
filters are costly pollution control devices
and they cannot remove sulphur dioxide present
in the flue gas.
|
| |
| |
|
| |
