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Reduction techniques and costs
Particles
Four main types of equipment can be
used to reduce the emissions of particles leaving the chimney.
These are cyclones, fabric
filters, electrostatic precipitators
and wet scrubbers.
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Cyclones
These
are effective on large particles (>15µm). This type of equipment
uses a mechanical force (gravity, inertia or centrifugal force)
to divert dust from the flow of carrier gas and separate them
out. This works because the relative density of dust is about
1,000 times greater than that of gases. This device can extract
dust down to 120-250 mg/m3 and it is often used as a preliminary
extraction method.
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Fabric filters
The flow
of gas to be treated passes through a filtering medium which
traps the dust. It gradually blocks the pores of the filter
and it needs to be regularly cleaned by removing the accumulated
dust.
This type of filter can extract dust down to 5-10 mg/m3 and
is highly effective on all kinds of dust, even very fine.
There are costs associated with using this equipment arising
from the need to periodically change the filter element. Also,
it is very sensitive to high temperatures (>250°C) and so
a system for cooling the gas before it enters the filter may
be required.
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Electrostatic precipitators
Dust present
in the waste gases is ionised and then trapped in a strong
electric field created between a transmitting electrode and
a receiving electrode. The receiving electrode is regularly
vibrated to shed the accumulated dust. This technology is
very effective on all particles and can achieve 50mg/m3 of
dust removal in the waste gas, or even 30 mg/m3 by adapting
the number of fields.
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Wet scrubbers
Wet scrubbers
are sometimes essential for dust removal, when the gases to be treated
are close to saturation in water, or when safety problems make it
very difficult to clean them by dry means. Scrubbers have the drawback
of transferring gas pollution into liquid pollution. A wet scrubber
produces sludge which then has to be processed or at least decanted.
The effectiveness of scrubbers depends on the level of energy employed.
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Investment
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Operating
cost
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Emissions
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Cyclones
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+
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+
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120 to <250 mg/m3
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Electrostatic
precipitators
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+++
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+
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30
to <50 mg/m3
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Fabric
filters
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+++
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++
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5
to <10 mg/m3
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Wet
scrubbers
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+
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++/+++
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<50
mg/m3
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Orders
of magnitude concerning the cost of these processes
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