Electrostatic Precipitator Sizing
Electrostatic precipitators have been used in many industries; several examples are cement,
refinery and petrochemical, pulp and paper and power generation. Although the physical operation
of a precipitator is simple and essentially the same for each industry, involving particle charging,
collection, dislodging and disposal, the sizing of a precipitator is more complex combining both
art and science.
The typical equation used in precipitator sizing is the modified Deutsch equation:
Where A is the collecting electrode surface area, V is the gas volume and w is the precipitation rate.
The exponent y is a variable based on test data for each specific application.
Factors that influence precipitator sizing are:
 | gas volume |
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| precipitator inlet loading |
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| precipitator outlet loading required |
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| outlet opacity required |
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| particulate resistivity |
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| particle size |
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 Resistivity is a term used to describe the resistance of a medium
to the flow of an electrical current.
By definition, resistivity, which has units of ohm-cm, is the electrical resistance of a dust sample 1
cm2 in cross sectional area and 1 cm thick.
| | Resistivity levels are generally broken down into three categories: |
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| | Particles in the medium resistivity range are the most acceptable for
electrostatic precipitators.
Particles in the low range are easily charged, however upon contact with the
collecting electrodes, they rapidly loss their negative charge and are repelled
by the collecting electrodes back into the gas stream to either escape or to be
recharged by the corona field.
Particles in the high resistivity category may cause back corona which is a
localized discharge at the collecting electrode due to the surface being coated
by a layer of non-conductive material.
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Resistivity is influenced by flue gas temperature and conditioning agents, such as flue gas
moisture and ash chemistry. Conductive chemical species, such as sulfur and sodium will tend
to reduce resistivity levels while insulating species, such as SiO2, AL2O3
and Ca will tend to increase resistivity.
In those cases where high resistivity is encountered, such as the utility industry when low
sulfur coal is being fired, flue gas conditioning with SO3 can reduce resistivity to a more
optimum value thus reducing the size of the precipitator needed.
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Particle size of the incoming particulate has a dramatic impact on the sizing of an electrostatic
precipitator. Applications such as Fluid Catalytic Cracking Units and Recovery Boilers, which have
particle resistivities in the medium range, exhibit very fine particulate. The size of the precipitator
must be increased in these cases because the fine particulate is easily re-entrained into the gas stream.
In the power industry, generally the higher the fuel ash content, the larger the ash particle size.
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When are Electrostatic Precipitators not a suitable solution?
As the size of the required precipitator increases, other technologies become more cost effective.
For low sulfur utility applications, fabric filters are an attractive alternative.
As part of the overall
precipitator/fabric filter cost evaluation, operating costs need to be included. Typically, the pressure
drop across a flange to flange fabric filter will be in the 6 to 8" w.c. range whereas an electrostatic
precipitator will have approximately a 1" w.c. pressure drop. This pressure drop penalty for a fabric
filter will be somewhat offset by its lower power consumption which can run as high as 2.0 watts per
square foot of collecting electrode area for a precipitator.
Another benefit of a fabric filter is high acid gas, SO2, chlorides, fluorides and Hg removal capability.
When operating downstream of a spray dryer absorber, removal efficiencies of 90% or greater can be attained
for some species when operating in conjunction with a fabric filter. The fabric filter dust layer acts as
a fixed bed where high acid gas removal efficiency can take place. Since most of the particulate is removed
from the collecting electrodes of a precipitator during normal operation, acid gas removal capability is much
reduced.
Quick links:
Electrostatic Precipitator General Description
Precipitator Operating Principle
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