The resistivity of the collected dust on the collection plate affects the acceptable current
density through the dust layer, dust removal from the plates, and indirectly, the corona
charging process. High resistivity conditions in utility fly ash applications have received
much attention. The optimum resistivity range for ESP operation is relatively narrow; both
high and low resistivity cause problems. Excursions outside the optimum resistivity range
are particularly a problem when a unit is designed with a modest amount of plate area, sectionalization,
and power-input capabilities. At industrial sources where resistivity changes
are intermittent, modification of operating procedures may improve performance temporarily.
Expensive retrofitting or modifications may be required if the dust resistivity is
vastly different than the design range.
High Resistivity
High dust resistivity is a more common problem than low dust resistivity. Particles
having high resistivity are unable to release or transfer electrical charge. At the collection
plate, the particles neither give up very much of their acquired charge nor easily
pass the corona current to the grounded collection plates. High dust resistivity conditions
are indicated by low primary and secondary voltages, suppressed secondary currents
and high spark rates in all fields. This condition makes it difficult for the T-R
controller to function adequately.
Severe sparking can cause excessive charging off-time, spark "blasting" of particulate
on the plate, broken wires due to electrical erosion, and reduced average current levels.
The reduced current levels generally lead to deteriorated performance. Because
the current level is indicative of the charging process, the low current and voltage levels
that occur inside an ESP operating with high resistivity dust generally reflect
slower charging rates and lower particle migration velocities to the plate. Particle collection
is reduced; consequently, the ESP operates as though it were "undersized." If
high resistivity is expected to continue, the operating conditions can be modified or conditioning agents can be used to accommodate this problem and thereby improve
performance.
High resistivity also tends to promote rapping problems, as the electrical properties of
the dust tend to make it very tenacious. High voltage drop through the dust layer and
the retention of electrical charge by the particles make the dust difficult to remove
because of its strong attraction to the plate. The greater rapping forces usually required
to dislodge the dust may also aggravate or cause a rapping reentrainment problem.
Important items to remember are (1) difficulty in removing the high-resistivity dust is
related to the electrical characteristics, not to the sticky or cohesive nature of the dust;
and (2) the ESP must be able to withstand the necessary increased rapping forces
without sustaining damage to insulators or plate support systems.
Low Resistivity
Low dust resistivity, although not as common, can be just as detrimental to the performance
of an ESP as high resistivity. When particles with low resistivity reach the collection
plate, they release much of their acquired charge and pass the corona current
quite easily to the grounded collection plate. Without the attractive and repulsive electrical
forces that are normally present at normal dust resistivities, the binding forces
between the dust and the plate are considerably weakened. Therefore, particle reentrainment
is a substantial problem at low resistivity, and ESP performance appears to
be very sensitive to contributors of reentrainment, such as poor rapping or poor gas
distribution.
Since there is lower resistance to current flow for particles with low resistivity (compared
to normal or high), lower operating voltages are required to obtain substantial
current flow. Operating voltages and currents are typically close to clean plate conditions,
even when there is some dust accumulation on the plate. Low-resistivity conditions,
are typically characterized by low operating voltages, high current flow, and low
spark rates.
Despite the large flow of current under low-resistivity conditions, the corresponding
low voltages yield lower particle migration velocities to the plate. Thus, particles of a
given size take longer to reach the plate than would be expected. When combined with
substantial dust reentrainment, the result is poor ESP performance. In this case, the
large flow of power to the ESP represents a waste of power.
Low-resistivity problems typically result from the chemical characteristics of the particulate
and not from flue gas temperature. The particulate may be enriched with compounds
that are inherently low in resistivity, either due to poor operation of the process
or to the inherent nature of the process. Examples of such enrichment include excessive
carbon levels in fly ash (due to poor combustion), the presence of naturally occurring
alkalis in wood ash, iron oxide in steel-making operations, or the presence of
other low-resistivity materials in the dust. Over-conditioning may also occur in some
process operations, such as the burning of high-sulfur coals or the presence of high
SO3 levels in the gas stream, which lower the inherent resistivity of the dust. In some
instances, large ESPs with SCAs greater than 750 ft2/1000 acfm have performed
poorly because of the failure to fully account for the difficulty involved in collecting a
low-resistivity dust. Although some corrective actions for low resistivity are available,
they are sometimes more difficult to implement than those for high resistivity.
density through the dust layer, dust removal from the plates, and indirectly, the corona
charging process. High resistivity conditions in utility fly ash applications have received
much attention. The optimum resistivity range for ESP operation is relatively narrow; both
high and low resistivity cause problems. Excursions outside the optimum resistivity range
are particularly a problem when a unit is designed with a modest amount of plate area, sectionalization,
and power-input capabilities. At industrial sources where resistivity changes
are intermittent, modification of operating procedures may improve performance temporarily.
Expensive retrofitting or modifications may be required if the dust resistivity is
vastly different than the design range.
High Resistivity
High dust resistivity is a more common problem than low dust resistivity. Particles
having high resistivity are unable to release or transfer electrical charge. At the collection
plate, the particles neither give up very much of their acquired charge nor easily
pass the corona current to the grounded collection plates. High dust resistivity conditions
are indicated by low primary and secondary voltages, suppressed secondary currents
and high spark rates in all fields. This condition makes it difficult for the T-R
controller to function adequately.
Severe sparking can cause excessive charging off-time, spark "blasting" of particulate
on the plate, broken wires due to electrical erosion, and reduced average current levels.
The reduced current levels generally lead to deteriorated performance. Because
the current level is indicative of the charging process, the low current and voltage levels
that occur inside an ESP operating with high resistivity dust generally reflect
slower charging rates and lower particle migration velocities to the plate. Particle collection
is reduced; consequently, the ESP operates as though it were "undersized." If
high resistivity is expected to continue, the operating conditions can be modified or conditioning agents can be used to accommodate this problem and thereby improve
performance.
High resistivity also tends to promote rapping problems, as the electrical properties of
the dust tend to make it very tenacious. High voltage drop through the dust layer and
the retention of electrical charge by the particles make the dust difficult to remove
because of its strong attraction to the plate. The greater rapping forces usually required
to dislodge the dust may also aggravate or cause a rapping reentrainment problem.
Important items to remember are (1) difficulty in removing the high-resistivity dust is
related to the electrical characteristics, not to the sticky or cohesive nature of the dust;
and (2) the ESP must be able to withstand the necessary increased rapping forces
without sustaining damage to insulators or plate support systems.
Low Resistivity
Low dust resistivity, although not as common, can be just as detrimental to the performance
of an ESP as high resistivity. When particles with low resistivity reach the collection
plate, they release much of their acquired charge and pass the corona current
quite easily to the grounded collection plate. Without the attractive and repulsive electrical
forces that are normally present at normal dust resistivities, the binding forces
between the dust and the plate are considerably weakened. Therefore, particle reentrainment
is a substantial problem at low resistivity, and ESP performance appears to
be very sensitive to contributors of reentrainment, such as poor rapping or poor gas
distribution.
Since there is lower resistance to current flow for particles with low resistivity (compared
to normal or high), lower operating voltages are required to obtain substantial
current flow. Operating voltages and currents are typically close to clean plate conditions,
even when there is some dust accumulation on the plate. Low-resistivity conditions,
are typically characterized by low operating voltages, high current flow, and low
spark rates.
Despite the large flow of current under low-resistivity conditions, the corresponding
low voltages yield lower particle migration velocities to the plate. Thus, particles of a
given size take longer to reach the plate than would be expected. When combined with
substantial dust reentrainment, the result is poor ESP performance. In this case, the
large flow of power to the ESP represents a waste of power.
Low-resistivity problems typically result from the chemical characteristics of the particulate
and not from flue gas temperature. The particulate may be enriched with compounds
that are inherently low in resistivity, either due to poor operation of the process
or to the inherent nature of the process. Examples of such enrichment include excessive
carbon levels in fly ash (due to poor combustion), the presence of naturally occurring
alkalis in wood ash, iron oxide in steel-making operations, or the presence of
other low-resistivity materials in the dust. Over-conditioning may also occur in some
process operations, such as the burning of high-sulfur coals or the presence of high
SO3 levels in the gas stream, which lower the inherent resistivity of the dust. In some
instances, large ESPs with SCAs greater than 750 ft2/1000 acfm have performed
poorly because of the failure to fully account for the difficulty involved in collecting a
low-resistivity dust. Although some corrective actions for low resistivity are available,
they are sometimes more difficult to implement than those for high resistivity.
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