Dust Layer Resistivity

Let’s take a closer look at the way resistivity affects electrical conditions in the dust
layer. A potential electric field (voltage drop) is formed across the dust layer as negatively
charged particles arrive at the dust layer surface and leak their electrical charges
to the collection plate. At the metal surface of the electrically grounded collection
plate, the voltage is zero. Whereas at the outer surface of the dust layer, where new
particles and ions are arriving, the electrostatic voltage caused by the gas ions can be
quite high. The strength of this electric field depends on the resistivity and thickness
of the dust layer.
In high resistivity dust layers, the dust is not sufficiently conductive, so electrical
charges have difficulty moving through the dust layer. Consequently, electrical
charges accumulate on and beneath the dust layer surface, creating a strong electric
field. Voltages can be greater than 10,000 volts. Dust particles with high resistivities
are held too strongly to the plate, making them difficult to remove and causing rapping
problems.
In low resistivity dust layers, the corona current is readily passed to the grounded collection
electrode. Therefore, a relatively weak electric field, of several thousand volts,
is maintained across the dust layer. Collected dust particles with low resistivity do not
adhere strongly enough to the collection plate. They are easily dislodged and become
reentrained in the gas stream.
The following discussion of normal, high, and low resistivity applies to ESPs operated
in a dry state; resistivity is not a problem in the operation of wet ESPs because of the
moisture concentration in the ESP.

Normal Resistivity
As stated above, ESPs work best under normal resistivity conditions. Particles with
normal resistivity do not rapidly lose their charge on arrival at the collection electrode.
These particles slowly leak their charge to grounded plates and are retained on the collection
plates by intermolecular adhesive and cohesive forces. This allows a particulate
layer to be built up and then dislodged from the plates by rapping. Within the
range of normal dust resistivity (between 107 and 1010 ohm-cm), fly ash is collected
more easily than dust having either low or high resistivity.

High Resistivity
If the voltage drop across the dust layer becomes too high, several adverse effects can
occur. First, the high voltage drop reduces the voltage difference between the discharge
electrode and collection electrode, and thereby reduces the electrostatic field
strength used to drive the gas ion - charged particles over to the collected dust layer.
As the dust layer builds up, and the electrical charges accumulate on the surface of the
dust layer, the voltage difference between the discharge and collection electrodes
decreases. The migration velocities of small particles are especially affected by the
reduced electric field strength.
Another problem that occurs with high resistivity dust layers is called back corona.
This occurs when the potential drop across the dust layer is so great that corona discharges
begin to appear in the gas that is trapped within the dust layer. The dust layer
breaks down electrically, producing small holes or craters from which back corona
discharges occur. Positive gas ions are generated within the dust layer and are accelerated
toward the "negatively charged" discharge electrode. The positive ions reduce
some of the negative charges on the dust layer and neutralize some of the negative
ions on the "charged particles" heading toward the collection electrode. Disruptions of
the normal corona process greatly reduce the ESP's collection efficiency, which in
severe cases, may fall below 50% (White 1974).

The third, and generally most common problem with high resistivity dust is increased
electrical sparking. When the sparking rate exceeds the "set spark rate limit," the automatic
controllers limit the operating voltage of the field. This causes reduced particle
charging and reduced migration velocities toward the collection electrode.
High resistivity can generally be reduced by doing the following:
• Adjusting the temperature
• Increasing moisture content
• Adding conditioning agents to the gas stream
• Increasing the collection surface area
• Using hot-side precipitators (occasionally)
Figure  shows the variation in resistivity with changing gas temperature for six different
industrial dusts (U.S. EPA 1985). For most dusts, resistivity will decrease as the
flue gas temperature increases. However, as can be seen from Figure  the resistivity
also decreases for some dusts (cement and ZnO) at low flue gas temperatures.

Resistivity of six different dusts at various

temperatures

The moisture content of the flue gas stream also affects particle resistivity. Increasing

the moisture content of the gas stream by spraying water or injecting steam into the

duct work preceding the ESP lowers the resistivity. In both temperature adjustment

and moisture conditioning, one must maintain gas conditions above the dew point to

prevent corrosion problems in the ESP or downstream equipment. Figure 3-2 shows

the effect of temperature and moisture on the resistivity of cement dust. As the percentage

of moisture in the dust increases from 1 to 20%, the resistivity of the dust dramatically

decreases. Also, raising or lowering the temperature can decrease cement

dust resistivity for all the moisture percentages represented.

Effect of temperature and moisture on the

resistivity of cement dust

The presence of SO3 in the gas stream has been shown to favor the electrostatic precipitation

process when problems with high resistivity occur. Most of the sulfur content

in the coal burned for combustion sources converts to SO2. However,

approximately 1% of the sulfur converts to SO3. The amount of SO3 in the flue gas

normally increases with increasing sulfur content of the coal. The resistivity of the

particles decreases as the sulfur content of the coal increases

Fly ash resistivity versus coal sulfur content

for several flue gas temperature bands

The use of low-sulfur western coal for boiler operations has caused fly ash resistivity

problems for ESP operators. For coal fly ash dusts, the resistivity can be lowered

below the critical level by the injection of as little as 10 to 30 ppm SO3 into the gas

stream. The SO3 is injected into the duct work preceding the precipitator. Figure 3-4

shows the flow diagram of a sulfur-burning flue gas conditioning system used to

lower resistivity at a coal-fired boiler.

Flow diagram of sulfur-burning flue gas conditioning system

Courtesy of Wahlco, Inc.

Other conditioning agents, such as sulfuric acid, ammonia, sodium chloride, and soda

ash, have also been used to reduce particle resistivity (White 1974). Therefore, the

chemical composition of the flue gas stream is important with regard to the resistivity

of the particles to be collected in the ESP. Table 3-5 lists various conditioning agents

and their mechanisms of operation (U.S. EPA 1985).

Two other methods that reduce particle resistivity include increasing the collection

surface area and handling the flue gas at higher temperatures. Increasing the collection

area of the precipitator will increase the overall cost of the ESP, which may not be

desirable. Hot-side precipitators, which are usually located in front of the combustion

air preheater section of the boiler, are also used to combat resistivity problems. However,

the use of conditioning agents has been more successful and very few hot-side

ESPs have been installed since the 1980s.