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 resistance and thickness of the dust layer. In high-resistance 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, volts.
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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 resistance and thickness of the dust layer. In high-resistance 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, volts. Dust particles with high resistance are held too strongly to the plate, making them difficult to remove and causing rapping problems.
In low resistance 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 resistance do not adhere strongly enough to the collection plate.
They are easily dislodged and become retained in the gas stream. The electrical conductivity of a bulk layer of particles depends on both surface and volume factors. Volume conduction, or the motions of electrical charges through the interiors of particles, depends mainly on the composition and temperature of the particles. Volume conduction also involves ancillary factors, such as compression of the particle layer, particle size and shape, and surface properties.
These films usually differ both physically and chemically from the interiors of the particles owing to adsorption phenomena. Theoretical calculations indicate that moisture films only a few molecules thick are adequate to provide the desired surface conductivity.
Surface conduction on particles is closely related to surface-leakage currents occurring on electrical insulators, which have been extensively studied. A sharp rise in current signals the formation of a moisture film on the glass.
This method has been used effectively for determining the marked rise in dew point, which occurs when small amounts of sulfuric acid vapor are added to an atmosphere commercial Dewpoint Meters are available on the market.
The following discussion of normal, high, and low resistance applies to ESPs operated in a dry state; resistance is not a problem in the operation of wet ESPs because of the moisture concentration in the ESP. The relationship between moisture content and resistance is explained later in this work.
Normal resistivity[ edit ] 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 and 2 x ohm-cm , fly ash is collected more easily than dust having either low or high resistivity. High resistivity[ edit ] 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.
When back corona is present, the dust particles build up on the electrodes forming a layer of insulation. Often this can not be repaired without bringing the unit offline.
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; Adding conditioning agents to the gas stream; Increasing the collection surface area; and Using hot-side precipitators occasionally and with foreknowledge of sodium depletion. Thin dust layers and high-resistivity dust especially favor the formation of back corona craters.
Severe back corona has been observed with dust layers as thin as 0. The Figure below and to the left shows the variation in resistivity with changing gas temperature for six different industrial dusts along with three coal-fired fly ashes. The Figure on the right illustrates resistivity values measured for various chemical compounds that were prepared in the laboratory.
These data are typical for a moderate to high combustibles content ash. Data for Fly Ash B are from the same sample, acquired during the descending temperature mode.
The differences between the ascending and descending temperature modes are due to the presence of unburned combustibles in the sample. Exactly how carbon works as a charge carrier is not fully understood, but it is known to significantly reduce the resistivity of a dust. Resistivity Measured as a Function of Temperature in Varying Moisture Concentrations Humidity Carbon can act, at first, like a high resistivity dust in the precipitator. Higher voltages can be required in order for corona generation to begin.
These higher voltages can be problematic for the TR-Set controls. The problem lies in onset of corona causing large amounts of current to surge through the low resistivity dust layer.
The controls sense this surge as a spark. As precipitators are operated in spark-limiting mode, power is terminated and the corona generation cycle re-initiates. Thus, lower power current readings are noted with relatively high voltage readings.
The same thing is believed to occur in laboratory measurements. Parallel plate geometry is used in laboratory measurements without corona generation. A stainless steel cup holds the sample. Another stainless steel electrode weight sits on top of the sample direct contact with the dust layer.
As voltage is increased from small amounts e. Then, a threshold voltage level is reached. At this level, current surges through the sample Low resistivity[ edit ] Particles that have low resistivity are difficult to collect because they are easily charged very conductive and rapidly lose their charge on arrival at the collection electrode.
The particles take on the charge of the collection electrode, bounce off the plates, and become re-entrained in the gas stream. Thus, attractive and repulsive electrical forces that are normally at work at normal and higher resistivities are lacking, and the binding forces to the plate are considerably lessened. Examples of low-resistivity dusts are unburned carbon in fly ash and carbon black.
If these conductive particles are coarse, they can be removed upstream of the precipitator by using a device such as a cyclone mechanical collector. The addition of liquid ammonia NH 3 into the gas stream as a conditioning agent has found wide use in recent years. It is theorized that ammonia reacts with H 2SO 4 contained in the flue gas to form an ammonium sulfate compound that increases the cohesivity of the dust.
This additional cohesivity makes up for the loss of electrical attraction forces. The table below summarizes the characteristics associated with low, normal and high resistivity dusts. 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.
The figure to the right shows the effect of temperature and moisture on the resistivity of a cement dust. Also, raising or lowering the temperature can decrease cement dust resistivity for all the moisture percentages represented. The presence of SO 3 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 SO 2. The amount of SO 3 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.
BETH Tar Electrostatic Precipitator
Gardalmaran The most marked effects of back corona on the current-voltage characteristics are:. Volume conduction, or the motions of electrical charges through the interiors of particles, depends mainly on the composition and temperature of the particles. NaCl — natural conditioner when mixed with coal. Also, raising or lowering the electrostatoc can decrease cement dust resistivity for all the moisture percentages represented. Electrostatic precipitator But once charged, they do not readily give up their acquired charge on arrival at the collection electrode. Mechanism is not clear, various ones proposed; Modifies resistivity. When gas that contains an aerosol dust, mist flows between the collecting plates and the discharge wires, the aerosol particles in the gas are charged by the ions.