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The basic principle of an electrostatic precipitator is to attract charged dust particles to the collecting plates where they can be removed from the gas stream.
Dust entering the precipitator is charged by a corona discharge leaving the electrodes. Corona is a plasma containing electrons and negatively charged ions. Most industrial electrostatic precipitators use negative discharge corona for charging dust.
When charged, the dust particles are driven toward the collecting plates by the electromagnetic force created by the voltage potential applied to the discharge electrodes. An electrostatic precipitator contains multiple mechanical fields located in series and parallel to the direction of the gas flow. Each mechanical field comprises a group of collecting plates that define a series of parallel gas passages. These passages run in the direction of gas flow. Bisecting the gas passage is a series of discharge electrodes, also running in the direction of gas flow.
A mechanical field contains one or more electrical fields. A single transformer rectifier serves each electrical field. Multiple electrical sections can be contained in a single electrical field.
Some form of mechanical cleaning device serves both the high-voltage and collecting system. These rappers can take the form of hammers mounted on a drive shaft, externally mounted pneumatic rappers, or electromagnetic impact devices. The basic intent is to impart a mechanical force to the collecting plates and discharge electrodes to cause dust to drop to the bottom of the precipitator for disposal.
During operation, AC is applied to the voltage control cabinet. Inside the cabinet is a voltage control and silicon control rectifier. The voltage controls the flow of current through the silicon control rectifier. Current from the silicon-control rectifier enters the current-limiting reactor, then the transformer rectifier. The current-limiting reactor serves to reduce distortion in the AC waveform and limit current flow during sparking. The transformer rectifier takes the AC and converts it to DC. In addition, the primary voltage is stepped up to significantly higher secondary voltages. Typical secondary voltages are in the range of 45,000-115,000 kV. Current exiting the transformer rectifier enters the electrical field where charging occurs.
Based on data measured within the electrical field, the voltage controls fire the silicon-control rectifier to introduce current into the field. The amount of time that current is applied to the field is a function of the voltage at which sparking occurs within the field. When a spark is detected within the electrical field, the voltage quenches the spark by turning power off or reducing power levels to a preset level. Once the quenching period is satisfied, the voltage control ramps up power applied to the field in search of the next spark.
Primary Mechanism Used
As indicated, dust must be charged to be attracted to the collecting plates. This charging occurs between the collecting plates where the discharge electrodes are located. The presence of charge in the gas passage is a function of the secondary voltage applied to the electrical field.
Creation of Charge
Applying secondary voltage to the discharge electrodes creates the corona discharge. The minimum secondary voltage at which current flow is created is called the corona onset voltage. Typical corona onset voltages range from 12,000 to 25,000 V. In general, the corona onset voltage is a function of the discharge electrode geometry, process gas characteristics, and dust characteristics. If the electrical field operates at a secondary voltage lower than the corona onset voltage, no charging will occur.
Two basic charging mechanisms occur within an electrostatic precipitator: field and diffusion charging. Particle size has a major impact on the type of charging that occurs. A discussion of each mechanism follows.
This charging mechanism generally dominates in particles of 1.5 pm and larger. Dust particles intercept negative ions and electrons emanating from the discharge electrode. Charge physically collects on the surface of the dust, reaching a saturation point. This type of charging is very rapid, occurring in the first few feet of the precipitator.
Particles less than 0.5 pm in diameter are charged using a diffusion mechanism. Diffusion charging is the result of commingling of particles and charge contained in the gas stream. Charging follows the pattern of Brownian movement is a gas stream; charge does not accumulate on the dust but acts upon it. This mechanism of charging is slow compared with field charging.
As seen from the explanation, neither of the two charging mechanisms dominates when particle diameter is between 0.5 and 1.5 pm. In this size range, the combination of field and diffusion charging occurs with neither mechanism dominating. As a result, the combined charging occurs at a rate much slower than either of the two mechanisms. When a precipitator experiences a dominant quantity of particles in this size range, performance is suppressed.
The relationship between operating parameters and collection efficiency is defined by the Deutsch-Anderson equation. There are several modifications to the original formula, but the basic equation is:
W = (E0 Ep a/2 n q)
Efficiency = Fractional percentage collected from gas stream A = Total collecting plate area
V = Volumetric flow rate in actual terms
W = Migration velocity of dust toward collecting plates
E0 = Charging field strength
Ep = Collecting field strength
A = Particle radius
H = Gas viscosity
П = Pi
The simple explanation of the Deutsch-Anderson equation is that the precipitator collection efficiency is defined by the speed of the dust toward the collecting plates and the amount of collecting plate area relative to the total gas volume.
Increasing the migration velocity of the dust will increase collection efficiency of the electrostatic precipitator. Increasing the amount of collecting plate area available to treat the gas volume will also increase collection efficiency.
Likewise, reductions in migration velocity or plate area, or an increase in gas volume, will cause collection efficiency to decrease.
As shown previously, removal efficiency of an electrostatic precipitator is largely determined by the ratio of the total collecting plate area to the gas volume treated. This ratio is called the SCA. The higher the value for SCA, the greater the removal efficiency for the electrostatic precipitator.
Also critical to precipitator performance is treatment time. Higher treatment time implies a larger precipitator available for gas treatment. This parameter is a function of the total length of the mechanical fields in the direction of gas flow and the velocity of the gas through the precipitator. High-efficiency electrostatic precipitators generally provide treatment times greater than 10 s.
Aspect ratio (treatment length divided by collecting plate height) should be greater than 0.8. If the collecting plate becomes too tall relative to the available treatment length, problems associated with dust distribution and reentrainment will increase.
Resistivity of Dust
There are two types of conduction characterized in dust: surface conduction and volume conduction.
Dust resistivity plays a major role in defining electrostatic precipitator collection efficiency. It is generally accepted that electrostatic precipitators operate most effectively when dust resistivity is in the range of 5 x Ю4 to 5 x 1010 ohm-cm.
When dust resistivity drops below this range, the dust releases its charge readily to the collecting surface. As a result, the dust migrates to the collecting plates, where it immediately loses its charge. The charge in conjunction with the cohesive nature of the dust keeps the dust on the collecting plates. If the charge is lost, the dust is likely to be reentrained back into the gas stream. Conversely, high-resistivity dust retains charge for extended periods. When the high- resistivity dust deposits on the collecting plates, charge does not dissipate. In fact, charge continues to accumulate due to the constant corona emanating from the discharge electrodes. As a result, high-resistivity dust is difficult to remove from the collecting plates. It is not uncommon for high-resistivity dust applications to require periodic manual cleaning to restore precipitator performance.
Figure 5.6 indicates relative dust resistivity for varying sulfur content of coal. Similar relationships exist between resistivity and process gas moisture content.
Flow of current through the dust layer occurs in one of two methods: surface conduction or volume conduction. The temperature at which the process operates defines the dominant method of conduction.
Volume conduction is the process of current flow through the particle. This conduction method occurs on the hot side of the resistivity curve. The hot side starts at the point on the resistivity curve where increasing temperature produces reduced resistivity.
Volume conduction is determined by the resistivity of the constituents at the process operating temperature. Changing the moisture content or adding conditioning agents to the process gas stream will have minimal impact on the hot side of dust resistivity.
Surface conduction occurs on the cold side of the resistivity curve. The cold side is defined as the peak on the resistivity curve toward the slope of decreasing resistivity with decreasing process temperature.
Surface conduction occurs across the surface of the dust particle. Current flow is largely determined by the quantity and type of gasses condensed on the surface of the particle. When operating on the cold side of the resistivity curve, addition of conditioning agents or moisture will generally improve operation.
Several activities are necessary to ensure effective operation of an electrostatic precipitator.
Air Load/Gas Load Testing
Air load/gas load testing is the process of operating the electrical fields under known conditions. The air load test occurs before startup or immediately after shutdown of the process. Before testing, each electrical field is isolated and confirmed to be ready for energization of the transformer rectifiers. Fans are set at a low flow rate, adequate to provide some ventilation of the electrostatic precipitator.
The voltage control is set in a manual condition. The secondary voltage levels applied to a single electrical field are increased incrementally from zero. At each increment, the measured secondary current is recorded. The secondary voltage at which secondary current is first observed is called the corona onset voltage. The secondary voltage is increased to the point at which the nameplate rating of the transformer rectifier is achieved or the field sparks. This process is repeated for each electrical field until all are complete.
As a practical matter, all air load tests should be performed from the outlet electrical field working toward the first field of the precipitator. Sparking generates ozone, which lowers the sparking threshold of a field.
The data derived from the air load test can be plotted to create a volt versus amps (V-I) chart. The air load V-I chart can then be compared with that achieved during operation. Most modern voltage controls contain an automatic air load function that will ramp the voltage and create the plot.
Tests like the air load can be accomplished during operation of the process. These tests are called gas load tests. The curve plotted from these process conditions can be used to diagnose electrostatic precipitator operating problems.
As indicated, the speed of the dust toward the collecting plates is a function of the applied field strength. The secondary voltage levels achieved largely determine field strength.
It is desirable to have the discharge electrodes centered within the gas passage and between collecting plate stiffeners. As the electrical clearance decreases due to changes in alignment, the voltage at which sparking will occur decreases. Bowed collecting plates, misaligned fields, and foreign objects in the gas passage will increase spark rates and decrease secondary voltage levels.
When the casing and internal components of a precipitator achieve operating temperature, thermal expansion may change the electrical alignment. In this condition, electrical conditions may be acceptable at ambient temperatures but not at operating temperatures.
It is essential to ensure that the components can accommodate growth associated with thermal expansion and still maintain acceptable electrical clearances.
As shown in the Deutsch-Anderson equation, collection efficiency is a function of SCA. If ambient air is leaking into a negative pressure gas stream, the precipitator is forced to treat a larger total gas volume. There are other reasons that air in-leakage reduces precipitator performance.
Ambient air generally contains a lower water content compared with flue gas. As shown in the resistivity section, increasing moisture content improves dust resistivity. When ambient air leaks into the gas stream, the average moisture content is reduced, and resistivity generally increases. This applies to those units operating on the surface conduction side of the dust resistivity curve.
The ongoing satisfactory performance of an electrostatic precipitator is a function of maintaining the collecting surfaces and discharge electrodes free from excessive dust layer.
Creation of an acceptable rapping program is an iterative process. There is no formula that establishes the correct program. As changes are implemented to the rapper program, they must be evaluated in terms of their impact on emissions and electrical conditions. It can take several hours for some rapper changes to begin showing impact on the precipitator performance.
It is desirable to have a slight buildup of dust on collecting plates. Dust depositing on the surface of the collecting plates will agglomerate with the dust already residing there. This reduces the potential for dust reentrainment during normal rapping. Generally, this dust layer should be less than inch thick and uniform across the surface of the panels.
If the dust layer is too thick, the potential exists for excessive amounts of dust to be dislodged during rapping. In addition, if the dust resistivity is high, the dust layer will create a voltage proportional to the resistivity of the dust. This will reduce performance of the unit.
The high-voltage system should not have a normal dust layer. It is desirable to keep the electrodes clean during operation. Dust depositing on the electrodes can create a voltage drop that will impair performance.
The high-voltage system is isolated from ground by support insulators. These insulators are exposed to process gas, which contains dust and moisture. Dust and moisture accumulating on the surface of insulators will cause them to track and carry current. This can result in loss of current necessary to charge dust and, in the extreme case, failure of the insulators.
In an electrostatic precipitator, there are insulators supporting the high- voltage system, stabilizing the lower high-voltage frames, and isolating the high-voltage rapping system. External to the process are insulators supporting the high-voltage bus and providing high-voltage termination from the transformer rectifier. All the insulators must be kept clean and free from carbon tracking.
Purge Heater and Ring Heater Systems
Most electrostatic precipitators operate under negative process pressure. As a result, air drawn into the penthouse or insulator compartment can cause condensation of moisture contained in the gas stream. The condensation results in accelerated corrosion and excessive sparking in the electrical field.
It is advisable to provide a blower filter heater arrangement that forces air into the insulator enclosure. This clean heated dry air will mix with the process gas without causing condensation.
If a purge heater system cannot be used, ring heaters installed around each support insulator will provide some protection.
It is essential that the purge heater or ring heater system be energized at least 4 hours before introducing process gas into the electrostatic precipitator.
As indicated in the resistivity section, elevated gas temperature on a cold- side precipitator will result in degraded performance. As a result, it is critical to minimize process temperatures entering the cold-side unit.
This can be accomplished by monitoring soot-blowing programs and maintaining the heat transfer efficiency of the air heater.
In the case of a precipitator operating on the hot side of the resistivity curve, it is beneficial to maximize gas temperature. When operating this type of unit at reduced load, high-resistivity dust may build up on the collecting plate and electrodes. This will result in excess emission during load ramp up. To avoid this problem, an aggressive rapping program should be initiated at reduced loads.
As coal composition changes, the resistivity of dust created can increase. Increased dust resistivity may result in reduced electrostatic precipitator performance. To alleviate this problem, it is common to increase the moisture content of the flue gas when operating on the cold side of the resistivity curve.
Moisture content of the process gas can be increased by operating the steam soot blowers or by installing an evaporative gas conditioning system ahead of the precipitator. If alternative coals are on site that have more favorable resistivity, they can be blended with the difficult coal to produce better precipitator operation. In severe cases, it may be necessary to install a flue gas conditioning system that injects SO, into the gas stream.