EXPERIMENT 4 - Electrostatic Precipitator

CCB 4213

Air Pollution Engineering

EXPERIMENT 4

ELECTROSTATIC PRECIPITATOR

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Introduction An electrostatic precipitator is a large, industrial emission-control unit. It is designed to trap and remove dust particles from the exhaust gas stream of an industrial process. Precipitators are used in these industries: Power/Electric Cement Chemicals Metals Paper     

In many industrial plants, particulate matter created in the industrial process is carried as dust in the hot exhaust gases. These dust-laden gases pass through an electrostatic precipitator that collects most of the dust. Cleaned gas then passes out of the precipitator and through a stack to the atmosphere. Precipitators typically collect 99.9% or more of the dust f rom the gas stream. Precipitators function by electrostatically charging the dust particles in the gas stream. The charged particles are then attracted to and deposited on plates or other collection devices. When enough dust has accumulated, the collectors are shaken to dislodge the dust, causing it to fall with the force of gravity to hoppers below. The dust is then removed by a conveyor system for disposal or recycling. Depending upon dust characteristics and the gas volume to be treated, there are many different sizes, types and designs of electrostatic precipitators. Very large power plants may actually have multiple precipitators for each unit. Data Acquisition National instrument Labview PC based data acquisition software Windows 7. Complete with PC-based software to eliminate paper & pen data logging and record Real-time data display by graphs & multiple window display of user preferences Printable data output Data able exported to other program such a notepad or MS excel for other analysis Computer: PC with I5 processor Mouse, keyboard, I9‖ LCD flat screen monitor. Licensed Windows OS Key board, optical mouse    

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Basic Principles Electrostatic precipitation removes particles from the exhaust gas stream of an industrial process. Often the process involves combustion, but it can be any industrial process that would otherwise emit particles to the atmosphere. Six activities typically take place: Ionization - Charging of particles Migration - Transporting the charged particles to the collecting surfaces Collection - Precipitation of the charged particles onto the collecting surfaces Charge Dissipation - Neutralizing the charged particles on the collecting surfaces Particle Dislodging - Removing the particles from the collecting surface to the hopper Particle Removal - Conveying the particles from the hopper to a disposal point      

Every particle either has or can be given a charge positive or negative. Let's suppose we impart a negative charge to all the particles in a gas stream. Then suppose we set up a grounded plate having a positive charge. What would happen? The negatively charged particle would migrate to the grounded collection plate and be captured. The particles would quickly collect on the plate, creating a dust layer. The dust layer would accumulate until we removed it, which we could do [email protected]

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by rapping the plate or by spraying it with a liquid. Charging, collecting, and removing —that's the basic idea of an ESP, but it gets more complicated. 1. Particle Charges Typical has thin wires called discharge electrodes, which are evenly spaced between large plates called collection electrodes, which are grounded. Think of an electrode as something that can conduct or transmit electricity. A negative, high -voltage, pulsating, direct current is applied to the discharge electrode creating a negative electric field. You can mentally divide this field into three regions (Figure 1). The field is strongest right next to the discharge electrode, weaker in the areas between the discharge and collection electrodes called the inter-electrode region, and weakest near the collection electrode. The region around the discharge electrode is where the particle charging process begins.

Figure 1: ESP Electric Field

2. Corona Discharge: Free Electron Generation Several things happen very rapidly (in a matter of a millisecond) in the small area around the discharge electrode. The applied voltage is increased until it produces corona discharge, which can be seen as a luminous blue glow around the discharge electrode. The free electrons created by the corona are rapidly fleeing the negative electric field, which repulses them. They move faster and faster away from the discharge electrode. This acceleration causes them to literally crash into gas molecules, bumping off electrons in the molecules. As a result of losing an electron (which is negative), the gas molecules become positively charged, that is, they become positive ions (Figure 2). So, this is the first thing that happens —gas molecules are ionized, and electrons are liberated. All this activity occurs very close to the discharge electrode. This process continues, creating more and more free electrons and more positive ions. The name for all this electron generation activity is avalanche multiplication. (Figure 3)

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Figure 2: Corona Generation

Figure 3: Avalance Multiplication of gas molecules The electrons bump into gas molecules and create additional ionized molecules. The positive ions, on the other hand, are drawn back toward the negative discharge electrode. The molecules are hundreds of times bigger than the tiny electrons and move slowly, but they do pick up speed. In fact, many of them collide right into the metal discharge electrode or the gas space around the wire causing additional electrons to be knocked off. This is called secondary emission. So, this is the second thing that happens. We still have positive ions and a large amount of free electrons. 3. Ionization of Gas Molecules As the electrons leave the strong electrical field area around the discharge electrode, they start slowing down. Now they're in the inter-electrode area where they are still repulsed by the discharge electrode but to a lesser extent. There are also gas molecules in the inter-electrode region, but instead of violently colliding with them, the electrons kind of bump up to them and are captured (Figure 4). This imparts a negative charge to the gas molecules, creating negative gas ions. This time, because the ions are negative, they too want to move in the direction opposite the strong negative field. Now we have ionization of gas molecules happening near the discharge electrode and in the inter-electrode area, but with a big difference. The ions near the discharge electrode are positive and remain in that area. The ions in the middle area are negative and move away, along the path of invisible electric field lines, toward the collection electrode.

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Figure 4: Negative gas ions formed in the inter electrode regions

4. Charging of Particles These negative gas ions play a key role in capturing dust particles. Before the dust particles can be captured, they must first acquire a negative charge. This is when and where it happens. The particles are traveling along in the gas stream and encounter negative ions moving across their path. Actually, what really happens is that the particles get in the way of the negatively charged gas ions. The gas ions stick to the particles, imparting a negative charge to them. At first the charge is fairly insignificant as most particles are huge compared to a gas molecule. But many gas ions can fit on a particle, and they do. Small particles (less than 1 μm diameter) can absorb ―tens‖ of ions. Large particles (greater than 10 μm) can absorb "tens of thousands" of ions (Turner et al. 1992). Eventually, there are so many ions stuck to the particles, the particles emit their own negative electrical field. When this happens, the negative field around the particle repulses the negative gas ions and no additional ions are acquired. This is called the saturation charge. Now the negatively-charged particles are feeling the inescapable pull of electrostatic attraction. Bigger particles have a higher saturation charge (more molecules fit) and consequently are pulled more strongly to the collection plate. In other words, they move faster than smaller particles. Regardless of size, the particles encounter the plate and stick, because of adhesive and cohesive forces. Let's stop here and survey the picture. Gas molecules around the discharge electrode are positively ionized. Free electrons are racing as fast as they can away from the strong negative field area around the discharge electrode. The electrons are captured by gas molecules in the inter-electrode area and impart a negative charge to them. Negative gas ions meet particles and are captured (Figure 5). And all this happens in the blink of an eye. The net result is negatively charged particles that are repulsed by the negative electric field around the discharge electrode and are strongly attracted to the collection plate. They travel toward the grounded collection plate, bump into it, and stay there. More and more particles accumulate, creating a dust layer. This dust layer builds until it is somehow removed. Charging, collecting, and removing.

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Figure 5: Particle charging

5. Particle Charging Mechanism Particles are charged by negative gas ions moving toward the collection plate by one of these two mechanisms: field charging or diffusion charging. In field charging (the mechanism described above), particles capture negatively charged gas ions as the ions move toward the grounded collection plate. Diffusion charging, as its name implies, depends on the random motion of the gas ions to charge particles. In field charging (Figure 6), as particles enter the electric field, they cause a local dislocation of the field. Negative gas ions traveling along the electric field lines collide with the suspended particles and impart a charge to them. The ions will continue to bombard a particle until the charge on that particle is sufficient to divert the electric lines away from it. This prevents new ions from colliding with the charged dust particle. When a particle no longer receives an ion charge, it is said to be saturated. Saturated charged particles then migrate to the collection electrode and are collected.

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Figure 6: Field charging Diffusion charging is associated with the random Brownian motion of the negative gas ions. The random motion is related to the velocity of the gas ions due to thermal effects: the higher the temperature, the more movement. Negative gas ions collide with the particles because of their random thermal motion and impart a charge on the particles. Because the particles are very small (submicrometer), they do not cause the electric field to be dislocated, as in field charging. Thus, diffusion charging is the only mechanism by which these very small particles become charged. The charged particles then migrate to the collection electrode. Each of these two charging mechanisms occurs to some extent, with one dominating depending on particle size. Field charging dominates for particles with a diameter >1.0 micrometer because particles must be large enough to capture gas ions. Diffusion charging dominates for particles with a diameter less than 0.1 micrometer. A combination of these two charging mechanisms occurs for particles ranging between 0.2 and 1.0 micrometer in diameter. A third type of charging mechanism, which is responsible for very little particle charging is electron charging. With this type of charging, fast-moving free electrons that have not combined with gas ions hit the particle and impart a charge. 6. Electric Field Strength In the inter-electrode region, negative gas ions migrate toward the grounded collection electrode. A space charge, which is a stable concentration of negative gas ions, forms in the inter -electrode region because of the high electric field applied to the ESP. Increasing the applied voltage to the discharge electrode will increase the field strength and ion formation until sparkover occurs. [email protected]

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Sparkover refers to internal sparking between the discharge and collection electrodes. It is a sudden rush of localized electric current through the gas layer between the two electrodes. Sparking causes an immediate short-term collapse of the electric field (Figure 7.) For optimum efficiency, the electric field strength should be as high as possible. More specifically, ESPs should be operated at voltages high enough to cause some sparking, but not so high that sparking and the collapse of the electric field occur too frequently. The average sparkover rate for optimum precipitator operation is between 50 and 100 sparks per minute. At this spark rate, the gain in efficiency associated with increased voltage compensates for decreased gas ionization due to collapse of the electric field.

Figure 7: Spark Generation Profile 7. Particle Collection When a charged particle reaches the grounded collection electrode, the charge on the particle is only partially discharged. The charge is slowly leaked to the grounded collection plate. A portion of the charge is retained and contributes to the inter -molecular adhesive and cohesive forces that hold the particles onto the plates (Figure 8). Adhesive forces cause the particles to physically hold on to each other because of their dissimilar surfaces. Newly arrived particles are held to the collected particles by cohesive forces; particles are attracted and held to each other molecularly. The dust layer is allowed to build up on the plate to a desired thickness and then the particle removal cycle is initiated.

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Figure 8: Particle collection at collection electrode

8. Particle Removal Dust that has accumulated to a certain thickness on the collection electrode is removed by one of two processes, depending on the type of collection electrode. As described in greater detail in t he next section, collection electrodes in precipitators can be either plates or tubes, with plates being more common. Tubes are usually cleaned by water sprays, while plates can be cleaned either by water sprays or a process called rapping. Rapping is a process whereby deposited, dry particles are dislodged from the collection plates by sending mechanical impulses, or vibrations, to the plates. Precipitator plates are rapped periodically while maintaining the continuous flue-gas cleaning process. In other words, the plates are rapped while the ESP is on-line; the gas flow continues through the precipitator and the applied voltage remains constant. Plates are rapped when the accumulated dust layer is relatively thick (0.08 to 1.27 cm or 0.03 to 0.5 in.). This allows the dust layer to fall off the plates as large aggregate sheets and helps eliminate dust reentrainment. Most precipitators have adjustable rappers so that rapper intensity and frequency can be changed according to the dust concentration in the flue gas. Installations where the dust concentration is heavy require more frequent rapping. Dislodged dust falls from the plates into the hopper. The hopper is a single collection bin with sides sloping to allow dust to flow freely from the top of the hopper to the discharge opening. Dust should be removed as soon as possible to avoid (dust) packing. Packed dust is very difficult to remove. Most hoppers are emptied by some type of discharge device and then transported by a conveyor. In a precipitator using liquid sprays to remove accumulated liquid or dust, the sludge collects i n a holding basin at the bottom of the vessel. The sludge is then sent to settling ponds or lined landfills for proper ultimate disposal.

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Spraying occurs while the ESP is on-line and is done intermittently to remove the collected particles. Water is generally used as the spraying liquid although other liquids could be used if absorption of gaseous pollutants is also being accomplished.

Types of Electrostatic Precipitators ESPs can be grouped, or classified, according to a number of distinguishing features in their design. These features include the following: The structural design and operation of the discharge electrodes (rigid-frame, wires or plate) and collection electrodes (tubular or plate) The method of charging (single-stage or two-stage) The temperature of operation (cold-side or hot-side) The method of particle removal from collection surfaces (wet or dry) 

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These categories are not mutually exclusive. For example, an ESP can be a rigid-frame, single stage, cold-side, plate-type ESP as described below. Plate ESP Plate electrostatic precipitators primarily collect dry particles and are used more often than tubular precipitators. Plate ESPs can have wire, rigid-frame, or occasionally, plate discharge electrodes. Figure 9 shows a plate ESP with wire discharge electrodes. Dirty gas flows into a chamber consisting of a series of discharge electrodes that are equally spaced along the center line between adjacent collection plates. Charged particles are collected on the plates as dust, which is periodically removed by rapping or water sprays. Discharge wire electrodes are approximately 0.13 to 0.38 cm (0.05 to 0.15 in.) in diameter. Collection plates are usually between 6 and 12 m (20 and 40 ft) high. For ESPs with wire discharge electrodes, the plates are usually spaced from 15 to 30 cm (6 to 12 in.) apart. For ESPs with rigid-frame or plate discharge electrodes, plates are typically spaced 30 to 38 cm(12 to 15 in.) apart and 8 to 12 m (30 to 40 ft) in height. Plate ESPs are typically used for collecting fly ash from industrial and utility boilers as well as in many other industries including cement kilns, glass plants and pulp and paper mills.

Figure 9: Gas flow through a plate precipitator

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ESP Basic Component All electrostatic precipitators, regardless of their particular designs, contain the following essential components: 1. Discharge electrodes 2. Collection electrodes 3. High voltage electrical systems 4. Rappers 5. Hoppers 6. Shell

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Discharge electrodes are either small-diameter metal wires that hang vertically (in the electrostatic precipitator), a number of wires attached together in rigid frames, or a rigid electrode made from a single piece of fabricated metal. Discharge electrodes create a strong electrical field that ionizes flue gas, and this ionization charges particles in the gas. Collection electrodes collect charged particles. Collection electrodes are either flat plates or tubes with a charge opposite that of the discharge electrodes. High voltage equipment provides the electric field between the discharge and collection electrodes used to charge particles in the ESP. Rappers impart a vibration, or shock, to the electrodes, removing the collected dust. Rappers remove dust that has accumulated on both collection electrodes and discharge electrodes. Occasionally, water sprays are used to remove dust from collection electrodes. Hoppers are located at the bottom of the precipitator. Hoppers are used to collect and temporarily store the dust removed during the rapping process. The shell provides the base to support the ESP components and to enclose the unit.

Figure10: Typical dry Electrostatic Precipitators

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Factors Affecting the Performance of E.S.P. The principle of electrostatic precipitator is described in 3 stages as charging of the suspended particles, collecting of particulates under the influence of electrostatic field and removal of the precipitate from the collector plate. Many factors influence these three fundamental steps but they are critical to the reliability and performance of high temperature precipitators which are listed below: Corona characteristics: Initiation of corona depends upon free electrons by random sources such as natural radioactivity. Under the influence of an electrical field, these electrons are accelerated to a terminal velocity. The rapidly moving electrons produce additional free electrons y colliding with the orbital electrons of gas molecules and by ionization. At higher temperatures, flue-gas density is reduced, resulting in a reduced starting potential. Thus, at higher temperatures, lower voltages initiate the corona to start the precipitation process, resulting in more collection for a given voltage than at lower temperatures. Electrostatic precipitators operated at maximum power input have steep corona characteristics; that is, the rate of change of corona current is much greater than the concurrent charge in precipitator-circuit voltage. The steeply rising corona current is further enhanced by increasing temperature of the stack gases. The net effect is to maximize power levels to achieve high efficiency. Resistivity of the particles Particulate resistivity is probably the most important basic variable influencing the precipitator and therefore is an important design consideration.A too high level of electrical resistivity or too low level causes collection difficulty. A high resistivity dust, such as sulphur, does not readily give up its negative charge and assumes a positive charge. This causes the particulate to be repelled back into the gas stream of negatively charged particles. A low resistivity dust can be collected and repelled in this manner many times before finally being emitted to the atmosphere. Therefore, the presence of large quantities of carbon in the ash can adversely affect the collection efficiency of a precipitator. One thumb rule followed by designer is to downgrade the efficiency of the unit by 1% for every 1% of carbon in the gas over 15%. Therefore, one always wishes a medium resistivity for good collection efficiency. In coal fired boilers, sulphur in the form of SO2 affects resistivity. Resistivity has two components, one related to the bulk of the material and another is related to the surface of the particle, absorbed layer of gas. As the temperature increases, the absorbed surface contaminants evaporate and surface resistivity increases. And with all insulating materials, the volume resistivity increases with decreasing temperature. Rapping behaviour This is perhaps the most complex among the three performance steps. Non electrical adhesive forces which play a significant role in plate rapping, vary inversely with particle diameter, but depend generally on the chemical and physical nature of the particle. Moisture can increase adhesion at lower temperatures. Particle resistivity has a critical effect on the electrical force causing particles to slick to the collection plates: the more resistive the particle, the greater the force. Operation at low temperatures and high resistivity requires considerably more rapping acceleration on the collection plates than it does under normal resistivity, and higher temperatures.

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Conventional practice limits maximum average gas velocity in high resistivity and low temperature operation to approximately 1.2 m/s. this limit avoids losses due to re-entrainment of particles which can occur when the dust layer is dislodged violently. In contrast, precipitators run at 1.7 m/s gas velocity at higher temperature. Gas velocity There are two forces acting on a particle having direct right angles to each other. First is due to the flow of gas and second is produced by the electric force on the ionized particle perpendicular to the motion of the gas. The path followed by the particle will take direction which is resultant of the two forces mentioned above. Therefore the efficiency of the collector decreases with an increase in velocity which can be compensated by increasing the voltage supplied to the plates.

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Layout Descriptions

(1) (2) (3) (4)

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(5) (13)

(6)

(14)

(16) (7)

(15) (9) (8)

(10) (11)

(12)

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1. A chimney for air and dust outlet with 20.5 cm Dia & 2 m L. 2. Air Blower 2 3. Anemometer 4. Acrylic cylinder for observation 5. ESP 3 6. ESP 2 7. ESP 1 8. Dust Feeding System (Motor) 9. Vertical Motor (used to devote the dust to feeding system) 10. Dust tanks made of mild steel with transparent acrylic plastic for dust level indications 11. Air Blower 1 12. A funnel like mild steel for air / gas inlet 13. Collection container for ESP 3 14. Collection container for ESP 2 15. Collection container for ESP 1 16. Electrical Control Panel 17. PC with DAQ Electrical Control Panel Layout

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1. Nameplate for equipment brand and model. 2. Voltage meter output for ESP 3 3. Voltage meter output for ESP 2 4. Voltage meter output for ESP 1 5. Main Power Switch

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6. Emergency Stop Switch 7. Dust Feed System Running Indication 8. Air Blower Run Indication 9. Dust Feed Vertical Motor Switc

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ESP Layout

(1)

(2)

(3)

1. Voltage Adjustment Controller 2. ESP Current Meter 3. ESP Switch and Trip and Run LED Indication

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Software System Layout

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(2)

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(4)

1. 2. 3. 4.

Anemometer or Air Flow Meter serves to measure the air flow rate. Air Blower Switch & Speed Control ESP Current Reading Dust Feed Switch and Speed Control

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Experiment 1 Title: Determine the collection efficiency of ESP. Objective: 1. To calculate the dust collection efficiency of ESP. Procedure: 1. Switch ON the main switch and run the ESP software on PC. 2. Switch ON the Air Blower switch (on software) and set the speed to 25 Hz. 3. Switch ON all three electrodes plates and set the voltage to 8 kW. 4. Weight the dust for 500 gram and manually feed it through air inlet. 5. Switch OFF the Air Blower and the main power. 6. Pull out the collection tray and collect the dust. (You may use vacuum cleaner as a method of collection because the dust is not entirely fall to collection tray.) 7. Weight the dust collection and determine the efficiency of the dust collection by:

Efficiency =

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Experiment 2 Title: The effect of varying ionization strength on the performance of the ESP Objective: 1. To study the effect of the ESP performance by varies the ionization strength. 2. To differentiate the collection efficiency with different ionization strength. Procedure: 1. Switch ON the main switch and run the ESP software on PC. 2. Switch ON the Air Blower switch (on software) and set the speed to 25 Hz. 3. Switch ON all three electrodes plates and set the voltage adjustment to 2 kW. 4. Weight the dust for 500 gram and manually feed it through air inlet. 5. Switch OFF the Air Blower and the main power. 6. Pull out the collection tray and collect the dust. (You may use vacuum cleaner as a method of collection because the dust is not entirely fall to collection tray.) 7. Weight the dust collection and determine the efficiency of the dust collection. 8. Record the efficiency of the dust collection in table given below. 9. Repeat step 1 – 8 with different ionization strength (4 and 6 kW) 10. Plot a graph of the results of ionization strength versus dust collection efficiency.

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Experiment 3 Title: The effect of varying air flow rate on the performance of the ESP Objective 1. To study the effect of the ESP performance by varies the air flow rate. 2. To differentiate the collection efficiency with different air flow rate. Procedure 1. Switch ON the main switch and run the ESP software on PC. 2. Switch ON all three electrodes plates and set the voltage to maximum. 3. Switch ON the Air Blower switch (on software) and set the speed to 10 Hz. 4. Weight the dust for 500 gram and manually feed it through air inlet. 5. Switch OFF the Air Blower and the main power. 6. Pull out the collection tray and collect the dust. (You may use vacuum cleaner as a method of collection because the dust is not entirely fall to collection tray.) 7. Weight the dust collection and determine the efficiency of the dust collection. 8. Record the efficiency of the dust collection in table given below. 9. Repeat step 1 – 8 with different air flow rate. ( 20, 30, and 40 Hz) 10. Plot a graph of the results of air flow rate versus dust collection efficiency.

Experiment 4 Title: The effect of varying the number of electrodes plates on the performance and efficiency of the ESP Objectives 1. To differentiate the collection efficiency of ESP with different number of electrodes plates. Procedure 1. Switch ON the main switch and run the ESP software on PC. 2. Switch ON the Air Blower switch (on software) and set the speed to 25 Hz. 3. Switch ON one (1) (ESP1) electrodes plates and set the voltage to maximum. 4. Weight the dust for 500 gram and manually feed it through air inlet. 5. Switch OFF the Air Blower and the main power. 6. Pull out the collection tray and collect the dust. (You may use vacuum cleaner as a method of collection because the dust is not entirely fall to collection tray.) 7. Weight the dust collection and determine the efficiency of the dust collection. 8. Record the efficiency of the dust collection in table given below. 9. Repeat step 1 – 8 with two (2) and three (3) electrodes plates with maximum voltage. 10. Plot a graph of the results of air flow rate versus dust collection efficiency.

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EXPERIMENT 4 - Electrostatic Precipitator

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