THE MEMBRANE VIBRATION AND THE DUST REMOVAL EFFICIENCY OF THE MEMBRANE BASED ELECTROSTATIC PRECIPITATOR. A thesis presented to.

Transcription

1 THE MEMBRANE VIBRATION AND THE DUST REMOVAL EFFICIENCY OF THE MEMBRANE BASED ELECTROSTATIC PRECIPITATOR A thesis presented to the faculty of the Fritz J. and Dolores H. Russ College of Engineering and Technology of Ohio University In partial fulfillment of the requirements for the degree Master of Science Bo Liao June 2003

2 This thesis entitled THE MEMBRANE VIBRATION AND THE DUST REMOVAL EFFICIENCY OF THE MEMBRANE BASED ELECTROSTATIC PRECIPITATOR BY BO LIAO has been approved for the Department of Mechanical Engineering and the Russ College of Engineering and Technology Hajrudin Pasic Professor of Mechanical Engineering Dennis Irwin Dean, Fritz J. and Dolores H. Russ College of Engineering and Technology

3 LIAO, BO. M.S. June Mechanical Engineering The Membrane Vibration and the Dust Removal Efficiency of the Membrane Based Electrostatic Precipitator (96pp.) Director of Thesis: Hajrudin Pasic As energy and material industries develop faster than ever in modern society, and air pollution has been one of the most important issues in environmental protection, the electrostatic precipitator (ESP) has become more important than ever in industry. The ESP technique has been developed over almost 400 years and is widely applied in industry plants at the present time. Recent research has focused on every characteristic of it. Cross-discipline technology and science were applied in its study: physics, electromagnetism, chemistry, electrics, mechanism, dynamics, fluid dynamics, etc. The bench ESP unit has been researched in the Department of Mechanical Engineering at Ohio University. Much research and analysis has been done in the areas of electricity properties, material of collection plate and discharge electrodes, vibration analysis, mechanical dust removal methods, dust collection and removal efficiency, etc. From these results, similar properties were extrapolated for the pilot ESP unit. The pilot ESP unit was constructed in 2001 in the Department of Mechanical Engineering at Ohio University. Its parts are larger size, and its facilities have a higher capacity than the bench ESP unit such as ESP mechanical structure, collection membrane, discharge electrode, TR unit, air fan, and fly ash feeder. In this thesis, research experiments were done on the pilot ESP unit, and it was expected that by inducting from the results from both the bench ESP unit and the pilot ESP unit, properties

4 of a larger scale ESP which would be installed in power plants could be extrapolated. This thesis mainly focuses on the pilot ESP unit, dealing with calibration of dust feeding, measurement of airflow velocity, collection membrane vibration induced by air flow and sonic horn blowing, dust collection and removal efficiency. Chapter 1 briefly introduces the ESP history, basic theoretical operations of ESP and important factors affecting ESP performance. Chapter 2, the literature review, describes the ESP research development in recent years, and membrane-based ESP and result analysis of the bench ESP unit experiment. Chapter 3 introduces the theory and methodology applied in the pilot ESP unit experiment including: the pilot ESP unit experiment setup in Ohio University, the sonic horn working principle, calibration of dust feeder and gas velocity in the ESP chamber, vibration and sound measurement, and dust removal efficiency measurement. Chapter 4 shows the results of calibration of the dust feeder and calibration of gas flow velocity inside the ESP chamber. Chapter 5 shows the results of vibration and sound measurement including: analytical natural frequency, sonic horn-induced vibration, flow-induced vibration, acceleration, velocity and displacement, loose membrane vibration, taut membrane vibration and sonic horn sound level dbs. Chapter 6 describes the experiment results of dust removal efficiency including: effects of different collection times, effects of airflow of different speeds, effects of taut membrane and loose membrane, and comparison between the pilot ESP unit and the bench ESP unit for the same collection time. Chapter 7 concludes the thesis. Approved: Hajrudin Pasic Professor of Mechanical Engineering

5 Acknowledgments 5 First and most importantly, thanks to my advisor, Dr. H. Pasic, who always scientifically, arduously and patiently guided me through the research. Not only in scientific research, but also in usual study did I learn many valuable things that will always help in my life and study. Thanks to other professors who always helped in the research whenever needed, Dr. K. Alam, Dr. J. Lew, and Dr. D. Bayless. Thanks to my classmates with whom I have worked: Z. Hasan Khan, T. Ramamoorthy and M. Qadeer. We helped each other during the experiment. Special thanks to Hasan with whom I have worked most often. His help was great. Thanks to my father and mother who always supported me so that I could finish my research work efficiently. Thanks to all of you, I appreciate it very much.

6 Table of contents 6 Abstract...3 Acknowledgments...5 List of Tables...8 List of Figures...10 Chapter 1. Introduction to ESP Brief history of ESP Basic theoretical operation of ESP Important factors influencing the ESP dust collection efficiency Interaction of the influencing factors...24 Chapter 2. Literature Review ESP research development in recent years The bench ESP unit in the laboratory Bench ESP unit experiment result...29 Chapter 3. Theory and Methodology Pilot ESP unit experiment setup in Ohio University Sonic horn working principle Calibration of dust feeder Calibration of gas velocity inside ESP chamber Vibration and sound measurement Dust removal efficiency measurement...47 Chapter 4. Results of Calibration of the Dust Feeder and Gas Velocity Calibration results of the dust feeder Calibration results of gas velocity...55 Chapter 5. Results of Vibration and Sound Measurement Analytical natural frequency Sonic horn-induced vibration Flow-induced vibration measurement results Acceleration, velocity and displacement Conclusion for loose-membrane Taut membrane vibration measurement Sonic Horn dbs measurement..73 Chapter 6. Dust Removal Efficiency Results and analysis...76

7 7 6.2 Conclusion for dust removal efficiency...91 Chapter 7. Conclusions...92 Bibliography...95

8 List of Tables Comparison of Results with Different Methods Dust feeding rate vs. controller index number Air flow velocity at the center position of the ESP chamber vs. fan controller reference number Frequencies and Correspondent Vibration Magnitude Vibration acceleration amplitude of the membrane in time and frequency domain Acceleration, velocity and displacement in frequency domain Vibration acceleration of membrane in timeand frequency domain Dust removed from the membrane by flowing air. Airflow speed: 1.0 m/sec; collection time: 10 minutes Dust removed from the membrane by horn blowing. Airflow speed: 1.0 m/sec; collection time: 10 minutes Dust remaining on the membrane and total dust collected. Airflow speed: 1.0 m/sec; collection time: 10 minutes Dust removed from the membrane by flowing air. Airflow speed: 1.0 m/sec; collection time: 20 minutes Dust removed from the membrane by horn blowing. Airflow speed: 1.0 m/sec; collection time: 20 minutes Dust remaining on the membrane and total dust collected. Airflow speed: 1.0 m/sec; collection time: 20 minutes Comparison of average percentage of dust removed by air flow, horn and dust remaining between 10-minute collection and 20-minute collection Dust removed from the membrane by flowing air. Airflow speed: 1.25 m/sec; collection time: 20 minutes Dust removed from the membrane by horn blowing. Airflow speed: 1.25 m/sec; collection time: 20 minutes Dust remaining on the membrane and total dust collected. Airflow speed: 1.25 m/sec; collection time: 20 minutes Dust removed from the membrane by flowing air. Airflow speed: 1.50 m/sec; collection time: 20 minutes Dust removed from the membrane by horn blowing. Airflow speed: 1.50 m/sec; collection time: 20 minutes Dust remaining on the membrane and total dust collected. Airflow speed: 1.50 m/sec; collection time: 20 minutes Removal efficiency comparison of different air speed Dust removed from the membrane by airflow. Airflow speed: 1.0 m/sec; collection time: 20 minutes; taut membrane Dust removed from the membrane by horn blowing. Airflow speed: 1.0 m/sec; collection time: 20 minutes; taut membrane Dust remaining on the membrane and total collected. Airflow speed: 1.0 m/sec; collection time: 20 minutes; taut membrane...85

9 Comparison between the loose membrane and the taut membrane; collection time: 20 minutes Percentage of dust removal and remaining on the pilot ESP unit and the bench ESP unit experiment Dust removed from the membrane by airflow. Airflow speed: 1.0 m/sec; collection time: 60 minutes; loose membrane Dust removed from the membrane by horn blowing. Airflow speed: 1.0 m/sec; collection time: 60 minutes; loose membrane Dust remaining on the membrane and total collected. Airflow speed: 1.0 m/sec; collection time: 60 minutes; loose membrane Comparison of 60-minute fly-ash collection between the bench ESP unit and the pilot ESP unit...89

10 List of Figures Schematic illustration of the separation process in the ESP Current-voltage relationship Principle of Corona discharge Impact of Dust resistivity on collection efficiency because the electrical charges have to pass through the dust layer Migration velocity and collection efficiency as a function of particle diameter Collection efficiency as a function of specific collection area Overall mass collection efficiency vs. resistivity Influencing factors and their interactions Performance comparisons among differentmethods of dust removal Overall ESP structure in R-Tech building in Ohio University Side view of the pilot ESP unit structure Principle of sonic soot cleaning Fly ash feeder Fly ash feeder controller Hotwire anemometer installed inside the ESP chamber Fan controller fixed on the wall Dynamic signal analyzer HP35670A Accelerometer attached in the center of the collection membrane Sonic horn inside the ESP chamber Microphone accessories together with DSA 35670A Tray underneath the membrane Dust collected on the membrane before horn blowing Remainder of dust on the membrane after horn blowing Membrane after cleaning the remainder of dust Trend line and the linear regression line of the dust feeding as a function of controller index number Flow velocities at the center position of the ESP chamber vs. fan controller reference number Flow speed at the left position inside the ESP chamber Flow speed of the right position inside the ESP chamber Comparison of the flow speed at the three positions inside the ESP chamber Schematic diagram of the membrane with the force acting on it Vibration acceleration of the membrane by sonic horn in time domain Power spectrum of the vibration of the membrane by sonic horn Average vibration acceleration of membrane in time domain Peak amplitude of membrane vibration acceleration of membrane in frequency domain (power spectrum) Average vibration acceleration amplitude of membrane in frequency domain (power spectrum) Average acceleration comparison of loose and taut membrane in time domain...72

11 Comparison of the average acceleration of membrane in frequency domain Sonic horn sound wave in time domain Sonic horn sound analyses in frequency domain (power spectrum) Comparison between 10-minute dust collection and 20-minute collection Comparison of removal efficiency of different airflow speeds (1.00, 1.25, 1.50 m/sec) Comparison between loose membrane and taut membrane (20 min. collection) Percentage of dust removal and dust remaining on the pilot ESP unit and the bench ESP unit experiment Comparison of the 60-minute fly-ash collection between the bench ESP unit and the pilot ESP unit...90

12 12 Chapter 1 Introduction to the ESP 1.1 Brief history of the ESP Cleaning the air is a constant battle for most industries. The ESP or electrostatic precipitator plays a very important role in the battle. It has been used to separate particulate matter from gas streams in power plants. Now with increased awareness of air pollution effects and more stringent legislation, the ESP plays a more important role than ever in industry. The basis for current precipitation theory and design includes: electrodynamics, fluid-dynamics, solid state electronics and microprocessor applications. Electrostatic precipitators have been studied for 400 years. In 1601, William Gilbert, the personal physician to Queen Elizabeth I of England, found that an electrically charged object could attract smoke while he was studying electricity and magnetism. Around 1824, Hohlfield, a physicist in England, found that a jar could be filled with fog by using an electrified point to draw it into the jar. In 1884, the Olive Lodge Factory in England found that it was possible to use electricity to control the fumes from a chemical process. In 1885 in the Walker, Parker and Co. Lead Factory, a Voss electrostatic machine was used to collect lead fumes in Bagillt, North Wales. In 1906, Frederick Gardner Cottrell applied the technique at Pinhole nearby Selby in California. The plant collected about two gallons per minute of euphoric acid from a volume flow rate of 5000 ft 3 /min of gas. The precipitator worked well due to the design of the discharge electrode and the power supply. One of Cottrell s students, Walter

13 13 Schmidt, developed and patented the fine wire discharge electrode. At this time, hundreds of tons of cement dust were collected each day in the Riverside Portland Cement Company. In 1923, the first application of electrostatic precipitators was in practice at Detroit, Michigan in the United States of America. Before it, the cyclone collectors were no longer efficient enough for their furnaces which used pulverized fuel. In 1929, Lodge- Cottrell Ltd at the North Metropolitan Electrical Company in London, England installed electrostatic precipitators. Around 1930 the wet-film collecting electrode became in use. This invention made the control of the emission from blast furnaces highly successful. Between 1940 and 1945 as carbon black began to be used extensively in producing rubber, application of electrostatic precipitation for collecting carbon black was expanded greatly. In the last thirty years, with the increasing demand for electricity, many large power stations burning coal demanded rapid advancement in the technology. This advancement made it possible for poorer quality coal to be used. This kind of coal gives out little heat and may consist of as much as 30 incombustible material. Thus, the electrostatic precipitators are needed to collect more fly ash than ever. The high electrical resistivity of the fly ash made poor quality coal collection even more difficult. To increase the conductivity of the fly ash, chemicals such as sulphur trioxide or ammonia were introduced into the flue gases of the precipitator. Other methods such as bag-filters and increasing the temperature improved conductivity and improved the high-voltage power supplies. Semiconductors such as silicon diodes improved the power supply and control.

14 14 New materials developed for the components in the ESP have given the precipitator and its components a longer life expectancy than before. Even though the construction cost was greater, the maintenance and renewing cost was lower. Around 1980, it was found that separating the electrodes with more space between each other would also enhance the fly-ash collection. Presently, collecting heavy metal from gases and particulate discharging are the main focus of research. Application of the microprocessors has facilitated the rapid development of the ESP control and its related equipment. 1.2 Basic theoretical operation of the ESPs In the ESPs, particles can be separated from flue gases by electrical means. Schematically the separation process consists of five essential steps as illustrated in Figure 1.1. They include: generation of charge carriers; charging of particles; deflection and separation of particles; depositions of dust; and removal of dust (Parker 1997). In plate-type precipitators, a row of discharge wires are positioned between parallel collecting plates that form a fly ash flow duct. It is also the type applied in the pilot ESP unit used in the laboratory at Ohio University. Dust is deposited on the collecting plates mainly due to electrical forces. The plates are cleaned by a mechanical method. The dust collected falls down to the hoppers underneath the collecting plates. In wet the ESP, the dust is removed by flushing liquid over the plate. The schematically separate process is introduced in section through

15 15 Figure 1.1 Schematic illustration of the separation process in the ESP (Parker, 1997) Ionizing When the electrical field strength is greater than the critical point, a large amount of charge carriers in the gas can be produced. When the high voltage applied exceeds a distinct value, an electrical current between two electrodes can be measured. This is a corona discharge. As shown in Figure 1.2 for the corona current vs. voltage, the corona onset point is at a certain voltage point instead of at zero point. The current increases when the voltage increases until spark-over happens.

16 16 Figure 1.2 Current-voltage relationship (Parker, 1997) Some molecules recombine immediately after ionization because of natural radioactivity and cosmic radiation. If an electrical field is present at the time of ionization, the electron will be accessed and separated quickly from the positive ion, as shown in Figure 1.3. In a short distance, the electron will hit another neutral gas molecule. The second electron will be produced if the ionization energy is strong enough. Thus, additional molecules will be ionized resembling the effect of an avalanche. When the moving electrons go inside a lower electrical strength zone, no additional molecules can be ionized; however, the electrons will attach to an electronegative gas molecule, such as O 2 or SO. Therefore negative gas ions are formed. The corona discharge continuously produces drifting charge carriers. The electrical discharge current actually consists of drifting charge carriers. 2

17 17 Figure 1.3 Principle of Corona discharge (Parker, 1997) The method for negative corona energizing is that the discharge electrodes are energized with negative high voltage and the collecting plates (positive) are grounded. In positive corona energizing the opposite effect takes place. The negative energizing has a higher spark-over voltage than the positive one does. Negative energizing is employed in most industries and is used in this pilot ESP laboratory in order to have a higher electrical strength field Charging of the particles The charging process consists of a field charging region and a diffusion charging region. The former is for dust particles > 1µm, and requires an electrical field. This field

18 18 can drive free movable charging carriers. The diffusion charging region is for particles<1µm, and it attracts randomly-moving gas ions caused by temperature increase Deflection and separation of the particles Different forces, including the momentum force, the electrical force and the drag force act on the particles in the ESP together. They play important roles in determining the particle migration velocity, flow field and particle trajectories. The charged particles will separate and deflect to the collection plate when the electrical force is big enough Depositions of dust When electrically charged particles drift to the collecting electrode, the electrical charge flows from the particles into the grounded electrode as shown in Figure 1.4. Because the electrical charges must pass through the dust layer, the dust resistivity has a very important impact on the electrical charge process. In the dust layer, the back-corona deteriorates particle precipitation. For highly resistive dusts, a lower charge carrier flow can sometimes have better collection effects than a high-voltage system, since a high voltage system decreases the back-corona effects.

19 19 Figure 1.4 Impact of Dust resistivity on collection efficiency because the electrical charges have to pass through the dust layer (Parker, 1997) Removal of dust In the dry ESP, dust is removed by mechanical impact such as hammers, a rapping mechanism or the sonic horn-blowing method. The dust layer breaks down from the collecting plates and settles into the hoppers. In the wet ESP, dust is washed off with water. 1.3 Important factors influencing the ESP dust collection efficiency ESP is a complicated system consisting of many different parts, including electrical discharges, fly ash particles, collection plate material, air flow duct and the ESP geometry. Some of these factors significantly influence dust collection efficiency. Here is a very brief introduction to these factors.

20 Effects of particle size distribution on the ESP performance The distribution of sizes of fly-ash particles flowing through the ESP has a significant effect on fly ash collection efficiency because different particle diameters have different effects on particle migration velocities and collection efficiencies in an ESP. Many experimental data results have shown that when the average fly ash particle size increases, the effective migration velocity also increases, and the collection efficiency increases too. (McDonald, 1982) Figure 1.5 shows the trend that when the particle diameter increases, the migration velocity and collection efficiency increase as well. Figure 1.5 Migration velocity and collection efficiency as a function of particle diameter (Dean, 1982)

21 1.3.2 Effects of specific collection area 21 Another important factor influencing the ESP collection efficiency is the specific collection area (SCA). This is the ratio of total collection area to total air volume flow rate. Adjusting either the plate collecting area and/or the air flowing rate can change the SCA, because by increasing the SCA, the time for the particle treatment in the ESP is increased, and the dust collection efficiency is also increased. Figure 1.6 shows the trend that when the specific collection area increases, the dust collection efficiency increases as well. Figure 1.6 Collection efficiency as a function of specific collection area (Dean, 1982)

22 1.3.3 Effects of voltage-current characteristics 22 The voltage-current curves are expected to cover a wide range so that a maximum discharge voltage can be applied to the ESP electrodes. When the discharge electrode voltage increases, the current will also increase. A higher discharge voltage and a higher current will result in higher collection efficiency. Discharge voltage can increase until it reaches the spark-over voltage. After the spark-over voltage, the discharge voltage will break down to zero. Therefore, the sparkover voltage must be high enough to create a sufficiently high discharge current so that the fly-ash collection efficiency can be increased. Low discharge voltage or low discharge current will result in lower fly ash collection efficiency. Because the discharge voltage plays the most important role in controlling the particle charge and the electrical field, it should be the first factor to be considered. If by increasing the voltage, the current will increase, the collection efficiency will also be increased. If when increasing the current, there is a much smaller change in voltage, there will be no pronounced improvement in fly-ash collection Effect of fly ash resistivity In many cases, the resistivity of the collected dust particle layer on the electrode and the collection plates has a significant effect on current density in the ESP. If the resistivity of the collected dust layer is sufficiently high, the electrical breakdown of the layer will occur at a much lower current. The breakdown of the collected dust layer will result in either sparking or stable back corona from the collected dust layer. Excessive

23 23 sparking and back corona will greatly decrease the collection efficiency. Figure 1.7 shows that the overall collection efficiency decreases as the dust resistivity increases. Figure 1.7 Overall mass collection efficiency vs. resistivity. (Dean, 1982) Effect of gas velocity distribution Non-uniform gas velocity distribution will reduce the ESP collection efficiency. Because of the non-uniform gas velocity distribution, particles in different velocity zones are treated unevenly. In high gas velocity regions, there is more potential for reentrainment of collected dust from the collection plate surface and the hoppers. The nonuniform particulate mass loading distribution will then result in excessive dust accumulations in certain regions of the ESP. To make gas velocity distribution more even, the following designs must be improved: the location and types of diffuser elements such as grids and perforated plates, the ductwork design, the fan design and location, and the configuration and location of turning vanes.

24 1.3.6 Effect of nonuniform temperature and dust accumulation 24 Nonuniform temperature and dust concentration in the ESP may also decrease collection effects. A nonuniform temperature may lead to variations in the electrical property of the gas, variations in the resistivity of the collected dust layer, and corrosion in low temperature regions. There will be excessive sparking in certain regions of the precipitator because of the first two effects. A nonuniform dust concentration may lead to excessive buildup of dust on corona wires, collection plates and beams. Excessive dust buildup on corona wires tends to suppress the corona and cause uneven corona emission. Excessive dust buildup on the collection electrodes may also cause significant particle re-entrainment and undesirable electrical condition. 1.4 Interaction of the influencing factors Many factors including the flow field, electrical conditions, dust condition and the factor s interaction determine the precipitator s operation state. Figure 1.8 shows the influencing factors and their interactions with respect to their impact on the dust particle transport. The interaction of the influencing factors plays a more important role than the influencing factors themselves in deciding the particle transport. For example, the turbulence intensities of the air flow change when the high discharge voltage is switched on. The dust particle dispersion and concentration are decided by the interaction between the flow field and the dust particles. The charging effects are decided by the interaction between electrical state and dust particles.

25 25 Figure 1.8 Influencing factors and their interactions (Parker, 1997) Almost all influencing factors and interactions are functions of the geometry of the precipitator. Similar geometry on different sizes of precipitators provides similar transport condition for particles. This means that, most of the time, the experiments done on small-scale size ESP can be inducted to a larger scale ESPs if they have the same structure geometry.

26 Chapter 2 26 Literature Review 2.1 ESP research development in recent years In 1975, Sem and Tsurubayashi in USA developed a new mass sensor for respirable dust measurement. They used an electrostatic precipitator to deposit particles as small as 10 µm onto a piezoelectric micro-balance censor. Digital readout and a built- In sensor cleaner were utilized in the instrument. In 1985, Rajala and Janka research group in the Netherlands studied the effect of ESPs on the behavior of radon decay products in laboratory conditions. It was found that the ESP could decrease the equilibrium factor of daughters and increase the unattached fraction of decay products. In clean air, an ESP could also decrease the activity of unattached daughters. In 1986, the Rajala research group also found an ESP could produce condensation nuclei that were observable when no other particles were present. It was found that probably the ozone produced by the corona discharge of the filter has a great effect on the production of these submicron particles. In 1989, Bigu and Grenier in Elliot Lake Laboratory, Canmet, Canada, applied an ESP based on charged water spray technology in an underground uranium mine to control long-lived radioactive dust and short-lived aerosol concentration in a mine gallery. The dust was discharged from where rocks were broken and transported. Two main sampling stations were established: one is set upstream of the dust precipitator and another on downstream. Experiments were conducted under a variety of air flow conditions. Finally it was found out that the ESP

27 27 3 could reduce 40 the radioactive dust at a ventilation rate of 0.61m /sec. This was obtained as a result of the combined action of water scrubbing and electrostatic precipitation by the charged water spray in the electrostatic precipitator. It was also found that this result was the optimum efficiency obtained within the range of ventilation rates and the dust removal efficiency decreased with increasing ventilation rates. In 1992, the Shiotsuka research group at the Brookhaven National Laboratory, USA, studied the inhalation toxicity by applying ESPs. They found that an electrostatic precipitator placed in the exhaust lines reduced the amount of dust delivered to highefficiency particulate air filters, therefore reducing the number of filter changes. In 1996, Leith and Boundy of the University of North Carolina, Chapel Hill, USA investigated the laboratory measurements of oil mist concentrations using filters together with an ESP. This study investigated the potential for mineral oil mist to evaporate during sampling from filters and ESP substrates. This study was used to assess personal exposure to mineral oil mist. If the sample evaporates, mist concentrations measured will underestimate true exposure. The study found out that less evaporation occurred for samples taken with an electrostatic precipitator, where mist droplets are separated from the air flow by electrostatic force and coalesce on the precipitator wall. 2.2 The bench ESP unit in the laboratory Conventional dust collectors were metal plates that were heavy and expensive. The plates were pushed or buckled to dislodge collected dust. The conventional rapping method, i.e. striking a hammer on the metal plate to dislodge dust, actually sends collected pollutants back into air.

28 28 At Ohio University, under the leadership of Professor H. Pasic, a new set of more efficient and low-cost ESP was developed. The concept is based on the replacement of the particle collection plates by membranes made from inexpensive advanced material. And the membrane was pulled to induce shear for dislodging the collected particles, cleaned by the sonic horn method or washed with water. The new ESP will help coal, steel, paper and other industries meet forthcoming U.S. Environmental Protection Agency emissions regulations. The experiments conducted with different membrane materials in the pilot ESP unit showed that the membranes woven from carbon fibers, silicon and similar fiberbased materials can be used to collect dust particles as efficiently as conventional steel plates. The membrane can trap fine air pollutants and toxic heavy metals. The new ESPs could make high-sulfur coal a more viable energy resource for the nation s power plants. Vibration of the membrane plays an important role in dislodging dust particles. The relationship between the dust collection/removal efficiencies and vibration parameters such as gas flow speed and membrane tension needed to be better understood. The experiments showed that membrane vibrations can be controlled by membrane characteristics and by adjusting its tension. A significant advantage of this design is the great reduction in weight of collector plates by one order of magnitude. It is 10 to 20 times lighter than the conventional metal collector plates. The new design also reduced cost of production, installation, transportation and repairs. Another advantage of the new ESP is that they can be fabricated from corrosion-resistant material. It was expected that the membrane could be

29 29 coated with catalytic materials to improve removal efficiency of gas pollutants and heavy metals. 2.3 The bench ESP experiment result Dust removal efficiency experiments have been done on the bench ESP unit. Three different experimental methods were applied, including the rapping mechanism on the collection membrane, the sonic horn on a metal plate and the sonic horn on the membrane. The sonic horn on collection membrane is the most efficient method for removing dust in the ESP. A comparison of the three different experimental results is summarized in Table 2.1 and pictorially shown in Figure 2.1, followed by explanations. Table 2.1 Comparison of Results with Different Methods Dust removing method Average % dust removed by air flow Average dust removed Average dust remaining on membrane/metal plate By sonic horn from the membrane By sonic horn from the metal plate By rapping mechanism from the membrane 34% 57% 9% 22% 53% 25% 31% 52% 17%

30 30 100% 90% 9% 25% 17% 80% 70% 60% 50% 40% 57% 53% 52% Remaining dust on membrane Dust removed by horn/rapping 30% 20% 10% 34% 22% 31% Dust removed by gas flow 0% Horn/membrane Horn/metal plate Rapping/membrane Figure 2.1 Performance comparisons among different methods of dust removal It can clearly be seen from the above that the method of sonic horn blowing on the membrane has a higher dust removal efficiency than on a metal plate. Dust removed by sonic horn from the membrane is around 57% and only 9% remains on the membrane. On the other hand, the amount of dust removed by the sonic horn from the metal plate is 53%, while 25 the total remains on the plate. The amount of dust removed from the membrane by fluid flow alone is up to 12% higher than the dust removed from the metal plate. This indicates that the membrane itself can vibrate and help with dust removal. In the pilot ESP unit, membranes are larger and less stiff than this membrane. This effect can, therefore, be expected to play an even more important role in removing dust. Furthermore, most of the dust removed from the

31 31 membrane is collected in the first two slots of the tray within the flow boundary layer, which is estimated to be about 1 inch thick, indicating the low re-entrainment level. Also, sonic horn blowing removes 57 total collected dust from the membranes, while it removes 53% from the metal plate. Thus, 4% more dust falls from the membrane due to induced wall vibrations. Hence, it can be concluded that the vibrations induced by sonic horn helps the membrane dislodge more dust. The effect of the sonic horn position and its operating time appears to have no substantial influence on such a small ESP unit. The membrane has the size of only 1ft 1ft; however this is not the case in larger scale ESP. The sonic horn-membrane method has a higher efficiency than the rapping mechanism-membrane method. The amount of dust removed from the membrane by the rapping mechanism is around 52% which is less than the 57% resulting from the sonic horn method, while 17 the dust remains on the membrane which is higher than the 9% left by the sonic horn method. The rapping mechanism dose work, however using it continuously risks the membrane tearing. In conclusion, the sonic horn blowing in conjunction with a membrane is the best choice from the standpoint of dust removal efficiency. Therefore this method was also applied in the pilot ESP unit experiment as described in Chapter 5 and 6.

32 Chapter 3 Theory and Methodology The pilot ESP unit experiment setup in Ohio University Figure 3.1 shows the overall ESP setup installed in the R-Tech Building at Ohio University. The chamber of this ESP unit is 5ft. wide, 7 ft. long and 10 ft. high. Installed inside the chamber is the collection membrane, the discharge electrode, the water duct pipe for wet ESP and other mechanical transmission parts. The collection membrane is 1.5 ft. wide and 6 ft. long. It is a carbon fabric membrane made by Fabric Development Inc. The discharge electrode is a metal mesh electrode that is almost the same size as the collection membrane. It is parallel to the collection membrane, and is 7.5 in. away from the collection membrane. The collection membrane and the electrode are connected to the Transformer Rectifier (TR) unit by cables on top of the ESP chamber as shown in Figure 3.1. The TR unit supplies sufficient discharge voltage for electrodes. The water duct pipe and other mechanical transmission parts are installed to prepare for the wet ESP experiment. In this thesis, all the experiments were conducted on the dry ESP. There are hoppers at the bottom of the ESP chamber for collecting fallen dust. The Transformer Rectifier (TR) unit is installed high above the ground over the ESP chamber as shown at the top center of Figure 3.1. It is connected by cables to the discharge electrode and the collection membrane, which are installed in the ESP

33 33 chamber. The TR unit is adjusted by a controller which can use either local or remote control to manipulate the TR unit. With the remote control, the controller can be operated far from the TR unit. Figure 3.1 Overall ESP structure in R-Tech building in Ohio University The fan is installed outside of the laboratory room and is connected to the ESP chamber by an air conduct transmitter. The transmitter can be seen in Figure 3.1 on the left side coming from the wall to the ESP chamber. When the fan is switched on, it will suck the air into the ESP chamber from the inlet of the ESP chamber, i.e. the other end of ESP on the right side in Figure 3.1. The fan can be adjusted to any desired air flow speed by a remote controller. The inlet is another air duct transmitter shown on the right side of Figure 3.1. Here fly ash is

34 34 also supplied and mixed together with the air, which will flow through the ESP collection membrane inside the ESP chamber. The dust feeder, made by the Vibra Company, is installed at the air inlet of the ESP for supplying fly ash to the ESP unit. In the bench ESP unit a fly ash fluidizer is used for feeding dust; however, the volume is insufficient. Presently the dust feeder can provide 10,000 gm of fly ash at one time. The fly-ash collection experiment can last for one hour without any interruption in the middle of the experiment to re-supply dust to the feeder. The feeder has a very stable dust feeding rate and is adjustable by a controller so that it can always supply a stable amount of dust per unit time to the ESP. Figure 3.2 shows the side view of the pilot ESP unit structure. The fan on the right side will suck air from the air inlet on the left side into ESP chamber. Figure 3.2 Side view of the pilot ESP unit structure

35 3.2 Sonic horn working principle 35 In this thesis, sonic horn blowing is the method applied to remove dust from the collection membrane in the ESP. By this means, all the internals including electrodes, walls and screens can be cleaned. There is no need for the mechanical rapping mechanism, and there is less re-entrainment produced than with the rapping mechanism. On the other hand, it also has an adverse effect on the precipitator because the horn blowing results in the re-dispersion of the dust in the ESP. Because of its characteristics, acoustic horns were used for dislodging soot or dust from the heat exchanger tubes or boilers 60 years ago in rubber and power plants. The actual physical reason for the influence of the sonic horn sound waves on dust layers is not well known. Now, only some hypotheses exist to explain the sound wave removing effect. K.E. Widell (1982) described four possible physical mechanisms of dust dislodgement from collecting electrodes. Figure 3.3 shows the four possible principles of sonic soot cleaning that include: (1) vibration of the collection plate or membrane induced by the sonic horn blowing which removes the dust down; (2) sound pressure fluctuations impact on the dust layer; whereby a porous dust layer resonated by the sound pressure might create high internal gas pressure; (3) particles collected on the surface layer are worn away by the acoustical movement of the gas; (4) particles at the surface are removed by the gas movement. More clarification about the physical mechanism needs to be developed.

36 36 Figure 3.3 Principle of sonic soot cleaning (K.E. Widell, 1982) a) Induced wall vibrations b) Internal pressure making dust layer explode c) Particle erosion of surface d) Gas friction acting on surface particles By using the sonic horn dust-removing method, the initial expenditure and maintenance costs can be reduced. While there is no more rapper wearing and fatigue life problems, they have problems with diaphragm corrosion and diaphragm-fatigue ruptures. It is believed that the optimized installation of horns in a precipitator may have a better effect. Optimization includes number and locations of the sonic horns installed inside the ESP. More experimental details about sonic horn are presented in Chapter 5 and Calibration of the dust feeder In ESP dust collection experiments, the dust feeder supplies fly ash to the ESP chamber at a constant rate. The feeder, made by Vibra Screw Inc., Totowa, New Jersey, was connected with the ESP unit by the air inlet of the ESP. Figure 3.4 shows the dust feeder.

37 37 Figure 3.4 Fly ash feeder The feeding rate can be controlled by adjusting the dust feeder controller. When the controller button was adjusted to a higher index number, the dust feeder increases its feeding rate and more fly ash per minute was supplied. The dust feeder controller fixed on the laboratory wall is shown in Figure 3.5.

38 38 Figure 3.5 Fly ash feeder controller Before using the dust feeder, it must to be calibrated so that in the experiment for testing dust removal efficiency it can be adjusted according to the calibration resort. To calibrate the feeding speed, the amount of dust collected over a certain period of time was weighed and recorded. When the feeding speed was low, much less dust was fed out, therefore to have a more accurate result, more feeding time was needed so as to supply enough dust to be weighed. The controller button was adjusted to the controlling index number of 14. The amount of fly ash supplied in two minutes was weighed and recorded. Then the controller was increased to a higher controlling index number, 16. Then the dust was weighed and the feeding rate was again calculated. For every measurement the controller index number was increased by 2 or 4. This process was repeated until the controller index number reached 60. To get a more precise calibration result, an average

39 39 of dust feeding rate was needed, therefore the entire calibration process was repeated again. 3.4 Calibration of gas velocity inside the ESP chamber The pilot ESP unit chamber was much larger than the small ESP chamber. The chamber s cross-section area was 4 ft. wide and 7 ft. high. The collection membrane, discharge electrode and other supporting accessories were all inside the chamber. Because the cross-section area inside the chamber was large and the collection membrane was 1.5 ft. by 6 ft., the air speed must be measured at different positions inside the chamber to check for uniformity of the flow. Whenever the fly ash collection experiment was initiated, the fan was switched on to bring air from the air inlet at another end of the ESP. The air goes through the ESP chamber. The speed of air flowing in the ESP chamber can be adjusted by controlling the fan speed. It was necessary to calibrate the airflow speed inside the ESP so that in the later dust collection experiment described in Chapter 6, airflow speed can be adjusted according to the calibration result. The hotwire anemometer, FMA 906-V, made by Omega Engineering Inc., was fixed inside the chamber, as shown in Figure 3.6, and was connected to a multi-digital meter. The number read from the meter was multiplied with a weighting factor to calculate the air speed.

40 40 Figure 3.6 Hotwire anemometer installed inside the ESP chamber The fan controller can control air flow speed by adjusting the controller reference number. Figure 3.7 shows the fan controller. By increasing the reference number the fan will run faster and the air speed will be increased. To calibrate, the controller reference number was first adjusted to 10, then the fan was switched on. After the airflow became stable, the speed of gas was measured and recorded. The reference number was then increased by 2 and the speed was measured again. The measurement process was repeated until the reference number reached 38. At 38, the air speed was so big as to produce a very intense noise; therefore it was not increased further.

41 41 Figure 3.7 Fan controller fixed on the wall The hotwire anemometer was fixed in the center position of the chamber, 3.5 ft high from the bottom, 2.0 ft distant from the side of the ESP chamber, and fixed in front of the collection membrane. At every fan controller reference number, the corresponding air speed was recorded. Then it was fixed to the left position of the membrane inside the chamber. It was 1 ft. from the center position, and the whole set of measurement process was repeated again. Finally it was moved to the right side position which was also 1 ft. from the center position. The measurement was repeated again. 3.5 Vibration and sound measurement Principle and objective During electrostatic precipitation when the fly ash was collected on the collection membrane, some of the dust collected slid down off the membrane due to the flow-

42 42 induced vibration of the membrane. After a certain amount of dust had collected on the membrane, the dust had to be removed. As mentioned earlier, there were several ways to remove the dust. In this experiment, a sonic horn was used as the way to remove dust because the vibration of the membrane induced by the sonic horn blowing has the highest dust removal efficiency. In the experiment performed on the small-size membrane in the small ESP, the loose-membrane condition was compared with the taut-membrane condition to determine which one had the higher removal efficiency as described in Chapter 2. About 5 lb/inch of stretching force was applied on the taut membrane. A similar test was also performed on the pilot ESP unit and the results were compared. Finally the sonic horn sound wave and its dbs were recorded for analysis Procedure Dynamic signal analyzer HP35670A was used to measure and record vibrations and the sonic horn sound. Figure 3.8 shows the HP35670A. It gathered signals from the accelerometer and did signal analyzing in the time domain and the frequency domain.

43 43 Figure 3.8 Dynamic signal analyzer HP35670A The accelerometer, model number 353B16, made by the Agilent Company, was attached in the center of the collection membrane inside the ESP as shown in Figure 3.9. It was fixed perpendicularly to the membrane by wax. It can pick up the vibration signals of the membrane and transfer them to the DSA 35670A to be recorded and analyzed. The DSA can do the real-time recording and FFT analysis(fast Fourier Transformation). In the power spectrum analysis, i.e. in frequency domain, the DSA can record acceleration, velocity and displacement as a function of frequency. The membrane was first adjusted to a loose condition with zero stretching force on it, but the membrane was still kept straight. As previously described in Chapter 2, it was found that this membrane state demonstrated the most beneficial dust cleaning efficiency.

44 44 Figure 3.9 Accelerometer attached in the center of the collection membrane The horn was fixed on the ceiling of the ESP chamber, facing down towards the ESP chamber as shown in Figure It was connected to a compressed air supplier. The horn, made by the Kockum Sonics AB Company in Sweden, was type MKT 75/440. Its main technical data includes: Frequencies: 440 Hz Sound pressure level at 1m >130 db Air consumption (free air) 7-10 l/sec Weight: 1.75kg (3.9lbs)

45 45 Figure 3.10 Sonic horn inside the ESP chamber First, the effect of the sonic horn on the membrane was tested. The membrane s vibration induced by the horn blowing was recorded. Then the flow induced vibration of the membrane was tested and recorded. The sonic horn was not blown this time. Since different air speeds had different impacts on the membrane vibration, the speed was changed gradually for each test. The fan was switched on, the air speed was first set at 0.8 m/sec, and the vibration of the membrane was recorded. Then the speed was increased by 0.2 m/sec, and the membrane vibration was recorded again. This test was repeated until a speed of 2.2 m/sec was reached. From 0.8 m/sec to 2.2 m/sec, there was total of eight recording steps. After the loose-membrane condition was tested, the membrane was slightly tightened to have a force of about 5 lbs/in. The membrane vibration was recorded again with regard to different air speeds. The procedure was the same as for the loose membrane condition.

46 46 Finally the sonic horn sound wave and the dbs were recorded with by the microphone accessories made by the Agilent Company. They include: Microphone: ACOJ-7013 XX Microphone Power Supply: ACOP-9200 XX Preamplifier: ACOP-4012 XX They were connected to each other and connected to the DSA 35670A as shown in Figure The DSA recorded the sound signal that the microphone sensed. When it was used to measure the sound, the microphone was placed inside the ESP chamber close to the collection membrane. The sound signal was recorded and analyzed by the DSA 35670A. Figure 3.11 Microphone accessories together with DSA 35670A

47 Dust removal efficiency measurement Principle and objective In the dust removal experiment, the membrane collected fly ash flowing inside the ESP chamber. Since it was expected that the pilot ESP unit would have a higher dust removal efficiency than the small ESP had, experiments on the pilot ESP unit were performed to verify this. In addition, by analyzing the results from the bench ESP and the pilot ESP unit, the dust removal efficiency of the larger scale ESP which would be installed in power plants, could be extrapolated. Some of the dust collected on the membrane can be removed by membrane vibration induced by airflow. However, some dust still remained on the membrane. After a period of time of dust collection, a certain amount of dust accumulated on the membrane. In this experiment, sonic horn blowing was used to clean the membrane and could remove most of the remaining dust on membrane. There was however still a slight amount of dust left on the membrane after horn blowing. To evaluate the dust removal efficiency, the amount of dust in the three periods during the collection was recorded. They include dust removed by airflow, by sonic horn, and dust which remained on the membrane Procedure To precisely measure the dust removed from the membrane, a tray was placed under the membrane as shown in Figure The tray had four slots to collect dust that

48 48 fell over the area underneath the membrane in order to differentiate the amount of dust which fell at different distances from the membrane. Figure 3.12 Tray underneath the membrane The procedure includes the following steps: 1. The membrane was adjusted and kept in loose condition. The tray was put underneath the membrane to prepare for collecting fallen dust from membrane. 2. Fly ash was fed into the dust feeder in order to supply enough fly ash for the ESP. During the dust collection experiment, fly ash evenly spread through the air due to flowing air and the ESP structure and was made to pass through the membrane inside the ESP chamber by air flow.

49 49 3. The electrode, controlled by the TR unit, was loaded with high voltage. The discharge voltage of the electrode can be adjusted from 0 to 80 kv by the TR unit controller. The discharge electrode voltage in this experiment was 45 kv. 4. The dust feeder was switched on to supply the fly ash for ESP. The feeder controller was adjusted to point to the reference controlling number of 16%, which would keep dust feeder supply fly ash at the rate of 4.17 g/sec. The feeding rate was kept stable during dust collection. 5. The fan was switched on to bring the air into the ESP chamber from the inlet of the ESP, thus air would flow through the collection membrane inside the ESP chamber. The airflow speed was 1.0 m/sec which was the same air speed applied in the bench ESP dust collection experiment. 6. Dust was collected continuously for 10 minutes. The total amount of dust supplied was 2502 gm (4.17gm/sec 60 sec 10 min). 7. After 10 minutes of dust collection, the fan was switched off to stop airflow, the dust feeder was switched off to stop fly ash supply, and finally the TR unit was switched off to stop supplying electrode discharging. Dust collected on the membrane at this period is shown in Figure 3.13.

50 50 Figure 3.13 Dust collected on the membrane before horn blowing 8. The tray was taken out carefully from underneath the membrane. The amount of dust in the four different slots of the tray was weighted on one electrical scale and recorded respectively. 9. The tray was put back carefully underneath the membrane not to shack some of the dust off of the membrane. 10. The valve between the sonic horn and the compressed air supplier was switched on to start sonic horn blowing. The horn blew for five seconds. In the test on the small ESP, five seconds of horn blowing had the same removal efficiency as seven seconds of horn blowing did. The air pressure for the horn was 80 psi. It was the same pressure as applied in the small ESP experiment. Most dust on the membrane was removed down after horn blowing and the membrane was much cleaner, as shown in Figure 3.14.

51 51 Figure 3.14 Remainder of dust on the membrane after horn blowing 11. The tray was taken out carefully again. The amount of dust in the slots removed by the sonic horn was carefully weighed and recorded. 12. The collection membrane had a large size of 6 ft. by 1.5 ft. To measure what was left on the membrane, the remaining dust had to be carefully collected by hand. The experiment operator went inside of the ESP, careful not to touch the membrane. Using a brush, the remaining dust on membrane was slowly brushed into the tray. The brushing started from the top of the membrane and moved gradually to the bottom of membrane. Finally the amount of dust was weighed and recorded. The clean membrane is shown in Figure 3.15.

52 52 Figure 3.15 Membrane after cleaning the remainder of dust 13. The experiment was repeated from step 1 through step 12 five times with all experimental conditions being kept the same. 14. In the first group of experiment, every experiment lasted for 10 minutes. In the second group, they lasted for 20 minutes. It was repeated again from step 1 through step 13. All together there were five repetitions of the experiment. 15. To compare the effects that different airflow speed had on the dust removal efficiency, the airflow speed was increased to 1.25 m/sec, step 1 through step 13 were followed, and the experiment was again repeated five times. 16. To compare with the loose membrane experiment result, the membrane was adjusted to a taut condition with a force of 5 lb/ft on the membrane. Dust collection lasted for 20 minutes. This was the same as in step Finally, to compare with the small ESP result, dust collection lasted for one hour, which was the same collection time applied in the small ESP experiment.