CHAPTER 1 INTRODUCTION

Transcription

1 1 CHAPTER 1 INTRODUCTION 1.1 GENERAL Rapid economic growth and industrialisation has led to significant reduction in ambient air quality. There is an ever increasing demand for power and to meet this demand more coal based power plants are being set up. Stack emissions from coal based power plant has increased off late and research is still going on to reduce stack emission to prescribed levels, and also to meet stringent air quality standards in future. Electrostatic Precipitator (ESP) is the most commonly used device to reduce stack emissions from coal based power plants by capturing micro particles suspended in flue gas stream. Though Electrostatic Precipitator (ESP) is being used commercially for more than 60 years now, thorough understanding of its operation still remains a topic of debate. The existence of electrostatic forces has been known for over 1000 years. Fredrick Gardner Cottrell combined the newly invented synchronous mechanical rectifier with the work of early researchers (notably Sir Oliver Lodge), and in 1907, designed the first commercial electrostatic precipitator. Along with its rapid commercial acceptance, the technology quickly developed its form and design characteristics that remain unchanged to this day. An electrostatic precipitator is an air pollution control device that removes particles from a flowing gas with electric forces.

2 2 The precipitation process involves 1) charging particles by means of ions produced in a corona discharge, 2) separating the charged particles from the gas stream in an imposed electric field, 3) collecting the particles on a grounded surface, and 4) removing the collected particles by either knocking them off or washing them off with water. The Clean Air Act Amendments give the U.S. Environmental Protection Agency (EPA) the power to enforce more stringent regulations on industrial pollutants. Particles of less than 10 µm in diameter (PM10) which reduce visibility, and of those less than 2.5 µm in diameter (PM2.5) which adversely affect human health. The need to provide effective removal of both particles and gaseous pollutants introduces new factors in the determination of the best technology for industrial air pollution control and has complicated the design of precipitators. Electrostatic precipitators (ESP) are well-known, highly efficient devices applied in power stations and many other large-scale industrial systems in order to reduce fly ash and fine particle emissions. 1.2 ELECTROSTATIC PRECIPITATOR An electrostatic precipitator is a device which removes particles from a gas stream. It accomplishes particle separation by the use of an electric field which (i) imparts a positive or negative charge to the particle, (ii) attracts the particle to an oppositely charged plate or tube, (iii) removes the particle from the collection surface to a hopper by vibrating or rapping the collection surface. One of the most important aspects characterising ESP is the fluid dynamics, as the velocity field throughout the collecting region controls dust deposition. The stringent emission regulations that have been stipulated in the recent years have set new targets for the ESP manufacturers. Efforts to improve ESPs through upgraded technologies and managing operational prob-

3 3 lems through careful improvement of the ESP operational practices have proved its success till date. Electrostatic precipitator is the most widely used device for particulate emission control. With the present growth of giant cement, super thermal power stations, fertilizer complexes, and oil refineries in India, and with the air pollution control regulation by the various local authorities, the application of ESPs has been increasing at a faster rate. 1.3 TYPES OF ESP Electrostatic precipitators may be classified according to the type of use for which they are designed. Industrial precipitators are used for collecting dust, smoke fume, mist, etc., from industrial gases and used to collect fly ash from pulverized coal-fired boilers, cement kiln dust, catalyst dust at oil refineries, metallurgical fumes, soda fume in pulp mills, and sulphuric acid mist. The ESPs are classified based on the design stages: Two stage ESPs. Two stage ESPs are designed so that the charging field and the collecting field are independent of each other. The charging electrode is located upstream of the collecting plates. Two stage ESPs are used in the collection of fine mists. Single stage ESPs. Single stage ESPs are designed so that the same electric field is used for charging and collecting particulates. Single stage ESPs are the most common type used for the control of particulate emissions and are either of tube or parallel plate type construction. A schematic view of the wire-plate and two-stage arrangement is shown in Figure 1.1. The wire/plate configuration (Figure 1.2 & Figure1.3)

4 4 consists of an array of high-voltage discharge wires located midway between grounded, parallel-plate collecting electrodes. A large ESP for a power plant would consist of many of these flow channels. (a) Wire-plate ESP (b) Two-stage ESP (c). Top: idealized electrical field lines; bottom: principle fluid flow; the ionic wind exerts a pressure on the gas flow, whereby due to continuity, vortices are created at the wall regions. [Riehle] Figure 1.1. Schematic of ESP.

5 5 Figure 1.2 Single channel Wire-plate ESP Figure 1.3 Multiple channel Wire-plate ESP The tube type precipitator is a pipe with a discharge wire running axially through it. Gas flows up through the pipe and collected particulate is discharged from the bottom. This type of precipitator is mainly used to handle small gas volumes. It possesses collection efficiency comparable to the parallel plate types, usually greater than 90 percent. Washing the ash with water is frequently used instead of rapping to clean the collecting surface. Parallel plate precipitators are the most commonly used precipitator type of precipitator. The plates are usually less than twelve inches apart with

6 6 the charging electrode suspended vertically between each plate. Gas flow is horizontal through the plates. The ESPs are classified based on the operating temperatures: Hot precipitator. A hot precipitator is designed to operate at gas temperatures above 315 o C and is usually of the single stage, parallel plate design. It has the advantage of collecting more particulate from the hot gas stream because particle resistance to collection decreases at higher temperatures. Cold precipitator. Cold precipitator is designed to operate at temperatures around 150 o C. The term "cold" is applied to any device on the low temperature side of the exhaust gas heat exchanger. Cold ESPs are also generally of the single stage, parallel plate design. Wet precipitator. A wet precipitator uses water to aid in cleaning the particulate collection plates. It may employ water spray nozzles directed at the collection plates, or inject a fine water mist into the gas stream entering the precipitator. 1.4 APPLICATIONS In 1991, Electric Power Research Institute reported that 95% of all utility air pollution controls include ESPs. They are used to control particulate emissions in industry (for example, steel mills, pulp/paper plants, cement kilns, and waste incineration), coal-fired electric power plants, and indoors in homes and offices. Boiler application. Parallel plate electrostatic precipitators are commonly employed in the utility industry to control emissions from coalfired boilers. Cold type precipitators are the prevalent type because they are

7 7 the most easily retrofitted. In the design of new installations, the use of hot precipitators has become more common, because of the greater use of lower sulfur fuels. Low sulfur fuels have higher particle resistivity and therefore particulate emissions are more difficult to control with cold precipitation Wood refuse boiler applications. An ESP can be used for particulate collection on a wood fired boiler installation if precautions are taken for fire prevention. The ESP should be preceded by some type of mechanical collection device to prevent hot glowing char from entering the precipitator and possibly starting a fire. Incinerator application. Until relatively recently, ESPs were used for pollution control on incineration units only in Europe. In the United States, however, the ESP is now being viewed as one of the more effective methods for the control of emissions from incinerators. The major problem associated with the use of precipitators on incinerators is high gas temperatures. Temperatures upto 1800 o F can be encountered at the incinerator outlet Steel industry. Dust emission problems in the sintering plant mainly arise from the exhaust gases from the combustion zones and from the ventilation air out of crushers, sieves, coolers, and loading stations. The average dust emission level is about kg/tonne of sinter which is returned to the process when collected. The dust contained in the exhaust gas extracted from the top of the blast furnace mainly consists of iron oxide, silica, and lime. Thermal power plants. In thermal power stations, suspended particulate matter (SPM), sulphur dioxide (SO 2 ), and oxides of nitrogen (NO x ) are the major emissions, resulting from fuel combustion during power generation. ESPs are used in all thermal power plants to control particulate

8 8 emissions. Ash generated in a power plant has nearly 30 percent by weight in 10 m range and 10 percent in 2 m range. Cement plant. The use of ESPs in cement plants has been prevalent since its invention. This is due to its recovery value and the finest and best cement can be collected through ESP. Use of ESP in cement plants has dominated every market in the world. The cement industries with the stringent air borne emission standards have begun to use conventional ESPs for control of emissions in kilns and coal mills. 1.5 ADVANTAGES The advantages of ESPs are listed below: Pressure drop rarely exceeds 15 mm, water gauge. Electrostatic precipitator can operate automatically. Can collect all types of dust, gas mist, droplets, etc. in both wet and dry conditions Can collect all sizes of particles, from microns to coarsers Offers the highest efficiency, can be designed in principle for any efficiency without excessive pressure drop Operates with low operation cost (though initial cost is more) Can operate over a wide range of inlet conditions, i.e., temperature, pressure, dust burden, humidity, etc. Can be built in multiple units, for almost any gas volume Has a long life, comparatively free from abrasion effect due to low operating velocity

9 9 The ESP can be designed to have 99.9 % percent collection efficiency. The high recovery value of the dust collected offsets the cost of the equipment. In steel making, for example, dust can be collected and added to the sinter mixer pelletised in a flying saucer and returned directly to the steel making vessel without treatment. The electrical force acts for the most part on the particles and not the gas. Thus, the pressure drop is very low compared to other control options. 1.6 PROBLEMS IN ESP The problems in ESPs are stated below. Due to the size of a typical ESP and the erratic nature of most processes (especially if frequent start-up and shutdowns occur) the temperature in different parts of the structure could at times drop below the acid dew point. Corrosion can cause structural damage and allow air leakage. An ESP is sensitive to its design parameters. A change in the type of coal used, for example, could drastically affect performance. High capital costs. If particulate emission concentrations are high, a mechanical precleaner may be necessary. High voltages are required. No SO 2 control is possible with an ESP.

10 MOTIVATION FOR PRESENT WORK Breathing clean air naturally makes a person feel better. It may increase the productivity of the person. People consistently exposed to PM 2.5 and PM 10 die prematurely due to lung related illness. To improve air quality standards either the particle emission should be reduced or improve methods to capture particles more efficiently such as ESP. As the former is not achievable under present circumstances, the latter means was adopted. As the ESP is the most widely used particle capturing device, research on avenues to improve its collection efficiency is very essential. 1.8 OBJECTIVES OF THE PRESENT WORK It was identified from the extensive literature survey that the following research works has not yet been carried out to Flow distribution inside an ESP using CFD. Effect of porosity and placement of perforated plates in a wide angle diffuser applicable to ESP s using CFD Modeling of the Discrete phase in an ESP using a UDF taking the mechanical force generated by electric field on particle The above factors are considered in the present work and discussed in the following sections.