High Temperature-High Pressure Effect on Performance of an Electrostatic precipitator

This paper presents a finite difference numerical model to calculate the effect of high temperature-high pressure on V-I curves for negative corona discharge of an electrostatic precipitator . The simultaneous solution for the governing Poisson's and current continuity equations helps in predicting the voltage –current characteristics of precipitator ,the results at atmospheric conditions are considered as a reference. Electrical characteristics were evaluated for an operating of (1,2,3and 4bar)pressures and temperatures (293,400,500,600 and 700 K). The numerical results are compared with experimental data from the literature, and the agreement is excellent. :ةصلاخلا ةريتلىفلا اري ح ىرلع طغرضلا و ةساشرحلا ةرجسد شيثأرت اردحل ايربايس ارجرىم جرحنلا اذرا هشرعي برسشملل ساريتلا اع ر ررت بلارردلا يلارهلا لريشفتلا ىررلع دارمتعتالا يكيتاترسوشهكلا رن تلل ةيىرس ساريتلا ةيساشمترسا ةرلداع و عىرساىلا ةرلد ةيتلىفلا صئاصخلا ارد رت جرحنلا ازرا يب. ةر سا ملل سذرصمك ةيرساي لا اوشرالا ذر ع دئارت لا دارمتعا رتو برسشملل سايتلا ةساشرحلا ارجسد يرب برسشملل ةريئالاشهكلا صئارصخلا (293,400,500,600 & 700 K) طغرضلاو bar 4 & 3 , 2,1 ) ت اضيأ . ا ذ ىلإ ةلىن ة سا ملا ت اكو ةيلمعلا دئات لا ع ةيلاادحلا دئات لا ة سا 1-Introduction Electrostatic precipitator (ESP) is the most widely used device for particulate emission control . ESP is a physical process by which particles suspended in gas stream are charged electrically ,and under the influence of electric field are separated from the gas stream . The precipitation system consists of appositively charged collecting surfaces and a high voltage discharge electrode wire are placed centrally between the plates. A corona discharge, occurs close the negative electrode, ,setting up an electric field between the emitter and the charged surface. The voltage-current ( V-I ) characteristics are one of the major diagnostic tools being frequently used to monitor an ESP's performance. Typical new equipment design efficiencies are between 99and 99.9%[1]. But several factories determine ESP collection efficiency, ESP size, maximizing electric field strength ,dust resistively, gas temperature ,chemical composition and particle size distribution[2]. The most economical design and operation of particles fall within certain desired limits. If the resistivity is low, particles give up their charge too easily and become re-entrained in the gas flowing past the collecting electrode . On the other hand ,high resistivity particles cannot lose their charge upon reaching the collector electrode because of low conducting of the material deposited earlier. As a result , the effective field strength is reduced and the efficiency falls. Particle resistivity can be controlled in many cases by selecting the proper operating temperature or by injecting water to reduce the resistivity. The disadvantage of injecting water is the increased complexity of the wash and the fact that the collected slurry must be handled more carefully than a dry product ,adding to the expensive . Thomas and Wong [3] studied the effects of temperature and pressure on electrical characteristics of ESP using stainless and platinum wires . They commented that the corona is affected by electrode material but did not quantify the effect. , Vol. 6 No.2 Scientific. 2008 Journal of Kerbala University 48 The variation in resistivity with changing gas temperature for six different industrial dusts was discussed in (U.S.Environmental Protection Agency 1985) [4] For most dusts, resistivity will decrease as the gas temperature increase. George Rinard et al. [5] measured the electrical operating characteristics and collection efficiencies for an operating pressure of (6.4 bar) and temperature near 870 ° C with both negative and positive corona of tube type ESP. In positive corona the curves were spark-limited and with negative corona the maximum voltage was limited by the power supply. Operating gas temperature of the dust is key factor influencing dust resistivity and must be carefully considered in the design of an ESP Awma [ 6 ]. Junhong Chen et al. [7] developed a numerical model of the negative DC corona plasma in a dry air.A parametric study is conducted to examine the effects of linear current density (0.1-100 μA per cm of wire length), wire radius (10-1000 μm ), and air temperatures (293800K) on the distribution of electrons and the Townsend second ionization coefficient, in the negative corona ,energetic electrons are beyond the ionization boundary and the number of the electron is greater than in the positive corona . The number of electrons increases with increasing temperature. A numerical model developed by Biskos et al. [8] to describe the performance of the ESP is used to predict the behavior of the charger when some operating conditions are changed, the numerical model show better agreement with the experimental results at sub-atmospheric pressure. It is shown that greater ion concentrations are achieved at lower pressure. 2-Description of the Negative DC Corona Discharge Several things happen very rapidly ( in a matter of a millisecond ) in the small area around the discharge electrode. Figure (1) illustrates the negative polarity.High negative voltage is applied to the small radius wire and the plate or cylinder is grounded. A high negative voltage is applied to the small radius wire and the plate or cylindrical is grounded. The corona discharge is initiated when the electric field near the wire is sufficient to ionize the gaseous species . The minimum electric field in a dry air is a function of the wire radius,the surface roughness of the wire, air temperature and pressure[9]. The corona discharge is normally used at atmospheric pressure. Negative corona is only possible in electronegative gases,such as oxygen, water vapor and carbon dioxide. It dose not occur in the pure gases such as nitrogen ,hydrogen,helium and argon that have no affinity for electrons. Figure ( 1 ) Corona Generation[7] , Vol. 6 No.2 Scientific. 2008 Journal of Kerbala University 48 In contrast to the uniform positive corona discharge ,negative corona discharge appear as discrete points or tufts along the wire . At voltages near the corona onset voltage , only a few tufts appear . They are irregularly spaced along the wire and preferentially appear at imperfections on the surface . As the voltage is increases and the distribution of tufts becomes more uniform[10] . Seed electrons to initiate the ionization process are produced by naturally occurring ionization events . The free electrons produced in the initial ionization process are accelerated away from the wire in the imposed electric field . Inelastic collisions of electron and neutral gas molecules produce more electron –positive ion pairs in a self-sustained process referred to as the electron avalanche[11]. Secondary electrons to sustain the discharge may be produced by photoemission from the discharge electrode ,bombardment of the discharge surface by positive ions or photo ionization in the gas[12] . The mean kinetic energy of ions in the corona plasma is on the order of (0.010.1ev). These ions are not energetic enough to knock out electrons from the electrode . On the other hand ,short –wavelength photons emitted in the corona discharge are energetic enough to either ionize the gaseous species or to extract electrons from the electrode surface . Since the work needed to remove electrons from the electrode surface ( approximately 4 to 5 eV [13] for metals most likely to be used in corona discharge devices ) is considerably less than the ionization energy of oxygen and nitrogen molecules ( 12.06 eV for O2 and 15.6 eV for N2[13] ) the yield of photoelectrons from the electrode surface is mach higher than that from the gas . As a result ,the most significant mechanism for the generation of the secondary electrons is photoemission from the discharge electrode surface[14]. The yield of photoelectrons depends on the wavelength of photons as well as the work function of the discharge electrode material. -Mathematical Model 3 The mathematical analysis is perform by solving simultaneously Poisson's and current continuity equations ,by Finite Difference Method. The equations describing the potential distribution and charge density are obtained from the following [15]. ( 1 )       V 2