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Numerical modelling in plate-type electrostatic precipitator supplied with pulse energization

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In thermoelectric power plants the electrostatic precipitators are used to dedusting of high gas flow resulted the coal's burning. For a plate-type electrostatic precipitator area was made an analysis upon charging of dust particles and a
  Numerical modelling in plate-type electrostatic precipitator supplied with pulse energization Gabriel Nicolae Popa, Member, IEEE, Corina Maria Dini ş , Sorin Ioan Deaconu, Member, IEEE DEPARTMENT OF ELECTROTECHNICAL ENGINEERING AND INDUSTRIAL INFORMATICS “POLITECHNICA” UNIVERSITY OF TIMISOARA 331128, Revolutiei str., no.5 Hunedoara, Romania Tel.: +40 / 2547541. Fax: +40 / 2547501. E-Mail: URL: Acknowledgements This paper was supported by the project "Development and support for multidisciplinary postdoctoral  programs in primordial technical areas of the national strategy for research - development - innovation" 4D-POSTDOC, contract nr. POSDRU/89/1.5/S/52603, project co-funded from the European Social Fund through the Sectorial Operational Program Human Resources 2007-2013. Keywords «high voltage power converters», «modelling», «power supply», «pulsed power», «simulation» Abstract In thermoelectric power plants the electrostatic precipitators are used to dedusting of high gas flow resulted the coal’s burning. For a plate-type electrostatic precipitator area was made an analysis upon charging of dust particles and a numerical modeling on electric potential and electric field strength for real field duct from ESP, with pulse energization. The numerical analysis was made without clean gas and with dusty gas dust operation of electrostatic precipitator. The analysis was made by finite element method for transversal cross-section of a duct from electrostatic precipitator field, when it is supply with pulse energization. Introduction To separating dust particles from process there are some methods with different principles: gravity separation, inertial separation with cyclones, impact in mechanical filtration, electrostatic charge  particles applied in electrostatic precipitators (tube type and plate type), contacting and impaction in the case of wet scrubbers. For the dedusting of the waste gases resulted further the coal’s burning in the boilers’ firing places, to each boiler is attached one or two plate-type electrostatic precipitators (ESPs) composed, usually, by three or four fields, for separating the dust from the waste gases, placed successively (one after another) each field being able to be sectioned in two zones. Each field or section can be supplied with different types of energization: classical D.C., intermittent energization, pulse energization, and with high frequency power supplies [1,2,3,4].  The configuration of the electrode system of ESP and the shape of the voltage has a strong influence of the charging process of the dust particles [3,5,6]. To obtain maximum electrical field for charging of dust particles it is important to apply maximum voltage between ESP electrodes for operation time.  Some separating methods can be very efficient in collecting the large dust particles, greater than 10 µ m, but the legislative emission levels for dust particles with 1 µ m or less is now essential for a large number of processes. For large process gas streams (over 50 m 3 N /s) is used plate-type electrostatic precipitators. This is the case of thermal power station. The plate-type electrostatic  precipitators collect dust particles between 0.01 µ m -1000 µ m and with dust resistivity between 10 6   Ω⋅ cm-10 14   Ω⋅ cm. Large plate-type electrostatic precipitators have from 2 to several sections, each of them are supply separately from a power supply [1,4]. The Corona discharge servers as ions source for dust particles charging. Near the discharge wires an intense electric field occurs, and along the conductors appears electric discharges (by Corona effect). The ions will be attached to dust particles and the particles will be migrate and deposit to the collecting plates. In an ESP a negative voltage applied between a discharge wires (connected at negative polarity) and a collecting plates (grounding, connected at positive polarity) produce a negative Corona at discharge wires (fig.1). Charging of the dust particles The mathematical modelling of charging dust particles must be taken the following conditions: -   the shape of dust particles is spherical; -   the particles diameters are much smaller than the distance between the particles; -   near the particles, the ion concentration and the electric field are constants. The charging rate for a dust particle with a radius r   p  is [5,6]: , q  p <q s  (1) , q  p ≥ q s  (2) where q  p  is particle charge, ρ i  is ionic space charge density, k  i  is ionic mobility, q s  is saturation charge, and ε 0 =8.854 ⋅ 10 -12 (F/m). The saturation charge is the limiting charge achievable by field charging, and depending on electric field strength and particle radius. (3) ε r   is the electric permittivity of dust particles. If ε r  =1 it is consider that is clean gas, and if ε r  >1 it is dusty gas. Fig. 1: Principle of negative Corona discharge 2 s p0 sii p qq14qk dt dq ⎟⎟ ⎠ ⎞⎜⎜⎝ ⎛  −⋅⋅⋅= ε  ρ  0dt dq  p =   Fig. 2: Electric charge depending on dust particle diameter (1-10) µ m and electric field strength 2 pr r 0 s  r  E 212q  ⋅⋅+⋅⋅⋅= ε ε ε π    The distribution of electric field strength it is non linear between the electrodes, and near the discharge wires it is possible to obtain 50-100 kV/cm. In fig.4,5 it is consider the discharge wire with constant (circular) cross-section and it is placed in the right side of the graph. In the left side of the graph is the collecting plate. Between the electrodes it is applied high voltage (50 kV or 92 kV). The saturation charge q s  is the limiting charge achievable by field charging, and depending on electric field strength E, particle radius r   p , and relative permittivity ε r  . The saturation charge of dust particles depends on the place of particles in the cross-section of the duct. If the particles are near the discharge wire the saturation charges it is much higher comparative with the particles in the middle of the cross-section and near the collecting plates. The collecting high resistance dusts with ESP it is a not well resolved problem. Classical DC energization of ESP To control the Corona power, the line voltage is regulated by a thyristor controller (through phase control) before it is applied to the primary of the high voltage transformer. The high voltage transformer have some turns ratio n to desired high voltage, to obtain Corona effect and than rectified  by a high voltage silicon bridge rectifier. This voltage (ussualy up to 60kV) is applied to a precipitator section without additional filtering. The high voltage is connected that the discharge electrodes have negative polarity and the plate electrodes are connected to the ground. The inductor connected on high voltage line is included to limit the current during sparking or short-circuit in section. The firing angle of the thyristors is determined by a control unit with microprocessor or microcontroller for every half-cycle of the voltage line. There is a low-pass filter and it is used to filter the current with the frequency higher then 50 Hz [2,4].  The following quantities are normally indicated on the high voltage supply: precipitator mean current (I 0nom ); precipitator peak voltage without load (V 0nom ); primary r.m.s. current (I  pnom ); line voltage (V lnom ); frequency (f); apparent input power (S). Fig. 3: Electric charge depending on dust particle diameter (10-100) µ m and electric field strength a. b. Fig. 4: The saturation charge for dust particle diameter 10 µ m depending on position of particles in the duct: a. V=50 kV and b. V=92 kV  Pulse energization of ESP The pulse energization has been used during the last 25 years for high dust resistivity between (10 11 ÷10 13 ) Ω⋅ cm.The pulse energization consists of short duration high voltage pulses superimposed on a D.C. voltage. There are two main circuits which can be used to supply the precipitator sections: first is based on switching at low potential (fig.6) and the second is based on switching at high potential (fig.7) [3]. The pulse energization are different compared with classical d.c. energization: - the high voltage pulses have a high amplitude; - the continous voltage is kept close to obtain the Corona power. The pulse current can be compute with: ( ) t  sin I  )t ( i 0 p p  ⋅⋅=  ω   (4) I  p  is the peak current value and ω  0  is angular frequency of the oscillation. The pulse precipitator voltage is: ∫ =  dt  )t ( i C 1 )t ( u  p F  p  (5) The migration velocity it is an important parameter that it is connected with the electrical operation of ESP fields. The enhancement factor H is:  DC  PE  ww H   =  (6) w PE  is the migration velocity in pulse energization, and w DC  is the migration velocity in classical DC energization. If H>1 than the collection efficiency is better in pulse energization then in classical DC energization, for the same dust [1,2].  The pulse length is between 0.5 µ s÷100 µ s and the pulse frequency is between 1Hz-400Hz at a voltage of approximately 80÷100 kV (fig.8). Fig. 6: Pulse energization - switching at low otential Fig. 7: Pulse energization - switching at high  potential Fig. 8: Voltage and current waveforms in pulse energization  Electric potential and electric field strength The main equations of monopolar Corona model with ions drift are: (7)  ( )  0V k  i  =∇⋅⋅⋅∇  ρ   (8) V is electric potential, ρ i  is ionic space charge density, and k is ionic mobility. A number of different techniques have been applied to compute (4) and (5): finite element method (FEM), finite difference method (FDM), charge simulation method (CSM) [3,4,5]. It was used the PDE toolbox from Matlab 7 [7,8]. The area from the duct was represented at scale, in transversal cross-section. The initial condition for the spatial charge was established according to literature at 10 -5  C/m 3  (low ionic space charge density)-fig.10,12,14,16, and 15 ⋅ 10 -5  C/m 3  (high ionic space charge density) – fig.11,13,15,17. The field voltage has the value of 50 kV and 92 kV (for high voltage pulse). In conditions of a gas with dust, it was evaluated ε r  =15, and in conditions of a clean gas (without dust) ε r  =1. In fig.9 was established the mesh of the duct cross-section with profiled collecting  plates and five discharge wires (rectangular shape, ISODYN B5 type) placed in the middle, from a duct of an industrial ESP. The mesh is detailed achieved near-by the discharge wires. In fig.10-13 was achieved the high electrical voltage distribution and in fig. 14-17 was achieved the electrical field strength distribution. In simulations, the duct it is composed by two collecting plates and five discharge wires (in the middle of the graph). In fig. 10-17 s is the distance between the collecting plates and discharge wires, and l is the length of the analyzed duct. The electric potential has some difference in shape, for the same voltage when the ionic space charge density it is different (10 -5  C/m 3  or 15 ⋅ 10 -5  C/m 3 ). When it is the dusty gas there it is not an essential difference between the electric potential at different ionic space charge density. For low ions space charge density and when the gas is dusty the high voltage it is much concentrated near the discharge wires. 0i2 V  ε  ρ  −=∇   Fig. 9: The mesh in transversal cross-section of the duct, consists of 77568 nodes and 154176 triangles   Fig. 10: Electric potential (3D) in transversal cross-section of the duct, with dust and without dust, V=50kV, and ρ i =10 -5  C/m 3  
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