Non-static Collection Process of the Electrostatic Precipitator

Transcription

1 Non-static Collection Process of the Electrostatic Precipitator 79 Non-static Collection Process of the Electrostatic Precipitator HAO Wenge, XIONG Haowang (Northeastern University, Shenyang, 114, China BAbstract: In orer to escribe non-static ust collection in electrostatic precipitator comprehensively an propose a non-static ust collection theory on it, the leaing accumulation an release of the ust layer electronic charge on the collection plate was researche accoring to the electrostatic principle. The calculation equation of the ynamic ust collection electric-fiel intensity that changes with the ust layer thickness was euce. Furthermore, the equation of the ynamic velocity of the charge particle coul also be obtaine. An finally, the non-static electrostatic ust collection theory was propose. The results inicate that the particle velocity was relate to many factors such as particle specific electric resistance, applie voltage an ust layer thickness uring electrostatic ust collection process. The collection efficiency ecreases with the increasing of the particle specific electric resistance. The collection efficiency can reach a maximum value when an optimal applie voltage is got. Multiple laboratory experiments were complete on many kins of particles with ifferent specific electric resistance. The experiment results obey to the theoretic results. The non-static ust collection theory coul explain contraictive points between the actual electrostatic ust collection process an the traitional electrostatic ust collection theory an provie a scientific theoretical founation for the esign of the electrostatic precipitator an the ecision of its operating parameters. 3BKeywors: electrostatic precipitator; accumulate ust layer on the collection plate; accumulate electronic charges; particle velocity; collection efficiency 1B1 INTRODUCTION As high effect ust-cleaning equipment electrostatic precipitator has been wiely applie. However comparative to the process of ust-cleaning technique it evelope slowly. Traitional theory on static uster neglecte influence on static ust collecting of ust eposite on electroe boar. It inicate that ust collecting was a stable process. Practical operation which conflicts with traitional theory is relational with the assumption. Influence of ust eposite on electroe boar on static ust collection has been a penent problem in static ust collection inustry (Zhao Zhibin, Zhang Guoquan, 199; Zhang Guoquan, ). At present, no relational formulas between ust-surface accumulate charge an working voltage, specific electric resistance an thickness of ust layer has been erive. In the operation of electrostatic precipitators the thickness an conuctance of ust layer on ust-cleaning electroe boar certainly will affect conuction an release of inpouring charge. Various amount accumulate charge are forme in ust layer Accumulate charge prouce aitional electric fiel, irection of which is contrary to original fiel. It changes the characteristic of original electric fiel. In fact the electrostatic precipitator is a non-steay-state process. (HAO Wenge 4).The stuy applie basic theory of electrostatics to analyses the inpouring, conuction an release processes of current in ust layer in the course of ust epositing to electroe boar. Finally the formula of particle velocity in non-static collection is obtaine. The experiments of non-static collection prove preciseness of non-static collection theory. The results coul explain many phenomenon that traitional theory conflict with reality, which has guie meaning on esign an selection of parameter in electrostatic precipitator. B THEORETICAL RESEARCH ON THE NON-STATIC COLLECTION PROCESS.1 Accumulation charge of plate seimentary ust layers In the process of collecting ust in ESP, charge particles in the electric fiel run to collecting plate uner the power of electric fiel. A certain thickness ust layer graually forme on electroe boar. Supposing collecting time is t(s), the resistivity of the particles is ρ (Ω m), the charge ust accumulate on the plate layer by the spee of f (m s -1 ), the charge ensity in internal ust is q (cm -3 ), electric fiel intensity is E (V m -1 ), thickness of ust layer is X (m). When the ust layer accumulates to the thickness of x(m), the following relationship come into existence (Bao Chongguang,1993;Blanchar D, Atten P, ): j q t (1) E ivεε E = εε = q () E j = ρ (3) ivj = = Supposing that the electric fiel intensity E ' in the junction between ust layer an ust collection space is a constant, accoring to the electric isplacement bounary conitions, it can be conclue that ε E = εε E '.Then the electric fiel intensity E ' in the junction between ust

2 8 11th International Conference on Electrostatic Precipitation layer an ust collection space is E E ' = (4) ε E electric fiel intensity of electric-fiel (V/m); ε relative ielectric constant of ust layer; ε Vacuum permittivity (C/Vm); j the ensity of electeic current in the ust (A/m ). With the growth of collecting time, the charge particles eposit in collecting boar by the force of electric-fiel. Accoring to Poisson equation the current ensity of the borer can be conclue: j ' = E '/ ρ (5) for j / f = q,it can be simplifie: E ' q ' = q (6) ρ f By-(6) the current ensity of the borer can be seen a constant. For q ' = q o Accoring to continuity equation an Ohm's law, the ifferential equation of ust layer charge ensity istribution can be establishe. q q = (7) t τ x X in the context of (7) points to a ust ensity of the internal charge-istribution function: j E q ( x) = ( )exp[ ( X x)/ fτ ] (8) f ερ f By-(8) can be known that when the ust thickness is for X, accumulate charge of ust in per area for the total Q (C/m ): X E Q= q() xx= ( j )[1 exp( X/ fτ )] τ (9) ερ j the current ensity in the ust-collecting space (A/m ); τ the constant of the ischarge (s), τ = ρεε.. Electric fiel intensity of electric-fiel for ustcollecting Accoring to the ESP characteristics, when the collecting plate is clean (Written by Hyter, Translate by Wang Chenhuan,1984): j = KU ( U U s ) (1) U operation voltage (V); U s ischarge inception voltage (V); ε k K = 3 b, k the mobility ratio (m /VS); b the istance between plates (m). When the ust in the plate is in a certain thickness: j = KU ( jρx)( U jρ X U s ) (11) By-(11), it can be seen that the rop of the voltage significantly reuce the size of the corona current. Then the collection fiel intensity reuces significantly. It affects the capability of electrostatic ust collection. By-(11) the change an size of corona current can be calculate uner a certain thickness ust layer. By-(9), it can be seen that plate seimentary layers of ust accumulation an charge of ust is a value relate to ust resistivity. The bigger ust resistivity is, the more accumulate charge is, an the bigger the strength of the anti-electric fiel is. They have negative infection for the ESP. The size of anti- electric-fiel generate by the surface accumulate charge can be calculate accoring to Gauss theorem: jρε E Ep = (1) Compose with original fiel the real ensity of fiel is ' 3 jρε jb j E = E = 3 ρ ε (13) πε k.3 Particles velocity of non-static electrostatic ust collection With reuce of fiel ensity, the particles velocity ecreases. By-(13), particles velocity can be obtaine uner ifferent collection time, ifferent particles, ifferent supply voltage (current ensity) εε p jb jρε ω = (3 ) (14) με ( + ) πε k ω particles velocity (m/s); p iameter of ust particle (m); μ Gas viscosity coefficient (Pa/s); Through the theory above process it can be seen that: in the electrostatic ust collection process, the accumulate charge generate by seimentary ust layers on base plate of an its voltage rop will reuce the particles velocity. Then the efficiency of ust collecting reuces. To the electrostatic ust collection, there is a best supply voltage, uner which voltage efficiency of ust collection is the highest. Uner the same voltage the collecting efficiency ecline with the rop of particles velocity. When the resistivity of the particles is higher, with the thickening layer of ust, the corona current significantly reuce. Then the particles velocity an collection efficiency reuce. 3 EXPERIMENTAL RESEARCHES ON THE NON- STATIC COLLECTION PROCESS 3.1 Experimental system an metho In orer to test the influence of ust layer for static ust collection efficiency, the efficiency is teste uner ifferent factors Test moel is shown in Fig. 1. Moel uses single-istrict lines. The length of the collecting plate is.7 m; height is.3 m, line-spacing of.1 m. four lines of Corona are establishe on access centers. ESP applies high-voltage power supply by CGD DC. Dust in the electric fiel is accoring to Membrane-law in min (GB/T16157). The usts, such as calcium hyroxie, ashes, aluminum oxie are use in the experiment. At normal temperature the resistivity of the particles were 1 9 Ω cm, Ω cm, Ω cm, respectively. Experiment tests

3 Non-static Collection Process of the Electrostatic Precipitator 81 relationship between working voltage an the collection efficiency, the collection efficiency an the ust thickness, ifferent resistivity of the particles an collection efficiency. 1 Dust Generator; Air istribution plate; 3 The moel of ESP; 4 High-voltage power; 5 Sampler; 6 Fan Fig. 1 Sketch map of experiment evice 3. Experimental results 3..1 Relationship between operating voltage an collecting efficiency In orer to verify the relationship between voltage an collection efficiency, the relationship between voltage an collection efficiency on three ifferent resistivity particles were stuie in Fig. : col l ect i on ef f i ci ency Fig. 1% 9% 8% 7% 5% 4% 3% Ca(OH) ash Al O 3 % wor ki ng voltage/ kv The relation between collection efficiency an the applie voltage From the Fig., to ifferent resistivity of the particles, there is a best-voltage power supply; high resistivity ust (aluminum oxie) is most obvious. Mainly reasons is: in process of electrostatic ust collection, the plate seimentary layer of ust will graually accumulate a certain amount charge, resulting in anti-electric fiel. It makes the spee of the charge particles is relative to a corona current, the resistivity of the particles, an other factors. Theoretical analysis an experimental results show that: when the corona current reache a certain value, particles velocity is largest an collection efficiency is the highest. 3.. Relationship between collection efficiency an resistivity It can be seen from Fig. 3: In the same voltage (3 kv), with the increase of the resistivity, collection efficiency ecrease. The reason is that with the increase of the resistivity, accumulate charge in the ust layer increase which makes anti-increasing electric fiel increase. At the same time the current corona reuces which weakens the intensity of collecting electric fiel. So collection efficiency is reuce. When the resistivity of the particles is very high (ρ ) Ω cm), the anti-electric-fiel coul seriously affect the collection process, making collection efficiency rop significantly. This is consistent with experimental results showe in Fig. 3. Fig. 3 The relation between the particle resistivity an the collection efficiency 3..3 Relationship between t thickness of ust layer an collection efficiency It can be seen from Fig. 4: at the same voltage (3 kv) an entrance ensity (. g/m 3 ) the collection efficiency of high resistivity reuces significantly with the increase of ust thickness. The reason i that ust plate seimentary layer will prouce a certain amount of accumulate charge which may exclue follow-up ust collection. Increase of ust layer will reuce corona current. Efficienc y Fig. 4 Efficiency 1% 95% 9% 85% 8% 75% 7% Ca(OH) ash Al O 3 65% Resistivity /1 Ω cm 7% 5% 4% 3% % 1% % t hi ckness/ mm Relation between thickness of the ust layer an the collection efficiency.

4 8 11th International Conference on Electrostatic Precipitation 3.3 Comparisons between classification efficiency an f theoretical results To calculate classification efficiency applies numerical moeling metho. Substitution non-static theoretical particle velocity into transport equation of charge particle: v ρ ρ + ω E ρ = (15) y y In orer to obtain accurate numerical solution, the bounary conition an turbulence coefficient must be ensure. Because the entrance concentration is homogeneous, the bounary conition on entrance is: ρ = ρ x =. Because the symmetrical characteristic of ust consistency of both sies of corona wire, in the center: ρ = y =. y Supposing laminar bounary layer is δ, length is x.accoring to conservation of particles, the following equation can be create (Miller J et al,1998; Robinson M, 1961; Solati A, 1993): ρ ρ υ ρv1δ ρv1δ + v1δ x + ωρx Ey x ωρx= x δ y t (16) v 1 average spee of Laminar bounary layer, (m/s); δ average thickness of the Laminar bounary layer (m); As ust concentration is a fixe value, then: c c νδ 1 = Ey y= b (17) y Synthesize turbulence mixing coefficient:.5.38 ( 1+ NEHD) ν y cf Ey = (18).4 b( [ 1+ NEHD) Re] c f friction coefficient; Re Reynols number of flui in fiel; Current number; N EHD N EHD i sρkv =, s Corona length(m); ρ g gas ensity(kg m -3 ). Parse (15), exit sectional consistency of ifferent particle iameter are obtaine. Then classification efficiency is figure out. Compare calculate result to experimental result. The experimental ust is fly ash. It can be seen from Fig. 5 that non-static -state theory of electrostatic collecting explains the problem that there is obvious bias between the traitional theoretical an experimental ata. Classification efficiency base on the non-static collection theory inosculates with experimental ata. This is ue to three main sally port on calculation metho of classification efficiency: the anti-electric fiel prouce by seimentary ust layer on polar plate is calculate accoring to non-static ust collection theory, which moify original electric fiel; the turbulence coefficient is euce when using numerical simulation methos to simulate the ust istribution; in the transport formula, the unreasonable points of original bounary conitions are revise. Efficiency Fig. 5 Comparison of the classification efficiency. 4 CONCLUSIONS (1) Base on basic principles of static electricity, relationship-formula about plate seimentary layers of ust accumulation of charge, thickness of ust an resistivity of the particles has been euce. The anti-electric fiel prouce by accumulate charge has been calculate accoring to Gauss theorem. Then the new ynamic particle velocity of charge particle is obtaine. Complete non-static ust collection theory is establishe. Theoretical stuy reveals: With the increase of resistivity in ust layer an thickness of the layer, the anti- electric fiel generate by the ust accumulate charge increases, an the particle velocity reuces. There is a maximum voltage of collecting efficiency () In orer to verify the non-steay-state theory of electrostatic ust collection relationships between collection efficiency an voltage, resistivity of particles an collection efficiency, ust thickness an collection efficiency are stuie by experiment The results show that: With the increase of voltage, the collection efficiency increases. Collection efficiency reaches the highest uner the best voltage. Since then with the increase of voltage, collecting efficiency starts to ecline. With the increase of resistivity of the particles, ust collection efficiency eclines. With the graual increase of the thickness of ust layer, the corona current an collection efficiency eclines. (3) To calculate classification efficiency applies numerical moeling metho. Its results basically inosculate with the experimental ata. Non-static -state theory of electrostatic collecting explains the problem that there is obvious bias between the traitional theoretical an experimental ata, which puzzle omestic an forgien scholastic. REFERENCES 1. Bao C G Theory of electric technique [M]. Beijing: Beijing University of Technology Press (in Chinese).. Blanchar D, Atten P.. Corrletion between current

5 Non-static Collection Process of the Electrostatic Precipitator 83 ensity an layer structure for fine particles eposition in a laboratory electrostatic precipitator[j]. IEEE transactions inustry applications. 38 (3): Miller J, Hoferer B, Schwab A J The Impact of Corona Electroe Precipitator Performance[J].Journal of Electrostatics. 44: Robinson M Movement of Air in the Electric Win of the Corona Discharge [J]. Trans AIEE. 8: Solati A, Aureussi P,Banerjee S Wang C H Electrostatic precipitator of inustry[m]. BeiJing: Metallurgical inustry press (in Chinese). 6. Zhan Guochuan.. ESP technology research in several new trens [J]. China s environmental protection inustry evelopment strategy experts essay Hao Wenge. 4. On ESP technology issues face by the new [J]. China Environmental Science Society in 4 acaemic year will collection Bao Chongguan Electrostatic Principle [M]. Beijing: Beijing Institute of Technology Press % 9% 8% 7% 多依奇数值模拟值 5% 4% 实验值 3% i amat er / μ m 多依奇 :Deutsch 数值模拟值 :Numerical simulation value 实验值 :Experimental value