Effects of particle space charge density and turbulent diffusivity on the performance of plate-plate electrostatic precipitators

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J ,f.,osol SCI.• Vol. 21, 5uppl. I. pp. 5199-5200. 1996

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Pergamon

PH: 80021-8502(96)00172-3

1996 Elsevier Science Ltd Copyright Printed in Great Bntain. All rights .....rved 0021.8502/96515.00+0.00

EFFECTS OF PARTICLE SPACE CHARGE DENSIlY AND TURBULENT DIFFUSIVITY ON THE PERFORMANCE OF PLATE-PLATE ELECTROSTATIC PRECIPITATORS S. J. PARK and S. S. KIM

Heat andMass Transfer Lab. Department ofMechanical Engineering Korea Advanced Institute of Science andTechnology 373-1, Kusong-Dong, Yusong-Gu, Taejon, 305-701, Korea KEYWORDS Plate-Plate Electrostatic Precipitator; ParticleSpace ChargeDensity; TurbulentDiffusivity

The behaviorof charged particles in an electrostatic precipitator is largely determined by the electric and gas flow field. Generally the electric field has been considered to be uniform in the collection part of two-stage electrostatic precipitators, which is reasonable if space chargedensity is negligible. However, the electric field may be enhanced as the particle space charge density increases (Palotai et aI., 1987). The enhanced nonuniform electric field affects the particle concentration profile in an ESP. And the gas flow field with large turbulent fluctuation activates the particle turbulent diffusion, so that the collection efficiency decreases. (Leonard et al., 1980, 1982) Understanding of these phenomena has to proceed for the optimal configuration design and the economical operating conditions in electrostatic precipitators. However, the interactions among the electric field, the particle space charge density and the turbulent particle diffusivity have not been investigated thoroughly, influencing the particle transport. In the present study, the particle concentration profileand the collection efficiency are calculated from the Poisson equation of the electrical potential and the modified convective diffusion model considering the particle space charge densityabout the plate-plate collection part of the two-stage ESP. The duct is Scm wide and SOcm long. The particles with a mean diameter of 3.SJ.ll11 precharged saturatelyare entered into the collector of the ESP. The electrical mobility of particles is 0.022cm2N-s, the electrical migration velocity is 0.3m/s, and the gas mean velocity is 3m/s. Particle turbulent diffusivities are 0.6 and 30 cm2/s and space charge densities are le-' - le-3 C/m3 • The space charge is due to ions and precharged .particles. The contribution of ions may be negligible in the collector of the two-stage ESP since the corona discharge is not occured. So the space charge density is proportional to the particle numberconcentration multiplied by the charge of a single particle for monodisperse particles; Pc.P =ql" For a given particle space charge density, governing equations are

V2V = - Pc.P and (u + pE). V" = DV 2n - PPc. Pn Eo

Eo

where V, Eo, U, p, E, and D are electrical potential, permittivity constant, gas velocity, particle mobility, electric field intensity, and particleturbulentdiffusivity, respectively. Figure I shows electrical potential profiles for turbulent diffusivity of O.6cm2/s. When the space charge d.ensi~ from entered pa~cles is le-3C/m3, the electrical potential is enhaced and the inner voltage IS higher than Vo apphed on the lower plate at inlet. However the particles are collected on the plates along the flow direction, so that the electrical potential is reduced to become uniform. The computational results show that the precipitation efficiency increases with 5199

5200

Abstracts of the 1996 European Aerosol Conference

the incoming particle space charge density regardless of the values of the particle turbulent diffusivity below a critical Deutsch number (De, nondimensional length of ESP in the flow direction). Onthe otherhand, overthe critical Deutsch number, the efficiency decreases andthen increases as the incoming particle space charge density increases for the small particle turbulent diffusivity. (Fig. 2)

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Fig. ) Electrical potential profiles; (a) on the incoming space charge density, SCDO=le3C/m3 at xl=5cm, x2=25cm, and x3=45cm; (b) at x=5cm on the different incoming space 3 charge densities, SCOI=le-6, SC02=le-5, SC03=le-4, and SCD4=)e-3 C/m ; particle 2/s. turbulent diffusivity, 0=0.6cm 1.0,....------------, 0 1 -_ _

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Fig. 2 Collection efficiencies on incoming space charge densities for different particle 2/s. turbulent diffusivities at x1d= 10; DI =0.6cm%, 02= 1.2cm%, D3=1Ocm 2/s, and D4=30cm REFERENCES

Kihm, K. D., Mitchner, M., and Self, S. A. (1987) Comparison of wire-plate and plate-plate electrostatic precipitators in turbulent flow. J. Electrostatics 19, 21. Leonard, G. L., Mitchner, M., and Self, S. A. (1980) Particle transport in electrostatic precipitators. Atmospheric Environment 14, 1289. Leonard, G. L., Mitchner, M., and Self, S. A. (1982) Experimental study of the effect of turbulent diffusion on precipitator efficiency. J. AerosolSci. 11,271. . Palotai, T., Berta, 1., Czeiler, A., Szedenik, N., and Koltai, M. (1987) Space charge effects m electrostatic precipitation. Proceeding of the 3rd international conference on electrostatic precipitation, International steering committee, Abano-Padova, 527.