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Electrostatic Precipitator (ESP)

Yang. Ying. Positive. Negative. Cation. Anion. Woman. Man. Love. Hate. Attraction. Repel. Heaven. Hell. Democrat. Republican. Peace. War. Electrostatic Precipitator (ESP). Electrical migration Electrical mobility Corona discharge ESP theory Charging mechanisms

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Electrostatic Precipitator (ESP)

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  1. Yang Ying Positive Negative Cation Anion Woman Man Love Hate Attraction Repel Heaven Hell Democrat Republican Peace War Electrostatic Precipitator (ESP) • Electrical migration • Electrical mobility • Corona discharge • ESP theory • Charging mechanisms • Ash resistivity • Flue gas conditioning • Power consumption Reading: Chap. 5

  2. Electrical Migration Electric Field • Coulomb’s law • Statcoulomb (stC): the charge that causes a repulsive force of 1 dyne when 2 equal charges are separated by 1 cm (3.3310-10C) • Unit charge: 4.8 10-10stC (1.610-19C) (q=ne)

  3. Millikan Experiment (Robert Millikan, US, 1868-1953; Nobel Prize Laureate, 1923) Hinds, Aerosol Technology, 1999 http://nobelprize.org/nobel_prizes/physics/laureates/1923/millikan-bio.html

  4. Electrical Mobility • Terminal velocity in an electrical field (electrical migration velocity/drift velocity) May the force be with the particles! (force balance) (for Re < 1) Q: Difference between cyclone and ESP in terms of forces acting on the system? What’s the effect? Q: What is the physical meaning of electrical mobility? Q: When does a particle have a higher mobility?

  5. Electron Molecule Corona Discharge Particle Positive Corona Negative Corona - + - + Step 1 - + + - Step 2 + + - + + - + Step 3 - - + + - + Step 4 - Electrode Collection Plate Electrode Collection Plate Q: How can we generate charges? Ozone generation - http://www.mtcnet.net/~jdhogg/ozone/ozonation.html

  6. Electrostatic Precipitator Turbulent Flow with Lateral Mixing Model 2 3 1 2 1 3 (20) (12) (8)

  7. Turbulent flow: uniformly mixing • Perfect Collection • The fraction of the particles removed in unit time = the ratio of the area traveled by drift velocity in unit time to the total cross-section • Deutsch-Anderson Equation Ac/Q: Specific Collection Area (SCA) Q: How to increase the efficiency?

  8. Q: An ESP that treats 10,000 m3/min of air is expected to be 98% efficient. The effective drift velocity of the particles is 6.0 m/min. (a) What is the total collection area? (b) Assuming the plates are 6 m high and 3 m long, what is the number of plates required? 6 m 3 m Internal Configuration: self-review

  9. (ci ~ 2.4104 cm/s) Charging Mechanism: Diffusion Charging • Random collisions between ions and particles The total number of charges on a particle Use esu, not SI units. The total charges on a particle Q: Does q depend on time? Does q depend on dp?

  10. Charging Mechanism: Field Charging • Bombardment of ions in the presence of a strong field Total number of charges by field charging (Zi ~ 450 cm2/stV•s) Saturation charge Q: Is the charging rate dependent on particle size? On field strength? On time? On material? Aerosol Technology, Hinds, W. C., John Wiley & Sons, 1999.

  11. Comparison of Diffusion & Field Charging • Nit = 107 s/cm3 • = 5.1 E = 5 KV/cm T = 298 K Q: Does collection efficiency increase as particle size increase (because of a higher number of charges)?

  12. Typical fly ash size distribution Q: If the ESP is used to collect the fly ash, how will the particle size distribution at ESP outlet look like?

  13. Resistivity/Conductivity • Impact of particles’ resistivity on ESP’s performance: • Factors: temperature, composition • Flue gas conditioning 109 - 1010ohm-cm is desired Q: How does resistivity affect an ESP’s performance?

  14. Effects of sulfur content and temperature on resistivity Q: Is S in coal good or bad?

  15. Flue Gas Conditioning Water spray for cement kiln dust

  16. Effective drift velocity as a function of resistivity by measurement Use the same Deutsch-Anderson Equation with new we. Q: Estimate the total collection area required for a 95% efficient fly-ash ESP that treats 8000 m3/min. The ash resistivity is 1.6×1010 ohm-cm.

  17. Good for moderate collection efficiency (90% ~ 95%)

  18. High Efficiency ESP (>95%) Q: In designing a high efficiency ESP, a smaller drift velocity is to be used. Why? Matts-Ohnfeldt Equation Use k = 1 for fly ash k = 0.5 or 0.6 for industrial category • Rule of Thumb • Below 95%, use Deutsch-Anderson Equation • Above 99%, use Matts-Ohnfeldt Equation • Between them, use an average

  19. Power Consumption • Corona power • Drift velocity • Efficiency vs. Corona Power Power density ~ 1-2 W/ft2 k = 0.55 for Pc/Q in W/cfs up to 98.5%

  20. Quick Reflection

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