ESP

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Presentation Transcript

Electrostatic Precipitator (ESP): 

Electrostatic Precipitator (ESP) Electrical migration Electrical mobility Corona discharge ESP theory Charging mechanisms Ash resistivity Flue gas conditioning Power consumption Reading: Chap. 5

Electrical Migration: 

Electrical Migration 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) Electric Field

Millikan Experiment: 

Millikan Experiment (Robert Millikan, US, 1868-1953; Nobel Prize Laureate, 1923) Hinds, Aerosol Technology, 1999 http://www.britannica.com/nobel/cap/omillik001a4.html

Electrical Mobility: 

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

Slide5: 

Q: How can we generate charges? Ozone generation - http://www.mtcnet.net/~jdhogg/ozone/ozonation.html

Slide6: 

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

Slide7: 

Deutsch-Anderson Equation Ac/Q: Specific Collection Area (SCA) 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 Q: How to increase the efficiency?

Slide8: 

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

Charging Mechanism: Diffusion Charging: 

Charging Mechanism: Diffusion Charging Random collisions between ions and particles Q: Does q depend on time? Does q depend on dp? The total number of charges on a particle The total charges on a particle Use esu, not SI units.

Charging Mechanism: Field Charging: 

Charging Mechanism: Field Charging Bombardment of ions in the presence of a strong field Total number of charges by field charging 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. Saturation charge (Zi ~ 450 cm2/stV•s)

Slide11: 

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

Slide12: 

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?

Resistivity/Conductivity: 

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

Slide14: 

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

Slide15: 

Water spray for cement kiln dust Flue Gas Conditioning

Slide16: 

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.

Slide17: 

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

Slide18: 

High Efficiency ESP (>95%) 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 Q: In designing a high efficiency ESP, a smaller drift velocity is to be used. Why?

Power Consumption: 

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

Quick Reflection: 

Quick Reflection