History of Electrical Precipitation

Date1600

1745

1824

1878

1885

SignificanceWilliam Gilbert, English court physician, publishes De Magnete

Benjamin Franklin describes the effects of points “in drawing and throwing off the electric fire.”

M. Hohlfeld, German mathematician, describes the precipitation of fog in a jar containing an electrified point

R. Nahrwold notes that the discharge from an electrified sewing needle surrounded by a tin cylinder greatly increases the collection of atmospheric dust. Nahrwold repeats the experiment with a glycerin coating to help particles adhere.

Sir Oliver Lodge attempts, unsuccessfully, to remove lead fume from from a smelting works in North Wales

History of Electrical Precipitation

• Frederick Cottrell– Incorporated more reliable rectifier

transformer circuits in ESP design - able to sustain higher voltages

– Successfully collected sulfuric acid mist in Berkeley, CA laboratory in 1906

– First successful commercial precipitator used to collect H2SO4 in Pinole, CA 200 cfm capacity

– 1912, large scale ESP used to collect cement kiln dust at 1,000,000 cfm in Riverside CA

Frederick Cottrell1877 - 1948Source: U.S. Department of Agriculture

Advantages to Electrical Precipitation

Electrostatic Precipitators (ESPs):- collect particles from 0.01 µm to 100 µm with 99% efficiency- operate at high temperatures, up to 1200° F (650° C)- operate at high gas pressures, up to 150 psi (10 atm)- operate at high flow rates, up to 3,000,000 cfm (1500 m3/s)- operate at high particle loadings, 500 grams/m3

- have low energy costs, 200 – 1000 Watts/1000cfm- have low pressure drop

ESPs can be used when:- large volumes of particulate air pollutants are produced- no explosion hazard exists- high efficiency needed- continuous processes (expensive to build but inexpensive to operate)

Industries and their pollutants where ESPs are commonly used

Process Principal Material CollectedElectrical Utility Fly Ash (SiO2, Al2O, Fe2O3)

Industrial Boiler Houses Fly Ash

Steelmaking Furnaces Iron Oxide (Fe2O3)

Cement Kilns Calcium Oxide, Silicon Oxide

Pulp and Paper Sodium Sulfate

Metal Machining Oil Mist

Multi-stage wire-plate ESP

Gas inlet

Collected dust to hopper

Flow straighteners

Collection plates

Corona wire electrodes

with rappers

Electrical Precipitators in use

Wire plate type design

Electrical Precipitators in use

Courtesy of Dr. Wayne T. Davis, Univ. of Tennesseehttp://members.aol.com/apcutk/esp.htm

Examples of discharge electrodes

Courtesy of Dr. Wayne T. Davis, Univ. of Tennesseehttp://members.aol.com/apcutk/esp.htm

Practical considerations:Removing collected dust

• Collected particles must be disposed of properly

• Dust coated electrodes can- lower electric field strength- increase likelihood of spark - cause back corona

• Result: Decreased collection efficiency

• Methods used to clean collecting plates- Wire - cylinder design: washing- Wire - plate design: rapping

Practical considerations:Removing collected dust: Rapping

Electrode rapping

Collecting plate rapping

Courtesy of Dr. Wayne T. Davis, Univ. of Tennesseehttp://members.aol.com/apcutk/esp.htm

Practical considerations: Dust resistively

• Highly insulated particles are poor conductors– Resistive to charging– Not easily collected

• Particle resistivity (Ω-cm) related to:– Elemental composition– Moisture content of air– Gas temperature– Above 1010 Ω-cm, particle collection becomes difficult

• Conditioners– Added to gas stream to increase particle conductivity– Examples include: H20, NH3, H2SO4

Practical Considerations: Particle re-entrainment

• Re-entrainment occurs when collected particles are re-released into the air stream

• Sources of re-entrainment:– Highly turbulent flow – velocity concentration– Rapping – observed as ‘puffs’ exiting the precipitator– Back corona

Take 5!

Personal ESP sampler developed at UNC

+dc power

corona collectionsubstrate

inlet

ionizing wire

ESP sampler current-voltage characteristics

0

50

100

150

200

250

300

0 1 2 3 4 5 6 7

Voltage, kV

Cur

rent

, µA

10 mil #1

10 mil #2

10 mil #3

Current ~ ion concentration, NiVoltage ~ electric field strength, E

Ozone output vs. ESP power

0

100

200

300

400

500

600

0 500 1000 1500 2000

Power, mW

Ozo

ne, p

pb

20 mil #110 mil #120 mil #210 mil #220 mil #310 mil #3

ESP collection efficiency vs. flow

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

0.01 0.1 1 10

Particle Diameter, microns

Col

lect

ion

Effic

ienc

y

2.04.0

6.08.0

Flow, Lpm

Review: Semi-Volatile Compounds

10-8 torr < vapor pressure < 10-2 torr

Semi-volatile aerosols:- exist in both particle and vapor phases- can readily transfer mass between phases- important for exposure – health effect studies- lung deposition behavior- atmospheric transport- emission regulations

evaporation

particlephase

adsorption/absorption

vapor phase

Filters cannot sample semi-volatile mists accurately

• Metalworking fluids are semi-volatile

• Particles evaporate from filter over time

• Underestimation of worker exposure

Mist Vapor loss

Filter

ESP Advantages, Disadvantages

Advantages• Collection substrate has low surface area

– lower vapor adsorption artifact• Collected particles coalesce together

– less potential for particle evaporation artifact

Disadvantages• Corona discharge generates O3

– some potential for chemical artifact

Comparison of sampling methods for mineral oil mist

PVC PTFE GF ESP DataRAM DustTRAK0.0

0.2

0.4

0.6

0.8

1.0

Mis

t Con

cent

ratio

n, m

g/m

³

filters

optical particle counters

Sampling semi-volatile aerosols

Sampling semi-volatile aerosols

Sampling semi-volatile aerosols