history of electrical precipitation - university of north ... · pdf filehistory of electrical...
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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
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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
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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)
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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
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Multi-stage wire-plate ESP
Gas inlet
Collected dust to hopper
Flow straighteners
Collection plates
Corona wire electrodes
with rappers
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Electrical Precipitators in use
Wire plate type design
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Electrical Precipitators in use
Courtesy of Dr. Wayne T. Davis, Univ. of Tennesseehttp://members.aol.com/apcutk/esp.htm
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Examples of discharge electrodes
Courtesy of Dr. Wayne T. Davis, Univ. of Tennesseehttp://members.aol.com/apcutk/esp.htm
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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
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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
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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
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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
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Take 5!
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Personal ESP sampler developed at UNC
+dc power
corona collectionsubstrate
inlet
ionizing wire
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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
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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
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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
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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
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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
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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
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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
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Sampling semi-volatile aerosols
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Sampling semi-volatile aerosols
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Sampling semi-volatile aerosols