Electrical Discharges in Liquid WaterElectrical Discharges in Liquid Water

Michael J. KirkpatrickMichael J. Kirkpatrick

Assistant Professor, SupéleAssistant Professor, Supélecc

24/06/200824/06/2008

OutlineOutline

A few words about plasma in generalA few words about plasma in general

Electrical discharges for water treatmentElectrical discharges for water treatment

• • Electric discharge in liquid waterElectric discharge in liquid water

• • Gliding Arc dischargesGliding Arc discharges

• • Hybrid water-gas discharge reactorsHybrid water-gas discharge reactors

Thermal Plasma

• Lightning and the Sun’s corona are examples of thermal plasma

1,000,000 K or 86eV30,000 K or 2.6 eV

What is a “Non-Thermal” Plasma?

• Defined as a system where free electrons have much higher energy (~12eV) than that of the bulk gas (~0.03eV)

• Can be produced in gases by an electrical discharge using AC, DC, or pulsed fields.

• To produce a discharge in liquid water, a very high and non-homogeneous electric field is required.

Pulsed electrical discharge in water

• Capacitor bank – spark gap circuit can produce pulses with ~20ns voltage rise time.

Plasma discharge in waterPulse forming circuit

Electric discharge in liquid water – physical-chemical aspects

• Chemical: produces reactive species such as hydroxyl radical, hydrogen radical, hydroperoxyl radical, hydrogen peroxide…

• Mechanical: Shockwave production• Radiative: UV and visible emissions

- Two very different discharge regimes: ‘corona’ and arc

Pulsed electrical discharge in water

StrongWeak to moderateUV emission

StrongWeak to moderateShock generation

10-5 – 10-6 s10-7 – 10-9 sPulse rise time

103 – 104 V104 – 106 VPeak voltage

103 – 104 A10 – 102 APeak current

10-2 – 10-3 Hz102 – 103 HzPulse frequency

Pulsed ArcPulsed CoronaParameter

Taken from: Locke et al. “Electrohydraulic discharge and non-thermal plasma for water treatment”, Industrial and Engineering Chemistry Research, 45, 2006.

Electric discharge in liquid water

2.5cm

Water Discharge in Motion

• Large bubbles are nitrogen purge gas for downstream hydrogen measurement.

• Very fine bubbles can be seen near the discharge (if you look very closely)

H2 and H2O2 Production

High voltage = 45 kV, Power = 67 WSolution conductivity = 150 μS/cm

0

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m H

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ol H

2O

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ppm hydrogenmmol hydrogen peroxide

Hydrogen rate = fraction hydrogen at steady state multiplied by carrier flow rate

Hydrogen peroxide rate = slope of concentration vs time line

Rates of Production of HRates of Production of H22, H, H22OO22, and O, and O22

H2

H2O2

O2 *

• HH22 : H : H22OO22 : O : O22 rates have a ratio of 4:2:1 rates have a ratio of 4:2:1

22222 246 OOHHOH

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270 370 470 570 670 770 870 970

Wavelength (nm)

Co

un

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·OH

H alpha

O

Light emissions

Change in hydrogen production with electrode material

Platinum (Pt)Nickel-Chromium (NiCr)

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m)

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ppm hydrogen

mmol H2O2

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pm

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ppm hydrogen

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Platinum

Before

After

Electrode erosion

100 micron100 micron

Nickel-Chromium

Liquid phase contaminantsLiquid phase contaminants

• • PhenolsPhenols• • TrichloroethyleneTrichloroethylene• • PCBsPCBs• • AtrazineAtrazine• • Chemical warfare simulants (H & G)Chemical warfare simulants (H & G)• • Explosives (RDX)Explosives (RDX)

In general, the application of this technique for water treatment is economically challenged, and therefore only interesting for the most recalcitrant compounds…

Physical effects - shockwaves

• Shockwaves can be Shockwaves can be focused to break up focused to break up kidney stones or kidney stones or detach zebra mussels detach zebra mussels from water intake from water intake pipes.pipes.

• Investigations are now Investigations are now being made into the being made into the treatment of ballast treatment of ballast water to kill invasive water to kill invasive species.species.

Taken from: Locke et al. “Electrohydraulic discharge and non-thermal plasma for water treatment”, Industrial and Engineering Chemistry Research, 45, 2006.

Gliding arc discharge for water treatment

Gliding arc with water spray

High voltage power supply

Water levelGround electrode

Hybrid gas-liquid discharge reactors: gas and liquid discharges

HV

HV

High voltageelectrode

Gas phaseplasma

Groundelectrode

LiquidDischarge(over exposed)

“Parallel” reactor

Thank you for your attention!