Electrical Discharges in Liquid Water

1. Electrical Discharges in Liquid Water Michael J. Kirkpatrick Assistant Professor, Supélec 24/06/2008

2. Outline A few words about plasma in general Electrical discharges for water treatment • Electric discharge in liquid water • Gliding Arc discharges • Hybrid water-gas discharge reactors

3. Thermal Plasma • Lightning and the Sun’s corona are examples of thermal plasma 30,000 K or 2.6 eV 1,000,000 K or 86eV

4. 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.

5. Pulsed electrical discharge in water Pulse forming circuit Plasma discharge in water • Capacitor bank – spark gap circuit can produce pulses with ~20ns voltage rise time.

6. 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

7. Pulsed electrical discharge in water - Two very different discharge regimes: ‘corona’ and arc Parameter Pulsed Corona Pulsed Arc Pulse frequency 102 – 103 Hz 10-2 – 10-3 Hz Peak current 10 – 102 A 103 – 104 A Peak voltage 104 – 106 V 103 – 104 V Pulse rise time 10-7 – 10-9 s 10-5 – 10-6 s Shock generation Weak to moderate Strong UV emission Weak to moderate Strong Taken from: Locke et al. “Electrohydraulic discharge and non-thermal plasma for water treatment”, Industrial and Engineering Chemistry Research, 45, 2006.

8. Electric discharge in liquid water 2.5cm

9. 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)

10. H2 and H2O2 Production Hydrogen rate = fraction hydrogen at steady state multiplied by carrier flow rate Hydrogen peroxide rate = slope of concentration vs time line High voltage = 45 kV, Power = 67 W Solution conductivity = 150 μS/cm

11. H2 H2O2 O2 * Rates of Production of H2, H2O2, and O2 • H2 : H2O2 : O2 rates have a ratio of 4:2:1

12. O H alpha ·OH Light emissions

13. Change in hydrogen production with electrode material Nickel-Chromium (NiCr) Platinum (Pt)

14. Electrode erosion Nickel-Chromium Platinum Before 100 micron 100 micron After

15. Liquid phase contaminants • Phenols • Trichloroethylene • PCBs • Atrazine • Chemical warfare simulants (H & G) • Explosives (RDX) In general, the application of this technique for water treatment is economically challenged, and therefore only interesting for the most recalcitrant compounds…

16. Physical effects - shockwaves • Shockwaves can be focused to break up kidney stones or detach zebra mussels from water intake pipes. • Investigations are now being made into the treatment of ballast water to kill invasive species. Taken from: Locke et al. “Electrohydraulic discharge and non-thermal plasma for water treatment”, Industrial and Engineering Chemistry Research, 45, 2006.

17. Gliding arc discharge for water treatment

18. Gliding arc with water spray

19. Hybrid gas-liquid discharge reactors: gas and liquid discharges Ground electrode Water level High voltage power supply

20. HV HV “Parallel” reactor High voltage electrode Gas phase plasma Ground electrode Liquid Discharge (over exposed)

21. Thank you for your attention!