Research and Technology Initiatives

Environmental and Biological Applications of (Micro)Plasmas

Kurt H. Becker Polytechnic Institute of New York University, Brooklyn, NY, USA

Thanks to many collaborators: • Christos Christodoulatos • Agamemnon Koutsospyros • Shu-Min Yin • Abe Belkind • Jose Lopez • WeiDong Zhu • Nina Abramzon • Sophia Gershman • Oksana Mozgina • Erich Kunhardt

Thanks to many sponsors: NSF, NASA, Ozonia, US Army, AFOSR, ONR, FCE Research and Technology Initiatives

Environmental Applications: Biological Applications: • Electrostatic Precipitators • Bio-decontamination • Ozonizers (briefly) • Biofilm inactivation • VOC Destruction (in low-flow applications) • Bio-medical application • Preparation of fuel cell feed gas • Pulsed electrical discharges in liquids

FROM: B.M. Penetrante and S.E. Schultheiss “Non-thermal Plasma Technologies for Pollution Control” Proc. NATO-ASI, Vol. 34, Plenum Press, New York (1993) “Non-thermal plasmas have an enormous potential of becoming the leading technology for the remediation of environmental pollutants in the near future”

15 Years Later: Much of the “enormous potential” remains unrealized - why ??? What are the challenges, where are the opportunities ??? Research and Technology Initiatives TO DATE: Only 2 commercial plasma-based technologies in the environmental field I. Electrostatic Precipitators (using corona discharge plasmas): Removal of particulates from gas streams

• mature technology • large industrial scale • economical & efficient • reliable • little unknown science • some engineering issues Research and Technology Initiatives II. Ozonizers (using dielectric barrier discharge, DBD, plasmas):

Generation of ozone (O3) for disinfection applications Research and Technology Initiatives Industrial Ozonizer

• mature technology • large industrial scale • fairly economical • fairly reliable

• not efficient (< 20% O3) • some unknown science • engineering issues

Science Issues (correlation with O3 generation efficiency): • effect of feed gas mixture, pure O2 vs. air (O2/N2); exact N2 admixture • effect of HC contaminants • effect of electrical power coupling to DBD • plasma – surface processes • materials issues (exposed electrode, dielectric coating) • degree of DBD filamentation Research and Technology Initiatives Filamentary DBD and O3 Generation (complex interplay between plasma chemistry & discharge physics)

• low power, many weak filaments

• low O3 generation efficiency at low O3 background concentrations

• high O3 generation efficiency at high O3 background concentration

• high power, few strong filaments

• low O3 generation efficiency at high O3 background concentrations

• high O3 generation efficiency at low O3 background concentration Research and Technology Initiatives One Solution: Intelligent Gap System (IGS) • Adjust microdischarge (gap, coating) • Adjust fraction of total applied power (gap, capacitance)

O3 Strong Microdischarges get weaker microdischarges

High O3 Low O generation, but also much less O destruction generation 3 3

Optimize O3 outlet concentration at >20% and maintain over time Research and Technology Initiatives Ozone Generation in DBDs

State of the Art:

• Larger ozonizers can produce up to 100 kg of O3 per hour • O3 concentrations are typically 18 wt% in O2 and 6 wt% in air • Use of O2 requires <50 ppm HC contamination • Energy for 1 kg of O3 is 8 kWh for O2 and up to 20 kWh for air • Cost is about $2 per kg of O3

Future Prospects:

• Novel concepts (e.g. the IGS) can push max. O3 concentration to >20% • Advances in power semiconductors (improved gate turn-off thyristors and insulated gate bipolar transistors which can switch 1 kA at 5 kV) will reduce size of ozonizers by eliminating the need for step-up transformers and allow use of more efficient excitation waveforms • Use of homogeneous self-sustained volume discharges may lead to

more favorable plasma conditions for O3 generation Research and Technology Initiatives Application of Low-T Plasmas in ‘High Potential’ Area:

Removal of VOCs, SOx, and NOx from Gaseous Streams

Low-T plasmas have been used in bench-scale applications to: • convert VOCs in gaseous waste streams

• convert SOx and NOx in gaseous waste streams • use in high-flow and low-volume applications • convert contaminants in Diesel exhaust • prepare feed gas for fuel cell

Possible show-stoppers preventing industrial-scale applications : • by-product formation (characterization, control) • carbon closure (accounting for fate of all C atoms) • competing technologies (advanced oxidation techniques, catalysts, …) • energy efficiency • economics (cost of manufacture, cost of operation, …) Research and Some Low-T Plasma Concepts Technology Initiatives Capillary Plasma Electrode (CPE) Concept metal dielectric (pulsed ) dc, ac, or rf voltage dielectric metal

Capillary Plasma Electrode (CPE) Realizations

Cylindrical Electrodes Solid Pin Electrodes Hollow Pin Electrodes (Longitudinal Flow) (Cross Flow) (Flow-Through) Experimental Setup Gas Preparation 0–1500ppm(v) 50 – Osindry air VOCs influent Plasma Reactor Measurement I-V, Power effluent On-Line Analysis Off-Line Analysis Off-Line GC-MS (gasphase) Solvent Extraction FTIR Absorption Carbon Trap GC-FID GC-MS

Exhaust Removal of n-Heptane in an Annular Plasma Reactor (residence time: 0.6 s; initial concentration: 700 ppm)

800 100

640 80

480 60

320 40

160 20 Removal Efficiency, % Effluent Concentration, ppm Effluent Concentration, 0 0 0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50

Specific Energy, J/cm3 Removal of Toluene in an Annular Plasma Reactor (residence time: 0.6 s; initial concentration: 490 ppm)

400 100

80 300

60

200

40

100

20 Removal Efficiency, % Effluent Concentration, ppm Effluent Concentration,

0 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 Specific Energy, J/cm3 Destruction of Toluene in a Cross-Flow Rectangular Capillary Plasma Reactor vs. Residence Time (specific energy 1.5 J/cm3; initial concentration 490 ppm)

150.0 100.0 140.0 90.0 130.0 120.0 80.0 110.0 70.0 100.0 90.0 60.0 80.0 50.0 70.0 60.0 40.0 50.0

30.0 Destruction Efficiency, % 40.0 Effluent Concentration, ppm Effluent Concentration, 30.0 20.0 20.0 10.0 10.0 0.0 0.0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Residence Time, s Benzene Destruction

Initial contaminant concentration: 200 - 1200 ppm(v) flow rate: 2 - 8 l/min; residence for maximum destruction efficiency

100

90

80

70

60 A-CPE Reactor 50 40 CF-CPE Reactor 30

20

Destruction Efficiency (%) 10

0 0246810 Energy Density (J/cm3) Destruction Efficiency of 4 compounds with Different Ionization Energies vs. Specific Energy (Identical Conditions) (shortest residence time for ~ 100% Xylene destruction)

100 Xylene IE = 8.44 eV 90 IE = 8.77 eV 80 Ethylbenzene IE = 8.83 eV 70

60 IE = 9.24 eV Toluene 50

40 Destruction Efficiency, % Benzene 30

20 0.5 1.0 1.5 2.0 2.5 3.0 Specific Energy, J/cm3 Destruction Efficiency of the four BTEX Compounds vs. Degree of Substitution and Ionization Energy

Benzene Toluene Ethylbenzene Xylene

Destruction Efficiency

Degree of Substitution

Ionization Energy Research and Technology Initiatives Kinetic Model

• Basic assumptions – Plug flow conditions prevail throughout the reactor (verified by Reynolds and Dispersion numbers) – The temperature remains constant, thus the density of the gaseous influent and effluent streams remain constant • Mass balance around the reactor

C = Co exp (-kd ES); E = 1 – exp (-kd ES)

Co, C = influent, effluent contaminant concentration E = contaminant degradation efficiency

ES = energy density Research and Technology Initiatives Kinetic Studies

• 3 model compounds: toluene, ethylbenzene, and m-xylene

• Influent flowrate and contaminant concentration tightly controlled (i.e. approximately constant to within ±3%))

• Five sets of experiments at input power: 20, 30, 40, 50, and 75 W

• At each power setting influent stream the flow rate was varied in the range from 2.0 - 8.0 l/s

• Influent target compound concentration was constant at 265 ppm, 270 ppm, and 155 ppm for toluene, ethylbenzene, and m-xylene

• Effluent concentration is plotted vs. energy density Research and Technology Initiatives Ethylbenzene Destruction 300

250

200

150

100 C = 275.2 exp(-0.519 E)

50 R2 = 0.953

Effluent Concentration (C), ppm Effluent Concentration 0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Energy Density E, J/cm3 -0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 Absorbance Plasma On Plasma Off 3300 3200 etuto fTlee–FTIRSpectra Destruction ofToluene – C-H str. Aromatic Ring 3100 Wavenumbers (cm-1) 3000 2900

C-H str. Alkanes 2800 2700 -0.00 0.11 0.12 0.13 0.14 0.15 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 Absorbance Plasma On Plasma Off 1600 1550 Wavenumbers (cm-1) Technology Initiatives 1500 Research and

C=C Aromatic Ring 1450 1400 Research and Technology Initiatives FTIR Spectra of a Pure Air Plasma and a 750 ppm(v) Toluene in Air Plasma

¾ Trace Concentrations of VOCs in Air Create a very Complex Plasma Chemistry SUMMARY: Low-T plasmas can be used effectively for the treatment of gaseous waste streams containing VOCs in a bench-scale R&D environment

Low-T plasmas for Environmental Applications: • High Percentage of VOC Destruction in Low-Flow Applications • Reasonable Destruction Efficiency in High-Flow Applications • Extensive Characterization of By-Products • High Level of Carbon Closure

But: Challenges Remain ¾ Scale-up to high gas flow is non-trivial ¾ Cost and energy efficiency (vs. competing technologies) ¾ Materials for long-term, maintenance-free operation ¾ Control of by-product formation ¾ Poorly understood plasma chemistry ¾ Coupling of discharge physics to plasma chemistry

Æ Large-scale industrial utilization is still some time away ! Research and Low-T Plasmas for Technology Initiatives Fuel Cell Systems

Idea: Use low-T plasma to generate hydrocarbon feed gas for cell

2 m Solid Oxide Fuel Cell Chemistry

300 kW Fuel Cell Research and Conventional SOFC Process Technology Initiatives AC Power Two Catalytic Reactors Power Conditioning

Exhaust Diesel Sulfur Pre- Carbonate Heat/Water Removal Reforming DFC/SOFC Recovery Steam Generation H2 steam air

Low-T Plasma Alternative

Diesel Plasma Reactor ZnO Cartridge Vaporization Clean Fuel Diesel Æ CH4,H2, HCs ZnO + H S Æ ZnS + H O 2 2 Cell Feed R-S + H2 Æ H2S + R Water/Steam Research and Various DBDs Technology Initiatives 1. Surface DBDs (S-DBDs) using Microrods

High Voltage (~10-15 KHz) Ground Wires Dielectric

Top electrode removed

Plasma

2. Parallel-Plate DBD (PP-DBD) Gas in Gas out 10” (1) Low & High Sulfur Fuel @ Steam/Fuel = 3 Low Sulfur High Sulfur Methane 27 % (v/v) 28 Good Stuff Hydrogen 23 23 Ethene 30 29 Acetylene 5 2 Not Bad Ethane/Propane 1/0 2/0 Higher Hydrocarbons 14 16 (2) Effect of Steam/Fuel Ratio for NATO 76 Diesel LOW (2:1) MEDIUM (3:1) HIGH (8:1) Methane 25 % (v/v) 27 25 Good Stuff Hydrogen 21 23 21 Ethene 28 30 33 Acetylene 4 5 4 Not Bad Ethene/Propane 2/1 1/0 1/0 Higher Hydrocarbons 19 14 16

Æ Work continues – at bench-scale R&D, no engineering realization yet Research and Technology Initiatives Pulsed Electrical Discharges in Bubbled Water • Pulsed electrical discharges in water to develop a reactor and process for the in-situ,

on-demand generation of oxidants (O, OH, H2O2, O3) for the disinfection of water • Produce high concentrations of oxidants and UV with low power consumption Æ use externally introduced oxygen bubbles in the water (bubbled water discharge) Typical Reactor Set-up Bubble locations

Bubble not touching electrodes: • High power required • Weak discharge, low efficiency • most bubbles

Bubble touching one electrode: • Medium power required • Stronger glow discharge, more efficient • some bubbles

Bubble touching both electrodes: • Low power required • Strong spark discharge, high efficiency • few bubbles

Most efficient scenario is difficult to realize and stabilize in practice !!! Research and Technology Initiatives Hydrogen peroxide and ozone generation 3 O Research and Production of H2O2 at low pH: Technology Initiatives more efficient Conditions: T = 25ºC; 200 ml of DI water; conductivity ~ 1.5mS/cm; voltage 15 kV; pulse repetition rate 20 Hz; power consumption ~ 2 W 2+ pH = 3 (H2SO4); [Fe ] – 1.5 mmol/l

30 H O production: 25 2 2

20 • is highest for air

15 • the same for Ar and O2

10 • has efficiency of ~ 3 mg/kWh concentration, mg/l concentration, 2 Ar, 200 ml/min O 2 O , 200 ml/min 5 2 H Air, 200 ml/min 0 020406080100 Time, min Research and HMX degradation: Technology Initiatives optimization of process parameters O2N N NO Octahydro-1,3,5,7-tetranitro 1,3,5,7-tetrazocine; N 2 N high melting explosive (HMX) O N 2 N Conditions: NO2 T = 25ºC; 200 ml of HMX solution; conductivity – 1.38 mS/cm; 1.0 voltage -15 kV; repetition rate – 20Hz;

C0 of HMX – 4 mg/l; pH is varied: 5.5 0.8 +2 and 3 (H2SO4); [Fe ] – 1.5 mmol/l 0 0.6 Results: ‰ HMX degradation:

0.4 ƒ is much faster at low pH HMX, C/C Ar, pH 5.5, no [Fe+2] ƒ is faster in O than Ar +2 2 O , pH 5.5, no [Fe ] 2 Ar, pH 5.5, [Fe+2]=1.5mM/l 0.2 +2 O , pH 5.5, [Fe ]=1.5mM/l 2 ‰ H2O2 is responsible for about 25% Ar, pH 3, [Fe+2]=1.5mM/l O , pH 3, [Fe+2]=1.5mM/l of HMX degradation 0.0 2 0 20 40 60 80 100 120 Time, min ‰ The remaining degradation is due to ozone and other radicals, UV Research and HMX decomposition at Technology Initiatives low pH with Fenton reagent Conditions:

ƒ pH = 3 (H2SO 4) 2+ ƒ [Fe ] – 1.5 mmol/l 1.0 Under these conditions: Ar, 200 ml/min O , 200 ml/min 2 0.8 ƒ Reactions continue after

0 discharge is turned off 0.6 ƒ HMX degradation rate is

0.4 higher for O2 than Ar HMX, C/C ƒ O plays a role in the 0.2 3 Power off HMX decomposition

0.0 0 20 40 60 80 100 120 Time, min Research and Technology Initiatives HMX decomposition at low pH with Fenton reagent with and without the discharge

1.0 Conditions:

0.8 ƒ pH = 3 (H2SO4) ƒ [Fe2+] = 1.5 mmol/l 0 Discharge 0.6 ƒ [H2O2] = 20 mg/l

0.4 HMX, C/C Results:

HMX degradation by H2O2 + 0.2 Ar, pH 3, [Fe+2]=1.5mM/l + H O 2 2 Fenton reaction (black) and by Ar, pH 3, [Fe+2]=1.5mM/l O , pH 3, [Fe+2]=1.5mM/l discharge in water bubbled 2 0.0 0 20 40 60 80 100 120 using Ar (red) and Oxygen Time, min (blue) + Fenton reaction

HMX degradation:

‰ H2O2 is responsible for about 25% of HMX degradation ‰ Discharge is essential for rapid degradation ‰ Ozone and other radicals and UV contribute as well Research and Summary Technology Initiatives

‰ The discharge in gas bubbles in DI water is a DBD: a high current, pulsed, self-terminating microplasma ‰ Fast space-charge propagation – streamer ‰ DBD discharge (SS electrodes are covered by a layer of water) ‰ Streamers in small bubbles and in large bubbles that occupy the space between the electrodes ‰ Large bubbles are more effective for radical production and water treatment ‰ Radicals (O, H, OH) and H2O2 and O3 are generated in-situ ‰ No significant difference in H2O2 production for O2 and Ar, but more effective in air

‰ HMX decomposition is not effective using only H2O2 and Fenton reaction ‰ HMX decomposition is effective using a discharge in bubbled water at low pH and Fenton reaction ‰ Degradation is due to O3, other radicals (O, OH) and/or UV (not clear yet) Research and Technology Initiatives Destruction of Bacteria

Known Facts: • Plasmas can inactivate (“kill”) individual microorganisms (cells, bacteria, spores, viruses; E.coli, Anthrax, etc. …) • Both low-pressure and high-pressure plasmas “work” (higher inactivation rates compared to conventional methods such as EtO, dry heat, steam heat, etc. ..) “Kill Agents”: “Kill Mechanisms”: • UV radiation • well understood at the cellular level • Reactive Radicals, Ions • fairly well understood how low-pressure plasmas • Heat induce cell death • Electric fields • less well understood for high-pressure plasma

??? Open Question (not really a plasma physics question) ??? Role of synergistic effects of the various “kill agents”, i.e. effect of the of simultaneous or sequential action of more than one “kill agent” !!! Research and Technology Initiatives Experimental Setup for Spore Inactivation Reactor Enclosure Gas Analyzer Gas Mixing and Control

Power Capillary Cold Plasma Supply Plate Jets

HV

Carrier Gases Sample Glass Plate Data Acquisition

Ground Plate

Ambient Gas Research and Technology Initiatives UV Absorption - A Qualitative Measure for Cell Destruction (Bacillus subtilis spores)

2

After Plasma Treatment

1

Absorbance Before Plasma Treatment

0 200 250 300 350 400

Wavelength (nm) 4x106

The Status in 2002 3x106 D-Value: 92 s 6 2x10 (Decimal Reduction Number)

6 Number of CFU/ml Number 1x10 Bacillus subtilis Spore Destruction by a CPE Plasma (Air, 760 Torr) 0 0 20406080100120 1000 Plasma Exposure Time (s)

Bacillus stearothermophilus 100 Spore Destruction by a CPE Plasma (He, 760 Torr)

10 D-Value: 101 s Number of CFU/ml D-values of 10s of seconds 1 0 30 60 90 120 150 180 210 240 Plasma Exposure Time (s) Research and Technology Initiatives The Status in 2007

1.E+00 3 mm 1.E-01 10 mm

1.E-02 y = 0.1e-0.6941x

1.E-03 CFU 1.E-04

1.E-05 y = 0.0773e-2.5065x R2 = 0.998 1.E-06

1.E-07 024681012 Treatment time, sec

D-values of (much) less than 10s (with additives in the feed gas) Research and Low-T Plasmas and Biofilms Technology Initiatives

A biofilm is a highly structured community of bacteria with a complex structure that can adhere to solid surfaces or interfaces.

• Biofilms can form when planktonic bacteria adhere to surfaces and begin to excrete exopolysaccharide that anchor them to the surface

• A biofilm may form on any surface exposed to bacteria and some water, but moisture is not always necessary Research and Technology Initiatives Biofilms Impact Many Aspects of Daily Life • Biofilms can cause serious • Biofilms impact on many industrial side effects associated with processes and have adverse many illnesses and implants effects on the environment Research and Technology Initiatives Biofilms are not easy to Inactivate • Biofilms have different properties than planktonic cells • Biofilms are highly resistant to antibiotics, germicides, and other conventional forms of inactivation – Concentrations of antibiotics required to destroy a biofilm would probably kill the patient – Concentrations of germicides required to sanitize equipment is usually environmentally detrimental.

!!! Most conventional inactivation approaches do not work well for biofilms !!! !!! BUT: Plasma Works !!! Research and Technology Initiatives Inactivation of Chromobacterium violaceum biofilm-forming cells (4-day old bioflm)

6 Rapid 2-order-of-magnitude reduction 10 (needs to be looked at further)

105 Slow 1-order-of-magnitude exponential reduction

104 Number of CFU/ml Number

103 0 102030405060 Plasma Exposure Time (min) Research and SUMMARY & OUTLOOK Technology Initiatives

• Electrostatic Precipitators & Ozone Generators are the only fully commercialized plasma-based environmental technologies

• Plasma-initiated degradation of contaminants (VOCs, NOx, SOx) in gaseous waste streams has been demonstrated in the lab • By-product characterization & control, carbon closure, and scale-up challenges remain obstacles to full commercialization

• Plasma-assisted generation of fuel cell feed gas mixtures and plasma- enabled degradation of biofilms are being studied in the lab • Unresolved science issues remain before engineering realization

• Pulsed electrical discharges in water and aqueous solutions represent a promising approach to disinfection and decontamination of liquids • Early-stage lab phase technology • Non-thermal plasmas have yet not fulfilled their full potential as a key technology in the area of environmental applications • Remaining science and/or serious engineering challenges have slowed down the translation of lab achievements into viable technologies for large-scale industrial applications Research and Medical and Biomedical Applications Technology Initiatives The Plasma Needle

Cleaning of Dental Cavities Other Applications • Bio Decontamination • Sterilization of Medical Instruments and Wounds

!!! BIG ISSUE: PLASMAS AND HUMANS !!!