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 Off-Line Analysis Gas Preparation Plasma Reactor Carbon Trap influent effluent Solvent Extraction GC-MS VOCs in dry air I-V, Power 50 – 1500 ppm(v) Measurement On-Line Analysis FTIR Absorption GC-FID GC-MS (gas phase) 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 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 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 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 0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Energy Density E, J/cm3 Research and Technology Initiatives 0.20 Plasma Off Destruction of Toluene – FTIR Spectra Plasma On 0.15 Plasma Off 0.18 0.14 Plasma On 0.16 0.13 0.12 0.14 0.11 C=C Aromatic Ring 0.10 0.12 C-H str. Alkanes 0.09 0.10 C-H str. Aromatic Ring 0.08 0.07 0.08Absorbance 0.06Absorbance 0.06 0.05 0.04 0.04 0.03 0.02 0.02 0.01 -0.00 -0.00 3300 3200 3100 3000 2900 2800 2700 1600 1550 1500 1450 1400 Wavenumbers (cm-1) Wavenumbers (cm-1) 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.