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MODELING OF H 2 PRODUCTION IN Ar/NH 3 MICRODISCHARGES Ramesh A. Arakoni a) , Ananth N. Bhoj b) , and Mark J. Kushner

MODELING OF H 2 PRODUCTION IN Ar/NH 3 MICRODISCHARGES Ramesh A. Arakoni a) , Ananth N. Bhoj b) , and Mark J. Kushner c) a) Dept. Aerospace Engr, University of Illinois, Urbana, IL 61801 b) Dept. Chemical and Biomolecular Engineering University of Illinois, Urbana, IL 61801.

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MODELING OF H 2 PRODUCTION IN Ar/NH 3 MICRODISCHARGES Ramesh A. Arakoni a) , Ananth N. Bhoj b) , and Mark J. Kushner

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  1. MODELING OF H2 PRODUCTION IN Ar/NH3 MICRODISCHARGES Ramesh A. Arakonia) , Ananth N. Bhojb), and Mark J. Kushnerc) a) Dept. Aerospace Engr, University of Illinois, Urbana, IL 61801 b) Dept. Chemical and Biomolecular Engineering University of Illinois, Urbana, IL 61801. c) Dept. Electrical and Computer Engineering Iowa State University, Ames, IA 50010 mjk@iastate.edu, arakoni@uiuc.edu, bhoj@uiuc.edu http://uigelz.ece.iastate.edu ICOPS 2006, June 4 – 8, 2006. * Work supported by NSF and AFOSR. ICOPS2006_arnh3_00

  2. AGENDA  Microdischarge (MD) devices for H2 production • Reaction mechanism • Scaling using plug flow modeling.  Description of 2-d model  Scaling considering hydrodynamics.  Concluding Remarks Iowa State University Optical and Discharge Physics ICOPS2006_arnh3_01

  3. MICRODISCHARGE PLASMA SOURCES  Microdischarges are dc plasmas leveraging pd scaling to operate at high pressures (10s-100s Torr) in small reactors (100s m). CW high power densities (10s kW/cm3) due to wall stablization enables both high electron densities and high neutral gas temperatures; both leading to molecular dissociation. High E/N, and non-Maxwellian character of electron energy distribution leads to a significant fraction of energetic electrons.  Energetic electrons in the cathode fall ionize and dissociate the gas. Flow direction Ref: D. Hsu, et al. Pl. Chem. Pl. Proc., 2005. Iowa State University Optical and Discharge Physics ICOPS2006_arnh3_02

  4. H2 GENERATION: MICRODISCHARGES  Storage of H2 is cumbersome and dangerous. Real-time generation of H2 using microdischarges is investigated here.  H2 can be produced from NH3 via the reverse of the Haber process1,2.  Applications include fuel cells where H2 storage is difficult.  Economic feasibility of such a fuel cell depends on the ability to convert enough NH3 to H2 for a power gain. 1 H. Qiu et al. Intl. J. Mass. Spec, 2004. 2 D. Hsu et al. Pl. Chem. Pl. Proc., 2005. Iowa State University Optical and Discharge Physics ICOPS2006_arnh3_03

  5. Ar/NH3: REACTION MECHANISM • Hformation by electron impact dissociation of NH3 in discharge. e + NH3  NH2 + H + e  Thermal decomposition is important at high gas temperatures (> 2000 °K)  3-body recombination of H in the afterglow produces H2. H + H + M  H2 + M, where M = Ar, NH3, NH3(v), H, H2. Iowa State University Optical and Discharge Physics ICOPS2006_arnh3_04

  6. SCALING OF H2 PRODUCTION • Investigation of H2 production in microdischarges to determine optimum strategies and efficiencies. • Power and gas mixture scaling: Plug flow model GLOBAL_KIN • Hydrodynamic issues: 2-d model nonPDPSIM. Iowa State University Optical and Discharge Physics ICOPS2006_arnh3_04a

  7. GLOBAL PLASMA MODEL • Time-independent plug flow model. • Boltzmann solver updates e-impact rate coefficients. • Inputs: • Power density vs positio • Reaction mechanism • Inlet speed (adjusted downstream for Tgas) • Assume no axial diffusion. Iowa State University Optical and Discharge Physics ICOPS2006_arnh3_05

  8. PLUG FLOW MODEL: ION DENSITIES • [H+], [Ar+], [NH3+], and [NH4+] are the primary ions in the discharge. • Plasma density exceeds 1014 cm-3  [NH4+] dominates in afterglow due to charge exchange. • [H-], [NH2-] < 1010 cm -3.  5 m/s, Ar/NH3=98/2, 100 Torr.  2.5 kW/cm3 (0.2 – 0.24 cm). Iowa State University Optical and Discharge Physics ICOPS2006_arnh3_06

  9. PLUG FLOW MODEL: NEUTRALS • 66% conversion of NH3 to H2 • For 100% conversion, only 2-3% of the input power required in these conditions. • Input energy = 0.39 eV per molecule. • Higher efficiency process desirable since energy recover is poor. • 5 m/s, 98:02 Ar/NH3 • 100 Torr.2.5 kW/cc (0.2 – 0.24 cm). Iowa State University Optical and Discharge Physics ICOPS2006_arnh3_07

  10. PLUG FLOW MODEL: H2 FLOW RATE  Conversion of NH3 to H2 is most efficient at lower [NH3] and lower flow rates where eV/molecule is largest. • To maximum throughput, higher [NH3] density and higher flow rate must be balanced by higher power deposition. • 2.5 kW/cm3, 200 Torr. Iowa State University Optical and Discharge Physics ICOPS2006_arnh3_08

  11. DESCRIPTION OF 2-d MODEL • To investigate hydrodynamic issues in microdischarge based H2 production, the 2-dimensional nonPDPSIM was used. • Finite volume method on cylindrical unstructured meshes. • Implicit drift-diffusion-advection for charged species • Navier-Stokes for neutral species • Poisson’s equation (volume, surface charge) • Secondary electrons by ion impact on surfaces • Electron energy equation coupled with Boltzmann solution • Monte Carlo simulation for beam electrons. Iowa State University Optical and Discharge Physics ICOPS2006_arnh3_09

  12. DESCRIPTION OF MODEL: CHARGED PARTICLE, SOURCES  Continuity (sources from electron and heavy particle collisions, surface chemistry, photo-ionization, secondary emission), fluxes by modified Sharfetter-Gummel with advective flow field.  Poisson’s Equation for Electric Potential:  Secondary electron emission: Iowa State University Optical and Discharge Physics ICOPS2006_arnh3_10

  13. ELECTRON ENERGY, TRANSPORT COEFFICIENTS • Bulk electrons: Electron energy equation with coefficients obtained from Boltzmann’s equation solution for EED. • Beam Electrons: Monte Carlo Simulation • Cartesian MCS mesh superimposed on unstructured fluid mesh. • Construct Greens functions for interpolation between meshes. Iowa State University Optical and Discharge Physics ICOPS2006_arnh3_11

  14. DESCRIPTION OF MODEL: NEUTRAL PARTICLE TRANSPORT • Fluid averaged values of mass density, mass momentum and thermal energy density obtained using unsteady, compressible algorithms. • Individual species are addressed with superimposed diffusive transport. Iowa State University Optical and Discharge Physics ICOPS2006_arnh3_12

  15. GEOMETRY OFMICRODISCHARGE REACTOR • Fine meshing near the cathode. • Anode grounded, cathode potential varied to deposit required power (up to 1 W). • 100 Torr Ar/NH3 mixture, with NH3 mole fraction from 2 – 10 %. • Flow rate 10 sccm. • Plasma diameter: 100 m near anode, 150 m near cathode. • Cathode, anode 100 m thick. • Dielectric gap 100 m. Iowa State University Optical and Discharge Physics ICOPS2006_arnh3_13

  16. BASE CASE: PLASMA CHARACTERISTICS [e] (cm-3 ) Pot (V) [e] sources(cm-3 s-1) • Ionization dominated by beam electrodes produces plasmas densities > 1014 cm-3. 1 100 0 -360 1 1000 Logscale Logscale  10 sccm, Ar/NH3=98/02  1 W, 100 Torr. Iowa State University Optical and Discharge Physics ICOPS2006_arnh3_14

  17. BASE CASE: PLASMA CHARACTERISTICS Tgas (°K)  (mg cm-3) [H] (1013 cm-3 ) [H2] (1013 cm-3 ) • High power densities (10s kW/cm3) produce significant gas heating. • H2 generation is maximum in discharge region prior to NH3 depletion. • Reduction of H in the afterglow due to recombination. 300 2 200 8 800 1600 0 0.22 Logscale Logscale Animation 0 – 0.1 ms  10 sccm, Ar/NH3=98/02  1 W, 100 Torr Iowa State University Optical and Discharge Physics ICOPS2006_arnh3_15

  18. AXIAL DISTRIBUTION OF H CONTAINING NEUTRALS • Conversion efficiency to H and H2 of 4%. • Conversion of H into H2 dominantly by 3-body collisions in afterglow. H + H + M  H2+ M • Small contribution from wall recombination. • N2H2 density small. • 10 sccm, Ar/NH3=98/02 • 1 W, 100 Torr Iowa State University Optical and Discharge Physics ICOPS2006_arnh3_16

  19. [e] (cm-3) 1 100 Ar/NH3 COMPOSITION: ELECTRON DENSITY  5% NH3  10% NH3  2% NH3 • With increasing [NH3] more power is expended in dissociation and gas heating, reducing [e]. • Plasma constricts due to more rapid electron-ion recombination. • 10 sccm, Ar/NH3, 1 W, 100 Torr logscale Iowa State University Optical and Discharge Physics ICOPS2006_arnh3_17

  20. [H2] (cm-3) 1 100 logscale Ar/NH3 COMPOSITION: H2 DENSITY  2% NH3  10% NH3  5% NH3 Max 2 x 1015 Max 3.7 x 1015 Max 6 x 1015 • Although fraction conversion of NH3 to H2 is larger at low mole fractions (larger eV/molecule), total throughput is larger at higher mole fraction. • 10 sccm, Ar/NH3, 1 W, 100 Torr Iowa State University Optical and Discharge Physics ICOPS2006_arnh3_18

  21. CONCLUDING REMARKS • Dissociation of NH3 in a microdischarge was investigated for scaling as a “real time” H2 source. • Maximizing eV/molecule increases conversion efficiency. • Large eV/molecule produces both more electron impact dissociation and larger thermal decomposition: • Larger power: Discharge stability an issue • Smaller NH3 fraction, lower flow: Total throughput of H2 may be small. •  3-body recombination of H dominates H2 production in the afterglow, whereas direct thermal dissociation of NH3 by dominate H2 production in the plasma. Iowa State University Optical and Discharge Physics ICOPS2006_arnh3_19

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