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.

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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 c) a) Dept. Aerospace Engr, University of Illinois, Urbana, IL b) Dept. Chemical and Biomolecular Engineering University of Illinois, Urbana, IL c) Dept. Electrical and Computer Engineering Iowa State University, Ames, IA ICOPS 2006, June 4 – 8, * Work supported by NSF and AFOSR. ICOPS2006_arnh3_00

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

Iowa State University Optical and Discharge Physics  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/cm 3 ) 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. ICOPS2006_arnh3_02 MICRODISCHARGE PLASMA SOURCES Ref: D. Hsu, et al. Pl. Chem. Pl. Proc., Flow direction

Iowa State University Optical and Discharge Physics H 2 GENERATION: MICRODISCHARGES  Storage of H 2 is cumbersome and dangerous. Real-time generation of H 2 using microdischarges is investigated here.  H 2 can be produced from NH 3 via the reverse of the Haber process 1,2.  Applications include fuel cells where H 2 storage is difficult.  Economic feasibility of such a fuel cell depends on the ability to convert enough NH 3 to H 2 for a power gain. ICOPS2006_arnh3_03 1 H. Qiu et al. Intl. J. Mass. Spec, D. Hsu et al. Pl. Chem. Pl. Proc., 2005.

 H formation by electron impact dissociation of NH 3 in discharge. e + NH 3  NH 2 + H + e  Thermal decomposition is important at high gas temperatures (> 2000 °K)  3-body recombination of H in the afterglow produces H 2. H + H + M  H 2 + M, where M = Ar, NH 3, NH 3 (v), H, H 2. Iowa State University Optical and Discharge Physics Ar/NH 3 : REACTION MECHANISM ICOPS2006_arnh3_04

 Investigation of H 2 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 SCALING OF H 2 PRODUCTION ICOPS2006_arnh3_04a

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

Iowa State University Optical and Discharge Physics PLUG FLOW MODEL: ION DENSITIES ICOPS2006_arnh3_06  [H + ], [Ar + ], [NH 3 + ], and [NH 4 + ] are the primary ions in the discharge.  Plasma density exceeds cm -3  [NH 4 + ] dominates in afterglow due to charge exchange.  [H - ], [NH 2 - ] < cm -3.  5 m/s, Ar/NH 3 =98/2, 100 Torr.  2.5 kW/cm 3 (0.2 – 0.24 cm).

Iowa State University Optical and Discharge Physics PLUG FLOW MODEL: NEUTRALS ICOPS2006_arnh3_07  66% conversion of NH 3 to H 2  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 PLUG FLOW MODEL: H 2 FLOW RATE ICOPS2006_arnh3_08  Conversion of NH 3 to H 2 is most efficient at lower [NH 3 ] and lower flow rates where eV/molecule is largest.  To maximum throughput, higher [NH 3 ] density and higher flow rate must be balanced by higher power deposition.  2.5 kW/cm 3, 200 Torr.

Iowa State University Optical and Discharge Physics DESCRIPTION OF 2-d MODEL  To investigate hydrodynamic issues in microdischarge based H 2 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. ICOPS2006_arnh3_09

Iowa State University Optical and Discharge Physics DESCRIPTION OF MODEL: CHARGED PARTICLE, SOURCES ICOPS2006_arnh3_10  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 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. ICOPS2006_arnh3_11

Iowa State University Optical and Discharge Physics  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. DESCRIPTION OF MODEL: NEUTRAL PARTICLE TRANSPORT ICOPS2006_arnh3_12

Iowa State University Optical and Discharge Physics GEOMETRY OF MICRODISCHARGE REACTOR  Fine meshing near the cathode.  Anode grounded, cathode potential varied to deposit required power (up to 1 W).  100 Torr Ar/NH 3 mixture, with NH 3 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. ICOPS2006_arnh3_13

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

200  High power densities (10s kW/cm 3 ) produce significant gas heating.  H 2 generation is maximum in discharge region prior to NH 3 depletion.  Reduction of H in the afterglow due to recombination. Iowa State University Optical and Discharge Physics BASE CASE: PLASMA CHARACTERISTICS ICOPS2006_arnh3_15  10 sccm, Ar/NH 3 =98/02  1 W, 100 Torr T gas (°K)  (mg cm -3 ) [H 2 ] (10 13 cm -3 ) 2 Logscale [H] (10 13 cm -3 ) 8008 Logscale Animation 0 – 0.1 ms

 Conversion efficiency to H and H 2 of  4%.  Conversion of H into H 2 dominantly by 3-body collisions in afterglow. H + H + M  H 2 + M  Small contribution from wall recombination.  N 2 H 2 density small.  10 sccm, Ar/NH3=98/02  1 W, 100 Torr Iowa State University Optical and Discharge Physics AXIAL DISTRIBUTION OF H CONTAINING NEUTRALS ICOPS2006_arnh3_16

 With increasing [NH 3 ] more power is expended in dissociation and gas heating, reducing [e].  Plasma constricts due to more rapid electron-ion recombination.  10 sccm, Ar/NH 3, 1 W, 100 Torr Iowa State University Optical and Discharge Physics Ar/NH 3 COMPOSITION: ELECTRON DENSITY ICOPS2006_arnh3_ [e] (cm -3 ) logscale  2% NH 3  5% NH 3  10% NH 3

Ar/NH 3 COMPOSITION: H 2 DENSITY Iowa State University Optical and Discharge Physics ICOPS2006_arnh3_ [H 2 ] (cm -3 ) logscale Max 2 x Max 3.7 x  2% NH 3  5% NH 3  10% NH 3 Max 6 x  Although fraction conversion of NH 3 to H 2 is larger at low mole fractions (larger eV/molecule), total throughput is larger at higher mole fraction.  10 sccm, Ar/NH 3, 1 W, 100 Torr

Iowa State University Optical and Discharge Physics CONCLUDING REMARKS ICOPS2006_arnh3_19  Dissociation of NH 3 in a microdischarge was investigated for scaling as a “real time” H 2 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 NH 3 fraction, lower flow: Total throughput of H 2 may be small.  3-body recombination of H dominates H 2 production in the afterglow, whereas direct thermal dissociation of NH 3 by dominate H 2 production in the plasma.