MODELING OF H2 PRODUCTION IN Ar/NH3

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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_at_iastate.edu, arakoni_at_uiuc.edu,
bhoj_at_uiuc.edu http//uigelz.ece.iastate.edu ICOP
S 2006, June 4 8, 2006. Work supported by
NSF and AFOSR.
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AGENDA

• ? Microdischarge (MD) devices for H2 production
• Reaction mechanism
• Scaling using plug flow modeling.
• ? Description of 2-d model
• ? Scaling considering hydrodynamics.
• ? Concluding Remarks

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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.
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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.
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Ar/NH3 REACTION MECHANISM

• H formation by electron
• impact dissociation
• of NH3 in discharge.
• e NH3 ? NH2 H e
• ? Thermal decomposition is
• important at high gas
• temperatures (gt 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.

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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.

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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.

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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- lt 1010 cm -3.
• ? 5 m/s, Ar/NH398/2, 100 Torr.
• ? 2.5 kW/cm3 (0.2 0.24 cm).

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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, 9802 Ar/NH3
• 100 Torr.2.5 kW/cc (0.2 0.24 cm).

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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.

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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
• Poissons equation (volume, surface charge)
• Secondary electrons by ion impact on surfaces
• Electron energy equation coupled with Boltzmann
  solution
• Monte Carlo simulation for beam electrons.

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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.
• ? Poissons Equation for Electric Potential
• ? Secondary electron emission

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ELECTRON ENERGY, TRANSPORT COEFFICIENTS

• Bulk electrons Electron energy equation with
  coefficients obtained from Boltzmanns equation
  solution for EED.

• Beam Electrons Monte Carlo Simulation
• Cartesian MCS mesh superimposed on unstructured
  fluid mesh.
• Construct Greens functions for interpolation
  between meshes.

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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.

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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.

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BASE CASE PLASMA CHARACTERISTICS
e (cm-3 )
Pot (V)
e sources(cm-3 s-1)
1000
100
1
-360
0
1
Logscale
Logscale

• ? 10 sccm, Ar/NH398/02
• ? 1 W, 100 Torr.

• Ionization dominated by beam electrodes produces
  plasmas densities gt 1014 cm-3.

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BASE CASE PLASMA CHARACTERISTICS
Tgas (K)
? (mg cm-3)
H2 (1013 cm-3 )
H (1013 cm-3 )
200
300
2
800
8
1600
0.22
0
Logscale
Logscale

• 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.

Animation 0 0.1 ms

• ? 10 sccm, Ar/NH398/02
• ? 1 W, 100 Torr

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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/NH398/02
• 1 W, 100 Torr

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Ar/NH3 COMPOSITION ELECTRON DENSITY
? 5 NH3
? 10 NH3
? 2 NH3
logscale

• 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

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Ar/NH3 COMPOSITION H2 DENSITY
? 2 NH3
? 5 NH3
? 10 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

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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.

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