Transcript Document

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
[email protected], [email protected], [email protected]
http://uigelz.ece.iastate.edu
ICOPS 2006, June 4 – 8, 2006.
* Work supported by NSF and AFOSR.
ICOPS2006_arnh3_00
AGENDA
 Microdischarge (MD) devices for H2 production
 Reaction mechanism
 Scaling using plug flow modeling.
 Description of 2-d model
 Scaling considering hydrodynamics.
 Concluding Remarks
ICOPS2006_arnh3_01
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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.
ICOPS2006_arnh3_03
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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 (> 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.
ICOPS2006_arnh3_04
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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.
ICOPS2006_arnh3_04a
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GLOBAL PLASMA MODEL
 Time-independent plug flow
model.
 Boltzmann solver updates eimpact rate coefficients.
 Inputs:
 Power density vs positio
 Reaction mechanism
 Inlet speed (adjusted
downstream for Tgas)
 Assume no axial diffusion.
ICOPS2006_arnh3_05
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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-] < 1010 cm -3.
 5 m/s, Ar/NH3=98/2, 100 Torr.
 2.5 kW/cm3 (0.2 – 0.24 cm).
ICOPS2006_arnh3_06
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PLUG FLOW MODEL: NEUTRALS
 66% conversion of NH3 to H2
 For 100% conversion, only 23% 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).
ICOPS2006_arnh3_07
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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.
ICOPS2006_arnh3_08
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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
 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
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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.
 
N i
     Si
t
 Poisson’s Equation for Electric Potential:
     V  S
 Secondary electron emission:
jS    ij j
j
ICOPS2006_arnh3_10
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ELECTRON ENERGY, TRANSPORT COEFFICIENTS
 Bulk electrons: Electron energy equation with coefficients
obtained from Boltzmann’s equation solution for EED.

ne  
5
 
2
 j  E  EEM  ne  Ni i       Te , j  qe
t
2

i
 Beam Electrons: Monte Carlo
Simulation
 Cartesian MCS mesh
superimposed on unstructured
fluid mesh.
 Construct Greens functions for
interpolation between meshes.
ICOPS2006_arnh3_11
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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.


   ( v )  ( inlets, pum ps)
t


 v 



   N i kTi     v v        qi N i Ei  S i mi  i qi E
t
i
 i



 
 c pT 

  T  v c pT   Pi   v f   Ri H i   ji  E
t
i
i
 Individual species are addressed with superimposed diffusive
transport.

 N i t  t   
   SV  S S
N i t  t   N i t      v f  Di NT 

N
T



ICOPS2006_arnh3_12
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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/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.
ICOPS2006_arnh3_13
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BASE CASE: PLASMA CHARACTERISTICS
[e] (cm-3 )
1
100
Logscale
[e] sources(cm-3 s-1)
Pot (V)
0
-360
 Ionization dominated by beam electrodes
produces plasmas densities > 1014 cm-3.
ICOPS2006_arnh3_14
1
1000
Logscale
 10 sccm, Ar/NH3=98/02
 1 W, 100 Torr.
Iowa State University
Optical and Discharge Physics
BASE CASE: PLASMA CHARACTERISTICS
 (mg cm-3)
Tgas (°K)
300
1600
0
0.22
[H] (1013 cm-3 )
[H2] (1013 cm-3 )
8
2
800
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.
ICOPS2006_arnh3_15
200
Logscale
Animation 0 – 0.1 ms
 10 sccm, Ar/NH3=98/02
 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  H 2+ M
 Small contribution from
wall recombination.
 N2H2 density small.
 10 sccm, Ar/NH3=98/02
 1 W, 100 Torr
ICOPS2006_arnh3_16
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Ar/NH3 COMPOSITION: ELECTRON DENSITY
 2% NH3
 5% NH3
 10% NH3
[e] (cm-3)
1
100
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
ICOPS2006_arnh3_17
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Ar/NH3 COMPOSITION: H2 DENSITY
 2% NH3
Max 2 x 1015
 5% NH3
 10% NH3
Max 3.7 x 1015
Max 6 x 1015
[H2] (cm-3)
1
logscale
100
 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
ICOPS2006_arnh3_18
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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.
ICOPS2006_arnh3_19
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