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

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 61801b) 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

Iowa State University

Optical and Discharge Physics

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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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.

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MICRODISCHARGE PLASMA SOURCES

Ref: D. Hsu, et al. Pl. Chem. Pl. Proc., 2005.

Flow direction



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.

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1 H. Qiu et al. Intl. J. Mass. Spec, 2004.

2 D. Hsu et al. Pl. Chem. Pl. Proc., 2005.

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.

Iowa State UniversityOptical and Discharge Physics

Ar/NH3: REACTION MECHANISM

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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.


SCALING OF H2 PRODUCTION

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GLOBAL PLASMA MODEL

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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.



PLUG FLOW MODEL: ION DENSITIES

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[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).



PLUG FLOW MODEL: NEUTRALS

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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).



PLUG FLOW MODEL: H2 FLOW RATE

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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.



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.

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ii St

N

SV

DESCRIPTION OF MODEL: CHARGED PARTICLE, SOURCES

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j

jijSj

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:



ELECTRON ENERGY, TRANSPORT COEFFICIENTS

Bulk electrons: Electron energy equation with coefficients obtained from Boltzmann’s equation solution for EED.

e

ieiie

2EM

e qj,T2

5NnEEj

t

n

Beam Electrons: Monte Carlo Simulation

Cartesian MCS mesh superimposed on unstructured fluid mesh.

Construct Greens functions for interpolation between meshes.

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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.

)pumps,inlets()v(t

iiiiiiii

iii EqmSENqvvkTN

t

v

i i

iiifipp EjHRvPTcvTt

Tc

DESCRIPTION OF MODEL: NEUTRAL PARTICLE TRANSPORT

SV

T

iTifii SS

N

ttNNDvtNttN

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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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1000

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


BASE CASE: PLASMA CHARACTERISTICS

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10 sccm, Ar/NH3=98/02

1 W, 100 Torr.

1001Logscale

[e] (cm-3 )

-3600

Pot (V) [e] sources(cm-3 s-1)

1Logscale

200

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. Iowa State University


BASE CASE: PLASMA CHARACTERISTICS

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10 sccm, Ar/NH3=98/02

1 W, 100 Torr

1600300

Tgas (°K)

0.220

(mg cm-3) [H2] (1013 cm-3 )

2Logscale

[H] (1013 cm-3 )

8008Logscale

Animation 0 – 0.1 ms

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


AXIAL DISTRIBUTION OF H CONTAINING NEUTRALS

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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 Iowa State UniversityOptical and Discharge Physics

Ar/NH3 COMPOSITION: ELECTRON DENSITY

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1001[e] (cm-3)

logscale

2% NH3 5% NH3 10% NH3

Ar/NH3 COMPOSITION: H2 DENSITY

Iowa State UniversityOptical and Discharge PhysicsICOPS2006_arnh3_18

1001[H2] (cm-3)

logscale

Max 2 x 1015 Max 3.7 x 1015

2% NH3 5% NH3 10% NH3

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



CONCLUDING REMARKS

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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.

Documents

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