optimization of h2 production in ar/nh3 microdischarges

OPTIMIZATION OF H2 PRODUCTION IN Ar/NH3MICRODISCHARGES

Ramesh A. Arakoni,a) Ananth N. Bhojb) and Mark J. Kushnerc)

a) Dept. Aerospace Engineering, 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]://uigelz.ece.iastate.edu

59th Gaseous Electronics Conference, October 2006

* Work supported by NSF and AFOSR.

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Iowa State UniversityOptical and Discharge Physics

AGENDA

• Microdischarge devices for H2 production

• Ar/NH3 reaction mechanism

• Scaling of H2 production

• Kinetics: Plug flow modeling

• Hydrodynamics: 2-d modeling

• Concluding Remarks

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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 µm).

• High power densities (10s kW/cm3) due to wall stablization enable high electron densities and high neutral gas temperatures; both leading to molecular dissociation.

• Dominance of cathode fall leading to energetic secondary electrons and non-Maxwellian electron distributions increases dissociation efficiency.

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MICRODISCHARGE PLASMA SOURCES

Ref: D. Hsu, et al. Pl. Chem. Pl. Process. (2005)

Flow direction

Fig. 1. Microhollow cathode and electrical circuit.


H2 GENERATION: MICRODISCHARGES• Storage of H2 for portable applications such as fuel cells is

difficult.

• Energy efficient real time production of H2 is of interest.

• H2 can be produced from NH3 via the reverse of the Haber process using micro-discharges as the dissociation source.

• Feasibility of such an approach requires an energy gain from power expended in dissociation to power produced by fuel cell.

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Refs: H. Qiu et al. Intl. J. Mass. Spec (2004).D. Hsu et al. Pl. Chem. Pl. Proc., (2005).

•H is produced by electron impact dissociation of NH3.

e + NH3 → NH2 + H + e

• Thermal decomposition is important at high gas temperatures (> 2000 °K)

M + NH3 → NH3-n + nH + M


Ar/NH3: REACTION MECHANISM

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• 3-body recombination of H in the afterglow produces H2. H + H + M → H2 + M

• Power dissipation is dominantly by dissociative excitation of NH3 at Te = 2-5 eV.

• Dissociative ionization produces additional H through subsequent dissociative recombination.

• Efficiency of H generation can be controlled through Ar/NH3mixture by reducing Te to ≈ 2 eV or less.


FRACTIONAL POWER DEPOSITION AND H2 PRODUCTION

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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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GLOBALKIN

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• Plug flow model.• Rate coefficients from solution

of Boltzmann’s equation as mole fractions change.

• Inputs:• Power density vs position• Reaction mechanism• Inlet speed (adjusted

downstream for Tgas)


NonPDPSIM: 2-d MODEL

• To investigate hydrodynamic issues in microdischarge based H2 production, the 2-d 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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GEOMETRY OFMICRODISCHARGE REACTOR

• Anode grounded, cathode potential varied to deposit specified power.

• 100 Torr Ar/NH3

• NH3 mole fraction: 1 – 20 %.

• Flow rate 5 - 50 sccm.

• Plasma diameter: 300 µm.

• Cathode, anode: 100 µm thick.

• Dielectric gap 1 mm.

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PLUG FLOW: ION DENSITIES

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• NH4+ dominates due to charge

exchangee + NH3 → NH3

+ + 2eNH3

+ + NH3 → NH4+ + NH2

• H-, NH2- are formed where Te is

small, and NH3 is not depleted by rarefaction.

• Increase in Ar+ downstream reflects decomposition of NH3.

• 100 Torr, 10 m/s, • 2.5 kW/cm3 (0.2 – 0.24 cm)• Ar/NH3=90/10


PLUG FLOW: NEUTRAL DENSITIES

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• Dominant neutral products are H2 and N2.

• Tgas ≈ 1000 K producing significant rarefaction and some thermal dissociation.

• Input energy corresponds to 15.2 eV per H2 produced, exceeding energy gain in fuel cell (2.5 eV).

• Higher efficiency process is needed.

• 100 Torr, 10 m/s, • 2.5 kW/cm3 (0.2 – 0.24 cm)• Ar/NH3=90/10


PLUG FLOW: H2 FLOW RATE

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• Absolute [H2] densities increased with increasing NH3 mole fraction and decreasing velocities.

• Conversion of NH3 to H2 is most efficient at lower [NH3] and lower flow rates where eV/NH3 molecule is largest.

• 100 Torr, 10 m/s, • 2.5 kW/cm3 (0.2 – 0.24 cm)


PLUG FLOW: H2 FLUX

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• For a fixed power, higher [NH3] density and lower flow rates return the maximum H2 throughput on which the power efficiency depends.

• 100 Torr, 10 m/s, • 2.5 kW/cm3 (0.2 – 0.24 cm)

• High power deposition in negative glow and positive column produce electron densities > 1014 cm-3.

• 10 sccm, 100 Torr, Ar/NH3=95/5, 1 W Iowa State UniversityOptical and Discharge Physics

BASE CASE: PLASMA CHARACTERISTICS

MIN MAX

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• Tg > 2000 K produces rarefaction.

• Significant H production by electron impact prior to hot zone.

• Conversion of H to H2downstream of discharge.

• 10 sccm, 100 Torr, Ar/NH3=95/5, 1 W


BASE CASE: NEUTRAL GASTgas 2050°K

[NH3] (2 decs) 2.3 x 1017 cm-3

[H] (3 decs) 1.4 x 1016 cm-3

[H2] (3 decs) 5.6 x 1016 cm-3

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MIN MAX

Flow direction

• Rapid consumption of H downstream by recombination produces a plume of H2.

• 10 sccm, 100 Torr, Ar/NH3=95/5, 1 W Iowa State UniversityOptical and Discharge Physics

BASE CASE: DOWNSTREAM PROPERTIES

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MIN MAX

Flow direction

4.5 mm

7.2 mm


Ar/NH3 COMPOSITION: ELECTRON DENSITY

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Flow direction

0.4 mm

1.2 mm

• 15%

• 10%

• 7%

• 5%

• NH3

MIN MAX

• With increasing NH3fraction, power dissipation per electron by dissociative excitation increases.

• For fixed power, electron density decreases.

• More resistive plasma at large NH3 fraction produces higher densities in positive column.

• 10 sccm, 100 Torr, Ar/NH3, 1 W

Ar/NH3 COMPOSITION: [H2], [e]

Iowa State UniversityOptical and Discharge PhysicsGEC2006_arnh3_18

• With increasing NH3 fraction, increasing rate of dissociation dominates over decrease in electron density.

• Net result is increase in H2 production.

• 10 sccm, 100 Torr, Ar/NH3, 1 W


CONCLUDING REMARKS

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• Dissociation of NH3 in a microdischarge was investigated for scaling as a “real time” H2 source.

• Electron impact processes efficiently produce H at Te < 3 eV with significant contribution from thermal decomposition.

• H2 production dominated in the afterglow by 3-body recombination.

• Tradeoff on total H2 densities with efficiency of conversion.

• Lower flow rates and lower NH3 mole fractions result in maximum conversion efficiency, whereas lower flow rates and higher NH3 mole fractions result in maximum throughput.

optimization of h2 production in ar/nh3 microdischarges

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