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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 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
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/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.
ICOPS2006_arnh3_02
MICRODISCHARGE PLASMA SOURCES
Ref: D. Hsu, et al. Pl. Chem. Pl. Proc., 2005.
Flow direction
Iowa State University
Optical and Discharge Physics
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.
ICOPS2006_arnh3_03
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
ICOPS2006_arnh3_04
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.
Iowa State UniversityOptical and Discharge Physics
SCALING OF H2 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 Tgas)
Assume no axial diffusion.
Iowa State University
Optical and Discharge Physics
PLUG FLOW MODEL: ION DENSITIES
ICOPS2006_arnh3_06
[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).
Iowa State University
Optical and Discharge Physics
PLUG FLOW MODEL: NEUTRALS
ICOPS2006_arnh3_07
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).
Iowa State University
Optical and Discharge Physics
PLUG FLOW MODEL: H2 FLOW RATE
ICOPS2006_arnh3_08
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.
Iowa State University
Optical and Discharge Physics
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
Iowa State University
Optical and Discharge Physics
ii St
N
SV
DESCRIPTION OF MODEL: CHARGED PARTICLE, SOURCES
ICOPS2006_arnh3_10
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:
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.
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.
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.
)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
ICOPS2006_arnh3_12
Iowa State University
Optical and Discharge Physics
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.
ICOPS2006_arnh3_13
1000
Ionization dominated by beam electrodes produces plasmas densities > 1014 cm-3.
Iowa State UniversityOptical and Discharge Physics
BASE CASE: PLASMA CHARACTERISTICS
ICOPS2006_arnh3_14
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
Optical and Discharge Physics
BASE CASE: PLASMA CHARACTERISTICS
ICOPS2006_arnh3_15
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
Iowa State UniversityOptical and Discharge Physics
AXIAL DISTRIBUTION OF H CONTAINING NEUTRALS
ICOPS2006_arnh3_16
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
ICOPS2006_arnh3_17
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
Iowa State University
Optical and Discharge Physics
CONCLUDING REMARKS
ICOPS2006_arnh3_19
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.