A Kinetic Model for the Reduction of CO2 in a Corona Plasma Discharge for Syngas production
Dr. Jaime LozanoProf. Ray AllenThe University of Sheffield, UK
Outline
• 4CU & Plasmolytic reduction
• What is a Plasma?
• CO2 Plasma Model─ Reaction Scheme─ Modelling Chart─ Results
4CU Programme Grant
• 4 year, £5.7m project funded by EPSRC started
September 2012
• Main aim is the sustainable conversion of CO2 to fuel
SP3 & SP4
SP7
SP5 & SP6
SP6 – Plasmolytic reduction
SP6 – Plasmolytic reduction of CO2 to CO
Aim: Activation of CO2 using non-thermal plasma technology to
produce Syngas
– Study of reactor geometries and impact on process parameters on
dissociation of CO2 molecules
– Development of in-situ spectroscopy techniques to evaluate reactions
mechanism
– Development of novel electrodes to generate plasma at lower voltage
– Development of a kinetic model to further advance
experimental work
Plasma generated in a ferroelectric packed bed reactor(Courtesy of Tom Butterworth, 4CU)
“A plasma is a quasineutral gas of charged and neutral particles which exhibits collective behavior”
Francis F. Chen
i.e. the fourth state of matter!
Plasma has many applications• Fusion research• Semiconductor industry• Lighting industry• Chemical industry• Gas cleaning• …..
What is a Plasma?
The most common types of plasma are:
• Inductively coupled plasmas (ICP)• Capacitively coupled plasmas (CCP)
• Corona discharges(already used industrially for large scale gas treatment)
Types of Plasma
(From http://www.plasmacenter.pl/corona.htm)
A plasma discharge consists of:
• Electrons• Neutral particles• Excited species• Ions (positive and negative)
Components of the Plasma
( COSI, Columbus, Ohio, US, by Steve Spanoudis)
The broad range of characteristic time scales for the different interactions between components in a plasma discharge creates numerical difficulties
• Plasma chemistry data is very hard to find or not exist at all...when available, come in different formats!
• Stiffness in space (charge separation needs to be resolved).
• Stiffness in time (different time scales)
• Large number of degrees of freedom (many species)
• Strong couplings between electron energy and electromagnetic fields, transport of charged species and electromagnetic fields, etc.
Hence, plasma processes are considered unpredictable and extremely difficult to model...
However, this is changing with the availability of new commercial software
Challenges in Plasma Modelling
• Non-equilibrium, where temperature of electron higher than gas temperature
• High electron temperature• Low gas temperature• Low currents• Selective tool (potential for greater efficiency)
“COLD” PLASMA
Potential benefits of non-thermal plasma
• Non-thermal.• CO2 molecule has resonant vibrational energy
levels (V-V relax high, V-T relax low).• Corona discharges can be tuned to the resonant
frequencies of CO2 molecules.• Despite transient phenomena, coronas are, in
good approximation, well behaved...Predictable!• Easy to design, build, operate and couple to
diagnostics systems
Why coronas for CO2 dissociation?
CO2 decomposition in a plasma
This is a truly multiphysics problem, because it involves:
• Electron dynamics (Boltzmann equation, energy equation)• Electron/heavy species mass transport equations• Fluid dynamics (flow equations)• Heat transfer• Ion transport• Reaction engineering (chemical reactions)• Electrodynamics • Interaction with external circuit (power supply)
Comsol Multiphysicschosen because it has a dedicated plasma
module that integrates all these physics modes into a single computational environment!
Creating a kinetic model for a CO2 plasma discharge
Widely thought of as being almost impossible!
Initial species: e + CO2 → CO, O, O2, O3, C, O-, O2-, CO3
-, CO4-, O2*, O*, O2*
Electron impact dissociatione + CO2 → CO + O + ee + O2 → O + O + ee + O3 → O + O2 + e e + CO → C + O + e
Ion-molecule reactionsO- + CO2 + CO2 → CO3- + CO2
O2- + CO2 + CO2 → CO4- + CO2
Reaction scheme
Electron attachmente + CO2 → O- + COe + O2 → O- + Oe + O3 → O- + O2
e + O3 → O2- + O
e + O2 + O2 → O2- + O2
Heavy – heavy reactionsO + O2 + O2 → O3 + O2
O + O2 + CO2 → O3 + CO2
O + O + CO2 → O2 + CO2
O(3P) + O3 → O2 + O2
O3 + O2(1Dg) → O2 + O2 + OO + CO + CO2 → CO2 + CO2
C + CO + CO2 → C2O + CO2
O + C2O → CO + CO
Collision Data
Boltzmann Solver
EEDFRate Calculator
Rates
Kinetic Model
Electron energy Equation
Operating Conditions
Species Evolution
• Begin with collision data, and use Boltzmann solver to find electron energy distribution (EEDF)
• Calculate reaction rates for kinetic model using COMSOL
• Obtain species evolution
Modelling chart
Solving the 0D model (no spatial dependencies)
Collision Data
Boltzmann Solver
EEDFRate Calculator
Rates
Kinetic Model
Electron energy Equation
Operating Conditions
Species Evolution
• Kinetic model can be used with fluid dynamics and electrostatics in COMSOL for a 1D model incorporating reactor geometry
• Spatial features are coupled to kinetic model
• EEDF can be determined experimentally and fed back into rate calculator
Modelling chart
A spatially resolved model
Temperature Field
Flow Regime
Potential Distribution
Collision Data
Boltzmann Solver
EEDFRate Calculator
Rates
Kinetic Model
Electron energy Equation
Operating Conditions
Species Evolution
• Using PSpice the reactor can be modeled as an electrical circuit based on data from the spatially resolved kinetic model giving power consumption
Modelling chart
An Electrical Model
Temperature Field
Flow Regime
Potential Distribution
Electrical Model
Power Usage
Collision Data
Boltzmann Solver
EEDFRate Calculator
Rates
Kinetic Model
Electron energy Equation
Operating Conditions
Species Evolution
Modelling chart - Summary
0D ModelStrongly coupled 0D variablesSpatially dependent model – also coupledAn Electrical Model
Temperature Field
Flow Regime
Potential Distribution
Electrical Model
Power Usage
1) Species Continuity Equation
,
j
j j l
l
nR
t
Describes conservation of the plasma species, j
Accumulation Flux term Reaction term
j = electrons, ions, neutrals
Kinetic Model
1) Species Continuity Equation
,
j
j j l
l
nR
t
Describes conservation of the plasma species, j
j = electrons, ions, neutrals
Kinetic Model
2) Drift Diffusion ApproximationDescribes movement of the plasma species, j
j n j jE Djn j
Drift Term – Depends on charge of species, and electric field
Diffusion Term – Introduces diffusivity of species
1) Species Continuity Equation
,
j
j j l
l
nR
t
Describes conservation of the plasma species, j
j = electrons, ions, neutrals
Kinetic Model
2) Drift Diffusion ApproximationDescribes movement of the plasma species, j
j n j jE Djn j
3) Electron Energy EquationDescribes distribution of electron energies
5 5
3 3
e
e e e e e N
nn D E Q
t
Electron energy flux Electron “heating” due to electric field
Collisional energy loss
1) Species Continuity Equation
,
j
j j l
l
nR
t
Describes conservation of the plasma species, j
j = electrons, ions, neutrals
Kinetic Model
2) Drift Diffusion ApproximationDescribes movement of the plasma species, j
j n j jE Djn j
3) Electron Energy EquationDescribes distribution of electron energies
5 5
3 3
e
e e e e e N
nn D E Q
t
4) Poisson EquationThe effect of charged species on the electric potential
0 j j
j
E q n Dielectric constant Charge distribution
Results – CO production
CO2 splits into CO, O3, CO3-, O2, O, CO4
-...But, CO dominates!
Conditions:Te=2.6 eV, Tg=300K
Results - Effect of CO2 / O2 ratio
0 20 40 60 80 100
0
10
20
30
40
50C
O [m
ol/m
^3
]
% CO2
CO
0 20 40 60 80 100
0
1
2
3
4
C [m
ol/m
^3
]
% CO2
C
0 20 40 60 80 100
0
5
10
15
20
25
O3 [m
ol/m
^3]
% CO2
O3
0 20 40 60 80 100
0
20
40
60
80
O [m
ol/m
^3
]
% CO2
O
Results - Pure H2O model
H2O splits into H2, O2, H2O2, HO2, OH and HBut, H2 and O2 species dominate!
Conclusions
• Plasma corona discharges are suitable for CO2 dissociation.For Te=2.6 eV, Tg=300 K, 2 Atm conditions, ≈48% conversion into CO can be achieved.
• Excitation of vibrational modes in CO2 molecules allow for selective transfer of energy. Hence, potential for greater efficiency than thermal plasma.
• Plasma simulation with new commercial software makes easier what was terribly difficult just few years ago.
• Alternative routes to syngas production:• CO2 dissociation with plasma corona reactors + parallel
H2O dissociation (also has resonant vibrational modes)• CO2 + H2O together ... Future work!
Acknowledgements
http://4cu.org.uk
Additional slides
CFD-ACE+:• general PDE solver• plasma physics module available• ESI Group (http://www.esi-group.com/)
ANSYS Fluent:• general fluid dynamics solver • applicable to low pressure CVD simulation• ANSYS Inc. (http://www.ansys.com/)
COMSOL Multiphysics:• FE (finite element) solver• ~20 pre-defined application modules from fluid dynamics to mechanics
• plasma module included in version 4.1• Comsol, Inc. (http://www.comsol.com/)
Available codes for plasma simulation
The wide spread use of plasmas means that modelling software has become commercially available. This opens up possibilities
for gaining basic understanding of plasma processes that did not exist until very recently!
Plasma Reactor
CO2 CG
MFC
MFC
VapourSystem
H2O
Power Supply
Products Out
Minimum experimental requirements to create plasma
Experimental schematic
Plasma Reactor
CO2 CG
MFC
MFC
VapourSystem
H2O
Power Supply
Products Out
Equipment required for characterisation
Experimental schematic
HV Probe
Current Monitor
Gas Chromatography
Spectroscopy
Plasma Reactor
CO2 CG
MFC
MFC
VapourSystem
H2O
Power Supply
Products Out
Experimental data obtained can then be used to refine model
Experimental schematic
HV Probe
Current Monitor
Gas Chromatography
Spectroscopy
Key Features:• Coaxial geometry with internal live
electrode• Powered by a pulsed high voltage
DC power source• Quartz windows allowing direct
optical access for spectroscopy
Corona discharge reactor
“A plasma is a quasineutral gas of charged and neutral particles which exhibits collective behaviour”. Francis F. Chen
i.e. the fourth state of matter!
What is a Plasma?
(Courtesy of DOE Fusion labs, NASA & Steve Albers)