a kinetic model for the reduction of co in a corona...

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



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



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


Electrical Model

Power Usage

Collision Data

Boltzmann Solver

EEDFRate Calculator

Rates

Kinetic Model



Species Evolution

Modelling chart - Summary

0D ModelStrongly coupled 0D variablesSpatially dependent model – also coupledAn Electrical Model

Temperature Field

Flow Regime


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


,

j

j j l

l

nR

t



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


,

j

j j l

l

nR

t



Kinetic Model


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


,

j

j j l

l

nR

t



Kinetic Model


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


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


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)

a kinetic model for the reduction of co in a corona...

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