A Kinetic Model for the Reduction of CO 2 in a Corona Plasma Discharge for Syngas production Dr. Jaime Lozano Prof. Ray Allen The University of Sheffield, UK
Outline • 4CU & Plasmolytic reduction • What is a Plasma? • CO 2 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 CO 2 to fuel SP5 & SP6 SP3 & SP4 SP7 SP6 – Plasmolytic reduction
SP6 – Plasmolytic reduction of CO 2 to CO Aim: Activation of CO 2 using non-thermal plasma technology to produce Syngas – Study of reactor geometries and impact on process parameters on dissociation of CO 2 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)
What is a Plasma? “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 • …..
Types of 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) (From http://www.plasmacenter.pl/corona.htm)
Components of the Plasma A plasma discharge consists of: • Electrons • Neutral particles • Excited species • Ions (positive and negative) ( COSI, Columbus, Ohio, US, by Steve Spanoudis)
Challenges in Plasma Modelling 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
Potential benefits of non-thermal plasma • 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
Why coronas for CO 2 dissociation? • Non-thermal. • CO 2 molecule has resonant vibrational energy levels (V-V relax high, V-T relax low). • Corona discharges can be tuned to the resonant frequencies of CO 2 molecules. • Despite transient phenomena, coronas are, in good approximation, well behaved...Predictable! • Easy to design, build, operate and couple to diagnostics systems
CO 2 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 Multiphysics chosen because it has a dedicated plasma module that integrates all these physics modes into a single computational environment!
Creating a kinetic model for a CO 2 plasma discharge Widely thought of as being almost impossible!
Reaction scheme Initial species: e + CO 2 → CO, O, O 2 , O 3 , C, O - , O 2 - , CO 3 - , CO 4 -, O 2 *, O*, O 2 * Electron impact dissociation Ion-molecule reactions e + CO 2 → CO + O + e O- + CO 2 + CO 2 → CO 3 - + CO 2 e + O 2 → O + O + e O 2 - + CO 2 + CO 2 → CO 4 - + CO 2 e + O 3 → O + O 2 + e e + CO → C + O + e Electron attachment Heavy – heavy reactions e + CO 2 → O - + CO O + O 2 + O 2 → O 3 + O 2 e + O 2 → O - + O O + O 2 + CO 2 → O 3 + CO 2 e + O 3 → O - + O 2 O + O + CO 2 → O 2 + CO 2 O( 3 P) + O 3 → O 2 + O 2 e + O 3 → O 2 - + O O 3 + O 2 ( 1 D g ) → O 2 + O 2 + O e + O 2 + O 2 → O 2 - + O 2 O + CO + CO 2 → CO 2 + CO 2 C + CO + CO 2 → C 2 O + CO 2 O + C 2 O → CO + CO
Collision Data Modelling chart Solving the 0D model (no spatial dependencies) Boltzmann Solver • Begin with collision data, and use Boltzmann solver to find electron energy distribution Rate Calculator EEDF (EEDF) • Calculate reaction rates for Rates kinetic model using COMSOL Electron energy • Obtain species evolution Equation Kinetic Model Operating Conditions Species Evolution
Collision Data Modelling chart A spatially resolved model Boltzmann Solver • Kinetic model can be used with fluid dynamics and electrostatics in COMSOL for Rate Calculator EEDF a 1D model incorporating reactor geometry • Spatial features are coupled Rates to kinetic model Electron energy • EEDF can be determined Equation experimentally and fed back Kinetic Model into rate calculator Operating Conditions Potential Temperature Distribution Field Species Flow Evolution Regime
Collision Data Modelling chart An Electrical Model Boltzmann Solver • Using PSpice the reactor can be modeled as an electrical circuit based on data from Rate Calculator EEDF the spatially resolved kinetic model giving power Rates consumption Electron energy Equation Electrical Model Kinetic Model Operating Conditions Power Usage Potential Temperature Distribution Field Species Flow Evolution Regime
Collision Data Modelling chart - Summary 0D Model Boltzmann Solver Strongly coupled 0D variables Spatially dependent model – also coupled An Electrical Model Rate Calculator EEDF Rates Electron energy Equation Electrical Model Kinetic Model Operating Conditions Power Usage Potential Temperature Distribution Field Species Flow Evolution Regime
Kinetic Model 1) Species Continuity Equation Describes conservation of the plasma species, j n j R j j l , t l j = electrons, ions, neutrals Accumulation Flux term Reaction term
Kinetic Model 1) Species Continuity Equation Describes conservation of the plasma species, j n j R j j l , t l j = electrons, ions, neutrals 2) Drift Diffusion Approximation Describes movement of the plasma species, j j n j j E D j n j Drift Term – Depends on charge of Diffusion Term – Introduces species, and electric field diffusivity of species
Kinetic Model 1) Species Continuity Equation Describes conservation of the plasma species, j n j R j j l , t l j = electrons, ions, neutrals 2) Drift Diffusion Approximation Describes movement of the plasma species, j j n j j E D j n j 3) Electron Energy Equation Describes distribution of electron energies n 5 5 e n D E Q e e e e e N t 3 3 Collisional energy loss Electron energy flux Electron “heating” due to electric field
Kinetic Model 1) Species Continuity Equation Describes conservation of the plasma species, j n j R j j l , t l j = electrons, ions, neutrals 2) Drift Diffusion Approximation Describes movement of the plasma species, j j n j j E D j n j 3) Electron Energy Equation Describes distribution of electron energies n 5 5 e n D E Q e e e e e N 3 3 t 4) Poisson Equation The effect of charged species on the electric potential E q n 0 j j j Dielectric constant Charge distribution
Conditions: Results – CO production Te=2.6 eV, Tg=300K - , O 2 , O, CO 4 - ... CO 2 splits into CO, O 3 , CO 3 But, CO dominates!
Results - Effect of CO 2 / O 2 ratio 50 25 CO O3 20 40 15 30 O3 [mol/m^3 ] CO [mol/m^3] 10 20 5 10 0 0 0 20 40 60 80 100 0 20 40 60 80 100 % CO2 % CO2 80 O 4 C 60 3 O [mol/m^3] C [mol/m^3] 40 2 20 1 0 0 0 20 40 60 80 100 0 20 40 60 80 100 % CO2 % CO2
Results - Pure H 2 O model H 2 O splits into H 2 , O 2 , H 2 O 2 , HO 2 , OH and H But, H 2 and O 2 species dominate!
Conclusions • Plasma corona discharges are suitable for CO 2 dissociation. For Te=2.6 eV, Tg=300 K, 2 Atm conditions, ≈48 % conversion into CO can be achieved. • Excitation of vibrational modes in CO 2 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: • CO 2 dissociation with plasma corona reactors + parallel H 2 O dissociation (also has resonant vibrational modes) • CO 2 + H 2 O together ... Future work!
Acknowledgements http://4cu.org.uk
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