Modeling plasma-based CO 2 conversion: From chemistry to plasma design

Transcription

1 Modeling plasma-based CO 2 conversion: From chemistry to plasma design Annemie Bogaerts T. Kozak, R. Snoeckx, G. Trenchev, K. Van Laer Research group PLASMANT, University of Antwerp, Belgium CO 2 + CH 4 Catalyst H 2 /CO, CH 3 OH, QDB Workshop, London, Sept 15, 2017

2 PLASMA for CO 2 conversion? Non-equilibrium plasma: - Energetic electrons: Dissociate inert molecules (thermodynamically unfavorable reactions can take place) - Role of vibrational excitation!! (~ energy efficiency) Possibly energy efficient alternative to classical processes Plasma = flexible: Easily switched on/off CO 2 + CH 4 Possible solution for 3 important problems - global warming - dependence fossil fuels - storage renewable energy However: more research needed to better understand underlying mechanisms CO/H 2, CH 3 OH, Computer modeling

3 DBD: Plasma types often used for research on CO 2 conversion - Atmospheric pressure, Simple design, Upscaling possible (O 3 ) - Limited E-efficiency (~ 10%), improved in packed bed DBD - Plasma catalysis (~ selectivity) Microwave plasma: - Highest E-efficiency (up to 90%), but at supersonic flow + reduced pressure (~ 100 Torr) (USSR, 1983) - More recent results (DIFFER): - up to 55% (supersonic flow + reduced pressure) (2013) - up to 50% (reverse vortex flow, atm.pressure) (2015) Gliding arc plasma: - Rather high E-efficiency (~ 43%) - Atmospheric pressure - Classical GA reverse vortex flow GA

4 CO 2 research within PLASMANT 1) Computer simulations: - Plasma chemistry: - Pure CO 2 (focus vibrational levels) - CO 2 /CH 4, CH 4 /O 2, CO 2 /H 2 O, CO 2 /N 2 - Plasma reactors: DBD, packed bed DBD, MW, GA - Plasma-catalyst interactions 2) Experiments: - DBD, Packed bed DBD, Plasma catalysis - Reverse vortex flow GA (GAP)

5 Modeling tools (1) 0D model: plasma chemistry Boltzmann solver EEDF Species kinetics Gas temperature: energy conservation equation EEDF, T e : BOLSIG+ 1 E.g. Global_kin 2 or ZDPlaskin 3 1 Hagelaar and Pitchford, Plasma Sources Sci. Technol. 14, 722 (2005) 2 A. C. Gentile and M. J. Kushner, J. Appl. Phys., 78 (1995) Pancheshnyi et al, 2008 (

6 Modeling tools (2) 2D/3D fluid models (Comsol): plasma reactors: - Species densities: Conservation equations: - Chemical source + loss terms - Transport by diffusion, migration, convection - Average electron energy: - Heating by electric field, loss by collisions - EM field distributions: Maxwell equations - Gas temperature + gas flow: Navier-Stokes equations 3D problems, but start with 2D Start with argon or helium chemistry

7 Plasma chemistry: CO 2 Molecules Charged species Radicals Excited species CO 2, CO CO 2+, CO 4+, CO +, C 2 O 2+, C 2 O 3+, C 2 O 4+, C 2+, C +, CO 3-, CO 4 - O 2, O 3 O +, O 2+, O 4+, O -, O 2-, O 3-, O 4 - electrons Vibrational levels of CO 2, CO and O 2 - CO 2 : 4 lowest symmetric mode levels, 21 asymmetric mode levels (up to dissociation limit) - CO: 10 vibrational levels - O 2 : 3 effective vibrational levels C 2 O, C, C 2 CO 2 (Va, Vb, Vc, Vd), CO 2 (V1 - V21), CO 2 (E1, E2), CO (V1 - V10), CO (E1 - E4) O O 2 (V1 - V3), O 2 (E1 - E2) Details: - Aerts, Martens, Bogaerts, J. Phys. Chem. C 116, (2012). - Kozák, Bogaerts, Plasma Sources Sci. Technol. 23, (2014). - Berthelot, Bogaerts, Plasma Sources Sci. Technol. 25, (2016). - Berthelot, Bogaerts, J. Phys. Chem. C 121, (2017).

8 Plasma chemistry: CO 2 /CH 4 (idem for CO 2 /H 2 O and CH 4 /O 2 ) Molecules CH 4, C 2 H 6, C 2 H 4, C 2 H 2, C 3 H 8, C 3 H 6, C 4 H 2, H 2, O 2, CO 2, CO, H 2 O, H 2 O 2, CH 2 O, CH 3 OH, CH 3 CHO, CH 2 CO Charged species CH 5+, CH 4+, CH 3+, CH 2+, CH +, C 2 H 6+, C 2 H 5+, C 2 H 4+, C 2 H 3+, C 2 H 2+, O 2+, O -, H 3 O +, OH -, electrons Radicals CH 3, CH 2, CH, C, C 2 H 5, C 2 H 3, C 2 H, C 3 H 7, H, O, OH, HO 2, CHO, CH 2 OH, CH 3 O, C 2 HO, CH 3 CO, CH 2 CHO, C 2 H 5 O 2 Excited species H 2 *, O*, CO 2 *, CO*, H 2 O* 17 molecules, 14 ions, 19 radicals, 5 excited species, electrons 121 electron impact reactions, 87 ion reactions, 290 neutral reactions Details: - Snoeckx, Aerts, Tu, Bogaerts, J. Phys. Chem. C 117, (2013). - De Bie, van Dijk, Bogaerts, J. Phys. Chem. C 119, (2015).

9 This talk 1) Plasma chemistry: pure CO 2 : (a) DBD (b) Comparison DBD MW, GA (~ role vibrational levels) 2) Plasma chemistry: CO 2 /CH 4 (DBD) 3) Plasma reactor modeling (a) Packed bed DBD, (b) GA

10 This talk 1) Plasma chemistry: pure CO 2 : (a) DBD (b) Comparison DBD MW, GA (~ role vibrational levels) 2) Plasma chemistry: CO 2 /CH 4 (DBD) 3) Plasma reactor modeling (a) Packed bed DBD, (b) GA

11 1(a) Pure CO 2 : DBD DBD: filamentary character: How treated in 0D model? Reactor length + gas flow residence time (spatial ~ temporal behavior) Filaments = Microdischarge pulses as f(time) 1(a) Pure CO 2 : DBD

12 1(a) Pure CO 2 : DBD 1 pulse + afterglow: 5 consecutive pulses: - During pulse: densities (except CO 2 ) - During afterglow: constant ( accumulation) or (if long afterglows) Aerts et al., J. Phys. Chem. C 116, (2012). 1(a) Pure CO 2 : DBD

13 1(a) Pure CO 2 : DBD Extended to many filaments Real residence time of gas in reactor Compare CO 2 conversion with experiments: reasonable agreement: Aerts, Somers, Bogaerts, ChemSusChem 8, 702 (2015). 1(a) DBD: pure CO 2

14 1(a) Pure CO 2 : DBD Because CO 2 model ~ experiments: Can be used to elucidate most important splitting mechanisms Reaction scheme : e - CO 2 e - O CO CO 2 + O 2 - O 2 O 3 O - e - e - In DBD: mostly electron impact - Dissociation ( CO + O) (O recombines to O 2, O 3 ) - Ionization ( CO 2+ ) (CO 2+ + e - CO + O) - Dissociative attachment ( CO + O - ) (O - O 2, O 3 ) 1(a) DBD: pure CO 2

15 1(b) Pure CO 2 : Comparison DBD MW, GA Conversion: Energy efficiency ~ 35% ~ 25% ~ 15% ~ 5% ~ 25% (max) ~ 5% MW >> DBD MW: T g,calc (~1000 K) < MW (300 K) Aim: keep T g in MW low (~ reduced pressure, fast gas flow ) Kozák, Bogaerts, PSST 23, (2014). 1(b) CO 2 : Comparison DBD - MW, GA

16 1(b) Pure CO 2 : Comparison DBD MW, GA Reason for higher E-efficiency in MW, GA: Vibrational kinetics! - CO 2 : efficiently excited to vibrational levels - Vibrational levels: efficient dissociation pathway Energy required for vibration-induced dissociation bond energy: 5.5 ev 5.5 ev Energy required for electron impact dissociation (~ 7-10 ev) > bond energy 1(b) CO 2 : Comparison DBD - MW, GA

17 This talk 1) Plasma chemistry: pure CO 2 : (a) DBD (b) Comparison DBD MW, GA (~ role vibrational levels) 2) Plasma chemistry: CO 2 /CH 4 (DBD) 3) Plasma reactor modeling (a) Packed bed DBD, (b) GA

18 2) CO 2 /CH 4 : Conversion + E-efficiency CO 2 + CH 4 conversion + E-efficiency, as f(sei): (exp: Tu et al., Univ. Liverpool) Snoeckx, Aerts, Tu, Bogaerts, J. Phys. Chem. C 117, 4957 (2013). Trade-off: conversion + E-efficiency Also calculated: Selectivities of formed products: At 50/50 CO 2 /CH 4 : - H 2 : 55% - CO: 48% - C 2, C 3 : 6%, 30% - Formaldehyde: 13% - Methanol: 3% Future: plasma catalysis 2) DBD: CO 2 /CH 4

19 Species densities 2) CO 2 /CH 4 : Reaction pathways C x H y, CH 2 O, CH 3 CHO, CH 3 OH Pathways: De Bie, van Dijk, Bogaerts, J. Phys. Chem. C 119, (2015). 2) DBD: CO 2 /CH 4

20 This talk 1) Plasma chemistry: pure CO 2 : (a) DBD (b) Comparison DBD MW, GA (~ role vibrational levels) 2) Plasma chemistry: CO 2 /CH 4 (DBD) 3) Plasma reactor modeling (a) Packed bed DBD, (b) GA

21 3) Plasma reactor modeling (a) Packed bed DBD (b) Gliding arc plasma 3) Plasma reactors

22 3(a) Packed bed DBD COMSOL (2D, He): - Enhanced electric field near contact points (~ polarization of dielectric material) - Higher T e (8 ev vs 2-3 ev): power more efficiently used for electron heating more e-impact ionization + excitation-dissociation of CO 2 E-efficiency Van Laer and Bogaerts, PSST, 25, (2016). 3(a) Packed bed DBD

23 3(b) GA plasma: GAP: reverse vortex flow COMSOL (3D, Ar) Gas flow pattern: - Outer vortex ( ) ~400 K - Inner vortex (smaller velocity, ) Arc movement 1.2 ev (constant) Arc stabilized within 1 ms Plasma confined by flow Efficient gas conversion. Trenchev, Kolev and Bogaerts, PSST, 25, (2016). 3(b) GA

24 Conclusion Plasma modeling for CO 2 conversion: - Plasma chemistry (~ conversion, E-efficiency, products): 0D modeling - Plasma reactors (packed bed DBD, MW, GA): 2D or 3D fluid modeling CO 2 : DBD: electron impact (ground state) MW, GA: vibrational kinetics MW, GA: better E-efficiency CO 2 /CH 4 : formation value-added chemicals But selectivity low Combine with catalysts: Plasma catalysis Plasma reactors: Packed bed DBD, GAP: improved E-efficiency General: Modeling useful for better insights To improve applications Conclusion

25 Thanks Thanks