Modelling Carbon Black

Transcription

1 Modelling Carbon Black Matthew Celnik, Tim Totton, Abhijeet Raj, Markus Sander, Markus Kraft 09/09/09

2 Soot Formation Burner Reaction Zone Temperature Flame Carbon Condensation Particle Addition Inception Coalescense Reactions of Aggregation PAHs by PAHs Oxidation by O 2 and OH,

3 Molecular soot structure Nanoparticles (3 nm 2 µm) Primary particles occur due to dimerisation of PAH molecules What is the critical PAH size to form a dimer in flame conditions? What is the morphology of a soot particle?

4 Soot model hierarchy Quantum Chemistry (DFT) Full representation of molecules Determine kinetic parameters PAH Aromatic Site Model (ARS) Functional site description PAH reactions as jump processes Kinetic Monte-Carlo (PAH KMC) Single planar PAH simulations Used to generate internal particle structure Population Balance (PAH-PP) Particle ensemble modelling Particles described by PAH-PP Model Inception, growth and coagulation

5 Oxidation processes in PAHs Investigated reactions: PAH: Polyaromatic hydrocarbons Oxidation process: Decomposition process:

6 Using structural information

7 Oxidation rates of different site types Zigzag next to zigzag (zz) Eact=156 kj/mole Zigzag next to free edge (zf) Eact=161 kj/mole Armchair next to free edge (af) Eact=173 kj/mole Units: k in cm3/(mole*s), T in K

8 PAH reactions (selection) S1 S2 S3 S4 S5 S6 Free-edge growth Free-edge desorption 5-member ring addition 5-member ring desorption Armchair growth 5- to 6-member ring Frenklach, Wang, Violi

9 PAH KMC growth simulation Seed molecule: Pyrene Growth of a PAH molecule kinetic Monte Carlo (KMC) simulation

10 PAH growth mechanism

11 Particle Model

12 PAH-PP model: State Space Structure of a particle: P = P p,..., p, ) ( 1 Cn n: number of primary particles p of particle P C: Matrix containing the sphericity of the neighbouring primaries C ij =0 if p i and p j are not touching Each primary particle p i is composed of m PAHs: p i = pi ( PAH1,..., PAH m)

13 PAH-PP model: Data structure Contains: [C] 2,5 Contains: [C] 5,6

14 PAH mass spectra J. Happold, H.-H. Grotheer, and M. Aigner. Rapid Commun. Mass Sp., 21: , C 2 H 4 -O 2 flame, Pressure = 120 mbar, C/O = 1, Cold gas velocity = 54 cm/sec Monomers Monomers Dimers Dimers Experimental Computed

15 Coagulation efficiency C E = 1+ exp 2 D M 3 min min 1 + M min C E D M min = Coagulation efficiency min = Diameter of smaller PAH = Mass of smaller PAH

16 PAH coagulation efficiency

17 Investigating structure How do PAH molecules form clusters? How do these clusters grow? Driven by intermolecular potentials Alston Misquitta, Aron Cohen, Dwaipayan Chakrabarti, Mark Miller, David Wales

18 Basin hopping Finds stable molecular clusters by searching for minima Based on potential energy landscape Uses Monte-Carlo criterion when jumping between minima Energy

19 Investigating structure How do PAH molecules form clusters? How do these clusters grow? Driven by intermolecular potentials Alston Misquitta, Aron Cohen, Dwaipayan Chakrabarti, Mark Miller, David Wales

20 Experimental comparison Experimental HR-TEM images of an aggregate sampled from a diesel engine. Indicated are length scales of structures within a primary particle (from Mosbach et al., 2009, Combustion and Flame). A TEM-style projection of a computed cluster of 50 coronene molecules

21 Thank you! Please visit our website:

22 DISI Engine Late injection produces stratified mixture. Fuel rich regions close to spark gap. Image from

23 Soot in DISI Engine λ = 1.0 EOI = -50 CAD ATDC Spark = -30 CAD ATDC

24 Soot in DISI Engine 2.6 CAD ATDC 12.6 CAD ATDC 32.6 CAD ATDC

25 PAH growth rates - sensitivity P. Weilmünster, A. Keller, and K. H. Homann. Combust. Flame, 116:62 83, 1999.

26 Global minimum clusters 2 Coronene molecules 5 Coronenes molecules 10 Coronene molecules E = kj/mol E = kj/mol E = kj/mol

27 Comparison of potentials Poor agreement between L-J potential and SAPT(DFT) results Good agreement with W99 potential Potential (kj/mol) LJ Gr LJ SP LJ X W99 Gr W99 SP W99 X SAPT Gr SAPT SP SAPT X Dimer Separation (Å) Graphite (Gr) Slipped Parellel (SP) Crossed (X)

28 Cluster Density Need to define cluster volume Used scaled van der Waals radii Define Volume in terms of scaling factor, α Determine critical α to determine density

29 Cluster density Volume varies non-linearly with α Choose critical α at minimum of dv/dα (α crit = 1.7) Corresponds to point where all intermolecular space is covered by overlapping spheres.

30 Varying density in our models For α crit = 1.7, coronene ρ = 1.1 g/cm 3, pyrene ρ = 1.0 g/cm 3 Standard soot density in models = 1.8 g/cm 3 Density has been shown to be an important parameter

31 HRTEM images of soot Some evidence for different soot structures based on different fuels Top: graphitic Bottom: amorphous Pictures from: Uitz, Cracknell, Jansma and Makkee, Impact of diesel fuel composition on soot oxidation Characteristics, SAE