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A Kinetic Modeling Study of Polycyclic
Aromatic Hydrocarbons (PAHs) and
Soot Formation in Acetylene Pyrolysis
Dipartimento di Chimica, Materiali e Ingegneria Chimica “G. Natta” Politecnico di Milano
C. Saggese, N. E. Sanchez, A. Callejas, A. Millera, R. Bilbao,
M. U. Alzueta, A. Frassoldati, A. Cuoci, T. Faravelli, E. Ranzi
in collaboration with:
COST Meeting on SOOT & PAHs – Sorrento – April 2013
Outline 2
Aim of the work: soot modeling in combustion and pyrolysis
Study of acetylene pyrolysis: primary reaction kinetics
(gas phase kinetic scheme)
C4H2 and C4H4 pyrolysis
C2H2 pyrolysis at lower severity of operating conditions
(Hidaka et al., Wu et al.)
Study of acetylene pyrolysis: successive addition and condensation
reactions to form heavy PAHs and soot formation
(coupling of gas phase kinetic scheme with soot kinetic model)
C2H2 pyrolysis at higher severity of operating conditions
(Colket et al., Norinaga et al., Sanchez et al.)
Conclusions
COST Meeting on SOOT & PAHs – Sorrento – April 2013
Soot formation – Mechanisms (1) 3
Main stages of SOOT formation
throughout the flame
POST FLAME ZONE
FLAME ZONE
BURNER
PREMIXED FUEL + OXIDIZER
Particle-particle
interactions
Particle
inception
Gas-phase
chemistry
Literature: S. Izvekov, A. Violi, J. Chem. Theory Comput. 2 (2006) 504-512.
PAH
chemistry
COST Meeting on SOOT & PAHs – Sorrento – April 2013
Soot formation – Mechanisms (2) 4
POST FLAME ZONE
FLAME ZONE
BURNER
PREMIXED FUEL + OXIDIZER
Particle-particle
interactions
Particle
inception
Gas-phase
chemistry
Literature: S. Izvekov, A. Violi, J. Chem. Theory Comput. 2 (2006) 504-512.
PAH
chemistry
Soot chemistry
Main stages of SOOT formation
throughout the flame
COST Meeting on SOOT & PAHs – Sorrento – April 2013
Soot formation – Mechanisms (3) 5
Particle-particle
interactions
Particle
inception
Gas-phase
chemistry
PAH
chemistry
Molecules as key precursor in soot formation
• Oligomers of aromatic compounds (OAC)
• Pericondensed aromatic compounds (PAC)
Literature: A. D‘Anna et al., Combust. Flame 157 (2010) 2106–2115.
CH CH
COST Meeting on SOOT & PAHs – Sorrento – April 2013
6
Aromatics
NOx
Chlorinated
species
N-propylbenzene
Acenaphthalene
Pyrene
Phenanthrene
Naphthalene
H2-O2
CO
CH4
C2
C3
C6
nC7-iC8
...
MAIN MECHANISMS
Ethylbenzene
Xylene
Toluene
Benzene
Methyl esters
Soot
Modular and
hierarchical
approach
High temperature mechanism
for PAH and soot
(HT1303s)
~300 species
~16500 reactions
Coupling gas phase and soot kinetic model
http://creckmodeling.chem.polimi.it/
C. Saggese et al., A wide range kinetic modeling study of pyrolysis and oxidation of benzene, Combust. Flame (2013),
http://dx.doi.org/10.1016/j.combustflame.2013.02.013.
COST Meeting on SOOT & PAHs – Sorrento – April 2013
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Fuel
Reactor
Temperature
(K)
Pressure
(atm)
Residence
time
Feed composition
References
C2H2
Shock Tube 1300-2200 1.1-2.6 0.8-2.5 ms 2.5% C2H2 in Ar Hidaka et al. (1996)
Shock Tube 2032-2534 0.3-0.5 0.75 ms 3.2% C2H2 in Ne/Ar Wu et al. (1987)
Shock Tube 1100-2400 8 0.7 ms 3. 7% C2H2 in Ar Colket (1986)
Plug Flow 873-1473 1 1.5-4 s 1-3% C2H2 in N2 Sanchez et al. (2012)
Plug Flow 1000-1400 0.08 0.5-2 s C2H2 Norinaga et al. (2008)
C4H4 Shock Tube 1100-2400 8 0.7 ms 1% C4H4 in Ar Colket (1986)
Shock Tube 1500-2000 0.2-0.5 0.75 ms 2% C4H4 in Ne Kiefer et al. (1988)
C4H2
Shock Tube 1882-1993 0.3-0.4 0.75 ms 1% C4H2 in Ne Kern et al. (1990)
Shock Tube 2158 0.4 0.75 ms 1% C4H2/1% C2H2 in Ne Kern et al. (1990)
Shock Tube 1987 0.34 0.75 ms 1% C4H2/1% H2 in Ne Kern et al. (1990)
Shock Tube 1300-2000 1.1-2.6 1.6-2.5 ms 1% C4H2 in Ar Hidaka et al. (2002)
Shock Tube 1300-2000 1.1-2.6 1.6-2.5 ms 1% C4H2/1-4% H2 in Ar Hidaka et al. (2002)
Aim of the work
Kinetic analysis of acetylene pyrolysis in a wide range of experimental
conditions, including the most severe ones.
Refinement and further extension of the kinetic model towards the formation
of Polycyclic Aromatic Hydrocarbons (PAHs) and soot in order to improve
its predictive ability.
COST Meeting on SOOT & PAHs – Sorrento – April 2013
8 Acetylene pyrolysis: primary kinetics
Radical path
Molecular path
The most important reactions considered in this analysis and constituting the
core mechanism of acetylene pyrolysis are:
C2H2 + C2H2 = C4H4
C2H2 + C2H2 = C4H2 + H2
C4H4 = C4H2 + H2
C2H2 + C2H2 = C4H3 + H
C4H4 = C4H3 + H
COST Meeting on SOOT & PAHs – Sorrento – April 2013
9 Acetylene pyrolysis: primary kinetics
The most important reactions considered in this analysis and constituting the
core mechanism of acetylene pyrolysis are:
C2H2 + C2H2 = C4H4
C2H2 + C2H2 = C4H2 + H2
C4H4 = C4H2 + H2
C2H2 + C2H2 = C4H3 + H
C4H4 = C4H3 + H
Radical path
Molecular path
COST Meeting on SOOT & PAHs – Sorrento – April 2013
10 Acetylene pyrolysis at lower severity
Experimental conditions
• 2.5% C2H2 pyrolysis in Ar
• Shock tube reactor
• 1300-2200 K
• 1.1-2.6 atm
• 0.8-2.5 ms
The C2H2 feed was carefully purified from possible acetone impurities.
These data permit to focus the attention on the importance of molecular or free
radical pathways.
At 1300 K molecular path involving C4H4 formation with a minor role of
dehydrogenation reactions.
At T > 1600 K free radical decomposition reactions become important.
At T > 2000 K the model predicts 10-20% of carbon selectivity towards heavy
PAHs and soot inside the reactor Hidaka, Y., Hattori, K., Okuno, T., Inami, K., Abe, T., Koike, T., Shock-Tube and Modeling Study of Acetylene Pyrolysis and Oxidation, Combustion and Flame 107:401-417 (1996).
COST Meeting on SOOT & PAHs – Sorrento – April 2013
11 Acetylene pyrolysis at lower severity
Experimental conditions
• 3.2% C2H2 pyrolysis in Ne/Ar
• Shock tube reactor
• 2000-2500 K
• 0.3-0.5 atm
• 0.75 ms
This is a study at high temperature conditions (2000-2500 K), where the free radical
pathway is favored by the formation of the very stable polyacetylenes (C4H2, C6H2, ..).
Molecular paths account for less than 20% of acetylene decomposition at 2032 K.
Low pressure and limited reaction times
Wu, C. H., Singh, H. J., Kern, R. D., Pyrolysis of Acetylene Behind Reflected Shock Waves, International Journal of Chemical Kinetics, Vol. 19, 975-996 (1987).
the carbon selectivity to heavier species passes
from 5% at 2032 K to 40% at 2534 K
COST Meeting on SOOT & PAHs – Sorrento – April 2013
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[1] K.H. Homann, H.G. Wagner, Eleventh Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1967, p. 3
Experimental data [1]
BIN C
BIN B
BIN A
Molecular weight [amu]
Soot kinetic model : Discrete sectional method
Using a discrete sectional approach, large PAHs and soot particles with diameters
of up to ~60 nm are defined as classes with increasing molecular mass.
Each class is represented by a combination of lumped pseudo-species (BINs),
each with an assigned H/C.
The first BIN is the species with 20 carbon atoms and mass of about 250 amu,
which is the corannulene.
The first particle of soot is considered of about 3000 amu, which is the BIN5.
COST Meeting on SOOT & PAHs – Sorrento – April 2013
Soot kinetic model: Soot Pseudo-species 13
BIN Mass
[amu]
CxHy Mean Diameter
σ [nm]
H/C
A B C
1 ~ 250 C20H16-C20H10-C20H6 0.76 0.8 0.5 0.3
2 ~ 500 C40H32-C40H20-C40H12 0.96 0.8 0.5 0.3
3 ~ 1000 C80H60-C80H36-C80H24 1.21 0.75 0.45 0.3
4 ~ 2000 C160H112-C160H64-C160H48 1.52 0.7 0.4 0.3
5 ~ 4000 C320H208-C320H64-C320H64 1.91 0.65 0.35 0.20
6 ~ 8000 C640H384-C640H224-C640H96 2.41 0.6 0.35 0.15
7 ~ 15500 C1250H688-C1250H375-C1250H125 3.01 0.55 0.3 0.1
8 ~ 30000 C2500H1250-C2500H625-C2500H250 3.78 0.5 0.25 0.1
9 ~ 61000 C5000H2250-C5000H1000-C5000H500 4.76 0.45 0.2 0.1
10 ~ 121000 C10000H4000-C10000H1500-C10000H1000 5.99 0.4 0.15 0.1
Different H/C ratios
for particles of the same class
the dehydrogenation reactions
the aging of the soot particles
the different degree of methylation of the
pericondensed aromatic species
First
soot
particle
COST Meeting on SOOT & PAHs – Sorrento – April 2013
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BIN Mass
[amu]
CxHy Mean Diameter
σ [nm]
H/C
A B
11 ~ 245000 C20000H7000-C20000H2000 7.55 0.35 0.1
12 ~ 490000 C40000H14000-C40000H4000 9.52 0.35 0.1
13 ~ 970000 C80000H24000-C80000H8000 11.98 0.30 0.1
14 ~ 1950000 C160000H48000-C160000H16000 15.10 0.30 0.1
15 ~ 3900000 C320000H80000-C320000H32000 19.01 0.25 0.1
16 ~ 7800000 C640000H128000-C640000H32000 23.92 0.20 0.05
17 ~ 15100000 C1250000H250000-C1250000H62500 29.90 0.20 0.05
18 ~ 30200000 C2500000H500000-C2500000H125000 37.67 0.20 0.05
19 ~ 60200000 C5000000H1000000-C5000000H250000 47.46 0.20 0.05
20 ~ 121000000 C10000000H2000000-C10000000H500000 59.80 0.20 0.05
Soot kinetic model : Soot Pseudo-species
S. Granata, F. Cambianica, S. Zinesi, T. Faravelli, E. Ranzi, “Detailed Kinetics of PAH and Soot Formation in Combustion Processes: Analogies and Similarities in Reaction
Classes”, presented at the European Combustion Meeting ECM2005, Louvain-la-Neuve, Belgium, April 3-6, 2005, Paper 035.
COST Meeting on SOOT & PAHs – Sorrento – April 2013
16 Acetylene pyrolysis at higher severity
Experimental conditions
• 3.7% C2H2 pyrolysis in Ar
• Shock tube reactor
• 1100-2400 K
• 8 atm
• 0.7 ms
Colket M. B., The pyrolysis of acetylene and vinylacetylene in a single-pulse shock tube. Symposium (International) on Combustion, Volume 21(1), 1986, Pages 851–864.
Experimental and kinetic study of PAHs formation in acetylene pyrolysis through
molecular reaction paths. Soot formation was not reported in these conditions.
The C2H2 feed contains 0.1 to
0.2 % of acetone.
Activation of the low temperature
chain radical process.
COST Meeting on SOOT & PAHs – Sorrento – April 2013
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Acetylene pyrolysis at higher severity
At lower temperatures, C2H2 decomposition mainly follows a radical reaction path, due to
the presence of acetone impurities.
Molecular reactions prevail at intermediate temperature.
At T > 2000 K, radical reaction paths supported by acetylene become dominant.
Compared acetylene flux analysis at three different conditions: 1300, 1600 and 2000 K.
COST Meeting on SOOT & PAHs – Sorrento – April 2013
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Recent studies on the pyrolysis of acetylene in flow reactors significantly affected by
soot formation:
Low pressure pyrolysis of Norinaga and coworkers.
Low temperature pyrolysis of Alzueta and coworkers.
These data report a great detail of several compounds, including species from
hydrogen and methane up to heavy PAHs, such as to dibenzo(ah)anthracene (C22H14),
benzo(ghi)perylene (C22H12), and coronene (C24H12).
The influence of residence time, pressure conditions, and fuel concentration on
acetylene conversion and soot formation is further investigated.
The proper analysis of these data, in which even more than 50% of the feed is
transformed into soot, requires the use of a kinetic scheme able to predict the
formation of heavy PAHs and soot.
Acetylene pyrolysis at higher severity
Norinaga, K., V. M. Janardhanan, O. Deutschmann (2008) , Int. J. Chem. Kinetics. 40(4):199208.
N. E. Sánchez, A. Callejas, A. Millera, R. Bilbao, M.U. Alzueta, Energy 43 (2012) 30-36.
N. E. Sánchez, A. Millera, R. Bilbao, M.U. Alzueta, J. Anal. Appl. Pyrol. (2012) in press.
COST Meeting on SOOT & PAHs – Sorrento – April 2013
19 Acetylene pyrolysis at higher severity Experimental conditions
Pure C2H2 - Flow reactor – T: 1000-1400 K – P: 0.08 atm – τ = 0.5-2 s
Due both to: - wide density variations
- non-isothermal temperature
profile along the reactor
HACA mechanism well explains the successive formation of the different PAHs.
At the highest severity, i.e. 1400 K and 2 s, model predicts a quasi-complete
acetylene conversion with a carbon selectivity to soot higher than 80%.
0.7 s
2 s
Modeling conditions
relevant uncertainty
on the effective
residence time
COST Meeting on SOOT & PAHs – Sorrento – April 2013
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Experimental conditions
3 % C2H2 in N2 - Flow reactor – T = 873 -1473 K – P = 1 atm - τ = 1.5-4 s
Acetylene pyrolysis at higher severity
These data are interesting both for the very severe conditions explored
and also for the accurate details on intermediate PAHs and soot.
τ = 4 s τ = 1.5 s
N. E. Sánchez, A. Callejas, A. Millera, R. Bilbao, M.U. Alzueta, Energy 43 (2012) 30-36.; N. E. Sánchez, A. Millera, R. Bilbao, M.U. Alzueta, J. Anal. Appl. Pyrol. (2012) in press.
COST Meeting on SOOT & PAHs – Sorrento – April 2013
21 Acetylene pyrolysis at higher severity
Carbon selectivity towards soot is lower than 10% in the first series of experiments at
1.5 s and becomes higher than 70% at the highest severity conditions.
N. E. Sánchez, A. Callejas, A. Millera, R. Bilbao, M.U. Alzueta, Energy 43 (2012) 30-36.; N. E. Sánchez, A. Millera, R. Bilbao, M.U. Alzueta, J. Anal. Appl. Pyrol. (2012) in press.
COST Meeting on SOOT & PAHs – Sorrento – April 2013
22 Conclusions
A vast amount of experimental data on acetylene pyrolysis reported in
the literature was collected and reviewed.
A further validation study of a comprehensive kinetic scheme of
pyrolysis and combustion of hydrocarbon fuels is presented in this work.
Simulating all these data covering a wide range of operating conditions
permits to:
- better understand the experimental results
- refine the mechanism and discover its limits and
possible extensions useful to improve its predictive
ability
- study the relative importance of radical and molecular
pathways