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

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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

15 Soot kinetic model: Reaction classes

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

COST Meeting on SOOT & PAHs – Sorrento – April 2013

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Thank you

for your attention