C. Saggese, N. E. Sanchez, A. Callejas, A. Millera, R. Bilbao, M. U. Alzueta, A. Frassoldati, A. Cuoci, T. Faravelli, E. Ranzi

Transcription

1 Dipartimento di Chimica, Materiali e Ingegneria Chimica G. Natta Politecnico di Milano in collaboration with: A Kinetic Modeling Study of Polycyclic Aromatic Hydrocarbons (PAHs) and Soot Formation in Acetylene Pyrolysis C. Saggese, N. E. Sanchez, A. Callejas, A. Millera, R. Bilbao, M. U. Alzueta, A. Frassoldati, A. Cuoci, T. Faravelli, E. Ranzi

2 Outline 2 Aim of the work: soot modeling in combustion and pyrolysis Study of acetylene pyrolysis: primary reaction kinetics (gas phase kinetic scheme) C 4 H 2 and C 4 H 4 pyrolysis C 2 H 2 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) Conclusions C 2 H 2 pyrolysis at higher severity of operating conditions (Colket et al., Norinaga et al., Sanchez et al.)

3 Soot formation Mechanisms (1) 3 Particle-particle interactions Main stages of SOOT formation throughout the flame POST FLAME ZONE FLAME ZONE BURNER Particle inception PAH chemistry Gas-phase chemistry PREMIXED FUEL + OXIDIZER Literature: S. Izvekov, A. Violi, J. Chem. Theory Comput. 2 (2006)

4 Soot formation Mechanisms (2) 4 Soot chemistry Particle-particle interactions Main stages of SOOT formation throughout the flame POST FLAME ZONE FLAME ZONE Particle inception PAH chemistry BURNER Gas-phase chemistry PREMIXED FUEL + OXIDIZER Literature: S. Izvekov, A. Violi, J. Chem. Theory Comput. 2 (2006)

5 Soot formation Mechanisms (3) 5 Molecules as key precursor in soot formation Particle-particle interactions HC CH Particle inception PAH chemistry Gas-phase chemistry Oligomers of aromatic compounds (OAC) Pericondensed aromatic compounds (PAC) Literature: A. D Anna et al., Combust. Flame 157 (2010)

6 Coupling gas phase and soot kinetic model 6 N-propylbenzene... Acenaphthalene Pyrene Phenanthrene Naphthalene Ethylbenzene Xylene Toluene Benzene Modular and hierarchical approach nc 7 -ic 8 C 6 Chlorinated species C 3 Methyl esters C 2 CH 4 CO H 2 -O 2 MAIN MECHANISMS Aromatics Soot High temperature mechanism for PAH and soot (HT1303s) ~300 species ~16500 reactions NO x C. Saggese et al., A wide range kinetic modeling study of pyrolysis and oxidation of benzene, Combust. Flame (2013),

7 Aim of the work 7 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. Fuel Reactor Temperature (K) Pressure (atm) Residence time Feed composition References Shock Tube ms 2.5% C 2 H 2 in Ar Hidaka et al. (1996) C 2 H 2 Shock Tube ms 3.2% C 2 H 2 in Ne/Ar Wu et al. (1987) Shock Tube ms 3. 7% C 2 H 2 in Ar Colket (1986) Plug Flow s 1-3% C 2 H 2 in N 2 Sanchez et al. (2012) Plug Flow s C 2 H 2 Norinaga et al. (2008) C 4 H 4 Shock Tube ms 1% C 4 H 4 in Ar Colket (1986) Shock Tube ms 2% C 4 H 4 in Ne Kiefer et al. (1988) Shock Tube ms 1% C 4 H 2 in Ne Kern et al. (1990) Shock Tube ms 1% C 4 H 2 /1% C 2 H 2 in Ne Kern et al. (1990) C 4 H 2 Shock Tube ms 1% C 4 H 2 /1% H 2 in Ne Kern et al. (1990) Shock Tube ms 1% C 4 H 2 in Ar Hidaka et al. (2002) Shock Tube ms 1% C 4 H 2 /1-4% H 2 in Ar Hidaka et al. (2002)

8 Acetylene pyrolysis: primary kinetics 8 Radical path Molecular path The most important reactions considered in this analysis and constituting the core mechanism of acetylene pyrolysis are: C 2 H 2 + C 2 H 2 = C 4 H 4 C 2 H 2 + C 2 H 2 = C 4 H 2 + H 2 C 4 H 4 = C 4 H 2 + H 2 C 2 H 2 + C 2 H 2 = C 4 H 3 + H C 4 H 4 = C 4 H 3 + H

9 Acetylene pyrolysis: primary kinetics 9 Radical path Molecular path The most important reactions considered in this analysis and constituting the core mechanism of acetylene pyrolysis are: C 2 H 2 + C 2 H 2 = C 4 H 4 C 2 H 2 + C 2 H 2 = C 4 H 2 + H 2 C 4 H 4 = C 4 H 2 + H 2 C 2 H 2 + C 2 H 2 = C 4 H 3 + H C 4 H 4 = C 4 H 3 + H

10 Acetylene pyrolysis at lower severity 10 Experimental conditions 2.5% C 2 H 2 pyrolysis in Ar Shock tube reactor K atm ms The C 2 H 2 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 C 4 H 4 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: (1996).

11 Acetylene pyrolysis at lower severity 11 Experimental conditions 3.2% C 2 H 2 pyrolysis in Ne/Ar Shock tube reactor K atm 0.75 ms This is a study at high temperature conditions ( K), where the free radical pathway is favored by the formation of the very stable polyacetylenes (C 4 H 2, C 6 H 2,..). Molecular paths account for less than 20% of acetylene decomposition at 2032 K. Low pressure and limited reaction times the carbon selectivity to heavier species passes from 5% at 2032 K to 40% at 2534 K Wu, C. H., Singh, H. J., Kern, R. D., Pyrolysis of Acetylene Behind Reflected Shock Waves, International Journal of Chemical Kinetics, Vol. 19, (1987).

12 Soot kinetic model : Discrete sectional method 12 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. BIN A BIN B BIN C Experimental data [1] Molecular weight [amu] [1] K.H. Homann, H.G. Wagner, Eleventh Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1967, p. 3

13 Soot kinetic model: Soot Pseudo-species 13 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 BIN Mass [amu] CxHy Mean Diameter σ [nm] H/C A B C 1 ~ 250 C 20 H 16 -C 20 H 10 -C 20 H ~ 500 C 40 H 32 -C 40 H 20 -C 40 H ~ 1000 C 80 H 60 -C 80 H 36 -C 80 H First soot particle 4 ~ 2000 C 160 H 112 -C 160 H 64 -C 160 H ~ 4000 C 320 H 208 -C 320 H 64 -C 320 H ~ 8000 C 640 H 384 -C 640 H 224 -C 640 H ~ C 1250 H 688 -C 1250 H 375 -C 1250 H ~ C 2500 H C 2500 H 625 -C 2500 H ~ C 5000 H C 5000 H C 5000 H ~ C H C H C H

14 Soot kinetic model : Soot Pseudo-species 14 BIN Mass [amu] CxHy Mean Diameter σ [nm] A H/C B 11 ~ C H C H ~ C H C H ~ C H C H ~ C H C H ~ C H C H ~ C H C H ~ C H C H ~ C H C H ~ C H C H ~ C H C H 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.

15 Soot kinetic model: Reaction classes 15

16 Acetylene pyrolysis at higher severity 16 Experimental and kinetic study of PAHs formation in acetylene pyrolysis through molecular reaction paths. Soot formation was not reported in these conditions. Experimental conditions 3.7% C 2 H 2 pyrolysis in Ar Shock tube reactor K 8 atm 0.7 ms The C 2 H 2 feed contains 0.1 to 0.2 % of acetone. Activation of the low temperature chain radical process. Colket M. B., The pyrolysis of acetylene and vinylacetylene in a single-pulse shock tube. Symposium (International) on Combustion, Volume 21(1), 1986, Pages

17 Acetylene pyrolysis at higher severity 17 Compared acetylene flux analysis at three different conditions: 1300, 1600 and 2000 K. At lower temperatures, C 2 H 2 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.

18 Acetylene pyrolysis at higher severity 18 Recent studies on the pyrolysis of acetylene in flow reactors significantly affected by soot formation: Low pressure pyrolysis of Norinaga and coworkers. Norinaga, K., V. M. Janardhanan, O. Deutschmann (2008), Int. J. Chem. Kinetics. 40(4): Low temperature pyrolysis of Alzueta and coworkers. N. E. Sánchez, A. Callejas, A. Millera, R. Bilbao, M.U. Alzueta, Energy 43 (2012) N. E. Sánchez, A. Millera, R. Bilbao, M.U. Alzueta, J. Anal. Appl. Pyrol. (2012) in press. 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 (C 22 H 14 ), benzo(ghi)perylene (C 22 H 12 ), and coronene (C 24 H 12 ). 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.

19 Acetylene pyrolysis at higher severity Experimental conditions Pure C 2 H 2 - Flow reactor T: K P: 0.08 atm τ = s 19 Modeling conditions 0.7 s 2 s Due both to: - wide density variations - non-isothermal temperature profile along the reactor relevant uncertainty on the effective residence time HACA mechanism well explains the successive formation of the different PAHs. At the highest severity, i.e K and 2 s, model predicts a quasi-complete acetylene conversion with a carbon selectivity to soot higher than 80%.

20 Acetylene pyrolysis at higher severity τ = 4 s These data are interesting both for the very severe conditions explored and also for the accurate details on intermediate PAHs and soot. Experimental conditions 3 % C 2 H 2 in N 2 - Flow reactor T = K P = 1 atm - τ = s 20 τ = 1.5 s N. E. Sánchez, A. Callejas, A. Millera, R. Bilbao, M.U. Alzueta, Energy 43 (2012) ; N. E. Sánchez, A. Millera, R. Bilbao, M.U. Alzueta, J. Anal. Appl. Pyrol. (2012) in press.

21 Acetylene pyrolysis at higher severity 21 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) ; N. E. Sánchez, A. Millera, R. Bilbao, M.U. Alzueta, J. Anal. Appl. Pyrol. (2012) in press.

22 Conclusions 22 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

23 23 Thank you for your attention