Kinetic Modeling of the Mutual Oxidation of NO and Larger Alkanes at

Jul 7, 2005 - Sierra Engineering, Inc., 3050 Fite Circle, Suite 212, Sacramento, California 95827. Y. Koshiishi, N. Matsunaga, and M. Hori. Department...
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Energy & Fuels 2005, 19, 1839-1849

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Kinetic Modeling of the Mutual Oxidation of NO and Larger Alkanes at Low Temperature P. A. Glaude* De´ partement de Chimie-Physique des Re´ actions, UMR 7630 CNRS, INPL-ENSIC, 1 rue Grandville, 54000 Nancy, France

N. Marinov Sierra Engineering, Inc., 3050 Fite Circle, Suite 212, Sacramento, California 95827

Y. Koshiishi, N. Matsunaga, and M. Hori Department of Mechanical System Engineering, Takushoku University, Hachioji, Tokyo 193-0985, Japan Received February 23, 2005. Revised Manuscript Received June 3, 2005

A detailed chemical kinetic modeling investigation of n-butane and n-pentane oxidation interaction with NO to NO2 conversion is presented. The model was validated against experiments obtained in an atmospheric-pressure, quartz flow reactor, which was used to examine the NO oxidation behavior for the temperature range of 600-1100 K and residence times of 0.16-1.46 s. Probe measurement of the species concentrations was performed in the flow reactor using a mixture NO (20 ppm)/air/hydrocarbon (10 ppm). In the chemical kinetic calculation, the time evolution of NO, NO2, hydrocarbons, and reaction intermediates were evaluated using two mechanisms of oxidation of n-butane and n-pentane, coupled with a nitrogen oxide submechanism for all temperatures. The model of the reaction of hydrocarbon uses the same set of parameters previously developed and validated by the DCPR for the oxidation of alkanes, without any specific adjustment. The reactions of coupling between the alkane oxidation and the nitrous compounds were added. The model reproduces the temperature dependence of the conversion of the reactants and the species concentration profiles versus the residence time in the reactor. The results show a strong coupling between the conversion of hydrocarbon in the low-temperature range and the NO-to-NO2 conversion. Both hydrocarbon-fuel oxidation systems are accelerated in the blends, in comparison to undoped or pure hydrocarbon oxidation systems. The NO + •HO2 ) NO2 + •OH and alkylperoxy + NO ) alkyloxy + NO2 reactions have a major role in converting NO to NO2 at the lower temperatures. The same reactions strongly accelerate the oxidation of hydrocarbon by converting •HO2 and alkylperoxy to •OH. Above 900 K, the decrease in NO2 concentration is attributed to HONO formation and reaction of NO2 with small radicals. Calculations with various initial concentrations of NO and n-pentane were performed and analyzed for main reaction channels and most sensitive reactions. The coupling between many mechanisms involving equilibriums such as R + O2, NO + •OH, or NO2 + •CH3 explain the very complex behavior of the mixtures versus temperature. Ignition delay times have been calculated for a mixture of n-pentane/ air with and without NO. Calculated results for conditions representative of engines show the importance of NO kinetics in new combustion modes.

Introduction New combustion modes involve conditions where interactions are possible between hydrocarbons and nitrogen oxides. The ignition and oxidation characteristics of the fuel are modified by the presence of NOx produced in a previous combustion cycle. For example, a strategy of limiting the production of pollutants in new engines is to dilute the reactants with exhaust gas (exhaust gas recirculation, EGR). This process allows reactions with lean mixtures and a lower final temper* Author to whom correspondence should be addressed. E-mail: [email protected].

ature, but can change auto-ignition characteristics, which is a critical parameter in HCCI engines, and the formation of pollutants, i.e., by oxidizing NO to NO2. The study of the NO2 formation mechanism began in 1970s, to explain the presence of NO2 in probe-sampled gases and in the exhaust from gas turbines and domestic combustion appliances. Originally, research showed that NO was oxidized to NO2 by a radical relaxation process mechanism, because of the rapid cooling of hot combustion gas.1-6 These studies suggested that the main NOto-NO2 oxidation route was the reaction with •HO2 radical in the low-temperature range. Although it was later shown that only a small concentration of fuels such

10.1021/ef050047b CCC: $30.25 © 2005 American Chemical Society Published on Web 07/07/2005

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Energy & Fuels, Vol. 19, No. 5, 2005

as hydrocarbons greatly promoted the NO-to-NO2 conversion at low and intermediate temperatures,7-12 whereas NO may be reduced to N2 and HCN at higher temperatures and for different equivalence ratios.13,14 Recent work showed that a small amount of NO conversely promoted the oxidation of fuels.8-12,15-18 Bromly and co-workers8,9 and Nelson and Haynes10 called the conversion process a “mutually sensitized oxidation of NO and fuel”. Because the NO-to-NO2 promotion effect due to fuel oxidation controls NO2 emission from combustion systems, additional research is needed to understand the promotion effect of each fuel type on the NO-to-NO2 conversion process. Furthermore, the promoting effect of NO concentration on the oxidation of fuels needs to be investigated for better prediction of auto-ignition delay times by models used in engine research. Experimental studies focused on the mutual oxidation of fuels and of NO to NO2 through the use of a hot combustion gas-cold air double concentric jets6,11,19 and atmospheric-pressure flow reactors.8-12,15-18 Flow reactor experiments and detailed chemical kinetic calculations were performed to examine the promotion effect of small C1-C3 hydrocarbons12 on the NO-to-NO2 conversion. Experimental work was also performed using n-butane and n-pentane as fuel17 with a preliminary modeling study of the behavior of n-butane. Some other kinetic models have also been developed for reproducing the coupling between the conversion of NO to NO2 and the oxidation of different fuels such as methane16,18 and ethylene,9 as well as C1-C4 alkanes, ethylene, propene, and methanol.20 Previously, no detailed chemical kinetic model has involved the interaction of NO with larger alkanes representative of fuel behavior in the negative temperature coefficient (NTC) region that is of primary importance for auto-ignition. The present paper reports a refined detailed modeling study of the promoting effect of the co-oxidation of (1) Kramlich, J. C.; Malte, P. C. Combust. Sci. Technol. 1978, 18, 91. (2) Cernansky, N. P. In Experimental Diagnostics in Gas Phase Combustion Systems; Zinn, B. T., Bowman, C. T., et al., Eds.; Progress in Astronautics and Aeronautics, Vol. 53; American Institute of Aeronautics and Astronautics (AIAA): New York, 1977; p 83. (3) Hori, M. Combust. Sci. Technol. 1980, 23, 131. (4) Bromly, J. H.; Barnes, F. J.; Johnston, J. C. R.; Little, L. H. J. Inst. Energy 1984, 57, 411. (5) Sano, T. Combust. Sci. Technol. 1984, 238, 129. (6) Hori, M. Proc. Combust. Inst. 1986, 21, 1181. (7) Lyon, R. K.; Cole, J. A.; Kramlich, J. C.; Chen, S. L. Combust. Flame 1990, 81, 30. (8) Bromly, J. H.; Barnes, F. J.; Mandyczewski, R.; Edwards, T. J.; Haynes, B. S. Proc. Combust. Inst. 1992, 24, 899. (9) Doughty, A.; Barned, F. J.; Bromly, J. H.; Haynes, B. S. Proc. Combust. Inst. 1996, 26, 589. (10) Nelson, P. F.; Haynes, B. S. Proc. Combust. Inst. 1994, 25, 1003. (11) Hori, M.; Matsunaga, N.; Malte, P. C.; Marinov, N. M. Proc. Combust. Inst. 1992, 24, 909. (12) Hori, M.; Matsunaga, N.; Marinov, N.; Pitz, W.; Westbrook, C. Proc. Combust. Inst. 1998, 27, 389. (13) Dagaut, P.; Luche, J.; Cathonnet, M. Energy Fuels 2000, 14, 712. (14) Dagaut, P.; Luche, J.; Cathonnet, M. Combust. Flame 2000, 121, 651. (15) Prabhu, S. K.; Li, H.; Miller, D. L.; Cernansky, N. P. SAE Paper No. 932757, 1993. (16) Bendtsen, A. B.; Glarborg, P.; Dam-Johansen, K. Combust. Sci. Technol. 2000, 151, 31. (17) Hori, M.; Koshiishi, Y.; Matsunaga, N.; Glaude, P. A.; Marinov, N. Proc. Combust. Inst. 2002, 29, 2219. (18) Dagaut, P.; Nicolle A. Proc. Combust. Inst. 2004, 30, in press. (19) Hori, M. Proc. Combust. Inst. 1988, 22, 1175. (20) Faravelli, T.; Frassoldati, A.; Ranzi, E. Combust. Flame 2003, 132, 188.

Glaude et al.

alkanes (n-butane and n-pentane) and NO on both conversions. The model is validated against experimental results that previously have been partially presented17 and investigates the behavior of these mixtures in the low-temperature range where auto-ignitions occur in the case of homogeneous charge compression ignition (HCCI) engines or knocking spark-ignited engines. In the chemical kinetic calculations, the time evolution of NO, NO2, hydrocarbons, and reaction intermediates were evaluated using two mechanisms of oxidation of n-butane and n-pentane, coupled with a nitrogen oxide submechanism, which also contains the coupling reactions between hydrocarbons and NOx. Experimental Results The experimental apparatus and some of the results have been previously described;12,15 therefore, the main features of the experimental procedure are summarized here. The dry air flowed into the bottom end of the flow reactor and was heated to a desired reaction temperature by an electric heater. NO (balance N2) and fuel (n-butane or n-pentane, balance N2) from gas cylinders were mixed, and a resultant NO/fuel mixture (900 K, had been reevaluated in ref 17 to better reproduce the decrease of the amount of NO2 in the high-temperature range. The new rate constant was 3.65 × 1013 exp[-8000/(RT)] cm3 mol-1 s-1, which was consistent with the experimental measurements at 298 and 1350 K. Table 1 displays the thermochemical data for some sensitive species involved in the nitrogen chemistry. The submechanism was extended to account for additional conversion pathways involving the alkylperoxy (C4H9O2• and C5H11O2•) and hydroperoxy-alkylperoxy radicals (•O2C4H8OOH and •O2C5H10OOH) with NO. The rate constant has been taken as 2.5 × 1012 exp[358/(RT)] by analogy with the reaction of CH3OO•.28 These reactions produce NO2 and alkoxy radicals RO•, which were not present in the mechanism of oxidation of alkanes. Their reactions have been added, involving the decomposition by β-scission, the isomerization, and the subsequent addition on O2 yielding to chain-branching agents. The rate parameters were estimated following the same rules as those used for other radicals in the automatic generation of the mechanism of oxidation of the alkanes,22,25 but for the decomposition of the RO• radical that was not taken into account previously. The values were obtained, in this case, from Choo and Benson.29 The reactions of resonance-stabilized secondary radicals (•C4H7 and •C5H9) with NO2 were also (26) Bowman, C. T.; Hanson, R. K.; Davidson, D. F.; Gardiner, W. C., Jr.; Lissianski, V.; Smith, G. P.; Golden, D. M.; Frenklach, M.; Goldenberg, M. GRI-Mech 2.11 http://www.me.berkeley.edu/gri_mech/. (27) Dean, A. M.; Bozzelli, J. W. In Combustion Chemistry; Gardiner, W., Jr., Ed.; Springer-Verlag: New York, 1997. (28) Atkinson, R.; Baulch, D. L.; Cox, R. A.; Hampson, R. F., Jr.; Kerr, J. A.; Troe, J. J. Phys. Chem. Ref. Data 1992, 21, 1125. (29) Choo, K. Y.; Benson, S. W. Int. J. Chem. Kinet. 1981, 13, 833.

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Table 2. Reactions Added for the Coupling between the Oxidation of n-pentane Oxidation and the Nitrogen Mechanisma reaction

A

Reactions of Peroxy Radicals with NO CH3CH2CH(OO•)CH2CH3 + NO f NO2 + CH3CH2CH(O•)CH2CH3 CH3CH2CH2CH2CH2OO• + NO f NO2 + CH3CH2CH2CH2CH2O• CH3CH2CH2CH(OO•)CH3+NO f NO2 + CH3CH2CH2CH(O•)CH3 CH3CH(OO•)CH(OOH)CH2CH3 + NO f NO2 + C2H5CHO + CH3CHO + •OH CH3CH2CH(OOH)CH2CH2OO• + NO f NO2 + HCHO + C4H8 + •HO2 CH2(OOH)CH2(OO•)CH2CH2CH3 + NO f NO2 + C3H7CHO + HCHO + •OH CH3CH2CH(OO•)/CH2CH2OOH + NO f NO2 + C2H5CHO + C2H4 + •HO2 CH3CH(OO•)/CH2CH2CH2OO + NO f NO2 + CH3CHO + •CH2CH2CH2OOH CH2(OOH)CH2CH2CH2CH2OO• + NO f NO2 + HCHO + •CH2CH2CH2CH2OOH CH2(OO•)CH(OOH)CH2CH2CH3 + NO f NO2 + C3H7CHO + HCHO + •OH CH3CH2CH(OO•)CH(OOH)CH3 + NO f NO2 + C2H5CHO + CH3CHO + •OH CH3CH(OO•)CH2CH(OOH)CH3 + NO f NO2 + CH3CHO + •CH2CH(OOH)CH3 CH2(OO•)CH2CH2CH(OOH)CH3 + NO f NO2 + HCHO + C2H4 + CH3CHO + •OH CH2(OO•)CH2CH(OH)CH2CH3 + NO f NO2 + HCHO + C4H8 + •OH CH3CH(OO•)CH2CH2CH2OH + NO f NO2 + CH3CHO + C2H4 + •CH2OH CH2(OO•)CH2CH2CH(OH)CH3 + NO f NO2 + HCHO + C2H4 + CH3CHO + •H

2.53 × 1012 2.53 × 1012 2.53 × 1012 2.53 × 1012 2.53 × 1012 2.53 × 1012 2.53 × 1012 2.53 × 1012 2.53 × 1012 2.53 × 1012 2.53 × 1012 2.53 × 1012 2.53 × 1012 2.53 × 1012 2.53 × 1012 2.53 × 1012

b 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

E

reaction number

-358 -358 -358 -358 -358 -358 -358 -358 -358 -358 -358 -358 -358 -358 -358 -358

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Reactions of Resonance-Stabilized Radicals with NO2 CH2)CH•CH2 + NO2 f •C2H3 + HCHO + NO 2.3 × 1013 CH2)CH•CHCH3 + NO2 f •CH3 + C2H3CHO + NO 2.3 × 1013 CH2)CH•CHCH2CH3 + NO2 f •C2H5 + C2H3CHO + NO 2.3 × 1013

0.0 0.0 0.0

Decomposition of RO• Radicals CH3CH2CH(O•)CH2CH3 f C2H5CHO + •C2H5 CH3CH2CH2CH2CH2O• f HCHO + •CH2CH2CH2CH3 CH3CH2CH2CH(O•)CH3 f CH3CHO + •CH2CH2CH3 CH3CH2CH2CH(O•)CH3 f C3H7CHO + •CH3 •CH CH CH(OH)CH CH f C H + C H CHO + •H 2 2 2 3 2 4 2 5 CH3CH•CH2CH2CH2OH f C3H6Y + C2H4 + •OH •CH CH CH CH(OH)CH f C H Y + C H + •OH 2 2 2 3 3 6 2 4

8.0 × 1013 5.0 × 1013 4.0 × 1013 3.0 × 1013 2.0 × 1013 2.0 × 1013 2.0 × 1013

0.0 0.0 0.0 0.0 0.0 0.0 0.0

13500 15600 13500 16800 27700 28700 28700

20 21 22 23 24 25 26

Isomerizations CH3CH2CH(O•)CH2CH3 ) •CH2CH2CH(OH)CH2CH3 CH3CH2CH2CH2CH2O•) CH3•CHCH2CH2CH2OH CH3CH2CH2CH(O•)CH3 ) •CH2CH2CH2CH(OH)CH3 CH2(OO•)CH2CH(OH)CH2CH3 ) CH2(OOH)CH2•C(OH)CH2CH3 CH3CH(OO•)CH2CH2CH2OH ) CH3CH(OOH)CH2CH2•CHOH CH2(OO•)CH2CH2CH(OH)CH3 ) CH2(OOH)CH2CH2•C(OH)CH3

1.0 × 1010 5.9 × 108 5.1 × 109 3.0 × 108 1.0 × 108 5.2 × 107

1.0 1.0 1.0 1.0 1.0 1.0

13900 6800 8600 20500 20000 17000

27 28 29 30 31 32

Additions on O2 • 2CH2CH(OH)CH2CH3 + O2 ) OOCH2CH2CH(OH)CH2CH3 CH3•CHCH2CH2CH2OH + O2 ) CH3CH(OO•)CH2CH2CH2OH •CH CH CH CH(OH)CH + O ) •OOCH CH CH CH(OH)CH 2 2 2 3 2 2 2 2 3 CH2(OOH)CH2•C(OH)CH2CH3 + O2 ) CH2(OOH)CH2C(OO•)(OH)CH2CH3 • CH3CH(OOH)CH2CH2 CHOH + O2 ) CH3CH(OOH)CH2CH2CH(OH)OO• CH2(OOH)CH2CH2•C(OH)CH3 + O2 ) CH2(OOH)CH2CH2C(OO•)(OH)CH3

2.2 × 1019 2.2 × 1019 2.2 × 1019 2.2 × 1019 2.2 × 1019 2.2 × 1019

-2.5 -2.5 -2.5 -2.5 -2.5 -2.5

Decomposition of Hydroperoxy Alkyl Radicals in Cyclic Ethers CH2(OOH)CH2•C(OH)CH2CH3 f cycloC5H9O(OH) + •OH 2.5 × 1010 CH3CH(OOH)CH2CH2•CHOH f cycloC5H9O(OH) + •OH 2.1 × 109 CH2(OOH)CH2CH2•C(OH)CH3 f cycloC5H9O(OH) + •OH 2.1 × 109

0.0 0.0 0.0

15250 6500 6500

39 40 41

Degenerate Branchings CH2(OOH)CH2C(OO•)(OH)CH2CH3 f •HO2 + HCHO + C2H4 + •CH2CHO + •OH CH3CH(OOH)CH2CH2CH(OH)OO•f •HO2 + CH3CHO + C2H4 + •CHO + •OH CH2(OOH)CH2CH2C(OO•)(OH)CH3 f •HO2 + HCHO + C2H4 + CH3•CO + •OH

1.0 1.0 1.0

23500 17000 20000

42 43 44

•CH

a

5.9 × 108 5.2 × 107 1.0 × 108

0.0 0.0 0.0

0.0 0.0 0.0 0.0 0.0 0.0

17 18 19

33 34 35 36 37 38

Rate constants are in the form ATb exp[-E/(RT)] in cm3, s, mol, K, cal units.

added, yielding NO and an unsaturated alkoxy free radical that was considered to decompose directly by β-scission: •

C4H7 + NO2 f •CH3 + acrolein + NO

•

C5H9 + NO2 f •C2H5 + acrolein + NO

The rate constant was 2.3 × 1013 by analogy with the reaction of allyl radical with NO2.30 Because of the higher temperature, many reactions of importance in atmospheric chemistry have been neglected here, such as the addition of alkyl radicals larger than CH3 to NO2 or the addition of peroxy radicals to NO yielding a stabilized adduct. Table 2 summarizes the (30) Slagle, I. R.; Yamada, F.; Gutman, D. J. Am. Chem. Soc. 1981, 103, 149.

reactions added to the n-pentane oxidation model and to the nitrogen submechanism for their connection. The detailed chemical kinetic model consisted of 870 reactions and 167 species in the case of n-butane and in 1143 reactions and 220 species in the case of n-pentane. The thermochemical data was calculated using THERGAS31 in the case of the hydrocarbons and obtained elsewhere32 in the case of nitrogen containing compounds. The numerical calculations were performed using the CHEMKIN-II/SENKIN computer programs,33 considering the reactor as an ideal plug flow reactor with negligible axial diffusion. A complete listing of the chemical kinetic mechanisms and thermodynamics used (31) Muller, C.; Michel, V.; Scacchi, G.; Come, G. M. J. Chim. Phys. 1995, 92, 1154. (32) Marinov, N. M. LLNL Report No. UCRL-JC-129372, Lawrence Livermore National Laboratories, Berkeley, CA, 1998.

Mutual Oxidation of NO and Larger Alkanes at Low T

Figure 1. Oxidation of a n-butane (10 ppm)/NO (20 ppm)/air mixture, as a function of the temperature at a residence time of 1.46 s: (a) [C4H10]/[C4H10]0 ratio and (b) [NO2]/[NOx] ratio. The points refer to experimental data and the lines refer to calculated data.

Energy & Fuels, Vol. 19, No. 5, 2005 1843

Figure 2. Oxidation of a n-pentane (10 ppm)/NO (20 ppm)/ air mixture, as a function of the temperature at a residence time of 1.46 s: (a) [C5H12]/[C5H12]0 ratio and (b) [NO2]/[NOx] ratio. The points refer to experimental data and the lines refer to calculated data.

in the modeling portion of the study can be obtained from the authors.

Results and Discussion Figure 1 compares the calculated and the experimental normalized concentration of n-butane (Figure 1a) and the NO2/NOx ratio (Figure 1b) for the mixture of 10 ppm of n-butane and 20 ppm of NO in air, as a function of temperature for a constant residence time of 1.46 s. Figure 2 shows the experimental and simulated results in the case of 10 ppm of n-pentane under the same conditions. The model reproduces the experimental results very well and clearly shows the promoting effect of the cooxidation: a strong coupling appears between the conversion of hydrocarbon in the low-temperature range and the NO-to-NO2 conversion. In the case of these blends, the oxidation of both the hydrocarbon fuel and NO is accelerated, when compared to the oxidation of the pure hydrocarbon fuel or NO. However, the coupling with C5H12 is more efficient than C4H10 for the entire temperature range. This can be explained by the higher reactivity of C5H12 in oxidation below the negative temperature coefficient (NTC) region by yielding peroxides, which are chain-branching agents. To ensure the model’s ability to reproduce the oxidation of hydrocarbons, ignition delay times measured by Minetti et al.34 in a rapid compression machine for the oxidation of n-pentane have been simulated. Figure 3 compares the calculations to the experimental ignition delay times. Both cool flame and auto-ignition delay times are correctly reproduced for two pressure ranges (33) Kee, R. J.; Rupley, F. M.; Meeks, E.; Miller, J. A. Report No. SAND 96-8216, Sandia National Laboratories, Albuquerque, NM, 1996. (34) Minetti, R.; Ribaucour, M.; Carlier, M.; Sochet, L. R. Combust. Sci. Technol. 1996, 113-114, 179-192.

Figure 3. Ignition delay times versus temperature for stoichiometric n-pentane/oxygen/nitrogen/argon mixtures in a rapid compression machine (from Minetti et al.34) at Pc values of (a) 6-10 bar for a load of 138.5 mol/m3 (initial pressure of 300 Torr) and (b) 8-11 bar for a load of 179.5 mol/m3 (initial pressure of 400 Torr).

(6-10 bar and 8-11 bar) and temperatures in the range of 650-900 K. At low temperature, i.e., 1000 K, the alkyl radicals R• produced by initiation and hydrogen abstraction reaction steps decompose in a unimolecular reaction to alkene and a smaller alkyl radical. The scheme is similar to that of thermal decomposition and only the small free radicals with longer lifetimes will react with oxygen. The most promoting chain-branching reaction responsible for strong combustion auto-acceleration is •

beyond 1100 K.

H + O2 ) •OH + •O•

Glaude et al.

Flow rate analyses reveal that the NO to NO2 conversion is strongly temperature-dependent. The reactions

NO + •HO2 ) NO2 + •OH NO + ROO• ) NO2 + RO• have a major role in converting NO to NO2 at the lower temperatures. The same reactions strongly accelerate the oxidation of hydrocarbon by exchanging unreactive radicals such as •HO2 and alkylperoxy ROO• into very reactive •OH and RO• free radicals. Resonance-stabilized radicals are produced from butenes and pentenes, which are primary products in the oxidation of n-butane and n-pentane, respectively. They react with NO2 and yield NO and an alkoxy radical. This reaction is sensitive in the low-temperature range by regenerating NO and exchanging the unreactive allylic radicals to reactive alkoxy radicals. At >900 K, NO + HO2 is still the main channel producing NO2, but new reaction paths become faster and cause a decrease of NO2 concentration. NO2 recycles back to NO, mostly through the sequence

NO2 + •HO2 ) HONO + O2 HONO (+ M) ) NO + •OH (+ M) The higher temperature also favors the formation of O atoms in the gas phase via the branching step: •

H + O2 ) •OH + •O•

O atoms then react with NO2, yielding NO:

NO2 + •O• ) NO + O2 Figure 4 compares the experimental and simulated normalized concentration of n-pentane and NO2/NOx ratio, as a function of residence time at 700, 900, and 1100 K. These temperatures correspond to high reactivity in the low-temperature region, the maximum conversion of NO to NO2 in the intermediate temperature range, and the high-temperature chemistry region, respectively. The model reproduces the time dependence in the experimental results well. For the highest temperature, the NO2 concentration reaches a maximum during the consumption of the fuel before being consumed by O atoms. A small shift exists between the simulations and the experiments for the initial reaction time. This may be explained by the difficulty of knowing precisely where the reaction begins in the mixing section of the flow reactor. Figure 5 displays a sensitivity analysis for the npentane consumption along the reactor at 700 (Figure 5a), 900 (Figure 5b), and 1100 K (Figure 5c) for the oxidation of the mixture n-pentane (10 ppm)/NO (20 ppm). Under each condition, the most promoting reaction is the hydrogen abstraction from the reactant by •OH. At 700 K, in the C/H/O submechanism, the isomerization of C5H11OO• into •C5H10OOH is a sensitive promoting reaction, whereas the disproportionation of •HO with •OH inhibits the reaction. As the residence 2 time increases, hydrogen abstraction from formaldehyde

Mutual Oxidation of NO and Larger Alkanes at Low T

Energy & Fuels, Vol. 19, No. 5, 2005 1845

Figure 4. Normalized NO2/NOx ratio and conversion of n-pentane in the oxidation of a mixture of n-pentane (10 ppm)/ NO (20 ppm)/air, as a function of the residence time at 700, 900, and 1100 K. Points refer to experimental data and lines refer to calculated data.

HCHO by •OH also inhibits promotion by exchanging the chain carrier •OH with the less-reactive •CHO. NO strongly promotes the consumption of n-pentane via the two reactions

NO + •HO2 ) NO2 + •OH NO + CH3OO• ) NO2 + CH3O• where unreactive •HO2 and CH3OO• converts to reactive and CH3O•, followed by decomposition to HCHO • and H. Interestingly, NO exhibits an inhibiting effect, because of the termination step with •OH, which yields HONO that is stable at this temperature. At 900 K, the branching step between •H + O2 ) •OH + •O• becomes very promoting, whereas the concurrent addition •H + O2 (+ M) ) •HO2 (+ M) strongly inhibits the reaction. The reactions of •OH with HCHO and •HO2 are still inhibiting. NO enhances hydrocarbon-fuel conversion via NO + •HO2 ) NO2 + •OH followed by fuel + •OH ) R• + H2O, whereupon NO2 is recycled back to NO through the reaction step •OH

Figure 5. Normalized sensitivity coefficients for the consumption of the n-pentane in the oxidation of a mixture of n-pentane (10 ppm)/NO (20 ppm)/air, as a function of the residence time at (a) 700, (b) 900, and (c) 1100 K.

promoting, whereas •H + O2 + M ) •HO2 + M and the reactions of •OH with •HO2 and formaldehyde inhibit conversion. Also, the promoting oxidation step of CO by •OH, yielding CO and H atoms, is of secondary impor2 tance. Nitrous species appear in NO + •HO2, which is sensitive at the first stage of the reaction, and in

NO2 + •CH3 ) NO + CH3O•

NO2 + •HO2 ) HONO + O2

This reaction step converts •CH3 to the reactive CH3O• species and regenerates NO. Nitrous species react via the autocatalytic cycle,

which accelerates the reaction rate by yielding HONO that decomposes immediately to NO and the chain carrier •OH. Last, the inhibiting step,

NO + •HO2 ) NO2 + •OH

NO2 + •O• ) NO + O2

NO2 + •CH3 ) NO + CH3O•

consumes O atoms and explains the decay of NO2 in the high-temperature range. The rate constant of the reactions sensitive to n-pentane and NO conversion are summarized in Table 3. The validated model has been used thereafter to study the effect of the variation of the concentrations of NO

and strongly accelerates hydrocarbon-fuel consumption at temperatures just above the NTC. At 1100 K, hydrogen abstraction from n-pentane and the branching reaction between •H and O2 are still

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Glaude et al.

Table 3. Selected Sensitive Reactions for the Co-oxidation of NO and n-Pentanea

a

reactions

A

b

E

O2 + H ) OH + O O2 + H (+ M) ) HO2 (+ M) k0 Troe fit third-body efficiency: O2, 0.4; N2, 0.67; Ar, 0.29; CO, 0.75; CO2, 1.5 HO2 + OH ) H2O + O2 HCHO + OH ) CHO + H2O CO + OH ) CO2 + H NO + HO2 ) NO2 + OH NO + CH3OO ) NO2 + CH3O NO + OH (+ M) ) HONO (+ M) k0 NO2 + HO2 ) HONO + O2 NO2 + O ) NO + O2 NO2 + CH3 ) CH3O + NO CH3NO2 (+ M) ) NO2 + CH3 (+ M) k0

9.80 × 4.52 × 1013 1.8 × 1018 0.5

0 0 -0.8 1

14800 0 0 1 × 108

2.9 × 1013 3.4 × 109 6.3 × 106 2.11 × 1012 2.53 × 1012 2.0 × 1013 2.33 × 1023 3.65 × 1013 3.9 × 1012 1.5 × 1013 1.78 × 1016 1.26 × 1016

0 1.18 1.5 0 0 0 -2.4 0 0 0 0 0

-500 -400 -500 -480 -358 0 0 8000 -240 0 58500 42000

1013

Rate constants are in the form ATb exp[-E/(RT)] in cm3, s, mol, K, cal units.

est reactivity is reached for 20 ppm of NO, even if the fuel is consumed at lower temperatures with 10 ppm of NO. As the amount of NO increases, fuel consumption is initiated only at higher temperatures, while the NTC region becomes smaller. With 100 ppm of NO, fuel consumption begins at >700 K and no NTC behavior is exhibited. At >800 K, reactivity increases with the concentration of NO. On the other hand, the maximum value of the NO2/NOx ratio in the high-temperature region decreases as the amount of NO increases, especially when NO is present in excess to the fuel concentration. As explained previously, the promoting effect at low temperature comes from the reactions of NO with •HO2 and peroxy radicals ROO•. When the concentration of NO increases, the concurrent termination with •OH becomes faster:

NO + •OH (+ M) ) HONO (+ M) In this low-temperature range, HONO reacts slowly with itself or with •OH: Figure 6. Simulation of the oxidation of 10 ppm of n-pentane in air without NO and with the adjunction of 10, 20, 40, and 100 ppm of NO, as a function of the temperature for a residence time of 1.46 s: (a) conversion of the n-pentane and (b) NO2/NOx ratio.

and of the fuel, particularly in the low-temperature range, which corresponds to the auto-ignition conditions. Figure 6 presents the simulated concentration of npentane (Figure 6a) and the NO2/NOx ratio (Figure 6b), as a function of temperature for the constant residence time of 1.46 s and different amounts of NO in the blend (without NO, 10 ppm, 20 ppm, 40 ppm, 100 ppm). The calculation without addition of NO exhibits the classical behavior of alkane oxidation, i.e., some reactivity in the low-temperature range with a maximum at ∼650 K, a strong negative temperature coefficient, and hightemperature reactivity. A small amount of NO strongly promotes the conversion below and in the NTC region. As the NO concentration increases, relative to a fixed hydrocarbon-fuel concentration, fuel consumption increases at first but decreases dramatically for larger NO concentrations in the low-temperature range. The high-

HONO + •OH ) NO2 + H2O 2HONO ) NO + NO2 + H2O These sequences of termination steps producing NO2 and H2O are consuming the chain carrier radical and strongly inhibit the reaction. For higher temperatures, at >800 K, the HONO molecules become less stable and decompose quickly in the reverse reaction:

HONO (+ M) ) NO + •OH (+ M) Under these conditions, HONO is produced mostly by NO2:

NO2 + •HO2 ) HONO + O2 The sequence is consequently converting unreactive • 2 to OH and promotes the reactivity for any initial amount of NO. To examine the possible kinetic effect role of NO2 on the oxidation of alkanes, Figure 7 displays the calculated normalized amounts of n-pentane and NO2, using initial conditions with NO2 instead of NO, as previously

•HO

Mutual Oxidation of NO and Larger Alkanes at Low T

Energy & Fuels, Vol. 19, No. 5, 2005 1847

Figure 7. Simulation of the ratios [C5H12]/[C5H12]0. and [NO2]/ [NOx] for the oxidation of 10 ppm of n-pentane in air with and without the addition of 20 ppm of NO2, as a function of the temperature and for a residence time of 1.46 s..

shown in Figure 2: 10 ppm of hydrocarbon and 20 ppm of NO2 in air with a residence time of 1.46 s. It seems that the addition of NO2 slightly promotes the conversion of C5H12 below the NTC and at >950 K. In the NTC region, a small conversion of NO2 to NO occurs, through reactions involving •CH3:

NO2 + •CH3 ) NO + CH3O• NO2 + •CH3 ) CH3NO2

Figure 8. Simulation of the [C5H12]/[C5H12]0 ratio for the oxidation of 10, 40, 100 and 1000 ppm of n-pentane in air with (a) and without (b) the addition of 20 ppm of NO, in function of the temperature and for a residence time of 1.46 s.

CH3NO2 reacts by hydrogen abstraction with radicals and then decomposes to HCHO and NO. The small amount of NO enhances hydrocarbon oxidation by converting •HO2 and CH3OO• to •OH and CH3O•. This promoting effect is in competition with the inhibiting cycle:

NO2 + •HO2 ) HONO + O2 HONO + •OH ) NO2 + H2O At higher temperatures, HONO decomposes directly to and NO, which may explain the accelerating effect of the addition of NO2 as that of NO. Figures 8 and 9 show the computational results of the co-oxidation of 20 ppm of NO with different amounts of n-pentane (10, 40, 100, and 1000 ppm) in air for a residence time of 1.46 s. Figure 8 displays the normalized concentration of the hydrocarbon with NO (Figure 8a) and without NO (Figure 8b). For the four mixtures, NO promotes the conversion of n-pentane, except in the case of the initial concentration of 1000 ppm of C5H12 at 850-900 K. The effect of NO is more dramatic for the smaller amounts of C5H12, for which the NTC becomes very small. The shape of the hydrocarbon profile, as a function of temperature, is very complex with the addition of NO. This is shown by the many changes in the slope appearing for 40 and 100 ppm of n-pentane. While the amount of n-pentane increases from 10 to 1000 ppm, the minimum temperature for which the reaction starts decreases from 550 K to 480 K in the case of the addition of NO. Without the addition of NO, this temperature is constant and equal to 590 K. The extent of the NTC region is also reduced by the addition of NO. This effect is more important for the smallest concentration of hydrocarbon. •OH

Figure 9. Simulation of the oxidation of 10, 40, 100, and 1000 ppm of n-pentane in air with the addition of 20 ppm of NO, in function of the temperature and for a residence time of 1.46 s: (a) concentration of NO2, (b) concentration of CH3NO2, and (c) concentration of HONO.

Figure 9 displays the amounts of NO2 (Figure 9a), CH3NO2 (Figure 9b), and HONO (Figure 9c), in units

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of ppm, for the four mixtures, as a function of temperature. For all mixtures, at the lowest temperature, NO is completely converted to NO2. When increasing the temperature, the amount of NO2 decreases and some HONO appears; its concentration at ∼600 K, which is similar to ∼2 ppm, does not change much with the alkane concentration. At 600-900 K, the main change is the competition between NO2 and CH3NO2. The concentration of CH3NO2 increases rapidly with the amount of n-pentane and is the major compound in this temperature range when the concentration of hydrocarbon is >100 ppm. At >900 K, CH3NO2 concentration decreases and new maxima are reached by the concentration of NO2 and HONO. For higher temperature, all concentrations decrease and NO is the major nitrous compound. The reactivity of the alkane and the reaction of the nitrous compounds are strongly dependent on the relative amounts of the reactant. Many interactions happen between the two mechanisms, as shown by the complex profiles. At