Reduced Mechanism for Hybrid NOx

Reduced Mechanism for Hybrid NOx...
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Energy Fuels 2009, 23, 5920–5928 Published on Web 09/16/2009

: DOI:10.1021/ef900666k

Reduced Mechanism for Hybrid NOx Control Process Yu Lv, Zhihua Wang,* Junhu Zhou, and Kefa Cen State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering, Zhejiang University, Hangzhou, 310027, Zhejiang, China Received June 30, 2009. Revised Manuscript Received August 25, 2009

To accurately predict hybrid NOx control process, a joint mechanism is initially developed based on the NOxOUT mechanism of Rota et al. and the A˚A scheme of Zebetta et al. Directed, related graphs and rateof-product were employed to make the joint mechanism simplified, finally leading to a scheme including 44 species and 150 elementary reactions. Validation of the joint mechanism is strictly conducted at selective noncatalytic reduction, reburning, and hybrid NOx reducing conditions, by comparing the predicted results of the joint mechanism with reported experimental data. Then in order to handle CFD simulation of practical problems, a reduced mechanism, including 18 global steps and 22 species, was accordingly established. The testing of the reduced mechanism was carried out by comparing its predicted results with those of the joint one. Perfect coherency between them is observed at different operating conditions, and the deviation is negligible compared with the general measuring uncertainty. The joint mechanism can serve as the main chemical scheme in simple flow computations and the reduced one is expected to be directly integrated to CFD modeling of industrial equipments with complex flow and geometry configuration.

kinetics, there are two ways to coherently unite these two techniques: advanced reburning and enhanced SNCR. A typical reburning process is to create a fuel-rich ambience where CHi can react with NOx and avoid being quickly oxidized, and advanced reburning is to further increase NOx reduction by injecting NHi into this ambience. As shown in Figure 1, after O2 is largely consumed by reburning, its scarcity hinders the reagent oxidization in the SNCR regime, benefiting NHi reacting with NOx. Moreover, Tree4 and Alzueta5 have experimentally proven that, compared with traditional SNCR, relatively high temperature in advanced reburning promotes the reducing effect of NHi on NOx. On the other hand, typical SNCR is applied in postcombustion zone located in oxygen-rich (fuel-lean) side, and its main drawback is the narrow working temperature window.6,7 Enhanced SNCR, through the addition of methane or natural gas, can enlarge the OH pool at oxygen-rich flue gas and widen the temperature window, especially toward the low temperature direction. Zhang8 and Bae9 have confirmed this phenomenon through sophisticated measurements, providing good proofs for kinetics modeling and verification. To keep pace with the developing trend of NOx control and better guide technical transformation and operation, kinetics investigation and CFD modeling on hybrid NOx control process play indispensable roles and have obtained some

1. Introduction Nowadays, environmental protection has become a strategic topic when individual government makes future planning for economic and social development. Effectively controlling pollutant emissions is one of the most recurrent problems related to environmental protection. Accordingly, the discharge standard of NOx has become stricter and stricter to the industries that utilize large amount of fossil fuel. Since the 1980s, encouraged by local governments and industries, many researchers have been engaged in creating new technologies for NOx reduction. Among these innovative technologies, reburning and SNCR (selective noncatalytic reduction) are the two most successful methods in term of reduction efficiency, system complexity, and project investment. Also, many demonstrational projects1-3 have been constructed and have shown their advantages and applicability. However, due to the more restricted emission and limited technical expense, the expected reduction will not be achieved by any single NOx control technique. Consequently, a hybrid NOx control technique has arisen and has been paid more attention. As called, hybrid NOx reduction defined in the present work is to use CHi and NHi (including HNCO), which are, respectively, the main reagent in reburning and SNCR, in combination to reduce NOx. Yet, these reagents have different working conditions and characteristics, so reburning and SNCR are normally relatively independent. The primary chemical channels involved are summarized in Figure 1. On the basis of the current understanding on this

(4) Tree, D. R.; Clark, A. W. Fuel 2000, 79, 1687–1695. (5) Alzueta, M. U.; Bilbao, R.; Millera, A.; Oliva, M.; Iba~ nez, J. C. Energy Fuels 1998, 12, 1001–1007. (6) Lyon R. K.; Benn D. Seventeenth Symposium (International) on Combustion; Combustion Institute: Pittsburgh 1979; pp 601-610. (7) Duo W.; Dam-Johansem K.; Østergaard K. Twenty-Third Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, 1990, pp 297-303. (8) Zhang, Y.; Cai, N.; Yang, J.; Xu, B. Chemosphere 2008, 73, 650– 656. (9) Bae, S. W.; Roh, S. A.; Kim, S. D. Chemosphere 2006, 65, 170–175.

*To whom correspondence should be addressed. Telephone: 86-57187952443-8027. Fax: þ86-571-87953162. E-mail: [email protected]. (1) Yang, W.; Zhou, Z.; Zhou, J; L.V., H.; Liu, J.; Cen, K. Environ. Eng. Sci. 2009, 26 (2), 311–317. (2) Kim, H. S.; Shin, M. S.; Jang, D. S.; Ohm, T. I. Appl. Therm. Eng. 2004, 24, 2117–2129. (3) Kuo, J.-H.; Tseng, H. H.; Srinivasa, R. P.; Wey, M. Y. Appl. Therm. Eng. 2008, 28 (17-18), 2305–2314. r 2009 American Chemical Society

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them by assuming their net variation rates to be zero. As a result, the corresponding mass balance equations will be degraded to normal algebraic equations and totally decoupled from initial differential system. Then, a reduced mechanism with several global chemical steps is finally formed. Nearly all reduced mechanisms were obtained according to this procedure, for example, the reduction by Xu et al.22,23 for ammoniabased SNCR and the work by Mallampalli et al.24 and Giral et al.25 for reburning. In the present work, our reduced mechanism is accordingly developed. There are three main goals expected in this paper: (1) to construct a coherent joint mechanism used for hybrid NOx control process through strict selection and certain simplification for industrial application; (2) to develop a reduced mechanism adaptable to CFD simulation from the joint one; (3) to test the aforementioned mechanisms under various operating conditions and demonstrate their accuracy and reliability.

Figure 1. Demonstrative chart for a hybrid NOx control technique. (Three main chemical pools are labeled; red and blue arrows respectively refer to outflows and inflows with regard to these pools).

preliminary achievements. Two typical kinetics used in reburning are GRI-mech 3.0 and A˚A mechanism, which was developed by Smith et al.10 and Zebetta et al.,11 respectively. Both well explain the interaction between CHi and NOx while including more than 50 species and over 300 elemental reactions. Previously, Miller and Bowman12 built a detailed kinetics for NHi-based NOx reduction, but more detailed mechanism concerning SNCR were recently discovered by Rota et al.,13 which encloses urea, ammonia, isocyanic acid, and 28 other species, and can predict urea-based SNCR. Nevertheless, when it comes to studying hybrid NOx control, especially in frame of industrious practice, there is no existing mechanism adaptable. Another related problem is how to incorporate such complicated mechanisms into CFD simulation with the limited computational resource. A detailed mechanism usually includes tens of species and hundreds of reactions and causally huge computational load makes CFD unmanageable. One smart way is to establish a reduced mechanism with merely a few steps by precluding the superfluous information involved in detailed kinetics and preserving the essence to ensure predictive accuracy. To obtain a reduced mechanism, a skeletal one should be first necessarily established as usual. Many skeletal reduction techniques, such as directed related graph,14-16 sensitivity analysis,17 rate-of-production analysis,18 and computational singular perturbation,19 can be used to accomplish this job. After a skeletal mechanism is acquired, it still maintains the stiffness and involves some species with short-time scale. Short-time species refers to the species always stabilized at low amount, empirically below ppm order, because of their fast consumption rates. Once those species are identified, quasi-steady-state approximation20,21 can be imposed on

2. Development of the Joint Mechanism In view of the practicality, the developed joint mechanism is in fact a skeletal mechanism based on previous researches. Elementary reactions involved in SNCR process are chosen from the work of Rota et al.13 while those included in reburning are selected from the A˚A mechanism of Zebetta et al.11 First, a directed related graph is used to directly exclude some superfluous species for both system, such as HCOH, CH, C, C4H2, C3H3, C3H2, NCN, and C2N2 in the reburning mechanism and N, N2H3, N2H4, H2O2, HNNO, and NO3, in the SNCR one. Second, rate-of-production analysis is conducted to get further reduction. The conditions used for mechanism reduction are consistent with the experiments of Alzueta et al.26 and Lee and Kim,27 as listed in Table 1. Plug flow cases are run for every 50K increment of each condition. As to one reaction, its contribution to a certain remaining species is quantified as: 2jυiK wi j ð1Þ εiK ¼ P jυiK wi j where wi is the reaction rate of the ith reaction and υiK refers to the stoichiometric coefficient of the Kth species in the ith reaction. If εiK is smaller than the chosen threshold value 0.03 in each case, then the ith reaction can be omitted as to the Kth species. After a careful analysis, many reactions with negligible effect on the individual system were detected and truncated. However, some important reactions that exist in the interface between two techniques are artificially rechecked and retained to ensure the hybrid control effect as revealed in Figure 1. Finally, a joint mechanism with 44 species and 150 reactions is established and listed in the Appendix. In all selected reactions, four reactions exploit the parameters modified by Zhang et al.,8 which are more adaptable at low temperature ambience. The validation of our joint mechanism is implemented by directly comparing the predicted results with typical experimental data. There are two reasons to testing in such

(10) Smith G. P.; Golden D. M.; Frenklach M.; Moriarty N. W.; Eiteneer B.; Goldenberg M.; Bowman C. T.; Hanson R. K.; Song S.; Gardiner W. C.; Lissianski, Jr., V. V.; Qin Z. GRI-Mech, version 3.0 2002, http://www.me.kerkeley.edu/gri_mech/. (11) Zabetta E. G.; Hupa M. Scheme A˚A 2006-02-20, http://www. abo.fi/tkf/cmc/research/r_schemes.html. (12) Miller, J. A.; Bowman, C. T. Prog. Energy Combust. 1989, 15, 287–338.  F.; Morbidelli, M. . Chem. Eng. (13) Rota, R.; Antos, D.; Zanoelo, E. Sci. 2002, 57, 27–38. (14) Lu, T. F.; Law, C. K. Proc.Combust. Inst. 2005, 30, 1333–1341. (15) Lu, T. F.; Law, C. K. Combust. Flame 2005, 144, 24–36. (16) Lu, T. F.; Law, C. K. Combust. Flame 2006, 146, 472–483. (17) Tomlin A. S.; Turanyi T.; Pilling M. J. Comprehensive Chemical Kinetics; Elsevier: Amsterdam, 1997; pp 293-437. (18) Tur anyi, T.; Berces, T.; Vajda, S. Int. J. Chem. Kinet. 1989, 21 (2), 83–99. (19) Massias, A.; Diamantis, D.; Mastorakos, E. Combust. Flame 1999, 117, 685–708. (20) Peters, N.; Kee, R. J. Combust. Flame 1987, 57, 89–94. (21) Lovas, T.; Nilsson, D.; Mauss, F. Proc. Combust. Inst. 2000, 28, 1809–1815.

(22) Xu, H.; Smoot, L. D.; Hill, S. C. Energy Fuels 1998, 12, 1278– 1289. (23) Xu, H.; Smoot, L. D.; Hill, S. C. Energy Fuels 1999, 13, 411–420. (24) Mallampalli H. P.; Fletcher T. H.; Chen J. Y. J. Engng. Gas Turbines Power Trans. ASME 1998, 120, 703-712. (25) Giral, I.; Alzueta, M. U. Fuel 2002, 81, 2263–2275. (26) Alzueta, M. U.; Glarborg, P.; Dam-Johansen, K. Combust. Flame 1997, 109, 25–36. (27) Lee, J. B.; Kim, S. D. J. Chem. Eng. Jpn. 1996, 29, 620–626.

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Table 1. Conditions Used to Screen Reactions in Rate-of-Production Analysis SNCR reburning

temperature (K)

reagent amount

950-1450 900-1500

NHi/NO = 1-2 CH4 = 2750 ppm, C2H6 = 255 ppm

residence time (s) -0.37 þ 646/T 181/T

ambient condition O2 = 4%, NO = 300 ppm, H2O = 1.5%, N2 balance. λ = 0.5-2, NO = 830 ppm, H2O = 2%, N2 balance.

Figure 2. Comparison between the results of the joint mechanism and the data of Alzueta et al.26 for reburning. Initial conditions: 2800 ppm CH4, 4830 ppm O2, 920 ppm NO, 2% H2O, N2 to balance, and residence time = 181/T s (black); 2770 ppm CH4, 260 ppm C2H6, 4885 ppm O2, 850 ppm NO, 2% H2O, N2 to balance and residence time = 170/T s (red).

a way: the predictive deviation between the joint mechanism and the original one is found to be negligible, less than 4% for all concerned species, separately in reburning and SNCR conditions; the applicability of our mechanism, in hybrid control process, is completely untested and should be guaranteed through comparison to experimental data. In the following testing, cases in plug flow reactor and perfectly stirred reactor are solved by using the PFR and PSR modules of CHEMKIN 4.0, respectively. To obtain a good mechanism used in hybrid NOx control process, the basic requirement is that the mechanism can accurately describe reburning and the SNCR process separately, just as originally analyzed in the Introduction. As shown in Figures 2 and 3, when the joint mechanism is used to predict reburning process, the changing tendencies of the concentrations of CO, CO2, NO, and HCN as the function of temperature is well traced in comparison to the experimental data. At high temperature region, the mechanism tends to undervalue CO concentration while overpredicting CO2 concentration, but the predicted HCN and NO concentrations are in good agreement with the measurements. The deviation could be inherited from A˚A mechanism because Glarborg28 got the same results even using an extended one, or probably stem from the experimental uncertainty involving reactant mixing, flow characteristics, residence time, and measurement accuracy. Because of the underestimation, the joint mechanism should be carefully handled when used to predict CO. In the SNCR condition, two reagents, ammonia and urea, are used to validate the joint mechanism, and these two SNCR processes are named thermal DeNOx and NOxOUT, respectively, which are both widely used in practice. In thermal DeNOx process predicted results in PFR and PSR (perfectly stirred reactor) are compared with the data of Lyon

Figure 3. Comparison between the results of the joint mechanism and the data of Alzueta et al.26 for reburning. Initial conditions: 2750 ppm CH4, 255 ppm C2H6, 830 ppm NO, 2% H2O, N2 to balance, and residence time = 165/T s. 3200 ppm O2 (black), 6210 ppm O2 (red).

et al.29 and Rota et al.13, respectively. Consequently, excellent coherency between prediction and measurement is observed in Figure 4a. Meanwhile, the predicted results of NOxOUT process are compared with the experimental data of Lee and Kim,27 and perfect agreement is observed in Figure 4b. Generally, the predictive accuracy of the joint mechanism, not only in reburning but also in SNCR, is reliable and it is applicable to these two individual processes. Due to the complexity of NOx involved chemical processes, the applicability of the joint mechanism in predicting hybrid NOx control process should be carefully verified. According to the above analysis, advanced reburning first reduces NOx by reburning and fulfills the secondary reduction through SNCR in ambience with low oxygen and high temperature.

(28) Glarborg, P.; Alzueta, M. U.; Dam-Johansen, K.; Miller, J. A. Combust. Flame 1998, 115, 1–27. (29) Lyon R. K. 194th Annual ACS Meeting. Div. Fuel Chem. 1987, 32, 433.

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Figure 4. Comparison between the results of the joint mechanism and experimental data13,27,29 in SNCR process. Ammonia as reagent (a): 600 ppm NO, NSR = 2, 2% O2, 19% H2O, N2 to balance, and residence time = 200/T s in PSR (Rota et al.13); 225 ppm NO, NSR = 2, 1.23% O2, and He to balance (Lyon29). Urea as reagent (b): 300 ppm NO, 4% O2, 1.5% H2O, N2 to balance, and residence time = -0.37 þ 646/T s; experimental data from Lee and Kim.27

Therefore, in the present study the adaptability of the joint mechanism in reburning has been singly tested at first, and then that in advanced reburning can be simply validated by observing the predicted effects of O2 and temperature on SNCR because NHi reagent may begin to work at various O2 concentrations and temperatures in this technique.30,31 As presented in Figure 5, in comparison with the data of Rota et al.,13 the predicted NO reduction by using the joint mechanism shows good agreement in both quality and quantity. As O2 concentration decreases, predicted NO reduction tend to be more efficient at higher temperature, which is supported by previous investigations.4,5 In other words, the predictive ability of our joint mechanism in advanced reburning is guaranteed. As for enhanced SNCR process, methane or natural gas is inputted with reagent as an additive. When NH3 is used as the reagent, the predicted results are compared with the data of Bae et al.9 and Hemberger et al.32 In the experiment by Bae et al.,9 the reactor specializes a very long residence time, so thermal equilibrium is assumed when doing kinetics modeling. As shown in Figure 6a, the predictions follow the variations of the measurements as CH4 concentration increases and nearly coincide with all the data given by Hemberger et al.32 However, the mechanism slightly underestimates NO concentration at the exit compared with the data of Bae et al.9 This most probably results from some underlying assumptions in modeling. For example, in a practical reactor there may be slight heat transfer, but there is completely no heat transfer in computation. In reality, reactants being mixed need time, whereas in modeling this mixing time is given no consideration. When urea is explored as the reagent, the mechanism correctly predicts the CH4 effect on SNCR at low temperature region while giving contrary trend at high temperature compared with the data

Figure 5. Comparison between the results of the joint mechanism and the data of Rota et al.13 in advanced rebuning. Initial conditions: 500 ppm NO, 600 ppm urea, 19% H2O, N2 to balance, and residence time = 200/T s in PSR.

by Gao et al.,33 as presented in Figure 6b. Zhang et al.8 conducted a similar measurement using ammonia as the reagent but found the CH4 effect coherent with our predictions. Empirically, these two reagents are expected to have similar characteristics in higher temperature, which is being quickly oxidized. As OH radical increases when CH4 is added, the reagent oxidization at higher temperature (>1000 °C) is improved in the experiment by Zhang et al.8 while showing a decrease in the conclusion by Gao et al.33 It may be a contradiction stemming from the experimental uncertainty because of the differences in reactor form, flow pattern, and mixing time between their experiments, or maybe there are a few subtle distinctions between the interaction of urea and CH4 and that of ammonia and CH4 that have not been described. Generally, the joint mechanism shows robust predictive ability and adaptability in most of the tested conditions, especially in hybrid NOx control process highlighted in the present work. Because there are still 44 species remaining in the joint

(30) Chen, S. L.; Lyon, R. K.; Seeker, W. R. Environ. Prog. 1991, 10 (3), 182–185. (31) Chen, S. L.; Kramlich, J. C. Control Tech. 1989, 39 (10), 1375– 1379. (32) Hemberger, R.; Muris, S.; Pleban, K. U.; Wolfrum, J. Combust. Flame 1994, 99, 660–668. (33) Gao, P.; Mei, L. C.; Han, K. H.; Niu, S. L.; Xiong, Z. B. NO removal by adding additives in the selective non-catalytic reduction process. J. Combust. Sci. Tech. 2008, 14(4), 333-337(in Chinese).

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Figure 6. Comparison between the results of the joint mechanism and experimental data9,32,33 in enhanced SNCR. Ammonia as reagent (a): 500 ppm NO, 750 ppm NH3, 2% O2, N2 to balance, and residence time = 525/T s (Hemberger et al.32); 300 ppm NO, 600 ppm NH3, 3% O2, 5% H2O, and N2 to balance (Bae et al.9). Urea as reagent (b): 600 ppm NO, NSR = 1.5, ≈18% O2, N2 to balance, and residence time = 1s; experimental data for the Gao et al.33 data. Natural gas compositions: 30.6% CH4, 2.9% C2H6, and 66.5% N2. Table 2. Conditions Used to Develop and Test Reduced Mechanism SNCR Reburn Hybrid

temperature (K)

input condition

residence time (ms)

ambient gas compositions

950-1450 1100-1600 1050-1450 900-1350

NHi/NO(NSR) = 1-2 λ = 0.5-2, CH4 = 1000 ppm NHi/NO = 2, O2% = 0.4-4% NHi/NO = 2, CH4/NO = 0.5-1

200 200 200 500

NO = 300 ppm, O2 = 4%, H2O = 15%, N2 balance NO = 300 ppm, H2O = 10%, N2 balance NO = 300 ppm, H2O = 15%, N2 balance NO = 300 ppm, O2 = 4%, H2O = 15%, N2 balance

mechanism, a further reduced one is of great necessity to meet the urgent need of handling CFD simulation.

during reaction process. Thus, these two species still remain in the reduced mechanism with the avoidance of losing much accuracy. When it comes to the construction of the reduced mechanism, one corresponding reaction should be first precluded for each QSS species because its reaction rate can be expressed by substitution. The chosen reaction should own the fastest rate at which some QSS species is consumed. Here these reactions are directly taken according to the results of rate-of-production analysis: (HO2, w10), (NH, w29), (NH2, w23), (NNH, w51), (N2H2, w45), (HNO, w36), (H2NO, w41), (CN, w80), (HOCN, w81), (NCO, w64), (HCO, w84), (CH3O, w103), (CH2OH, w108), (C2H5, w122), (C2H3, w127), (CH2CO, w133), (HCCO, w134), (CH2(s), w114), (CH2, w113), (H2CN, w137), (HCNO, w142). The subscripts of wi here are consistent with those in the joint mechanism listed in the Appendix. Then based on the left elementary reactions, Chen’s36 algebraic procedure is adopted to find a small group of independent reactions to repreent the global stoichiometry of the whole chemical system. Finally, one reduced mechanism was completely constructed, including 23 species and 19 global reactions. Its reactions are listed in Table 3, and the corresponding rate expression for each reaction, a combination of elementary rates, is given in Table 4. Additionally, Alzueta et al.5 and Brouwer et al.37 suggested that urea, when used as reagent, can be deemed as equal parts of ammonia and isocyanic acid, so actually R6 can be omitted and only 22 species and 18 steps remain. By referring to some typical reduced mechanism,22-25 the scale of our mechanism is acceptable.

3. Development of the Reduced Mechanism To obtain a reduced mechanism with better adaptability, four operating conditions, including one for SNCR, one for reburning, and two for hybrid NOx control process, are designed for reducing and testing as shown in Table 2. Before the construction of reduced mechanism, it is of great importance to correctly identify quasi-steady-state (QSS) species. In the present work, the wisdom approach proposed by Smooke34 is explored and species concentration is introduced into selection criterion to offset its effect on related reaction rates. If the symbolic value P j υjK wK j j P ð2Þ δK ¼ XK jυjK wK j j is less than 10-8, a predefined limit in all studied conditions, the Kth species belongs to QSS species. Here, where XK is the mole fraction of the Kth species, and wK j refers to the rate of the jth reaction including the Kth species. Consequently 23 species were detected to satisfy this standard: HO2, NH, NH2, NNH, N2H2, HNO, H2NO, CN, HOCN, NCO, HCO, CH3O, CH2OH, C2H5, C2H3, CH2CO, HCCO, CH2(s), CH2, H2CN, HCNO, CH3, and H. However, H plays a critical role in competitive reactions, H þ O2 = OH þ O and H þ O2 þM=HO2 þ M, and should not be arbitrarily omitted as Peter35 proposed. CH3 is another important species taking part in CH4 oxidation and NO deoxidation, so its information should be completely recorded

(36) Chen, J. Y. Combust. Sci. Technol. 1988, 57, 89–94. (37) Brouwer, J.; Heap, M. P. A model for prediction of selective noncatalytic reduction of nitrogen oxides by ammonia, urea, and cyanuric acid with mixing limitations in the presence of CO. Twenty-Sixth Symposium (International) on Combustion; Naples, Italy, 1996; pp 2117-2124.

(34) Smooke, M. D. Reduced Kinetic Mechanisms and Asymptotic Approximations for Methane Air Flames; Springer-Verlag: New York, 1991. (35) Peters, N. Lect. Notes Phys. 1991, 384, 48–67.

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Table 3. The Reduced Mechanism for Hybrid NOx Control

OH þ CO = H þ CO2 O þ OH = H þ O2 O þ H2 = H þ OH OH þ H2 = H2O þ H NH3 þ NO þ OH = 2H2O þ N2 NH2CONH2 = NH3 þ HNCO O þ HNCO = HCN þ O2 NO2 þ NO þ CO = N2O þ O þ CO2 HNCO þ NO þ H = H2O þ N2 þ CO H þ OH þ CO þ CH4 = O2 þ 2CH3

1 2 3 4 5 6 7 8 9 1.

11 12 13 14 15 16 17 18 19

OH þ C2H6 = C2H4 þ H þ H2O OH þ C2H4 = C2H2 þ H þ H2O 2OH þ C2H2 = 2H þ CO þ CH2O NO þ CH3 = H þ OH þ HCN NO þ OH þ CO þ N2 = N2O þ O2 þ HCN NO þ H þ CO = O2 þ HCN OH þ CH3 = CH3OH CH3OH þ OH = H þ CH2O þ H2O CH3OH þ H þ O2 = OH þ H2O þ CH2O

Table 4. The Rate of Global Chemical Steps in the Reduced Mechanisma R1 = þ w1 - w24 þ w31 þ w32 - w33 - w34 - w56 þ w60 þ w62 þ w65 þ w67 - w68 þ w72 - w118 R2 = þ w2 þ w6 þ w9 þ w11 þ w14 þ w18 þ w24 þ w25 þ w26 - w31 þ w33 þ 2w34 - w35 - w39 þ w42 þ w43 þ w44 þ w46 - w48 - w53 þ w56 þ w61 þ w63 - w65 w66 - w67 - w70 - w73 þ 2w74 þ w75 - w76 þ w77 þ w78 þ w79 þ w82 þ 2w86 þ 2w87 þ w88 þ w90 þ w91 þ w92 - w96 þ w98 - 2w99 - w100 - w101 þ w106 þ w107 þ 2w109 - w111 þ w112 þ w115 - w116 - w118 þ w119 - w125 þ w128 þ 2w129 þ 2w130 þ w132 þ w138 þ 2w141 þ w146 R3 = þ w3 - w5 - w6 - w9 - w11 þ w12 - w14 þ w15 - w17 þ w18 - w19 - w20 - w24 - w25 - 2w26 - 2w27 - w28 þ w31 þ w32 - w33 - w34 þ w35 þ w37 þ w39 - w43 - w44 - w46 þ w47 þ w48 þ w52 - 2w56 þ w60 - w61 - w62 - w63 þ w66 þ w67 - w68 þ w70 þ w73 þ w75 þ w76 - w82 - 2w86 - 2w87 - w88 - w90 - w91 þ w96 þ w97 - w98 þ w99 þ w100 þ w101 - w106 - w107 - 2w109 þ w111 - w112 þ w116 þ w118 - w119 þ w125 - w129 - w130 - w132 - w138 - w141 - w146 R4 = þ w4 þ w5 þ w6 þ w11 - w12 - w15 - w20 - w22 þ w24 þ w25 þ w26 þ w27 þ w28 - w31 - w32 þ w33 þ w34 þ w38 þ w40 þ w43 þ w46 þ w49 þ w50 þ w56 w60 þ w61 þ w63 - w66 - w67 þ w68 - w75 - w76 þ w82 þ w86 þ 2w87 þ w88 þ w91 - w97 þ w98 - w99 - w100 - w101 þ w106 þ w107 þ w109 - w111 þ w112 w116 - w118 þ w119 - w120 - w124 - w125 þ w129 þ w130 þ w132 þ w138 þ w146 - w147 - w148 R5 = þ w12 þ w13 - w21 - w40 - w50 - w63 - w72 R6 = þ w71 þ w72 R7 = þ w14 þ w17 - w18 þ w19 þ w20 þ w24 þ w25 þ w26 þ w27 þ w28 - w31 - w32 þ w33 þ w34 - w47 þ 2w56 þ w61 þ w62 þ w63 - w66 - w67 þ w68 - w70 w75 - w77 - w78 þ w82 þ w83 R8 = þ w24 - w31 - w32 þ w33 þ w34 þ w56 þ w68 þ w69 R9 = - w14 - w17 þ w18 - w19 - w20 - w24 - w25 - w26 - w27 - w28 þ w31 þ w32 - w33 - w34 þ w47 - 2w56 þ w59 þ w60 þ w66 þ w67 - w68 þ w72 þ w75 R10 = - w89 þ w90 þ w91 þ w92 þ w93 þ w94 R11 = - w119 þ w120 þ w121 R12 = - w102 - w119 þ w123 þ w124 þ w125 þ w1 R13 = - w102 - w119 þ w123 þ w128 þ w129 þ w130 þ w1 R14 = þ w86 þ w87 þ w88 - w89 þ w90 þ w91 þ w92 þ w93 þ w95 - w96 þ w98 þ w109 - w110 - w111 - w116 - w118 þ w119 þ w129 þ w130 þ w138 þ w139 R15 = þ w15 þ w16 - w19 þ w21 - w25 - w26 - w27 - w28 - w30 þ w31 þ w32 - w33 - w34 þ w47 þ w52 - w53 - w54 - w55 - w56 þ w60 þ w66 - w68 þ w75 þ w140 R16 = - w14 - w15 - w16 - w17 þ 18 - w20 - w21 - w24 þ w30 - w52 þ w53 þ w54 þ w55 - w56 - w60 - w61 - w62 - w63 þ w67 þ w70 þ w73 - w74 - w75 þ w76 - w82 - w86 - w87 - w88 þ w89 - w90 - w91 - w92 - w93 - w95 þ w96 - w98 - wm þ wm þ wln þ wm þ wm - wn9 þ w123 - w129 - wm þ wU3 þ w1M R17= - w86 - w87 - w88 þ w97 þ w99 þ w100 þ w101 þ w102 - w106 - w109 þ w111 þ w116 þ w118 þ w119 - w123 - w129 - w130 - w132 þ w145 R18 = - w6 - w7 - w8 - w14 - w24 - w25 - w26 - w27 - w28 þ w31 - w33 - w34 þ w35 - w42 - w43 - w44 - w46 þ w48 þ w53 - w56 - w61 - w62 - w63 þ w66 þ w67 þ w70 þ w73 - w74 þ w76 - w82 - w85 - 2w86 - 2w87 - 2w88 - w90 - w91 - w92 - w93 þ w96 þ w97 - w98 þ 2w99 þ w100 þ 2w101 þ 2w102 - w104 þ w105 - w106 - 2w109 þ 2w111 - w112 þ 2w116 þ 2w118 þ w119 - 2w123 þ w125 - w128 - 3*w129 - 3w130 - 2w132 þ w135 - w138 - w141 - w146 þ w148 þ w149 R19 = þ w6 þ w7 þ w8 þ w14 þ w24 þ w25 þ w26 þ w27 þ w28 - w31 þ w33 þ w34 - w35 þ w42 þ w43 þ w44 þ w45 - w48 - w53 þ w56 þ w61 þ w62 þ w63 - w66 w67 - w70 - w73 þ w74 - w76 þ w82 þ w85 þ w86 þ w87 þ w88 þ w90 þ w91 þ w92 þ w93 - w95 þ w98 - w99 - w101 - w102 þ w104 - w105 þ w109 - w111 þ w112 - w116 - w118 þ w123 - w125 þ w128 þ 2w129 þ 2w130 þ w132 - w135 þ w138 þ w141 þ w147 þ w150 a

Numbers in each expression refer to the reaction numbers in the joint mechanism given in theAppendix.

shown in Figures 9 and 10. In the advanced reburning case, the trend that as O2 concentration decreases the optimistic temperature for NOx reducing rises up is correctly predicted by both mechanisms. Maximum error of the reduced one relative to the joint one is detected when NH3 is used as reagent at 1.0% O2 mole fraction, and its value is not over 0.18. As for the case run for enhanced SNCR, the reduced mechanism explains well the promoting effect of CH4 on NOx reduction in low temperature, just as is done by the joint one. Predictive errors are comparatively larger when obvious NOx reduction appears, but still negligible. Another phenomenon worth mentioning is that as CH4/NO increases, a peak gradually forms at around 1100 K based on the testing condition, which could be the results of the competitive effect of CHi and NHi to OH. Although this phenomenon is captured by our mechanism, further experiments should be conducted to validate it. As seen, the reduced mechanism shows good accuracy in all testing cases, therefore, its adaptability in various application

Because net rates of QSS species are assumed to be zero, their concentrations can be explicitly solved through iteration. 4. Testing of the Reduced Mechanism Validation of our reduced mechanism was carried out in a homogeneous reactor with the initial conditions listed in Table 2. The predicted results were compared with those obtained from the joint mechanism. Figure 7 and 8 shows the accuracy of our reduced mechanism when it is applied in SNCR and reburning cases, respectively. In a large portion of the temperature region, the predicted curves of the two mechanisms nearly coincide with each other. Although certain deviations are observed during rapid change of curves, the predicted results of the reduced mechanism follows the changing tendency quite well. From the perspective of differential system theory, the deviation may rise from the effect of simplification on the stiffness. On average, the relative errors in reburning cases are slightly larger. Anyway, compared with the general uncertainty of the relevant data, (10% as Alzueta et al.26 and Glarborg et al.28 mentioned, the observed errors in the reduced mechanism can be considered to be negligible or acceptable. The predicting power of the reduced mechanism in hybrid NOx control process is also very satisfactory, as

(38) Lamnaouer, M. Implementation of Law’s 19 Species Methane Mechanism into FLUENT. Access-date: 2009-8-15. http://www.clemson. edu/scies/UTSR/FellowLamnourSUM.pdf.

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Figure 7. Comparison between the results of the joint mechanism and reduced one in SNCR process. Initial conditions are in accordance with the first row in Table 2. Ammonia as reagent (a); urea as reagent (b).

Figure 8. Comparison between the results of the joint mechanism and reduced one in reburning process. Initial conditions are in accordance with the second row in Table 2.

Figure 9. Comparison between the results of the joint mechanism and the reduced one in advanced reburning process. Initial conditions are in accordance with the third row in Table 2. Ammonia as reagent (a); urea as reagent (b).

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Figure 10. Comparison between the results of the joint mechanism and reduced on in enhanced SNCR process. Initial conditions are in accordance with the forth row in Table 2. Ammonia as reagent (a); urea as reagent (b).

situations, not only in reburning and SNCR separately but also in hybrid NOx control process, is warranted.

simplification is implemented by identifying and exerting quasisteady-state on short-time species. The applicability of the reduced mechanism is tested by comparing the predicted results with those of the joint one. Satisfactory coherency in testing is observed in all studied conditions, and the relative errors are proven to be negligible compared with the general experimental uncertainty. Many popular CFD codes, for example FLUENT, have provided user-defined-interface (UDI) with various useful functions which can effectively integrate our reduced mechanism. One real case has appeared on a website38 to demonstrate the detailed procedures. Furthermore, the fluid parameters of all species involved can be found on websites or in the CHEMKIN package. In future work, CFD modeling combined with our reduced mechanism could be conducted to study hybrid NOx control technique more practically.

5. Conclusions A joint mechanism for hybrid NOx control process has been developed from two detailed ones which are used in NOxOUT and in reburning, respectively. The joint mechanism is a skeletal one, including 44 species and 150 elemental reactions, simplified based on directed related graph and rate-of-production analysis. After strict validation, it is demonstrated that the joint mechanism developed has good predictive accuracy and adaptability to different operating conditions, such as separately in reburning, SNCR, and hybrid NOx control process mainly involving advanced reburning and enhanced SNCR. To efficiently predict practical NOx control process, a reduced mechanism, including only 22 species and 18 global steps, is subsequently given and ready to be integrated into CFD simulations. Mechanism

Appendix Table 5. Established Joint Mechanism NH3 CO2 CO(NH2)2 CH3O

N2O N2 NH CH2OH

NO2 HCN NH2 C2H5

NO HNCO NNH C2H3

H CH2O N2H2 CH2CO

involved species O CH4 HNO HCCO

OH CH3 H2NO CH2(S)

H2 C2H6 CN CH2

O2 C2H2 HOCN H2CN

H2O HO2 NCO HCNO

CO CH3OH HCO

Reactions Taken from the NOxOUT Mechanism of Rota et al.

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

reaction a

A

CO þ OH = CO2 þ H O þ OH = H þ O2 O þ H2 = OH þ H OH þ H2 = H2O þ H OH þ OH = H2O þ O H þ OH þ M = H2O þ Mb H þ O2 þ M = HO2 þ Mb H þ O2 þ N2 = HO2 þ N2 HO2 þ H = H2 þ O2 HO2 þ H = OH þ OH HO2 þ OH = H2O þ O2 NH3 þ O = NH2 þ OH NH3 þ OH = NH2 þ H2O NH2 þ O = HNO þ H NH2 þ O = NH þ OH NH2 þ OH = NH þ H2O

1.50  10 2.00  1014 5.10  1004 2.10  1008 4.30  1003 8.40  1021 2.10  1018 6.70  1019 4.30  1013 1.70  1014 2.90  1013 9.40  1006 2.00  1006 6.60  1014 6.80  1012 4.00  1006 07

β

Ea

1.30 -0.40 2.67 1.52 2.70 -2.00 -1.00 -1.42 0.00 0.00 0.00 1.94 2.04 -0.50 0.00 2.00

-765 0 6290 3450 -2486 0 0 0 1411 875 -497 6460 566 0 0 1000

reaction a 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52

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HNO þ O = NO þ OH HNO þ OH = NO þ H2O HNO þ O2 = NO þ HO2 HNO þ NH2 = NO þ NH3 H2NO þ M = HNO þ H þ M H2NO þ O = HNO þ OH H2NO þ OH = HNO þ H2O H2NO þ NO = HNO þ HNO N2H2 þ M = NNH þ H þ Mb N2H2 þ OH = NNH þ H2O N2H2 þ NO = N2O þ NH2 NNH = N2 þ H NNH þ OH = N2 þ H2O NNH þ NH2 = N2 þ NH3 NNH þ NO = N2 þ HNO NNH þ O = NH þ NO

β

A 1.00  10 3.60  1013 1.00  1013 2.00  1013 5.00  1016 3.00  1007 2.00  1007 2.00  1007 5.00  1016 1.00  1013 3.00  1012 1.00  1014 5.00  1013 5.00  1013 5.00  1013 5.00  1013 13

0.00 0.00 0.00 0.00 0.00 2.00 2.00 2.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Ea 0 0 25 000 1000 50 000 2000 1000 13 000 50 000 1000 0 0 0 0 0 0

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Table 5. Continued Reactions Taken from the NOxOUT Mechanism of Rota et al. reaction

a

β

A

17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

NH2 þ HO2 = NH3 þ O2 H2NO þ O = NH2 þ O2 NH2 þ NH = N2H2 þ H NH2 þ NH2 = N2H2 þ H2 NH2 þ NH2 = NH3 þ NH NH2 þ NO = NNH þ OH NH2 þ NO = N2 þ H2O NH2 þ NO2 = N2O þ H2O NH þ O = NO þ H NH þ OH = HNO þ H NH þ O2 = HNO þ O NH þ O2 = NO þ OH NH þ NO = N2O þ H NH þ NO = N2 þ OH NO þ HO2 = NO2 þ OH NO þ O þ M = NO2 þ Mb NO2 þ H =NO þ OH NO2 þ O =NO þ O2 HNO þ M= H þ NO þ Mb HNO þ H = NO þ H2

73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89

3.60  1008 CN þ H2 = HCN þ H HCN þ O = NCO þ H 1.40  1004 HCN þ O = NH þ CO 3.50  1003 8.00  1012 CN þ H2O = HCN þ OH HCN þ OH = HOCN þ H 5.90  1004 HCN þ OH = HNCO þ H 2.00  10-03 CN þ OH = NCO þ H 6.00  1013 7.50  1012 CN þ O2 = NCO þ O HOCN þ H = HNCO þ H 2.00  1007 6.40  1005 HOCN þ OH = NCO þ H2O HNCO þ CN = HCN þ NCO 1.50  1013 1.90  1017 HCO þ M = H þ CO þ Mb 7.60  1012 HCO þ O2 = CO þ HO2 2.20  1008 CH2O þ H = HCO þ H2 3.40  1009 CH2O þ OH = HCO þ H2O c 1.00  1014 CH2O þ O2 = HCO þ HO2 6.00  1016 CH3 þ H þ M = CH4 þ Mb low-pressure limit 8.00  1026 1.30  1004 CH4 þ H = CH3 þ H2 c 1.26  1010 CH4 þ OH = CH3 þ H2O 1.02  1009 CH4 þ O = CH3 þ OHc 4.00  1013 CH4 þ O2 = CH3 þ HO2 4.30  1012 CH4 þ CH2 = 2CH3 1.00  1016 CH3 þ M = CH2 þ H þ M 7.20  1013 CH2s þ H2 = CH3 þ H 8.40  1013 CH3 þ O = CH2O þ H 5.00  1013 CH3 þ OH = CH2(s) þ H2O 1.10  1013 CH3 þ O2 = CH3O þ O 5.65  1011 CH3 þ O2 = CH2O þ OHc 2.00  1013 CH3 þ HO2 = CH3O þ OH 4.20  1013 CH3 þ CH2 = C2H4 þ H 1.90  1026 CH3O þ M = CH2O þ H þ M 4.00  1010 CH3O þ O2 = CH2O þ HO2 CH2OH þ M = CH2O þ H þ M 1.10  1043 1.00  1014 CH2OH þ H = CH3 þ OH 1.00  1013 CH2OH þ OH = CH2O þ H2O 2.20  1014 CH2OH þ O2 = CH2O þ HO2 8.30  1015 CH2O þ M = CO þ H2 þ M 2.00  1013 HCO þ CH2 = CH3 þ CO 3.00  1013 CH2 þ OH = CH2O þ H 8.00  1012 CH2 þ O2 = CO þ H2O 1.70  1013 CH2 þ O2 = CO þ OH þ H

90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113

1.00  10 2.50  1014 5.00  1013 8.50  1011 5.00  1013 8.92  1012 1.26  1016 3.20  1018 9.20  1013 2.00  1013 4.60  1005 1.30  1006 2.90  1014 2.20  1013 2.10  1012 7.50  1019 8.40  1013 3.90  1012 1.50  1016 4.40  1011

reaction a

Ea

A

53 N2O þ M = N2 þ O þ M 54 N2O þ H = N2 þ OH 55 N2O þ H = N2 þ OH 56 NH2 þ NO2 = H2NO þ NO 57 NH2 þ NO = N2 þ H2O 58 NH þ NO = N2O þ H 59 HNCO þ H = NH2 þ CO 60 HNCO þ O = NH þ CO2 61 HNCO þ OH = H2O þ NCO 62 HNCO þ O2 = HNO þ CO2 63 HNCO þ NH2 = NH3 þ NCO 64 NCO þ O = NO þ CO 65 NCO þ O2 = NO þ CO2 66 NCO þ NO = N2O þ CO 67 NCO þ NO = N2 þ CO2 68 NCO þ NO2 = 2NO þ CO 69 NCO þ NO2 =N2O þ CO2 70 NCO þ HNO = HNCO þ NO 71 CO(NH2)2 f NH3 þ HNCO 72 CO(NH2)2 þ H2O f 2NH3 þ CO2 Reactions Taken from the A˚A Mechanism of Zebetta et al. 13

0.00 0.00 0.00 0.00 0.00 -0.35 -1.25 -2.20 0.00 0.00 2.00 1.50 -0.40 -0.23 0.00 -1.41 0.00 0.00 0.00 0.72

b

0 0 0 0 10 000 0 0 0 0 0 0 100 0 0 -480 0 0 -238 48 680 650

1.55 2.64 2.64 0.00 2.40 4.00 0.00 0.00 2.00 2.00 0.00 -1.00 0.00 1.77 1.18 0.00 -1.00 -3.00 3.00 2.00 1.50 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 -2.70 0.00 -8.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

3000 4980 4980 7450 12 500 1000 0 -389 2000 2560 0 17 020 400 3000 -447 39 981 0 0 8047 14 012 8642 56 908 10 034 90 607 0 0 0 27 818 8938 0 0 30 600 2126 42 999 0 0 4709 69 545 0 0 1490 1490

114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150

4.00  10 4.40  1014 3.30  1010 1.50  1019 -8.92  1012 -2.20  1013 2.25  1007 9.60  1007 6.40  1005 1.00  1012 5.00  1012 4.70  1013 2.00  1012 6.20  1017 7.80  1017 1.30  1013 5.40  1012 1.80  1013 1.27  1004 6.13  1010

CH2(s) þ M = CH2 þ Mb CH2(s) þ O = CO þ 2H CH2(s) þ OH = CH2O þ H CH2(s) þ O2 = CO þ OH þ H CH2(s) þ CO2 = CH2O þ CO 2CH3 þ M = C2H6 þ M low-pressure limit C2H6 þ H = C2H5 þ H2 C2H6 þ OH = C2H5 þ H2O C2H4 þ H þ M = C2H5 þ Mb low-pressure limit C2H5 þ H = 2CH3 C2H4 þ M = C2H2 þ H2 þ M C2H4 þ M = C2H3 þ H þ M C2H4 þ OH = C2H3 þ H2O C2H2 þ H þ M = C2H3 þ Mb C2H3 þ O = CH2CO þ H C2H2 þ O = CH2 þ CO C2H2 þ O = HCCO þ H C2H2 þ OH = H þ CH2CO CH2CO þ H = CH3 þ CO CH2CO þ OH = CH2O þ HCO HCCO þ H = CH2(s) þ CO HCCO þ OH = HCO þ CO þ H HCCO þ O2 = 2CO þ OH H2CN þ M = HCN þ H þ M CH3 þ NO = HCN þ H2O CH3 þ NO = H2CN þ OH CH2 þ N2 = HCN þ NH CH2 þ NO = NCO þ H2 HCNO þ H = HCN þ OH CH2(s) þ NO = HCN þ OH HCCO þ NO = HCNO þ CO OH þ CH3 þ M = CH3OH þ Mb low-pressure limit H þ CH2OH þ M = CH3OH þ Mb low-pressure limit CH3OH þ H = CH2OH þ H2 CH3OH þ H = CH3O þ H2 CH3OH þ OH = CH3O þ H2O CH3OH þ OH = CH2OH þ H2O

14

1.00  1013 3.00  1013 3.00  1013 3.10  1013 6.60  1012 3.60  1013 3.20  1041 1.40  1009 7.20  1006 2.20  1013 6.40  1027 1.00  1014 1.50  1015 1.40  1016 1.20  1014 5.50  1012 3.30  1013 7.00  1003 1.50  1004 2.20  10-04 3.60  1012 1.00  1013 1.50  1014 1.00  1013 1.60  1012 3.00  1014 5.30  1011 5.30  1011 1.00  1013 3.50  1012 1.00  1014 1.00  1014 2.00  1013 2.79  1018 4.00  1036 1.06  1012 4.36  1031 3.20  1013 8.00  1012 1.00  1006 7.10  1006

β

Ea

0.00 0.00 0.00 -2.20 -0.35 -0.23 1.70 1.41 2.00 0.00 0.00 0.00 0.00 -1.73 -1.73 0.00 0.00 0.00 0.00 0.00

56 100 19 254 4729 0 0 0 3800 8520 2560 35 000 6200 0 20 000 763 763 0 0 0 15 540 20 980

0.00 0 0.00 0 0.00 0 0.00 0 0.00 0 0.00 0 -7.03 2762 1.50 7411 2.00 854 0.00 2066 -2.60 54 0.00 0 0.00 55 437 0.00 81 268 0.00 6140 0.00 2404 0.00 0 2.80 497 2.80 497 4.50 -994 0.00 2345 0.00 0 0.00 0 0.00 0 0.00 854 0.00 21 857 0.00 14 902 0.00 14 902 0.00 73 519 0.00 -1093 0.00 11 915 0.00 0 0.00 0 -1.43 1330 -5.92 3140 0.50 86 -4.65 5080 0.00 6095 0.00 6095 2.10 497 1.80 -596

a The rate constants are in the form of k = AT βe-Ea/RT ; A units of mol cm s K; Ea units of cal/mol. b Enhanced third-body efficiencies: (6) N2/2.6, H2O/16.5 (7) N2/0.0, H2/1.5, H2O/10.0; (32) N2/1.7, O2/1.5, H2O/10; (35) N2/2.0, H2/2.0, O2/2.0/, H2O/10.0; (45) N2/2.0, H2/2.0, O2/2.0/, H2O/15.0; (53) N2/1.5, O2/1.5, H2O/10; (84) N2/1.5, O2/1.5, CO/1.9, CO2/3.0, H2O/5.0; (89) H2/2.0, CO/2.0, CO2/3.0, H2O/5.0; (114) H/20.0, H2O/3.0, C2H2/4.0; (122) H2/2.0, CO/2.0, CO2/3.0, H2O/5.0; (127) H2/2.0, CO/2.0, CO2/3.0, H2O/5.0; (145) H2/2.0, H2O/6.0, CH4/2.0, CO/1.5, CO2/2.0, C2H6/3.0; (146) H2/2.0, H2O/6.0, CH4/2.0, CO/1.5, CO2/2.0, C2H6/3.0; c Adopting the modified parameters according to Zhang et al.8

Acknowledgment. The authors would like to express gratitude to National Natural Science Foundation of China (50806066)

and National Science Foundation for Distinguished Young Scholar (50525620) for their financial supports. 5928