Study on the Improvement of Chemical Reaction Mechanism of

The only feasible way for a verification of reaction mechanism in OEC would be a .... chain reaction (R38) H + O2 ↔ O + OH contribute mostly in a po...
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Energy & Fuels 2007, 21, 3202–3207

Study on the Improvement of Chemical Reaction Mechanism of Methane Based on the Laminar Burning Velocities in OEC Ji-Woong Han,*,† Chang-Eon Lee,‡ Sung-Min Kum,§ and Yong-Soo Hwang† Nuclear Fuel Cycle Strategy Research Lab., Korea Atomic Energy Research Institute, P.O. Box 105, Yuseong, Deajeon 305-600, Korea; School of Mechanical Engineering, Inha UniVersity, 253, Yonghyun-dong, Nam-ku, Inchon 402-751, Korea; and School of Mechanical and AutomotiVe Engineering, Halla UniVersity, 66, Heungup, Wonju, Gangwon 220-712, Korea ReceiVed April 9, 2007. ReVised Manuscript ReceiVed September 17, 2007

In the present research a series of investigations have been performed for an improvement of the chemical reaction mechanisms of methane in the oxygen-enriched condition. Among the representative combustion characteristics, the burning velocity was selected as a reference for the improvement. The burning velocities of methane/air and methane/oxygen-enriched flame according to various equivalence ratios were determined by an analysis of the Schlieren images. The propriety of present experimental data was inspected by comparison with other experimental results. Based on the experimental burning velocity data, performance of the existing reaction mechanisms was assessed for the prediction of burning velocity. Also from the sensitivity analysis a modified GRI 3.0 reaction mechanism was suggested, in which the reaction rate coefficients of (R38) H + O2 T O + OH in GRI 3.0 reaction mechanisms were corrected. Modified reaction mechanisms showed a good agreement for predicting the burning velocity and number density of NO in methane/oxygenenriched flame as well as methane/air flame.

Introduction Recently, many nations are putting intensive restrictions on greenhouse gases including CO2, which contribute to an increase of the earth’s average temperature. Because CO2 is an inevitable byproduct generated from a fossile fuel combustion, one of the best strategies for reducing the amount of CO2 is to increase the thermal efficiency of the combustion systems. In accordance with the cost reduction of the oxygen production, oxygenenriched combustion (OEC) is recommended as another promising technique for a highly efficient combustion system. OEC is a combustion method which uses oxygen as an oxidizer partly or entirely. In the OEC system, many benefits are expected such as an increased thermal efficiency, increased processing rates, reduced flue gas volumes, etc.1 Even though a lot of information related to oxygen-enriched combustor designs and global combustion characteristics have been acquired from experiments, a higher combustion temperature makes it very difficult to obtain detailed information by using the traditional experiment methods. Recently, in accordance with the development of the computer and numerical analysis techniques, more numerical * Corresponding author: Tel 82-42-868-4853; Fax 82-42-868-2035; e-mail [email protected]. † Korea Atomic Energy Research Institute. ‡ Inha University. § Halla University. (1) Baukal, C. E., Jr. Oxygen-Enhanced Combustion; CRC: Boca Raton, FL, 1998. (2) Hedley, J. T.; Pourkashanian, M.; Williams, A.; Yap, L. T. NOx formation in large-scale oxy-fuel flames. Combust. Sci. Technol. 1995, 108, 311–322. (3) Han, J. W.; Lee, C. E. Numerical study on flame structure and NO formation characteristics in oxidizer-controlled diffusion flames. Trans. Korean Soc. Mech. Eng. 2002, 26, 742–749. (4) Sung, C. J.; Law, C. K. Dominant Chemistry and Physical Factors Affecting NO Formation and Control in Oxy-Fuel Burning. Proc. Combust. Inst. 1998, 27, 1411–1418.

investigations on OEC are in progress and being scheduled. Unfortunately, most of the existing numerical investigations on OEC2–4 have been performed by using the ordinary reaction mechanisms without any verification processes; therefore, verifying and improvement investigations about the reaction mechanisms in OEC need to be proceeded more than any others for more reliable results. In a previous study, Lee et al.5 evaluated the performance of the conventional reaction mechanisms in OEC on the basis of Jahn6 and Morgan’s7 experimental data, and they suggested modified reaction mechanisms (LKY mechanisms) for OEC. However, the direct photograph techniques which were utilized to assess the burning velocities in Jahn’s experiments are known to be insufficient to provide exact values.8 Also, the burning velocity data from Jahn’s experiments show some discrepancies in a comparison with that of Morgan’s. Therefore the exactness of these experimental data should be evaluated in advance before a utilization of them as a reference for the verification of any reaction mechanism in OEC. It is most desirable to investigate various flame characteristics such as structures, extinction limits, and burning velocities under various conditions in order to verify or modify the reaction mechanisms.9 However, a limited amount of experimental data10 about a flame structure has been reported because of the experimental difficulties in OEC caused by a high temperature and soot generation. It is also difficult to detect the extinction (5) Lee, K. Y.; Nam, T. H.; You, H. S.; Choi, D. S. The Flame Structure of Freely Propagating CH4/O2/N2 Premixed Flames on the O2 Enrichment. Trans. Korean Soc. Mech. Eng. 2002, 32, 555–560. (6) Jahn, G. Der Zundvorgang in Gasgemischen, Ph.D. Thesis, Oldenbourg, Berlin, 1934. (7) Morgan, G. H.; Kane, W. R. Some Effects of Inert Diluents on Flame Speeds and Temperature. Proc. Combust. Inst. 1953, 4, 313–320. (8) Kanury, A. M. Introduction to Combustion Phenomena; Gordon and Breach Science Publishers: Langhorne, PA, 1975.

10.1021/ef070175v CCC: $37.00  2007 American Chemical Society Published on Web 10/27/2007

Chemical Reaction Mechanism of Methane

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Figure 2. Schematics of the flow and Schlieren measurement system.

Figure 1. Schematics of the premixed burner.

limit from the experiments by using a counterflow flame burner with a small nozzle diameter because the flame will be sustained at a turbulent flow field with high Reynolds numbers. The only feasible way for a verification of reaction mechanism in OEC would be a burning velocity measurement. In the present study, detailed experimental studies with Schlieren optical apparatus are conducted to determine the burning velocities under various equivalence ratios and oxygen enrichments. Based on the experimental data, the exactness of the conventional reaction mechanisms for a prediction of the burning velocities is evaluated. A newly modified reaction mechanism is proposed, and it shows a good agreement with methane/oxygen-enriched flame as well as the conventional methane/air flame through a sensitivity analysis for the elementary reactions related with methane. Measurement of Burning Velocity Figure 1 shows a schematic of the premixed nozzle type burner, designed by Morel’s suggestion,11 to make a flame front without a curvature. The burner consists of a diffuser and a straight and contraction section. The inclination of the diffuser section is designed to be less than 7° to avoid a flow separation. The straight part is designed to be long enough for a flow to be fully developed. Since the burning velocity of a flame is increased steeply with the oxygen enrichment increment, a smaller nozzle exit diameter for a high laminar flow velocity is expected for a burning velocity measurement in the oxygenenriched condition. However, too much of a downsizing for a nozzle exit diameter could raise the disturbances for obtaining a highly resolved Schlieren image, hence calculating the exact (9) Peters, N.; Rogg, B. Reduced Kinetic Mechanisms for Applications in Combustion Systems; Springer-Verlag: Berlin, 1993. (10) Naik, S. V.; Laurendeau, N. M. Quantitative Laser-Saturated Fluorescence Measurements of Nitric Oxide in Counter-flow Diffusion Flames under Sooting Oxy-Fuel Conditions. Combust. Sci. Technol. 2002, 129, 112–119. (11) Morel, T. Comprehensive Design of Axisymmetric Wind Tunnel Contraction. J. Fluid Eng. 1975, 225–233.

Figure 3. (a) Typical photographs of the Schlieren image. (b) Schematics of a laminar burning velocity evaluation in a nozzle-type premixed burner.

burning velocity. In the present study the optimal nozzle exit diameter is decided as a 4 mm which satisfies the range of the laminar flow velocity and image resolution after various preliminary measurements. Figure 2 shows a schematic of the flow system with the Schlieren optical apparatus. The flow rate of methane (chemically pure level) and N2, O2 of a purity of more than 99.9% is regulated precisely by using a mass flow controller (MFC, Tylan FS280 series) system. The burning velocity is measured by using a traditional z-shaped laser Schlieren technique. The tungsten– halogen lamp is adopted as a source of light. Figure 3 shows a traditional Schlieren image and a simplified geometry.12 In the present experiments the Schlieren images obtained via a digital camera with additional filters and additional lenses are processed to determine the laminar burning velocity in a data acquisition system. That is, the laminar flame thickness is, generally, negligible when compared with the radius of a curvature, and thus the formed flame front can be regarded as a simplified geometry in Figure 3b. Then the laminar burning velocity is expressed as SL ) Vu,n ) Vu sin(R/2).13 The oxygen enrichment is expressed as the following equation Ω)

QO2 QO2 + QN2

(1)

where Ω and Qi represent the total contents of the oxygen in the oxidizer stream and the flux of i species, respectively. From the definitions Ω ) 0.21 and Ω ) 1.0 we can describe the

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Table 1. Various Detailed Reaction Mechanisms reaction mechanism

species

elementary reaction

GRI 3.0 reaction mechanism (GRI 3.0 Mech.) Miller–Bowman reaction mechanism (MB Mech.) Lee–Ki–Yong reaction mechanism (LKY Mech.)

53

325 reversible reaction

51

219 reversible reaction

53

632 forward reaction

conventional methane/air and methane/pure oxygen flame, respectively. Numerical Calculation In the present study, two kinds of flame geometry are utilized. One is a premixed flame for the laminar burning velocity evaluation and sensitivity analysis, and the other is an opposed diffusion flame for the major and minor species’ concentrations comparison. The formal and the latter are calculated by the PREMIX code14 and OPPDIF code,15 respectively. The ChemkinII16 and Tranfit17 codes are used for the calculation of the thermodynamic and transport properties, respectively, in all the flames, and a simple radiation heat loss model18 is adopted. In order to evaluate the performance of the existing reaction mechanisms in OEC, three kinds of reaction mechanisms are selected and listed in Table 1. GRI 3.0 Mech.19 is the most recently modified one on the basis of GRI 2.11 reaction mechanism. MB Mech.20 is one of the popularly adopted reaction mechanisms together with GRI 3.0 Mech. LKY Mech.5 is a modified GRI 3.0 Mech. for OEC on the basis of the results from Jahn and Morgan’s experiments. Results and Discussion Figure 4 shows the experimental results of the laminar burning velocity according to the equivalence ratio for a conventional methane/air flame. In order to validate the present experimental results, various results are compared with each other. The inverse triangle is the experimental results based on the LDV techniques by using the counterflow burner by Law et al.21 The rectangular is the one based on the same techniques as the present experiments by using the 20 mm diameter nozzle type burner by Lee et al.,12 and the (12) Lee, C. E.; Oh, C. B.; Jung, I. S.; Park, J. A Study on the Determination of Burning Velocities of LFG and LFG-mixed Fuels. Fuel 2002, 81, 1679–1686. (13) Kee, R. J.; Grcar, J. F.; Smooke, M. D.; Miller, J. A. A Fortran Program for Modeling Steady Laminar One-Dimensional Premixed Flame, SAND, 85-8240; 1994. (14) Lutz, A. E.; Kee, R. J.; Grcar, J. F.; and Rupley, F. M. OPPDIF: A Fortran Program for Computing Opposed-Flow Diffusion Flames, SAND, 96-8243, 1997. (15) Kee, R. J.; Rupley, F. M.; Miller, J. A. Chemkin-II: A Fortran Chemical Kinetics Package for the Analysis of Gas Phase Chemical Kinetics, SAND, 89-8009B, 1989. (16) Kee, R. J.; Dixon-Lewis, G.; Warnatz, J.; Coltrin, M. E.; Miller, J. A. A Fortran Computer Code Package for the Evaluation of Gas-Phase Multi-component Transport, SAND86-8246, 1994. (17) Ju, Y.; Guo, H.; Maruta, K.; Liu, F. On the Extinction Limit and Flammability Limit of Non-adiabatic Stretched Methane-Air Premixed Flames. J. Fluid Mech. 1997, 342, 315–334. (18) GRI Mech. Ver. 3.0, Web address: http://www.me.berkeley.edu/ gri_mech/version30/ text30.html. (19) Miller & Bowman Mech. Web address: http://www.galcit.caltech.edu/ EDL/mechanisms/ library/library.html. (20) Law, C. K. A Compilation of Experimental Data on Laminar Burning Velocities. In Peters, N.; Rogg, B. Reduced Kinetic Mechanisms for Applications in Combustion Systems; Springer-Verlag: Berlin; pp 15– 26. (21) Turns S. R. An Introduction to Combustion, 2nd ed.; McGrawHill: New York, 2000.

Figure 4. Comparison of the measured burning velocities according to the equivalence ratio in the CH4/air flame.

Figure 5. Comparison of the measured burning velocities according to the equivalence ratio in the CH4/O2-enriched flame.

circle is the one based on the direct photograph techniques by Jahn. The peak value and overall variation trend with the equivalence ratio increments of the present experimental data are seen to be in very good agreement (probably within the experimental uncertainty in most cases) with two of the previous experimental ones. However, Jahn’s experimental data, which has often been referred to as a systematically measured laminar burning velocity extended over a wide oxygen-enrichment, shows some discrepancies in a comparison with the others, especially near the equivalence ratio of 1.0. From the experimental results it is difficult to decide which experimental result has been measured more accurately. However, it is found that the results of the present experiments are more satisfactory in all range of equivalence ratio, and it is also expected that present experimental techniques would provide more accurate data in the oxygenenriched conditions in a comparison with the ones adopted in Jahn’s experiments. In Figure 5, the present experimental results of the laminar burning velocities are compared with Jahn’s ones in the range of the oxygen-enrichment ratio from 0.40 to 0.60. There are few experiments which report on the systematic laminar burning velocity data of the oxygen enriched flame according to the equivalence ratio. Because of a transition from a laminar flow to a turbulent one in the nozzle type burner with a small exit diameter, a laminar burning velocity exceeding Ω ) 0.60 could not be measured in the full range of the equivalence ratio. The present experimental data, however, are expected to provide provisions for the design of a practical oxygen-enriched combustor because the adiabatic flame temperature of Ω ) 0.60 amounts to 2900 K. In the oxygen-enriched flame the results of Jahn’s experiments are a little bit lower than those of the present ones as well as in methane/air flame.

Chemical Reaction Mechanism of Methane

Figure 6. (a) Comparison of the burning velocities between the various experimental data and numerically predicted data by using the conventional reaction mechanisms in the CH4/air flame and (b) between the present experimental data and the numerically predicted data by using the conventional reaction mechanisms in CH4/O2-enriched flame.

In the case of the direct photograph techniques adopted by Jahn’s experiments, the luminous zone is recorded. Since the luminous zone is produced mainly by the hot incandescent products of a combustion, the flame surface recorded by a direct photography is closer to the “postflame” region. In this way the R in Figure 3b would be underestimated so the burning velocity would be underestimated as well.8 On the other hand, Schlieren, which indicates the region close to where d2T/dx2 is a maximum, is produced at the face of the flame which is closest to the fresh mixture. Because the fundamental burning velocity is defined as the speed of the flame front relative to the unburned mixture, it is natural that a direct photographs yields on erroneous value and the Schlieren image is more suitable for evaluating the burning velocity. From the description of Figures 4 and 5 it can be concluded that Jahn’s experiments underestimate the laminar burning velocity and the present experiments produce a better result to within the experimental uncertainty. It is, therefore, adjudged to be reasonable that the present results provide more reliable information on the laminar burning velocity in a wide range of the oxygen enrichment ratio and the equivalence ratio than Jahn’s one and that they can be used as one of the references for the verification of the reaction mechanisms in OEC. In order to evaluate the performance of the various reaction mechanisms numerically, the predicted burning velocities are compared to the experimental ones for methane/air flame in Figure 6a and methane/oxygen-enriched flame in Figure 6b. As shown in Table 1, three reaction mechanisms are selected in this study. The solid line is the calculated result by using GRI 3.0 Mech., and the dotted and dashed-dotted lines are the ones using MB Mech. and LKY Mech., respectively. The terminology

Energy & Fuels, Vol. 21, No. 6, 2007 3205

of a symbol is the same as that of Figure 4. In Figure 6a, methane/air flame case, all the numerically predicted results are satisfactorily consistent with the experimental results, even though the result by using GRI 3.0 Mech. is predicted a little bit lower in a comparison with the ones by the MB mech. and LKY Mech. near the peak point. At Ω ) 0.4 in Figure 6b, however, numerical results by using LKY Mech. overpredict and those using GRI 3.0 and MB underpredict the experimental results. These differences between the numerical and experimental results appear at Ω ) 0.6, and they are expected to be very large at Ω ) 0.985. From these results it is revealed that the conventional reaction mechanisms are good at predicting the burning velocities of methane/air flame. However, they turned out to be poor at predicting the burning velocities of a highly oxygen-enriched flame, and thus modifications of the conventional reaction mechanism are needed for a more accurate prediction of the burning velocity in methane/oxygen-enriched flame. As a preliminary investigation for the reaction mechanism modification, a sensitivity analysis is performed to discriminate the dominant elementary reaction which affects the laminar burning velocity. In the present study GRI 3.0 Mech. is used as a basic reaction mechanism for an improvement. In Figure 7, the major dominant elementary reactions on the burning velocity are selected and listed by various oxygen enrichment ratios. The sensitivity values are normalized with respect to the maximum one at each condition. In methane/air flame chain reaction (R38) H + O2 T O + OH contribute mostly in a positive way to the burning velocity increment in the whole range of the equivalence ratios. (R99) OH + CO T H + CO2, which contribute a lot to the heat release, considerably affect an increase of the burning velocity especially in the fuellean condition. However, (R35) H + O2 + H2O T HO2 + H2O and (R52) H + CH3 (+M) T CH4 (+M) are only effective in decreasing the burning velocity in a fuel-lean and fuel-rich condition, respectively. These overall sensitivity tendencies of the elementary reactions in methane/air flame are almost the same in methane/ oxygen-enriched flame. Contributions of them, however, decrease considerably in a comparison with those of (R38). Even though the contribution of (R99) is exceptionally high for the oxygen-enriched flame, this trend is only restricted to a fuellean condition. From the sensitivity analysis about the burning velocity (R38) is revealed as a commonly contributing major elementary reaction for a burning velocity increment in the whole range of the oxygen enrichment and that of the equivalence ratio. In the present research various reaction coefficients of (R38) were investigated from the literature and tested by using the PREMIX code. Finally, the modified GRI 3.0 Mech. where the reaction coefficient of (R38) is substituted by that of Yetter et al.22 is showed the best performance in burning velocity prediction in all the conditions. The original and modified values of the reaction coefficients are listed in Table 2. Figure 8 shows the numerically calculated centerline profiles of the major and minor species’ concentration in a counterflow diffusion flame with Ω ) 0.90 and ag ) 20. The solid line and the line with a symbol represent the results by using GRI 3.0 Mech. and the present Mech., respectively. As shown in Figure (22) Yetter, R. A.; Dryer, F. L.; Rabitz, H. A Comprehensive Reaction Mechanism for Carbon Monoxide/Hydrogen/Oxygen Kinetics. Combust. Sci. Technol. 1991, 79, 97–128.

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Figure 8. Comparison of the numerically predicted profiles in CH4/ O2-enriched flame between the present reaction mechanism and GRI 3.0 reaction mechanism for (a) major species and (b) minor species.

Figure 7. Comparison of the normalized burring velocity sensitivities according to the equivalence ratio in CH4/O2-enriched flame. Table 2. Modified Reaction and Related Constant Information k* ) A*Tn exp(–E/RT) (R38) H + O2 T O + OH

A

n

Figure 9. Comparison of the burning velocities according to the equivalence ratio by using the present reaction mechanism in CH4/O2enriched flame.

E

original value reversible reaction 2.650 × 1016 -0.6707 17041.0 modified value forward reaction 1.940 × 1014 0.00 16440.0 backward reaction 1.528 × 1013 0.00 408.75

8, a modification of the reaction coefficient has little effect on the performance of GRI 3.0 Mech. for the flame structure prediction. In Figure 9 the numerically predicted burning velocities by using the present reaction mechanism are compared to those by using the conventional reaction mechanisms. Present experimental burning velocities are also presented as a reference one. The calculation by using the present Mech. is shown to describe the experimental results quite well from Ω ) 0.21 to Ω ) 0.60, for which the conventional reaction mechanisms do not. It can also be expected that present reaction mechanism would predict the burning velocity of a highly oxygen-enriched flame (Ω ) 0.985) as well as showing a superior performance in predicting the burning velocity of various oxygen-enriched flames. Figure 10 plots the numerical and experimental results of NO of a counterflow diffusion flame. Symbol represents the

Figure 10. Comparison of NO number density by using present reaction mechanism in CH4/O2-enriched flame.

experimental results of Naik et al.10 Other experimental conditions are the same as those of Figure 8. Except for the results by using MB Mech., and those using the other reaction mechanisms including the present one are in accord with the measured NO number density quite well.

Chemical Reaction Mechanism of Methane

Even though there is more room for improvement based on the additional detailed experiments about the various combustion characteristics such as an ignition delay, extinction limits, etc., it is worthy of note that the present experimental data and reaction mechanism can present relatively good references for an industrial oxygen-enriched combustor design. Conclusions Experimental and numerical studies on detailed methane reaction mechanisms in the oxygen-enriched flame have been conducted. It is seen, in the cases of the experimental laminar burning velocities in the oxygen enriched condition, that the present data by using the Schlieren technique is more reliable than Jahn’s one by using the direct photograph technique. On the basis of the present experimental data, investigations on the performance of the conventional reaction mechanisms have been performed numerically, and modified reaction mechanisms which predict the burning velocity in the oxygen-enriched flame quite well are proposed. The modified reaction mechanisms are also in accord with the measured NO number density as well as the laminar burning velocity. Acknowledgment. This project was financially supported by both CERC (Combustion Engineering Research Center) and the midand long-term nuclear R&D program in KOREA by Ministry of Science and Technology through KOSEF.

Nomenclature ag A

global strain rate cross-sectional area

Energy & Fuels, Vol. 21, No. 6, 2007 3207 A* cp cpk E hk k k* K M˘ q˘r r R SL T u Vk Vu Vu,t Vu,n w˘k Wj Wk x Yk

frequency factor constant pressure specific heat of a mixture constant pressure specific heat of species k activation energy specific enthalpy of species k species k reaction rate constant Kth species mass flow rate radiative heat loss radial distance universal gas constant burning velocity temperature unburned axial velocity normal to flame surface diffusion velocity of species k velocity of unburned mixture tangential component of the velocity of an unburned mixture normal component of the velocity of an unburned mixture production rate of species k average molecular weight of a mixture molecular weight of species k axial distance mass fraction of species k

Greek Letters R λ µ F Φ EF070175V

angle of a premixed reaction cone thermal conductivity of a mixture viscosity density of a mixture equivalence ratio