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Nov 15, 2007 - San 100, Yongdang-dong, Nam-gu, Busan 608-739, Korea, EnVironment .... methane/hydrogen-air laminar diffusion flames are numerically...
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Energy & Fuels 2008, 22, 278–283

Preferential Diffusion Effects on NO Formation in Methane/ Hydrogen-Air Diffusion Flames Jeong Soo Kim,† Jeong Park,*,‡ Oh Boong Kwon,‡ Jin Han Yun,§ Sang In Keel,§ and Tae Kwon Kim| School of Mechanical and Aerospace Engineering, Sunchon National UniVersity, 315 Maegok, Suncheon, Jeonnam 540-742, Korea, School of Mechanical Engineering, Pukyong National UniVersity, San 100, Yongdang-dong, Nam-gu, Busan 608-739, Korea, EnVironment and Energy Research DiVision, Korea Institute of Machinery and Materials, 171 Jang-dong, Yuseong-gu, Daejeon 305-343, Korea, and School of Mechanical and AutomotiVe Engineering, Keimyung UniVersity, 1000 Singdang-dong, Dalseo-gu, Daegu 704-701, Korea ReceiVed August 23, 2007. ReVised Manuscript ReceiVed October 8, 2007

A numerical study in methane/hydrogen diffusion flames has been conducted to clarify the preferential diffusion effects of H2 and H with detailed chemistry. The composition of fuel is systematically changed from pure methane to pure hydrogen through the molar addition of H2 to methane. A comparison was made by employing three species diffusion models, i.e., mixture-averaged species diffusion and the suppression of the diffusivities of H and H2. The behavior of maximum flame temperatures with the three species diffusion models is not explained by the scalar dissipation rate but the nature of chemical kinetics such as the behaviors of chain carrier radicals of H, O, and OH. It is found that the preferential diffusion of H radical into the reaction zone curbs the populations of the chain carrier radicals and then flame temperature while that of H2 into the reaction zone produces the reduction of the scalar dissipation rate and the population of chain carrier radicals and these force the flame temperature to decrease. These preferential diffusion effects of H2 and H are also compared among NO emission behaviors through the three species diffusion models. Under all flame conditions, Fenimore NO is much more remarkable compared to thermal NO. It is also seen that the preferential diffusion of H radical into the reaction zone suppresses the thermal and Fenimore NO while that of H2 into the reaction zone increases them. To facilitate the details of those NO behaviors through preferential diffusion effects of H2 and H, importantly contributing reaction steps to the production and destruction of Fenimore NO are addressed.

1. Introduction Natural gas, which is mainly composed of methane, offers significant advantages over other fuels. Methane has lower emission levels compared with other fuels, and utilizing it in leaner combustion of methane can further reduce these levels. Leaner combustion of methane results in higher thermal efficiency and reduced emissions such as CO2 and NOx. However, the low ignitability and lower burning velocity, which may cause combustion instabilities and lower power output, are the major difficulties in achieving satisfactory combustion. One of the reasonable solutions to overcome the difficulties is addition of more reactive fuels such as hydrogen. There have been several investigations on the effects of the addition of hydrogen into methane,1–9 and these studies have shown * To whom correspondence should be addressed. E-mail: jeongpark@ pknu.ac.kr. Phone: +82-51-620-1601. Fax: +82-51-620-1531. † Sunchon National University. ‡ Pukyong National University. § Korea Institute of Machinery and Material. | Keimyung University. (1) Yu, G.; Law, C. K.; Wu, C. K. Combust. Flame 1986, 63, 339. (2) Karim, G. A.; Wierzba, I.; Al-Alousi, Y. Int. J. Hydrogen Energy 1996, 21, 625. (3) Fotache, C. G.; Kreutz, T. G.; Law, C. K. Combust. Flame 1998, 112, 522. (4) Karbasi, M.; Wierzba, I. Int. J. Hydrogen Energy 1998, 23, 123. (5) Bade Shresha, S. O.; Karim, G. A. Int. J. Hydrogen Energy 1999, 24, 577.

experimentally and numerically that hydrogen addition results in substantial enhancement in the ignitability and lean flammability limits of methane/air mixtures. On the contrary, the use of hydrocarbons as an additive to hydrogen flames could be susceptible to the suppression of explosion hazards in storage as well as knock-limited operation when supercharging is required to increase the power density.10 Even if this study supported the potential of propane as a suppressant of both diffusive-thermal cellular and hydrodynamic instabilities, the realization of a fully developed hydrogen-economy society may be still quite distant due to stringent problems of safety. Furthermore, an enormous investment cost is needed for the replacement of fossil fuels by hydrogen in existing combustion systems. Thus, the use of methane/hydrogen mixtures could be an interim solution toward a fully developed hydrogen-economy society.11,12 Meanwhile, it has been known that flames are affected by preferential diffusion especially when the fuel includes species (6) Bauer, C. G.; Forest, T. W. Int. J. Hydrogen Energy 2001, 26, 55. (7) Ren, J.-Y.; Qin, W.; Egolfopoulos, F. N.; Tsotsis, T. T. Combust. Flame 2001, 124, 717. (8) Halter, F.; Chauveau, C.; Djebaili-Chaumeix, N.; Gokalp, I. Proc. Combust. Inst. 2005, 30, 201. (9) Dagaut, P.; Nicolle, A. Proc. Combust. Inst. 2005, 30, 2631. (10) Law, C. K.; Kwon, O. C. Int. J. Hydrogen Energy 2004, 29, 867. (11) Di Sarli, V.; Di Benedette, A. Int. J. Hydrogen Energy 2007, 32, 637. (12) Park, J.; Keel, S. I.; Yun, J. H. Energy Fuels, in press.

10.1021/ef700505a CCC: $40.75  2008 American Chemical Society Published on Web 11/15/2007

Preferential Diffusion Effects on NO Formation

of significantly different diffusivity such as H2 and H. Evidence of the preferential diffusion effects has been presented in flame tip opening of laminar jet diffusion flames13 and superadiabatic flame temperatures in rich premixed hydrocarbon flames14–17 and even for the case of an extremely small strain rate of 0.1 in a CO/H2/N2 laminar opposed-flow diffusion flame.18 The previous study14 suggested that superadiabatic flame temperature occurs only in rich premixed hydrocarbon flames but not in hydrogen flames and addressed that the nature of superadiabatic flame temperature is chemical kinetics.14,15 The other insisted that the nature in rich hydrocarbon premixed flames is the preferential diffusion of H2 from the reaction zone to the preheat zone and its preferential oxidation compared to hydrocarbons.16 The previous study17 showed in freely propagating rich CH4/ air and CH4/O2 flames that the preferential diffusion of H2 from the reaction zone to the preheat zone had negligible effects on the phenomenon of superadiabatic flame temperature, and clarified that the chemical nature of the superadiabatic flame temperature was identified to be the relative scarcity of H radical. The study19 in tubular diffusion flames showed that positive curvature strengthened the preferential diffusion and negative curvature weakened the preferential diffusion. Similar tendencies were observed in H2/N2-air laminar counterflow diffusion flames strained by steadily impinging microfuel or microair jets.20 However, those previous researchers who studied the preferential diffusion effects of H and H2 in diffusion flames were likely to fail to notice the important role of chemical kinetics through the preferential diffusion effects of H2 and H. Although the importance of chemical kinetics through preferential diffusion of H2 and H in rich premixed hydrocarbon flames has been speculated on the nature of superadiabatic flame temperature,14,15,17 attempts on the important nature of chemical kinetics have been seldom made in diffusion flames; thus, further insight into these phenomena might be required. Furthermore, few studies on the preferential diffusion effects of H2 and H in NOx emission behaviors have been reported. This study is motivated by the lack of understanding of the preferential diffusion effects of H2 and H in flame structure and NOx emission characteristics of laminar diffusion flames. In the present study, the structures and NO emission behavior in methane/hydrogen-air laminar diffusion flames are numerically examined for three species diffusion models with detailed chemistry. To clearly display the preferential diffusion effects of H2 and H in flame structure and NO emissions, the diffusivities of H2 and H are suppressed. The objectives of the present numerical work are, therefore, (i) to compare flame structures among pure CH4 flames, pure H2 flames, and CH4/ H2 flames with and without preferential diffusion effects; (ii) to clarify the chemical nature of the preferential diffusion effects of H2 and H on the modification of flame structure; and (iii) to examine the impact of preferential diffusion of H2 and H to NOx emission behaviors. Especially preferential diffusion effects (13) Ishizuka, S.; Sakai, Y. Proc. Combust. Inst. 1986, 25, 1821. (14) Liu, F.; Guo, H.; Smallwood, G. J.; Gu¨lder, Ö. Proc. Combust. Inst. 2002, 29, 1543. (15) Ruf, B.; Behrendt, F.; Deutchmann, O.; Kleditzsch, S.; Warnatz, J. Proc. Combust. Inst. 2000, 28, 1455. (16) Zamashchikov, V. V.; Namyatov, I. G.; Bunev, V. A.; Babkin, V. S. Combust., Explos. Shock WaVes 2004, 40, 32. (17) Liu, F.; Gu¨lder, Ö. Combust. Flame 2005, 143, 264. (18) Drake, M. C.; Blint, R. J. Combust. Sci. Technol. 1988, 61, 187. (19) Wang, P.; Hu, S.; Pitz, R. Proc. Combust. Inst. 2007, 31, 989. (20) Takagi, T.; Yoshikawa, Y.; Komiyama, M.; Kinoshita, S. Proc. Combust. Inst. 1996, 26, 1103.

Energy & Fuels, Vol. 22, No. 1, 2008 279

in the dominant reaction contributions to thermal and Fenimore mechanisms of NO are examined with three species diffusion models. 2. Numerical Methods and Strategies The conservation equations of mass, momentum, energy, and chemical species for laminar counterflow diffusion flames are solved using OPPDIF code.21 The thermochemical and transport properties of chemical species are attained from the CHEMKIN II22 and TPLIB23 databases. The only difference between this work and those conducted by them is the energy conservation equation, in which we retain the sink term relevant to the thermal radiation. All of these conservation equations are transformed into a set of ordinary differential equations as follows: G(x) ) H-2 Fu Fu

dF(x) dx

(1)

d d FG 3G2 d G + + µ )0 dx F F dx dx F

[ ( )]

( )

dYk d + (FY V ) - w˙kWk ) 0, dx dx k k

(2)

k ) 1, ... , K

(3)

q˙ ( ) ∑ c V Y dTdx + c1 ∑ h w˙ - c ) 0

F dT 1 d dT λ + dx cp dx dx cp

r

p k k

k

k k

p k

p

(4) where G(x) ) - (Fv/r), F(x) ) (Fu/2), and the radial pressure gradient, H ) (1/r)(∂p/∂r), is constant and is an eigenvalue of the problem. In this formulation, the axial and radial velocity components at the nozzle exit can be independently specified and the pressure eigenvalue is computed as part of the solution. The boundary conditions for the fuel and oxidizer streams at the nozzles are as follows: FFuF , G ) 0, T ) TF, FuYk + FYkVk ) (FuYk)F 2 (5) FOuO , G ) 0, T ) TO, FuYk + FYkVk ) (FuYk)O x ) L: F ) 2

x ) 0: F )

The main contribution to radiative heat loss is given to CO2, H2O, CO, and CH4, and the radiative heat loss, based on the optically thin approximation,24 is expressed as q˙r ) -4σKp(T 4 - T ∞4 )

(6)

4

Kp )

∑P K i

i

i ) CO2, H2O, CO, CH4

(7)

i)1

where σ is the Stefan-Bolzmann constant, T and T∞ are the local and ambient temperatures, respectively, and Kp is the Plank mean absorption coefficient. Pi and Ki are the partial pressure and the Plank mean absorption coefficient of a species, respectively. The Plank mean absorption coefficient is approximately obtained as a polynomial function of temperature. The separation distance of the two opposed jets is 2.0 cm, and the flame zone is located at the position which momentum fluxes of fuel and oxidizer sides balance each other. The global strain rate is obtained as follows:25 (21) Lutz, A. E.; Kee, R. J.; Grcar, J. F.; Rupley, F. M. Sandia Natl. Lab. [Tech. Rep.] 1997, SAND 96-8243. (22) Kee, R. J.; Rupley, F. M.; Miller, J. A. Sandia Natl. Lab. [Tech. Rep.] 1989, 89-8009B. (23) Kee, R. J.; Dixon-Lewis, G.; Warnatz, J.; Coltrin, M. E.; Miller, J. A. Sandia Natl. Lab. [Tech. Rep.] 1994, SAND86-8246. (24) Ju, Y.; Guo, H.; Maruta, K.; Liu, F. J. Fluid Mech. 1997, 342, 315. (25) Chellian, H. K.; Law, C. K.; Ueda, T.; Smooke, M. D.; Williams, F. A. Proc. Combust. Inst. 1990, 23, 503.

280 Energy & Fuels, Vol. 22, No. 1, 2008

Kim et al.

Figure 1. Behavior of maximum flame temperature according to methane mole fraction at global strain rates of 157 and 400 s-1 in methane/hydrogen flames.

ag )

[

2(-uO) uF 1+ L (-uO)

] FF FO

(8)

Here, the subscripts F and O mean the fuel and oxidizer, respectively. The present reaction model adopts the GRI v-3.0 mechanism,26 since it was validated by comparing experimentally and numerically determined flame properties for various fuel mixtures in hydrogen-enriched CH4/air premixed flames.27 The flame structure of the CH4/air diffusion flame is systematically changed into those of the blending fuels through molar additions of H2 in the fuel stream. The ambient pressure is 1 atm, and the temperature in the fuel and oxidizer sides is 300 K. To investigate preferential diffusion effects of H and H2 in flame structure, calculations are conducted using the following three treatments of chemical species diffusion: (1) mixture average (M-A) allowing preferential diffusion of H and H2, (2) mixture average with the assumption of DH ) DN2, and (3) mixture average with the assumption of DH2 ) DN2. Thermal diffusion of chemical species was not taken into account.

3. Results and Discussion 3.1. Behavior of Maximum Flame Temperature. Figure 1 shows the variations of maximum flame temperatures calculated using the three species diffusion models and adiabatic flame temperature with methane mole fraction at global strain rates of 157 and 400 s-1 in methane/hydrogen flames. In all cases, maximum flame temperatures are smaller than adiabatic flame temperatures; thus, spueradiabatic flame temperatures do not appear, since selected global strain rates are appropriately high.18 Maximum flame temperatures become small according to the increase of global strain rate, as has been well-known. Therefore, maximum flame temperatures at a strain rate of 157 s-1 are higher than those at a strain rate of 400 s-1 for all of the individual three species diffusion models. Furthermore, the tendencies for the three species diffusion models at a strain rate of 157 s-1 are similar to those at a strain rate of 400 s-1. In general, the reaction rate of the chain branching reaction H + O2 f O + OH, which is an indicator of overall reaction rate, is relatively less than those between H atoms and hydrocarbons, and both of the reactions compete with each other for H atoms. Thus, the increase of the population of hydrocarbons forces the maximum reaction rate and maximum flame temperature to decrease through the relative enfeeblement of the reaction rate of the principal chain branching reaction H + O2 f OH + (26) Smith, G. P.; Golden, D. M.; Frenklach, N. W.; Eiteneer, M. B.; Goldenberg, M.; Bowman, C. T.; Hanson, R. K.; Dong, S.; Gardiner, W. C.; Lissianski, V. V., Jr.; Qin, Z. http://www.me.Berkeley.edu/gri_mech/. (27) Schefer, R. W.; Wicksall, M.; Agrawal, A. K. Proc. Combust. Inst. 2002, 29, 843.

O.28 Therefore, the reasonable behavior is going forward to the reduction of maximum flame temperature in the increase of methane mole fraction. Previous studies in rich hydrocarbon premixed flames showed the contrary results that superadiabatic flame temperature was due to the preferential diffusion of H2 from the reaction zone to preheat zone16 while the preferential diffusion of H played an important role in the occurrence of superadiabatic flame temperature.17 The present study in methane/ hydrogen diffusion flames displays in Figure 1 that suppression of the diffusivity of H increases maximum flame temperature and that of H2 decreases maximum flame temperature. There is a possibility that these complicated behaviors of maximum flame temperatures might be explained by the scalar dissipation rate if these are not the nature of chemical kinetics. Furthermore, in Figure 1, the suppression of the diffusivity of H2 indicates an anomalous flame temperature behavior which decreases and then increases after showing a minimum at a methane mole fraction of 0.2 in the increase of methane mole fraction. This anomalous behavior has to also be explained by the traditional flame behavior according to the scalar dissipation rate if the nature of flame behavior is not responsible for chemical kinetics but physical processes. Figure 2 describes (a) the distributions of scalar dissipation rates with the three species diffusion models at a global strain rate of 157 s-1 for a pure methane, a pure hydrogen, and a methane/hydrogen flame and (b) the variations of maximum scalar dissipation rates using the three species diffusion models with methane mole fraction at global strain rates of 157 and 400 s-1. The mixture fraction suggested by Bilger28 was used to calculate the scalar dissipation rate. For the pure methane flame in Figure 2a, an insignificant difference among the distributions of scalar dissipation rates using the three species diffusion models appears. For the pure hydrogen and methane/ hydrogen flames, the distributions of scalar dissipation rates with DH2 ) DN2 are significantly deviated from those with the species diffusion models of M-A and DH ) DN2. Maximum scalar dissipation rates are displayed to clarify them for all flame conditions in Figure 2b. The tendency of maximum scalar dissipation rate at a strain rate of 157 s-1 is similar to that at at a strain rate of 400 s-1. That is, an insignificant difference among the distributions of scalar dissipation rates with the species diffusion models of M-A and DH ) DN2 appears for all of the flame conditions. Figure 2b clearly shows that the degree of deviation of maximum scalar dissipation rate with DH2 ) DN2 becomes gradually significant with the decrease of methane mole fraction compared to those with the species diffusion models of M-A and DH ) DN2. For all of the flame conditions, maximum scalar dissipation rates decrease with an increase of methane mole fraction. From the comparison of Figures 1 and 2, the decrease of maximum flame temperature with mixtureaveraged species diffusion with the decrease of maximum scalar dissipation rate might be meaningless, since each methane mole fraction implies different fuel flames. Nevertheless, different maximum scalar dissipation rates using the three species diffusion models should affect maximum flame temperature at the same methane mole fraction. However, lower maximum flame temperatures with DH2 ) DN2 might be mainly attributed to higher scalar dissipation rate for the same methane mole fractions. Meanwhile, the points that maximum flame temperatures with DH ) DN2 are higher than those with the species diffusion model of M-A and the tendency of maximum flame temperature with DH2 ) DN2 are significantly different from (28) Westbrook, C. K.; Dryer, F. L. Prog. Energy Combust. Sci. 1984, 10, 1.

Preferential Diffusion Effects on NO Formation

Energy & Fuels, Vol. 22, No. 1, 2008 281

Figure 2. (a) Distributions of scalar dissipation rates with the three species diffusion models at a global strain rate of 157 s-1 for a pure methane, a pure hydrogen, and a methane/hydrogen flame and (b) the variations of maximum scalar dissipation rates using the three species diffusion models with methane mole fraction at global strain rates of 157 and 400 s-1.

those with the species diffusion models of M-A and DH ) DN2 and are still unclear. Then, these complicated behaviors of maximum flame temperatures are not explained by scalar dissipation rate but might be addressed to the nature of chemical kinetics. Figure 3 depicts the variations of maximum mole fractions of (a) H, (b) O, and (c) OH radicals using the three species diffusion models with methane mole fraction at a global strain rate of 157 s-1. It is evident that the maximum mole fractions of these radicals substantially exceed their equilibrium values under all flame conditions. The superequilibrium phenomena of these radicals have been well-known in stretched diffusion flames,18 and are a consequence of fast two-body radicalformation reactions in the reaction zone followed by slow threebody recombination reactions. Except for the maximum mole fractions of these radicals with DH2 ) DN2, the maximum mole fractions of these radicals decrease with an increase of methane mole fraction including the results from equilibrium calculations. Similar to the behavior of maximum flame temperature, maximum mole fractions of chain carrier radicals of H, O, and OH with the species diffusion model of M-A are larger than those with DH2 ) DN2 and smaller than those with DH ) DN2. A previous study18 in CO/H2/N2 laminar diffusion flames showed that the maximum flame temperature decreases and the maximum mole fractions of the chain carrier radicals such as H, O, and OH increase as the flame stretch increases, but both decrease steeply near flame extinction and finally flame extinction occurs. This inferred that the increase of the population of chain carrier radicals, which might result in the increase of the overall reaction rate through the principal chain branching

Figure 3. Variations of the maximum mole fractions of (a) H, (b) O, and (c) OH radicals using the three species diffusion models with methane mole fraction at a global strain rate of 157 s-1.

reaction H + O2 f OH + O, was complementary to the decrease of flame temperature according to flame stretch. It was then understood that flame extinction occurs when both the rapid reductions of chain carrier radical and flame temperature could not overcome the situation of weakened flame strength. In cases of DH ) DN2, the maximum flame temperatures in Figure 1 and maximum mole fractions of chain carrier radicals (H, O, and OH) in Figure 3 are larger than those with mixture-averaged species diffusion even though both scalar dissipation rates with mixture-averaged species diffusion and DH ) DN2 are almost identical. It is evident that a higher population of these chain carrier radicals increases the flame temperature because of the increased overall reaction rate through the principal chain branching reaction. Meanwhile, the maximum flame temperature and maximum mole fractions of these chain carrier radicals with DH2 ) DN2 are smaller than those with mixture-averaged species diffusion. Furthermore, scalar dissipation rates with DH2 ) DN2 are much higher than those with the species diffusion models of M-A and DH ) DN2. As a result, a lower population of these chain carrier radicals tends to decrease flame temperature due

282 Energy & Fuels, Vol. 22, No. 1, 2008

Figure 4. Variations of emission indices of NO through the full and thermal mechanisms using the three species diffusion models with methane mole fraction at a global strain rate of 157 s-1 in methane/ hydrogen flames.

Kim et al.

Figure 5. Variations of the NO production rate of the precursor reaction step of thermal NO using the three species diffusion models with methane mole fraction at a global strain rate of 157 s-1.

to the decreased overall reaction rate and much higher scalar dissipation rates with DH2 ) DN2 also tend to decrease flame temperature. This was the reason why the maximum flame temperature showed such a tendency in Figure 1. Consequently the suppression of the diffusivity of H increases the population of the chain carrier radicals and thus results in the increase of flame temperature, and this implies that on the contrary the preferential diffusion of H radical into the reaction zone curbs the populations of the chain carrier radicals and then flame temperature. Furthermore, the constraint of the diffusivity of H2 decreases the population of the chain carrier radicals and thus flame temperature. Conversely, the preferential diffusion of H2 into the reaction zone produces the reduction of the scalar dissipation rate and the population of chain carrier radicals, and these force the flame temperature to decrease. These present results in diffusion flames are not consistent with those of Liu and Gl¨der17 but are consistent with those of Zamashchikov et al. in rich hydrocarbon premixed flames.16 These changes of flame temperatures and chain carrier radicals through preferential diffusion effects of H2 and H may also affect the production and destruction of NOx. 3.2. Preferential Diffusion Effects on NOx Emission Behavior. Figure 4 describes the variations of emission indices of NO through the full and thermal mechanisms with methane mole fraction at a global strain rate of 157 s-1 in methane/hydrogen flames. Both the emission indices of NO by the thermal and full mechanisms decrease with the increase of methane mole fraction. In Figure 4, the difference between the full and thermal mechanisms is due to the Fenimore mechanism, since the contributions by the N2O mechanism and NO2 mechanism are negligible in the present study. This tendency is consistent with the results in methane flame of the previous study.30 The difference between the full and thermal mechanisms in the pure hydrogen flame (XCH4 ) 0) is due to the contribution of NH-, NNH-, and HNO-related reactions. Figure 4 clearly shows that Fenimore NO is larger than thermal NO for large methane mole fractions in CH4/H2 flames and for the pure methane flame. This tendency is consistent with the results in diffusion flames of the previous study.30 It is, therefore, seen that the full NO is mainly a direct outcome from Fenimore NO at large methane mole fractions of the CH4/H2 flame. On the contrary, the contribution of thermal NO in full NO becomes important as the methane mole fraction decreases. It has been known that thermal NO (Zeldovich NO) is closely relevant to flame temperature. The tendencies of thermal NO using the species

diffusion models of M-A and DH ) DN2 are similar to those of maximum flame temperatures in Figure 1. However, the thermal NO with the species diffusion model of DH2 ) DN2 decreases with methane mole fraction differently from the tendency of maximum flame temperature in Figure 1. This implies that in the case with DH2 ) DN2 the thermal NO, which has been understood by the Zeldovich mechanism and thus explained by the relevance to flame temperature, does not follow the tendency of flame temperature. It is also confirmed in Figure 5 that the production rates of the precursor reaction step, N2 + O f NO + N, for thermal NO shows a similar tendency to those of emission indices of NO in Figure 4. Despite this anomalous behavior in thermal NO with DH2 ) DN2, the emission indices of thermal NO with the species diffusion model of M-A are larger than those with DH2 ) DN2 and smaller than those with DH ) DN2 for the same methane mole fractions. These tendencies are similar to those of maximum flame temperatures in Figure 1. As a result, the preferential diffusion of H radical into the reaction zone suppresses thermal NO while that of H2 plays a role in the enhancement of thermal NO. Figure 6 compares the production and destruction rates of importantly contributing reaction steps to Fenimore NO at a global strain rate of 157 s-1 for a pure methane flame and a CH4/H2 flame. The NO production through the Fenimore mechanism is mainly relevant to the following reaction steps:7

(29) Bilger, R. W. Proc. Combust. Inst. 1988, 22, 475. (30) Nishioka, M.; Nakagawa, S.; Ishikawa, Y.; Takeno, T. Combust. Flame 1994, 98, 127.

NH + O f NO + H NNH + O f NH + NO HNO + H f H2 + NO

(R190) (R208) (R214)

HNO + OH f NO + H2O

(R215)

NCO + O f NO + CO (R222) The major NO destruction through the Fenimore mechanism7 is contributed by H + NO + M f HNO + M C + NO f CN + O C + NO f CO + N CH + NO f HCN + O CH + NO f H + NCO CH + NO f N + HCO CH2 + NO f H + HNCO

(R212) (R244) (R245) (R246) (R247) (R248) (R249)

CH2 + NO f H + HCNO

(R251)

CH3 + NO f HCN + H2O

(R255)

HCCO + NO f HCNO + CO

(R274)

Preferential Diffusion Effects on NO Formation

Energy & Fuels, Vol. 22, No. 1, 2008 283

diminished according to the addition of H2. It is also noted that the production rate of the reaction step R222, which is one of the major sources for NO production, and the destruction rate of the HCN recycle route reaction step R274 change little or even decrease according to the addition of H2. In all cases, the production and destruction rates of NO with the species diffusion model of M-A are smaller than those with DH ) DN2 and larger than those with DH2 ) DN2 except that the destruction rate of Fenimore NO through the HCN recycle route with DH ) DN2 is smaller than that with M-A and larger than that with DH2 ) DN2. These were the main reasons why Figure 4 showed such a tendency of emission indices of NO for the three species diffusion models. As a result, the preferential diffusion of H radical into the reaction zone suppresses Fenimore NO while the preferential diffusion of H2 plays a role in the enhancement of Fenimore NO. 4. Concluding Remarks

Figure 6. Production and destruction rates of importantly contributing reaction steps to Fenimore NO using the three species diffusion models at a global strain rate of 157 s-1 for a pure methane flame and a CH4/ H2 flame with a methane mole fraction of 0.6.

For the pure methane flame in Figure 6a, the HNO-related reaction step R214 dominantly contributes to the NO formation compared to those of other NO formation reaction steps while the HNO-related reaction step R212 consumes NO. The importance of the HNO-related reactions in Fenimore NO has already been described for oxyfuel combustion and methane flames in previous studies.30–32 The importantly contributing reaction steps to NO destruction are R244, R245, R246, R249, R255, and R274. The role of the reaction steps, R246 and R255, as a reburning mechanism was addressed in a previous study.30 The importance of the reaction step R274, known as the HCN recycle route, leading to the consumption of NO has been well described.30,31 Globally, the production and destruction rates of the above-mentioned reaction steps increase gradually according to the addition of H2. However, the importance of the NH-related reaction steps such as R190 in NO production is relatively (31) Miller, J. A.; Bowman, C. T. Prog. Energy Combust. Sci. 1989, 15, 287. (32) Park, J.; Park, J. S.; Kim, H. P.; Kim, J. S.; Kim, S. C.; Choi, J. G.; Cho, H. C.; Cho, K. W.; Park, H. S. Energy Fuels 2007, 21, 121.

A numerical study on methane/hydrogen diffusion flames has been conducted to compare flame structures and NO emission behaviors for three species diffusion models with detailed chemistry. The following conclusion can be drawn: In pure methane, pure hydrogen, and methane/hydrogen flames, maximum flame temperatures with the species diffusion model of M-A are smaller than those with DH2 ) DN2 and larger than those with DH2 ) DN2. These tendencies are not explained by the typical flame behavior with scalar dissipation rate but rather by the behavior of the chain carrier radicals such as H, O, and OH, which impacts directly the global reaction rate through the principal chain branching reaction H + O2 f O + OH. This can be one piece of evidence that the nature of flame temperature behavior through the preferential diffusion of H2 and H into the reaction zone is chemical kinetics. Furthermore, these behaviors imply that the preferential diffusion of H radical into the reaction zone curbs the populations of the chain carrier radicals and then flame temperature while that of H2 into the reaction zone produces the reduction of the scalar dissipation rate and the population of chain carrier radicals, and these force the flame temperature to decrease. These changes of flame temperatures and chain carrier radicals through the preferential diffusion effects of H2 and H also affect the production and destruction of NOx. That is, the preferential diffusion of H radical into the reaction zone suppresses the thermal NO and Fenimore NO while that of H2 increases them. Acknowledgment. Financial support of our work by the funds with fundamental business of Korea Institute of Machine and Materials is gratefully acknowledged. EF700505A