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Energy & Fuels 2007, 21, 3216–3224
Addition Effects of H2 and H2O on Flame Structure and Pollutant Emissions in Methane–Air Diffusion Flame Jeong Park,*,† Sang In Keel,‡ and Jin Han Yun‡ School of Mechanical Engineering, Pukyung National UniVersity, San 100, Yongdang-dong, Nam-gu, Busan 608-739, Korea, and EnVironment & Energy Research DiVision, Korea Institute of Machinery and Materials, 171 Jang-dong, Yuseong-gu, Daejeon, 305-343, Korea ReceiVed April 24, 2007. ReVised Manuscript ReceiVed July 12, 2007
Addition effects of H2 and H2O on flame structure and NOx emission behavior are numerically studied with detailed chemistry in methane–air counterflow diffusion flames. The discernible differences in flame structure and the behaviors of pollutant emissions such as CO, CO2, and NOx are compared among a pure methane flame, CH4–H2 flames, and CH4–H2–H2O flames. The important role of chemical effects of added H2O in flame structure and pollutant emissions is also discussed. It is seen that chemical effects of added H2O increase the maximum flame temperature for small H2O mole fraction, and this is relevant to the enhanced OH radical through the reaction step O + H2O f OH + OH. Emission indices of CO increase and then decrease after showing a maximum in the increase of methane mole fraction for CH4–H2 flames and in the increase of H2O mole fraction for CH4–H2–H2O flames, while those of CO2 increase monotonously. These behaviors are caused by the competition of the production through the reaction step HCO + H2O f H + CO + H2O with the destruction of CO by the reaction step CO + OH f CO2 + H. It is also found that chemical effects of added H2O reduce the CO emission index and increase the CO2 emission index. The changes of thermal NO and Fenimore NO are also analyzed for pure methane, CH4–H2 flames, and CH4–H2–H2O flames. In all flames, the contribution of the Fenimore mechanism in NO production is much more important. It is also shown that chemical effects of added H2O suppress NO formation mainly through the Fenimore mechanism. To facilitate the details of those NO behaviors, importantly contributing reaction steps to thermal NO and Fenimore NO are addressed for pure methane, CH4–H2 flames, and CH4–H2–H2O flames.
Introduction Natural gas, which is mainly composed of methane, offers significant advantages over other fuels in the economic and environmental sides. That is, methane has lower CO2 emission levels compared with other hydrocarbons because of its lower carbon-to-hydrogen ratio, and thus, the application to a leaner combustion system can further reduce these levels. It is well known that the leaner combustion system suppresses NOx emission. However, the leaner combustion system may cause combustion instabilities and lower power output. Hydrogen blending could be a key to overcome both difficulties. This is the main reason why extensive fundamental research has been conducted on the combustion characteristics of methane–air mixtures with a small amount of hydrogen addition.1–4 Hydrogen–methane blends are also receiving attention as alternative fuels for power generation applications because of concerns about global warming and the prospect of using hydrogen in fuel cells and combustors of the next generation. However, the use of pure hydrogen may still be quite distant * To whom correspondence should be addressed. Phone: +82-51-6201601. Fax: +82-51-620-1531. E-mail:
[email protected]. † Pukyung National University. ‡ Korea Institute of Machinery and Materials. (1) Yu, G.; Law, C. K.; Wu, C. K. Combust. Flame 1986, 63, 339–347. (2) Karim, G. A.; Wierzba, I.; Al-Alousi, Y. Int. J. Hydrogen Energy 1996, 21 (7), 625–631. (3) Karbasi, M.; Wierzba, I. Int. J. Hydrogen Energy 1998, 23 (2), 123– 129. (4) Fotache, C. G.; Kreutz, T. G.; Law, C. K. Combust. Flame 1998, 112, 522–532.
due to stringent problems of safety and storage and the enormous investment cost for the replacement of fossil fuels by hydrogen in existing power plant systems. To bypass these difficulties for a moment, blending hydrogen into methane or other hydrocarbons might be a proposal as an intermediate solution towards a fully developed hydrogen-economy society.5,6 Production of hydrogen by electrolysis of water may be preferred because of the safety and cost considerations of handling and storing hydrogen as a fuel additive. Practical applications of the addition of electrolysis products into other fuels are already available in practical combustors such as IC engines.7 The produced hydrogen through this process contains water vapor, and thus, the utilization of the blended fuels of hydrocarbons, hydrogen, and water vapor may make combustion phenomena complicated. In addition to understanding the complicated combustion characteristics of the blended fuels of hydrocarbons and hydrogen, the chemical effects of the addition of water vapor to a combustion system have two sides: the maximum flame temperature could increase due to chemical effects of added H2O under high temperature flame conditions, but it could also decrease due to diluted effects of added H2O under low temperature flame conditions.8,9 A previous study9 (5) Law, C. K.; Kwon, O. C. Int. J. Hydrogen Energy 2001, 26, 867– 879. (6) Ilbas, M.; Crayford, A. P.; Yilmaz, I.; Bowen, P. J. Int. J. Hydrogen Energy 2006, 312, 1768–1779. (7) Bade Shresha, S. O.; Karim, G. A. Int. J. Hydrogen Energy 1999, 24, 577–586. (8) Park, J.; Kim, S. C.; Keel, S. I.; Noh, D. S.; Oh, C. B.; Chung, D. Int. J. Energy Res. 2004, 28, 1075–1088.
10.1021/ef700211m CCC: $37.00 2007 American Chemical Society Published on Web 10/12/2007
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Energy & Fuels, Vol. 21, No. 6, 2007 3217
only difference between the present 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 and are written as G(x) ) H-2 Fu Fu
dF(x) dx
(1)
d FG d 3G2 d G + + µ )0 dx F F dx dx F
[ ( )]
( )
dYk d + (FY V ) - w˙k dx dx k k
F dT 1 d dT λ + dx cp dx dx cp
( )
Wk ) 0; k ) 1, ... , K (3)
∑c V Y
p k k
k
(2)
dT 1 + dx cp
q˙r
∑ h w˙ - c k k
k
)0
p
(4) Figure 1. Schematic of the configuration of the counterflow diffusion flame.
clarified that remarkably increased production of OH radical due to chemical effects, which was the direct outcome of the reaction step O + H2O f OH + OH, modified flame structure sufficiently to produce higher flame temperature under high temperature flame conditions. It was therefore found that these chemical effects also affected NOx emission behavior considerably. Methane flame is well described by a fuel consumption layer and a H2–CO consumption layer in the side of the flame structure.10 The addition of hydrogen to methane results in intensifying the characteristic of the H2–CO consumption layer, and it was shown that these affected flame structure and NOx emission considerably.11 Up to now, most past research efforts have been given to study flame structure and NO emissions in blended fuels of methane and hydrogen only with low hydrogen contents. Indeed, combustion characteristics and NO emission behavior are not yet understood so much at intermediate and high hydrogen contents. Especially considering the extensive application of the utilization of electrolysis products in the future, some fundamental research efforts are still needed to better understand the flame structure and NOx emission of these mixtures, i.e., mixtures of methane, hydrogen, and water vapor as a fuel. The objectives of the present numerical work are, therefore, (i) to compare the difference of flame structures among a pure CH4 flame, CH4–H2 flames, and CH4–H2–H2O flames; (ii) to provide the effects of added hydrogen and water vapor on pollutant emissions such as CO, CO2, and NOx; (iii) to clarify the chemical effects of added H2O on flame structure and the behavior of pollutant emissions in methane–hydrogen–H2O flames; and (iv) finally to examine the dominant reaction steps in reaction contribution to the production and destruction of CO, and also to provide importantly contributing reaction steps to the thermal and Fenimore mechanisms of NO under various conditions by varying the contents of methane, hydrogen, and water vapor in the blend. Numerical Strategies A laminar opposed jet diffusion flame is established between two opposed jets impinging on each other, as shown in Figure 1. The mathematical description near the stagnation point is onedimensional, and the model adopted in the study is that developed by Kee et al.12 and extended by Lutz et al.13 The (9) Hwang, D. J.; Choi, J. W.; Park, J.; Keel, S. I.; Oh, C. B.; Noh, D. S. Int. J. Energy Res. 2004, 28, 1255–1267. (10) Seshadri, K.; Peters, N. Combust. Flame 1988, 73, 23–44.
where G(x) ) –(Fν/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 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 )
where the fuel-side velocity is given to be equal to the oxidizerside one. 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, is as follows:14 q˙r ) -4σKp(T4 - T∞4 )
(6)
4
Kp )
∑PK, i i
i ) CO2, H2O, CO, CH4
(7)
i)1
where σ is the Stefan–Boltzmann constant, T and T∞ the local and ambient temperatures, respectively, and Kp 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 governing equations are solved using a CHEMKIN-based code15 and a transport-based code.16 An adaptive grid redistributes a weighting function of the first and second derivatives of the temperature, and the system of algebraic equations is solved by a damped Newton algorithm.13 In the strategy for a converged solution, if the Newton algorithm fails to converge, the solution estimate is conditioned by a period of time integration. This provides a new starting point for the Newton algorithm that is close to the solution. (11) Park, J.; Hwang, D. J.; Park, J. S.; Kim, J. S.; Keel, S. I.; Cho, H. C.; Noh, D. S.; Kim, T. K. Int. J. Energy Research 2007, in press. (12) Kee, R. J.; Miller, J. A.; Evans, G. H.; Dixon-Lewis, G. Proc. Combust. Inst. 1988, 1479. (13) Lutz, A. E.; Kee, R. J.; Grcar, J. F.; Rupley, F. M. Sandia Natl. Lab. [Tech. Rep.] SAND 1997, SAND 96-8243. (14) Ju, Y.; Guo, H.; Maruta, K.; Liu, F. J. Fluid Mech. 1997, 342, 315. (15) Kee, R. J.; Rupley, F. M.; Miller, J. A. Sandia Natl. Lab. [Tech. Rep.] SAND 1989, SAND 89-8009B. (16) Kee, R. J.; Dixon-Lewis, G.; Warnatz, J.; Coltrin, M. E.; Miller, J. A. Sandia Natl. Lab. [Tech. Rep.] SAND 1994, SAND86-8246.
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The separation distance of the two opposed jets is 2.0 cm, and the flame zone is located in the position at which momentum fluxes of fuel and oxidizer sides balance each other. The global strain rate is obtained as follows:17 ag )
[
2(-uO) uF 1+ L (-uO)
] FF FO
(8)
Here, the subscripts F and O mean the fuel and oxidizer, respectively. In the previous study,18 the GRI v-3.0 mechanism19 was validated by comparing experimentally and numerically determined flame properties for various fuel combinations in hydrogen-enriched CH4–air premixed flames. Accordingly, the present reaction model adopts the GRI v-3.0 mechanism. An artificial species, referred to as X hereafter, is introduced to clearly identify the chemical effects of added H2O.8,9 The artificial species X is defined in a manner that it has exactly the same thermochemical, transport, and radiation properties as those of added H2O but it is not allowed to participate in any chemical reaction. Therefore, X is strictly regarded as a chemically inert species. Numerical calculations are conducted twice with X and the real H2O. The difference between the results calculated with the artificial species and the real H2O is then wholly attributed to the chemical effects of added H2O. The flame structure of the CH4–air diffusion flame is systematically changed into those of the blending fuels (methane–hydrogen–H2O) through molar additions of H2 and H2O in the fuel stream. The boundary temperature of both the fuel and air sides is 300 K. Results and Discussion Figure 2 shows the variations of maximum flame temperatures (a) in the increase of methane mole fraction for CH4–H2 flames and (b) in the increase of H2O mole fraction (or X mole fraction) for CH4–H2–H2O flames. In the figures, Xi in the horizontal axis represents the mole fraction of chemical species i and X means the artificial species which is introduced to facilitate the chemical effects of added H2O, as was mentioned above. The zero in methane mole fraction means pure hydrogen, and the unity represents pure methane in Figure 2a. The increase of H2O (or X) mole fraction implies the decrease of added hydrogen for a fixed methane mole fraction in Figure 2b. In general, the reaction rate of the principal 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.20 Moreover the reaction rate of the chain branching reaction is much smaller than those between H atoms and hydrocarbons. As a result, populous hydrocarbons consume the H atoms rigorously, and this enfeebles the chain branching reaction. This is the reason why the maximum flame temperature decreases with an increase in the methane mole fraction for CH4–H2 flames, as shown in Figure 2a. In Figure 2b, the maximum flame temperature decreases with the increase of the mole fraction of H2O or the artificial species X for a fixed methane mole fraction. (17) Chellian, H. K.; Law, C. K.; Ueda, T.; Smooke, M. D.; Williams, F. A. Proc. Combust. Inst. 1990, 503. (18) Ren, J.-Y.; Qin, W.; Egolfopoulos, F. N.; Tsotsis, T. T. Combust. Flame 2001, 124, 717–720. (19) 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. The “GRI-Mech 3.0” chemical kinetic mechanism 2007. http://www.me.berkeley.edu/gri_mech/. (20) Westbrook, C. K.; Dryer, F. L. Prog. Energy Combust. Sci. 1984, 10, 1–57.
Figure 2. Variation of maximum flame temperature at a global strain rate of 157 s-1 (a) with methane mole fraction in CH4–H2 flame and (b) with H2O mole fraction in CH4–H2–H2O flame.
This is so because the addition of H2O or the artificial species reduces the population of the reactive species. This is confirmed by the fact that flames are extinguished by excessive addition of H2O in Figure 2b. Figure 2b also shows that maximum flame temperatures with H2O addition are higher than those with the addition of the artificial species for XH2O ) 0.1. However, the tendency is reversed as the methane mole fraction increases. These complicated behaviors might be, essentially, explained through the inspection of the behaviors of chain carrier radicals such as H, O, and OH and of the principal chain branching reaction, since the principal chain branching reaction has been known to be an indicator of overall reaction rate. Figure 3 shows the variation of maximum mole fractions of H, O, and OH with mole fraction of added H2O or artificial species for (a) XCH4 ) 0.1 and (b) XCH4 ) 0.7. Maximum mole fractions of H and O with added H2O are smaller than those with artificial species, but maximum OH mole fractions with added H2O are oppositely larger than those with artificial species. This implies that OH is remarkably produced due to the chemical effects of added H2O and it may modify flame structure sufficiently as much as higher flame temperatures with the addition of H2O are obtained in comparison to those with the addition of artificial species. Dominant H2O-related reaction steps relevant to the production of OH radical were examined to verify the behavior of the OH radical produced chemical effects of added H2O, and the following was the most
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Figure 3. Behavior of chain carrier radicals according to H2O addition and their chemical effects for methane mole fractions of (a) 0.1 and (b) 0.7 at a global strain rate of 157 s-1 in CH4–H2–H2O flame.
importantly contributing reaction step (these tendencies were consistent with those of the previous studies8,9,21). O + H2O f OH + OH
(R86)
Figure 4 displays the variations of (a) the reaction rate of the principal chain branching reaction and (b) that of the reaction step (R86) with the additions of H2O and artificial species. The principal chain branching reaction rates with the addition of H2O are smaller than those with the addition of the artificial species in all cases in Figure 4a. The reaction rates of the reaction step (R86) with H2O addition are oppositely larger than those with the addition of the artificial species in Figure 4b, and those differences due to the chemical effects are reduced with an increase of the added methane mole fraction. Consequently, the reason why maximum flame temperatures with the addition of H2O are higher than those with the artificial species for small quantities of methane mole fraction but the tendency is reversed as the methane mole fraction is increased is addressed by the following explanation. That is, the behavior of maximum flame temperature generally follows that of the principal chain branching reaction H + O2 f O + OH. By the way, maximum mole fractions of H and O with added H2O are smaller than those with artificial species, but maximum OH mole fractions with added H2O are oppositely larger than those with artificial species. Indeed, the chemical effects of added H2O should have a tendency to reduce the overall reaction rate, since the mole fraction of H radical decreases. However, the remarkably produced OH radical has a tendency to enhance the overall reaction rate and thus maximum flame temperature through the OH-related (21) Zhao, D; Yamashita, H.; Kitagawa, K.; Arai, N. Combust. Flame 2002, 130, 352.
Figure 4. Variation of reaction rates of the reaction steps (a) O + H2O f OH + OH and (b) H + O2 f OH + O with methane mole fraction and their chemical effects at a global strain rate of 157 s-1 in CH4–H2–H2O flame.
reaction pathway. Moreover, the reaction rates between hydrocarbons and H atoms are much larger than the principal chain branching reaction, and thus, the increased population of hydrocarbons forces the overall reaction to be enfeebled. For small methane mole fractions such as XCH4 ) 0.1, the produced OH radical due to the chemical effects of added H2O modifies the flame structure sufficiently as much as higher flame temperatures can be attained in comparison to those with artificial species. However, the enhancement of the reaction rate of R86 due to chemical effects becomes gradually weakened, as shown in Figure 4b, and the principal chain branching reaction also becomes enfeebled due to the populous hydrocarbons according to the increase of methane mole fraction. The fact that maximum flame temperatures with the addition of H2O become smaller than those with the artificial species implies that the latter effects become dominant according to the increase of methane mole fraction. Figure 5 describes the variation of emission indices of CO and CO2 (a) with methane mole fraction for CH4–H2 flames and (b) with mole fraction of added H2O or artificial species for CH4–H2–H2O flames. The use of mole fraction is not appropriate to grasp the behavior of CO, CO2, and NOx, since they include convection and diffusion terms, and the following emission indices of CO, CO2, and NOx, suggested by Nishioka et al.,22 are adopted:
∫ W w˙ dx EI ) - ∫ W w˙ dx L
i
0 L
0
i i
where i ) CO, CO2, and NO
(9)
CH4 CH4
(22) Nishioka, M.; Nakagawa, S.; Ishikawa, Y.; Takeno, T. Combust. Flame 1994, 98, 127–136.
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Figure 6. (a) Reaction contribution of important reaction steps to CO at a global strain rate of 157 s-1 in CH4–H2 flame with XCH4 ) 0.2 and (b) reaction contribution of reaction steps R166, R167, and R99 according to methane mole fraction at a global strain rate of 157 s-1 in CH4–H2 flame.
CO2 but decrease emission indices of CO for CH4–H2–H2O flames. These complicated behaviors may be addressed only through the inspection of important contribution reaction steps to the production and destruction of CO. The importantly contributing reaction steps to CO production are as follows:
Figure 5. (a) Behavior of emission indices of CO and CO2 according to methane mole fraction at a global strain rate of 157 s-1 in CH4–H2 flame and chemical effects in emission indices of (b) CO and (c) CO2 at a global strain rate of 157 s-1 in CH4–H2–H2O flame.
Here, Wi is the molecular weight of chemical species i and w˘ i is the molar production rate of chemical species i. For CH4–H2 flames in Figure 5a, the emission index of CO increases and decreases after showing a maximum at the methane mole fraction of 0.3, while those of CO2 increase monotonously with the increase of methane mole fraction. For CH4–H2–H2O (or X) flames in Figure 5b and c, emission indices of CO also increase and then decrease with the increase of mole fraction of added H2O or artificial species for methane mole fractions of 0.1, while those emission indices decrease for the mole fraction of 0.7. Meanwhile, emission indices of CO2 increase monotonously with the increase of H2O and its artificial species. Chemical effects of added H2O increase emission indices of
O + C2H2 f CO + CH2
(R23)
H + HCO f H2 + CO
(R55)
H + HCCO f CH2(s) + CO
(R79)
HCO + H2O f H + CO + H2O
(R166)
HCO + M f H + CO + M O + CH3 f H + H2 + CO
(R167) (R284)
Most of the produced CO2 results from the following reaction step: CO + OH f CO2 + H
(R99)
Figure 6a displays importantly contributing reaction steps to the production and destruction of CO for XCH4 ) 0.2 and XH2 ) 0.8 as a representative case of CH4–H2 flame. As shown in Figure 6a, most of the CO is produced through reaction steps R166 and R167 and are consumed through reaction step R99. The reaction rates of reaction steps R99, R166, and R167, which are dominantly contributing to the production and destruction of CO, are displayed with the increase of methane mole fraction in Figure 6b. The CO production rates through both reaction steps R166 and R167 increase with the increase of methane mole fraction. However, the CO consumption rate through
Addition Effects of H2 and H2O on Flame Structure
Figure 7. (a) Reaction contribution of important reaction steps to CO at a global strain rate of 157 s-1 in CH4–H2–H2O flame with XCH4 ) 0.7, XH2 ) 0.05, and XH2O ) 0.25; (b) reaction contribution of the reaction steps R166, R167, and R99 according to H2O mole fraction at a global strain rate of 157 s-1 in CH4–H2–H2O flame with XCH4 ) 0.1; and (c) those at a global strain rate of 157 s-1 in CH4–H2–H2O flame with XCH4 ) 0.7.
reaction step R99 increases much more steeply with increasing methane mole fraction. This implies that the produced CO is immediately converted to CO2 through reaction step R99. This is the direct reason why the CO emission index in Figure 5a increases and then decreases. The reaction rates of reaction steps R99, R166, and R167, which are dominantly contributing to production and destruction of CO, are displayed in the increase of H2O (or X) mole fraction for (a) XCH4 ) 0.1 and (b) XCH4 ) 0.7 of CH4–H2–H2O (or X) flame in Figure 7. Even if we do not provide all of the data, the above-mentioned reaction rates relevant to CO production increase and then decrease with the increase of H2O mole fraction except for reaction step R166.
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The reason why the tendency is opposite to those of the other reaction steps is relevant to the fact that the reaction rate of reaction step R166 is directly increased by the increase of H2O mole fraction. Figure 7a and b also shows for XCH4 ) 0.1 and XCH4 ) 0.7 that in both cases the CO consumption rates through reaction step R99 are much larger than the CO production rates though reaction steps R166 and R167 and the CO consumption rate, moreover, increases much more steeply in comparison to the CO production rates. These cause an increase in the CO2 emission indices according to H2O mole fraction for the cases of both XCH4 ) 0.1 and XCH4 ) 0.7 in Figure 5c. However, the CO2 emission index for XCH4 ) 0.1 increases more sensitively with increasing H2O mole fraction in comparison to that for XCH4 ) 0.7. This implies that CO2 is produced much more for the blended fuels, in which hydrogen is more populous relative to methane. As a result, the CO emission index increases steeply and decreases rapidly for the case of XCH4 ) 0.1, while it decreases mildly for XCH4 ) 0.7. Meanwhile, the CO production rates through reaction step R166 with H2O addition are larger than those with X addition, as shown in Figure 7, while those through reaction step R167 with H2O addition are smaller than those with X addition. This is so because H2O addition increases the reaction rate of reaction step R166, as can be understood from the reaction equation. It is also found that the CO consumption rates with H2O addition are larger than those with X addition, and this is due to the increase of OH radical population by chemical effects of added H2O, as was shown in Figure 3. These complicated behaviors according to the addition of H2 and H2O (or X) may affect the production and destruction of NOx considerably. Figure 8 describes the variation of emission indices of NO through the full and thermal mechanisms with methane mole fraction for CH4–H2 flame and with methane mole fraction and/or fraction of added H2O (or X) for CH4–H2–H2O (or X) flame at a strain rate of 157 s-1. The emission indices of NOx by both the thermal and full mechanisms decrease with an increase of methane mole fraction for CH4–H2 flame and with an increase of H2O (or X) mole fraction. In Figure 8, 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.22 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 8a clearly shows that Fenimore NO is larger than thermal NO, and this tendency is consistent with the results in diffusion flames of the previous study,22 and also clarifies that the full NO is mainly a direct outcome from Fenimore NO at large methane mole fractions for CH4–H2 flame. In general, thermal NO (Zeldovich NO) is closely relevant to flame temperature. Flame temperature decreased with an increase of methane mole fraction in Figure 2. As a result, thermal NO is shown to decrease with an increase of methane mole fraction. For the CH4–H2–H2O flame of Figure 8b, thermal NO decreases with an increase of H2O mole fraction, while Fenimore NO increases and then decreases at XCH4 ) 0.1. For large H2O mole fractions, Fenimore NO approaches full NO, and this also implies that Fenimore NO becomes predominated as the H2O mole fraction increases. Figure 8b also shows that chemical effects of added H2O reduce the emission index of NO mainly through Fenimore
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Figure 9. Comparison of the reaction contribution of reaction steps R178, R179, and R180 to thermal NO among (a) pure methane flame and CH4–H2 flames with XCH4 ) 0.1 and XCH4 ) 0.7 and (b) that among CH4–H2–H2O flames with various compositions.
Fenimore NO in Figure 8b and c are not clearly addressed. More cautious examination may be required to understand the NO behavior through the comparison to pure methane flame for CH4–H2 flames and CH4–H2–H2O flames to clarify those behaviors. The Zeldovich mechanism relevant to thermal NO production is generally as follows: N + NO T N2 + O N + O2 T NO + O
Figure 8. Contribution of Fenimore NO and thermal NO in full NO emission index (a) according to methane mole fraction at a global strain rate of 157 s-1 in CH4–H2 flame, according to (b) H2O mole fraction at a global strain rate of 157 s-1 in CH4–H2–H2O flame with XCH4 ) 0.1, and (c) that in CH4–H2–H2O flame with XCH4 ) 0.7.
NO. Figure 8c shows for XCH4 ) 0.7 of CH4–H2–H2O flame that both Fenimore NO and thermal NO decrease with an increase of H2O mole fraction and the role of Fenimore NO in full NO is much more important in comparison to that at XCH4 ) 0.1 in Figure 8b. It is also seen that chemical effects of added H2O also suppress NO formation. Meanwhile, the behavior of thermal NO can be understood on the basis of the relevance to flame temperature, but the tendencies of
(R178) (R179)
N + OH T NO + H (R180) Figure 9 displays the reaction contribution to the production and destruction of thermal NO (a) for pure methane flame and CH4–H2 flames and (b) for CH4–H2–H2O flames. The main source of thermal NO is reaction step R180, and thermal NO is consumed through reaction step R178 for CH4–H2 flame and CH4–H2–H2O flames, as shown in Figure 9. These global features are very similar to those in the previous study on CO2-added methane flames except for the point that reaction step R178 contributes to the production of thermal NO in CH4–H2 flames with large hydrogen mole fraction such as the condition of XCH4 ) 0.1 and XH2 ) 0.9. It is also seen in Figure 9b that chemical effects of added H2O suppress the production of thermal NO for CH4–H2–H2O flames. Figure 10 also displays the production and destruction rates of the importantly contributing reaction steps to Fenimore NO for pure methane flame, CH4–H2 flames, and CH4–H2–H2O flames. The NO production through the Fenimore mechanism
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fraction. These characteristics according to the addition of H2 are much more remarkable. The importance of the NH-related reaction steps such as R190 in NO production is diminished according to the addition of H2. It is also noted that reaction step R222, which is one of the major sources for NO production, becomes less important in NO formation according to the addition of H2. The importantly contributing reaction steps to NO destruction are R244, R245, R246, R249, R255, and R274. The role of reaction steps R246 and R255 as a reburning mechanism was addressed in a previous study.22 The importance of reaction step R274, known as a HCN recycle route, leading to the consumption of NO has been well described.22,23 As can definitely be seen in Figure 10b, the contribution of NO consumption is through the reburning process and the HCN recycle route for the cases of small H2 mole fraction and through the HNO-related reaction R212 for the cases of large mole fraction. It is also found for CH4–H2–H2O flame that the reaction contribution of all the major reaction steps to the production and destruction of NO is suppressed according to the addition of H2O. Chemical effects of added H2O repress both the production and destruction of NO, as shown in Figure 10b. Conclusion
Figure 10. Comparison of the reaction contribution of important reaction steps to Fenimore NO among (a) pure methane flame and CH4–H2 flames with XCH4 ) 0.1 and XCH4 ) 0.7 and (b) that among CH4–H2–H2O flames with various compositions.
in methane flame is mainly relevant to the following reaction steps:18 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 mechanism18 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) For CH4–H2 flame and CH4–H2–H2O flame, the HNO-related reaction steps such as R214 and R215 importantly contribute to the NO formation, while the HNO-related reaction step R212 consumes NO remarkably according to the increase of H2 mole
Numerical study was conducted to investigate the effects of the addition of hydrogen and steam in methane–air diffusion flame. The following conclusion can be obtained. Maximum flame temperature increases with an increase in hydrogen mole fraction and with a decrease of H2O mole fraction. Chemical effects of added H2O could modify flame structure through the complicated behavior of principal chain carrier radicals. Some outstanding features of these chemical effects are as follows: (1) maximum flame temperatures with H2O addition are higher than those with the addition of the artificial species for small H2O mole fractions such as the condition of XH2O ) 0.1 and (2) the chemical effects inhibit the radicals of H and O but augment OH radical through the reaction O + H2O f OH + OH. For CH4–H2 flames, the emission index of CO increases and then decreases after showing a maximum, while that of CO2 increases monotonically with an increase in methane mole fraction. For CH4–H2–H2O flames, the emission index of CO increases and then decreases after showing a maximum, while that of CO2 increases monotonically with an increase in the fraction of added H2O. It is also found that chemical effects of added H2O reduce the CO emission index and increase the CO2 emission index. These complicated behaviors are mainly caused by the competition of the production of CO through the reaction step R166 with the destruction of CO by the reaction step R99. For CH4–H2 flame and CH4–H2–H2O flame, Fenimore NO is larger than thermal NO, and the full NO is mainly originated from Fenimore NO at large methane mole fractions. Chemical effects of added H2O suppress NO formation mainly through the Fenimore mechanism. The main source of thermal NO is the reaction N + OH f NO + H. Reaction step R178 contributes to the consumption of NO for pure methane flame but the production of NO for large hydrogen mole fractions of CH4–H2 flame and CH4–H2–H2O flame. The HNO-related reaction steps such as R212 and R214 importantly contribute to the destruction and production of NO remarkably, respectively. These characteristics according to the addition of H2 are (23) Miller, J. A.; Bowman, C. T. Prog. Energy Combust. Sci. 1989, 15, 287.
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much more remarkable. The reaction step NCO + O f NO + CO, which is one of the major sources for NO production, becomes less important in NO formation according to the addition of H2 and H2O. The contribution of NO consumption is mainly through the reburning process and the HCN recyle route for the cases of small H2 mole fraction, while it is almost through the HNO-related reaction R212 for the cases of large mole fraction. It is also found that chemical effects of added
Park et al.
H2O inhibit both thermal NO and Fenimore NO except that chemical effects of added H2O increase themal NO only under the conditions of XH2O ) 0.1. Acknowledgment. Financial support of our work by the funds with fundamental business of Korea Institute of Machine and Materials is gratefully acknowledged. EF700211M
