addition effects of h 2 and h 2 o on flame...

Addition Effects of H2 and H2O on Flame Structure and PollutantEmissions 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 withdetailed chemistry in methane–air counterflow diffusion flames. The discernible differences in flame structureand the behaviors of pollutant emissions such as CO, CO2, and NOx are compared among a pure methaneflame, CH4–H2 flames, and CH4–H2–H2O flames. The important role of chemical effects of added H2O inflame structure and pollutant emissions is also discussed. It is seen that chemical effects of added H2O increasethe maximum flame temperature for small H2O mole fraction, and this is relevant to the enhanced OH radicalthrough the reaction step O + H2O f OH + OH. Emission indices of CO increase and then decrease aftershowing a maximum in the increase of methane mole fraction for CH4–H2 flames and in the increase of H2Omole fraction for CH4–H2–H2O flames, while those of CO2 increase monotonously. These behaviors are causedby the competition of the production through the reaction step HCO + H2O f H + CO + H2O with thedestruction of CO by the reaction step CO + OH f CO2 + H. It is also found that chemical effects of addedH2O reduce the CO emission index and increase the CO2 emission index. The changes of thermal NO andFenimore NO are also analyzed for pure methane, CH4–H2 flames, and CH4–H2–H2O flames. In all flames, thecontribution of the Fenimore mechanism in NO production is much more important. It is also shown thatchemical effects of added H2O suppress NO formation mainly through the Fenimore mechanism. To facilitatethe details of those NO behaviors, importantly contributing reaction steps to thermal NO and Fenimore NOare addressed for pure methane, CH4–H2 flames, and CH4–H2–H2O flames.

Introduction

Natural gas, which is mainly composed of methane, offerssignificant advantages over other fuels in the economic andenvironmental sides. That is, methane has lower CO2 emissionlevels compared with other hydrocarbons because of its lowercarbon-to-hydrogen ratio, and thus, the application to a leanercombustion system can further reduce these levels. It is wellknown that the leaner combustion system suppresses NOxemission. However, the leaner combustion system may causecombustion instabilities and lower power output. Hydrogenblending could be a key to overcome both difficulties. This isthe main reason why extensive fundamental research has beenconducted on the combustion characteristics of methane–airmixtures with a small amount of hydrogen addition.1–4

Hydrogen–methane blends are also receiving attention asalternative fuels for power generation applications because ofconcerns about global warming and the prospect of usinghydrogen in fuel cells and combustors of the next generation.However, the use of pure hydrogen may still be quite distant

due to stringent problems of safety and storage and the enormousinvestment cost for the replacement of fossil fuels by hydrogenin existing power plant systems. To bypass these difficultiesfor a moment, blending hydrogen into methane or otherhydrocarbons might be a proposal as an intermediate solutiontowards a fully developed hydrogen-economy society.5,6

Production of hydrogen by electrolysis of water may bepreferred because of the safety and cost considerations ofhandling and storing hydrogen as a fuel additive. Practicalapplications of the addition of electrolysis products into otherfuels are already available in practical combustors such as ICengines.7 The produced hydrogen through this process containswater vapor, and thus, the utilization of the blended fuels ofhydrocarbons, hydrogen, and water vapor may make combustionphenomena complicated. In addition to understanding thecomplicated combustion characteristics of the blended fuels ofhydrocarbons and hydrogen, the chemical effects of the additionof water vapor to a combustion system have two sides: themaximum flame temperature could increase due to chemicaleffects of added H2O under high temperature flame conditions,but it could also decrease due to diluted effects of added H2Ounder low temperature flame conditions.8,9 A previous study9* To whom correspondence should be addressed. Phone: +82-51-620-

1601. 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.

(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. HydrogenEnergy 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.

Energy & Fuels 2007, 21, 3216–32243216

10.1021/ef700211m CCC: $37.00 2007 American Chemical SocietyPublished on Web 10/12/2007

clarified that remarkably increased production of OH radicaldue to chemical effects, which was the direct outcome of thereaction step O + H2O f OH + OH, modified flame structuresufficiently to produce higher flame temperature under hightemperature flame conditions. It was therefore found that thesechemicaleffectsalsoaffectedNOxemissionbehaviorconsiderably.

Methane flame is well described by a fuel consumption layerand a H2–CO consumption layer in the side of the flame structure.10

The addition of hydrogen to methane results in intensifying thecharacteristic of the H2–CO consumption layer, and it was shownthat these affected flame structure and NOx emission consider-ably.11 Up to now, most past research efforts have been given tostudy flame structure and NO emissions in blended fuels of methaneand hydrogen only with low hydrogen contents. Indeed, combustioncharacteristics and NO emission behavior are not yet understoodso much at intermediate and high hydrogen contents. Especiallyconsidering the extensive application of the utilization of electrolysisproducts in the future, some fundamental research efforts are stillneeded to better understand the flame structure and NOx emissionof these mixtures, i.e., mixtures of methane, hydrogen, and watervapor as a fuel.

The objectives of the present numerical work are, therefore,(i) to compare the difference of flame structures among a pureCH4 flame, CH4–H2 flames, and CH4–H2–H2O flames; (ii) toprovide the effects of added hydrogen and water vapor onpollutant emissions such as CO, CO2, and NOx; (iii) to clarifythe chemical effects of added H2O on flame structure and thebehavior of pollutant emissions in methane–hydrogen–H2Oflames; and (iv) finally to examine the dominant reaction stepsin reaction contribution to the production and destruction ofCO, and also to provide importantly contributing reaction stepsto the thermal and Fenimore mechanisms of NO under variousconditions by varying the contents of methane, hydrogen, andwater vapor in the blend.

Numerical Strategies

A laminar opposed jet diffusion flame is established betweentwo opposed jets impinging on each other, as shown in Figure1. The mathematical description near the stagnation point is one-dimensional, and the model adopted in the study is thatdeveloped by Kee et al.12 and extended by Lutz et al.13 The

only difference between the present work and those conductedby them is the energy conservation equation, in which we retainthe sink term relevant to the thermal radiation. All of theseconservation equations are transformed into a set of ordinarydifferential equations and are written as

G(x)) dF(x)dx

(1)

H- 2ddx(FG

F )+ 3G2

F+ d

dx[µ ddx(G

F )]) 0 (2)

FudYk

dx+ d

dx(FYkVk)- wk Wk ) 0; k) 1, ... , K (3)

FudTdx

- 1cp

ddx(λ dT

dx )+ Fcp∑

k

cpVkYkdTdx

+ 1cp∑

k

hkwk -qr

cp) 0

(4)

where G(x) ) –(Fν/r), F(x) ) (Fu/2), and the radial pressuregradient, H ) (1/r)(∂p/∂r), is constant and is an eigenvalue ofthe problem. In this formulation, the axial and radial velocitycomponents at the nozzle exit can be independently specifiedand the pressure eigenvalue is computed as part of the solution.The boundary conditions for the fuel and oxidizer streams atthe nozzles are

x) 0: F)FFuF

2, G) 0, T) TF, FuYk +FYkVk ) (FuYk)F

x+ L: F)FOuO

2, G) 0, T) TO, FuYk +FYkVk ) (FuYk)O

(5)

where the fuel-side velocity is given to be equal to the oxidizer-side one. The main contribution to radiative heat loss is givento CO2, H2O, CO, and CH4, and the radiative heat loss, basedon the optically thin approximation, is as follows:14

qr )-4σKp(T4 - T∞

4 ) (6)

Kp )∑i)1

4

PiKi, i)CO2, H2O, CO, CH4 (7)

where σ is the Stefan–Boltzmann constant, T and T∞ the localand ambient temperatures, respectively, and Kp the Plank meanabsorption coefficient. Pi and Ki are the partial pressure andthe Plank mean absorption coefficient of a species, respectively.The Plank mean absorption coefficient is approximately obtainedas a polynomial function of temperature. The governingequations are solved using a CHEMKIN-based code15 and atransport-based code.16 An adaptive grid redistributes a weight-ing function of the first and second derivatives of the temper-ature, and the system of algebraic equations is solved by adamped Newton algorithm.13 In the strategy for a convergedsolution, if the Newton algorithm fails to converge, the solutionestimate is conditioned by a period of time integration. Thisprovides a new starting point for the Newton algorithm that isclose to the solution.

(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.

(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.

Figure 1. Schematic of the configuration of the counterflow diffusionflame.

Addition Effects of H2 and H2O on Flame Structure Energy & Fuels, Vol. 21, No. 6, 2007 3217

The separation distance of the two opposed jets is 2.0 cm,and the flame zone is located in the position at which momentumfluxes of fuel and oxidizer sides balance each other. The globalstrain rate is obtained as follows:17

ag )2(-uO)

L [1+uF

(-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 numericallydetermined flame properties for various fuel combinations inhydrogen-enriched CH4–air premixed flames. Accordingly, thepresent reaction model adopts the GRI v-3.0 mechanism. Anartificial species, referred to as X hereafter, is introduced toclearly identify the chemical effects of added H2O.8,9 Theartificial species X is defined in a manner that it has exactly thesame thermochemical, transport, and radiation properties asthose of added H2O but it is not allowed to participate in anychemical reaction. Therefore, X is strictly regarded as achemically inert species. Numerical calculations are conductedtwice with X and the real H2O. The difference between theresults calculated with the artificial species and the real H2O isthen wholly attributed to the chemical effects of added H2O.The flame structure of the CH4–air diffusion flame is systemati-cally changed into those of the blending fuels(methane–hydrogen–H2O) through molar additions of H2 andH2O in the fuel stream. The boundary temperature of both thefuel 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 flamesand (b) in the increase of H2O mole fraction (or X mole fraction)for CH4–H2–H2O flames. In the figures, Xi in the horizontal axisrepresents the mole fraction of chemical species i and X meansthe artificial species which is introduced to facilitate the chemicaleffects of added H2O, as was mentioned above. The zero inmethane mole fraction means pure hydrogen, and the unityrepresents pure methane in Figure 2a. The increase of H2O (orX) mole fraction implies the decrease of added hydrogen for afixed methane mole fraction in Figure 2b. In general, the reactionrate of the principal chain branching reaction H + O2 f O +OH, which is an indicator of overall reaction rate, is relativelyless than those between H atoms and hydrocarbons, and bothof the reactions compete with each other for H atoms.20

Moreover the reaction rate of the chain branching reaction ismuch smaller than those between H atoms and hydrocarbons.As a result, populous hydrocarbons consume the H atomsrigorously, and this enfeebles the chain branching reaction. Thisis the reason why the maximum flame temperature decreaseswith an increase in the methane mole fraction for CH4–H2

flames, as shown in Figure 2a. In Figure 2b, the maximum flametemperature decreases with the increase of the mole fraction ofH2O or the artificial species X for a fixed methane mole fraction.

This is so because the addition of H2O or the artificial speciesreduces the population of the reactive species. This is confirmedby the fact that flames are extinguished by excessive additionof H2O in Figure 2b.

Figure 2b also shows that maximum flame temperatures withH2O addition are higher than those with the addition of theartificial species for XH2O ) 0.1. However, the tendency isreversed as the methane mole fraction increases. These com-plicated behaviors might be, essentially, explained through theinspection of the behaviors of chain carrier radicals such as H,O, and OH and of the principal chain branching reaction, sincethe principal chain branching reaction has been known to bean indicator of overall reaction rate.

Figure 3 shows the variation of maximum mole fractions ofH, O, and OH with mole fraction of added H2O or artificialspecies for (a) XCH4 ) 0.1 and (b) XCH4 ) 0.7. Maximum molefractions of H and O with added H2O are smaller than thosewith artificial species, but maximum OH mole fractions withadded H2O are oppositely larger than those with artificialspecies. This implies that OH is remarkably produced due tothe chemical effects of added H2O and it may modify flamestructure sufficiently as much as higher flame temperatures withthe addition of H2O are obtained in comparison to those withthe addition of artificial species. Dominant H2O-related reactionsteps relevant to the production of OH radical were examinedto verify the behavior of the OH radical produced chemicaleffects of added H2O, and the following was the most

(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 kineticmechanism 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 strainrate of 157 s-1 (a) with methane mole fraction in CH4–H2 flame and(b) with H2O mole fraction in CH4–H2–H2O flame.

3218 Energy & Fuels, Vol. 21, No. 6, 2007 Park et al.

importantly contributing reaction step (these tendencies wereconsistent with those of the previous studies8,9,21).

O+H2OfOH+OH (R86)

Figure 4 displays the variations of (a) the reaction rate of theprincipal chain branching reaction and (b) that of the reaction step(R86) with the additions of H2O and artificial species. The principalchain branching reaction rates with the addition of H2O are smallerthan those with the addition of the artificial species in all cases inFigure 4a. The reaction rates of the reaction step (R86) with H2Oaddition are oppositely larger than those with the addition of theartificial species in Figure 4b, and those differences due to thechemical effects are reduced with an increase of the added methanemole fraction. Consequently, the reason why maximum flametemperatures with the addition of H2O are higher than those withthe artificial species for small quantities of methane mole fractionbut the tendency is reversed as the methane mole fraction isincreased is addressed by the following explanation. That is, thebehavior of maximum flame temperature generally follows thatof the principal chain branching reaction H + O2f O + OH. Bythe way, maximum mole fractions of H and O with added H2Oare smaller than those with artificial species, but maximum OHmole fractions with added H2O are oppositely larger than thosewith artificial species. Indeed, the chemical effects of added H2Oshould have a tendency to reduce the overall reaction rate, sincethe mole fraction of H radical decreases. However, the remarkablyproduced OH radical has a tendency to enhance the overall reactionrate and thus maximum flame temperature through the OH-related

reaction pathway. Moreover, the reaction rates between hydrocar-bons and H atoms are much larger than the principal chainbranching reaction, and thus, the increased population of hydro-carbons forces the overall reaction to be enfeebled. For smallmethane mole fractions such as XCH4 ) 0.1, the produced OHradical due to the chemical effects of added H2O modifies the flamestructure sufficiently as much as higher flame temperatures can beattained in comparison to those with artificial species. However,the enhancement of the reaction rate of R86 due to chemical effectsbecomes gradually weakened, as shown in Figure 4b, and theprincipal chain branching reaction also becomes enfeebled due tothe populous hydrocarbons according to the increase of methanemole fraction. The fact that maximum flame temperatures withthe addition of H2O become smaller than those with the artificialspecies implies that the latter effects become dominant accordingto the increase of methane mole fraction.

Figure 5 describes the variation of emission indices of COand CO2 (a) with methane mole fraction for CH4–H2 flamesand (b) with mole fraction of added H2O or artificial speciesfor CH4–H2–H2O flames. The use of mole fraction is notappropriate to grasp the behavior of CO, CO2, and NOx, sincethey include convection and diffusion terms, and the followingemission indices of CO, CO2, and NOx, suggested by Nishiokaet al.,22 are adopted:

EIi )∫0

LWiwi dx

-∫0

LWCH4

wCH4dx

where i)CO, CO2, and NO (9)

(21) Zhao, D; Yamashita, H.; Kitagawa, K.; Arai, N. Combust. Flame2002, 130, 352.

(22) Nishioka, M.; Nakagawa, S.; Ishikawa, Y.; Takeno, T. Combust.Flame 1994, 98, 127–136.

Figure 3. Behavior of chain carrier radicals according to H2O additionand 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.

Figure 4. Variation of reaction rates of the reaction steps (a) O + H2Of OH + OH and (b) H + O2f OH + O with methane mole fractionand their chemical effects at a global strain rate of 157 s-1 inCH4–H2–H2O flame.


Here, Wi is the molecular weight of chemical species i and wi

is the molar production rate of chemical species i. For CH4–H2

flames in Figure 5a, the emission index of CO increases anddecreases after showing a maximum at the methane molefraction of 0.3, while those of CO2 increase monotonously withthe increase of methane mole fraction. For CH4–H2–H2O (orX) flames in Figure 5b and c, emission indices of CO alsoincrease and then decrease with the increase of mole fractionof added H2O or artificial species for methane mole fractionsof 0.1, while those emission indices decrease for the molefraction of 0.7. Meanwhile, emission indices of CO2 increasemonotonously with the increase of H2O and its artificial species.Chemical effects of added H2O increase emission indices of

CO2 but decrease emission indices of CO for CH4–H2–H2Oflames. These complicated behaviors may be addressed onlythrough the inspection of important contribution reaction stepsto the production and destruction of CO. The importantlycontributing reaction steps to CO production are as follows:

O+C2H2fCO+CH2 (R23)

H+HCOfH2 +CO (R55)

H+HCCOfCH2(s)+CO (R79)

HCO+H2OfH+CO+H2O (R166)

HCO+MfH+CO+M (R167)

O+CH3fH+H2 +CO (R284)

Most of the produced CO2 results from the following reactionstep:

CO+OHfCO2 +H (R99)

Figure 6a displays importantly contributing reaction steps tothe production and destruction of CO for XCH4 ) 0.2 and XH2

) 0.8 as a representative case of CH4–H2 flame. As shown inFigure 6a, most of the CO is produced through reaction stepsR166 and R167 and are consumed through reaction step R99.The reaction rates of reaction steps R99, R166, and R167, whichare dominantly contributing to the production and destructionof CO, are displayed with the increase of methane mole fractionin Figure 6b. The CO production rates through both reactionsteps R166 and R167 increase with the increase of methanemole fraction. However, the CO consumption rate through

Figure 5. (a) Behavior of emission indices of CO and CO2 accordingto 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.

Figure 6. (a) Reaction contribution of important reaction steps to COat 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 R99according to methane mole fraction at a global strain rate of 157 s-1 inCH4–H2 flame.


reaction step R99 increases much more steeply with increasingmethane mole fraction. This implies that the produced CO isimmediately converted to CO2 through reaction step R99. Thisis the direct reason why the CO emission index in Figure 5aincreases and then decreases. The reaction rates of reaction stepsR99, R166, and R167, which are dominantly contributing toproduction and destruction of CO, are displayed in the increaseof 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 donot provide all of the data, the above-mentioned reaction ratesrelevant to CO production increase and then decrease with theincrease of H2O mole fraction except for reaction step R166.

The reason why the tendency is opposite to those of the otherreaction steps is relevant to the fact that the reaction rate ofreaction step R166 is directly increased by the increase of H2Omole fraction.

Figure 7a and b also shows for XCH4 ) 0.1 and XCH4 ) 0.7that in both cases the CO consumption rates through reactionstep R99 are much larger than the CO production rates thoughreaction steps R166 and R167 and the CO consumption rate,moreover, increases much more steeply in comparison to theCO production rates. These cause an increase in the CO2

emission indices according to H2O mole fraction for the casesof both XCH4 ) 0.1 and XCH4 ) 0.7 in Figure 5c. However, theCO2 emission index for XCH4 ) 0.1 increases more sensitivelywith increasing H2O mole fraction in comparison to that forXCH4 ) 0.7. This implies that CO2 is produced much more forthe blended fuels, in which hydrogen is more populous relativeto methane. As a result, the CO emission index increases steeplyand decreases rapidly for the case of XCH4 ) 0.1, while itdecreases mildly for XCH4 ) 0.7.

Meanwhile, the CO production rates through reaction stepR166 with H2O addition are larger than those with X addition,as shown in Figure 7, while those through reaction step R167with H2O addition are smaller than those with X addition. Thisis so because H2O addition increases the reaction rate of reactionstep R166, as can be understood from the reaction equation. Itis also found that the CO consumption rates with H2O additionare larger than those with X addition, and this is due to theincrease of OH radical population by chemical effects of addedH2O, as was shown in Figure 3.

These complicated behaviors according to the addition ofH2 and H2O (or X) may affect the production and destructionof NOx considerably. Figure 8 describes the variation ofemission indices of NO through the full and thermalmechanisms with methane mole fraction for CH4–H2 flameand with methane mole fraction and/or fraction of added H2O(or X) for CH4–H2–H2O (or X) flame at a strain rate of 157s-1. The emission indices of NOx by both the thermal andfull mechanisms decrease with an increase of methane molefraction for CH4–H2 flame and with an increase of H2O (orX) mole fraction. In Figure 8, the difference between thefull and thermal mechanisms is due to the Fenimore mech-anism, since the contributions by the N2O mechanism andNO2 mechanism are negligible in the present study. Thistendency is consistent with the results in methane flame ofthe previous study.22 The difference between the full andthermal mechanisms in the pure hydrogen flame (XCH4 ) 0)is due to the contribution of NH-, NNH-, and HNO-relatedreactions. Figure 8a clearly shows that Fenimore NO is largerthan thermal NO, and this tendency is consistent with theresults in diffusion flames of the previous study,22 and alsoclarifies that the full NO is mainly a direct outcome fromFenimore NO at large methane mole fractions for CH4–H2

flame. In general, thermal NO (Zeldovich NO) is closelyrelevant to flame temperature. Flame temperature decreasedwith an increase of methane mole fraction in Figure 2. As aresult, thermal NO is shown to decrease with an increase ofmethane mole fraction. For the CH4–H2–H2O flame of Figure8b, thermal NO decreases with an increase of H2O molefraction, while Fenimore NO increases and then decreasesat XCH4 ) 0.1. For large H2O mole fractions, Fenimore NOapproaches full NO, and this also implies that Fenimore NObecomes predominated as the H2O mole fraction increases.Figure 8b also shows that chemical effects of added H2Oreduce the emission index of NO mainly through Fenimore

Figure 7. (a) Reaction contribution of important reaction steps to COat 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 thereaction steps R166, R167, and R99 according to H2O mole fraction ata 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 flamewith XCH4 ) 0.7.


NO. Figure 8c shows for XCH4 ) 0.7 of CH4–H2–H2O flamethat both Fenimore NO and thermal NO decrease with anincrease of H2O mole fraction and the role of Fenimore NOin full NO is much more important in comparison to that atXCH4 ) 0.1 in Figure 8b. It is also seen that chemical effectsof added H2O also suppress NO formation. Meanwhile, thebehavior of thermal NO can be understood on the basis ofthe relevance to flame temperature, but the tendencies of

Fenimore NO in Figure 8b and c are not clearly addressed.More cautious examination may be required to understandthe NO behavior through the comparison to pure methaneflame for CH4–H2 flames and CH4–H2–H2O flames to clarifythose behaviors. The Zeldovich mechanism relevant tothermal NO production is generally as follows:

N+NOTN2 +O (R178)

N+O2TNO+O (R179)

N+OHTNO+H (R180)

Figure 9 displays the reaction contribution to the produc-tion and destruction of thermal NO (a) for pure methane flameand CH4–H2 flames and (b) for CH4–H2–H2O flames. Themain source of thermal NO is reaction step R180, and thermalNO is consumed through reaction step R178 for CH4–H2

flame and CH4–H2–H2O flames, as shown in Figure 9. Theseglobal features are very similar to those in the previous studyon CO2-added methane flames except for the point thatreaction step R178 contributes to the production of thermalNO in CH4–H2 flames with large hydrogen mole fraction suchas the condition of XCH4 ) 0.1 and XH2 ) 0.9. It is also seenin Figure 9b that chemical effects of added H2O suppressthe production of thermal NO for CH4–H2–H2O flames.

Figure 10 also displays the production and destruction ratesof the importantly contributing reaction steps to Fenimore NOfor pure methane flame, CH4–H2 flames, and CH4–H2–H2Oflames. The NO production through the Fenimore mechanism

Figure 8. Contribution of Fenimore NO and thermal NO in full NOemission index (a) according to methane mole fraction at a global strainrate of 157 s-1 in CH4–H2 flame, according to (b) H2O mole fractionat 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.

Figure 9. Comparison of the reaction contribution of reaction stepsR178, R179, and R180 to thermal NO among (a) pure methane flameand CH4–H2 flames with XCH4 ) 0.1 and XCH4 ) 0.7 and (b) that amongCH4–H2–H2O flames with various compositions.


in methane flame is mainly relevant to the following reactionsteps:18

NH+OfNO+H (R190)

NNH+OfNH+NO (R208)

HNO+HfH2 +NO (R214)

HNO+OHfNO+H2O (R215)

NCO+OfNO+CO (R222)

The major NO destruction through the Fenimore mechanism18

is contributed by

H+NO+MfHNO+M (R212)

C+NOfCN+O (R244)

C+NOfCO+N (R245)

CH+NOfHCN+O (R246)

CH+NOfH+NCO (R247)

CH+NOfN+HCO (R248)

CH2 +NOfH+HNCO (R249)

CH2 +NOfH+HCNO (R251)

CH3 +NOfHCN+H2O (R255)

HCCO+NOfHCNO+CO (R274)

For CH4–H2 flame and CH4–H2–H2O flame, the HNO-relatedreaction steps such as R214 and R215 importantly contributeto the NO formation, while the HNO-related reaction step R212consumes NO remarkably according to the increase of H2 mole

fraction. These characteristics according to the addition of H2

are much more remarkable. The importance of the NH-relatedreaction steps such as R190 in NO production is diminishedaccording to the addition of H2. It is also noted that reactionstep R222, which is one of the major sources for NO production,becomes less important in NO formation according to theaddition of H2. The importantly contributing reaction steps toNO destruction are R244, R245, R246, R249, R255, and R274.The role of reaction steps R246 and R255 as a reburningmechanism was addressed in a previous study.22 The importanceof reaction step R274, known as a HCN recycle route, leadingto the consumption of NO has been well described.22,23 As candefinitely be seen in Figure 10b, the contribution of NOconsumption is through the reburning process and the HCNrecycle route for the cases of small H2 mole fraction and throughthe HNO-related reaction R212 for the cases of large molefraction. It is also found for CH4–H2–H2O flame that the reactioncontribution of all the major reaction steps to the productionand destruction of NO is suppressed according to the additionof H2O. Chemical effects of added H2O repress both theproduction and destruction of NO, as shown in Figure 10b.

Conclusion

Numerical study was conducted to investigate the effects ofthe addition of hydrogen and steam in methane–air diffusionflame. The following conclusion can be obtained.

Maximum flame temperature increases with an increase inhydrogen mole fraction and with a decrease of H2O molefraction. Chemical effects of added H2O could modify flamestructure through the complicated behavior of principal chaincarrier radicals. Some outstanding features of these chemicaleffects are as follows: (1) maximum flame temperatures withH2O addition are higher than those with the addition of theartificial species for small H2O mole fractions such as thecondition of XH2O ) 0.1 and (2) the chemical effects inhibit theradicals of H and O but augment OH radical through the reactionO + H2O f OH + OH.

For CH4–H2 flames, the emission index of CO increasesand then decreases after showing a maximum, while that ofCO2 increases monotonically with an increase in methanemole fraction. For CH4–H2–H2O flames, the emission indexof CO increases and then decreases after showing a maxi-mum, while that of CO2 increases monotonically with anincrease in the fraction of added H2O. It is also found thatchemical effects of added H2O reduce the CO emission indexand increase the CO2 emission index. These complicatedbehaviors are mainly caused by the competition of theproduction of CO through the reaction step R166 with thedestruction of CO by the reaction step R99.

For CH4–H2 flame and CH4–H2–H2O flame, Fenimore NOis larger than thermal NO, and the full NO is mainly originatedfrom Fenimore NO at large methane mole fractions. Chemicaleffects of added H2O suppress NO formation mainly throughthe Fenimore mechanism. The main source of thermal NO isthe reaction N + OH f NO + H. Reaction step R178contributes to the consumption of NO for pure methane flamebut the production of NO for large hydrogen mole fractions ofCH4–H2 flame and CH4–H2–H2O flame. The HNO-relatedreaction steps such as R212 and R214 importantly contributeto the destruction and production of NO remarkably, respec-tively. These characteristics according to the addition of H2 are

(23) Miller, J. A.; Bowman, C. T. Prog. Energy Combust. Sci. 1989,15, 287.

Figure 10. Comparison of the reaction contribution of importantreaction steps to Fenimore NO among (a) pure methane flame andCH4–H2 flames with XCH4 ) 0.1 and XCH4 ) 0.7 and (b) that amongCH4–H2–H2O flames with various compositions.


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 theaddition of H2 and H2O. The contribution of NO consumptionis mainly through the reburning process and the HCN recyleroute for the cases of small H2 mole fraction, while it is almostthrough the HNO-related reaction R212 for the cases of largemole fraction. It is also found that chemical effects of added

H2O inhibit both thermal NO and Fenimore NO except thatchemical effects of added H2O increase themal NO only underthe conditions of XH2O ) 0.1.

Acknowledgment. Financial support of our work by the fundswith fundamental business of Korea Institute of Machine andMaterials is gratefully acknowledged.

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