Kinetic Effects of Hydrogen Addition on the Thermal Characteristics of

Article pubs.acs.org/EF

Kinetic Effects of Hydrogen Addition on the Thermal Characteristics of Methane−Air Premixed Flames Qingfang Li,†,‡ Ge Hu,*,‡ Shiyong Liao,*,†,§ Qian Cheng,§ Chi Zhang,† and Chun Yuan§ †

State Key Laboratory of Coal Mine Disaster Dynamics and Control and ‡School of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400030, People’s Republic of China § Department of Power Engineering, Chongqing Communication Institute, Chongqing 400035, People’s Republic of China ABSTRACT: A numerical investigation was implemented to evaluate the kinetic effects of hydrogen addition on the thermal characteristics of lean and stoichiometric premixed methane−air flames, on the basis of the detailed kinetic reaction mechanism GRI-Mech 3.0. The flame temperature profiles and the distributions of reactive species were predicted as a function of flame height by solving a freely propagating laminar premixed flame model. Some global properties of the premixed flame, i.e., the flame temperature gradient, the inner layer flame temperature, and the heat release rate, were estimated. Results showed that hydrogen enrichment in methane−air flames led to increases in the peak flame temperature, the peak temperature gradient, and the peak heat release rate, but a decrease in the inner layer flame temperature. Analyses of the interactions among heat release rates, radical production, and reaction progress rates were conducted. Analysis of the contribution of the heat release rate showed that the reactions concerning CH3 consumption consistently released a large proportion of heat, while significant contributions were found from the reactions of OH + H2 ⇔ H + H2O, H + O2 + H2O ⇔ HO2 + H2O, and H + HO2 ⇔ 2OH, when hydrogen was added. The rates of radical production were computed on the basis of the predicted species profiles. It was suggested that OH and H were the two important radicals, which had significant relevance to the heat release in the hydrogen-enriched flame. The analyses of the formation pathway were conducted for OH and H. It was shown that radical H was mainly produced in the reactions OH + H2 ⇔ H + H2O and O + H2 ⇔ H + OH, and the dominant reactions contributing to the production of radical OH were O + H2 ⇔ H + OH, O + CH4 ⇔ OH + CH3, and H + HO2 ⇔ 2OH. The progress rates of these preceding reactions were compared. Numerical results indicated that hydrogen had its kinetic effects on methane−air flames through promoting the formations and consumptions of OH and H, and the dominant reactions in terms of the contribution to heat release were OH + H2 ⇔ H + H2O, H + O2 + H2O ⇔ HO2 + H2O, and H + HO2 ⇔ 2OH. lengths and flammability range of stretched methane−air premixed flames. Results showed that the effective Markstein lengths of blended flames are a nonmonotonic function of blending ratio due to the competition effect between the Zeldovich and Lewis number. Ranzi et al.9 found that premixed methane−air flames with hydrogen addition presented the increased flame speed due to the enhanced thermal, kinetic, and transport effects of hydrogen. Sarli and Benedetto10 numerically studied the laminar burning velocities of premixed hydrogen− methane−air flames with GRI kinetic mechanism. They found that the laminar burning velocities of blended fuels are always smaller than those obtained by averaging method based on the molar proportions of gaseous fuels. Coppens et al.11 measured the adiabatic burning velocities of methane−hydrogen−air flames with the heat flux method. Moccia and D’Alessio12 studied the effects of the initial pressure and the hydrogen content on the laminar burning velocity, the Markstein length, and the flame instability by means of the measurements of spherical expanding flames. Hu et al.13,14 made systematic measurements and computations and explored the variations in laminar burning velocities for methane−air flames with hydrogen addition under different pressures. They reported

1. INTRODUCTION Gaseous methane blended with hydrogen is considered to be one of the most promising gaseous fuels because both methane and hydrogen can be uniformly mixed with air for complete combustion with lower pollutant emissions, and hydrogen has a relatively fast flame propagating speed and a wide flammability range. Hydrogen added into methane can effectively improve the combustion efficiency and reduce emissions by accelerating the flame propagation rate and expanding the flame flammability range.1−4 Ma et al.5,6 made engine tests and reported that hydrogen addition into methane was an effective method for reducing exhaust emissions and fuel consumption. Fundamental combustion research is a key way to realize combustion with high efficiency and low pollution. Up to now, numerous numerical and experimental studies on the combustion characteristics of the mixtures of methane, hydrogen, and air have been made. The corresponding chemical kinetic mechanism has therefore received much attention in the combustion research field over the past decades. Hawkes and Chen7 numerically studied the effect of hydrogen addition on the stability and pollutant formation of lean premixed methane−air flames based on a reduced chemical mechanism. They reported that hydrogen-enriched flame was less diffusive-thermally stable and more resistant to quenching than the pure methane−air flame. Sankaran and Im8 studied the effect of hydrogen addition on the Markstein © 2014 American Chemical Society

Received: July 10, 2013 Revised: April 27, 2014 Published: May 12, 2014 4118

dx.doi.org/10.1021/ef500263v | Energy Fuels 2014, 28, 4118−4129

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Figure 1. Mechanism checking curves: (a) temperature profiles; (b) mole fraction profiles of species CH4, H2, H, and OH.

be considered as the accumulation of an exothermic or endothermic amount for all elementary reactions. That is to say, it is convenient to make heat contribution analyses to identify dominant reactions. Lafay et al.19 made a heat release analysis to study the effects of hydrogen on the flame thickness of the methane−air mixture. The estimated contribution ratios of reactions were used to quantitatively interpret the decrease in the flame thickness when methane was enriched by hydrogen. Hu et al.20 computed the heat release rates of methane−hydrogen−air flames stabilized on a McKenna burner on the basis of the measurements of flame temperature profiles. The computation of heat release rate was performed for every reaction to obtain their contributions to the global heat released by the flame. It was confirmed that the reaction OH + H2 ⇔H+ H2O was one of the most important elementary reactions contributing to the early heat release. In this work, based on the correlation analyses on the heat release rate against reaction progress and radical formation, we propose a new view of exploring the effects of hydrogen addition on the oxidation process of premixed methane−air flames. The flame structure computations were carried out for one-dimensional freely propagating premixed laminar methane−hydrogen−air flames with PREMIX code. The variations in the global thermal parameters of flames, such as the flame temperature gradient, the inner layer flame temperature, and heat release rate, were estimated with the variation of hydrogen enrichment. Reaction analyses in terms of the contributions to the heat release and reactive species formation were conducted to identify the dominant radicals and the governing reactions in the oxidation of hydrogen-enriched methane−air flames.

that the laminar burning velocities increased with the increase of initial temperature and the hydrogen fraction and decreased with the increase of the initial pressure. Generally, most of the above studies were performed from a macroview to measure or compute the global parameters of hydrogen and methane mixtures in air, such as the laminar burning velocities, the Markstein lengths, the flammability limits, and flame stability. Recently, as one of the key approaches to develop flame chemistry, flame structure analysis is becoming a study hotspot. Wang et al.15 made the computational study and explored the kinetic effects of hydrogen addition on the oxidation process of premixed methane−air flames. They predicted the distributions of some reactive radicals with PREMIX code and reported that the reaction pathways of methane oxidation would move toward the lower carbon reaction routes when hydrogen was available. They believed that the promotion of chemical reaction with hydrogen addition is caused by the increases of H, O, and OH mole fractions in flames. Bougrine et al.16 evaluated the laminar burning velocities and flame thicknesses of methane−hydrogen−air premixed flame under high pressure and high temperature with detailed flame chemistry. They found that the effect of hydrogen addition was achieved through the formation of H. Park and Oh17 made a numerical simulation on partially premixed methane−hydrogen−air flames established in a one-dimensional counterflow field, in which the flame structure, the heat release rate, and flame speed were investigated against the mixture equivalence ratio and flame strain rate. Recently, de Ferrières et al.18 implemented measurements and predictions of the chemical structures of premixed natural gas−hydrogen flames. Following the analyses of reaction paths, they suggested that hydrogen enhanced Habstraction reactions by H atoms under lean conditions. Up to now, the measurements and predictions of the methane− hydrogen−air flame structure were extensively studied to analyze the relationship among reactive species distribution, reaction rate, and laminar burning velocity and identify which radicals or reactions are the most important factors in governing flame propagation. The heat release rate is one of the global flame properties that can be used to characterize flame chemistry, as well as laminar burning velocity. Compared to the analysis of laminar burning velocity, the analysis of heat release rate shows remarkable advantage because the heat released by flames can

2. NUMERICAL SIMULATION METHODS The numerical simulation was performed by means of solving the governing conservation equations for one-dimensional freely propagating premixed flame with the PREMIX code of CHEMKIN II. Onedimensional freely propagating premixed laminar flame is often used in modeling the flame of a gas mixture under the specified conditions. This flame configuration is idealized and fully developed without heat losses, and the propagation of flame is generally governed by the properties of fuel−oxidizer mixtures.21 Chemical kinetic mechanism is of great importance in such flame simulations. Figure 1 shows some typical predicted curves for methane−air flame enriched by 30% hydrogen obtained with GRI-Mech 3.0.22 The predictions with kinetic schemes of Lutz et al.23 and Lindstedt et al.24 were given for comparison as well. It was clear that the bulk predictions were consistent, including the temperature profiles and the methane and 4119


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Figure 2. Effects of the determination of transport properties on flame simulation.

Table 1. Properties of the Flames Studied in the Present Work no.

ϕ

P0 (MPa)

T0 (K)

CH4 (%)

H2 (%)

Ṁ (g·cm−2·s−1)

qmax (J·cm−3·s−1)

Tmax (K)

(dT/dz)max (K·mm−1)

1 2 3 4 5 6 7 8 9 10 11 12

0.8 0.8 0.8 0.8 0.8 0.8 1.0 1.0 1.0 1.0 1.0 1.0

0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1

298 298 298 298 298 298 298 298 298 298 298 298

100 90 80 70 50 30 100 90 80 70 50 30

0 10 20 30 50 70 0 10 20 30 50 70

0.0307 0.0326 0.0347 0.0374 0.0457 0.0611 0.0428 0.0453 0.0484 0.0522 0.0638 0.0871

2325 2518 2734 2998 3810 5245 4055 4404 4783 5274 6739 9496

1996 2000 2006 2012 2029 2053 2219 2223 2235 2240 2256 2280

3169 3301 3471 3658 4175 4892 4341 4538 4747 4999 5662 6523

hydrogen distributions, while for the intermediate species, OH and H, slight scatters were observed. Since the accuracy of the GRI-Mech 3.0 scheme had been validated by numerous experimental measurements of hydrocarbon and hydrogen,10,15−17,19,20,25,26 this detailed kinetic reaction, which consists of 325 reactions and 53 species, was finally adopted in this work. The hybrid time-integration Newton-iteration technique was used in PREMIX code to solve the steady-state mass, species, and energy conservation equations for a freely propagating flame.21 In the freely propagating flame modeling, the flame temperature profile is computed from the energy equation, and the mass flow rate Ṁ is an eigenvalue and should be determined as part of the solution. Ṁ = ρuA, where ρ denotes the mass density; u the velocity of the fluid mixture; and A the cross-sectional area of the tube encompassing the flame, by default, which is taken to be constant and equal to unity. The adaptive mesh technique is adopted in the flame simulation. The initial grid points are defined as 10, and a maximum of 500 grids are allowed. The gradient and the curvature are both set to be 0.1 to control the adaptive grid. Transport properties play a key role in the simulations of laminar premixed flame. It is well-known that hydrogen is a special gaseous fuel with significant diffusion ability. Figure 2 shows the effects of the determination of transport properties of mixtures on the predicted results. The mixture-averaged approach and multicomponent formulation were used to estimate the differential diffusion of mixtures, and the Soret effect was taken into account when multicomponent formulation was adopted. It was clear that the determination of the transport properties of a mixture primarily affected the early hydrogen consumption, while no obvious variation was observed in the profiles of methane, OH, and H. Multicomponent formulation shows significant accuracy advantage in flame modeling, but requires too long computation time. In the present work, the mixture diffusion was

computed with mixture-averaged transport to minimize the computational expense. The present simulation commenced at 298 K and 0.1 MPa and covered the range of hydrogen enrichment rates α from 0.0 to 0.7 with flame equivalence ratio ϕ at 0.8 and 1.0. The detailed information on the tested flame can be found in Table 1. The hydrogen enrichment rate α is defined by the percentage of hydrogen in fuels as

α=

nH2 nH2 + nCH4

(1)

where nH2and nCH4 are the mole amounts of hydrogen and methane in fuel. And the mixture equivalence ratio is

ϕ=

ṁ fuel /ṁ air FAR stoic

(2)

where ṁ fuel and ṁ air are the mass flow rates of fuel and air in the initial unburned mixture and FARstoic is the mass ratio of fuel to air for a stoichiometric mixture and expressed as 16(1 − α) + 2α (2 − 1.5α)(32 + 3.76 × 28) 16 − 14α = 274.56 − 205.92α

FAR stoic =

(3)

The flame characteristic properties, e.g., the temperature, composition, and aerodynamic profiles, can be obtained from the above solution. With the average axial gas velocity υ, average gas density ρ, average molecular weight W̅ , and species molecular fraction f i as a function of the distance z through the flame zone, and an approximation of molecular diffusion velocity Vi, the molecular flux of species i is 4120


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determined as (ρ(v + Vi)/W̅ )f i.27 The species production rate ω̇ i is hence written as

∂ ⎛ ρ(v + Vi ) ⎞ fi ⎟ ⎜ ⎠ ∂z ⎝ W̅

ωi̇ =

(4)

The production rate of species i, ω̇ i, can also be considered as the summation of the production or consumption rates of all reactions involving species i. Thereby, the contribution ratio of reaction k to the production of species i, σik, can be given as ωik̇ ωi̇

σik =

(5)

where ω̇ ik denotes the production or consumption rate of species i in reaction k, which can be estimated from the rate of progress of reaction k.21 Based on the energy conservation equation of flat laminar flame, the global heat release rate of flame, q, is hence given as

Figure 3. Temperature profiles of methane−air flames with or without hydrogen addition.

K

q=

∑

Hi0ωi̇ Wi

i=1

(6)

Figure 4 shows the temperature gradient profiles of methane−air flames with or without hydrogen addition. It is

H0i

is the formation enthalpy of species i, Wi is the molecular where weight, and K the species number in the kinetic scheme. Meanwhile, the heat release rate of reaction k, qk, can also be determined as Kk

qk =

∑ Hi0ωik̇ Wi i=1

(7)

where Kk is the species number in reaction k. Integration of eqs 6 and 7 through the flame zone yields

Q=

∫flame q dz

Qk =

∫flame qk dz

(8) (9)

where Q denotes the accumulated heat released by flame and Qk the heat released by reaction k, respectively. Hence, the heat contribution of reaction k to the global heat release, ηk, can be estimated as Qk

(10)

Figure 4. Profiles of temperature gradients for methane−air flames with hydrogen addition.

3. RESULTS AND DISCUSSION 3.1. Flame Temperature and Temperature Gradient. Flame temperature is one of the most important parameters to characterize the flame combustion process. Figure 3 shows typical temperature profiles of six laminar premixed methane− air flames with different equivalence ratios and different hydrogen fractions. The peak flame temperature is increased by about 200 K when the equivalence ratio varied from 0.8 to 1.0 for the mixtures with the same hydrogen enrichment. Hydrogen addition only leads to a slight rise in the peak flame temperature. As depicted in Figure 3, with the same equivalence ratio of 1.0, compared to the pure methane−air flame, the flame enriched by 30% hydrogen only gives an increase of about 10 K in the peak temperature. Besides, the increase is also insignificant for the flames with different hydrogen enrichment rates. For the flames with the higher hydrogen addition proportion (70%), the peak temperature is only about 20 K greater than that of flame with 30% hydrogen addition proportion. With the same equivalence ratio, all the flames with or without hydrogen addition give the nearly overlapped temperature curves in the early stage, as illustrated in Figure 3.

obvious that all of the profiles showed parabola-like curves. The less the added hydrogen is, the flatter the parabolic curve is. The temperature gradient is closely related to the heat release rate of flame. It is well-known that the faster burning speed can lead to the more rapid heat release rate and consequently result in the more rapid increase in the flame temperature, i.e., the greater temperature gradient. That is to say, Figure 4 also indicates that hydrogen addition has promoted the combustion chemistry of flame. Figure 5 summarized the variations in the peak temperature gradient with the hydrogen-enrichment rate. The results indicate that the peak temperature gradient increases with the increase in hydrogen addition. The premixed flame with the equivalence ratio of 0.8 and the hydrogen enrichment rate of 70% presents the peak temperature gradient of nearly 4800 K·mm−1, while the stoichiometric flame (ϕ = 1.0) with the same hydrogen enrichment shows the higher value of about 6500 K·mm−1. Both of the two values are about 1.5 times greater than those of the pure methane−air flames with the same equivalence ratio. In Table 1, for 12 flames involved in the numerical investigation, the maximum temperature and the maximum temperature gradient are provided, including basic information on the predicted flames, such as the

ηk =

Q

× 100%

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According to the concentration distributions of radicals CH3 and O (Figure 6e,f), CH3, which is directly produced from the cracking reactions of methane, is one of the most important intermediate species. Thereby, the appearance of CH3 in flame can almost be considered as the indication of the initiation of methane oxidation. As shown in Figure 6e, compared to the pure methane−air flame, the enriched flame presents an earlier appearance and earlier disappearance of CH3, and the flame location at which CH3 is initially formed is also the position at which the methane concentration decreases. The concentration distribution curves of radical O are very similar to those of radical OH and H. 3.3. Heat Release Rate. Figure 7 shows six typical curves of global heat release rate for the flames discussed previously. Obviously, equivalence ratio is one of the determinant parameters of flame heat release rate. The stoichiometric methane−air flame shows a peak heat release rate of about 4055 J·cm−3·s−1, which is almost 1700 J·cm−3·s−1 higher than that of methane−air flame with the equivalence ratio of 0.8. The temperature at which the peak heat release rate is attained also increases with the increase of the equivalence ratio because the stoichiometric flame has a faster burning velocity and releases more combustion heat. The comparison of the heat release rates among the flames with different hydrogenenrichment ratios is also presented in Figure 7. It is indicated that the larger hydrogen addition can result in the higher peak heat release rate. The peak heat release rate of the flame enriched by 70% hydrogen is more than two times larger than that of the pure methane−air flame under the same equivalence ratio. The peak heat release rates are summarized in Table 1. However, Hu et al.20 reported that hydrogen enrichment into premixed methane−air flame resulted in a slight increase in the peak rate of heat release. The difference is attributed to the measured flames that were stabilized in a burner with a fixed volumetric flow, in which the variation of combustion heat was not as significant as that in the freely propagating flames. 3.4. Contribution Ratios of the Reactions to Global Heat Release of Flames. The global heat released by flames is the total heat of all endothermic and exothermic reactions involved in kinetic schemes. On the basis of the predicted species distributions, the heat release rate and heat production can been estimated for each reaction with eqs 7 and 9. The heat contribution ratio consequently can be obtained with eq 10. Table 2 shows the reactions with relatively large contribution ratios, and the contribution ratios are indicated by percentages. For premixed methane−hydrogen−air flames, the reactions with the major contribution ratios to the global heat release include the following:

Figure 5. Maximum temperature gradient and the inner layer flame temperature.

initial temperature, pressure, mass flow rate, equivalence ratio, and hydrogen-enrichment rate. The variation in the inner layer flame temperature T0, which was defined as the temperature corresponding to the peak temperature gradient, was also explored, as shown in Figure 5. It is well-known that the flame temperature at which the peak temperature gradient is achieved can be considered as the average temperature of the reaction zone, to some extent. The curves of the inner layer temperature show a declining tendency with the increase of hydrogen enrichment rate, indicating that hydrogen addition can promote the combustion to a lower temperature zone. 3.2. Reactive Species Distribution. The fuel and species concentration distributions are the key properties of flame in the analysis of reaction routines. Herein fuel consumptions, such as the concentrations of methane and hydrogen, had been predicted and presented against flame distance in Figure 6a,b. The comparison results of the distribution profiles indicate that hydrogen-enriched flames give an early consumption of methane. After hydrogen addition, methane consumption can be advanced to about 0.02 cm in flame height. The more the added hydrogen is, the shorter the flame distance at which methane is consumed. Figure 6a also indicates that the flame distance at which methane is consumed decreases with the increase of hydrogen addition rate. Hu et al.20 reported that both the flame thickness and the thickness of the reaction zone decrease with the increase of hydrogen addition. All of these characterizations are attributed to the fact that hydrogen has faster burning velocity than methane. Figure 6b shows the hydrogen consumption curves. The flames with 30% hydrogen enrichment give a steady hydrogen concentration profile. For the flame with the larger hydrogen fraction (70%), hydrogen consumption shows no difference with methane in the pure methane−air flames because hydrogen addition becomes the primary reactant in the mixture. It was suggested that the radicals H, OH, and O are the most reactive species in the hydrocarbon oxidation system.9,27,28 The analyses on such radicals are of practical importance in developing flame reaction kinetics. Parts c and d of Figure 6 show the concentration profiles of H and OH radicals. It was clear that the flame with the larger hydrogen enrichment (70%) gives the relatively larger production of OH and H. 4122

O + CH3 ⇔ H + CH 2O

(R10)

H + O2 + H 2O ⇔ HO2 + H 2O

(R35)

H + HO2 ⇔ 2OH

(R46)

OH + H 2 ⇔ H + H 2O

(R84)

OH + CH4 ⇔ CH3 + H 2O

(R98)

OH + CO ⇔ H + CO2

(R99)

OH + CH 2O ⇔ HCO + H 2O

(R101)

HCO + O2 ⇔ HO2 + CO

(R168)

O + CH3 ⇒ H + H 2 + CO

(R284)


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Figure 6. Mole fraction profiles of fuel and species.

decreased with the increase of the hydrogen-enrichment rate. The change is consistent with the prediction reported by Hu et al.20 and slightly different from that of Lafay et al.19 Lafay et al. found that, for the methane−air flame with the equivalence ratio of 0.7, the two reactions showed a nearly constant contribution to the overall heat release rate when the hydrogenenrichment rate varied from 0 to 20%. The constant contribution could be attributed to the smaller concentration of methane in the lean mixture (ϕ = 0.7), which would lead to less CH3 cracked from methane. One can conclude that R10

The preceding reactions were selected from the 325 reaction GRI-Mech 3.0 scheme adopted in this work (see section 2), and the reaction numbers correspond to those from that scheme. Figure 8 shows their contribution ratios under different flame conditions. More than 25% of the heat release is from reactions R10 and R284 in the pure methane−air flame, indicating that the consumption reaction of CH3 plays a dominant role in the combustion process, while, for hydrogen-enriched flames, the total contribution ratio of R10 and R284 is significantly 4123


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Figure 7. Global heat release rates of methane−air flames with different hydrogen enrichments.

Figure 8. Contributions of reactions to the overall heat release of flames.

Table 2. Contributions of Reactions to the Global Heat Release of Flames flame: 100% CH4 + 0% H2 reaction R10 (15.2%) R284 (9.9%) R168 (8.4%) R101 (7.5%) R98 (6.1%) R99 (6.1%) R84 (6.1%) R46 (4.3%) R35 (4.1%) R10 (15.3%) R284 (10.1%) R84 (8.2%) R99 (6.9%) R98 (5.5%) R168 (4.4%) R101 (3.9%) R46 (3.3%) R35 (2.7%)

flame: 70% CH4 + 30% H2 ϕ = 0.8 reaction R10 (15.2%) R284 (10.0%) R84 (8.9%) R99 (6.5%) R168 (5.6%) R98 (5.4%) R46 (4.9%) R35 (4.7%) R101 (4.4%) ϕ = 1.0 R10 (15.0%) R84 (10.5%) R284 (9.9%) R99 (5.7%) R98 (4.9%) R168 (4.2%) R46 (3.8%) R101 (3.3%) R35 (3.2%)

different with those of the reactions R35, R46, and R84. One can find that R101, R98, and R99 are related to the consumption of CH2O, CH4, and CO. CH2O and CO are the intermediate species of methane pyrolysis. Both of them could be influenced by the initial concentration of methane in the mixture. The contributions of R101, R98, and R99 show gradual decreases with the increase of hydrogen addition, because the concentrations of CH2O and CO show gradual decrease when hydrogen is added. 3.5. Production Rate of the Key Radical. Reactions of R35, R46, and R84 were found to present increased heat contribution ratios with the increase of hydrogen addition, indicating that H and OH are the two most important radicals that could promote flames chemistry. Hu et al.20 conducted the reaction heat contribution analyses for burner-stabilized methane−hydrogen−air flames and pointed out that R84 was one of the most governing reactions in the early stage of flame oxidation. They inferred that OH might be the key radical in flame chemistry because reaction R84 was directly related to radical OH consumption. Regarding the complexity of flame chemistry, the preceding inference might be somewhat assertive. Thereby, the production rates of species OH, H, O, and CH3 were systematically analyzed. In Figure 9a−d, a positive number denotes a production rate, while a negative one represents the consumption. The formations of OH, O, and H first show almost concurrent increases and then achieve their peak values at the approximate temperature, while the variation trend of CH3 is like a sine curve, which shows a positive peak first and then a negative extreme value. It is clear that the first production peak of CH3 appears earlier than those of radicals OH, H, and O, confirming that the first step of methane oxidation is to crack methane into CH3.29 Besides, the production promotion roles of OH, H, and O are also found in Figure 9. The flames with the larger enrichments present the greater production rates of OH, H, and O radicals, compared to those of pure methane−air flames and the flames with the smaller enrichments. Moreover, it is deserved to be emphasized that the temperatures at which the peak values of OH, H, and O production appear gradually drop with the increase of hydrogen enrichment in the mixture. The change trend is consistent with the above conclusion of the global heat release rate plotted in Figure 7.

flame: 30% CH4 + 70% H2 reaction R84 (15.0%) R10 (11.7%) R35 (7.9%) R284 (7.7%) R46 (7.1%) R168 (4.0%) R99 (3.9%) R98 (3.7%) R101 (2.4%) R84 (16.7%) R10 (12.3%) R284 (8.1%) R46 (5.8%) R35 (5.6%) R99 (3.5%) R98 (3.4%) R168 (3.1%) R101 (1.8%)

and R284 are primarily affected by the concentration of methane in the mixture, and the effect of hydrogen addition is very small. Reactions R35, R46, and R84 can be considered as the secondary contributing reactions to the overall heat release of flame. Reactions R35, R46, and R84 provide about 15% contribution to pure methane−air flames. However, their contribution ratios increase with increase of the hydrogen addition. When 70% hydrogen is added, the contribution ratios of all the three reactions are above 30%. These reactions are related to the consumption of H and OH, indicating that hydrogen addition can promote the formation of such radicals and enhance the heat release. Other reactions, such as R101, R98, and R99, also make substantial contributions to the overall heat release. However, the ratios of the heat release contribution decrease with the increase of hydrogen addition. The change is significantly 4124


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Figure 9. Production rates of some typical radicals for premixed flames with hydrogen addition.

Figure 10. Maximum production rates of species versus hydrogen-enrichment rates.

Figure 11 shows the correlation analyses between the peak mole fraction of some reactive species and the maximum heat release rate. It is confirmed that OH and H have the dominant influences on the global heat release of flame because their peak mole fractions show apparent relevancies with the maximum heat release rate. The consumption of CH3 releases a large quantity of heat during flame combustion, but weak correlations have been found in the curve of the peak mole fraction versus the maximum heat release. For the radical of O, a relatively strong correlation is achieved in the lean flame,

Figure 10 shows the relevancies of the maximum production rates of CH3, O, OH, and H to hydrogen-enrichment rate. For lean flames, the radicals OH, O, and H show significant increases in their peak production rates with the increase of hydrogen addition. For stoichiometric flames, it is observed that the increase in the maximum production rate of O is not so significant as those of OH and H, indicating that there might be some kinetic changes in the reaction pathway. The results also indicate that hydrogen addition has little effect on the production rate of CH3 for lean or stoichiometric flames. 4125


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Figure 11. Correlations of peak mole fraction of species with the maximum heat release rate.

Figure 12. Contributions of reactions to the productions of OH and H.

Figure 13. Variations in contributions of reactions to the productions of OH and H.

while the correlation becomes weak in the stoichiometric flame, indicating that the production of radical O is primarily dominated by the concentration of oxygen in the mixture. After the reaction progress analyses, Lafay et al.19 also found that O and OH are the most sensitive radicals of the global heat release rate of flame. 3.6. Contributing Reactions for the Productions of the Key Radicals. The above analyses on heat contribution

identified that hydrogen enhances the heat release through promoting the consumptions of OH and O. It is then necessary to confirm the reactions that play crucial roles in the formations of these radicals. Figure 12 shows the comparisons of the reaction contributions to the productions of OH and H of premixed flames with or without hydrogen. In Figure 12, it is clear that 4126


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Figure 14. Variations in the progress rates of some contributing reactions.

O + H 2 ⇔ H + OH

the contribution of diffusion and thermal effect, further studies are still required.

(R3)

O + CH4 ⇔ OH + CH3

(R11)

H + O2 ⇔ O + OH

(R38)

4. CONCLUSION The numerical study on freely propagating premixed methane− hydrogen−air flames was conducted at atmospheric conditions for premixed methane−air with the equivalence ratios of 0.8 and 1.0. The flame temperature profiles and the species distributions had been predicted to investigate the effects of hydrogen addition on the flame thermal characteristics. Analyses of the interactions among heat release, radical formation, and the progress rate of reaction were carried out, and the dominant radicals and governing reactions had been explored for hydrogen-enriched methane−air flames. (1) The peak flame temperature, the peak temperature gradient, and heat release rate increase with the increase of hydrogen fraction, while the inner layer temperatures decreases with the increase of hydrogen addition. (2) The consumption reaction of CH3 plays a dominant role in the global heat release. The heat contribution analyses indicate that more than 25% of heat is released by O + CH3 ⇔ H + CH2O and O + CH3 ⇒ H + H2 + CO for pure methane− air flames, while for hydrogen-enriched flames, their contribution ratios become smaller. (3) The heat contributions of reactions H + O2 + H2O ⇔ HO2 + H2O, H + HO2 ⇔ 2OH, and OH + H2 ⇔ H + H2O are found to increase with the increase of hydrogen addition, indicating that hydrogen addition can promote the formation of such radicals, and thus enhance heat release. Correlation analyses between the peak mole fraction of species and the maximum heat release rate confirm the conclusion. (4) The analyses of reaction contribution to the productions of OH and H were conducted. Reactions of O + H2 ⇔ H + OH, H + HO2 ⇔ 2OH, and OH + H2 ⇔ H + H2O play dominant roles in the generation of OH and H. The reaction of OH + H2 ⇔ H + H2O has a dual role in the contributions of heat release and H production.

and R46 are the main contributing reactions to the production of OH. Especially, R11 is the dominant reaction and provides about 60% contribution ratio, no matter whether hydrogen is added. For the larger hydrogen enrichment flame, such as 70% enrichment rate, the most contributing reactions to the production of OH are found to be R3 and R46. In contrast, there are more reactions involved in the production of H radical, for example, R84, R3, and R99 play the dominant roles. Reaction R84 plays an important role in the flame chemistry and provides more than 30% (methane−air flame) or even 50% (when 70% hydrogen added) contribution to the production of H, as well as a major heat contribution. When hydrogen is added, R3 presents an increasing contribution because hydrogen becomes the reactant. Figure 13 shows the effect of hydrogen addition on the variations in the contribution ratios of reactions for OH and H productions. The contribution percentages of pure methane− air flame were used as the baseline of 100% for the estimation, It is clear that R3 shows the significant contribution increments in both OH and H productions when hydrogen is added. Generally, the contribution ratio is only a relative value, which cannot fully characterize the combustion enhancement by hydrogen addition. Figure 14 shows some typical curves of the progress rates of reactions. It is indicated that, for pure methane−air flame, the progress rates of R84, R11, R46, and R3 have the magnitude of 10−3, while for the enriched flame, such rates show the magnitude of 10−2. The peak rate of R84 is enlarged by about 5 times, but R11 only shows about 2 times the increment, which suggests that most of the reactions have been promoted by hydrogen addition, but only several reactions present significant changes in reaction rates. It is known that hydrogen is distinctly different from other hydrocarbons in its higher chemical reactivity and greater transport ability. The combustion promotion should be ascribed to the combined effects of all properties of hydrogen, when hydrogen is added into methane−air mixtures. This work is based on elementary reaction analyses, but strictly speaking, it still does not separate the kinetic role of hydrogen from the effects of diffusion and thermal properties. In order to identify

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AUTHOR INFORMATION

Corresponding Authors

*(S.L.) Tel.: +86(023)86798126. E-mail: [email protected]. *(G.H.) Tel.: +86(023)65106756. E-mail: [email protected]. Notes

The authors declare no competing financial interest. 4127


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ACKNOWLEDGMENTS This study was supported by the Natural Science Foundation Project of Chongqing CSTC (Grant No. cstc2012jjjq90002), the Open Fund of State Key Laboratory of Coal Mine Disaster Dynamics and Control (Grant No. 2011DA105287KF201304), and the National Natural Science Foundation of China (Grant No 50706058 and 51076167).

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NOMENCLATURE A = cross-sectional area of the burner encompassing the flame, cm2 FARstoic = mass ratio of fuel to air for a stoichiometric mixture f i = mole fraction of species i, % H0i = formation enthalpy of species i, J·g−1 K = species number in kinetic scheme Kk = species number in reaction k; see eq 7 Ṁ = mass flow rate, Ṁ = ṁ fuel + ṁ air, g·cm−2·s−1 ṁ fuel, ṁ air = mass flow rates of fuel and air in the initial unburned mixture respectively, g·cm−2·s−1 n = mole amount; see eq 1 P = pressure, MPa Q = accumulated heat released by flame, J·cm−2·s−1 Qk = heat release of reaction k, J·cm−2·s−1 q = global heat release rate of flame, J·cm−3·s−1 qk = heat release rate of reaction k, J·cm−3·s−1 T = temperature, K T0 = inner layer temperature of flame, K u = velocity of mixture, cm·s−1 Vi = molecular diffusion velocity, cm·s−1 W̅ = average molecular weight, g·mol−1 Wi = molecular weight of species i, g·mol−1 z = distance coordinate through the flame height, cm

Greek Symbols

α = Hydrogen enrichment rate ηk = heat contribution of reaction k to the global heat release of flame, % ρ = Mass density of mixture, g·cm−3 σik = contribution ratio of reaction k to the production of species i, % υ = Axial gas velocity, cm·s−1 ϕ = Equivalence ratio of mixture ω̇ i = production rate of species i, mol·cm−3·s−1 ω̇ ik = production rate of reaction k to species i, mol·cm−3·s−1

Superscripts and Subscripts

CH4 = methane i = species i H2 = hydrogen k = reaction k max = maximum value 0 = reference condition

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Kinetic Effects of Hydrogen Addition on the Thermal Characteristics of

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