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Numerical study of the chemical, thermal and diffusion effects of H2 and CO addition on the laminar flame speeds of methane–air mixture
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Numerical study of the chemical, thermal and diffusion effects of H2 and CO addition on the laminar flame speeds of methaneeair mixture Jie Liu a,b,*, Xin Zhang a,b, Tao Wang a,b, Xiaosen Hou a,b, Jibao Zhang a,b, Shizhuo Zheng a,b a b
School of Mechanical, Electronic and Control Engineering, Beijing Jiaotong University, Beijing 100044, PR China Beijing Key Laboratory of New Energy Vehicle Powertrain Technology, PR China
article info
abstract
Article history:
In this study, the chemical, thermal and diffusion effects of H2 and CO addition on the
Received 9 March 2015
characteristics of methane laminar flame are examined numerically by using the CHEMKIN
Received in revised form
2.0 code with a modified GRI-Mech 3.0 mechanism. The results reveal that a better
13 April 2015
agreement between the measured and predicted laminar flame speed is obtained by using
Accepted 25 April 2015
the modified mechanism. The effect of H2 addition to flame speed is mainly due to
Available online 16 May 2015
chemical effect at lean and stoichiometric conditions. However, with the addition of CO, around 75% of the increase in laminar flame speed is due to thermal effect. Soret diffusion
Keywords:
of H have an obviously effect on laminar flame speed, lowing it around the peak segment.
Syngas
Furthermore, the combined Soret effect of H and H2 basically accounts for the total Soret
Hydrogen
diffusion effect. With the addition of H2 and CO, the lean flammability limit is extended to a
Laminar flame speed
leaner mixture at atmospheric pressure. A linear correlation between the laminar flame speed and the relative amount of H2 was found in tertiary CH4/CO/H2 mixtures. However, this linear correlation is undermined when CO mole fraction is over than 45%. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction With increasing demand on energy and concerning on air pollution, research on clean alternative fuel and renewable energies has attracted more and more attentions [1e3]. Synthesis gas offers a promising opportunity for sustainable development in the energy and transportation sectors. Harnessing energy from syngas is not only proving to be economical, but is also environmentally benign [4]. Synthesis
gas, produced from coal, biomass, waste, landfill and alcohol fuels via gasification, pyrolysis or fermentation processes, has been widely used in power plant or other combustion engines [5]. The most serious obstacles in developing combustion engines using synthesis gas result from a large variety of compositions from different feedstock source and gasification process. The main compositions of the synthesis gas are carbon monoxide (CO), hydrogen (H2) and methane (CH4). The remainder is made up of non-combustible gases, primarily nitrogen (N2) and carbon dioxide (CO2). Owning to the small
* Corresponding author. School of Mechanical, Electronic and Control Engineering, Beijing Jiaotong University, Beijing 100044, PR China. Tel./fax: þ86 10 51684279. E-mail address: [email protected] (J. Liu). http://dx.doi.org/10.1016/j.ijhydene.2015.04.133 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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lower heating value, synthesis gas can be blended with natural gas to achieve higher thermal efficiency in combustion engines [6]. Laminar flame speed, which is the fundamental parameter that characterizes the flame propagation, provides information about flame shape as well as important flame stability characteristics such as blowout and flashback. Moreover, the correlations of laminar flame speed are also used in CFD (computational fluid dynamic) simulations for accurate predictions. Understanding how CH4/H2/CO blended fuel combustion characteristics are influenced by the fuel variations is therefore important. There are many studies on the laminar flame speed of premixed synthesis gas. In particular, experimental works on the effect of H2 addition on the laminar flame speed and flame stability of CH4/H2/air mixture were expensively investigated [7,8]. These researches indicate that the addition of H2 promote the flame propagation speed and decrease the Markstein length. The addition of CO2 directly participates in the chemical reaction through the elementary reaction OH þ CO 4 H þ CO2 and inhibits the combustion process [9e11]. Kinetics analysis indicates that CO2 has a stronger chemical effect than H2O [12]. The combustion characteristics of CO are different from those of hydrogen and hydrocarbons. It is difficult to ignite and sustain a dry COeO2 flame for the reason that it has high activation energy for the direct reaction between CO and O2. However, with the presence of a small amount of hydrogen, CO oxidation process can be significantly accelerated via the reaction CO þ OH 4 CO2 þ H. Wu et al. also found that as the CO content in the fuel is increased from 0 % to 80 %, the laminar flame speed of the premixed CH4/air increase significantly [13]. The flame structures of the laminar premixed stoichiometric H2/CO/CH4 air opposed-jet flames were experimentally and numerically investigated [14]. The result showed that the increase in the laminar speed with H2 addition was most likely due to an increase in active radicals, rather than from change in the adiabatic flame. More recently, Lapalme et al. found the laminar flame speed of H2/CO mixture decreases with the addition of CH4. Meanwhile, the location of the maximum flame speed shifts to leaner mixtures with the addition of CH4 due to its inherent slower flame speed [15]. It is also found that the hydrogen content in H2/CO/ CH4 syngas has the main effect on the flame temperature [16]. High multi-mode combustion instability was observed at some particular compositions of H2/CH4/CO [17]. There are several motivations for the present study. First, since CO and H2 are not only the fuel, but also the intermediate species in the hydrocarbon flames, the intrinsic interaction between the CO and H2 from the fuel and those produced from the hydrocarbon combustion process and it is interaction on the flame characteristics of the tertiary fuel mixtures are still not very clear. Developing and validating a new tertiary fuel oxidation mechanism is very important. Second, the adiabatic flame temperatures of the pure fuel gas of CO and H2 are higher than that of the pure CH4 under the same equivalence ratio, which will lead to a corresponding increase in the mixture reactivity. Moreover, hydrogen has a much higher diffusivity than any other gases. Therefore, apart from the enhanced chemical reactivity, the thermal effect and diffusion effect also need to be assessed. Finally, an intriguing
issue that needs to be addressed is whether the linear correlation between the laminar flame speed and the relative amount of H2 addition (RH), which are identified in binary hydrocarbon/hydrogen mixtures, still holds in tertiary CH4/ CO/H2 mixtures, which will facilitate using laminar flame speed in CFD. In this study, the laminar flame speeds of the CH4/CO/H2 mixtures are firstly calculated and compared with the experiment results. Then, a new mechanism is developed and validated against the experiment data. After that, the chemical effect, thermal effect and diffusion effect of H2/CO addition on methane laminar flame speed will be investigated. Finally, a new effective tertiary fuel/air equivalence ratio and the relative hydrogen addition parameter are introduced.
Numerical approach The flame speed and temperature of the freely propagating premixed laminar flame are calculated by the PREMIX code [18], which is integrated with CHEMKIN II [19] and TRANSPORT [20] subroutine libraries. Moreover, the multicomponent species diffusion and the thermal diffusion of H and H2 are included in the transport model. The convective terms are approximated by windward difference and the diffusive terms are approximated by central difference. Additionally, the adiabatic flame temperatures are computed by using equilibrium program EQUIL [21]. The calculations are performed with unburned mixture temperature of 298 K at atmospheric pressure. The solution is obtained with the gradient and curvature values of 0.025 and provides solutions with at least 500 grid points. The beginning and end of the computational boundary are x ¼ 2.0 cm and x ¼ 10.0 cm, respectively. In order to investigate the chemical aspect of the flammability limit, the thermal radiation loss is added to the PREMIX code. Furthermore, the PREMIX code has to be modified to include the capability of solving the singular Jacobian matrix at the extinction turning point. The one point temperature controlling continuation method is used to solve the singular Jacobian problem [22]. Radiative heat losses from the most important radiating species of CO2, H2O, CH4 and CO are considered with the optically thin model [23]. In the fuel-air mixture with hydrogen and carbon monoxide addition there are three fuels and one oxidizer in the system, therefore three parameters should be designated to present its composition, such as the overall equivalence ratio, the mole fractions of hydrogen and carbon monoxide in the fuel mixture. The equivalence ratio used by most of the previous researchers is determined based on the ratio of air needed at stoichiometric condition to actual air in the system: f¼
CCH4 ðCCH4 CA Þst þ CH2 CH2 CA st þ CCO ðCCO =CA Þst CA
(1)
where Ci represent the mole concentration of species i; and the subscript st designates the value at stoichiometric condition. For instance, ðCCH4 =CA Þst is 0.105, ðCH2 =CA Þst is 0.42 and ðCCO =CA Þst is 0.42, respectively.
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The mole fractions of hydrogen and carbon monoxide in the fuel mixture, respectively defined as: XH2 ¼
CH2 CCO ; XCO ¼ CCH4 þ CH2 þ CCO CCH4 þ CH2 þ CCO
(2)
Sensitivity analysis is conducted to understand quantitatively how the solution depends on the chemical kinetics in the model. In this study, the normalized sensitivity coefficient in the form of logarithmic is expressed as: Fsen;R ¼
Ai vX X vAi
(3)
where Ai is the “A-factors” of the reaction rate coefficients, and X is the flame speed or species mole concentration. The chemical kinetic model employed in this study is GRIMech 3.0 [24]. It consists of 36 species and 219 elementary chemical reactions with associated rate coefficient expressions, where the associated reactions with NO are deleted from the original reaction mechanism. It is an optimized mechanism designed to model natural gas combustion. It includes the detailed oxidation reaction mechanism of hydrogen and CO and has been widely used as the combustion mechanism of the CH4/H2/CO blended fuels.
Result and discussion In order to investigate the effects of H2 and CO addition on the characteristics of the CH4 laminar flame, six CH4/H2/CO mixtures with 0% H2, 15% H2, 30% H2, 30% H2 þ 15% CO, 30% H2 þ 30% CO and 40% H2 þ 40% CO are selected in this study, as shown in Table 1. The chemical, thermal and diffusion effects are analyzed separately in the following sections.
Effect of syngas addition on laminar flame speed Figs. 1 and 2 compare the experimental and calculation results of the laminar flame speed of CH4/H2eair and CH4/H2/COeair mixtures, where the numerical results are given as lines and the experimental results are given as symbols. For CH4/H2eair mixtures, the flame speeds calculated by GRI-Mech 3.0 are in good agreement with experimental ones [15,25,26]. For CH4/ H2/COeair mixtures, both the prediction data from Dryer's model [27] and GRI-Mech 3.0 are consistent with the experimental ones at lean to stoichiometric flames, however, the deviation are obviously at rich flames. Moreover, GRI-Mech 3.0 provided a better match for the experimental results than Dryer's model. As an effort to improve the predictive ability of the GRI-Mech 3.0, a sensitivity analysis with respect to the
Fig. 1 e Laminar flame speed as a function of equivalence ratio. Lines: numerical results. Symbols: experimental results [25,26].
laminar flame speed was performed for two tertiary CH4/CO/ H2 fuel mixtures based on the GRI-Mech 3.0 mechanism as it provided a better prediction. Fig. 3 gives the 21 elementary reactions exhibiting the largest normalized sensitivity coefficient with respect to the flame speed. As the large deviation is presented at rich side, the sensitivity calculations are performed with the equivalence ratio of 1.5. This deviation also observed by Vu et al. [5] and Lapalme et al. [15]. A modified GRI-Mech 3.0 mechanism with 12 updated reaction rate coefficients is recommend by them using the sensitivity analysis for species and flame speed, respectively, as suggested in Ref. [5]. Most of the updated reactions are presented in our sensitivity analysis with respect to the flame speed as shown in Fig. 3. However, reaction 41, which is: 2H þ H2O 4 H2 þ H2O, was shown with a very small sensitivity coefficient in our calculation, which means it has little effect on the laminar flame speed. Therefore, only 11 reactions are updated in our
Table 1 e Compositions of the fuels used in this study (vol. %). Number
Fuel name
CH4
H2
CO
Blend Blend Blend Blend Blend Blend
CH4 M85H15 M70H30 M55H30C15 M40H30C30 M20H40C40
100 85 70 55 40 20
e 15 30 30 30 40
e e e 15 30 40
0 1 2 3 4 5
Fig. 2 e Calculated laminar flame speeds vs equivalence ratio for different CH4/CO/H2/air mixtures. Lines: numerical results. Symbols: experiment results [15].
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Fig. 3 e Normalized logarithmic sensitivity coefficients of the laminar flame speed.
new modified GRI-Mech 3.0 mechanism, as shown in Table 2. The modified reaction rate parameters are collected from combustion modeling in literature [5,27e30]. Reaction 41 is the same as in the original mechanism. Fig. 4 illustrates the laminar flame speed predicated by this new modified model, along with the results from the model updated by Vu et al. and the experimental results. It is shown that both the modified mechanism fit the experimental data well, and they coincide with each other. Specifically, the maximum flame speed of M40H30C30 is predicted to occur at equivalence ratio of 1.2 by the new modified mechanism, matching the experimental data. With the addition of CO and H2, the experimental results show that the peak laminar flame speed appears at the equivalence ratio of 1.3 for the fuel of M20H40C40, which is also successfully captured by the new modified mechanism. Moreover, there is a higher level of agreement between the predictions and the experimental
data on the rich side compared with the original mechanism. So, in the following discussion, for tertiary CH4/CO/H2 fuel mixtures, the new modified GRI-Mech 3.0 model with 11 updated reactions will be used. For binary CH4/H2 fuel mixtures, the original GRI-Mech 3.0 mechanism will be used.
Effect of thermal effect on laminar flame speed Considering the fact that the increase of the flame speed with the addition of syngas is owing to both chemical effect (increase in active radicals) and thermal effect (changes in the adiabatic flame temperature), the method detailed in Ref. [31] is used in this study to separate the chemical effect and the thermal effect. Namely, the blend 1 to 4 fuel mixtures are adjusted by replacing part of the nitrogen with carbon dioxide, in order to reduce the temperature of the baseline mixture. It should be noted that the mole fraction of the inert gas is
Table 2 e Reaction rate coefficients. Units are mole-cm-s-cal-k. No. 1 2 3 4 5 6 7 8 9 10 11
Name
Reaction
R3 R10 R38 R45 R46 R52 R53 R55 R84 R98 R166
O þ H2 ⇔H þ OH O þ CH3 ⇔H þ CH2 O H þ O2 ⇔O þ OH H þ HO2 ⇔O2 þ H2 H þ HO2 ⇔2OH H þ CH3 ðþMÞ⇔CH4 ðþMÞ H þ CH4 ⇔CH3 þ H2 H þ HCO⇔H2 þ CO OH þ H2 ⇔H þ H2 O OH þ CH4 ⇔CH3 þ H2 O HCO þ H2 O⇔H þ CO þ H2 O
A 3.82E 8.43E 3.55E 1.66E 1.70E 1.27E 5.47E 5.00E 1.17E 5.72E 2.24E
þ 12 þ 13 þ 15 þ 13 þ 14 þ 16 þ 07 þ 13 þ 09 þ 06 þ 18
n
Ea
Ref.
0.000 0.000 0.406 0.000 0.000 0.630 1.970 0.000 1.300 1.960 1.000
7948.00 0.00 16599.00 823.00 875.00 383.00 11210.00 0.00 3635.28 2639.00 17000.00
[29] [27,28] [27] [27,28] [5] [27,28] [27] [28] [28] [27] [30]
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Fig. 4 e New calculated laminar flame speeds vs equivalence ratio for different CH4/CO/H2/air mixtures. Lines: numerical results. Symbols: experiment results [15].
maintained constant. The thermal effect on the laminar flame speed can be determined by comparing the results of the baseline mixture and that obtained by compensating the baseline flame. Fig. 5 compares the net percentage increase in laminar flame speed due to thermal effect with the addition of H2 and CO. Blend 1 and 2 fuels are used to illustrate the effect of H2 addition on laminar flame speed. And blend 3 and 4 fuels are used to illustrate the effect of CO addition on laminar flame speed. It is shown that with the addition of H2, the increase of flame speed is mainly due to chemical effect at stoichiometric conditions. At lean mixture conditions, thermal effect on laminar flame speed is comparable to that of the chemical effect. However, with the addition of CO, thermal effect has an obvious role for the increase of the laminar flame speed under lean and stoichiometric conditions. Specifically, around 75% of the increase in laminar flame speed is due to thermal effect under lean and stoichiometric conditions.
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the baseline condition (CH4 flame). The data reveal that the addition of H2 and CO can significantly increase the radical concentrations. Fig. 6 shows the normalized logarithmic sensitivity coefficients of the OH mole fraction with respect to the reaction rate coefficients of each elementary reaction. The main reactions with positive sensitivities listed here is almost the same as in Fig. 3, however, the values are relative small. For the main reactions with negative effects, there are some differences with that of the flame speed. Compared with the sensitivity coefficients of the flame speed, the reactions of R36, R38, R85 and R179 appear to have higher priority to the decrease of the OH mole fraction. OH radical is consumed through the chain termination reaction R181, from which two stable species will be produced. The consumption of OH radicals by reaction R36 and R88 leads to the production of the HO2 radical, which is much less reactive than OH radical. Reaction R85 consumes two OH radicals and forms one hydrogen peroxide, which is less reactive in low temperature conditions.
H þ O2 þ N2 4 HO2 þ N2
(R36)
2OH (þM) 4 H2O2 (þM)
(R85)
OH þ H2O2 4 HO2 þ H2O
(R88)
OH þ HO2 4 O2 þ H2O
(R181)
With the addition of both H2 and CO, there is an obviously increase in the normalized logarithmic sensitivity coefficients of the major reactions, which means these reactions become more important for the production and consumption of the OH radicals.
Dependence of laminar flame speed on the transport properties
Peak mole fractions of H, O and OH radicals Table 3 lists the percentage increase in peak mole fractions of H, O and OH radicals with the addition of H2 and CO relative to
While molecular diffusion due to concentration gradient, that is Fickian diffusion, is the dominant mode of transport. Thermal diffusion caused by the presence of temperature
Fig. 5 e Comparison of the total effect and the thermal effect on laminar flame speed.
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Table 3 e Percent increase in peak mole fractions of H, O and OH. f ¼ 0.6
f ¼ 1.0
H O OH H O OH
M85H15
M70H30
M55H30C15
M40H30C30
19.39 10.34 4.37 9.16 3.55 4.22
48.69 25.52 4.56 23.77 12.03 9.76
87.88 57.50 14.25 46.15 44.71 13.93
147.07 98.10 25.44 64.10 72.52 16.87
gradient, that is Soret diffusion, is often seen as the secondary mass diffusion. Fig. 7 presents the laminar flame speeds of the C40H30C30eair mixtures as a function of the equivalence ratio. Results are given for situations in which the Soret diffusion effects are either activated for all species (Total Soret) or suppressed (No Soret), and only the Soret effect of H (H Soret), H2 (H2 Soret) or both of them are activated (H þ H2 Soret). It is shown the following result. Firstly, the H2 Soret diffusion has only minute effects on the laminar flame speed, slightly lowering it around the peak segment of the corresponding curve with no Soret effects. Secondly, Soret diffusion of H does have an obviously effect on laminar flame speed, lowing it around the peak segment with much wider equivalence ratios than H2 Soret. Furthermore, the combined Soret effect of H and H2 basically accounts for the total Soret diffusion effect, as the H þ H2 Soret effect curve agrees closely with that of the total Soret diffusion effects. In order to get a satisfactory explanation of the above results, an overall consideration of the transport, composition and reaction effects is required. Fig. 8 gives the diffusion flux of the H, H2 and CO at stoichiometric condition for demonstration; results are qualitatively similar for other equivalence ratios. Furthermore, Fig. 9 compares the H, H2 and CO mole fractions with and without Sore diffusion, and Fig. 10 gives the heat release rates of the major exothermic and endothermic elementary reactions for the case of no and total Soret diffusion. Fig. 8 shows that the H Soret diffusion is in the opposite direction to the Fickian diffusion in the range of 0.35e0.77 cm.
Fig. 6 e Normalized logarithmic sensitivity coefficients of the OH mole fractions.
Both of them peak at about 0.55 cm, which is also the location of the peak heat release rates. Therefore, Soret diffusion drives more H radicals transport to the downstream portion (high temperature region), which leads to the reduction of the overall reaction intensity and the reduction in the laminar flame speed. However, for H2, both the Soret diffusion and Fickian diffusion transport it to the downstream region. The Soret diffusion for H2 is almost eliminated beyond 0.55 cm, which is the peak region of the main reaction. Consequently, Soret diffusion of H2 is basically spatially decoupled from the major reaction region. This is therefore explains the small sensitivity of H2 Soret diffusion to the laminar flame speed. The CO Soret diffusion, however, hardly affect the overall diffusion coefficient, as it is very little compared with the CO Fickian diffusion. Therefore, the insensitivity CO Soret diffusion to the laminar flame speed is confirmed.
Lean flammability limits Considering the fact that the gaseous fuel is usually used under lean mixture conditions, combustion instability can occur when the combustion take place near the lean flammability limit. It is, therefore, of interest to investigate to what extent the lean flammable limit of the CH4eair mixture can be extended owing to the H2 and CO addition. Fig. 11 presents the laminar flame speed as a function of the equivalence ratio for the radiative CH4/CO/H2 flames. In the present study, the flammability limit is identified as the exhibition of the extinction turning point, where the steady flame propagation is failure. As illustrates in Fig. 11, with the addition of H2 and CO, the lean flammability limit is extended to leaner mixture at atmospheric pressure.
Fig. 7 e H2 and H Soret effect on laminar flame speed.
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Fig. 8 e Spatially-resolved Fickian and Soret diffusion fluxes of H, H2 and CO.
Liner correlation of the laminar flame speed The above definition of the overall equivalence ratio implies that the oxidizer is equally consumed by methane, hydrogen and carbon monoxide. However, for small to moderate amount of hydrogen addition, there should be enough air to facilitate its complete oxidation. Moreover, as the oxidation of carbon monoxide is the last step of the hydrocarbon oxidation process, therefore, carbon monoxide should have lower (or same) priority to react with oxygen than that of the hydrocarbon.
Fig. 9 e Spatially-resolved mole fraction of H2, CO and H.
Following this concept, CH2 =ðCH2 =CA Þst amount of air is needed to oxidize CH2 amount of hydrogen. If the remaining air is used to oxidize the hydrocarbon and the carbon monoxide, we can then define an effective fuel/air equivalence ratio as: CCH4 ðCCH4 CA Þst þ CCO ðCCO =CA Þst fF ¼ CA CH2 CH2 CA st
(4)
This definition is different from the expression introduced
Fig. 10 e Spatially-resolved heat release rate of the main reactions.
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Fig. 11 e Lean flammability limit.
Fig. 13 e Linearity coefficient with the increase of CO mole fractions.
in Ref. [32], where the hydrogen and carbon monoxide are considered as the same priority to react with oxygen. The relative amount of hydrogen addition is defined as: RH ¼
CCH4
CH2 þ CH2 CH2 CA st þ CCO þ CA CH2 CH2 CA st
(5)
The numerator of RH is the amount of hydrogen plus the air needed for complete oxidation, while the denominator is the amount of the hydrocarbon and carbon monoxide plus the amount of air left for their oxidation. Fig. 12 displays the calculated laminar flame speeds of methaneeair mixture with hydrogen addition at various carbon monoxide mole fractions for both lean and stoichiometric cases. With the same CO mole fraction in the fuel mixture, the laminar flame speed increase almost linearly with the increase of RH. However, this linear correlation is undermined with CO mole fractions large than 45% (or more). A possible explanation for this is that the chemistry of CO consumption shifts to dry oxidation kinetics when CO content is too much [13].
Fig. 12 e Calculated laminar flame speed with hydrogen addition.
This linear result in tertiary fuel mixtures is consistent with that of the binary fuel mixture for hydrogen addition in methaneeair, propaneeair and butaneeair mixtures [33]. Following Ref. [33], we then correlate the calculated laminar flame speed with RH by a linear expression: Su ðfF ; XCO ; RH Þ ¼ Su ðfF ; XCO ; 0Þ þ kðfF ; XCO ÞRH
(6)
Such that the coefficient kðfF ; XCO Þ can be considered as the sensitivity of hydrogen addition on the flame speed. As shown in Fig. 13, the data indicate that k actually depends on CO mole fractions, where the minimum is at the smallest CO mole fraction conditions.
Conclusions In this study, the effect of H2 and CO addition to methane laminar flame is examined systematically. The laminar flame speed of the premixed CH4/H2/CO/air mixture under various equivalence ratios are calculated by using the CHEMKIN 2.0 code with the original and modified GRI-Mech 3.0. The chemical effect, thermal effect and diffusion effect of H2 and CO addition to methane laminar flame speed are investigated under atmospheric conditions. The main results are summarized as follows: 1. A new modified GRI-Mech 3.0 mechanism with less updated reaction coefficients is proposed, which improve the prediction of the laminar speed at rich flames. 2. The effect of H2 addition to laminar flame speed is mainly due to chemical effect at lean to stoichiometric conditions. However, with the addition of CO, thermal effect plays an obvious role for the increase of flame speed. 3. Soret effect increases the transportation of H radical to the downstream direction, and consequently reduces its concentration in the active reaction region, which lead to a reduction in laminar flame speed.
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4. Under the same CO mole fraction conditions, the laminar flame speed increase linearly with the relative amount of hydrogen.
Acknowledgment This work was supported by“the Fundamental Research Funds for the Central Universities” (No. 2014JBM102).
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[15] Lapalme D, Seers P. Influence of CO2, CH4, and initial temperature on H2/CO laminar flame speed. Int J Hydrog Energy 2014;39:3477e86. [16] Joo S, Yoon J, Kim J, Lee M, Yoon Y. NOx emissions characteristics of the partially premixed combustion of H2/ CO/CH4 syngas using artificial neural networks. Appl Therm Eng 2015;80:436e44. [17] Lee M, Yoon J, Joo S, Kim J, Hwang J, Yoon Y. Investigation into the cause of high multi-mode combustion instability of H2/CO/CH4 syngas in a partially premixed gas turbine model combustor. Proc Combust Inst 2015;35:3263e71. [18] Kee J, Grcar K, Smooke MD, Miller JA. Premix: A Fortran program for modelling steady laminar one dimensional premixed flames. Report No. SAND85-8240. Livermore, CA: Sandia National Laboratories; 1985. [19] Kee RJ, Rupley FM, Miller JA. CHEMKIN-II: A Fortran chemical kinetics package for the analysis of gas-phase chemical kinetics. Livermore, CA (USA): Sandia National Labs; 1989. [20] Kee RJ, Warnatz J, Miller JA. A Fortran computer code package for the evaluation of gas-phase viscosities, conductivities, and diffusion coefficient's. Sandia National Laboratories Report SAND83e8209. 1983. [21] Lutz AE, Rupley FM, Kee RJ, Reynolds WC. EQUIL: A CHEMKIN implementation of STANJAN for computing chemical equilibria. 1998. [22] Zhang Y, Qiu X, Li B, Zhang H, Li S. Extinction studies of nearlimit lean premixed syngas/air flames. Int J Hydrog Energy 2013;38:16453e62. [23] Wang J, Huang ZH, Kobayashi H, Ogami Y. Laminar burning velocities and flame characteristics of COeH2eCO2eO2 mixtures. Int J Hydrog Energy 2012;37:19158e67. [24] Smith GP, Golden DM, Frenklach M, Moriarty NW, Eiteneer B, Goldenberg M. Gri-Mech 3.0. 1999. http://www.me.berkeley. edu/gri_mech. [25] Hermanns RTE. Laminar burning velocities of methanehydrogen-air mixtures [Ph.D. thesis]. Technische Universiteit Eindhoven; 2007. [26] Coppens FHV, Ruyck JD, Konnov AA. The effects of composition on burning velocity and nitric oxide formation in laminar premixed flames of CH4 þ H2 þ O2 þ N2. Combust Flame 2007;149:409e17. [27] Li J, Zhao Z, Kazakov A, Chaos M, Dryer FL, Scire JJ. A comprehensive kinetic mechanism for CO, CH2O, CH3OH combustion. Int J Chem Kinet 2007;39:109e36. [28] Saxena P, Williams FA. Testing a small detailed chemical kinetic mechanism for the combustion of hydrogen and carbon monoxide. Combust Flame 2006;145:316e23. http:// maeweb.ucsd.edu/combustion/cermech/. [29] Sun H, Yang SI, Jomaas G, Law CK. High-pressure laminar flame speeds and kinetic modeling of carbon monoxide/ hydrogen combustion. Proc Combust Inst 2007;31:439e46. [30] Davis SG, Joshi AV, Wang H, Egolfopoulos F. An optimized kinetic model of H2/CO combustion. Proc Combust Inst 2005;30:1283e92. [31] Law CK. Combustion physics. New York: Cambridge University Press; 2006. [32] Sung CJ, Huang Y, Eng JA. Effects of reformer gas addition on the laminar flame speeds and flammability limits of nbutane and iso-butane flames. Combust Flame 2001;126:1699e713. [33] Tang CL, Huang ZH, Law CK. Determination, correlation, and mechanistic interpretation of effects of hydrogen addition on laminar flame speeds of hydrocarboneair mixtures. Proc Combust Inst 2011;33:921e8.
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Numerical study of the chemical, thermal and diffusion effects of H2 and CO addition on the laminar flame speeds of methaneeair mixture Jie Liu a,b,*, Xin Zhang a,b, Tao Wang a,b, Xiaosen Hou a,b, Jibao Zhang a,b, Shizhuo Zheng a,b a b
School of Mechanical, Electronic and Control Engineering, Beijing Jiaotong University, Beijing 100044, PR China Beijing Key Laboratory of New Energy Vehicle Powertrain Technology, PR China
article info
abstract
Article history:
In this study, the chemical, thermal and diffusion effects of H2 and CO addition on the
Received 9 March 2015
characteristics of methane laminar flame are examined numerically by using the CHEMKIN
Received in revised form
2.0 code with a modified GRI-Mech 3.0 mechanism. The results reveal that a better
13 April 2015
agreement between the measured and predicted laminar flame speed is obtained by using
Accepted 25 April 2015
the modified mechanism. The effect of H2 addition to flame speed is mainly due to
Available online 16 May 2015
chemical effect at lean and stoichiometric conditions. However, with the addition of CO, around 75% of the increase in laminar flame speed is due to thermal effect. Soret diffusion
Keywords:
of H have an obviously effect on laminar flame speed, lowing it around the peak segment.
Syngas
Furthermore, the combined Soret effect of H and H2 basically accounts for the total Soret
Hydrogen
diffusion effect. With the addition of H2 and CO, the lean flammability limit is extended to a
Laminar flame speed
leaner mixture at atmospheric pressure. A linear correlation between the laminar flame speed and the relative amount of H2 was found in tertiary CH4/CO/H2 mixtures. However, this linear correlation is undermined when CO mole fraction is over than 45%. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction With increasing demand on energy and concerning on air pollution, research on clean alternative fuel and renewable energies has attracted more and more attentions [1e3]. Synthesis gas offers a promising opportunity for sustainable development in the energy and transportation sectors. Harnessing energy from syngas is not only proving to be economical, but is also environmentally benign [4]. Synthesis
gas, produced from coal, biomass, waste, landfill and alcohol fuels via gasification, pyrolysis or fermentation processes, has been widely used in power plant or other combustion engines [5]. The most serious obstacles in developing combustion engines using synthesis gas result from a large variety of compositions from different feedstock source and gasification process. The main compositions of the synthesis gas are carbon monoxide (CO), hydrogen (H2) and methane (CH4). The remainder is made up of non-combustible gases, primarily nitrogen (N2) and carbon dioxide (CO2). Owning to the small
* Corresponding author. School of Mechanical, Electronic and Control Engineering, Beijing Jiaotong University, Beijing 100044, PR China. Tel./fax: þ86 10 51684279. E-mail address: [email protected] (J. Liu). http://dx.doi.org/10.1016/j.ijhydene.2015.04.133 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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lower heating value, synthesis gas can be blended with natural gas to achieve higher thermal efficiency in combustion engines [6]. Laminar flame speed, which is the fundamental parameter that characterizes the flame propagation, provides information about flame shape as well as important flame stability characteristics such as blowout and flashback. Moreover, the correlations of laminar flame speed are also used in CFD (computational fluid dynamic) simulations for accurate predictions. Understanding how CH4/H2/CO blended fuel combustion characteristics are influenced by the fuel variations is therefore important. There are many studies on the laminar flame speed of premixed synthesis gas. In particular, experimental works on the effect of H2 addition on the laminar flame speed and flame stability of CH4/H2/air mixture were expensively investigated [7,8]. These researches indicate that the addition of H2 promote the flame propagation speed and decrease the Markstein length. The addition of CO2 directly participates in the chemical reaction through the elementary reaction OH þ CO 4 H þ CO2 and inhibits the combustion process [9e11]. Kinetics analysis indicates that CO2 has a stronger chemical effect than H2O [12]. The combustion characteristics of CO are different from those of hydrogen and hydrocarbons. It is difficult to ignite and sustain a dry COeO2 flame for the reason that it has high activation energy for the direct reaction between CO and O2. However, with the presence of a small amount of hydrogen, CO oxidation process can be significantly accelerated via the reaction CO þ OH 4 CO2 þ H. Wu et al. also found that as the CO content in the fuel is increased from 0 % to 80 %, the laminar flame speed of the premixed CH4/air increase significantly [13]. The flame structures of the laminar premixed stoichiometric H2/CO/CH4 air opposed-jet flames were experimentally and numerically investigated [14]. The result showed that the increase in the laminar speed with H2 addition was most likely due to an increase in active radicals, rather than from change in the adiabatic flame. More recently, Lapalme et al. found the laminar flame speed of H2/CO mixture decreases with the addition of CH4. Meanwhile, the location of the maximum flame speed shifts to leaner mixtures with the addition of CH4 due to its inherent slower flame speed [15]. It is also found that the hydrogen content in H2/CO/ CH4 syngas has the main effect on the flame temperature [16]. High multi-mode combustion instability was observed at some particular compositions of H2/CH4/CO [17]. There are several motivations for the present study. First, since CO and H2 are not only the fuel, but also the intermediate species in the hydrocarbon flames, the intrinsic interaction between the CO and H2 from the fuel and those produced from the hydrocarbon combustion process and it is interaction on the flame characteristics of the tertiary fuel mixtures are still not very clear. Developing and validating a new tertiary fuel oxidation mechanism is very important. Second, the adiabatic flame temperatures of the pure fuel gas of CO and H2 are higher than that of the pure CH4 under the same equivalence ratio, which will lead to a corresponding increase in the mixture reactivity. Moreover, hydrogen has a much higher diffusivity than any other gases. Therefore, apart from the enhanced chemical reactivity, the thermal effect and diffusion effect also need to be assessed. Finally, an intriguing
issue that needs to be addressed is whether the linear correlation between the laminar flame speed and the relative amount of H2 addition (RH), which are identified in binary hydrocarbon/hydrogen mixtures, still holds in tertiary CH4/ CO/H2 mixtures, which will facilitate using laminar flame speed in CFD. In this study, the laminar flame speeds of the CH4/CO/H2 mixtures are firstly calculated and compared with the experiment results. Then, a new mechanism is developed and validated against the experiment data. After that, the chemical effect, thermal effect and diffusion effect of H2/CO addition on methane laminar flame speed will be investigated. Finally, a new effective tertiary fuel/air equivalence ratio and the relative hydrogen addition parameter are introduced.
Numerical approach The flame speed and temperature of the freely propagating premixed laminar flame are calculated by the PREMIX code [18], which is integrated with CHEMKIN II [19] and TRANSPORT [20] subroutine libraries. Moreover, the multicomponent species diffusion and the thermal diffusion of H and H2 are included in the transport model. The convective terms are approximated by windward difference and the diffusive terms are approximated by central difference. Additionally, the adiabatic flame temperatures are computed by using equilibrium program EQUIL [21]. The calculations are performed with unburned mixture temperature of 298 K at atmospheric pressure. The solution is obtained with the gradient and curvature values of 0.025 and provides solutions with at least 500 grid points. The beginning and end of the computational boundary are x ¼ 2.0 cm and x ¼ 10.0 cm, respectively. In order to investigate the chemical aspect of the flammability limit, the thermal radiation loss is added to the PREMIX code. Furthermore, the PREMIX code has to be modified to include the capability of solving the singular Jacobian matrix at the extinction turning point. The one point temperature controlling continuation method is used to solve the singular Jacobian problem [22]. Radiative heat losses from the most important radiating species of CO2, H2O, CH4 and CO are considered with the optically thin model [23]. In the fuel-air mixture with hydrogen and carbon monoxide addition there are three fuels and one oxidizer in the system, therefore three parameters should be designated to present its composition, such as the overall equivalence ratio, the mole fractions of hydrogen and carbon monoxide in the fuel mixture. The equivalence ratio used by most of the previous researchers is determined based on the ratio of air needed at stoichiometric condition to actual air in the system: f¼
CCH4 ðCCH4 CA Þst þ CH2 CH2 CA st þ CCO ðCCO =CA Þst CA
(1)
where Ci represent the mole concentration of species i; and the subscript st designates the value at stoichiometric condition. For instance, ðCCH4 =CA Þst is 0.105, ðCH2 =CA Þst is 0.42 and ðCCO =CA Þst is 0.42, respectively.
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The mole fractions of hydrogen and carbon monoxide in the fuel mixture, respectively defined as: XH2 ¼
CH2 CCO ; XCO ¼ CCH4 þ CH2 þ CCO CCH4 þ CH2 þ CCO
(2)
Sensitivity analysis is conducted to understand quantitatively how the solution depends on the chemical kinetics in the model. In this study, the normalized sensitivity coefficient in the form of logarithmic is expressed as: Fsen;R ¼
Ai vX X vAi
(3)
where Ai is the “A-factors” of the reaction rate coefficients, and X is the flame speed or species mole concentration. The chemical kinetic model employed in this study is GRIMech 3.0 [24]. It consists of 36 species and 219 elementary chemical reactions with associated rate coefficient expressions, where the associated reactions with NO are deleted from the original reaction mechanism. It is an optimized mechanism designed to model natural gas combustion. It includes the detailed oxidation reaction mechanism of hydrogen and CO and has been widely used as the combustion mechanism of the CH4/H2/CO blended fuels.
Result and discussion In order to investigate the effects of H2 and CO addition on the characteristics of the CH4 laminar flame, six CH4/H2/CO mixtures with 0% H2, 15% H2, 30% H2, 30% H2 þ 15% CO, 30% H2 þ 30% CO and 40% H2 þ 40% CO are selected in this study, as shown in Table 1. The chemical, thermal and diffusion effects are analyzed separately in the following sections.
Effect of syngas addition on laminar flame speed Figs. 1 and 2 compare the experimental and calculation results of the laminar flame speed of CH4/H2eair and CH4/H2/COeair mixtures, where the numerical results are given as lines and the experimental results are given as symbols. For CH4/H2eair mixtures, the flame speeds calculated by GRI-Mech 3.0 are in good agreement with experimental ones [15,25,26]. For CH4/ H2/COeair mixtures, both the prediction data from Dryer's model [27] and GRI-Mech 3.0 are consistent with the experimental ones at lean to stoichiometric flames, however, the deviation are obviously at rich flames. Moreover, GRI-Mech 3.0 provided a better match for the experimental results than Dryer's model. As an effort to improve the predictive ability of the GRI-Mech 3.0, a sensitivity analysis with respect to the
Fig. 1 e Laminar flame speed as a function of equivalence ratio. Lines: numerical results. Symbols: experimental results [25,26].
laminar flame speed was performed for two tertiary CH4/CO/ H2 fuel mixtures based on the GRI-Mech 3.0 mechanism as it provided a better prediction. Fig. 3 gives the 21 elementary reactions exhibiting the largest normalized sensitivity coefficient with respect to the flame speed. As the large deviation is presented at rich side, the sensitivity calculations are performed with the equivalence ratio of 1.5. This deviation also observed by Vu et al. [5] and Lapalme et al. [15]. A modified GRI-Mech 3.0 mechanism with 12 updated reaction rate coefficients is recommend by them using the sensitivity analysis for species and flame speed, respectively, as suggested in Ref. [5]. Most of the updated reactions are presented in our sensitivity analysis with respect to the flame speed as shown in Fig. 3. However, reaction 41, which is: 2H þ H2O 4 H2 þ H2O, was shown with a very small sensitivity coefficient in our calculation, which means it has little effect on the laminar flame speed. Therefore, only 11 reactions are updated in our
Table 1 e Compositions of the fuels used in this study (vol. %). Number
Fuel name
CH4
H2
CO
Blend Blend Blend Blend Blend Blend
CH4 M85H15 M70H30 M55H30C15 M40H30C30 M20H40C40
100 85 70 55 40 20
e 15 30 30 30 40
e e e 15 30 40
0 1 2 3 4 5
Fig. 2 e Calculated laminar flame speeds vs equivalence ratio for different CH4/CO/H2/air mixtures. Lines: numerical results. Symbols: experiment results [15].
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Fig. 3 e Normalized logarithmic sensitivity coefficients of the laminar flame speed.
new modified GRI-Mech 3.0 mechanism, as shown in Table 2. The modified reaction rate parameters are collected from combustion modeling in literature [5,27e30]. Reaction 41 is the same as in the original mechanism. Fig. 4 illustrates the laminar flame speed predicated by this new modified model, along with the results from the model updated by Vu et al. and the experimental results. It is shown that both the modified mechanism fit the experimental data well, and they coincide with each other. Specifically, the maximum flame speed of M40H30C30 is predicted to occur at equivalence ratio of 1.2 by the new modified mechanism, matching the experimental data. With the addition of CO and H2, the experimental results show that the peak laminar flame speed appears at the equivalence ratio of 1.3 for the fuel of M20H40C40, which is also successfully captured by the new modified mechanism. Moreover, there is a higher level of agreement between the predictions and the experimental
data on the rich side compared with the original mechanism. So, in the following discussion, for tertiary CH4/CO/H2 fuel mixtures, the new modified GRI-Mech 3.0 model with 11 updated reactions will be used. For binary CH4/H2 fuel mixtures, the original GRI-Mech 3.0 mechanism will be used.
Effect of thermal effect on laminar flame speed Considering the fact that the increase of the flame speed with the addition of syngas is owing to both chemical effect (increase in active radicals) and thermal effect (changes in the adiabatic flame temperature), the method detailed in Ref. [31] is used in this study to separate the chemical effect and the thermal effect. Namely, the blend 1 to 4 fuel mixtures are adjusted by replacing part of the nitrogen with carbon dioxide, in order to reduce the temperature of the baseline mixture. It should be noted that the mole fraction of the inert gas is
Table 2 e Reaction rate coefficients. Units are mole-cm-s-cal-k. No. 1 2 3 4 5 6 7 8 9 10 11
Name
Reaction
R3 R10 R38 R45 R46 R52 R53 R55 R84 R98 R166
O þ H2 ⇔H þ OH O þ CH3 ⇔H þ CH2 O H þ O2 ⇔O þ OH H þ HO2 ⇔O2 þ H2 H þ HO2 ⇔2OH H þ CH3 ðþMÞ⇔CH4 ðþMÞ H þ CH4 ⇔CH3 þ H2 H þ HCO⇔H2 þ CO OH þ H2 ⇔H þ H2 O OH þ CH4 ⇔CH3 þ H2 O HCO þ H2 O⇔H þ CO þ H2 O
A 3.82E 8.43E 3.55E 1.66E 1.70E 1.27E 5.47E 5.00E 1.17E 5.72E 2.24E
þ 12 þ 13 þ 15 þ 13 þ 14 þ 16 þ 07 þ 13 þ 09 þ 06 þ 18
n
Ea
Ref.
0.000 0.000 0.406 0.000 0.000 0.630 1.970 0.000 1.300 1.960 1.000
7948.00 0.00 16599.00 823.00 875.00 383.00 11210.00 0.00 3635.28 2639.00 17000.00
[29] [27,28] [27] [27,28] [5] [27,28] [27] [28] [28] [27] [30]
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Fig. 4 e New calculated laminar flame speeds vs equivalence ratio for different CH4/CO/H2/air mixtures. Lines: numerical results. Symbols: experiment results [15].
maintained constant. The thermal effect on the laminar flame speed can be determined by comparing the results of the baseline mixture and that obtained by compensating the baseline flame. Fig. 5 compares the net percentage increase in laminar flame speed due to thermal effect with the addition of H2 and CO. Blend 1 and 2 fuels are used to illustrate the effect of H2 addition on laminar flame speed. And blend 3 and 4 fuels are used to illustrate the effect of CO addition on laminar flame speed. It is shown that with the addition of H2, the increase of flame speed is mainly due to chemical effect at stoichiometric conditions. At lean mixture conditions, thermal effect on laminar flame speed is comparable to that of the chemical effect. However, with the addition of CO, thermal effect has an obvious role for the increase of the laminar flame speed under lean and stoichiometric conditions. Specifically, around 75% of the increase in laminar flame speed is due to thermal effect under lean and stoichiometric conditions.
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the baseline condition (CH4 flame). The data reveal that the addition of H2 and CO can significantly increase the radical concentrations. Fig. 6 shows the normalized logarithmic sensitivity coefficients of the OH mole fraction with respect to the reaction rate coefficients of each elementary reaction. The main reactions with positive sensitivities listed here is almost the same as in Fig. 3, however, the values are relative small. For the main reactions with negative effects, there are some differences with that of the flame speed. Compared with the sensitivity coefficients of the flame speed, the reactions of R36, R38, R85 and R179 appear to have higher priority to the decrease of the OH mole fraction. OH radical is consumed through the chain termination reaction R181, from which two stable species will be produced. The consumption of OH radicals by reaction R36 and R88 leads to the production of the HO2 radical, which is much less reactive than OH radical. Reaction R85 consumes two OH radicals and forms one hydrogen peroxide, which is less reactive in low temperature conditions.
H þ O2 þ N2 4 HO2 þ N2
(R36)
2OH (þM) 4 H2O2 (þM)
(R85)
OH þ H2O2 4 HO2 þ H2O
(R88)
OH þ HO2 4 O2 þ H2O
(R181)
With the addition of both H2 and CO, there is an obviously increase in the normalized logarithmic sensitivity coefficients of the major reactions, which means these reactions become more important for the production and consumption of the OH radicals.
Dependence of laminar flame speed on the transport properties
Peak mole fractions of H, O and OH radicals Table 3 lists the percentage increase in peak mole fractions of H, O and OH radicals with the addition of H2 and CO relative to
While molecular diffusion due to concentration gradient, that is Fickian diffusion, is the dominant mode of transport. Thermal diffusion caused by the presence of temperature
Fig. 5 e Comparison of the total effect and the thermal effect on laminar flame speed.
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Table 3 e Percent increase in peak mole fractions of H, O and OH. f ¼ 0.6
f ¼ 1.0
H O OH H O OH
M85H15
M70H30
M55H30C15
M40H30C30
19.39 10.34 4.37 9.16 3.55 4.22
48.69 25.52 4.56 23.77 12.03 9.76
87.88 57.50 14.25 46.15 44.71 13.93
147.07 98.10 25.44 64.10 72.52 16.87
gradient, that is Soret diffusion, is often seen as the secondary mass diffusion. Fig. 7 presents the laminar flame speeds of the C40H30C30eair mixtures as a function of the equivalence ratio. Results are given for situations in which the Soret diffusion effects are either activated for all species (Total Soret) or suppressed (No Soret), and only the Soret effect of H (H Soret), H2 (H2 Soret) or both of them are activated (H þ H2 Soret). It is shown the following result. Firstly, the H2 Soret diffusion has only minute effects on the laminar flame speed, slightly lowering it around the peak segment of the corresponding curve with no Soret effects. Secondly, Soret diffusion of H does have an obviously effect on laminar flame speed, lowing it around the peak segment with much wider equivalence ratios than H2 Soret. Furthermore, the combined Soret effect of H and H2 basically accounts for the total Soret diffusion effect, as the H þ H2 Soret effect curve agrees closely with that of the total Soret diffusion effects. In order to get a satisfactory explanation of the above results, an overall consideration of the transport, composition and reaction effects is required. Fig. 8 gives the diffusion flux of the H, H2 and CO at stoichiometric condition for demonstration; results are qualitatively similar for other equivalence ratios. Furthermore, Fig. 9 compares the H, H2 and CO mole fractions with and without Sore diffusion, and Fig. 10 gives the heat release rates of the major exothermic and endothermic elementary reactions for the case of no and total Soret diffusion. Fig. 8 shows that the H Soret diffusion is in the opposite direction to the Fickian diffusion in the range of 0.35e0.77 cm.
Fig. 6 e Normalized logarithmic sensitivity coefficients of the OH mole fractions.
Both of them peak at about 0.55 cm, which is also the location of the peak heat release rates. Therefore, Soret diffusion drives more H radicals transport to the downstream portion (high temperature region), which leads to the reduction of the overall reaction intensity and the reduction in the laminar flame speed. However, for H2, both the Soret diffusion and Fickian diffusion transport it to the downstream region. The Soret diffusion for H2 is almost eliminated beyond 0.55 cm, which is the peak region of the main reaction. Consequently, Soret diffusion of H2 is basically spatially decoupled from the major reaction region. This is therefore explains the small sensitivity of H2 Soret diffusion to the laminar flame speed. The CO Soret diffusion, however, hardly affect the overall diffusion coefficient, as it is very little compared with the CO Fickian diffusion. Therefore, the insensitivity CO Soret diffusion to the laminar flame speed is confirmed.
Lean flammability limits Considering the fact that the gaseous fuel is usually used under lean mixture conditions, combustion instability can occur when the combustion take place near the lean flammability limit. It is, therefore, of interest to investigate to what extent the lean flammable limit of the CH4eair mixture can be extended owing to the H2 and CO addition. Fig. 11 presents the laminar flame speed as a function of the equivalence ratio for the radiative CH4/CO/H2 flames. In the present study, the flammability limit is identified as the exhibition of the extinction turning point, where the steady flame propagation is failure. As illustrates in Fig. 11, with the addition of H2 and CO, the lean flammability limit is extended to leaner mixture at atmospheric pressure.
Fig. 7 e H2 and H Soret effect on laminar flame speed.
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Fig. 8 e Spatially-resolved Fickian and Soret diffusion fluxes of H, H2 and CO.
Liner correlation of the laminar flame speed The above definition of the overall equivalence ratio implies that the oxidizer is equally consumed by methane, hydrogen and carbon monoxide. However, for small to moderate amount of hydrogen addition, there should be enough air to facilitate its complete oxidation. Moreover, as the oxidation of carbon monoxide is the last step of the hydrocarbon oxidation process, therefore, carbon monoxide should have lower (or same) priority to react with oxygen than that of the hydrocarbon.
Fig. 9 e Spatially-resolved mole fraction of H2, CO and H.
Following this concept, CH2 =ðCH2 =CA Þst amount of air is needed to oxidize CH2 amount of hydrogen. If the remaining air is used to oxidize the hydrocarbon and the carbon monoxide, we can then define an effective fuel/air equivalence ratio as: CCH4 ðCCH4 CA Þst þ CCO ðCCO =CA Þst fF ¼ CA CH2 CH2 CA st
(4)
This definition is different from the expression introduced
Fig. 10 e Spatially-resolved heat release rate of the main reactions.
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Fig. 11 e Lean flammability limit.
Fig. 13 e Linearity coefficient with the increase of CO mole fractions.
in Ref. [32], where the hydrogen and carbon monoxide are considered as the same priority to react with oxygen. The relative amount of hydrogen addition is defined as: RH ¼
CCH4
CH2 þ CH2 CH2 CA st þ CCO þ CA CH2 CH2 CA st
(5)
The numerator of RH is the amount of hydrogen plus the air needed for complete oxidation, while the denominator is the amount of the hydrocarbon and carbon monoxide plus the amount of air left for their oxidation. Fig. 12 displays the calculated laminar flame speeds of methaneeair mixture with hydrogen addition at various carbon monoxide mole fractions for both lean and stoichiometric cases. With the same CO mole fraction in the fuel mixture, the laminar flame speed increase almost linearly with the increase of RH. However, this linear correlation is undermined with CO mole fractions large than 45% (or more). A possible explanation for this is that the chemistry of CO consumption shifts to dry oxidation kinetics when CO content is too much [13].
Fig. 12 e Calculated laminar flame speed with hydrogen addition.
This linear result in tertiary fuel mixtures is consistent with that of the binary fuel mixture for hydrogen addition in methaneeair, propaneeair and butaneeair mixtures [33]. Following Ref. [33], we then correlate the calculated laminar flame speed with RH by a linear expression: Su ðfF ; XCO ; RH Þ ¼ Su ðfF ; XCO ; 0Þ þ kðfF ; XCO ÞRH
(6)
Such that the coefficient kðfF ; XCO Þ can be considered as the sensitivity of hydrogen addition on the flame speed. As shown in Fig. 13, the data indicate that k actually depends on CO mole fractions, where the minimum is at the smallest CO mole fraction conditions.
Conclusions In this study, the effect of H2 and CO addition to methane laminar flame is examined systematically. The laminar flame speed of the premixed CH4/H2/CO/air mixture under various equivalence ratios are calculated by using the CHEMKIN 2.0 code with the original and modified GRI-Mech 3.0. The chemical effect, thermal effect and diffusion effect of H2 and CO addition to methane laminar flame speed are investigated under atmospheric conditions. The main results are summarized as follows: 1. A new modified GRI-Mech 3.0 mechanism with less updated reaction coefficients is proposed, which improve the prediction of the laminar speed at rich flames. 2. The effect of H2 addition to laminar flame speed is mainly due to chemical effect at lean to stoichiometric conditions. However, with the addition of CO, thermal effect plays an obvious role for the increase of flame speed. 3. Soret effect increases the transportation of H radical to the downstream direction, and consequently reduces its concentration in the active reaction region, which lead to a reduction in laminar flame speed.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 8 4 7 5 e8 4 8 3
4. Under the same CO mole fraction conditions, the laminar flame speed increase linearly with the relative amount of hydrogen.
Acknowledgment This work was supported by“the Fundamental Research Funds for the Central Universities” (No. 2014JBM102).
references
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