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 3 4 ( 2 0 0 9 ) 6 5 4 5 – 6 5 5 7
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Numerical study on laminar burning velocity and NOformation of premixed methane–hydrogen–air flames
Erjiang Hu, Zuohua Huang*, Jianjun Zheng, Qianqian Li, Jiajia He
State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an, 710049, People’s Republic of China
a r t i c l e i n f o
Article history:
Received 12 April 2009
Received in revised form
15 May 2009
Accepted 16 May 2009
Available online 23 June 2009
Keywords:
Methane
Hydrogen
Laminar burning velocity
NO formation
* Corresponding author. Tel.: þ86 29 8266507E-mail address: [email protected]
0360-3199/$ – see front matter ª 2009 Interndoi:10.1016/j.ijhydene.2009.05.080
a b s t r a c t
Numerical study on laminar burning velocity and NO formation of the premixed methane–
hydrogen–air flames was conducted at room temperature and atmospheric pressure. The
unstretched laminar burning velocity, adiabatic flame temperature, and radical mole
fractions of H, OH and NO are obtained at various equivalence ratios and hydrogen frac-
tions. The results show that the unstretched laminar burning velocity is increased with the
increase of hydrogen fraction. Methane-dominated combustion is presented when
hydrogen fraction is less than 40%, where laminar burning velocity is slightly increased
with the increase of hydrogen addition. When hydrogen fraction is larger than 40%,
laminar burning velocity is exponentially increased with the increase of hydrogen fraction.
A strong correlation exists between burning velocity and maximum radical concentration
of HþOH radicals in the reaction zone of premixed flames. High burning velocity corre-
sponds to high radical concentration in the reaction zone. With the increase of hydrogen
fraction, the overall activation energy of methane–hydrogen mixture is decreased, and the
inner layer temperature and Zeldovich number are also decreased. All these factors
contribute to the enhancement of combustion as hydrogen is added. The curve of NO
versus equivalence ratio shows two peaks, where they occur at the stoichiometric mixture
due to Zeldovich thermal-NO mechanism and at the rich mixture with equivalence ratio of
1.3 due to the Fenimore prompt-NO mechanism. In the stoichiometric flames, hydrogen
addition has little influence on NO formation, while in rich flames, NO concentration is
significantly decreased. Different NO formation responses to stretched and unstretched
flames by hydrogen addition are discussed.
ª 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights
reserved.
1. Introduction realized in the spark-ignition engines. However, due to the
With increasing concern about energy shortage and stringent
emission regulation, the development of alternative fuel
engines has attracted more and more attention in engine
community. Natural gas is a clean fuel in which methane is its
major component, is considered to be one of the favorable
fuels for engines. The naturalgasfuelled engines have been
5; fax: þ86 29 82668789.n (Z. Huang).ational Association for H
slow burning velocity of natural gas and its poor lean-burn
capability, the natural gas spark-ignition engine still remains
its disadvantages like low thermal efficiency, large cycle-by-
cycle variation, and poor lean-burn capability, and these will
decrease the engine power output and increase the fuel
consumption [1,2]. One of the effective methods to solve the
problem is to mix the natural gas with a fuel that possesses
ydrogen Energy. Published by Elsevier Ltd. All rights reserved.
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 3 4 ( 2 0 0 9 ) 6 5 4 5 – 6 5 5 76546
high burning velocity. Hydrogen is regarded as the best
gaseous candidate for natural gas due to its very high burning
velocity, and the combination of natural gas with hydrogen is
expected to improve the lean-burn characteristics and
decrease the engine emissions (mainly HC and CO), but NOx
emissions will be increased [3–6]. Thus, the investigation on
combustion mechanism of hydrogen addition into methane
and decreasing NOx has become one of the most important
topics in combustion.
Flame propagation is strongly related to the improvement
of combustion in spark ignition (SI) engines. In conventional SI
engine, combustion initiates from the spark plug where the
flame front will develop and propagate to the combustion
chamber volume [7]. By the time when 5–10% of fuel/air
mixture is burned, the combustion process in SI engines is
well established and the flame front moves quickly in the
engine cylinder [8]. The burning velocity directly affects the
flame propagation speed and hence, the operation of the SI
engine. Faster burning in SI engines leads to a more robust and
repeatable combustion and permits engine operation with
substantially larger amount of exhaust gas recirculation (EGR),
bringing the reduction in NOx emission. Fast burning results
in the decreased combustion duration and the compact
combustion process will improve engine thermal efficiency
and decrease fuel consumption [5–7].
Laminar burning velocity is the fundamental parameter in
combustion and it is base for determining turbulent burning
velocity. Laminar burning velocity can be used to validate the
chemical reaction mechanisms [9,10] and is of practical
importance in the design and optimization of internal
combustion engines and power plant burners [11]. Experi-
mental and numerical studies of methane–hydrogen fuel
blends have been conducted on laminar burning velocity
measurement, intermediate species measurement and
chemical kinetics simulation. Yu et al. [12] investigated the
laminar burning velocity of methane–hydrogen mixtures and
showed that the laminar burning velocities of methane–
hydrogen mixtures increased linearly with the increase of
hydrogen fraction in the fuel blends. Law and Kwon [13]
studied the hydrogen flame doped with small fraction of
hydrocarbon fuel and revealed the inhibition of hydrocarbon
fuel in addition to the hydrogen combustion. The results
showed that the laminar burning velocity and flame temper-
ature decreased remarkably by adding a small fraction of
hydrocarbon fuel. Halter et al. [14] investigated the effect of
initial pressure and hydrogen fraction on laminar burning
velocity of methane–hydrogen flame and their results showed
that laminar burning velocity was increased with the increase
of hydrogen fraction in fuel blends and was decreased with
the increase of initial pressure. Naha et al. [15] conducted the
numerical study of hydrocarbon–hydrogen flames and
revealed the influence of hydrogen addition on emissions by
analyzing the mole fraction profiles of NO, C2H2 and CO. Using
isotope shift/planar laser induced fluorescence spectroscopy,
Katoh et al. [16] detected two types of OH radicals in the
methane–hydrogen–air flame. These two OH radicals are
detected separately from the methane–air flame and the
hydrogen–air flame.
In the numerical study, most of simulations were con-
ducted with the CHEMKIN [17] and/or the COSILAB [18]
laminar premixed flame codes where detailed kinetic
schemes can be implemented [19,20]. However, these simu-
lation and chemical kinetics analysis mostly focused on
methane-rich flames [21–27] or hydrogen-rich flames [13,28],
and few literatures reported on the combustion mechanism in
the case of hydrogen addition over wide range of hydrogen
fractions. Previous studies in engines and flame showed that
NOx concentration of methane combustion was increased as
hydrogen was added especially at large hydrogen fraction
[29–32]. This is regarded as the increase of combustion
temperature when hydrogen is added. However, the chemical
mechanism of hydrogen addition on NOx formation in pre-
mixed methane–air flames has not well been understood.
The objectives of this study will focus on the chemical
kinetic mechanisms of hydrogen addition on burning velocity
and NO formation in the methane–air flames. Flames with
various hydrogen fractions and equivalence ratios at room
temperature and atmospheric pressure are included. The
study will broaden the understanding of methane–hydrogen–
air mixture flames and provide new guidance to low emission
combustion.
2. Computational methods and chemicalmechanism validation
A freely propagating adiabatic, premixed, unstretched planar
flame was simulated by using PREMIX [17], a steady laminar
one-dimensional flame code. PREMIX uses a hybrid time-
integrating/Newton iteration technique to solve the steady-
state mass, species and energy conservation equations and
can simulate the propagating flame. Equations were solved by
using the TWOPNT, a boundary value problem solver in the
CHEMKIN package [33]. Also built in the CHEMKIN package are
a transport property processor and a gas-phase interpreter
which provide the species transport properties and process
the chemical reaction mechanism.
One of the critical elements for simulation is the proper
reaction mechanism that can describe the essential funda-
mental reaction paths followed by the overall reaction. GRI-
Mech is an optimized detailed chemical reaction mechanism
for the calculation of natural gas chemical reaction process
and the latest version is GRI 3.0 [19]. GRI 3.0 consists of 325
elementary chemical reactions with associated rate coeffi-
cient expressions and thermochemical parameters for 53
species. It includes the detailed combustion reaction mecha-
nism for hydrogen. The ranges of GRI 3.0 are 1000–2500 K in
temperature, 10 Torr to 10 atm in pressure and 0.1–5.0 in
equivalence ratio.
To simulate and interpret the effect of hydrogen addition
on methane–air chemical reactions, the chemical kinetics
mechanism used in the calculation must be capable of the
calculation of pure methane and methane–hydrogen fuel
blends. Prior to the calculation, the GRI 3.0 mechanism needs
to be validated by the experimental results. Ren et al. [21,22]
implemented the GRI-Mech in the PREMIX code and validated
this kinetic mechanism for hydrogen–methane blends by
comparing the computed laminar burning velocities with
those from experiments at low equivalence ratios (0.63–0.73)
and hydrogen mole fractions in the fuel less than 8%. In this
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 3 4 ( 2 0 0 9 ) 6 5 4 5 – 6 5 5 7 6547
study, the validation of reaction mechanism was attempted
for the stoichiometric and rich methane–air mixture as well as
for methane–hydrogen–air mixture with high hydrogen
fraction.
Fig. 1 shows the comparison between the computed
laminar burning velocities by using GRI-Mech 3.0 and experi-
mental results from literature [34]. Good agreement is found
except at high hydrogen fraction, and this is probably related
to the used kinetic scheme. Maximum deviation of 16% is
presented in the stoichiometric hydrogen flame. However,
this deviation will not affect the results and conclusions of the
analysis. The results show that GRI 3.0 can well reproduce the
laminar burning velocity of methane–hydrogen–air mixtures
at stoichiometric mixture flame and rich mixture flame as
well as methane–hydrogen–air flame. GRI 3.0 was also vali-
dated in the calculation of laminar flame speed [21–23,35] and
ignition delay time [36,37] for methane–hydrogen–air flames
in the previous literatures.
In this study, at the upstream boundary, the initial
temperature and initial pressure are set at 303 K and 0.1 MPa,
respectively. Mole fraction of hydrogen in the fuel blends (XH2 )
is defined as,
XH2¼ nH2
nCH4þ nH2
(1)
where nCH4 and nH2are the mole fraction of methane and
hydrogen in the fuel blends, respectively. Hydrogen fractions
in the fuel are varied from 0% to 100% with interval of 10%. The
composition of air is 21% of oxygen and 79% of nitrogen in
volumetrically.
Fig. 2 – Unstretched laminar burning velocity versus
equivalence ratio for methane–air and hydrogen–air
mixtures.
3. Results and discussions
3.1. Unstretched laminar burning velocity
Unstretched laminar burning velocity (ul) is plotted as a func-
tion of equivalence ratio for methane–air and hydrogen–air
mixtures as shown in Fig. 2. In the case of methane–air flame,
Fig. 1 – Comparison between computed burning velocities
with GRI-Mech 3.0 and those from experiment for
methane–hydrogen–air mixtures.
the unstretched laminar burning velocity of fuel-lean
mixtures is increased with the increase of equivalence ratio
and it decreases as the mixtures become fuel-rich. Peak value
of unstretched laminar burning velocity of methane–air
mixture is presented at the equivalence ratio of 1.1 and that of
hydrogen–air mixture is presented at equivalence ratio of 1.8.
The unstretched laminar burning velocities from other liter-
atures obtained by Yu [12], Law [13], Hu [34] and Tanoue [38]
for methane–air mixture and Hu [39], Takahashi [40], Sun [41]
and Dowdy [42] for hydrogen–air mixture are also plotted in
Fig. 1. The results show that the data of the present study
show good agreement with those of literatures.
Unstretched laminar burning velocity versus hydrogen
fraction at three equivalence ratios (f ¼ 0:8, 1.0 and 1.2) are
plotted in Fig. 3. The figure clearly shows that the values of
unstretched laminar burning velocity are below the averaged
values estimated by molar proportions. This reveals the
strong non-linear influence from hydrogen addition on
chemical kinetics. When hydrogen fraction is less than 40%, it
is the regime of methane-dominated combustion. Here
laminar burning velocity is slightly almost linearly increased
with the increase of hydrogen addition. When hydrogen
fraction is larger than 40%, laminar burning velocity shows an
Fig. 3 – Unstretched laminar burning velocity versus
hydrogen fraction at different equivalence ratios.
Table 1 – Summary of variables (0.1 MPa, 303 K).
f A B C D t
0.8 0.28462 0.00229 0.37426 0.00200 15.41042
1.0 0.39836 0.00367 0.49842 0.00812 18.28432
1.2 0.34798 0.00467 0.44489 0.01452 19.47500
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 3 4 ( 2 0 0 9 ) 6 5 4 5 – 6 5 5 76548
exponential increase with the increase of hydrogen fraction.
This is consistent with the phenomenon that engines fuelled
with methane–hydrogen mixtures appear backfire and
knocking due to the rapid increasing burning velocity when
the hydrogen fraction becomes high, especially above 50–60%
[43].
On the basis of the numerical results, the following corre-
lation is proposed:
ul ðm=sÞ ¼ Aþ B� XH2;�0 � XH2
� 40%�; (2)
ulðm=sÞ ¼ Cþ D� exp�XH2
=t�;�40 < XH2
� 100%�: (3)
The values of A, B, C, D and t are tabulated in Table 1.
3.2. Flame structure
As demonstrated in Fig. 1, the measured and calculated
unstretched laminar burning velocities were in reasonably
good agreement. Therefore, the calculation can gain better
understanding of the effect of hydrogen addition on laminar
burning velocity. The approach involved numerical simula-
tions of methane–air flames with various fractions of
hydrogen addition.
Typical predicted structures of plane unstretched
methane–hydrogen–air flames at the stoichiometric condi-
tions are illustrated in Fig. 4. Results in Fig. 4a and b provide
the base line flame structure without hydrogen addition.
Fig. 4b–h shows a similar pattern of flame structure for
mixtures with hydrogen fractions of 40%, 80% and 100%. In
each pair of figures, the left graph gives the temperature
distribution and stable concentrations of reactants and
products (CH4, H2, O2, H2O, CO, CO2), whereas the right graph
shows the concentration profiles of radical species (H, O, OH,
CH3), as function of distance through the flame. Maximum
mole concentrations of H, O and OH radicals are also plotted in
the figure.
In these figures, the consumption of CH4, the formation of
intermediate species CO, and the oxidation of CO to CO2 can
be clearly observed. CO concentration gives its peak value
almost at the same position where CH4 concentration falls to
zero, while CO2 concentration postpones to CO and will
increase as CO is oxidized. Fuel is completely consumed at
1 mm or slight less than 1 mm and main temperature rising
(approximately 70%) will be achieved at this position, and then
gradually reaches the equilibrium condition after this posi-
tion. Actually, complete equilibrium will not reach even at
4 mm. The slow approaching to equilibrium is primarily
a consequence of the dominance of three-body recombination
reactions in this region [44].
In the stoichiometric methane–air flame, the radical OH has
the largest peak concentration in the flame, with H has roughly
10–30% lower than that of OH in peak concentration.
Concentration of O is less than half of H concentration and CH3
gives the lowest concentration. In hydrogen–air flame, the
largest concentration is H radical. In methane–hydrogen–air
flame, the mole fractions of H, O and OH are increased when
hydrogen is added and this will promote the combustion of
methane–air flame. As reported by Kwon and Faeth [45], the
increase of peak concentration of H would lead a correspond-
ing increase of flame laminar burning velocity. For pure
Fig. 4 – Predicted flame structure of premixed stoichiometric methane–hydrogen–air flame.
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 3 4 ( 2 0 0 9 ) 6 5 4 5 – 6 5 5 7 6549
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 3 4 ( 2 0 0 9 ) 6 5 4 5 – 6 5 5 76550
hydrogen–air flame and methane–hydrogen–air flame with
hydrogen fraction of 80%, the increase of O2 concentration
near the cold boundary of the flame prior to O2 decreasing
approaches the reaction zone of the flame. This reflects the
dominate effect from the preferential diffusion of fast-
diffusing reactant, H2, compared to the slow-diffusing reac-
tants, CH4 and O2 [46], resulting in a rapid reduction in H2
concentration as mixture approaches the active reaction zone.
3.3. H and OH radical behaviors
The flame structure in Fig. 4 indicates that H and OH radicals
have the highest concentrations among all radicals in the
premixed methane–hydrogen–air flames. This behavior is due
to the strong correlation between laminar burning velocity
and peak concentration of H in the reaction zone of premixed
flames as observed by Padley [47] and Butler [48]. Based on
flame structure analysis, this correlation is also to be observed
in the premixed flames of this study.
Fig. 5 gives laminar burning velocity and maximum mole
fraction of H, OH and HþOH in the reaction zone for the
methane–air flames. The results show that, comparing with
the maximum H and OH concentrations, the most robust
Fig. 5 – Laminar burning velocities and peak H, OH and
H D OH mole fraction in the reaction zone of methane–air
flames.
correlation between laminar burning velocities and radical
concentrations for laminar premixed methane–air flames is
demonstrated when using maximum mole fraction of HþOH
in the flames. The scattering degree for the correlation of
laminar burning velocity as a function of maximum mole
fraction of HþOH is small in this study. Therefore, these
results demonstrate an existing best-fitting correlation
between laminar burning velocity and maximum mole frac-
tion of HþOH for methane–air flames as following,
ulðm=sÞ ¼ 0:07929þ 22:92321ðXH þ XOHÞmax (4)
where XH and XOH are mole fractions of H and OH in the
reaction zone, respectively.
Fig. 6 shows laminar burning velocity and peak mole
fraction of HþOH at different hydrogen fractions at three
equivalence ratios. Enhancement of chemical reaction with
hydrogen addition is resulted from the increase of H, O and OH
mole fractions as hydrogen is added. Good linear-fit correla-
tion is existed between laminar burning velocity and peak
HþOH mole fraction. Laminar burning velocity is increased
with the increase of peak HþOH mole fraction. The formulas
are as follows,
Fig. 6 – Laminar burning velocity and peak H D OH mole
fraction for different hydrogen fractions at three
equivalence ratios.
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 3 4 ( 2 0 0 9 ) 6 5 4 5 – 6 5 5 7 6551
ulðm=sÞ ¼ �0:16786þ 53:65323ðXH þ XOHÞmax for f ¼ 0:8 (5)
ulðm=sÞ ¼ �0:26365þ 48:21428ðXH þ XOHÞmax for f ¼ 1:0 (6)
ulðm=sÞ ¼ �0:24373þ 44:85874ðXH þ XOHÞmax for f ¼ 1:2 (7)
3.4. Adiabatic flame temperature and Zeldvoich number
Hydrogen addition will increase the laminar burning velocity.
One reason is the increase of H radical concentration as
hydrogen is added, and another is the increase of adiabatic
flame temperature Tad as hydrogen is added. Adiabatic flame
temperature through Arrhenius kinetics exerts an influence
on laminar burning velocity. The clear evidence of such strong
dependence is the close correlation between ul and Tad as
shown in Fig. 7 for methane–air and hydrogen–air flames. In
methane–air mixture, the two factors not only give the same
pattern but they give their peak on the rich side with close
equivalence ratios. At the same Tad, laminar burning velocity
of fuel-lean mixtures and fuel-rich mixtures almost give the
same value except at high rich conditions. However, in
hydrogen–air mixture, this correspondence is offset in which
Tad shows its peak at f ¼ 1:1 while ul gives peaks at f ¼ 1:8, as
shown in Fig. 7b. This sufficiently off-stoichiometric peak of ul
at rich side is a consequence of the highly diffusive nature of
hydrogen. Specially, since ulwffiffiffiffiffiLep
and the freestream Le in
Fig. 7 – Unstretched laminar burning velocity and adiabatic
flame temperature of methane–air and hydrogen–air
mixtures.
sufficiently lean and rich hydrogen–air mixtures are 0.33 and
2.3 respectively. The effect of Le is to reduce ul on the fuel-lean
side but to increase ul on the fuel-rich side, leading to the peak
value of ul toward the rich side. The gradient of ul on the fuel-
lean side shows larger decreasing compared with that on the
fuel-rich side [49].
The temperature profiles of the stoichiometric methane–
hydrogen–air flames are given in Fig. 8a. The figure shows that
at small hydrogen fraction, hydrogen addition has a little
influence on the temperature. The equilibrium adiabatic
flame temperature will increase only 15 K from methane–air
flame to methane–hydrogen–air flame with 40% hydrogen
fraction but another 120 K increase in temperature will reach
when hydrogen fraction is increased from 40% to pure
hydrogen–air flame. The curves of ul and Tad versus hydrogen
fraction also give the same trend as shown in Fig. 8b.
Asymptotic analyses of Zeldovich and co-workers [50]
expressed laminar burning velocity in terms of the square root
of Arrhenius expression in adiabatic flame temperature Tad
with overall activation energy, Ea. Peters and Williams [51]
have derived an asymptotic structure of the flame that intro-
duced the inner layer temperature T0 in fuel consumption.
The inner layer temperature characterizes the balance
between chain-branching reactions and chain-breaking effect
of the fuel consumption and recombination reactions, and is
interpreted as the critical temperature ‘‘at and above which
chemical reactions take place’’ [52]. Peters and Williams [51]
Fig. 8 – Temperature profiles and comparison of ul and Tad
at different hydrogen fractions.
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 3 4 ( 2 0 0 9 ) 6 5 4 5 – 6 5 5 76552
expressed the activation temperature Ea=R in terms of mass
flux(ruul) and adiabatic flame temperature(Tad) as following,
Ea
R¼ �d2½lnðruulÞ�
dð1=TadÞ(8)
where R denotes universal gas constant, and ru, ul are the
unburned gas density and the unstretched laminar burning
velocity, respectively. The differential is evaluated by calcu-
lating ruul at given hydrogen fraction and equivalence ratio,
and by slightly varying its value through the substitution of
a small quantity of nitrogen by inert argon [53].
Integration of Eq. (8) gives the Eq. (9),
2lnðruulÞ ¼ �Ea
R1
Tadþ C (9)
where C is the integration constant.
Therefore, ul can be expressed as,
ul ¼expð0:5CÞ
ru
exp
�� Ea
2R1
Tad
�(10)
The activation temperature Ea=R can be derived from the
linear plot of 2lnðruulÞ against 1=Tad. Fig. 9 gives the variation
of 2ln(ruul) with 1=Tad at different hydrogen fractions for the
stoichiometric flame. The value of Ea=R is decreased with the
increase of hydrogen fraction. C is also the function of
hydrogen fraction. Values of Ea=R and C are tabulated in
Table 2.
As mentioned above, is the inner layer temperature which
represents the crossover temperature between chain-
branching and chain-termination reactions. Within the
temperature profile of a premixed flame, it demonstrates
a transition position from inert preheat zone to reaction zone,
and this is also the position where the second derivative
vanishes and the gradient gives the maximum value. Fig. 10
gives the temperature profiles, the first derivative of temper-
ature and the second derivative of temperature at various
hydrogen fractions. The results show that T0 is decreased with
the increase of hydrogen fraction and this is consistent with
the tendency of overall activation energy (Ea). Based on
Fig. 9 – Variation of 2lnðruulÞ with 1/Tad at different
hydrogen fractions.
fundamental combustion theory [54], the easier occurrence of
combustion reaction of H2–air mixture than that of CH4–air
mixture is due to low activation energy of hydrogen compared
with that of methane. With the increase of hydrogen fraction,
the activation energy of methane–hydrogen mixture is
decreased because the critical temperature or transition
temperature (T0) is decreased. The values of Ea and T0 at
different hydrogen fractions are also tabulated in Table 2.
From the value of Ea=R, the Zeldovich number, Ze can be
calculated [53,55,56] by,
Ze ¼ Ea
RTad � Tu
T2ad
(11)
Zeldovich number (Ze) is a dimensionless form of overall
activation energy. It represents the sensitivity of chemical
reactions to the variation of maximum flame temperature,
and the inverse of it physically denotes an effective dimen-
sionless width of the reaction zone [57]. Fig. 11 gives the
variation of Zeldovich number against hydrogen fraction for
the stoichiometric mixtures. The figure shows that Ze is
largely influenced by hydrogen addition. The increase of
hydrogen fraction results in the decreasing of Ze because of
the decrease of inner flame temperature (T0). This behavior
reflects the controlling influence of the flame temperature
which increases with the increase of XH2. This facilitates the
temperature-sensitive two-body branching reactions relative
to the temperature-insensitive three-body termination reac-
tions [53], leading to overall faster reactions and reducing
overall activation energy with the increase of XH2.
3.5. Effect of hydrogen addition on NO formation
In the combustion of fuels that contain no nitrogen, nitric
oxide (NO) is formed by three chemical mechanisms or routes
that involve nitrogen from the air. They are thermal or Zel-
dovich mechanism, prompt or Fenimore mechanism, and
N2O-intermediate mechanism [54]. Thermal mechanism
dominates in high-temperature combustion over a wide range
of equivalence ratios, while prompt mechanism is particularly
important in fuel-rich combustion. It appears that the N2O-
intermediate mechanism plays an important role in the
production of NO in the high lean and low-temperature
combustion process.
Fig. 12 gives NO mole fraction profiles as a function of
distance at different equivalence ratios. The figure demon-
strates that near the stoichiometric mixture (f ¼ 0:9 and
f ¼ 1:0), formation of nitric oxide is almost completed at the
distance greater than 8 cm from the flame front. Because of
the high flame temperature near the stoichiometric mixtures,
the NO formation is resulted from the Thermal NO mecha-
nism. While in fuel-rich flames (f ¼ 1:2 and f ¼ 1:3), there are
a lot of CH radicals in the flame front and the flame temper-
ature is lower, so NO formation is resulted from the Fenimore
prompt-NO mechanism which occurs within the flame front.
At fuel-lean premixed combustion (f ¼ 0:6 andf ¼ 0:7), NO
formation is also almost completed at the distance of 8 cm
from the flame front. NO is generated via N2O mechanism,
which is analogous to the thermal mechanism in that O-atom
attacks molecular nitrogen. This reaction has been often over-
estimated since it is usually an insignificant contributor to the
Table 2 – Summary of calculation conditions (0.1 MPa, 303 K, f[1:0).
XH2 /% Tad/K T0/K ul/m/s Ea/R/K C Ea/kJ/mol Ze
0 2230 1332 0.406 35,183 14.191 292.46 13.63
20 2236 1308 0.463 34,491 14.034 286.71 13.34
40 2245 1245 0.555 34,255 14.144 284.74 13.20
60 2259 1188 0.725 33,404 14.131 277.67 12.80
80 2289 1130 1.147 31,300 13.734 260.18 11.86
100 2365 764 2.432 22,846 11.009 189.90 8.42
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 3 4 ( 2 0 0 9 ) 6 5 4 5 – 6 5 5 7 6553
total NO. However, fuel-lean conditions can suppress the
formation of CH, and hence lead to less Fenimore NO while
low temperatures can suppress the Zeldovich NO. What
remains is NO generated via N2O. All of these circumstances
lead to the N2O route to be the major source of NO in fuel-lean
premixed combustion [58].
Fig. 13 shows the prediction of NO concentrations at 10 cm
downstream from flame front as a function of equivalence
ratio for methane–air flames. The dependence of NO
concentration as a function of equivalence ratio gives two
peak values, in the stoichiometric mixtures due to the Zel-
dovich thermal-NO mechanism and in fuel-rich mixture at
equivalence ratio around 1.3 due to the Fenimore prompt-NO
mechanism, and this is consistent with the results in the
literatures [59,60].
Fig. 14 gives NO mole fraction profiles of methane–
hydrogen–air mixtures at the stoichiometric and rich flames.
In the case of f ¼ 1:0, formation of NO is almost completed at
the distance of 8 cm from the flame front due to the Zeldovich
thermal-NO mechanism. While in the rich flames, the most of
NO is formed within the flame front due to the Fenimore
prompt-NO mechanism. NO is not increased with hydrogen
Fig. 10 – Temperature profiles at different hydrogen
fractions.
addition in both the stoichiometric flames or the rich flames
seem to be some counterintuitive. Usually, one may easily
think that with the addition of hydrogen which itself has high
adiabatic flame temperature, mixtures of methane and
hydrogen will burn at higher temperature than pure methane,
and lead to increasing in NO.
Fig. 15 presents NO mole fraction at 10 cm downstream
from flame front in methane–hydrogen–air flames with
different hydrogen fractions. The figure shows little variance
of NO concentration in the stoichiometric flames with
different hydrogen fractions. As discussed above, the adia-
batic temperature at 10 cm downstream from flame front
gives little variation with the increase of hydrogen fraction.
The same behavior of small temperature variation with
hydrogen addition was found by Gauducheau et al. [26] in
their study of premixed flames.
It is obviously seen that the behavior of premixed but
stretched flames of methane–hydrogen–air mixtures differs
significantly from that of unstretched flames studied in this
study. It was observed experimentally and confirmed
computationally that if the equivalence ratio is kept constant,
the addition of hydrogen to the methane–air flames in a stag-
nation flow, or on the cylinder burner, would increase flame
temperatures significantly [21,32,61]. This leads eventually to
the increase of the NO emissions in stretched flames [32]. This
difference between stretched flame and unstretched flames
can be explained from the preferential diffusion of atomic and
molecular hydrogen, as being analyzed in the literatures [61]
and [62].
Fig. 11 – Zeldovich number versus hydrogen fraction.
Fig. 12 – NO mole fraction of methane–air flame versus
distance at different equivalence ratios.
Fig. 14 – NO mole fraction of methane–hydrogen–air
flames.
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 3 4 ( 2 0 0 9 ) 6 5 4 5 – 6 5 5 76554
In fuel-rich mixtures (f ¼ 1:3), NO concentration at 10 cm
downstream from flame front is notably decreased with
hydrogen addition. This decrease is apparently due to the
reduced availability of hydrocarbon radicals, which are major
precursors of the Fenimore prompt-NO mechanism.
Considering different responses of stretched and
unstretched flames of both the stoichiometric and rich
mixtures with respect to enrichment by hydrogen, it can be
assumed that, in the real burners, an overall change of NO
emissions is defined not only by mixing mode (premixed or
diffusion flames) but also by turbulent intensity. When the
structure of the real flame approaches the quasi-laminar case,
Fig. 13 – NO mole fraction of methane–air flame at 10 cm
downstream of flame front versus equivalence ratio.
little change or decrease of NO formation with enrichment by
hydrogen is presented. This behavior is supported by the
experimental study of hydrogen addition to methane in
different burners [63–65].
Fig. 15 – NO mole fraction at 10 cm downstream from flame
front versus hydrogen fraction.
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 3 4 ( 2 0 0 9 ) 6 5 4 5 – 6 5 5 7 6555
4. Conclusions
Numerical study on laminar premixed methane–hydrogen–air
flames was conducted at room temperature and atmospheric
pressure. The unstretched laminar burning velocity, adiabatic
flame temperature, and radical mole fraction of H, OH and NO
are obtained at various hydrogen fractions and equivalence
ratios. The main conclusions are summarized as follows.
Laminar burning velocity is increased with the increase of
hydrogen fraction. In the case of hydrogen fraction less than
40%, it is the regime of methane-dominated combustion. Here
laminar burning velocity is slightly increased with the
increase of hydrogen addition. When hydrogen fraction is
larger than 40%, laminar burning velocity is exponentially
increased with the increase of hydrogen fraction.
Enhancement of chemical reaction with hydrogen addition
is due to the increase of H, O and OH concentrations in the
flame as hydrogen is added. A strong correlation exists
between burning velocity and peak radical concentrations of
HþOH radicals in the reaction zone of premixed flames. High
burning velocity corresponds to high radical concentration in
the reaction zone.
With the increase of hydrogen fraction, the adiabatic flame
temperature is increased. The overall activation energy of
methane–hydrogen mixture is decreased, and the inner layer
temperature and Zeldovich number are also decreased. This
behavior reflects the controlling influence of the flame
temperature which is increased with the increase of XH2 . This
facilitates the temperature-sensitive two-body branching
reactions relative to the temperature-insensitive three-body
termination reactions.
The curve of NO versus equivalence ratio shows two peaks,
presenting at the stoichiometric mixtures due to the Zeldo-
vich thermal-NO mechanism and at fuel-rich mixtures due to
the Fenimore prompt-NO mechanism. In the stoichiometric
flame, hydrogen addition has little effect on NO, while in the
fuel-rich flames, NO concentration decreased significantly.
The behavior of premixed but unstretched flames of
methane–hydrogen–air mixtures differs significantly from
that of stretched flames.
Acknowledgments
This study is supported by National Basic Research Program of
China (Grant No. 2007CB210006) and National Natural Science
Foundation of China (Grant Nos. 50636040 and 50821604).
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