Chemical Engineering Journal xxx (2012) xxx–xxx
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Chemical Engineering Journal
journal homepage: www.elsevier.com/locate/cej
Experimental study and kinetics modeling of partial oxidation reactions in heavily
sooting laminar premixed methane flames
Qingxun Li, Tiefeng Wang ⇑, Yefei Liu, Dezheng Wang
Beijing Key Laboratory of Green Reaction Engineering and Technology, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China
h i g h l i g h t s
" Temperature and concentrations of heavily sooting methane flames were measured.
" Detailed chemistry mechanisms were used to simulate this process.
" Curran and Wang-Frenklach mechanisms gave reasonable predictions.
" Recombination and oxidation reactions were responsible for acetylene depletion.
a r t i c l e
i n f o
Article history:
Available online xxxx
Keywords:
Partial oxidation
Kinetics modeling
Soot formation
Acetylene reactions
Fuel rich methane flame
a b s t r a c t
The partial oxidation (POX) of methane in heavily sooting laminar premixed methane/oxygen flames was
studied with an emphasis on acetylene formation and depletion. The flame temperature profiles were
measured with a Pt/Pt–Rh thermocouple coated with Y2O3–BeO ceramic. Gas species along the flame axis
were sampled by a quartz probe for their concentrations to be measured by a mass spectrometer. The
problem of soot deposition on the sampling probe was overcome by in situ cleaning of the nozzle orifice.
The mole ratios of O2/CH4 in the experiments were 0.55, 0.60, 0.65 and the STP (standard temperature
and pressure) reactant flow velocity was fixed at 4 cm/s. Computational results based on the Curran,
Wang–Frenklach and GRI 3.0 detailed chemistry mechanisms were compared with the experimental
results. The values predicted by the Curran and Wang-Frenklach mechanisms for the reaction conditions
of this study were within the acceptable range. The maximum concentrations of acetylene were positioned in the flame area at 4–8 mm distance from the burner, and were behind the positions of the maximum mole fractions of ethane and ethylene. Much more diacetylene and benzene were generated in the
post-flame area than in the flame. Recombination reactions to larger hydrocarbon molecules and oxidation with hydroxyl radicals in the post-flame region were the main reactions responsible for acetylene
depletion in the fuel rich methane flame.
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
As crude oil reserves decrease in the coming years and advanced gas exploration techniques that are developed and applied
discover new gas reservoirs such as shale gas fields [1–3], natural
gas and shale gas will become increasingly important in the energy
and chemical supplies of the future. Acetylene (C2H2) is an important hydrocarbon used in chemical manufacture. Among the various acetylene production processes, the partial oxidation (POX)
of natural gas has been an important acetylene production process
since the 1950s [4–6]. The process is catalyst free, thus long term
stable production is guaranteed. The recent development of using
the co-production of synthesis gas (H2/CO) with it has enhanced
⇑ Corresponding author. Tel.: +86 10 62794132; fax: +86 10 62772051.
E-mail address: [email protected] (T. Wang).
its economic advantage over the polluting calcium carbide production process. When natural gas becomes the main chemical feedstock, the POX process can be the main route to produce
acetylene [7].
The partial oxidation process produces acetylene by burning
hydrocarbons with an insufficient supply of oxygen that is just enough to provide the energy for the pyrolysis of the remaining
hydrocarbons. The reactor for acetylene production is operated under non-equilibrium conditions since carbon and not acetylene is
the main product at equilibrium. The industrial reactor is
quenched at the position where acetylene has its maximum concentration, otherwise, acetylene will be depleted by recombination
and pyrolysis reactions. Therefore, for reactor design and operation, it is critical to know the axial profile of the C2H2 concentrations. The fuel rich sooting flame comprises several thousand
reactions of several hundred species in a complex chemical mech-
1385-8947/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cej.2012.06.093
Please cite this article in press as: Q. Li et al., Experimental study and kinetics modeling of partial oxidation reactions in heavily sooting laminar premixed
methane flames, Chem. Eng. J. (2012), http://dx.doi.org/10.1016/j.cej.2012.06.093
2
Q. Li et al. / Chemical Engineering Journal xxx (2012) xxx–xxx
anism. Understanding their chemical kinetics is essential to optimizing reactor performance and increasing acetylene yield in the
POX reactor. In an early study, Basevich et al. [8] investigated the
kinetics of acetylene formation during the combustion of methane/oxygen mixtures and pointed out that it is necessary to refine
the chemical mechanism before a sufficiently accurate quantitative
description of acetylene formation is possible. Although much
more is now known about the formation mechanism of acetylene
in combustion systems, a deeper understanding of acetylene formation flame chemistry is still essential for reactor design and
the specification of the optimal operating conditions.
The experimental study and kinetic modeling of the POX process is very challenging because its flame characteristics include
the formation of polycyclic aromatic hydrocarbons (PAHs) and
soot, which is an umbrella term that covers many complex particulate species. This has made a quantitative description of acetylene
production difficult. In addition, turbulent reacting flows increase
the complexity due to the coupling of turbulent flow and chemical
reactions. Most of the studies on methane fuel-rich premixed
flames have been confined to non-sooting or lightly sooting flames
[9–17], since even small soot concentrations rapidly clog sample
probe orifices. Moreover, soot deposition can cause a large increase
in the error in temperature measurement [18,19]. Also, with heavily sooting flames that are nearly opaque, optical techniques cannot be used in the flame measurements [19]. These problems and
the overall scarcity of measurements have left significant gaps in
our knowledge of the chemical species and their distributions in
heavily sooting premixed flames. The absence of experimental data
has hindered the development of the kinetics of acetylene formation for the conditions used in the industrial POX process and hindered the evaluation of whether current chemical mechanisms can
be extrapolated to those conditions. A new experimental study
should prove useful in elucidating the mechanism of acetylene formation and reactions. The significance of this work is that it provides a basis for using fundamental data and mechanisms in the
literature for the special conditions specific to acetylene
production.
Most studies reported in the literature used a reactant mixture
that was highly diluted with Ar and acetylene was not studied as
the target product, and therefore the condition for the maximum
production of acetylene cannot be easily derived from literature
data. In this paper we report the measurement of the distributions
of the temperatures and species concentrations in atmospheric
pressure laminar methane/oxygen premixed flames with no dilution, and showed how the maximum acetylene yield depended
on the O2/CH4 ratio and distance from the burner (reaction time).
Laminar flow conditions were used to decouple the turbulent flow
from the flame chemistry so that only the effects of the reaction
and molecular transport on the distributions of the temperature
and species concentrations had to be considered in the mathematical analysis. Gas samples were extracted from the flame with a
quartz probe and analyzed online by an electron impact quadrupole mass spectrometer (MS). McEnally and Pfefferle [18] had
developed a self-cleaning probe for sampling heavily particle-laden gases, and we used a modified version of this to enable the
measurement of species concentrations throughout the flame despite the presence of local dense soot concentrations. Some popular reaction mechanisms [20–22] were evaluated for their ability to
model the process of acetylene production by comparing their simulations with our experiments. This was considered useful because
the different mechanisms were developed for different reactant
systems, and it was not clear that they would be appropriate for
use for C2H2 production, that is, although most current CH4 models
can predict well the major species in fuel lean methane flames, it
was not clear that they can do so also for fuel rich methane flames.
2. Experimental methods
2.1. Burner assembly
Measurements were carried out using a burner with a methane/
oxygen flame stabilized downstream of a brass honeycomb element at the burner exit. The setup is shown in Fig. 1. The honeycomb comprised 0.6 mm cells with a length of 5 mm. The burner
was 100 mm long and had a 45 mm diameter port for the reactant
mixture flow. An inert gas cocurrent flow shroud was used to keep
the flame from the room air. This used the effect of buoyancy to
keep the flame free from air disturbances. All the flames were
heavily sooting and gave off a yellow–orange luminosity in the
Soot
cleaning
Positioning
System
Flame
Stabilizer
Probe
Flame
Burner
Valve
Mass
Spectormeter
Flowmeter
Molecularturbo
Pump
Fuel
Oxidizer
Mechanical
Pump
Fig. 1. Schematic of the experimental setup.
Please cite this article in press as: Q. Li et al., Experimental study and kinetics modeling of partial oxidation reactions in heavily sooting laminar premixed
methane flames, Chem. Eng. J. (2012), http://dx.doi.org/10.1016/j.cej.2012.06.093
3
Q. Li et al. / Chemical Engineering Journal xxx (2012) xxx–xxx
soot formation region. The reactant mixture velocity used was relatively large so that the flame was stabilized just beyond the honeycomb and the burner can be operated without the need for
cooling. The flame was stabilized by a stagnation plate with a
20 mm diameter hole centered on the flame axis and placed
40 mm above the burner exit.
The flow rates of methane and oxygen were controlled and
measured by mass flow controllers. Gas purities were methane,
99.5% and oxygen, 99.9%. The reactant flows were fed into a manifold where they passed through a mixer specially designed to give
a uniform mixture at the burner inlet. The gas temperatures and
concentrations of detectable gas species were measured along
the axis of the flame in the region between the burner exit and stabilizing plate. In order to have accurate axial positioning for the
temperature and concentration measurements, a positioning system was used to move, respectively, the sampling probe and thermocouple into the required sampling position.
2.2. Temperature measurements
Gas temperatures were measured with a 400 lm diameter Pt/
Pt–Rh thermocouple coated with Y2O3–BeO ceramic. The thermocouple was inserted vertically into the flame to minimize the disturbance to the temperature field along the flame centerline. The
thermocouple voltage was amplified and read by a personal computer-based A/D board, and the temperature was obtained from
tabulated relationships between voltage and temperature. Soot
rapidly deposited onto the junction in the flame zone and postflame zone, which increased its diameter and emissivity, and to
counter this, the method developed by McEnally and Pfefferle
[18] was used to clean deposited soot before the junction was
moved to the next measurement location for each measured point.
Heat conduction away from the junction along the wires was
considered negligible compared to radiation loss, which caused
the thermocouple junction temperature to be different from the
actual gas temperature. Radiation loss was taken to be the main
source of temperature measurement error. There is much uncertainty in the temperature measurement when soot is formed on
the thermocouple junction, which can cause large errors. In much
of the literature, the uncertainty of the measured temperature was
estimated to be ±100 K [23]. In this work, the junction temperature
was corrected using Eq. (1) which was based on a monograph recommendation [24]. This method was also used by Slavinskaya and
Frank [25]. The junction temperature was correlated to the gas
temperature by an energy balance at the junction that equated
the heat gained by transfer from the gas through a boundary film
and heat loss by radiation, which gave
Tg ¼ Tt
er
Nuk
T 3t þ 1 ; with a ¼
d
a
2.3. Gas sampling and analysis
The compositions of the combustion gas at different axial positions were sampled online with a probe and analyzed by a MS. The
sample probe that extracted gas samples from the flame was made
from a 9 mm od, 7 mm id quartz tube with a 500 lm diameter orifice at the probe tip. Fig. 1 schematically illustrates the vibrating
platinum wire mechanism that prevented soot from clogging the
orifice. One end of a 200 lm diameter platinum wire, which was
coated with high temperature cement and aligned along the probe
centerline, was extended through the orifice, while the other end
was attached to a piece of elastic rubber band of 1.6 mm diameter.
The wire extended roughly 2 mm beyond the tip and almost filled
the orifice so that as it vibrated, the wire grinded away soot deposits on the orifice and slowed their accumulation. The sampling
cone was specially designed with a 40° angle since Biordi et al.
[26] reported that this would reduce the disturbance to the flow
field.
The gas chemical species were sampled through the annular
space between the wire and nozzle at the probe tip. The residence
time in the cone was less than 0.001 ms, since it was estimated
that the flow in the probe rapidly reached sonic velocity using
the method discussed in Roth [27]. Thus little C2H2 would be consumed in the sampling probe. The pressure in the probe tube was
lower than 103 Pa and except for a very short section in the flame,
its temperature was several hundred degrees lower than the flame
temperatures, so the reactions were quenched in the probe and
there was no depletion of C2H2. The probe walls did not need to
be cooled to prevent further reaction of the sampled gas inside
the probe because the high speed of the gas allowed only a very
short residence time in the hot region of the probe. All radicals
were annihilated on the wall of the probe and only stable species
concentrations could be measured. The measured major species
were CH4, O2, H2, H2O, CO, CO2 and C2H2, and minor species were
C2H4, C2H6, C4H2 and C6H6. The concentration of a gas was the measured intensity of the peak divided by the response coefficient. The
response coefficients were obtained by calibration experiments
that used mixtures with known compositions using Ar as a reference species. The mole fractions were obtained by normalizing
where the sum of species concentrations was one. The maximum
relative error that occurred in the low concentration range was
10% for major species and 20% for minor species.
ð1Þ
2.4. Operating conditions
where Tt was the thermocouple junction temperature, Tg was the
gas temperature, e was the emissivity of the wire, r was the Stephen–Boltzmann constant, k was the thermal conductivity of the
gas, and Nu was the Nusselt number based on the welded point
diameter d. The manufacturer gave the emissivity of the thermocouple as 0.1. Assuming the welded junction of the thermocouple
was a spherical bead in a flowing stream, the Nusselt number was
Nu ¼ 2:0 þ 0:03Re0:54 Pr0:33 þ 0:35Re0:58 Pr0:36
sivity of 0.95, which is the value characteristic of soot. Slavinskaya
and Frank [25] pointed out that the temperature correction in their
study could exceed 200 K, which would have a large impact on the
calculation of reaction rates.
ð2Þ
with the Reynolds and Prandtl numbers, Re and Pr, calculated from
the local gas composition [24]. In a soot-free flame region, the
parameters in Eq. (1) were easily obtained. However, in a soot-containing region, soot rapidly deposited on the junction, which would
increase the diameter and emissivity, and consequently depressed
the measured temperature. The thermocouple measurements were
corrected for the soot layer on the thermocouple by using the emis-
The standard temperature and pressure (STP) flow speed of the
cold premixed methane/oxygen mixture was fixed at 4 cm/s. Mixtures having three molar O2/CH4 ratios in the range 0.55–0.65, as
Table 1
Summary of reaction parametersa.
RO2/CH4
Fuel-equivalence ratio
Methane flow rate(mg/s)
Oxygen flow rate(mg/s)
Reactant composition (% CH4 by volume)
0.55
3.64
29.30
32.23
64.52
0.60
3.33
28.39
34.06
62.50
0.65
3.08
27.53
35.78
60.61
a
Atmospheric pressure premixed methane/oxygen flames stabilized on a 45 mm
inside diameter honeycomb burner (0.6 mm cell size) with a shroud with cocurrent
Ar flow.
Please cite this article in press as: Q. Li et al., Experimental study and kinetics modeling of partial oxidation reactions in heavily sooting laminar premixed
methane flames, Chem. Eng. J. (2012), http://dx.doi.org/10.1016/j.cej.2012.06.093
Q. Li et al. / Chemical Engineering Journal xxx (2012) xxx–xxx
summarized in Table 1, were used. Their flames roughly corresponded to premixed industrial flames that have O2/CH4 ratios in
the range 0.51–0.58 [5]. The flow speed in the inert gas cocurrent
flow shroud was varied accordingly with that of the reactant flow.
The flow speed of the heated reactant mixture was about half of
the speed of the high temperature products, which was two to five
times that of the low temperature reactant mixture.
3. Computational methods
Steady state, laminar, one-dimensional premixed flame simulations were carried out using CHEMKIN 4.1.1 [28]. To simulate the
chemical structure of laminar premixed flames from which there
was significant heat loss by radiation, the experimental temperature profiles were used as input to the simulations. A computational grid of over 300 points was used. It was verified that this
was sufficient by increasing the grid points to over 600 points
and seeing that the results had less than 1% difference. Mixtureaveraged transport properties were used for the multicomponent
mixtures.
In fuel rich flames, heat radiation cannot be ignored and an energy balance that does not include radiation loss would be erroneous, especially in the post-flame region where there is much
radiation heat loss because there is a lot of soot. Due to this, we
chose the ‘‘fixed temperature’’ model for the simulation calculations, and so the accuracy of the temperature was very important.
Flame properties were calculated with three detailed chemistry
mechanisms, namely, the Curran, GRI 3.0 and Wang-Frenklach
mechanisms listed in Table 2, to see how well these could fit the
data. The thermodynamic and transport data used were those provided on the web site for the mechanism. The Curran mechanism
[20] has been used to describe acetylene formation and depletion
in our previous work [7]. The GRI 3.0 mechanism was used with
thermodynamic and transport data taken from Smith et al. [21].
Although GRI 3.0 does not include soot formation chemistry, it
was still considered for the prediction of C2H2 concentrations because a wide range of equivalence ratios from 0.1 to 5 had been
used in its validation. The third mechanism used, the Wang–Frenklach mechanism [22], has been used for the oxidation of methane,
ethane, ethylene and acetylene at different flame temperatures,
and an attractive feature in it is that aromatic species chemistry
up to the formation of pyrene has been included. It is based on
GRI-Mech 1.2, and was extended by a consistent set of rate coefficients, thermodynamic data, and transport data for the reactions of
aromatics by Wang and Frenklach [22]. The mechanism has been
tested against a number of literature reports on laminar premixed
flames of acetylene and ethylene, and the measured species profiles of C1, C2 and one ring aromatic compounds were reproduced
very well. The Wang-Frenklach mechanism had since been updated several times [29], but the acetylene reactions were not significantly affected.
4. Results and discussion
4.1. Flame temperature
The experimental temperature profiles of the flames are shown
in Fig. 2. These were the values obtained after the correction of the
Table 2
Summary of detailed chemistry natural gas oxidation mechanisms considered.
Mechanism
Num species
Num reactions
Published time
Ref.
Curran
GRI 3.0
Wang-Frenklach
289
53
99
1580
325
527
2008
1999
1997
[20]
[21]
[22]
1800
Temperature (K)
4
1500
1200
RO2/CH4
0.55
0.60
0.65
900
600
0
10
20
Distance from burner (mm)
30
Fig. 2. Measured axial flame temperatures for different operating conditions.
thermocouple junction temperature for radiation loss and soot
deposition. The temperature corrections were less than 100 K,
but which still made an obvious difference in the calculation of
the species profiles. In Figs. 2–5, curves have been drawn through
the data points to indicate the experimental trends. In Fig. 2, the
flame temperature rose steeply to a maximum temperature due
to the exothermic oxidation reactions, and then decreased in the
post-flame zone due to the endothermic pyrolysis reactions and
radiation heat loss from soot. The presence of soot particles in
the flames was responsible for significant radiation heat loss, with
values of 10 K, 50 K, 80 K in the preheating zone, flame zone, and
post-flame zone, respectively. Endothermic pyrolysis reactions
and radiation heat losses caused the flame temperatures, illustrated in Fig. 2, to decrease from 1700 – 1800 K at 5 mm distance
to 1500 – 1600 K at 30 mm distance. Temperatures were higher
with larger values of the O2/CH4 ratio because there was more heat
of combustion and less soot radiation. The flames in this work have
larger temperature increases and a more reactive environment because there was no diluent. In the post-flame region, the flame
temperatures were not sensitive to the changes in O2/CH4 ratios.
For a flame with a higher O2/CH4 ratio, the laminar flame propagation velocity increased and the flame front moved nearer to the
burner exit, and accordingly, the maximum flame temperature position was nearer the burner exit.
4.2. Gas species concentrations
The measurement errors for the experimental data sets are
shown by how reproducible the data points were for experiments
performed on successive days (labeled Series 1, 2, and 3 in Fig. 3).
Most of the data points had good reproducibility and they can be
seen to almost overlap on the plots, but there were much larger
measurement uncertainties at very low concentrations (roughly
mole fractions <0.01) where these were estimated to be 10% for
major species and 20% for minor species. The major species were
H2, CH4, H2O, C2H2, CO, O2, CO2, and the minor species were
C2H6, C2H4, C4H2, C6H6. The curves shown in the figures were
drawn using average values from three repeated experiments that
gave data points that would almost overlap on the plots.
The measured concentrations of major and minor gas species
along the axis of the flames with O2/CH4 = 0.55, 0.60 and 0.65 are
plotted as a function of the distance from the burner in Figs. 4
and 5. Methane and oxygen concentrations decreased with distance from the burner for the three O2/CH4 ratios, but large composition changes were confined to only the lower regions of the
flames. In the oxidation region, all the oxygen was consumed.
Unreacted methane was still present in the post-flame zone. As
the O2/CH4 ratio was increased, the concentration of residual
Please cite this article in press as: Q. Li et al., Experimental study and kinetics modeling of partial oxidation reactions in heavily sooting laminar premixed
methane flames, Chem. Eng. J. (2012), http://dx.doi.org/10.1016/j.cej.2012.06.093
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Q. Li et al. / Chemical Engineering Journal xxx (2012) xxx–xxx
0.05
0.05
(b) RO2/CH4=0.60
(a) RO2/CH4=0.55
t=29.8ms
0.03
0.02
Series No.
1
2
3
Average
C2H2 mole fraction
0.04
C2H2 mole fraction
cC2H2=4.54%
cC2H2=4.10%
0.04
t=21.8ms
Series No.
1
2
3
Average
0.03
0.02
Maximum relative uncertainty=6.68%
0.01
Maximum relative uncertainty=5.32%
0.01
0
0.05
10
20
Distance from burner (mm)
30
0
10
20
Distance from burner (mm)
0.0010
(c) RO2/CH4=0.65
(d) RO2/CH4=0.60
0.0008
0.04
t=15.0ms
Series No.
1
2
3
Average
0.03
0.02
Maximum relative uncertainty=2.05%
10
20
Distance from burner (mm)
30
Series No.
1
2
3
Average
0.0006
0.0004
Maximum relative uncertainty=14.17%
0.0002
0.01
0
C6H6 mole fraction
C2H2 mole fraction
cC2H2=4.43%
30
0
10
20
Distance from burner (mm)
30
Fig. 3. Measurement fluctuations of the mole fractions of major species and minor species, (a–c) C2H2 (d) C6H6, for three replicate measurements.
methane decreased, which was attributable to the methane oxidation and pyrolysis reactions. At a higher O2/CH4 ratio, there was
more oxygen for methane oxidation and the larger heat release
at the same time gave more methane pyrolysis (thermal decomposition). In the post-flame region, since there were no oxygen and
lower temperatures, the mole fractions of residual methane decreased very slightly, which indicated that the conversion of residual methane there was slow.
The partial combustion of hydrocarbons in a flame is an important method to produce acetylene and synthesis gas (syngas). The
coproduction of syngas is an important part of the POX process because it gives more complete utilization of the combustion products. Syngas is composed of hydrogen (H2) and carbon monoxide
(CO), and is a feedstock for the production of bulk chemicals (acetic
acid, methanol, DME, isocyanates, ammonia) and synthetic fuels
[30]. The H2 to CO ratio in the syngas is important in its downstream application, e.g., syngas for synthetic fuel production by
the Fisher–Tropsch process should have a H2 to CO molar ratio
slightly larger than 2 [31]. In the rich flames in this work, the H2
to CO ratio was a little larger than 2 at the higher O2/CH4 ratio
due to the hydrogen produced from methane thermal decomposition. The results for this ratio and three other calculated ratios are
listed in Table 3. The concentration of carbon dioxide increased
gradually with increasing oxygen ratio, which showed that the
O2/CH4 ratio was an essential factor in optimal syngas production
because complete oxidation reactions increased with an increased
oxygen concentration. Water and carbon monoxide concentrations
decreased after reaching a maximum value, which indicated the
occurrence of the water gas shift reaction at the high temperature.
Acetylene (C2H2) is an important intermediate species in combustion chemistry. Fig. 4 shows that the measured acetylene concentrations along the flame axis showed rise-decay profiles at
4–8 mm distance from the burner in the flame area, and their maxima were behind the positions of the maximum mole fractions of
ethane and ethylene. Diacetylene and benzene were generated in
the post-flame area. Acetylene was produced in the flame where
it accumulated and increased in concentration in the oxidation region and it decreased in concentration in the post-flame zones. Alfè
et al. reported that the maximum mole fraction of acetylene approached 5.5% for the lower O2/CH4 ratios that they used [17].
The source of acetylene was methane thermal decomposition.
The overall reactions in the pyrolysis of methane at high temperature are the stepwise dehydrogenation reactions:
CH4 ! C2 H6 ! C2 H4 ! C2 H2 ! C4 H2 ; C6 H6 ! C
ð3Þ
The free radical mechanism and the reaction parameters for the
primary reactions are now well defined but the details of the later
consecutive reactions that depleted acetylene and the formation of
soot are not yet fully understood.
In most existing reaction networks [20–22,32,33], the main
C2H2 formation channels are the recombination of two C1 radicals
and H abstraction from C2H3, while the consumption of C2H2 is
mainly by C2H2 oxidation, C2H3 formation, and C2H2 recombination
to form higher hydrocarbon radicals like C3H4, C3H3 and C4H2. C2H2
oxidation occurs by O atom, oxygen molecule and OH radical attack. Calculations showed that the C2H2 mole fraction in the
post-flame was flat because the temperature used was relatively
low and there was 10% unburned CH4 which through pyrolysis
reactions compensated for the depletion reactions of C2H2, that
is, that it was flat was not because there was little influence of
the temperature. The results reported by Gersen et al. [11] showed
that the decrease of C2H2 concentration was more significant in the
post-flame zone at a higher temperature.
Please cite this article in press as: Q. Li et al., Experimental study and kinetics modeling of partial oxidation reactions in heavily sooting laminar premixed
methane flames, Chem. Eng. J. (2012), http://dx.doi.org/10.1016/j.cej.2012.06.093
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Q. Li et al. / Chemical Engineering Journal xxx (2012) xxx–xxx
0.10
0.5
(a) RO2/CH4=0.55
(a) RO2/CH4=0.55
0.08
H2
0.3
H2O
CO
CH4
0.2
Mole fraction
Mole fraction
0.4
0.06
C6H6x100
C4H2x50
C 2H 2
C2H4x5
0.04
0.02
0.1
CO2
O2
0.0
0
10
20
C2H6x20
0.00
0
30
10
20
30
Distance from burner (mm)
40
Distance from burner (mm)
0.10
0.5
(b) RO2/CH4=0.60
(b) RO2/CH4=0.60
0.08
H2
0.3
H2O
0.2
CO
0.1
Mole fraction
Mole fraction
0.4
C6H6x100
0.04
C4H2x50
C 2H 2
C2H4x5
0.02
CH4
CO2
O2
0.0
0.06
C2H6x20
0.00
0
0
10
20
30
10
20
30
Distance from burner (mm)
40
Distance from burner (mm)
0.10
(c) RO2/CH4=0.65
0.5
(c) RO2/CH4=0.65
0.08
H2
0.3
H2O
0.2
CO
Mole fraction
Mole fraction
0.4
0.06
C6H6x100
C4H2x50
0.04
C 2 H2
0.02
0.1
CH4
CO2
O2
0.0
C2H4x5
C2H6x20
0.00
0
0
10
20
30
Distance from burner (mm)
Fig. 4. Measured concentration profiles of reactants and main products in the
methane flame (RO2/CH4 = 0.55, 0.60, 0.65).
In the flame zone, the main consumption pathways were oxidation reactions. In the post-flame zone, recombination reactions
were also responsible for the depletion of acetylene. Acetylene reacts with C2 and C4 radicals to form larger molecules like C4H2 and
C6H6. That there were contributions by these reactions was supported by the following results: (1) further away from the burner,
the H/C ratio of all smaller molecules increased, which indicated
some carbon loss to form large PAH molecules and soot, and (2)
the maximums for the mole fractions of C4H2 and C6H6 were where
the maximum mole fraction of C2H2 was, which suggested there
was consumption of C2H2 by recombination reactions to larger
hydrocarbon molecules in the post-flame region. In the industrial
process, the depletion reactions of acetylene must be quenched
in order to get a high yield of acetylene, and the knowledge of
where to quench is important.
10
20
30
Distance from burner (mm)
40
Fig. 5. Measured concentration profiles of intermediate species in the methane
flame (RO2/CH4 = 0.55, 0.60, 0.65).
Table 3
Measured and calculated H2/CO ratios.
RO2/CH4
Experiment
Curran
GRI 3.0
Wang-Frenklach
0.55
1.97
1.67
1.70
1.78
0.60
1.95
1.79
1.81
1.86
0.65
2.05
1.88
1.96
1.95
4.3. Effect of O2/CH4 ratio on major gas species concentrations
The measurements showed that the O2/CH4 ratio has a large
influence on the concentrations of the major species, especially
acetylene and soot. Soot growth and nucleation rates measured
here were larger than those reported with less fuel rich mixtures.
The underlying chemical mechanism in sooting flames is of much
Please cite this article in press as: Q. Li et al., Experimental study and kinetics modeling of partial oxidation reactions in heavily sooting laminar premixed
methane flames, Chem. Eng. J. (2012), http://dx.doi.org/10.1016/j.cej.2012.06.093
7
Q. Li et al. / Chemical Engineering Journal xxx (2012) xxx–xxx
Mole fraction of C2H2
0.08
(a) RO2/CH4=0.55
0.06
0.04
Experiment
Curran
GRI 3.0
Wang-Frenklach
0.02
0.00
0
0.08
10
20
Distance from burner (mm)
30
interest. The measured and calculated concentrations of acetylene
in the flames in this work are plotted as a function of the O2/CH4
ratio in Fig. 6. The calculations were carried out using the Curran,
Wang-Frenklach and GRI 3.0 mechanisms. There was unavoidable
disturbance of the flames by the sample probe that led to that the
axial position of the measured acetylene concentrations could not
be exactly matched with the axial position of the calculated concentrations. When this was taken into account, it can be accepted
that the acetylene concentrations calculated using the Curran and
Wang-Frenklach mechanisms were in agreement with the experiment data. However, there were large deviations in the concentrations calculated by GRI 3.0. This was probably because the GRI 3.0
mechanism was developed for use with fuel lean flames or with
fuels with a stoichiometric ratio, and it underestimated the importance of depletion of acetylene by C2H2 + OH ? CH2CO + H [11],
(b) RO2/CH4=0.60
0.04
Experiment
Curran
GRI 3.0
Wang-Frenklach
0.02
0.00
0
0.3
Experiment
Curran
GRI 3.0
Wang-Frenklach
0.2
30
0.1
0
0.08
Mole fraction of C2H2
10
20
Distance from burner (mm)
Mole fraction of H2
(a) RO2/CH4=0.55
(c) RO2/CH4=0.65
0.06
10
20
Distance from burner (mm)
30
(b) RO2/CH4=0.60
0.4
0.04
Experiment
Curran
GRI 3.0
Wang-Frenklach
0.02
0.00
0
10
20
Distance from burner (mm)
30
Mole fraction of H2
Mole fraction of C2H2
0.4
0.06
0.3
Experiment
Curran
GRI 3.0
Wang-Frenklach
0.2
0.1
Fig. 6. Variations of C2H2 mole fraction as a function of the distance from the
burner.
0
30
0.5
(c) RO2/CH4=0.65
0.10
Experiment
Curran
GRI 3.0
Wang-Frenklach
0.08
0.06
Mole fraction of H2
Maximum C2H2 mole fraction
0.12
10
20
Distance from burner (mm)
0.4
0.3
Experiment
Curran
GRI 3.0
Wang-Frenklach
0.2
0.04
0.55
0.60
0.65
O2/CH4 ratio
Fig. 7. Measured and predicted maximum acetylene concentrations for different
operating conditions.
0.1
0
10
20
Distance from burner (mm)
30
Fig. 8. Variations of H2 mole fraction as a function of the distance from the burner.
Please cite this article in press as: Q. Li et al., Experimental study and kinetics modeling of partial oxidation reactions in heavily sooting laminar premixed
methane flames, Chem. Eng. J. (2012), http://dx.doi.org/10.1016/j.cej.2012.06.093
8
Q. Li et al. / Chemical Engineering Journal xxx (2012) xxx–xxx
and it was no longer updated after 2000. In the Curran mechanism,
the rate constant of the reaction C2H2 + OH ? CH2CO + H was obtained by Kaiser, who adjusted the rate coefficient to fit the measured C2H2 profile obtained for a C3H8/air flame. The obtained
rate constant was roughly 10 times larger than that used in the
GRI 3.0 Mech. Using this rate constant with GRI 3.0 gave reasonable
predictions of the C2H2 concentration. The maximum concentration of acetylene and the optimal residence time that gives this
are the most important data needed in the industrial process.
The Curran and Wang-Frenklach mechanisms gave calculated
trends for the acetylene concentrations that were in agreement
with the measured acetylene concentrations, which was because
they accounted for acetylene depletion in their reaction networks.
Fig. 7 shows that the maximum acetylene concentrations calculated by the Curran and Wang-Frenklach mechanisms were close
to the measured values. Acetylene decomposes under high temper-
ature conditions, but the acetylene concentrations calculated by
GRI 3.0 were almost constant since depletion of acetylene by
C2H2 + OH ? CH2CO + H was underestimated and there were no
larger molecules in its network whose production would deplete
acetylene, so GRI 3.0 without changes is not suitable for use for
the simulation of heavily sooting flames.
Figs. 8 and 9 show the measured and calculated concentrations
of hydrogen and carbon monoxide in the flames. These concentrations deviated and, in particular, the calculated hydrogen concentrations were lower than the experimental concentrations at the
lower O2/CH4 ratios. In the physical reaction system, the burner absorbs a lot of heat, which caused the pyrolysis of methane to be less
than it would be. The deviations at low O2/CH4 ratios indicated that
the three mechanisms underestimated thermal decomposition
products such as hydrogen. A tentative conclusion is that the three
detailed chemistry mechanisms did not include some important
0.5
(a) RO2/CH4=0.55
0.25
Mole fraction of CO
0.20
0.15
0.10
Experiment
Curran
GRI 3.0
Wang-Frenklach
0.05
Mole fraction of CH4
(a) RO2/CH4=0.55
0.4
0.3
0.2
0.1
0
0.00
0
10
20
Distance from burner (mm)
30
10
20
Distance from burner (mm)
30
(b) RO2/CH4=0.60
0.25
0.4
0.20
0.15
0.10
Experiment
Curran
GRI 3.0
Wang-Frenklach
0.05
Mole fraction of CH4
(b) RO2/CH4=0.60
Mole fraction of CO
Experiment
Curran
GRI 3.0
Wang-Frenklach
0.3
Experiment
Curran
GRI 3.0
Wang-Frenklach
0.2
0.1
0.00
0
0
10
20
Distance from burner (mm)
30
10
20
Distance from burner (mm)
30
0.4
(c) RO2/CH4=0.65
Mole fraction of CO
0.25
0.20
0.15
Experiment
Curran
GRI 3.0
Wang-Frenklach
0.10
0.05
Mole fraction of CH4
(c) RO2/CH4=0.65
0.3
Experiment
Curran
GRI 3.0
Wang-Frenklach
0.2
0.1
0.0
0
0
10
20
Distance from burner (mm)
30
Fig. 9. Variations of CO mole fraction as a function of the distance from the burner.
10
20
Distance from burner (mm)
30
Fig. 10. Variations of CH4 mole fraction as a function of the distance from the
burner.
Please cite this article in press as: Q. Li et al., Experimental study and kinetics modeling of partial oxidation reactions in heavily sooting laminar premixed
methane flames, Chem. Eng. J. (2012), http://dx.doi.org/10.1016/j.cej.2012.06.093
9
Q. Li et al. / Chemical Engineering Journal xxx (2012) xxx–xxx
(a)
(b)
RO2/CH4
Expt
0.55
0.60
0.65
C2H6 mole fraction
Wang-Frenklach
0.55
0.60
0.65
1E-3
RO2/CH4
0.020
1E-4
Wang-Frenklach
C2H4 mole fraction
0.01
0.55
0.60
0.65
0.015
Expt
0.55
0.60
0.65
0.010
0.005
1E-5
0
10
20
Distance from burner (mm)
0
30
(c)
30
(d)
1E-3
1E-3
C6H6 mole fraction
C4H2 mole fraction
10
20
Distance from burner (mm)
1E-4
RO2/CH4
Wang-Frenklach
1E-5
0.55
0.60
0.65
Expt
0.55
0.60
0.65
1E-4
RO2/CH4
Wang-Frenklach
0.55
0.60
0.65
1E-5
Expt
0.55
0.60
0.65
1E-6
0
10
20
Distance from burner (mm)
30
0
10
20
Distance from burner (mm)
30
Fig. 11. Variations of mole fraction as a function of the distance from the burner.
reactions. However, any detailed revised validation of the chemical
mechanism can only be carried out when the experimental methods can be made more precise and the experimental uncertainties
are further reduced. The H2 and CO mole fractions calculated by
the Curran mechanism at the O2/CH4 ratio of 0.65 were larger than
the measured values and the values calculated by the other mechanisms because the decomposition of methane was significantly
influenced by temperature in this network. Table 3 also shows that
both the measured and calculated H2/CO ratios were all approximately 2, which indicated that the off-gas can be used as syngas
after separating off C2H2 and CO2.
Fig. 10 shows the consumption of methane in the experiment
and the calculated values from the three mechanisms. The amount
of unreacted methane calculated by the Curran mechanism was
more in agreement than the other two mechanisms, which further
pointed out that the Curran mechanism would be the best for the
simulation of heavily sooting methane flames. At the O2/CH4 ratio
of 0.55, there was more unreacted methane than at the other two
O2/CH4 ratios, and the selectivity to acetylene was higher. In industrial production, selectivity and yield need to be both considered.
The experiments suggested that the O2/CH4 ratio of 0.60 was a better choice than the other two ratios. This value was larger than the
industrial optimal ratio of 0.55, which was because there was no
preheating of the reactant mixtures in the experiments here. The
Wang–Frenklach mechanism gave reasonable predictions for
C2H6, C2H4 and C4H2, but the C6H6 predictions had larger deviations, as shown in Fig. 11. It is important to know how to inhibit
soot generation and lessen carbon black production in the industrial process, but for this, the mechanism of generation and depletion of larger molecules such as benzene and PAH is needed, which
will be our next study.
5. Conclusions
Major gas species concentrations in the flat flames of atmospheric pressure fuel-rich methane/oxygen mixtures were experimentally determined. The data are a contribution to the database
for further development of the chemical mechanism for these
experimental conditions. Flame temperatures in the axial direction
were measured and corrected for soot deposition, and it was discussed that accurate temperature measurements are very important for kinetics modeling. All the oxygen was consumed in the
oxidation region, but unreacted methane was still present in the
post-flame zone. As the O2/CH4 ratio was increased, the concentration of residual methane decreased. The coproduction of syngas is
an important part of the POX process because it gives more complete utilization of the combustion products. The optimal acetylene
production was obtained with the O2/CH4 ratio of 0.6, which was
higher than that in industrial production because preheating was
not used in the experiments here. Recombination and oxidation
reactions were both important for acetylene consumption in the
post-flame region. The Curran and Wang-Frenklach mechanisms
gave satisfactory calculated concentrations, but the GRI 3.0 mechanism without change is not suitable for heavily sooting flames.
Acknowledgements
The authors gratefully acknowledge the financial supports by
the National Natural Science Foundation of China (Nos.
20976090, 21173125) and the Foundation for the Author of National Excellent Doctoral Dissertation of PR China (No. 200757).
Please cite this article in press as: Q. Li et al., Experimental study and kinetics modeling of partial oxidation reactions in heavily sooting laminar premixed
methane flames, Chem. Eng. J. (2012), http://dx.doi.org/10.1016/j.cej.2012.06.093
10
Q. Li et al. / Chemical Engineering Journal xxx (2012) xxx–xxx
References
[1] S. Paltsev, H.D. Jacoby, J.M. Reilly, Q.J. Ejaz, J. Morris, F.O. Sullivan, S. Rausch, N.
Winchester, O. Kragha, The future of US natural gas production, use, and trade,
Energy Policy 39 (2011) 5309–5321.
[2] C.K. Thomas, The economic impact of shale gas extraction: a review of existing
studies, Ecol. Econ. 70 (2011) 1243–1249.
[3] G. Moritis, World shale gas resources outside US assessed, Oil Gas J. 109 (2011)
22.
[4] Y.Y. Turov, G.A. Parshina, Acetylene production in the principal capitalist
countries, Chem. Tech. Fuels Oils 3 (1967) 376–378.
[5] R.E. Gannon, R.M. Manyik, C.M. Dietz, H.B. Sargent, R.O. Thribolet, R.P. Schaffer,
Acetylene, in: Kirk-Othmer (Ed.), Enc. of Chem. Tech, fifth ed., John Wiley &
Sons, Inc., New York, 2008, pp. 177–178.
[6] . Pässler, W. Hefner, K. Buckl, H. Meinass, A. Meiswinkel, H.J. wernicke, G.
Ebersberg, R. Muller, J. Bässler, H. Behringer, D. Mayer, Acetylene, in:
Ullmann’s (Ed.), Enc. of Ind. Chem, seventh ed., Wiley-VCH Verlag GmbH &
Co. KGaA, Weinheim, 2008, pp. 9–36.
[7] S. Cao, D.Z. Wang, T.F. Wang, Simulation of partial oxidation of natural gas with
detailed chemistry: influence of addition of H2, C2H6 and C3H8, Chem. Eng. Sci.
65 (2010) 2608–2618.
[8] V.Y. Basevich, S.M. Kogarko, Kinetic of acetylene formation in combustion of
methane–oxygen mixtures, Combust. Explo. Shock Waves 14 (1978) 46–52.
[9] E.W. Kaiser, Measured and computer-simulated hydrocarbon and OH radicals
profiles in fuel-rich atmospheric-pressure flat flames: reactions of CH4 and
C2H2, J. Phys. Chem. 94 (1990) 4493–4499.
[10] M. Musick, P.J.V. Tiggelen, J. Vandooren, Experimental study of the structure of
several fuel-rich premixed flames of methane, oxygen and argon, Combust.
Flame 105 (1996) 433–450.
[11] S. Gersen, A.V. Mokhov, H.B. Levinsky, Extractive probe/TDLAS measurements
of acetylene in atmospheric-pressure fuel-rich premixed methane/air flames,
Combust. Flame 143 (2005) 333–336.
[12] A.V. Mokhov, S. Gersen, H.B. Levinsky, Spontaneous Raman measurements of
acetylene in atmospheric-pressure methane/air flames, Chem. Phys. Lett. 403
(2005) 233–237.
[13] F. Xu, K.C. Lin, G.M. Faeth, Soot formation in laminar premixed methane/
oxygen flames at atmospheric pressure, Combust. Flame 115 (1998) 195–209.
[14] F. Xu, G.M. Faeth, Structure of the soot growth region of laminar premixed
methane/oxygen flames, Combust. Flame 121 (2000) 640–650.
[15] S. Senkan, M. Castaldi, Formation of polycyclic aromatic hydrocarbons (PAH) in
methane combustion: comparative new results from premixed flames,
Combust. Flame 107 (1996) 141–150.
[16] T.R. Melton, A.M. Vincitore, S.M. Senkan, The effects of equivalence ratio on the
formation of polycyclic aromatic hydrocarbons and soot in premixed methane
flames, Proc. Combust. Inst. 27 (1998) 1631–1637.
[17] M. Alfè, B. Apicella, J.-N. Rouzaud, A. Tregrossi, A. Ciajolo, The effect of
temperature on soot properties in premixed methane flames, Combust. Flame
157 (2010) 1959–1965.
[18] C.S. McEnally, L.D. Pfefferle, Aromatic and linear hydrocarbon concentration
measurements in a non-premixed flame, Combust. Sci. Tech. 116–117 (1996)
183–209.
[19] C.S. Mcenally, U.O. Koylu, L.D. Pfefferle, D.E. Rosner, Soot volume fraction and
temperature measurements in laminar nonpremixed flames using
thermocouples, Combust. Flame 109 (1997) 701–720.
[20] G. Bourque, D. Healy, H.J. Curran, C. Zinner, D. Kalitan, J. de Vries, C. Aul, E.
Petersen, Ignition and flame speed kinetics of two natural gas blends with high
levels of heavier hydrocarbons, Proc. ASME Turbo Expo. 3 (2008) 1051–1066.
[21] G.P. Smith, D.M. Golden, M. Frenklach, N.W. Moriarty, B. Eiteneer, M.
Goldenberg, C.T. Bowman, R.K. Hanson, S. Song, W.C. Gardiner, Jr., V.V.
Lissianski, Z.W. Qin <http://www.me.berkeley.edu/gri_mech/>.
[22] H. Wang, M. Frenklach, A detailed kinetic modeling study of aromatics
formation in laminar premixed acetylene and ethylene flames, Combust.
Flame 110 (1997) 173–221.
[23] R.M. Fristrom, A.A. Westenberg, Flame Structure, McGraw-Hill Book Co., New
York, 1965. pp. 150–152.
[24] V.A. Grigoriev, V.M. Zorin, Heat and Mass Transfer. Thermotechnical
Experiment, Reference book, Energoisdat publishing house, Moscow, 1982.
[25] N.A. Slavinskaya, P. Frank, A modelling study of aromatic soot precursors
formation in laminar methane and ethene flames, Combust. Flame 156 (2009)
1705–1722.
[26] J.C. Biordi, C.P. Lazzara, J.F. Papp, Molecular beam mass spectrometry applied
to determining the kinetics of reactions in flames. I. Empirical characterization
of flame perturbation by molecular beam sampling probes, Combust. Flame 23
(1974) 73–82.
[27] A. Roth, Vacuum technology, Amsterdam; New York: North-Holland Pub. Co.;
New York: Sole distributors for USA and Canada, Elsevier, North-Holland,
1982, pp. 72–76.
[28] R.J. Kee, F.M. Rupley, J.A. Miller, M.E. Coltrin, J.F. Grcar, E. Meeks, H.K. Moffat,
A.E. Lutz, G. Dixon-Lewis, M.D. Smooke, J. Warnatz, G.H. Evans, R.S. Larson, R.E.
Mitchell, L.R. Petzold, W.C. Reynolds, M. Caracotsios, W.E. Stewart, P. Glarborg,
C. Wang, C.L. McLellan, O. Adigun, W.G. Houf, C.P. Chou, S.F. Miller, P. Ho, P.D.
Young, D.J. Young, D.W. Hodgson, M.V. Petrova, K.V. Puduppakkam, CHEMKIN
Release 4.1.1, Reaction Design, San Diego, CA, 2007.
[29] J. Appel, H. Bockhorn, M. Frenklach, Kinetic modeling of soot formation with
detailed chemistry and physics: laminar premixed flames of C2 hydrocarbons,
Combust. Flame 121 (2000) 122–136.
[30] T.S. Christensen, I.I. Primdahl, Improve syngas production using autothermal
reforming, Hydrocarbon Process. 73 (1994) 39–46.
[31] B.A. Albrecht, J.B.W. Kok, N. Dijkstra, T.V.D. Meer, Prediction and measurement
of the product gas composition of the ultra rich premixed combustion of
natural gas: effects of equivalence ratio, residence time, pressure, and oxygen
concentration, Combust. Sci. Technol. 181 (2009) 433–456.
[32] J. Warnatz, U. Maas, R.W. Dibble, Combustion: Physical and Chemical
Fundamentals, Modeling and Simulation, Experiments, Pollutant Formation,
second ed., Springer-Verlag, Berlin, 1999.
[33] C.K. Law, Combustion Physics, Cambridge University Press, NewYork, 2006.
Please cite this article in press as: Q. Li et al., Experimental study and kinetics modeling of partial oxidation reactions in heavily sooting laminar premixed
methane flames, Chem. Eng. J. (2012), http://dx.doi.org/10.1016/j.cej.2012.06.093