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Experimental study and kinetics modeling of partial oxidation reactions in heavily sooting laminar premixed methane flames
Chemical Engineering Journal 207–208 (2012) 235–244
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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 23 July 2012 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). 1385-8947/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2012.06.093
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-
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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.
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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.
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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)
238
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
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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.
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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
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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.
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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.
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(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).
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Contents lists available at SciVerse ScienceDirect
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 23 July 2012 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). 1385-8947/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2012.06.093
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-
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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.
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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.
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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)
238
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
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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.
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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
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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.
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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.
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(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).
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