Experimental and Numerical Study on the Combustion Characteristics of a Laminar Non-Premixed Methane Jet Flame in Oxygen/Carbon Dioxide Coflow

The combustion characteristics of laminar non-premixed CH₄ jet flame in an O₂/CO₂ coflows with different oxygen mole fractions were studied experimentally. The flame heights at different oxygen concentrations and fuel jet velocity were obtained. The experimental observation shows that the luminosity of the CH₄ jet flame in O₂/CO₂ coflow is different from that of the flame in air stream. A two-dimensional numerical study of a laminar non-premixed CH₄ jet flame in the O₂/CO₂ coflow with the O₂ mole fraction of 0.35 was conducted to analyze the effects of CO₂ dilution on the flame. The distribution of OH radicals in the flame was measured experimentally using planar laser-induced fluorescence (PLIF) to validate the computational method adopted in this work, and the computational and experimental results of the OH distributions showed good consistency at various fuel flow velocities. Three artificial species were created in the numerical experiment to analyze the effects of the chemical reactions, third-body collisions, and transport properties of CO₂ on the height, width, and temperature distribution of the flame. The results showed that CO₂ participation in chemical reactions exerts significant effects on the flame. However, the influences of the third-body effects and transport properties of CO₂ on the jet flame are unremarkable. The global reaction pathways and distributions of important species in the laminar non-premixed CH₄ jet flame were analyzed in detail to investigate the influence mechanisms of CO₂ on the flame height and temperature. The entire flame can be divided into two oxidation parts, which separated by the boundary of the HCCO. The H, O, and OH concentrations and distributions in different parts of the flame were influenced by CO₂ dilution, resulting in different flame heights and temperature distributions.

Introduction

The reduction of CO₂ emissions during fossil fuel utilization is an important task for human society. The injection of purified CO₂ from flue gas into underground reservoirs is believed to help achieve this goal (Buhre et al., 2005; Wall, 2007). However, CO₂-rich flue gas is required to increase the efficiency and economy during carbon capture and storage. Several new advanced combustion technologies that use O₂/CO₂ as an oxidizer, such as oxy-fuel combustion technology (Buhre et al., 2005; Wall, 2007; Hjärtstam et al., 2009; Rathnam et al., 2009; Scheffknecht et al., 2011; Taniguchi et al., 2011; Chen et al., 2012; Dhaneswar and Pisupati, 2012; Luo et al., 2015; Moroń and Rybak, 2015; Liu et al., 2016; Ge et al., 2017; Seddighi, 2017; Menage et al., 2018; Zhang et al., 2018), moderate or intense low-oxygen dilution (MILD) oxy-combustion (Li et al., 2013; Tu et al., 2015; Mardani and FazlollahiGhomshi, 2016; Mao et al., 2017; Gładysz et al., 2018), gaseous fuel-fired oxy-fuel combustion technology (Yin et al., 2011; Seepana and Jayanti, 2012a; Seepana and Jayanti, 2012b; Oh et al., 2013a; Oh et al., 2013b; Oh and Noh, 2013; Oh and Noh, 2014; Giménez-López et al., 2015; Oh and Noh, 2015; Oh and Hong, 2016; Bürkle et al., 2018), and high-temperature oxygen combustion technology (Li et al., 2014; Li et al., 2015; Li et al., 2016), have been developed to meet this requirement. However, many other challenges must be solved before these combustion technologies can be widely adopted in the industry, since the differences in the physical properties and chemical characteristics of CO₂ and N₂ can lead to important distinctions in flame structure. More information on the combustion characteristics of different fuels in various O₂/CO₂ environments must be obtained for the development of new combustion approaches.

Experimental and numerical studies on the fundamental combustion characteristics of O₂/CO₂-fired flames have been conducted previously. The effect of CO₂ on the free-propagation speed of laminar premixed CH₄/O₂/CO₂ flames was investigated numerically (Liu et al., 2003). The speeds of laminar CH₄/O₂/CO₂ flames at different equivalence ratios and pressures were measured and computed (Xie et al., 2013). The CH₄ oxidation under CO₂-rich conditions has also been investigated using an atmospheric pressure flow reactor (Glarborg and Bentzen, 2008). The effects of environmental pressure (Maruta et al., 2007) and oxidizer temperature (Li et al., 2014) on the stretch extinction characteristics of CH₄/CO₂ versus O₂/CO₂ counterflow non-premixed flames have been studied through experiments and numerical computations. The ignition temperature of a stoichiometric CH₄/O₂/CO₂ mixture was examined by using a micro flow reactor with a controlled temperature profile (Li et al., 2015). The fundamental studies (Liu et al., 2003; Maruta et al., 2007; Glarborg and Bentzen, 2008; Xie et al., 2013; Li et al., 2014; Li et al., 2015; Li et al., 2016) show that CO₂ exerts an inhibitory effect on flames. Consequently, a higher oxygen concentration is recommended for the O₂/CO₂-fired flames to achieve a comparable combustion characteristics as the air flames.

The previous fundamental studies have provided an essential understanding of flames in O₂/CO₂ environments (Liu et al., 2003; Maruta et al., 2007; Glarborg and Bentzen, 2008; Xie et al., 2013; Li et al., 2014; Li et al., 2015; Li et al., 2016). However, the three-dimensional structures of flames in O₂/CO₂, which are important for the development of industrial burner, deserves more attention. The combustion characteristics of turbulent premixed CH₄/air/CO₂ and CO/H₂/CO₂/O₂ flames was investigated experimentally (Kobayashi et al., 2007; Kobayashi et al., 2009; Wang et al., 2013). And it was observed that local wrinkled structures become sharp and propagate deeply into the burned mixture with addition of CO₂. It was also proposed that CO₂ addition is effective for restraining combustion oscillation (Kobayashi et al., 2007). Experimental studies of the swirl-stabilized turbulent CH₄/air and CH₄/O₂/CO₂ flames show that the average length of air flames is longer than that of oxy-flames. Moreover, it was found that one-dimensional laminar flame properties could not be used to explain the intense burning of turbulent oxy-flames (Watanabe et al., 2016).

Recently, the influences of CO₂ on the combustion characteristics of multidimensional flames has been studied in detail. The physical and chemical effects of CO₂ dilution on a CH₄/H₂ jet flame in the MILD oxy-combustion regime have been investigated using two-dimensional numerical computations with a detailed kinetic mechanism (Tu et al., 2016; Tu et al., 2017). It was found that the chemical effects of CO₂ play a comparable role in the suppression of temperature rise to the physical effects (Tu et al., 2016). The influences of CO₂ dilution on the shape and structure of laminar CO/H₂ diffusion flames in an O₂/N₂/CO₂ coflow was examined (Xu et al., 2017), and the chemical effects of the thermal and transport properties of CO₂ on the flames were discussed in detail. The previous numerical studies provide important information on the effects of CO₂ under MILD (Tu et al., 2016; Tu et al., 2017) and syngas (Xu et al., 2017) combustion regimes. However, the effects of CO₂ on the flame shape and temperature distribution of gaseous-fuel-fired flames in an O₂/CO₂ environment with a high O₂ concentration needs special attention and was not clarified.

The target of the present work is to study on the combustion characteristics of laminar non-premixed methane jet flames in oxygen/carbon dioxide coflows. The laminar flame heights at different oxygen concentrations were measured by experiments. The OH distributions of typical laminar methane jet flames in a O₂/CO₂ coflow were measured by OH-PLIF technique. Two-dimensional numerical computations with a detailed kinetic mechanism were conducted, and three artificial species modified from CO₂ were employed in the numerical experiment to clarify the effects of CO₂. The influences through the chemical reactions, third-body collisions, and transport properties of CO₂ on the combustion characteristics of a laminar non-premixed CH₄ jet flame in an O₂/CO₂ environment with a high O₂ concentration was distinguished in detail.

Experiment

Experimental Setup and Method

Figure 1 shows a schematic diagram of the experimental system. The jet-flame experimental apparatus includes three parts, namely, a quartz tube, a fuel tube, and a stainless-steel chamber. The length and height of the rectangular quartz tube are 10 and 60 cm, respectively, and the wall thickness is 4 mm. The fuel stream was supplied through a stainless-steel tube with an inner diameter (d) of 1 mm and a length of 15 cm. The fuel tube was installed at the center of a rectangular quartz tube. The rectangular quartz tube and the stainless-steel chamber were connected together, and a ceramic honeycomb (diameter, 12 cm; height, 10 cm) and ceramic beads (diameter, 2 mm) were arranged in the stainless-steel chamber to achieve a uniform coflow velocity. A well-mixed O₂/CO₂ stream was fed into the rectangular quartz tube from the bottom of the chamber. Three MKS digital mass flow controllers were employed to control the volumetric flow rates of gases, and a wet gas meter (Shinagawa, W-NK-2) was used to calibrate the digital mass flow controllers. The rectangular quartz tube was adopted in the experiment because it is convenient for optical measurements. In addition, the similarity of non-premixed laminar CH₄ jet flames in a circular tube with an inner diameter of 9.2 cm to those in a rectangular quartz tube has been confirmed by preliminary experiment. The laminar CH₄ jet flames in O₂/CO₂ colfows with different oxygen mole fractions (X_O2 = 0.30, 0.35 and 0.40) were tested by the present experimental system.

FIGURE 1

FIGURE 1. Schematic of the experimental system.

A single-lens reflex digital camera (Nikon D-610, f/4) was fixed in front of the flame to record the flame images. The ISO and exposure time were set as 1,250 and 1/40 s, respectively. The qualitative distribution of OH radicals in laminar non-premixed CH₄ jet flames in an O₂/CO₂ coflow was measured using the OH-PLIF technique. The experimental apparatus was fixed on a lifter with a scale which can adjust the position of the flame. Consequently, the OH distribution of the different parts of the flame can be measured. The distribution of OH radicals at the center plane of the flame was measured by the OH-PLIF system. A beam with a wavelength of 355 nm was provided by a Nd:YAG laser (Quanta-Ray Pro-230). This beam was then transformed into a new laser beam by a dye laser (Sirah PSCAN-G-30) to excite the OH radicals in the flame. The wavelength of the dye laser beam was around 283.565 nm for the present OH-PLIF system. And the height of the laser sheet was about 40 mm and the thickness was less than 100 μm at the location of the flame. An ICCD camera (LAVISION VC-IRO and VC-Imager Pro X 4M) with an OH filter was used to obtain the OH image of the flame. The finest pixel resolution of the ICCD camera at the flame position was approximately 50 µm. A detailed description of the OH-PLIF system and the selected wavelength is provided elsewhere (Li et al., 2017). Raw OH-PLIF images were directly used for qualitative comparison with the numerical results because the measured OH intensity is proportional to the computational OH molar concentration within a 10% error (Yamamoto et al., 2009). The OH-PLIF results at each fuel flow velocity were averaged from every 100 OH-PLIF images. As the length of the laser sheet at the location of the flame was approximately 40 mm, which cannot cover the overall scope of the flame, the OH distributions in different parts of the flame were measured by adjusting the height of the experimental apparatus. The overall OH distribution of a jet flame was assembled from the OH distributions at different parts of the flame.

Experimental Results

The flame height and width which are determined by oxygen concentration and fuel flow rate are important parameter for the design of the industrial burner. Consequently, the laminar CH₄ jet flames in O₂/CO₂ coflows with oxygen mole fractions (X_O2) of 0.30, 0.35 and 0.40 were studied experimentally. The coflow velocity was kept at 0.1 m/s for different cases. The laminar CH₄ jet flames in the air coflow were also tested for the comparison. The images of CH₄ laminar jet flames in the O₂/CO₂ coflow (X_O2 = 0.35) with different fuel jet velocities are shown in Figure 2. It can be seen that the flame height of the laminar CH₄ jet flame increase linearly with the fuel velocity when the O₂/CO₂ is used as the oxidizer.

FIGURE 2

FIGURE 2. The images of CH₄-jet-flames in O₂/CO₂ coflows with an oxygen mole fraction of 0.35. (The number on the top denotes the corresponding fuel flow velocity in the unit of m/s).

The experimental results in Figures 2, 3 show that the luminosity of the flame in the O₂/CO₂ coflow is different from that of the laminar CH₄ jet flame in air. Both the laminar CH₄ jet flames in the air and O₂/CO₂ coflows can be divided into two sections, bottom and top partitions, based on the flame luminosity. The bottom section of the flame is dominated by the blue color luminosity, while the top partition of flame shows orange and yellow colors for the flame in O₂/CO₂ and air respectively. It can be seen that the bottom section of the flame with O₂/CO₂ coflow show dark blue color, however, the flame in air is more bright. The luminosities of the laminar CH₄ jet flames in O₂/CO₂ and air coflows suggests that the effect of CO₂ on the flame structure is remarkable, which needs further investigation.

FIGURE 3

FIGURE 3. The images of CH₄-jet-flames in air coflow. (The number on the top denotes the corresponding fuel flow velocity in the unit of m/s).

The flame height of laminar CH₄ jet flames in O₂/CO₂ (X_O2 = 0.30, 0.35 and 0.40) and air coflows were measured, as shown in Figure 4. It can be seen that the flame height is smoothly decreased as the increase of oxygen mole fraction. The flame in air is taller than those of the flames in O₂/CO₂ coflows. The theoretical study on the flame length of non-premixed laminar jet flame was conducted in the early studies (Roper, 1977; Roper et al., 1977), and the formula of the flame length for circular port burner was obtained and shown as follows,

$L_{\text{f}}=\frac{Q\left(\raisebox{1ex}{$T_0$}\!\left/ \!\raisebox{-1ex}{$T_F$}\right.\right)}{4\text{π}{D}_0\text{In}\left(1+\frac{1}{\mathit{S}}\right)}{\left(\raisebox{1ex}{$T_0$}\!\left/ \!\raisebox{-1ex}{$T_{\mathrm{f}}$}\right.\right)}^{0.67}(1)$

Where T₀, T_F and T_f are the ambient temperature, the fuel flow temperature and the average temperature of the flame respectively. Q is the volumetric flow rate of fuel stream, S is the stoichiometric molar ratio of oxidizer to fuel. D₀ is the diffusion coefficient at T₀. The increase of oxygen concertation leads a decrease of the stoichiometric molar ratio of oxidizer to fuel and an increase of the flame temperature. Consequently the laminar flame height in the O₂/CO₂ coflow with a higher oxygen concentration is lower.

FIGURE 4

FIGURE 4. Measured flame heights of CH₄-jet-flames in O₂/CO₂ and air coflows with 5% error bars.

Numerical Computation and Analysis

Physical Model and Computation Method

Numerical computation with a detailed reaction mechanism was conducted to clarify the effects of CO₂ on the combustion characteristics of the laminar CH₄ jet flame in O₂/CO₂ colfow. Figure 5 illustrates the physical model and boundary conditions. A CH₄ stream was injected into the O₂/CO₂ coflow through a tube with an inner diameter (d) of 1 mm and a length (L) of 15 cm. The length (L) and width (D/2) of the computation domain are 50 and 4.6 cm, respectively. An O₂/CO₂ mixture with an O₂ mole fraction (X_O) of 0.35 was used in the study, since the previous work has shown that the counterflow non-premixed CH₄ flame in an O₂/CO₂ coflow with X_O = 0.35 has a combustion intensity comparable with that of an air flame (Maruta et al., 2007; Li et al., 2014). The pressure in the system was 1 atm, and the inlet temperature of the oxidizer and fuel streams was 300 K. Laminar non-premixed CH₄ jet flames with two different fuel jet velocities (V_F = 5 and 10 m/s, Re = 293 and 586) were investigated. The inlet flow velocity of the oxidizer stream was fixed at 0.1 m/s. The laminar non-premixed jet flame was simplified to a two-dimensional axisymmetric swirl model. All walls were considered to be in a no-slip state. A convective heat transfer boundary (ambient temperature, 300 K; convective heat transfer coefficient, 6 W/m²·K) based on the experimental conditions was used for the wall of the computational domain, and an adiabatic wall condition was applied to the fuel tube wall. While the velocity inlet boundary was used at the fuel and oxidizer inlets, the pressure outlet boundary was applied at the outlet.

FIGURE 5

FIGURE 5. Physical model and boundary conditions.

The governing equations included the mass, momentum, energy, and species-conservation equations, as well as the ideal gas equation of state. A detailed description of the governing equations can be found elsewhere (Li et al., 2017). The governing equations were discretized based on the finite-volume method, and the open-source framework, OpenFOAM (OpenFOAM, 2016) was employed to conduct the numerical study. The SIMPLE algorithm was used. The finite rate chemistry model and the GRI-Mech 3.0 mechanism (Smith et al., 2020) were also used to compute the chemical reactions. Previous studies on CO₂-diluted flames (Liu et al., 2003; Maruta et al., 2007; Xie et al., 2013; Li et al., 2014; Li et al., 2015) have shown satisfactory performance of the mechanism. The diffusion coefficient was calculated using the Maxwell–Stefan equations. The thermal diffusion effect was also included in the calculations. Mesh independence was confirmed by preliminary numerical computations. Grids with 136320 cells were used, and the finest grid size was 45 µm. The values of 1.0 × 10^–6 and 1.0 × 10^–8 were used as convergence criteria for the mass and energy conservation equations and chemical reactions, respectively. Radiation heat transfer from the gases was not included in the computation, since the present work primarily focuses on comparisons of the effects of the chemical reactions, third-body collisions, and transport properties of CO₂. A discussion on the radiation of CO₂ on non-premixed flames can be found elsewhere (Maruta et al., 2007).

Additional computations using three artificial species modified from CO₂ were conducted to analyze the effects of the chemical reactions, third-body reactions, and transport properties of CO₂ on the combustion characteristics of the laminar non-premixed jet flame. The properties of the artificial species are listed in Table 1. The first artificial species is KCO₂, which does not participate in chemical reactions but participates in third-body collisions and has thermal and transport properties identical to those of CO₂. The second artificial species is XCO₂, which has the same thermal and transport properties as CO₂ but does not participate in chemical reactions and third-body collisions. The third artificial species is DCO₂, which has the same chemical reactions, third-body effects, and thermal properties as CO₂ but transport properties (i.e., thermal conductivities, viscosities, diffusion coefficients, and thermal diffusion coefficients) identical to those of N₂. The participation or not of CO₂ in the chemical reactions can lead to differences in the computational results between the CO₂ and KCO₂ conditions. Differences in the combustion characteristics between KCO₂ and XCO₂ can be attributed to the third-body effects of CO₂. The effects of the transport properties of CO₂ on the jet flame can be attributed to differences in the computational results between the DCO₂ and CO₂ cases.

TABLE 1

TABLE 1. Properties of artificial species.

Validation of the Computational Method

The computational OH distributions of laminar non-premixed CH₄ jet flames at V_F = 5 and 10 m/s were compared with the experimental results, as shown in Figure 6. The total OH distributions in the flames at fuel jet velocities of 5 and 10 m/s consist of three and five parts, respectively. It can be seen that the shape and height determined from the computational OH distributions are consistent with the results obtained from the experiment. The small difference between the computational and experimental OH distributions may be attributed to two reasons: the laser sheet is not perfectly set at the center of the flame and is slightly inclined and the computational method assumes a two-dimensional physical model and neglects radiation heat loss. Despite minor differences can be observed between the measured and computational OH distributions, the comparison of the overall results indicates that the present computational method and kinetic mechanism are reasonable for the study of the effects of CO₂ on the combustion characteristics of laminar non-premixed CH₄ jet flames in an O₂/CO₂ coflow with a high O₂ concentration.

FIGURE 6

FIGURE 6. Measured and computated OH distributions of laminar non-premixed CH₄ jet flames in an O₂/CO₂ coflow at fuel jet velocities of 5 and 10 m/s. (A) 5 m/s. (B) 10 m/s.

Effects of CO₂ on Flame Temperature and Shape

The computational results of jet flames in O₂/CO₂, O₂/KCO₂, O₂/XCO₂, and O₂/DCO₂ coflows were obtained and compared. Figures 7, 8 respectively show the temperature and OH distributions in laminar non-premixed jet flames at V_F = 10 m/s. The flames in the O₂/CO₂ and O₂/DCO₂ coflows are nearly identical, and the flames in O₂/KCO₂ and O₂/XCO₂ coflows are almost the same. Comparison of flames in the O₂/KCO₂ and O₂/XCO₂ coflows indicates that the contribution of the third-body effects of CO₂ on the shape and temperature distribution of the laminar jet flame is insignificant. The results of flames in the O₂/CO₂ and O₂/DCO₂ coflows reveal that the difference in transport properties between CO₂ and N₂ also does not lead to a significant change in the flames. However, the difference between the flames in the O₂/CO₂ and O₂/KCO₂ coflows suggests that the flame shape and temperature distribution of laminar non-premixed jet flames are significantly changed by the chemical effects of CO₂. Specifically, the flame height is decreased, whereas the maximum flame width is increased, when the chemical effects of CO₂ are excluded from consideration. The decrease in the maximum flame temperature by the chemical effect of CO₂ is approximately 230 K.

FIGURE 7

FIGURE 7. Computational temperature distributions in flames in different oxidizers (V_F = 10 m/s).

FIGURE 8

FIGURE 8. Computational OH radical distributions in flames in different oxidizers (V_F = 10 m/s).

The detailed computational results of laminar non-premixed CH₄ jet flames in O₂/CO₂, O₂/KCO₂, O₂/XCO₂, and O₂/DCO₂ coflows at a fuel flow velocities of 5 and 10 m/s were analyzed. The flame heights and widths based on the OH distribution in flames in different oxidizers at V_F = 5 and 10 m/s are illustrated in Figures 9A,B, respectively. The flame boundaries are judged by 99% decrease of the maximum OH molar concentration. The variations in the flame height and width of laminar non-premixed CH₄ jet flames at V_F = 5 m/s in different oxidizer coflows are similar to those of flames at V_F = 10 m/s. Although not shown here, the numerical results indicate that the maximum flame temperatures at V_F = 5 m/s in O₂/CO₂ and O₂/DCO₂ are nearly identical, i.e., 2470 K, and the maximum flame temperatures in O₂/KCO₂ and O₂/XCO₂ are similar, i.e., approximately 2670 K. The computational OH distributions show that flame height decreases, whereas flame width increases, when the chemical effect of CO₂ is suspended.

FIGURE 9

FIGURE 9. Computational flame height and width based on OH distributions in laminar non-premixed CH₄ jet flames in different oxidizers. (A) Maximum flame height. (B) Maximum flame width.

Mechanism of CO₂ on the Laminar Non-Premixed CH₄ Jet Flames

The reaction pathways and flame structures in O₂/CO₂, O₂/KCO₂, O₂/XCO₂ and O₂/DCO₂ were analyzed to interpret the mechanism of the chemical effect of CO₂ on laminar non-premixed CH₄ jet flames. The primary reaction pathways of flames in the O₂/KCO₂, O₂/XCO₂, and O₂/DCO₂ coflows are nearly identical to those of flames in the O₂/CO₂ coflow. Figure 10 shows the global reaction pathways of laminar non-premixed flames in the O₂/CO₂ coflow at V_F = 10 m/s. CH₄ is transformed into CH₃ through H abstraction by H, O, and OH. Most of the CH₃ radicals are transformed into C₂H₅, CH₂(S), and C₂H₆, and only a small amount of the CH₃ radicals react with O atoms to produce CH₂O. Most of the C₂H₆ generated is transformed into C₂H₅ through third-body reactions. C₂H₅ radicals decompose rapidly into C₂H₄, and H abstraction by H and OH, which are important sources of C₂H₄ consumption, produces C₂H₃ radicals. The reaction C₂H₃(+M) = C₂H₂ + H (+M) consumes C₂H₃ radicals to produce C₂H₂. Most of the C₂H₂ is consumed by reaction with O to produce HCCO. The HCCO radicals generate CH and CO via the third-body reaction HCCO(+M) = CH + CO(+M). Finally, CO is oxidized to CO₂ by the reaction OH + CO = H + CO₂.

FIGURE 10

FIGURE 10. Integral reaction flow in a laminar non-premixed CH₄ jet flame in an O₂/CO₂ coflow at V_F = 10 m/s (thick lines show the major reactions).

Figure 11 shows the distribution of important species in a laminar non-premixed jet flame in an O₂/CO₂ environment at V_F = 10 m/s. The CH₄-rich region can be observed near the exit of the tube, and the CH₃-rich region can be observed around the region with a high CH₄ concentration. The C₂H₂-rich concentration region is located at nearly the same area as the CH₃-rich region. The HCCO is found on the margins of the C₂H₂-rich region, and theHCO only exists at the two sides of the CH₃-rich region. The molar concentration of the HCCO is higher than that of the HCO. The CO-rich region is located downstream of the HCCO-rich region.

FIGURE 11

FIGURE 11. Molar concentrations of important species in a laminar non-premixed CH₄ jet flame in an O₂/CO₂ coflow (V_F = 10 m/s).

The analysis of the computational results of flames in the O₂/KCO₂, O₂/XCO₂, and O₂/DCO₂ coflows shows that the distributions of important species are similar to those of flames in O₂/CO₂. Moreover, laminar non-premixed CH₄ jet flames in different oxidizers can be divided into two parts based on the global oxidation process. The results of flames in the O₂/CO₂ coflow at V_F = 10 m/s are illustrated in Figure 12; here, the white dotted line indicates the boundary between the first and second parts of the flame. The first part of the flame includes the oxidation process from CH₄ to CO through the reaction pathway CH₄ → CH₃ → C₂H₆ → C₂H₅ → C₂H₄ → C₂H₃ → C₂H₂ → HCCO → CO, while the primary reaction in the second part is the oxidation of CO to CO₂. Consequently, the top of the HCCO region is used as the boundary separating the first and second parts of the flame. Generation of H₂ and H₂O is mainly completed in the first part of the flame. The distribution of the heats of reaction indicates that the first and second parts of the flame make remarkable contributions to heat generation. A distinct endothermic region is located in the first part of the flame. The effects of CO₂ on the first and second typical parts of non-premixed laminar jet flames result in different combustion characteristics.

FIGURE 12

FIGURE 12. The structure of the laminar non-premixed CH₄ jet flame (V_F = 10 m/s) in an O₂/CO₂ coflow. (Temp: temperature; HR: heat of reaction).

The distributions of important species of flames in the O₂/CO₂ and O₂/KCO₂ coflows were compared to enable interpretation of the chemical effect of CO₂ on the laminar non-premixed CH₄ jet flame, since the effect of third-body collisions with CO₂ on the laminar non-premixed jet flame has been confirmed to be negligible. The distributions of the molar concentrations of HCCO, CO, O, and H in the flames in O₂/CO₂ and O₂/KCO₂ coflows are shown in Figures 13A–D. The heights of the HCCO-, CO-, O-, and H-rich regions in the flame in the O₂/KCO₂ coflow are shorter than those in the flame in the O₂/CO₂ coflow. Interestingly, the difference between the heights of the HCCO-rich region in the flames in the O₂/CO₂ and O₂/KCO₂ coflows is nearly identical to that between the heights of the OH-rich regions (as shown in Figure 8). The difference between the flame heights based on the OH distribution is related to differences in HCCO distribution in the first part of the flame. The analysis of the reaction pathways shows that HCCO is primarily produced by the reaction O+ C₂H₂ = H + HCCO. As O atoms are mainly generated by the reaction H + O₂ = O+ OH, the flame height essentially depends on the H distribution. The contours of the H molar concentration shown in Figure 13D indicate that the concentration of H atoms of the flame in O₂/KCO₂ is significantly higher than that of the flame in O₂/CO₂. Pathway analysis of the entire flame region shows that the reaction OH + CO = H + CO₂ is an important source of H atoms. This finding suggests that the chemical effect of CO₂ on H formation and distribution ultimately leads to a difference in flame heights.

FIGURE 13

FIGURE 13. Distributions of the molar concentration of important species in flames in O₂/CO₂ and O₂/KCO₂: (A) HCCO, (B) CO, (C) O, and (D) H.

The results shown in Figure 12 indicate that the high-temperature region and the OH-rich region are coincident, this finding can also be observed from the results shown in Figures 7, 8. This phenomena is attributed to the fact that the high-temperature region is located in the second part of the laminar non-premixed flame, where the reaction OH + CO = H + CO₂ is the primary exothermic reaction. The results shown in Figure 7 indicate that the maximum temperature of the laminar non-premixed flame in the O₂/CO₂ coflow is approximately 230 K lower than that of the flame in the O₂/KCO₂ coflow. Although not shown here, analysis of the computational results shows that the net reaction rates of OH + CO = H + CO₂ and H + O₂ = O + OH of the flame in the O₂/CO₂ coflow are lower than those of the flame in the O₂/KCO₂ coflow. The net reaction rate of OH + CO = H + CO₂ in the second part of the flame is significantly reduced by the chemical effect of CO₂, which results in a low flame temperature.

Conclusion

The heights of the laminar CH₄ jet flames in O₂/CO₂ coflows with different oxygen mole fractions of 0.30, 0.35 and 0.40 were obtained by experimental study. The height of the flame in O₂/CO₂ coflow decreases as the increase of oxygen mole fraction. And the height of the laminar CH₄ jet flame in O₂/CO₂ coflow with oxygen mole fraction of 0.30 is shorter than that of the laminar CH₄ jet flame in air. Moreover, the luminosity of the CH₄ jet flame in O₂/CO₂ coflow is totally different from that of the flame in air stream.

The chemical reactions, third-body effects, and transport properties of CO₂ on the combustion characteristics of a laminar non-premixed CH₄ jet flame in an O₂/CO₂ coflow with a high O₂ concentration (X_O = 0.35) were investigated using a two-dimensional numerical computational method with a detailed kinetic mechanism. The computational OH distributions showed good consistency with the results obtained from the experiments. Whereas the third-body effect and transport properties of CO₂ do not exert remarkable effects on the laminar non-premixed flame, the chemical effect of CO₂ on the laminar non-premixed jet flame is significant. The chemical effect of CO₂ increases the flame height but decreases the maximum flame temperature. The primary oxidation pathway in the jet flame is CH₄ → CH₃ → C₂H₆ → C₂H₅ → C₂H₄ → C₂H₃ → C₂H₂ → HCCO → CO → CO₂. The CH₄ non-premixed laminar jet flame can be divided into two parts based on the global oxidization process; here, the top of the HCCO-rich region is the boundary between these parts. The decrease in the concentration of O atoms by the chemical effect of CO₂ in the first part of the flame can explain the observed decrease in flame height, and the inhibitory effect of CO₂ on the reaction rate of OH + CO = H + CO₂ in the second part of the flame induces a decrease in maximum flame temperature.

Data Availability Statement

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.

Author Contributions

FZ: Laser diagnostic, Data analysis, Reviewing and editing. XL: Conceptualization, Supervision, Methodology, Investigation, Reviewing and editing, Funding acquisition. SX: Data Curation, Reviewing and editing. JW: Data Curation, Reviewing and editing. XW: Reviewing and editing.

Funding

The supports of National Natural Science Foundation of China (No. 52176139) and the DNL Cooperation Fund, CAS (DNL202006) are gratefully acknowledged.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s Note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors, and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

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