Research on Combustion Characteristics of Air–Light Hydrocarbon Mixing Gas

: Air–light hydrocarbon mixing gas is a new type of city gas which is composed of light hydrocarbon with the main component of n -pentane and air mixed in a certain proportion. To explore the dominant reactions for CO production of air–light hydrocarbon mixing gas with di ﬀ erent mixing degrees at the critical equivalence ratios, a computational study was conducted on the combustion characteristics, including the ignition delay time, laminar ﬂame speed, extinction residence time, and emission of air–light hydrocarbon mixing gas at atmospheric pressure and room temperature in the present study. The calculated results indicate that the ignition delay time of air–light hydrocarbon mixing gas at temperatures of 1000–1118 K is greater than that of n -pentane, while the opposite at temperatures of 1118–1600 K. From the study of the laminar ﬂame speed and ignition delay time, it was found that the essence of air–light hydrocarbon mixing gas is that its attribute parameter is determined by the ratio of n -pentane to the total amount of air at the moment of combustion. The changes in the extinction residence time and the CO emission index of air–light hydrocarbon mixing gas are not synchronized, that is the CO emission index is not necessarily small for air–light hydrocarbon mixing gas with excellent extinction residence time. CO sensitivity analysis and CO rate of production identiﬁed key species and reactions that are primarily responsible for CO formation and annihilation. The mixing degree plays a key role in the CO emission index of air–light hydrocarbon mixing gas, which has a constructive opinion on the future experiment and application of air–light hydrocarbon mixing gas.


Introduction
Energy is the key to global prosperity, well-being, and the foundation of human life and industrial activities. With the rapid growth of the global economy and the increasing demand for heating and cooling in certain parts of the world, according to statistics from the International Energy Agency (IEA), global energy consumption increased by 2.3% year-on-year in 2018, almost twice the average growth rate since 2010 [1]. In recent years, global energy supply and demand and governance methods have undergone profound changes, and major countries and regions in the world have also adjusted their mid-and long-term energy development strategies in a timely manner [2][3][4]. China, on the one hand, is actively constructing new energy structures, such as the development of wind energy, water energy, nuclear energy, and geothermal energy; on the other hand, it is seeking the comprehensive utilization of traditional energy, that is the mode of energy utilization is changing from extensive to intensive, such as recycling by-products generated during oilfield mining. Part of these recovered by-products can be separated to extract a light hydrocarbon with the main component of n-pentane, experimental and computational studies on the basic combustion characteristics of methane-ammonia mixtures, developed a new reaction mechanism, and optimized it.
In summary, methane research is very extensive and in-depth, including the improvement and optimization of kinetic mechanisms, experiments and simulations of premixed combustion characteristics, and the exploration and innovation of combustion methods and combustors. These series of studies are of great significance to the promotion and application of natural gas. For, the research on n-pentane is analyzed. Pilcher and Chadwick [20] measured the heat of gaseous combustion of pentane isomers at a temperature of 298 K and a pressure of 1 atm. Knox and Kinnear [21] studied the initial stage of the slow reaction of gaseous n-pentane with oxygen under static conditions of 523-673 K. Hughes et al. [22] studied the reaction of n-pentane with oxygen in the temperature range of 530-553 K, including the slow oxidation zone and the cold flame zone. Westbrook et al. [23] used a numerical model and a detailed chemical kinetic reaction mechanism to study the oxidation reaction of n-pentane in a stirred reactor, including 53 chemical species and 326 basic reactions. Chakir et al. [24] studied the oxidation reaction of n-pentane in a jet-stirred reactor in the temperature range of 950-1050 K, which is suitable for a wide range of fuel-oxygen equivalent ratios (0.2-2.0). Zhukov et al. [25] determined the ignition delay time of n-pentane when the equivalence ratio was 0.5, the temperature was 867-1534 K, and the pressure was 11-530 atm. Bugler et al. [26,27] studied the ignition delay time of n-pentane in two shock tubes and the chemical products of the oxidation process of n-pentane in two jet-stirred reactors under a wide range of temperatures and pressures. Kelley et al. [28] innovatively designed the high temperature, high pressure, and constant pressure combustion chamber environment to determine the experimental data of the laminar flame speed of C 5 toC 8 n-alkanes. It can be seen that the practical application research of n-pentane is lacking, and the study of air-light hydrocarbon mixing gas can fill this gap.
The research history of methane indicates that using the detailed mechanism of a fuel to study its characteristic parameters is indispensable. Therefore, the purpose of this study was to investigate the combustion reaction characteristics, including the ignition delay time, laminar flame speed, extinction residence time, and emission of air-light hydrocarbon mixing gas with different mixing degrees at the critical equivalence ratios. The present study provides effective reference suggestions for the next experiments of air-light hydrocarbon mixing gas and has practical significance for the rich study of the air-fuel mixed combustion characteristics of hydrocarbon fuels.
According to the extensive research on n-pentane by Jiang et al. [29], the pentane isomer model of National University of Ireland, Galway has a better agreement with the experimental results. Thus, the NUI Galway pentane isomer model, including 697 species and 3214 reactions, was selected as the detailed chemical mechanism for n-pentane, which was the base fuel in this study.

Numerical Models
The ignition delay time is determined by the closed homogeneous batch reactor model in ANSYS CHEMKIN 17.0 [30] at temperatures of 1000-1600 K and a pressure of 1 atm. According to the experiment of Bugler et al., the ignition delay time is defined as auto-ignition.
The laminar flame speed is calculated at a pressure of 1 atm by using the premixed laminar flame-speed calculation model in ANSYS CHEMKIN 17.0 [30]. The unburned gas temperature is mentioned in the specific simulation calculation below.
The calculation of extinction residence time and emission at a pressure of 1 atm and an inlet temperature of 298 K is performed using the perfectly stirred reactor (PSR) provided by ANSYS CHEMKIN 17.0 [30]. It is assumed that the mixing is infinitely fast and the extinction is achieved by reducing the residence time until there is not enough time to react in a PSR [31,32]. As a civilian fuel, the CO produced by air-light hydrocarbon mixing gas poses a threat to human safety, and the generation of more CO also indicates low combustion efficiency. Thus, to determine the main reaction for CO production, the sensitivity analysis of CO production and the CO rate of production (ROP) are studied in PSR [33].

Computational Cases
First, understand the laminar flame speed of n-pentane-air at different equivalence ratios, which involves the combustion limit of lean and rich fuel. Figure 1 shows the laminar flame speeds of n-pentane-air at equivalent ratios of 0.4-2.5 and unburned gas temperatures of 298, 353, and 400 K, respectively. The experimental data were measured by Kelley et al. [28] at an unburned gas temperature of 353 K. As can be seen, no matter what kind of unburned gas temperature, the laminar flame speed is close to 0 cm/s when the equivalent ratio is 0.4 or 2.5. In particular, the unburned gas temperature in this study is 298 K, and the investigated range of equivalent ratio is 0.4-2.5.

Computational Cases
First, understand the laminar flame speed of n-pentane-air at different equivalence ratios, which involves the combustion limit of lean and rich fuel. Figure 1 shows the laminar flame speeds of npentane-air at equivalent ratios of 0.4-2.5 and unburned gas temperatures of 298, 353, and 400 K, respectively. The experimental data were measured by Kelley et al. [28] at an unburned gas temperature of 353 K. As can be seen, no matter what kind of unburned gas temperature, the laminar flame speed is close to 0 cm/s when the equivalent ratio is 0.4 or 2.5. In particular, the unburned gas temperature in this study is 298 K, and the investigated range of equivalent ratio is 0.4-2.5. Next, the essence of air-light hydrocarbon mixing gas is n-pentane mixed with air in a certain ratio, and then it is regarded as a whole fuel. According to the research of Fan et al. on air-light hydrocarbon mixing gas, the mixing degree of n-pentane and air is defined as: that is, 1 mol of n-pentane mixed with n mol of air is the air-light hydrocarbon mixing gas with a mixing degree of Z. This indicates that air-light hydrocarbon mixing gas is a dilution of n-pentane. Therefore, when air-light hydrocarbon mixing gas reacts with air, the above-mentioned laminar flame speed problem of lean and rich fuel must be considered. Table 1 summarizes the computational cases of air-light hydrocarbon mixing gas and air combustion. All data of computational cases were obtained based on the above situation combined with the actual project. Although there may be slight deviations in the calculation results, the retention of 6 or 7 decimal places is considered according to specific cases, so that the accuracy can be improved as much as possible. The equivalent ratio refers to the equivalent ratio of n-pentane to air. 1/ZΦ refers to the reciprocal of Z at Φ. For example, the equivalent ratio of n-pentane to air is 1.2, thus the maximum mixing degree of n-pentane and air can only reach Z1.2 = 1:2 = 1/2, i.e., 1/Z1.2 = 2. In this way, the specific mole fractions of n-pentane, nitrogen, and oxygen when air-light hydrocarbon mixing gas and air are combusted at this mixing degree can be obtained. 1/ZΦ = 0 means pure n-pentane without mixed air, in order to provide reference data. After many simulation tests of the laminar flame speed, the equivalent ratios of 0.8, 1.2, 1.7, and 2.1 Next, the essence of air-light hydrocarbon mixing gas is n-pentane mixed with air in a certain ratio, and then it is regarded as a whole fuel. According to the research of Fan et al. on air-light hydrocarbon mixing gas, the mixing degree of n-pentane and air is defined as: that is, 1 mol of n-pentane mixed with n mol of air is the air-light hydrocarbon mixing gas with a mixing degree of Z. This indicates that air-light hydrocarbon mixing gas is a dilution of n-pentane. Therefore, when air-light hydrocarbon mixing gas reacts with air, the above-mentioned laminar flame speed problem of lean and rich fuel must be considered. Table 1 summarizes the computational cases of air-light hydrocarbon mixing gas and air combustion. All data of computational cases were obtained based on the above situation combined with the actual project. Although there may be slight deviations in the calculation results, the retention of 6 or 7 decimal places is considered according to specific cases, so that the accuracy can be improved as much as possible. The equivalent ratio refers to the equivalent ratio of n-pentane to air. 1/Z Φ refers to the reciprocal of Z at Φ. For example, the equivalent ratio of n-pentane to air is 1.2, thus the maximum mixing degree of n-pentane and air can only reach Z 1.2 = 1:2 = 1/2, i.e., 1/Z 1.2 = 2. In this way, the specific mole fractions of n-pentane, nitrogen, and oxygen when air-light hydrocarbon mixing gas and air are combusted at this mixing degree can be obtained. 1/Z Φ = 0 means pure n-pentane without mixed air, in order to provide reference data. After many simulation tests of the laminar flame speed, the equivalent ratios of 0.8, 1.2, 1.7, and 2.1 correspond to the maximum mixing degree of n-pentane and air of 1, 1/2, 1/3, and 1/4, respectively. It can also be understood that, when the equivalence ratio is less than 0.8, n-pentane cannot mix air, and, when the equivalence ratio is greater than 2.1, n-pentane can mix air at any mixing degree. However, it should be noted that the final mole fraction of n-pentane cannot be higher than the mole fraction of n-pentane when the equivalent ratio is 2.5.

Ignition Delay Time
Figures 2-5 show ignition delay times of air-light hydrocarbon mixing gas at different mixing degrees and n-pentane at equivalent ratios of 0.5, 1.2, 1.7, and 2.1, respectively. As shown in Figure 2, there is a temperature node. When the temperature is lower than this node, the ignition delay time of air-light hydrocarbon mixing gas is longer than that of n-pentane. When the temperature is higher than this node, the ignition delay time of air-light hydrocarbon mixing gas is shorter than that of n-pentane. There is also a temperature node in Figure 3; the above rule still exists, but it is also found that the smaller is the mixing degree of air-light hydrocarbon mixing gas, the smaller is the ignition delay time. Figures 4 and 5 also obey the above two rules. Next, Figures 2-5 are summarized in a graph to verify whether the temperature nodes that appear in the ignition delay time study of air-mixed light hydrocarbon gas with different mixing degrees are the same.        Figure 6 shows ignition delay times of air-light hydrocarbon mixing gas at different mixing degrees and n-pentane at equivalence ratios of 0.8, 1.2, 1.7, and 2.1, respectively. Obviously, the temperature nodes are the same. This temperature node can be calculated to be approximately 1118 K. Then, the above rules are summarized as: in the temperature range of 1000-1118 K, compared with the ignition delay time of n-pentane, the smaller is the mixing degree, the longer is the ignition delay    Figure 6 shows ignition delay times of air-light hydrocarbon mixing gas at different mixing degrees and n-pentane at equivalence ratios of 0.8, 1.2, 1.7, and 2.1, respectively. Obviously, the temperature nodes are the same. This temperature node can be calculated to be approximately 1118 K. Then, the above rules are summarized as: in the temperature range of 1000-1118 K, compared with the ignition delay time of n-pentane, the smaller is the mixing degree, the longer is the ignition delay Figure 5. Comparison of the ignition delay times of n-pentane at an equivalence ratio of 2.1 and air-light hydrocarbon mixing gas at mixing degree of 1, 1/2, 1/3, and 1/4, respectively. Figure 6 shows ignition delay times of air-light hydrocarbon mixing gas at different mixing degrees and n-pentane at equivalence ratios of 0.8, 1.2, 1.7, and 2.1, respectively. Obviously, the temperature nodes are the same. This temperature node can be calculated to be approximately 1118 K. Then, the above rules are summarized as: in the temperature range of 1000-1118 K, compared with the ignition delay time of n-pentane, the smaller is the mixing degree, the longer is the ignition delay time of air-light hydrocarbon mixing gas. In the temperature range of 1118-1600 K, compared with the ignition delay time of n-pentane, the smaller is the mixing degree, the shorter is the ignition delay time of air-light hydrocarbon mixing gas. Figure 7 plots the ignition delay time as a function of the reciprocal of the mixing degree at temperatures of 1000 and 1600 K, respectively. Since the four equivalence ratios selected are the critical values of each mixing degree, the ignition delay time of the maximum mixing degree at each equivalence ratio tends to be the same regardless of the entire temperature line or single temperature point. Expanded, this indicates that the problem of the maximum mixing degree of different equivalence ratios is essentially the problem of the minimum mole fraction required for the combustion reaction of n-pentane and air. time of air-light hydrocarbon mixing gas. In the temperature range of 1118-1600 K, compared with the ignition delay time of n-pentane, the smaller is the mixing degree, the shorter is the ignition delay time of air-light hydrocarbon mixing gas. Figure 7 plots the ignition delay time as a function of the reciprocal of the mixing degree at temperatures of 1000 and 1600 K, respectively. Since the four equivalence ratios selected are the critical values of each mixing degree, the ignition delay time of the maximum mixing degree at each equivalence ratio tends to be the same regardless of the entire temperature line or single temperature point. Expanded, this indicates that the problem of the maximum mixing degree of different equivalence ratios is essentially the problem of the minimum mole fraction required for the combustion reaction of n-pentane and air.   Figure 8 plots the laminar flame speed as a function of the reciprocal of the mixing degree at an unburned gas temperature of 298 K. Combining with Figure 1, it can be concluded that the laminar flame speed of air-light hydrocarbon mixing gas changes according to the ratio of the mole fraction of n-pentane to the total mole fraction of air. At each equivalence ratio, as shown in Figure 8, the laminar flame speed of the maximum mixing degree tends to be the same. It is further verified that the final conclusion of the ignition delay time study, whether air-light hydrocarbon mixing gas can  flame speed of air-light hydrocarbon mixing gas changes according to the ratio of the mole fraction of n-pentane to the total mole fraction of air. At each equivalence ratio, as shown in Figure 8, the laminar flame speed of the maximum mixing degree tends to be the same. It is further verified that the final conclusion of the ignition delay time study, whether air-light hydrocarbon mixing gas can produce a combustion reaction is fundamentally whether the mole fraction of n-pentane in the whole gas at the moment of ignition of air-light hydrocarbon mixing gas is greater than the critical value of the combustion reaction.

Laminar Flame Speed
of each mixing degree at temperatures of 1000 and 1600 K, respectively. Figure 8 plots the laminar flame speed as a function of the reciprocal of the mixing degree at an unburned gas temperature of 298 K. Combining with Figure 1, it can be concluded that the laminar flame speed of air-light hydrocarbon mixing gas changes according to the ratio of the mole fraction of n-pentane to the total mole fraction of air. At each equivalence ratio, as shown in Figure 8, the laminar flame speed of the maximum mixing degree tends to be the same. It is further verified that the final conclusion of the ignition delay time study, whether air-light hydrocarbon mixing gas can produce a combustion reaction is fundamentally whether the mole fraction of n-pentane in the whole gas at the moment of ignition of air-light hydrocarbon mixing gas is greater than the critical value of the combustion reaction.

Extinction Residence Time and Emission
Figures 9-12 show relationship curves (C-curves) of the residence time and the temperature of air-light hydrocarbon mixing gas at different mixing degrees and n-pentane at equivalent ratios of 0.5, 1.2, 1.7, and 2.1, respectively. It can be seen that, in the equivalence ratios where the minimum mixing degree is not less than 1/2, the extinction residence time of air-light hydrocarbon mixing gas is increasing, and, in the equivalence ratios where the minimum mixing degree is not more than 1/3, the extinction residence time of air-light hydrocarbon mixing gas at the minimum mixing degree is decreasing. Figure 13 shows C-curves of air-light hydrocarbon mixing gas at different mixing degrees and n-pentane at equivalence ratios of 0.8, 1.2, 1.7, and 2.1, respectively. Within the residence time of 0.1-100 ms, the lower temperature region of C-curves of air-light hydrocarbon mixing gas is approaching a limit. According to the essence of air-light hydrocarbon mixing gas described above, this limit is the limit of the lower temperature region of n-pentane lean fuel combustion. Figure 14 plots the extinction residence time as a function of the reciprocal of the mixing degree and the CO emission indices of the corresponding the reciprocal of the mixing degree at the residence time of 20 ms. The residence time of 20 ms was chosen because it lies between the extinction and equilibrium of all computational cases. The CO emission index refers to the mass ratio of CO generated to fuel reacted in the combustion reaction. Different from the other studied parameter laws, the CO minimum emission index is neither at the minimum mixing degree nor at the maximum mixing degree, but at the position prior to the minimum mixing degree. The most striking contrast is that the reciprocal of the mixing degree is 2, the extinction residence time of B is less than A, but the CO emission index of C is greater than D. In other words, the optimal values of extinction residence time and CO emission index do not overlap. This is one of the key issues to be considered in the research and application of air-light hydrocarbon mixing gas in the future. and n-pentane at equivalence ratios of 0.8, 1.2, 1.7, and 2.1, respectively. Within the residence time of 0.1-100 ms, the lower temperature region of C-curves of air-light hydrocarbon mixing gas is approaching a limit. According to the essence of air-light hydrocarbon mixing gas described above, this limit is the limit of the lower temperature region of n-pentane lean fuel combustion.          the CO minimum emission index is neither at the minimum mixing degree nor at the maximum mixing degree, but at the position prior to the minimum mixing degree. The most striking contrast is that the reciprocal of the mixing degree is 2, the extinction residence time of B is less than A, but the CO emission index of C is greater than D. In other words, the optimal values of extinction residence time and CO emission index do not overlap. This is one of the key issues to be considered in the research and application of air-light hydrocarbon mixing gas in the future.  Figure 15 shows CO sensitivity analysis of air-light hydrocarbon mixing gas at the maximum mixing degree for equivalence ratios of n-pentane. Figure 16 shows CO ROP of air-light hydrocarbon mixing gas at the maximum mixing degree for equivalence ratios of n-pentane. Table 2 lists the reactions that play important roles in the production and annihilation of CO. Combining the analysis of Figures 15 and 16, the formation of CO in the combustion of air-light hydrocarbon mixing gas is mainly dominated by HCO + O2 <=> CO + HO2 (R32). It can also be considered that HCO radicals make the greatest contribution to the formation of CO. The main pathway for the consumption of CO is CO + OH <=> CO2 + H (R28), that is CO reacts with OH radicals to finally generate stable CO2. In particular, when the equivalence ratio is 1.7 and the maximum mixing degree of air-light hydrocarbon mixing gas is 1:3, the most sensitive reaction to CO is H + O2 <=> O + OH (R1). In connection with the analysis in Figure 14, under this condition, the CO emission index is the highest Figure 14. Comparison of the extinction residence times of air-light hydrocarbon mixing gas at the reciprocal of each mixing degree and the CO emission indices corresponding to the residence time of 20 ms. Figure 15 shows CO sensitivity analysis of air-light hydrocarbon mixing gas at the maximum mixing degree for equivalence ratios of n-pentane. Figure 16 shows CO ROP of air-light hydrocarbon mixing gas at the maximum mixing degree for equivalence ratios of n-pentane. Table 2 lists the reactions that play important roles in the production and annihilation of CO. Combining the analysis of Figures 15 and 16, the formation of CO in the combustion of air-light hydrocarbon mixing gas is mainly dominated by HCO + O 2 <=> CO + HO 2 (R32). It can also be considered that HCO radicals make the greatest contribution to the formation of CO. The main pathway for the consumption of CO is CO + OH <=> CO 2 + H (R28), that is CO reacts with OH radicals to finally generate stable CO 2 . In particular, when the equivalence ratio is 1.7 and the maximum mixing degree of air-light hydrocarbon mixing gas is 1:3, the most sensitive reaction to CO is H + O 2 <=> O + OH (R1). In connection with the analysis in Figure 14, under this condition, the CO emission index is the highest among air-light hydrocarbon mixing gas, which illustrates that R1 plays a key role in promoting the generation of CO.

Conclusions
At atmospheric pressure, a detailed mechanism was used to study the ignition delay time, laminar flame speed, extinction residence time, and CO emission of air-light hydrocarbon mixing gas with different mixing degrees at the critical equivalence ratios. Comparing the ignition delay time of air-light hydrocarbon mixing gas with n-pentane, it was found that there is a temperature node of Figure 16. Comprehensive comparison of CO ROP of air-light hydrocarbon mixing gas at the maximum mixing degree for equivalence ratios of n-pentane.