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air mixtures in a closed duct
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Effect of N2 and CO2 on explosion behavior of syngas/air mixtures in a closed duct Kai Zheng a, Xufeng Yang a, Minggao Yu a,b,*, Rongjun Si c, Lei Wang c a
State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing University, Chongqing, 400044, PR China b School of Safety Science and Engineering, Henan Polytechnic University, Jiaozuo, 454003, PR China c China Coal Technology and Engineering Group Corp Chongqing Research Institute, Chongqing, 400039, PR China
highlights Effect of N2 and CO2 on explosion behavior of syngas/air was experimentally investigated. Flame structure, flame front speed, and overpressure history were provided and discussed. Addition of N2 and CO2 both reduce flame speed and retard the flame distortion. Compare with N2, CO2 displayed a stronger effect on explosion flame behavior of syngas/air.
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abstract
Article history:
The explosion behavior of syngas/air mixtures under the effect of N2 and CO2 addition is
Received 11 July 2019
experimentally investigated in three cases of N2 and CO2 volume fractions (0, 20% and 40%).
Received in revised form
Tests are performed for syngas/air mixtures with varying equivalent ratios (from 0.8 to 2.5)
20 August 2019
and hydrogen fractions (from 25% to 75%). The effects of N2 and CO2 addition on flame
Accepted 4 September 2019
structure evolution, flame speed and overpressure histories are analyzed. The results
Available online xxx
showed that the tulip shaped flames appear in all cases regardless of whether N2 and CO2 are added. After flame inversion, the appearance of tulip shaped flame distortion can be
Keywords:
observed in syngas/air without N2 and CO2 addition and meanwhile the oscillations are
Syngas/air
seen in the flame front position and speed trajectories. The flame distortion becomes less
Explosion behavior
pronounced with N2 and CO2 addition, and the oscillation amplitude of the flame front
CO2 and N2
position and speed reduce accordingly. Both addition of N2 or CO2 decrease the flame speed
Flame speed
and the maximum overpressure. Therefore, it increases the time required for flame
Overpressure
arriving to the discharge vent. Whereas CO2 has evidently better inhibition performance for syngas/air explosion. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction With the increasing environmental pollution and rapid demand for energy, the search for clean alternative energy
sources has become a crucial topic. Among these fuels, syngas, which can be produced from gasification of coal and other solid fuels, has recently drawn considerable attention [1]. Syngas is the blend of main hydrogen and carbon monoxide.
* Corresponding author. 174 Shazhengjie, Shapingba, Chongqing, 400030, PR China. E-mail address: [email protected] (M. Yu). https://doi.org/10.1016/j.ijhydene.2019.09.053 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Zheng K et al., Effect of N2 and CO2 on explosion behavior of syngas/air mixtures in a closed duct, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.053
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Syngas composition is significantly different from the hydrogen fraction variation [2]. Numerous studies have been carried out to explore the combustion characteristics of premixed syngas/air mixtures. The main contents include detailed mechanism on syngas combustion, laminar flame speed, turbulent flame characteristic and flame instability, etc [3e10]. Apart from the fundamental combustion properties, the explosion risk analysis of syngas also has been the subject in many studies. As a combustible gas, the syngas may be subjected to issues like leakage, fire and explosion in the process of transportation and utilization [11]. Obviously, the substantial variations in composition may bring a challenge to study the explosion behavior of syngas. Recently, the investigations on explosion behavior of syngas has attracted increasing attention due to its high reactivity and variation in composition. Sun et al. [12] investigated the turbulent explosion characteristics of stoichiometric syngas/air mixture in a spherical vessel and found that the maximum overpressure, maximum rate of overpressure and deflagration index rise rapidly as the hydrogen fraction rise. The effect of syngas composition and H2O addition on pressure development in syngas explosion has been experimentally studied by Xie et al. [13], and the explosion parameters, such as explosion pressure, explosion time, maximum rate of pressure rise, and deflagration index have been obtained and discussed. Furthermore, Xie et al. [14] experimentally studied the explosion processes of H2/CO/air mixtures with diluents addition, and the flame instabilities during the explosion process have been discussed in details. Yu et al. [15,16] experimentally studied the explosion behavior of syngas/air and found that the hydrogen volume fraction has a prominent influence on explosion syngas/air flame shape evolution. The premixed syngas/air flame behavior in a half-open duct has been experimentally studied by Yang et al. [17], who declared that the pressure waves cannot change the flame front shape directly, and also they have conducted an experimental study to make a comparison of different flame behaviors in a closed duct and a half-open duct [18]. Wang et al. [19] studied the detonation behaviors of three stoichiometric syngas-oxygen mixtures in three tubes with various cross-sections and analyzed the experimental velocity deficits and the cellular structures that are near to the limits. Tran et al. [20] experimentally and numerically investigated the explosion behavior of syngas/air mixtures in a cylindrical chamber. It is found that increased H2 volume fraction in the fuel blends significantly increases the maximum rate of overpressure rise and deflagration index. Based upon reports, it is known that the variation of components plays an important role in influencing the explosion behavior of syngas. Although the main constituents are H2 and CO, there are still some diluent gases, e.g., CO2 and N2 in syngas. The fundamental combustion characteristics of syngas under the effect of CO2 and N2 are studied in details due to the importance for further development of combustion equipment at low emissions [21e25]. However, in the publications related to explosion suppression, CO2 and N2 are also considered as important inhibitors. Wang et al. [26] performed a series of experiments to investigate the effects of N2/CO2 on explosion behaviors of methane/air mixture and found that both the
explosion strength and limited oxygen concentration decrease with the volume fraction of N2/CO2. Du et al. [27] experimentally studied the suppression of the gasoline-air mixture explosion with non-premixed nitrogen in a closed tunnel, and the results indicated that values of maximum overpressure and overpressure rise rate decrease under the effect of non-premixed suppression. Recently, preliminary study on the effect of N2 and CO2 addition on explosion behaviors of syngas has been also conducted. Di Benedetto et al. [28] experimentally and theoretically studied the explosion behaviors of CH4/O2/N2/CO2 and H2/O2/N2/CO2 mixtures in the peak overpressure, and the maximum rate of overpressure rise and laminar burning velocity are measured in a closed cylindrical vessel. Salzano et al. [29] studied the combined effects of CO2 and O2 on the explosion behavior of H2/CO/O2/ N2/CO2 mixtures in a closed cylindrical vessel through experiment and numerical calculation. Liu et al. [30] computationally and theoretically analyzed the overpressuretemperature explosion limits of H2/CO/O2/CO2/H2O mixtures and found that the explosion temperatures are increased with the increase of CO2 mole fractions around all the three explosion limits. However, the current studies about the effect of N2 and CO2 addition on syngas/air explosion only focus on the fundamental parameters in a closed chamber. But the specific behavior of syngas/air explosion in the duct with the effect of N2 and CO2 addition, which is essential for understanding the explosion flame acceleration, is seldom reported. The purpose of this study is therefore to investigate the explosion behavior of syngas/air with the effect of N2 and CO2 addition in a closed duct. In this study, the syngas/air mixtures with five cases of equivalent ratios (0.8, 1.0, 1.5, 2.0 and 2.5) and three cases of hydrogen fractions (25%, 50% and 75%) are studied, and three volume fractions (0, 20% and 40%) of N2 and CO2 in syngas fuel are selected. In the experiment, flame structure, flame speed and overpressure history are obtained and the effect of N2 and CO2 addition is analyzed. This work may help to ensure the safety utilization of syngas in the industry.
Experimental set-up In this paper, the explosion behaviors of syngas-air, which includes flame structure, flame speed and overpressure histories, are observed in the experimental platform as being adopted in our prior studies [15,16]. The experimental platform is made up of an explosion duct, a spark igniter, a high speed camera, a high frequency piezoresistive overpressure sensor, a photoelectric sensor and five mass flow controllers (MFC). The explosion duct is 1.0 m long with a cross-section of 80 mm 80 mm, producing an aspect ratio of 12.5. Both the left and right ends of the duct are sealed by a TP304 stainless plate, forming a closed duct. For the purpose of safety, a round discharge vent, which is closed by the PVC membrane with an inner diameter of 30.293 mm, is mounted at the point with 100 mm distance from the right end. The explosion is initiated by the spark igniter mounted in the center of the left end and the ignition energy is about 0.1J. The process of explosion flame evolution is captured by a “phantom” (Miro M310) highspeed camera and the acquisition frequency is 3200 frame/s.
Please cite this article as: Zheng K et al., Effect of N2 and CO2 on explosion behavior of syngas/air mixtures in a closed duct, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.053
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The overpressure-time history is recorded by a piezoresistive overpressure transducer, which is made by Shanghai Mingkong Sensor Technology CO., Ltd. The transducer is installed on the ignition end and closed to the spark igniter. The overpressure data is obtained at a sampling rate of 15 kHZ. A FSN18 N photoelectric transducer, which is used to mark the ignition time, is located at the outside of the duct and points to the spark igniter. The sketch of experimental setup is shown in Fig. 1. Experiments are conducted for syngas/air mixtures with five equivalent ratios (0.8, 1.0, 1.5, 2.0, and 2.5) and three hydrogen fractions (25%, 50% and 75%). In order to examine the effect of N2 and CO2 addition on explosion behavior of syngas/air, three volume fractions (0, 20% and 40%) are selected. The equivalent ratio, F, hydrogen fraction, XH2 , N2 and CO2 volume fraction, XN2 ðCO2 Þ , are defined by Wang et al. [20]: F¼
ðF=AÞ ðF=AÞStioch
XH2 ¼
VH2 VH2 þ VCO
XN2 ðCO2 Þ ¼
VN2 ðCO2 Þ VH2 þ VCO þ VN2 ðCO2 Þ
(1)
(2)
(3)
where F/A is the volume ratio of fuel to air, VH2 and VCO are the volume fractions of H2 and CO in syngas fuel. Similarly, VN2 ðCO2 Þ is the volume fraction of N2 or CO2 in syngas fuel. The CO, H2, N2, CO2 and Air used in the experiment are produced by Jining XieLi Special Gas Co. Ltd., and the purity exceeds 99.99%. During the gas filling, the fuel and air flow in and out the duct through the valve mounted at the left end and wall of duct which are close to the right end (shown in Fig. 1), and the volume flow rates are controlled by the mass flow controllers. To purge air from duct, the filling process lasts about 6 min and supply the volume of the duct at least 4 times. After resting for 30s, the combustible mixture is ignited by the spark igniter. All the experiments are performed at about ambient temperature (298 K) and overpressure (1 atm). Each of the tests is repeated at least 3 times so as to guarantee the accuracy of experimental results.
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Results and discussion Effect of N2 and CO2 addition on the explosion flame structure The typical effects of equivalent ratio, hydrogen fraction, N2 and CO2 volume fraction varying on explosion flame propagation of the syngas/air are presented in Figs. 2e5. In this paper, the images of explosion flame taken with high speed camera are cropped to eliminate the unwanted portions. And because the venting induced flow after pass through the discharge vent, which would significantly affect the explosion flame behavior, both the structure and speed of the flame downstream the discharge vent are not discussed here. For the premixed flame propagation in a closed duct with aspect ratio that is larger than 2, the “tulip” flame, which has been analyzed experimentally and numerically by many scholars, would appear [31e35]. The formation process of tulip shaped flame has been divided into four stages by Clanet and Searby [32], i.e., hemisphere flame, t0 < t < tsphere; finger flame, tsphere < t < twall; the skirt of flame touching the sidewalls, twall < t < ttulip; tulip shaped flame, ttulip < t. And the different characteristic times, tsphere, twall and ttulip, increase inversely with the laminar flame speed of fuel/air mixture. Additionally, for some fuel/air mixtures (e.g., hydrogen/air and methane/ hydrogen/air), the distortion would appear in the original tulip lips after the tulip shaped flame is well developed [16,37e40]. Xiao et al. [39] regarded this feature as distorted “tulip” flame. Recent experimental and numerical results proved that the formation of distorted “tulip” flame can be attributed to the interaction between the flame front and pressure wave [16,38e42]. In Figs. 2e5, for all the cases, it can be seen that the flame propagates with hemispherical shape flame after ignition. At this stage, the flame expands freely and can not be affected by the sidewalls. Then the flame shape changes from hemispherical one to finger shaped one and the flame grows exponentially due to the restriction of sidewalls [36]. With the flame touching the sidewalls, the flame surface area decreases and consequently a plane shaped flame forms. The flame begins to inverse after the plane flame appears. This process
Fig. 1 e Sketch of experimental setup. Please cite this article as: Zheng K et al., Effect of N2 and CO2 on explosion behavior of syngas/air mixtures in a closed duct, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.053
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Fig. 2 e Flame evolution structure for syngas/air mixtures with F ¼ 1.0 and XN2 ðCO2 Þ ¼ 0. (a)XH2 ¼ 25%; (b)XH2 ¼ 75%.
continues and eventually results in the formation of tulip shaped flame. In this work, due to the aspect ratio of the duct (12.5) is far greater than 2, the tulip shaped flame can be observed in all cases and the addition of N2 and CO2 only affects those characteristic times of tsphere, twall and ttulip. After the tulip shaped flame is well established, the flame distortion could be observed in the two tulip lips in some syngas/air mixtures. Fig. 2 showed the evolution process of syngas/air explosion flame with F ¼ 1.0,XN2 ðCO2 Þ ¼ 0 but XH2 ¼ 25% and 75% respectively. As expected, for the syngas/air mixtures with fixed F and XN2 ðCO2 Þ , due to the increased flame speed [5,6], when planed shape flame forms, ttulip, would decrease with
the increases of XH2 . In Fig. 2(a) and (b), the plane shaped flame is fully settled at about 18.60 ms and 8.68 ms. Laminar flame speed is one of the most important parameters of the empirical model to predict the four stages of the tulip flame [5,6]. The laminar flame speed increases with hydrogen fraction in syngas mixtures, and therefore the time, ttulip, decreases as hydrogen fraction increases. Then after the completed development of the tulip shaped flame, the flame distortion occurs at 23.25 ms of Figs. 2(a) and 13.33 ms of Fig. 2(b) with two smaller cusp generated in the two tulip lips. The two cusps become more pronounced as the flame grows and become fully developed at 25.42 ms and 14.57 ms. Obviously, the flame distortion is more distinguished when XH2 ¼ 75%.
Fig. 3 e Flame evolution structure for syngas/air mixtures with XH2 ¼ 50% and XN2 ðCO2 Þ ¼ 0. (a) F ¼ 0.8; (b) F ¼ 1.5; (c) F ¼ 2.0; (d) F ¼ 2.5. Please cite this article as: Zheng K et al., Effect of N2 and CO2 on explosion behavior of syngas/air mixtures in a closed duct, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.053
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Fig. 4 e Flame evolution structure for syngas/air mixtures with F ¼ 1.5 and XH2 ¼ 50%. (a)XN2 ¼ 20%; (b)XCO2 ¼ 20%; (c)XN2 ¼ 40%; (d)XCO2 ¼ 40%.
This result is consistent with our previous work, in which the flame distortion becomes pronounced with the XH2 increasing [16]. This trend can also be observed in other syngas/air with fixed F and XN2 ðCO2 Þ . It is caused by the increase of the interaction between the flame front and overpressure wave due to the increase of laminar flame speed. According to the reported finding of Xiao et al. [42], when the finger shaped flame touches the sidewalls, a pressure wave will be generated due to the sudden reduction of flame area. The generated pressure wave propagates to the right end and becomes reflected, and the reflected pressure wave travels to the left side and becomes reflected again. This process leads to the interaction between the flame front and pressure wave and consequently
results in the occurrence of the flame distortion. Apparently, an increase in laminar flame speed can result in a faster flame speed and in turn produce a lager reduction of the flame area. This leads to the enhancement of interaction between the flame front and pressure wave, consequently results in the appearance of more distinguished flame distortion. As the flame propagates, the two cusps gradually grow toward each other and meanwhile another two cusps appear at the tulip lips again. As a result, the original distorted tulip shaped flame collapses and then a new one is well established at 29.45 ms and 17.67 ms. This process accompanied by the flame and overpressure oscillation repeats in the rest propagation process. But at each repeated process, the regenerated
Fig. 5 e Flame evolution structure for syngas/air mixtures with XH2 ¼ 25% and XCO2 ¼ 40%. (a) F ¼ 0.8; (b) F ¼ 2.5. Please cite this article as: Zheng K et al., Effect of N2 and CO2 on explosion behavior of syngas/air mixtures in a closed duct, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.053
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two cusps would become less pronounced than the former one. The two cusps are almost disappeared at 33.48 ms in Fig. 2(a), but they always exist in Fig. 2(b) until the flame arrives to discharge vent, suggesting that the flame distortion is more vulnerable to disappear with the XH2 decreasing. This result can also be observed in other syngas/air mixtures with fixed F and XN2 ðCO2 Þ but different XH2 . It may be because the interaction between flame front and pressure wave is weakened as the laminar flame speed decreases. In Fig. 2, it can be seen that the flame propagates in symmetric structures after ignition. But after flame inversion, the tulip lips always keep in symmetric structure in Fig. 2(a), but gradually become asymmetric in Fig. 2(b), which the upper lip propagates faster than the lower one. This phenomenon can be attributed to the buoyancy effect, which is enhanced in a slower-propagating flame [39]. The effect of equivalent ratio on flame evolution can be examined in Fig. 3, in which the explosion flame images are collected in syngas/air with XH2 ¼ 50% and XN2 ðCO2 Þ ¼ 0, but different equivalent ratios (F ¼ 0.8, 1.5, 2.0 and 2.5) are presented. As mentioned above, the larger the laminar flame speed is, the shorter the characteristic time ttulip is. For the syngas/air with fixed XH2 and XN2 ðCO2 Þ , the maximum laminar flame speed locates at the fuel-rich side and the equivalent ratio corresponding to the maximum laminar flame speed ranges from 1.5 to 2.0 [5,6]. Thus the shortest ttulip is found in syngas/air with F ¼ 1.5 (8.37 ms). Compared with Fig. 3(b) and (c), the flame distortion becomes unnoticeable and disappears more quickly after the flame forms in Fig. 3(a) and (d). This phenomenon is similar with the results observed in Fig. 2, which the flame distortion becomes less pronounced and more vulnerable to disappear with XH2 decreasing. Both of them are caused by the laminar flame speed decreasing. Fig. 4 presented the flame evolution structure for syngas/ air mixtures with F ¼ 1.5 andXH2 ¼ 50% but different XN2 ðCO2 Þ , representing the typical effect of N2 and CO2 addition on syngas/air explosion flame propagation. In Fig. 4, asXN2 ðCO2 Þ increases, the time ttulip increases accordingly and the flame distortion becomes less pronounced after the flame inverts. For example, only two small cusps can be observed in the original tulip lips at 26.66 ms in Fig. 4(c) and even there is virtually no evidence of flame distortion in Fig. 4(d). This may be caused by the laminar flame speed decreasing under the effect of N2 and CO2 addition [20]. The effect of N2 and CO2 addition on the formation process of tulip flame distortion shows to be different as the F and XH2 vary. Generally, as the XN2 ðCO2 Þ increases, due to the decrease of laminar flame speed, the flame distortion becomes less pronounced. And for syngas/air with F approaching to 1.5 and XH2 larger than 50% (laminar flame speed is faster), the flame distortion can always be observed regardless of N2 and CO2 addition. However, when the equivalent ratio locates at the extreme fuel-lean or fuel rich side (close to 0.8 or 2.5), XH2 is smaller than 50% or equal to 50%. Due to the slower laminar flame speed, the flame distortion will not appear in syngas/air when XN2 ðCO2 Þ ¼ 40%, as shown in Fig. 5, in which the flame structure of F ¼ 0.8 and 2.5 with XH2 ¼ 25% and XCO2 ¼ 40% are presented. In Fig. 4, with the XN2 ðCO2 Þ increasing, the ttulip increases accordingly and one can easily see that they are much larger under the effect of N2 for the syngas/air with the sameXN2 ðCO2 Þ .
It seems obvious from the above results that CO2 has a better suppression effect to syngas/air explosion than N2. In fact, the main effects of both CO2 and N2 on syngas/air explosion are on the specific heat of the mixture [43]. Increasing the XN2 ðCO2 Þ can increase the specific heat and in turn decrease the laminar flame speed. The CO2 has greater thermal capacities than N2 and reduces the laminar flame speed to a greater extent. In addition, CO2 also influences the chemical kinetics, because CO2 is a major product of syngas combustion, and the addition of CO2 will chemically reduce radical concentration of H, leading to the decrease in concentration of other free radicals, such as O and OH [43e45]. Thus, CO2 addition can further reduce laminar flame speed through chemical kinetics, as a result, the suppression effect of CO2 will be better than N2 in syngas/air mixtures. What's more, for the explosion flame images collected by the “phantom” (Miro M310) high-speed camera, a brighter flame denotes a higher temperature of the flame front. In Fig. 4, as expected, the addition of N2 and CO2 results in a decrease of the luminance of the flame. Additionally, it is clear that the flame becomes darkened as theXN2 ðCO2 Þ increases. For the syngas/air with the sameXN2 ðCO2 Þ , a darker color appears under the effect of CO2, implying a lower temperature in both reaction and burned area. This result shows again that the CO2 has a better suppression effect on syngas/air explosion than N2.
Effect of N2 and CO2 addition on the flame speed Fig. 6 presented the measured flame front positions of syngas/ air with varying F,XH2 andXN2 ðCO2 Þ . In this paper, the flame front position is defined as the distance between the flame leading tip and the ignition spark. In this experiment, after the flame inversion occurs, the flame front close to the sidewalls is considered to be the leading tip and the one with higher flame speed is used to calculate flame front position. As expected, the larger the laminar flame speed is, the faster the flame propagation will be. In Fig. 6, for all the syngas/air mixtures, as the flame accelerates with the hemisphere and finger shapes, it's obvious that the flame front position increases exponentially after ignition. At those stages, the flame propagates in the form of laminar flow and the acceleration process is dominated by the laminar flame speed [36]. The exponential growth speeds up in syngas/air with higherXH2 but slows down under the effect of CO2 and N2 addition. With flame touching the sidewalls, the finger shaped flame acceleration ends and consequently the curve slope changes. After this, withXH2 andXN2 ðCO2 Þ varying, as the different structures appear in flame front, the happened flame front positions show different features. For the appearance of cases with the tulip shaped flame distortion, the flame front position presents ladder form rise because of the flame oscillation. In Fig. 6, for the syngas/air with XN2 ðCO2 Þ ¼ 0, the ladder form rise can be observed in all cases. The ladder form rise begins to steepen as theXH2 increases. This result can be attributed to the weakened interaction between pressure wave and flame front as the laminar flame speed decreases. For the syngas/air with fixed XN2 ðCO2 Þ , the most pronounced ladder shape appears in the cases with F ¼ 1.5 and XH2 ¼ 75%, which the syngas/air has the fastest laminar flame speed. Then under the effect of
Please cite this article as: Zheng K et al., Effect of N2 and CO2 on explosion behavior of syngas/air mixtures in a closed duct, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.053
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Please cite this article as: Zheng K et al., Effect of N2 and CO2 on explosion behavior of syngas/air mixtures in a closed duct, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.053
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addition CO2 and N2, the flame distortion becomes less pronounced and even disappears before it approaches to the discharge vent. As a result, the ladder shape gradually becomes steady as the XN2 ðCO2 Þ increases. Eventually, when XN2 ðCO2 Þ ¼ 40%, the ladder form rise disappears completely in some cases with smaller laminar flame speed. The flame speed can be calculated based on the flame front positions in Fig. 6. Fig. 7 showed the flame speed of syngas/air with F ¼ 1.5 but different XH2 and XN2 ðCO2 Þ . Obviously, the flame speed increases exponentially due to the acceleration of finger shaped flame after ignition. The exponential increase appearing in Fig. 7 is mainly caused by the increase of flame surface area and it has been discussed by many previous studies [32,36]. As the flame shape changes from hemisphere to finger shaped, due to the restriction of the sidewalls, the flame surface area grows exponentially. The increase of the flame speed terminates with the initial contact of flame with the sidewalls. Accompanied by the flame front position curve slope changes, the flame speed reaches the first peak. Subsequently, the flame speed decrease as a result of reduction of the flame area. This process continues until the flame is flattened out. The flame accelerates again with the tulip shaped flame forming. Due to the flame distortion formation, the oscillation appears in the flame speed trajectories. The oscillation amplitude becomes stronger as the XH2 increases but decreases dramatically with XN2 ðCO2 Þ increasing. In Fig. 7, when the XN2 ðCO2 Þ is small, although the amplitude of flame oscillation decreases, the oscillation can always be seen in the flame speed histories until the flame reaches the discharge vent. But
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as the XN2 ðCO2 Þ becomes higher, the flame speed oscillation gradually disappears before the flame approaches to the discharge vent and the distance between the points of oscillation disappearing and discharge vent increasing. In this work, for the other fuel/air mixtures with F locating at the fuel-lean or -rich side (close to 0.8 or 2.5) (not displayed here), the flame oscillation disappears more quickly with XN2 ðCO2 Þ increasing and completely disappears when XN2 ðCO2 Þ ¼ 40%. Those results are consistent with the evolution of flame structure. In Fig. 7, it's evident that both the maximum flame speed and oscillation amplitude decrease with XN2 ðCO2 Þ increasing. The decreased trend is significantly more pronounced under the effect of CO2 addition. The overall effect of N2 and CO2 addition on flame speed can be analyzed through comparing the average time required for flame arriving to the discharge vent, tv and the increasing ratio, as summarized in Fig. 8 and Fig. 9. In Fig. 8, for a given XH2 and XN2 ðCO2 Þ , the shortest tv is found at F ¼ 1.5. For a given XH2 and F, w, the time tv increases and the increasing ratio grows accordingly as the XN2 ðCO2 Þ increases. What's more, for a given XN2 ðCO2 Þ , both the time tv and the corresponding increasing ratio are increased under the effect of N2 or CO2 and the maximum value appears atXCO2 ¼ 40%, suggesting again that the explosion suppression effect of CO2 is better than N2. Because N2 just has the dilution effect on syngas/air explosion suppression. However, the addition of CO2 into syngas mixtures can be attributed to the reduction of fuel volumetric fraction (dilution/thermal effect) and the chemical effect of CO2, resulting a reduction of net reaction rate of important
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Fig. 8 e Time required for flame arrived to the vent, tv. Please cite this article as: Zheng K et al., Effect of N2 and CO2 on explosion behavior of syngas/air mixtures in a closed duct, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.053
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Increasing ratio of tv/%
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Fig. 9 e Increasing ratio of tv.
reactions that contain major species [43e45]. Then for a given XH2 and XN2 ðCO2 Þ , the increasing ratio grows with F, representing that the suppression effect becomes stronger for the syngas/air at fuel-rich side. But for a given XN2 ðCO2 Þ and F, the increasing ratio decreases as theXH2 increases. The result showed that the effectiveness of N2 and CO2 on syngas/air explosion suppression reduces when the XH2 is higher.
Effect of N2 and CO2 addition on the overpressure history The addition of N2 and CO2 has strong effect on the overpressure and diminishes significantly with XN2 ðCO2 Þ increasing. For syngas/air with different F, the variation trend of overpressure under the effect of N2 and CO2 addition is similar. Thus only the typical explosion overpressure histories of syngas/air with F ¼ 1.5 are presented, as shown in Fig. 10. In Fig. 10, after ignition, the overpressure obtained in all cases increases exponentially due to the dramatic growth of flame surface area. As expected, the overpressure growth slows down with theXN2 ðCO2 Þ increasing. Thereafter, with the flame touching the sidewalls, because of reduction of flame surface area, the overpressure trajectories show different behaviors as theXN2 ðCO2 Þ andXH2 varies. WhenXH2 ¼ 25%, in the case of XN2 ðCO2 Þ ¼ 0, because of overpressure discharge caused by the rupture of the PVC membrane, the overpressure shows a slight (at about 13 ms) drop after reaching the first peak and subsequently presents a ladder form to continuous rise. The ladder form growth of overpressure is caused by the repeated pressure wave reflection in the duct [39e42]. The overpressure drop would disappear as theXH2 increases from 25% to 75%, but
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becomes more distinguished with XN2 ðCO2 Þ increasing. In Fig. 10, in the cases ofXN2 ðCO2 Þ ¼ 0 and 20%, the overpressure grows continuously and eventually reaches to the maximum value. But when XH2 ¼ 25% and XN2 ðCO2 Þ ¼ 40%, the overpressure increases very slowly after a long period decreases, it would reach the second peak that is much smaller than the first one. This trend gradually disappears as the laminar flame speed increases. WhenXH2 ¼ 75%, it only appears in syngas/air withXCO2 ¼ 40%. The flame speed dynamic is closely related to the overpressure, as shown in Fig. 11, in which the flame speed and overpressure obtains in syngas/air with F ¼ 0.8,XH2 ¼ 25%,XCO2 ¼ 40%; F ¼ 2.5,XH2 ¼ 50%,XN2 ¼ 40% and F ¼ 1.5,XH2 ¼ 75%,XN2 ðCO2 Þ ¼ 0 are compared. In Fig. 11(a), (b) and (c), the laminar flame speeds of syngas/air mixtures are 0.25 m/s, 0.81 m/s and 2.34 m/s respectively [20]. As discussed in details by Sarli et al. [46], the overpressure generation is dominated by the competition between the combustion rate and venting rate. In theory, a larger combustion rate means a faster generation rate of combustion and it could lead to a faster growth of overpressure. Conversely, the venting rate contributes to overpressure decrease after the rupture of the PVC membrane. In Fig. 11, at the finger stage, the flame speed increases exponentially, representing the combustion rate increases so rapid that the overpressure increase rate is mainly dominated by the laminar flame speed. With the flame inversion, the flame speed begins to decrease and subsequently increases again due to the tulip shaped flame formation. Thus, in Fig. 11(a) and (b), when laminar flame speed is relatively small, after the tulip shaped flame forms, the
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Fig. 10 e The measured overpressure history at F ¼ 1.5. Please cite this article as: Zheng K et al., Effect of N2 and CO2 on explosion behavior of syngas/air mixtures in a closed duct, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.053
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Fig. 11 e The relationship between flame speed and overpressure dynamic. (a) F ¼ 0.8, XH2 ¼ 25%, XCO2 ¼ 40%; (b) F ¼ 2.5, XH2 ¼ 50%, XN2 ¼ 40%; (c) F ¼ 1.5,XH2 ¼ 75%, XN2 ðCO2 Þ ¼ 0.
flame propagates so slowly that the combustion rate may be smaller than the venting rate, consequently the overpressure drops more quickly. But for the syngas/air with larger laminar flame speed, as shown in Fig. 11(c), the flame also propagates with a relative fast speed and the combustion rate is larger than the venting rate. The overpressure continues to maintain the growth trend until reaching the maximum value. In Fig. 11(c), accompanied by the occurrence of flame distortion, the flame front positon and overpressure present ladder form rising after they reach the first peak, at the same time a violent oscillation appears in flame speed history. As the laminar flame speed decreases, the ladder shape can also be observed in flame front position and overpressure histories in Fig. 11(b). But the overpressure slows down and the flame speed oscillates with smaller amplitude compared with the one in Fig. 11(c). In Fig. 11(b), before the flame approaches to the discharge vent, with the disappearance of flame distortion, both the ladder forms decrease and oscillations vanish in overpressure, flame front position and flame speed histories. What's more, in Fig. 11(a), for the case with smallest laminar flame speed, without forming the flame distortion, there is no oscillation in the flame speed history. The curve of overpressure and flame front position are smooth. The measured maximum overpressure is presented in Fig. 12. As shown, for all the cases, the maximum overpressure decreases with XN2 ðCO2 Þ increasing. Generally, the largest maximum overpressure is observed when the F ranges from 1.5 to 2.0. When XH2 ¼ 25%, for the syngas/air with fixed F, the maximum overpressure decreases quickly as XN2 ðCO2 Þ increases from 0 to 20%. But the decline is very small with further XN2 ðCO2 Þ increasing. This trend is obviously different with those of XH2 ¼ 50% and 75%, which the maximum overpressure decreases steadily as the XN2 ðCO2 Þ increases. What's more, for all the syngas/air with fixed F, when XN2 ðCO2 Þ increases from 0 to 40%, it is clear that the most rapid decline always occurs when the largest maximum overpressure of F appears. On the contrary, the slowest decline appears at the fuel-lean side, representing that the inhibiting effect of N2 and CO2 on overpressure is the worst. The trend is similar with the one observed in Fig. 9, in which the increasing ratio of tv, is basically the smallest at the fuel-lean side. In the case with sameXN2 ðCO2 Þ , the maximum overpressure decreases more quickly under the effect of CO2. This result again demonstrates that the suppression effect of CO2 is better than that of N2.
Fig. 12 e The measured maximum overpressure. Please cite this article as: Zheng K et al., Effect of N2 and CO2 on explosion behavior of syngas/air mixtures in a closed duct, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.053
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Conclusions In this paper, the explosion behavior of syngas/air under the effect of N2 and CO2 addition in a closed duct is investigated. The syngas/air mixtures with five equivalent ratios (0.8, 1.0, 1.5, 2.0 and 2.5) and three hydrogen fractions (25%, 50% and 75%) are used, and three volume fractions (0, 20% and 40%) of N2 and CO2 in syngas fuel are selected. The formation process of tulip shaped flame can be observed in all cases. In the syngas/air without N2 and CO2 addition, the flame distortion occurs accompanied by the appearance of the flame oscillation. The flame distortion becomes more pronounced with the hydrogen fraction increasing. However, with the N2 and CO2 volume fraction increasing, the flame distortion becomes less pronounced and even disappears in the syngas/air with smallest laminar flame speed. For all the cases, both the flame speed and overpressure increase exponentially after ignition. The flame acceleration terminates with flame touching the sidewalls. With the flame distortion occurring, the oscillation appears in the flame speed trajectories. Both the flame speed and amplitude of oscillation decrease dramatically with N2 and CO2 addition. With the N2 and CO2 volume fraction increasing, the time needed for flame arriving to the discharge vent increases and the maximum overpressure decreases accordingly. For the case with same N2 and CO2 volume fraction, the CO2 has a better suppression effect to syngas/air explosion compared with N2.
Acknowledgments This research was funded by National Key R&D Program of China (No. 2018YFC0807900), National Natural Science Foundation of China (No. 51804054, 51774059), China Postdoctoral Science Foundation (No. 2018M631065).
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Available online at www.sciencedirect.com
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Effect of N2 and CO2 on explosion behavior of syngas/air mixtures in a closed duct Kai Zheng a, Xufeng Yang a, Minggao Yu a,b,*, Rongjun Si c, Lei Wang c a
State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing University, Chongqing, 400044, PR China b School of Safety Science and Engineering, Henan Polytechnic University, Jiaozuo, 454003, PR China c China Coal Technology and Engineering Group Corp Chongqing Research Institute, Chongqing, 400039, PR China
highlights Effect of N2 and CO2 on explosion behavior of syngas/air was experimentally investigated. Flame structure, flame front speed, and overpressure history were provided and discussed. Addition of N2 and CO2 both reduce flame speed and retard the flame distortion. Compare with N2, CO2 displayed a stronger effect on explosion flame behavior of syngas/air.
article info
abstract
Article history:
The explosion behavior of syngas/air mixtures under the effect of N2 and CO2 addition is
Received 11 July 2019
experimentally investigated in three cases of N2 and CO2 volume fractions (0, 20% and 40%).
Received in revised form
Tests are performed for syngas/air mixtures with varying equivalent ratios (from 0.8 to 2.5)
20 August 2019
and hydrogen fractions (from 25% to 75%). The effects of N2 and CO2 addition on flame
Accepted 4 September 2019
structure evolution, flame speed and overpressure histories are analyzed. The results
Available online xxx
showed that the tulip shaped flames appear in all cases regardless of whether N2 and CO2 are added. After flame inversion, the appearance of tulip shaped flame distortion can be
Keywords:
observed in syngas/air without N2 and CO2 addition and meanwhile the oscillations are
Syngas/air
seen in the flame front position and speed trajectories. The flame distortion becomes less
Explosion behavior
pronounced with N2 and CO2 addition, and the oscillation amplitude of the flame front
CO2 and N2
position and speed reduce accordingly. Both addition of N2 or CO2 decrease the flame speed
Flame speed
and the maximum overpressure. Therefore, it increases the time required for flame
Overpressure
arriving to the discharge vent. Whereas CO2 has evidently better inhibition performance for syngas/air explosion. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction With the increasing environmental pollution and rapid demand for energy, the search for clean alternative energy
sources has become a crucial topic. Among these fuels, syngas, which can be produced from gasification of coal and other solid fuels, has recently drawn considerable attention [1]. Syngas is the blend of main hydrogen and carbon monoxide.
* Corresponding author. 174 Shazhengjie, Shapingba, Chongqing, 400030, PR China. E-mail address: [email protected] (M. Yu). https://doi.org/10.1016/j.ijhydene.2019.09.053 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Zheng K et al., Effect of N2 and CO2 on explosion behavior of syngas/air mixtures in a closed duct, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.053
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Syngas composition is significantly different from the hydrogen fraction variation [2]. Numerous studies have been carried out to explore the combustion characteristics of premixed syngas/air mixtures. The main contents include detailed mechanism on syngas combustion, laminar flame speed, turbulent flame characteristic and flame instability, etc [3e10]. Apart from the fundamental combustion properties, the explosion risk analysis of syngas also has been the subject in many studies. As a combustible gas, the syngas may be subjected to issues like leakage, fire and explosion in the process of transportation and utilization [11]. Obviously, the substantial variations in composition may bring a challenge to study the explosion behavior of syngas. Recently, the investigations on explosion behavior of syngas has attracted increasing attention due to its high reactivity and variation in composition. Sun et al. [12] investigated the turbulent explosion characteristics of stoichiometric syngas/air mixture in a spherical vessel and found that the maximum overpressure, maximum rate of overpressure and deflagration index rise rapidly as the hydrogen fraction rise. The effect of syngas composition and H2O addition on pressure development in syngas explosion has been experimentally studied by Xie et al. [13], and the explosion parameters, such as explosion pressure, explosion time, maximum rate of pressure rise, and deflagration index have been obtained and discussed. Furthermore, Xie et al. [14] experimentally studied the explosion processes of H2/CO/air mixtures with diluents addition, and the flame instabilities during the explosion process have been discussed in details. Yu et al. [15,16] experimentally studied the explosion behavior of syngas/air and found that the hydrogen volume fraction has a prominent influence on explosion syngas/air flame shape evolution. The premixed syngas/air flame behavior in a half-open duct has been experimentally studied by Yang et al. [17], who declared that the pressure waves cannot change the flame front shape directly, and also they have conducted an experimental study to make a comparison of different flame behaviors in a closed duct and a half-open duct [18]. Wang et al. [19] studied the detonation behaviors of three stoichiometric syngas-oxygen mixtures in three tubes with various cross-sections and analyzed the experimental velocity deficits and the cellular structures that are near to the limits. Tran et al. [20] experimentally and numerically investigated the explosion behavior of syngas/air mixtures in a cylindrical chamber. It is found that increased H2 volume fraction in the fuel blends significantly increases the maximum rate of overpressure rise and deflagration index. Based upon reports, it is known that the variation of components plays an important role in influencing the explosion behavior of syngas. Although the main constituents are H2 and CO, there are still some diluent gases, e.g., CO2 and N2 in syngas. The fundamental combustion characteristics of syngas under the effect of CO2 and N2 are studied in details due to the importance for further development of combustion equipment at low emissions [21e25]. However, in the publications related to explosion suppression, CO2 and N2 are also considered as important inhibitors. Wang et al. [26] performed a series of experiments to investigate the effects of N2/CO2 on explosion behaviors of methane/air mixture and found that both the
explosion strength and limited oxygen concentration decrease with the volume fraction of N2/CO2. Du et al. [27] experimentally studied the suppression of the gasoline-air mixture explosion with non-premixed nitrogen in a closed tunnel, and the results indicated that values of maximum overpressure and overpressure rise rate decrease under the effect of non-premixed suppression. Recently, preliminary study on the effect of N2 and CO2 addition on explosion behaviors of syngas has been also conducted. Di Benedetto et al. [28] experimentally and theoretically studied the explosion behaviors of CH4/O2/N2/CO2 and H2/O2/N2/CO2 mixtures in the peak overpressure, and the maximum rate of overpressure rise and laminar burning velocity are measured in a closed cylindrical vessel. Salzano et al. [29] studied the combined effects of CO2 and O2 on the explosion behavior of H2/CO/O2/ N2/CO2 mixtures in a closed cylindrical vessel through experiment and numerical calculation. Liu et al. [30] computationally and theoretically analyzed the overpressuretemperature explosion limits of H2/CO/O2/CO2/H2O mixtures and found that the explosion temperatures are increased with the increase of CO2 mole fractions around all the three explosion limits. However, the current studies about the effect of N2 and CO2 addition on syngas/air explosion only focus on the fundamental parameters in a closed chamber. But the specific behavior of syngas/air explosion in the duct with the effect of N2 and CO2 addition, which is essential for understanding the explosion flame acceleration, is seldom reported. The purpose of this study is therefore to investigate the explosion behavior of syngas/air with the effect of N2 and CO2 addition in a closed duct. In this study, the syngas/air mixtures with five cases of equivalent ratios (0.8, 1.0, 1.5, 2.0 and 2.5) and three cases of hydrogen fractions (25%, 50% and 75%) are studied, and three volume fractions (0, 20% and 40%) of N2 and CO2 in syngas fuel are selected. In the experiment, flame structure, flame speed and overpressure history are obtained and the effect of N2 and CO2 addition is analyzed. This work may help to ensure the safety utilization of syngas in the industry.
Experimental set-up In this paper, the explosion behaviors of syngas-air, which includes flame structure, flame speed and overpressure histories, are observed in the experimental platform as being adopted in our prior studies [15,16]. The experimental platform is made up of an explosion duct, a spark igniter, a high speed camera, a high frequency piezoresistive overpressure sensor, a photoelectric sensor and five mass flow controllers (MFC). The explosion duct is 1.0 m long with a cross-section of 80 mm 80 mm, producing an aspect ratio of 12.5. Both the left and right ends of the duct are sealed by a TP304 stainless plate, forming a closed duct. For the purpose of safety, a round discharge vent, which is closed by the PVC membrane with an inner diameter of 30.293 mm, is mounted at the point with 100 mm distance from the right end. The explosion is initiated by the spark igniter mounted in the center of the left end and the ignition energy is about 0.1J. The process of explosion flame evolution is captured by a “phantom” (Miro M310) highspeed camera and the acquisition frequency is 3200 frame/s.
Please cite this article as: Zheng K et al., Effect of N2 and CO2 on explosion behavior of syngas/air mixtures in a closed duct, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.053
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The overpressure-time history is recorded by a piezoresistive overpressure transducer, which is made by Shanghai Mingkong Sensor Technology CO., Ltd. The transducer is installed on the ignition end and closed to the spark igniter. The overpressure data is obtained at a sampling rate of 15 kHZ. A FSN18 N photoelectric transducer, which is used to mark the ignition time, is located at the outside of the duct and points to the spark igniter. The sketch of experimental setup is shown in Fig. 1. Experiments are conducted for syngas/air mixtures with five equivalent ratios (0.8, 1.0, 1.5, 2.0, and 2.5) and three hydrogen fractions (25%, 50% and 75%). In order to examine the effect of N2 and CO2 addition on explosion behavior of syngas/air, three volume fractions (0, 20% and 40%) are selected. The equivalent ratio, F, hydrogen fraction, XH2 , N2 and CO2 volume fraction, XN2 ðCO2 Þ , are defined by Wang et al. [20]: F¼
ðF=AÞ ðF=AÞStioch
XH2 ¼
VH2 VH2 þ VCO
XN2 ðCO2 Þ ¼
VN2 ðCO2 Þ VH2 þ VCO þ VN2 ðCO2 Þ
(1)
(2)
(3)
where F/A is the volume ratio of fuel to air, VH2 and VCO are the volume fractions of H2 and CO in syngas fuel. Similarly, VN2 ðCO2 Þ is the volume fraction of N2 or CO2 in syngas fuel. The CO, H2, N2, CO2 and Air used in the experiment are produced by Jining XieLi Special Gas Co. Ltd., and the purity exceeds 99.99%. During the gas filling, the fuel and air flow in and out the duct through the valve mounted at the left end and wall of duct which are close to the right end (shown in Fig. 1), and the volume flow rates are controlled by the mass flow controllers. To purge air from duct, the filling process lasts about 6 min and supply the volume of the duct at least 4 times. After resting for 30s, the combustible mixture is ignited by the spark igniter. All the experiments are performed at about ambient temperature (298 K) and overpressure (1 atm). Each of the tests is repeated at least 3 times so as to guarantee the accuracy of experimental results.
3
Results and discussion Effect of N2 and CO2 addition on the explosion flame structure The typical effects of equivalent ratio, hydrogen fraction, N2 and CO2 volume fraction varying on explosion flame propagation of the syngas/air are presented in Figs. 2e5. In this paper, the images of explosion flame taken with high speed camera are cropped to eliminate the unwanted portions. And because the venting induced flow after pass through the discharge vent, which would significantly affect the explosion flame behavior, both the structure and speed of the flame downstream the discharge vent are not discussed here. For the premixed flame propagation in a closed duct with aspect ratio that is larger than 2, the “tulip” flame, which has been analyzed experimentally and numerically by many scholars, would appear [31e35]. The formation process of tulip shaped flame has been divided into four stages by Clanet and Searby [32], i.e., hemisphere flame, t0 < t < tsphere; finger flame, tsphere < t < twall; the skirt of flame touching the sidewalls, twall < t < ttulip; tulip shaped flame, ttulip < t. And the different characteristic times, tsphere, twall and ttulip, increase inversely with the laminar flame speed of fuel/air mixture. Additionally, for some fuel/air mixtures (e.g., hydrogen/air and methane/ hydrogen/air), the distortion would appear in the original tulip lips after the tulip shaped flame is well developed [16,37e40]. Xiao et al. [39] regarded this feature as distorted “tulip” flame. Recent experimental and numerical results proved that the formation of distorted “tulip” flame can be attributed to the interaction between the flame front and pressure wave [16,38e42]. In Figs. 2e5, for all the cases, it can be seen that the flame propagates with hemispherical shape flame after ignition. At this stage, the flame expands freely and can not be affected by the sidewalls. Then the flame shape changes from hemispherical one to finger shaped one and the flame grows exponentially due to the restriction of sidewalls [36]. With the flame touching the sidewalls, the flame surface area decreases and consequently a plane shaped flame forms. The flame begins to inverse after the plane flame appears. This process
Fig. 1 e Sketch of experimental setup. Please cite this article as: Zheng K et al., Effect of N2 and CO2 on explosion behavior of syngas/air mixtures in a closed duct, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.053
4
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Fig. 2 e Flame evolution structure for syngas/air mixtures with F ¼ 1.0 and XN2 ðCO2 Þ ¼ 0. (a)XH2 ¼ 25%; (b)XH2 ¼ 75%.
continues and eventually results in the formation of tulip shaped flame. In this work, due to the aspect ratio of the duct (12.5) is far greater than 2, the tulip shaped flame can be observed in all cases and the addition of N2 and CO2 only affects those characteristic times of tsphere, twall and ttulip. After the tulip shaped flame is well established, the flame distortion could be observed in the two tulip lips in some syngas/air mixtures. Fig. 2 showed the evolution process of syngas/air explosion flame with F ¼ 1.0,XN2 ðCO2 Þ ¼ 0 but XH2 ¼ 25% and 75% respectively. As expected, for the syngas/air mixtures with fixed F and XN2 ðCO2 Þ , due to the increased flame speed [5,6], when planed shape flame forms, ttulip, would decrease with
the increases of XH2 . In Fig. 2(a) and (b), the plane shaped flame is fully settled at about 18.60 ms and 8.68 ms. Laminar flame speed is one of the most important parameters of the empirical model to predict the four stages of the tulip flame [5,6]. The laminar flame speed increases with hydrogen fraction in syngas mixtures, and therefore the time, ttulip, decreases as hydrogen fraction increases. Then after the completed development of the tulip shaped flame, the flame distortion occurs at 23.25 ms of Figs. 2(a) and 13.33 ms of Fig. 2(b) with two smaller cusp generated in the two tulip lips. The two cusps become more pronounced as the flame grows and become fully developed at 25.42 ms and 14.57 ms. Obviously, the flame distortion is more distinguished when XH2 ¼ 75%.
Fig. 3 e Flame evolution structure for syngas/air mixtures with XH2 ¼ 50% and XN2 ðCO2 Þ ¼ 0. (a) F ¼ 0.8; (b) F ¼ 1.5; (c) F ¼ 2.0; (d) F ¼ 2.5. Please cite this article as: Zheng K et al., Effect of N2 and CO2 on explosion behavior of syngas/air mixtures in a closed duct, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.053
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5
Fig. 4 e Flame evolution structure for syngas/air mixtures with F ¼ 1.5 and XH2 ¼ 50%. (a)XN2 ¼ 20%; (b)XCO2 ¼ 20%; (c)XN2 ¼ 40%; (d)XCO2 ¼ 40%.
This result is consistent with our previous work, in which the flame distortion becomes pronounced with the XH2 increasing [16]. This trend can also be observed in other syngas/air with fixed F and XN2 ðCO2 Þ . It is caused by the increase of the interaction between the flame front and overpressure wave due to the increase of laminar flame speed. According to the reported finding of Xiao et al. [42], when the finger shaped flame touches the sidewalls, a pressure wave will be generated due to the sudden reduction of flame area. The generated pressure wave propagates to the right end and becomes reflected, and the reflected pressure wave travels to the left side and becomes reflected again. This process leads to the interaction between the flame front and pressure wave and consequently
results in the occurrence of the flame distortion. Apparently, an increase in laminar flame speed can result in a faster flame speed and in turn produce a lager reduction of the flame area. This leads to the enhancement of interaction between the flame front and pressure wave, consequently results in the appearance of more distinguished flame distortion. As the flame propagates, the two cusps gradually grow toward each other and meanwhile another two cusps appear at the tulip lips again. As a result, the original distorted tulip shaped flame collapses and then a new one is well established at 29.45 ms and 17.67 ms. This process accompanied by the flame and overpressure oscillation repeats in the rest propagation process. But at each repeated process, the regenerated
Fig. 5 e Flame evolution structure for syngas/air mixtures with XH2 ¼ 25% and XCO2 ¼ 40%. (a) F ¼ 0.8; (b) F ¼ 2.5. Please cite this article as: Zheng K et al., Effect of N2 and CO2 on explosion behavior of syngas/air mixtures in a closed duct, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.053
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two cusps would become less pronounced than the former one. The two cusps are almost disappeared at 33.48 ms in Fig. 2(a), but they always exist in Fig. 2(b) until the flame arrives to discharge vent, suggesting that the flame distortion is more vulnerable to disappear with the XH2 decreasing. This result can also be observed in other syngas/air mixtures with fixed F and XN2 ðCO2 Þ but different XH2 . It may be because the interaction between flame front and pressure wave is weakened as the laminar flame speed decreases. In Fig. 2, it can be seen that the flame propagates in symmetric structures after ignition. But after flame inversion, the tulip lips always keep in symmetric structure in Fig. 2(a), but gradually become asymmetric in Fig. 2(b), which the upper lip propagates faster than the lower one. This phenomenon can be attributed to the buoyancy effect, which is enhanced in a slower-propagating flame [39]. The effect of equivalent ratio on flame evolution can be examined in Fig. 3, in which the explosion flame images are collected in syngas/air with XH2 ¼ 50% and XN2 ðCO2 Þ ¼ 0, but different equivalent ratios (F ¼ 0.8, 1.5, 2.0 and 2.5) are presented. As mentioned above, the larger the laminar flame speed is, the shorter the characteristic time ttulip is. For the syngas/air with fixed XH2 and XN2 ðCO2 Þ , the maximum laminar flame speed locates at the fuel-rich side and the equivalent ratio corresponding to the maximum laminar flame speed ranges from 1.5 to 2.0 [5,6]. Thus the shortest ttulip is found in syngas/air with F ¼ 1.5 (8.37 ms). Compared with Fig. 3(b) and (c), the flame distortion becomes unnoticeable and disappears more quickly after the flame forms in Fig. 3(a) and (d). This phenomenon is similar with the results observed in Fig. 2, which the flame distortion becomes less pronounced and more vulnerable to disappear with XH2 decreasing. Both of them are caused by the laminar flame speed decreasing. Fig. 4 presented the flame evolution structure for syngas/ air mixtures with F ¼ 1.5 andXH2 ¼ 50% but different XN2 ðCO2 Þ , representing the typical effect of N2 and CO2 addition on syngas/air explosion flame propagation. In Fig. 4, asXN2 ðCO2 Þ increases, the time ttulip increases accordingly and the flame distortion becomes less pronounced after the flame inverts. For example, only two small cusps can be observed in the original tulip lips at 26.66 ms in Fig. 4(c) and even there is virtually no evidence of flame distortion in Fig. 4(d). This may be caused by the laminar flame speed decreasing under the effect of N2 and CO2 addition [20]. The effect of N2 and CO2 addition on the formation process of tulip flame distortion shows to be different as the F and XH2 vary. Generally, as the XN2 ðCO2 Þ increases, due to the decrease of laminar flame speed, the flame distortion becomes less pronounced. And for syngas/air with F approaching to 1.5 and XH2 larger than 50% (laminar flame speed is faster), the flame distortion can always be observed regardless of N2 and CO2 addition. However, when the equivalent ratio locates at the extreme fuel-lean or fuel rich side (close to 0.8 or 2.5), XH2 is smaller than 50% or equal to 50%. Due to the slower laminar flame speed, the flame distortion will not appear in syngas/air when XN2 ðCO2 Þ ¼ 40%, as shown in Fig. 5, in which the flame structure of F ¼ 0.8 and 2.5 with XH2 ¼ 25% and XCO2 ¼ 40% are presented. In Fig. 4, with the XN2 ðCO2 Þ increasing, the ttulip increases accordingly and one can easily see that they are much larger under the effect of N2 for the syngas/air with the sameXN2 ðCO2 Þ .
It seems obvious from the above results that CO2 has a better suppression effect to syngas/air explosion than N2. In fact, the main effects of both CO2 and N2 on syngas/air explosion are on the specific heat of the mixture [43]. Increasing the XN2 ðCO2 Þ can increase the specific heat and in turn decrease the laminar flame speed. The CO2 has greater thermal capacities than N2 and reduces the laminar flame speed to a greater extent. In addition, CO2 also influences the chemical kinetics, because CO2 is a major product of syngas combustion, and the addition of CO2 will chemically reduce radical concentration of H, leading to the decrease in concentration of other free radicals, such as O and OH [43e45]. Thus, CO2 addition can further reduce laminar flame speed through chemical kinetics, as a result, the suppression effect of CO2 will be better than N2 in syngas/air mixtures. What's more, for the explosion flame images collected by the “phantom” (Miro M310) high-speed camera, a brighter flame denotes a higher temperature of the flame front. In Fig. 4, as expected, the addition of N2 and CO2 results in a decrease of the luminance of the flame. Additionally, it is clear that the flame becomes darkened as theXN2 ðCO2 Þ increases. For the syngas/air with the sameXN2 ðCO2 Þ , a darker color appears under the effect of CO2, implying a lower temperature in both reaction and burned area. This result shows again that the CO2 has a better suppression effect on syngas/air explosion than N2.
Effect of N2 and CO2 addition on the flame speed Fig. 6 presented the measured flame front positions of syngas/ air with varying F,XH2 andXN2 ðCO2 Þ . In this paper, the flame front position is defined as the distance between the flame leading tip and the ignition spark. In this experiment, after the flame inversion occurs, the flame front close to the sidewalls is considered to be the leading tip and the one with higher flame speed is used to calculate flame front position. As expected, the larger the laminar flame speed is, the faster the flame propagation will be. In Fig. 6, for all the syngas/air mixtures, as the flame accelerates with the hemisphere and finger shapes, it's obvious that the flame front position increases exponentially after ignition. At those stages, the flame propagates in the form of laminar flow and the acceleration process is dominated by the laminar flame speed [36]. The exponential growth speeds up in syngas/air with higherXH2 but slows down under the effect of CO2 and N2 addition. With flame touching the sidewalls, the finger shaped flame acceleration ends and consequently the curve slope changes. After this, withXH2 andXN2 ðCO2 Þ varying, as the different structures appear in flame front, the happened flame front positions show different features. For the appearance of cases with the tulip shaped flame distortion, the flame front position presents ladder form rise because of the flame oscillation. In Fig. 6, for the syngas/air with XN2 ðCO2 Þ ¼ 0, the ladder form rise can be observed in all cases. The ladder form rise begins to steepen as theXH2 increases. This result can be attributed to the weakened interaction between pressure wave and flame front as the laminar flame speed decreases. For the syngas/air with fixed XN2 ðCO2 Þ , the most pronounced ladder shape appears in the cases with F ¼ 1.5 and XH2 ¼ 75%, which the syngas/air has the fastest laminar flame speed. Then under the effect of
Please cite this article as: Zheng K et al., Effect of N2 and CO2 on explosion behavior of syngas/air mixtures in a closed duct, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.053
7
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(e) Fig. 6 e The measured flame front position histories. (a) F ¼ 0.8; (b) F ¼ 1.0; (c) F ¼ 1.5; (d) F ¼ 2.0; (e) F ¼ 2.5.
Please cite this article as: Zheng K et al., Effect of N2 and CO2 on explosion behavior of syngas/air mixtures in a closed duct, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.053
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addition CO2 and N2, the flame distortion becomes less pronounced and even disappears before it approaches to the discharge vent. As a result, the ladder shape gradually becomes steady as the XN2 ðCO2 Þ increases. Eventually, when XN2 ðCO2 Þ ¼ 40%, the ladder form rise disappears completely in some cases with smaller laminar flame speed. The flame speed can be calculated based on the flame front positions in Fig. 6. Fig. 7 showed the flame speed of syngas/air with F ¼ 1.5 but different XH2 and XN2 ðCO2 Þ . Obviously, the flame speed increases exponentially due to the acceleration of finger shaped flame after ignition. The exponential increase appearing in Fig. 7 is mainly caused by the increase of flame surface area and it has been discussed by many previous studies [32,36]. As the flame shape changes from hemisphere to finger shaped, due to the restriction of the sidewalls, the flame surface area grows exponentially. The increase of the flame speed terminates with the initial contact of flame with the sidewalls. Accompanied by the flame front position curve slope changes, the flame speed reaches the first peak. Subsequently, the flame speed decrease as a result of reduction of the flame area. This process continues until the flame is flattened out. The flame accelerates again with the tulip shaped flame forming. Due to the flame distortion formation, the oscillation appears in the flame speed trajectories. The oscillation amplitude becomes stronger as the XH2 increases but decreases dramatically with XN2 ðCO2 Þ increasing. In Fig. 7, when the XN2 ðCO2 Þ is small, although the amplitude of flame oscillation decreases, the oscillation can always be seen in the flame speed histories until the flame reaches the discharge vent. But
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as the XN2 ðCO2 Þ becomes higher, the flame speed oscillation gradually disappears before the flame approaches to the discharge vent and the distance between the points of oscillation disappearing and discharge vent increasing. In this work, for the other fuel/air mixtures with F locating at the fuel-lean or -rich side (close to 0.8 or 2.5) (not displayed here), the flame oscillation disappears more quickly with XN2 ðCO2 Þ increasing and completely disappears when XN2 ðCO2 Þ ¼ 40%. Those results are consistent with the evolution of flame structure. In Fig. 7, it's evident that both the maximum flame speed and oscillation amplitude decrease with XN2 ðCO2 Þ increasing. The decreased trend is significantly more pronounced under the effect of CO2 addition. The overall effect of N2 and CO2 addition on flame speed can be analyzed through comparing the average time required for flame arriving to the discharge vent, tv and the increasing ratio, as summarized in Fig. 8 and Fig. 9. In Fig. 8, for a given XH2 and XN2 ðCO2 Þ , the shortest tv is found at F ¼ 1.5. For a given XH2 and F, w, the time tv increases and the increasing ratio grows accordingly as the XN2 ðCO2 Þ increases. What's more, for a given XN2 ðCO2 Þ , both the time tv and the corresponding increasing ratio are increased under the effect of N2 or CO2 and the maximum value appears atXCO2 ¼ 40%, suggesting again that the explosion suppression effect of CO2 is better than N2. Because N2 just has the dilution effect on syngas/air explosion suppression. However, the addition of CO2 into syngas mixtures can be attributed to the reduction of fuel volumetric fraction (dilution/thermal effect) and the chemical effect of CO2, resulting a reduction of net reaction rate of important
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Fig. 8 e Time required for flame arrived to the vent, tv. Please cite this article as: Zheng K et al., Effect of N2 and CO2 on explosion behavior of syngas/air mixtures in a closed duct, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.053
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Increasing ratio of tv/%
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Fig. 9 e Increasing ratio of tv.
reactions that contain major species [43e45]. Then for a given XH2 and XN2 ðCO2 Þ , the increasing ratio grows with F, representing that the suppression effect becomes stronger for the syngas/air at fuel-rich side. But for a given XN2 ðCO2 Þ and F, the increasing ratio decreases as theXH2 increases. The result showed that the effectiveness of N2 and CO2 on syngas/air explosion suppression reduces when the XH2 is higher.
Effect of N2 and CO2 addition on the overpressure history The addition of N2 and CO2 has strong effect on the overpressure and diminishes significantly with XN2 ðCO2 Þ increasing. For syngas/air with different F, the variation trend of overpressure under the effect of N2 and CO2 addition is similar. Thus only the typical explosion overpressure histories of syngas/air with F ¼ 1.5 are presented, as shown in Fig. 10. In Fig. 10, after ignition, the overpressure obtained in all cases increases exponentially due to the dramatic growth of flame surface area. As expected, the overpressure growth slows down with theXN2 ðCO2 Þ increasing. Thereafter, with the flame touching the sidewalls, because of reduction of flame surface area, the overpressure trajectories show different behaviors as theXN2 ðCO2 Þ andXH2 varies. WhenXH2 ¼ 25%, in the case of XN2 ðCO2 Þ ¼ 0, because of overpressure discharge caused by the rupture of the PVC membrane, the overpressure shows a slight (at about 13 ms) drop after reaching the first peak and subsequently presents a ladder form to continuous rise. The ladder form growth of overpressure is caused by the repeated pressure wave reflection in the duct [39e42]. The overpressure drop would disappear as theXH2 increases from 25% to 75%, but
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becomes more distinguished with XN2 ðCO2 Þ increasing. In Fig. 10, in the cases ofXN2 ðCO2 Þ ¼ 0 and 20%, the overpressure grows continuously and eventually reaches to the maximum value. But when XH2 ¼ 25% and XN2 ðCO2 Þ ¼ 40%, the overpressure increases very slowly after a long period decreases, it would reach the second peak that is much smaller than the first one. This trend gradually disappears as the laminar flame speed increases. WhenXH2 ¼ 75%, it only appears in syngas/air withXCO2 ¼ 40%. The flame speed dynamic is closely related to the overpressure, as shown in Fig. 11, in which the flame speed and overpressure obtains in syngas/air with F ¼ 0.8,XH2 ¼ 25%,XCO2 ¼ 40%; F ¼ 2.5,XH2 ¼ 50%,XN2 ¼ 40% and F ¼ 1.5,XH2 ¼ 75%,XN2 ðCO2 Þ ¼ 0 are compared. In Fig. 11(a), (b) and (c), the laminar flame speeds of syngas/air mixtures are 0.25 m/s, 0.81 m/s and 2.34 m/s respectively [20]. As discussed in details by Sarli et al. [46], the overpressure generation is dominated by the competition between the combustion rate and venting rate. In theory, a larger combustion rate means a faster generation rate of combustion and it could lead to a faster growth of overpressure. Conversely, the venting rate contributes to overpressure decrease after the rupture of the PVC membrane. In Fig. 11, at the finger stage, the flame speed increases exponentially, representing the combustion rate increases so rapid that the overpressure increase rate is mainly dominated by the laminar flame speed. With the flame inversion, the flame speed begins to decrease and subsequently increases again due to the tulip shaped flame formation. Thus, in Fig. 11(a) and (b), when laminar flame speed is relatively small, after the tulip shaped flame forms, the
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Fig. 10 e The measured overpressure history at F ¼ 1.5. Please cite this article as: Zheng K et al., Effect of N2 and CO2 on explosion behavior of syngas/air mixtures in a closed duct, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.053
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Fig. 11 e The relationship between flame speed and overpressure dynamic. (a) F ¼ 0.8, XH2 ¼ 25%, XCO2 ¼ 40%; (b) F ¼ 2.5, XH2 ¼ 50%, XN2 ¼ 40%; (c) F ¼ 1.5,XH2 ¼ 75%, XN2 ðCO2 Þ ¼ 0.
flame propagates so slowly that the combustion rate may be smaller than the venting rate, consequently the overpressure drops more quickly. But for the syngas/air with larger laminar flame speed, as shown in Fig. 11(c), the flame also propagates with a relative fast speed and the combustion rate is larger than the venting rate. The overpressure continues to maintain the growth trend until reaching the maximum value. In Fig. 11(c), accompanied by the occurrence of flame distortion, the flame front positon and overpressure present ladder form rising after they reach the first peak, at the same time a violent oscillation appears in flame speed history. As the laminar flame speed decreases, the ladder shape can also be observed in flame front position and overpressure histories in Fig. 11(b). But the overpressure slows down and the flame speed oscillates with smaller amplitude compared with the one in Fig. 11(c). In Fig. 11(b), before the flame approaches to the discharge vent, with the disappearance of flame distortion, both the ladder forms decrease and oscillations vanish in overpressure, flame front position and flame speed histories. What's more, in Fig. 11(a), for the case with smallest laminar flame speed, without forming the flame distortion, there is no oscillation in the flame speed history. The curve of overpressure and flame front position are smooth. The measured maximum overpressure is presented in Fig. 12. As shown, for all the cases, the maximum overpressure decreases with XN2 ðCO2 Þ increasing. Generally, the largest maximum overpressure is observed when the F ranges from 1.5 to 2.0. When XH2 ¼ 25%, for the syngas/air with fixed F, the maximum overpressure decreases quickly as XN2 ðCO2 Þ increases from 0 to 20%. But the decline is very small with further XN2 ðCO2 Þ increasing. This trend is obviously different with those of XH2 ¼ 50% and 75%, which the maximum overpressure decreases steadily as the XN2 ðCO2 Þ increases. What's more, for all the syngas/air with fixed F, when XN2 ðCO2 Þ increases from 0 to 40%, it is clear that the most rapid decline always occurs when the largest maximum overpressure of F appears. On the contrary, the slowest decline appears at the fuel-lean side, representing that the inhibiting effect of N2 and CO2 on overpressure is the worst. The trend is similar with the one observed in Fig. 9, in which the increasing ratio of tv, is basically the smallest at the fuel-lean side. In the case with sameXN2 ðCO2 Þ , the maximum overpressure decreases more quickly under the effect of CO2. This result again demonstrates that the suppression effect of CO2 is better than that of N2.
Fig. 12 e The measured maximum overpressure. Please cite this article as: Zheng K et al., Effect of N2 and CO2 on explosion behavior of syngas/air mixtures in a closed duct, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.053
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Conclusions In this paper, the explosion behavior of syngas/air under the effect of N2 and CO2 addition in a closed duct is investigated. The syngas/air mixtures with five equivalent ratios (0.8, 1.0, 1.5, 2.0 and 2.5) and three hydrogen fractions (25%, 50% and 75%) are used, and three volume fractions (0, 20% and 40%) of N2 and CO2 in syngas fuel are selected. The formation process of tulip shaped flame can be observed in all cases. In the syngas/air without N2 and CO2 addition, the flame distortion occurs accompanied by the appearance of the flame oscillation. The flame distortion becomes more pronounced with the hydrogen fraction increasing. However, with the N2 and CO2 volume fraction increasing, the flame distortion becomes less pronounced and even disappears in the syngas/air with smallest laminar flame speed. For all the cases, both the flame speed and overpressure increase exponentially after ignition. The flame acceleration terminates with flame touching the sidewalls. With the flame distortion occurring, the oscillation appears in the flame speed trajectories. Both the flame speed and amplitude of oscillation decrease dramatically with N2 and CO2 addition. With the N2 and CO2 volume fraction increasing, the time needed for flame arriving to the discharge vent increases and the maximum overpressure decreases accordingly. For the case with same N2 and CO2 volume fraction, the CO2 has a better suppression effect to syngas/air explosion compared with N2.
Acknowledgments This research was funded by National Key R&D Program of China (No. 2018YFC0807900), National Natural Science Foundation of China (No. 51804054, 51774059), China Postdoctoral Science Foundation (No. 2018M631065).
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