Short CommunicationTransient temperature evolution of pulverized coal cloud deflagration in a methane–oxygen atmosphere
Graphical abstract
Introduction
Despite the rapid growth in coal yield owing to advancement of mining technology in recent decades, coal mine gas/dust explosions are still a major hazard in coal production [1,2]. According to reports [[3], [4], [5]], death in China caused by gas/dust explosions occurred in more than 50% of the coal mine accidents from 2001 to 2016. Worse still, the high temperature, high pressure, and toxic substances such as methane released in these explosions result in secondary disasters, severe pollution, and mass mortality [5]. Therefore, much research has been done on the mechanism and causes of explosion of gas/dust mixtures to improve safety.
Current studies mainly focus on explosion parameters [[6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]], flammability limits [[17], [18], [19]], ignition properties [20,21], combustion behaviors [[22], [23], [24]], burning velocity [[25], [26], [27]], and experimental flame behaviors [[28], [29], [30], [31], [32], [33], [34], [35]] of methane/coal mixtures on the basis of imposed restrictions and budget expenditures. Quantitative studies have also been done on external and internal conditions of methane/coal explosions, such as initial pressure [36], solid additives [[37], [38], [39]], ignition source [16,40], obstacle/water mist [[41], [42], [43], [44], [45], [46]], equivalence ratio [13], oxygen level [47], and initial turbulence [40,48] Their findings indicate that both external and internal conditions strongly affect the explosive properties of a gas/dust mixture.
However, none of the above studies has involved the temperature evolution of particles during deflagration. In fact, the coal dust cloud experiences intricate heat-mass transfer accompanied by transient chemical reactions throughout the hybrid explosion. Meanwhile, some research has concluded that the particle morphology is directly affected by thermal accumulation among particles [[49], [50], [51], [52], [53]]. Coal is a kind of brittle organic matter whose particle morphology is easily affected by flame temperature and thermal shock. Studying the formation mechanism of various types of solid residues in homogeneous–heterogeneous combustion of coal would provide valuable information on dust-involved explosions [54,55]. Thus, insights into the temperature development of a coal dust cloud would help in understanding its explosion [[56], [57], [58]].
Only a few recent attempts have been made to reveal the temperature effects on an exploding particle cloud. Different techniques have been proposed [[59], [60], [61]] and various measurements conducted [[62], [63], [64], [65], [66]] to capture the temperature development and distribution of particles during heat-mass transfer [[67], [68], [69], [70]]. For instance, Choi et al. [71] investigated the effect of microgravity on formation and motion of soot particles using a diffusion flame at three different temperatures. Their results showed a nonlinear relationship between the volume fraction and wall temperature. Majid et al. established a model for the particle–wall interaction and used the Euler-Lagrangian approach to analyze the effect of the wall on the particle trajectory and particle surface [72]. Golovin et al. used the Shvab-Zel'dovich method, which considers particle heat diffusion, to conclude that both particle forces and viscosity affect heat and mass transfer insignificantly [73]. Bu et al. studied the ignition behavior of a single coal particle under O2/N2 and O2/CO2 atmospheres using a volatile flame. They found that the ignition delay under an O2/CO2 atmosphere was longer than that in an O2/N2 atmosphere [74]. Si et al. [75] experimented with the combustion behaviors of individual coal particles. Their results showed that three single coal particles undergo particle heating, volatile release and combustion, and volatile and char oxidation during combustion. Wu et al. [76] characterized the holographic fringes of burning coal particles using concentric rings. They reported some typical modes of a volatile flame and found that the burning of one coal particle was affected by another particle.
Owing to the limits of experimental tests, numerical modeling provides a convenient, efficient, and swift method for predicting the temperature variation of an exploding particle cloud. Moissette and Boulet used a commercial computational fluid dynamics (CFD) code and advanced numerical methods to establish a dispersion model to predict particle behavior and temperature via the Lagrangian approach [77]. Manovic et al. analyzed the temperatures of a coal char particle in a hot bubbling fluidized bed considering heat and mass transfer, heterogeneous reaction, and homogeneous reaction [78]. Fattahi et al. studied gas–particle heat transfer for fluidized and spouting regimes using an Eulerian-Eulerian two-fluid model, finding that specularity coefficients greatly affect particle behavior [79]. Gerhardter et al. calculated the convective in-flight heating of non-spherical particles using the Euler-Lagrangian approach [80]. Luo et al. found that high temperature thickens the boundary layer and strengthens turbulence in the motion of charged particles [81]. Chen et al. revealed the relationship between flame velocity and particle behavior, concluding that the overall dust cloud expansion velocity falls behind the flame velocity [82]. Moreover, the CFD discrete element method (CFD-DEM) was used to calculate the configurational temperature [83], particle heat conduction [[83], [84], [85]], flow behavior [86,87], mass-heat transfer [88], and particle temperature distribution [89,90] during particle ignition.
Heat-mass transfer among particles is not well understood owing to the scarcity of studies on temperature variation of a particle cloud during a gas/dust hybrid explosion. This makes it difficult to determine particle cloud temperature accurately. Numerical methods and CFD software on particle flow reactions make it possible to accurately calculate temperature variations during deflagration [37,56,57,82].
Therefore, we implemented a three-dimensional numerical model using the commercial code FLUENT to study the temperature evolution and other particle cloud phenomena to obtain information on heat-mass transfer and coupling interactions in gas/dust hybrid explosions.
Section snippets
Mathematical models
Hybrid explosions can be modeled on the basis of mass and energy conservation and chemical reaction balance [56,57].
Temperature development and combustion behaviors of coal dust cloud
Fig. 5 presents the temperature development for discrete particles during the hybrid explosion at different moments. The maximal temperature of the coal dust cloud fluctuates peculiarly throughout the explosion process owing to gas–particle interaction, heat absorption by gaseous products, and heat conduction by the gaseous flame. The particle cloud dispersion is irregular and the particle temperature distribution uneven owing to turbulent flow, particle collision, gas–particle interaction,
Conclusions
We used a three-dimensional model to investigate the transient evolution of particle temperature for an explosion of a methane/coal dust mixture with a certain concentration. The maximal temperature of the coal dust cloud fluctuated throughout the explosion owing to the gas–particle interaction, heat absorption by gaseous products, and heat conduction by the gaseous flame. The dispersion of the particle cloud was irregular and its particle temperature distribution uneven owing to turbulent
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This work was supported by the National Key R&D Program of China (Grant no. 2018-YFC-0807-900, 2016-YFC-0800-100), the General Program of National Natural Science Foundation (Grant no. 5167-4191, 5197-4236), the Cultivation Fund for Excellent Doctoral Dissertation of the XUST, the Program of CSC (Grant no. 2018-0861-0249) funded by the China Scholarship Council, and the Key R&D Program of Shaanxi Province (Grant no. 2017-DCXL-GY-010203). Besides, the first author thanks a lot to Zhiqian Xue,
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2022, FuelCitation Excerpt :In recent years, frequent occurrences of methane-coal dust hybrid mixture explosions have aroused substantial attention, wide public concern, and special interest from domestic and foreign scholars. Numerous investigations into methane/coal dust explosions have focused on the ignition properties [10,14–18], explosion mechanisms [19–22], explosion behaviors [5,23–27], flame propagation [24,28–31], influence factors [11,23,25,32,33–36], explosion suppression [25,37–42], explosion assessment [43], and explosion monitoring [44,45]. The aforementioned studies can not only greatly promote the precaution and prevention of gas and coal dust explosion accidents, but could also guide the assessment and diagnosis of coal-dust-involved explosions.