Study On Propagation Characteristics And Mechanism Of Methane/Coal Dust Mixture Flame

The energy supplied by coal will always be an important part of the energy sources in our country in a long term. Although the administration and techniques in the coal mines were enhanced, the methane and coal dust explosion accidents happens frequently in the coal mine. These explosion accidents always carry great damages and losses to the coal miners and our country. Many facts indicate that coal mine explosions were almost induced by the hybrid system of methane and coal dust. However, few investigations focus on the hybrid system of methane and coal dust. The studies on the propagation characteristics and mechanism of methane/coal dust mixture will form the theoretical bases to prevent and control the methane and coal dust explosions in the coal mines. In this study, flame propagating through methane/coal dust hybrid was observed experimentally. The coal dusts were dispersed by premixed methane/air mixture in a vertical rectangle chamber and ignited by an electric spark. High-speed video camera was used to record the images of the propagating flame, photodiode was used to study the emitting light characteristics of the flame, micro-thermocouples and ion current probes were used to measure the flame temperature profile and the reaction behavior of the combustion zone respectively. Combustion behaviors and flame structure of methane/coal dust hybrid propagating flame were analyzed in detail.First, the propagation characteristics of the premixed methane/air flames propagating through a closed tube are explored. The techniques of high-speed Schlieren photography etc are used to study the propagating flames of the premixed gases with different methane concentration in the closed tube. Based on experimental results, Combustion behaviors and characteristics of the premixed gases are analyzed. The results show that, the transformation process of the flame front bend to unburned zone turning into bend to burned will happen during the premixed flame propagation while the equivalence ratio volume approaches to 1, thus the typical tulip flame structure form during the transformation. The premixed flame show typical laminar combustion and the transformation process of the flame front will not happen while the equivalence ratio volume deviates from 1 to a certain extent. The results show also that, the tulip flame structure is a middle stage during the transformation from laminar combustion to turbulent combustion of the premixed flame. The tulip flame structure forms during the period with velocity decrease greatly and it forms only while the acceleration absolute value of the velocity decrease period is larger than a certain quantity (about 156.8m/s² in this experimental condition).The effects of the fuel ingredient constitutionλ(λ=the theoretical oxygen consumed by the methane in the mixture/the theoretical oxygen comsumed by the coal dust in the mixture), the diameter distribution of coal dust and coal category on the propagation characteristics of the mixture flame are explored. The parameters of the flame shape, flame temperature and velocity in different experimental conditions are got. Based on the experimental results, the conversion rules between the fuel ingredient constitutionλ, the diameter distribution of coal dust and coal category and the flame propagation characteristics are analyzed in detail.Based on the expmerimantal results, the combustion reaction characteristics of the methane/coal dust mixture are anylized and the flame structures of the methane/coal dust mixture flames are concluded. The effects of the fuel ingredient constitution X, the diameter distribution of coal dust and coal category on the combustion reaction characteristics and the flame structure are explored, and the conversion rules between the fuel ingredient constitution X, the diameter distribution of coal dust, coal category and the combustion reaction characteristics, the flame structure are analyzed in detail.The propagation processes of the methane/coal dust mixture flame could be described by: the temperature of the preheated zone (Z₀) increases according to the thermal convection and radiation from the combustion front; The gaseous combustion zone (Z₁) consists of methane combustion mainly, and the heat produced by Z₁ not only increases the temperature of the preheated zone but also increases that of the coal dusts; Multiple combustion zone (Z₂) consists of heterogeneous coal dust combustion mainly, in which the coal dusts pyrolyze and the volatile matters combust quickly; The coal chars formed in the last combustion process keep combusting till the carbon is exhausted in the combustion zone Z₃.The onset time of the methane/coal dust mixture flame temperature is affected by the methane comsbustion and the maximum volume of that is affected by the coal dust combustion. Comparing with the single methane combustion or single coal dust combustion, the methane participation make the mixture flame propagate with a higer velocity and the coal dust participation make the mixture flame combust in a longer time and a larger zone.