Effect of the initial oxidized status of coal dust on the deflagration severities and flame behaviors of pulverized coal explosion in various methane-air atmospheres

Abstract

In this study, the influence of pre-oxidized status of coal dust on the deflagration severities and flame behaviors of methane–pulverized coal mixtures was experimentally investigated. To achieve this purpose, coal dust samples with six pre-oxidized status explosion in four various methane–air atmospheres were firstly explored by employing a modified 20-L spherical explosion reactor. Meanwhile, the flame propagation behaviors of the methane/pre-oxidized coal dust mixtures were qualitatively and quantitatively analyzed based on the sequential flame configurations. Furthermore, the morphological structure and the formation mechanisms of the post-explosion solid residues were qualitatively and theoretically discussed, the surface morphology features and the fractal characteristics of the solid particulate residues were quantitatively characterized using the graphical processing technology and the fractal theory. Results showed that the pre-oxidization of coal dust would promote the explosion severity but prolong the burning time of the methane/coal dust mixtures, as the pre-oxidization temperature increased from 25 °C to 300 °C, the explosion parameters Pmax, (dP/dt)max, and Kst presented a favourable linear variation under those four different methane–air atmospheres, whereas tc exhibited an exponentially increasing trend. The mixtures presented the most remarkable explosion severity as the methane equivalent ratio was 0.9, the Pmax raised from 0.618 MPa to 0.652 MPa while the (dP/dt)max decreased from 21.75 MPa/s to 15.46 MPa/s, suggesting an increasing amplitude of 5.5% and −28.92%. Whereas, the tc of the mixtures increased evidently from 68.98 ms to 140.47 ms, demonstrating an extension of 103.6%. Besides, the pre-oxidized coal dust would distinctively affect the evolutions of flame morphology and flame brightness, the higher the pre-oxidized status of coal, the smaller the maximum for flame radius and flame velocity. In addition, quantitative and qualitative analysis for morphological structural evolution of solid residues suggested that the pre-oxidation of coal dust would aggravate the cumbersome and complex features. The increasing pre-oxidization temperature of coal dust resulted in intricate changing tendency in fractal dimension for particle size of solid residues. Finally, the evident increase in structural parameters indicated that the pre-oxidized coal dust would aggravate the complexity for roundness distribution of solid residues.