Deflagration explosions of coal dust clouds and flammable gases are a major safety concern in coal mining industry. Accidental fire and explosion caused by coal dust cloud can impose substantial losses and damages to people and properties in underground coal mines. Hybrid mixtures of methane and coal dust have the potential to reduce the minimum activation energy of a combustion reaction. In this study the Minimum Explosion Concentration (MEC), Over Pressure Rise (OPR), deflagration index for gas and dust hybrid mixtures (K st ) and explosive region of hybrid fuel mixtures present in Ventilation Air Methane (VAM) were investigated. Experiments were carried out according to the ASTM E1226-12 guideline utilising a 20 L spherical shape apparatus specifically designed for this purpose. Results obtained from this study have shown that the presence of methane significantly affects explosion characteristics of coal dust clouds. Dilute concentrations of methane, 0.75–1.25%, resulted in coal dust clouds OPR increasing from 0.3 bar to 2.2 bar and boosting the K st value from 10 bar m s −1 to 25 bar m s −1 . The explosion characteristics were also affected by the ignitors’ energy; for instance, for a coal dust cloud concentration of 50 g m −3 the OPR recorded was 0.09 bar when a 1 kJ chemical ignitor was used, while, 0.75 bar (OPR) was recorded when a 10 kJ chemical ignitor was used. For the first time, new explosion regions were identified for diluted methane-coal dust cloud mixtures when using 1, 5 and 10 kJ ignitors. Finally, the Le-Chatelier mixing rule was modified to predict the lower explosion limit of methane-coal dust cloud hybrid mixtures considering the energy of the ignitors.

Effects of ignition energy on fire and explosion characteristics of dilute hybrid fuel in ventilation air methane

An experimental study has been conducted to investigate the effects of hydrogen addition on the fundamental propagation characteristics of methane/air premixed flames at different equivalence ratios in a venting duct. The hydrogen fraction in the methane–hydrogen mixture was varied from 0 to 1 at equivalence ratios of 0.8, 1.0 and 1.2. The results indicate that the tendency towards flame instability increased with the fraction of hydrogen, and the premixed hydrogen/methane flame underwent a complex shape change with the increasing hydrogen fraction. The tulip flame only formed when the fraction of hydrogen ranged from 0 to 50% at an equivalence ratio of 0.8. It was also found that the flame front speed and the overpressure increased significantly with the hydrogen fraction. For all equivalence ratios, the stoichiometric flame ( Φ  = 1.0) has the shortest time of flame propagation and the maximum overpressure.

Effects of hydrogen addition on propagation characteristics of premixed methane/air flames

Effect of concentration and obstacles on methane-air mixture deflagration to detonation transition (DDT) was investigated in a long circular duct with lengths of 40 m and inner diameter of 350 mm. Five different concentrations 6%, 8%, 10%, 12% and 14% (in this paper, concentration specifically refers to volume concentration unless otherwise specified) were selected for the various investigations. Experimental results show that flame reaches the maximum velocity when mixtures are near the stoichiometric concentration without obstacles while deflagration to detonation transition (DDT) may occur when obstacles are arranged in the duct. Four kinds of obstacles with blockage ratio of 0.3, 0.45, 0.6 and 0.75 are used in the experiment. The effect of obstacles on the flame velocity was investigate by inserting different number of obstacles (3, 6, 9 and 12) and adjusting spacing of obstacles (175 mm, 350 mm, 525 mm and 700 mm). The blockage ratio of obstacles as well as their spacing and number has great effects on the flame velocity of the mixtures. A high number of obstacles in the duct can increase flame turbulence and lead to flame acceleration. At a mixture volume concentration of 8%, flame propagates faster with an increase in obstacle spacing and DDT could happen. The larger the obstacle blockage ratio, the stronger the interaction of the unburned mixture with a shock wave, which is more beneficial to the acceleration of the flame, while more heat is dissipated with an increase of the obstacle blockage ratio and this is not good for flame acceleration and propagation, so final flame acceleration is determined by the two competing factors.

Effect of concentration and obstacles on flame velocity and overpressure of methane-air mixture

Study of flame distribution laws and the hazard effects in a tunnel gas explosion accident is of great importance for safety issue. However, it has not yet been fully explored. The object of present work is mainly to study the effects of premixed gas concentration on the distribution law of the flame region and the hazard effects involving methane-air explosion in a tube and a tunnel based on experimental and numerical results. The experiments were conducted in a tube with one end closed and the other open. The tube was partially filled with premixed methane-air mixture with six different premixed methane concentrations. Major simulation works were performed in a full-scale tunnel with a length of 1000 m. The first 56 m of the tunnel were occupied by methane–air mixture. Results show that the flame region is always longer than the original gas region in any case. Concentration has significant effects on the flame region distribution and the explosion behaviors. In the tube, peak overpressures and maximum rates of overpressure rise (dp/dt) max for mixtures with lower and higher concentrations are great lower than that for mixtures close to stoichiometric concentration. Due to the gas diffusion effect, not the stoichiometric mixture but the mixture with a slightly higher concentration of 11% gets the highest peak overpressure and the shock wave speed along the tube. In the full-scale tunnel, for fuel lean and stoichiometric mixture, the maximum peak combustion rates is achieved before arriving at the boundary of the original methane accumulation region, while for fuel rich mixture, the maximum value appears beyond the region. It is also found that the flame region for the case of stoichiometric mixture is the shortest as 72 m since the higher explosion intensity shortens the gas diffusion time. The case for concentration of 13% can reach up to a longest value of 128 m for longer diffusion time and the abundant fuel. The “serious injury and death” zone caused by shock wave may reach up to 3–8 times of the length of the original methane occupied region, which is the widest damage region.

Effects of premixed methane concentration on distribution of flame region and hazard effects in a tube and a tunnel gas explosion

The flame deflagration of hybrid methane coal dusts in a large-scale detonation tube (LSDT)

In this work, the effect of various effective dimensionless numbers and moisture contents on initiation of instability in combustion of moisty organic dust is calculated. To have reliable model, effect of thermal radiation is taken into account. One-dimensional flame structure is divided into three zones: preheat zone, reaction zone and post-flame zone. To investigate pulsating characteristics of flame, governing equations are rewritten in dimensionless space-time (ξ, η, τ) coordinates. By solving these newly achieved governing equations and combining them, which is completely discussed in body of article, a new expression is obtained. By solving this equation, it is possible to predict initiation of instability in organic dust flame. According to the obtained results by increasing Lewis number, threshold of instability happens sooner. On the other hand, pulsating is postponed by increasing Damkohler number, pyrolysis temperature or moisture content. Also, by considering thermal radiation effect, burning velocity predicted by our model is closer to experimental results.

Moslem Safari Ghahsareh

Papers

Investigation of effective dimensionless numbers on initiation of instability in combustion of moisty organic dust