Theoritical And Experimental Study On Explosibility Parameters Of Mg Powders In Inerting Conditions

Fire and explosion of magnesium dust are both major obstacles for the development of magnesium powder industry. Using inerting techniques to prevent and mitigate magnesium dust explosions have attracted more interesting in the world for the time being. For four kinds of spherical magnesium powder with different diameters, which are widely used in industry, experimental and theoretical study was systematically conducted on the ignition & explosion parameters and their effecting factors in different inerting conditions. Especially, a heat release rate model was constructed for the igniting process of magnesium powders in oxygen-nitrogen- argon ambience according to synchronous reaction mechanism. Based on this key model, the theoretical studies on ignition & explosion parameters of magnesium powders in inerting conditions were done.At first, the ignition process of magnesium dust layer at constant temperature of the heated plate was modeled according to numerical heat transfer and chemical reaction kinetics. The regularities of temperature distribution and’preferential ignition point’inside the layer were founded using finite-difference method. The results showed that the two stages existed during ignition process of the magnesium dust layer. The first is temperature- rising stage, the other one is long- time temperature keeping stage. How long the second stage existed was determined by the temperature of the heated plate. The results also showed that the heat release of chemical reaction was a main factor for inducing smoldering combustion of dust layer on hot surface.The minimum ignition temperature of magnesium powders layer in nitrogen or argon inerting condition was higher evidently than the one in Air. Nitrogen gas can inert Mg powder layer, but its effect was lower than that of argon. The "inert factor" put forward in this paper can describe their inerting effects quantitatively.The energy equations of magnesium powders cloud in G-G furnace was established based on two-phase flow, heat transfer and chemical reaction kinetics, and then instantaneous temperature model and thermal explosion model were put forward. The effect of dust concentration, particle diameter and dispersing pressure on the minimum ignition temperature of magnesium powders cloud was calculated by the two models. There was a good agreement between the calculation results and the experiments. The inerting effect of nitrogen and argon to the minimum ignition temperature of magnesium dust cloud was that the grade of minimum ignition temperature increased with the content of inert gas, which was similar to that of the dust layer.Finally, a set of numerical model based on chemical reaction kinetics and thermodynamics were set up for the evolutionary processes of magnesium explosion in 20L sphere. Oxygen and nitrogen were dealt with an admixed oxidant to simplify the calculating process of magnesium conversion. Heat loss through the wall of the sphere was taken into account, which improved the calculating accuracy compared to the traditional method. A ’corrective coefficient’ was defined and introduced in order to analyse the effect of turbulence to explosion process. The results showed that the inert effect of nitrogen to Mg powder explosion in confined vessel was similar to argon, but they showed different effect on the explosion overpressure. Argon has stronger inerting effect than nitrogen with the decreasing of oxygen concentration.The results were useful for the application and development of inert techniques of Mg powders in the industry. The given models in the paper were also suitable to other combustible dusts, especially for the one which can react with two or more oxidants simultaneously.