On explosibility of melamine dust/air mixture

Abstract

The work is mainly devoted to the analysis of dust hazard features, whose test in a standard blasting chamber with a volume of 20 liters to 1 m³ leads to a maximum pressure P_max comparable to a value of 100 kPa. Such dust is suggested to be considered dust with a low explosion hazard, in contrast to dust with high explosion hazard ( P_max >> 100 kPa). The choice of the critical pressure level (»100 kPa), indicating the flame propagation over a considerable distance from the ignition source, is based on the analogy between the models of percolation theory and the simplified combustion scheme for a macroscopically inhomogeneous dust/air mixture. For dust with low explosion hazard (Poletaev, 2017), the limiting oxygen concentration (LOC) is close to the normal oxygen content in the atmosphere C_OX,0 = (20.7 ± 0.1) % by vol. It was proposed to use the parameter D_OX = C_OX,0 - LOC to evaluate the dust explosion hazard along with the standard parameter Pmax. It is shown that for dust with a low explosion hazard 0 < D_OX < 0.5 % by vol. An illustration of the conclusions of the work was carried out using the known results of a study in 20-l and 1000-l blasting chambers of three melamine samples with an average particle size dm of less than 10, 19 and 52 μm, respectively. It is shown that the mixtures of all the examined melamine samples with atmospheric air are non-explosive under normal conditions (absolute pressure 100 kPa, temperature 25 °C). To explain the non-explosive of melamine dust having a high calorific value (15.67 MJ/kg), it has been suggested that the volatile component of its thermal decomposition products (ammonia) is burned, which allows an analogy between the combustion of melamine and coal particles. On the basis of the fact that a study of dust in a 20-liter chamber is actually performed at elevated initial values of temperature and pressure (Cashdollar and Chatrathi, 1993, Poletaev, 2017), the temperature at which a melamine sample ( d_m = 19 μm) forms dust/air mixture with a high explosive hazard was determined (68 °C). The importance of this result is due to the fact that in the production of dispersed melamine, the finished product can be transported by air with a temperature of over 100 °C. In European standards EN 14034, the correction of the value of P_max obtained in a 20-liter chamber is proposed with the aim of predicting the value of this parameter, expected from the results of tests in a 1000-liter chamber. The above test feature in a 20-liter chamber makes such an adjustment erroneous in the case of dust with low explosion hazard. Taking into account this peculiarity and a number of empirical regularities of turbulent combustion of an dust/air mixture, an estimate of the minimum value of the index K_st , corresponding to the case of an explosion in a 20-liter chamber, is performed. It is shown that for a subcritical non-explosive dust/air mixture (LOC » C_OX,0) the transition to a state of high explosion hazard is possible with heating by only 10…20 °C.