Prediction of the thermal ignition of hazardous materials from heat flow studies by using advanced kinetic analysis

摘要

The method of the prediction of the thermal behaviour of the energetic materials such as temperature and time to the ignition during cook-off experiments depends on the sample mass due to the significant influence of the heat and pressure generated during the reaction course. This study presents the application of the appropriate decomposition kinetics and proper heat balance calculated by numerical analysis for the determination of the effect of scale and geometry as well as the heat transfer, thermal conductivity and surrounding temperature on the heat accumulation in the sample and time to ignition. In this study, the exothermic decomposition parameters of a propellant were obtained using differential scanning calorimeter (DSC) tests conducted at various heating rates. The DSC signals were processed using differential isoconversional method to compute the apparent activation energy of the rate of a propellant's decomposition as a function of conversion. There was excellent agreement between the experimental and the simulation plots, which confirms the validity of the kinetic model used to describe the propellant's exothermic decomposition. The kinetic parameters and heat balance were subsequently analyzed and used for a simulation of slow cook-off experiments. Three temperature modes were chosen for experiments and simulations: (i) hold temperature 40 C, hold time 6h followed by the temperature ramp of 3.3 Kh-1 according to STANAG 4383, (ii) hold temperature 100 C, hold time 9h followed by the 1 Kh-1 temperature ramp, (iii) heat-wait-search mode (H-W-S) similar to those applied in Accelerating Rate Calorimetry. In this mode, the sample was heated to the pre-selected initial temperature 109 C, slightly lower than the ignition temperature recorded during the slower cook-off (lKh-1) experiment, and held a period of time (1.8 days) to achieve thermal equilibrium. A search was than conducted to measure the rate of heat gain (self-heating) of the sample. If the rate of self-heating was so slow that the temperature of the sample stayed constant, the temperature was increased by 4K and the heat-wait-search sequence was repeated. This routine was continued until the significant temperature jump was observed. The cook-off experiments were carried out in cylindrical steel tube with: ID 47 mm, length 200 mm, wall thickness 4 mm and the volume of 0.35L (armasuisse and AKTS in-house construction for data acquisition) equipped with thermo elements and pressure transducers. The pressure evolution, the fragments' size and number give the qualitative information on the violence level. During modelling the geometry, dimension of the container and, additionally, the amount, properties and thickness of the layer of the cylindrical steel tube and sample mass of the ammunition contained have been taken into account. The results of the simulation of the time-to-ignition of the 'slow cook-off experiment indicated the very good fit of the simulated and the experimental data. The proposed method was illustrated for a propellant but it can be applied for any hazardous materials [1,2].