Determination of Kinetic Parameters of Rapid Pyrolysis of Ammonium Perchlorate

Abstract

A laser pyrolyser setup is developed to achieve isothermal conditions using a heating rate of up to ~10 5 º C.s -1 for rapid pyrolysis of a sample. It provides mechanistic information about the decomposition behavior of the sample during combustion, typically in the condensed phase. Data about the kinetics of chemical reactions occurring in the condensed phase during combustion is not derived from ab initio techniques, but rather by fitting the variation of linear burning rate with pressure with appropriate kinetic parameters. This leads to a wide variation of chemical kinetic parameters governing the decomposition of the same solid propellant ingredient, such as the commonly used oxidizer, ammonium perchlorate (AP). Therefore, the laser pyrolyser/Fourier transform infrared (FTIR) spectroscopy was used to study the rapid pyrolysis of AP at 1 bar. The concentration profiles of the evolved gases were used to develop the global reaction mechanism. The global reaction mechanism was found as follows: 𝑁𝐻 4 𝐶𝑙𝑂 4 → 1.09 𝐻 2 𝑂 + 0.06 𝐻𝑁𝑂 3 + 0.56 𝑁𝑂 2 + 0.18 𝑁 2 𝑂 + 0.5 𝐶𝑙 2 + 0.71 𝑂 2 + 0.88 𝐻 2 with kinetic parameters as pre-exponential factor (A) = 6.6x10 7 s -1 , activation energy (Ea) = 108 kJ.mol -1 . Kinetic parameters of the same AP from thermogravimetric analysis are A = 5.6 x 10 5 s -1 , and Ea = 100 kJ.mol -1 . This suggests that kinetic parameters obtained from slow heating experiments predict a reaction rate significantly lower than that observed under high heating rates. Therefore, the raw data generated from rapid pyrolysis would be useful to develop detailed chemical reaction mechanisms for the chosen ingredients which would further improve the fidelity of the solid propellant combustion models.