Lumped kinetic study of non-isothermal decomposition of urea using combined model-free and Coats-Redfern methods

Abstract

Kinetic modeling of urea decomposition under very high heating rates is essential for a wide range of applications. In this work, a lumped kinetic analysis of the decomposition process is presented. Thermogravimetric analysis (TGA) was employed to measure the mass loss of urea at various heating rates. Based on TGA and XRD results, the decomposition pathway was classified into two principal stages: (i) pyrolysis of urea to biuret, followed by subsequent decomposition, and (ii) decomposition of cyanuric acid. For each stage, kinetic parameters were evaluated using Coats–Redfern (CR) integral approximations in conjunction with Flynn–Ozawa–Wall (FWO) and distributed activation energy model (DAEM) methods. The average activation energies derived from the FWO and DAEM approaches were 97.4 and 93.9 kJ/mol for the first stage, and 200.2 and 190.7 kJ/mol for the second stage. Utilizing these values, CR-based kinetic analysis indicated that the first stage follows a first order reaction model (n = 1), with an average activation energy of 94.9 kJ/mol, whereas the second stage is best described by a third order reaction model (n = 3), with an average activation energy of 196.9 kJ/mol.