Thermal transformation of tobelite from coal at high temperatures and the kinetics and mechanism of dehydroxylation and deamination process

Abstract

Tobelite from coal was processed to remove impurities and submitted to thermal treatments. The thermogravimetric analyzer (TGA) was employed to investigate the thermal behavior of tobelite between 25 and 1500°C. X-ray diffraction (XRD) and in-situ diffuse reflectance infrared Fourier transformation (DRIFT) spectroscopy were used to examine the structural change of tobelite and the dehydroxylation and deamination process. The results show that the thermal transformation of tobelite can be divided into three stages: −350°C (stage I), 350–1000°C (stage II), and 1000–1500°C (stage III). The following kinetic triplet in the dehydroxylation and deamination process at stage II were calculated: Eα=199.67kJmol−1, f(α)=(1/3)(1−α)[−ln(1−α)]−2 and A=6.76×1013min−1. Meanwhile, the thermodynamic functions (ΔS, ΔH, ΔG) calculated using activated complex theory shows that dehydroxylation and deamination process is endothermic. The proposed mechanism of tobelite decomposition includes multi-step reactions: (1). Condensation of water molecule in the octahedral layer; (2) Transmission of water molecules through the tetrahedral ring; (3) Migration of water and ammonia molecules through the interlayer region and recombination of silica and alumina tetrahedron into metatobelite; (4) Metatobelite transformation into mullite at around 1030°C. Besides, the deamination and dehydroxylation process proceeds simultaneously, but the former occurs more easily.