Carbothermal reactions of tobelite with coal char at high temperatures under N2 atmosphere

Abstract

The main aim of this investigation is to present a comprehensive analysis of carbothermal reactions with tobelite at high temperatures under N2. The mixtures with mass ratios of demineralized coal: tobelite = 1:3, 1:5, 1:7, 1:9 were subjected to thermogravimetric analysis (TGA) up to 1500 °C, the mineral composition and morphology of the product after carbothermal reaction were evaluated by X-ray diffraction (XRD) and scanning electron microscope with energy dispersive spectroscopy (SEM-EDS). FactSage equilibrium calculations also used for predicting mineral transformation process. It was found that thermal transformation of the tobelite in demineralized coal includes four stages: stage I (0–200 °C), stage II (200–900 °C), stage III (900–1200 °C), stage IV (1200–1500 °C). The processes of coal devolatilization and tobelite decomposition are independent processes when the temperature is below 1000 °C, the contact mode among minerals and coal matrix affect the thermal behavior of minerals. The carbothermal reactions start at 1200 °C. When coal: tobelite = 3:7, the products include incompletely reacted Al6Si2O13 (mullite), Al2O3, SiC, β-Si3N4 and Al6Si6N8O9; When coal: tobelite = 5:5, the reaction products include SiAl6O2N, Al2O3 and SiC, Al2O3 and SiC are the main phases; When coal: tobelite = 7:3, the composition is Si6AlO6N5, AlN, SiC and SiO2, SiC and AlN are the main phases. Carbon structure may strongly influence the carbothermal reaction. In addition, the formation and disappearance of SiC could releases CO by reaction with carbon during the carbothermal reaction, thus promoting char gasification.