Gasification reactivity and morphology of coal chars formed in N2 and CO2 atmospheres

Abstract

In this study, chars derived from Zhundong coal were prepared in a tubular reactor under CO2 and N2 atmospheres in the temperature range 500–800 °C. The structure features of char were proven using scanning electron microscopy, X-Ray diffraction, N2 adsorption analyzer and Raman spectroscopy. Besides, the correlations of structure features and gasification reactivity of coal char were also investigated. The results reveal that char yield during devolatilization under CO2 atmosphere is relatively lower than that under N2 atmosphere, which could be attributed to heterogeneous reactions of CO2 with tar and char under CO2. The char formed in CO2 atmosphere exposes more pores in char matrix accompanied by the rough surface, resulting in a higher specific surface than that obtained under N2. Additionally, the char formed under CO2 atmosphere has lower degree of graphitization and higher ratio of small to large aromatic ring systems, compared to that obtained under N2. As a result, the gasification in a thermo-gravimetric analyzer indicates that the char prepared under CO2 is more reactive, due to more amorphous carbon structures formed under CO2. The homogeneous and heterogeneous reactions of CO2 with gases, tar and char occur when CO2 is introduced in the pyrolysis process. Moreover, the introduction of CO2 changes morphology of char, since CO2 participates in reaction process on the char surface, resulting in promoting the gasification reactivity.