Char reactivity and kinetics based on the dynamic char structure during gasification by CO₂

Coal char reactivity is the essential for gasifier design and operation. Char structure and reactivity are dynamic with reaction during gasification. To obtain the relationship between dynamic structure and reactivity, char samples at different carbon conversion were prepared at 1223 K by a rapid heating thermogravimetric analyzer (R-TGA). The ex-situ char reactivity was investigated in TGA between 1073 and 1223 K, and was correlated with the dynamic structure. Besides of XRD and SEM results, nano-computed tomography (nano-CT) revealed the morphology of carbon matrix and minerals during gasification. Results showed that reaction rate of in-situ char significantly increased firstly and then decreased, which was attributed to the gas switch step, the increase of graphitization degree and inhibition effect of minerals. In contrast, the reactivity of ex-situ char was initially controlled by chemical structure and later by the abundant catalytic activity of minerals. The reactivity decreased slightly and then increased with increasing carbon conversion. The unreaction core model (URCM) was more suitable to interpret the gasification kinetics based on the dynamic char structure and fitting coefficient. Meanwhile, the results by iso-conversional method also supported the validity of URCM, but also indicated that the activation energy of char gasification gradually increased during in-situ gasification.