Char structural ordering during pyrolysis and combustion and its influence on char reactivity

In the present study, two processes, thermal treatment and oxidation, were separated for a fundamental study of structural evolution during pyrolysis and combustion, as well as for the study of the influence of such evolution on char reactivity. Chars were prepared at different temperatures and heating rates from a size-graded low volatile bituminous coal. The reactivity of resultant chars was measured in Kinetic Regime I using a fixed bed reactor. The structure of fresh and partly burnt chars was characterized using quantitative XRD analysis (QXRDA), high-resolution TEM (HRTEM), high-resolution FESEM, and multi-point gas adsorption. XRDA and HRTEM observations show that char structure becomes more ordered with increasing pyrolysis temperature and decreasing heating rate. Char structure was also investigated as a function of char burnoff. The QXRDA results show that the amorphous concentration of char decreases during combustion while the aromaticity and average crystallite size of char increase. As a result, char structure becomes more ordered during combustion. This is in agreement with HRTEM observations. Due to the low reaction temperature (about 673 K), which is much lower than that for char preparation (1473 K), it was believed that oxidation, instead of thermal effect, contributed to the structural ordering observed during combustion. The structural parameters obtained from QXRDA were then correlated to char reactivity. Structural ordering was found to be responsible for char deactivation during thermal treatment and oxidation. Since the amorphous concentration and aromaticity of char are two strongest indicators of char reactivity, a structural disorder index, DOI, was defined based on them to describe char structural evolution, and further correlated to char reactivity.