Variation of the crystalline structure of coal char during pyrolysis and its effect on gasification reactivity

The effects of pyrolysis temperature and heating rate on crystalline structures of Shenfu char were studied by using X-Ray Diffraction (XRD) technique. Shi et al. 's model[12,13] was adopted to quantitatively process the data of XRD patterns of chars and obtain the crystalline structure parameters of Basic Structure Unit (BSU). The characteristics and transformation of chars' crystalline structure in the temperature range of 950° ~1400° were examined . Char gasification reactivity was investigated by using TGA. Under the conditions used in this study, it is found that the average stacking height (Lc) increases apparently and the carbon crystallite size (La) of the BSU changes little with the increase of pyrolysis temperature. It means that the coalescence growth of longitudinally adjacent BSUs dominates the growth process of BSUs in the temperature range used. Crystalline structure of chars becomes ordering with the increase of pyrolysis temperature but the graphitization is still not reached. Good correlation is also found between the heating rate and the char gasification reactivity. The chars generated from slow pyrolysis demonstrate a distinct low gasification reactivity as compared with ones from rapid pyrolysis. During slow pyrolysis, relative long resident time at high temperature leads to the further coalescence growth of longitudinally adjacent BSUs and the loss of lateral active sites of aromatic units. Crystalline structure of char reaching unity further would cause the chars less active.