Study on structure and gasification reactivity of lignite char from different pyrolysis reactor

Char gasification reactivity plays a vital role in the design of a gasifier. In this paper, a series of char were prepared by pyrolysis at 900−1100 ℃ in a fluidized bed and a drop tube reactor with a lignite as the sample, and the physico-chemical structure and CO₂ gasification reactivity of chars were studied by X-ray diffractometer (XRD), Raman spectrometer, static physical adsorption instrument and fixed-bed reactor. The results showed that the gasification reactivity of char in the fluidized bed and drop tube reactors mainly depended on its chemical structure. In the two reactors, as pyrolysis temperature increased, the polycondensation reactions of char deepened, resulting in the increasing crystalline size (aromatic sheet stacking height L_c, average diameter L_a) and larger ring to smaller ring ratio $ {I_{\rm{D}}}/{I_{({{\rm{G}}_{\rm{r}}}{\rm{ + }}{{\rm{V}}_{\rm{r}}}{\rm{ + }}{{\rm{V}}_{\rm{1}}})}} $ of char. Therefore, the gasification reactivity of char decreased with increasing pyrolysis temperature. Compared with in the drop tube reactor, the crystalline size and larger ring to smaller ring ratio $ {I_{\rm{D}}}/{I_{({{\rm{G}}_{\rm{r}}}{\rm{ + }}{{\rm{V}}_{\rm{r}}}{\rm{ + }}{{\rm{V}}_{\rm{1}}})}} $ of char in the fluidized bed reactor were lower at the same pyrolysis temperature, and their variations with increasing pyrolysis temperature were smaller, resulting in the lower gasification reactivity of char and the smaller change of gasification reactivity of char with increasing pyrolysis temperature. Those were mainly due to the long residence time of char and strong interactions of char and volatiles in the fluidized bed reactor, deepening the degree of polycondensation reaction of char.