Reactivity and susceptibility to porosity development of coal maceral chars on steam and carbon dioxide gasification

Abstract

Chars obtained at 1173 K from macerals of low rank Janina bituminous coal, i.e. exinite, vitrinite and fusinite, were gasified in steam and carbon dioxide to 50% d.a.f. burn-off (BO). The morphological changes of chars upon gasification, examined using optical and scanning electron microscopy, revealed characteristic external zonal BO of exinite and vitrinite char grains gradually diminishing from the outer to the inner part. The extent of external BO evaluated from mercury density data of chars and gasification products showed that the reaction of steam proceeds preferentially at the internal surface of the char pore structure (70% d.a.f. of total BO), while the reaction of carbon dioxide proceeds more readily at the external char surface (60% d.a.f. of total BO). Porosity parameters of investigated chars and gasification products evaluated from sorption data of CO 2 and C 6H 6 at 298 K showed that vitrinite is the most susceptible coal constituent for the preparation of porous carbon. Its char exhibits the highest reactivity towards the studied gasification agents as well as gives gasification products with the highest pore structure parameters. On the contrary, the values of the corresponding parameters for exinite char are the lowest. The influence of petrographic constituent mineral substances on the reactivity and porosity development of their chars has been assessed from their behaviour in the reactions of model composite chars (obtained from brown coal humic acids (HA) doped with the macerals low temperature ash (LTA)). It was shown that the reactivity of composite chars proceeds in parallel to the sum concentration of elements, strongly enhancing the rates of reaction for steam and carbon dioxide. Catalytic LTA enhancement of composite char reaction rate resulted in gasification products of higher mesoporosity and lower microporosity development.