VARIATION OF SODIUM FORMS AND CHAR REACTIVITY DURING GASIFICATION OF A SOUTH AUSTRALIAN LOW-RANK COAL

摘要

Experiments were performed to determine the gasification rate of South Australian Lochiel coal in CO_2 at temperatures between 750 and 950°C. Five samples were prepared, namely, raw coal, acid-washed coal, and acid-washed coal doped with 5 wt % Na_2CO_3, 5 wt % CaCO_3, and 2.5 wt % Na_2CO_3 + 2.5 wt % CaCO_3, and gasified in a TGA apparatus. While the acid-washed coal incurred a low but invariant reactivity, the raw and carbonate-doped samples experienced an increase in gasification rate with carbon conversion up to ca. 90% conversion, after which the gasification rate rapidly fell to zero at 100% conversion. It was speculated that transformation of the cations in samples during gasification might be responsible for the observed variation in the gasification rate. Further experiments using the 5 wt % Na_2CO_3―doped coal sample identified that the rising concentration of the active cations in the char was the primary reason for the increasing gasification rate before 90% conversion, but the lack of carbon availability was responsible for the rapid fall in gasification rate above 90% conversion. Before gasification, sodium in the 5 wt % Na_2CO_3―doped coal was about 30% water soluble and 70% water insoluble but acid soluble. Upon gasification, the water-insoluble sodium but acid-soluble sodium decreased to about 25% of the total sodium and remained invariant with time and temperature. The acid-insoluble sodium incurred an initial increase to 15% and then continuously reacted with silicates in the presence of coal. Heat treatment rapidly destroyed carboxylic functional groups and thus reduced the ion-exchange capacity of the acid-washed coal sample. The changes in sodium forms and quantity and the effect of heat treatment on the content of carboxylic functional groups and ion-exchange capacity explained the variation of gasification rate with carbon conversion. The change in the total surface area of the char during gasification was not the dominating factor influencing the char gasification rate.