Effects of metal ions on the thermal decomposition of brown coal

Pyrolysis experiments of Loy Yang brown coal from Victoria, Australia, were performed to examine the influence of coexisting metal species. Sodium, magnesium, calcium, barium and nickel were loaded either by ion exchange or by impregnation. The metal-loaded samples were heated under helium at the rate of 5 degrees C/min up to 900 degrees C. Exchanged metal ions affected the course of decomposition in various ways. The temperature of maximum rate of weight reduction was lower for sodium, barium and nickel exchanged brown coal than that for acid-washed coal by 50 degrees C, while the presence of magnesium and calcium did not influence the decomposition temperature compared to that for acid-washed coal. The volatile matter yield decreased and the yield of C-1 gases increased with the amount of nickel or calcium loading. The char yields were almost the same regardless of the cations present, but the surface area of chars varied with metal species and metal loading. In particular, the surface area of char obtained from sodium exchanged coal decreased dramatically with increasing sodium content. The influence of impregnated metal species was found to be different somewhat from those of ion exchanged species, suggesting different interaction with coal surfaces.