Abstract: Zhundong coal field is the largest coal field in China. However,the content of Na,Ca,Mg and Fe in the coal is usually very high,which has caused severe slagging and fouling problems during Zhundong coal combustion. In order to solve the slagging and fouling problems, the transformation mechanism of inorganic elements during combustion should be well understood. This study aims to investigate the transformation of inorganic elements in different forms in- to ash particles during Zhundong coal combustion. A set of coal samples were prepared by water washing,ammonium acetate washing and diluted hydrochloric acid washing. All the coals were burned in a drop tube furnace at 1 300 ℃ in air condition. The air flow rate was 10 L / min. Coal was fed at the rate of 0. 1 g / min. The combustion generated ash particles were collected by a low pressure impactor ( LPI). The mass of particles was measured by a precision bal- ance. The composition and morphology were analyzed by SEM-EDS. The results showed that Na was mainly in water soluble form. Ca and Mg were mainly in the ammonium acetate soluble and hydrochloric acid soluble form. While Fe was mainly in the hydrochloric acid soluble and insoluble form. After combustion,most of the ash particles were dis- tributed in the range of 1-10 μm. These particles mainly came from the ammonium acetate soluble and hydrochloric acid soluble Ca and Mg in the coal. Most of them were irregular in shape with high content of Ca and Mg. The remai- ning particles were Ca / Mg contained alumino-silicates with spherical shapes. Na mainly contributed to the formation of PM1(especially to PM0. 1 ),and the rest was presented in PM1-10 . Fe was mainly distributed in PM1-10 ,and the contri- bution of acid soluble Fe was significant. From this study,it can be found that inorganic elements in the form of ammo- nium acetate soluble and hydrochloric acid soluble were the key that resulted in the ash deposition of Zhundong coal. The use of acid to remove the inorganics in the coal is a way to mitigate the ash deposition problem.