Sequential extraction was employed to determine the modes of occurrence of calcium in Zhundong coal.The samples from Zhundong coals before and after extraction were ashed in a high temperature atmosphere furnace.The calcium content of the coal and ash samples and the calcium-contained minerals in the samples were determined using an inductively coupled plasma atomic emission spectroscopy and X-ray diffraction analysis.The results indicated that the calcium contained in Zhundong coal was in the forms of water-soluble

ammonium acetate-soluble

hydrochloric acid-soluble and insoluble species

and the main calcium modes were the ammonium acetate-soluble and hydrochloric acid-soluble species

while the proportions of the water-soluble and insoluble-calcium were low.During the combustion process

the chemical reactions among the calcium-containing minerals

the silicon-containing minerals and aluminum-containing minerals in Zhundong coal occurred to form fukalite

mayenite

kilchoanite and rankinite.The base/acid ratio decreased and the ash melting temperature increased after water extraction.The hydrochloric acid-soluble calcium species reacted with the silicon-containing and aluminum-containing minerals

leading to form mayenite

kilchoanite and rankinite.The insoluble-calcium species existed in the form of margarite was stable in the coal during combustion process.