Abstract: Zhundong coal,which is produced in Zhundong district,Xinjiang,is a kind of coal featured low ash and sulfur contents,and high reactivity.However,the alkali metals tend to release into the flue gas in the form of gas phase during combustion,which leads to changes in physicochemical properties of the fly ash particles,such as reducing their ash melting point and enhancing their cohesiveness and corrosivity.Furthermore,it will cause serious problems such as fouling,slagging,and corrosion on the surface of the boilers and heat exchange equipments.The occurrence modes of alkali metals determine the release behavior,thus in-depth study of occurrence forms and contents of alkali metals would be beneficial to accurately predict their release form and content,and it would be meaningful for clean and efficient use of Zhundong coal.Zhundong coal with high Na content was employed in this paper.The surface distribution characteristics of different elements were analyzed by scanning electron microscopy-energy dispersive spectrometry( SEM-EDS).Simultaneously,the occurrence modes and contents were investigated by inductively coupled plasma optical emission spectrometer( ICP-OES).X-ray photoelectron spectroscopy( XPS) and X-ray diffraction( XRD) were employed for calculating surface elemental content and crystalline structure of alkali metals,respectively.The SEM-EDS results show that Ca and S are obviously enriched in same regions,indicating that CaSO_4 is presented in these regions; no highlight region for Cl was observed during the scanning process,suggesting that Cl is evenly distributed in Zhundong coal; In addition,a small number of highlight regions in Na co-exist with S,implying that the Na_2SO_4 crystal is also presented in Zhundong coal.The XRD results illuminate that Na exists in the form of crystalline structure of Na Cl and Na_2SO_4 after drying at 105 ℃.The corresponding peak intensity decreases or even disappears after extraction via deionized water.The ICP-OES results show that Na in Zhundong coal is mainly presented in the form of water-soluble,ammonium acetate and acid-soluble Na. with mass fraction of 78.42%,11.57%,and 6. 44%,respectively.And the remaining 3.57% is the insoluble-Na in the form of aluminosilicate; the cations in the extract liquid are mainly sodium,calcium,and magnesium ions,and the anions are mainly chloride,sulfate,and hydrogencarbonate ions.The total charge numbers of cations and anions were calculated to be 5.369 mmol/L and 5.385 mmol/L,respecctively.This indicates that the charge numbers of anions and cations are basically conserved in the solution.The water-soluble Na is considered to exist mainly in the form of crystals or hydrated ions of Na Cl,Na_2SO_4,and NaHCO_3,with mass fraction of 5.32%,85.7% and 8.98%,respectively( uniformly distributed).Besides,XPS was used to analyze samples dried at different temperatures,and the results indicate that the water-soluble Na can migrate to the surface with the water during drying process and then release with the evaporating water when temperature reached to 100 ℃.The Na/C ratio on the surface of the sample increases first and then decreases with the migration of moisture.The released Na through this process can be defined as water-volatile Na.