Transformation of Sodium during the Ashing of Zhundong Coal

Zhundong coalfield in Xinjiang is the largest integrated coal basin newly found in China. Due to the unique properties such as high volatile content, low ash and sulfur content, and high reactivity, Zhundong coal can be potentially used to supply gas fuels, liquid fuels, hydrogen, electricity and other chemical materials. However, according to practical utilization, the high alkali (especially sodium) content in Zhundong coal could induce severe problems during the combustion process, such as fouling and slagging, which influence the safe operation of the plants. It is well known that alkali metals would be volatilized at relatively low temperature. However, the existing national test standard for coal ash composition cannot completely reflect the actual content and their effect on fouling and slagging. Therefore, it is important to investigate the transformation of sodium in Zhundong coal during the ashing process. In this work, thermogravimetric analysis was adopted to determine the ignition and burnout temperatures of Zhundong coal. Coal samples were ashed at 500oC, 600oC, 700oC, 800oC, 850oC and 900oC, respectively. The element contents, phase components and microstructure of the ash were characterized with X-ray fluorescence (XRF), X-ray diffraction (XRD) and scanning electron microscope & energy dispersive spectrometry (SEM-EDS). The results show that the ignition and burnout temperatures of Zhundong coal samples are 386.5oC and 487oC, respectively, indicating that the selected ashing temperatures can ensure the complete combustion of Zhundong coal samples. The ashing temperature has a significant influence on the content and composition of sodium in ash. The total contents of sodium and chlorine decrease with the increase in the ashing temperature. At 500oC and 600oC, the main form of sodium is NaCl. At 700oC, NaAlSi3O8 is observed. Further increasing the ashing temperature will promote the appearance of NaAlSiO4. 500oC can be chosen as the ashing temperature of Zhundong coal for alkali detection. Useful foundation has been laid for further study on the advanced technologyfor the clean and efficient utilization of Zhundong coal resource.