Study on the Migration of Sodium during the Combustion and Gasification of Zhundong Coal

<>Study on the Migration of Sodium during the Combustion and Gasification of Zhundong Coal

Haixia Zhang^1*, Xiaoyu Zhang^1,2, Yongjie Na¹, Zhiping Zhu¹

¹Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing, China

²University of Chinese Academy of Sciences, Beijing, China

*Mailing Address: 11 Beisihuanxi Road, Beijing 100190, China

E-mail of Corresponding Author: zhanghaixia@iet.cn

Abstract:

Zhundong coalfield in Xinjiang, with reserves of 390 billion tons, is the largest integrated coal basin in China. With low mining costs, good reactivity and great reserves, Zhundong coal can be potentially used to generate electricity and supply chemical products. However, practical utilization of Zhundong coal in power plants shown that, the high sodium content in Zhundong coal induced to agglomeration and erosion influencing safety and efficiency of the equipment operation, and there is no fundamental solution till now. Circulating fluidized bed (CFB) gasification is one of the key technologies for the high efficiency and clean use of coal resources, but the high sodium content in Zhundong coal would also induce to agglomeration or even to de-fluidization phenomenon during gasification process. It has significant research and practical value to study the migration of sodium during combustion and gasification of Zhundong coal, and then provide basic data for the design and operation of CFB gasifier with the utilization of such a cheap but important coal resource.

In this work, the migration of sodium during combustion and gasification of Zhundong coal was investigated and the cause of agglomeration phenomenon was also studied. Thermogravimetric analysis was adopted to analyze the combustion and gasification characteristics of Zhundong coal. Coal sample was ashed at 575^oC, 815^oC and 900 ^oC, respectively, following the slow ashing process in a muffle furnace to obtain combustion residual. Gasification residual was obtained with a horizontal tube furnace at 900 ^oC in CO₂ atmosphere. The element contents, phase components, morphology and sodium content of the residuals were characterized with X-ray fluorescence (XRF), X-ray diffraction (XRD), scanning electron microscope & energy dispersive spectrometry (SEM-EDS).

The results show that Zhundong coal has good combustion and gasification reaction activities. The sodium content decreases with the increase of the ashing temperatures. At 900 ^oC, the mass contents of combustion and the gasification residuals are almost the same but the compositions are quite different. The Si and Fe contents of the gasification residual are much higher than that of the combustion residual, but the Na content is slightly lower. The S content of the combustion residual is much higher than that of the gasification residual. On the phase composition, both of the residuals contain SiO₂ and NaAlSiO₄; Fe₂O₃ and CaSO₄ shows up in the combustion residual while Fe₃O₄ and Ca₂SiO₄ in the gasification residual. NaAlSi₃O₈ is also detected in the gasification residual. The agglomeration phenomenon is more obvious for the gasification residual. The sodium-containing compounds seem much easier to form low temperature eutectic by reacting with oxide, such as SiO₂ and Al₂O₃, increasing agglomeration phenomenon.

Keywords: Zhundong coal; sodium; combustion; gasification; agglomeration

Acknowledgements:

This work was financially supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (No. XDA07030100) and the National Natural Science Foundation of China (No. 21306193).