Occurrence of sodium in high alkali coal and its transformation during combustion

With the discovery of super-huge coalfields located in Xinjiang province, the features of large coal reserves, sodium-rich in coal and the problems of boiler fouling during coal thermal utilization are paid more and more widespread attention. Also, more comprehensive investigation on the characteristics of sodium volatilization and its influencing factors in the combustion can provide an important reference for the efficient and clean utilization of high alkali coal. Therefore, a review was made to collect and analyze the relevant data about sodium volatilization characteristics during high alkali coal combustion from published materials. It is found that the sodium species in most of high alkali coal is dominated by water-soluble sodium (WS-Na), excepting few coals with insoluble sodium (HIS-Na) as main sodium species, e.g. Shenhua Kuanggou coal and Houxia coal. The coal ontaining higher HIS-Na has higher hydrochloride-soluble sodium (AS-Na) and ammonium acetate-soluble sodium (HS-Na) contents. Four influence factors including chlorine content, ash composition and combustion temperature, sodium form and content, which affects the sodium migration and transformation, were compared. It is concluded that the temperature has the greatest influence on sodium volatilization. The volatilization of sodium can be significantly increased with the increase of temperature, and the evaporation rate of sodium will be accelerated after 900 ℃. For the sodium content in 2000-4000 μg/g, there is a well positive correlation between the volatilization amount and the total amount of sodium, but it is irrelevant to the soluble sodium content. Chlorine can promote the volatilization of sodium as the molar ratio of Na to Cl is less than 3.5, while it has an inhibition as the molar ratio of Na to Cl exceeds 10. Moreover, a significant negative correlation is observed between the amount of sodium volatilization and the molar ratio of Na to(Si+Al)-(Ca+Mg). According to the existing research results, the migration and transformation behavior of sodium in coal combustion process is summarized into 3 stages consisting of internal conversion, external volatilization and conversion, as well as condensation, and 4 paths.