Effect of atmosphere on sodium migration during conversion of high sodium coals

The sodium release and transformation of Yili (YL) and Hutubi (HTB) coal were examined at different temperatures under N₂, CO₂ and H₂O atmospheres. By using sequential extraction and wet digestion separately, the occurrence mode and content of sodium in raw coal and chars were measured. The migration of sodium in two coals was studied qualitatively and quantitatively. The results show that water soluble sodium in YL coal accounts for 80.08%, while in HTB, insoluble sodium is 61.54%. The volatilization of sodium in two coals increases with increasing temperature. Water soluble sodium in semi coke shows a decrease trend; ammonium acetate soluble sodium increases and then decreases; hydrochloric acid soluble sodium exhibits an increasing tendency. For YL volatilization of sodium is inhibited by CO₂ while it is promoted by H₂O compared with N₂. At 900℃ volatilization of sodium under CO₂ and H₂O is 50.25% and 111.45%, respectively, of that under N₂. CO₂ atmosphere can inhibit volatilization of sodium in HTB. At 900℃ volatilized sodium under CO₂ is 80.91% of that under N₂. In H₂O atmosphere, volatilization of sodium is higher than that in N₂ before 800℃ and slightly lower than that in N₂ at 900℃. The predominant sodium in YL coal is water soluble which volatilized during increasing temperature and transformed to hydrochloric acid soluble and insoluble sodium simultaneously. While for HTB, insoluble sodium is the dominant occurrence mode, followed by water soluble one. CO₂ and H₂O can promote transformation of insoluble sodium to soluble one in HTB coal.