Efficient and accurate removal of Na from Xinjiang Zhundong coal with low concentration ammonium salts

The Zhundong region in Xinjiang has abundant high-Na coal resource, which can be used as high-quality fuel and chemical raw material. However, the high-Na content can lead to severe equipment pollution and ash slagging during combustion and gasification processes, thereby significantly restricting its large-scale and efficient utilization. The efficient, economical, and environmental friendly removal of Na is of great significance for the clean utilization of the Zhundong coal. This study investigates the rule and mechanism of Na removal from low-rank high-Na coal in the Zhundong region using a series of low-concentration ammonium salts. The results show that the deionized water (DI) can only remove about 52% of Na in the coal. When some low-concentration ammonium salts such as NH₄Ac, NH₄Cl, (NH₄)₂SO₄, NH₄NO₃ and TMACl with a concentration of 0.006 mol/L are used, the total Na removal rate and organic Na removal rate can reach to 90%−97% and 80%−95%, respectively. The Na₂O mass fraction in coal ash is significantly reduced from 6.21% in raw coal, 2.77% in water-treated coal and 0.27%−0.81% in coal treated with low-concentration ammonium salts. Among the various ammonium salts mentioned above, the NH₄Ac shows the best Na removal effect. The size of the cation in the Na removal agent plays an important role in the removal of organic Na, and the Na removal capacity follows the order: H⁺ > NH₄⁺ > TMA⁺, which is due to the smaller cations are more favorable for entering coal pores and interacting with organic Na through ion exchange. Ammonium salts can remove various types of Na carboxylates through conventional ion exchange mechanisms, but the removal mechanism of phenolic Na may involve ion exchange, complex decomposition of phenolic ammonium salts, and hydrolysis of NH₄⁺. Increasing the temperature, prolonging the time, and increasing the amount of ammonium salts significantly enhance the removal of total Na, water-soluble inorganic Na, and organic Na in coal. A correlation between the amount of ammonium salt added and the removal of total Na and organic Na in coal has been established. For NH₄Ac, the correlation is as follows: y_{(total Na)} = 12479x_{(ammonium amount)} + 2200, y_{(organic Na)} = 12394x_{(ammonium amount)} + 136. In order to meet the national coal combustion standard (w(Na₂O) < 2.0%, mass fraction by ash) for the Zhundong coal, about 70% of the total Na needs to be removed, about 4.26 kg of NH₄Ac should be added per ton of the Zhundong coal.