Selective Removal of Sodium from Low-Rank Xinjiang Coal upon Multistage Countercurrent Water Washing: Experimental Investigation and Kinetics Modeling

Prior washing of coal is an essential step for the minimization of ash-related slagging and fouling inside a coal-fired boiler. With regard to the coal washing process, the recyclability of the washing reagent, usually water, is critical from both cost-effective and efficiency perspectives. This paper addresses the washing kinetics of water-soluble Na+ from two different low-rank Xinjiang coals using both fresh and used/recycled water to alleviate the ash-related fouling in low-rank coal-fired boilers. Apart from once-through fresh water washing, washing using recycled water via both multicycle single-stage and three-stage countercurrent processes was studied in detail to investigate the recyclability of water. Additionally, a modified shrinking core model (SCM) was developed to reveal the Na+ removal mechanism under all washing conditions. Our experimental results showed that the Na+ removal extent decreased with the recycling of used water due to an increase in the Na+ concentration in the recycled water and/or a decrease in the Na+ content in the washed coal. The saturation point of Na+ in the used water, beyond which the water can no longer remove Na+, is far below the solubility of NaCl in water. The modeling approach further confirmed that the overall rate for the removal of water-soluble Na+ is dominated by the intraparticle diffusion within the coal matrix. The effective diffusion coefficient of Na+ was within the range of 0.28 x 10(-6) to 3.75 x 10(-6) cm(2)/s, which agrees with reported values in the literature. Additionally, a novel iterative calculation method integrating the modified SCM into the three-stage countercurrent washing process has been proposed to predict the Na+ removal at each stage for each cycle. The results show that the water can be recycled a maximum of 15 times in the three-stage countercurrent process.