Investigation on release and migration characteristics of sodium during the combustion of high-sodium coal with phosphorite

Abstract

To mitigate slagging, fouling, and corrosion from burning high-sodium coal, phosphorite was introduced during combustion. The migration and release characteristics of Na were analyzed at different temperatures and phosphorite ratios. Results show that Na in Shaerhu coal mainly exists in water-soluble (68.75 %) and insoluble forms (22.35 %). As the temperature increases from 700 °C to 1100°C, the water-soluble sodium in the coal ash decreases by 1151 μg∙g−1, and the insoluble sodium increases by 98 μg∙g−1. Phosphorite can react with water-soluble sodium at high temperatures. Adding 15 % phosphorite at 900°C will reduce the water-soluble sodium content by 455 μg∙g−1, increase the insoluble sodium content by 1408 μg∙g−1, and reduce the sodium release ratio by 12.99 %. The softening temperature of Shaerhu coal is 1158°C. After adding phosphorite, the softening temperature of coal ash can be increased to more than 1200°C. Compared with SiO2 used in traditional processes, phosphorite as an additive can improve anti-scaling performance and facilitate ash discharge in solid form. Meanwhile, recycling phosphorite co-combustion with coal for using in thermal phosphoric acid process can increase the conversion ratio of phosphorite by 10 %–20 %.