Correlation of sodium releasing and mineral transformation characteristics with ash composition of typical high-alkali coals

Abstract

Understanding the sodium migration mechanism during combustion process is critical for the clean and efficient utilization of high-alkali coals. In this paper, sodium releasing characteristics of five typical high-alkali coals are experimentally investigated based on occurrence mode and X-ray diffraction analysis. The experiments are conducted in a tube-furnace to investigate the sodium releasing and mineral transformation characteristics. The silica-alumina minerals in coal ash have a sodium fixation effect and therefore reduce the sodium releasing rate. However, calcium-containing minerals can weaken this effect. The mole fraction ratio of (Ca + Mg)/(Al + Si) in coal ash correlate well with the sodium release rate (considering occurrence forms), which is further proved by experiments. A new prediction formula for sodium release rate is proposed and validated therefore. It also demonstrates that, the sodium release rate of coal with a high‑calcium content is high, and much sodium is released in the gas phase at high temperatures, thus retaining the calcium minerals in coal ash. The sodium release rate of coal with high silicon and aluminum contents is low, and sodium remains in the coal ash in the form of aluminosilicate at high temperatures. Finally, the migration and transformation mechanism of sodium-containing minerals in high-alkali coals is concluded.