Removal Of Alkali By Kaolin During High-Sodium Coal Combustion

Xinjiang is one of the biggest coal reserve areas in China. However, most of the Xinjiang coals are low-rank lignite with high sodium content. Release of alkali species during high-sodium coal combustion in a boiler would cause many problems such as fouling, slagging and high-temperature corrosion, which puts a huge obstacle for their broad and efficient utilization. The method of using solid sorbents to clear alkali vapors in hot flue gas is very simple and effective. In this work, kaolin was chosen to conduct alkali adsorption experiments both in a high-temperature muffle furnace and in a drop-tube furnace to evaluate kaolin as a sodium-binding additive for the high-sodium coal. And influences of kaolin content, temperature, particle sizes of kaolin and fuel on sodium emission and the sodium capture performance of kaolin were investigated. In addition, the sodium capture mechanism of kaolin was elucidated with various analyses.The content of sodium release increased with increasing temperature during high-sodium coal combustion. The sodium emission ratio was 51.73% at 1100°C. But the fusion of ash would accelerate sodium release when the softening temperature of ash was reached. 61.11% of sodium was released at 1400°C. Firing large coal particles would depress the sodium emission. The sodium emission ratio was 60.14% for firing 13μm coal at 1100°C, while it was only 43.29% for the 48μm coal.The sodium capture performance of kaolin was better when it was used to adsorb sodium vapor in a muffle furnace than that in a drop-tube furnace. However, kaolin could decrease sodium concentration in the flue gas and thus fouling and slagging to some extent even during the drop-tube combustion. Therefore, it is suitable for kaolin to be a sodium-binding additive for the high-sodium coal. The sodium capture mechanism of kaolin was the reactions between metakaolin, SiO2 and gaseous sodium species to form sodium aluminosilicates such as nepheline(NaAlSiO4) and albite(NaAlSi3O8), among which nepheline was the predominant reaction product.Increasing kaolin addition or using fine kaolin particles could both improve the sodium capture performance of kaolin by increasing the available contact areas needed for reaction. In the static combustion tests, the sodium capture efficiency of 3μm kaolin(6wt.% addition) was 83.55%, which was higher than 78.47% of the 12 wt.% kaolin(18μm) addition. This suggested the poor utilization of kaolin in high dosages and gave us a guideline for practical utilization of kaolin. The sodium capture efficiency decreased as temperature was increased, which could be attributed to the increasing sodium release and the decomposition of albite at high temperatures. Approximately 59% of sodium was retained in ash by kaolin at 1400°C in the static combustion experiments, while it was only 50% for the drop-tube tests. In the tested size range, size effect of coal on sodium capture performance of kaolin could be neglected, which might be ascribed to the adequate sodium vapor in the flue gas despite firing different coal particles. In the drop-tube experiments, the sodium retention with 6wt.% kaolin(11μm) addition at 1200°C could attain 70% of the total sodium in the combusted coal.