Ash deposition characteristics and mechanism of coal with high Na and Fe contents during combustion in circulating fluidized bed

Abstract The ash deposition characteristics and mechanism of coal with high Na and Fe contents during combustion were studied based on experimental and theoretical analysis. Therein, the ash deposition experiment was performed in a 30 kW self-sustained circulating fluidized bed (CFB), while the theoretical analysis of mineral transformation was carried out by Factsage thermodynamic software. The morphological analysis indicates that the deposited ash formed within the furnace has a layered structure. The inner and outer layers have grayish-white and reddish-brown color, respectively. This morphology is related to the enrichment of Na and Fe. A portion of Na will deposit on the low-temperature heating surface and form a sticky sub-layer through “vaporization-condensation” mechanisms during the coal combustion, while other Na is retained in the ash as stable aluminosilicate compounds. Because of the low volatility of Fe and the relatively complex atmosphere within the furnace, Fe probably exists in the ash in the form of Fe 2 O 3 , FeO, Fe 3 O 4 and FeS, etc. These Na-and Fe-containing compounds both cause the decrease of ash melting point, thereby leading to the fly ash being captured more easily. Besides, the deposited Na and Fe will react with Si, Ca, Mg and Al in the high-temperature flue gas, furthermore promoting the continued growth of deposited ash.