Ash deposition behavior in co-combusting high-alkali coal and bituminous coal in a circulating fluidized bed

Abstract

High contents of alkali and alkali earth metals (AAEMs) induce severe ash slagging and fouling during coal combustion. In this regard, co-combustion is an effective method to mitigate ash deposition. The ash deposition behavior in co-combusting high-alkali Zhundong (ZD) coal and a bituminous coal, i.e. Shenhua (SH) coal was tested in a 30 kW circulating fluidized bed test system. The results revealed that the condensation of Na2SO4 triggered the slag formation of ZD coal in the furnace. The Na-Ca-Fe eutectics were responsible for the melting or partial melting of slags at high temperatures. After blending SH coal with ZD coal, the volatilization of Na was effectively suppressed, and compacted slags practically disappeared at 20% blending ratio of SH coal. In the convective backpass, fine particles (<10 µm) tended to be deposited on the leeward side of the heating surface because of the eddy impaction. As the blending ratio of SH coal increased, the size distribution of fly ash shifted to a large size range, because of the dilution effect and capture of AAEMs by silicates or aluminosilicates. The minimum blending ratio of SH coal was recommended as 20% to mitigate the ash deposition tendency of ZD coal.