Experimental comparative study on ash fusion characteristics of Ningdong coal under oxidizing and reducing atmosphere by means of SiO2-Al2O3-(CaO + MgO + Na2O + K2O) pseudo-ternary diagrams

Concern about the serious slagging during combustion and the troublesome liquid slag tapping during gasification has motivated the study on ash fusion characteristics of Ningdong coal under oxidizing and reducing atmosphere by SiO2-Al2O3-(CaO + MgO + Na2O + K2O) pseudo-ternary diagrams of ash fusion temperatures (AFTs). The AFTs contours in the pseudo-ternary diagrams presented inverted V-shape distributions. Al2O3 showed positive effect on the AFTs; with unchanged Al2O3 content (less than 0.5), the AFTs went down first and then up with increasing SiO2 content; whereas with consistent AAEM (CaO + MgO + Na2O + K2O), the AFTs decreased with increasing SiO2 content. By doping SiO2 and Al2O3 or the mixtures into the coal, the AFTs increased significantly due to the formation of high-melting quartz, corundum, silicates, and aluminosilicates, which impeded melt-induced slagging under both oxidizing and reducing atmosphere. On the contrary, when doping AAEM, the AFTs plunged because of the formation of low-melting akermanite, which favored liquid slag tapping during gasification. The pseudo-ternary diagrams indicated that the AFTs under reducing atmosphere were higher than that under oxidizing atmosphere, with an exception of around the limited region of SiO2:Al2O3:AAEM = 0.25:0.45:0.3. Ningdong coal located in the singularity region, where the existing forms of Fe as low-melting FeS and FeO under reducing atmosphere and high-melting Fe2O3 under oxidizing atmosphere resulted in slightly higher AFTs under oxidizing atmosphere. Besides, the atmosphere difference had more influence on IDT, ST, and HT, and less influence on FT. This research provides valuable practical guidance for the selection and dosage of additives during the combustion and gasification of Ningdong coal.