Investigation on the ash characteristics and AAEM migration during co-combustion of Zhundong coal and shale char in a fixed bed

Abstract

Co-firing with high Si/Al additives has been considered as an effective method to mitigate ash-related issues of Zhundong coal during combustion. Thus, in this paper, shale char rich in Si/Al obtained from pyrolysis of oil shale was selected as an additive of Zhundong coal to investigate the AAEM migration and ash characteristics. The results revealed that after blending shale char during combustion, the sodium/calcium retention ratios reached 72%-88% and 90%-98%, respectively. When the proportion of shale char was 5%, the sodium/calcium retention ratios were the highest, and the fouling and slagging tendency was the lowest. The sodium/calcium elements in Zhundong coal were fixed in the ash by shale char, and their volatilization was inhibited. With an increase in the blending ratio of shale char, the AFTs of mixed fuels exhibited an upward trend. When the proportion of shale char increased from 0% to 25%, the ST and DT of mixed ash increased by 79℃ and 150℃. Shale char had a excellent effect on enhancing the AFTs of Zhundong coal. Further, the primary minerals in Zhundong coal were mainly calcite, anhydrite and quartz, as well as a small amount of thenardite, periclase and rankinite. After blending shale char into Zhundong coal, some silicoaluminates in different forms, such as nepheline, anorthite and akermanite etc. formed during combustion. More AAEM-containing minerals in Zhundong coal transformed to aluminosilicate phase, which effectively reduced their corrosivity. During individual combustion of Zhundong coal, the ash particle presented spherical in the molten state, and formed large cluster and sintered ash (>100 μm). When the shale char was added during combustion, a large amount of refractory minerals with larger particle size generated, such as silicoaluminates or mullite. These minerals could maintain their inherent shape in large particle size rather than sintering. Likewise, the fine ash particles (<10 μm) of Zhundong coal were dominant during combustion at 575℃. And, the cluster ash particles (>100 μm) accounted for 80.69% at final temperature of 1150℃. When the shale char was added during combustion, the content of cluster ash decreased to less than 5%, suggesting that the slagging issues had been effectively alleviated.