Fate of Na/As along the flue gas path during coal combustion with kaolin addition

Abstract

During the combustion of high-Na coal, kaolin was a commonly used additive to alleviate the fouling and slagging problems. It was also proved to be effective in capturing As2O3 in our previous lab-scale experiment. In this study, the effect of kaolin on the fate of Na/As-related ashing behavior was investigated by determining the concentration and binding forms of Na/As in the ash along the flue gas path on a 3 MWth pilot-scale furnace. The mechanism involved was also explored through DFT calculation, the spatial distribution of Na/Si/Al in single-particle, and the ash size classification. The results showed that in the coal-fired furnace, metakaolin derived from kaolin dehydroxylation promptly captured Na/As-containing substances, and enhanced As adsorption capacity of metakaolin-NaCl was observed. Part of the adsorption products went downstream with the flue gas, leading to a decrease of 45% and 27% Na content in ash at the burner (BR, 1683 K) and bottom ash hopper (BT, 1473 K). Meanwhile, another part of the products was left at the two positions, contributing to an increment to 1.7/2.1 μg/g As in BT/BR ash. With the flue gas temperature decreasing to 923 K, ash containing kaolin with Na/As adsorbed was deposited at the low-temperature reheater. Aluminosilicate-bound Na and poorly crystalline Al-bound As increased by 5.68% and 22%, respectively. When the temperature dropped to 473 K, more fine particles with kaolin (aerodynamic diameter < 10 μm) were deposited at the air preheater, which resulted in 102% of increase in total As content and 40.7% in poorly crystalline Al-bound As content. In general, it is a promising way to use kaolin for co-capturing Na- and As-substances during coal combustion since Na could promote the adsorption of As by kaolin instead of competing at high temperatures.