Research on the effects of the fly ash reburning on element migration and ash deposition characteristics of high-alkali coal in a full-scale slag-tapping boiler

Abstract

Burning high-alkali coals in boilers can cause severe slagging problems and affect boiler safety, which restricts the utilization of high-alkali coals. Some studies have shown that high-alkali coals can be burned well in the laboratory or medium-scale slag-tapping boilers, but few studies have been conducted in a full-scale boiler. The effects of the fly ash reburning on element migration and ash deposition characteristics of high-alkali coal in a full-scale slag-tapping boiler were investigated in this work. The results showed that the primary components in different samples were SiO2, Al2O3, Fe2O3 and CaO. The liquid slag had high Si and Fe contents and low Na content, while the ash deposition showed an opposite trend. Moreover, element Cl was only found in the ash deposition. After the fly ash reburning system was put in, the element content changed greatly. The Si and Al contents increased, and the Fe and Ca contents decreased in the liquid slag and fly ash samples. At the same time, the Na content in #1 liquid slag (LS) and #2 LS increased by 10.6% and 8.86% and decreased by 14.76%, 8.05%, 2.27% and 21.49% at the fifth superheater, the secondary reheater, the primary reheater and the economizer, respectively. Fly ash reburning can enhance the absorption of alkali metals by liquid slag and reduce alkali metals in the flue gas. Microscopic morphology analysis showed that the ash particles had a smooth surface after fly ash reburning. NaCl acted as a binder between ash particles, causing particle aggregation in the ash deposition. Mineral composition analysis indicated that fly ash reburning could affect the mineral composition contents in the ash samples. The sodium in the flue gas mainly existed in the form of NaCl. This work can guide industrial boilers to burn high-alkali coal directly and is beneficial for improving boiler design and optimizing combustion operation.