Ash deposition behavior of a high-alkali coal in circulating fluidized bed combustion at different bed temperatures and the effect of kaolin

Abstract

High alkali and alkali earth metals (AAEMs) content in coal causes severe slagging and fouling during combustion in a boiler. In this study, the ash deposition behavior of a high-alkali coal at different bed temperatures and the effect of kaolin were investigated in a 30 kW circulating fluidized bed (CFB) test system using an ash slagging probe and deposition probe. The results show that the ash deposition tendency increases with the bed temperature. The condensation of Na₂SO₄ is an important inducement for slag formation in the furnace. The melting or partial melting of slags is attributed to Na–Fe–Ca eutectics. At 920 °C, Na₂SO₄ will react with CaSO₄ to form the low-melting compound of Na₂SO₄–CaSO₄. The deposited ash on the convection-heating surface consists of granular particles. On the windward side, the layered-structure ash deposits, i.e. the inner and outer layers, are formed at the bed temperature of 920 °C but are absent at lower temperatures (820 °C and 870 °C). The formation of the inner layer consists of fine particles (<2 μm) and is closely related to Na₂SO₄. The size of the deposited ash in the outer layer is larger than 10 μm, while that on the leeward side is less than 10 μm. By adding kaolin in the coal, the slags are replaced by loose particles due to the absorption reactions between kaolin and alkali metals. The ash deposition tendency is improved and the optimal result is achieved when kaolin is added at an addition ratio of 3%.