Experimental study on the effects of calcium-based additives to control ammonium bisulfate and ash mixture deposition with an on-line digital image technique

Abstract

The Ash deposition caused by ammonium bisulfate (NH4HSO4, ABS) and fly ash blocks and corrodes air preheaters in coal-fired power plants, affecting heat transfer efficiency and boiler operation safety. In this work, the effects of three calcium-based additives (Ca(OH)2, CaO, and CaCO3) on ash deposition were studied, and the effect of the molar ratio of Ca(OH)2 to ABS was studied. A probe was applied to accumulate ash deposition while an on-line digital image technique was used to record the morphology of ash throughout its deposition. New research ideas and methods were used in this work. The results showed that all calcium-based additives could inhibit the deposition of ash, among which the effect of Ca(OH)2 was the best, and as the molar ratio was increased, the thickness of ash deposition decreased. These results were observed because although ABS was sticky and easily adsorbed fly ash, causing ash deposition, additives reacted with ABS to reduce its content, thereby reducing the ash deposition. As the molar ratio was increased, the amount of ABS further reduced, leading to less ash deposition. When the molar ratio of additives to ABS was 2:1, the maximum thicknesses of ash deposition were 0 mm, 0.81 mm and 1.24 mm for Ca(OH)2, CaO and CaCO3, respectively; however, the maximum thickness of ash deposition was 2.37 mm without additives. Using Ca(OH)2 as an additive, the maximum thicknesses of ash deposition were 1.43 mm, 1.22 mm, and 0 mm at molar ratios of 0.5:1, 1:1, and 2:1, respectively. For a better understanding of ash deposition, the deposition was tested by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectrometry (EDS). This work provided reliable suggestions on the choice of additives to reduce ash deposition in coal-fired power plants.