Sintering mechanism of the coal fly ash particle revealed from the compressive strength

Abstract

The deposition of fly ash particles can lead to significant blockages in the cross hangers, reduced heat exchange efficiency, and increased exit temperatures during the operation of an entrained-flow coal gasifier. A thorough understanding of the sintering mechanisms of fly ash can help mitigate the ash deposition issue and provide guidance for the long-term operation of an entrained-flow coal gasifier. In this study, the sintering behavior of fine fly ash particles (< 65 μm) under various conditions (residence time, atmosphere, particle size, and temperature) was characterized by compressive strength, and the sintering mechanism was elucidated through phase evolution, element distribution, and liquid properties (viscosity and surface tension). The sintering process of fine fly ash particles was not governed by the pressure gradient (ΔP) of the curved surfaces of the ash particles or the surface tension at 900–1000 °C. Fly ash particles <45 μm exhibit a moderate Si/Al ratio (the mass ratio of SiO₂ to Al₂O₃) and relatively low viscosity, which promotes anorthite crystallization. The viscosity of the residual liquid increased with anorthite crystallization. The high compressive strength of these fly ash particles is attributed to the bonding facilitated by the viscous liquid. In contrast, fly ash particles in the range of 45–65 μm possess a high Si/Al ratio and high viscosity, which inhibit mineral crystallization. The high compressive strength arises from the dense structure formed by the fusion of the unsolidified liquid. This study demonstrated that decreasing the alkali component in coal ash and maintaining a low syngas cooling temperature could prevent the viscous surface caused by mineral crystallization, potentially alleviating the ash deposition problem.