Combined effect of ball-milled ash and ammonium dihydrogen phosphate on fouling, slagging, and combustion characteristics of high-sodium zhundong coal

Phosphorus-based additives are recognized as an effective strategy for alleviating slagging during Zhundong coal combustion. However, their efficacy to mitigate fouling issues caused by the substantial sodium release is constrained. In this work, ball-milled ash was combined with ammonium dihydrogen Phosphate (NH₄H₂PO₄) to jointly address the challenges of fouling and slagging. The sodium release behavior, ash fusion temperatures (AFTs), and combustion characteristics were comprehensively investigated by using simultaneous thermal analyzer and other characterizing methods. The findings show that ball-milled ash, owing to its high reactivity, substantially enhanced the sodium retention percentage. The sodium retention mechanism of ball-milled ash is mainly attributed to solid–solid reactions, whereas NH₄H₂PO₄ primarily serves to increase AFTs through gas–gas and gas–solid reactions. The highly reactive components within ball-milled ash also facilitate the reactions with NH₄H₂PO₄ to form high melting-point phosphorus-based minerals. In terms of combustion characteristics, the inclusion of ball-milled ash leads to a noteworthy reduction in the burnout temperature, while the addition of NH₄H₂PO₄ contributes to a moderate increase in the burnout index. Taking into account the three aspects of sodium release, AFTs, and combustion characteristics, the optimal combination strategy is the mass addition ratio of 0.05 for ball-milled ash and the PO ₄ ³⁻ /Na molar ratio of 1.0 for NH₄H₂PO₄.