Investigation on co-combustion characteristics of Fe/Ca-rich Zhundong coal with high-ash low-calorific-value coal: Combustion behavior, ash fusion and slagging properties

Abstract

To address the severe slagging issue during the combustion of Fe/Ca-rich Zhundong coal, this study employed high-ash, low-calorific-value coal (HAC, rich in silicon and aluminum components) discovered in the Jiangjunmiao mining area of the Zhundong region as a single additive, and combined it with Kaolin (KL) to form a composite additive. Co-combustion experiments of Zhundong coal, HAC, and KL were conducted on a 0.2 MW one-dimensional furnace drop tube furnace platform. Combined with characterization techniques including X-ray fluorescence (XRF), X-ray diffraction (XRD), and scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), the differences in elemental distribution, mineral phase transformation, and slagging severity in the high-temperature zone (1300 °C) and low-temperature zone (1000 °C) under varying additive blending conditions were analyzed emphatically. The results indicate that the combustion characteristic temperatures shifted toward the high-temperature range after HAC blending. The higher the HAC blending ratio, the higher the temperature required for complete combustion of the blended coal samples. This phenomenon is mainly attributed to the dual role of HAC high ash content: diluting the relative concentration of combustibles and impeding oxygen diffusion and heat transfer during combustion. The blended coal sample with 20 % HAC blending exhibited the poorest ash melting performance, with the temperature difference between flow temperature (FT) and deformation temperature (DT) being only 29 °C, showing the highest risk of slagging. Further analysis of slagging differences in different temperature zones reveals that the formation of complex multi-component eutectic phases was the core mechanism in the high-temperature zone. The Fe/Ca introduced by HAC forms synergistic enrichment with the inherently abundant Fe/Ca in Zhundong coal, accelerating the generation of low-melting-point phases such as Haüynite and Pyroxene. This causes the deposits to exhibit a continuous molten state, significantly exacerbating slagging. In the low-temperature zone, the mineral phases are dominated by high-melting-point binary/ternary oxides. The silicon-aluminum inert components in HAC can reduce the relative concentration of Na through physical dilution and adsorb gaseous sulfate precursors, thereby exerting a certain inhibitory effect on sulfate-type deposition. In comparison, the HAC-KL composite blending demonstrates significantly superior slagging mitigation capability compared to single HAC blending. KL enhances the inert matrix through the enrichment of silicon-aluminum components, and at the same time, relies on active Al₂O₃ to convert free Fe³⁺ and Ca²⁺ in the system into high-melting-point stable phases such as Hercynite and Anorthite. This process reduces the formation of low-melting-point eutectics and effectively lowers the risk of slagging.