The mineralogy, morphology and sintering characteristics of ash deposits on a probe at different temperatures during combustion of blends of Zhundong lignite and a bituminous coal in a drop tube furnace

• Zhundong lignite ash deposits were rich in fine-sized and sintered fluxing minerals.
• The Australian bituminous coal ash deposits were rich in coarse refractory minerals.
• Increasing the bituminous in the blends inhibited nepheline and diluted fine particles.
• Sintering temperature increased as the ratio of bituminous in the blend increased.
• The deposit mineralogy and morphology changed as the probe temperature increased.

Ash deposits formed on an air-cooled probe at different temperatures during combustion of coal blends of a Zhundong lignite (ZL) and an Australian bituminous (AB) were studied in the present work. The blends with various AB ratios were combusted in a drop tube furnace in air at 1400 °C, and the ash deposits formed on the probe top surface maintained at 500 °C, 600 °C, 700 °C, and 750 °C, were collected and characterised using XRD and SEM-EDS for their mineralogy, morphology and elemental composition. The size distributions and sintering temperatures of the ash deposits were also determined. At the probe temperature of 500 °C, the ZL ash deposit was dominated by fluxing minerals where significant amounts of fine particles (< 10 μm) and their aggregates existed, leading to low ash sintering temperatures. As the AB ratio increased to 20 wt% and above, the amount of the fluxing minerals in the deposits decreased while the amount of refractory quartz and mullite minerals increased. Morphologically, the refractory ash particles from AB coal diluted the Na-bearing mineral phases and the contacts between fine ash particles, leading to the absence of nepheline and, correspondingly, an increase in the ash sintering temperature. In addition, as the probe temperature increased, mineral interactions and ash sintering were facilitated when the AB ratio was less than 20 wt%, leading to the formation of new mineral phases and an increase in ash particle sizes. However, no significant mineralogical and morphological changes were observed when the AB ratio was higher than 40 wt%, indicating that these ash deposits were more refractory and resistant to the temperature change.