Characterisation of ash deposits on a probe at different temperatures during combustion of a Zhundong lignite in a drop tube furnace

• Ashes deposited on probes during Zhundong lignite combustion systematically studied
• Deposits on probe top were wide in sizes, rich in fluxing mineral, and sintered
• Low melting-point mineral, hauyne, formed at 973 K, causing significant sintering
• Deposits on probe side surface were fine in size, rich in Ca Mg S and sintered
• Dominant ash deposition mechanisms on the probe tip and side surfaces identified

Characteristics of ash deposits formed on an air-cooled stainless steel probe simulating superheater surfaces at different temperatures during combustion of Zhundong lignite in a drop tube furnace were systematically studied. Zhundong lignite in a size fraction of 50–100 μm was combusted in air in the drop tube furnace at 1673 K. Ash deposits formed on both the top tip surface and side surface of the probe maintained at 773 K, 873 K and 973 K, respectively, were collected and characterised using XRD and SEM-EDS for their mineralogy, morphology and chemical composition. The particle sizes and sintering temperatures of the top surface deposits were also determined. At probe temperatures of 773 K and 873 K, the ash deposits on the top surface composed of particles with sizes varying from sub-microns to 100 μm, where fine particles (< 10 μm) had agglomerated and stuck to coarse ash particles (> 10 μm), indicating sintering had occurred. The mineral phases were dominated by anhydrite, lime, nepheline, hematite, quartz, periclase, and mullite. At 973 K, fine particles had melted and incorporated into the coarse particles, leading to increased particle sizes and the formation of a new mineral phase with low melting-point, hauyne (Al6Ca2Na6O32S2Si6), suggesting significant sintering. The ash deposits on the probe side surface, however, were also sintered but composed of fine particles and their aggregates, nominally < 10 μm in size. The mineral phases mainly consisted of anhydrite, lime and periclase, being much less complex than those in the top surface deposits. The fine ash rich in Na, Ca, S, and Mg on the probe due to condensation and thermophoresis is believed to be responsible for the severe ash deposition during combustion of Zhundong lignite.