Morphological and Mineralogical Characterization of Ash Deposits during Circulating Fluidized Bed Combustion of Zhundong Lignite

The mineralogical and morphological characteristics of ash deposits collected from a circulating fluidized bed (CFB) burning Zhundong lignite were investigated. Zhundong lignite with particle sizes less than 3 mm was combusted in a 0.2 t/h CFB combustor at 950 degrees C with four air-cooled probes installed at the combustion chamber, cyclone outlet, and tail flue to simulate ash deposition on heat transfer surfaces. Ash deposits on these probes were extracted after 6 h of experimentation and analyzed using scanning electron microscopy energy-dispersive X-ray spectroscopy, X-ray diffraction, inductively coupled plasma optical emission spectroscopy, and a laser particle analyzer for their morphological, mineralogical, and chemical characterization. Results show that the ash deposit on the probe within the CFB chamber was a thin layer enriched in Na-bearing condensates and Ca/Mg fumes in the mineral forms of NaAlSi2O6, Na2SiO3, Na2SO4, and CaSO4, which were believed to be responsible for initiating the ash deposition by condensation or thermophoresis. The ash deposit at the cyclone outlet was composed of a sintered inner ash cone enriched in Na, Ca, S, and a coarse-grained outer layer of silicates and aluminosilicates. The presence of Na/Ca sulfates was responsible for the initiation and sintering of the deposit. For ash deposition on the two probes in the tail flue, both windward and leeward ash deposits on the probes did not sinter due to the low surface and flue gas temperatures. However, the leeward deposits were finer in size than the windward deposits because the inertial impaction was absent on the leeward side. Na in the deposits on the probes in the tail flue was identified as mainly NaAlSiO4 and Na2Si2O5, which differed from Na2SO4 identified in the deposits in the chamber or at the cyclone outlet. This indicates that Na was captured by silicates or aluminosilicates of high melting points, and the deposits in the tail flue were thus less sticky and not sintered.