Investigation into scavenging of sodium and ash deposition characteristics during co-combustion of Zhundong lignite with an oil shale semi-coke of high aluminosilicate in a circulating fluidized bed

Scavenging of sodium and ash deposition characteristics during co-combustion of Zhundong lignite with an oil shale semi-coke in a circulating fluidized bed (CFB) were investigated. Zhundong lignite (ZD) and its blends with the semi-coke (SC) of 10 and 20 wt% were combusted in the CFB furnace at 950 degrees C. Two air-cooled probes were placed in the furnace chamber (P-1) and at the cyclone outlet (P-2), respectively, to simulate ash deposition. The bottom ash, fly ash and ash deposits on the probes were collected and analysed by using XRD, SEM-EDX, ICP-OES and laser particle analyser. Results show that the fly ash burning ZD was comprised of sub-micron particles and agglomerates enriched in CaSO4 and SiO2, with Na being NaAlSi3O8, Na2SiO3, Na2Si2O5 and NaAlSiO4. When SC of high Si/Al was added, the fly ash was comprised of sub-micron particles with less agglomerates, where Na-bearing minerals were also evidently reduced. Likewise, the bottom ash was coarse-grained particles rich in SiO2, (Ca, Na)(Si, Al)(4)O-8 and CaSiO3 during ZD combustion but changed to SiO2, (Na, Ca) Al(Si, Al)(3)O-8 or NaAlSi3O8 as SC was added, suggesting that Na was captured by SC and retained in the bottom ash. The deposit on P-1 was enriched in Na2SO4 and CaSO4 when ZD was burned but turned into coarse-grained particles rich in Ca/Na aluminosilicates while SC presented, showing a decreased sintering propensity. Likewise, the P-2 deposit was of Ca/Na sulphates agglomerates, but became refractory SiO2, Ca sulphate and aluminosilicates as SC added. Ash deposition in CFB burning ZD is consequently mitigated with SC addition.