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 °C. Two air-cooled probes were placed in the furnace chamber (P₁) and at the cyclone outlet (P₂), 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)₄O₈ and CaSiO₃ during ZD combustion but changed to SiO₂, (Na,Ca)Al(Si,Al)₃O₈ or NaAlSi₃O₈ as SC was added, suggesting that Na was captured by SC and retained in the bottom ash. The deposit on P₁ 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 P₂ 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.