An experimental investigation into bed particle agglomeration and ash deposition during circulating fluidized bed gasification of Zhundong lignite

Bed particle agglomeration and ash deposition in a 0.2 t/d circulating fluidized bed (CFB) gasifying high sodium Zhundong lignite (ZD) at different air equivalence ratios (ERs) were experimentally investigated. With quartz as bed material in the CFB at 900 °C, ZD within 3 mm in size was fed into the furnace and gasified at ERs 0.73–0.33. Two air-cooled probes were inserted in the furnace chamber (P₁) and placed at the cyclone outlet (P₂) respectively to simulate ash deposition. The bottom ash (BA), fly ash and ash deposited on probes were collected and analysed by using XRD, SEM-EDS, and an ash fusion analyser. At ER0.73 during gasification, agglomerates of bed particles were observed, since low melting-points sodium silicates sticking the bed particles together. Meanwhile, in the presence of NaFeSi₂O₆ and sodium silicate, char particles were found to have been coated by quartz and ash particles. Independent temperature-programmed ESEM imaging analysis showed these minerals became molten even at 800 °C, confirming their low melting-points. Agglomeration in BA became even severe as ER decreased, suggesting an aggravated agglomeration tendency. The fly ash mainly composed of calcium sulphate and Na-bearing minerals, whose morphological and mineralogical features were independent of ERs. In addition, ash deposited on P₂ under all gasification conditions was found to be rich in Na₂SO₄. However, the contents of NaCl and unburned carbon increased significantly as ERs decreased. This decreased the sintering tendency of deposited ash but promoted corrosion of the P₂ probe tip as observed.