Alkali metal transformation and ash deposition performance of high alkali content Zhundong coal and its gasification fly ash under circulating fluidized bed combustion

•Zhundong (ZD) coal and its gasification fly ash are high alkali fuels.•The transformation of Na and K during high alkali fuels combustion was studied.•The release of Na during fly ash combustion was less pronounced than it was in the ZD coal.•Variation in the bed temperature did not greatly affect the conversion of K.•The ash deposition behavior during ZD coal and fly ash combustion were different. Alkali metal transformation and ash deposition during combustion of Zhundong (ZD) coal and its gasification fly ash (ZDf) were studied in a 0.4 t/d circulating fluidized bed. The Na in ZD coal was present primarily as Na2SO4. NaAlSi3O8 and KAlSi3O8 were the main compounds of Na and K in ZDf. Variation in the bed temperature had a significant effect on the transformation of Na, but it did not greatly affect the conversion of K. During combustion of the ZD coal, Na was mainly in flue gas, the Na in fly ash was mainly in the water soluble form, but in bottom ash it was mainly in the insoluble form. During combustion of the ZDf, Na was found mainly in the fly ash and flue gas, insoluble Na and K accounted for above 80.0% of Na and K in ashes. The release of Na during the ZDf combustion was less pronounced than that during the ZD coal combustion. The ash deposition during the ZD coal combustion occurred primarily as a result of the agglomeration and bonding of the ash particles that were rich in Na2SO4, and the deposition propensity was high. However, the ash deposition during the ZDf combustion was primarily caused by the accumulation of fine fly ash particles with a low Na content, the deposition propensity was low.