Mechanisms and characteristics of ash layer formation on bed particles during circulating fluidized bed combustion of Zhundong lignite

In this paper, the bed particles and their agglomerates that formed in circulating fluidized bed(CFB) during Zhundong lignite combustion were characterised to reveal the role of Na and ash-bed interaction on agglomeration. Experiments were conducted in a lab-scale CFB, within which either quartz or steel slag enriching in Mg was used as bed material to burn ZD at 950 °C. The particle size distribution, chemistry, mineralogy, morphology and in-situ sintering behaviour of the obtained samples were analysed in detail. Results showed that the discrete quartz particles were coated by a layer of submicron ash particles enriched in Ca and Mg. Na was less than 3% across the coating layer, but was enriched as Na silicates locally between quartz particles and initiated agglomeration. While heated in ESEM, Na silicates and those coated ashes became soft and even melted at 950-1100 °C, presenting a relatively low melting-point for agglomeration initiation. When steel slag was used as the bed material, Na in bottom ash became a negligible amount of 0.84 wt% as its chemical reaction with steel slag would not occur. The ash coating layer was also not formed on the steel slag, resulting in these particles in discrete state for agglomeration mitigation.