Microstructural development of product layer during limestone sulfation and its relationship to agglomeration in large-scale CFB boiler

Agglomeration is one of the major operational problems in fluidized bed system. Agglomeration in a fluidized bed heat exchanger of a 550-MWe circulating-fluidized-bed boiler was investigated, and the particles were found to be agglomerated by two types of microstructures: layer joint and bridge joint with two- and one-dimensional structures, respectively. Both the layer and bridge joints were found to be formed from the product of the sulfation reaction. The product-layer development in a lab-scale bubbling fluidized bed reactor was found very fast during the initial stage of sulfation, while the product-layer thickness was found to be on the order of micrometers, which is suggested to be the reason for the formation of layer joint. The bridge joint was found to be formed owing to the island overgrowth of the CaSO4 crystal. Both the layer and bridge joints were verified in a demonstration in a fixed-bed reactor, while the transmission-electron-microscopy investigation showed oriented and misoriented growth, respectively, of the bridge and layer joints. The present study not only elucidated the microstructural mechanism of the product-layer development during the gas–solid limestone sulfation reaction, but also demonstrated its relationship to the agglomeration phenomenon in a large-scale circulating-fluidized-bed boiler. [Display omitted] •Microstructure of the agglomerates is classified into layer and bridge joints.•Limestone sulfation is found to contribute significantly on the agglomeration.•The layer joint is speculated to be formed during rapid product layer development.•The island joint is formed due to island overgrowth of product.•The agglomeration is demonstrated in a lab-scale reactor.