In this study, 3 types of circulating fluidized bed combustion(denoted as CFBC) boiler fly ashes, an original fly ash (denoted as O-FA), a hydrated fly ash (denoted as H-FA) and a carbonated fly ash (denoted as C-FA), were used to produce granules of 600 ㎛~1.7 mm in diameter with a high intensive mixer, and the granulation mechanism of each fly ash was analyzed by examining the shear stress acting on the impeller and the size distribution of the granules obtained according to time. The C-FA with a high specific surface area had a long early stage of wetting and nucleation because a longer time was required to wet the particles, while the O-FA and H-FA with relatively lower specific surface areas had a shorter early stage. In the middle stage, the granule grew by fusing several nuclei, and as time passed, the granule became dense; the water inside was squeezed out to the surface which acted as a bridge between the granules, promoting the consolidation and growth of the granule. However, for the coarse particles from the O-FA, grain growth did not occur by the water-squeeze-out and bridging mechanism. In addition, the O-FA had higher levels of free-CaO causing high heat of hydration; therefore, the granule was quickly dried and easily broken at the final stage, which makes granulation of O-FA difficult. However, H-FA and C-FA, which are in a stable state, could be easily granulated by controlling the operating time.

Keywords: CFBC fly ash, High intensive mixer, Granulation, Shear stress, Granular size.