Mixing behaviors of cold–hot particles in the downer of a triple-bed combined circulating fluidized bed

In a triple-bed combined circulating fluidized bed (TBCFB) gasifier, consisting of a downer for rapid pyrolysis of coal, a bubbling fluidized bed for gasification of char, and a riser for combustion of unreacted char to produce heat, it is necessary to transfer the heat carried by inert particles from the riser rapidly and effectively to coal particles injected into the downer. In order to evaluate the quality of mixing between injected coal from the inlet of the downer and circulating inert solids in the TBCFB, hot sand particles which are substitute of coal were injected into the downer through two kinds of injection nozzle arrangements, i.e., normal and tangential arrangements. The hot sand particles were mixed with the cold sand particles circulated in a large-scale TBCFB, and the temperatures of the sand were measured using thermocouples below the injection nozzle. A mixing index was suggested to quantify the degree of mixing. Simultaneously, Eulerian–Eulerian method was adopted to simulate the hydrodynamics of hot sand, cold sand and air, as well as the heat transfer among them. The mixing index was also calculated based on the temperature distributions, and was compared with the experimental data. In experiment, it was found that the tangential arrangement gave better mixing just below the injection nozzle than the normal arrangement. This may be due to the fact that the hot sand branches into two flows at the inlet in the tangential arrangement. However, uniform temperature distribution was observed along the downer in the normal arrangement and gave better mixing than in the tangential arrangement. The simulation result agreed well with the experimental observations in the normal arrangement just below the injection nozzle. Tangential arrangement gave better mixing indices as mixing between the hot sand and cold sand proceeded.