Gas-phase combustion in the freeboard of a fluidized bed-freeboard characterization

Abstract

The prediction of propane autoignition in the freeboard of a fluidized bed is complicated by the presence of solids, intermediate products and non-homogeneities (solids, temperature and species gradients) that should be accounted for in a reaction model. However, the simultaneous characterization of these parameters during combustion is very challenging. An experimental study of propane combustion inside the freeboard (I.D. = 0.2 m) of a fluidized bed of sand (dp = 290 μm) was performed at a low superficial gas velocity (Ug = 0.24 m/s). Propane was injected inside the fluidized bed (TBed = 650°C) through a downward-facing sparger. Also, solids flux and species volume fractions were measured using a non-isokinetic sampling probe. The results showed an exponential decrease with height of the upward solids flux (GSU) -GSU was zero at 0.17 m above the bed surface, which was taken as the inflection point of the GSU curve. GSUo measurements were significantly higher than the values given by the correlation of Wen et al. (1982). The bed surface (boundary condition) and freeboard were characterized by measuring pressure, solids flux, species volume fractions and temperature at several radial and axial positions. During the experiments, the fluidized bed achieved a pseudo steady-state operation that ensured that the measured temperature profile corresponded to the solids flux and species fractions. Partial propane combustion in the fluidized bed (71%) produced CO and cracking species that were transported in the freeboard. Complete combustion occurred within 0.15 m of the bed surface and the propane induction time in the freeboard (< 0.25 s) was on the same order as the values given by three induction time correlations for homogeneous systems.