An investigation into the effects of variation of particle size on the fluidization and de-aeration characteristics of powders to assess dense-phase pneumatic conveyability

Abstract

This thesis results from an ongoing investigation carried out into the effects of differences in particle size distribution on the fluidization and de-aeration characteristics of seven different samples of fly ash (d50: 21-139 µm) collected from different ESP hoppers of same unit of a coal fired thermal power station. An experimental facility was developed for testing the fluidization and de-aeration characteristics. It was found that the minimum fluidization velocity decreased quite considerably with the reduction in median particle size from 139 to 102 µm. Further reduction in particle size to 21 µm resulted in an increment in minimum fluidization velocity because of the stronger inter-particle cohesive forces that are prevalent in smaller sized particles. De-aeration test results showed that the sample with largest particle size (139 µm) has the least ability to retain air due to higher permeability. The smallest particle sized particles (21µm) showed a gradual decrease in relative bed height and pressure drop per unit length with time indicating higher air retention capacities. It was found that the experimental value of minimum fluidization velocity for the finer 21µm sized fly ash (Geldart Group C powder) was larger than that predicted by the popular existing models. It is concluded that the 139 µm sized fly ash is a dilute-phase product, whereas fly ash samples with particle size varying from 69 to 21 µm have dense-phase conveyability due to higher air retention capacities.