Bulk flow properties of pulverized coal systems and the relationship between inter-particle forces and particle contacts

Abstract

Two groups of lignite powders characterised by different particle size distributions were prepared herein to investigate the effect of particle size distribution (PSD) on the flow properties of fine powders. The first group was composed of samples with a narrow PSD and prepared using an air classifier. The second group was made of two different samples of the same lignite material characterised by industrial-grade particle size distributions, which were much wider in range than the samples of the first group. The experimentally determined flow properties were used to understand the effect of the PSD. The packing properties and the flow behaviour of all the coal powder samples were characterised in terms of compressibility and flow properties using an FT4 powder flow rheometer. Furthermore, the Brunauer, Emmett and Teller surface areas and the dispersive surface energies were determined using a surface energy analyzer. The samples with similar mean particle sizes, but different particle size distributions, provided significantly different results. Accordingly, a micro-scale approach inspired by the Rumpf and Molerus approach was proposed to obtain a theoretical insight of the effect of the particle size distribution on the bulk flow properties of these powders. This approach was modified to account for wide particle size distributions. A procedure to estimate the tensile strength of the industrial lignite powders starting from the data relative to the narrow-size samples was also developed. The approach validity was demonstrated by the acceptable agreement between the values of the isostatic tensile strength for industrial pulverized coals estimated with the model application and the values directly calculated from the measured flow properties. The bulk flow properties of the industrial grade pulverized lignite were investigated based on the model results to highlight the most significant phenomena affecting the powder flow properties.