An Investigation into the Flowability and Conveyability of Fly Ash

Abstract

This thesis presents the results of an ongoing investigation into the pneumatic fly ash conveying systems in thermal power plants that often cannot transport ash as per their expected duty due to either variability of ash characteristics and/or inadequate system sizing, resulting in generation loss and reduced ash utilization. Based on a comprehensive test program, including the pneumatic conveying (in a pilot plant) and flow property testing of 23 ash samples obtained from five different power stations, predictions for conveyability and flowability have been made using bulk property characterization. Of all the different particle and bulk parameters investigated, the angle of repose is the significant bulk parameter linking conveyability and flowability. A newly developed design tool based on the angle of repose is expected to assist designers and operational engineers predict the flow condition and appropriate sizing of equipment/system with suitable operating parameters. Accurately predicting the flow mode is essential for the design of reliable pneumatic conveying systems. The existing popular powder classification diagrams use particle or loose poured bulk density and average particle diameter. An evaluation of powder characterization and conveying data of 59 powders reveals that all the existing classification diagrams have overlapping zones between fluidized dense- and dilute-phase. Such uncertainty significantly limits the use of existing classification diagrams. A novel classification diagram has been developed using the powder characterization and conveying data of 59 powders for fluidized dense to dilute-phase regime using a modified particle Froude number term (based on loose poured bulk density) and particle size distribution. The novelty of this classification diagram is that it uses particle size distribution (instead of average particle size) and quantitatively marks the uncertain zone in the classification diagram, ensuring design reliability. Accurate blockage conditions or the minimum transport boundary prediction is essential for the reliable design and operation of a pneumatic powder conveying system. Many existing empirical models for minimum transport boundary do not consider essential powder properties and operating conditions, such as loose poured bulk density, particle size, and air density. Based on the conveying results of 13 different powders, this paper has developed a new empirical model for the minimum transport boundary. The model includes a Froude number based on particle size and bulk density and a dimensionless gas density term, which makes the model inherently adaptable to variations in powder properties and operating conditions. Results of validation show that the new model provides a significantly improved prediction of minimum Froude Number (in the range of 7 to 13% relative error only) compared to the existing models, which provided relative errors in the range of 19 to 67%. A new approach for estimating the force of adhesion has been developed by using the angle of repose and flow function test data of 23 fly ash samples and modifying an existing approach. Adhesion force has been used to determine the Bond number, which has been used subsequently to predict powder flowability by considering particle size distribution. The predicted values using the developed model for ash flowability have been validated against 10 other fly ash data, which provided a correlation coefficient value of 91% (indicating a good fit). The new adhesion model resulted in a correlation coefficient value of 95% when the predicted values (using this model) were compared with the experimental data of other researchers, thus indicating a good fit.