Sorbent Transport and Dispersion
Three topics are covered in this chapter: powder characterization, powder dispersion, and modeling of powder attrition and dispersion. In the powder characterization section, powder surface properties, the results of powder transport under the influence of attraction forces, the effects of additives and temperature, and the powder mechanical properties are presented. Four sorbents are characterized—calcite, dolomite, hydrated lime and dolomitic hydrate. Calcite and dolomite have comparable median particle sizes and dolomite has a wider size distribution than the calcite. The dolomite hydrates are composed of fine particles. All the sorbents have similar morphology. It is found that the van der Waals force is dominant, compared to the electrostatic and gravitational forces, in powder agglomeration. Powder dispersion experiments show that the sorbents with the lower average van der Waals forces have greater dispersibility. Temperature has an effect on the dispersion of powders. The properties of hydrate can be modified by additives, such as lignosulfanate. SEM results show that the agglomerate size of the modified hydrate is smaller than that of the pure hydrate and the modified hydrates are less cohesive. Experimental results indicate more agglomeration and/or loss of fines at high temperatures for hydrates.
The second section compares three types of nozzles in terms of the local solid concentration and velocity at the entrance region. A particle image velocimetry (PIV) technique is developed to quantify these parameters. PIV is used to analyze the recorded agglomerate images. The uniqueness of this PIV system is its capability to measure the instantaneous characteristics of particle motions; e.g., velocity, solids concentration, and size distribution. It is found that powder dispersion is mainly due to the shear stress generated by high turbulence intensity inside the nozzles. From the experimental study on the agglomerate size distribution and the numerical simulation on the flow structures in different nozzles, the expansion nozzle with two booster jets has the optimum performance.
Attrition of sorbents can take place during handling, transport, and injection of sorbent powders, and attrition may be due to thermal, chemical, static mechanical and kinetic stresses. A stochastic model for sorbent attrition is developed. Variations of particle size distribution along with the mean and variance of the distribution during the attrition process are also presented. To simulate the powder dispersion in the nozzle, an integral model is developed. In this model, particle-particle interaction force and the hydrodynamic stress on the powder agglomerates are taken into account. The interparticle force is mainly van der Waals force. The flow structure in the nozzle is solved via computational fluid dynamics. The agglomerate size distribution is simulated by this model. Simulation results show that this model can reasonably fit the experimental data.
KeywordsParticle Image Velocimetry Hamaker Constant Powder Dispersion Hydrated Lime Interparticle Force
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