(28c) Attrition Prediction of Grid Jets in Fluidized Bed Systems

The development of novel carbon capture and commodity chemical production implementing fluidized bed and circulating fluidized bed (CFB) systems has garnered much attention in recent years. These emerging technologies include chemical looping combustion (CLC), continuous temperature swing adsorption, and transport gasifiers. However, a major uncertainty for determining the economic feasibility and system operations of these technologies is the attrition of the solid bed materials. Attrition in these systems is caused by the mechanical attrition of the particulate material, undergoing high-velocity impacts with walls and with other particles, and low-velocity wear of the surface of particles, . While mechanical attrition can occur in a variety of areas in CFB’s such as cyclones, L-valves, impactors, standpipes, risers, etc., the fluidized bed has shown to be an area where attrition is severe. A fluidized bed has two regions where attrition occurs, the jetting region and bubbling region. The bubbling region tends to cause particles to rub together and attrit through a wearing mechanism, whereas the jetting region entrains particles in a high velocity jet and impacts them against a bed of particles at the top of the jet. Experimental data and models predict the jetting region to have a much more severe impact of attrition than the bubbling region.

A common grid-jet attrition model in the literature is that by Werther and Xi, where they show that the attrition rate is a function of orifice velocity, orifice diameter, gas density, and an unknown proportionality constant based on material properties. The issue in using this model is that it requires extensive experimental testing to obtain this constant. The goal of this work is to determine the value of this constant based on fundamental material properties by developing an attrition model for the jetting region in the fluidized bed based on Ghadiri’s impact model (Eq. 1):.

where α is a proportionality constant, ρ_{p ­}isparticle density, U_p is particle velocity, d_{p ­}is diameter of the particle, H is hardness, and K_c is the fracture toughness of the material. Since Eq.1 only considers the fractional impact attrition of a single particle, a term accounting for the mass of particles entrained by the jet must also be considered. This paper presents the model development and compares predictions to data from the literature.