Flow Characteristics of Countercurrent Spray Tower

Abstract

In most large coal-fired power plants an absorption process with a limestone suspension is applied today. The flue gas proceeds upwards through a series of spray headers that introduce a uniform liquid flux of droplets of the limestone suspension. These droplets resist the gas flow and provide a large mass transfer surface area required for the SO 2 removal process. During the spray overlapping the collision of the droplets may lead to a coagulation or a separation process depending on certain collision parameters such as surface tension, impact velocity and collision geometry. A model for droplet collisions was developed and implemented in a two-phase flow simulation by Euler-Lagrange. The model is based on experimental investigations with overlapping sprays.With the emission of pollutants into the air increasing, it is necessary to control the amount of particles released into the air. An understanding of filtration phenomena is therefore required, particularly of the main mechanisms of particle capture through a filter.This work consists in determining the combined efficiency of particle capture through the filter when several mechanisms are acting in synergy. The mechanisms of Brownian diffusion and direct interception allow particles to be captured on the pore surface of the filter; the first one concerns the smallest particles, while the second concerns the largest. A theoretical study has been carried out in the intermediate cross-over regime near the most penetrating particle size. The effects of slip are also taken into account, leading to an increase of the theoretical efficiency compared with the case for which slip is neglected. A finite coefficient of adsorption at the wall of the pore has been introduced, which implies that the particle concentration at the wall is not necessarily equal to zero, as is usually assumed.Our theory, which is based on a cylindrical pore geometry, can be adapted to ceramic filters by modeling their granular porous structure by a simple cubic lattice. Convergent-divergent pores in this lattice are assimilated to a cylindrical pore whose length is determined by the conservation of the membrane hydraulic permeability.The adjustment of only two parameters, namely the effective number of pores, which takes into account the tortuosity of the granular structure, and the coefficient of adsorption, allows us to obtain good agreement between the theory and experiment.