The Simulation of an Industrial Wet Flue Gas Desulfurization Absorber

A comprehensive computational fluid dynamics (CFD) model was developed to study the process of desulfurization in the absorption tower of a wet flue gas desulfurization (WFGD) unit at a coal fired power station. This study was motivated by the need for South African coal-fired power stations to meet internationally aligned compliance levels w.r.t. SO₂ emissions, and the fact that current WFGD designs have been largely tailored for European power stations. Currently, the design of WFGD absorbers are based on empirical and semi-empirical correlations which address a limited set of design parameters; therefore, CFD modeling is considered to be essential for further enhancement of WFGD design to ensure compliance with the stringent environmental requirements w.r.t. SO₂ emissions. The main objective was to develop a comprehensive CFD model for an industrial (plant size) WFGD to provide an in-depth analyses of (i) the slurry injection dynamics and the droplet behavior with the inclusion of droplet properties such as size distribution, droplet distortion, wall-droplet interaction and evaporation, (ii) the hydrodynamics of the respective slurry and the gas phases to analyze the velocity distribution, the L/G ratio and temperature profiles, and (iii) the SO₂ absorption efficiency by the limestone slurry. The model was subsequently validated against plant test data of an operational WFGD absorber, with respect to the flue gas velocity, temperature and desulfurization efficiency.

The modelling of the injectors was successfully achieved with a statistical model based on an optimized number of parcel streams to ensure converged results. The detailed modelling of the injector assembly enabled accurate description of the gas and droplet behaviour with particular reference to the velocity fields, the L/G ratio, and the droplet size distribution. The gas and droplet velocities were found to be within the ranges suitable for controlling conditions to avoid carry-over of the smaller droplets in the outlet gas stream, and thus to ensure effective operation of the mist eliminator. The distortion of droplets was found to be very small with the largest effect occurring at the gas inlet to the tower. The inclusion of evaporation in the model proved to be important to accurately describe the temperature and moisture concentration profiles within the column. Consequently, the model predicted that saturation is achieved relatively close to the flue gas inlet, and that saturated conditions are prevalent over most of the column. The dependence of the desulfurization efficiency on the slurry pH as predicted by the model were found to correlate well with the plant test results.