Multiphase Euler–Lagrange CFD simulation applied to Wet Flue Gas Desulphurisation technology Luca Marocco a, * , Fabio Inzoli b a Independent Researcher, via Luigi Barzini, 18, 20125 Milano, Italy b Department of Energy, Politecnico di Milano, P.zza Leonardo da Vinci, 32, 20133 Milano, Italy article info Article history: Received 13 July 2008 Received in revised form 8 September 2008 Accepted 14 September 2008 Available online 15 October 2008 Keywords: CFD Desulphurisation Absorption Multiphase abstract The gas and liquid hydrodynamics inside an Open Spray Tower has been simulated using a commercial CFD code, while a model that treats the absorption process of SO 2 has been developed and implemented in the software through dedicated modules. Besides SO 2 absorption also evaporation of slurry droplets and droplet-wall interaction are considered, the latter modeled with an empirical correlation and imple- mented in a sub-module. The continuous gas phase has been modeled in an Eulerian framework, while the dispersed liquid phase with a Lagrangian approach by tracking a large number of particles through the computational domain. Physical absorption of SO 2 has been modeled using dual-film theory and appropriate empirical and semi-empirical correlations. The model for aqueous phase chemistry considers instantaneous equi- librium reactions of eight dissolved species into a slurry droplet, namely: SO 2;aq ; CO 2;aq ; H þ ; OH  , HSO 3  ; SO 3 2 ; HCO 3  ; CO 3 2 . The empirical droplet-wall interaction model handles impact, deposition and splashing events occurring when a liquid particle hits an internal element of the scrubber. A pilot plant OST has been simulated and the numerical results show good agreement with the experimental val- ues of pressure drop, temperature and sulphur dioxide removal efficiency. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Sulfur dioxide removal from flue gases has probably been the subject of more research than any other gas purification operation. Since the vast majority of SO 2 emissions are from fossil fuel-fired boilers at power stations, these sources have been widely con- trolled. The limestone forced oxidation system is the most exten- sively used wet scrubbing process in the world today, comprising roughly one third of the wet scrubbing systems and has been the subject of numerous studies and publications (Klingspor and Cope, 1987). Among the available desulphurisation equipments, Open Spray Towers (OST) are the most frequently installed scrubber types for limestone Wet Flue Gas Desulphurization (WFGD) plants and they cover the major part of the market today. In the last decade and especially in Europe, the SO 2 emissions regulation has become more stringent necessitating very high scrubbing performances, only achievable through optimization of the WFGD equipment design. This is nowadays mainly based on empirical and/or semi-empirical correlations between process parameters that can be easily measured and controlled. Previous CFD studies (Heiting et al., 2004; Hofelsauer et al., 2008) are lim- ited to the analysis of the gas and liquid hydrodynamics inside an OST. Sulphur dioxide removal efficiency is a complex function of chemistry and fluid dynamics and can be properly evaluated with the model developed in this work, which computes the local and global SO 2 mass transfer between the gas and liquid phase. Different simulations of a pilot plant OST have been performed. The CFD code ANSYS-FLUENT 6.3.26 has been used to simulate the gas–liquid hydrodynamics, while the developed model has been implemented in the software through dedicated modules. Further- more, besides SO 2 absorption, the model also simulates evaporation of slurry droplets and droplet-wall interaction, the latter modeled with an empirical correlation and implemented in a sub-module. The numerical results obtained with three different grid discret- izations have been analyzed and compared with the measured data. 2. Governing equations The fluid dynamics inside an OST can be described as a two- phase flow consisting of a carrier gas and a large number of dispersed liquid droplets and has been modeled with an Euler–La- grange approach (Lapin and Lübbert, 1994). The main advantage of using a Lagrangian framework for dispersed phase flows is that particle-level phenomena can be modeled rigorously, thus allow- ing to accommodate complicated forms of interphase physical pro- cesses. Furthermore, droplet size distribution and droplet-wall 0301-9322/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijmultiphaseflow.2008.09.005 * Corresponding author. Tel.: +39 328 2321735. E-mail address: luca.marocco@polimi.it (L. Marocco). International Journal of Multiphase Flow 35 (2009) 185–194 Contents lists available at ScienceDirect International Journal of Multiphase Flow journal homepage: www.elsevier.com/locate/ijmulflow