CFD simulation of dilute phase gas-solid riser reactors: part II—simultaneous adsorption of SO2-NOx from flue gases

Simultaneous adsorption of SO₂-NO_x in a riser configuration is a novel route for flue gas cleaning. The riser operates at a low flux of small diameter Na-γ-Al₂O₃ sorbent particles. The reaction scheme is adopted from previous work (Ind. Eng. Chem. Res. 40 (2001) 119), without adjusting any of the kinetic parameters. The significant concentration gradient between the gas and solid phase mainly arises from the low solid fraction (typically 5×10⁻⁴) in the riser. Enhancing the fluctuating kinetic motion of gas and solid phase increases the SO₂ adsorption, whereas the NO adsorption is decreased marginally. The solid recirculation in the top section of the riser, induced by the abrupt T outlets, significantly decreases the NO and NO₂ removal, while the SO₂ removal remains mostly unaffected. Therefore, it is desirable to avoid recirculation for a maximum NO_x removal. A comparison of the 3D and a 1D model shows that higher SO₂ and NO removal efficiencies are predicted by the 3D model in the major part of the riser. However, these positive effects are largely neutralized by the negative effects of the outlet-induced recirculation, resulting in similar overall removal efficiencies calculated by the two models. Unlike the 1D model, the 3D simulation shows a considerable axial variation in the solid fraction and slip velocity. The 3D simulation also allows to calculate the effects of outlet geometry on the flow and reaction fields. The reactor efficiency can be improved by modifying the outlet configuration to minimize the recirculation.