Numerical investigation of sewage sludge combustion in a fluidized bed reactor: A comparison of 2D and 3D simulations

Combustion of sewage sludge is a technology with high relevance both, regarding energy efficiency and recovery of valuable substances like phosphor. The combustion is usually performed in a fluidized bed. CFD modelling is a central element of the optimization of this process. However, there are a number of modelling approaches available, with different advantages and disadvantages concerning accuracy and computational power. In this study, a new simplified reactor design for sewage sludge fluidized bed combustion was analyzed by CFD simulations based on the Eulerian multiphase approach and a formulated model for thermal conversion. The CFD simulations were carried out in 2D as well as 3D and the numerical results were utilized for a methodological comparison. This enables a scientifically sound recommendation on a suitable numerical approach based on the evaluation parameters of interest. As a conclusion of the investigations, the computationally more expensive 3D approach regarding simulation time should be used if the quantitative flow evaluation of the entire reactor is in focus. The 2D approach is well suitable to resolve fluidization and sludge mixing behavior within the sand bed, if the general combustion behavior is of interest. In the light of the significantly faster simulation, it can be recommended as a suitable method for preliminary analyses. The numerical results for typical evaluation parameters like gas temperature or the residual oxygen content at the gas outlet are presented. Local flow differences within the freeboard are deduced between the 2D and 3D approaches so that expectable differences are better assessable during the reactor design phase.