CFD modeling of sewage sludge combustion in an experimental entrained flow reactor with integrated slag flow dynamics

This study addresses the important role of slag flow predictions in sewage sludge combustion systems. From the typically high ash content, valuable elements such as phosphorus can be recovered. Two advanced computational fluid dynamics (CFD) modeling approaches are evaluated in terms of their predictive capabilities. A sophisticated Eddy Dissipation Model/Volume of Fluid (EDM/VOF) approach serves as the state-of-the-art benchmark for evaluating a novel Steady Diffusion Flamelet/Eulerian Wall Film (SFM/EWF) method. Both models feature user-defined coupling with the discrete phase model (DPM) and show general agreement on the evolution and final characteristics of the slag flow on the walls of a newly developed entrained flow reactor. With an input of 170 kg/h of sludge, the models predict a maximum slag thickness of approximately 0.6 to 1.0 mm and a maximum slag flow velocity of about 37 mm/s. It was found that the slag flow is mostly unaffected by the swirling gas and follows a pattern primarily driven by gravity. In summary, the straightforward SFM/EWF method can deliver high-quality predictions at a fraction of the computational cost compared to the EDM/VOF approach. It generates statistically steady slag flow results in less than two days on a standard workstation, significantly faster than the 35-day minimum required by the EDM/VOF model. This speed-up is achieved through reduced mesh requirements and fewer transport equations. The paper also highlights the limitations of this computationally efficient model regarding film growth normal to the walls, a critical aspect in freeze-lining applications. • First application of UDF-based, 3D slag flow CFD model in sewage sludge combustion. • A new, computationally inexpensive approach is developed as an alternative. • New model predicts slag flow similarly but cuts computation time by 95%. • Detailed slag flow simulation using both models, featuring a complex geometry. • Study highlights balance between computational efficiency and model accuracy.