A CFD-based porous medium model for simulating municipal solid waste incineration grates: A sensitivity analysis

This study starts from a generalized CFD-based porous medium model developed in previous work simulating the combustion of municipal solid waste (MSW) on incineration grates. In this paper, three specific bed models are compared: a newly developed whole moving bed model, a newly developed multi-zone model, and a fixed bed model. Although the whole moving bed model covers the entire gas flow field, this advantage is offset by a high computational cost. In the multi-zone model, different zones of the waste bed are computed consecutively. This helps to reduce the computational time by about 75% but causes a loss of accuracy in predicting the gas flow field at the transition between the zones. For the fixed bed model, the walking column approach is needed. Additional accuracy loss is introduced but the computational time is significantly reduced even further compared to the multi-zone model. Because of the trade-off between computational cost and accuracy and the great uncertainty of the model parameters in industrial practice, the fixed bed model is suggested for simulating industrial grate-firing systems. In addition to these moving bed modeling strategies, this study also investigates how sensitive model results are to the assumed minimum and maximum solid fractions, which determine the evolution of porosity along the grate. The results show that the conversion rates are relatively insensitive to these criteria within the investigated range. Conversely, the effective surface area greatly influences the combustion characteristics as it affects the drying and char conversion rate and governs the temperature difference between the solid and gas phases. This finding calls for a better understanding of the evolution of the effective surface area and equivalent particle diameter during the combustion of MSW.