Optimization of thermal waste processing: CFD study on aspects of flow, heat and mass transfer, chemical kinetics and thermodynamics

The general aim of this dissertation is to study the flow, heat and mass transfer, kinetics, and thermodynamics of thermal waste processing using a numerical approach. More specifically, this thesis seeks to provide tools that assist engineers of Waste to energy (WtE) plants in determining optimal operation in terms of burnout and energy recovery with a minimum risk of corrosion. To this end, two specific objectives are defined. The primary objective is to develop numerical models for the thermal degradation of low rank solid fuels (mainly municipal solid waste) on a particle and packed bed level. The secondary objective is to explore the potential of employing these numerical models to study the formation and transformation of corrosive elements (i.e., Cl and S species).

Three numerical models have been developed in this thesis, namely a 2D pyrolysis model for thermally thick particles, a 2D fixed bed model, and a 2D moving waste bed model. All three models employed the porous medium approach, in which the solid phase was modeled as a porous zone with local volume-averaged properties. The gas flow was modeled using a commercial CFD code, while the solid phase was solved using an in-house code written in C/C++.

First, the 2D pyrolysis model for thermally thick particles was validated with experimental data from the literature. Then, it was used to study the anisotropy of cylindrical woody particles and the effect of external heating conditions. Next, the 2D fixed bed model was developed to simulate the combustion of wood chips in a fixed bed reactor. The model takes into account the heterogeneous composition of solid fuels, as well as the turbulent gas flow in porous media. Furthermore, the solid mass movement approach was employed to simulate bed compaction. A comparison of simulation results with data from the literature was presented, and a good agreement was obtained. Later, the model was extended to simulate the release behavior of Cl and S species. The results of this investigation are not subject to generalization but rather provide a basis for the next steps for studying corrosion in full-scale WtE plants. Finally, the 2D moving waste bed model was developed to simulate MSW incineration on a grate. The model includes realistic descriptions of waste heterogeneity, particle movement, flow in the packed bed, and coupling between the waste bed and the freeboard. More importantly, the model is robust and can run at a low computational cost, which makes it suitable for industrial use.

In general, the developed models lay the groundwork for further applications, including diagnosis, optimization, and design of WtE plants. The insights gained from this thesis will be of assistance to engineers in determining the optimal operation of WtE plants and in increasing energy and material efficiency. Furthermore, the models can be used for further investigation into the field of solid fuel combustion, especially in studies on the effect and behavior of inorganic compounds.