A single particle model of the fluidized bed combustion of a char particle with a coherent ash skeleton: application to granulated sewage sludge

A single particle model of the fluidized bed combustion and of the parallel course of attrition of a fuel particle characterized by a coherent ash skeleton is presented. The model combines a shrinking core feature to represent carbon combustion with a shrinking particle feature to represent attrition of the carbon-depleted ash layer. Moreover, the establishment of non-uniform temperature profiles within the particle as a consequence of the resistance exerted to thermal conduction from the unreacted core across the ash layer is taken into account. The inherent stiffness of the model has been dealt with by developing a solution procedure that proved to be efficient and not time-consuming. The model has been applied to the description of the fluidized bed combustion of pelletized sewage sludge. Experiments consisting of the fluidized bed combustion of single pellets in a bench scale reactor have been purposely carried out with the aim of determining the values of selected model parameters and of enabling model validation. The experiments confirmed the relevance of the formation of a coherent ash skeleton to combustion and attrition. In particular, it appeared that attrition departs significantly from the combustion-assisted attrition pattern typical of fuels characterized by incoherent ash. Model results highlight the role of the coherent ash skeleton to the establishment of oxygen concentration and temperature fields within the particle. The interplay of the different processes that contribute to the apparent kinetics of char combustion, namely boundary layer and intraparticle diffusion of oxygen and heterogeneous reaction between oxygen and carbon at the core surface, is analyzed and discussed. The relevance of model variables to the time-temperature history of the particle is assessed.