Use of a transient model to simulate fluidized bed incineration of sewage sludge

This paper presents a mathematical model dedicated to fluidized bed incineration of sludge. The main assumptions as well as the principal equations of the model are presented. Basically, this model relies on instantaneous pyrolysis of sludge and on a five zones description of the hydrodynamics into the incinerator. Its originality is essentially due to the introduction of a two-dimensional buffer zone between the bubble and the emulsion phase. The chemistry of the gaseous system is described by chemical equilibrium. Results of a start up procedure have been presented in order to illustrate the ability of the model to describe transient behavior. To validate the model from an energetic point of view, different types of sludge (primary, activated and digested) have been computed and compared to industrial data and to results obtained from a simple calculation. The lower dryness limit corresponds to self incineration of the sludge. The higher one corresponds to maximal thermal level that should not be exceeded within the furnace.