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The main goal of this paper is to elaborate a mathematical model that represents the physics and chemistry involved when a small particle of wet sewage sludge is incinerated. Compared to existing models, our study includes both drying and heterogenous combustion of the pyrloysis residue of the processed sludge. This model relies on the assumption of homogeneous composition and temperature for the particle under study. It includes drying, pyrolysis (controlled by a four successives steps reaction pathway) and combustion of the resulting char. The ability of the model is illustrated using it in two different process conditions (representing thermogravemetric analysis and fluidized bed conditions) in order to investigate the influence of the surrounding atmosphere. It is found, that fluidized bed conditions reduce the burnout time of a small particle by enhancing the rate at which heat is transferred to that particle. It is also shown that high heating rates enhance the tar yield.

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.