Modelling attrition of limestone during calcination and sulfation in a fluidized bed reactor

The attrition of limestone during calcination and sulfation in a fluidized bed reactor is modelled by taking into account the parallel occurrence of abrasion and gas-solid reaction. To this end, the particle is represented as a two-region domain over which equations expressing diffusion and reaction of gaseous reactants and calcium oxide depletion by reaction and attrition are written. Constitutive equations of different complexity are introduced to express the relationship between the attrition rate and the status of the particle surface. Experiments consisting of batchwise calcination and sulfation of limestone samples have been carried out in a bench-scale atmospheric bubbling fluidized bed reactor. Experimental data provided the basis for the evaluation of constitutive parameters and relationships to be embodied in the model. Model computations were directed towards reproducing experimental data points. In spite of its descriptive nature, the model provides useful insight into important features of the interactive processes of sorbent attrition and chemical reactions. First, calcium lost as elutriated fines is made almost entirely of attrited lime, the contribution of sulfate being limited. Second, attrition decays rapidly over a time scale depending on the parallel progress of particle round-off and lime sulfation. In particular, the progress of sulfation dramatically reduces attrition, presumably via the enhancement of particle connectivity and strength associated with the occurrence of 'molecular cramming'. Third, the ability of attrition to enhance calcium utilization via the removal of impervious sulfate layers which build up at the particle surface is rather limited, at least with the sorbent and under the operating conditions considered in the work. (C) 1998 Elsevier Science S.A.