Almerinda Di Benedetto

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

Abstract 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.

CFD Modeling and Simulation of a Catalytic Micro-Monolith

In this work, a first step in modeling and simulating the thermal behavior of an entire catalytic micro-monolith was performed. In particular, a Computational Fluid Dynamics (CFD) model was developed for simulating three-channel and five-channel micro-combustors. For both configurations, the operating maps were built as functions of the inlet gas velocity and compared to the operating map of a single-channel configuration. Results show that, due to the relevance of heat losses in micro-devices, it is not possible to extrapolate the behavior of the multi-channel configurations from that of the single channel. Therefore, simulation of the entire catalytic micro-monolith is needed. However, this is computationally demanding: it has been found that the CPU time almost linearly increases with the number of channels simulated. Finally, for a fixed total mass flow rate, it has been demonstrated the opportunity to maximize the overall fuel conversion by means of a non-uniform distribution of mass flow rates among the channels.