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Communication Dans Un Congrès Année : 2014

A multiscale approach for the kinetic modeling of CaO carbonation

Résumé

Thermal analysis is a useful tool to determine the solid-gas reactions kinetics. Several methods have been proposed to interpret experimental data. Nevertheless these methods must be used with low sample quantity (typically few milligrams of solid) to interpret for example thermogravimetric data. However some powders are structured in aggregates which involve that, even with low sample quantity, the gas access to the solid phase may be limited due to the gas diffusion inside the intra-aggregates porosity. Several models have been developed to account for this complexity based on the grain model or the pore model approaches. However most of these models are based on the Sohn's law of additive reaction times which is submitted to restrictive conditions: kinetic order with respect to the reaction gas equal to one, and shrinking-core type kinetic models. We propose here another approach offering more diversity in the applications. In the case of CaO carbonation by CO2, thermogravimetric data present some particularities showing a complex behaviour. Indeed a strong slowing down of the reaction is observed on thermogravimetric curves. Moreover these curves exhibit a sigmoid shape and an induction period which can be as long as 20 minutes. In order to explain the kinetic slowing down, changes in texture and microstructure during processing were regarded as essential. Reaction at the grain scale induces a volume increase from CaO to CaCO3 which modifies the porosity characteristics, thus limiting the gas access inside the aggregates. So carbonation reaction must be described by coupling the kinetics of the solid-gas reaction at the grains scale ("micro-scale") and the heat and mass transfers at the aggregate scale ("meso-scale"). The kinetic modeling was realized thanks to CIN4 software which is able to couple both micro- and meso-scales. Induction period is linked to the nuclei formation mechanism. The presence of large induction periods indicates that the nucleation process can be sluggish depending on experimental conditions. This observation can be correlated with an ab initio study of the calcite nucleation at the CaO surface which has shown that some surfaces appear unfavorable for nucleation due to strong structural instabilities related with CO2 insertion. So at the micro-scale, the kinetic model was based on the assumption of one nucleus by dense grain with isotropic growth. At meso-scale, we considered spherical aggregates with pores of the same size which depends on the fractionnal conversion. Exchanges of heat and mass are governed by diffusion equations of heat and the gaseous species. By finite difference it was possible to calculate the reaction rate for given T and PCO2. This rate allows to evaluate heat and mass sources produced by the reaction. Microscopic reaction thus influences the spatial and temporal evolution of the thermodynamic processes at the aggregate scales. Inversely since it modifies the kinetic constants, thermodynamic influences fractional conversion and reaction rate of the microscopic reaction. Using this approach, it was thus possible to reproduce the experimental data from the induction period up to the kinetic slowing down.
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Dates et versions

hal-00929755 , version 1 (13-01-2014)

Identifiants

  • HAL Id : hal-00929755 , version 1

Citer

Loïc Favergeon, Lydie Rouchon, Michèle Pijolat. A multiscale approach for the kinetic modeling of CaO carbonation. 41st Annual Conference of NATAS (North American Thermal Analysis), Aug 2013, Bowling Green (Kentucky), United States. A paraître. ⟨hal-00929755⟩
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