Calcium Looping for Thermochemical Storage: Assessment of Intrinsic Reaction Rate and Estimate of Kinetic/Transport Parameters for Synthetic CaO/Mayenite Particles from TGA Data

Abstract

Mayenite-supported CaO represents an affordable and safetycompliant candidate material for thermochemical storage processes. We here analyze the thermogravimetric analysis (TGA) performance of synthetic CaO/mayenite micrometric powder under carbonatation/calcination looping and develop a model to interpret and analyze the experimental results. In the experimental campaign, calcination is run at 900 °C, while the carbonatation temperature is varied between 600 and 800 °C. For the carbonatation reaction, a generalized shrinking core model assuming a thermodynamically consistent first-order kinetic and a conversion-dependent diffusivity of CO 2 inside the porous CaCO 3 layer is validated through TGA carbonatation tests conducted with CO 2 /N 2 mixtures at different compositions. Interestingly, the kinetic constant of this reaction is found to be relatively insensitive to the temperature in the interval considered. In contrast, diffusion-limited regimes are never found for the calcination reaction so that this phase of the cycle can be predicted based on a single kinetic constant of the heterogeneous reaction. This constant is found to follow the typical Arrhenius-type dependence on temperature. Sizably different kinetic and transport parameters are obtained in the first carbonation performed on virgin CaO/mayenite particles with respect to those associated with subsequent cycles. When different parameters are afforded for the first and following cycles, the shrinking core model proposed closely predicts the TGA data over five CaO/CaCO 3 cycles. The results found constitute an essential preliminary piece of information for designing equipment geometry and operating conditions of industrial-scale reactors. In this respect, knowledge of the parameters defining the intrinsic reaction rates and diffusive transport is essential in defining the optimal conversion of the material associated with minimal looping time.

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