Search

A novel CaO-based material supported with Ca3Al2O6 and Dy2O3 was found to show excellent performance as a thermochemical energy storage material for use in solar thermal power plants. It retains a carbonation conversion capacity of 82.7% for a period of 40 cycles, as well as exothermic heats of reaction of 582.2 kJ kg−1, up to seven times greater than other materials found in the literature. The improved performance was attributed to the greater prevention of sintering and retention of high surface area by the addition of two inert supports: Ca3Al2O6 and Dy2O3. Long-term effectiveness of the novel material was also evaluated by using a sintering model. It retains an energy storage utilization of 6.2 kg kWh−1 after 30 years of cycling, while commercial limestone would require 81 tons kWh−1 equivalent. Limestone requires replacement every six thermal cycles, making it impractical for real thermochemical energy storage implementation. The extra cost associated with the addition of supports in this CaO-based material is justified by the long-term durability, which would imply a reduction in the overall capital and operational expenditure of the plant.

The calcium carbonate looping cycle is an important reaction system for processes such as thermochemical energy storage and carbon capture technologies, which can be used to lower greenhouse gas emissions associated with the energy industry. Kinetic analysis of the reactions involved (calcination and carbonation) can be used to determine kinetic parameters (activation energy, pre-exponential factor, and the reaction model), which is useful to translate laboratory-scale studies to large-scale reactor conditions. A variety of methods are available and there is a lack of consensus on the kinetic parameters in published literature. In this paper, the calcination of synthesized CaCO3 is modeled using model-fitting methods under two different experimental atmospheres, including 100% CO2, which realistically reflects reactor conditions and is relatively unstudied kinetically. Results are compared with similar studies and model-free methods using a detailed, comparative methodology that has not been carried out previously. Under N2, an activation energy of 204 kJ mol−1 is obtained with the R2 (contracting area) geometric model, which is consistent with various model-fitting and isoconversional analyses. For experiments under CO2, much higher activation energies (up to 1220 kJ mol−1 with a first-order reaction model) are obtained, which has also been observed previously. The carbonation of synthesized CaO is modeled using an intrinsic chemical reaction rate model and an apparent model. Activation energies of 17.45 kJ mol−1 and 59.95 kJ mol−1 are obtained for the kinetic and diffusion control regions, respectively, which are on the lower bounds of literature results. The experimental conditions, material properties, and the kinetic method are found to strongly influence the kinetic parameters, and recommendations are provided for the analysis of both reactions.

Reaction kinetics is an important field of study in chemical engineering to translate laboratory-scale studies to large-scale reactor conditions. The procedures used to determine kinetic parameters (activation energy, pre-exponential factor and the reaction model) include model-fitting, model-free and generalized methods, which have been extensively used in published literature to model solid-gas reactions. A comprehensive review of kinetic analysis methods will be presented using the example of carbonate looping, an important process applied to thermochemical energy storage and carbon capture technologies. The kinetic parameters obtained by different methods for both the calcination and carbonation reactions are compared. The experimental conditions, material properties and the kinetic method are found to strongly influence the kinetic parameters and recommendations are provided for the analysis of both reactions. Of the methods, isoconversional techniques are encouraged to arrive at non-mechanistic parameters for calcination, while for carbonation, material characterization is recommended before choosing a specific kinetic analysis method.