The Composition of Fluidised Bed Inventory Influences the Performance of Limestone Sorbent in Thermochemical Energy Storage Based on Calcium Looping

Concentrated Solar Power (CSP) integrated with ThermoChemical Energy Storage (TCES) systems represents a promising technology to obtain an effective exploitation of solar energy and overcome its intrinsic intermittency. Since solid-gas reactions are favoured in TCES processes, fluidised bed reactors appear to be excellent particle receptors thanks to the good mass/heat transfer coefficients ensured by FB fluid-dynamics.

An interest reversible reaction for the above-mentioned purpose is represented by the calcination/carbonation of limestone seen as Ca-based sorbent thanks to the high reaction enthalpy (ΔH_r°=178 kJ/mol), and low-cost of the raw material. Conversely, this process is penalised by the rapid deactivation of the limestone induced by loss of porosity following thermochemical sintering, pore plugging and low absorption of solar radiation, due to the high reflectance of limestone.

In this research work experiments were carried out in a lab-scale fluidised bed reactor, electrically heated. An Italian limestone was present in the bed alone, or in combination with silica sand/silicon carbide. SiC was chosen as representative of a high-absorbance bed material.

Calcination/carbonation cycles were carried out under operating conditions resembling the “closed-loop” scheme (calcination at 950°C, carbonation at 850°C, fluidising atmosphere composed by pure CO₂). Carbonation degree, particle size distribution and bulk density were measured as cycles progressed, together with the application of a model equation to relate carbonation degree to the number of cycles.

Mutual relationships between nature of bed material and possible interactions, degree of CaO carbonation, generation of fragments and changes in particle density were critically discussed. The possibility of separate bed particles at different carbonation degree by segregation patterns is also investigated. To this end, the segregation behaviour of binary mixtures of limestone and lime particles was scrutinized through tests in a fluidisation column equipped with a capacitive probe able to measure the composition of bed solids along the bed height.