Multiscale modeling of an entrained flow solar calciner for thermochemical energy storage via calcium looping

Abstract

Calcium looping has been proposed as an alternative route addressing the intermittent nature of solar energy in terms of thermochemical energy storage, with the added benefit of CO2 capture. In this work, a multiscale approach aims to predict and optimize the performance of a pilot-scale solar calciner operating at realistic conditions, incorporating a diffusion–reaction model with multiphase flow considerations in a downer reactor. The model has been successfully validated with experimental data from the literature studying the effect of particle size on the observed rate, and with limestone decomposition experiments carried out in a lab -scale downer reactor. It was revealed that particles larger than 100 μm exhibit internal mass transport resistances, due to CO2 accumulation within the particle, whereas heat transport limitations are not expected in the operating conditions of interest. A reference scenario demonstrated 92 % limestone conversion at realistic operating conditions, feasible in concentrated solar power plants. The solar flux density on the reactor wall influences the efficiency of the system critically since heat loss to the environment leads to greater solar input. To tackle the low efficiency, a partially insulated, indirectly irradiated reactor was investigated: the efficiency increased from 8 % to 26 %, while integration in a Calcium-Looping system can further boost the efficiency to 34 %. This approach constitutes a basis for future scale-up considerations.