The share of renewable energies is rapidly growing partly in response to the urgent need of mitigating CO2 emissions from fossil fuel power plants. However, cheap and efficient large-scale energy storage technologies are not yet available to allow for a significant penetration of renewable energies into the grid. Recently, it has been proposed a potentially low-cost and efficient thermochemical energy storage (TCES) system based on the integration of the Calcium Looping (CaL) process into Concentrated Solar Plants (CSP).
The CaL process relies on the multicycle carbonation/calcination of CaO, which can be derived from calcination of widely available, cheap and non-toxic natural limestone (CaCO3). This work explores the effect on the multicycle activity of limestone derived CaO of thermal pretreatment under diverse atmospheres and the addition of nanosilica, which would expectedly hinder CaO grain sintering.
Importantly, optimum CaL conditions for CSP energy storage differ radically from those used in the application of the CaL process for CO2 capture. Thus, calcination should be ideally carried out under low CO2 partial pressure at moderate temperature (below 750ºC) whereas CO2 concentration and temperature should be high for carbonation in order to maximize thermoelectric efficiency.
When subjected to carbonation/calcination cycles at these conditions, limestone performance is critically dependent on the type of pretreatment. Our results indicate that the multicycle CaO activity is correlated with the size of the particles and the CaO pore size distribution.
Thus, CaO activity is impaired as particle size is increased and/or CaO pore size is decreased. These observations suggest that pore plugging poses a main limitation to the multicycle performance of limestone derived CaO at the optimum CaL conditions for TCES in CSP, which is supported by SEM analysis. Strategies to enhance the performance of natural limestone at these conditions should be therefore oriented towards minimizing pore plugging rather than CaO grain sintering, which stands as the main limitation at CaL conditions for CO2 capture.