A multi-scale experimental study on calcium-looping for thermochemical energy storage using the CO2 captured from power generation systems

Abstract

The reversible carbonation/calcination reaction of calcium-rich particles (i.e., so-called calcium-looping), whether in an open loop configuration or in a closed cycle, has attracted significant attention in recent years for utility-scale thermochemical energy storage. The calcium-looping process and its variants can be used in conjunction with solid fuel-based conversion systems (e.g., combustors, gasifiers, pyrolysers, etc.) to store thermal energy in chemical form using the CO2 captured from the flue gas streams of these systems with CO2 concentrations typically ranging between 5 and 20 vol%. The calcium-looping-based processes, however, are very much scale dependant. What we know today about the application of the calcium-looping-based processes for thermochemical energy storage is largely based on small-scale experiments reported in the open literature. A multi-scale study was therefore conducted over that past four years to address this shortcoming by conducting a series of complementary experiments at instrument-scale (e.g., thermogravimetric analysers “TGA”), bench-scale, laboratory-scale, and pilot-scale.It was found that the results at different scales were not linearly correlated. For instance, while the calcination and carbonation temperatures had an impact on the reactivities of the calcium-rich particles, the extent of the impact was different at the instrument-scale (e.g., TGA), bench-scale, laboratory-scale, and pilot-scale. The differences between instrument-scale results and those of other scales are largely assigned to the absence of diffusional limitations for the transport of heat and mass at instrument-scale. Diffusional limitations, however, depend on the size and volume of a given system and as such underpin the discrepancies that we have seen among the experimental data collected at the bench, laboratory, and pilot-scales. For example, in our study the low calcination and carbonation temperatures typically resulted in higher reactivities at bench and laboratory-scales while the reactivity of calcium-rich particles at the pilot-scale remained largely unaffected by the calcination and carbonation temperature. Also, the extent of the deactivation of calcium-rich particles over the first few calcium-looping cycles differed at bench, laboratory, and pilot-scales. Hence, any industrial scale use of calcium-looping-based processes for thermochemical energy storage must be backed up by multi-scale studies like that reported here so that the scaling effects are well understood, and proper mitigation measures are implemented.