Carbon Dioxide Shuttling Thermochemical Storage Using Strontium Carbonate
Phase I concludes with significant progress made towards the SunShot ELEMENTS goals of high energy density, high power density, and high temperature by virtue of a SrO/SrCO 3 based material. A detailed exploration of sintering inhibitors has been conducted and relatively stable materials supported by YSZ or SrZO 3 have been identified as the leading candidates. In 15 cycle runs using a 3 hour carbonation duration, several materials demonstrated energy densities of roughly 1500 MJ/m 3 or greater. The peak power density for the most productive materials consistently exceeded 40 MW/m 3 —an order of magnitude greater than the SOPO milestone. The team currently has a material demonstrating nearly 1000 MJ/m 3 after 100 abbreviated (1 hour carbonation) cycles. A subsequent 8 hour carbonation after the 100 cycle test exhibited over 1500 MJ/m 3 , which is evidence that the material still has capacity for high storage albeit with slower kinetics. Kinetic carbonation experiments have shown three distinct periods: induction, kinetically-controlled, and finally a diffusion-controlled period. In contrast to thermodynamic equilibrium prediction, higher carbonation temperatures lead to greater conversions over a 1 hour periods, as diffusion of CO 2 is more rapid at higher temperatures. A polynomial expression was fit to describe the temperature dependence of the linear kinetically-controlled regime, which does not obey a traditional Arrhenius relationship. Temperature and CO 2 partial pressure effects on the induction period were also investigated. The CO 2 partial pressure has a strong effect on the reaction progress at high temperatures but is insignificant at temperatures under 900°C. Tomography data for porous SrO/SrCO 3 structures at initial stage and after multiple carbonation/decomposition cycles have been obtained. Both 2D slices and 3D reconstructed representations have been obtained from the raw data. Porosity and surface area have been computed based on tomography. The two parameters, permeability and Forchheimer constants, in the Darcy’s law for linear and non-linear range have been obtained from the fluid dynamics simulation. The structure is nearly isotropic from the hydrodynamics point of view. Thermal conductivity at elevated temperature has been obtained from the 3-D pore-scale simulations. The computed thermal conductivities of the structure compare well with an existing correlation. Computational models have been developed for solving the energy and mass transport equations with chemical reaction on the surface and inside the particles. The chemical kinetics or rate law is needed on the particle scale; however it is generally measured using a lumped model based on the average of reacted as well as unreacted mass. Many simulations have been conducted on a 2-D model to determine the effects of those pore scale parameters on the heat release based on the measured data. The simulation results using 2-D model show that average volumetric heat flow rate of 4 MW/m 3 in the first 5 minutes can be obtained. The insight gained from the 2-D simulation will be useful for the next stage simulation based on realistic 3-D structures. A probabilistic analysis has been conducted on the baseline system of recovering heat from the hot CO 2 evolved from the endothermic reaction and compressing the cooled CO 2 into a storage tank at 20-21 bar. Using a Monte Carlo simulation using 2,000,000 samples and both uniform and triangular probability distributions of four variables (heat exchanger pinch points, compressor efficiency, and power block efficiency), exergetic efficiency of 90% appears to be very likely.