The thermochemical cycle of calcium oxide looping is well suited for use with concentrated solar irradiation for carbon dioxide (CO2) capture. The cycle consists of the reversible calcination–carbonation heterogeneous solid–gas reaction pair. CO2 is chemically absorbed from a dilute source by a calcium oxide (CaO) sorbent to form solid calcium carbonate (CaCO3) in the exothermic, non-solar carbonation step:
CaO + CO2 → CaCO3, Dh0 = –178 kJ mol-1
The sorbent is regenerated and a stream of concentrated CO2is released in the endothermic, solar-driven calcination step:
CaCO3 → CaO + CO2, Dh0 = 178 kJ mol-1
Length scales involved in the realization of solar thermochemical technologies such as calcium oxide looping carbon dioxide capture typically range from 101–103 m at the solar plant level, to 10-1–101 m at the reactor level, to 10-2–10-1 m at the reactor components level, and to 10-8–10-2m at the component features level. Thermal transport models developed at the distinct length scales inform the design of the materials, reactors, and the overall process.
At the intra-particle level, a transient thermal transport model is used. It predicts heat and mass transfer coupled to chemical kinetics inside sorbent particles undergoing cycling. Both the solid and fluid phases are considered in the model. The model is used to investigate effects of the physical parameters such as particle size, and reactor operational parameters such as incident solar irradiation and carbon dioxide concentration. At the reactor component level, heat and mass transfer coupled to chemical kinetics is investigated for a packed bed of sorbent particles using computational fluid dynamics. The packed-bed dimensions are varied to study their effect on pressure drop, temperature distribution, and heat transfer in the packed-bed. On the reactor level, radiative exchange within the reactor cavity is analyzed using the net radiation method. The radiative exhange model is coupled to the heat and mass transfer model of the packed-bed to investigate the thermal transport phenomena occuring in the reactor.
A 1 kWth reactor concept is proposed for solar CaO looping. The concept consists of a beam-up oriented, dual-cavity reactor wherein a receiver cavity is surrounded by an annular packed bed of sorbent particles. During carbonation, a CO2-containing gas flows through the packed-bed reaction zone. The CO2 chemically reacts with the sorbent particles, forming CaCO3, while CO2-depleted gas leaves the system. During calcination, concentrated solar energy enters an aperture located at the bottom of the reactor. The concentrated solar energy is captured by the inner cavity and transferred by conduction through a diathermal cavity wall to the packed-bed reaction zone, driving the calcination reaction. CO2 liberated from the calcination reactor is removed from the reactor in a concentrated stream appropriate for storage or further use and processing. The reactor prototype design is informed by numerical thermal transport models at the intra-particle, packed-bed and reactor levels. The final reactor design is presented.
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