Calcium oxide, a natural sorbent of CO2, has received significant attention for its potential in carbon mitigation technologies. The reaction of CaO with CO2, carbonation, is a reversible process that theoretically can be employed to selectively capture CO2 from gaseous mixes. The resulting calcium carbonate, CaCO3, is subsequently processed to regenerate CaO and release CO2 for storage purposes. The regeneration conditions cause CaO to lose its practical CO2 sorption capacity due to internal sintering; the loss in capacity increases with each subsequent capture/release cycle.
For our research, we dispersed CaO as a thin layer on a high-surface area inert aluminum oxide, by employing the incipient wetness technique utilized in the catalytic industry to micro-distribute metallic catalytic materials. The resulting sorbent, CaO on Al2O3, was studied with the thermogravimetric analyzer over a large number of cycles and a variety of experimental conditions. The sorbent was found to be more physically stable and chemically reactive than pure CaO over multiple CO2 capture/ release cycles. The performance of the sorbent was superior to that of naturally sourced and chemical grade CaO: over 20 cycles, the sorbent retained 85% of its initial sorption capacity, as measured in the first capture cycle. Pure CaO only retained 52% of the initial capacity over 20 cycles and naturally-sourced CaO is expected to lose even higher fraction of its capacity, as predicted by literature models.
In addition to improved multi-cycle performance, our method of utilizing CaO offers a kinetic advantage over the bulk oxide. The surface reaction of CO2 with CaO is very rapid initially; however, once a layer of CaCO3 forms on the surface, the rate is limited by the product layer diffusion. Dispersing CaO as a thin layer maximizes the reactive surface area. Furthermore, the layer of CaO is on the comparable scale with the critical CaCO3 product layer thickness, thus the diffusion limitations are diminished.
While CaO reacts with CO2 very slowly at temperatures below 550° C, the sorbent introduced here, CaO on Al2O3, is capable of the CO2 capture with rapid kinetics at temperatures as low as 100°C. The capture extent is inversely related to temperatures. Furthermore, regeneration of CaO is also feasible at much lower temperatures than the calcination of CaCO3. The two facts together suggest that the CO2 capture on the sorbent occurs via a sorption process other than the full carbonation of CaO.
The benefit of utilizing CaO is not limited to its potential in carbon capture. When coupled with the CO2-generating processes, such as steam methane reforming (SMR) and water gas shift (WGS), the continuous withdrawal of CO2 on CaO beneficially shifts the thermodynamic equilibrium of the processes to allow for higher product yields. We proposed a new system for utilizing the CaO-based sorbent within reactor systems, where the sorbent was washcoated onto a monolithic support, prototyped on the automotive catalytic converters. The monolith coated with the sorbent was studied in a flow-through reactor system in the operational regimes of the SMR and WGS processes. We have demonstrated the reversible capture of CO2 from a simulated gas stream at both 275° and 600° C, operational conditions of the SMR and WGS. For future studies, the same monolithic support is to house the SMR and WGS catalysts. Potential benefits of the integrated catalytic/CO 2 capture system for electricity generation are outlined with the focus on the flexibility in real-time response to grid pricing and the realization of the economies of scale, given the small size of the integrated system.