The establishment of advanced global CCUS (Carbon Capture, Utilization and Storage) platforms is a crucial challenge to accomplish the Paris agreement, limiting global warming well below 2 ºC compared to pre-industrial level, and thus lowering the global emission of greenhouse gases to net negative by the end of this century. Carbon capture from the extensive industrial carbon sources for power generation and/or for the heavy manufacturing processes such as cement or steel production is the most crucial step in CCUS, and recognized widely that the lowering of its operation cost is the key breakthrough to accelerate the technology developments. Herein, we will present a new insight to allow the drastic cost reduction for carbon capture by introducing a new strategy, “energy recoverable carbon capture”, and the first high temperature operative liquid phase CO2 absorbents based on the molten ionic alkali-metal borates.
Carbon capture, more precisely, the separation of CO2 from the flue gases by fossil fuels combustion or conversion requires large amounts of heat energy, and its energy penalty is the major drawback on the operation of carbon capture processes. The heat is mainly consumed for the endothermic regeneration to release CO2 from the absorbents, and the capture of CO2 by the absorbents itself is the exothermic reaction. The quantity of heat liberated by the CO2 capture is almost equivalent to that required for the regeneration, as the absolute value of enthalpy of formation (ΔH) in the reversible carbonation-decarbonation reaction is identical. It means that if it is possible to recover the exothermic heat on the capture of CO2, we can operate the system with minimum amounts of net energy input. In the conventional carbon capture processes, such as the reference amine-scrubbing, operating at low temperature, it is difficult to recover the heat under the limitation of Carnot’s rule. High temperature operative carbon capture systems based on the solid adsorbents, such as CaO have the advantage in this context. However, these solids-based systems suffer several technical difficulties due to the limited heat exchange rate from the solid granular adsorbents to the heat transfer fluids for power generation in the pneumatic fluidized bed system. For these reasons, we explored the liquid phase CO2 absorbents operative at high temperatures for high-efficiency heat recovery, and focused on the molten ionic alkali-metal borates, which can be written as A3BO3 (A: Li, Na), as the first high temperature operative liquid phase CO2 absorbents. It shows high CO2 uptake performances at the temperature ranges from 500-700 ºC with remarkably fast reaction kinetics and the excellent intrinsic cyclic regenerability. The superior CO2 uptake performances of the novel oxide melts are explained by the high concentration of oxide ions (O2-) dissociated from the borate ions at the presence of alkali-metal cations in the eutectic mixed oxide melts, where the gaseous CO2 can be introduced efficiently by the bubbling or the liquid spraying, and the carbonate ions (CO32-) can diffuse rapidly in the melts through the rapid ionic diffusion and liquid convection. No rigid product layer formation, nor the sintering or pulverization of particle grains during the reaction are also important factors for high uptake performances of the melts.
In this seminar, the peculiar CO2 uptake performances of this new class of high temperature operative liquid phase absorbents will be presented, and followed by the discussions on its materials compatibility, the influence of contaminant acidic gases, and the reactor design with the techno-economical evaluations to ensure the versatilities of new carbon capture system. The future perspective for the advanced integration of CO2 capture and the conversion processes will also be noted.
 T. Harada et al., J. Mater. Chem. A, 2019, 7, 21827-21834