Development of typical processes and key devices for direct air CO₂ capture

Conventional carbon capture and storage technologies and carbon capture, utilization and storage technologies mainly target CO₂ emissions from fixed sources, while direct air capture CO₂ (DAC) technology, as an emerging negative carbon emission technology, can capture CO₂ emissions from distributed sources and further reduce the global atmospheric CO₂ concentration. In this paper, the development process of DAC's typical liquid absorption process, solid adsorption process and the construction of relevant demonstration projects are introduced, the technical characteristics of emerging DAC processes are analyzed, and the key equipment schemes and future development trends of existing DAC processes are discussed. The DAC liquid absorption process has the characteristics of low cost of absorbent raw materials and high selectivity, which can realize a large-scale continuous capture, but high energy consumption in the regeneration process. The DAC solid adsorption process has the characteristics of modularity, low investment cost, and relatively low energy consumption in the regeneration process, but requires a regular replacement of adsorption materials and maintenance of adsorption equipment, which is suitable for small-scale DAC application scenarios. Two typical DAC process absorption/adsorption materials are reviewed. In the DAC electric oscillation adsorption process, CO₂ is chemically captured in the solid electrode, and CO₂ desorption is achieved by changing the polarity of the solid electrode with applied electric field. This process has a higher efficiency than that of the heat or pressure-based separation process. The CO₂ in the air is selected through the DAC separation membrane to achieve an efficient carbon capture. The DAC process achieves the CO₂ absorption and desorption through the change of humidity, which breaks through the high energy consumption limit of conventional variable temperature/pressure swing adsorption. The DAC bio-absorption process absorbs and fixes CO₂ through the photosynthesis of algae organisms. The DAC process based on bifunctional catalyst can capture and catalyze CO₂ in one integrated process, saving the transportation and storage cost of CO₂ capture. The key devices of DAC liquid absorption process are air contactor, particle reactor, calciner and curing device, among which the core of air contactor development is to improve gas-liquid contact efficiency, reduce water loss during spray process and reduce equipment corrosion, while the key of particle reactor and curing development is to improve the contact efficiency of solid-liquid two-phase materials and the solid-liquid separation efficiency after reaction. The DAC solid adsorption process is composed of a modular device consisting of an induced air module, an adsorption/regeneration module, an energy supply regeneration module and a CO₂ compression module. The core of optimizing the adsorption module is to improve the gas-solid mass transfer rate, adjust the CO₂ capture efficiency and reduce the pressure drop. It is very important to select suitable regeneration system or use clean energy based on the process requirements of different application scenarios, optimize DAC process and develop high-performance DAC core devices.