常规的碳捕集与封存技术和碳捕集、利用与封存技术多针对固定源排放CO₂，直接空气捕集CO₂(Direct air capture, DAC)技术作为一种新兴的负碳排放技术可对分布源排放的CO₂进行捕集，进一步降低全球大气CO₂体积分数。介绍了DAC典型液体吸收工艺、固体吸附工艺的发展过程及相关示范项目建设情况，分析了新兴DAC工艺的技术特点，探讨了现有DAC工艺关键装置方案和未来发展趋势。DAC液体吸收工艺具有吸收剂原料成本较低、选择性较高的特点，可实现大规模连续化捕集，但再生过程中能耗较高。DAC固体吸附工艺具有模块化、投资成本较低的特点，且再生过程能耗相对较低，但需要定期对吸附材料更换和吸附设备维护，适用于较小规模的DAC应用场景。对2种典型DAC工艺吸收/吸附材料进行了概述。DAC电振荡吸附工艺中CO₂在固体电极中发生化学反应被捕集，并通过外加电场改变固体电极极性实现CO₂脱附，该工艺具有比基于热量或压力的分离过程更高的效率。空气中CO₂选择透过DAC分离膜从而实现了高效碳捕集。DAC变湿吸附工艺通过湿度的改变实现CO₂的吸脱附，突破了常规变温/变压吸附的高能耗限制等问题。DAC生物吸收工艺通过藻类生物的光合作用将CO₂吸收固定。基于双功能催化剂的DAC工艺可以在一个综合过程中实现CO₂的捕集与催化，节省了CO₂捕集后的运输与存储成本。DAC液体吸收工艺的关键装置为空气接触器、颗粒反应器、煅烧炉和熟化器，其中空气接触器开发的核心在于提高气液接触效率，减少喷淋过程中的水分损失和减轻设备腐蚀，颗粒反应器和熟化器开发的关键在于提高固液两相物料的接触效率以及反应后的固液分离效率。DAC固体吸附工艺由引风模块、吸附/再生模块、供能再生模块和CO₂压缩模块组成的模块化装置组成，其中优化吸附模块的核心在于提高气固传质速率、调谐CO₂捕集效率、降低压降；并基于不同应用场景工艺需求选择合适的再生系统或利用清洁能源，优化DAC工艺过程和开发高性能的DAC核心装置至关重要。

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.