Introduction of integrated CO2 capture and utilisation

Integrated CO₂ capture and utilisation (ICCU), also known as integrated CO₂ capture and conversion, or CO₂ capture and in-situ conversion, is a relatively new concept. Unlike conventional CO₂ capture and utilisation (CCU), the capture of CO₂ and the conversion of the captured CO₂ happen in one reactor and at the same temperature. As shown in the following figure, CO₂ sources (e.g. flue gas) pass through dual functional materials to remove CO₂. When the materials are saturated with CO₂ or kinetics unfavoured for CO₂ capture, the inlet gas is switched to a reducing stream (e.g. H₂ or CH4). The introduction of a reducing agent can regenerate the materials, by releasing CO₂ and/or reacting with the carbonated materials directly to produce CO, H₂ or other products.

The following figure indicates typical gas concentrations of an ICCU process using a lab-scale fixed bed reactor. CO₂ is captured in the 1^st stage. Then inert gas (N₂) is used to remove the remaining CO₂ inside the reactor (this step might not be necessary for large-scale applications). After the introduction of H₂, CO is clearly produced until the captured CO₂ is depleted.

The advantages of ICCU are that 1) avoiding the transportation of solid materials which are normal for conventional CO₂ capture through adsorption; 2) avoiding the storage of captured CO₂; 3) increasing the instant molar ratio between reducing agent (e.g. H₂) and CO₂ during the regeneration/utilisation stage, thus resulting in the high conversion of CO₂; 4) enabling a possibility of removing transition metals for catalytic CO₂ conversion and thus simplifying materials development.

The concept can be applied to different CO₂ sources such as flue gas, syngas, biogas and air. Key process parameters include reaction temperature, sorbent/catalyst materials, reducing agents, and targeted products. Furthermore, these parameters are internally closely connected. For example, CaO sorbent can only be used at high temperatures (~600 °C), while capturing CO₂ from the air might use room temperature and polymers as the adsorbents. The materials are called dual functional materials. However, if the materials are physically mixed, they might be called combined functional materials. In any case, the materials need to have two functions, including 1) CO₂ adsorption sites and 2) catalyst sites for converting captured CO₂. However, we have reported that pure CaO can have these two functions for ICCU integrated with reverse water gas shift reaction (https://doi.org/10.1016/j.ccst.2021.100001). This is exceptional and will be discussed in other articles. Normally, the preparation of dual functional materials is to combine an adsorbent and a catalyst (e.g. the mixture of Ni/Al₂O₃ and CaO). The development of dual functional materials is a key research focus for ICCU. A review paper is available for dual functional materials development (https://doi.org/10.1016/j.ccst.2022.100052).

H₂ is normally used as the reducing agent for ICCU. The key products are CH₄ and CO. The production of CO has more interest, as the produced syngas can be further converted to liquid fuels through the commercial F-T process. Therefore, there is increasing research about ICCU integrated with RWGS. However, researchers might argue that H₂ is a high-quality energy carrier, and it might not be feasible to use H₂ to produce CO. This is arguable. Thus recently, more reports are related to the use of methane as the reducing agent, focusing on ICCU integrated with dry methane reforming. Different ICCU processes are reviewed in this paper (https://doi.org/10.1039/D1SE00797A).

The ICCU process can be integrated with microwave heating, plasma heating and photocatalysis, to manipulate the process temperature and products. Thus it has a great potential for future research.

It is noted that ICCU does not have to happen at the same temperature. The capture of CO₂ and the conversion of the captured could be separated at different temperatures. However, this will normally require the transportation of materials between two reactors.