Strategies to improve CaO absorption cycle stability and progress of catalysts in Ca-based DFMs for integrated CO2 capture-conversion: A critical review

The capture or utilization of CO2 from industrial sources (i.e., fossil-fueled power plants, cement plants and industrial furnaces) is the priority for achieving carbon reduction. The calcium looping (CaL) process is a promising technology for CO2 capture, where CaO exhibits excellent potential for high temperature CO2 absorbent applications in terms of low cost, abundant storages and high theoretical capture capacity. In addition, integrated CO2 capture-conversion (ICCC) technology is an emerging integrated process that achieves cost reduction and efficiency by capturing CO2 directly from industrial flue gases using Ca-based bifunctional materials (i.e., absorbents and catalysts, DFMs) and simultaneously converting them into high value-added chemicals. In this work, the research progress of Ca-based materials as CO2 absorbents and bifunctional materials are respectively reviewed. The sintering of CaO during the carbonation/calcination cycle caused its absorption capacity to decrease rapidly with the number of cycles, thus limiting the application of Ca-based absorbents. The first section summarizes the incorporation of alkali metals, the optimization of inert components, and the synthesis of highly porous structures to slow down the sintering of CaO particles. Meanwhile, the effects of reaction temperature (carbonation and calcination temperatures) and reaction atmosphere (SO2, H2O and concentrated CO2) on the sintering of CaO pellets are also reviewed. Furthermore, in response to the research progress of ICCC process, the second section analyses and discusses the influence of reaction conditions such as temperature and impurity gases (i.e., NOx, SOx, H2O and O-2, etc.) in the flue gas on ICCC for guidance of future research. Subsequently, the development of Ca-based DFMs categorized by the catalytic component (i.e., Ni, Ru, etc.) is reviewed, including their performance and potential reaction mechanisms. The interaction mechanism between catalyst and absorbent in Ca-based DFMs is reviewed. Finally, the future development of Ca-based CO2 absorbents and bifunctional materials are envisaged. It is also hoped that this work will help researchers to provide effective guidelines in the field of the preparation and reaction conditions of Ca-based materials.