Materials and Systems Design for Energy Conversion with CO2 Separation and Utilization Using Chemical-looping Technology

The latest report of the Intergovernmental Panel on Climate Change (IPCC) has provided unequivocal evidence that human influence has warmed the atmosphere, ocean, and land, and demands further reduction of CO2 emissions1). Therefore, innovative new technologies, including separation, storage, and utilization of CO2, are indispensable for achieving net-zero emissions by 2050. Chemical looping (CL) technologies have the potential for reducing CO2 emissions worldwide. The basic concepts of chemical looping combustion (CLC) processes2) were presented in a patent by Lewis and Gilliland in 19543). The term “Chemical-Looping Combustion (CLC)” was first proposed by Ishida et al. in a thermodynamic study4). The main features of CL systems are sequential redox reactions and CO2 separation using multi-reactor systems with circulating metal oxide particles acting as oxygen carriers (OCs). The redox cycles form CO2, H2, and N2 separately in each reactor as well as generate high-grade heat. Consequently, CL systems can separate CO2 and reduce CO2 emissions. Therefore, redox reactions involving metal oxides are important for application to CL systems in power generation, hydrogen production, and energy storage5)~7). Various CL systems are shown in Fig. 1. In a typical CL system, the OCs circulate between the fuel reactor (FR) and the air reactor (AR), i.e., the OCs are reduced (Eq. (1)) and reoxidized (Eq. (2)) repeatedly. During these reactions, carbonaceous fuels (CmHn) are converted to CO2 in the FR and generate high-grade heat in the AR to produce electricity using a steam turbine, as shown in Fig. 1(a). A CL hydrogen production system with CO2 separation using three reactors was proposed for partially oxidizing OCs with steam to form H2 (Eq. (3)), as shown in Fig. 1(b). In addition, an advanced CL system involving energy conversion and storage with a reversible solid oxide fuel cell/solid oxide electrolytic cell (SOFC/SOEC) system, the CLtype air battery, has been proposed, as shown in Fig. 1(c). The H2H2O system acts as a redox mediator (Eq. (3))8),9). (2n + m)MO + CnH2m → (2n + m)M + nCO2 + mH2O (1)