Chemical looping combustion (CLC) has emerged as an efficient and promising combustion technique for fossil fuels during the past few decades. The main advantages of CLC lie in its inherent CO 2 sequestration and cascade energy utilization, being primarily benefited from the in situ reactive separation facilitated by the circulation of a solid intermediate. Up to date, the research on the CLC-related oxygen carrier, reactor, and system has made extensive and in-depth development worldwide. CLC units with thermal power ranging from the kW th to MW th scale were demonstrated with fuels of different types (gaseous, liquid, and solid fuels). Over the past 20 years, Chinese researchers have made significant progress in chemical looping technologies, extending from fundamental oxygen carrier studies to the implementation of pilot-scale CLC units. For the use of solid fuels, such as coal, in CLC, it is a rather challenging task but a lot of opportunities also remain. As a result of the particular "rich coal, meager oil, and deficient gas" energy reserve characteristics, China has become the main research battlefield on CLC of coal these years. In this paper, the main advances and research status on CLC of coal in China are reviewed and appraised. The contents in this paper cover most of, if not all, the research hotspots on CLC of coal, i.e., oxygen carrier screening, reactor design/construction/operation, pollutant emission, reaction kinetics, and numerical simulation. Chinese researchers have made substantial contributions to two bottleneck issues faced by the coal-derived CLC technique, i.e., developing and preparing a low-cost while well-performing oxygen carrier and promoting the slow char gasification process in the fuel reactor. In addition, remaining challenges that constrain the development of large-scale CLC units and indeed deserve in-depth investigation are analyzed. Particular attention is paid to the following three key challenges: severe mismatch of reaction rates in CLC of coal, difficulty in attaining a good balance between the oxygen carrier performance and cost, and challenge in controlling solid circulation to manage heat and mass transfer. Accordingly, potential opportunities for future research and scaling-up of the coal-derived CLC technique are discussed. The academic thoughts that are highlighted here include (1) achieving a good compromise between the oxygen carrier cost and performance through the rational design of a multifunctional and composite oxygen carrier and its scalable preparation using cheap raw materials, (2) coordination among reactor modeling− reactor design−reactor operation to attain effective management of heat and mass transfer in the CLC reactor, and (3) a complex while effective matching matrix among coal type, oxygen carrier particle, and reactor configuration to acquire the optimal performance of the whole CLC system. Overall, this review summarizes the contributions of Chinese scholars to CLC of coal and presents how these research achievements benefit the commercial-sca...

Cu-based materials have been regarded as suitable oxygen carrier (OC) candidates in chemical looping combustion as a result of their high reactivity. Because Cu is a common reduction product in the fuel reactor and can be reoxidized to CuO in the air reactor, obtaining insights into the complete oxidation process of Cu at a microcosmic level is critical for exploring the intrinsic oxidation mechanism and kinetics. In this work, the detailed oxidation steps have been investigated by density functional theory calculations. First, the most likely dissociative adsorption pathways of oxygen molecules on the Cu(111) surfaces are determined. On the basis of the Mulliken charge analysis and partial density of state analysis, the oxygen uptake process would preferentially produce a CuO nano-island rather than Cu2O on the surface. Then, ions (O2– and Cu2+) diffusion pathways are examined to explore the details of oxide growth. Oxygen inward diffusion leads to the formation of Cu2O, however with quite high energy barriers. On the contrary, the horizontal diffusion of copper atoms on the surface is quite easy in kinetics and thermodynamics, corresponding to an epitaxial growth of the CuO oxide nano-island and then the formation of an exterior thin CuO layer. Furthermore, the continuous outward diffusion (replenishing copper atoms for the epitaxial growth) of copper atoms in the Cu or Cu2O bulk is also considered. Results show that the energy barrier of each diffusion step in the Cu(111) bulk is smaller than that in the Cu2O­(111) bulk, indicating that the bulk Cu2O phase preferentially forms as a result of copper outward diffusion in the Cu(111) bulk and then is oxidized to generate the bulk CuO phase eventually. In such a way, a complete oxidation mechanism of Cu → Cu2O → CuO is elucidated, which has been validated by a well-designed thermogravimetric experiment of controllable oxidation of pure copper.

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CuO-based materials as oxygen carrier (OC) always exhibit a weak sintering resistance at high temperature, which leads to a significant decrease of reactivity in chemical looping processes. Inert component is usually added to enhance the thermal stability and increase the specific surface area of OC particles. Detailed knowledge on the sintering mechanism of CuO nanograins within the bulk of OC particles and the interactions between active component and inert support materials is thus of considerable importance. In this study, molecular dynamics (MD) method was conducted to explore the fundamental understanding of CuO sintering mechanism and the effects of different support materials (TiO 2 , ZrO 2 , and SiO 2 ) on the sintering resistance of supported CuO nanograins. The sintering simulations of pure CuO nanograins show that CuO particle with smaller diameter or at higher temperature tends to be more amorphous. With respect to the sintering of two unsupported nanograins, it can be concluded that the neck growth during sintering is the joint effect of surface diffusion and grain boundary diffusion. Among these three composite OCs (CuO supported by TiO 2 , ZrO 2 , or SiO 2 ), CuO/ZrO 2 shows a better sintering resistance. The enlarged discrepancy on the surface area loss between different supported CuO nanograins with the rising of temperature emphasizes the importance of rational selection of support materials at high temperature.

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