Abstract
Combining chemical looping with a traditional fuel conversion process yields a promising technology for low-CO2-emission energy production. Bridged by the cyclic transformation of a looping material (CO2 carrier or oxygen carrier), a chemical looping process is divided into two spatially or temporally separated half-cycles. Firstly, the oxygen carrier material is reduced by fuel, producing power or chemicals. Then, the material is regenerated by an oxidizer. In chemical looping combustion, a separation-ready CO2 stream is produced, which significantly improves the CO2 capture efficiency. In chemical looping reforming, CO2 can be used as an oxidizer, resulting in a novel approach for efficient CO2 utilization through reduction to CO. Recently, the novel process of catalyst-assisted chemical looping was proposed, aiming at maximized CO2 utilization via the achievement of deep reduction of the oxygen carrier in the first half-cycle. It makes use of a bifunctional looping material that combines both catalytic function for efficient fuel conversion and oxygen storage function for redox cycling. For all of these chemical looping technologies, the choice of looping materials is crucial for their industrial application. Therefore, current research is focused on the development of a suitable looping material, which is required to have high redox activity and stability, and good economic and environmental performance. In this review, a series of commonly used metal oxide-based materials are firstly compared as looping material from an industrial-application perspective. The recent advances in the enhancement of the activity and stability of looping materials are discussed. The focus then proceeds to new findings in the development of the bifunctional looping materials employed in the emerging catalyst-assisted chemical looping technology. Among these, the design of core-shell structured Ni-Fe bifunctional nanomaterials shows great potential for catalyst-assisted chemical looping.
Highlights
To bolster the rapid economic growth of modern society, the entire world is facing increasing demands on its energy sources
A Ni-Fe bifunctional nanomaterial (Fe/Zr@Zr-Ni@Zr) was designed, for the catalyst-assisted chemical looping process. It consists of a ZrO2 -coated Ni outer shell, encapsulating a Fe2 O3 /ZrO2 @ZrO2 core (Figure 16a), showing multiple benefits: (1) the Ni-based shell and the Fe-based core serve as reforming catalyst and oxygen carrier, respectively, and achieve catalytic reforming and oxygen transfer in a single nanoscale unit; (2) the hollow sphere surrounding the core prevents the aggregation of core particles [152], while providing space for gas-solid contact; (3) at the same time, the sphere offers a large specific surface area, leading to fine dispersion of Ni particles and the adequate deposition of the outer ZrO2 protective layer, both of which contribute to a high catalytic activity [153]
Chemical looping technology has recently emerged as a promising “clean energy conversion technology” due to the advantages of manageable inherent CO2 capture
Summary
To bolster the rapid economic growth of modern society, the entire world is facing increasing demands on its energy sources. Based on this concept, catalyst-assisted chemical looping dry reforming (CCDR) was recently proposed as an intensified process with the goal of maximum utilization of two main greenhouse gases, CH4 and CO2 , in terms of CO yield. Catalyst-assisted chemical looping dry reforming (CCDR) was recently proposed as an intensified process with the goal of maximum utilization of two main greenhouse gases, CH4 and CO2 , in terms of CO yield This process is generally implemented over a bifunctional reactor bed, composed of either a physical mixture of a reforming catalyst and a metal oxide looping material, or a bifunctional looping material [15,23,30]. During the early stages of development, research focused on the metal oxide materials with a single function as CO2 accepter or oxygen storage, which are mainly used for conventional chemical looping combustion and reforming processes. An update of the previous overview of chemical looping materials is given, together with an extension focusing on new insights in the development of bifunctional looping materials based on the latest studies
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