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Chemical looping steam methane reforming (CL-SMR) is a promising and efficient method to produce hydrogen and syngas. However, oxygen carrier (OC) prepared by synthesis are complex, expensive and poor mechanical performance, while natural ore OCs are low activity and poor selectivity. In order to avoid these problems, Ni/Fe modification of natural ores were proposed to improve the reactivity and stability of OC to CL-SMR. The results indicated that the modified calcite recombined and improved the structural phase during the reaction, enhancing performance and inhibiting agglomeration. Moreover, high ratio of iron to nickel was easy to sinter and decline the OC performance. In addition, with the increase of steam flow, both CH 4 conversion and carbon deposition decreased. Thereinto, the highest H 2 concentration, CH 4 conversion efficiency and H 2 yield were obtained when the ratio of steam to OC was 0.05. Furthermore, CH 4 flow rate had a great impact on CL-SMR performance. When the ratio of CH 4 to OC was 0.04, it achieved the highest CH 4 conversion efficiency of 98.96%, the highest H 2 concentration of 98.83% and the lowest carbon deposition of 3.23%. However, the carbon deposition increased with the increase of CH 4 flow rate. After a long-time chemical looping process, the Ni/Fe modified calcite showed a consistently stable performance with average H 2 concentration of 93.08%, CH 4 conversion efficiency of 88.03%, and carbon deposition of 2.15%. • Natural ores show good performance of oxygen carrier in chemical looping reforming. • High Iron/Nickel is easy to sinter and decline the OC performance. • Steam and CH 4 content have an obvious effect on H 2 production and efficiencies. • Ni/Fe improves the abilities of anti-agglomeration and anti-sintering of calcite. • Ni/Fe modified calcite exhibits an excellent stability performance to produce H 2. [ABSTRACT FROM AUTHOR]

In chemical looping gasification (CLG), as one of the most commonly used oxygen carrier (OC), agglomeration is one of the most crucial triggers of Fe 2 O 3 /Al 2 O 3 OC deactivation. However, the formation mechanism of agglomeration is not clear due to the complexity of the interaction between biomass and OC. In this study, we investigated agglomeration process of Fe 2 O 3 /Al 2 O 3 OC in CLG from two dimensions of time and space. The results showed that in the time dimension, after ten cycles, the DT (deformation temperature) of the spent OC decreased by 5.39%, showing melting point was lower after cycles. Meanwhile, it caused the significant increase of the average particle size (59.73%) and the agglomeration degree (120.13%), indicating more severe agglomeration. Further, it led an obvious decrease in the CLG performance of efficiencies and gas yield. In the space dimension, when agglomeration occurred, low melting point K and Na compounds would first melt on the OC surface to form the K–Na inner layer (KAlSi 2 O 6 and NaAlSiO 4). As the reaction continued, high melting point Ca and Mg compounds would also be adhered to the surface of inner layer to form the Ca–Mg outer layer (CaAl 2 Si 2 O 8 and Mg 4 Al 10 Si 2 O 23), resulting in the continuous increase of OC particle size. In addition, more molten substances of KAlSi 2 O 6 and NaAlSiO 4 would lead to more adhesion between the OC particles. [Display omitted] • The molten layer on the OC surface thickened as the number of cycles increased. • The agglomeration of OC could be divided into the inner and outer layer. • K and Na compounds would first melt on the OC surface to form the K–Na inner layer. • Ca and Mg compounds would be adhered to form the Ca–Mg outer layer. • More molten substances would lead to more adhesion between the OC particles. [ABSTRACT FROM AUTHOR]

[Display omitted] • Temperature greatly influences the formation of agglomeration. • Mechanisms, detection methods and countermeasures of agglomeration were reviewed. • The agglomeration mechanism of chemical looping technology was speculated. • The directional design can improve the sintering resistance of OC. • Coating-induced and melt-induced together lead to the ash-related agglomeration. This review discusses the influence of three key factors on agglomeration of chemical looping technology from the aspects of reaction conditions, the oxygen carrier and biomass composition. In addition, the existing agglomeration mechanisms are extended, and the technologies of detection and prevention of the agglomeration is reviewed. In the process of chemical looping reaction, high reaction temperature, the oxygen carrier materials with poor anti-sintering ability and the reaction system containing more alkali metals and silicon-based materials will easily cause agglomeration. Previous literatures have summarized the agglomeration produced by two mechanisms, and both of them pointed out that the alkali metal in the biomass ash is the main component to induce the agglomeration. For the SiO 2 -based oxygen carrier material, it is speculated that the alkali metal will react with the support to inactivate the active metal component through the inclusion of the molten material. However, the oxygen carrier without SiO 2 -based material is mainly produced through the mechanism of melt-induced, but not the coating-induced agglomeration. The predication of agglomeration focuses on the biomass ash test before the reaction and the monitoring of some variables during the reaction process, such as temperature and pressure analysis. Moreover, the agglomeration can be prevented or controlled by condition adjustment, reactor design, oxygen carrier design and biomass pretreatment. In addition, the use of additives is also a reliable method to reduce the agglomeration. [ABSTRACT FROM AUTHOR]

• HY zeolite is the best carrier to convert wood vinegar into H 2 -rich syngas by CLR. • Ni/HY has excellent redox performance and gasification efficiency during CLR. • The optimal Ni loading is 20% with the highest H 2 yield and gasification efficiency. • +15% is the best water volume for CLR on crude wood vinegar. • Carbon deposition of Ni/HY at 900 °C for long-time reaction is the lowest. The crude wood vinegar contained a large amount of water and a small amount of complex organic compounds, which was difficult to industrialize practically. An efficient oxygen carrier was prepared by Ni-modified HY zeolite (Ni/HY) to produce hydrogen-rich syngas by chemical looping reforming (CLR) on crude wood vinegar. The yield of gas, conversion efficiency, H 2 /CO, oxygen carrier structure and carbon deposition were investigated to obtain the optimal preparation of oxygen carrier and the optimal CLR performance. The results showed that HY zeolite was the most suitable carrier for CLR among five different molecular sieves. Compared with HY zeolites modified by different metallic oxides, Ni/HY had the highest H 2 yield of 64.68%, gasification efficiency of 85.29% and structural characteristics. Further, water volume had a great impact on the CLR performance. The optimal water volume was + 15% with H 2 yield of 68.4%, carbon conversion efficiency of 37.58% and gasification efficiency of 92.38%. Furthermore, the particle size of oxygen carrier increased with the increase of Ni loading. The best performance with gasification efficiency of 96.17% and carbon conversion efficiency of 39.41% could be obtained when the Ni loading was 20%. Moreover, the CO yield, gasification efficiency and carbon conversion efficiency gradually increased with temperature increasing. After CLR for the long-term reaction time, 900 °C was the best reaction temperature to achieve gasification efficiency of 107.29% and the lowest carbon deposition of 3.71%. [ABSTRACT FROM AUTHOR]