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[Display omitted] • Agglomeration is affected by OC preparation of methods, carriers and Al:Fe ratios. • Co-precipitation method has better dispersion and lower agglomeration degree. • The carrier plays a role in dispersing active metals and supporting the OC structure. • Fe content will influence the active sites and the metal-carrier interaction. • Excessive Fe content causes serious agglomeration and lower performance. Iron-based oxygen carrier (OC) plays a crucial role in biomass chemical looping gasification (BCLG) to produce high quality syngas. However, a significant challenge during BCLG is the OC deactivation due to agglomeration, which is an urgent problem to be solved. In this study, the agglomeration behavior of iron-based OC preparation during the application for BCLG was obtained under the investigation of different preparation methods, carriers and ratios of active metal to carrier. The results revealed that preparation method affected the dispersion, crystal structure and stability of OC, thereby affecting the agglomeration. Among four preparation methods, the OC prepared by co-precipitation method achieved a 58.57 % lower agglomeration degree and 38.38 % smaller particle size than that of mechanical mixing method. Moreover, the carrier played a role in structural support and lattice oxygen transport, affecting crystalline structure and the agglomeration. Among four carriers, the OC prepared with Al 2 O 3 carrier exhibited 36.18 % lower agglomeration degree and 25.15 % smaller particle size than that of CeO 2 carrier. In addition, higher Fe content would weaken the metal-carrier interaction and reduce the structural stability of OC, causing serious agglomeration. With the decrease of Al:Fe molar ratio, the average particle size and agglomeration degree of spent OC increased by 43.56 % and 78.26 %, respectively. With the increase of cycles, the average particle size and agglomeration degree of spent OC increased gradually, while the reaction performance decreased obviously. [ABSTRACT FROM AUTHOR]

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]

• CaMn 0.5 Ti 0.375 Fe 0.125 O 3-δ perovskite prepared by spray drying granulation method. • FB-TGA was used to evaluate the characteristics of the OC. • After several redox cycles the crystal phase of perovskite was distorted. • The OC reacted with the air distribution plate and quartz tube wall in redox cycle. • The high oxygen transfer capacity is the root cause of particles agglomeration. The development of oxygen carrier particles that can be prepared on an industrial scale with good cycling stability remains a challenge in the field of chemical looping. In this work, a perovskite oxygen carrier (CaMn 0.5 Ti 0.375 Fe 0.125 O 3-δ) was prepared by an industrial-scale spray drying granulation and tested on a fluidized bed thermogravimetric analyzer for long-term chemical looping combustion. The results showed that the attrition of oxygen carrier particles originated from irregular parts of the fresh particle surface early in the cycle, and the chemical stress on particles gradually increased with the number of redox cycles, increasing the attrition and sintering of the particles. In addition, as the cycle proceeds, the perovskite structure was destroyed and the oxygen carrier reacted chemically with the quartz tube wall and the air distribution plate leading to bonding. The deep redox cycling reactions indicated that agglomeration of particles occurred mainly in the reduction stage, with the bed entering repeated periods of defluidization during the 37th-43rd and 47th-55th cycles due to the changes in particle bonding and bed pressure. Fixed bed experiment showed that particle activation came from an increase in the active area of the particles during the cycle, independent of whether they were fluidized or not. In addition, agglomeration started at the contact surface between the air distribution plate and the particles, resulting in blocked oxidation of the particles and deepening agglomeration due to excessive reduction of the particles. [ABSTRACT FROM AUTHOR]

[Display omitted] • Composite hematite and copper ore OC prepared by spray drying granulation method. • FB-TGA was used to evaluate the characteristics of the OC. • Copper and iron migrate to the particle surface after several cycles. • The high temperature is the direct factor leading to the sintering of OC particles. • The high oxygen transfer capacity is the root cause of particles agglomeration. A single natural ore oxygen carrier (OC) has a limited industrial application due to its weak reactivity and cyclic stability. For the composite hematite and copper ore OC prepared by spray drying granulation, long-term chemical looping combustion (CLC) cycle tests were performed on a fluidized bed thermogravimetric analyzer (FB-TGA). The results showed that a substantial mechanical stress and the presence of partially enriched fine particles on the surface of new OC particles caused a high attrition rate of OC at the early stage of fluidization. After 6 min, the attrition rate of the particle was stabilized at 0.28 %/h. The OC was subjected to chemical and thermal stresses during the redox cycle reaction, which decreased the mechanical strength and increased particle attrition and sintering. The particle sintering also hindered the oxidation stage, which led to a deep reduction of OC to a lower valence state, which drove particle agglomeration and eventual defluidization. Therefore, lowering the oxygen transfer capacity and residence time in the fuel reactor is beneficial for the safe operation of OC in industry. [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]

The chemical looping gasification (CLG) of rice husk was conducted in a small fixed-bed reactor to investigate the effects of oxygen carrier preparation methods, active components, Fe loadings, water inflow and successive redox cycles. The gas content, the relative gas concentration, lower heating value of syngas (LHV), carbon conversion efficiency, gasification efficiency, H 2 -TPR, XRD, BET and SEM were analyzed to obtain CLG reaction characterization and comprehensive performance of oxygen carrier. The results showed that compared with several methods and active components, the coprecipitation method with a Fe:Al ratio of 2 was the best preparation method for oxygen carrier. Furthermore, the H 2 concentration was 57.29%, while the gasification efficiency and carbon conversion efficiency were 95.79% and 51.56%. Meanwhile, the oxygen carrier prepared by coprecipitation had a greater performance of oxygen release and reduction. There was an optimal water inflow to obtain the highest gas content, carbon conversion efficiency and gasification efficiency. Thereinto, the H 2 concentration was stable at 54.07–57.65%. And the highest gas content of 78.33% and carbon conversion efficiency of 54.42% were obtained at a water inflow of 14.0 mL/h, while the highest gasification efficiency of 95.79% was obtained at 16.8 mL/h. In addition, after ten successive redox cycles, the gasification performance, crystal size, specific surface area and pore structure were relatively stable. Moreover, the material composition remained basically unchanged. • Coprecipitation is the best preparation method to gain the highest H 2 concentration. • Fe is the optimal active component to gain a highest H 2 concentration and efficiency. • Al:Fe of 2:1 is the best Fe loading in oxygen carrier for a great performance. • Water inflow is beneficial to increase the H 2 concentration. • Reaction performance and structure are stable after ten successive redox cycles. [ABSTRACT FROM AUTHOR]

[Display omitted] • Ca in biomass can significantly improve the CLG performance. • The excessive Ca will cause obvious agglomeration and sintering. • Ca reduces the activation energy of CLG under the reasonable concentration. • The improvement of (CH 3 COO) 2 Ca is better than that of CaCl 2. • Gasification efficiency increases by 45.61% after adding 6% (CH 3 COO) 2 Ca. As the most abundant trace element in agricultural wastes, Ca can improve the reaction performance, but aggravate the agglomeration and sintering of reaction materials at high temperature during the process of thermochemical conversion. In this paper, the effect of Ca in rice husk was studied in chemical looping gasification (CLG) under different conditions of Ca concentration, Ca solution, temperature and steam/carbon (S/C). The results indicated that Ca would significantly improve the CLG performance, but cause obvious agglomeration and sintering phenomenon at high Ca concentration. Furthermore, the improvement for CLG performance of (CH 3 COO) 2 Ca was greater than that of CaCl 2. After adding 6% (CH 3 COO) 2 Ca, gas yield, carbon conversion efficiency and gasification efficiency increased by 53.30%, 47.06% and 45.61%, respectively. By contrast, they respectively increased by 28.11%, 17.27% and 26.79% after adding CaCl 2. Thermogravimetric analysis showed that the presence of Ca was beneficial to reduce the activation energy of the reaction. In the pyrolysis stage, the maximum reduction of activation energy was 37.43%, while in the gasification stage, the maximum reduction of activation energy was 65.36% after adding Ca in the rice husk. Compared with CaCl 2 , (CH 3 COO) 2 Ca decreased more activation energy of CLG process. Moreover, with the increase of temperature and S/C, the CLG performance increased firstly and then decreased after adding 6% (CH 3 COO) 2 Ca by impregnation on the pickling rice husk. 800℃ and 6.9 was the optimal temperature and S/C to achieve the best gasification efficiency of 65.03% and gas yield of 0.69 Nm3/kg in the CLG process. [ABSTRACT FROM AUTHOR]