Chemical looping combustion (CLC) is an advanced technology with inherent CO 2 capture in which a solid oxygen carrier circulates between an air reactor and a fuel reactor. For coal-fueled CLC, the existence of solid impurities requires the oxygen carrier not only to have good reactivity but also to be contaminant-resistant, low-cost, and readily available. Therefore, the development of cost-effective and well-performing oxygen carriers is very meaningful for the coal-fueled CLC process. Natural red mud, a byproduct from the aluminum industry, was found to function well as an oxygen carrier and has also been found to have in situ coal catalytic gasification behavior. A thorough study on the longterm cyclic performance of red mud with coal char in a fluidized reactor was conducted in this work. For the purpose of comprehensively understanding the functions of inert supports as well as the sodium content in red mud, the effect of various inert oxides (Al 2 O 3 , SiO 2 , TiO 2 , and CaO) and the addition of sodium was evaluated. It has been proven that inert supports, Al 2 O 3 , SiO 2 , and TiO 2 , have a positive effect on both the reduction and oxidation reactivity of iron-based oxygen carriers by developing a porous structure in the particle. Al 2 O 3 and SiO 2 show the ability to stabilize the reactivity of iron oxide with a gaseous reductant (CO), even under fluidized conditions. Both Al 2 O 3 and TiO 2 can assist in maintaining the mechanical strength of the oxygen carrier after many cycles in a fluidized-bed reactor. The addition of sodium (Na) to red mud does not exhibit much effect on the reactivity of OC with CO as the fuel. However, it can significantly enhance the char gasification rate due to its catalytic function. Additionally, interaction between the active iron oxide and inert supports or sodium in the form of red mud at high temperatures leads to the formation of spinel phases. The growth of spinel phases results in the reduction of the oxygen carrying capacity. However, it helps fix sodium as a relatively stable chemical compound (NaAlSiO 4 or NaFe 0.25 Al 0.75 O 2 ). Both inert supports and sodium in natural red mud play critical roles in the performance of red mud as an oxygen carrier from either physical or chemical aspects.

Chemical looping combustion (CLC) is an advanced technology developed to achieve highly efficient fuel combustion with in-situ CO2 capture. In this process, metal oxide particles are used as an oxygen carrier (OC) to transport lattice oxygen for fuel combustion. In this process, a stream of CO2 and steam is produced by successful separation of atmospheric N2 and the gaseous product of combustion. In CLC of solid fuel, metal oxide particles are physically mixed and react with solid fuel at high temperature using gasification enhancer, such as steam, or CO2. A full understanding of the reaction mechanism between the OC and solid fuel is vital for OC development and the fuel reactor design. Several reactions may be involved in solid-fueled CLC when an iron-based OC is used, including (1) solid fuel devolatilization /gasification, (2) OC reduction with intermediate syngas, (3) the solid-solid reaction between OC and solid fuel via direct contact, and (4) the homogeneous water-gas shift reaction. The former two reactions have been extensively studied in recent years. This study focuses on the third reaction, the solid-solid reaction, which occurs thermodynamically at typical operational temperatures of CLC. The direct solid-solid reaction between coal char and two iron-based OCs via random particle collision in a fluidization bed regime was investigated and focuses on the reaction kinetics and the carbon conversion at different temperature. The contribution of the solid-solid reaction to the global carbon conversion was estimated for steam-gasified CLC at different temperature. The solid-solid reaction via static contact in a thermal-gravimetric analyzer (TGA) was also tested to evaluate the role of different OCs and to better understand the reaction mechanism between the two solid particles.

Solid-fueled chemical-looping gasification (SF-CLG) is a chemical looping technology integrated with gasification for poly-generation, i.e. syngas and power production. This flexible platform could avoid costly air separation unit, providing low cost H 2-rich syngas for the production of fuel chemicals, ammonia, methanol, etc. One key to successful SF-CLG is the development of a multi-functional cyclic solid material that can be treated in large quantities. The solid circulating between the two reactors is the means for oxygen, heat, and catalyst transport. This study demonstrates a cost-effective oxygen carrier (OC) developed from Bauxite residual of alumina industry, containing sufficient active content, multi-supporting materials, and promoter. It is proposed to be a cyclic material to transport oxygen and heat from the air reactor to gasifier to promote gasification, and in its reduced form to catalyze internal syngas reforming. The catalytic functions of this OC for char gasification and syngas reforming were validated. Syngas composition and yield, gasification rate, and OC behavior were investigated at different fuel/OC ratio, and by successive redox cycle in a fluidized bed reactor. The compatibility of new composite materials with the proposed auto-thermal SF-CLG was confirmed based on thermodynamics and the material's physical and chemical properties. Keywords: H 2 rich syngas production; chemical looping gasification; solid fuel; iron-based oxygen carrier With gaseous fuel feed stock, the integration of chemical looping with syngas production yields several types of technologies, such as steam reforming chemical-looping combustion (SR-CLC), auto-thermal chemical looping reforming (a-CLR), and partial oxidation for syngas production [3-7]. These technologies using Nibased oxygen carrier (OC), have been investigated and successfully demonstrated in continuous facilities with thermal inputs ranging from 500 W to 140 kW [4, 8-9]. Here the Ni-based OCs functions as lattice oxygen transport materials and catalysts for fuel combustion, decomposition and steam reforming.

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Ilmenite ore is one of the promising oxygen carriers (OCs) used for coal-fueled Chemical-Looping Combustion (CLC) for electricity generation and CO 2 capture. However, the low reactivity and natural activation of ilmenite OC are two major constrains impeding its application. This effort is to improve ilmenite OC's performance by introducing a small amount of foreign elements, including alkali or alkaline earth metals (K and Ca) and transition metals (Cu, Mn, and Ni). Coating and re-granulation methods were used to prepare OCs where ilmenite ore was the primary constituent. The reactivity, transport capacity, and selectivity of these ilmenite-based OCs with wet syngas, as well as their performances in coal char-fueled CLC were investigated using a TGA, fixed-and fluidized-bed reactors. The addition of K-element significantly improved the OC's reactivity with wet syngas and coal char. The strong catalytic function for WGSR from K-added OCs was found to play a vital role. Cu-coating hindered effectively Fe-element segregation on the surface of ilmenite OC during cyclic reaction, and the OC structural integrity was well maintained. Ca-, Ni-and Mn-added ilmenite OCs did not show promising prospects. The gasification rate and combustion efficiency could be respectively correlated to the reactivity and selectivity of different OCs except for the K-added samples.