## 1 Introduction  
The production of pure CO\({}_{2}\) was proposed by [PERSON] and [PERSON] [1], which was similar to the process of chemical looping combustion. [PERSON] and [PERSON] adopted this concept to improve the thermal efficiency of fossil fuel fired power plants [2]. Later, the chemical looping combustion (CLC) was firstly named and used for CO\({}_{2}\) capture by [PERSON] et al. [3, 4], and since then the concept of chemical looping has been used in many fields [5, 6, 7, 8, 9]. As an inherent CO\({}_{2}\) separation technology without extra energy penalty, CLC has been considered as one of the most promising technologies to capture high-concentration CO\({}_{2}\)[8, 10]. In the CLC process, the oxygen carrier (OC) circulates between the air reactor and fuel reactor to transfer the oxygen from air to the fuel. Usually, the oxygen carrier is a kind of metallic oxide (M\({}_{x}\)O\({}_{y}\)). In the fuel reactor, the solid fuel is gasified herein with the gasification agent (e.g., H\({}_{2}\)O, CO\({}_{2}\)). The oxygen carrier reacts with the gasified gases or fuel gas to realize the non-flame combustion of the fuel (C\({}_{n}\)H\({}_{2m}\)), described as reaction Equation (1). Then, the reduced oxygen carrier particles (M\({}_{x}\)O\({}_{y-1}\)) are carried by the flue gas into the separator, where the oxygen carrier is separated. Subsequently, the reduced metallic oxide circulates into the air reactor and is reoxidized by the air, shown as Equation (2). \[(2n+m)\text{M}_{x}\text{O}_{y}+\text{C}_{n}\text{H}_{2m}\rightarrow(2n+m) \text{M}_{x}\text{O}_{y-1}+m\text{H}_{2}\text{O}+n\text{CO}_{2}, \tag{1}\]  
\[\text{M}_{x}\text{O}_{y-1}+0.5\text{O}_{2}\rightarrow\text{M}_{x}\text{O}_{y}, \tag{2}\]As a significant role in CLC, the oxygen carrier requires proper reactivity, mechanical and thermal performances. Currently, many successful operations have been conducted using synthetic OCs in pilot-scale gaseous fuel CLC systems. Synthetic OCs such as NiO, CoO, CuO, Mn\({}_{3}\)O\({}_{4}\) and Fe\({}_{2}\)O\({}_{3}\) presented favorable performances with high reactivity, high mechanical strength and low agglomeration [11; 12; 13; 14; 15; 16]. To further enhance the reaction and/or mechanical performances, inert materials (e.g., Al\({}_{2}\)O\({}_{3}\), SiO\({}_{2}\), TiO\({}_{2}\)) were widely used as supports for the synthetic OCs [17; 18; 19; 20; 21; 22]. However, it should be considered that the OC loss takes place continuously in the operation of pilot-scale CLC reactors, which is caused by the attrition along the flow channel [23; 24]. The attrition and toxication of OCs are more severe in the solid fuel CLC process owing to the solid-solid contact between solid fuels and OC particles. Additionally, regular drain of fuel ash in CLC process is always accompanied by the OCs loss [25; 26; 27]. The synthetic OCs always need high-grade metal oxides and/or pure compounds, which increases the cost of raw materials. This would be a constraint factor of synthetic OCs for commercial applications. In recent years, finding low-cost OCs with relatively high reactivity and strong mechanical strength has become a dominant research focus. Abundant ore resources are possible oxygen carriers, such as natural copper, manganese and iron minerals [28; 29; 30; 31]. Due to their low cost, environmental friendliness and abundance, iron-based minerals (e.g., ilmenite, hematite, magnetite, limonite, etc.) have been some of the most attractive OCs and have been widely used [31; 32]. Their uncontrollable compositions and structures made the natural iron-based ores exhibit variable reactivities and mechanical performances in the process of CLC. As for reaction mechanism study, studies in Chalmers University of Technology demonstrated that the ilmenite offered attractive performances in CLC with high conversion of fuel gas [33; 34]. The ilmenite also has been investigated in University of Kentucky and it showed satisfying reactivity, mechanical strength and oxygen transport capacity [23; 35].