## 1 Introduction
OC is the most important part of the CLC technique. The ideal OC should be highly reactive. It should immediately oxidize in the air reactor, as well as reduce in the fuel reactor for good combustion efficiency. It should also be stable, and show good resistance towards fragmentation and attrition. In addition, it should have good fluidization properties so that it does not agglomerate. The materials that can be used for OC are copper, nickel, manganese and iron oxides, but these are not limited, as other elements have also been developed [7]. Several numerical studies have described the reduction and oxidation behavior of oxygen carriers [8; 9; 10; 11; 12; 13]. Various researchers have performed simulations on CLC using different OCs [14; 15; 16; 17; 18; 19; 20; 21]. [PERSON] et al. [14] developed a reaction kinetic model of a fuel reactor using a mixed OC of calcium sulphate and hydrogen (CaSO\({}_{4}\)+H\({}_{2}\)). The computational result showed a low fuel conversion rate with this OC. [PERSON] et al. [20] evaluated the performances of different manganese ores as OCs for CLC. They found that the high char conversion rate of manganese makes it a good choice as an OC. [PERSON] and [PERSON] [21] simulated  
Figure 1: Conceptual diagram of CLC. the fuel reactor to compare the performances of two OCs, namely, ferric oxide (Fe\({}_{2}\)O\({}_{3}\)) and copper oxide (CuO), as regards the interaction between particle and particle, and fluid and particle, and the reaction rates. They found that CuO has a better performance when the ash agglomeration effect is not considered, and Fe\({}_{2}\)O\({}_{3}\) performed better when the ash agglomeration effect was considered. [PERSON] et al. [22] experimentally investigated the cold flow performance of a dual circulating fluidized bed system to understand the hydrodynamic behavior of a pilot-scale advanced CO\({}_{2}\) capture technology (PACT) facility at Cranfield, UK. FE100 and molochite were used as the OC. They have concluded that both the risers share similar pressure and density profiles. The circulation rate of OC can be changed by altering the fluidization velocity and bed inventory. However, a stable circulation rate was achieved using pneumatic transport. CFD modeling techniques and the fundamental equations used in the development of the CFD models have been reviewed by [PERSON] et al. [23]. [PERSON] et al. [24] developed a 2D CFD model for CLC using inter-connected fluidized beds. They used an Eulerian continuum and two fluid models for both the gas phase and the solid phase. Using their model, they successfully described the hydrodynamics of gas and solid particles in the CLC process. Recently, [PERSON] et al. [25] performed a numerical simulation using the multi-phase particle-in-cell (MP-PIC) method for a biomass-fueled CLC. They found that the fluidization velocity of the air reactor greatly affects the hydrodynamic behavior. Based on their results, they also stated that the pressure balance is very important in order to minimize the gas leakage from the reactor. From the above discussion of the literature, it can be concluded that in the development of CLC, various researchers have opted for computational approaches to study various parameters in the operation of CLC. The computational approach mainly consists of CFD-based simulation, numerical simulation, thermodynamic computation, ASPEN Plus-based simulation, the DSMC model, the direct simulation Monte Carlo method, etc., to model the process and to study the behavior resulting from variations in design and operating parameters [26]. During their research, various OCs have been investigated, but the cold flow simulation of an air reactor using hematite as the OC has not been performed. In this work, a numerical investigation using the cold flow simulation of the air reactor is performed to understand the hydrodynamic behavior of the system.