# Performance of Granulated Ti-Dopped Manganese Oxide Oxygen Carriers in Chemical Looping Processes
## 1 Introduction  
In order to reach the Paris Agreement goals, a clean energy transition from massive use of fossil fuels to sustainable energy sources must be undertaken. To navigate this transition successfully, it is required to focus beyond a single solution or technology. Low-carbon energy sources considered in this transition are solar, wind, and energy storage systems, together with hydrogen and other sustainable fuels, and carbon capture, utilization, and storage (CCUS). In recent decades, the Chemical Looping (CL) technology has positioned itself as one of the most flexible alternatives to facilitate this transition to a decarbonized economy [1]. CL processes can be integrated into combustion, gasification, and reforming schemes [2]. They can be the basis of oxygen generation production processes and be used in the production of chemicals [3]. They can also be used as energy storage technologies or in combination with the use of solar energy [4]. A recent classification distributed this versatility of CL process in three categories [5]:  
* systems for fuel full oxidation (Chemical Looping Combustion, CLC; _in situ_ Gasification-CLC, iG-CLC; chemical looping with oxygen uncoupling, CLOU)
* systems for fuel partial oxidation (Chemical Looping Reforming, CLR; Chemical Looping Gasification, CLC; Chemical Looping Oxidative Dehydrogenation, CL-ODH)3. systems without fuel (Chemical Air Separation, CLAS; thermal chemical energy storage, TCES) The basic principle behind Chemical Looping is the use of a solid carrier material to cyclically transport oxygen from one part of a chemical process to another. One of the first configurations to be investigated was that corresponding to combustion. Chemical Looping Combustion (CLC) is classified as a Carbon Capture and Storage (CCS) technology. It has attracted attention due to its potential to reduce the cost associated with CO\({}_{2}\) capture [6]. In CLC, the oxygen carrier is normally a metal oxide (M\({}_{\text{x}}\)O\({}_{\text{y}}\)) [7]. It circulates between two interconnected reactors, commonly fluidized beds, as it is shown in Figure 1. First, the oxygen in the oxygen carrier is transferred to the fuel while the oxygen carrier is reduced (fuel reactor). Subsequently, the reduced oxygen carrier reoxidizes with air (air reactor). To develop an adequate oxygen carrier is the cornerstone of the CLC technology. Commonly, the mechanism for oxygen transfer from the oxygen carrier to the fuel is based on the lattice oxygen of the metal oxide. However, additionally to this mechanism, some oxygen carriers exhibit oxygen uncoupling capability (r1), i.e., the capacity to release molecular oxygen while they are reduced. This is known as chemical looping with oxygen uncoupling (CLOU). Under CLOU combustion, molecular oxygen released by the metal oxide in reaction (r1) can also be used in the combustion of the fuel, improving the combustion efficiency. \[2\,\text{M}_{\text{x}}\text{O}_{\text{y}}\leftrightarrow 2\,\text{M}_{\text{x}} \text{O}_{\text{y}-1}+\text{O}_{2}\] (r1)  
The possibility of generating O\({}_{2}\) during reduction and subsequently regenerating in air is also the basis for the development of other CL processes such as Chemical Looping Air Separation (CLAS) and thermochemical energy storage (TCES) [8]. Since the oxygen carrier is decomposed in steam, the CLAS process can provide a pure oxygen stream once water is condensed at the outlet of the reduction reactor. After that, the reduced oxygen carrier is regenerated with air in an exothermic reaction in the oxidation reactor. Comparing conventional cryogenic air separation to CLAS, the operation cost can be reduced by 40-60% [9]. Based on the CLAS process, the concept of a solar chemical looping air separation (Sol-CLAS) system (Figure 2) was introduced for TCES application [10]. In this process, the energy required for the heating and reduction of the oxygen carrier particles in the reduction reactor (charging step) is provided by concentrated solar thermal radiation.