# Biomass-Based Chemical Looping Gasification: Overview and Recent Developments
## 3 Looping Materials in Chemical Looping Gasification of Biomass
### Type of Oxygen Carrier
An oxygen carrier performs in a chemical looping system depending on a combination of reaction kinetics, a ratio of oxygen carrier to fuel, contact time, and process design. To select oxygen carriers for chemical-looping gasification applications, there are two approaches to achieve partial oxidation of fuels [9]. Firstly, metal oxides in zone B are used to mainly produce CO and H\({}_{2}\), which cannot be completely oxidized due to thermodynamic restrictions. The other approach is to use under stoichiometric quantities of materials in zone A. According to the basis of desired characteristics, various types of oxygen carriers have been investigated for chemical looping gasification of biomass including Fe-based [26; 32; 67; 68; 69; 70], Ni-based [25; 44], Cu-based [71; 72], Mn-based [28], Zn [73], bimetallic oxygen carriers (Fe-Ni [31; 74], Fe-Cu [75; 75], etc.). They can be classified as follows:  
1. Ni-based oxygen carriers. 2. Cu-based oxygen carriers. 3. Fe-based oxygen carriers. 4. Mn-based oxygen carriers. 5. Co-based oxygen carriers. 6. Perovskite-type complex metal oxides
7. Other oxygen carriers. Several kinds of research have been conducted to compare a large number of oxygen carriers. These results show the general reactivity of the commonly used metal oxides follows orders [9; 14; 76]:  
NiO \[>\] CuO \[>\] Mn \[{}_{3}\] O \[{}_{4}>\] Fe \[{}_{2}\] O \[\text{Reaction line 1: 2 CO}+\text{O}_{2}\to 2\text{CO}_{2} \tag{19}\]0 (22)  
One of important characteristics of an oxygen carrier is oxygen transport capacity, R\({}_{O}\), which is defined as the usable oxygen in the oxygen carrier during one redox cycle, defined by  
\[\text{Reaction line 1: 2 CO}+\text{O}_{2}\to 2\text{CO}_{2} \tag{19}\]1  
where \(m_{ox}\) and \(m_{red}\) represent the weight of fully oxidized and reduced oxygen carrier in the redox cycle, respectively. The oxygen transport capacity, \(R_{O}\), determines the fuel conversion and the amount of solid circulation rate. Table 2 presents the \(R_{O}\) values of different redox couples of oxygen carriers. It can be seen that CaSO\({}_{4}\), Co\({}_{3}\)O\({}_{4}\), NiO, and CuO have higher \(R_{O}\) values, whereas reduction of Fe\({}_{2}\)O\({}_{3}\) to Fe\({}_{3}\)O\({}_{4}\) produces only around 3 wt.% of oxygen, the lowest one in the list. The support materials are commonly used to improve the oxygen carrier's performance. The interaction of the metal oxide and the support material can influence the \(R_{O}\) value of oxygen carriers. By using Al\({}_{2}\)O\({}_{3}\) supported Fe-based oxygen carriers, more Fe\({}_{2}\)O\({}_{3}\) can be reduced to FeO in form of FeAl\({}_{2}\)O\({}_{4}\) with R\({}_{O}\) of 0.045, allowing it to almost completely combust to CO\({}_{2}\) and H\({}_{2}\)O in a chemical looping system. Among oxygen carriers, the Nikel-based oxygen carriers perform the best reactivity of the reduction and oxidation reactions in chemical looping gasification of biomass [25; 31; 34]. Due to high oxygen transport capacity, very high chemical reactivity, and almost complete conversion of hydrocarbons, NiO/Ni should be utilized in sub-stoichiometric quantities for chemical looping gasification applications. Additionally, Ni-based oxygen carriers exhibit the potentials for chemical looping process applications at high temperatures in the range of 900-1100 \({}^{\circ}\)C, but significant challenges include accumulative chemical and thermal stress, and mechanical degradation with high cycle number [77] along with low reaction rate, high cost, and sulfur deactivation have hindered for their commercial applications. Copper-based oxygen carriers perform good properties in the chemical-looping process, e.g., high reactivity, oxygen transport capacity, cyclical, and mechanical stability [78]. Reactions of CuO and the gasification products are exothermic thus could promote biomass reforming [71; 75]. However, the main challenges of Cu-based oxygen carriers are sintering and de-fluidization due to the low melting temperature of metallic Cu.