# Biomass-Based Chemical Looping Gasification: Overview and Recent Developments
## 3 Looping Materials in Chemical Looping Gasification of Biomass
### Type of Oxygen Carrier
The advantages of the materials are excellent regeneration ability and less agglomeration at temperatures above 100 \({}^{\circ}\)C, as well as high thermal stability, good mechanical properties, and high selectivity to synthesis gas. Along with transition metal oxides, i.e., Ni, Cu, Co, Mn, Fe have been investigated as possible oxygen carriers for the chemical looping gasification process, synthetic oxygen carriers are widely researched, while natural minerals are potentially developed due to their low cost. The low-cost natural minerals are used as oxygen carriers including iron ore, ilmenite, manganese ore, and waste materials from the steel industry and alumina production [63]. Synthetic materials generally comprise single metal oxides, as well as their blend with inert support (Al\({}_{2}\)O\({}_{3}\), SiO\({}_{2}\), MgAl\({}_{2}\)O\({}_{4}\), TiO\({}_{2}\), ZrO\({}_{2}\), CaO, etc.) [11] to improve their reactivity, mechanical properties, and stability, as well as multifunctional properties, including catalytic function, tar decomposition, and CO\({}_{2}\) adsorbent in BCLG. Many studies have been carried out to investigate experimentally the reactivity and stability of different oxygen carriers with various supporting materials, e.g., Al\({}_{2}\)O\({}_{3}\)[25; 26; 33; 42; 45; 78; 84], SiO\({}_{2}\)[30], CaO [43; 85; 86], MgO [45], TiO\({}_{2}\)[69; 84]. It was found that the reactivity and stability of oxygen carriers increase significantly during the multiple cycles when supported with Al\({}_{2}\)O\({}_{3}\). Additionally, TiO\({}_{2}\) is used as a supporting material with iron oxides for chemical looping processes. The compounds performed high reactivity and stability in various studies [32; 69; 84]. Ilmenite performed well in the continuous operations of a 1.5 kW\({}_{\text{th}}\) pilot reactor with >90% of biomass conversion and carbon conversion efficiency obtained [32]. It was reported that the presence of MgO and ZrO\({}_{2}\) can increase thermal and chemical stability, as well as specific heat capacity of oxygen carriers [45; 87]. The addition of CaO to oxygen carrier has emerged in the chemical looping gasification process for continuous hydrogen production [43; 45; 88; 89; 90]. Calcium oxide (CaO) has been known as a catalyst and CO\({}_{2}\) sorbent, which favors hydrogen-rich gas production [43; 85; 91]. Furthermore, the presence of CaO also works as tar cracking catalyst for hydrogen production in a fluidized bed reactor [92; 93]. High hydrogen content (approximately 63%) and hydrogen yield (23.07 mol/kg rice straw) at 800 \({}^{\circ}\)C were obtained with the presence of calcium-iron oxide oxygen carriers [86], while [PERSON] et al. [93] reported the maximum hydrogen yield reaches 7.12 mol/kg pine wood at 850 \({}^{\circ}\)C.  
Many kinds of research have been carried out by mixing different active metal oxides or mixing different oxygen carriers composed of single metal oxides. The major aims of using mixed oxides are desired as follows [11; 31; 34; 78; 94; 95]:  
1. Increase the reactivity and/or stability of particles. 2. Improve the conversion of the fuel gas. 3. Improve the mechanical strength and resistance to the attrition of particles. 4. Improve tar decomposition. 5. Improve carbon dioxide adsorption. 6. Decrease the carbon deposition. 7. Decrease the preparation cost of the oxygen carrier. 8. Decrease the use of toxic metals. Many studies have been carried out to evaluate the performance of many metal oxides as oxygen carrier for hydrogen production. It was found that the Fe-based oxygen carrier shows the most attractive application prospect for the chemical looping applications, particularly for hydrogen production due to its high-temperature stability, low cost, environmentally friendly effects [14; 96]. Composite materials of iron oxide and perovskite have been proposed as a potential oxygen carrier material for hydrogen production due to their excellent oxygen transport properties and stability under cycling.