# Biomass-Based Chemical Looping Gasification: Overview and Recent Developments
## 3 Looping Materials in Chemical Looping Gasification of Biomass
### Type of Oxygen Carrier  
Oxygen carrier plays a key role as the chemical intermediate to indirectly transfer pure oxygen from the air to the fuels via redox reactions in the chemical looping processes. In chemical looping gasification, an oxygen carrier is used to not only provide the oxygen needed for gasification to extremely improve the quality of syngas but also as a thermal carrier that increases heat balance between the two reactors [27]. Furthermore, some metal oxide oxygen carriers may have a catalytic effect on biomass tar cracking [60; 61; 62]. Thus, the selection of an appropriate oxygen carrier is one of the most important criteria for the good performance of the chemical looping process. The preferable properties of an oxygen carrier for system performance should be as follows [63; 64; 65; 11; 12]:  
1. Sufficient oxygen transport capacity. 2. Favorable thermodynamics and reactivity regarding the solid fuel for the reduction reactions. 3. High reactivity in the oxidation reactions. 4. Selectivity towards CO and \(\text{H}_{2}\). 5. Resistance to attrition to minimize losses of elutriated solid. 6. Minimal carbon deposition. 7. Good fluidization characteristics (no presence of agglomeration) and high melting points. 8. Reasonable cyclability/circulation for using over several redox reactions. 9. Low cost and long lifetime. 10. Environmentally friendly properties. 11. High mechanical strength and resistance to frictional stresses. 12. Capability of converting biomass to gaseous products. 13. Propensity to convert methane  
In the chemical looping systems, the lattice oxygen in metal oxides can oxidize partially or fully carbonaceous fuels; therefore, the reduction behavior of the metal oxides significantly affects the performance of chemical looping gasification systems. A modified Ellingham diagram is shown in Figure 8a, which depicts the standard Gibbs free energies of reactions as a function of temperature according to the following reaction:  
\[\text{aMe}_{\text{x}}\text{O}_{\text{y}}+\text{O}_{2}\rightleftharpoons \text{aMe}_{\text{x}}\text{O}_{\text{(y+2/a)}} \tag{18}\]  
The diagram shows the change of the Gibbs free energy with temperature variation, which can be used to evaluate the redox potentials of common oxygen carrier materials. According to the thermodynamic analysis, metal oxides can be mainly categorized in three zones according to their potential to oxidize the fuel or desired applications. In Figure 8b, three zones are bound by three reaction lines as follows:  
\[\text{Reaction line 1: 2 CO}+\text{O}_{2}\to 2\text{CO}_{2} \tag{19}\]  
\[\text{Reaction line 2: 2H}_{2}+\text{O}_{2}\to 2\text{H}_{2}\text{O} \tag{20}\]  
\[\text{Reaction line 3: 2C}+\text{O}_{2}\to 2\text{CO} \tag{21}\]Zone A: metal oxides located in the area above lines 1 and 2 have strong oxidizing properties and can completely or near completely convert the fuel to yield 95% of CO\({}_{2}\)[66]. Metal oxides in this zone consist of CuO, Co\({}_{3}\)O\({}_{4}\), Fe\({}_{2}\)O\({}_{3}\), Cu\({}_{2}\)O, NiO, CoO, and Fe\({}_{3}\)O\({}_{4}\). Zone B: the region is bound in between lines 1 and 3, the materials in this region can partially oxidize the fuel to produce CO or H\({}_{2}\) and an excessive quantity of oxygen  
Figure 8: Modified Ellingham diagram for oxygen carrier material: (a) standard Gibbs free energy changes of some oxidation reactions and (b) reaction zones for syngas production applications [9]. carrier does not yield full oxidation. Thus, metal oxides in zone B are theoretically ideal for chemical-looping gasification applications with 90% of syngas yield [66]. Zone C: the materials in this zone below line 3 cannot be used as oxygen carriers due to their low oxidizing ability; thus, they are considered inert for chemical looping applications. Ellingham diagrams allow effective evaluation of redox pairs thermodynamically, which can provide theoretical indications for the oxygen carrier's selection.