# Biomass-Based Chemical Looping Gasification: Overview and Recent Developments
## 3 Looping Materials in Chemical Looping Gasification of Biomass  
### Looping Materials  
In the BCLG process, oxygen carriers are used to transfer lattice oxygen from the air to the fuel via redox reactions to produce syngas, which can avoid the direct contact of air and fuel. Metal oxide materials play a key role in the chemical looping redox processes. These metal oxide materials can be classified as chemical looping gasification (BCLG) and oxygen uncoupling chemical looping gasification (OU-BCLG) based on the properties of looping materials as shown in Figure 3. As can be seen in Figure 7a, biomass fuel is decomposed and cracked down at high temperatures mainly into three products of gas, tar, and char, which can be simplified into volatiles and char. The volatile matter in the biomass, which is reduced to the release of hydrocarbon gases, is constituted by complex organic substances and can be condensed at a sufficiently low temperature to liquid tars. The gaseous fraction is an incondensable mixture of gases at ambient temperature and accounts typically for 70-90 wt.% of the feedstock [55,56]. This mixture of gases consists mainly of hydrogen, carbon monoxide, carbon dioxide, and light hydrocarbon. Afterward, these products react with oxygen carrier particles. Two main types of reactions occur simultaneously in the fuel reactor: homogeneous and heterogeneous. There are two reaction pathways proposed between the oxygen carrier and biomass in the fuel reactor: direct reduction of oxygen carrier by biomass and reduction of oxygen carrier by gaseous biomass gasification products [9]. The first pathway consists of two main reaction types, reactions of volatile matter released from biomass with oxygen carrier (see Reaction (7)), and direct solid-solid reactions. Since solid-solid reactions are very limited due to low contact efficiency, the impact on the final gas composition is negligible in comparison to heterogeneous reactions. Due to a relatively high fraction of volatile matter in biomass, it is a benefit for biomass that a higher proportion of the fuel can directly react with oxygen carrier in a CLG system [9]. The second pathway is an indirect reaction between biomass and oxygen carrier. Firstly, Biomass is gasified with H\({}_{2}\)O/CO\({}_{2}\) to produce mainly H\({}_{2}\)/CO (see Reactions (8) and (9)), and then gaseous products can readily react with oxygen carrier (see Reactions (6), (10) and (11)). The general reactions occur in the fuel reactor:  
\[\text{Biomass}\rightarrow\text{Volatiles + Char (mainly C) + H}_{2}\text{O} \tag{6}\]  
\[\text{Volatiles + Me}_{\text{x}}\text{O}_{\text{y}}\rightarrow\text{CO}_{2}+ \text{H}_{2}\text{O}+\text{Me}_{\text{x}}\text{O}_{\text{y}-1} \tag{7}\]  
\[\text{Char + CO}_{2}\to 2\text{CO} \tag{8}\]  
\[\text{Char + H}_{2}\text{O}\rightarrow\text{H}_{2}+\text{CO} \tag{9}\]  
\[\text{CO}+\text{Me}  
The other concept proposed is oxygen uncoupling biomass chemical looping gasification (OU-BCLG). The system is based on the use of an oxygen carrier that can release gaseous oxygen in the fuel reactor to oxidize the fuel as shown in Figure 7b.  
\[2\text{Me}_{\text{x}}\text{O}_{\text{y}}\rightleftharpoons 2\text{Me}_{\text{x}}\text{O}_{\text{y}-1}+\text{O}_{2} \tag{12}\]  
\[\text{Char}+\text{O}_{2}\rightarrow\text{CO}+\text{CO}_{2} \tag{13}\]  
\[\text{Volatiles}+\text{O}_{2}\rightarrow\text{CO}+\text{CO}_{2}+\text{H}_{2} \text{O} \tag{14}\]  
In the OU-BCLG process, the oxygen carrier releases oxygen according to Reaction (12), and the biomass fuel can be decomposed simultaneously into volatiles and char as shown in Reaction (6). Afterward, the devolatilization products are oxidized to CO, CO\({}_{2}\), and H\({}_{2}\)O according to Reactions (13) and (14). The remaining char is gasified by CO\({}_{2}\) and H\({}_{2}\)O to produce CO and H\({}_{2}\). Moreover, the fuel reactor in both concepts should be fluidized by H\({}_{2}\)O, CO\({}_{2}\), or their mixture, which also acts as gasifying agents to accelerate biomass gasification.