# Biomass-Based Chemical Looping Gasification: Overview and Recent Developments
## 2 Process of Chemical Looping Gasification of Biomass
### Process Classification
#### 2.2.1 System Configuration  
The biomass chemical looping gasification concept is to produce high-quality syngas for further applications. The contact between the fuel and the oxygen carrier plays a key role in the chemical looping system, especially BCLG. Hence, the selection of the reactor configuration is an important criterion for chemical looping processes. The essential requirements for selecting an appropriate BCLG with a continuous operation are as follows [12; 17; 23]:  
1. There should be sufficient particle circulation between the FR and the AR. 2. There should be sufficient contact between the fuel/air and the solid oxygen carriers to achieve maximum conversion. 3. High temperature and high-pressure operations must be carried out to achieve higher overall efficiency. 4. There should be limited gas leakage between the FR and the AR. As a part of a chemical looping system, a reactor is a crucial factor that affects process performance. Two common types of reactors have been proposed for chemical looping \begin{table}
\begin{tabular}{l l l} \hline \hline
**No.** & **Name of Reaction** & **Chemical Reaction** \\ \hline
1 & Boudouard reaction & C + CO\({}_{2}\) [\(\leftrightarrow\)] 2 CO \\
2 & Char reforming/water gas & C + H\({}_{2}\)O [\(\leftrightarrow\)] CO + H\({}_{2}\) \\
3 & Methanation & C + 2H\({}_{2}\) [\(\leftrightarrow\)] CH\({}_{4}\) \\
4 & Water gas shift reaction & CO + H\({}_{2}\)O [\(\leftrightarrow\)] CO\({}_{2}\) + H\({}_{2}\) \\
5 & Steam reforming of methane & CH\({}_{4}\) + H\({}_{2}\)O [\(\leftrightarrow\)] CO + 3H\({}_{2}\) \\
6 & Dry reforming & CH\({}_{4}\) + CO\({}_{2}\) [\(\leftrightarrow\)] 2 CO + 2H\({}_{2}\) \\
7 & Oxygen carrier reduction & CO + Me\({}_{5}\)O\({}_{y}\) \(\rightarrow\) CO\({}_{2}\) + Me\({}_{5}\)O\({}_{y-1}\) \\
8 & Oxygen carrier reduction & H\({}_{2}\) + Me\({}_{5}\)O\({}_{y}\)\(\rightarrow\) H\({}_{2}\)O + Me\({}_{5}\)O\({}_{y-1}\) \\
9 & Oxygen carrier reduction & CH\({}_{4}\) + Me\({}_{5}\)O\({}_{y}\)\(\rightarrow\) 2H\({}_{2}\) + CO + Me\({}_{5}\)O\({}_{y-1}\) \\
10 & Oxygen carrier reduction & C + Me\({}_{5}\)O\({}_{y}\)\(\rightarrow\) CO + Me\({}_{5}\)O\({}_{y-1}\) \\
11 & Oxygen carrier reduction & C + 2 Me\({}_{5}\)O\({}_{y}\)\(\rightarrow\) CO + 2 Me\({}_{5}\)O\({}_{y-1}\) \\
12 & Tars’ reforming & Tars + H\({}_{2}\)O [\(\leftrightarrow\)] CO + H\({}_{2}\) + CO\({}_{2}\) + hydrocarbons +... \\
13 & Hydrocarbon reforming & Hydrocabons + H\({}_{2}\)O [\(\leftrightarrow\)] CO + H\({}_{2}\) + CO2\({}^{2+}\)... \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chemical reactions in fuel reactor [19; 21; 22]. Figure 3: Classification of biomass chemical looping gasification process. The two-reactor system is the most popular configuration for BCLG as shown in Figure 4a [25; 26; 27; 28]. The configuration consists of two fluidized bed reactors as the AR and FR, respectively. In the AR, the oxygen carrier materials are oxidized by oxygen from air. The oxidized form of the oxygen carrier is transferred to the FR to react with biomass to produce a gaseous mixture and the reduced form of metal oxides, then they are returned to the AR for regeneration. Additionally, two loop-seal devices are installed between the AR and FR to prevent gas mixing between two reactors. This configuration is based on the reactivity of the oxygen carrier considering that the residence time of the  
Figure 4: BCLG configurations for hydrogen enrich gas production: (**a**) Two-reactor system and (**b**) Three-reactor system. oxygen carrier required for the reduction reaction is higher than for the oxidation. The AR, a fast fluidized bed reactor, has two objectives: to give the driving force for the solid material circulation and provide sufficient oxygen and heat for fuel conversion in the FR [11]. Interconnected fluid fluidized bed reactors, a type of two-reactor system, comprise mainly two fluidized bed reactors.