# Biomass-Based Chemical Looping Gasification: Overview and Recent Developments
## 2 Process of Chemical Looping Gasification of Biomass
### Process Classification
#### 2.2.1 System Configuration
This configuration normally consists of a high-velocity riser and a low velocity bubbling fluidized-bed as the AR and FR, respectively, being the most popular configuration among all the various types [26; 28; 29; 30; 31; 32; 33]. Biomass gasification takes place in the FR while the oxygen carrier is oxidized inside the AR. The loop-seals are installed to prevent gas leakage between the AR and the FR. Additionally, cyclones are used to remove solid particles from the gas stream. The three-reactor system can generate pure hydrogen and syngas separately and simultaneously. It shares similar principles with the chemical looping reforming of methane configuration [14] as shown in Figure 4b. A simplistic mechanism of the three-reactor system for BCLG combined with water splitting is illustrated in Figure 5. Biomass is partially oxidized by the lattice oxygen of the metal oxides in the FR to produce syngas as shown in Reaction (4), but the reduced oxygen carrier is oxidized by steam to regenerate lattice oxygen and produce H\({}_{2}\) in the steam reactor Reaction (5) instead of in the AR. Afterward, the oxygen carrier is fully oxidized in the AR Reaction (3) before continuing the next cycle. The reaction in the steam reactor can be illustrated as follows:  
\[\text{Steam reactor: Me}_{\text{x}}\text{O}_{\text{y}-2}+\text{H}_{2}\text{O} \rightarrow\text{H}_{2}+\text{Me}_{\text{x}}\text{O}_{\text{y}-1} \tag{5}\]  
where Me\({}_{\text{x}}\)O\({}_{\text{y}}\) is an oxygen carrier, Me\({}_{\text{x}}\)O\({}_{\text{y}-1}\) and Me\({}_{\text{x}}\)O\({}_{\text{y}-2}\) are the corresponding reduced form of oxygen carriers with different reduction degrees, i.e., the strongly reduced oxygen carriers (Me\({}_{\text{x}}\)O\({}_{\text{y}-2}\)) leaving the FR are partially oxidized in the steam reactor (Me\({}_{\text{x}}\)O\({}_{\text{y}-1}\)) before full regeneration in the AR. Oxygen carrier material used in the process demands a sufficiently high reactivity with biomass, good performance for water splitting to generate hydrogen, and high stability during redox cycles. Additionally, the material should have good resistance to carbon deposition because it may cause contamination of the hydrogen produced. Some metal oxides have been considered as possible oxygen carriers for this configuration such as Fe\({}_{3}\)O\({}_{4}\), WO\({}_{3}\), SnO\({}_{2}\), Ni-ferrites, (Zn, Mn)-ferrites, Cu-ferrites, and Ce based oxides [14]. A study was developed in a fixed bed reactor by [PERSON] et al. [34] to combine BLCG and water/CO\({}_{2}\) splitting using NiFe\({}_{2}\)O\({}_{4}\) as oxygen carrier. In this study, experimental investigations were  
Figure 5: Process scheme of three-reactor system for BCLG. carried out separately in three steps. Firstly, biomass was reduced by NiFe\({}_{2}\)O\({}_{4}\) to generate syngas in the presence of steam/CO\({}_{2}\). Afterward, the reduced form of oxygen carrier was oxidized partially by steam/CO\({}_{2}\) to produce H\({}_{2}\)/CO, then it was fully oxidized by air in the oxidation step. During the investigations, syngas and H\({}_{2}\)/CO were obtained separately in different steps. The authors also proposed phase transitions corresponding to different reduction degree of oxygen carrier at different steps as follows:  
\[\mathrm{NiFe_{2}O_{4}}\rightarrow\mathrm{metallic\;Fe(Ni)/FeO_{x}} \rightarrow\mathrm{Ni_{1.43}Fe_{1.7}O_{4}/Ni}\rightarrow\mathrm{NiFe_{2}O_{4}}\]  
The three-reactor configuration has been considered a promising approach since it can produce syngas and pure hydrogen simultaneously. However, there have been very few studies on this configuration. Some research related to the three-reactor system has been focused on gaseous fuels, coal, and FeO/Fe\({}_{3}\)O\({}_{4}\)/Fe\({}_{2}\)O\({}_{3}\) materials as looing materials [35; 36; 37; 38; 39; 40; 41]. Packed and Fluidized-Bed  
The packed bed configuration reactor can be applied for chemical looping gasification of biomass.