# Biomass-Based Chemical Looping Gasification: Overview and Recent Developments
## 1 Introduction
[10] firstly proposed the term \"chemical looping\" for the process, where a metal oxide is used as an oxygen transport medium to perform a redox reaction scheme for an increase of exergy efficiency in power generation. In the chemical looping concept, oxygen carriers (e.g., Me\({}_{\text{x}}\)O\({}_{\text{y}-1}\)/Me\({}_{\text{x}}\)O\({}_{\text{y}}\)) [5; 6; 7] are applied for oxygen transport, avoiding direct contact between fuels and air. Chemical looping processes can be used for power generation, production of syngas, chemicals, and liquid fuels through chemical looping combustion (CLC) [9; 11; 12], chemical looping reforming (CLR) [13; 14], and chemical looping gasification (CLG) [9; 15]. In the CLC process, metal/metal oxide as oxygen carrier circulates between two reactors to completely combust fuels (gaseous and solid fuels), while the CLR is a process for the partial oxidation of hydrocarbon fuels to produce hydrogen. The CLC shares similar principles with the CLC and CLR, but the CLG can produce useful combustible gas from gaseous and solid carbonaceous materials through the partial oxidation process. The typical mechanism operation of CLC of the biomass process is illustrated in Figure 1. The configuration mainly consists of an air reactor (AR) and a fuel reactor (FR), where oxidation and reduction reactions take place, respectively. In the FR, a metal oxide as an oxygen carrier (OC) is reduced to provide oxygen for fuel conversion. Then, the reduced metal oxide is circulated to the AR to be re-oxidized before a new cycle. The general chemical reactions in the FR and AR are shown as follows. \[\text{Biomass}+\text{Me}_{\text{x}}\text{O}_{\text{y}}\rightarrow\text{Me}_ {\text{x}}\text{O}_{\text{y}-1}+\text{H}_{2}+\text{CO}+\text{CO}_{2}+\text{CH }_{4} \tag{1}\]  
\[\text{Me}_{\text{x}}\text{O}_{\text{y}-1}+\frac{1}{2}\text{O}_{2}\rightarrow \text{Me}_{\text{x}}\text{O}_{\text{y}} \tag{2}\]  
where Me\({}_{\text{x}}\)O\({}_{\text{y}}\) is the oxidized and Me\({}_{\text{x}}\)O\({}_{\text{y}-1}\) represents the reduced form of oxygen carrier [16]. Biomass-derived chemical looping gasification is a novel technology to convert biomass into renewable hydrogen-enriched syngas. Since most reactions in the FR are endothermic, they require a large energy supply for stable operation. On the other hand, oxidation reactions in the AR are exothermic. Thus, the oxygen carrier, which can circulate between two reactors in the CLG process, can transfer not only oxygen but also heat from the AR to the FR through oxidation-reduction reactions [9; 17; 18]. Additionally, looping materials (metal oxides) can reduce considerably the amount of tar that causes serious problems in a biomass gasification system. Metal oxides act as an effective catalyst for tar cracking and reduction in tar formation during chemical looping gasification. [PERSON] et al. [19] found that the tar content was reduced around 2.4% per degree Celsius with the presence of iron ore. The NiFe\({}_{2}\)O\({}_{4}\) showed a dual-function of oxidation-catalyst for toluene reduction and significantly promotes toluene converted into carbon and H\({}_{2}\)[20]. Although biomass-based chemical looping gasification currently remains several challenges, the potential achievements may outweigh the challenges. Many studies have been carried out to investigate its nature and to solve operational challenges for the commercialization of this technology. The efforts focus on the development of biomass gasification processes for large-scale applications, improvement in reactivity and stability, as well as the multifunctional nature of looping materials, holistic evaluation for the economic feasibility of biomass-based chemical looping systems, and various types of biomass feedstock for chemical looping processes [9]. Therefore, biomass chemical looping technology has notable potentials to convert biomass-based materials into valuable products and effectively mitigate CO\({}_{2}\) emissions in the atmosphere.