# Biomass-Based Chemical Looping Gasification: Overview and Recent Developments
## 3 Looping Materials in Chemical Looping Gasification of Biomass
### Type of Oxygen Carrier
The oxygen transport capacity of Fe-based oxygen carriers is relatively low, and it cannot be reduced further than Fe\({}_{3}\)O\({}_{4}\) in a fluidized bed reactor because of the thermodynamic limitation [25]. Thus, to improve the reaction rate and characteristics of metal oxides, alkali, and alkaline compounds are added to the oxygen carrier. [PERSON] et al. [79] investigated that the addition of alkali metals to Fe\({}_{2}\)O\({}_{3}\) can improve the reaction rate of solid fuel, and [PERSON] et al. [80] reported that K\({}_{2}\)CO\({}_{3}\) -added iron ore showed stable catalysis properties for coal chemical looping combustion, whereas calcium oxide exhibited not only catalytic activity but also the ability to capture CO\({}_{2}\) and sulfur compounds [81]. The known benefits of iron oxide are low price, chemical stability, non-toxicity, low sintering temperature needed, and low degree de-fluidization problems, the Fe-based materials are promising oxygen carriers in chemical looping processes. Fe-based oxygen carriers perform differently in their reactivity with various fuels, their reaction rate varies with the following order: H\({}_{2}\) > CO > CH\({}_{4}\) > solid fuels [69; 82]. Mn-based oxygen carriers are probably classified into the group of oxygen uncoupling materials with Mn\({}_{2}\)O\({}_{3}\)/Mn\({}_{3}\)O\({}_{4}\) phases, but Mn\({}_{2}\)O\({}_{3}\) is not stable at relevant temperatures; therefore, Mn\({}_{3}\)O\({}_{4}\)/MnO can be the main phase transition in the chemical looping processes which is not possible to release oxygen [28]. Mn-based materials show low-cost and environmentally friendly properties that are similar to Fe-based oxygen carriers. MgO addition improved the oxygen release ability of the oxygen carrier and increased the performance of biomass chemical looping gasification [45]. The theoretical oxygen transport capacity of Mn-based oxygen carriers is higher than that of Fe-based materials, but there are a few reports that have been published regarding these materials in the chemical looping gasification of biomass. Perovskite-type complex metal oxides have a general formula ABO\({}_{3}\), where A is a lanthanide ion and/or alkaline earth metal and B is a transition metal ion [18]. Many studies pointed out that two different oxygen species being in the perovskite oxides, such surface absorbed oxygen and bulk lattice oxygen which play a different role in chemical looping processes [83]. The surface absorbed oxygen combusts completely methane, whereas the other is responsible for partial oxidation of methane to H\({}_{2}\) and CO. Redox activity of the \begin{table}
\begin{tabular}{c c c c c c} \hline \hline Redox Couples & CaSO\({}_{4}\)/CaS & Co\({}_{3}\)O\({}_{4}\)/Co & CuO/Cu & CuO/Cu\({}_{2}\)O & NiO/Ni \\ \hline R\({}_{\rm o}\) & 0.47 & 0.27 & 0.20 & 0.1 & 0.21 \\ \hline Redox couples & Mn\({}_{2}\)O\({}_{3}\)/MnO & Mn\({}_{2}\)O\({}_{3}\)/Mn\({}_{3}\)O\({}_{4}\) & Fe\({}_{2}\)O\({}_{3}\)/FeO & Fe\({}_{2}\)O\({}_{3}\)/Fe\({}_{3}\)O\({}_{4}\) & Fe\({}_{2}\)TiO\({}_{5}\)/FeTiO\({}_{3}\) \\ \hline R\({}_{\rm o}\) & 0.1 & 0.034 & 0.1 & 0.034 & 0.05 \\ \hline Redox couples & CuAl\({}_{2}\)O\({}_{4}\)/Cu.Al\({}_{2}\)O\({}_{3}\) & CuAlO\({}_{2}\)/Cu.Al\({}_{2}\)O\({}_{3}\) & CuAl\({}_{2}\)O\({}_{4}\)/Cu.AlO\({}_{2}\) & Fe\({}_{2}\)O\({}_{3}\)/Al\({}_{2}\)O\({}_{3}\)/FeAl\({}_{2}\)O\({}_{4}\) & FeAl\({}_{2}\)O\({}_{4}\)/Ni.Al\({}_{2}\)O\({}_{3}\) \\ \hline R\({}_{\rm o}\) & 0.089 & 0.066 & 0.044 & 0.045 & 0.091 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Oxygen transport capacity of some Redox couples, R\({}_{\rm O}\)[11]. perovskite materials is dependent on the mixed-conductivity of the support. Many types of research found that the materials released between 3.44 and 8.23 wt.% of oxygen at 600 \({}^{\circ}\)C [18]. Oxygen transport within iron oxide particles can play an important role at the final stage of the reduction, whereas higher Ni\({}^{2+}\) shows a greater reduction ability.