# Biomass-Based Chemical Looping Gasification: Overview and Recent Developments
## 4 Effect of Process Parameters on Biomass Chemical Looping Gasification
### Effect of Operating Conditions on Tar Formation  
Tar is a product of the gasification process which is defined as any matter in the product stream that is organic condensable compounds produced in thermochemical reactions [52]. Due to the high content of volatile matters in biomass, a large amount of tar derived from biomass gasification may cause many serious problems for gasifier or downstream processing steps as blockages, clogged filters, and efficiency reduction, but there are few investigations into its formation and effects in biomass chemical looping gasification.  
High temperatures can thermally crack tar components efficiently, but they may reduce process efficiency. It was observed that naphthalene reduces by 47% when temperature increases from 820 to 940 \({}^{\circ}\)C [33]. [PERSON] et al. [120] reported that the tar content declines from 18.58 to 9.03 g/Nm\({}^{3}\) in the range of operating temperature between 740 and 940 \({}^{\circ}\)C. Another study found that about 86% of tar content is eliminated when temperature increases from 500 to 900 \({}^{\circ}\)C [121].  
Furthermore, the addition of steam in the gasifier can also abate an amount of tar in the product gas through steam reforming reactions of tar. Some publications found that steamcan effectively crack down large tar molecules [15; 32; 33]. Increasing the SBR would promote cracking reactions. However, this promotion can be less effective at a relatively high value of SBR. Moreover, excess amount of steam causes a temperature reduction in the reactor, resulting in low tar decomposition by temperature effect. [PERSON] et al. [32] investigated the variation of the amount of tar formed as a function of the steam-to-biomass ratio. They found that an increase in the steam-to-biomass ratio results in a decrease in the amount of tar produced with a reduction of approximately 27% at 820 \({}^{\circ}\)C when the ratio increases from 0.06 to 0.9. A downward trend of tar yield was observed with a reduction of 73% when steam addition was further increased to the steam-to-biomass of 1.25 in an experimental investigation of biomass chemical looping gasification with Cu\({}_{5}\)Fe\({}_{5}\) at 800 \({}^{\circ}\)C [12].  
Figure 12 presents the tar yields of biomass chemical looping gasification under different gasification conditions and various oxygen carriers. The feedstocks used in these investigations were pine wood and pine wood sawdust. Without an oxygen carrier, the highest amount of tar was produced with about 22 and 17 g/Nm\({}^{3}\). In the presence of an oxygen carrier as catalyst and oxidation agent, tar yield obtained reduces significantly to the lowest yield of around 2 g/Nm\({}^{3}\). A similar trend was observed that tars were obtained between 0.9 and 3.0 g/Nm\({}^{3}\) at different operating conditions with Fe\({}_{2}\)O\({}_{3}\)/Al\({}_{2}\)O\({}_{3}\) as oxygen carrier [33]. It is noteworthy that increasing the oxygen carrier-to-biomass ratio promotes the tar decomposition, but it can decline the reduction degree of the oxygen carrier, resulting in the low catalytic reactivity of the oxygen carrier and reduction in tar removal efficiency. [PERSON] et al. [71] investigated the effect of oxygen carrier-to-biomass ratio on the reforming of tar using Cu\({}_{5}\)Fe\({}_{5}\) as oxygen carrier. They found that the total amount of tar obtained reduces strongly by approximately 67% when the oxygen carrier-to-biomass ratio increases from 0 to 0.8. The results also perform that the presence of oxygen carrier can promote the transformation of macromolecular components into smaller molecular species.  
Hence, operating conditions have significant influences on tar formation, including its yield and constituents. Increasing operating parameters, i.e., gasification temperature, steam-to-biomass ratio, and oxygen carrier-to-biomass ratio, can be beneficial for tar removal from biomass chemical looping gasification.