# Biomass-Based Chemical Looping Gasification: Overview and Recent Developments
## 4 Effect of Process Parameters on Biomass Chemical Looping Gasification
### Oxygen Carrier-to-Biomass Ratio (OBR)  
In the CLG process, metal/metal oxide provides lattice oxygen as a gasifying agent for solid fuel gasification. Oxygen carrier-to-biomass ratio (OBR) is defined as the mass ratio of the amount of oxygen carrier per biomass fed into the gasifier being a key factor in a BCLG process. OBR strongly influences the composition of the product gas, gas yields, and the performance of the BCLG process. Many studies have been carried out to evaluate the effect of oxygen carrier (in both type and quantity used) on the performance of BCLG, especially syngas production. It is noted that an increase in the amount of oxygen carrier used leads to further char gasification; therefore, high OBRs are beneficial for carbon conversion efficiency. However, it was found that a rise in OBR cause low content and yield of H\({}_{2}\) and CO in the product gas. It is due to the higher quantity of oxygen carriers that would promote the oxidation reactions of combustible gases including H\({}_{2}\) and CO. On the other hand, a high amount of oxygen carrier could result in a higher gasification temperature, which can enhance the gasification and improve the quality of the product gas to some extent.  
[PERSON] et al. [117] reported that a high amount of oxygen carrier could keep gasification temperature stable under the experimental conditions. Additionally, carbon conversion efficiency increased corresponding to the increase in the hematite mass percentage from 40 to 60 wt.%. However, H\({}_{2}\) and CO concentrations declined significantly by around 9% and 5%, respectively, resulting in the reduction in the syngas yield from 0.74 to 0.52 Nm\({}^{3}\)/kg. An experimental study on BCLG using BaFe\({}_{2}\)O\({}_{4}\)/Al\({}_{2}\)O\({}_{3}\) as an oxygen carrier [42] figured out that CO reduced with the increase of OBR for 30 AF, whereas it increased for 30 ABF at OBR of 0.6, then dropped down at higher OBRs. [PERSON] et al. [67] used iron ore as an oxygen carrier to oxidize biomass char in a fixed bed reactor. They evaluated the effect of iron ore excess number, \(\Omega\), which refers to the ratio of oxygen provided by oxygen carrier to the oxygen required for the complete oxidation of the fuel, on the performance of char gasification. Carbon conversion efficiency increased by approximately 23%, but H\({}_{2}\) and CO contents reduced by 7% and 10%, respectively, when the OBR increased from 0.46 to 1.17. [PERSON] et al. [25] showed an increase in carbon conversion efficiency while large amounts of H\({}_{2}\) and CO could be consumed with the rise in NiO content resulting in a significant decrease in syngas yield. An interesting result was found in a study on chemical looping co-gasification of biomass and polyethylene [43]. It was noted that there was an opposite variation of the contents of H\({}_{2}\) and CO with the increment of OBR. The concentration of H\({}_{2}\) increased from 34.78 vol.% to 38.15 vol.%, whereas CO and CH\({}_{4}\) contents declined with increasing OBR from 0 to 1.0. Additionally, more syngas yield also was generated with the elevated OBR. It can be considered that syngas production could be promoted by the presence of the oxygen carrier.