Biomass gasification using chemical looping in a 25 kWth reactor with natural hematite as oxygen carrier

Abstract

Biomass gasification using chemical looping (BGCL) presents a promising route to meet syngas production with lattice oxygen instead of molecular oxygen. And natural hematite is an attractive oxygen carrier material catalyzing tar cracking. A 25kWth prototype was made to investigate the performance of BGCL with hematite oxygen carrier in this work. The new prototype configuration is composed of a high velocity fluidized bed as an air reactor, a cyclone, a bubbling fluidized bed as a fuel reactor, and a loop-seal. The effects of gasification temperature, steam-to-biomass (S/B) ratio and hematite mass percentage were investigated. And the optimal operating parameters were determined. Because of the intensive endothermic reactions in the fuel reactor, the gasification temperature decreased sharply for bed materials of 100wt.% silica sand. Only when the hematite mass percentages reached 40wt.%, the gasification temperature was stable. The experimental results also showed that 860°C was the optimal gasification temperature corresponding to higher carbon conversion efficiency and maximal syngas yield. With respect to the effect of S/B ratio, carbon conversion efficiency reached the maximums at the S/B ratio of 1.0, while syngas yield increased firstly and then maintained almost unchanged. The optimal S/B ratio was 1.0 for higher syngas yield without sacrificing the maximal carbon conversion efficiency. When hematite mass percentage was varied in the range of 40–60wt.%, carbon conversion efficiency showed a monotonously increasing trend. However, a contrary trend was observed for gas yields, which reached the maximum when hematite mass percentage was set to 40wt.%. To get a higher syngas yield was the primary goal for BGCL. Therefore, the optimal hematite mass percentage seemed to be 40wt.%.