Effect of Mg in rice husk on promoting reaction and causing agglomeration during chemical looping gasification

Abstract

Mg is one of the most abundant trace elements in biomass, which plays a contradictory role in biomass conversion. However, the evolution behavior and mechanism of Mg in biomass chemical looping gasification (CLG) are still unclear. In this paper, the comprehensive influence of Mg on the CLG performance is investigated under different Mg-type solutions, Mg contents, temperatures and ratios of steam/biomass (S/B). The results show that Mg added on pickling rice husk (AH) can promote CLG reaction, but cause more agglomeration by forming MgSiO3 around particles at high Mg content. Moreover, the CLG performance of (CH3COO)2Mg-AH is better than that of MgCl2-AH. Compared with pickling rice husk, the gasification efficiency, gas yield and carbon conversion efficiency of 6% (CH3COO)2Mg-AH are increased by 28.15%, 33.77% and 26.20%, respectively. Meanwhile, the smaller particle size, grain size and agglomeration degree are achieved in the CLG on (CH3COO)2Mg-AH. Furthermore, Mg-addition will reduce the activation energy in CLG process. Compared with rice husk, the activation energy of 6% (CH3COO)2Mg-AH decreases by 21.88% in pyrolysis stage and 61.90% in gasification stage. In addition, the activation energy of (CH3COO)2Mg-AH is lower than that of MgCl2-AH, especially the gasification stage (20.89%). With the increase of temperature and S/B, the CLG performance increases first and then decreases. The highest gasification efficiency of 70.26% and gas yield of 0.72 Nm3/kg are achieved under the optimal temperature of 850℃ and S/B of 3 during the CLG on 6% (CH3COO)2Mg-AH.