Effect of alkaline earth metal Ca in rice husk during chemical looping gasification process

Abstract As the most abundant trace element in agricultural wastes, Ca can improve the reaction performance, but aggravate the agglomeration and sintering of reaction materials at high temperature during the process of thermochemical conversion. In this paper, the effect of Ca in rice husk was studied in chemical looping gasification (CLG) under different conditions of Ca concentration, Ca solution, temperature and steam/carbon (S/C). The results indicated that Ca would significantly improve the CLG performance, but cause obvious agglomeration and sintering phenomenon at high Ca concentration. Furthermore, the improvement for CLG performance of (CH3COO)2Ca was greater than that of CaCl2. After adding 6% (CH3COO)2Ca, gas yield, carbon conversion efficiency and gasification efficiency increased by 53.30%, 47.06% and 45.61%, respectively. By contrast, they respectively increased by 28.11%, 17.27% and 26.79% after adding CaCl2. Thermogravimetric analysis showed that the presence of Ca was beneficial to reduce the activation energy of the reaction. In the pyrolysis stage, the maximum reduction of activation energy was 37.43%, while in the gasification stage, the maximum reduction of activation energy was 65.36% after adding Ca in the rice husk. Compared with CaCl2, (CH3COO)2Ca decreased more activation energy of CLG process. Moreover, with the increase of temperature and S/C, the CLG performance increased firstly and then decreased after adding 6% (CH3COO)2Ca by impregnation on the pickling rice husk. 800℃ and 6.9 was the optimal temperature and S/C to achieve the best gasification efficiency of 65.03% and gas yield of 0.69 Nm3/kg in the CLG process.