Experimental investigation of hematite oxygen carrier decorated with NiO for chemical looping combustion of coal

Experiments on chemical looping combustion were conducted in a 1 kW_th interconnected fluidized bed reactor using a natural hematite as oxygen carrier and Shenhua bituminous coal and Huaibei anthracite as fuel. An evaluation of hematite oxygen carrier decorated with NiO by mechanical mixing and impregnation methods was also performed. The results indicate that coal gasification rate is a time limiting step in chemical looping combustion of coal and the coal type has a great impact on CO₂ capture efficiency. When fuel reactor temperature is 970 ℃ and hematite as oxygen carrier, CO₂ capture efficiency for Shenhua bituminous coal is 81.7%, whereas the one for Huaibei anthracite is 65%. Hematite shows stable reactivity in a process of long-term operation and it should be a good candidate of oxygen carrier for chemical looping combustion of coal. It is an effective way to improve the reactivity of hematite oxygen carrier as well as CO₂ capture efficiency by mechanical mixing with NiO/Al₂O₃ oxygen carrier. Impregnating NiO on hematite shows a negative effect of reaction performance between product of coal gasification and oxygen carrier due to the poor microstructure after calcination. Further investigations on method and process of impregnation should be done.