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

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