Performance of Ca-Based Oxygen Carriers Decorated by K_2CO_3 or Fe_2O_3 for Coal Chemical Looping Combustion

摘要

Chemical looping combustion (CLC) of coal is an attractive technology with inherent CO_2 separation and high energy utilization efficiency. The large-scale preparation of a cheap oxygen carrier with high attrition resistance challenges the scale-up step of CLC reactor systems. To improve the reactivity between the CaSO_4/bentonite (CaBen) oxygen carrier and coal, CaSO_4-Fe_2O_3/bentonite (CaFeBen) and CaSO_4-K_2CO_3/bentonite (CaKBen), decorated by Fe_2O_3 and K_2CO_3, respectively, were prepared in this work. The active component content, multi-cycle reactivity, and enhancement mechanism of two decorated oxygen carriers were investigated in a fluidized bed with steam as the gasification-fluidization medium. Finally, three types of coals, including lignite, bitumite, and anthracite, were used as fuel. The addition of Fe_2O_3 and K_2CO_3 can improve the reactivity of the CaBen oxygen carrier but degrade the attrition resistance slightly. The multiple-cycle experiments indicated that Fe_2O_3 itself is the oxygen carrier for coal CLC with high reactivity, while K_2CO_3 acts as the catalysis for coal gasification. The carbon conversion rate of the three coals that reacted with the CaKBen oxygen carrier was higher than that with CaFeBen as the oxygen carrier because of catalysis of potassium on the coal gasification reaction. However, the CaKBen oxygen carrier particles were seriously sintered, and the potassium content in the oxygen carrier reduced with the increasing redox cycles. The coals with high volatile and ash contents have a high instantaneous rate of carbon conversion reacted with two decorated oxygen carriers.