Reactivity of a NiO/Al_2O_3 oxygen carrier prepared by impregnation for chemical-looping combustion

摘要

The reactivity of a Ni-based oxygen carrier prepared by hot incipient wetness impregnation (H1WI) on α-A1_2O_3 with a NiO content of 18 wt% was studied in this work. Pulse experiments with the reduction period divided into 4-s pulses were performed in a fluidized bed reactor at 1223 K using CH_4 as fuel. The number of pulses was between 2 and 12. Information about the gaseous product distribution and secondary reactions during the reduction was obtained. In addition to the direct reaction of the combustible gas with the oxygen carrier, CH_4 steam reforming also had a significant role in the process, forming H_2 and CO. This reaction was catalyzed by metallic Ni in the oxygen carrier and H_2 and CO acted as intermediate products of the combustion. No evidence of carbon deposition was found in any case. Redox cycles were also carried out in a thermogravimetric analyzer (TGA) with H_2 as fuel. Both tests showed that there was a relation between the solid conversion reached during the reduction and the relative amount of NiO and NiAl_2O_4 in the oxygen carrier. When solid conversion increased, the NiO content also increased, and consequently NiAl_2O-4 decreased. Approximately 20% of the reduced nickel was oxidized to NiAl-2O-4, regardless ΔX_S. NiAl_2O_4 was also an active compound for the combustion reaction, but with lower reactivity than NiO. Further, the consequences of these results with respect to the design of a CLC system were investigated. When formation of NiAl_2O_4 occurred, the average reactivity in the fuel reactor decreased. Therefore, the presence of both NiO and NiAl-2O_4 phases must be considered for the design of a CLC facility.