Fuel, Vol.233, 133-145, 2018
Determination of redox pathways of supported bimetallic oxygen carriers in a methane fuelled chemical looping combustion system
The incipient wetness impregnation technique was used to synthesize supported bimetallic precursors (Ni and Co, Cu or Fe) by Al2O3, CeO2, TiO2 and ZrO2 in order to understand both oxidation and reduction reactions (redox) pathways during the methane chemical looping combustion (CH4-CLC). Understanding the reaction pathways helps to enable proper modifications of oxygen carriers, which later assists in choosing chemically stable precursors that could enhance the overall redox reaction rates. A higher solid conversion rate and fewer interactions with supports will produce highly reactive and thermally stable oxygen carriers. The BET surface area results showed highest increase in Ni-Fe/ZrO2 sample by 17.73% and highest decrease in Ni-Fe/CeO2 sample by 78.80%. The internal mass transfer results revealed that the reaction and internal diffusion for Ni-Fe/TiO2 and Ni-Fe/ZrO2 samples were similar; however, the effectiveness factor of the Ni-Cu/TiO2 sample showed isothermal surface reaction was the controlling step. Under the operating condition presented in this study (oxidation with air at 20 ml/min, and reduction with CH4 balanced with N-2 at 20 ml/min), most stable samples for CLC practical deployment were Ni-Co/ZrO2, Ni-Cu/ZrO2, and Ni-Fe/ZrO2 due to their exhibition of stable oxygen transport capabilities, and nonexistence of interaction between precursors and supports.