A Novel Mixed Metallic Ni Based Oxygen Carrier for Chemical Looping Combustion
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This study reports the development of a high surface Ni based oxygen carrier (OC) and the investigation of its reactivity with gaseous fuel for Chemical-Looping Combustion (CLC). High surface area facilitates high dispersion of metal particle and increases the oxygen carrying capacity. On the other hand , high surface area also enhances the formation of irreducible species like NiAl2O4 , which minimize the reactivity and oxygen transport capacity. Therefore , to achieve the high surface OC while preserving its reactivity , stability and fluidizability , a Ni based OC was developed in this study over Co promoted La modified γ-Al2O3 using incipient wetness impregnation technique. The γ-Al2O3 was first modified with addition of 5wt% La. The resulting La/γ-Al2O3 was then impregnated with 1wt% Co. Finally , a total of 20wt% Ni was loaded to the OC precursor via successive impregnations of 5wt% Ni. The resultant paste after each successive metal loading was reduced under three controlled gas flow rates containing 10% H2 with the balance being He. The surface area was found to vary considerably based on the gas flow rates used during the preparation stages. 50-120 m2/g surface area was found for OCs prepared under low reducing gas flow. These synthesized OCs display excellent reducibility and stability in successive TPR/TPOs and H2 chemisorptions. TPR profiles for the two OCs consistently show 95% reducibility at/below 750 °C. These results are encouraging given that they demonstrate the oxygen in the OC is very close to the expected amount. These results also show that the OC reducible phase is primarily NiO with small fractions of NiAl2O4. The controlled gas flow rates help the formation of easily reducible NiO species minimizing Ni-support interactions and formation of NiAl2O4. It was also found that the reduction behavior of these OCs resemble the chemical properties of δ- and θ-Al2O3 for low gas flow rates. In contrast , the structural integrity and characteristic chemical properties of Ni over γ-Al2O3 was observed in the case of high gas flow rates. Considerable amount of Ni was observed to be present as NiAl2O4 in this case. However , the absence of θ- and/or δ-phase of Al2O3 in the XRD experiments suggest that the phase transformation might occur at the interfaces between the Ni crystallites and alumina support surfaces , whereas the bulk structure of the support remained in the γ-phase. The SEM/EDX and H2 pulse chemisorption results confirmed that all the three prepared OCs were highly stable in consecutive redox cycles. A well dispersed state of Ni crystals and the absence of agglomerations were observed for both the fresh and used OCs. The reactivity of these prepared OCs was studied under the expected conditions (turbulent fluidized bed and high temperature) of an industrial scale CLC unit in CREC Riser Simulator. The CREC Riser Simulator is a bench scale mini-fluidized bed reactor capable of simulating circulating fluidized bed reactions conditions. The reactivity study was performed with stoichiometric amount of CH4. The CH4 conversion was found to be 81% (ABET = 50 m2/g) and 86% (ABET = 120 m2/g) for the OCs prepared under low gas flow rates. Maximum 88% CH4 conversion was observed for the OC with high surface area (ABET = 140 m2/g). The increased conversion of CH4 was resulted from the increased oxygen transport capacity with the surface area of the carrier particles. However , the selectivity of CO2 was found to decrease from 97% to 82 % with the increasing surface area. This was mainly due to the increased catalytic effect resulting from highly dispersed Ni crystals over high surface area. This systematic investigation of the reactivity of the Ni particles over wide range of crystal sizes reveals the fact that the low surface and low dispersed Ni based OC is favorable for CLC operation , whereas highly dispersed Ni based OC is suitable for CLR process. The oxygen transport capacity can be significantly increased with increasing surface area.