Gasification and Combustion of Biomass In Chemical Looping Combustion Over A Ni Based Oxygen Carrier
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This study reports the development of a Ni based oxygen carrier and the investigation of an Integrated Gasification (IG) and Combustion of solid biomass as a fuel for Chemical-Looping Combustion (CLC). The aim is to develop a highly reactive, stable and fluidizable oxygen carrier (OC): a cobalt (Co) promoted Ni/γ-Al2O3. Every OC impregnation step was accomplished using incipient wetness impregnation. The OC was modified first by stabilizing the γ-Al2O3 with a 5% La1 addition. The resulting La-γAl2O3 was then impregnated with 1wt% Co. Finally, a total of 20% Ni was loaded to the OC precursor via successive impregnations of 2.5wt% Ni. The synthesized OC displays excellent reducibility (94%) and stability, as demonstrated in successive TPR/TPOs and H2 chemisorptions. TPR profiles for this OC consistently show essentially complete reduction at 700 °C. These results are encouraging given that they demonstrate that 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 NiAl2O4 fractions. The addition of Co helps the formation of easily reducible NiO species minimizing Ni-support interactions and formation of NiAl2O42,3,4. It was also found that the small TPR peak assigned to NiAl2O4 remained unchanged over repeated TPR/TPO cycles, with this suggesting that NiAl2O4 always remains as a small fraction in the OC. Pulse chemisorption results were valuable to further confirm the stable behavior of the sample in consecutive redox cycles, showing a well dispersed state of Ni crystals as well as the absence of Ni crystal agglomeration.
The prepared OC was used for integrated gasification and combustion of biomass in the CREC Fluidized Riser Simulator5under the expected conditions (turbulent fluidized bed and high temperature) of an industrial scale CLC unit. Glucose was employed as the model compound for biomass. The product analysis of IG-CLC (Integrated Gasification-Chemical looping combustion) experiments shows that H2O, CO2, CO and H2 are the main products under the studied reaction conditions. A small amount of CH4 was also detected with no glucose found in the formed products. The combustion of the gasified biomass was further confirmed by comparing products composition with the ones of thermal gasification. Results showed 7 % CH4, 35 % CO and 44 % H2 and 15 % CO2 for thermal gasification and 7 % CH4, 8 % CO and 0.5 % H2 and 86 % CO2 for the IG-CLC with OC/glucose close to the stoichiometric amounts for complete combustion. It can be concluded from these findings that during the IG-CLC most of the chemical species produced during gasification are burnt in the subsequent combustion step involving both product gases and OC. As a result, the CO2 fraction in the IG-CLC product significantly increases. The only exception is CH4, whose composition remains at almost the same level, both in thermal and IG-CLC runs. Thus, it appears that the CH4 formed during the gasification step neither reacts with the oxygen carrier to give CO2 and H2O nor reforms with H2O to produce H2 and CO. In order to further investigate this matter, subsequent IG-CLC experiments were carried out using both an excess and a deficient stochiometric amount of OC under the same reaction conditions. It was shown, that the product analysis of these experiments displays significant variations of H2, CO and CO2 with the varying amounts of the OC., and with the CH4 remaining close to a constant value. Thus, product selectivity using a supported metal oxide is closely associated with the degree of the reduction of the oxygen carrier. At the beginning of the reaction (or short contact times), the fully oxidized oxygen carrier favors the total oxidation of the gasification products forming CO2 and H2O. As the reaction proceeds, the partially reduced oxygen carrier starts catalyzing the reaction products (CO+H2). Consequently, the methane in the oxygen deficient runs decreased due to the reforming reaction while CO and H2 remained unreacted. Regarding the stability of the oxygen carrier, repeated combustion-regeneration experiments show that NiO/Al2O3 particles display excellent reactivity and stability during the cyclic process.
References: (1) Hossein M., Quddus M., de Lasa H., “Reduction Kinetics of Lanthanum Modified Ni/γ-Al2O3 Oxygen Carrier for CLC” Ind.Eng.Chem.Res. Vol.49, N 21, 11009-11017 (2010); (2) Hossein M., de Lasa H., “Reduction Kinetics of Co-Ni/γ-Al2O3 involved in a chemical-looping combustion cycles” Chem.Eng.Sci.,65 98-106 (2010); (3) Hossein M., de Lasa H., “Reduction and oxidation kinetics of Co-Ni/Al2O3 oxygen carrier involved in a chemical looping combustion cycles”, Chem.Eng.Sci, Vol.65 98-106 (2009); (4) Hossein M., de Lasa H., “Chemical Looping Combustion (CLC) for Inherent CO2 Separations- A Review”, Chem.Eng.Sci., Vol.63, 4433-4451 (2008); (5) de Lasa H., “Riser Simulator for Catalytic Cracking Studies”, US Patent 5,102,628,(1992).