Gasification and Combustion of Biomass In Chemical Looping Combustion Over A Ni Based Oxygen Carrier

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 NiAl₂O₄^2,3,4. 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).