Catalyst-enhanced autothermal chemical looping reforming of methane : experimental kinetics and reactor modeling
(2022)
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- Authors
- Supervisors
- De Wilde, Juray
- Abstract
- Hydrogen and syngas production from natural gas is the fundamental process for ammonia production, methanol synthesis, and the petrochemical industry. However, the most widely deployed technology for hydrogen production technology, i.e., steam methane reforming (SMR), has several drawbacks including low energy efficiency and CO2 emission from its furnace that requires CO2 capture under the carbon-constrained scenario. Autothermal chemical looping reforming (aCLR) is proposed to address these two problems. The aCLR has been successfully tested from bench-scale semi-continuously operated fluidized beds to 140kWth continuously operated dual fluidized bed facilities. However, environmental concerns arising from the use of Ni-based oxygen carriers (OC). On the other hand, other investigated OCs, e.g., Cu- and Fe-based materials have lower reactivity towards methane reforming, which leads to low CH4 conversion. Modeling and simulation accounting for detailed reaction mechanisms and reactor hydrodynamics plays important role in designing, understanding, upscaling, and eventually optimizing the aCLR. Limited by the computational power, scale-bridging strategies are required to efficiently evaluate the reactor performance while understanding the relative importance of different phenomena, e.g., intrinsic reaction kinetics, gas-solid mass heat, mass transfer, and the residence time of solids in the bubbling fluidized bed (BFB). This thesis is aimed at investigating the feasibility of a more environmentally compatible aCLR. This process considers using the environmentally friendly Fe-based OC for transferring oxygen between the two reactors and PGM-based SMR catalyst for enhanced methane conversion, which is called catalyst-enhanced autothermal chemical looping reforming (CE-aCLR). CE-aCLR is studied by experimentally investigating the reaction kinetics of the potential Fe-based oxygen carrier and the PGM-based SMR catalysts, and performing modeling and simulation of the aCLR unit accounting for physical-chemical phenomena at different scales by applying scale-bridging strategies. Firstly, a 1-D aCLR model consisted of a riser air reactor (AR) and BFB fuel reactor (FR) was developed. This model accounts for the essentials of the hydrodynamics of the riser and BFB, and couples the detailed reaction kinetics. The model for the BFB is an improved version of the classic two-phase model that allows for simulating the methane reforming reactions. The aCLR model was validated against the experimental results of the 140kWth pilot scale test from the literature. Then, based on the simulation results of the 1-D model, a commercial scale aCLR unit using Ni-based OC was designed and the performance was used as the reference for CE-aCLR. In addition, the coke formation risk was evaluated. Besides, the influence of main operation parameters was investigated. The intrinsic kinetics of SMR reactions on two commercial PGM-based catalysts were investigated in a micro-packed bed reactor with carefully designed reactor dimensions and operation conditions that guarantee plug flow, negligible interfacial and intraparticle mass transfer, and isothermal operation. The two PGM-based SMR catalysts are seen to be ~100 times more active than the traditional SMR catalysts. Yet, the intrinsic catalytic reaction kinetics were successfully estimated. A novel Fe-based, MgAl2O4-supported OC suitable for chemical looping reforming was synthesized considering the temperature limits in the AR after adiabatic oxidation by air. The oxidation reaction was studied in a micro-fixed bed reactor using a transit step response approach. The oxidation was found extremely fast that only the minimum oxidation rate could be estimated. Nevertheless, the estimated minimum oxidation rate is the fastest one in the literature and is at least the same magnitude of mass transfer in the fixed bed reactor. For the modeling of CE-aCLR, the reduction kinetics of iron oxides, i.e., Fe2O3, Fe3O4, and FeO under H2, CO, and CH4 were reviewed and some reliable reduction kinetics from the literature were adopted to form a complete reduction kinetics model. Finally, for the simulation of the CE-aCLR, two major modifications were made to the developed 1-D aCLR model in Chapter 2, which to account for the mass transfer between catalyst particles and the residence time distribution of the OC particles in the FR. The simulation results show that the CE-aCLR is feasible that can achieve ~96% of methane conversion with reasonable reactor dimensions while using 5%wt. of catalysts. The sensitivities of the reactor performance on the reactor dimension, catalyst activity, and reactivity of the OC were also studied.
- Affiliations
Citations
He, Z. (2022). Catalyst-enhanced autothermal chemical looping reforming of methane : experimental kinetics and reactor modeling. https://hdl.handle.net/2078.5/101168