Catalyst and reactor design for carbon dioxide methanation
[eng] The transformation of the current energy model towards a more sustainable mix, independent of fossil fuels, requires the exploration of new technologies that are capable of taking advantage of excess electricity derived from renewable energy sources and to use new alternative sources of carbon...
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| Tipo de recurso: | tesis doctoral |
| Estado: | Versión publicada |
| Fecha de publicación: | 2021 |
| País: | España |
| Institución: | Universidad de Barcelona |
| Repositorio: | Dipòsit Digital de la UB |
| OAI Identifier: | oai:diposit.ub.edu:2445/177835 |
| Acceso en línea: | https://hdl.handle.net/2445/177835 http://hdl.handle.net/10803/671781 |
| Access Level: | acceso abierto |
| Palabra clave: | Catàlisi Cinemàtica Reactors químics Diòxid de carboni Metà Catalysis Kinematics Chemical reactors Carbon dioxide Methane |
| Sumario: | [eng] The transformation of the current energy model towards a more sustainable mix, independent of fossil fuels, requires the exploration of new technologies that are capable of taking advantage of excess electricity derived from renewable energy sources and to use new alternative sources of carbon for the generation of clean fuels. An alternative that combines both is the Power-to-Gas (P2G) technology, whose concept is based on a two-stage process. In the first stage, excess electricity from renewable energies is converted to hydrogen by electrolysis. Then, in a second stage, the H2 produced is transformed to CH4 through methanation with CO2. The CH4 produced is referred to as synthetic natural gas (SNG) and allows large amounts of renewable energy to be distributed from the energy sector to the end-use sectors. The thermo-chemical CO2 methanation process is considered the most efficient route for large-scale SNG production. However, developing a cost-effective CO2 methanation technology is one of the biggest challenges facing the P2G concept. In this context, this thesis focused on the catalyst and reactor design for CO2 methanation. The thesis objectives were addressed in three main aspects, which are: i) design a high-performance catalyst based on metal oxide-promoted Ni/γ-Al2O3 and determine its reaction mechanism; ii) evaluate the stability of the catalyst and the tolerance to sulfur for its implementation in a relevant industrial environment (CoSin project); and finally, iii) develop a CFD model based on experimental kinetic data to understand the role of operating conditions and propose a new reactor configuration. In the first Chapter of this thesis it is presented a general introduction of the SNG production through CO2 methanation process. In the second Chapter, the addition of a promoter (X) on a system composed by Ni and γ-Al2O3 microspheres was studied as the design strategy to develop a micro-sized Ni-X/γ-Al2O3 catalyst. The catalysts based on Ni-CeO2/γ-Al2O3 was proposed as the most feasible due to its high catalytic performance in relation to its economic competitiveness. The optimal composition of each component of the Ni-CeO2/γ-Al2O3 was found through a systematic experimental design. The catalyst composed by 25wt.%Ni, 20wt.%CeO2 and 55wt.%γ-Al2O3 proved to be the most active and stable thanks to its enhanced Ni dispersion and reduction, its high metallic area, and the formation of moderate base sites. In Chapter three, the thermal stability and tolerance to sulfur impurities on the Ni-CeO2/γ-Al2O3 catalyst was further studied using high temperatures and the presence of H2S on the reactants. The strong metal-promoter interaction and the favourable formation of Ce2O2S were revealed as the main causes of its high stability and tolerance to H2S, respectively. Additionally, the implementation of Ni-CeO2/γ-Al2O3 in a two-stage industrial methanation process was performed to evaluate its technical feasibility. The desired gas composition (≥92.5%CH4) was successful obtained using a decreasing temperature profile (T=450-275°C) and P=5bar·g. The high stability recorded during the 2000h of experimentation demonstrated that Ni-CeO2/γ-Al2O3 can be a competitive catalyst for CO2 methanation. Regarding to reactor design, in Chapter four, the design of a fixed-bed multitubular reactor on a Ni-CeO2-Al2O3 catalyst was evaluated for mid-scale SNG production. A CFD mathematical model based on experimental kinetic data was developed. A reactor tube with a diameter of 9.25mm and a length of 250mm was proposed, which should be operated at Tinlet=473K, Twall=373K, GHSV=14,400h-1 and P=5atm to achieve XCO2=99% with Tmax of 673K. On the other hand, a reactor tube (di=4.6mm and L=250mm) with a heat management approach based on free convection was proposed for small-scale SNG production. The optimal conditions were found at GHSV=11,520h-1, Tinlet=503K, P=5atm, and Tair=298K. The feasibility of the simulated reactor proposal was experimentally validated over the micro-sized Ni-CeO2/γ-Al2O3 (XCO2=93% and T=830-495K). |
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