Model analysis and catalysts study of CO2 methanation in fluidized bed reactor

With the increasing greenhouse gas carbon dioxide (CO2) emission due to the consumption of fossil fuels, various methods have been investigated for the capture and recycle of CO2. In these processes, catalytic conversion of CO2 into chemicals and fuels is an alternative to alleviate climate change and ocean acidification. This thesis contains mainly three parts: Firstly, considering the catalytic reduction of CO2 by H2 can lead to the formation of various products: carbon, carbon monoxide, carboxylic acids, aldehydes, alcohols and hydrocarbons, a comprehensive thermodynamics analysis of CO2 hydrogenation is conducted using the Gibbs free energy minimization method. The results show that CO2 reduction to CO needs a high temperature and H2/CO2 ratio to achieve a high CO2 conversion. However, synthesis of methanol from CO2 needs a relatively high pressure and low temperature to minimize the reverse water-gas shift reaction. Direct CO2 hydrogenation to formic acid or formaldehyde is thermodynamically limited. On the contrary, production of CH4 from CO2 hydrogenation is the thermodynamically easiest reaction with nearly 100 % CH4 yield at moderate conditions. In addition, complex reactions with more than one product are also calculated in this project. The thermodynamic calculations are partially validated with some experimental results, suggesting that the Gibbs free energy minimization method is effective for thermodynamically understanding the reaction network involved in the CO2 hydrogenation process, which is helpful for the development of high-performance catalysts. Second, through above thermodynamics analysis, it is known that the reduction of carbon dioxide to methane by hydrogen (CO2 + 4H2 → CH4 + 2H2O, termed CO2 methanation) from renewable energy is a promising process for CO2 recycling. However, both the development of better catalysts and better reactors for the subsequent implementation are critical for the practical application of CO2 methanation. Towards large-scale implementation, (i) fluidized beds, which have excellent heat transfer, are promising for the highly exothermic reaction; and (ii) catalysts suitable for long-term use in fluidized beds are needed. This project focused on the former, specifically on the understanding of the operating parameters affecting CO2 methanation in the highly efficient fluidized bed reactor. A fluidized bed reactor model was developed based on an earlier one reported for CO methanation. The reaction kinetics of the Ni-Mg-W catalyst, which has been reported to exhibit superior catalytic performance, was experimentally measured. The fluidized bed model results indicated that the Ni-Mg-W was indeed superior to two other catalysts reported earlier in terms of faster depletion of reactants and higher concentrations of product CH4 throughout the reactor. Moreover, regarding the effect of operating parameters, the overall productivity of CH4 increases with decreased inlet reactant flow rate, increased temperature, increased H2/CO2 ratio, decreased catalyst particle diameter and decreased catalyst particle sphericity. The results presented in this part are expected to be valuable for both the further development of catalysts and of the reactors needed for practical CO2 methanation processes. Last part focuses on the catalyst study for carbon dioxide (CO2) methanation. In this project, a novel Ni-Co bimetal catalyst supported on TiO2-coated SiO2 spheres (NiCo/TiO2@SiO2) was rationally designed and evaluated for CO2 methanation in fluidized bed reactor. The results demonstrated that NiCo/TiO2@SiO2 exhibited high CO2 conversion with CH4 selectivity of greater than 95%. Moreover, the superior performance was sustained for more than 100 hours in the fluidized bed reactor, affirming the long-term stability of the catalyst. Comprehensive characterizations were conducted to understand the relationship between structure and performance. This study is expected to be valuable for the potential implementation of the CO2 methanation process in fluidized beds. In all, this thesis would be a useful guidance for the process development of CO2 utilization through hydrogenation process.Doctor of Philosoph