Turquoise hydrogen generation through catalytic decomposition of methane by modeling approach

• Main Author: Tong, Sirui
• Other Authors: Chan Siew Hwa
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2024
• Subjects: Engineering; Chemical engineering; Catalytic decomposition of methane
• Online Access: https://hdl.handle.net/10356/175766
• Institution: Nanyang Technological University

Hydrogen stands as a pivotal clean energy vector, offering a versatile and environmentally sustainable alternative to conventional fossil fuels. Its potential to enable deep decarbonization across diverse sectors like transportation, industry, and power generation is increasingly recognized. Traditional methods of hydrogen production, however, pose inherent challenges, chiefly the unavoidable emissions of CO2. Enterprising solutions, particularly within the realm of "turquoise hydrogen," spotlight the catalytic decomposition of methane (CDM) as a transformative technology. This process directly converts methane into hydrogen and solid carbon (CH4 → C + 2H2), distinctly curbing emissions and positioning CDM as a critical conduit toward the zero-emission "green hydrogen" phase. While extant experimental studies have offered insights into catalyst development, reaction kinetics, and mechanisms, a comprehensive exploration of varied reaction conditions' impact on performance remains uncharted territory. To bridge this gap, numerical simulations emerge as a potent tool capable of amassing intricate reaction-related data, proposing optimization strategies, and, in turn, guiding subsequent experimental endeavors. This thesis primarily focuses on developing multiphase flow reaction models to describe the CDM process in various reactors. Beginning with the relatively simple and mature fixed bed reactor, a three-dimensional porous media model is constructed on a computational fluid dynamics platform to simulate the CDM reaction. Considering the catalyst deactivation and diffusion effects within the bed layer, the model’s accuracy is validated under different reaction conditions. The variations in catalyst lifespan are discussed based on the maximum carbon yield before deactivation. Furthermore, parametric studies examine the effects of inlet gas composition and operating pressure on the CDM performance. These parameters are crucial for the future industrialization of CDM technology and can provide some guidance for improving reaction efficiency. With benefits from the good heat/mass transfer characteristics and the potential for catalyst regeneration cycling, applying the CDM reaction in fluidized bed reactors is gaining popularity. To simulate this process precisely, a novel two-dimensional model based on a multiphase Euler-Euler framework is proposed. As a comprehensive reactor simulation, it not only effectively captures the details of the bed layer motion but also incorporates an Arrhenius-based deactivation kinetics model to describe catalyst deactivation due to carbon depositions. The model validation confirms the minimum fluidization velocity (Umf) of the specific catalyst bed layer and demonstrates its predictive capabilities for CDM reactions under different temperatures and methane inlet concentrations. By analyzing methane conversion and carbon formation rates, the impact of gas flow rate on the reaction performance is further investigated, along with potential optimization strategies. These findings highlight the improvement in CDM operating conditions, providing valuable insights for subsequent experimental design and industrial-scale development. Furthermore, rotary bed reactors that allow for continuous catalyst input/output and achieve better gas-solid contact through rotation are also considered suitable for the CDM process. To address the complex operational modes, a small experimental apparatus is constructed without reactions, and the results are compared with the simulations under the Euler-Euler multiphase flow model. Apart from obtaining good fittings of velocity distributions in the bed layer at varied rotating speeds, the effects of different loading ratios on bed layer motion are also discussed. These results effectively predict the flow pattern in rotary bed reactors under various conditions and lay the foundation for further exploration of applying CDM reactions in them.