CO2 Methanation from micro to macro scale

The present dissertation undertakes a comprehensive investigation of the CO2 methanation process, addressing it holistically across scales, from micro to macro, describing the intimate connections between the disciplines of catalysis and chemical engineering. The thesis starts from the micro-scale, concerning the design, synthesis and functional assessment of heterogeneous catalysts to then deal with reactor modelling and design, representing a meso-scale, and a plant-scale investigation of the techno-regulatory viability of the methanation plant. The adoption of a multi-scale logic is grounded in the need to foster the adoption of multi-disciplinary and complementing approaches to respond to the challenges posed by the energy transition and the overall decarbonisation efforts. The fundamental insights into catalyst design and assessment, representative of the micro-scale level of the present dissertation, are based on a concise yet strategically significant experimental campaign that was conducted at the Institute for Advanced Energy Technologies "Nicola Giordano" of the Italian National Research Council (CNR-ITAE). The campaign was carried out under the expert supervision of Dr. Antonio Vita and Dr. Cristina Italiano, within the Hydrogen Production Technologies and Catalytic Materials (FPMAT) research group and was focused on the testing of a Ni/CeO2-ZrO2 catalysts for CO2 methanation. On the other hand, the methodological framework devised by Professors Marcello De Falco and Mauro Capocelli of the Process Engineering Research Unit at Università Campus Bio‑Medico di Roma is at the base of the entire pathway from the micro- to the macro- scale. This framework encompasses system modelling and simulation, economic and environmental assessment and its application throughout the entire dissertation demonstrates the inherent versatility across scales and application contexts, confirming its pivotal role in identifying optimal pathways towards a sustainable, low-carbon economy model [29]. The thesis is divided into four chapters, briefly described below: -) Chapter 1 provides a comprehensive overview of the historical development of the methanation process, identifying the main drivers and the strategic relevance in energy transition scenarios. The chapter positions methanation within the broader landscape of sustainable energy technologies, emphasising its role as an Energy Storage Technology. The fundamental thermodynamic principles of the methanation process are discussed to properly understand the main challenges faced in the development of the process. -) Chapter 2 focuses on the preparation, the characterization and the testing of a Ni/CeO2-ZrO2 catalyst prepared through Solution Combustion Synthesis (SCS), capable to operate under mild operating conditions for a prolonged period. CO2 methanation tests were performed in a FBR equipped with the 25 wt.% Ni/20 wt.% CeO2-55 wt.% ZrO2 catalyst in powder form (50–70 mesh, ~250 µm) diluted with inert quartz particles of similar size (1:9 ratio of approximately 1:1 by weight) to prevent hot spots formation and operate in isothermal conditions. The activity tests were performed in the temperature range between 175 °C and 400 °C and at pressures ranging from 1 to 10 bar. The activities herein discussed served as a precursor to a broader investigation aimed at determining the intrinsic kinetics of the reaction. The chapter also proposes an extension of the catalyst assessment to encompass its environmental footprint during manufacture and operation through a pre-commercial Life Cycle Assessment (LCA). -) Chapter 3 addresses the challenge of managing the significant exothermicity of the methanation reaction through the conceptualisation, modelling and simulation of a Multi Tubular Heat Exchange Reactor (MT-HER), designed to maintain control over temperature gradients and avoid localised hot spots. The MT-HER has been designed as a potential candidate technology for large-scale SNG production, with an estimated output of approximately 1,000 Nm3 h-1. Therefore, the developed model has been therefore integrated into a plant-scale simulation aimed at assessing the feasibility of deploying a streamlined and compact methanation plant to produce pipeline-grade SNG. The discussion encompasses state-of-the-art reactor modelling and plant designs, beside the role of process intensification strategies to advance the technical maturity of certain solutions. -) Chapter 4 synthesises the findings of the preceding chapters, thereby articulating the contributions of this research to advancing CO2 methanation technologies across scales. Furthermore, the chapter provides a more extensive reflection on the implications of the methanation process and the replacement of fossil NG with SNG, outlining potential roads for future research and development. The Annex constitutes a compendium of detailed materials that complement and enrich the main body of the dissertation, providing both broader context and deeper technical understanding. The overall methodological framework employed in the research is further elaborated, including a section devoted to the LCA methodology. Beyond this methodological contribution, the section includes additional notes and observations on SNG grid compliance requirements.