Engineering reaction pathways in molten bimetallic alloys to achieve near-unity CO₂ conversion and tunable product selectivity

국문 초록 (Abstract)

본 논문은 용융 주석 기반 이원합금(Sn–Zn, Sn–Mg)을 이용한 금속 연소 기반고온 CO₂전환 공정을 구축하고, 기포 컬럼 반응기에서의 반응 특성, 유동 거동, 재생 및 에너지·배출 특성을 통합적으로 평가하였다. 목표는 용융 금속을 활용하여 높은 CO₂ 전환율과 조절 가능한 생성물 선택성을 달성하고, 금속 재생을 포함한 폐쇄형 순환 CCU 공정으로의 확장 가능성을 검증하는 데 있다. 실험 결과, Sn–Zn 시스템은 CO₂ + Zn → CO + ZnO 단일 단계 부분 환원 경로를 따라 650 °C 이상에서 거의 완전한 CO₂전환과 높은 CO 선택도를 보였으며, CO가 추가적으로 고체 탄소로 환원되지 않아 선택적 CO 생성 매질로 기능함을 확인하였다. Sn–Mg 시스템은 CO₂ + Mg → CO + MgO 및 CO + Mg → C + MgO의 연속 이종 반응을 통해 CO와 고체 탄소를 동시에 생성하였고, 온도·체류시간에 따라 두 생성물의 분배가 뚜렷이 달라졌다. 가스 유량, 용융층 높이, 가스 주입 방식(단일 노즐·다공 스파저)을 변화시키는 유동 특성 실험을 통해, 체류시간과 계면적 증가는 두 시스템 모두에서 CO₂ 전환율을 향상시키지만, Sn–Mg에서만 CO/탄소 선택성이 강하게 변화함을 보였다. 다공 스파저는 전환율을 높였으나 MgO와 C 축적으로 인한 막힘을 유발하여, 계면적 극대화와 장기 운전 사이의 공정 상충 관계를 제시하였다. 장기 운전 및 스케일업 시험에서 Sn–Zn 14 wt% 시스템은 550 °C, 165 h 연속 운전 동안 높은 CO₂ 전환과 CO 선택도를 유지하다가 ZnO 축적이 약 30 wt%에 도달한 이후 성능 저하를 보여, 산화물 축적에 따른 실질 용량 한계를 제시하였다. Sn–Mg 14 wt% 벤치 규모 실험에서는 고유량·고농도 CO₂조건에서도 높은 전환율이 유지되었고, 발열 반응에 따른 자가열 거동이 관찰되어 외부 열 공급 저감 가능성을 시사하였다. 공정의 순환 운전을 위해 Sn–Zn에는 메탄 기반 화학 루핑을, Sn–Mg에는 고온 수소 환원을 적용하여 각각 ZnO/SnO₂와 MgO/SnO₂를 금속으로 재활성화할 수 있음을 보였으며, 에너지·배출 분석을 통해 저탄소 전력·그린 H₂와의 결합이 순 CO₂감축에 중요함을 확인하였다. 마지막으로, 1차원 반응기 모델을 구축하여 Sn–Zn의 단일 단계 CO₂→ CO, Sn–Mg의 CO₂→ CO → C 및 MgCO₃분기 경로를 유동·계면 특성과 함께 해석함으로써, 실험에서 관찰된 온도·유량·높이 의존성을 재현하고 향후 공정 설계 및 재생 최적화를 위한 기반을 마련하였다. 따라서 본 연구는 Sn–Zn 및 Sn–Mg 용융 합금을 이용한 버블 컬럼 반응기는 CO₂를 고효율로 전환하면서 CO 또는 고체 탄소를 선택적으로 생산할 수 있는 액체금속 기반 CCU 플랫폼으로서, 적절한 재생 전략과 저탄소 에너지원과의 결합을 통해 지속가능한 CO₂활용 기술로 발전할 수 있음을 제시한다.

다국어 초록 (Multilingual Abstract)

This thesis examines the thermochemical conversion of CO2 in molten metal bubble column reactors utilizing Sn–Zn and Sn–Mg bimetallic alloys, with the objective of achieving high CO2 conversion and adjustable selectivity between carbon monoxide (CO) and solid carbon. In the Sn–Zn system, CO₂ is primarily converted through a single-step partial reduction (CO2 + Zn → CO + ZnO), resulting in nearly complete conversion and high CO selectivity with minimal carbon deposition. Conversely, the Sn–Mg system follows a consecutive reaction scheme (CO2 + Mg → CO + MgO; CO + Mg → C + MgO), where the distribution between CO and solid carbon is significantly influenced by temperature and gas residence time. Systematic variations in gas flow rate, melt height, and gas injection configuration revealed that residence time and interfacial area enhanced CO2 conversion in both systems, while only the Mg-containing alloy exhibited notable selectivity shifts between CO and solid carbon. Microscopic and spectroscopic analyses confirmed the presence of ZnO-dominated powders in the Sn–Zn system and MgO–carbon composites containing turbostratic carbon in the Sn–Mg system, aligning with the proposed reaction pathways. Extended operation of Sn–Zn and bench-scale tests with Sn–Mg demonstrated stable performance, practical oxide loading limits, and auto-thermal behavior due to reaction exothermicity. To facilitate closed-loop operation, methane-based chemical looping for Sn–Zn and high-temperature hydrogen reduction for Sn–Mg were assessed as regeneration routes, and an energy–emissions assessment underscored the importance of integrating these cycles with low-carbon energy sources. These regeneration strategies, combined with the intrinsic buoyancy-driven separation of reaction products, position molten metal systems as viable candidates for sustainable CO2 capture and utilization. A one-dimensional reactor model incorporating gas–liquid hydrodynamics and distinct kinetic schemes for the two alloys replicates the observed trends and provides a foundation for designing molten-metal CO2 utilization processes with controllable product streams. Collectively, these findings demonstrate that molten bimetallic alloys can serve as adaptable and sustainable platforms for engineering CO2-to-carbon pathways in future low-carbon energy and circular carbon systems.

목차 (Table of Contents)

• Chapter 1. Introduction 1
• Chapter 2. Theoretical Background 8
• 2.1 Thermodynamics and Kinetics of CO2 Conversion 8
• 2.1.1 Reverse Water-Gas Shift (RWGS) Reaction 9
• 2.1.2 CO2 Methanation 10
• 2.1.3 CO2 Hydrogenation to Methanol 12
• 2.1.4 Dry Reforming of Methane (DRM) 13
• 2.2 Fundamentals and Applications of Metal Combustion in CO2 Atmospheres 16
• 2.2.1 Thermodynamics and Mechanisms of Metal-CO2 Combustion 16
• 2.2.2 Technologies Utilizing Metal-CO2 Reaction Systems 17
• 2.3 Molten Metal Bubble Column Reactor 19
• 2.3.1 Physicochemical Properties of Molten Metals 19
• 2.3.2 Hydrodynamics and Mass Transfer in Bubble Columns 22
• 2.4 Thermodynamics of Molten Bimetallic Alloys 26
• 2.4.1 Theory of Mixing and Activity Modulation 27
• 2.4.2 Tunability of Oxide Stability and Regeneration Efficiency 28
• 2.5 Interfacial Oxides and Carbon Products 31
• 2.5.1 SnSnO: Dynamic Semi-Permeable Interfaces 32
• 2.5.2 MgMgO : Strongly Passivating Limit 36
• 2.5.3 ZnZnO : Volatility-Driven Anisotropic Oxides and Semi-Passivating Interfaces 40
• 2.6 Carbon Products: Amorphous and Turbostratic Carbon 43
• 2.6.1 Kinetic Trapping and Amorphous Carbon Formation 44
• 2.6.2 Liquid-Mediated Growth and Turbostratic Carbon 45
• 2.6.3 Spectroscopic Distinctions and Engineering Implications 46
• 2.7 Theoretical Basis of CO 47
• 2.7.1 Thermodynamic Selectivity and the Boudouard Equilibrium 47
• 2.7.2 Dynamic Desorption and Purity Advantages 48
• 2.7.3 Industrial Integration and Syngas Applications 49
• Chapter 3. Experimental 50
• 3.1 Materials 50
• 3.2 Experimental Configuration 50
• 3.3 Experimental Procedure 52
• 3.3.1 High-Throughput Screening Using High-Frequency Induction Heating 52
• 3.3.2 CO2 Conversion in Molten Metal Bubble Column 54
• Chapter 4. Results and Discussion 59
• 4.1 High-Throughput Screening of Binary Alloy Candidates Materials 59
• 4.1.1 Preliminary Investigation of Binary Alloy Candidates 59
• 4.1.2 Screening Results and Candidate Selection 61
• 4.2 Lab-Scale CO2 Conversion in a Molten Metal Bubble Column Reactor 65
• 4.2.1 CO2 Conversion Behavior of Pure Molten Tin 66
• 4.2.2 Comparative CO₂ Conversion Behavior in Molten SnZn and SnMg Alloys 69
• 4.2.3 Characterization of Solid Products 75
• 4.2.4 Microstructural Analysis and Crystal Growth Mechanisms 78
• 4.3 Process Optimization and Operational Stability 80
• 4.3.1 Impact of Hydrodynamic Parameters 82
• 4.3.2 Long-Term Durability and Operational Limits 85
• 4.4 Scale-Up Effects on Reaction and Hydrodynamics 87
• 4.4.1 Hydrodynamic Control of Product Selectivity 87
• 4.4.2 Buoyancy-Driven Product Separation and Coking Resistance 89
• 4.4.3 Thermodynamic Characteristics and Auto-Thermal Behavior 91
• 4.5 Regeneration Strategies and Mechanistic Comparison 94
• 4.5.1 Hydrogen-Based Regeneration 94
• 4.5.2 Methane Reforming and Chemical Looping Pathways 95
• 4.5.3 Mechanistic Insights and Strategy Selection 96
• 4.6 Kinetic Modeling of CO2 Conversion in Molten SnZn and SnMg Systems 97
• 4.6.1 Objectives and Modeling Scope 97
• 4.6.2 Reactor Geometry, Assumptions, and Hydrodynamics 99
• 4.6.3 Zn system: single-step CO2 CO kinetic model 103
• 4.6.4 Mg System: Consecutive CO2 CO C Kinetics with an MgCO3 Branch 106
• 4.6.5 Parameter Estimation and Identifiability 109
• 4.6.6 Progressive Activation of Physical Models 111
• 4.7 Energy Requirements and Net CO₂ Reduction 123
• Chapter 5. Conclusion 125
• References 131
• 논문요약 142