Green hydrogen is produced by water electrolysis with renewable energy sources, as illustrated in the photo of windmills. Excess renewable energy captured in the form of hydrogen provides a clean and efficient form of energy storage. The hydrogen can be utilized in the hydrogenation process to produce synthetic methane so that CO₂ captured can be used as a valuable fuel. The Sabatier reaction (CO₂ + 4H₂ → CH₄ + 2H₂O) does the CO₂ to high-energy-density methane (CH₄) conversion, which can be fed into existing natural gas grids.
It is a form of CCU (carbon capture and utilization), and it creates a closed carbon loop: instead of releasing CO₂ into the environment, it is converted into a product with greater energy content and is stored in the form of methane. Here, in this paper, we will discuss the way novel experimental techniques incorporate adsorbing solids in reactor systems to achieve simultaneous CO₂ capture and hydrogen/methane production. We will cover the fundamental operating principles, technological choices, and advanced concepts presented in this field.
Combined CO₂ Capture Technologies
Various technological solutions seek to combine CO₂ capture with H₂ and CH₄ production. Key approaches include:
Adsorption-enhanced reforming (Sorption-Enhanced Reforming):This approach combines the gas steam reforming with the simultaneous adsorption of the produced CO₂ by limestone-type or other solid materials (e.g., CaO). Thus, the reaction shifts towards the production of more H₂ yield (up to 4 moles of H₂ per mole of CH₄) because the CO₂ is trapped as CaCO₃.
Adsorption-enhanced water-gas shift:In this process (CO + H₂O → H₂ + CO₂), elimination of generated CO₂ formed in situ by concurrent adsorption (e.g., by applying adsorbent such as Li₄SiO₄, NaHCO₃) enhances yield of H₂. This is one technique available for clean production of hydrogen from CO-containing gas mixtures.
Dual-Function Materials (DFMs):These are composite materials on which both an absorbent (e.g., CaO, MgO, Na₂CO₃) and a catalytic metal (e.g., Ru, Ni) are supported. CO₂ is first adsorbed over the alkaline material, and then, on introduction of H₂, the captured CO₂ is hydrogenated over the metal catalyst to CH₄. These systems allow for isothermal operation for simultaneous CO₂ capture and methanation.
Separation processes (PSA/TSA, membrane reactors): Although they don't directly produce fuel, technologies like pressure swing adsorption (PSA) or temperature swing adsorption (TSA) are used to separate H₂ and CO₂ after reaction, yielding pure hydrogen. Moreover, catalytic membrane reactors selectively pass H₂ or CO₂ between reaction zones, enhancing conversion rates.
The diagram shows a dual-function material (DFM) operating in two stages: In Stage 1, flue gas is introduced into the reactor, where CO₂ is captured and the exit stream is almost CO₂-free. In Stage 2, renewable hydrogen is introduced, and the captured CO₂ is catalytically hydrogenated into methane (CH₄) and water. The process operates isothermally (~320°C) and produces high-purity CH₄ that can be recycled as fuel, while the produced water is removed.
Experimental Techniques and Absorbent Materials
At the laboratory level, systems are being studied where specialized solid absorbents cooperate with catalysts to achieve integrated CO₂ capture and conversion into H₂ or CH₄. Key elements and techniques include:
Absorbent materials: The best results have been achieved with materials like CaO (limestone cycle), carbonates (e.g., Na₂CO₃, K₂CO₃), alkaline oxides (MgO, Li₂O), as well as zeolites (types 13X, 4A) and MOFs (e.g., ZIF-8, MIL-101). These absorbents exhibit high CO₂ capacity and can be regenerated through thermal CO₂ release cycles.
Catalytic materials: Ni or noble metals (Ru, Rh) supported on carriers (Al₂O₃, ZrO₂, SiO₂) are often used to accelerate methanation or water-gas shift reactions. These catalysts must withstand high temperatures and water vapor environments.
Experimental setups: Tests are conducted in tubular or fixed-bed reactors with flow switching. Initially, CO₂ adsorption occurs (without H₂) at ~300–400°C to capture CO₂. Then, H₂ is introduced into the reactor, and the captured CO₂ is catalytically hydrogenated. Gas composition (CH₄, H₂O, etc.) is analyzed (e.g., GC analysis). Furthermore, thermogravimetric analysis (TGA) measures CO₂ capacity per cycle, while spectroscopic techniques (e.g., in situ FTIR) evaluate surface species during the reaction.
Practical Application Examples
Several applications of combined CO₂ capture and fuel production have been practically or pilot-scale implemented:
Power-to-Gas plants: Audi’s E-Gas plant (Germany) produces synthetic methane by combining hydrogen from renewables with CO₂, demonstrating a closed carbon cycle.
Biomethane from biogas: In anaerobic digestion units, excess H₂ (e.g., from wind parks) is injected to convert the CO₂ in biogas into additional methane, improving biomethane yield and quality.
Industrial processes: CO₂ methanation from flue gases (e.g., from cement plants) produces renewable synthetic natural gas for energy use, closing the carbon loop.
Energy storage: Experimental Power-to-Methane systems convert surplus electricity into hydrogen and then into methane, stored for later use, helping to stabilize renewable energy supply.
Space applications: On the International Space Station (ISS), the Sabatier reaction converts respiratory CO₂ into water and CH₄. The water is recycled (split into H₂/O₂), while CH₄ is vented, showcasing the practical value of this technology.
Challenges and Future Outlook
Although research developments show great promise, there are significant challenges to be addressed:
Material stability and durability: Repeated adsorption–desorption cycles lead to sintering of CaO and reduced CO₂ capacity. Selecting proper additives and improving absorbent structures is vital for long-term system stability.
Heat management: The Sabatier reaction is highly exothermic, while CO₂ release is endothermic. Effective management of thermal flows within the reactor (e.g., heat exchangers, sequential reactor layers) is needed to maintain stable operating temperatures.
Conversion capability: Complete conversion requires high purity and pressure or highly active catalysts. Reaction rates can be limited by mass transfer phenomena, as CO₂ adsorption and chemical conversion should not compete with each other.
Costs and energy efficiency: Producing "green" hydrogen remains expensive and energy-intensive. Additionally, catalysts (e.g., Ru, high-purity Ni) and thermal systems (coolers/heat exchangers) increase system costs. Techno-economic optimization is critical for widespread deployment.
Scale and integration: While pilot plants have been built, scaling to industrial levels faces difficulties. Reactor design, integration with existing grids, and safe handling of produced methane are fields of ongoing research.
Future research: Promising directions include the development of new composite materials (e.g., hybrid absorbents with catalyst nanostructures), use of simulation and AI for reactor optimization, and integration of advanced renewables (e.g., high-temperature electrolysis). Future advances aim to reduce energy intensity and costs, making these technologies commercially viable.
In conclusion, processes combining CO₂ capture with green hydrogen and synthetic methane production offer highly promising solutions for the energy sector. Utilizing captured CO₂ to produce high-energy-density fuels effectively creates a closed carbon cycle, significantly reducing emissions. Experimental methods employing dual-function absorbent materials have already demonstrated significant conversion efficiency improvements, enhancing process effectiveness.
Future research will focus on optimizing materials (e.g., increasing surface area, durability over cycles) and thermal integration of processes. Despite technical and economic challenges, the rapid progress in this field highlights the potential for "combined CO₂ capture" technologies to play a crucial role in the energy transition and sustainable fuel production.