Analysis Of The Progress And Application Of Carbon Dioxide Methanation Technology

To cope with climate change and achieve sustainable development, the international community has taken a series of measures to reduce CO2 emissions, one of which is carbon capture technology. In recent years, carbon capture technology has been rapidly developed and widely used, becoming an important means to cope with climate change and reduce greenhouse gas emissions. However, the development of carbon capture technology still faces many challenges, such as operating costs, energy consumption of modern chemical equipment and storage and transportation of CO2. Using CO2 as a raw material and preparing valuable industrial products through chemical synthesis methods is considered to be an effective way to solve the limitation of carbon capture. The methanation reaction of CO2 was proposed by Paul Sabatier. The reaction of CO2 and H2 to produce CH4 is considered to be an effective technology for the recycling of CO2.

Some researchers have proposed an innovative business model: using CO2 methanation technology and green hydrogen technology to produce CH4 to complete the storage of electrical energy into chemical energy. This model can convert excess electricity into CH4 for storage, solving the problem of renewable energy volatility; using existing natural gas storage and transportation facilities can significantly reduce investment costs; promoting the development of carbon capture and utilization technology, and contributing to the global green energy transition. This paper introduces the development of CO2 methanation catalysts and analyzes the industrial application scenarios of power-to-gas (PIG) technology using methanation technology.

Keywords: carbon dioxide, methanation, power-to-gas, carbon capture, energy storage, hydrogen feedstock, water electrolysis

The following reactions mainly occur between H₂ and C0₂ during the methanation process.

Main reaction: C0₂ + 4H₂ ⇄ CH₄ + 2H₂ O

Δ H = -165.0 kJ/mol

Side reaction: C0₂ + H₂ ⇄ C0 + H₂ O

Δ H = 41.1 kJ/mol

The side reaction, also known as the water-gas shift reaction, is endothermic. When the reaction reaches a certain temperature, the selectivity of the byproduct CO increases, and the selectivity of CH₄ decreases. Therefore, the research focus of CO₂ methanation catalysts is to develop catalysts with high activity at low temperatures. The effect of pressure and temperature on the reaction products, where the volume ratio of reactants H₂ to CO₂ is 4:1.

The superiority of the methanation reaction is mainly reflected in its thermodynamic properties and the theoretical potential for high conversion rate at room temperature and pressure. Since the C0₂ molecule has been completely oxidized and the carbon atom and the oxygen atom form a covalent double bond, the activation energy required for the reaction is very high, and a catalyst is needed to reduce the activation energy and thus increase the conversion rate.

The metals of Group VIII can improve the conversion rate and selectivity in the methanation reaction, and their activity is in descending order: Ru, Ir, Rh, Ni, CO, Os, Pt, Fe, Mo, Pd, Ag. When only the most important factors (activity and selectivity) of the methanation reaction are considered, the activity is in descending order: Ru, Fe, Ni, Co, Mo, and the selectivity is in descending order: Ni, Co, Fe, Ru.

The performance comparison of different metal catalysts is shown in Table 1. Catalytic systems based on transition metals (Ru, Rh, Pd, etc.) have been widely developed. They have excellent catalytic activity and selectivity in CO₂ methanation reactions! In particular, Ru-based catalysts show high activity and selectivity under low temperature and moderate conditions. Studies have shown that under the same reaction conditions, 3%Ru/AI₂O₃ (3% Ru loaded on the AI₂O₃ carrier, the same below) has a higher selectivity for CH₄ than 20%Ni/AI₂O₃. Precious metals (such as Pt) also have high activity and selectivity in CO₂ methanation reactions, but their high cost has become the main obstacle to their large-scale application in CO₂ methanation. Other transition metals (such as Fe and Co) also have certain chemical reactivity, but their selectivity is low, and they are mainly used as additives for other bimetallic catalysts. Due to their low price and abundant raw materials, most research and industrial projects use Ni-based catalysts.

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Different carriers play a great role in the preparation and improvement of catalysts. Commonly used carriers are mainly metal oxidant carriers. These carriers mainly increase the adsorption capacity of reactants by changing the existence state of the active phase, including surface morphology, metal dispersion, and the main exposed crystal faces, thereby improving the catalytic performance. Oxides with specific properties are often used as catalyst carriers, such as AI₂O₃, SiO₂, ZrO₂, CeO₂, etc. Among them, AI₂O₃ is one of the most commonly used catalyst carriers because it has the advantages of low price, large surface area, and developed pores. However, the active phase of Ni/AI₂O₃ catalyst is easy to sinter and carbonize at high temperature, resulting in catalyst deactivation. Therefore, the preparation of Ni/AI₂O₃ catalyst with good activity and high stability has become the current research focus.

For Ni-based catalysts, additives are key modification components that can significantly change the properties and performance of the catalyst. Catalyst performance is optimized through different mechanisms, such as bimetallic synergy, changing the metal electronic environment, inhibiting the formation of spinel species, enhancing the adsorption of reactants and intermediates, and improving metal dispersion. Sidik et al. found that by adding Co as a binder, the prepared Ni-Co/MSN catalyst showed higher activity and stability than Ni/MSN. Analysis confirmed that Ni-Co alloy helps to reduce Ni particle size and provides better metal dispersion performance. Paviotti et al. prepared Ni-Ru catalysts and found that Ru improved the reducibility of Ni-based catalysts and enhanced Ni's adsorption capacity for H₂, thereby improving catalyst performance.

At present, although the development of renewable energy has achieved remarkable results, it also faces some challenges. The bottleneck lies mainly in the unpredictability of renewable resources, which may lead to power surplus and bring challenges to the stable operation of the power grid.

PtG technology uses CO₂ methanation reaction, uses green hydrogen produced by renewable energy and CO₂ as raw materials to generate synthetic natural gas products, and the by-product oxygen can be purified and sold. Making full use of the high-quality reaction heat can reduce the cost of PtG technology. Since the main component of the generated synthetic natural gas is CH₄, it can be directly sent to the natural gas pipeline network, realizing the long-term and large-scale storage of CH₄.

The main methods of industrial hydrogen production include natural gas hydrogen production, water electrolysis hydrogen production, coal hydrogen production, and industrial by-product hydrogen production. Except for water electrolysis, the hydrogen production, other methods require the use of fossil energy. Fossil energy hydrogen production technology is mature, and the cost is lower than water electrolysis hydrogen production, but a large amount of CO₂ will be generated during the production process.

Ideally, the hydrogen raw material of PtG technology is H2 (green hydrogen) produced by coupling water electrolysis technology with renewable energy. At present, the main water electrolysis technologies are alkaline water electrolysis (AEL) technology, proton exchange membrane (PEM) technology, anion exchange membrane (AEM) technology, and solid oxide electrolyzer (SOEC) technology. Among them, AEL technology is mature and suitable for large-scale industrial hydrogen production, but the production rate is low, and the power consumption is high. Before 2014, domestic and foreign water electrolysis projects used AEL technology. Since 2015, the installed capacity of PEM has gradually increased. This is mainly due to the fact that PEM electrolyzers are suitable for working conditions with large power fluctuations, have shorter startup times, and respond more quickly to fluctuations in renewable energy. AEM technology and SOEC technology are not mature: they are currently unable to adapt to industrial-scale green hydrogen production.

The CO₂ in the methanation reaction comes from CO₂-rich oil and gas fields, large coal-fired power plants, cement plants, etc. These plants usually emit a large amount of CO₂ during operation, and high-purity CO₂ can be obtained using carbon capture technology. At present, post-combustion capture technology has been widely used in factories, mainly using chemical absorbents to capture CO₂ in flue gas after combustion.

Carbon capture technology has basically matured, and a large number of projects have been implemented around the world. Although carbon capture technology can reduce the carbon tax costs of enterprises, no valuable chemical products are produced in the whole process. At present, some CO₂ utilization and storage technologies have potential risks, such as CO₂ flooding technology and underground storage technology.

At present, there are no large-scale commercial PtG device cases in my country, and there are not many studies in this field. The research on PtG technology is currently mainly in European countries, and the United States and Japan have also increased their research and development of this technology in recent years.

Chehade et al. conducted research and analysis on 192 power-to-X demonstration projects in 32 countries. 91% of the projects are still in operation, and 27% of the projects are considering expanding production scale: 99 projects first use renewable energy to produce H₂, and then use hydrogen fuel cell technology or gas turbines to convert H₂ into electricity and directly inject it into the power grid; 69 projects produce green hydrogen and use methanation technology to convert H2 into CH₄ and inject it into the natural gas pipeline network; there are 154 PtG projects in Europe, of which nearly one-third are located in Germany.

Wulf et al. investigated Power-to-X projects in Europe. As of June 2020, a total of 220 such projects have been implemented or are under construction and planning in Europe, with the fastest growth in projects in Germany and France. In 2020, 20 countries implemented Power-to-X demonstration projects, of which 44% were located in Germany. Most projects directly used renewable electricity. One-third of Power-to-X projects converted H2 into other gas products for sale. Most projects used biomass gas as CO2 feedstock, while others used flue gas from natural gas or coal-fired power plants as CO2 feedstock. 66% of all projects used chemical catalytic methanation reactions.

Australia is also actively exploring ways to convert CO₂ into CH₄. Southern Green Gas plans to implement a renewable CH₄ project to produce CH4 using small modules. These modules contain solar panels that supply electricity to electrolyzers. The H2 produced by the electrolyzers reacts with the captured CO₂ in a methanation reactor to produce CH₄, which is then injected into the Australian natural gas pipeline network for storage or commercial use. The project uses the existing pipeline system as a transportation and storage facility, significantly reducing the cost of infrastructure construction: the skids can be self-sufficient, greatly reducing operating costs; and renting low-cost land further improves the project's competitiveness.