What is the secret weapon for reducing carbon dioxide in the atmosphere?

In recent years, people seem to have become accustomed to global warming, and scientists have been constantly warning of the "global greenhouse effect". Due to the impact of human activities, the carbon dioxide (CO2) content in the atmosphere has exceeded the limit of natural changes. Analysis of ice cores taken from the Vostok Glacier in Antarctica shows that the current concentration of carbon dioxide in the atmosphere has risen to a level never seen in the history of the Earth, which will trigger a series of environmental problems.

Scientific and technological workers from all over the world have tried to fix carbon dioxide in various ways to minimize its negative impact on climate warming. Forest fixation methods are affected by changes in land use and its own absorption capacity, such as the variability of wood production, weather, climate, the impact of carbon dioxide as fertilizer, and concentrated afforestation methods, which make wood production full of uncertainty. After trying, the capacity of using abandoned natural gas reservoirs to store carbon dioxide is only less than the storage capacity of the ocean. Ocean storage may have an impact on marine ecology, and ocean storage of carbon dioxide is not suitable for countries and regions with large carbon dioxide production but far away from the ocean. Scientists believe that storing carbon dioxide in biogenic natural gas fields can use the inherent anaerobic archaea in the gas fields - methanogens (Methanogenus) (Figure 1) to convert carbon dioxide into methane and achieve energy regeneration at the same time.

Application research of carbon dioxide fixation by methanogens

In 1999, Japanese scientists proposed that carbon dioxide be injected under a gas hydrate layer or under a permafrost layer. The deep and low-temperature aquifer will automatically seal the carbon dioxide, thereby achieving the purpose of storing carbon dioxide underground. Autotrophic bacteria fix carbon dioxide in deep water and sunlight-free environments. Methanogens can convert carbon dioxide into methane under deep water and oxygen-free conditions. After carbon dioxide is injected underground for dozens or hundreds of years, it is expected to form an underground hydrocarbon layer.

Gas hydrates often exist under the deep ocean crust at high pressure and low temperature. Gas may accumulate under gas hydrate layers or under permafrost, which are natural gas separators. The concern is that global warming will cause the permafrost to melt and the hydrates to warm, which will accelerate the release of methane accumulated there. Injecting carbon dioxide under the permafrost and methane hydrate caps can extract the accumulated methane and prevent accelerated global warming, because the injected carbon dioxide will strengthen the permafrost and methane hydrate caps. Carbon dioxide hydrates are more stable than ice at high temperatures and more stable than methane hydrates at high pressures. So in theory, this is an effective method.

Underground storage of carbon dioxide

If CO2 is injected into an aquifer and dissolved in groundwater with active methanogens, the methanogens will convert CO2 into methane as long as hydrogen is present. Moreover, methane tends to separate from the water and migrate upward. Therefore, methane often accumulates at the top of the reservoir. Due to the diversity of underground microorganisms and environments, if CO2 is to be fixed and recycled by microorganisms, extensive and careful research on suitable bacterial species and ecological environments is required.

In 2000, the U.S. Department of Energy proposed that there are many methanogens in nature that can convert carbon dioxide into methane. Some methanogens survive in extreme environments of high temperature and high pressure. Therefore, useful methanogens should be cultivated and selected, and microbial design or biomimicry systems should be established through the intersection of biology, chemistry, and geophysics. In this system, carbon dioxide can be converted into methane.

There is a lot of evidence that biogenic natural gas can be the main body of a gas field, such as the Terang-Sirusan gas field in the East Java Sea, the Qaidam Basin biogas field in my country, and the Luliang Basin biogas field in Yunnan. According to statistics, 20% of the natural gas on Earth is produced by methanogens, of which 2/3 is produced by acetate fermentation and 1/3 is formed by carbon dioxide fixation.

Developing genetic decoding, gene sequencing, identifying new enzymes and selecting good methanogens have played a great role in accelerating the conversion of carbon dioxide to methane. Scientists have isolated some organisms that live in extreme environments of high temperature and pressure from deep-sea chimneys, and then put them into layers suitable for oil and natural gas production, trying to get them to further help produce natural gas.

In Canada, scientists have already developed the technology to use carbon dioxide to enhance oil recovery, inject carbon dioxide into deep aquifers or inject carbon dioxide into abandoned oil and gas wells, although the amount of carbon dioxide deposited in these ways is very large, but no economic benefits have been generated.

Injecting carbon dioxide into deep coal seams can increase the coal seam methane recovery rate while depositing carbon dioxide. The carbon dioxide waste gas produced by power plants that use coal seam methane as raw material can be recycled and injected into the coal seams, thereby generating more coal seam methane to supply power plants, which can greatly increase the industrial added value of carbon dioxide.

The role of methanogens

Methanogens are a type of strictly anaerobic archaea that can survive under conditions of high temperature, high salt, and high pressure. They have the characteristic of producing methane using carbon dioxide as a substrate. They are located at the end of the anaerobic food chain of the carbon cycle in nature and play an important role in the material cycle in nature. They are different from various macroscopic and microscopic organisms, and the biological community once called them "third organisms." Most methanogens can use hydrogen as a reducing agent for carbon dioxide to synthesize organic matter. At the same time, they also use special anaerobic respiration, methane fermentation, and carbonate respiration to obtain the energy needed for life activities.

In 1997, the Key Open Laboratory of Anaerobic Microorganisms of the Ministry of Agriculture and Rural Affairs of China believed that the ecological environment of methanogens can be roughly divided into three categories: (1) Rural biogas digesters and anaerobic sewage treatment systems, which undergo all four stages of hydrolysis and fermentation of complex organic matter, hydrogen and acetogenesis, methanogenesis, and homoacetogenesis. (2) Ruminants, which only undergo two stages: hydrolysis and fermentation and methanogenesis. Various fatty acids are fermented in the rumen to promote rapid absorption by the gastrointestinal tract wall, and there is a lack of hydrogen and acetogenesis stages. (3) Represented by hot springs and submarine volcanic hydrothermal vents, hydrogen and carbon dioxide are mainly produced through geochemical processes. The production of methane includes at least the methanogenesis stage and the homoacetogenesis stage.

The study of geological fixation of carbon dioxide using methanogens can be conducted from two aspects: first, by utilizing the characteristics and diversity of methanogens, selecting suitable strains for cultivation and developing characteristics suitable for application; second, providing suitable conditions for the existing strains in the underground ecological environment and maintaining the original ecological environment. The second method has attracted the attention of countries around the world and has gradually become a new science. Earth scientists at Peking University conducted preliminary experimental research in the Qaidam Basin and achieved quite satisfactory results, laying a good foundation for the industrial promotion of this method.

A model of success

In 2002, in order to meet the requirements of the Kyoto Protocol, the European Union planned to reduce European greenhouse gas emissions by 8% from 2008 to 2012. This requires an average annual reduction of 6 million tons of carbon dioxide emissions. Short-term measures are to improve energy efficiency and switch from fossil fuels to renewable fuels. However, the United Nations' long-term goal of stabilizing the concentration of greenhouse gases in the atmosphere requires further reductions in carbon dioxide emissions.

In January 2003, Europe established an organization dedicated to coordinating the joint efforts of various enterprises and scientific research institutions to reduce the concentration of carbon dioxide in the atmosphere, including oil companies, contractors, geological sciences and other professional and technical colleges in European countries.

The objectives of the programme are: (1) to promote joint research and support the implementation of CO2 storage, capture and reduction plans on the European continent; (2) to evaluate and revise strategies for oil exploration and development; (3) to provide relevant information for policy making in Europe and in individual countries; and (4) to increase public awareness and recognition of different CO2 capture and storage technologies through technical events, conferences and website publications.

According to the European Commission's Fifth Framework Program, six programs will be launched to assess the safety and environmental impact of carbon dioxide storage, including injecting carbon dioxide into underground saline aquifers, coal seam fissures, abandoned oil fields and geological reservoirs above sea level, and analyzing the causes of natural accumulation of carbon dioxide in the formation. In addition, the program also supports the study of carbon dioxide injection into the Weyburn oil field in Saskatchewan, Canada. Scientists are working on geochemical analysis and modeling to gain a deeper understanding of the results and significance of this study.

Author: Excited