Why people have come to know a little about carbon dioxide gas (CO₂) capture and separation technology

According to information previously reported1), the first indication that global warming was a problem is thought to have been a remark made by James Hansen, of America's National Aeronautics and Space Administration (NASA), at a United States Senate Public Hearing, in which he said that recent abnormal weather, particularly hot weather conditions, were 99% certain to be related to global warming. That same year, an Intergovernmental Panel on Climate Change (IPCC) was established and a search for the cause of the warming began. After that, through the adoption of the United Nations Framework Convention on Climate Change (UNFCCC) at a summit held in Rio de Janeiro in 1992, the Kyoto Protocol was then adopted in Kyoto in 1997 at a Conference of the Parties (COP3) of the UNFCCC summit, and a reduction in the emission reduction of greenhouse gases (not limited to carbon dioxide gas) became obligatory.

More recently, in 2004, an American Paleoclimatologist, Michael Mann, presented a graph that estimated temperature changes over the last 1,000 years; it demonstrated that temperatures in the latter half of the 20th century were the highest for the last 1,000 years and that the rise in those temperatures was more drastic than had ever been seen before. At the same time, a research group lead by Charles Keeling of the Scripps Institution of Oceanography pointed out that since the International Geophysical Year of 1957, they had made atmospheric observations at the Mauna Loa Observatory in Hawaii, and that these showed that the concentration of CO₂ in the atmosphere has been steadily increasing.

It is thought that the idea that CO₂ is the leading cause of warming originated when the IPCC, which retrieved these results, expressed the opinion that the leading cause of global warming was likely to be due to the rise in CO2. Following this, attention was paid to technology that could capture and separate the CO₂ that electrical and steel plants, etc, emitted in great quantity.

Is the leading cause of global warming truly CO₂?

The media started to back the IPCC's claim, and the theory that carbon dioxide gases were the leading cause of global warming became widely believed. However, papers suggesting a different cause have also been published. It has been suggested that water vapor is also a greenhouse gas, and that about 90% of the world's greenhouse gases are water vapor, with carbon dioxide gases making up as little as 10%. Furthermore, six types of gas have been declared by the Environment Ministry to be greenhouse gases contributing to the annual emission rates: carbon dioxide gas (CO₂), methane (CH₄), nitrous oxide (N₂O), hydro-fluorocarbons (HFCs), perfluorocompounds (PFCs) and sulfur hexafluoride (SF₆). According to another report²⁾, it has been estimated that nitrous oxide causes 310 times the greenhouse effect of carbon dioxide gas. Moreover, it is suggested that the following are other primary causes of the earth warming.

1. The Sun's activity level → fluctuation of the Sun's energy → fluctuation of solar radiation
2. Changes in the geomagnetic field → changes in cosmic radiation → fluctuation of the amount of clouds →fluctuation of solar radiation
3. Volcanic eruptions → increase in fine powder volcanic ash, etc, → fluctuation of solar radiation
4. Milankovitch cycle → changes in the inclination of the earth's axis → fluctuation of solar radiation
5. Greenhouse gases (As stated above)

Historically, the temperature of the earth has changed depending on the extent of the sun's activity, and it is known that there have been hot periods and cold periods throughout history¹⁾. However, considering that the length of the fluctuation cycles is at least several decades, it is suggested that conclusions about the earth's temperature changes cannot be made in just a few years.

So, the idea that the leading cause of global warming is carbon dioxide gas cannot be verified and a variety of causes have been considered. At this point in time, it would not be certain whether the increase in carbon dioxide gas is the main cause of global warming.

Even so, technologies and materials for the capture and separation of carbon dioxide gas (CO₂) seem necessary.

Regardless of whether carbon dioxide gas is the main cause of global warming, it is certain that carbon dioxide gas does have a greenhouse effect. I would like to explain about technologies for the capture and separation of carbon dioxide gases that are available now and developing technology that will probably be useful in the future, including carbon dioxide gas absorbents that I am investigating in my laboratory.

The first one is the capture and separation of carbon dioxide gas from mixed gases (including carbon dioxide gas) which occurs in thermal power generation. Thermal power generation is a method in which oil, gas, natural gas, waste matter, etc, are burned (reacted with oxygen); water is boiled by heat produced, and electricity is generated through the turning of turbines using that steam. When oil and natural gas are burned (power generation), water vapor (H₂O) and carbon dioxide gas (CO₂) are produced. So by using only carbon dioxide gas absorbents, CO₂ can be captured and separated. Taking the example of methane, the reaction formula becomes the following:

The second is a method that does not involve burning hydrocarbons, which are constituents of the natural gas and oil used in thermal power generation. This technology, using a catalyst and water, splits the hydrocarbons into carbon dioxide gas and hydrogen, extracting only the hydrogen. This is known as a steam-forming reaction of hydrocarbons. Diagram 2 (below) takes methane (CH₄) as an example of a hydrocarbon. When methane (CH₄) and water (H₂O) are made to react, they change into carbon monoxide (CO) and hydrogen (H₂). Then, by making the remaining carbon monoxide (CO) further react with water, it changes to carbon dioxide gas (CO₂) and hydrogen (H₂). At this time, carbon dioxide gas absorbents are used when speeding up the reaction, or separating the hydrogen. Showa Denko Group has been separating carbon dioxide gas (CO₂) by this method, and has been creating ammonia (NH₃) using the hydrogen (H₂) extracted at this last stage. This ammonia (NH₃) has then been sold.

The carbon dioxide gas absorbents, which are actually being used, or whose application is expected in the near future, are amines³⁾, and K₂CO₃⁴) dissolved in water as liquid form absorbents. On the other hand, solid form absorbents are soda lime (a mixture of Ca (OH)₂ with a negligible amount of NaOH) and CaO, and Lithium complex oxides (Li₄SiO₄⁵), Li₂CuO₂⁶)), etc. K₂CO₃ dissolved in water, as stated earlier, is currently being used as the carbon dioxide gas absorbent for steam forming reactions4). The reaction from this is presented in Diagram 3. Carbon dioxide gas reacts with K2CO3 dissolved in water and taken into the aqueous solution. On the other hand, in order to emit carbon dioxide gas already used, the temperature is raised when the reaction is run in the opposite direction(←).

K₂CO₃(Aqueous solution)+H₂O(water)+CO₂(Gas)⇔ 2KHCO₃(Aqueous solution)

Diagram 3: Aqueous solution system carbon dioxide gas absorption material. This cannot be used at high temperature as it is aqueous solution.

In contrast, solid form oxides react with carbon dioxide gas by the reaction formula presented in diagram 4, and solidify the carbon dioxide gas (CaCO₃ & Li₂CO₃). Because they are solid, it is possible to make them extremely hot in places such as power plant outlets, etc, and, they are also compact. In carbon dioxide gas emissions, due to heating, the temperature increases, bringing about a reaction in the opposite direction(←). When solid materials are used as absorbents, granular or fine powder forms can be considered, but the problem is that heat is necessary at the time of emitting the carbon dioxide gas that has been previously absorbed, and at this time energy is required. What is more, Ca(OH)₂ & CaO are inexpensive, but they are strong bases, and because the rate of expansion at the time of carbon dioxide gas absorption is large, when they plug up an outlet of power plant, then some special devices are needed. Also, as the temperature for emitting carbon dioxide gas that has previously once been absorbed(returning CaCO₃ to CO₂ & CaO)is high, there is a possibility that it may be difficult to put this to practical use. With Li complex oxides, the decarbonation temperature is relatively low compared to that of CaCO₃, they have an weak point that Li is expensive. For this reason, we can consider that its applications will be difficult unless we think of some kind of special configuration and method of use.

Ca(OH)₂(solid) + CO₂(gas) ⇔ CaCO3(solid) + H2O(water)
CaO(solid)+ CO₂(gas) ⇔ CaCO₃(solid)
Li₄SiO₄(solid) + CO₂(gas) ⇔ Li₂CO₃(solid) + Li₂SiO₃(solid)
Li₂CuO₂(solid) + CO₂(gas) ⇔ Li₂CO₃(solid) + CuO(solid)

Diagram 4: Solid form carbon dioxide gas absorbents. This can be used at high temperatures because they are solid.

In order to resolve the problems above, I have been devising a solid state carbon dioxide gas absorbent with a special configuration such as that shown in diagram 5. Li₄SiO₄ and Li₂CuO₂ include Si and Cu as central metals. So focusing on these metals, I formed a layer of silicon oxide (SiO₂) and copper oxide (Cu₂O, CuO) on the surface of Si or Cu metals and then on top of that have been able to form a structure that forms a layer of Li₄SiO₄ and Li₂CuO₂. With this construction, the emitting of previously absorbed carbon dioxide gas, by running electric current through the central metals, heats the absorbents themselves and becomes a process of increasing the temperature. Also, by turning wired shaped absorbents, shown in diagram 5, into a spiral such as that shown in diagram 6, it is conceivable that it will become possible to expand the surface area to absorb carbon dioxide gas even within a small space. In doing this, I expect it will be possible to use a new carbon dioxide gas absorbent that doesn't take up space and makes the best use of the compactness of the solid.

Metal-Air Battery, one of the electric car batteries that will be available the near future, uses oxygen, which exists in abundance in the atmosphere, as one of its fuels. This battery induces oxygen from the atmosphere but at the same time, atmospheric carbon dioxide gas also enters the battery, and as carbon dioxide gas has a bad effect on electrolyte of batteries, we must reduce it with absorbents. For this reason, I would like to expect that the solid state form absorbents shown in diagrams 5 & 6 will be used in batteries.

Reference

• Shigenori Maruyama, 90% of Scientists know that the suspect theory of CO2 regarding the "Global warming" is a lie. (Takarajima Shinsho)
• 2) Institute for Global Environmental Strategies, IGES policy report 2011
• 3) Tomio Mimura, Hidehiro Kumazawa, Yasuyuki Yagi, Toru Takashina, Ryuji Yoshiyama, Akihiro Honda, Journal of Chemical Engineering of Japan, 32, 236 (2006)
• 4) Showa Denko K. K., Chemicals Business Sector, Kawasaki Office pamphlet
• 5) M. Kato, and K. Nakagawa, J. Ceram. Soc. Japan, 2001, 109, 911.
• 6) Y. Matsukura, T. Okumura, R. Kobayashi, K. Ōh-ishi, Chem. Lett., 2010, 39, 966.