JPH0549838A - Method for conversion of carbon dioxide - Google Patents

Abstract

PURPOSE:To efficiently convert carbon dioxide in exhaust gas for effective use by refining carbon dioxide from carbon dioxide contg. gas by pressure varying adsorption method and converting the obtd. carbon dioxide into carbon by using an oxygen-lacking magnetite and/or strontium ferrite. CONSTITUTION:A carbon dioxide-contg. gas is pressurized over the adsorption pressure by a compressor 1 and introduced to a refining tower 2 for carbon dioxide filled with an adsorbent for carbon dioxide. The residual gas from which carbon dioxide is separated by adsorption is discharged to outside of the system through a tube 7, while a valve 10 is closed and a valve 3 is opened to desorb the adsorbed carbon dioxide and to release the carbon dioxide to a carbon dioxide storage tank 4. Then, the carbon dioxide is introduced to a carbon dioxide conversion tower 6 by opening a valve 5. In this tank, carbon dioxide is brought into contact with a catalyst to decompose into carbon and oxygen.

Description

Detailed Description of the Invention

[0001]

[Industrial application] In recent years, from the viewpoint of global environmental protection,
The importance of carbon dioxide removal and conversion technology is increasing, and research is being conducted from various perspectives. Carbon dioxide itself is not very toxic, but it is said that emission control is required as a greenhouse gas. Carbon dioxide is generated when fossil fuels such as coal and oil are burned, and is generated by a basic industrial process such as power generation. At present, various attempts have been made to reduce carbon dioxide emissions. In order to implement carbon dioxide emission restrictions, it is possible to promote energy conservation and suppress the absolute amount of carbon dioxide produced, but in this case there is concern that it may adversely affect economic growth, and it is impossible to prevent it completely. Is. On the other hand, a method of storing generated carbon dioxide in the sea after solidifying or liquefying it is also being studied. However, with this method, the effect of carbon dioxide on the marine biological system is not clear, and further, the reuse of the preserved carbon dioxide is difficult, and it cannot be said to be the optimum method.

Recently, attempts have been made to convert carbon dioxide into various compounds. However, it is needless to say that carbon dioxide is the most stable compound among carbon compounds, and thus energy is required to convert it into another carbon compound. For example, a method of electrochemically reducing carbon dioxide is known. As an electrode catalyst,
In addition to metal catalysts such as nickel, iron and cobalt, various studies have been conducted using an electrode catalyst carrying a catalyst such as cobalt tetraporphyrin, cyclam-Ni complex, isocitrate dehydrogenase, malate enzyme and the like. However, in these methods, the overvoltage of carbon dioxide reduction is high, and there are many problems in terms of current efficiency and reaction selectivity. Therefore, a relatively large amount of electric energy required for reduction is required, and there is a problem of what to seek for an electric energy source, which is still not a preferable method from the viewpoint of environmental problems. Also, a method of photoelectrochemically reducing carbon dioxide is known. This is a method of converting carbon dioxide into alcohol using a photoelectrode catalyst such as p-type amorphous silicon carbide or p-type copper oxide.

However, in this method, the light to be irradiated is ultraviolet light and a large amount of energy is required to obtain this light. Further, all of the above reactions are reactions in the electrolytic solution, and the carbon dioxide concentration and the electrode area are limited, so that the reaction rate is greatly restricted.

On the other hand, Tamaura et al. Of Tokyo Institute of Technology found that oxygen-deficient magnetite reduces carbon dioxide to carbon, and Inaga et al. Of Okayama University strontium ferrite selectively selects carbon dioxide. I found that it was reduced to carbon. These reactions occur at relatively low temperatures of 250-350 ° C. and all reaction products are characterized by solid carbon. Since the reaction proceeds at a relatively low temperature, exhaust heat from a thermal power plant or the like can be used, and no extra energy is required for carbon dioxide conversion, so it is expected to be a catalyst suitable for solving global environmental problems. Furthermore, the reaction rate is high and the reaction selectivity is almost 100%.

[0005]

However, in order to apply these catalysts to the actual conversion reaction of carbon dioxide in the exhaust gas, a technology for purifying carbon dioxide from the exhaust gas is necessary. This utilizes the strong reducing property of the above catalyst for the reduction of carbon dioxide, but if an oxidizing gas such as oxygen is present in the reaction gas, it selectively reacts and the reaction efficiency of carbon dioxide is remarkably high. Fall to. Known methods for purifying and separating carbon dioxide include a method using a carbon dioxide selective permeable membrane, and a pressure fluctuation adsorption method in which carbon dioxide is adsorbed on and desorbed from an adsorbent by a pressure change for purification. However, no attempt has yet been made to combine these purification methods with a carbon dioxide conversion method and apply it to the conversion of carbon dioxide in exhaust gas.

In order to respond to the above-mentioned demand, it is an object of the present invention to propose a method for efficiently converting carbon dioxide in exhaust gas generated from a thermal power plant or the like to effectively utilize it.

[0007]

According to the present invention, carbon dioxide is purified from a carbon dioxide-containing gas by a pressure fluctuation adsorption (hereinafter abbreviated as PSA) method, and then the carbon dioxide is converted into carbon using a catalyst. The main point is the method of converting carbon dioxide. By applying the carbon dioxide conversion method provided by the present invention, it becomes possible to efficiently convert the carbon dioxide in the exhaust gas to be purified at a thermal power plant or the like with almost no extra energy required.

The carbon dioxide conversion method of the present invention will be described below with reference to the drawings. The numbers in parentheses correspond to the numbers in the drawing.

Examples of the carbon dioxide-containing gas to which the present invention is applicable include carbon dioxide-containing exhaust gas discharged from a thermal power plant, cement production, or the like. It is desirable to reduce the water content of such exhaust gas to 0.1 mg / liter or less, particularly 0.03 mg / liter or less in advance. This is because water is not desorbed during regeneration, and the amount of water in the adsorbent increases and the carbon dioxide adsorption capacity decreases as the adsorption / desorption is repeated. The carbon dioxide-containing gas is pressurized to a pressure equal to or higher than the adsorption pressure by a pressurizer (1) and introduced into a carbon dioxide purification tower (2) having an adsorbent for carbon dioxide. Various adsorbents for carbon dioxide used in PSA are currently known, and all of them are applicable to the present invention. For example, as the carbon dioxide adsorbent used for PSA,
Silica gel, activated carbon and synthetic or natural zeolites are known. In terms of the effective adsorption amount of carbon dioxide,
It is generally said that synthetic zeolites are excellent. Synthetic zeolites are usually sodium X-type zeolites, which are commercially known as 13X.

When a synthetic zeolite is used as the adsorbent, the pressure during adsorption is 1 to 50 atm, and the pressure during desorption is 2 to 0.1 atm, but lower than the pressure during adsorption. Further, since the effective adsorption amount of carbon dioxide can be increased as the pressure difference at the time of adsorption / desorption increases, desorption is usually performed at 1 atm or less. In recent years, various processes have been proposed for continuously purifying and desorbing carbon dioxide by using a double tower as a refining tower. Although the drawing shows the method that uses only one CO2 purification tower, which is the most basic system,
Also in the present invention, the carbon dioxide purification can be continuously carried out by using two or more carbon dioxide purification towers.

The residual gas from which carbon dioxide has been separated by adsorption is discharged to the outside of the system through the conduit (7) and is recovered if necessary. After adsorbing carbon dioxide, the exhaust gas supply to the purification tower (2) is stopped and the valve (10) is closed. Less than,
The steps so far are called purification steps.

Next, by opening the valve (3), the adsorbed carbon dioxide is desorbed and released into the carbon dioxide storage tank (4), and then by opening the valve (5), the carbon dioxide is desorbed. Carbon dioxide can be reduced while regenerating the adsorbent. By adopting such a system, it is not necessary to provide a decompressor such as a vacuum pump normally used in the PSA system. The carbon dioxide that has reached the carbon dioxide conversion tower (6) through the valve (5) is brought into contact with the catalyst and decomposed into carbon and oxygen, and the oxygen is occluded by the catalyst to reduce its volume. This is because the pressure inside the conversion tower is kept low as long as it has a carbon dioxide conversion capacity, and the carbon dioxide purification tower is also maintained at a desorbable pressure. That is, the energy required in this conversion step is only the thermal energy required to maintain the reaction temperature in the carbon dioxide conversion tower (6). This heat energy can utilize exhaust heat which has not been used conventionally. Moreover, since the carbon dioxide supplied to the conversion tower is sufficiently purified, the catalyst is efficiently used for carbon dioxide conversion.

It is essential that the carbon dioxide conversion tower (6) has at least one catalyst capable of converting carbon dioxide into carbon. Known catalysts for converting carbon dioxide into carbon include iron, oxygen-deficient magnetite, and strontium ferrite. Any of these can be applied to the system of the present invention. Of these, oxygen-deficient magnetite, strontium ferrite, and the like are preferable since they exhibit activity at 400 ° C. or lower or 300 ° C. or lower, and can utilize exhaust heat.

In the drawing, the carbon dioxide conversion tower (6) is
Although only the tower is described, it can be composed of a double tower, and in the case of a double tower, the conversion of carbon dioxide can be carried out more efficiently. Carbon dioxide contacts the catalyst inside the carbon dioxide conversion tower (6) and converts it into carbon. During the conversion, the inside of the tower is heated to the temperature required for the reaction. For example, when oxygen-deficient magnetite is used as a catalyst,
The reaction proceeds at 50 ° C to 400 ° C, preferably 300 ° C to 350 ° C.

As the reaction progresses, carbon atoms in carbon dioxide are deposited as carbon on the surface of or inside the catalyst, and oxygen atoms are incorporated inside the crystal structure of oxygen-deficient magnetite or the like. In this way, the pressure in the system drops. The catalyst applied in this system has a limited conversion of carbon dioxide. Therefore, the reaction stops when a certain amount of carbon dioxide is converted. For example, in oxygen-deficient magnetite,
It is believed that the reaction stops when the oxygen deficiency disappears and it becomes magnetite. The end point of this reaction can be judged by stopping the pressure drop inside the system. The step of converting this carbon dioxide with a catalyst is hereinafter referred to as a carbon dioxide conversion step.

The catalyst which has completed the reaction synthesizes carbon into a carbon compound other than carbon dioxide by the following operation, and
Regeneration of catalytic ability is possible. That is, for example, the valve (5) of the carbon dioxide conversion tower after the reaction is closed and the gas containing oxygen is introduced into the tower. At this time,
If the system is kept at the same temperature as in the carbon dioxide conversion step, the generated carbon is converted to carbon monoxide by oxygen. Various methods have been proposed for the synthesis of organic compounds using carbon monoxide, and they can be effectively reused. After removing the carbon, hydrogen is supplied to the carbon dioxide conversion tower (6) to bring the temperature inside the tower to 300 to 500 ° C., whereby the catalytic ability can be regenerated. It is believed that this is because oxygen vacancies are regenerated in the catalyst by this step. The regenerated catalyst can be reused in the above carbon dioxide conversion step. Further, if hydrogen is supplied into the column instead of oxygen and the temperature inside the column is set to 500 to 700 ° C., carbon is converted into methane and the catalyst is regenerated. There are various methods for effective use of methane. By repeating the above carbon dioxide refining step and carbon dioxide converting step alternately or in parallel, carbon dioxide can be continuously treated.

[0017]

EFFECTS OF THE INVENTION By applying the present invention, carbon dioxide in exhaust gas can be efficiently converted and used effectively. The carbon dioxide conversion system and conversion method according to the present invention are superior to the conventional methods in the following points. First of all, a decompressor such as a vacuum pump was used during desorption for purification and separation of carbon dioxide by conventional PSA.
However, according to the present invention, the inside of the reactor for converting carbon dioxide is decompressed by the conversion reaction, and thereby the inside of the adsorption device connected to the reactor is also decompressed, so that the decompressor is not required. Moreover, since the pressure becomes lower as compared with a normal vacuum pump due to the reaction, the effective adsorption amount of the adsorbent can be made extremely high.

Secondly, the catalyst used in the present invention has been used so far only for conversion of pure carbon dioxide, but the system of the present invention enables conversion of carbon dioxide in exhaust gas. This is because carbon dioxide is generated by the adsorbent in the previous step. Moreover, since the catalyst operates at a relatively low temperature, it is possible to utilize exhaust gas, which has been conventionally discarded, as a heat source for the conversion of carbon dioxide.

[0019]

Embodiments will be described with reference to the drawings. The specifications of the equipment and raw materials were as follows.

[0020]diameter height volume Remark  cm cm liter carbon dioxide purification tower 18 65 16.5 with heating jacket carbon dioxide storage tank 14 65 10.0 carbon dioxide conversion tower 30 30 21.2 with heater Raw material gas: 0.03 mg of water from boiler exhaust gas /
Excluding up to 1 liter Composition (vol%): Carbon dioxide Nitrogen Oxygen 25 70 5 Adsorbent: Synthetic zeolite compact (13X) 10 kg Catalyst: Oxygen-deficient magnetite 20 kg Compress the raw material gas to about 4 atm with a compressor (1) And carbon dioxide
It was supplied to the elementary purification column (2). This adsorption operation is also the next desorption operation
The work was also performed at 25 ° C. Standard amount of raw material gas supply
Supply of raw material gas at the time of conversion of 2000 liters
Stop, close valve (7) and open valve (3) to remove diacid
Carbon dioxide was introduced into the carbon dioxide storage tank (4). Diacid
The pressure in the carbon dioxide purification tower dropped from 4 atm to 3.7 atm
did. Then, the valve (5) is opened and the diacid in the tank is
Introducing carbon dioxide into the carbon dioxide conversion tower (6)
Is discharged from the valve (9), and the oxygen concentration is 1 vol% or less.
And the valve (9) was closed. carbon dioxide
The pressure in the purification column dropped to 3.1 atm. Then convert
The column was heated to 350 ° C and held at that temperature,
The pressure in the carbon dioxide purification tower began to gradually decrease. 0.1 qi
When the pressure was reached, the pressure did not drop and the reaction stopped
Since found, valves (3) and (5) were closed.

Then, the temperature of the conversion tower is kept at 350 ° C.,
When oxygen was introduced into the conversion tower (6), the conduit (9)
As a result, 380 liters of carbon monoxide equivalent to the standard state were obtained.

Next, when the supply of oxygen was stopped and hydrogen gas was supplied, water vapor was detected through the conduit (9). Further, the supply of hydrogen was stopped and the conversion tower was shut off from the outside air.

When the above operation was repeated, almost the same result was obtained.

[Brief description of drawings]

FIG. 1 is a diagram showing a flow of an example of the present invention.

[Explanation of symbols]

 1: Compressor 2: Carbon dioxide purification tower 3, 5, 7, 8, 9: Valve 4: Carbon dioxide storage tank 6: Carbon dioxide conversion tower

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Internal reference number FI technical display location C01B 31/02 101 Z 7003-4G

Claims (3)

[Claims] 1. A method for converting carbon dioxide comprising purifying carbon dioxide from a carbon dioxide-containing gas by a pressure fluctuation adsorption method and then converting the carbon dioxide into carbon using a catalyst. 2. An oxygen-deficient magnetite and / or a catalyst.
Alternatively, the carbon dioxide conversion method according to claim 1, wherein strontium ferrite is used. 3. The carbon dioxide conversion method according to claim 1 or 2, wherein during desorption in the pressure fluctuation adsorption system, the adsorption equipment and the equipment for converting carbon dioxide are communicated with each other without a pressure reducer. ..