JPH03295175A - Removing method for carbon dioxide - Google Patents

Abstract

PURPOSE:To increase the hydrogen partial pressure in the hydrogen raw material reform gas and improve the removing efficiency of carbon dioxide by bringing an alkaline aqueous solution into direct contact or indirect contact via a permeable gas diffusing film in advance with the hydrogen raw material reform gas fed to a hydrogen electrode. CONSTITUTION:A gas separator 5 is formed with hydrogen feed grooves 5a and alkaline water feed grooves 5b in turn on a plate-shaped main body made of a metal, the hydrogen feed grooves 5a are communicated by two gas communicating paths 5c, and the alkaline water feed grooves 5b are communicated by two alkaline water communicating paths 5d respectively. When the hydrogen raw material reform gas is fed to the hydrogen feed grooves 5a and a KOH aqueous solution is fed to the alkaline water feed grooves 5b, the hydrogen raw material reform gas is once fed into a gas diffusing film 4A then brought into contact with the KOH aqueous solution in the adjacent alkaline water feed grooves 5b. The carbon dioxide in the hydrogen raw material reform gas is made K2CO3 by reaction and removed to the outside of the system together with the KOH aqueous solution.

Description

[Detailed Description of the Invention] <Industrial Application> The present invention provides a method for removing carbon dioxide that can increase the partial pressure of hydrogen in reformed hydrogen gas supplied to a hydrogen electrode in a fuel cell or an electrolyzer. Regarding.

<Prior Art> It is well known that hydrogen gas, which is effective as a reducing gas for metals, etc., can also be used as a reaction gas for fuel cells. This fuel cell does not require the use of fossil fuels, which have resource depletion issues, generates almost no noise, and has excellent features such as being able to recover energy much more efficiently than other energy engines. Because of this, it is used as a relatively small power generation plant for each building or factory, for example.

In recent years, it has been considered to use this fuel cell as a power source for a motor that operates in place of an internal combustion engine in a vehicle, and to use this motor to drive a vehicle or the like. What is important in this case is that it is natural to reuse the substances produced by the reaction as much as possible, and as it is clear from the fact that it is for automotive use, although a large output is not required, It is desirable to be as small as possible.

By the way, when hydrogen gas is obtained from methanol, which is commonly used as a hydrogen raw material for fuel cells, a multi-tubular fixed bed reactor is usually used as a reformer, and hydrogen gas is produced through a high temperature heat medium obtained from a heat medium boiler etc. methanol is reformed with steam through an endothermic reaction. This modification reaction is expressed by the following formula.

C) (30H+nH20-=(1-n)C0eCO2+
(2+n)H2 However, 0 < n < 1 In other words, there is carbon monoxide (CO) in the hydrogen raw material reformed gas.
and carbon dioxide (Co2).

Here, catalysts such as Pt contained in the electrodes of solid polymer electrolyte membrane fuel cells are easily poisoned by CO, especially when operated at low temperatures of about 100°C, so generally CO2 in the hydrogen raw material reformed gas Processing is being done to change it.

<Problems to be Solved by the Invention> Therefore, in general, the hydrogen raw material reformed gas obtained by the methanol reforming reaction contains about 25% CO2.

However, since cell performance improves when the hydrogen partial pressure in the hydrogen raw material reformed gas supplied to the fuel cell is as high as possible, especially in solid noble electrolyte membrane fuel cells aiming at miniaturization, hydrogen raw material reformed gas is supplied to the fuel cell. It is desired to improve the hydrogen partial pressure in quality gas. Furthermore, in alkaline fuel cells, it is necessary to completely remove CO.

In view of these circumstances, an object of the present invention is to provide a carbon dioxide removal method that can increase the hydrogen partial pressure in the reformed hydrogen raw material gas.

Means for Solving the Problems> The present invention is characterized in that the hydrogen raw material reformed gas supplied to the hydrogen electrode is brought into contact with an alkaline aqueous solution in advance, either directly or indirectly through an air-permeable gas diffusion membrane.

<Function> When the reformed hydrogen raw material gas is brought into contact with an aqueous alkali solution directly or indirectly, the carbon dioxide in the reformed hydrogen raw gas is removed by reacting with the alkali in the aqueous alkaline solution, and the reformed hydrogen raw gas is removed. hydrogen partial pressure increases.

EXAMPLE Hereinafter, an example in which the present invention is applied to a solid polymer electrolyte membrane fuel cell will be described.

FIG. 1 conceptually shows an example of a solid polymer electrolyte membrane fuel cell main body.

In FIG. 1, 1 is a solid polymer electrolyte membrane and 2A.

2B is a gas diffusion electrode, and 2A is a gas diffusion electrode.

2B are reaction membranes 3A, 3B and gas diffusion membrane 4A, respectively.
Consists of 4B. Further, 5 and 6 are gas separators.

The gas separator 5 alternately has hydrogen supply grooves 5a for supplying hydrogen to the gas diffusion electrode 2A to become hydrogen, and alkaline water supply grooves 5b for flowing a potassium hydroxide (KOH) aqueous solution as an alkaline aqueous solution. The gas separator 6 has an oxygen supply groove 6a for supplying oxygen to the gas diffusion electrode 2B serving as an oxygen electrode.

Here, the gas diffusion electrodes 2A and 2B contain platinum with an average particle size of 50, hydrophilic carbon black with an average particle size of 450, and polytetrafluoroethylene with an average particle size of 0.3μ.
Hydrophilic reaction membrane 3A with a ratio of 7: 7: 3, 3.8
and hydrophobic carbon black with an average particle size of 420
Polytetrafluoroethylene with a l diameter of 0.3 μ is 7:
It is composed of hydrophobic gas diffusion membranes 4A and 4B having a ratio of 3:3. Hydrophilic reaction membranes 3A, 3B and hydrophobic gas diffusion membrane 4A.

4B can be obtained by mixing raw material powders other than platinum with a solution of solvent naphtha, alcohol, water, two hydrocarbons, etc., and then compression molding the mixture. The gas diffusion electrodes 2A and 2B are manufactured by stacking these and rolling them to support PtO, 56W@, and Ci on the hydrophilic reaction membranes 3A and 3B using a platinum chloride redox method. Ru.

On the other hand, as the solid polymer electrolyte membrane 1, 0.1711
1111FJ perfluorosulfonic acid polymer membrane (
Nafion 117 (manufactured by DuPont) was used.

Then, the solid polymer electrolyte membrane 1 is sandwiched between the gas diffusion electrodes 2A and 2B and hot pressed to form a bonded body, which is further sandwiched between two gas separators 5 and 6 to form a fuel cell body.

Here, an example of the gas separator 5 will be explained with reference to FIG. 2. FIG. 2 shows the appearance of the joint surface of the gas separator 5 with the gas diffusion electrode 2A. As shown in the figure, the gas separator 5 has hydrogen supply grooves 5a and alkaline water supply grooves 5b alternately formed in a plate-shaped body made of, for example, metal, and each hydrogen supply groove 5a has two gas The communication passages 5C and each alkaline water supply groove 5b are communicated with each other by two alkaline water communication passages 5d.

Note that the gas separator 6 also has a similar structure to the gas separator 5.

In such a configuration, the hydrogen supply groove 55L and the alkaline water supply groove 5b of the gas separator 5 are connected to each other through a hydrophobic gas diffusion membrane 4A that contacts the gas separator 5, that is, a gas diffusion membrane that is water-impermeable and air permeable. This means that they are communicating. That is, when the hydrogen raw material reformed gas is supplied to the hydrogen supply groove 5a and the KOH aqueous solution is supplied to the alkaline water supply groove 5b, the hydrogen raw material reformed gas is supplied to the gas diffusion membrane 4.
After that, it comes into contact with the KOH aqueous solution in the adjacent alkaline water supply groove 5b. Then, carbon dioxide in the hydrogen raw material reformed gas is converted to 2C by the reaction shown in the following equation.
It is removed from the system as O3 together with the KOH aqueous solution.

2KOH + CO2 → 2CO3 + H20 In other words, with such a configuration, the CO2 concentration in the hydrogen raw material reformed gas supplied to the interface with the solid polymer electrolyte membrane 1 decreases, and conversely, the hydrogen partial pressure increases. It turns out.

FIG. 3 shows the entire system of a solid polymer electrolyte membrane fuel cell using the cell body just described.

As shown in the figure, the methanol reformer 13 is supplied with methanol reformed gas supplied to the hydrogen electrode 12 of the fuel cell main body 11.
Manufactured in The methanol reformer 13 consists of a reforming section 14 and a preheating section 15. The reforming section 14 is heated by combustion of combustion gas consisting of unreacted gas and air from the hydrogen electrode 12, and the preheating section 15 is heated by combustion of combustion gas consisting of unreacted gas and air from the hydrogen electrode 12. The mass part 14 is heated by the exhaust gas of the combustion gas that has heated it. This preheating section 15 is connected to a methanol tank 17 via a reforming methanol supply pipe 16, and in the middle of the reforming methanol supply pipe 16, methanol 18 in the methanol tank 17, which is a raw material for reformed gas, is connected. A pump 20 driven by a motor 19 for pumping the methanol to the methanol reformer 113
is installed. Further, the other end side of a water supply pipe 22 whose one end side communicates with a water tank 21 is connected to the middle of the reforming methanol supply pipe 16.
A pump 25 driven by a motor 24 is installed in the middle of the methanol 18 for pumping water 23 in the water tank 21, which serves as a raw material for reforming, into the reforming methanol supply pipe 16.

Therefore, while the reforming raw material consisting of methanol 18 and water 23 passes through the preheating tube 26 in the preheating section 15, it loses heat between it and the high temperature combustion exhaust gas generated by the combustion of the above-mentioned combustion gas. Preheat to about 200°C to 500°C by exchanging. Then, the preheated reforming raw material is gasified in the reforming section 14, passes through the reformed gas generation '1127, and comes into contact with the reforming catalyst filled in the reformed gas generation pipe 27 under heating. Therefore, it is modified by the following modification reaction.

C) (30H+nH20-(1-n)CO+nCO2+
(2+n)H2 However, O < n < 1 In such reforming, the mixing ratio of methanol 18 and water 23 is 0.05 water to 1 mole of methanol.
It is desirable to set the amount to about 5 moles. In addition, in order to carry out the reforming reaction of the raw material gas efficiently (in order to carry out the reforming reaction, the pressure inside the reformed gas generation pipe 27 must be set to l per square centimeter).
It is desirable to set the weight to about 100 kg to 20 kg, and to set the temperature inside the reformed gas generation tube 27 to about 200°C to 600°C.

Note that the reforming catalyst includes, for example, one containing at least one element among platinum (pt), balanrum (Pd), rhodium (Rh), and nickel (Ni), or copper (Cu) and zinc (Zn). and chromium (Cr).

Further, when the methanol reformer 13 is started, methanol 18 in the methanol tank 17 is supplied in place of the unreacted gas from the battery main body 11 used as combustion gas.

That is, a starting methanol supply pipe 28 is provided which connects the 4th methanol tank 17 to the reforming section.
9 is provided. This starting device 29 includes a starting fuel supply pump (not shown) for pressure-feeding methanol 18 in the methanol tank 17 to a nozzle section (not shown) in the reforming section 14, and methanol 18 supplied from the starting fuel supply pump. It is equipped with methanol vaporizers (not shown) for evaporating and vaporizing the methanol and sending it to a nozzle part (not shown).

On the other hand, a first CO reduction device 30 is provided so as to communicate with the reformed gas outlet side of the methanol reformer 13. This first CO reduction device 30 is filled with a CO shift catalyst for reducing CO in the reformed gas generated by the reforming reaction within the reformed gas generation pipe 27. Note that examples of the CO shift catalyst include those containing at least one of copper (Cu) and zinc (Zn).

Here, in the CO shift process in the first CO reduction device 30, CO is converted to CO2 by reaction with HO, and CO
The concentration is reduced to about 1%.

Further, a reformed gas supply pipe 31 communicating with the first CO reduction device 30 is connected to a second CO reduction device 32. In this second CO reduction device 32, air is introduced into the reformed gas to further reduce CO, which has reached about 1% as described above, to about 100 ppm (
selective oxo) is being carried out.

The reformed gas with reduced CO in this way is usually humidified by the humidifier 33 and then introduced into the hydrogen electrode 12 side of the fuel cell main body 11 .

Of the reformed gas sent to the hydrogen electrode 12 of the fuel cell main body 11 in this way, excess unreacted gas (well, the fuel cell main body 11 and the reforming section 14 of the methanol reformer 13 are The unreacted gas is supplied to the reforming section 14 via the unreacted gas supply pipe 34.

On the other hand, the air supply pipe 3 is connected to the oxygen electrode 35 of the fuel cell main body 11.
6, a blower 37 is connected to the blower 37.
The pressurized air from 7 is forced to be sent to the oxygen electrode 35 side.

Then, this air becomes a state containing reaction product water on the oxygen electrode 35 side in the fuel cell main body 11 and is supplied to the air/moisture IllN 38 connected to the oxygen electrode 35, and the water in this air flows through the water recovery pipe 39. The gas is collected in the water tank 21 via the water tank 21, and the gas is discharged to the outside through the exhaust pipe 40.

Here, the blower 37 is driven by a blower drive motor 42 supplied with electricity from a storage battery 41 serving as a power source. Note that the storage battery 4] is equipped with a first CO reduction device 30.
and the second CO reduction device 32.
Electricity generated by a generator 44 installed in the exhaust turbine 43 and driven by an exhaust turbine 43 is stored. Further, a second air supply pipe 45 branching from the air supply pipe 36 from the blower 37 is connected to the methanol supply pipe 28.
The second air supply pipe 45 communicates with the reforming section 1 of the methanol reformer 13 as described above.
Air, which becomes the combustion gas in step 4, is supplied.

Note that the motors 19 and 24 are also driven by electricity supplied from the storage battery 41, similar to the blower drive motor.

In this embodiment, K in the KOH aqueous solution tank 46 is
The OH aqueous solution 47 is passed through the KOH circulation pipe 48 to the alkaline water communication passage 5d of the gas separator 5 of the fuel cell main body 11 (see FIG. 1).

(see Figure 2).

Further, the circulation of this KOH aqueous solution 47 is carried out in the fuel cell main body 11.
It also serves as a cooling for the KOH released from the gas separator 5.
The aqueous solution 47 is heated. After the KOH aqueous solution 47 is introduced into the humidifier 33, it is returned to the KOH aqueous solution tank 46. In the humidifying device 133, the reformed gas flowing through the reformed gas supply pipe 31 and the heated KOH aqueous solution 47 are in contact with each other via a gas diffusion membrane, and the water vapor corresponding to the temperature of the heated KOH aqueous solution 47 is Steam is added to the reformed gas under pressure. Incidentally, such circulation of the KOH aqueous solution 47 is performed by a pump 50 driven by a motor 49, and the motor 49 is connected to a storage battery 41.
It is designed to be driven by electricity.

When power is generated using such a device, the hydrogen partial pressure in the hydrogen raw material reformed gas sent to the hydrogen electrode 1z increases, so power generation performance is improved.

FIG. 4 shows the case where CO2 is removed with a KOH aqueous solution in the solid polymer electrolyte membrane fuel cell according to the above example, and the case where K
This is a comparison of the battery performance with a case where the OH aqueous solution is not flowed and CO2 is not removed (comparative example) (reformed gas composition: H275%, CO□25%), but the battery performance is different when CO2 is removed. It is clear that this has improved.

In addition, in the above-mentioned apparatus, the KOH aqueous solution tank 4
The KOH aqueous solution 47 in 6 needs to be replaced periodically.

In addition, when carrying out the method of the present invention, the alkaline aqueous solution is K
Not limited to OH aqueous solution, sodium hydroxide (NaOH
) aqueous solution, calcium hydroxide [Ca(OH)2] aqueous solution, etc., as long as they remove CO2. At this time, C
When using a(OH)2 aqueous solution, C02 is CaC0,
Since it becomes a solid as Ca (OH
) 2 aqueous solution can be regenerated.

Furthermore, in the above embodiment, CO2 is removed in the gas separator 5, but for example, the hydrogen raw material reformed gas is brought into contact with an alkaline aqueous solution such as a KO)l aqueous solution in the middle of the reformed gas supply pipe 31, so that CO2 is removed. The hydrogen raw material reformed gas with improved hydrogen partial pressure may be sent to the gas separator 5.

In this case, CO2 is removed by, for example, bubbling hydrogen raw material reformed gas into an alkaline aqueous solution, or introducing hydrogen raw material reformed gas into an alkaline aqueous solution filled in a container partially formed with a gas diffusion membrane. The CO2 concentration may be reduced by taking out the hydrogen reformed gas through a gas diffusion membrane and bringing the hydrogen raw material reformed gas into direct contact with the alkaline aqueous solution.

For example, as shown in FIG. 5, by passing a reformed hydrogen raw material gas containing CO2 together with an alkaline aqueous solution of KOH or NaOH into a chamber 52 whose both sides are formed with gas diffusion membranes 51, only H is absorbed through the gas diffusion membrane. 51 for separation.

In addition, the hydrogen raw material reformed gas and the alkaline aqueous solution are made to come into indirect contact through a gas-permeable gas diffusion membrane.
7 concentration may be reduced.

For example, as shown in FIG. 6, a double tube consisting of an outer tube 53 that does not pass both gas and liquid and an inner tube 54 that allows only gas to pass through is used, and a KOH or Na
While the alkaline aqueous solution 55 consisting of OH is held or circulated, the hydrogen raw material reformed gas containing CO is passed into the inner pipe 54. As a result, the alkaline aqueous solution 55 and C
Since the O-containing hydrogen raw material reformed gas comes into contact with the gas through the gas diffusion membrane 54, CO2 dissolves into the alkaline aqueous solution 55. Also, in this case, alkaline water soluble lol! jL5
The water vapor from 5 is supplied into the hydrogen raw material reformed gas, that is, the excess or deficiency in the water vapor pressure of the reformed gas is added to the alkaline aqueous solution 5.
It also has the effect of being adjusted by the temperature adjustment in step 5.

<Effects of the Invention> As explained above, in the present invention, the hydrogen raw material reformed gas brought into contact with an alkaline aqueous solution directly or indirectly through a gas diffusion membrane is supplied to the hydrogen electrode of a fuel cell, etc. The hydrogen partial pressure in the reformed gas is increased, making it possible to improve the performance of fuel cells and the like.

[Brief explanation of drawings]

Figure 1 is a conceptual diagram showing the solid polymer electrolyte membrane fuel cell main body for carrying out the method of the present invention, Figure 2 is an external view showing the gas separator, and Figure 3 is the entire solid polymer electrolyte membrane fuel cell. FIG. 4 is a conceptual diagram showing the system, FIG. 4 is a graph showing its power generation performance, and FIGS. 5 and 6 are explanatory diagrams showing specific examples of the CO2 removal method. In the drawing, indicates a solid polymer electrolyte membrane, A and 2B are gas diffusion electrodes, A and 3B are reaction membranes, and A and 4B are gas diffusion membranes 1. 6; Yo gas separator, a is the hydrogen supply groove, b is the KOH aqueous solution supply groove, a is the oxygen supply groove, 1 is the fuel cell body, 2 is the hydrogen electrode, 3 is the methanol reformer, 4 is the reforming section, 5 is a preheating section, 6 is a methanol supply pipe for reforming 1, 7 is a methanol tank, 8 is methanol, 1 is a water tank, 3 is water, 0 is the first CO reduction device, 1 is a reformed gas supply pipe, 2 is 2nd CO reduction device, 3 is a humidifying device, 5 is an oxygen electrode, 6 is an air supply pipe, (: is a KOH aqueous solution tank, 7 is a KOH aqueous solution, 8!'1 is a KOH circulation pipe. A B Patent Applicant Mitsubishi Heavy Industries, Ltd. Agent

Claims (1)

[Claims] A method for removing carbon dioxide, characterized in that a reformed hydrogen raw material gas supplied to a hydrogen electrode is brought into contact with an alkaline aqueous solution in advance, either directly or indirectly through an air-permeable gas diffusion membrane.