JP3096686B1 - Fuel cell reformer - Google Patents

Abstract

Abstract: PROBLEM TO BE SOLVED: To provide a reformer for a fuel cell which has excellent startability and transient response and can be downsized. SOLUTION: The fuel cell has a fuel reforming catalyst supported on activated carbon having an electric resistance of 500 Ω / 125 cm ³ or less, and at least one pair of electrodes is provided with the fuel reforming catalyst supported on the activated carbon interposed therebetween. A fuel cell reformer that generates resistance heat by direct energization.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a reformer for a fuel cell, which reforms a hydrocarbon fuel into hydrogen gas. The reformer has excellent startability and transient response and can be downsized. Regarding porcelain.

[0002]

2. Description of the Related Art In recent years,
With the tightening of regulations on harmful exhaust gas (CO _x , NO _x , HC, etc.) in the global environment, research and development of exhaust gas reduction technology and energy utilization high efficiency technology have been activated. Among them, as a technology having the highest energy efficiency and a very small amount of harmful exhaust gas, there is a fuel cell power generation utilizing an electrochemical reaction between hydrogen gas and oxygen gas.

[0003] Hydrogen fuel required for power generation by a fuel cell is usually produced by reforming natural gas, petroleum, coal and the like with steam. For example, when hydrogen is generated from methane, the following formulas (A) and (B): CH ₄ + H ₂ O → 3H ₂ + CO−49.3 kcal / mol (A) CO + H ₂ O → H ₂ + CO ₂ + 9.8 kcal / mol ··· Pass through the process of steam reforming reaction (A) and denaturation reaction (B) as shown in (B). The steam reforming reaction (A) requires a high temperature of about 800 ° C., and a catalyst in which metal Ni is supported on a carrier such as alumina is used as a reforming catalyst. Such a reforming catalyst is installed in a reformer for a fuel cell in the form of a pellet, a powder, a honeycomb monolith, or the like, and the reformer is heated to a catalyst activation temperature region by heating the reformer. A hydrogen fuel can be obtained by passing a gasified hydrocarbon-based fuel and steam at a fixed ratio through a catalyst layer.

Here, as a method of heating the catalyst to the catalyst activation temperature, a method of separately burning fuel and transmitting the obtained heat to the catalyst directly by heat exchange or a heat exchanger, or a method of conducting heat by heat conduction with an electric heater. Is used. However, in the conventional heating method, since it takes a long time from the start of heating to the generation of hydrogen,
The startability and the transient response were inferior to other power generation devices.

On the other hand, a method of shortening the heating time by greatly increasing the amount of heat applied to the catalyst in order to increase the responsiveness of the reformer can be considered. However, there is a problem that the size of the apparatus needs to be increased in order to propagate heat to the heater, and that the heating distribution becomes non-uniform. In addition, the method of generating heat by high-frequency dielectric requires an increase in the size of the apparatus.

On the other hand, a method of generating Joule heat by directly passing an electric current to a substance to be heated requires only providing electrodes at both ends of a substance to be energized, so that a rapid rise in temperature is possible with a compact apparatus. However, the conventional catalyst-carrying carrier (alumina, zeolite, etc.) is an electric insulator (10 ⁶ Ω or more), and the method of direct energization cannot be used.

Accordingly, it is an object of the present invention to provide a fuel cell reformer which is excellent in startability and transient response and which can be downsized.

[0008]

Means for Solving the Problems As a result of intensive studies in view of the above-mentioned objects, the present inventors have found that by using highly conductive activated carbon as a carrier for a fuel reforming catalyst, direct energization is possible and the amount of catalyst carried Based on the knowledge that can be increased, while filling or attaching a highly conductive activated carbon carrying a fuel reforming catalyst to the reformer, and at least a pair of electrodes to provide resistance heating of the highly conductive activated carbon, by providing at least a pair of electrodes, The present inventors have found that it is possible to provide a reformer having a smaller size and higher responsiveness than before, and have reached the present invention.

That is, the reformer for a fuel cell of the present invention comprises:
Just providing electrodes at both ends of the fuel reforming catalyst allows current to flow directly into the carrier itself to generate Joule heat, enabling rapid temperature rise with a compact device and improving startability and transient response. is there.

Furthermore, highly conductive activated carbon used as the carrier, 700 ~2000 m ² / g stuff because it has a large specific surface area, conventional carriers (alumina support: 200 m ² / g, zeolite carrier: to 500m ² / As compared with g), the amount of catalyst carried per unit weight can be increased, and a more compact fuel cell reformer with excellent reforming efficiency can be realized.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION A reformer for a fuel cell according to the present invention is designed to charge or attach a highly conductive activated carbon carrying a fuel reforming catalyst to a reformer and to cause the highly conductive activated carbon to generate resistance heat. At least one pair of electrodes is provided. According to a preferred embodiment of the present invention, as shown in FIG. 1, the reformer 1 is provided with a fuel gas inlet 11 and a gas outlet 12 containing reformed hydrogen gas. An activated carbon 13 loaded with or attached to a catalyst for the purpose of the measurement, and a pair of electrodes 14 for raising the temperature of the activated carbon 13 by resistance heating are provided.

(A) Fuel reforming catalyst A known fuel reforming catalyst can be used without any particular limitation. Specifically, Ni, NiO, NiS, Cu, CuO, Fe ₃ C
_{4, Pt, Pd, Au,} Ru, Cr 2 O 3, ZnO, MgO, CaO, K
Catalysts such as ₂ O, BaO, La ₂ O ₃ , CeO ₂ , and Al ₂ O ₃ can be used. Particularly preferred of these are:
Ni, Cu and Ru. The fuel reforming catalyst may be used alone or in combination of two or more.

(B) Highly conductive activated carbon Highly conductive activated carbon is used as a carrier for supporting a fuel reforming catalyst. In this specification, “highly conductive activated carbon” means activated carbon having an electric resistance of 500 Ω / 125 cm ³ or less. More preferably, the electric resistance is 100 Ω / 125 cm ³ or less.

When the electric resistance of the activated carbon is large, it is preferable to increase the conductivity by heat treatment. The heat treatment is performed before carrying the catalyst.

Activated carbon having a large electric resistance has an amorphous carbon structure similar to graphite exhibiting anisotropic conductivity, and as shown schematically in FIG. 2 (a), has a turbostratic structure and a large amount of non-graphitized carbon. . On the other hand, in the case of graphitized carbon that has been subjected to heat treatment, as shown in FIG.
The conductivity increases in the growth direction. It has been found that an activated carbon having a higher ratio of graphitized carbon tends to have a higher electric resistance.

It is considered that the electric conduction in graphite is transmitted by the wave of π electrons in the sp ² hybrid orbital of the polycyclic aromatic crystal as shown in FIG. 3 (b). On the other hand, as shown in FIG. 2 (b), the activated carbon structure has a large number of functional groups on the surface that inhibit conductivity, so that the activity of π electrons in the polycyclic aromatic crystal is inhibited. It is thought that there is.

Therefore, in order to minimize the number of functional groups that inhibit the activity of π electrons and efficiently transmit the wave generated between π electrons, the activated carbon is graphitized to grow polycyclic aromatic crystals. This is effective for improving electric conductivity.

That is, by using activated carbon whose electric resistance has been reduced to a desired level for the carrier for supporting the catalyst, the temperature of the fuel reforming catalyst can be rapidly raised by the heat generated by energization, thereby reducing the time until hydrogen generation. Can be shortened. In addition, since the start-up time of the fuel cell can be shortened, it is possible to reduce the size of the electric supply (battery, capacitor) required for running before the fuel cell system starts up, and to improve the space efficiency in the engine room. I do. Further, the response to the transient response during acceleration during traveling can be accelerated. Hereinafter, the heat treatment method for activated carbon will be described in detail.

(I) Raw material activated carbon The activated carbon is not particularly limited, and various commercially available activated carbons can be used as they are. Raw materials for activated carbon include wood, coal, petroleum pitch, olives, coconut shells, special phenols, and the like.

Activated carbon is usually produced through a carbonization and activation step involving heating. Examples of the activated carbon activation method include a chemical activation method using phosphoric acid, zinc chloride, an alkali chemical, or the like, and a steam activation method. The maximum heating temperature during the activation treatment is about 600 to 800 ° C.

Of these, activated carbon made from raw materials such as wood and olive has an electric resistance of usually 10 ³ to 10 ⁶ Ω / 125 cm ³ , and is not suitable for electric heating as it is. Meanwhile, coal,
Activated carbon made from raw materials such as coconut shells has an electrical resistance of several tens of ohms
/ 125cm ³ lower than wood-derived activated carbon, but higher than the electrical resistance suitable for resistance heating. Therefore, it is preferable to reduce the electric resistance of any of the activated carbons by heat treatment.
The electric resistance of the activated carbon was measured using the container 2 (50 mm × 50 mm) shown in FIG.
It is a value measured by filling the sample into an mm × 50 mm and an internal volume of 125 cm ³ ). Table 1 shows specific examples of the raw material activated carbon.

Table 1 Raw material activated carbon Electric resistance symbol Raw material activation method T _max (Ω / 125 cm ³ ) a wood phosphoric acid 650 5 × 10 ⁵ b wood phosphoric acid 650 5 × 10 ⁴ c olive zinc chloride 650 2.5 × 10 ⁴ d wood Zinc chloride 650 2 × 10 ³ e Petroleum coke Alkaline 750 50 f Coconut shell Steam 800 40 g Coal steam 800 20 Note: T _max = maximum heating temperature (° C.) during production of activated carbon.

(Ii) Heat treatment Conductive activated carbon is produced by heat treatment in an inert gas atmosphere. By the heat treatment, graphite crystals in the structure of the activated carbon grow, and the electric resistance decreases. The heat treatment temperature needs to be higher than the graphitization temperature of the activated carbon. At a heat treatment temperature lower than the graphitization temperature, graphite crystals do not grow sufficiently in a short time, and a remarkable decrease in electric resistance cannot be obtained. The graphitization temperature of activated carbon is generally about 700 to 1200 ° C.

Generally, the heat treatment time (holding time at the maximum temperature) is preferably 3 hours or less. If the heat treatment time is longer than 3 hours, not only the mechanical strength of the activated carbon decreases, but also the adsorptivity thereof decreases, which is not preferable. A more preferred heat treatment time is 0.25 to 2 hours.

Since the growth of graphite crystal becomes faster as the heat treatment temperature rises, the heat treatment temperature can be shortened by increasing the heat treatment temperature. Therefore, the heat treatment temperature is preferably equal to or higher than the production temperature of activated carbon + 100 ° C. Since the production temperature of activated carbon is usually 600 to 800 ° C, the heat treatment temperature should be 700
The temperature may be set to 900 ° C or higher. There is no particular limitation on the heating rate to the heat treatment temperature and the cooling rate after the heat treatment. To prevent damage to the activated carbon due to thermal stress, the heating time is 0.5 hours or more, and the cooling time to 250 ° C is 2 hours. It is preferable to make the above.

In order to suppress the oxidation reaction of the activated carbon during the heat treatment, the heat treatment is performed in an inert gas atmosphere such as a nitrogen gas atmosphere. Examples of usable inert gas include nitrogen, argon and the like.

(Iii) Characteristics of conductive activated carbon The conductive activated carbon produced by the above method has lower electric resistance than the original activated carbon, and also has improved hardness and mechanical strength.

The electric resistance of the activated carbon after the heat treatment is
Depends on type and heat treatment conditions (temperature, time, etc.)
However, especially when heat-treated at a temperature of 800 ° C or more, 100 Ω /
125cm ^ThreeIt becomes below. For example, wood or olive
When activated carbon is heat-treated at a temperature of 900 ° C, its electrical resistance
Is 10 before heat treatment^Three~Ten⁶Ω / 125cm^ThreeFrom about 100 Ω / 12
5cm^ThreeIt falls below. In addition, coal and coke are used as raw materials.
Electrical resistance of activated carbon is reduced by more than 20% than before heat treatment.
You.

As a specific example, the raw material activated carbon (φ2
(a) Alumina container, (b) set in a firing furnace (KDF75, manufactured by Denken Co., Ltd.), and (c)
While flowing nitrogen gas at a rate of 5 L / min, the temperature was raised to the heat treatment temperature shown in Table 2 over 1 hour, and maintained for the heat treatment time shown in Table 2, (d) over 3 to 4 hours. 25
Table 2 shows the electrical resistance when the activated carbon cooled to 0 ° C. or lower and (e) heat-treated was taken out of the firing furnace and left at room temperature for 3 hours or more.

[0030] Table 2 heat treatment conditions of the activated carbon and electrical resistance materials heat treatment heat treatment time electrical resistance No. Activated carbon Temperature ° C. ^{(time) (Ω / 125cm 3) 1} a 900 2 16 2 a 900 1 16 3 a 650 2 2 × 10 5 4 b 900 2 1 5 c 900 2 2 6 d 900 2 2.7 7 e 900 2 10 8 f 900 2 309 g 900 2 9 10 e 750 2 46.5 11 f 800 238

Since the highly conductive activated carbon obtained by the heat treatment has excellent mechanical strength and hardness, the possibility of breakage even when densely packed in a reformer is extremely small. Therefore, the reformer of the present invention has good durability.

The activated carbon is used after being formed into an appropriate shape such as a pellet, a block, a honeycomb, a powder, a crushed or a sphere according to the use.

[0033]

The present invention will be described in more detail with reference to the following Examples, but it should not be construed that the invention is limited thereto.

Example 1 A molecular sieve carbon having an electrical resistance of 30 Ω / 125 cm ³ was prepared by heat-treating coconut husk activated carbon at 900 ° C. for 2 hours and formed into a pellet having a diameter of 2 mm and a length of 4 mm (hereinafter referred to as MC).

Nickel nitrate hexahydrate [Ni (NO ₃ ) _2.
H ₂ O] 145 parts by weight was dissolved to prepare an aqueous solution of nickel nitrate. After immersing 88 parts by weight of MC in the nickel nitrate aqueous solution for 1 hour, water was removed by drying under reduced pressure using a rotary evaporator to obtain MC containing nickel nitrate. MC containing nickel nitrate at 200 ° C. for 1 hour,
Next, after baking at 420 ° C. for 2 hours, reduction was carried out in hydrogen gas at 200 ° C. for 2 hours to obtain Ni-supported MC (hereinafter, Ni-MC).

Comparative Example 1 Ni-supported PAl was obtained in the same manner as in Example 1 except that alumina having a purity of 96% was formed into a pellet having a diameter of 2 mm and a length of 4 mm (hereinafter referred to as PAl), and MC was changed to PAl. (Hereinafter Ni-PAl).

Evaluation of Physical Properties Table 3 shows the specific surface area, Ni content and average pore diameter of Ni-MC and Ni-PAl obtained in Example 1 and Comparative Example 1. 5 and 6 show the results of the TEM analysis.

Table 3 Evaluation of physical properties Example 1 Example 2 Ni-MC Ni-PAl specific surface area (BET measurement result) 1200 m ² / g 230 m ² / g Ni content (ICP analysis result) 24 wt% 14 wt% Average pore size (XRD analysis result) 30 Å Measurement not possible

According to Table 3, the specific surface area of alumina (Comparative Example 1) is small, so the amount of Ni supported is limited to 14 wt%. Since the alumina surface is entirely covered with Ni and laminated, it is effective for reforming. It can be seen that a fine crystal cannot be formed. In contrast, it can be seen that the highly conductive activated carbon (Example 1) has a larger specific surface area than alumina, so that the amount of supported Ni is large and fine crystals are maintained.

Reforming test As shown in FIG. 7, a test tube column I was filled with 50 mL of Ni-MC, attached to the apparatus shown in FIG.
C was energized, and the temperature of the Ni-MC was set to 800 ± 50 ° C. FIG. 11 shows the relationship between the heating time and the temperature at this time. While maintaining the water bath at a constant temperature of 60 ° C, bubbling methane gas through column I at a flow rate of 10 L / min (humidity (98%) is maintained at a constant temperature by maintaining the temperature at 60 ° C). Was analyzed by gas chromatography. Table 4 shows the analysis results.

As shown in FIG. 9, Ni-PAl was added to test tube column II.
Was charged to the apparatus of FIG. 10, and the column was heated by a heater from the outside of the column to set the Ni-PAl temperature to 800 ± 50 ° C. FIG. 11 shows the temperature rise by the heating conduction method. Also, Ni
A similar apparatus was assembled except that -PAl was changed to Ni-MC, and the temperature rise property was also evaluated. 60 water baths
Bubble methane gas while maintaining constant ℃
Flow through column II filled with Ni-PAl at 10 L / min (temperature 60
Humidity (98%) is also kept constant by keeping the temperature at ℃), and the reformed gas was analyzed by gas chromatography. Table 4 shows the analysis results.

[0042]

From Table 4, it can be seen that Ni-MC has a higher hydrogen gas concentration and is superior in reforming efficiency as compared with Ni-PAl. Also, from FIG.
It can be seen that when heating is performed by the direct energization method, the temperature rise is significantly improved. Therefore, with a compact device,
The starting time of the fuel cell can be reduced.

[0044]

According to the present invention, since the highly conductive activated carbon is used for the carrier of the fuel reforming catalyst, the amount of catalyst carried can be increased as compared with the conventional alumina carrier. Can be reduced in size. Further, the temperature of the fuel reforming catalyst can be rapidly raised by the heat generated by the energization, and the starting time of the fuel cell can be shortened. Accordingly, it is possible to reduce the size of an electric supply device (battery, capacitor) required for traveling before the fuel cell system starts up, and space efficiency in the engine room is improved. Further, the response to the transient response during acceleration during traveling can be accelerated.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view of an example of a preferred embodiment of a reformer for a fuel cell of the present invention.

FIG. 2 shows the structure of activated carbon before heat treatment, (a) schematically shows an amorphous carbon structure, and (b) shows an example of a chemical structure of a polycyclic aromatic crystal in the activated carbon.

FIG. 3 shows a graphitized crystal structure in conductive activated carbon;
(a) schematically shows a graphitized carbon structure, and (b) shows an example of a chemical structure of a polycyclic aromatic crystal in a conductive activated carbon.

FIG. 4 is a perspective view showing an apparatus used to measure the electric resistance of activated carbon.

FIG. 5: TEM of Ni after supporting molecular sieve carbon
It is a photograph (× 800,000).

Fig. 6 TEM photograph of high-purity alumina after supporting Ni (× 80
0,000).

FIG. 7 is a cross-sectional view of a test tube column I filled with a catalyst-supporting activated carbon.

FIG. 8: Test tube column filled with catalyst-supported alumina II
FIG.

FIG. 9 is a schematic diagram of a reforming test device.

FIG. 10 is a schematic diagram of a reforming test device.

FIG. 11 is a graph showing a relationship between a heating time and a temperature of a catalyst.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Reformer 11 ... Inlet 12 ... Outlet 13 ... Activated carbon carrying catalyst 14 ... Electrode 2 ... Container 21 ... Aluminum electrode 22 ... Insulating plate 23・ ・ Electrical resistance meter 3 ・ ・ ・ Test tube column I 31 ・ ・ ・ Catalyst-supported activated carbon 32 ・ ・ ・ Electrode 33 ・ ・ ・ Electrode line 4 ・ ・ ・ Test tube column II 41 ・ ・ ・ Catalyst-supported alumina 51,61 ・・ ・ Methane gas cylinder 52, 62 ・ ・ ・ Flow meter 53, 63 ・ ・ ・ Humidifier 54, 64 ・ ・ ・ Water 55 ・ ・ ・ Power supply 56, 66 ・ ・ ・ Gas chromatograph 65 ・ ・ ・ Temperature control external heater

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H01M 8/04 H01M 8/06 B01J 23/755 C01B 3/40

Claims (5)

(57) [Claims] 1. A fuel cell reformer for reforming a hydrocarbon fuel into hydrogen gas, comprising a fuel reforming catalyst supported on activated carbon having an electric resistance of 500 Ω / 125 cm ³ or less, and A reformer for a fuel cell, wherein at least a pair of electrodes are provided with a fuel reforming catalyst supported on the activated carbon being interposed therebetween, and resistance heat is generated by directly energizing the activated carbon. 2. The reformer for a fuel cell according to claim 1, wherein the activated carbon has a surface area of 700 m ² / g or more. 3. The reformer for a fuel cell according to claim 1, wherein the activated carbon is heat-treated before carrying a catalyst, thereby at least partially growing graphite crystals to reduce the electric resistance. A reformer for a fuel cell, characterized in that: 4. The reformer for a fuel cell according to claim 3, wherein the activated carbon is heat-treated in an inert gas at a temperature higher than a graphitization temperature. 5. The reformer for a fuel cell according to claim 3, wherein the heat treatment time of the activated carbon is 3 hours or less.