JP2005302631A - Fuel cell generating system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem that it is impossible to secure long-time stable operation when impurities, such as sulfur oxide, hydrogen sulfide, and flame retardant organic compound, are mixed in air for anode air bleeding of a fuel cell, air for selective oxidation, air for purging during system suspension, and air for heating a shift catalyst. <P>SOLUTION: A fuel cell system comprising a hydrogen generating apparatus is structured such that the cathode off-gas of a fuel cell 4 is used for at least any one of the air for anode air bleeding, air for selective oxidation, air for purging during system suspension, and air for heating a shift catalyst. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a fuel cell power generation system. More specifically, in a fuel cell power generation system having a fuel cell and a hydrogen generator that generates a hydrogen-rich gas containing carbon monoxide by flowing a raw material containing an organic compound composed of at least carbon and hydrogen and water through a catalyst. Further, the present invention relates to the removal of impurities contained in the air flowing through the anode of the hydrogen generator or the fuel cell.

  A polymer electrolyte fuel cell (PEFC) generates electricity from oxygen and hydrogen. Since impurities in the air sent to the cathode of the PEFC cause a voltage drop, a catalytic combustor that removes impurities such as organic solvents has been proposed (see, for example, Patent Document 1).

  Industrial production of hydrogen includes electrolysis of water, and other methods include steam reforming of hydrocarbon gas, partial oxidation, and an autothermal method combining both. In these reforming methods, as exemplified by methane, ethane, propane, butane, city gas, LP gas, other hydrocarbon gases (including a mixed gas of two or more hydrocarbons), and alcohols such as methanol. A raw material containing an organic compound composed of at least carbon and hydrogen is reformed to generate a hydrogen rich gas. In either case, a reformer is used. Carbon monoxide (CO) content in the hydrogen-rich gas supplied to PEFC is limited to about 50 ppm (capacity, the same applies hereinafter), and battery performance deteriorates significantly beyond this, so before CO is introduced into PEFC It should be removed as much as possible. In the reformed gas generated in the steam reforming, in addition to unreacted methane, unreacted steam, and carbon dioxide, about 8 to 15% (capacity, the same applies hereinafter) of CO is by-produced. For this reason, the hydrogen-rich gas is introduced into the shift reaction unit in order to remove the by-product CO. In the shift reaction section, CO is changed to carbon dioxide and hydrogen by the shift reaction (1). Also for the hydrogen-rich gas obtained through the shift reaction part, CO is not completely removed, and a trace amount of CO is contained. For this reason, an oxidant gas such as air is added, and CO is reduced to 50 ppm or less, preferably 10 ppm or less by the CO selective oxidation reaction (2) in the CO selective oxidation unit. The purified hydrogen-rich gas is supplied to the PEFC anode. However, in order to prepare for a case where the CO concentration increases, such as when the load changes, air breathing is often performed to further supply air to the anode and suppress CO poisoning of the anode electrode catalyst.

CO + H ₂ O → CO ₂ + H ₂ (1)
CO + 1 / 2O ₂ → CO ₂ (2)
In a domestic fuel cell, in order to improve efficiency, it is desirable to stop the device when power consumption is small. At this time, if fuel or hydrogen remains in the system, there is a safety problem, so it is necessary to purge with nonflammable gas. In home fuel cells, it is difficult to permanently install N ₂ cylinders, etc., so a hydrogen generation device, a method of purging air in the temperature range where water vapor is purged after the anode flow path is not condensed, and the like have been proposed. Yes.

  Other than the fuel cell cathode, the air in the hydrogen generator, the fuel cell electrode catalyst, air is sent to the anode air breathing air, selective oxidation air, purge air when the system is stopped, shift catalyst heating Air. The amount of air used in these is less than that of cathode air. For example, in the case of a 1 kW fuel cell, the amount of air supplied to the cathode is 65 Nl / min (hereinafter referred to as 0 ° C., 1 atm equivalent value). In contrast, the supply amount of anode air breathing air, selective oxidation air, purge air when the system is stopped, and shift catalyst heating air is 0.3 Nl / min, 0.5 Nl / min, 20-100 Nl / times, The order is 0.5NL / min. Examples of impurity components contained in the air include inorganic gases such as sulfur oxide, hydrogen sulfide, nitrogen oxide, and ammonia, and organic gases such as amines, fatty acids, aromatic compounds, and aldehydes. The concentration of these gases is generally several tens of ppm to several ppb. In addition, since the reaction that occurs at the anode during air breathing and the reaction that occurs at the CO selective oxidation unit is an oxidation reaction, there is little influence in the case of an easily combusted substance such as acetaldehyde. The catalyst operating temperatures are about 700 ° C., about 300 ° C., and about 150 ° C. for the reforming catalyst, shift catalyst, and selective oxidation catalyst, respectively, which are higher than the cathode electrode catalyst operating temperature (70-80 ° C.). . In general, adsorption is less likely to occur at higher temperatures, and therefore, compared to the temperature alone, poisonous substances are more likely to be adsorbed to the cathode electrode catalyst than the catalyst of the hydrogen generator device. However, when a permanent poisonous substance such as sulfur oxide or hydrogen sulfide comes to each catalyst in the hydrogen generator, the active site of the catalyst is covered and subjected to long-term exposure such as tens of thousands of hours. Eventually it appears as a characteristic degradation. In addition, a flame-retardant organic substance does not burn even if it has a high oxidation activity such as a noble metal unless it is 200 ° C. or higher. For example, when toluene or the like is contained in the air, it remains on the anode electrode catalyst or the selective oxidation catalyst and acts as a poisoning substance.

For this reason, the inventors have proposed that in a fuel cell power generation system including a hydrogen generator and an air supply unit that mixes air between fuel cells, an impurity removing unit that removes impurities contained in the air is provided. (For example, see Japanese Patent Application No. 2003-405016 of an unpublished company application).
JP 2000-277139 A

  However, the invention described in Patent Document 2 has a problem that it is necessary to separately provide an impurity removing means for removing impurities contained in the air.

  Even when the air that has passed through the impurity removal filter sent to the cathode is branched and used, the amount of air used for the cathode is large (for example, 65 Nl / min), so it is completely in the air unless the filter is frequently replaced. There is also a problem that it is difficult to remove the impurities.

In order to solve the above problems, a fuel cell system of the present invention includes a reforming unit that generates a hydrogen-rich gas containing carbon monoxide from a raw material and water, hydrogen and carbon dioxide from carbon monoxide and water in the hydrogen-rich gas. A hydrogen generating device comprising: a shift reaction unit that generates a carbon monoxide; and a carbon monoxide removal unit for reducing carbon monoxide in the hydrogen-rich gas that has not been removed in the shift reaction unit;
A fuel cell that generates power using the hydrogen-rich gas supplied from the hydrogen generator and an oxidant gas containing oxygen;
An air supply unit for supplying air to the hydrogen generator,
A cathode off gas of the fuel cell is supplied to the air supply unit.

The fuel cell system of the present invention includes a reforming unit that generates a hydrogen-rich gas containing carbon monoxide from a raw material and water, and a shift reaction that generates hydrogen and carbon dioxide from the carbon monoxide and water in the hydrogen-rich gas. And a hydrogen generator having a carbon monoxide removing unit for reducing carbon monoxide in the hydrogen-rich gas that has not been removed in the shift reaction unit,
A fuel cell that generates power using the hydrogen-rich gas supplied from the hydrogen generator and an oxidant gas containing oxygen;
(1) Between the reforming section and the shift reaction section, (2) Between the shift reaction section and the carbon monoxide removal section, and (3) Between the carbon monoxide removal section and the fuel cell. An air supply part for supplying air to at least one place in between,
A cathode off gas of the fuel cell is supplied to the air supply unit.

  According to the fuel cell power generation system of the present invention, high characteristics can be maintained over a long period of time by using the cathode offgas of the fuel cell from which impurities are sufficiently removed in the air sent to the anode and the hydrogen generator. .

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

(Embodiment 1)
FIG. 1 is an explanatory diagram showing a schematic configuration of a fuel cell system according to Embodiment 1 of the present invention.

  In FIG. 1, fuel that has passed through an adsorbent that adsorbs and removes sulfur components in the fuel is mixed with water, heated, and introduced into the reforming reaction section 1 filled with a Ru / alumina catalyst. Although the temperature of the reforming catalyst depends on the fuel, in the case of city gas, the temperature immediately after the reforming catalyst outlet is maintained at about 650 ° C. A shift reaction unit 2 filled with a Pt / CeZrOx catalyst, which is an oxidation-resistant water gas shift reaction catalyst, is installed on the downstream side of the reforming reaction unit 1. A shift catalyst heating air supply unit 13 is installed downstream of the reforming reaction unit 1. A CO selective oxidation air supply unit 8 is installed downstream of the shift reaction unit 2, and a CO selective oxidation reaction unit 3 filled with Ru / alumina catalyst is installed downstream of the CO selective oxidation air supply unit 8. Has been. An anode air breathing air supply unit 9 is installed downstream of the CO selective oxidation reaction unit 3, and a polymer electrolyte fuel cell 4 is installed downstream of the anode air breathing air supply unit 9. . Examples of the fuel cell anode catalyst include a Pt—Ru / C catalyst. The cathode air passes through the impurity removing means 5, passes through the total heat exchanger 11, is humidified, and then sent to the cathode of the fuel cell 4. Oxygen in the air sent to the cathode is used for the reaction and discharged as cathode offgas 15. Depending on the fuel cell operating conditions, the cathode off-gas has an oxygen concentration of 13-14% (dry base) and a dew point of 65 ° C. The cathode off-gas is heat-exchanged with the air supplied to the cathode in the total heat exchanger 11, dehumidified, and then through the CO selective oxidation air supply unit 8 and the anode air breathing air supply unit 9, the CO selective oxidation reaction unit. 3. It is sent to the anode electrode of the polymer electrolyte fuel cell 4 respectively. For example, the supply amounts here are 0.5 Nl / min and 0.3 Nl / min, respectively. In addition, since it is dangerous if flammable gas such as hydrogen or city gas remains in the system when the fuel cell is stopped, it is necessary to purge the system system with air. In the present invention, after the apparatus is cooled to a temperature at which Ru of the reforming catalyst is not oxidized at the time of stoppage, the valve 7 is opened, and the cathode off gas is supplied to the reforming section reaction section 1 through the purge air supply section 6. Part 2, CO selective oxidation reaction part 3, and polymer electrolyte fuel cell 4 are sequentially replaced with air. The amount of air to be replaced here is, for example, 2 Nl / min for 10 minutes. Here, the cathode off gas is stored in the reservoir tank 14, and purge air is sent from the reservoir tank 14 through the pump 12 when stopped. In addition, when the load is low, such as 300 W, the amount of gas flowing in the hydrogen generator is reduced, making it difficult to keep the shift catalyst at the reaction temperature. In such a case, the shift catalyst is heated through the shift catalyst heating air supply unit 13. The amount of air added here is, for example, 0.3 Nl / min. The anode off-gas 16 is sent to the reforming and heating burner 17 together with air after the moisture is removed, burned, and used for heating the hydrogen generator 18.

  When considering permanent poisoning to the noble metal catalyst, it is said that the poisoning substance will cause remarkable poisoning if it covers about 1 to 1/2 of the exposed surface of the noble metal. In a 1 kW class fuel cell, Pt and Ru used in the anode catalyst are about 0.02 mol each. When air containing 0.5 ppm of hydrogen sulfide is sent as air breathing at 0.3 Nl / min, thousands to tens of thousands Accumulated amount that affects the catalyst over time. In the case of the selective oxidation reaction, it is necessary to send air of 0.5 Nl / min in consideration of processing a gas containing several thousand ppm of CO in a 1 kW class fuel cell, and 200 cc of a Ru 6 g / l honeycomb was used. In this case, when air containing 0.5 ppm of hydrogen sulfide is sent, the accumulated amount affects the catalyst in several hundred to several thousand hours. In addition, in order to heat the precious metal shift catalyst during low load operation, air may be supplied to the shift catalyst to cause catalytic combustion. However, when a Pt / CeZrOx catalyst is used as the precious metal shift catalyst, Since it is in a reducing atmosphere, the sulfur compound is discharged as hydrogen sulfide and poisons the selective oxidation catalyst.

The influence of SO ₂ on the selective oxidation catalyst was almost the same. The influence on the shift catalyst depends on the temperature and gas atmosphere, but in the oxidizing atmosphere, it is accumulated on the carrier as sulfate, and in the reducing atmosphere, it is discharged as hydrogen sulfide.

  When the hydrogen generator purge air is sent from the reforming reaction section inlet, the Ru catalyst as the reforming catalyst is most significantly affected. Considering the case of using 300 g of a 2 wt% Ru / alumina catalyst, the Ru amount is 0.06 mol, and when 100 Nl of air containing 0.5 ppm of hydrogen sulfide is sent in one purge, several hundred to several thousand times The amount of accumulation that affects the catalyst by purging. If the activation is stopped every day, the activation is stopped 3650 times in 10 years, so this effect cannot be ignored for long-term use. Although hydrogen sulfide has been described here, in the case of hydrogen sulfide, the concentration in the air can be 0.05 to 10 ppm only in a volcanic area or a hot spring area. However, since sulfur oxides are easily converted to hydrogen sulfide in a reducing atmosphere in which hydrogen is present on the noble metal catalyst, the influence of sulfur oxides may be equal to hydrogen sulfide depending on the catalyst type and gas atmosphere. Incidentally, since the environmental standard for the concentration of sulfur oxide is 0.04 ppm, in the case of a fuel cell installed in the vicinity of a road with a lot of traffic, there is a possibility that it will be affected in the long term.

  In addition, toluene contained in paints and the like is flame retardant, and even if a noble metal catalyst is used, it hardly oxidatively decomposes up to 200 ° C. Therefore, it remains on the catalyst at a temperature at which the anode electrode catalyst or the selective oxidation catalyst operates, Acts as a poisonous substance. The regulation standard of toluene in the Odor Control Law is 10 ppm in the first type area, and the influence is great in places where the odor of paint is always floating. Similarly, when flame retardant organic substances (including amines and fatty acids) are always present, it becomes a factor of catalyst poisoning. The influence of nitrogen oxides on the catalyst is reversible, and even if nitrogen oxides are present on the catalyst, they are easily desorbed, so that they do not remain on the catalyst and the influence is small.

  In the cathode, a Pt / C catalyst is usually used, and the potential is maintained around 0.7V. High potentials generally tend to cause oxidation reactions. For example, in the case of a sulfur compound, the standard potential (vs. SHE) represented by the following formula is obtained.

Since the cathode potential is about 0.7 V as described above, the reaction proceeds to the left when the influence of pH and the like is excluded and only the potential is considered, and the cathode electrode does not function as a poison for the sulfur compound. In the cathode electrode catalyst, SO ₄ ^2- does not particularly act as a poisoning substance. Further, no decrease in potential is observed even when SO ₂ is supplied to the cathode.

  Since oxygen is present at the cathode, organic compounds and the like are oxidized into oxygen-containing compounds such as aldehydes and fatty acids. Since the oxygen-containing compound is easily hydrogen bonded, it is easily dissolved in water. Since a large amount of water is present at the cathode, even if impurities have passed through the impurity removal means of the cathode, they are eventually dissolved in the water in the battery. Further, since the catalyst used for the cathode is Pt, aldehyde or the like may be adsorbed on the Pt surface depending on the potential. Thus, the impurities in the cathode off gas are extremely reduced.

  The Ru catalyst used as the reforming catalyst is easily oxidized, but oxidation is less likely to occur when air with a low oxygen concentration is used, such as cathode offgas, and oxidative deterioration due to air purge can be suppressed.

  In particular, when a cathode off gas is used for purging a hydrogen generator, it is desirable to remove water. This is to prevent the catalyst of the hydrogen generator from being immersed in water and resulting in a decrease in activity.

  Furthermore, when air is put into the shift catalyst, the cathode offgas is warm (several tens of degrees) due to the reaction heat of the fuel cell, and because it is humidified, the thermal efficiency is slightly increased, and CO is low from the viewpoint of reaction equilibrium. This direction is desirable.

  Hereinafter, the fuel cell power generation system and the operation method thereof according to the present invention will be described more specifically based on examples.

(Example 1)
To the Pt / C catalyst, water and a perfluorosulfonic acid ionomer ethanol solution (Flemion: 9 wt% perfluorosulfonic acid ionomer) manufactured by Asahi Glass were added to prepare a catalyst ink. The weight ratio between Flemion and carbon black was set to 1. This catalyst ink was applied to carbon paper by a doctor blade method so as to be Pt 0.3 mg / cm ^2, and dried at 60 ° C. to prepare cathode side carbon paper. On the other hand, the anode side carbon paper was prepared by the same method so that Pt was 0.3 mg / cm ² by 30 wt% Pt-24 wt% Ru / C. The Nafion 112 membrane (manufactured by DuPont) was sandwiched between the two gas diffusion electrode layers thus prepared, and hot-pressed at 130 ° C. to prepare a membrane / electrode assembly (MEA). Using air and hydrogen, the battery was operated at 0.2 A / cm ² at an oxygen utilization rate of 40%, a hydrogen utilization rate of 70%, a cell temperature of 75 ° C., a cathode dew point of 65 ° C., and an anode dew point of 70 ° C. After the start of power generation, the air supplied to the cathode of the fuel cell to generate electricity by adding SO ₂ of 50 ppb, were analyzed SO ₂ in the cathode off-gas. As a result, it was confirmed by gas chromatograph that SO ₂ decreased to an analysis limit value (20 ppb) or less. A similar experiment was conducted with 100 ppb of NO ₂ , but no decrease in voltage was observed even after 1000 hours. As described above, SO ₂ and NO ₂ were effectively removed in the cathode off gas.

In the same preparation method, a 1 kW class stack is created, air containing 500 ppm of SO ₂ is supplied to the cathode, a simulated off gas containing 50 ppm of CO is flowed to the anode, and the cathode off gas is used as the air for anode air breathing. It was. Even after 1000 hours, no voltage drop was observed due to the SO ₂ poisoning of the anode catalyst.

Also, a fuel cell system in which the cathode off-gas of the stack after the SO ₂ is removed is used as anode air breathing air, selective oxidation air, purge air when the system is stopped, and shift catalyst heating air. Prototype. The system was repeatedly started and stopped 200 times, and the cathode off gas was used for the above four applications. After starting and stopping 200 times, each catalyst (reforming catalyst, shift catalyst, selective oxidation catalyst, anode catalyst) of the hydrogen-rich gas distribution path to which the cathode off gas was supplied was taken out, and the characteristics were examined with a microreactor. No deterioration in characteristics due to SO ₂ was observed.

  As described above, since the concentration of impurities in the air supplied to the cathode of the fuel cell decreases in the cathode off gas, this cathode off gas can be used for supplying air to the hydrogen-rich gas distribution path in the fuel cell system. The permanent poisoning of each catalyst in the hydrogen-rich gas distribution path in the system is suppressed, and a more stable long-term operation of the fuel cell becomes possible.

  The fuel cell power generation system of the present invention is useful as a fuel cell power generation system capable of stable operation for a long period of time.

The figure which shows the outline of the fuel cell power generation system in Embodiment 1 of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Reforming reaction part 2 Shift reaction part 3 CO selective oxidation reaction part 4 Solid polymer fuel cell 5 Impurity removal means 6 Purge air supply part 7 Valve 8 CO selective oxidation air supply part 9 Anode air breathing air supply part DESCRIPTION OF SYMBOLS 10 Cathode air supply part 11 Total heat exchanger 12 Pump 13 Air supply part for shift catalyst heating 14 Reservoir tank 15 Cathode off gas 16 Anode off gas 17 Burner 18 Hydrogen generator

Claims (3)

Removal at the reforming unit that generates a hydrogen-rich gas containing carbon monoxide from the raw material and water, the shift reaction unit that generates hydrogen and carbon dioxide from the carbon monoxide and water in the hydrogen-rich gas, and the shift reaction unit A hydrogen generator having a carbon monoxide removal unit for reducing carbon monoxide in the hydrogen-rich gas that has not been formed;
A fuel cell that generates power using the hydrogen-rich gas supplied from the hydrogen generator and an oxidant gas containing oxygen;
An air supply unit for supplying air to the hydrogen generator when the fuel cell operation is stopped,
A fuel cell power generation system, wherein a cathode off gas of the fuel cell is supplied to the air supply unit. Removal at the reforming unit that generates a hydrogen-rich gas containing carbon monoxide from the raw material and water, the shift reaction unit that generates hydrogen and carbon dioxide from the carbon monoxide and water in the hydrogen-rich gas, and the shift reaction unit A hydrogen generator having a carbon monoxide removal unit for reducing carbon monoxide in the hydrogen-rich gas that has not been formed;
A fuel cell that generates power using the hydrogen-rich gas supplied from the hydrogen generator and an oxidant gas containing oxygen;
On the basis of the flow direction of the fuel, (1) between the reforming section and the shift reaction section, (2) between the shift reaction section and the carbon monoxide removal section, and (3) the monoxide. An air supply unit that supplies air to at least one location between the carbon removal unit and the fuel cell;
A fuel cell power generation system, wherein a cathode off gas of the fuel cell is supplied to the air supply unit. The fuel cell power generation system according to claim 1, further comprising a moisture removing unit that removes moisture in the cathode offgas.

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