JPH08188783A - Method for removing carbon monoxide in reformed gas - Google Patents

Abstract

PURPOSE: To widely and easily reduce the CO content in a reformed gas even in a small-sized apparatus by mixing the reformed gas with oxygen and subsequently passing the mixed gas through a layer of a specific CO-selective oxidation catalyst. CONSTITUTION: At first, a reformed gas obtained from a hydrocarbon raw fuel (e.g. methanol) is mixed with oxygen in a ratio of <=20vol.% based on the reformed gas. Subsequently, the mixed gas is passed through a layer of CO-selective oxidation catalyst consisting of copper oxide and manganese oxide to reduce the CO content in the reformed gas to <=0.1%. The CO-selective oxidation catalyst layer is preferably operated at 50-180 deg.C. Further, the catalyst is preferably used in a state in which the surface activity is lowered in advance. The reduction of catalyst activity is achieved e.g. by exposing the catalyst to the atmosphere for a long time.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention selectively converts CO contained in a reformed gas obtained by reforming a hydrocarbon raw fuel into C
The present invention relates to a method of greatly reducing the CO concentration in a reformed gas by oxidizing it to O ₂ . In particular, in a hydrocarbon reforming system used as a fuel supply for a low temperature operation type fuel cell, the case where it is necessary to reduce the CO content in the reformed gas as much as possible at the final stage is a main field of use thereof. is there.

[0002]

2. Description of the Related Art Hydrocarbon reforming technology is widely used in applications such as chemical plants. A general reforming method is as follows. First, a reforming gas containing H ₂ as a main component is generated from a hydrocarbon as a raw material by a steam reforming reaction or a thermal decomposition reaction using a reforming catalyst. Usually, the target gas is H ₂ , so
Next, a CO conversion reaction in which a large amount of CO contained in the reformed gas is reacted with H ₂ O to convert it into H ₂ and CO ₂ is used. As described above, it is general to obtain a reformed gas having a low CO content and a high H ₂ content by the two kinds of reactions.

Typical reaction formulas of steam reforming reaction, thermal decomposition reaction and CO conversion reaction when methanol is used as a raw material are shown in formulas (1), (2) and (3), respectively. In reality, various reactions other than those shown here are thought to occur in parallel. In any case, the reformed gas obtained as a result of such a reaction contains H ₂ as a main component (70
-80%), and CO ₂ and H ₂ O, CO
Etc. are included.

[0004]

Embedded image

In the reforming carried out in an actual chemical plant or the like, it is often necessary to obtain H ₂ of higher purity. In this case, CO 2 is produced by using a separation technique such as a pressure swing adsorption method (PSA method). Impurity gas such as is removed. Since such reforming of hydrocarbons that is generally performed is reforming with a large-scale apparatus, a reformed gas that does not contain CO or high-purity hydrogen can be obtained by using a large-scale method such as the PSA method. You can

On the other hand, in recent years, hydrocarbons have been used as a raw fuel for fuel cells and reformed to be used as a hydrogen-rich gas. Among the fuel cells, a molten carbonate fuel cell, a high temperature fuel cell such as a solid electrolyte fuel cell,
Since O can also be used as a fuel, it is not necessary to remove CO in the reformed gas. However, in a phosphoric acid fuel cell with an operating temperature of around 200 ° C., CO contained in the reformed gas poisons the platinum catalyst of the electrode, so it is necessary to reduce the CO concentration to at least 0.5% or less. Will occur. If the level is still at this level, the CO concentration can be sufficiently dealt with only by the CO shift reaction by the present advanced CO shift catalyst.

However, in a fuel cell typified by a polymer electrolyte fuel cell which operates at a lower temperature (room temperature to around 100 ° C.), it is necessary to reduce the CO concentration to the ppm order. This is because when the operating temperature of the fuel cell is lowered, the adsorption and poisoning of CO on the platinum catalyst used for the electrode of the fuel cell is likely to occur. In recent years, 100ppm to cope with such phenomenon
Platinum catalysts having excellent resistance to CO levels have been developed. However, it is extremely difficult to reduce the CO concentration below this level only by a general CO conversion reaction.

Also in the solid polymer electrolyte fuel cell,
If the system supplies a fuel to a fuel cell having a large scale to some extent, for example, an output of 10 kW or more, a hydrogen separation method using a palladium membrane, for example, can be adopted in the subsequent stage of CO conversion. However, in this case,
Since it is necessary to pressurize the reformed gas and to purge and remove the separated impurity gas such as CO, it is necessary to perform a batch operation or a switching operation of a plurality of hydrogen separation systems. Further, when the palladium film is used, there are problems that the palladium film itself is deteriorated by high concentration CO, CO _{2 and the} like contained in the reformed gas, and there is a problem of cost.

On the other hand, low-temperature operation type fuel cells such as solid polymer electrolyte fuel cells are expected to be used in various fields due to their advantages such as compactness and high output density. For example, when used with a small reformer, it can be used as a portable power source or a mobile robot power source. Also, utilization as a power source for electric vehicles is being considered. However, when considering the application and development of these relatively small-sized systems, what to do with the hydrogen source, which is the fuel, is practically used, except for the extremely limited use where a hydrogen cylinder or the like can be used as a fuel source. It's a big problem. That is, even if an attempt is made to reform a hydrocarbon to obtain a hydrogen-rich gas, there is a problem that the CO concentration can be reduced to about 0.1% at most by the CO shift reaction. On the other hand, when the separation technique as described above is used, there is a problem that an expensive and large-scale device is unavoidable, which makes it difficult to realize a small system. There has been no CO removal method capable of greatly reducing the CO concentration in an extremely small-scale reformer capable of overcoming such a situation and being excellent in compactness, convenience, and safety.

[0010]

When the application of hydrocarbon reforming to low-temperature fuel cells is considered as described above, the CO concentration in the reformed gas is 0.1% or less, and even 10 ppm order. It is desired to reduce Therefore, an object of the present invention is to provide a simple method for reducing the CO concentration in the reformed gas to the low concentration as described above even in a small-scale apparatus.

[0011]

The CO removal method of the present invention comprises:
Air is mixed with a reformed gas obtained by reforming a hydrocarbon-based raw fuel at a ratio of 20 vol% or less (20 vol with respect to 100 vol of reformed gas) based on the reformed gas, and the mixed gas is oxidized with copper oxide. By passing through the CO selective oxidation catalyst layer made of manganese, the CO concentration in the reformed gas was reduced to 0.
It is to be reduced to 1% or less. Here, it is preferable to operate the CO selective oxidation catalyst layer in a temperature range of 50 to 180 ° C. Further, it is preferable to use the CO selective oxidation catalyst in a state where the surface activity is reduced in advance.

Further, in the CO removal method of the present invention, air is mixed with a reformed gas obtained by reforming a hydrocarbon-based raw fuel at a ratio of 20 vol% or less based on the reformed gas, and the mixed gas is mixed with gold. By passing through a CO selective oxidation catalyst layer having a temperature of 50 to 230 ° C. and containing iron oxide, the CO concentration in the reformed gas is reduced to 0.1% or less.

[0013]

When a copper oxide / manganese oxide-based catalyst is used as the CO selective oxidation catalyst, this catalyst alone has an excellent ability to selectively oxidize CO, and functions even if O _{2 and the} like do not coexist. However, in reality, the main component of the reformed gas, H
_{Since 2} is also oxidized to some extent, this is also consumed by CO oxidation, and oxygen atoms in manganese oxide are rapidly lost.
The catalyst thus completely loses its effectiveness in a very short time. However, when the reformed gas is mixed with air at a ratio of 20 vol% or less and then passed through the catalyst layer, the oxygen in the mixed air is used for the oxidation of CO and the like via the selective oxidation catalyst. Can be used for a long time. In addition, by operating the catalyst layer in a limited temperature range of 50 to 180 ° C. and using the catalyst with its activity lowered in advance, CO selectivity can be maintained at a high level and oxidation by the oxidation reaction can be performed. It can prevent abnormal overheating and runaway.

When a gold / iron oxide-based CO selective oxidation catalyst is used, the catalyst itself has no ability to oxidize CO, and when O _{2 and the} like coexist, it is used to selectively CO. Can be oxidized to Generally, since almost no O ₂ is present in the reformed gas, CO oxidation does not occur even though the reformed gas is passed through this catalyst.
Since the air is mixed at a ratio of not more than vol% and is then passed through the catalyst layer, CO oxidation occurs. In this way CO
_The reformed gas oxidized to ₂ can reduce the CO concentration to around 10 ppm when the conditions are adjusted. On the other hand, a part of H ₂ is also oxidized, but its amount is acceptable, and the CO ₂ produced by the oxidation of CO is originally contained in the reformed gas at about 20%, and is high. The adverse effect on the molecular solid oxide fuel cell and the like can be ignored, and the fuel can be used as it is as a fuel.

[0015]

EXAMPLES Examples of the CO removal method according to the present invention will be described below. [Embodiment 1] FIG. 1 shows a schematic configuration of a CO removing apparatus used in this embodiment. In a glass tube 1 having a diameter of 15 mm, a reforming section 2 using methanol as a raw fuel, a CO conversion section 3 following the reforming section 2,
And a CO removal unit 4 is installed. An air mixing nozzle 4 is provided in the flow path between the CO shift conversion unit 3 and the CO removal unit 4, and predetermined air is mixed according to the flow rate of the reformed gas.
In this embodiment, as a simple method, a Venturi method is used to have a structure in which a fixed amount of air is mixed according to the flow rate of the reformed gas. The CO removing unit 4 has a cylindrical shape and is filled with 70 g of granular CO selective oxidation catalyst. In this example, a mixture of 25 wt% CuO and 75 wt% MnO ₂ was used as the copper oxide / manganese oxide catalyst for the CO selective oxidation catalyst. This type of catalyst is commonly known as hopcalit and has been used for a long time in CO gas masks and the like.

The temperature of the reforming section 2 is 350 ° C., and the CO shift conversion section 3
The temperature of 220 ° C and the molar ratio of methanol and water is 1: 2.
The mixed solution mixed in the ratio of was vaporized and supplied to the reforming section. The CO removal unit 4 is at room temperature, and the air mixing amount is changed between 0 and 20 vol% (value for reformed gas 100 vol, the same below) with respect to the reformed gas flow rate (excluding steam), and the CO concentration at the outlet and The H ₂ concentration was measured.
Further, since the catalyst itself was gradually consumed and lost its activity, and in that case it was necessary to replace the catalyst, the duration until the CO removal action was remarkably lowered was measured in each case. The results are shown in Table 1. The composition of the exit gas from the CO shift section is about 75% H _{2 and} 0.3 CO.
5 to 0.4%, that is, 3500 to 4000 ppm, CO ₂ is around 24%, and the dry gas flow rate is about ₂ L.
/ Min.

[0017]

[Table 1]

As can be seen from the results, although the value varies depending on the mixing ratio of air, the CO concentration is greatly reduced by the treatment with the CO selective oxidation catalyst of copper oxide / manganese oxide system. However, the catalyst CO
Much of the oxygen is consumed in the oxidation of hydrogen because the selectivity is not perfect. Further, there is a large difference in the duration of the catalyst depending on the mixing ratio of air. Compared with the fact that when air is not mixed, there are only oxygen atoms that the catalyst itself has as oxygen atoms that can be used for CO and H ₂ oxidation, but when air is mixed, O ₂ is oxidized. It is possible to delay the consumption of the catalyst itself because it can be used for. Judging the above results comprehensively, the air mixing amount is 10 vo
When it is set to around 1%, the CO concentration drops to a sufficiently low value of around 10 ppm, while the H ₂ concentration on one side can be maintained at around 70%, so it can be said that this is the optimum mixing ratio.

In the case of this embodiment, the basic requirements can be satisfied, but there is a problem. That is, local abnormal heat generation in the upstream, which is considered to be caused by excessive catalytic activity or local flow or bias in gas composition, rarely occurs, and a considerable amount of H ₂ oxidation and rapid catalyst deterioration occur. This is a point that is recognized. Further, depending on the conditions such as the outside air temperature and the flow rate of the reformed gas, the steam contained in the reformed gas and the steam generated by the H ₂ oxidation inevitably caused by the CO selective oxidation catalyst are condensed in the catalyst layer. There is a point to do. Regarding the mixing of air into the reformed gas containing H ₂ as a main component, it is necessary to consider the danger of explosion, but the structure is such that it is introduced into the catalyst layer immediately after gas mixing, and the piping system By reducing the free space inside, there is almost no danger of explosion. Moreover, there is no case where an explosion occurred even when abnormal heat was generated, and it is considered that safety is also verified in this respect. In addition, by reducing the air mixing amount as much as possible, the power of the explosion can be suppressed to a range that the piping system can withstand in case of emergency.

[Embodiment 2] In this embodiment, some of the disadvantages found in Embodiment 1 are improved. Reforming section and C
The O conversion section was the same as in Example 1, but a heater and a heat insulating material were installed in the CO removal section and the temperature was kept at 150 ° C. Similarly, the air mixing amount was changed from 0 to 20 vol% with respect to the reformed gas flow rate, and the CO concentration and H ₂ concentration at the outlet were measured. Table 2 shows the results. By keeping the catalyst layer temperature at 150 ° C., the reaction rate is increased and C
Oxidation removal effect of O is further enhanced. In addition, Example 1
The inconvenience such as the condensation of water vapor seen in 1) did not occur at all, and this point was also improved. However, on the other hand, the consumption amount of H ₂ is also increased, and there is a demerit that the duration is decreased in a form accompanying this.

[0021]

[Table 2]

[Embodiment 3] This embodiment improves the disadvantages found in Embodiments 1 and 2. Although the configuration of each element portion is the same as that of the second embodiment, the copper oxide / manganese oxide-based catalyst to be filled in the CO removing portion is intentionally 2
It differs greatly in that it is used after being exposed for a week and its activity is reduced to some extent.

The reasons and advantages of using a catalyst whose activity is reduced to some extent will be described below. It is generally known that the copper oxide / manganese oxide based catalyst used as the CO selective oxidation catalyst is poisoned by water vapor and its activity is lowered. For this reason, in applications such as CO gas masks, care must be taken such as sealing when storing, and such caution allows the catalyst to be used in a highly active state. However, this is in the case of oxidizing and removing CO contained in a very small amount in the oxidizing gas, which is different from the case of removing CO in the reformed gas, which is the object of the present invention. In other words, H ₂ which is the main component of the reformed gas has a characteristic that it is easily oxidized if there is a substance having some catalytic activity, an oxygen source and a slight temperature rise. For this reason, the copper oxide / manganese oxide-based catalyst originally has a high CO selective oxidative property, but the catalyst itself is an oxygen source, and the target reactant has a CO concentration of 1% or less and a H ₂ concentration. Under the conditions of 75%, a few% of O ₂ and a temperature of 150 ° C., it is actually difficult to suppress the H ₂ oxidation reaction as a side reaction.

However, as a result of some tests, when the copper oxide / manganese oxide based catalysts having different activities under such circumstances were used, the local heat generation and the local heat generation as described in Example 1 were induced. It was also found that there is a large difference in the probability of abnormal temperature rise of the entire catalyst layer and early deterioration of the catalyst. That is, when a highly active catalyst is used, the probability of abnormal heat generation and early deterioration is as high as 20 to 30%. On the other hand, if the activity is lowered to some extent by leaving it in the atmosphere for two weeks, the probability of such abnormal heat generation becomes significantly small. It is presumed that this is because the catalyst was partially poisoned by water vapor in the atmosphere, so that the oxidation reaction did not occur intensively in the upstream portion of the catalyst. However, the CO concentration at the outlet of the CO removal section was equivalent to that when a catalyst with high activity was used, and no reduction in CO oxidation capacity was observed as a whole. That is, it is presumed that CO oxidation is mildly occurring in the entire catalyst layer. Looking at each catalyst particle, it is assumed that the part where the surface activity is left reacts in the initial stage, and thereafter the reaction diffuses into the highly active part inside, causing the reaction as a whole and mildly. To be done. Since poisoning by water vapor basically does not change the oxygen content in the catalyst, it has almost no effect on the duration of the catalyst itself.

As described above, by reducing the activity of the copper oxide / manganese oxide based catalyst to be filled in the CO removing portion to some extent, the problems such as abnormal heat generation and early deterioration can be greatly improved. it can. In this example, the catalyst was exposed to the atmosphere for 2 weeks in order to reduce the activity of the catalyst to some extent. However, basically, the activity of the catalyst surface was changed without changing the composition and oxygen content of the catalyst itself. Just lower it. Therefore, the treatment may be carried out for a short period of time while the steam partial pressure or temperature is raised. If the surface activity can be controlled at the catalyst preparation stage, the activity may be controlled at that stage.

[Embodiment 4] In this embodiment, a gold / iron oxide catalyst is used as a CO selective oxidation catalyst. In this example, the Au ultrafine particle-supported α-Fe ₂ O ₃ catalyst was used. This catalyst is a catalyst commercialized by Toyo CCI Co., Ltd. based on the technology of Osaka Industrial Technology Research Institute, and is a catalyst that exhibits high CO selective oxidation activity at low temperatures. The reforming section and the CO shift section were the same as in Example 1, the temperature of the CO shift section was 120 ° C., the air mixing amount was changed between 0 and 20 vol% with respect to the reformed gas flow rate, and C at the outlet was used.
O concentration and H ₂ concentration were measured. The results are shown in Table 3.

[0027]

[Table 3]

As can be seen from these results, the effect of reducing the CO concentration is not so great as compared with the copper oxide / manganese oxide based catalyst. However, when the copper oxide / manganese oxide system is used, the catalyst itself is also consumed, but since this catalyst functions as the original catalyst, the time it functions as a catalyst becomes very long. With the configuration of this embodiment, about 500
A continuous operation test of time was conducted, but no deterioration in catalyst performance was observed. In addition, a phenomenon such as a local abnormal temperature rise was not observed, and stable operation was performed. That is, it is understood that the CO removal method according to the present embodiment has an advantage that stable operation can be performed for a long period of time, although the CO concentration level is slightly high. Therefore, when the required level for the CO concentration is not so high, it is sufficiently practical as a method for removing CO in the reformed gas.

In the above-mentioned embodiments, the case of methanol reforming has been described as an example, but this may be reforming of other hydrocarbons, and it may be applied to a reformer for city gas or the like. Is also good. Regarding air mixing, the air may be supplied via a flow rate controller, and the mixing ratio may be changed according to the state of the system. Further, when it is necessary to replace the flow rate control method or the catalyst, the CO removing unit may be used as a cartridge to improve the convenience. Furthermore, the temperature of the CO removing unit is not limited to the temperature in the embodiment.

[0030]

As described above, the method for removing CO in the reformed gas according to the present invention has a compact and simple structure and greatly reduces the CO concentration in the reformed gas in a small-scale hydrocarbon reforming system. It can be used for the purpose. The CO concentration can be lowered to about 10 ppm by selecting the conditions. For example, by using it in a fuel reforming system for a polymer electrolyte fuel cell, hydrogen can be directly separated from the reformed gas without purification. It becomes possible to supply to the battery. Further, it is possible to control the processing amount by the amount control.
Further, by making methanol, pure water, and a gas purification column into a cartridge, stable operation and reduction of CO concentration in the gas are possible, and reliability and safety are improved. Further, by supplying the hydrogen-rich gas having a sufficiently low CO concentration generated in the methanol reformer to the low temperature operation fuel cell, it becomes possible to construct a compact and movable power supply system.

[Brief description of drawings]

FIG. 1 is a vertical cross-sectional view showing a schematic configuration of a CO removing device used in an embodiment of the present invention.

[Explanation of symbols]

 1 Glass Tube 2 Reforming Section 3 CO Shifting Section 4 CO Removing Section 5 Air Mixing Nozzle 6, 7 Heater

Claims (4)

[Claims] 1. A reformed gas obtained by reforming a hydrocarbon-based raw fuel is mixed with air at a ratio of 20 vol% or less based on the reformed gas, and the mixed gas is CO-selective consisting of copper oxide and manganese oxide. A method for removing CO in a reformed gas, wherein the CO concentration in the reformed gas is reduced to 0.1% or less by passing through the oxidation catalyst layer. 2. The CO in the reformed gas according to claim 1, wherein the CO selective oxidation catalyst layer is operated in a temperature range of 50 to 180 ° C.
Removal method. 3. The method for removing CO in a reformed gas according to claim 2, wherein the CO selective oxidation catalyst is used in a state where the surface activity is reduced in advance. 4. A reformed gas obtained by reforming a hydrocarbon raw fuel is mixed with air at a ratio of 20 vol% or less based on the reformed gas, and the mixed gas is composed of gold and iron oxide at a temperature of 50.
A method for removing CO in a reformed gas, which comprises reducing the CO concentration in the reformed gas to 0.1% or less by passing the CO selective oxidation catalyst layer at 230 ° C.