JP2003183005A - Operating method for reformer for fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an operating method for such a reformer used in combination with a fuel cell as is contrived to achieve high efficiently, to down-size, and to control the reforming temperature. <P>SOLUTION: The operating method for the reformer which is used in combination with a fuel cell and which manufactures hydrogen by the steam reforming of a hydrocarbon-based fuel containing no oxygen atom in the molecular structure, is characterized in that the whole quantity or large portion of the heat required at the reformer is covered with the combustion heat of the off-gas out of the fuel cell by letting a reforming conversion ratio of the hydrocarbon- based fuel at the reformer to less than 90%. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a reformer used in combination with a fuel cell, that is, a method for operating a reformer for a fuel cell.

[0002]

2. Description of the Related Art Hydrogen is a basic raw material used for various purposes and is also used as a fuel for polymer electrolyte fuel cells (PEFC) and phosphoric acid fuel cells (PAEC). The steam reforming method, which is one of hydrogen production methods, is a method of reforming hydrocarbons or alcohols with steam to generate a hydrogen-rich reformed gas. In the steam reforming method, those hydrocarbons and alcohols are converted by catalytic reaction in the reformer.
Fuels are converted to hydrogen-rich reformed gas.

FIG. 1 is a diagram schematically showing a steam reforming apparatus. In general, it is composed of a combustion section (heating section) in which a burner or a combustion catalyst is arranged and a reforming section in which a reforming catalyst is arranged. In the reforming section, hydrocarbons and alcohols (hereinafter referred to as hydrocarbons) react with steam to generate hydrogen-rich reformed gas. Since the reaction that takes place in the reforming section is accompanied by a large endotherm, it is necessary to supply heat from the outside in order to proceed the reaction. Therefore, the combustion heat (ΔH) generated by the combustion of the fuel gas in the combustion section by the air (combustion air) is supplied to the reforming section.

The combustion heat is supplied to the reforming section indirectly via the heat transfer surface between the combustion section and the reforming section. Note that the combustion gas combusted with air is supplied to the combustion section of the reformer, and the hydrocarbon-based fuel that is reformed with steam is supplied to the reforming section. , And the combustion gas supplied to the combustion unit as the fuel gas,
The hydrocarbon fuel supplied to the reforming section is referred to as a raw material gas.

FIG. 2 is a diagram showing an example of the process from the source gas to PEFC using the steam reforming apparatus as described above. FIG. 3 shows the desulfurizer, steam generator, and
It is the figure which showed the reformer (combustion part + reforming part), CO shift converter, and CO selective oxidizer collectively as a reformer, and showed the general operation example. Odorants such as mercaptans, sulfides, and thiophene are added to city gas and LP gas. City gas or LP as raw material gas
When a gas is used, the reforming catalyst is poisoned by the sulfur compounds and deteriorates in performance, so that the reforming catalyst is introduced into the desulfurizer to remove the sulfur compounds. Next, steam from a separately provided steam generator is added and mixed and introduced into the reformer of the reformer, and the reforming reaction by the steam of the raw material gas in the reforming section causes a hydrogen-rich reforming. Gas is produced.

The reforming reaction when the source gas is methane is represented by "CH ₄ + 2H ₂ O → CO ₂ + 4H ₂ ". In the reformed gas produced, unreacted methane, unreacted water vapor, produced carbon dioxide (CO ₂ ), produced hydrogen (H ₂ ), and carbon monoxide (CO) as a byproduct of 8 to 15%. (% =% By volume, the same applies hereinafter). For this reason, the reformed gas is applied to a CO shift converter in order to remove the by-product CO by converting it into CO ₂ and H ₂ . The unreacted residual steam in the reformer is used as the steam required for the reaction in the CO shift converter, that is, the shift reaction “CO + H ₂ O → CO ₂ + H ₂ ”.

The reformed gas discharged from the CO shift converter is composed of hydrogen and carbon dioxide except for unreacted methane and excess steam. Of these, hydrogen is the target component, but C
Also in the reformed gas obtained through the O-transformer, CO is not completely removed, and the CO content is about 1% or less.
It is included. When the fuel cell is PAFC, the permissible concentration of CO in fuel hydrogen is about 1%, so the reformed gas that has passed through the CO shift converter can be used as it is as fuel hydrogen for PAFC.

On the other hand, when the fuel cell is PEFC, PEF
The allowable concentration of CO in fuel hydrogen supplied to C is 100 pp
m (ppm = capacity ppm, the same applies hereinafter), about 10 ppm depending on the constituent materials of the fuel electrode,
If it exceeds this range, the battery performance is remarkably deteriorated, so the CO component needs to be removed as much as possible before being introduced into the PEFC. Therefore, the reformed gas is converted to CO by the CO converter.
After reducing the concentration to about 1% or less, it is subjected to a CO selective oxidizer. In the CO selective oxidizer, an oxidant such as air is added, and CO is removed by changing CO to CO ₂ by the oxidation reaction of CO, and the CO concentration is 100 ppm or less, 1
It is reduced to 0 ppm or less, or 5 ppm or less.

By the way, in a reformer for PEFC which produces steam by itself and reforms a raw material gas with steam to produce hydrogen, (1) the hydrogen utilization rate (PEF) in PEFC
The flow rate of hydrogen consumed in C / the total flow rate of hydrogen in the hydrogen-rich gas x 100) is 70 to 95%, and (2) the operating load factor of PEFC is -100% (that is, PEF according to the demand amount of electric power).
Adjust the amount of power generation at C. Regardless of the operating condition of maximum load factor = 100%, the reforming temperature is set to obtain a sufficient methane conversion rate, that is, the temperature at which methane can be converted to hydrogen as much as possible (methane is still a little PEF with hydrogen-rich gas containing a little residual methane
Supply to C.

As a result, the exhaust gas from the PEFC, that is, the off gas, is insufficient for the required heating input of the reformer, that is, the required heating amount in the reformer and the steam generator. A general operation method is to supply and supplement the raw material gas. Specifically, as shown in FIGS. 2 and 3, raw material gas such as city gas is branched from the conduit and supplied as fuel gas for the combustion section. However, in this operating method, it is difficult to increase the efficiency of the reforming device for the following reasons, in addition to the fact that the reforming raw material gas needs to be supplied to make up for it.

Since the set reforming temperature is high, it is effective in improving the hydrogen production rate, but it is difficult to reduce the heat radiation loss. Since the set reforming temperature is high, a large amount of CO is produced, and it is difficult to reduce the CO selective oxidation air that leads to the reduction of the oxidation of the produced hydrogen. That is, in the CO selective oxidizer, only CO in the reformed gas needs to be oxidized, but hydrogen is also oxidized. Since the set reforming temperature is high, a large amount of CO is produced, which is effective for reducing heat loss, but it is difficult to operate at a low S / C ratio which further increases the amount of CO produced. Since the reforming is carried out by raising the set reforming temperature, the heat transfer area from the combustion section to the reforming section becomes large and the reforming apparatus itself becomes large. Since the set reforming temperature is high, it is necessary to use an expensive reforming device having high heat resistance.

[0012]

SUMMARY OF THE INVENTION The above-mentioned problems are caused by the conventional technical orientation of the reformer itself and the method of operating the reformer to improve the hydrogen production rate from the raw material gas as much as possible. It is due to being directed to. Even when the reformer is used in combination with a fuel cell, the operation is naturally performed on the assumption that it is used.

The present invention is intended to solve the above problems in the conventional method for operating a reformer, particularly the method for operating a reformer used in combination with PEFC. The idea is totally different from the above, and instead of improving the hydrogen production rate in the reformer as much as possible as in the past, on the contrary, by lowering the hydrogen production rate, the above problems can be solved all at once. An object of the present invention is to provide a method for operating a reformer for a fuel cell.

[0014]

The present invention is directed to a reformer for use in combination with a fuel cell and for steam-reforming a hydrocarbon fuel containing no oxygen atoms in its molecular structure to produce hydrogen. In the operating method, the reforming conversion rate of the hydrocarbon-based fuel in the reformer is set to less than 90% so that all or most of the required heat amount in the reformer is converted into combustion heat of off-gas from the fuel cell. Provided is a method for operating a reformer for a fuel cell, which is characterized by the provision.

Here, the reforming conversion rate (%) is expressed by the following formula (1) from the composition ratio of each component in the reformed gas when the hydrocarbon fuel is methane (CH ₄ ). . In the formula (1), CH ₄ is the amount of residual methane contained in the reformed gas without being reformed by the reformer. When the hydrocarbon-based fuel is ethane or other hydrocarbon or a mixed gas thereof, CH ₄ in the formula (1), that is, the amount of residual methane is the amount of residual hydrocarbon.

[0016]

[Equation 1]

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION In the conventional operating method, the reforming conversion rate in the reformer is generally set to 90% or more regardless of the hydrogen utilization rate or operating load rate in the fuel cell. , Further research and development are being continued to improve it. Taking the example where the hydrocarbon fuel is methane,
In the reformer, methane is reformed as much as possible to form reformed gas in which hydrogen is as rich as possible. Speaking in the above formula (1),
Development efforts are being made to reduce CH ₄ as much as possible and further improve the reforming conversion rate.

Then, the amount of residual methane in the off-gas discharged from the fuel cell is reduced, and when all or most of the off-gas is used as the fuel gas for the combustion section in the reformer, it is necessary for the reforming section. There is a shortage of heat. In the conventional operation method, in order to make up for this shortage, as shown in FIGS. 2 and 3, the raw material gas is branched from the conduit and supplied as the fuel gas for the combustion section.

FIG. 4 shows the relationship between the reforming conversion rate of methane and the hydrogen utilization rate of PEFC in the conventional operating method of the reformer and the operating method of the reformer in the present invention when PEFC is used as the fuel cell. It is the figure which showed based on the actual measurement at the time of constant load factor operation. Experimental conditions and measurement conditions are the same as those in the examples described later. As shown in FIG. 4, in the conventional operation method, since the reforming conditions are set in advance, the residual CH ₄ concentration in the reformed gas is 1.9% and the reforming conversion rate is 91%, which are almost constant. The reforming conversion rate remains almost constant even if the hydrogen utilization rate in PEFC is increased.

On the other hand, in the operating method of the present invention, the residual CH ₄ in the reformed gas is increased as the hydrogen utilization rate in the PEFC increases.
The concentration increases to 2.6%, 3.9%, 4.7%, and 6.4%, and the reforming conversion rate in the reformer further decreases in the range of less than 90%. It is lowered the reforming conversion i.e., is meant to be a lot of residual CH ₄ in the reformer [Formula (1)], the remaining CH ₄ in the reformed gas is PE
Since it is not used in FC but is contained in the off-gas as it is, it shows that it can be effectively used as fuel gas in the combustion section.

FIG. 5 shows the residual amount of methane (raw material gas) which is not reformed in the reforming device and is contained in the reformed gas and the offgas from the PEFC, respectively, which is also based on the actual measurement at the time of constant load operation. Percentage of heat due to CH ₄ and PEF
It is a figure which shows the relationship with the hydrogen utilization rate in C. In addition to hydrogen as reformed gas, residual CH ₄ (that is, unreformed CH ₄ )
And CO ₂ and the like are included, also other hydrogen off-gas in the unused in PEFC from PEFC, although etc. residual CH ₄ and CO _2, focusing on them in Fig residual CH _4, focus on this The amount of heat is plotted.

In the case of the conventional operation method, the reforming conversion rate is 90.
% Is almost constant in the range of not less than%, and correspondingly, the residual CH ₄ concentration in the reformed gas is almost constant, and its absolute amount does not change even when the off gas from PEFC is used. And PE
Since hydrogen is consumed in the FC, the residual CH ₄ concentration in the offgas relatively rises accordingly, and the residual C ₄ is correspondingly increased.
The heat quantity ratio due to H ₄ also rises relatively. That is, since the absolute amount of residual CH ₄ does not change, even if the hydrogen utilization rate in PEFC is increased, the heat amount ratio due to residual CH ₄ does not increase significantly. As shown in FIG. 5, even if the hydrogen utilization rate in PEFC is increased to 94%, for example, the heat quantity ratio due to the residual CH ₄ in the offgas remains at 57%.

On the other hand, in the operating method of the present invention, P
As the hydrogen utilization rate of EFC increases, the reforming conversion rate decreases and the absolute amount of residual CH ₄ in the reformed gas increases. Correspondingly, the ratio of heat quantity due to the residual CH ₄ in the reformed gas also increases. . Since hydrogen is consumed in the PEFC, the amount of residual CH ₄ in the off gas further increases accordingly, and the amount of heat due to the residual CH ₄ in the off gas further increases correspondingly. For example, as shown in FIG. 5, residual CH in the reformed gas
_The ratio of heat quantity by ₄ is 10.0%, 14.3%, 16.9
%, 21.9%, and in response to this, the proportion of heat due to residual CH ₄ in the offgas is also 32%, 47%, 60
%, 83%.

That is, in the operating method of the present invention, decreasing the reforming conversion rate means that the amount of residual CH ₄ increases as an absolute value [see the above formula (1)],
The amount of heat generated by the residual CH ₄ in the offgas also increases significantly, but the residual CH ₄ is not used in PEFC and is contained in the offgas as it is. Therefore, by using this as the fuel gas for the combustion part, the combustion part It can cover all or most of the heat requirement in. Although FIG. 4 and FIG. 5 show application examples of the present invention in the case where the hydrogen utilization rate of PEFC increases under a constant operating load rate,
Even when the PEFC hydrogen utilization rate is constant and the operating load rate decreases, the reforming conversion rate is reduced according to the change in the heat balance of the reforming unit, so that all or most of the required heat amount in the combustion unit is reduced. You can do the same.

The above facts are utilized in the operating method of the present invention. That is, in the reformer, the reforming conversion rate is 90% depending on the hydrogen utilization rate or the operating load rate in the fuel cell.
The operation is performed on the premise that the required heating input of the reformer, that is, the total amount of heat required as the amount of heat in the combustion section of the reformer or most of it is covered by the amount of off-gas of the fuel cell. According to the present invention, this makes it possible to improve the performance of the reformer as described in (1) to (5) below.

(1) Since the reforming temperature is suppressed, heat dissipation loss can be reduced. (2) Since the reforming temperature is suppressed, the amount of CO produced can be reduced, and the amount of air for CO selective oxidation to the CO selective oxidizer, which leads to a reduction in the oxidation of produced hydrogen, can be reduced. That is, C
In the O-selective oxidizer, not only CO in the reformed gas but also hydrogen is oxidized, but by reducing the amount of air for CO selective oxidation, it is possible to reduce or suppress the oxidation of hydrogen. (3) Since the reforming temperature is suppressed, the amount of CO produced can be reduced, and the operation at a low S / C ratio, which is effective for improving the efficiency of the reforming device, becomes easy. (4) The heat transfer area from the combustion section to the reforming section can be reduced by lowering the reforming temperature for reforming. As a result, the reformer itself can be downsized. (5) Since the reforming temperature is low, an inexpensive material having low heat resistance can be used as a constituent material of the reforming apparatus.

The reformer according to the present invention is basically composed of a combustion section in which a burner or a combustion catalyst is arranged and a reforming section in which a reforming catalyst is arranged. As the reforming catalyst, any catalyst can be used as long as it has a function of reforming a raw material gas to generate a hydrogen-rich gas. For example, a Ni-based catalyst (for example, a catalyst in which Ni is supported on alumina) or a Ru-based catalyst is used. (For example, a catalyst in which Ru is supported on alumina) can be mentioned. When arranging the combustion catalyst in the combustion section, for example, a precious metal catalyst such as platinum or a combustion catalyst such as alumina hexaneate is used.

As the source gas, any hydrocarbon type fuel containing no oxygen atom in its molecular structure can be used.
Examples thereof include methane, ethane, propane, butane, city gas, LP gas, natural gas, and other hydrocarbon gases (including mixed gas of two or more kinds of hydrocarbons).

The reformer includes a steam generator with or without a steam generator, and the operating method of the present invention is applicable to any of those reformers, but the required heating of the steam generator is required. As the input is added, the application to the one including the steam generator capable of making the off gas more methane rich and lowering the reforming temperature is more effective. Further, any fuel cell that uses hydrogen as a fuel can be used as the fuel cell in the present invention. Above all, the application to the PEFC in which the steam generator is required to be included in the reforming apparatus is effective. . Offgas from PEFC does not have to cover all of the required heating input of the reformer,
It is also possible to improve the operability of the reforming device by supplementing a part of the heat amount with the raw material gas and further operating the raw material gas flow rate to control the reforming conversion rate, that is, the reforming temperature.

[0030]

The present invention will be described in more detail based on the following examples, but it goes without saying that the present invention is not limited to the examples. In this example, the reformer set as shown in FIG. 6 was used. PEFC is connected to the reformer. A part of the reformer is configured by connecting a desulfurizer, a steam generator, a reformer, a CO shift converter and a CO selective oxidizer as shown in FIG.

The burner of the reformer uses a burner, the reformer uses a Ni-based catalyst whose carbon deposition resistance is improved by adding an appropriate amount of rare earth metal, and the CO shifter uses a copper-zinc catalyst (Cu / Zn). (Zn-based catalyst) was used, and in the CO selective oxidizer, a catalyst in which Pt was supported on alumina was used. Regarding the operating conditions, the flow rate of the raw material gas is constant and the air ratio λ in the combustion section is
Was set to 1.1, the S / C ratio in the reforming section was set to each value in the range of 3.1 to 2.6, and the reforming temperature was set to each temperature in the range of 680 to 620 ° C. Maximum DC output of about 1.5 for PEFC
A kW one was used. Further, natural gas was used as the raw material gas, and air was used as the oxidant supplied to the CO selective oxidizer.

FIG. 7 is a diagram showing the relationship between the hydrogen utilization rate in PEFC and the reforming temperature and S / C ratio in the reforming apparatus, among the results obtained in this example. FIG. 8 is a diagram showing the relationship between the hydrogen utilization rate in PEFC and the air flow rate to the CO selective oxidizer, which corresponds to the relationship of FIG. 7. Figure 9
FIG. 9 is a diagram showing the relationship between the hydrogen utilization rate in PEFC and the fuel processing efficiency of the reforming device, which corresponds to the relationship of FIGS. 7 and 8. Here, the fuel processing efficiency (%) is represented by the following formula (2) and is a value that directly indicates the performance of the reformer used in combination with the fuel cell.

[0033]

[Equation 2]

First, in FIG. 7, the reforming temperature of the conventional operating method is constant at 700 ° C., whereas it is less than 700 ° C. in the operating method of the present invention, and the hydrogen utilization rate in the PEFC increases further. It is descending. As described above, the operating method according to the present invention leads to a decrease in the reforming temperature, and the degree thereof tends to increase as the hydrogen utilization rate in PEFC increases. That is, the reforming temperature can be lowered according to the hydrogen utilization rate in PEFC, and it is not necessary to raise it as in the conventional case. This fact makes it possible to reduce the heat dissipation loss, reduce the size of the reformer by reducing the heat transfer area from the combustion section to the reforming section, and use inexpensive materials with low heat resistance as its constituent materials. Shows.

In addition, in FIG. 7, the conventional S / C ratio is 3.
While it is constant at 1, the S / C ratio of the present invention is 3.1 or less (S / C <3.1), and it can be operated at a lower value as the hydrogen utilization rate in PEFC increases. . this is,
Conventionally, the operation at a low S / C ratio, which was difficult to realize because the fuel processing efficiency is improved but the amount of by-product CO is increased, is that the amount of by-product CO is decreased as the reforming temperature of the present invention is lowered. It has been made possible by.

Next, referring to FIG. 8, the conventional air flow rate is 1.7.
While it is constant at 8 NL / min (min = minute),
The air flow rate of the present invention is 1.7 NL / min or less, and P
As the hydrogen utilization rate in the EFC increases, the CO concentration can be suppressed to 10 ppm or less with a smaller air flow rate. This fact indicates that the reforming temperature drop has a greater effect on the amount of CO produced as a byproduct than the low S / C ratio operation, and as a result, the reduction in oxidation of the produced hydrogen can be suppressed.

Further, as shown in FIG. 9, the fuel processing efficiency according to the present invention is higher than the conventional one in any hydrogen utilization rate, and the difference is further expanded as the hydrogen utilization rate in PEFC increases. This is a result that the effects of reducing the heat dissipation loss, realizing the low S / C ratio operation, and reducing the selective oxidizing air flow rate described in FIGS. The efficiency advantage of the present invention has been demonstrated.

[0038]

According to the present invention, in a method of operating a reformer used in combination with a fuel cell, all or most of the required heating input of the reformer combustion section is covered by methane-rich offgas from the fuel cell. As a result, the reforming temperature can be suppressed according to the upper limit hydrogen utilization rate and operating load rate that can be set in the fuel cell. Further, by doing so, various useful effects such as high efficiency and miniaturization of the reformer can be obtained.

[Brief description of drawings]

FIG. 1 is a diagram schematically showing a steam reformer.

FIG. 2 PEFC from raw material gas using a steam reformer
The figure which shows the example of the way to

FIG. 3 is a diagram showing an example of a conventional operation method of a reformer.

FIG. 4 is a diagram showing the relationship between the reforming conversion rate and the hydrogen utilization rate of PEFC for the conventional operation method and the operation method of the present invention for the reformer.

FIG. 5 is a diagram showing the relationship between the ratio of the amount of heat due to CH ₄ contained in the reformed gas without being reformed in the reformer, that is, CH ₄ contained in the offgas from the PEFC, and the hydrogen utilization rate in the PEFC.

FIG. 6 is a diagram showing an apparatus and an operating method used in Examples.

FIG. 7 is a diagram showing the results of Examples.

FIG. 8 is a diagram showing the results of Examples.

FIG. 9 is a diagram showing the results of Examples.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Jun Komiya             1-5-20 Kaigan, Minato-ku, Tokyo Tokyo Gas             Within the corporation (72) Inventor Naohiko Fujiwara             1-5-20 Kaigan, Minato-ku, Tokyo Tokyo Gas             Within the corporation F-term (reference) 4G040 EA03 EA06 EB03 EB14 EB31                       EB32 EB42 EB43 EB44                 5H026 AA06                 5H027 AA06 BA01 BA09 BA16 BA17                       KK42 KK52 MM12

Claims (6)

[Claims] 1. A method of operating a reformer, which is used in combination with a fuel cell and which produces hydrogen by steam-reforming a hydrocarbon-based fuel that does not contain oxygen atoms in its molecular structure,
90% conversion rate of hydrocarbon fuel in reformer
By setting the amount to be less than the above, all or most of the required amount of heat in the reformer is covered by the combustion heat of the off gas from the fuel cell. 2. The method for operating a reformer for a fuel cell according to claim 1, wherein the reformer includes a steam generator. 3. The method for operating a reformer for a fuel cell according to claim 1, wherein the setting of the reforming conversion rate is changed according to the hydrogen utilization rate of the fuel cell. 4. The setting of the reforming conversion rate is changed according to the operating load rate of the fuel cell.
4. The method for operating the fuel cell reformer according to any one of 3 above. 5. The reforming conversion rate is controlled by compensating a part of the required heat quantity with the hydrocarbon fuel and manipulating the flow rate of the hydrocarbon fuel for compensation. 4. The method for operating the fuel cell reformer according to any one of 1 to 4. 6. The method of operating a reformer for a fuel cell according to claim 1, wherein the fuel cell is a polymer electrolyte fuel cell.