JPH10255830A - Operating method for fuel cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method wherein deposition of carbon can be suppressed and a fuel cell can be operated for a long time stably, when methanol, ethanol or dimethyl ether is supplied as its fuel to a fuel electrode of the fuel cell. SOLUTION: A fuel cell contains electrolyte, a fuel electrode and an oxidizer electrode to put the electrolyte mentioned above between, and an operating temperature is set at 550-750 deg.C. In this case, a mol mixing rate for water and fuel is set for, (1) When fuel containing methanol is used, 0.250+0.287 P-1.08×10<-2> P<2> <=S/C<=1.994+0.724P-2.96×10<-2> P<2> , (2) When fuel containing ethanol or dimethyl ether is used, 1.500+0.574P-2.15×10<-2> P<2> <=S/C<=4.993+1.451 P-5.96×10<-2> P<2> , (In the above, P is an operating pressure for each fuel cell (atm)), and the fuel is supplied to the fuel electrode of this fuel cell.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a method for controlling the operating temperature of 550.
The present invention relates to a method for operating a fuel cell having a temperature of up to 750 ° C., in particular, a molten carbonate fuel cell.

[0002]

2. Description of the Related Art As a method for obtaining electric energy, a conventional method using thermal power generation has been adopted. In thermal power generation, fuel is burned to convert chemical energy into heat energy, and the heat energy is used to operate a steam turbine or a generator to obtain electric energy. Therefore, in this method, the thermal efficiency of Carnot is limited, and even with advanced thermal power generation, the conversion efficiency reaches only 35 to 40%.

On the other hand, a fuel cell that directly converts chemical energy into electrical energy is not subject to the thermal efficiency limitation of Carnot, and therefore can be expected to have high conversion efficiency. Also,
Fuel cells have low pollution, are free from noise, and can utilize waste heat, so that their potential is expected.

[0004] From such a background, various fuel cells have been studied, and as the fuel, fossil fuel is often used. This fossil fuel is not usually used as it is as fuel gas, but is subjected to a reaction with water in a reformer provided outside the fuel cell or a reforming catalyst layer provided in the fuel cell body. It is usually converted (reformed) into a fuel gas containing hydrogen and then subjected to a reaction.

[0005] For example, when methane is the fuel, the following equilibrium reaction exists in the reformer or the reforming catalyst layer.

Embedded image

[0006] Of these equilibrium reactions, reaction (3) is called a Boudouard reaction. Under an atmosphere of about 800 ° C. or more, the Boudouard reaction has a high rate of CO generation reaction (reaction to the left in the above reaction formula), a relatively low rate of generation of carbon (C), and a low rate of carbon generation. However, as a condition for actually operating such a fuel cell, it is difficult to select such a condition exceeding about 800 ° C., and the fuel cell is generally operated at a temperature lower than about 800 ° C. For this reason, carbon deposition may occur, and the deposited carbon causes clogging of the flow path, deterioration of the catalytic function due to the carbon adhering to the catalyst, and other obstacles, and causes the fuel cell to be damaged for a long time. This is an obstacle when trying to drive stably. To prevent such problems,
The steam / methane ratio where carbon deposition due to the reaction (3) is small is adjusted empirically, and methane and steam are supplied at that ratio.

On the other hand, in a fuel cell using an alcohol such as methanol as a fuel, the alcohol is converted into a fuel gas containing hydrogen on the fuel electrode, so that an external reformer or a reforming catalyst layer is not required. There is a feature. However, as far as the present inventors know, suitable operating conditions of a fuel cell using an alcohol as a fuel, particularly the relationship between the steam / fuel ratio and the pressure, have not been known so far. For this reason, if the ratio of alcohols to water vapor is the same as the water vapor / fuel ratio (S / C) employed when using conventional fuels, it is difficult to avoid carbon precipitation due to the Boudouard reaction, It was difficult to set appropriate operating conditions, and it was necessary to adjust the relationship between the steam / fuel ratio and the pressure each time the fuel cell device was designed.

[0008]

From such a background,
In a fuel cell without an external reformer or reforming catalyst, using methanol, ethanol, or dimethyl ether as a fuel, when operating at 550-750 ° C., no carbon deposition or at least suppression of carbon deposition There was a need for a driving method that would do it.

[0009]

[Means for Solving the Problems]

[Summary of the Invention] <Summary> A first method of operating a fuel cell according to the present invention includes an electrolyte, a fuel electrode and an oxidant electrode sandwiching the electrolyte, and has an operating temperature of 550 to 750 ° C. In a fuel cell, a fuel containing methanol is used, and the molar mixing ratio S / C of water and methanol contained in the fuel is 0.250 + 0.287P-1.08 × 10 ⁻² P ² ≦ S
/C≦1.994+0.724P−2.96×10 ⁻² P
² (where P is the operating pressure (atm) of the fuel cell)
The fuel is supplied to the fuel electrode.

[0010] A second method of operating a fuel cell according to the present invention relates to a fuel cell having an electrolyte, a fuel electrode and an oxidant electrode sandwiching the electrolyte, and having an operating temperature of 550 to 750 ° C. And a fuel containing ethanol or dimethyl ether, and the molar mixing ratio S / C of water and ethanol or dimethyl ether contained in the fuel is 1.500 + 0.574P-2.15 × 10 ⁻² P ² ≦ S
/C≦4.993+1.451P−5.96×10 ⁻² P
² (where P is the operating pressure (atm) of the fuel cell)
The fuel is supplied to the fuel electrode.

DETAILED DESCRIPTION OF THE INVENTION <Fuel Cell> The fuel cell used in the present invention has an operating temperature of 5 ° C.
Any temperature can be used as long as the temperature is 50 to 750 ° C. and a fuel containing methanol, ethanol, or dimethyl ether can be used as the fuel.

As such a fuel cell, there is a so-called high-temperature fuel cell. Among them, the fuel cell used in the method of the present invention is particularly preferably a molten carbonate fuel cell. This molten carbonate fuel cell uses, as an electrolyte, a molten carbonate, for example, a mixture of lithium carbonate and potassium carbonate (molar ratio: about 60: about 40), and uses a nickel-based metal on a fuel electrode holding the electrolyte. In general, nickel oxide is used for an oxidant electrode.

When a fuel other than hydrogen gas is used as the fuel, the fuel is generally reformed by an external reformer or a reforming catalyst layer and then supplied to the fuel cell. On the other hand, in the method of the present invention, since the operating temperature of the fuel cell is as high as 550 to 750 ° C. and the fuel used has a relatively low molecular weight, a special external reformer or reforming catalyst is not necessarily used. do not need. However, if necessary, various external reformers can be used together, or a noble metal or the like can be used as an electrode catalyst.

<Fuel> In the first method, the fuel used in the method of the present invention is a fuel containing methanol and a second fuel.
Is a fuel comprising ethanol or dimethyl ether. Methanol, ethanol and dimethyl ether have relatively low molecular weights and are easily converted to hydrogen gas on the electrode of the fuel cell used in the present invention, and they are easy to store because they are liquid at room temperature. Having.

The fuel containing methanol used in the first method of the present invention preferably has a methanol content of 10 mol% or more. Methanol content is 10
If it is less than mol%, the hydrogen concentration in the reformed fuel becomes insufficient. In addition, carbon deposition may occur due to carbon compounds which are impurities other than water. Therefore, it is preferable that the fuel used in the first method of the present invention consists essentially of only methanol and water.

The fuel containing ethanol or dimethyl ether used in the second method of the present invention preferably contains only ethanol or contains only dimethyl ether, but is a mixture of ethanol and dimethyl ether. You may. The fuel used in the method of the present invention preferably has a total content of ethanol or dimethyl ether of 5 mol% or more. When the content of ethanol or dimethyl ether is less than 5 mol%, the hydrogen concentration in the reformed fuel becomes insufficient.
Further, carbon deposition may occur due to a carbon compound as an impurity. Therefore, it is preferable that the fuel used in the second method of the present invention consists essentially of only ethanol or dimethyl ether and water.

<Operating Conditions of Fuel Cell> In the first and second methods of the present invention, the operating temperature is 550 to 750 ° C.
Is used. Here, if the operating temperature of the fuel cell is 550 to 750 ° C., the effects of the present invention can be obtained. However, it is preferable that the operating temperature be 580 to 680 ° C. from the viewpoint of battery performance and battery life.

FIG. 1 is a correlation diagram between the operating pressure and the steam / fuel ratio in the first method. In the first method using a fuel containing methanol, water and methanol are mixed at a molar mixing ratio S / C of 0.250 + 0.287P-1.08 × 10 ^-2 P ² ≦ S
/C≦1.994+0.724P−2.96×10 ⁻² P
² (where P is the operating pressure (atm) of the fuel cell) and mixed and supplied to the fuel electrode of the fuel cell (hatched portion in the figure). During the actual operation of the fuel cell, the fuel gas supply pressure fluctuates and the operating temperature fluctuates. Therefore, the molar mixing ratio S / C of water and methanol is set to 0.275 + 0.316P-1.19 × 10 ^-2 P ² ≤ S
/C≦1.795+0.652P−2.66×10 ⁻² P
It is preferably ² .

FIG. 2 is a correlation diagram between the operating pressure and the steam / fuel ratio in the second method. In the second method using a fuel containing ethanol or dimethyl ether, water and ethanol or dimethyl ether are mixed at a molar mixing ratio S / C of 1.500 + 0.574P-2.15 × 10 ⁻² P ² ≦ S
/C≦4.993+1.451P−5.96×10 ⁻² P
² (where P is the operating pressure (atm) of the fuel cell), and the mixture is supplied to the fuel electrode of the fuel cell (hatched portion in the figure). During the actual operation of the fuel cell, the fuel gas supply pressure fluctuates, the operating temperature fluctuates, and the like.
/ C 1.650 + 0.631P-2.36 × 10 ^-2 P ² ≦ S
/C≦4.494+1.306P−5.36×10 ⁻² P
It is preferably ² .

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First, a case where methanol is used as a fuel will be described. Using a molten carbonate fuel cell as the fuel cell, water vapor and methanol are supplied to the fuel electrode at a water vapor / fuel ratio between two solid lines (hatched portion) shown in FIG. At this time, at the fuel electrode, a hydrogen-containing gas is generated by the following reactions (4) and (5), and then a cell reaction (6) proceeds.

Embedded image At this time, steam and methanol are
By being supplied at the fuel ratio (S / C ratio), carbon deposition due to the Bouard-Aal reaction is prevented.

Next, the case where ethanol or dimethyl ether is used as the fuel will be described. Using a molten carbonate fuel cell as the fuel cell, water vapor and ethanol or dimethyl ether are supplied to the fuel electrode at a water vapor / fuel ratio between two solid lines (hatched portion) shown in FIG. At this time, the following (4 ′) and / or (4 ″)
And a hydrogen-containing gas is produced by the reaction of (5),
Next, the battery reaction (6) proceeds.

Embedded image At this time, by supplying steam and ethanol or dimethyl ether at a controlled steam / fuel ratio, carbon deposition due to the Boudouard reaction is suppressed.

According to the method of the present invention, when methanol, ethanol or dimethyl ether is supplied as a fuel to the fuel cell anode, carbon deposition can be suppressed, and the fuel cell can be operated stably for a long period of time. it can.

[Brief description of the drawings]

FIG. 1 is a correlation diagram of a steam / fuel ratio and an operating pressure when methanol is supplied as a fuel to a fuel cell according to the method of the present invention.

FIG. 2 is a correlation diagram of a steam / fuel ratio and an operating pressure when ethanol or dimethyl ether is supplied as a fuel to a fuel cell according to the method of the present invention.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Keiji Murata 1 Komukai Toshiba-cho, Saiwai-ku, Kawasaki-shi, Kanagawa Prefecture Toshiba R & D Center (72) Inventor Masakuni Sasaki 1-chome Shibaura, Minato-ku, Tokyo No. 1-1 Inside Toshiba Corporation Head Office (72) Inventor Masafumi Fukuda 1-1-1 Shibaura Minato-ku, Tokyo Inside Toshiba Corporation Head Office

Claims (4)

[Claims] 1. A fuel cell having an electrolyte, a fuel electrode and an oxidant electrode sandwiching the electrolyte, and operating at a temperature of 550 to 750 ° C., wherein a fuel containing methanol is used. The molar mixing ratio S / C of the contained water and methanol is 0.250 + 0.287P-1.08 × 10 ^-2 P ² ≦ S
/C≦1.994+0.724P−2.96×10 ⁻² P
² (where P is the operating pressure (atm) of the fuel cell)
A method for operating a fuel cell, comprising supplying the fuel to the fuel electrode. 2. The method according to claim 1, wherein the fuel cell is a molten carbonate fuel cell. 3. A fuel cell having an electrolyte, a fuel electrode and an oxidizer electrode sandwiching the electrolyte, and operating at a temperature of 550 to 750 ° C., wherein a fuel containing ethanol or dimethyl ether is used. Molar mixing ratio of water contained in fuel and ethanol or dimethyl ether S /
C is 1.500 + 0.574P-2.15 × 10 ^-2 P ² ≦ S
/C≦4.993+1.451P−5.96×10 ⁻² P
² (where P is the operating pressure (atm) of the fuel cell)
A method for operating a fuel cell, comprising supplying the fuel to the fuel electrode. 4. The method according to claim 3, wherein the fuel cell is a molten carbonate fuel cell.