JPH088114B2 - Fuel cell anode waste gas combustion method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for burning anode waste gas of a combustion cell.

[Prior art and its problems]

As a method of burning a lean fuel gas to obtain a high temperature of 1000 ° C or higher, a catalyst is filled in the preceding stage, where a gas in which a total amount of air and a part of the fuel are mixed is burned, and this excess air combustion gas is used as a fuel. Advanced Combustion or Hybrid Catalytic Co is a method of raising the temperature above the ignition temperature of the above and mixing the rest of the fuel with this gas in the latter stage to perform gas phase combustion (two-stage combustion method).
It is a well-known technique called mbustion. In general, a highly active combustion catalyst is packed in the front stage, and vapor phase combustion is performed in the empty column in the rear stage. As the high-activity catalyst in the preceding stage, a catalyst in which the surface of a cordierite honeycomb is washcoated with alumina and a noble metal such as Pt or Pd is highly dispersed and supported thereon is usually used.

On the other hand, in a fuel cell, hydrocarbons and methanol are reacted with water vapor to produce a gas containing hydrogen, and this gas is introduced into the anode of the fuel cell to cause hydrogen to react and then the residual gas is removed from the fuel cell. It is discharged as anode waste gas. This anode waste gas is combustible and can be used as a fuel for various heat sources. However, using it as a fuel for a heat source of the hydrogen production device (steam reformer) reduces the process cost of the fuel cell. Most preferred.

As a combustion method for utilizing the anode waste gas of the fuel cell as a heat source for the steam reformer, a two-stage combustion method combining the conventional catalytic combustion step and gas phase combustion step can be adopted, but Applying the prior art as it is does not yield advantageous results.

First, since the catalyst in the former stage used in the above-mentioned prior art is a noble metal dispersion type, it deteriorates severely when it is used at 800 ° C. or higher at which the noble metal causes sintering. If the temperature exceeds 1000 ° C, cordierite reacts with the wash-coated alumina to reduce the strength of the catalyst and volatilize noble metal. Further, if the melting point of cordierite exceeds 1430 ° C for a moment, the catalyst will melt. In this way, since the pre-stage catalyst is weak against high temperature, even a slight operational error such as generation of high temperature cannot be tolerated.For this purpose, the composition and temperature of the anode waste gas of the fuel cell, and the oxygen-containing gas are required. Since it is required to always maintain the flow rate of the fuel cell within a certain range, the operation of the fuel cell becomes extremely difficult.

Further, when trying to apply the above-mentioned conventional technique as a combustion method of anode waste gas of a fuel cell, at the time of start-up of the fuel cell, the starting gas fuel is burner ignited and burned, thereby heating the pre-stage catalyst, and the starting fuel gas is After raising the temperature of the catalyst layer to the temperature at which catalytic combustion is started, catalytic fuel is burnt with starter fuel gas containing excess air, and the fuel gas is burned in the vapor reformer by adding starter fuel gas again in the latter stage. It is necessary to operate so as to obtain the high temperature gas of 1100 ° C or higher required for operation. However, when the conventional catalyst is filled in the former stage, the combustion gas in the former stage must not be heated to 800 ° C. or higher as described above, and therefore the vapor-phase combustion start temperature in the latter stage is 800 ° C. or lower. However, methane gas does not practically burn at this temperature, and a fuel cell using natural gas containing methane gas, LNG, or the like has a disadvantage that another kind of fuel must be used as a starting fuel for the steam reformer.

Further, the gas-phase combustion of the latter stage in a simple empty column is not advantageous as the combustion method of the anode waste gas of the fuel cell.
The fuel concentration of the anode waste gas discharged from the fuel cell fluctuates, and the fuel concentration may sometimes decrease. In such a case, if the latter stage is vapor phase combustion of the empty column, it is extremely easy to misfire, and it is easy to cause a trouble in operation.

[Problems of the Invention]

An object of the present invention is to provide a method for effectively burning anode waste gas of a fuel cell for effective use as a heat source for a steam reformer.

[Means for solving the problem]

The present inventors have completed the present invention as a result of earnest studies to solve the above problems.

That is, according to the present invention, (1) when burning the anode waste gas of the fuel cell, a part of the waste gas and the total amount of the oxygen-containing gas necessary for burning the entire amount of the waste gas are mixed, A first combustion step in which this is combusted at 600 to 900 ° C. through a packed bed of a catalyst in which palladium powder is supported on a heat-resistant carrier, and the remaining amount of the waste gas is mixed with the combustion gas from the first combustion step. , Contact with the heat-resistant ceramics packed layer
A second combustion step of burning in the range of 1100 ° C. to 1400 ° C., and a heat-resistant ceramics filling layer used in the second combustion step is provided in the pores of the non-oxide ceramics made of silicon carbide or silicon nitride. In a packed bed of heat-resistant metal or metal oxide-containing non-oxide ceramics with a pore volume of 0.001 cc / g or less made by containing a metal or metal oxide that does not substantially melt at the combustion temperature of the second combustion step A two-stage combustion method is provided, characterized in that

The catalyst used in the first-stage catalytic combustion step of the present invention is a palladium powder catalyst supported on a heat-resistant carrier. As the heat resistant carrier, ceramics having a melting point or a sublimation point of 1800 ° C. or higher, preferably 2000 ° C. or higher is used. Examples of such a material include silicon carbide, silicon nitrogen, alumina titanate, and alumina zirconia. In the present invention, the metal catalyst supported on the heat resistant carrier is palladium powder. The average particle size of this palladium powder is at least 1 μm, preferably 5
~ 30 μm. The amount of palladium powder supported is 0.1 to 10% by weight, preferably 0.5 to 5% by weight, based on the whole catalyst.
The catalyst of the present invention may have various shapes such as a spherical shape, a cylindrical shape, and a cylindrical shape, but a honeycomb shape is preferable.

As a method for supporting the Pd powder on the heat-resistant carrier, various conventionally known methods are adopted. For example, using a honeycomb carrier as an example, Pd powder is mixed with heat-resistant ceramic powder containing alumina, silica, titania or the like as a main component in water or an organic solvent to form a slurry having an appropriate viscosity. At this time, an inorganic binder such as sodium silicate or acidic aluminum salt may be added to the slurry. This Pd-containing slurry is transferred to a closed container, and a honeycomb-shaped carrier is immersed therein to reduce the pressure of the whole. When the slurry has sufficiently penetrated into the cells of the honeycomb-shaped carrier, the honeycomb-shaped carrier is taken out, and the slurry clogged in the cells of the honeycomb-shaped carrier is blown off by air blow. Then, the carrier is dried and then calcined. In this way, a catalyst having a Pd powder layer on the surface is obtained. In this case, Pd
The thickness of the powder layer can be controlled by the viscosity and solid content concentration of the slurry prepared first.

Since the combustion catalyst used in the present invention uses a heat-resistant carrier having a melting point of 1800 ° C. or higher, unlike a conventional cordierite carrier, at the time of operation of the combustion device, some misoperation or generation of high temperature due to uneven fuel concentration Even if there is, it does not cause the melting phenomenon. In addition, since Pd powder, which does not easily volatilize at high temperatures, is used as the metal catalyst, the methane / air mixed gas at the start of combustion can be used only in the catalytic combustion process.
It is possible to raise the temperature to 00 ° C, and increase the fuel cell waste gas supply to a level higher than usual to raise the combustion temperature to 90
It can also be raised to 0 ° C. The weak point of this Pd catalyst used in the present invention as compared with the conventional catalyst is that it lacks low temperature ignitability. Conventional combustion catalysts use hydrogen-air mixed gas at 10
It can ignite below 0 ° C. On the other hand, with this catalyst, ignition of the hydrogen-air mixed gas is required to be about 250 ° C. However, since the fuel cell anode waste gas is 300 ° C or higher in the phosphoric acid type fuel cell and 450 ° C or higher in the molten carbonate type fuel cell even when air-mixed, there is no problem in combustion by this catalyst.

As the ceramics used in the second stage catalytic combustion step of the present invention, those having a high melting point, generally those having a melting point of 1800 ° C. or higher, like the carrier of the combustion catalyst are used. However, since it is used at a higher temperature than the combustion catalyst, it is preferable that it has excellent thermal shock resistance. The first role of this ceramic is to prevent misfire due to a change in fuel concentration by utilizing the fact that the ceramic emits infrared rays at high temperature. The second role is that, even if a misfire occurs once, its re-ignition becomes much easier than in the case of an empty tower.
Further, the third role is that the heat loss during combustion can be suppressed to a considerably low level as compared with the case of the empty tower. Since the ceramics used in the present invention are used under severe conditions, it is important that they have excellent durability. That is, the ceramics suitable for use in the present invention are required to have the following properties.

 Great thermal shock resistance.

 High heat resistance (high melting point).

 Great resistance to oxidation.

There is currently no single substance that meets all these requirements. Oxide ceramics are heat resistant
It is excellent in oxidation resistance, but inferior in thermal shock resistance due to its low thermal conductivity and high coefficient of thermal expansion. Among oxide-based ceramics, Celsian, Wilmite, Beryl, Cordierite, Spongemen,
There are quartz glass, alumina titanate, etc., but all of them cause phase change at high temperature, low melting point,
It has some drawbacks such as easy volatilization at high temperature and weak physical strength, and is inferior in practicality. On the other hand, non-oxide ceramics, especially the typical materials such as silicon carbide and silicon nitride, are excellent in thermal shock resistance and have a sublimation point of 20%.
It is as high as 00 ℃ or higher. However, the crystals of both silicon carbide and silicon nitride gradually oxidize from their surfaces, and at high temperatures,
Practicality is inferior because destruction due to oxidative expansion occurs. As a result of intensive studies to improve the oxidation resistance of the non-oxide ceramics, the present inventors have found that the second oxide is formed in the pores of the non-oxide ceramics made of silicon carbide or silicon nitride.
By containing a metal or metal oxide that does not substantially melt at the combustion temperature of the combustion process, the pore volume is substantially zero (0.001c
It has been found that the heat-resistant metal or non-oxide ceramics containing a metal oxide, which is c / g or less), has significantly improved oxidation resistance. In silicon carbide and silicon nitride having pores (pores), the outside air, especially oxygen, freely diffuses inside the pores, so that silicon carbide and silicon nitride including the bulk phase should be oxidized throughout the structure during use. become. On the other hand, in the case of silicon carbide or silicon nitride whose pores are filled with molten metal or molten metal oxide, oxygen only comes into contact with the outer surface of the metal and there is no diffusion of oxygen into the bulk phase. The oxidation rate of the entire structure is much slower than that of the material having pores, because it progresses only. Therefore, the life thereof is dramatically extended, and it can be practically used.

The shape of the ceramic used in the present invention may be cylindrical, cylindrical, plate-shaped, Raschig ring-shaped, etc., but is preferably honeycomb-shaped.

Next, a method for producing ceramics suitable for use in the second stage catalytic combustion step in the present invention will be described. Silicon carbide,
The sublimation point of ceramics made of silicon nitride is 2000 ° C.
It is structurally stable even when immersed in a melt at 1300 to 1800 ℃. Therefore, a ceramic container is placed in a pressurizable high-temperature furnace, and a metal or metal oxide having a melting point in the range of 1300 to 1800 ° C is put in powder form in this, and a ceramic compact is embedded in it and The metal or metal oxide powder is heated to its melting point in an active gas atmosphere, and after confirming that the metal or metal oxide is in a molten state, the system is pressurized with an inert gas to form a ceramic. The molten metal or the molten metal oxide is filled inside the pores of the body. A ceramic molded body whose pores are filled with a melt is taken out from the melt and cooled to obtain a ceramic molded body having improved oxidation resistance. It is not limited only to how to make it. Any method may be used as long as the pore volume of the final ceramic molded body is substantially zero (0.001 cc / g or less). As the metal or metal oxide, those having a melting point of 1300 to 1800 ° C. may be used,
From the viewpoint of ease of operation, a material having a melting point of 1300 to 1600 ° C is selected. Examples of such a metal include Si (1430
℃) Fe (1536 ℃) Co (1495 ℃), Ni (1453 ℃) Y (15
09 ℃), Sc (1539 ℃), rare earth elements (Gd, Tb, Dy, Ho, etc.)
And alloys thereof. Further, other alloys may be used as long as the alloy has a melting point of 1300 to 1600 ° C. 1300
Examples of the metal oxide having a melting point at 1600 ° C. include cordierite (2MgO · 2Al ₂ O ₃ · 5SiO ₂ , melting point 1470 ° C), calcia-titania-silica (CaO · TiO ₂ · SiO ₂ , melting point 135).
2 ° C), etc., and can be selected appropriately from the table on pages 541-542 of the Fine Ceramics Handbook (Asakura Shoten).
However, since pure silicon carbide compacts and silicon nitride compacts have poor wettability with these oxides, it is preferable to use one obtained by mixing and firing some oxide ceramics in the manufacturing process of these ceramic compacts, or It is preferable to use a ceramic molded body which has been exposed to a mixed gas of steam and air at a high temperature to oxidize the crystal surface of the ceramic molded body.

Next, the combustion method of the present invention will be described with reference to the drawings. First
The figure is an explanatory cross-sectional view of one example of a combustion apparatus used for carrying out the method of the present invention.

This combustion device is formed into a tubular shape as a whole, and the outer tubular body 1
Insulating materials A-1, A-2, and A-3 are attached to the inner peripheral surface of the cylinder, and a pre-combustion chamber B and an anode waste gas mixing chamber B are formed in the cylinder by partition walls made of the insulating materials A-4 and A-5. And the latter combustion chamber C. Further, heat insulating materials A-6 and A-7 are provided on the front surface and the rear surface of the outer tubular body 1, respectively.

In the combustion chamber B, two circular ceramic perforated plates 1 and 2 are erected. These perforated plates are cordierite,
It is composed of ceramics such as alumina and mullite, and promotes uniform mixing of the anode waste gas and air. Also,
In the combustion chamber B, plate-like catalysts (honeycomb plates) 4 and 5 having a palladium powder layer on the surface are erected, and between the ceramic porous plate 2 and the plate-like catalyst 4, anode waste gas /
A circular honeycomb plate 3 for rectifying the flow of the oxygen-containing gas mixture is erected. This honeycomb plate 3 is cordierite,
It is composed of ceramics such as alumina and mullite.

In the latter combustion chamber D, a plurality of circular ceramic plates 11 to 16 having a honeycomb structure are erected.

In the middle anode waste gas mixing chamber C, the pre-stage combustion chamber B
A heat-resistant pipe 6 is provided to connect the outlet of the anode and the inlet of the latter-stage combustion chamber D, and the pipe 6 is provided with a side pipe 7 for supplying anode waste gas.

In order to burn the anode waste gas using this combustion device, a part of the anode waste gas passing through the pipe 20 is passed through the pipe 22 and the oxygen-containing gas from the pipe 23 into the pre-stage combustion chamber B from the inlet 8 thereof. Introduce. In this case, the temperatures of the anode waste gas and the oxygen-containing gas are kept at least at 300 ° C in advance. The anode waste gas and the oxygen-containing gas introduced into the combustion chamber B are mixed here, and the plate-shaped catalysts 4,5
Is contacted with and burned to produce a combustion gas containing oxygen. The temperature of this combustion gas is usually 600 to 900 ° C.

The combustion gas containing oxygen enters the pipe 6 from the combustion chamber B through its outlet, is mixed with the remaining anode waste gas supplied through the pipe 21 and the side pipe 7, and is mixed in the latter stage combustion chamber D. Introduced therein, where it contacts and burns with the ceramic plates 11-16. In this case, as the oxygen as the combustion supporting gas, the excess oxygen contained in the combustion gas generated in the front combustion chamber B is used.

The combustion gas generated in the latter combustion chamber D has a temperature of 1100
~ 1400 ° C, which is used as a heat source for steam reformers to produce hydrogen.

As the oxygen-containing gas used in the present invention, oxygen, air or oxygen-enriched air is used. In the present invention, the oxygen-containing gas is supplied into the pre-stage combustion chamber B so as to be in excess of the amount of the anode waste gas supplied into the combustion chamber B. Generally, the pre-stage combustion chamber B and the anode waste gas mixing chamber C
Oxygen-containing gas sufficient to completely burn all the anode waste gas supplied to and is supplied into the pre-stage combustion chamber B.

〔The invention's effect〕

According to the present invention, the anode waste gas of the fuel cell can be efficiently combusted to obtain a high temperature combustion gas having a temperature of 1100 to 1400 ° C. Moreover, in the case of the present invention, the ceramic filling in the second combustion step is performed. The long life of the layer has the great advantage that it can be carried out continuously over a long period of time. This combustion gas is advantageously used as a heat source for a steam reformer for hydrogen production used in a fuel cell.

(Example) Next, the present invention will be described in more detail with reference to examples.

Reference Example 1 50 parts by weight of Pd fine powder having an average particle diameter of 3 μm, 50 parts by weight of γ-alumina powder, and 10 parts by weight of aluminum nitrate were mixed with 100 parts by weight of water and sufficiently stirred with a homogenizer to obtain a slurry-like substance. In this slurry-like substance, diameter 50 mm, thickness 25 m
After dipping two alumina titania (FeO added) honeycomb discs with m and cell number 200 / in ² and confirming that enough slurry was contained in the cells (when the generation of bubbles stopped), the honeycomb discs Was taken out, and the slurry clogged in the cell was removed by air blowing. The two honeycomb disks having the slurry adhered to the surface were dried at 150 ° C. for 3 hours, placed in an electric furnace and fired at 1200 ° C. for 3 hours. The finished plate-like catalysts are both Pd about 0.8 wt% from the weight increase rate.
Found to include. This catalyst was designated as the former catalyst.

Reference Example 2 A pressureless sintered silicon carbide honeycomb disc having a diameter of 40 mm, a thickness of 25 mm, and a cell number of 20 / in ² impregnated with a melt of Si and having a pore volume of substantially zero (ceramic plate I) Was obtained from the manufacturer. Also, a carbon honeycomb disc with a diameter of 40 mm, a thickness of 25 mm, and a cell number of 16 / in ² obtained by molding pitch from another manufacturer into a honeycomb disc and firing it in a non-oxidizing atmosphere was used. After being immersed in a silicon melt, a reaction-sintered silicon carbide plate (ceramic plate II) was obtained in which the carbon portion was converted into silicon carbide by sintering in an inert gas at 1600 ° C.
The ceramic plate II is also made of Si between silicon carbide particles during the manufacturing process.
Remains, the pore volume is substantially zero.

As other samples, four types of pressureless sintering type silicon carbide honeycomb disks III to VI were collected. These are diameter 40
It is in the range of ~ 55mm and all thicknesses are 25mm. Different from the ceramic plates I and II, these have pores (pores).

Example 1 The front-stage combustion chamber B of the two-stage type combustion apparatus shown in FIG. 1 was filled with two front-stage catalysts prepared in Reference Example 1, and the rear-stage combustion chamber D was filled.
Was simultaneously filled with the six types of ceramic plates I to VI shown in Reference Example 2, and a combustion test was conducted by flowing a mixed gas simulating the anode waste gas of the molten carbonate fuel cell. The composition of the simulated anode waste gas in this case is shown below.

Simulated anode waste gas 580 Nl / hr and air 2600 Nl / hr were mixed, preheated to 350 ° C., and allowed to flow through the inlet 8 of the apparatus shown in FIG. The gas burned in the first stage rises to 800 ℃, and the resulting combustion waste gas is the simulated anode waste gas in the second stage.
It is mixed with 00Nl / hr in the mixing chamber C, burned again in the ceramic packed bed in the latter stage, and heated to 1200 ° C. This experiment is 2000
It was driven smoothly on time. Table 1 shows the physical properties of the ceramic plate filled in the latter stage, the weight increase after 2000 hours of operation, and the results of visual observation of its appearance. The two ceramic plates I and II whose pores were filled with the melt had little increase in oxidation and showed almost no change in their appearance, while the other four ceramics III-VI having pores differed in size. In any case, it is suggested that each of them has a large oxidative increase and has expanded or collapsed, and cannot be used for a long time.

[Brief description of drawings]

FIG. 1 shows an explanatory sectional view of an example of a combustion apparatus used in the present invention. 1,2 ... Porous plate, 3 ... Honeycomb plate, 4,5 ... Plate catalyst,
11 to 16 ... Ceramic plate, A-1 to A-7 ... Heat insulating material.

Claims (2)

[Claims] 1. When burning anode waste gas of a fuel cell, a part of the waste gas and the total amount of oxygen-containing gas required to burn the entire amount of the waste gas are mixed, and this is mixed with a heat resistant palladium powder. A first combustion step in which the catalyst is supported on a porous carrier and burned at 600 to 900 ° C through a packed bed, and the residual amount of the waste gas is mixed with the combustion gas from the first combustion step to fill the heat-resistant ceramics. A second combustion step in which the layer is brought into contact with the layer and burned in the range of 1100 ° C. to 1400 ° C., and the heat-resistant ceramic filling layer used in the second combustion step is made of a non-oxide ceramic made of silicon carbide or silicon nitride. The volume of the pores is 0.001c because the pores contain a metal or metal oxide that does not substantially melt at the combustion temperature of the second combustion step.
A two-stage combustion method, which is a packed layer of a non-oxide ceramic containing a heat-resistant metal or a metal oxide of c / g or less. 2. A honeycomb in which the palladium powder is composed of a single crystal having an average particle size of 1 μm or more, and the heat-resistant carrier carrying the palladium powder contains alumina titanate, alumina zirconia, silicon carbide or silicon nitride as a main component. The method according to claim 1, which comprises: