WO2008093292A1

WO2008093292A1 - Combined solid oxide fuel cell

Info

Publication number: WO2008093292A1
Application number: PCT/IB2008/050359
Authority: WO
Inventors: Sadig Kuliyev; Beycan Ibrahimoglu; Rafig Alibeyli; Ibrahim Pamuk; Yalçin SEZEN; Deniz Kozlu
Original assignee: Vestel Elektronik Sanayi Ve Ticaret A.S.
Priority date: 2007-02-02
Filing date: 2008-01-31
Publication date: 2008-08-07
Also published as: TR200700587A2

Abstract

A combined solid oxide fuel cell is designed to convert the hydrogen generated by decomposing methane, natural gas, or other hydrocarbon gases into electrical energy by means of an electrochemical method, wherein the fuel cell and the gas recycling reformer are designed together in a single unit. Here, the reformer (8) is designed at the lower section of the fuel cell; catalyst (7) is provided in the reformer, and membranes (1) are placed on the reformer. Hydrogen generated by decomposing gases in the reformer generates current at the anode surface of membranes, forms water with oxygen ions coming from the cathode region (2), and such generated water contacts the reformer catalyst in the form of vapor. Water vapor generated at the anode region (3) reacts with the coke (carbon) formed on the reformer catalyst's surface, yielding carbon monoxide and hydrogen. Thanks to this feature, the carbonization of catalyst is avoided so as to prolong its operation time and generate extra current from such generated hydrogen.

Description

DESCRIPTION COMBINED SOLID OXIDE FUEL CELL

TECHNICAL FIELD The present invention relates to a Solid Oxide Fuel Cell (SOFC) designed in a combined manner. Such combined solid oxide fuel cell designed to convert the hydrogen generated by decomposing hydrocarbon gases and fossil fuels into electrical energy by means of an electrochemical method is characterized in that the fuel cell and the gas conversion reformer are designed together as a single unit.

TECHNICAL PROBLEMS THE PRESENT INVENTION AIMS TO SOLVE

There is a gradual increase in researches towards clean energy resources and technologies in various industrial fields in recent years. Among such researches, those focused on Solid Oxide Fuel Cells (SOFC) have been more widely encountered with respect to industrial applications. The operation principle of fuel cells is based on converting the chemical reaction energy directly into electrical energy [1 ,2]. As long as fuel is fed, the fuel cells shall keep producing electrical energy, heat, and water. This reaction is conducted by contacting the membrane within fuel cells to reducing and oxidizing agents. The amount of energy produced from fuel cells depends typically on the number, area, and quality of membranes. The fuel cells can be listed as following based on their types:

^■ Proton Exchange Membranes (PEM),

^■ Cylindrical Proton Exchange Membranes (CPEM),

^■ Alkaline fuel cells (AFC),

^■ Phosphoric acid fuel cells (PAFC),

■ Molten carbonate fuel cells (MCFC),

^« Solid oxide fuel cells (SOFC),

^■ Direct methanol fuel cells (DMFC), and

^■ Regenerative fuel cells (RFC), etc.

The US Patents 5.366.819, 5.712.055, and 5.763.114 may be referred to for reviewing such fuel cells. Fuel cells may provide low-, medium-, and large-capacity electrical power production. They have the following advantages over conventional power producing systems:

very low environmental pollution rates,

- very high energy production efficiency (above 90% theoretically),

- operability with different type of fuels such as hydrogen, natural gas, methanol, ethanol, naphtha, coal dust, etc.,

recoverability of wasted heat (cogeneration),

very high improvement potential,

no solid waste pollution problems,

- no noise creation during operation,

operability together with or separately from the city network.

Nowadays researches are being made on Solid Oxide Fuel Cells for utilization in military and civil fields such as houses, vehicles, submarines, and other equipments and industrial fields, etc.. SOFC of varying sizes have already been manufactured [3, A]. SOFCs operate with hydrogen obtained as a result of cracking fossil fuels at high temperatures [4].

There are available low-, medium-, and high-temperature solid fuels depending on the working temperature. High-temperature fuel cells are classified as Molten Carbonate Fuel Cells (MCFC) and Solid Oxide Fuel Cells (SOFC) according to the electrolytes they employ. Hydrogen gas (H₂) is required to operate all fuel cells. Hydrogen gas is produced by various means and supplied to fuel cells. The H₂ gas required for SOFCs is obtained by various catalytic methods from coal, diesel oil, gasoline, natural gas, methane, LPG, etc. [1 ,3]. The hydrogen gas generation process is conducted by means of the following catalytic methods at 650-850⁰C temperatures:

1. Reaction of methane with water vapor:

CH₄ + H₂O → CO + 3H₂

2. Partial oxidation reaction of methane with oxygen:

CH₄ + Vz O₂ → CO + 2 H₂ 3. Direct decomposition reaction of methane:

CH₄ → C + 2 H₂ Although each of said methods have the following advantages and/or disadvantages, they are still in use:

1. The significant advantage of the conversion method of methane with water vapor is based on that even the one hydrogen molecule in water resulting from the reaction is made use of, so as to enhance the efficiency.

The disadvantage of this method, however, is the need of continuous water vapor supply to the system, requiring additional equipment, control systems, and fittings for this purpose. In brief, the size of the cell becomes large, its cost increases, and the system becomes complicated.

2. In the method of partial oxidizing methane with oxygen, the system is fed with oxygen or air to bring about the oxidation reaction. Therefore, this method also requires additional equipment, control systems, and fittings. In brief, the size of the cell becomes large, its cost increases, and the system becomes relatively more complicated.

3. In the direct decomposition reaction of methane, the carbon produced from the reaction covers the catalyst's surface so that the hydrogen generation rate is gradually lowered and after a certain period of time, it renders the catalyst completely inoperable. In order to avoid this process, it becomes indispensable to regenerate the catalyst with oxygen at high temperatures after a certain period of time. While the catalyst is activated, two reformers are required to continuously run the fuel cell. Periodically, in one of said reformers the catalyst is activated, while in the other the hydrogen is converted. As known, extra system developments are required in order to conduct all such processes.

As illustrated in the solid oxide fuel cell system's diagram in Figure 1 , the reformer is built separately from the cell [1 , 2, 3].

The reference numbers of parts in this figure are as follows: a) Reformer section, 1- Reformer,

2- Catalyst

3- Heater, b) Fuel cell section,

4- Hydrogen chamber, 5- Membrane electrode group,

6- Membrane electrode group, 7- Oxygen chamber,

8- Water pipe.

First, the fuel is supplied to the reformer and is converted to hydrogen at high temperatures. The obtained hydrogen gas is fed to the fuel cell at high temperature.

There are various solid oxide fuel cells available now. The distinction among fuel cells are generally based on their constructions. For instance:

1. Tubular solid oxide fuel cells 2. Planar solid oxide fuel cells

3. Monolithic solid oxide fuel cells, etc.

The advantages and disadvantages of such or other solid oxide fuel cells are known. The tubular solid oxide fuel cells, for instance, allow for simple cell stacking and do not require gaskets. However the manufacturing of such fuel cells are difficult, expensive, and labor- intense. Since it also involves a long current collector path, it presents a lower power density as compared to other type of fuel cells.

Whilst the monolithic solid oxide fuel cells present a relatively high power density, they require sintering of each layer as of their raw forms, resulting in severe contraction and breaking problems. The plating of such type of fuel cells are cumbersome either.

As for the planar solid oxide fuel cells, whilst they have the advantage of being proper to be cast in the form of stripes, they present "thermal shock resistance" due to high temperatures.

Solid oxide fuel cells need to be operated at around 1000⁰C to have a low internal electrical resistance.

The operating temperature of solid oxide fuel cells is in principle at an adequate level for vapor reforming within the solid oxide stacks for hydrocarbon fuels. The internal vapor reforming shall simplify the balance of the solid oxide fuel cell's power system and enhance the operation efficiency. During the reforming of hydrocarbon fuel within the stack, however, many problems emerge.

As known, the following processes occur in solid oxide fuel cells: 1- Hydrogen gas is generated from fuels (natural gas, methane, LPG and similar gases) by means of catalytic methods,

2- Hydrogen and oxygen gases are reduced and oxidized in the fuel cell,

3- Direct current is generated by collecting electrons from anode collectors, 4- The heat resulting from reactions is utilized.

The decomposition of methane, the first of such processes, is conduced out of the fuel cell for hydrogen generation. The remaining processes are conducted within the fuel cell.

With respect to the Solid Oxide Fuel Cell (SOFC) according to the present invention, the methane converter (i.e. reformer) to generate hydrogen is not designed as a separate unit, instead, it is devised as a single unit together with the fuel cell. Differing from conventional methods, all processes in this design are conducted concurrently and in sequence within a single unit comprising the fuel cell and reformer, as illustrated below.

During the reforming process in the fuel cell according to the present invention, in addition to the direct decomposition of methane, the conversion reactions of methane and carbon with water vapor are conducted in parallel. The water required for the latter two reactions is generated at the surface of anode as a result of ongoing reactions within the solid oxide fuel cell, and the water is converted to steam with the high temperature in the cell.

H₂ + O^"2→ H₂O + 2e^"

Therefore, in the reformer section of the combined solid oxide fuel cell claimed as a single unit, the following serial-parallel reactions are conducted:

a) Direct decomposition of methane: CH₄ + H₂O → CO + 3H₂

b) Conversion of methane with water vapor:

CH₄ + H₂O ^■» CO + 3H₂

c) Conversion of carbon with water vapor: C + H₂O -» CO + H₂ Another feature of the combined solid oxide fuel cell according to the present invention is that the water required to convert methane and carbon by means of water vapor is completely compensated with the water generated at the anode section of the fuel cell membrane. In other words, no additional and external water is supplied to the system. Therefore the reactions occurring in the system come to balance by itself.

As seen, a further significant feature of the combined solid oxide fuel cell according to the present invention is that no extra oxidative regenerator is required in converting the coke (carbon) formed on the catalyst's surface in the direct decomposition process of methane with water vapor, in other words, in eliminating the coke formed this way.

Figure 2 illustrates the basic elements of the fuel cell and the operation principle thereof. The reference numbers of parts in Figure 2 are as following: 1- Fuel,

2- Reformer,

3- Interconnector,

4- Combustion products,

5- Electrolyte, 6- Exhaust,

7- Air inlet,

8- Load.

A SOF cell is composed of two electrodes separated by an electrolyte membrane. Oxygen supplied from air or by direct means is reduced to oxygen ion at the cathode region.

O₂ + 4e^" → 2O^"2

The electrolyte ensures the transfer of oxygen ions from the cathode to the anode. At the anode region, hydrogen and carbon monoxide are oxidized with the O^"2 ions coming from the cathode.

H₂ + O^"2→ H₂O + 2e^" CO + O^"2 → CO₂ + 2e^"

As is seen, while carbon monoxide is oxidized at the SOFC two electrons are released, enhancing the system's efficiency. In result, both the toxic CO gas is used as a fuel in the SOFC system run at low temperatures, and the diffusion of this gas to the atmosphere is avoided.

Thus, a different SOFC is designed based on the reactions occurring at the cathode and anode. The direct decomposition approach of methane is envisaged for the Combined Solid Oxide Fuel Cell according to the present invention. The cell design is simple, its efficiency is high, and the coke formation on the catalyst, along with other problems are avoided without necessitating additional equipments and devices. The subject SOFC is also more compact and is run at a relatively higher efficiency.

As shown in Figure 3, the reformer required for the fuel cell is not designed out of the cell, but is designed in the form of a plate at the lower section of the cell. The reference numbers of parts in Figure 3 are as following: 1- Catalyst, 2- Catalyst slot,

3- Membrane,

4- Reformer,

5- Inlet channel of the methane gas,

6- Methane distribution tank.

OPERATION OF THE COMBINED SOLID OXIDE FUEL CELL

The combined solid oxide fuel cell according to the present invention differs from other solid oxide fuel cells with respect to design and operation procedure.

Figure 4 illustrates a stack of combined fuel cell.

The reference numbers of parts in Figure 5 are as following:

1- Screw with an insulated exterior,

2- Nut,

3-Spacer,

4-Spring cover,

5-Spring,

6- Upper cover,

7-lnsulation, 8-Cells,

9- Hydrogen outlet channel,

10- Oxygen outlet channel,

11 -Tube,

12- Lower cover,

13- Reformer.

Figure 5 gives a cross-section of a combined fuel cell stack. The reference numbers of parts in Figure 5 are as following: 1- Membrane,

2- Oxygen flow channel,

3- Hydrogen flow channel,

4- Inter-cell flow channel (hydrogen),

5- Inter-cell flow channel (oxygen), 6- Catalyst slot,

7- Catalyst

8- Reformer,

9- Methane tank.

10- Gas distributor plate.

As seen, a plurality of cylindrical tubes (6) are provided within the reformer (8) located at the lower section of the fuel cell, and the interior of said tubes are filled with catalyst (7). The catalyst (7) is accurately filled so as to occupy the two third of the volume of cylindrical tubes (6), leaving a space for the enlargement of catalyst (7) in use. Methane gas supplied externally to the reformer (8) is collected at the lower tank (9) and is transferred by means of channels (5) to cylindrical tubes (6) containing the catalyst (7). The methane gas undergoes the decomposition reaction here on the catalyst (7):

CH₄→ C + 2H₂

yielding the hydrogen required for the solid oxide fuel cell. The hydrogen gas generated in cylindrical tubes (6) filled with catalyst contacts the anode surface of the membrane (1) assembled on the tubes. Oxygen supplied from air or by direct means is reduced to oxygen ion on the cathode surface.

O₂ + Ae → 2O^"2

The electrolyte membrane group provides the transfer of oxygen ions from the cathode to the anode. At the anode region, hydrogen and carbon monoxide are oxidized with the O^"2 ions coming from the cathode.

H₂ + O^"2 → H₂O + 2e^"

CO+O^"2 -> CO₂ + 2 e^'

Meanwhile, the water vapor generated at the anode surface enters into the following reactions with the coke (carbon) generated on the catalyst and the methane at the reformer region (vapor reforming):

CH₄ + H₂O -» CO +3H₂

C + H₂O → CO+ H₂

releasing hydrogen and carbon monoxide. In this manner, the rate of hydrogen generation is increased and the fuel cell's efficiency enhanced. The hydrogen generated at the reformer section is transferred to serially-connected fuel cells by means of inlet (4) and outlet (5) channels at the anode side, so that hydrogen (3) and oxygen (2) gases are uniformly distributed along the entire area by helical channels formed on the gas distributor plate (10). Thus, the hydrogen requirement to operate the fuel cell is met. Depending on the fuel cell's power, there may be designed more than one, i.e. several reformers in the combined solid oxide fuel cell.

In the combined solid oxide fuel cell according to the present invention, various hydrocarbon gases such as C₁-C₄ and their mixtures can be used as fuel.

The content of natural gas (from the city network) used in our researches and obtained by gas chromatography are given in Table 1 below.

Table 1.

The content of gases obtained from test results conducted at different temperatures are given below: Experiment 1 : Operation temperature: 600⁰C

Experiment 2: Operation temperature: 650°C

Claims

1. A combined solid oxide fuel ceil for converting the hydrogen, generated by decomposing hydrocarbon gases and fossil fuels, into electrical energy by means of an electrochemical method, characterized in that the fuel cell and the gas converting reformer (8) are designed together as a single unit.

2. A combined solid oxide fuel cell according to Claim 1 , characterized in that said reformer (8) is designed in the lower section of the fuel cell wherein the catalyst (7) is provided in the reformer, and membranes (1) are provided on the reformer .

3. A combined solid oxide fuel cell according to Claim 1 , characterized in that the hydrogen generated in the reformer (8) generates current on the anode surface of membranes and produces water with oxygen ions supplied by the cathode section and the generated water contacts the reformer catalyst in the form of vapor.

4. A combined solid oxide fuel cell according to Claim 1 , characterized in that the water vapor generated at the anode section reacts with coke (carbon) formed on the surface of the reformer catalyst (7), yielding carbon monoxide and hydrogen, and avoids the carbonization (coke formation) of the catalyst, prolonging the operation period of the catalyst, and in that it allows for extra current generation from the generated hydrogen.

5. A combined solid oxide fuel cell according to Claim 1 , characterized in that some portion of fuel (methane etc.) supplied to the reformer (8) reacts with water vapor generated at the anode region, yielding carbon monoxide and hydrogen, and in that it allows for extra current generation from the generated hydrogen.

6. A combined solid oxide fuel cell according to Claim 1 , characterized in that the carbon monoxide formed in the reformer (8) are oxidized up to carbon dioxide at the anode surface of membranes with oxygen ions from the cathode section so as to allow for extra current generation.

7. A combined solid oxide fuel cell according to Claim 1 , characterized in that any desired number of fuel cells are horizontally assembled on the reformer section in order to form a stack configuration.