JP5157264B2 - Fuel cell power generation system - Google Patents

Description

The present invention includes a power generation stack composed of solid oxide fuel cells, and reforms hydrocarbon fuels such as city gas, natural gas, propane gas, gasoline, light oil, kerosene, etc., as fuel gas, if necessary. It is related to a fuel cell power generation system that generates power using fuel gas containing hydrocarbons and carbon monoxide, and in particular, it prevents the deposition of carbonaceous matter at the fuel electrode of the fuel cell stack, thereby reducing the power generation performance. The present invention relates to a fuel cell power generation system that can prevent the above.

When the fuel cell power generation system is a power generation system using a solid oxide fuel cell (SOFC), from another type, for example, a polymer electrolyte fuel cell (PEFC) using a proton conductive solid polymer electrolyte Compared to the system which consists of, the advantage of the diversity of fuel used can be mentioned.
For example, in the case of a polymer electrolyte fuel cell, the electrolyte is a proton conductor and the operating temperature is as low as around 100 ° C., so that the fuel that can be supplied to the fuel cell is limited to pure hydrogen. This is because when the electrolyte is a proton conductor, the fuel cell reaction does not proceed even if the hydrocarbon fuel is directly supplied, and the hydrocarbon fuel is reformed inside the fuel cell at a temperature around 100 ° C. This is because hydrogen cannot be taken out.

The electrode reaction at the fuel electrode in this case is expressed by the following equation.
H ₂ → 2H ⁺ + 2e ⁻
Therefore, the fuel supplied to the fuel electrode is hydrogen using a high-pressure tank or the like, or the fuel is reformed outside the fuel cell to once generate hydrogen and supply the hydrogen.

However, the high-pressure tank for hydrogen becomes very large.
Moreover, since the electrode is poisoned by carbon monoxide when the operating temperature is around 100 ° C., even if hydrogen is generated by an external reformer, the concentration of carbon monoxide in the generated gas (reformed gas) Needs to be lowered to the limit, which leads to an increase in the reforming reactor.

On the other hand, in the case of a solid oxide fuel cell, a hydrocarbon fuel that can be transported in liquid without using a large high-pressure tank or a large external reformer, such as gasoline, light oil, kerosene, liquefied propane, etc. It is possible to construct a system using fuel as a fuel source.
When an oxygen ion conductor is used as the electrolyte made of a solid oxide, hydrocarbon fuel can be directly supplied to the fuel cell, and the electrode reaction at the fuel electrode in that case is expressed by the following equation.
CnHm + (2n + m / 2) O ²⁻ → nCO ₂ + (m / 2) H ₂ O + (4n + m) e ⁻

An internal reforming reaction can also be utilized. The internal reforming reaction can be expressed by the following formula.
CnHm + (q + x + 2y) H ₂ O → pH ₂ + qH ₂ O + xCO + yCO ₂

H ₂ produced here is a reaction represented by the following formula: H ₂ + 1 / 2O ²⁻ → H ₂ O + e ⁻
In addition, the electrode reaction of CO proceeds by a reaction represented by the following formula.
CO + 1 / 2O ²⁻ → CO ₂ + e ⁻

  Therefore, the fuel cell reaction can be performed by direct supply of hydrocarbon fuel or by internal reforming reaction.

Further, when a proton conductor is used as the solid oxide electrolyte, the electrode reaction proceeds with the hydrogen generated by the internal reforming by the reaction represented by the following formula.
H ₂ → 2H ⁺ + 2e ⁻
Furthermore, after CO is further converted into hydrogen by a shift reaction expressed by the following equation inside the fuel cell, the same electrode reaction proceeds.
CO + H ₂ O → CO ₂ + H ₂

As described above, the solid oxide fuel cell is compatible with various fuels. However, when power generation using a hydrocarbon fuel is performed, carbon deposition on the electrode may be a problem. The carbon deposition reaction proceeds according to the following reaction formula.
CnHm → nC + (m / 2) H ₂
2CO → C + CO ₂

If a large amount of carbon generated by such a reaction is deposited on the electrode surface, the reaction active sites on the electrode surface may be blocked, and the activity of the electrode reaction may be reduced.
In order to prevent such carbon deposition, there is a method of introducing H ₂ O into the fuel electrode. If H ₂ O is present, even if C is precipitated, it can be removed by the following reaction.
C + H ₂ O → CO + H ₂

Therefore, in order to prevent such carbon deposition, for example, Patent Document 1 proposes that a steam supply device is provided to supply steam to the fuel electrode, whereby the steam reforming reaction is performed. Generating and decomposing hydrocarbons into carbon monoxide (CO) and hydrogen (H ₂ ).

Further, in Patent Document 2, in order to reduce the supply of water to no or much, a fuel combustor and a plurality of solid oxide fuel cell modules are sequentially connected in series so that the fuel is completely burned in the combustor. The fuel electrode off-gas discharged from the first-stage module by mixing the combustion gas generated by the partial combustion that generates hydrogen, carbon monoxide, and water vapor with the fuel and supplying the fuel to the first-stage module In this document, the fuel is mixed and supplied to the second-stage module, and then the fuel is mixed with the fuel electrode off-gas from the previous-stage module and supplied to the subsequent-stage module.
JP 2003-86225 A JP 2004-247268 A

However, the solid oxide fuel cell described in Patent Document 1 requires a water vapor supply device, and requires a water tank for supplying water to the device and a vaporizer for converting water into water vapor. Thus, the complexity and size of the system are inevitable.
Furthermore, when steam is supplied excessively, it causes the steam oxidation of the electrode, so it is necessary to consider the control of the steam supply amount. When such a control device is added, the system becomes even larger, There is a problem of increasing complexity.

  On the other hand, in the fuel cell system described in Patent Document 2, since it is necessary to supply extra fuel to generate water, fuel efficiency is deteriorated, and water generated in the fuel cell modules at each stage is directly used. Since it is discharged to the module of the next stage, a large amount of water accumulates in the later stage, and there is a problem that steam oxidation of the electrode is unavoidable.

The present invention has been made in view of the above-described problems related to carbon deposition in a fuel cell system using a fuel gas containing hydrocarbons, and the object thereof is not to provide a water vapor supply device, Even if hydrocarbons and carbon monoxide are contained in the fuel gas without increasing the size and complexity of the device, it is possible to prevent the deposition of carbonaceous matter on the fuel electrode, and to improve battery performance over a long period of time. It is an object of the present invention to provide a fuel cell power generation system that can be held for a long time.

  As a result of intensive studies to achieve the above object, the present inventors have arranged a small fuel cell stack as a moisture adjusting means upstream of the fuel cell stack in the fuel supply path of the fuel cell power generation system. Thus, the inventors have found that the above object can be achieved and have completed the present invention.

That is , the fuel cell power generation system of the present invention includes a fuel cell stack composed of solid oxide fuel cells, fuel supply means for supplying fuel gas containing hydrocarbons and / or carbon monoxide to the fuel cell stack, An oxidant gas supply means for supplying a gas containing oxygen to the fuel cell stack; a sub-stack that is composed of a solid oxide fuel cell and is arranged in the middle of a fuel supply path from the fuel supply means to the fuel cell stack; The operation state detection means for detecting the operation state of the fuel cell stack, and the operation control means for adjusting the operation condition of the sub stack based on the state of the fuel cell stack detected by the operation state detection means , the operation control The means includes the operating state of the fuel cell stack detected by the operating state detecting means, the upstream side of the fuel gas flow path of the fuel cell stack, and Based on the oxygen partial pressure in the flow side, is calculated of H _{2 O} deficiency amount with respect to H _{2 O} required amount for preventing carbon deposition in the fuel cell stack, H _{2 O} emissions from the sub-stack H ₂ It is characterized by controlling so as to compensate for the O deficiency .

  According to the present invention, the sub-stack composed of the solid oxide fuel cell is disposed upstream of the fuel supply path to the fuel cell stack composed of the solid oxide fuel cell, and the exhaust fuel gas from the sub-stack is supplied to the sub-stack. Since it is introduced into the fuel cell stack, the sub stack functions as a moisture adjusting means. Then, by adjusting the operating conditions of the sub-stack according to the operating status of the fuel cell stack, the amount of water discharged from the sub-stack becomes an amount that is sufficient to prevent carbon deposition of the fuel cell stack. Therefore, even if hydrocarbons and carbon monoxide contained in the fuel gas are decomposed into carbon, they are removed by reaction with water without causing steam oxidation of the electrode.

  Hereinafter, the operation and control method of the fuel cell power generation system of the present invention, as well as its configuration, will be described more specifically and in detail.

  FIG. 1 is a block diagram showing an embodiment of a fuel cell power generation system according to the present invention. The fuel cell power generation system shown in the figure is a fuel cell stack 1 (hereinafter referred to as a sub-cell described later) composed of solid oxide fuel cells. A fuel cell stack (main stack) 1, a fuel supply means 2 for supplying fuel gas to the fuel cell stack (main stack) 1, and the fuel supply means 2 and the main stack 1. A piped fuel supply path 3, which is also composed of a solid oxide fuel cell, is arranged on the upstream side of the main stack 1 in the fuel supply path 3, and the main stack 1 and the substack 4 have oxygen in them. Is mainly composed of an oxidant gas supply means 5 for supplying an oxidant gas containing air (for example, air). An operation status detection means 6 for grasping the operation status of the main stack 1 based on outputs from a sensor, an ammeter, a voltmeter, and an oxygen sensor (not shown), and the main stack 1 grasped by the operation status detection means 6 On the basis of the state, the calculation control means 7 for adjusting the operating condition of the sub stack 4 is provided.

In addition, the arithmetic control means 7 is used for the relationship between the hydrocarbon flow rate in the main stack 1 and the sub stack 4 and the power generation amount, the H ₂ O generation amount, etc., the steam / carbon ratio (S / C value), and the carbon deposition amount. The relationship is accumulated as a database, the required fuel gas flow rate from the required power generation amount of the main stack 1, the amount of H ₂ O generated at that time, the amount of H ₂ O necessary to prevent carbon deposition, The amount of power required to generate the insufficient amount of H ₂ O by the sub stack 4 can be calculated, and the operating conditions of the sub stack 4 are controlled based on the calculation result and arranged in the sub stack 4. The operating status of the sub stack 3 can be monitored by the temperature sensor, ammeter, voltmeter and the like.

In the fuel cell power generation system according to the present invention, for example, city gas or natural gas is used as a fuel gas containing hydrocarbons or carbon monoxide, but these gases contain almost no H ₂ O. Since the fuel is inherently susceptible to carbon deposition, the sub-stack 4 effectively functions as a moisture adjusting means.

  In addition, propane gas, gasoline, light oil, kerosene, and partially reformed gas of alcohols also contain CO and unreacted hydrocarbons. Precipitation can be prevented. The partially reformed gas contains a small amount of moisture, but is not sufficient for preventing carbon deposition.

The fuel cell stack 1, that is, the main stack 1 and the sub stack 4 are each provided with one or more cell elements of a solid oxide fuel cell comprising a fuel electrode, an air electrode, and an electrolyte, The electrolyte material made of a material is not particularly limited, and examples thereof include YSZ (yttria stabilized zirconia), SDC (samaria doped ceria), SSZ (scandia stabilized zirconia), and LSGM (lanthanum rate). Examples of the fuel electrode material include Ni-YSZ, Ni-SDC, Ni-SSZ, and the like.
Examples of air electrode materials include electron / oxygen ion conductive oxides such as LSM (LaSrMnO), SSC (SrSmCoO), LSC (LaSrCoO), and LSCF (LaSrCoFeO), or metal materials such as Pt and Ag. Although it can, it is not necessarily limited to these.

It is desirable that the sub stack 4 has a structure in which carbon deposition hardly occurs even when hydrocarbons flow. If possible, such fuel species can be removed in the sub stack 4 and supplied to the main stack 1. If not, the durability of the main stack 1 is improved.
Therefore, it is desirable that the substack 1 has the following configurations.

For example, the operating temperature of the sub-stack 4 is desirably higher than the operating temperature of the main stack 1, the operating temperature of the sub-stack 4 is better not high, carbon deposition due to hydrocarbons and carbon monoxide is less likely to occur.
Similarly, in order to suppress the carbonization precipitation of the substack 4, it is desirable that the content of the oxygen ion conductive material in the fuel electrode is larger than the content of the electrode catalyst as the configuration of the fuel electrode of the substack 4. That is, even if hydrocarbons and carbon monoxide are thermally decomposed into carbon, they can be immediately oxidized by oxygen ions surrounded by the electrode catalyst, so that carbon deposition occurs in the substack 4. It becomes difficult. In addition, content of an oxygen ion conductive material and an electrode catalyst said here means volume ratio.

For the same purpose, the air electrode reducing ability of the sub stack 4 can be made higher than the air reducing ability of the main stack air electrode. That is, when the reducing ability of the air electrode is high, oxygen ions are easily generated at the air electrode, and oxygen ions flow more and more toward the fuel electrode side, so that the deposited carbon is immediately oxidized.
Note that the reducing ability of the air electrode material described above is in the order of SSC>LSCF>LSC> LSM.

Furthermore, it is also desirable that the oxygen ion conductivity in the electrolyte of the substack 4 be higher than the oxygen ion conductivity of the electrolyte in the main stack 1, which makes it easier for oxygen ions to flow to the fuel electrode side, and precipitates carbon. Oxidation of the electrode is promoted, and the active deterioration of the electrode is suppressed.
Since the oxygen ion conductivity of the electrolyte material is in the order of LSGM>SDC>SSZ> YSZ, among these, the oxygen ion conductivity of the electrolyte of the sub stack 4 and the main stack 1 is as described above. Each electrolyte material can be selected so as to have a magnitude relationship.

As described above, the arithmetic control means 7 is based on the operating state of the main stack 1 detected by the operating state detecting means 6 through the temperature sensor, ammeter, voltmeter, oxygen sensor, etc. arranged in the main stack 1. , and H _{2 O} produced amount in the main stack 1 at that time, calculated of H _{2 O} amount required to prevent carbon deposition in the main stack 1, H _{2 O} amount discharged is calculated from the sub-stack 4 In addition, the operating condition of the sub stack 4 is controlled so as to compensate for the shortage of the generated amount with respect to the necessary amount of H ₂ O.

At this time, the operating conditions of the sub stack 4 can be adjusted by increasing or decreasing the operating temperature, the load current, and the operating voltage alone or in combination, and the H ₂ O generation amount can be controlled.
The operation (power generation) temperature of the sub stack 4 can be adjusted by overheating with a burner or a heater, or by stopping overheating. The load current is adjusted by, for example, connecting a variable resistor and increasing or decreasing the resistance value.

In general, when the system is started, H ₂ O is in shortage, so the oxygen partial pressure on the fuel electrode side is not so high, whereas in steady operation, H ₂ O is generated one after another. Particularly in the downstream of the main stack 1, an increase in oxygen partial pressure due to an increase in H ₂ O is observed.
Therefore, oxygen sensors are provided on the upstream side and the downstream side of the fuel gas flow path of the main stack 1, and the operating state of the main stack 1 detected by the operating state detection means 6 and the upstream side of the fuel gas flow path in the main stack 1. The amount of H ₂ O deficiency is calculated in consideration of both the oxygen partial pressure on the downstream side and the downstream side, and the power generation amount of the sub stack 4 is adjusted so that the amount of H ₂ O flowing into the main stack 1 does not become excessive. Is desirable.

Further, when calculating the amount of H ₂ O deficiency by the arithmetic control means 7, the amount of water contained in the fuel gas can be obtained, and this amount of water can be included in the amount of generated water, thereby enabling more accurate moisture control. It becomes possible.
Note that the amount of water contained in the fuel gas is obtained in advance by analyzing the fuel gas, or, if a reformer is used, is known by calculating from the operating conditions of the raw material gas and the reformer. be able to.

  Hereinafter, although the present invention will be described in detail with respect to the configuration of the main stack 1 and the sub stack 4 and the operation procedure based on examples, the present invention is not limited to such examples.

(1) Fuel gas As a fuel gas, a mixed gas of 10% CH ₄ -2% C ₃ H ₈ -45% CO-10% CO ₂ -33% H ₂ is used as a simulated gas of partially reformed gas of isooctane. used.

(2) Substack A solid oxide electrolyte made of SSZ (Sc-doped zirconia) is formed on a fuel electrode made of Ni-YSZ (Ni: 35 vol%, YSZ: 65 vol%), and further on the electrolyte layer A sub-stack 4 is formed by stacking five stages of fuel electrode supporting cells having a diameter of 100 mm in which an air electrode made of SSC (SmSrCoO) is formed through an intermediate layer made of SDC.
The substack 4 has a power density of 100 mW / cm ² and an operating temperature of 700 ° C.

(3) Main stack (fuel cell stack)
A solid oxide electrolyte made of YSZ is formed on a fuel electrode made of Ni—YSZ (Ni: 70 vol%, YSZ: 30 vol%), and further, an LSC (intermediate layer made of SDC) is formed on the electrolyte layer via LSC ( 10 stages of fuel electrode-supporting cells having a diameter of 100 mm, on which air electrodes made of LaSrCoO) were formed, were used as the main stack 1.
The main stack 1 has a power density of 200 mW / cm ² and an operating temperature of 600 ° C.

When the fuel cell power generation system as shown in FIG. 1 having the sub stack 4 and the main stack 1 as described above is started, first, the power generation amount required for the main stack 1 and the operating condition detection means 6 are detected. Based on the temperature (600 ° C.) of the main stack 1, the map stored in the arithmetic control means 7, that is, the relationship between the reformed gas flow rate of the main stack 1 and the power generation amount at the temperature is shown in FIG. From the data shown in FIG. 5, the fuel gas supply amount necessary for power generation is calculated, and the H ₂ O amount generated in the main stack 1 by such a reformed gas flow rate is also stored in the arithmetic control means 7. It is calculated from the data map as shown in FIG.

Next, for such a supply amount of fuel, the relationship between the S / C value (steam / carbon ratio) and the amount of carbon generated was determined for the amount of H ₂ O necessary for preventing carbon from being deposited at the operating temperature. It is calculated from a data map as shown in FIG.

Then, the H _{2 O} amount for preventing the precipitation of the calculated carbon by subtracting of H _{2 O} amount generated by the main stack 1, determines the H _{2 O} content to replenish the sub-stack 4, sub-stack 4 From the data map as shown in FIG. 2D in which the relationship between the power generation amount at (700 ° C.) and the H ₂ O amount is obtained, the power generation amount of the sub stack 4 for generating the H ₂ O amount to be supplemented is obtained. Based on this, the power generation amount of the sub stack 4 is controlled to be the calculated value.
In the case of using a fuel gas containing moisture, the H _{2 O} amount required in order not to deposit carbon, with H _{2 O} amount generated by the main stack 1, H _{2 O} content in the fuel gas It is necessary to obtain the amount of H ₂ O to be replenished from the sub stack 4 by subtracting.

When the system shifts from the startup phase to the steady operation state, power generation continues, so that H ₂ O is generated more and more, and the generated H ₂ O flows from upstream to downstream. In the fuel gas flow path, H ₂ O concentrates downstream from the upstream side, and as a result, H ₂ O tends to be insufficient on the upstream side of the stack. Therefore, it is necessary to supply H ₂ O to the upstream side. Become.
However, if the amount of H ₂ O introduced constantly increases, the electrode may cause steam oxidation. Therefore, the oxygen partial pressure on the fuel electrode side upstream and downstream of the stack is measured, and the upstream and downstream oxygens are measured. When the partial pressure exceeds a predetermined value, it is preferable to perform control so as to reduce the amount of power generated by the substack 4 and reduce water supply from the substack 4.

  In the fuel cell power generation system, the sub stack functioning as the moisture adjusting means is disposed in front of the main stack, and by performing the control as described above, carbon deposition in the main stack can be suppressed, and excess moisture Since there was no supply, deterioration of the electrode due to steam oxidation could be prevented, moisture management was easy and system complexity could be avoided, and desired power generation performance could be maintained over a long period of time.

It is a block diagram which shows the structure of the fuel cell power generation system by one Embodiment of this invention. (A)-(d) is a graph which shows an example of the various data maps accommodated in the calculation control means in the fuel cell power generation system of this invention.

Explanation of symbols

1 Fuel cell stack (main stack)
2 Fuel supply means 3 Fuel supply path 4 Sub-stack 5 Oxidant gas supply means 6 Operating condition detection means 7 Calculation control means

Claims (8)

A fuel cell stack comprising solid oxide fuel cells;
Fuel supply means for supplying a fuel gas containing hydrocarbons and / or carbon monoxide to the fuel cell stack;
An oxidant gas supply means for supplying a gas containing oxygen to the fuel cell stack;
A sub-stack composed of a solid oxide fuel cell and disposed in the middle of a fuel supply path from the fuel supply means to the fuel cell stack;
An operating condition detecting means for detecting the operating condition of the fuel cell stack;
Computation control means for adjusting the operating conditions of the sub-stack based on the state of the fuel cell stack detected by the operating status detection means ,
The calculation control means is configured to determine the carbon state in the fuel cell stack based on the operating state of the fuel cell stack detected by the operating condition detection means and the oxygen partial pressures on the upstream side and the downstream side of the fuel gas flow path of the fuel cell stack. A fuel cell characterized by calculating a H ₂ O deficiency with respect to a H ₂ O necessary amount for preventing precipitation, and controlling so that a H ₂ O discharge amount from the sub-stack compensates for the H ₂ O deficiency. Power generation system. The arithmetic control means is based on the operating state of the fuel cell stack detected by the operating condition detecting means, and the amount of H ₂ O generated at that time and the required amount of H ₂ O for preventing carbon deposition in the fuel cell stack. is calculated, and the fuel cell power generation system according to claim 1, characterized in that H _{2 O} emissions from the sub-stack is controlled to compensate for the shortage of the H _{2 O} formation versus H _{2 O} required amount . Said operation control means, according to claim 1 or 2 by deducting of H _{2 O} content in the fuel gas supplied from the fuel supply means and calculates of H _{2 O} emissions from the sub-stack The fuel cell power generation system described in 1. Said operation control means, according to claim 1, wherein the controller controls the sub-stack temperature, the load current and by adjusting at least one condition selected from the group consisting of the operation voltage H _{2 O} emissions The fuel cell power generation system according to any one of the above items. The fuel cell power generation system according to any one of claims 1 to 4 , wherein an operating temperature of the sub-stack is higher than an operating temperature of the fuel cell stack. The fuel cell power generation system according to any one of claims 1 to 5 , wherein the content of the oxygen ion conductive material is greater than the content of the electrode catalyst in the fuel electrode of the sub stack. The fuel cell power generation system according to any one of claims 1 to 6 , wherein the reduction capacity of the air electrode of the sub stack is higher than the reduction capacity of the air electrode of the fuel cell stack. The fuel cell power generation system according to any one of claims 1 to 7 , wherein oxygen ion conductivity in the electrolyte of the sub-stack is higher than that of the fuel cell stack.

Images