KR20130065607A - Fuel cell system and method for operating the same - Google Patents

Abstract

PURPOSE: A fuel cell system comprises recirculation system of an anode exhaust gas, thereby simply guaranteeing control of the recirculation system. CONSTITUTION: A fuel cell system comprises an anode region, cathode region, a fuel cell with a recirculation unit(30,34), and a high temperature fuel cell. Fuel gas is supplied to the anode region, and anode exhaust gas is discharged from the anode region. Oxygen-containing gas is supplied to the cathode region, and cathode exhaust gas is discharged from the cathode region. At least a part of the anode exhaust gas is re-supplied to the anode region, by the recirculation unit.

Description

FUEL CELL SYSTEM AND METHOD FOR OPERATING THE SAME}

The present invention relates to a fuel cell system according to the preamble of the independent claim, a method of operation and use thereof.

CO ₂ at current supply Due to the potential for reduced emissions, fuel cell systems play an important role in future use. The same is true in the field of cogeneration (CHP), where the fuel cell system generates heat in addition to the current. The fuel cell system is formed for example on the basis of a ceramic cell, a so-called solid oxide fuel cell (SOFC) operating at high temperatures of 650-1000 ° C.

Such a cogeneration fuel cell system converts chemical energy contained in a fuel such as natural gas into current or heat. This is done during a chemical reaction in the fuel cell stack, in which case the fuel as a reducing agent reacts with an oxidant, for example oxygen contained in air. The fuel cell stack is divided into an anode region and a cathode region, which regions are separated from each other by an electrolyte. Energy generation in relation to current and heat is substantially achieved through the chemical reactions described below.

Anode side:

Cathode side:

Common natural gas consists mostly of alkanes. The aforementioned reactions represented by equations (1) and (2) are presupposed in the presence of hydrogen or carbon monoxide as extract. Therefore, pretreatment of natural gas supplied to the cogeneration fuel cell system is required. This conversion takes place directly in part within the fuel cell stack itself, but most of it is carried out by a reactor connected forward in the cogeneration fuel cell system, a so-called reformer which carries out the reforming reaction described below, which is basically a fuel Even within the cell stack itself, it can be made at the anode surface of the SOFC-fuel cell stack.

Steam-methane reforming:

Water gas shift reaction:

It can be seen that water is also required as the extract for the reforming reactions (3) and (4). Basically, converting any kind of fuel into hydrogen and carbon monoxide in the presence of water is called steam reforming.

The general mode of operation of a fuel cell system aims to provide more fuel than is theoretically required for the fuel cell stack. Therefore, the quality of the fuel cell stack is based on the ratio of the amount of fuel used in the fuel cell itself to the total amount of fuel supplied to the fuel cell stack. This ratio is called fuel utilization (FU). The manufacturer of the fuel cell stack assigns an upper limit FU _max for the characterization of the fuel cell stack. The upper limit is 65 to 85% in a typical SOFC-fuel cell stack. Theoretically, the actual fuel utilization (FU) is obtained as follows: For the given current I, the required electron molar flow is given.

Where n _c is the number of cells in the fuel cell stack and F is a Faraday constant. Current is generated when the fuel in the fuel cell stack provides enough electrons to enable the electrochemical reactions (1) and (2). The number of electrons provided is calculated as follows:

_NG는 천연가스 몰 유동이고, x_{i , NG}는 연료인 천연가스(NG) 내 성분 i의 몰 양이고, N_{e -j}는 천연가스 내 성분 i의 원자가이다.

_{e_, IST} 와

_{e_ moeglich} 사이의 비는 연료 이용률(FU)을 제공한다. In this formula,

_NG is the molar flow of natural gas, x _{i , NG} is the molar amount of component i in natural gas (NG) that is the fuel, and N _{e -j} is the valence of component i in natural gas.

_{e_, with IST}

The ratio between _{e_ moeglich} gives the fuel utilization (FU).

Unused fuel in the fuel cell stack, characterized by the total fuel flow multiplied by (1-FU), is generally combusted in the post-combustion apparatus downstream in the flow direction to the fuel cell stack. The energy obtained is supplied to a fuel cell stack, reforming reactor and / or natural gas, air and water via heat exchangers.

In addition to methane, natural gas contains heavier alkanes such as ethane, butane or propane. The modification of the molecules in the fuel cell stack can damage the fuel cell stack. Typically the reforming of the alkanes takes place in a preliminary reformer prior to the fuel cell stack:

Ethane reforming reaction:

Propane modification reaction:

Butane Modification Reaction:

The general reforming reaction of alkanes C _n H _{2n +2} is as follows:

Common reforming reactions:

The chemical reactions described above indicate steam reforming, in which water reacts with alkanes to produce hydrogen and carbon monoxide. Alkanes may alternatively be modified by oxygen as well. This is called partial oxidation (POX). For methane the chemical reaction is:

Partial methane reforming reaction

Typical partial alkanes reforming reactions are as follows:

Partial alkane modification reaction

Depending on the type of reforming reaction, air or water must be supplied before the preliminary reformer.

In addition, for effective operation of the fuel cell system, it should be ensured that no carbon precipitation takes place during the reforming reaction in the reformer as well as in the fuel cell stack. To this end, a fuel mixture is provided in a preliminary reformer or fuel cell stack, which has defined limits in relation to the oxygen to carbon ratio or the O / C ratio or the oxygen to carbon ratio. The O / C ratio is specified as follows:

_o는 O-원자 몰 유동이고,

_c는 C-원자 몰 유동이다. 예비 개질기 이전에 임계적 O/C-값의 미달시 탄소 침전의 위험이 있다. 즉: O/C_krit = F(T)이다. 일반적으로 안전한 시스템 작동을 위해서는 다음 식이 성립한다: O/C_krit = 1.8.In this formula,

_o is O-atomic mole flow,

_c is the C-atomic mole flow. There is a risk of carbon precipitation if the critical O / C-value is below the preliminary reformer. That is: O / C _krit = F (T). In general, the following equation holds for safe system operation: O / C _krit = 1.8.

In addition, at least partly the anode exhaust of the fuel cell, which is produced during operation of the fuel cell system and which contains a limited amount of unburned fuel gas and a greater amount of water vapor, is limited in efficiency, and the gas components contained therein It is known to be able to supply for recycling in a fuel cell stack.

In this connection, for example, DE10 2009 036 197 A1 discloses a fuel cell system with control electronics which makes it possible to omit the sensor in the anode recycle zone. Similar solutions are disclosed, for example, in DE 10 2006 071 614 A1, DE 10 2006 017 616 A1 and DE 10 2006 017 617 A1, in which case in order to be able to more accurately measure the temperature of the fuel cell in the anode region, For example, a temperature sensor is arranged in the anode outlet region of the fuel cell described in the above publication.

Another fuel cell system is disclosed in DE 10 2006 029 451 A1; The fuel display system includes a lambda sensor in addition to the temperature sensor, so that the lambda value of the anode exhaust can be measured as intended.

Another way of operating the fuel cell system with high accuracy is disclosed in DE 11 2006 002 715 T5, in which case the gas composition in contact with the anode of the fuel cell is controlled by a flow sensor and a temperature sensor. Common to all the solutions described above is a relatively complex structure and hence the complexity of the control.

SUMMARY OF THE INVENTION An object of the present invention is to provide a fuel cell system including a recycling system of anode exhaust gas of a fuel cell, and in which control of the recycling system is simply guaranteed.

The problem is preferably solved by a fuel cell system or method of operation thereof comprising the features of the independent claims.

Control of the recirculation unit for recirculation of at least a portion of the anode exhaust of the fuel cell is made based on the simply available characteristic values of the fuel cell. The characteristic value in this case is the actual temperature of the anode region of the fuel cell, the humidity associated with the fuel gas to be supplied to the anode region of the fuel cell and / or the current applied to the anode region of the fuel cell.

Control is achieved by detecting fuel gas utilization or so-called fuel utilization (FU) and / or control values in the form of the actual oxygen to carbon ratio (O / C) in the fuel gas to be supplied to the anode region of the fuel cell. The fuel utilization, or FU, is the ratio of the amount of fuel gas actually used in the anode region of the fuel cell to the total fuel gas supplied to the anode region of the fuel cell. The oxygen to carbon ratio is also the ratio of the total amount of glass and combined oxygen to the total amount of glass and combined carbon in the fuel gas to be supplied to the anode region of the fuel cell. This ratio is especially measured at the front of the preliminary reformer of the fuel cell system in the fuel gas, in which case the oxygen to carbon ratio O / C must be greater than the critical or minimum oxygen to carbon ratio (O / C _{krit .} ).

This is because the temperature inside the anode region of the fuel cell can be measured by one temperature sensor or because the absolute humidity associated with the fuel gas to be supplied to the anode region of the fuel cell can be measured by one humidity sensor. As a result, the control of the recirculation unit for the fuel cell can be implemented by one sensor. The same applies to the current applied to the fuel cell, and the current can be measured by one current sensor.

Other preferred embodiments of the invention are subject of the dependent claims.

The amount of anode exhaust re-supplied to the anode region of the fuel cell via the recirculation unit is based on the actual temperature of the anode region of the fuel cell, based on the actual current applied to the fuel cell and, if the fuel gas composition is known, the actual fuel It is desirable to be controlled based on the amount of fuel gas supplied to the cell since the measured value can be simply detected and tracked.

In the above embodiment, the gas composition of the anode exhaust gas discharged from the anode region of the fuel cell is substantially regarded as the temperature of the anode region, the current applied to the fuel cell, the amount of fuel gas supplied to the anode of the fuel cell, and the constant. It is based on the fact that it is a function of the gas composition of the fuel gas. If the composition of the anode exhaust is known, the amount of recycled or replenished anode exhaust which is provided back to the anode region of the fuel cell can be optimized because the fuel gas mixture provided to the anode region of the fuel cell This is because the composition of can be precisely controlled as a mixture of fresh fuel gas and recycled anode exhaust.

Another preferred embodiment of the present invention relates to a fuel cell system based on absolute humidity associated with a fuel gas mixture to be supplied to the fuel cell and based on the current applied to the fuel cell and the volume flow rate of the fuel gas supplied to the fuel cell. To control the recycling unit. This embodiment shows that the gas composition of the anode exhaust gas discharged from the anode region of the fuel cell system is characterized by the absolute humidity of the fuel gas supplied to the fuel cell system, the current actually applied to the fuel cell and the fuel gas actually supplied to the fuel cell. Take advantage of the fact that it is a function of volumetric flow. Further, the gas composition depends on the discharge gas composition of the fuel gas supplied to the fuel cell system, and the discharge gas composition can be regarded as almost constant. The actual composition of the anode exhaust thus calculated is subject to the control of the recycle unit to set the volume flow of the recycled anode exhaust to provide the fuel cell with an optimized fuel gas mixture of fresh fuel and recycled anode exhaust. Used by.

According to a particularly preferred embodiment of the present invention, only the current applied to the fuel cell is used as a measure to control the recirculation unit of the fuel cell system. This method applies to a fuel cell system in which the volumetric flow of anode exhaust gas resupplied to the anode region of the fuel cell by the recirculation unit is regarded as the set oxygen to carbon ratio or set fuel utilization and the current and constant actually applied to the fuel cell. It is based on the fact that it is a function of the composition of the fuel gas supplied. In addition, the volumetric flow of fresh fuel gas supplied to the fuel cell system is a function of the set oxygen to carbon ratio or set fuel gas utilization, the current actually flowing in the fuel cell and the composition of the fuel gas that can be regarded as a constant. Thus the set oxygen to carbon ratio, set fuel gas utilization and the composition of the fuel gas supplied to the fuel cell system can be regarded as constants, and the volumetric flow of recycled anode exhaust gas is controlled as a function of the current applied to the fuel cell. . This embodiment can be implemented particularly technically simply.

Embodiments of the invention are shown in the drawings and described below.

1 is a schematic diagram of a fuel cell system according to a first embodiment of the present invention;
2 shows a schematic view of a fuel cell as part of the fuel cell system according to FIG. 1;
3 is a schematic view of a recirculation unit according to a first embodiment of the fuel cell system according to FIG. 1;
4 is a schematic view of a recirculation unit according to a second embodiment of the fuel cell system according to FIG. 1;
5 is a schematic view of a recirculation unit according to a third embodiment of the fuel cell system according to FIG. 1;

_NG에 의해 연료전지(12)에 공급된다. 연료인 천연가스 NG는 천연가스 조성 X_{i , NG}를 갖는다. 천연가스는 일반적으로 안전상의 이유로 냄새가 나는 황화합물을 포함하기 때문에, 연료전지 시스템(10)은 추가로 바람직하게 탈황 유닛(16)을 포함한다. 상기 탈황 유닛은 연료전지(12)에서 사용되는 촉매제의 황에 의한 중독을 예방한다. 1 shows a preferred embodiment of a fuel cell system according to the invention. The fuel cell system 10 includes a fuel cell 12, to which fuel such as natural gas NG, hydrogen or methanol is supplied via a compressor 14. Natural gas NG, which is a fuel, is, for example, volumetric flow in fuel cell system 10.

_It is supplied to the fuel cell 12 by _NG . Natural gas NG as a fuel has a natural gas composition X _{i , NG} . Since natural gas generally contains odorous sulfur compounds for safety reasons, the fuel cell system 10 further preferably includes a desulfurization unit 16. The desulfurization unit prevents poisoning by sulfur of the catalyst used in the fuel cell 12.

Since natural gas NG is not directly used for the electrochemical reaction in the fuel cell 12, the fuel cell system 10 includes, for example, a preliminary reformer 18, in which the alkane contained in the natural gas in the reformer. The reforming of is done. This can be done, for example, in the form of a steam reforming reaction with the addition of water. However, reforming in the form of partial oxidation (POX) using oxygen is also possible. A humidity sensor 20, for example, is integrated into the supply path of natural gas NG as fuel, which sensor is preferably arranged in the natural gas supply path between the desulfurization unit 16 and the pre-reformer 18. The absolute humidity aH of the fuel in the form of natural gas NG to be supplied to the fuel cell by the humidity sensor 20 can be measured.

Since oxygen is also needed for the electrochemical oxidation of the fuel in the fuel cell 12, the fuel cell system 10 includes a compressor 22 for supplying air to the fuel cell 12. The exhaust gas generated during the electrochemical reaction in the fuel cell 12 is supplied to the combustion device 24 and used for heat generation, for example. The exhaust gas of the combustion device 24 is discharged from the fuel cell system 10 via the exhaust gas line 26, for example. The heat of combustion generated by the combustion device 24 is used to heat components of the fuel cell system 10, such as fuel cell 12 or pre-reformer 18, which operate at high temperatures, for example.

In general, since the fuel cell 12 of the fuel cell system 10 is operated with an excess amount of fuel gas, the anode exhaust gas in the anode exhaust gas line 28 discharged from the fuel cell 12 may react with hydrocarbons capable of reacting. Or a substantial portion of hydrogen. The fuel components may be resupplied to the fuel supply 32 of the fuel cell 12 via the recirculation path 30. The amount of recycled anode exhaust of the fuel cell 12 is controlled, for example, by a suitable three-way valve, not shown and integrated in the compressor 34 or anode exhaust line 28 integrated in the recirculation path 30. do. Compressor 34 may alternatively be integrated into fuel supply 32 or anode exhaust line 28 as well.

Partial recirculation of the anode exhaust gas regulates the characteristic oxygen to carbon ratio O / C at the fuel supply 32 after the inlet of the recirculation path 30 leading into the fuel supply 32 in the flow direction. This can be measured or adjusted in particular in the fuel supply located in front of the reformer 18 when viewed in the flow direction.

Variables characterizing the operation of the fuel cell 12 include a direct current I applied to the fuel cell 12 in addition to the temperature T _an and fuel utilization FU in the anode region of the fuel cell 12, the direct current being for example DC / To the AC converter 36. The resulting alternating current AC can be supplied to electrical loads inside and outside the fuel cell system 10. In addition, the fuel cell system 10 includes a housing 40 to protect and thermally insulate the components of the fuel cell system 10 that are operated at particularly elevated temperatures.

FIG. 2 schematically shows a fuel cell structure as part of the fuel cell system according to FIG. 1. Like reference numerals denote the same parts as in FIG.

The fuel cell 12 is preferably embodied as a so-called high temperature fuel cell having an operating range of 650 to 1000 ° C. The fuel cell includes, in particular, a cathode region 60 in which cathode reduction of atmospheric oxygen takes place and an anode region 62 in which anode oxidation of hydrocarbons, carbon monoxide and hydrogen is carried out. Cathode region 60 includes cathode 64 and anode region includes anode 66. The cathode 64 includes, for example, lanthanum manganese oxide, which is supported on yttrium stabilized zirconia, for example. The anode region 62 and the cathode region 60 are in contact with each other via an ion transfer electrolyte 68, in which case the electrolyte 68 is embodied, for example, of iridium stabilized zirconia. The current applied between the cathode 64 and the anode 66 is consumed through the electrical load 70, for example.

3 shows a part of the fuel cell system according to the invention according to FIG. 1 in the recirculation path region. Like reference numerals denote the same parts as in FIGS. 1 and 2.

The fuel cell system 10 includes a recirculation path 30, and anode exhaust gas discharged from the anode region 62 of the fuel cell 12 may be recirculated by the recirculation path.

_recy 양이 최적화되는 방식으로 상기 압축기의 제어가 최적화된다. The control of the compressor 34 is preferably effected by a suitable control device, not shown, and preferably in the recirculation path such that the mixture of optimum composition at the fuel supply 32 is supplied to the anode region 62 of the fuel cell 12. Anode exhaust gas supplied to the fuel supply part 32 through the 30.

The control of the compressor is optimized in such a way that the amount of _recy is optimized.

_NG, 원래의 천연가스 조성 x_{i , NG}, 연료전지(12)에 인가되는 직류 I 및 연료전지(12)의 애노드(66)의 온도 T_an.The gas composition x _i after the anode 66 depends only on four variables. That is, natural gas mole flow

_NG , the original natural gas composition x _{i , NG} , the direct current I applied to the fuel cell 12 and the temperature T _an of the anode 66 of the fuel cell 12.

The calculation of x _i relates to the thermodynamic equilibrium of the reforming reaction (11) and the water gas shift reaction:

Water gas shift reaction

The calculation can be explained by the formula:

Assuming that the temperature of the anode 66 of the fuel cell 12 is sufficiently high (T _an > 700 ° C.), the chemical reaction 11 can be considered complete: all alkanes are completely modified. In this case the equilibrium of reactions 11 and 15 can be analytically achieved to calculate the gas composition x _i . That is, the gas composition x _i is not calculated by the method of interaction.

Another method for calculating the gas composition x _{i is} to measure the absolute humidity aH of the fuel to be supplied to the anode region 62 of the fuel cell 12 using, for example, the humidity sensor 20. This variant is described in detail in FIG. 4. Like reference numerals denote the same parts as in FIGS. 1 to 3.

The information obtained on the absolute humidity aH and C-atoms, O-atoms and H-atomic mass balances of the anode exhaust gas of the anode exhaust line 28 allows for a complete analytical calculation of the gas composition x _{i in} the anode exhaust gas. do:

After calculating the gas composition x _i , the variables O / C and FU can be calculated as follows:

_recy와

_NG에 따라 답이 얻어진다:Where n _c is the number of cells in the stack, F is a Faraday constant, N _{e , i} is the valence of the component, and N _{i , C} is the number of C-atoms in component i. In addition, x _CO , x _{CO 2} , x _{H 2} , and x _{H 2 O} are mole fractions of carbon monoxide, carbon dioxide, hydrogen or water in the recirculation path 30. The system of equations (18) and (19)

_{with recy}

_The answer is obtained according to _NG :

_NG의 양과 재순활될 애노드 배기가스

_recy의 양은 제 1 방법에 따라 바람직하게 애노드(66)의 온도에 기초하여서만 또는 제 2 방법에 따라 바람직하게 연료전지(12)에 공급될 연료 혼합물 내의 습도 aH에 기초하여서만 검출될 수 있고, 이 경우 연료 혼합물은 바람직하게 측정 위치에서 재순환된 애노드 배기가스의 적어도 일부를 포함한다. Therefore, natural gas to be supplied to the fuel cell

_Amount of _NG and anode exhaust to be recycled

can only be detected the amount of _recy hayeoseo first method in preferably, the anode 66 is preferably according to the only or the second method hayeoseo based on the temperature of the fuel cell 12 based on the humidity of aH in the fuel mixture to be supplied to the result, In this case the fuel mixture preferably comprises at least a portion of the anode exhaust which is recycled at the measuring position.

_NG의 양 또는 재순환될 애노드 배기가스

_recy 양의 제어를 위해 애노드(66) 영역에서 온도 T_an의 측정 또는 유동 방향으로 볼 때 연료전지(12)의 애노드 영역(62) 앞의 연료 공급부(32) 내의 절대 습도 aH의 측정을 전제로 하기 때문에, 변수 O/C와 FU는 연료전지에 공급될 천연가스

_NG 의 양 또는 재순환될 애노드 배기가스

_recy의 양을 계산하기 위한 보조 변수로서 검출될 수 있다. The two methods described above provide natural gas to be supplied to a fuel cell.

Amount of _NG or anode exhaust to be recycled

_For the control of the amount of _{recy, assuming} the measurement of the temperature T _an in the anode 66 region or the absolute humidity aH in the fuel supply 32 in front of the anode region 62 of the fuel cell 12 in the flow direction. Therefore, the variable O / C and FU are the natural gas to be supplied to the fuel cell.

Amount of _NG or anode exhaust to be recycled

It can be detected as an auxiliary variable for calculating the amount of _recy .

The third method described below preferably omits the two measurement methods for detecting the variables O / C and FU. Only detection of the direct current I applied to the fuel cell is used. The modification is described in detail in FIG. 5. Like reference numerals denote the same parts as in FIGS. 1 to 4.

_NG 또는

_recy을 야기한다:On the one hand, the material resin of the component reaction at the anode 66 which takes into account the difference in the composition of the fuel mixture in the fuel supply 32 and on the other hand the composition of the anode exhaust gas is applied to the direct current I and the set value O applied to the fuel cell. Certain control parameters depending on / C and FU

_NG or

causes _recy :

Where

Thus the following equation is given:

In the above formula, the direct current I applied to the fuel cell 12 is the only measurement variable.

_recy의 양 또는 연료전지에 공급될 천연가스

_NG의 양이 연료전지(12)의 애노드 영역(62)의 온도 T_an, 연료전지에 인가되는 전류 I 및 연료전지(12)의 애노드 영역(62)에 공급된 연료가스 양에 기초하여서만 제어되는 것이 가능해진다. Thus, the recycle unit or the anode exhaust to be recycled

amount of _recy or natural gas to be supplied to the fuel cell

The amount of _NG is controlled only based on the temperature T _an of the anode region 62 of the fuel cell 12, the current I applied to the fuel cell and the amount of fuel gas supplied to the anode region 62 of the fuel cell 12. It becomes possible.

_recy의 양 또는 연료전지에 공급될 천연가스

_NG의 양은 연료전지(12)의 애노드 영역(62)에 공급된 연료가스의 습도 aH, 연료전지(12)에 인가되는 전류 I 및 연료전지(12)의 애노드 영역(62)에 공급된 연료가스의 양에 기초하여서만 제어되는 것이 가능해진다.Also, as an alternative, a recirculation unit, anode exhaust to be recycled

amount of _recy or natural gas to be supplied to the fuel cell

The amount of _NG is the humidity aH of the fuel gas supplied to the anode region 62 of the fuel cell 12, the current I applied to the fuel cell 12, and the fuel gas supplied to the anode region 62 of the fuel cell 12. It can be controlled only on the basis of the amount of.

_recy 의 양 또는 연료전지에 공급될 천연가스

_NG의 양이 연료전지(12)에 인가되는 전류 I에 기초하여서만 제어되는 것이다. A third alternative is a recycle unit or anode exhaust to be recycled.

amount of _recy or natural gas to be supplied to the fuel cell

The amount of _NG is controlled only based on the current I applied to the fuel cell 12.

12 fuel cells
30, 34 recirculation unit
60 cathode areas

Claims (8)

A fuel cell system comprising a fuel cell 12, in particular a high temperature fuel cell, having an anode region 62, a cathode region 60, and a recirculation unit 30, 34, wherein fuel gas is supplied to the anode region 62. An anode exhaust gas is discharged, an oxygen-containing gas is supplied to the cathode region 60, a cathode exhaust gas is emitted, and an anode discharged from the anode region 62 of the fuel cell 12 by the recirculation unit In a fuel cell system in which at least a portion of exhaust gas can be resupplied to the anode region 62 of the fuel cell 12,
A control unit is provided, wherein the recirculation units 30, 34 are controlled by the control unit, and the control of the recirculation units 30, 34 is based on fuel gas utilization FU and / or the anode area ( Based on the oxygen to carbon ratio (O / C) of the fuel gas supplied to 62, in which case the amount of gas used in the anode region 62 to the anode region 62 as a fuel utilization rate (FU). The ratio of the amount of gas to be used is used, in which case the oxygen to carbon ratio (O / C) is the free and combined total oxygen to free and combined total of fuel gas supplied to the anode region 62 of the fuel cell 12. The ratio of carbon is used and the fuel gas utilization rate (FU) and / or the oxygen to carbon ratio (O / C) of the fuel gas supplied to the anode region 62 of the fuel cell 12 is determined by the fuel cell 12. and the basis of the temperature (T _an) of the anode region 62, an open feed to the anode region 62 On the basis of the absolute humidity (aH) for the gas and / or fuel cell system, it characterized in that it is determined on the basis of the current (I) applied to the fuel cell 12. 2. A fuel cell system according to claim 1, wherein the control of the recirculation units (30, 34) is further based on the composition of the fuel gas supplied to the anode region (62) of the fuel cell (12). . The method of claim 1, wherein the control of the recirculation unit (30, 34) is the temperature (T _an ) in the anode region 62 of the fuel cell 12, the current (I) applied to the fuel cell 12 And depending on the amount of fuel gas supplied to the anode region (62) of the fuel cell (12). The method of claim 1, wherein the control of the recirculation unit (30, 34) is applied to the fuel cell 12, the absolute humidity (aH) of the fuel gas to be supplied to the anode region 62 of the fuel cell 12 And the amount of fuel gas supplied to the anode region (62) of the fuel cell (12). 2. A fuel cell system according to claim 1, wherein the control of the recirculation unit (30, 34) is based only on the current (I) applied to the fuel cell (12). The anode exhaust gas discharged from the anode region of the fuel cell 12 includes the anode exhaust gas line 28 of the anode region 62. Is supplied to the fuel supply part 32 of the anode region 62 of the fuel cell via a recirculation path 30 connected to the fuel supply part 32 of the fuel cell, in which case the recirculation pump 34 is supplied to the recirculation path. And a fuel supply (32) downstream of the supply of anode exhaust gas (30) or into the fuel supply (32). A method of operating a fuel cell system according to any one of claims 1 to 6,
The anode exhaust gas discharged from the anode region 62 of the fuel cell 12 is resupplied to the fuel supply part 32 of the fuel cell 12, and the volume flow of the anode exhaust gas supplied back is the fuel cell ( Depending on the temperature T _an of the anode region 62 of 12), and / or depending on the absolute humidity aH of the fuel gas supplied to the anode region 62 of the fuel cell 12 And is controlled in dependence on the current (I) applied to the fuel cell (12). Use of a fuel cell system according to any one of claims 1 to 6 or a method according to claim 7 in a cogeneration plant for obtaining electricity and thermal energy.

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