Chemistry: electrical current producing apparatus – product – and – Cell support for removable cell
Reexamination Certificate
2001-02-22
2003-03-11
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Cell support for removable cell
C429S010000, C429S010000, C429S010000
Reexamination Certificate
active
06531243
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a method of operating a fuel cell, wherein a hydrocarbon containing fuel is converted in the fuel cell into a synthesis gas with which the fuel cell is operated to generate electricity.
A fuel cell comprises a cathode, an electrolyte, and an anode. An oxidant such as air is supplied to the cathode and fuel such as hydrogen is supplied to the anode.
Various types of fuel cells are known as, for example, the SOFC fuel cell disclosed in the printed publication DE 44 30 958 C1 as well as the PEM fuel cell disclosed in the printed publication DE 195 31 852 C1.
The SOFC fuel cell is also called high temperature fuel cell since its operating temperature is up to 1000° C. In the presence of the oxidant oxygen ions are formed at the cathode of a high temperature fuel cell. The oxygen ions pass through the electrolyte and combine at the anode side with the hydrogen and/or carbon monoxide from the fuel. Electrons are released in the process whereby electric energy is generated. These electrochemical reactions are highly exothermic.
The operating temperature of a PEM fuel cell is about 80° C. At the anode of a PEM fuel cell, protons are formed by means of a catalyst in the presence of the fuel. The protons pass through the electrolyte and combine at the cathode side with the oxygen from the oxidant to form water. In this process electrons are released whereby electrical energy is generated.
For achieving a large electrical power output generally, several fuel cells are interconnected by connecting elements electrically and mechanically. An example for such a connecting element is the bipolar plate known from DE 44 10 74 C1. With bipolar plates, fuel cells, which are connected electrically in series, are stacked on top of one another. Such an arrangement is called a fuel cell stack.
As fuel, among others, methane and methanol may be provided. The fuels mentioned are converted for example to hydrogen or hydrogen-rich gas by reformation or oxidation.
It is known from DE 195 198 47 C1 to reform a fuel-like methane internally that is directly at, or within, the anode of a SOFC fuel cell. Alternatively, fuel is reformed in the interior of a fuel cell stack that is in additional chambers of a fuel cell stack (integrated reformation). With the internal or respectively, integrated reformation reaction, the endothermic reformation reaction is to draw the required heat from the exothermic electrochemical reaction. In this way, high efficiencies are to be achieved.
The printed publication DE 196 46 354 A1 discloses that fuel such as methanol can be oxidized at the anode of a PEM fuel cell by means of a catalyst such as platinum whereby hydrogen is released. The effects are comparable with those of the internal or integrated reformation.
It is practically impossible to convert a fuel such as hydrogen and/or carbon monoxide in a fuel cell completely to an electric current. The exhaust gas, which leaves the anode space of a fuel cell, will always contain a rest of the electrochemically active fuels mentioned above. The experts however are striving to minimize this rest of fuel in the exhaust gas.
A measure herefor is the fuel utilization degree, which is defined as follows:
BN
[
%
]
=
1
-
(
2
n
.
H2
+
2
n
.
CO
+
8
n
.
CH4
)
exit
(
2
n
.
H2
+
2
n
.
CO
+
8
n
.
CH4
)
entrance
Wherein {dot over (n)} indicates the respective mole flows in mol/h.
From EP 0 398 111 A a fuel cell arrangement is known, wherein O
2
and ballast gases as well as fuel are introduced into the fuel cell in excess and the gases are partially recirculated. In this way, fuel utilization degrees of about 90% can be achieved.
In the carbonate fuel cell known from DE 690 06 458 T2 natural gas is reformed internally within cooling plates which are arranged within the fuel cell. The fuel utilization degree achieved in this way is about 85%.
It is further known to introduce pre-reformed fuel into a SOFC fuel cell with internal reformation. Fuels such as natural gas whose main component is methane is partially externally reformed. A mixture of methane and the hydrogen-rich synthesis gas obtained by the pre-reformation is supplied to a fuel cell. Within the fuel cell, the methane contained in the mixture is also converted to a hydrogen-rich synthesis gas. By the pre-reformation, the higher hydrocarbons are disintegrated. This occurs because they are disadvantageously chemically unstable and therefore disintegrate easily while forming carbon. Besides the higher hydrocarbons, also the methane itself reacts to form carbon particularly by thermal cracking of methane:
CH
4(g)
⇄2H
2(g)
+C
(s)
This reaction takes place preferably at temperatures above 650° C. (J. R. Rostrup-Nielson, Catalytic Steam Reforming, Springer Verlag, 1984). The carbon formation should preferably be avoided in the fuel cell.
An external procedure for generating hydrogen by catalytic reformation has the disadvantage of high heat input requirements. The required heat is provided for example by additional combustion of hydrocarbons. The process is therefore relatively expensive.
The endothermic reactions occurring in a fuel cell cannot compensate for the heat generated by the exothermic electrochemical reactions. In the state-of-the-art techniques, the heat balance in a fuel cell always provides for excess heat. A fuel cell therefore needs to be cooled by a coolant as it is shown for example in the printed publication DE 196 36 908 A1. In the high-temperature fuel cell substantial amounts of cooling air must be provided in order to remove the heat released by the electrochemical reactions as can be taken from the printed publications “Fuel Cell Systems L. Blomen, M. Mugerwa, Plenum Press 1993” or “Optimization of 200 KW SOFC co-generation plant, Part II: variation of the flow sheet; E. Riensche, J. Meusinger U. Stimming, G. Unverzagt; Journal of Power Sources 71 (1998), pp. 306-314”. This results in large efficiency losses and increasing electric power generating expenses.
To cool a fuel cell requires energy. The efficiency of a fuel cell is reduced thereby. It would be advantageous to strive for an isothermal process wherein the heat budget of the fuel cell is balanced.
From the state of the art, combined processes are known wherein for example a fuel cell and a thermal power plant are coupled for the utilization of the excess heat of the fuel cell (DE 196 36 738 A1) or a fuel cell is coupled with a gas turbine (DE 40 32 993 C1).
A process for the manufacture of highly pure hydrogen or, respectively, synthesis gas which includes a fuel cell is also known from DE 196 36 068 C1. In this process, the isolated reaction space is brought for the reformation in thermal contact with the interior of a high-temperature fuel cell, which is operated at 300° C., but preferably above 500° C.
Based on the printed publication “Ch. Rechenauer, E. Achenbach, Dreidimensionale mathematische Modellierung des stationären und instationären Verhaltens oxid-keramischer Hochtemperatur-Brennstoffzellen (three-dimensional mathematical modelling of the stationary and instationary behavior of oxide-ceramic high temperature fuel cells) Jül-2752” known typical operating data of a single fuel cell are given in the following table:
Cooling by air
Average current flow density [mA/cm
2
]
300
Cell voltage [V]
0.72
Fuel gas utilization degree [%]
80.5
Active electrode surface [cm
2
]
78.2
Gas entrance temperatures [° C.]
750
Total pressure [bar]:
1.5
Air exit temperature [° C.]
851.7
Watervapor-methane-ratio
2.9
Natural gas-pre-reformation degree [%]
30
Airflow: air ratio[−]
6.8
Fuel gas flow [mol/h] (entrance)
0.938
Material volume content: (entrance)
CH
4
0.131
H
2
O
0.487
H
2
0.269
CO
0.025
CO
2
0.053
N
2
0.035
Fuel gas flow [mol/h] exit
1.188
Material volume content
CH
4
—
H
2
O
0.705
H
2
0.102
CO
0.024
CO
2
0.143
N
2
0.026
It is the object of the present i
Bach Klaus J.
Chaney Carol
Forschungszentrum Jülich GmbH
Yuan Dah-Wei D.
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