Fuel cell power plant

A fuel cell power plant utilizing hydrogen and carbon oxide rich feed gas (4) and comprising a hydrogen-carbon oxide consuming fuel cell (2) with an anode compartment (2a) and a cathode compartment (2c), means for supplying feed gas (8, 10, 12) to the anode compartment (2a), compression means (41) for supplying the cathode compartment (2c) with compressed cathode oxidant gas (44), and means for recirculating fuel cell exhaust gas (46, 50, 42) to the cathode compartment (2c), which fuel cell power plant further comprises a methanation unit (9) for converting the hydrogen and carbon-oxide rich feed gas (4) into a methanated gas, a reforming catalyst bed (13) adapted to receive and reform the methanated gas to anode process gas (14) by absorbing waste heat from said fuel cell (2).

FIELD OF THE INVENTION 
The present invention concerns a fuel cell power plant employing hydrogen 
and carbon oxide rich feed gas, and in particular, an improvement of the 
overall efficiency of such a fuel cell power plant by reducing the demand 
of cooling the fuel cell of the plant. 
BACKGROUND OF THE INVENTION 
In a fuel cell power plant chemical energy contained in the feed gas is 
converted into electrical energy by electrochemical reactions in the fuel 
cell. The feed gas is electrochemically oxidized at the anode of the fuel 
cell to give up electrons, which are combined with oxidant reactant gas in 
the cathode of the cell. Typical fuel cells widely employed in the known 
fuel cell power plants operates on hydrogen-carbon oxide fuel and air 
oxidant. Such a fuel cell is the known molten carbonate fuel cell and the 
solid oxide fuel cell, wherein H.sub.2 -O.sub.2 -fuel contained in the 
feed gas and the cathode oxidant gas is converted into water by the 
electrochemical reactions. 
When using a molten carbonate fuel cell CO.sub.2 is required in the cathode 
to maintain an appropriate ion transport through an electrolyte matrix, 
which is in contact with the anode and the cathode. CO.sub.2 is produced 
at the anode according to the following reactions: 
EQU H.sub.2 +CO.sub.3.sup.2- .fwdarw.H.sub.2 O+CO.sub.2 +2e (1) 
EQU CO+CO.sub.3.sup.2- .fwdarw.2 CO.sub.2 +2e.sup.- ( 2) 
and consumed at the cathode by the reaction: 
EQU 1/2O.sub.2 +CO.sub.2 +2e.sup.- .fwdarw.CO.sub.3.sup.2- ( 3) 
The theoretical thermal efficiency of a H.sub.2 -O.sub.2 fuel cell is 
determined by the ratio of free energy and heat of reaction of the overall 
cell reaction. 
EQU H.sub.2 +1/2O.sub.2 .fwdarw.H.sub.2 O; .DELTA.H=-241,8 kJ/mole(4) 
Though the free energy of the oxidation of H.sub.2 and CO decreases with an 
increase in temperature, giving a decreased reversible voltage of the 
cell, the performance of a practical fuel cell is kinetically controlled 
and benefits from a temperature increase. Mechanical properties and 
constraints of the materials used in the cell components, however, limit 
the operating temperature of the cells to a rather narrow temperature 
interval in order to avoid structural stress of the electrode material or 
electrolyte degradation, caused by sintering or crystallization of the 
electrolyte matrix. The operating temperature of e.g. a conventional 
molten carbonate fuel cell is confined within the range of 
600.degree.-700.degree. C. Thus surplus of heat generated by the 
exothermic electrochemical processes and polarization loss in the cell has 
to be removed from the cell. 
Cooling of the fuel cells, which in a fuel cell power plant are piled up to 
a stack of many individual cells, is provided by heat exchanging plates, 
or channels with a stream of a coolant to keep the stack at its optimum 
operating temperature. In the known fuel cell power plants cathode oxidant 
gas is used as coolant in the stack. Hot cathode exhaust gas is cooled and 
mixed with air along with CO.sub.2 from anode exhaust gas before it is 
recycled to the cathode compartment. The flow of the mixed recycle gas is 
adjusted at an appropriate rate depending on the cooling demand of the 
fuel cell stack in order to provide sufficient cooling. 
A drawback of the known fuel cell power plants using coal gas or 
hydrogen-carbon oxide rich gases as feed is the high cooling demand of the 
fuel cell stack caused by the strongly exothermic conversion reactions in 
the fuel cells. 
In particular, the recycle and gas supply system of the cathode gas loop 
has to be designed for a high gas flow to meet the cooling demand 
resulting in piping with considerable sectional areas and large 
compression units with high energy consumption for providing a sufficient 
gas flow. 
It is an object of the present invention to provide a fuel cell power plant 
employing hydrogen and carbon oxide rich gases as feed and having an 
improved overall efficiency by reducing the demand of cooling the fuel 
cell of the plant. 
It is further an object of the present invention to simplify the cathode 
gas supply loop of such a fuel cell power plant. 
SUMMARY OF THE INVENTION 
In accordance with the present invention there is provided a fuel cell 
power plant utilizing hydrogen and carbon oxide rich feed gas and 
comprising a hydrogen-carbon oxide consuming fuel cell with an anode 
compartment and a cathode compartment, means for supplying fuel gas to the 
anode compartment, compression means for supplying the cathode compartment 
of said fuel cell with compressed cathode oxidant gas, and means for 
recirculating fuel cell exhaust gas, said fuel cell power plant further 
comprises a methanation unit for converting the hydrogen and carbon oxide 
rich gas to a gas rich in methane, and means for reforming the methane 
rich gas, which means is adapted to absorb waste heat from said fuel cell 
by converting said methane rich gas to anode process gas. 
The methanation unit may comprise any of the known methanation reactors, 
such as an adiabatic methanation reactor and a boiling water methanation 
reactor. 
When using an adiabatic methanation reactor it is advantageous to connect a 
series of such reactors with recycling of processed gas to one or more of 
the reactors, as mentioned in e.g. U.S. Pat. No. 4,130,575. 
In a preferred embodiment of the invention the methanation-reaction is 
carried out in an adiabatic reactor connected to a boiling water 
methanation reactor, as disclosed in U.S. Pat. No. 4,298,694, which is 
incorporated herein by reference. Thereby waste heat generated during the 
exothermic methanation process is utilized to produce superheated steam, 
which may be used for generating additional electricity in a steam 
turbine. 
Methane rich gas from the methanation unit is converted to the hydrogen 
rich anode process gas by endothermic steam reforming in a reforming 
catalyst bed. By arranging the reforming catalyst bed in heat conducting 
relationship with the fuel cell necessary heat for the endothermic 
reforming process is supplied by the waste heat, which is generated during 
the exothermic electrochemical reactions in the fuel cell. 
In still a preferred embodiment of the invention the reforming catalyst bed 
is an integrated part of the fuel cell, such as in the known internal 
reforming molten carbonate fuel cell. 
In general the demand of cooling the fuel cell of a power plant according 
to the invention is reduced by consuming heat of reaction contained in the 
hydrogen-carbon oxide feed gas in a methanation unit and by absorbing 
waste heat from the fuel cell in a reforming catalyst bed by the 
endothermic steam reforming process. As a result the cathode gas supply 
loop of the fuel cell power plant according to the invention can be 
simplified in terms of diminished proportion of piping and reduced 
compression work for circulating cathode gas. 
Waste heat removal is thereby advantageously moved from the cathode gas 
loop as in the known hydrogen-carbon oxide feed employing fuel cell power 
plants to the methanation unit, which further results in an improved steam 
production due to a high gas pressure and resulting higher heat transfer 
coefficients in the methanation unit as compared to the corresponding 
parameters in the cathode gas loop. 
Furthermore, as an advantageous feature of the invention hot cathode 
exhaust gas may be circulated by means of an ejector, thereby saving costs 
for expensive hot gas compressors.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to the Figure the fuel cell power plant according to one 
embodiment of the invention comprises a hydrogen consuming fuel cell 2 
with an anode compartment 2a and a cathode compartment 2c held in contact 
by an electrolyte matrix 2e. As used herein and discussed above the fuel 
cell comprises a stack of several individual cells provided with heat 
exchanging plates (not shown in the Figure) for cooling the stack. 
Anode process gas supplied on line 14 is prepared by passing hydrogen and 
carbon oxide-feed gas in line 4 along with steam supplied on line 6 
successively through a methanation unit 9 and a reformer unit 13a. Before 
passing the combined stream of feed gas and steam to methanation unit 9 
the stream is adjusted to a temperature of about 350.degree. C. by heat 
exchange with anode exhaust gas 16 in heat exchanger 7 arranged in line 8. 
In methanation unit 9 hydrogen and carbon oxides contained in the combined 
stream of feed gas and steam, are methanated in the presence of a 
methanation catalyst by the following reactions: 
EQU CO+3H.sub.2 &lt;=&gt;CH.sub.4 +H.sub.2 O .DELTA.H=-206,15 kJ/mole(5) 
EQU CO.sub.2 +4H.sub.2 &lt;=&gt;CH.sub.4 +2H.sub.2 O .DELTA.H=-164,96 kJ/mole(6) 
Waste heat, which is formed during the strongly exothermic methanation 
reactions (5) and (6), may thereby be used to produce steam. As mentioned 
above, methanation unit 9 may comprise an adiabatic methanation reactor 
and connected thereto a boiling water methanation reactor wherein 
superheated steam is produced during the methanation process. 
The methanated gas leaving methanation unit 9 at a temperature of about 
350.degree. C. through line 10, consists mainly of methane, carbon dioxide 
and steam. The gas is preheated in heat exchanger 11 to a temperature of 
about 600.degree. C. with hot exhaust gas in line 16 from anode 
compartment 2a. 
The preheated gas is then passed in line 12 to reformer unit 13a, 
containing reforming catalyst 13. The reforming catalyst 13 is in heat 
conducting relationship with the fuel cell 2 by heat conducting separator 
plate 15. 
As mentioned above the reforming reactions and the electrochemical 
oxidation reactions may be integrated in the fuel cell (not shown in the 
Figure), such as by the known internal-reforming molten carbonate fuel 
cell. Thereby the need for an external reforming unit 13a is eliminated. 
In the reformer unit 13a methane and steam contained in the preheated gas, 
are reformed to anode process gas, supplied on line 14 to the anode 
compartment 2a, in the presence of reforming catalyst 13 by the reaction: 
EQU CH.sub.4 +H.sub.2 O&lt;=&gt;3H.sub.2 +CO .DELTA.H=+206,15 kJ/mole(7) 
EQU CH.sub.4 +H.sub.2 O&lt;=&gt;4H.sub.2 +CO.sub.2 .DELTA.H=+164,96 kJ/mole(8) 
Necessary heat for the endothermic steam reforming reaction (7) and (8) is 
thereby provided in fuel cell 2 by the exothermic electrochemical reaction 
(4) of the anode process gas with cathode oxidant gas supplied on line 44. 
The anode process gas flows at a temperature of about 650.degree. C. 
through the anode compartment 2a and is electrochemical reacted resulting 
in anode exhaust gas of mainly carbon dioxide and water along with minor 
amounts of unused hydrogen, carbon monoxide and methane, which leaves the 
anode compartment 2a through line 16. 
The anode exhaust gas in line 16 is cooled in heat exchangers 11 and 7 as 
described above and further cooled by cooling unit 21. The main part of 
water contained in the cooled exhaust gas 22 is removed in drain separator 
23. The dried exhaust gas 24 is then compressed in blower 27 and passed to 
combustion unit 25. In combustion unit 25 unused hydrogen, carbon monoxide 
and methane contained in the dried exhaust gas 24 is combusted to carbon 
dioxide and water, mixed with a mixture of cathode recycle gas and 
compressed air passed on line 42 to the combustion unit as further 
described below. 
Cathode oxidant gas, consisting mainly of air and carbon dioxide is 
prepared by mixing exhaust gas leaving the cathode compartment 2c through 
line 46 with compressed air on line 40 and combining the gas mixture with 
combusted anode exhaust gas in combustion unit 25. A part of the cathode 
exhaust gas is thereby circulated in recycle line 50 after cooling in 
waste heat boiler 47 arranged in line 50. 
The cathode exhaust gas in line 50 is circulated by means of ejector 51, 
which is driven by compressed air supplied on line 40 and compressed by 
compression unit 41. Before mixing with cathode exhaust gas with air in 
ejector 51, the air is preheated by heat exchange in heat exchanger 49 
with the residue of the cathode exhaust gas in line 48. 
The mixed gas in line 42 is then combined with combusted anode exhaust gas 
in combustion unit 25. The cathode oxidant gas thus obtained is passed in 
line 44 to the cathode compartment 2c, where it is reacted with electrons 
formed during the electrochemical reaction in the anode compartment 2a. 
Cooling of the fuel cell 2 is obtained by adjusting the temperature of the 
oxidant gas passed in line 44 to e.g. about 570.degree. C., which is lower 
than the operating temperature (650.degree. C.) of the fuel cell 2. The 
stream of oxidant gas has further to be adjusted to a flow rate at which a 
sufficient transport of waste heat out of the fuel cell 2 is ensured. The 
flow of the cathode oxidant gas 44 is adjusted by regulating the recycle 
flow of cathode exhaust gas in line 50, which is controlled by ejector 51 
and by the compressed air 42 according to the known principles of an 
ejectorpump. 
Waste heat is as mentioned above mainly absorbed by the reforming process, 
the residue is removed by the stream of cathode oxidant gas and hot 
cathode exhaust gas in line 46 and 48. 
In the following computation model shown in the Examples below the 
performance of a fuel cell power plant provided with a methanation unit 
and internal reforming molten carbonate fuel cell according to a preferred 
embodiment of the invention (Example 2) will be compared with a 
conventional fuel cell power plant (Example 1). 
For the purpose of comparison the following process parameters are assumed 
to be the same in both cases. 
The feed gas to the power plant is a hydrogen and carbon oxide rich gas 
having the following composition in mole %: 
______________________________________ 
H.sub.2 
33,6 
N.sub.2 
0,1 
CO 50,3 
CO.sub.2 
15,9 
CH.sub.4 
0,1 
______________________________________ 
The fuel cell comprises a stack of 300 individual internal reforming molten 
carbonate fuel cells, which operate at a temperature of 650.degree., 
giving a net power output of 105 kW. 
EXAMPLE 1 
In this Example a stream of hydrogen-carbon oxide feed gas combined with 
steam is directly passed through line 14 to the anode compartment 2a of 
the fuel cell 2 and converted to electricity as described above. Cooling 
of the fuel cell is provided by cathode oxidant gas supplied on line 44, 
which is prepared by combining a mixture of compressed air in line 40 and 
cathode recycle gas 50 with dry anode exhaust gas in line 24, burnt in 
combustion unit 25. 
Relevant process parameters and gas compositions will be apparent from 
Table 1 below, in which the position numbers refer to the lines and units 
shown in the Figure. 
TABLE 1 
__________________________________________________________________________ 
Composition 
Pos. No. 
Mole % 14 16 24 40 41 50 42 44 46 47 
__________________________________________________________________________ 
H2 19.10 
5.41 
8.41 
0.00 0.00 0.00 0.00 0.00 
H2O 43.24 
40.52 
7.42 
1.90 9.09 8.24 8.74 9.09 
N2 0.06 
0.04 
0.07 
76.56 65.19 
66.54 
62.70 
65.19 
CO 28.51 
3.39 
5.27 
0.00 0.00 0.00 0.00 0.00 
CO2 9.03 
50.65 
78.83 
0.03 20.34 
17.93 
22.11 
20.34 
CH4 0.06 
0.00 
0.00 
0.00 0.00 0.00 0.00 0.00 
O2 0.00 
0.00 
0.00 
20.59 4.60 6.50 5.69 4.60 
AR 0.00 
0.00 
0.00 
0.92 0.78 0.80 0.75 0.78 
Temp. Deg. C. 
600.00 
650.00 
50.73 
330.00 570.00 
545.96 
570.14 
650.00 
Pres. Bar g 
0.01 
0.01 
0.03 
0.20 0.01 0.02 0.02 0.01 
Flow Nm3/h 
159. 
216. 
139. 
250. 1856. 
2105. 
2235. 
2149. 
Duty kcal/h 58453 
Power W 4200 
__________________________________________________________________________ 
EXAMPLE 2 
This example illustrates the improvement of the overall efficiency of a 
hydrogen-carbon oxide employing fuel cell power plant according to a 
preferred embodiment of the invention compared to the conventional fuel 
cell power plant of Example 1. 
Feed gas combined with steam is supplied on line 8 and converted to a gas 
rich in methane by methanation of hydrogen and carbon oxide in methanation 
unit 9. The methanated gas from the methanation unit is passed in line 12 
to the anode compartment 2a, where it is reconverted to anode process gas 
by contact with an internal reforming catalyst arranged in the fuel cell 2 
and by utilizing waste heat from. the fuel cell. As seen in Table 2 below 
the feed gas in line 8 of this Example has the same composition as the 
feed gas 14 of Example 1, which in Example 1 is passed directly to the 
anode compartment 2a. 
By utilizing waste heat from the electrochemical reactions to the heat 
consuming reforming reaction the flow rate in the cathode gas loop 50, 42, 
44, 46 is appreciably reduced compared to the cathode gas loop of Example 
1. 
Relevant process parameters and gas compositions will be apparent from the 
Table 2 below, in which the position numbers refer to the lines and units 
shown in the drawing. 
TABLE 2 
__________________________________________________________________________ 
Composition 
Pos. No. 
Mole % 8 9 12 16 24 40 41 50 42 44 46 47 
__________________________________________________________________________ 
H2 19.10 0.99 
5.40 
8.41 
0.00 0.00 
0.00 
0.00 
0.00 
H2O 43.24 49.34 
40.52 
7.42 
1.90 9.09 
4.23 
7.54 
9.09 
N2 0.06 0.08 
0.04 
0.07 
76.56 65.19 
72.87 
54.03 
65.19 
CO 28.51 0.03 
3.39 
5.27 
0.00 0.00 
0.00 
0.00 
0.00 
CO2 9.03 33.70 
50.65 
78.83 
0.03 20.34 
6.63 
28.27 
20.34 
CH4 0.06 15.36 
0.00 
0.00 
0.00 0.00 
0.00 
0.00 
0.00 
O2 0.00 0.00 
0.00 
0.00 
20.59 4.59 
15.40 
9.51 
4.59 
AR 0.00 0.00 
0.00 
0.00 
0.92 0.78 
0.88 
0.65 
0.78 
Temp. Deg. C. 
350.00 600.00 
650.00 
50.73 
338.00 570.00 
421.12 
570.35 
650.00 
Pres. Bar g 
25.00 0.01 
0.01 
0.03 
0.03 0.01 
0.02 
0.02 
0.01 
Flow Nm3/h 
159. 122. 
216. 
139. 
250. 120. 
370. 
499. 
414. 
Duty kcal/h 54273 3787 
Power W 900 
__________________________________________________________________________ 
As seen from the above computation model enlisted in Table 1 and Table 2, 
the flow rate in recycle line 50 of the cathode gas loop of the fuel cell 
power plant according to a preferred embodiment of the invention is 
reduced by a factor of about 15, and a factor of about 6 in supply line 
42, resulting in diminished compression work by a factor about 5 as 
compared to a conventional hydrogen-carbon oxide employing fuel cell power 
plant as described in Example 1. 
Having thus described the invention in detail with respect to a specific 
embodiment of the invention it is to be understood that various changes 
which will be readily apparent to those skilled in the art are 
contemplated as within the scope of the present invention, which is 
limited only by the following claims.