Fuel-cell power plant

In a fuel-cell power plant according to this invention, a fuel gas system for supplying a fuel gas to an internal reforming type fuel cell is furnished with a fuel processor, fuel containing hydrocarbon or alcohols as its principal ingredient and supplied externally is partly or wholly passed through the fuel processor, and a fuel gas containing hydrogen and generated in the fuel processor is supplied to the internal reforming type fuel cell.

BACKGROUND OF THE INVENTION 
This invention relates to fuel-cell power plants, and more particularly to 
a fuel-cell power plant which employs an internal reforming type fuel 
cell. 
FIG. 1 shows a prior-art power plant which employs an internal-reforming 
molten-carbonate type fuel cell. Referring to the figure, numeral 1 
designates an internal-reforming molten-carbonate type fuel cell 
(hereinbelow, simply termed "fuel cell") which is made up of one or more 
laminated cell structures. A combustor 2 serves to oxidize a fuel gas 
unreacted in the fuel cell 1. Hezt exchangers 3a and 3b serve to preheat a 
reaction gas which is supplied to the fuel cell 1. 
Owing to the above construction, the fuel gas 7 which consists of fuel 5, 
containing hydrocarbon or alcohols as its principal ingredient, and steam 
6 is preheated to a predetermined temperature, for example, 550.degree. C. 
by the heat exchanger 3a and is thereafter supplied to the fuel cell 1. On 
the other hand, air 8 is mixed with an exhaust gas at the outlet of the 
combustor 2, the major ingredient of which is carbon dioxide perfectly 
oxidized by the combustor 2. The air 8 mixed with the exhaust gas is 
preheated by the heat exchanger 3b, and is thereafter supplied to the fuel 
cell 1. 
Here, the fuel cell 1 uses as its fuel the fuel gas principally containing 
hydrocarbon or alcohols and operates at a temperature of or near 
650.degree. C. by way of example. In a gas passage and an electrode on the 
fuel gas side of the fuel cell 1, there are carried out chemical reactions 
(Formulas (1)-(4) given below) of decomposing the hydrocarbon or alcohols 
to produce hydrogen and an electrochemical reaction (Formula (5) given 
below) of consuming the hydrogen to create electricity. Besides, in an 
electrode on the oxidizing gas side, an electrochemical reaction (Formula 
(6) given below) is conducted. The fuel cell 1 converts chemical energy 
inherent in the fuel gas into electric energy and its attendant thermal 
energy as a whole. 
(Gas Passageand Electrode on Fuel Gas Side) 
EQU Hydrocarbon+H.sub.2 O.fwdarw.H.sub.2, CO, CO.sub.2, CH.sub.4 ( 1) 
EQU Alcohol+H.sub.2 O.fwdarw.H.sub.2, CO, CO.sub.2, CH.sub.4 ( 2) 
EQU CH.sub.4 +H.sub.2 O.revreaction.CO+3H.sub.2 ( 3) 
EQU CO+H.sub.2 O.revreaction.CO.sub.2 +H.sub.2 ( 4) 
EQU H.sub.2 +CO.sub.3.sup.2- .fwdarw.H.sub.2 O+CO.sub.2 +2e (5) 
(Electrode on Oxidizing Gas Side) 
EQU 1/2O.sub.2 +CO.sub.2 +2e.fwdarw.CO.sub.3.sup.2 ( 6) 
For the purpose of steadily decomposing the hydrocarbon or alcohols at the 
temperature of or near 650.degree. C. by way of example so as to produce 
the hydrogen in accordance with Formulas (1)-(4), catalysts for 
accelerating the reactions, for example, a reforming catalyst and the heat 
of reactions necessary for the reactions of Formulas (1)-(4) need to be 
supplied. In the conventional internal-reforming molten-carbonate type 
fuel cell, accordingly, the steady operation thereof is made possible, for 
example, in such a way that the reforming catalyst is disposed within the 
fuel gas side gas passage and in adjacency to the electrode and that 
thermal energy produced accessorily in the electrochemical reactions of 
Formulas (5) and (6) is supplied as the heat of the reactions of Formulas 
(1)-(4). Therefore, it is important for operating the fuel cell 1 steadily 
and stably to maintain the activity of the catalyst and to supply 
sufficient heat of the reactions at all times. 
The above catalyst exemplified as the reforming catalyst is such that an 
active material such as nickel is carried on a support whose principal 
ingredients are alumina and magnesia. For keeping the activity of this 
catalyst, the oxidation of the nickel or the like active material as 
indicated by the following formula (7) needs to be prevented: 
EQU Ni+H.sub.2 O.revreaction.NiO+H.sub.2 ( 7) 
The conventional decomposition of hydrocarbon or alcohols/production of 
hydrogen employing the reforming catalyst is performed by adding steam as 
indicated by Formulas (1)-(4). Since the oxidation of the active material 
such as nickel is prevented by the produced hydrogen, the activity of the 
catalyst is kept. In the fuel cell 1 of this type, however, the hydrogen 
produced by Formulas (1)-(4) is consumed to produce steam as indicated by 
Formula (5). Therefore, the concentration of hydrogen lowers to incur the 
oxidation of the active material, e.g., nickel of the catalyst, so that 
the activity of the catalyst is prone to lower. Such a tendency of the 
active material toward the oxidation differs depending upon the kind of 
the active material. As to the catalyst employing nickel as the active 
material, it is known as one criterion that when the ratio of the water 
vapor concentration to the hydrogen concentration becomes 10-20 or above, 
the oxidation of the nickel takes place to lower the activity of the 
catalyst. In order to keep the activity of the catalyst, accordingly, the 
ratio of the water vapor concentration to the hydrogen concentration needs 
to be held at or below a certain value (for example, at or below 10-20). 
As illustrated in FIG. 1, in the prior-art fuel-cell power plant, hydrogen 
is not included in the fuel gas 7 which is supplied to the fuel cell 1. 
Accordingly, in a case where the electrochemical reaction of Formula (5) 
is excessively conducted, that is, in a case where current is derived from 
the fuel cell 1 in large quantities, the hydrogen concentration lowers in 
the fuel gas passage especially at an inlet part for the fuel gas, and the 
activity of the catalyst disposed in the fuel gas passage lowers, so that 
the power plant cannot be continuously operated. 
In general, the fuel cell 1 has the property that in case of diminishing 
the amount of current to be derived, cell characteristics are enhanced, in 
other words, the proportion of conversion of the chemical energy into the 
electric energy increases, whereas the occupying proportion of the thermal 
energy to be accessorily produced decreases. Accordingly, in the case 
where the amount of current to be derived is diminished, the proportion of 
the decrease of the thermal energy to be accessorily produced by the 
electrochemical reactions (Formulas (5) and (6)) becomes greater than the 
decrease of the heat of the reactions required for the decomposition of 
the hydrocarbon or alcohols (Formulas (1)-(4)). This results in the 
situation that the heat of the reactions necessary for the decomposition 
of the hydrocarbon or alcohols is insufficient, so the power plant cannot 
be operated with a light load for a long time. 
With the prior-art fuel-cell power plant as described above, the operation 
of the fuel cell under a high load is impossible because of the lowering 
of the activity of the catalyst attributed to the oxidation of the active 
material of the catalyst. To the contrary, in case of a low load, the heat 
of the reactions necessary for the decomposition of the hydrocarbon or 
alcohols cannot be supplied by only the thermal energy accessorily 
produced by the fuel cell. Thus, the operable condition of the fuel-cell 
power plant is limited to a very narrow range. This leads to the problem 
that an efficient operation following a demand load, which ought to be the 
feature of the power plant of the specified type, cannot be stably 
performed for a long term. 
SUMMARY OF THE INVENTION 
This invention has been made in order to solve the problem as described 
above, and has for its object to provide a fuel-cell power plant which 
realizes the high load operation of a fuel cell without lowering the 
activity of a catalyst and which is capable of a partial load operation in 
a wide range. 
In a fuel-cell power plant according to this invention, a fuel gas system 
for supplying a fuel gas to an internal reforming type fuel cell is 
furnished with a fuel processor, fuel containing hydrocarbon or alcohols 
as its principal ingredient and supplied externally is partly or wholly 
passed through the fuel processor, and a fuel gas containing hydrogen and 
generated in the fuel processor is supplied to the internal reforming type 
fuel cell.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Now, one embodiment of this invention will be described with reference to 
the drawing. FIG. 2 is an arrangement connection diagram of a power plant 
which utilizes an internal-reforming molten-carbonate type fuel cell (fuel 
cell 1). In the figure, numeral 4 designates a fuel processor which is 
incorporated in a fuel gas system for supplying a fuel gas to the fuel 
cell 1. This processor is a chemical reactor in which the fuel gas 
containing hydrocarbon or alcohols as its major ingredient and supplied to 
the fuel-cell power plant is partly or wholly decomposed thereby to be 
modified into a fuel gas 13 containing hydrogen as its principal 
ingredient. Besides, the same aymbols as in FIG. 1 indicate identical 
portions. 
Owing to the above construction, part 9 of the fuel gas principally 
containing the hydrocarbon or alcohols and supplied to the fuel-cell power 
plant is supplied to the fuel processor 4 along with steam 10 necessary 
for the decomposition of the fuel gas, thereby to be modified into the 
fuel gas 13 whose principal ingredient is hydrogen. Since the 
decomposition of the hydrocarbon or alcohols is an endothermic reaction, 
the fuel processor 4 needs to be supplied with the heat of reaction. In 
this fuel-cell power plant, as indicated by a broken line in FIG. 2, 
combustion heat inherent in a fuel side exhaust gas not utilized yet, 
obtained with the combustor 2 is utilized as the reaction heat necessary 
for the fuel processor 4. Concretely, this utilization of heat can be 
readily realized in such a way that the combustor 2 is disposed inside the 
fuel processor 4 and is used for heating or that the combusion gas at a 
high temperature exhausted from the combustor 2 is introduced into the 
fuel processor 4 and is subjected to heat exchange with the fuel gas 
inside the fuel processor 4. 
On the other hand, the remainder 11 of the fuel gas supplied to the 
fuel-cell power plant is mixed with steam 12 necessary for the 
decomposition of the fuel gas in the fuel cell 1, and it is directly 
supplied to the fuel cell 1 after being preheated to an appropriate 
temperature in the heat exchanger 3a. 
The fuel gases 13 and 14 supplied to the fuel cell 1 and containing 
hydrogen, carbon monoxide, carbon dioxide, methane, steam, hydrocarbon or 
alcohols, etc. are consumed according to Formulas (1)-(5) in the fuel gas 
side gas passage and the fuel gas side electrode, to create the electric 
energy and the attendant thermal energy. 
In the above embodiment, the hydrogen in an amount sufficient to prevent 
the activity of the catalyst from lowering due to the oxidation of the 
active material of the catalyst is already contained in the fuel gas 
supplied to the fuel cell 1, so that the operation of the fuel cell at a 
high load becomes possible. Moreover, the decomposition of the fuel gas 
externally supplied is partially carried out inside the fuel processor 4, 
thereby to reduce the reaction heat for the decomposition of the fuel gas 
as required in the fuel cell 1 and to permit the partial load operation of 
the fuel cell in a wide range. 
For operating the fuel-cell power plant efficiently and effectively, it is 
important to adjust the ratio at which the fuel supplied from outside is 
distributed as the fuel 9 and as the fuel 11. In case of increasing the 
proportion of the fuel 9 to be fed to the fuel processor 4, the partial 
pressure of the hydrogen contained in the fuel gas to be supplied to the 
fuel cell 1 increases, which is desirable from the aspect of the stability 
of the activity of the catalyst disposed inside the fuel cell. From the 
viewpoint of the balance between the reaction heat necessary for the fuel 
gas in the fuel cell 1 and the heat accessorily produced by the 
electrochemical reactions, no problem is posed under the rated load of the 
fuel cell because the heat accessorily produced is ordinarily in excess of 
the required reaction heat. In contrast, as the load of the fuel cell is 
lowered, the balance between the accessorily produced heat and the 
reaction heat collapses, and as the partial load factor is lowered more, 
the difference between them expands more. In this case, therefore, the 
proportion of the fuel 9 to be fed to the fuel processor 4 needs to be 
enlarged. 
One of the merits of the fuel cell 1 of this type is that the fuel gas is 
decomposed within the fuel cell, whereby the heat accessorily produced by 
the electrochemical reactions is efficiently utilized as the reaction heat 
of the decomposition of the fuel gas, while at the same time the cooling 
load of the fuel cell is relieved. Besides, an independent fuel processor 
is not required outside. It is accordingly possible to enhance the 
efficiency of the power plant and to miniaturize the plant. 
In the fuel-cell power plant, the proportion of the fuel 9 to be fed to the 
fuel processor 4 should desirably be minimized from the viewpoint of 
exploiting the merits of the fuel cell 1 to the utmost. Thus, the 
fuel-cell power plant is permitted to operate stably for a long term. 
Such a proportion of the fuel 9 relative to the whole fuel supplied changes 
depending upon the operating condition, the partial load factor, etc. of 
the fuel-cell power plant. By way of example, in a case where the average 
current density of the fuel cell in the rated operation is 160 
mA/cm.sup.2, where the hydrocarbon being the fuel is methane and where the 
steam-to-methane ratio is 3.0, approximately 3-30% is appropriate as the 
proportion of the fuel 9 relative to the whole fuel supplied. 
While the above embodiment has referred to the case of employing the 
internal-reforming molten-carbonate type fuel cell, a different kind of 
internally-reforming fuel cell may well be employed, and effects similar 
to those of the embodiment are achieved. 
The above embodiment has referred to the case where the reaction heat 
required for the fuel processor 4 is supplied by the combustion heat 
generated in the combustor 2. However, the reaction heat to be supplied 
may well be sensible heat inherent in the fuel gas or oxidizing gas of 
high temperature discharged from the internal reforming type fuel cell, or 
it may well be any other waste heat obtained in the fuel-cell power plant. 
Alternatively, fuel supplied anew from outside may well be burnt within 
the fuel processor 4 so as to use the combustion heat thereof as the 
aforementioned reaction heat. 
Further, while the above embodiment has illustrated the example in which 
the independent reactor is disposed as the fuel processor, the fuel cell 
and the fuel processor may well be unitarily constructed by disposing the 
latter in a gas manifold for supplying or discharging the fuel gas or 
oxidizing gas to or from the former. As a concrete example, FIG. 3 shows 
another embodiment in which the fuel processor 4 is disposed within a fuel 
gas inlet manifold 15. In this case, the fuel processor 4 is supplied with 
the reaction heat by the heat of radiation from the side surface of the 
fuel cell 1. 
FIG. 4 shows still another embodiment in which the fuel processor 4 is 
disposed within an oxidizing gas outlet manifold 16. In this case, the 
fuel processor 4 is heated by the heat of radiation from the side surface 
of the fuel cell 1 and the oxidizing gas of high temperature discharged 
from the fuel cell 1. The fuel processor 4 may well be disposed within a 
fuel gas outlet manifold 17, and similar effects are achieved. 
As apparent from the above description, this invention consists in that a 
fuel gas system for supplying a fuel gas to an internal reforming type 
fuel cell is furnished with a fuel processor and that a fuel gas supplied 
from outside is previously decomposed into hydrogen at least partly and is 
thereafter supplied to the internal reforming type fuel cell. Accordingly, 
this invention brings forth the effects that the high load operation of 
the internal reforming type fuel cell which is stable over a long term is 
permitted and that even in case of a low load, the thermal balance of the 
internal reforming type fuel cell does not collapse, so the load 
following-up property is good as s whole, and the reliability can be 
enhanced.