Process for cooling molten carbonate fuel cell stacks and apparatus therefor

In a fuel cell power plant comprising a plurality of stacks of molten carbonate fuel cells the CO.sub.2 produced at the anodes of the cells is combined with the process air for the cathode side of the stacks. The CO.sub.2 rich mixture is fed from stack to stack in series through the cathode sides thereof with heat being removed from the cathode side exhaust streams between consecutive stacks. This process improves cell performance by increasing the CO.sub.2 partial pressure within the stacks. The process also makes it possible to reduce the required heat exchanger heat transfer area needed to cool the cell exhaust products.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to molten carbonate fuel cell power plants. 
2. Description of the Prior Art 
Molten carbonate fuel cells are well know in the art. These cells comprise 
an ion conducting molten carbonate electrolyte sandwiched between an anode 
and cathode electrode. Cells of this type are somewhat more fully 
described in commonly owned U.S. Pat. Nos. 3,878,296 Vine et al. and 
3,615,839 Thompson et al. 
The design of any commercial power plant has as one of its major 
considerations the cost per kilowatt of electricity produced. Cost per 
kilowatt includes both capital expense and operating expense. Obviously, 
it is extremely important that the fuel cells operate efficiently, but the 
cost for obtaining high efficiency cannot be excessive. Cell efficiency is 
directly tied to the suitability of the fuel and oxidant compositions 
supplied to the anode and cathode, respectively. The anode reaction in a 
molten carbonate fuel cell is shown by the following equation: 
EQU CO.sub.3 .sup.= + H.sub.2 .fwdarw. CO.sub.2 + H.sub.2 O + 2e.sup.- 
The cathode reaction is shown by the following equation: 
EQU 1/2 O.sub.2 + CO.sub.2 + 2e- .fwdarw. CO.sub.3.sup.= 
the unique difference between the anode and cathode reactions of molten 
carbonate fuel cells as compared to the anode and cathode reactions of 
more conventional acid or base electrolyte fuel cells is that the molten 
carbonate cells consume carbon dioxide at the cathode and produce carbon 
dioxide at the anode. 
Good performance (high current density at high cell voltage) in a molten 
carbonate fuel cell necessarily requires reasonably high partial pressures 
of both carbon dioxide and oxygen at the cathode. It is generally agreed 
that a practical molten carbonate fuel cell system requires the carbon 
dioxide produced at the anode be returned to the cathode since the air 
used as the oxidant at the cathode does not naturally contain enough 
carbon dioxide for efficient performance. The aforementioned Thompson et 
al patent discloses one technique for accomplishing this task: 
transferring the CO.sub.2 in the anode exhaust to the cathode inlet air 
stream by means of a membrane selectively permeable to CO.sub.2. Another 
technique is to combine the entire anode exhaust with the cathode inlet 
air stream, such as shown in U.S. Pat. No. 3,436,271 Cole et al. Ideally, 
it is desirable to have twice as much CO.sub.2 as O.sub.2 in the oxidant 
stream since two moles of CO.sub.2 are consumed for each mole of O.sub.2. 
Another important consideration in the design of commercially feasible 
molten carbonate fuel cell power plants is cell cooling. Molten carbonate 
cells typically operate at temperatures of about 1200.degree. F. 
Considerable quantities of heat are produced during operation which must 
be removed to prevent overheating of the cells. Cooling the cells in an 
economical manner can make the difference between commercial success and 
failure. 
One method of cooling a plurality of molten carbonate fuel cell stacks is 
to use the process air stream for cooling as well as for delivery of the 
cathode side fuel cell reactants. With this method the air, enriched with 
CO.sub.2 from the anode exhaust, is divided into equal portions and passes 
in parallel through the cathode side of the stacks. This cooling method is 
hereinafter referred to as single pass process air cooling. (Process air, 
as that term is used herein, simply means the air used in the 
electrochemical process.) Single pass process air cooling has the drawback 
that, for a system with "n" stacks, the process air stream for each stack 
is limited to receiving only 1/n times the amount of CO.sub.2 produced by 
the stacks. Diluting this amount of CO.sub.2 with the necessary amount of 
air for cooling results in CO.sub.2 partial pressures which are often less 
than satisfactory, resulting in a power plant which may not be 
commercially attractive. 
From a fuel cell performance point of view, the best method for cooling the 
fuel cells is with a cooling stream which passes through the cells but is 
separate from the process air stream. This permits the lowest possible air 
flow rate at the cathode since the process air is not needed for cooling; 
CO.sub.2 partial pressure is therefore maximized. This method, however, 
requires separate cooling fluid passageways within the stack, which adds 
considerable expense to the stacks and is a complex technical problem. 
SUMMARY OF THE INVENTION 
One object of the present invention is a molten carbonate fuel cell system 
which is economically attractive. 
Another object of the present invention is a method for cooling molten 
carbonate fuel cells which is economically attractive. 
Accordingly, a process for operating a fuel cell power plant, including a 
plurality of stacks of molten carbonate fuel cells, comprises the steps of 
introducing the CO.sub.2 produced at the anode side of the stacks into the 
cathode side process air supply upstream of the stacks, feeding the 
CO.sub.2 rich process air from stack to stack in series, and removing heat 
from the cathode side exhaust stream between the stacks. 
In the process of the present invention (hereinafter referred to as 
"multi-pass process air cooling") the quantity of air needed for cooling 
the plurality of stacks in the series is nearly the same as that needed 
for cooling each stack in a single pass process air cooling system. This 
is made possible by removing heat from the cathode exhaust stream between 
the stacks. In addition note that, in the system of the present invention, 
all of the CO.sub.2 produced at the anodes of all the stacks is present in 
the process air stream supplied to the cathode side of the first stack in 
the series. The first stack in the series will, therefore, have a 
considerably higher CO.sub.2 partial pressure than any stack of a single 
pass process air cooling system. Furthermore, in a single pass process air 
cooling system any CO.sub.2 not consumed in each stack is lost; in the 
present invention the unconsumed CO.sub.2 from the first stack is made 
available to succeeding tacks (and so on), thereby boosting CO.sub.2 
partial pressure throughout the system. It is believed that in many 
instances, depending on the number of stacks in series and other factors, 
this will result in an improvement in cell performance coupled with either 
a reduction in equipment cost, or an increase in equipment cost which may 
be more than offset by improved performance. However, as will hereinafter 
be explained, the cost of adding additional stacks (due to required 
additional manifolding, controls, heat exchangers, etc.) may, at some 
point, override any resulting performance gain. 
Economic and performance advantages or disadvantages of the present 
invention as compared to, for example, single pass process air cooling, 
will, in the final analysis, depend upon the constraints placed upon the 
system such as the desired temperature gradient across the cells, the 
pressures of the reactants, hydrogen utilization levels, thermal 
efficiency of the power section, type of feedstock used, and equipment 
cost. It is believed that, when all things are taken into consideration, 
there will be an optimum number of stacks to be connected in series for 
any given set of constraints. Generally speaking, however, if the process 
air flow rate is kept at the minimum needed to maintain an acceptable 
temperature gradient across all the stacks, the optimum number of stacks 
will be determined by the point at which the addition of another stack 
adds such a small performance benefit that the additional expense 
associated with the added stack is not warranted.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, a schematic of a fuel cell power plant according to 
the present invention is shown. The power section is shown as comprising 
three stacks of fuel cells, the stacks having the reference numerals 14, 
16, and 18, respectively. Each stack comprises a plurality of molten 
carbonate fuel cells connected electrically in series. Although three 
stacks are shown, this is by way of example only, the invention not being 
limited thereto. 
Referring to FIG. 2, a more detailed representation of the stack 14 is 
shown. The stacks 16 and 18 may be identical to the stack 14. For the 
purposes of simplicity, the stack of FIG. 2 comprises only three cells; 
but, in an actual power plant each stack may contain upward of 100 or more 
cells. Preferably (but not necessarily), each stack should have the same 
number of cells. Each cell comprises an anode electrode 20 spaced apart 
from a cathode electrode 22. The space between the electrodes is filled 
with a molten carbonate electrolyte such as a mixture of potassium 
carbonate and lithium carbonate. The electrolyte is a solid at room 
temperature, but has a putty-like consistency at operating temperatures, 
which may be anywhere above the melting point of the electrolyte. Typical 
molten carbonate fuel cells operate in the range of 
1100.degree.-1300.degree. F. Adjacent cells are separated from each other 
by gas impermeable plates 26 which are configured to define reactant gas 
spaces or compartments adjacent the nonelectrolyte side of the electrodes, 
such as the fuel or anode compartments 28 adjacent the anode electrodes 
20, and the oxidant or cathode compartments 30 adjacent the cathode 
electrodes 22. In the schematic of FIG. 1 the anode compartments 28 or 
"anode side" of the stacks 14, 16, and 18 are represented by the reference 
numerals 14A, 16A, and 18A, respectively; the cathode compartments 30 or 
"cathode side" of the stacks are represented by the reference numerals 
14C, 16C, and 18C. 
The power plant also includes a steam reforming reactor 10. In the reactor 
10 hydrogen is formed by reacting steam and a suitable carbonaceous 
feedstock, such as naphtha or natural gas, in the presence of a suitable 
catalyst such as nickel. Heat for the endothermic reaction is supplied by 
a burner 12. 
In operation, reformed fuel (essentially hydrogen) from the reactor 10 is 
fed in parallel to the stacks 14, 16, and 18 via conduits 32, 34, and 36, 
respectively. The fuel passes through the anode sides 14A, 16A, and 18A of 
the stacks, reacting at the anode electrodes 20 to produce water, carbon 
dioxide, electricity and heat. The anode compartment exhaust, which 
includes unconsumed hydrogen, carbon dioxide, and water, leaves the stacks 
via the conduits 38, 40, and 42 and are fed to the burner 12. 
In the burner 12 the unconsumed hydrogen in the anode exhaust burns with 
air supplied to the burner via a conduit 44 from a suitable source 46. If 
necessary, the burner may also have a separate fuel supply. The heat 
produced is transferred to the reactor 10 as represented by the heat 
exchanger 48. Additional heat in the burner exhaust is used to convert 
water to steam in a boiler 50. The steam from the boiler 50 is combined 
with the feedstock as at 52 and is introduced into the steam reforming 
reactor 10 via a conduit 53. The water in the anode exhaust is removed in 
a condenser 54 and is fed to the boiler 50 via a conduit 56. The anode 
exhaust, which still includes the carbon dioxide produced in the stacks, 
is thereupon combined at 58 with the process air from the air supply 46 
for use at the cathode electrodes of the cells. 
The process air mixed with the anode exhaust and therefore rich in CO.sub.2 
(the mixture hereinafter referred to as the cathode feed) is fed in series 
through the cathode sides 14C, 16C, and 18C of the stacks via the conduits 
60, 62, and 64, respectively. At the cathode electrodes of the stacks 14, 
16, and 18, the oxygen and CO.sub.2 in the process air and the CO.sub.2 
from the anode exhaust react at the cathode electrodes according to 
equation (2) above. The depleted cathode feed is exhausted from the last 
stack in the series and vented to atmosphere as represented by the arrow 
66. 
As the cathode feed passes through each stack it picks up heat thereby 
cooling the stack. This heat is removed between stacks by passing the 
stack cathode exhaust through suitably sized heat exchangers, such as the 
heat exchangers represented by the numerals 68 and 70. Assuming stacks of 
equal size, it is preferable that the heat exchangers be designed to 
provide the same cathode feed inlet temperature to each stack in the 
series. Heat exchanger bypass conduits and other suitable controls (not 
shown) may be used if necessary. Although the heat removed from the 
cathode exhaust of each stack may be used for any suitable purpose, (or 
may even be thrown away) in this embodiment it is used to preheat the 
cathode feed to the first stack in order that it, too, will have the same 
inlet temperature as the other stacks in the series. In this embodiment 
the heat in the cathode exhaust from the last stack in the series is also 
used to preheat the cathode feed to the first stack by means of another 
heat exchanger 72. It should be apparent that, depending upon the heat 
available and required at other locations in the power plant, many 
different heat exchanger arrangements for accomplishing the results set 
forth above could be devised. 
A study was made to determine, for one particular set of constraints, the 
advantages and/or disadvantages of the present invention as compared to a 
power plant having stacks cooled by either single pass (i.e., parallel 
flow) process air cooling or by separate air cooling. The power plant 
studied was considered to be designed essentially as shown in FIG. 1, with 
the stack cooling scheme being varied as required for the purposes of the 
study. The constraints which were critical to this analysis were as 
follows: 
Feedstock: light naphtha 
Average stack temperature: 1200.degree. F 
Average .DELTA.T across the tacks: 200.degree. F 
Power section thermal efficiency: 63% 
Hydrogen utilization (overall): 
(H.sub.2 converted to electricity /H.sub.2 fed to fuel cell) = 86% 
Reactant pressure: atmospheric 
"Power section," as that term is used above, includes only the stacks and 
their attendant plumbing; heat exchangers are not considered part of the 
power section. 
The results of the study indicated that multi-pass process air cooling 
according to the present invention yields the lowest power plant cost per 
kilowatt by permitting the combining of the best features of single pass 
process air cooling and separate air cooling; i.e., simple stack design to 
minimize cost (single pass process air cooling feature) and high partial 
pressure of CO.sub.2 at the cathode to maximize cell performance (separate 
air cooling feature). 
To be more specific, at the power section design point of 63% thermal 
efficiency, cell performance (in terms of watts per square foot of 
electrode surface) with two and three stacks in series according to the 
present invention was estimated to be 46% and 77% higher, respectively, 
than cell performance in a single pass process air cooling system. 
Actually, cell performance of three stacks in series is about equal to 
cell performance using separate air cooling, but without the complexity 
and cost. Four (or more) stacks in series provide little or no improvement 
in performance as compared to three stacks. 
The study also showed that the multi-pass process air cooling system of the 
present invention increases the number of heat exchangers required as 
compared to other cooling modes, but decreases total heat exchanger heat 
transfer surface area requirements. For example, at the design point of 
63% power section thermal efficiency the required heat transfer area of a 
three stack system designed according to the present invention is only 
about 15% of the area required in a single pass process air cooling 
system; and is about half that required by a two stack multi-pass process 
air cooling system. This reduction in required heat transfer area 
contributes substantially to the economic attractiveness of the present 
invention. 
From the foregoing it is apparent that, at least for a power plant with the 
constraints as set forth above, the present invention is an improvement 
over the other cooling systems herein discussed. It is also apparent that, 
for these constraints, three stacks in series produce optimum results. 
Although the invention has been shown and described with respect to a 
preferred embodiment thereof, it should be understood by those skilled in 
the art that other various changes and omissions in the form and detail 
thereof may be made therein without departing from the spirit and the 
scope of the invention.