FUEL CELLS

“MECHANICAL ENGINEERING”

ACKNOWLEDGEMENT This article appeared in the March 1959 issue of “Mechanical Engineering.”

The authors are Everett Gorin and Howard L, Recht, of the Research and Development Division, Consolidated Coal Company, Library, Pennsylvania.

RESEARCH on the fuel cell has been carried out sporadically for almost a hundred years, but there has been a decided awakening of interest in recent years. By definition, a fuel cell is a primary electro- chemical device which effects the oxidation of a fuel with conversion of a substantial portion of the heat of combustion directly to electrical energy. A true fuel cell must operate continuously with a steady and constant output of electrical energy and with continuous supply of fuel to the negative electrode and of oxidant to the positive electrode. Only re- cently have fuel cells been devised which begin to meet this criterion of steady output.

Why is there such a widespread interest in the fueI cell? In a broad sense, the main attraction is the potentially higher efficiency that can be achieved in the conversion of natural fuels to electrical energy. It does not, in itself, introduce a new source of energy such as in the case of nuclear fission and fusion. Its main function would be to extend our resources of fossil fuels.

A number of different types of fuel cells have been under development. All cells for which any success has been claimed operate with gaseous fuels, and discussion will be limited to these. Ultimately, of course, the authors’ company is interested in the use

of solid fuels, particularly coal, which is by far our most abundant fossil fuel for generation of power. Prior gasification of coal can be used to produce fuel gas for use in the fuel cell. As will be discussed further, this operation can be conducted in integration with the fuel-cell process itself to produce a high potential efficiency for generation of power from coal as well as from other natural fuels.

There are really two basic types of fuel cells that have been under development. The low and medium- temperature cells operate broadly in the temperature range of room temperature to about 25OOC. These cells are characterized by the fact that high output is only achieved when relatively pure hydrogen is used as the fuel and pure oxygen as the oxidant.

The most highly developed cell of this type operates at moderately elevated temperatures and at relatively high pressures of about 600 psi. This is the cell developed by F. T. Bacon’ in England, whose use in this country is being developed by the Patter- son Moos Division of the Universal Winding Com- pany-

These hydrogen-oxygen cells have achieved considerable success in the course of their laboratory development. They are of interest for specialized military and possibly civilian applications. In their

A.S.N.E. Journal. Augurt 1957 449

FUEL CELLS “MECHANICAL ENGINEERING”

present state of development, however, they cannot be regarded seriously as a major power source. This is because the cost of the fuel and of the oxidant is as great as the cost of power generated directly from natural fuels by more conventional methods. Even the apparent advantage of the fuel cell for generation of power at a high efficiency level tends to disappear with this kind of cell. This can be shown by reference to Table I. Here we see the over-all balance for a scheme where electric power is generated from methane by way of its prior conversion to hydrogen and its final combustion in a high-pressure Bacon type cell. It is seen that the over-all electrical efficiency. based on the methane feed, is somewhat smaller even than one obtains by more conventional power-generation methods.

TABLE I. Energy bala~tce in operation of hydrogen-oxggett cell at 600 psi I Ib. mol, C H , as primary fuel

Gross heating value, Btu Methane feed . . . . . . . . . . . . . . . . . . . . .383,000

Electrical energy in Btu equivalents at

Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.02 0.905 Produced in cell . . . . . . . . . . . . . . . . . . . . . . . . . .160,800 142.500 Consumed in oxygen plant . . . . . . . . . . . . . . . . 21,500 21,500 Consumed for gas compression . . . . . . . . . . . . 14,400 14,400 Net electrical energy . . . . . . . . . . . . . . . . . . . . . .124,900 106.600 Over-all electrical efficiency, in per cent . . . 32.7 27.0

HIGH-TEMPERATURE CELLS

H2 product at 100 psig. . . . . . . . . . . . . ,233,000

Current density, amp per sq. ft. . . . . . . . . . . . . 9.3 93

The other type of cell that has been under development in various laboratories in the world is the so- called high-temperature cell. This cell is characterized by the fact that the operating temperature is above 500’. principally in the range of 500” to 850°C; that operation is carried out at atmospheric pressure; that air can be used as the oxidant; and that ordinary fuel gases which can be generated directly from coal or other fossil fuels such as water-gas can be used. The most extensive work in this fieid has been carried out in Holland at the University of Amsterdam under Ketelaar.‘.3 Considerable success has been obtained in this work and cells have been devised which show steady operation at operating temperatures for periods of six months.

EXPERIMENTAL WORK

The authors’ company has been working for several years under Signal Corps sponsorship on the development of a high-temperature fuel cell. The work paralleled fairly closely that performed at Amsterdam. The type of fuel cell that was used is shown: in Figure 1. Any commercial development that arises would, in all likelihood, operate on a similar principle.

The fuel cell is composed of two flanges between which an electronically insulating but electroly- tically conducting electrolyte matrix is disposed. This matrix is composed of a porous-magnesia refractory. It is impregnated with a molten carbonate mixture which serves as the electrolyte.

VYCOl) INSULATOR\

GAYET-

CENTER1 LUGS

LEAD IVE

SCREW CLAW

LEA0 rivE

~ U E L GAS OUT

Figure 1. Experimental high-temperature fuel cell.

Two porous electrodes are disposed in the gas chambers on either side of the electrolyte matrix. Fuel gas is passed in and out of one of these chambers while air passes in and out through the other one. The air electrode is composed either of a porous sintered semi-conducting nickel oxide or of silver- wire gauze. Equally good results can be obtained with either. The fuel-gas electrode can be porous sintered-nickel metal, although other materials, such as porous iron, can be used. Figure 1 illustrates one of the authors’ earlier fuel-cell models. Work is now being conducted with pressurized double-porosity- type electrodes for which much better performance is anticipated.

Good operating characteristics have been attained with these cells using carbon-monoxide carbon-dioxide mixtures as the fuel gas with an iron-activated fueI electrode. Broers and Ketelaar have obtained very good performance with a similar system where the iron activator is backed up with a silver-gauze electrode. The porous-nickel, single-porosity electrode gives fair performance with CO-CO, mixtures. This is illustrated in Figure 2 where the cell voltage under steady operating conditions is shown as a function of the carbon-monoxide carbon-dioxide ratio and the current density and the temperature.

la-

Figure 2. Cell performance data with CO-CO, fuel gas.

The theoretical open-circuit voltage for these mixtures is of the order of one volt.

450 A 5 N E. Journal August IPS9

“MECHANICAL ENGINEERING” FUEL CELLS

Nickel, however, operates very well as a fuel electrode when hydrogen is present. Figure 3 shows similar data on cell operating characteristics at 750°C for a hydrogen-steam mixture as a fuel gas. The open-circuit voltage of the cell changes with current density due to change of gas composition as a result of the cell reaction. This change was cal- culated by means of the Nernst equation and the corrected voltage is shown as the upper curve. The IR drop in the cell is the difference in voltage between these two curves. It is found that the IR drop is equal to the resistance measured with an a-c bridge. We therefore conclude that the cell operates without polarization with a hydrogen fuel gas. No polarization was observed at the air electrode at the current densities up to 125 milliamperes per sq. cm., using either lithiated nickel oxide or silver gauze.

-.. ---. 7 .-“-,.” -. Figure 3. Cell performance data with H,-H,O fuel gas at 750°C

POTENTIAL APPLICATION

What is the potential of the high-temperature fuel-cell system as a primary energy source? Several factors will determine the over-all economics of such an operation. First, the fuel cost which is a reflection of the thermal efficiency of the cell. Second, the in- vestment and operating costs must be considered. These latter two costs are very much a function of the cost of the individual fuel-cell elements and of their life. It is difficult, at the present time, to get a realistic estimate of the cost of the elements in the fuel cell since many of the materials are either not manufactured at all or made on a very small scale. Cell life, it would appear from the authors’ work and that of Ketelaar,’,3 can be of the order of at least six months. However, the ultimate life must be larger for such a system to be practical. The cell output per unit of electrode area also will have a great bearing on the economics of the operation of the system since the cost of the individual electrode area is fairly well fixed.

The authors’ work and that of Ketelaar2.3 have shown that a current output of the order of 35 amp. per sq. ft. can be obtained at an operating voltage of (1.75, but a detailed theoretical analysis shows that

0

Figure 4. Integrated fuel-cell systems.

further cell development can yield a current output of 200 amp. per sq. ft. at the operating voltage of 0.75. Such a current output would, if the cell were sufficiently long-lived, approach that required for commercial operation.

The other important factor in the fuel cell is the over-all thermal efficiency for conversion of the orig- inal fossil fuel to electrical energy. An integrated system for effecting this conversion using either coal or natural gas as fuel is shown in Figure 4. The scheme shown is based on the premise, which has been demonstrated by prior work, that carbon dioxide addition to the air is necessary to achieve a high-output fuel cell. We see here a high-temperature cell composed of rectangular elements disposed in par- allel through which the fuel gas is passed counter- current to the air. The system shown at A and B is a circulating-carbon-dioxide-acceptor system which operates countercurrently and adiabatically to effect the transfer of CO, from the exhaust gases leaving the cell to the air entering the cell. The cells have re-forming catalysts disposed in the gas channel to convert methane into CO and hydrogen. The heat released during the cell operation is available to carry out this endothermic reaction. The gas leaving the CO, acceptor system is rich in hydrogen and may be either simply recirculated where natural gas is used as fuel or passed through a coal hydrogenation or hydrodevolatilization zone to produce the gas required for fuel-cell operation.

An over-all heat material balance has been made around the operation of such a system based on rather extensive knowledge of the coal hydrogenation-gasification operation. This balance was made around a fuel cell operating at 0.75 volt. The efficien- cies for three sources of fuel to the cell are shown in Table 11. The first is gas produced by hydrodevolatilization of coal; the second is gas from complete coal hydrogenation; and the third is natural gas. It is seen that the efficiency attainable ranges up to 70 per cent for complete conversion of the coal. Another system which can be used for complete coal conversion which was not illustrated involves gasification of the coal with steam in indirect heat-exchange re-

A.S.N.E. Journdl. August 1959 451

FUEL CELLS “MECHANICAL ENGINEERING”

TABLE 11. Over-all marimum electrical efliciency n T COMBUS-

TION, BTU

A. Cool devolatilization 100 Ib. coal to devolatilization . . . . . . . . . . . . . . . . . . . . . .1,438,OOO

devolatilization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70,000

1,=,m 58.1 Ib. char. product from devolatilization . . . . . . . . . . 844,000

Net fuel consumed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 664,000 Etficiency =

4.8 Ib. char, burned to supply endothermic heat of

100 X Btu equivalent of electrical energy at 0.75 volt - _ _ _ ~ ~ - Net -fuel consumed

?EYE = 59.7 per cent 664,ooo

B. Complete coal conversion 100 Ib. coal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1,438,ooO’ Btu equivalent of electrical energy generated at 0.75

volt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . =,OOO

Electrical efficiency 998’ooo = 69.5 per cent 1,438;i

C. Methane combustion 1 Ib. mol. methane in . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 383,034 Btu equivalent of electrical energy at 0.75 volt . . . . . . 249,000

Electrical efficiency 2491000 = 65 per cent 383,034

lationship with cell operation. Such a system shows a potential electrical efficiency as high as 75 per cent.

As already pointed out, nothing can be said as yet about the ultimate economics of the fuel cell. How- ever, it is clear that the fuel cell is an extremely promising method for generation of power and that there is every indication that by further development work a cell could be developed with sufficient stability and high enough output for commercial use. It is possible to make a comparison of the fuel costs that can be achieved by use of a fuel cell operating at 70 per cent over-all thermal efficiency as com- pared with fuel costs involved in generation of nuclear power and by conventional steam plants based on coal. This is given in Table 111.

The fuel costs for nuclear plants are extremely uncertain and one must take the best estimate from the many available. The first generation of nuclear power plants is expected to show a fuel cost in the

TABLE 111. Comparison of estimated fuel costs, nuclear and coal-based power plants

Nuclear fuel cost, mills per kwhr.. . . . 5.8 1.7 to2.0 1.0 Competitive coal costs. cents per million

Steam plant heat rate, 8500 Btu per

Fuel cell heat rate, 4850 Btu per

Btu

kwhr ............................ 68 20to24 12

kwhr . . . . . . . . . . . . . . . . . . . . . . . . . . . ,120 35to41 21

neighborhood of 5 to 6 mills per k ~ h . ~ . ~ The second generation of nuclear plants is expected to show a much lower fuel cost which is generally not too well defined. The figures shown in Table 111, of 1.7 to 2.0 mills per k ~ h , ~ . ’ are the lowest estimates known to the authors for such second-generation plants. Sim- ilarly, ultimate nuclear fuel cost is even more indefinite, but a value of 1 mill per kwhr has been arbitrarily assigned.

It is seen that the fuel cell would permit coal to remain competitive with nuclear power on the basis of fuel cost over a much longer period than would be the case for a conventional steam plant. As a mat- ter of fact, the figures indicate that the fuel cell could show a lower fuel cost than a nuclear plant at a loca- tion close to the coal mine for the indefinite future.

REFERENCES

1. F. T. Bacon, Beama Journal, vol. 61, 1954, pp. 6-12. 2. J. A. A. Ketelaar, Die Ingenieur, vol. 66, August 20, 1954,

3. G. H. J. Broers, Ph.D. thesis, University of Amsterdam, The

4. C. W. Zielke and E. Gorin, Industrial and Engineering

5. W. Kenneth Davis, Louis H. Roddis, Jr., and Clark Good-

6. Paul R. Kasten and H. C. Claiborne, Nucleonics, vol. 14,

7. Paul R. Kasten and Russell E. Hoen, Industrial and Engi-

pp. Es8-91.

Netherlands, 1958.

Chemistry, vol. 47, 1955, pp. 820-825.

man, Nucleonics, vol. 15, 1957, pp. 90-93.

1956, p. 88.

neering Chemistry, vol. 47,1955, pp. 820-825.

NOTE: T w o recent review articles on fuel cells are: F. T. Bacon and J. S. Forrest, “High-Pressure Hydrogen Fuel Cell,” Engineer, vol. 202, July 20, 1956, pp. 93-94, from Fifth World Power Conference Paper No. 119K/4, Vienna, Austria, 1956; and E. K. Rideal, Zeitschrift fiir Elektrochemie, vol. 60, 1958.

452 A.S.N.E. Journal, August I959

Documents

FUEL CELLS