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(C02, water, hydrogen, CO) cathode: off-gas (N2, 02. CO,J 1-7 bar, 700"C

anode off gas: catalytic combustion of H2/CO lean gas; C02 removal for recirculation; heat recovery. cathode off gas: heat recovery; off gases disposal to stack DC output: conversion to AC shaft power, electricity generator

These concepts have been formalized in the approach presented in [4], which serves to identify options for the integration of energy conversion devices in chemical plants. In this approach, which may be used to identify novel market opportunities, the fuel cell stack is viewed as a multifunctional chemical unit operation, with specified functions, inputs and outputs. The approach comprises three steps. In the first step all functions of the fuel cell are described (the primary function is DC-power production, one of the secondary functions is hydrogen-depletion of a gas). The second step involves a description of all the functions of the auxiliary equipment present in a stand-alone fuel cell system (input processing, output processing etc.). The third and most elaborate step is the selection of chemical processes which can perform a number of functions of the fuel cell systems's auxiliary equipment. As stated preferably the chemical processes selected also benefit from the integration of the fuel cell, since this may provide the competitive edge required.

An example of such a fuel cell stack and fuel Cell system description is given in above table (adapted from [4]).

Trigeneration processes

■ An option for the application of fuel cells in the chemical industry which yet goes one step further is the development of trigeneration systems: process systems designed for the combined production of power, heat and chemicals. Both chemical and power may be sold or such a system may be auxiliary to a plant utility system. The trigeneration system may be connected to a common grid where it is a part of a distributed power generation system.

An example involves the conventional synthesis of ammonia, a process which includes steam reforming. Use of the hydrogen-rich purge-stream is an option for fuel cell integration identified by Haldor Topsoe [5]. This option was evaluated together with a the design of an MCFC-based trigeneration system. In this case excess syngas production capacity is used to feed an arrangement of fuel cell stacks. Thus the trigeneration of ammonia, heat and power is realized. No special treatment of the fuel cell off-gases is required because they are sent to the primary reformer furnace as fuel and high temperature oxydant [7],

In a similar fashion a trigeneration system for methanol, heat and power could be developed. The synthesis gas could be taken from a variety of sources. Taking the example of methanol synthesis a step further, the combination of power production and methanol production/dissociation could be used for load-leveling purposes [10].

Evaluation fuel cell applications in the chemical industry

The application of fuel cells in the chemical industry is hampered by the status of the technology, which is not considered to be 'industrially proven' in the chemical industry, except for the PAFC. Any particular plant operator obviously would prefer proof of the technology in a similar or even the same application as in his plant. Power generation by process-integrated and trigeneration fuel cell systems, however, may be seen as advantageous compared to stand-alone fuel cell systems because of the reduced amount of auxiliary equipment required, which improves both power generation economics and system operational reliability. The economics of fuel cell applications, however, need to become more favourable which would be achieved. A reduction in stack cost and an increase in operational lifespan are desirable. The consequence of a limited lifespan obviously implies increased costs, both in capital and labor. These replacement costs may be capitalized, i.e. the net present capital cost be calculated. One way of calculating this amount is by determination of the amount of capital which needs to be set aside to allow the annuities required for replacement to be paid out of the capital and the interest earned. It is obvious that the total capitalized cost required for a fuel cell stack decreases incase its lifespan is lengthened. This effect is enforced in case interest-rates are low. Doubling the expected lifespan would yield a dramatic improvement in project economics.

In case trigeneration systems are considered, the added value of the power generated by the fuel cell needs to exceed that of the production of the chemical product. Often this is only the case if single-train syngas capacity installed exceeds single-train chemical reactor loop. In this case, both chemical production and power generation benefit from the reduction in capital per unit syngas produced.

Inclusion of fuel cells will increase the complexity and operational characteristics of the chemical process system. Thus the perceived risk of unintended plant shutdown increases. As mentioned, the integrated fuel cell system is expected to be more simple than a stand-alone system. In any case a system design which allows continued operation of the chemical plant in case the fuel cell shuts down is to be preferred. Also basic control preferably is straightforward .

The fuel cell stack may perform functions which could also be performed by other technology. To arrive at succesful commercial fuel cell development therefore it is desired the combination of functions offered by the fuel cell cannot be adequately competed with.

Concluding remarks

Not only simple application of fuel cells to utilize waste gases is an option, also more sophisticated applications can be developed which may be seen as options presenting market-niches unique to fuel cells. Proper design will result in high system operational reliability, which exceeds the reliability of stand-alone systems and which is accpetable to chemical plant operators.

The conceptual method introduced may help market-developers identify options for fuel cell application in the chemical industry. Also, trigeneration systems may be considered. Possibly this will help to bring a substantial fleet of commercial fuel cell systems on-stream, thereby entering a spiral of cost-reductions because of increased stack-volumes produced.

References

1. Hirschofer, J.H, Stauffer, D.B and Engleman, R.R., "Fuel Cells. A Handbook (revision 3)", US-DOE report METC-94/1006, Moigantown, 1994

2. Schinkel, J.N., Dijkema, G.P.J., Weijnen, M.P.C., "Novel opportunities for fuel cell commercialization: process integrated applications", Proc. Int. Conference "Bringing Fuel Cells Down to Earth", Los Angeles, 1994.

3. Yang, W.C., Westinghouse Electric Corporation, Personal Communication, 1996 ("trigeneration")

4. Dijkema, G.P.J., Luteijn, C.P. and Weijnen, M.P.C., "Process design for combined generation of power, heat and chemicals: process integrated applications of eneigy conversion systems in chemical plants", Proc. 5th World Congress of Chemical Engineering, Vol. Ill, pp. 28-34, San Diego, 1996

5. Fiydenlund, F.A., EP. 0423177 to Haldor Topsoe 1993.

6. Lance J.R. et. al„ Int. J. Hydrogen Eneigy 8, 219-224, 1984

7. Dijkema, G.P.J., Vervoort, J., Daniels, R.J.E., Luteijn, C.P., Ammonia synthesis and er-mcfc-technology - a profitable combination?". Fuel Cell Seminar, Orlando, 1996 (to appear).

8. Goossens, J.C.J.M., "Conceptual design of a fuel cell system for an electrothermal phosphorus process", TwAIO study report. Faculty of Chemical Engineering and Material Science, Delft, 1995.

9. Dijkema, G.P.J., Veenstra, A.F., Vriesendoip, M. and de Lathouder, H.C., "Design of a MCFC-upgrade kit for a steam-reforming methanol plant", "1994 Fuel Cell Seminar, Program and abstracts", San Diego, 1994.

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