Fuel Cell Systems For A Sustainable Energy Production

Timo Kivisaari Royal Institute of Technology Department of Chemical Engineering and Technology/ Chemical Technology S-100 44 Stockholm Sweden

Introduction

When talking about fuel cell systems for stationary applications, two of the advantages are claimed to be a high inherent efficiency and environmentally favourable characteristics. It should, however, be obvious to everybody that in order to call an energy production Toute environmentally benign, it is not enough that just the energy production step itself has a low negative environmental impact, but that all steps involved (e.g. fuel pre-treatment. fuel processing etc.) should be subjected to the same constraints if the overall production process is to be considered environmentally friendly.

In order to evaluate the technical possibilities of a biomass fuelled MCFC unit for stationary applications a system study of a 40 MWe biomass-fired MCFC system is currently carried out at The Royal Institute of Technology, as part of the international co-operation within the IEA Advanced Fuel Cell Programme Annex I, Balance of Plant of MCFC Systems. In addition to the present work, other recent studies involving biomass and fuel cells can be found in literature [1,2,3,4,5].

Fuels for fuel cells

The main requirement for a fuel to be applicable for stationary fuel cell use is that is should be possible to gasify in order to be fed to the fuel cell. Therefore the easiest fuels to use in fuel cell power plants are those that in their natural state are gaseous (e.g. natural gas and LNG). The simplicity arises from that fact that the fuels are in the gaseous state at the temperatures and pressures where they are used, and therefore the necessary pre-treatment is limited to removal of impurities followed by catalytic steamreforming, either externally or internally within the fuel cell stack. Another class of possible fuels are those that in their natural state occur as liquids (e.g. ethanol [6], kerosene and other liquid fuels). Prior to purification and steamreforming an additional step is needed to transfer the liquid fuel into a gaseous state for all of these. Since the fuels characterised as liquids usually have a low boiling point, the transformation can be categorised as evaporation. The use of these fuels will probably be confined to smaller scale stationary systems. The last, but not less important, category of fuels are the ones that are more or less solid in their natural state (i.e. heavy fuel oils, coals, peat and biomass). Similarly to liquid fuels a phase transformation is needed before the final fuel cleanup. They therefore need to undergo a thermo-chemical conversion to render a gaseous mixture that can be used in the fuel cell. Since the gasification results in a partial oxidation process of the original solid fuel, no further processing, except for gas purification, is needed if the now gaseous fuel is to be used in an MCFC unit. When looking at the fuels applicable for larger scale applications, it can be recognised that the only fuel meeting the critérium of renewability is biomass, whereas the others are either manufactured from, or are themselves, fossil fuels. These fuels can therefore only be considered renewable in a multi million year time-span, and the supply is usually considered finite. In addition to being a renewable fuel, with infinite fuel supply, the use of biomass has another considerable advantage. It does not result in any net production of CO, to the atmosphere, at least if coupled with active silviculture. Therefore, the use of biomass as a fuel in an MCFC system will favourably influence the greenhouse effect, resulting in a fuel cell system for a sustainable energy production.

Biomass gasification

As in the case with coal fired MCFC power plants, the first process step of a biomass-fuelled MCFC power plant is the gasification of the primary fuel. The main difference compared to coal systems is the fact that biomass requires a considerably lower gasification temperature than coal. The reason for this is the higher reactivity of biomass under gasification conditions which results in adequate reaction rate and conversion at lower temperatures. Coal requires a gasifier temperature of more than 1000 °C, whereas biomass only needs gasification temperatures in the range of 500- 1000 °C.

As a result, biomass gasifiers can be operated with air as the primary gasification medium, whereas a coal gasifier requires oxygen for its operation. As a consequence, biomass gasification systems does not need the cryogenic units necessary for the oxygen production which is required in coal gasification.

A major drawback resulting from the, so far, low interest in biomass is that information about contaminant levels acceptable for an MCFC unit are scarce. For coal, on the other hand, there are several continuing evaluation programs related to these issues [7, 8]. To some extent the pollutants are the same and the coal-related data can be used directly, in other cases additional research may become necessary.

Layout of a biomass fuelled-MCFC unit

The flowsheet envisioned for a 40 MWe MCFC power plant with biomass gasification can be seen in figure 1.

Figure 1. Envisioned layout of a 40 MWe biomass-fired MCFC unit

The first step is the gasifier, where the pressurised thermochemical decomposition of biomass takes place in presence of air. The resulting gas consists mainly of CH,. CO, CO,, N, and H,. The dust removal in a cyclone is followed by a syngas cooler where the gas is cooled before subsequent purification. The first step of the purification is a water-wash used to remove ammonia and tars, which is followed by a conventional sulphur removal step (e.g. Selexol™, Purisol™, Rectisol™ etc.). Another possibility would be the utilisation of a high temperature purification technique, but since these are at the development stage, it has been considered safer to study a system with conventional purification.

Depending on the capabilities of the conventional purification, a ZnO bed may become necessary to remove trace amounts of sulphur.

After completed cleaning, the gas is reheated before entering the MCFC anodes.

The spent fuel is mixed with air and passed trough a catalytic combustor. The combusted anode off-gas can then be fed to the MCFC cathodes, thus ensuring adequate CO, supply.

Cathode off-gas leaving the fuel cell is divided into two streams, one to be recirculated back to the fuel cell cathodes, and another stream which is passed through an expansion turbine.

The large amount of heat that can be extracted both from the syngas cooler as well as from the expansion turbine effluent can be used for steam generation and additional power production in steam turbines.

Results

A technical assessment of the power plant described above is currently in progress at The Royal Institute of Technology, as part of the international co-operation within the IEA Advanced Fuel Cell Programme Annex I. Balance of Plant of MCFC Systems, and the results of this assessment is to be presented during the 1996 Fuel Cell Seminar in Kissimee, Florida.

Acknowledgements

This work is financially supported by the Swedish National Board for Technical and Economical Development (NUTEK).

References

1. G. Liberati and G. Spazzafumo, "MCFC versus SOFC in biomass gasifier/fuel cell power plants", Abstracts - Fourth Grove Fuel Cell Symposium. 19-22 September 1995. London.

2. S. Kartha, E.D. Larson, J.M. Ogden and R.H. Williams, "Biomass Integrated Gasifier/Fuel Cell Electric Power Generation and Cogeneration", 1994 Fuel Cell Seminar, Program and Abstracts, November 28 - December 1, 1994, San Diego, CA.

3. T. Mäkinen, J. Leppälahti, E. Kurkela and Y. Solantausta. "Electricity produced from biomass by gasification and a solid oxide fuel cell", 8th European Conference on Biomass for Energy,. Environment, Agriculture and Industry, 3-5 October, 1994, Vienna.

4. A. Ramsköld. "Biobränslebaserade bränsleceller". Report from Vatlenfall Research. Stockholm, 1993.

5. G. Liberati and G. Spazzafumo, "Biomass Gasifier/MCFC Plants - Performance Perspec lives". 1992 Fuel Cell Seminar, Program and Abstracts. November 29 - December 2, 1992. Tucson, AZ.

6. Information available at Energy Research Corporation's Internet WorldWideWeb-page: http://www.ercc.com/fuelcell.html, August 28.1996.

7. M.C. Wiliams and D.A. Berry," Overview of the DOE-Funded Fuel Cells Contaminants R&D Program", 1990 Fuel Cell Seminar, Program and Abstracts. November 25 - 28. 1990, Phoenix, AZ.

8. J.H. Hirschenhofer, D.B. Stauffer and R.R. Engleman, "Fuel Cell. A Handbook (Revision 3)". - DOE Report. DOE/METC-94/1006 (DE94004072).

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