Demonstration Of Direct Internal Reforming For Mcfc Power Plants

*K. Aasberg-Petersen, **P. Simonsen, *P.S. Christensen, *S.K. Winther, *E. Jorn, *C. Olsen, *E.J. Jensen *HALDOR TOPS0E A/S, Nymolievej 55, DK-2800 Lyngby, Denmark **EIkraft AmbA, Lautruphoj 5, DK-2750 Ballerup, Denmark

1. Introduction

The conversion of methane into hydrogen for an MCFC by steam reforming is accomplished either externally or internally in the stack. In the case of external reforming the plant electrical efficiency is 5 % abs. lower mainly because more parasitic power is required for air compression for stack cooling [1], Furthermore, heat produced in the stack must be transferred to the external reformer to drive the endothermic steam reforming reaction giving a more complex plant layout.

A more suitable and cost effective approach is to use internal steam reforming of methane. Internal reforming may be accomplished either by Indirect Internal Reforming (IIR) and Direct Internal Reforming (DIR) in series or by DIR-only as illustrated in Fig. 1. To avoid carbon formation in the anode compartment higher hydrocarbons in the feedstock are converted into hydrogen, methane and carbon oxides by reaction with steam in an adiabatic prereformer [2] upstream the fuel cell stack.

This paper discusses key elements of the design of both types of internal reforming and presents data from pilot plants with a combined total of more than 10,000 operating hours. The project is being carried out as part of the activities of the European MCFC Consortium, ARGE [3].

2. The IIR/DIR-system

Depending upon the operating temperature and the steam-to-carbon ratio 60-90% of the methane fed to the MCFC is steam reformed in the IIR-chambers. The remaining methane must be steam reformed in the anode chamber to reach a high fuel utilization and a high plant electrical efficiency. Catalyst pellets may be used but a more elegant solution allowing easier cell assembly is catalyzed hardware [4] where the surfaces of the cell hardware are covered with a thin layer of steam reforming calalyst. The performance of catalyzed hardware has been demonstrated in the IIR-chambers of a pipeline natural gas (PNG) fuelled 7 kW MCFC pilot plant at Elkraft's facilities in Kyndby with no deactivation as illustrated in Fig. 2.

The catalyst in the anode chamber, whether as pellets or catalyzed hardware, is susceptible to poisoning by alkali resulting in loss of catalyst activity. In the

IIR/DIR system the catalyst in the anode chamber must retain a few percent of its original activity to reacli a 95% methane conversion for the lifetime of the fuel cell stack [4]. Data from a PNG fuelled 8 kW pilot plant from 1994 operated at Elkraft's facilities in Kyndby (Kyndby II) show that long catalyst life in the anode chamber can be achieved as illustrated in Fig. 3.

3. The DIR-only System

The stack cost can be substantially reduced by omitting the IIR-chambers and performing the internal reforming only by DIR. However, in this case the amount of catalyst that can be placed in the anode chamber is limited. A large amount of catalyst will result in a severe and stack damaging temperature drop in the anode inlet area during the initial period of operation when the catalyst is highly active. On the other hand, a small amount of catalyst will result in short catalyst lifetime and, thereby, low stack efficiency.

A novel catalytic system which avoids the stack damaging temperature drop while maintaining sufficient catalyst lifetime has been developed. A mathematical model has been derived for the MCFC stack. Model computer simulations were performed to define an optimal catalyst (activity) distribution in the anode chamber. Fig. 4 shows the calculated temperature distribution for a DIR-only MCFC stack at typical start of run conditions. The calculated methane conversion exceeds 99%.

A new type of internal reforming catalyst is used in the anode inlet area. The start-of-run activity of the catalyst has been tuned to match the required activity as defined by the model calculations. Accelerated out of cell experiments in an alkali poisoning reactor indicate that the activity close to stack end-of-life is similar to catalysts used in the anode chamber of the IIR/DIR-system. The calculated temperature profile close to the end-of-life is shown in Fig. 5. The corresponding methane conversion is above 97%.

An 8 kW DIR-only pilot plant experiment for validation of the model is planned at Elkraft's facilities in Kyndby (Kyndby III) during the fall of 1996.

4. Acknowledgements

Part of this work lias been supported by the Danish Ministry of Energy and another part by the Joule-Programme of the European Union.

5. References

1. J.R. Rostrup-Nielsen and L.J. Christiansen, Proc. Int. Symp. on Energy and Environment, Helsinki, August, 1991, p. 52.

2. T.S. Christensen, "Adiabatic prereforming of hydrocarbons - an important step in syngas production", Appl.Cat.A: General 138(1996), 282.

3. P. Kraus, A. Heiming and K. Aasberg-Petersen, Proc. this conference.

4. J.R. Rostrup-Nielsen and L.J. Christiansen: "Internal steam reforming in fuel cells and alkali poisoning", Appl.Cat. A: General 126 (1995) 381.

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