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TECHNICAL BRIEFS

Highly Efficient Conversion of Ammonia in Electricity by Solid Oxide Fuel Cells

[+] Author and Article Information
N. J. J. Dekker

 Department of Fuel Cell Technology, Division SOFC, Energy Research Centre of the Netherlands (ECN), P.O. Box 1, 1755 ZG Petten, The Netherlandsn.dekker@ecn.nl

G. Rietveld

 Department of Fuel Cell Technology, Division SOFC, Energy Research Centre of the Netherlands (ECN), P.O. Box 1, 1755 ZG Petten, The Netherlands

www.InDECpp.com.

J. Fuel Cell Sci. Technol 3(4), 499-502 (Mar 27, 2006) (4 pages) doi:10.1115/1.2349536 History: Received November 30, 2005; Revised March 27, 2006

Hydrogen is the fuel for fuel cells with the highest cell voltage. A drawback for the use of hydrogen is the low energy density storage capacity, even at high pressures. Liquid fuels such as gasoline and methanol have a high energy density but lead to the emission of the greenhouse gas CO2. Ammonia could be the ideal bridge fuel, having a high energy density at relative low pressure and no (local) CO2 emission. Ammonia as a fuel for the solid oxide fuel cell (SOFC) appears to be very attractive, as shown by cell tests with electrolyte supported cells (ESC) as well as anode supported cells (ASC) with an active area of 81cm2. The cell voltage was measured as function of the electrical current, temperature, gas composition and ammonia (NH3) flow. With NH3 as fuel, electrical cell efficiencies up to 70% (LHV) can be achieved at 0.35Acm2 and 60% (LHV) at 0.6Acm2. The cell degradation during 3000 h of operation was comparable with H2 fueled measurements. Due to the high temperature and the catalytic active NiYSZ anode, NH3 cracks at the anode into H2 and N2 with a conversion of >99.996%. The high NH3 conversion is partly due to the withdrawal of H2 by the electrochemical cell reaction. The remaining NH3 will be converted in the afterburner of the system. The NOx outlet concentration of the fuel cell is low, typically <0.5ppm at temperatures below 950°C and around 4ppm at 1000°C. A SOFC system fueled with ammonia is relative simple compared with a carbon containing fuel, since no humidification of the fuel is necessary. Moreover, the endothermic ammonia cracking reaction consumes part of the heat produced by the fuel cell, by which less cathode cooling air is required compared with H2 fueled systems. Therefore, the system for a NH3 fueled SOFC will have relatively low parasitic power losses and relative small heat exchangers for preheating the cathode air flow.

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Copyright © 2006 by American Society of Mechanical Engineers
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Figures

Grahic Jump Location
Figure 1

Cell voltage as function of the current density, fuel composition, and temperature (ESC with LSM cathode, 900–1000°C, high fuel flow rate (Uf=100% at 0.7A∕cm2))

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Figure 2

Cell voltage as function of the current density, fuel composition, and temperature (ASC with LSCF cathode, 700–800°C, high fuel flow rate (Uf=100% at 0.7A∕cm2))

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Figure 3

Measured and equilibrium NH3 outlet concentration as function of the temperature (ESC and ASC cells, 75% fuel utilization, nominal NH3 flow)

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Figure 4

Cell voltage and efficiency as function of the current density (ASC, 800°C, nominal and high NH3 fuel flow rate (Uf=100 at 0.4 and 0.7A∕cm2))

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Figure 5

Efficiency as function of the temperature and current density (ASC, 800°C, nominal and high NH3 fuel flow rate (Uf=100 at 0.4 and 0.7A∕cm2))

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Figure 6

Cell voltage and efficiency with time (ASC with LSCF cathode, 750°C, high flow, 0.55A∕cm2, 75% fuel utilization)

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Figure 7

Possible system configuration with large (a) and smaller (b) air heat exchanger. (A-HE: air heat exchanger, F-HE: fuel heat exchanger).

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