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Research Papers

Simulation of the Postcombustor for the Treatment of Toxic and Flammable Exhaust Gases of a Micro-Solid Oxide Fuel Cell

[+] Author and Article Information
N. B. Raberger, M. J. Stutz, N. Hotz

Department of Mechanical and Process Engineering, Laboratory of Thermodynamics in Emerging Technologies, ETH Zurich, CH-8092 Zurich, Switzerland

D. Poulikakos1

Department of Mechanical and Process Engineering, Laboratory of Thermodynamics in Emerging Technologies, ETH Zurich, CH-8092 Zurich, Switzerlanddimos.poulikakos@ethz.ch

The averaging is performed over the mass flow rate times the specific heat capacity cp.

1

Corresponding author.

J. Fuel Cell Sci. Technol 6(4), 041002 (Aug 05, 2009) (11 pages) doi:10.1115/1.3080812 History: Received March 27, 2007; Revised December 26, 2008; Published August 05, 2009

This work investigates numerically a catalytic postcombustor for a micro-solid oxide fuel cell (SOFC) system. The postcombustor oxidizes toxic and explosive carbon monoxide (CO) and hydrogen exiting a solid oxide fuel cell to carbon dioxide and water. A single 1 mm diameter monolith reactor channel coated with platinum catalyst is modeled in this work. The inlet stream composition is provided by a semi-analytical 2D model of a detailed SOFC system. The model of the postcombustor includes the 2D axisymmetric Navier–Stokes equations, heat conduction in the channel wall, and a multistep finite-rate mechanism for the surface reactions. It is shown that under the operation conditions considered, the influence of homogeneous (gas phase) reactions can be neglected. The model predicts the expected adiabatic temperatures at the postcombustor outlet correctly and can be used for dimensioning and optimization. Postcombustor performance varies significantly with the choice of the operating parameters of the fuel cell. The most critical molecule at the SOFC outlet is shown to be CO because its depletion is slower than that of H2 for the entire operating range of the SOFC. It can be shown that the postcombustor is able to reduce the level of CO below the toxicity threshold of 25 ppm. Although higher voltages of the fuel cell lead to faster CO conversion in the postcombustor, they also result in a significant increase in wall temperature of the catalyst device. Furthermore, the percentage of SOFC power output used for pump work is lowest for the voltage where the maximum power is reached. For postcombustion the optimal operation point of the SOFC is at the voltage for maximum power of the SOFC system.

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Copyright © 2009 by American Society of Mechanical Engineers
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Figures

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Figure 1

Schematic of the micro-SOFC system including vaporizer (Vap), two preheaters (Pre1 and Pre2), the partial oxidation reformer (POX), the SOFC, and postcombustor (PC). Mass flows (--), heat flows (-), and electric power output P are shown.

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Figure 2

Monolith reactor with one of the channels enlarged. The washcoat fills the edges of the square channel resulting in a circular shape.

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Figure 3

(a) Power output (-) and exergetic efficiency (--) of entire fuel cell system consisting of three cells for different voltages of a single cell E. The values used as inlet conditions for the postcombustor in this study are marked with ∇, (b) current density-voltage curve for one SOFC fuel cell, and (c) hydrogen mole fraction at SOFC anode outlet.

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Figure 4

Computational domain of the axisymmetric postcombustor model

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Figure 5

(a) Oxygen conversion and (b) water production for an inlet velocity v=3 m s−1

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Figure 6

(a) H2 (-) and CO (--) conversion for first half of the channel (the arrow denotes increasing SOFC voltage from 0.6 V to 0.9 V) and (b) average channel temperature (-) and wall temperature (--), (× marks the calculated adiabatic flame temperature for isobaric conditions at 1 bar) for v=3 m s−1

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Figure 7

(a) CO conversion and (b) average channel temperature (-) and wall temperature (--) distribution for mixture 4 (E=0.7 V), × marks the calculated adiabatic flame temperature for isobaric conditions at 1 bar (the arrow denotes the direction of increasing inlet velocity from 3 m s−1 to 20 m s−1), (c) oxygen mean mole fraction (-), and surface mole fraction (--).

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Figure 8

(a) CO conversion at channel outlet, (b) average channel temperature at outlet (× marks the calculated adiabatic flame temperature for isobaric conditions at 1 bar), and (c) wall temperature maximum for different mixtures and speeds

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Figure 9

(a) Average level of CO at channel outlet, the dashed line denotes the 25 ppm limit and (b) maximum wall temperature versus total channel CO conversion (arrows denote direction of increasing SOFC voltage E from 0.6 V to 0.9 V)

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Figure 10

(a) Total pump work for the postcombustor flow and (b) ratio of total postcombustor pump work to total SOFC power output

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