Fuels for fuel cells: Fuel and catalyst effects on carbon formation Page: 2 of 5
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Fuels for Fuel Cells: Fuel and Catalyst Effects on Carbon Formation
Rod Borup, Michael A. Inbody, W. Lee Perry and W. Jerry Parkinson
Los Alamos National Laboratory
MST-11; P.O. Box 1663
Los Alamos, NM 87545
The goal of this research is to explore the effects of fuels, fuel constituents, additives and
impurities on the performance of on-board hydrogen generation devices and consequently on the
overall performance of fuel cell systems using reformed hydrocarbon fuels. Different fuels and
components have been tested in automotive scale, adiabatic autothermal reactors to observe their
relative reforming characteristics with various operating conditions. Carbon formation has been
modeled and was experimentally monitored in situ during operation by laser measurements of the
effluent reformate. Ammonia formation was monitored, and conditions varied to observe under
what conditions NH3 is made.
Autothermal Reforming of Gasoline and Diesel Fuels
Fuel partial oxidation and reforming are being explored for onboard production of hydrogen.
For the chemical conversion of fuel hydrocarbons, air is combusted with fuel, typically over a
catalyst to produce hydrogen and carbon monoxide. Eqn. 1 shows the partial oxidation of a
generic hydrocarbon for an air stoichiometry exactly correct (O/C = 1) for the production of
hydrogen and carbon monoxide. If the oxygen-fuel ratio is more fuel rich (O/C < 1) such as in
eqn 2, without sufficient residence time and water content, unconverted hydrocarbons will be
present in the reformate stream. These hydrocarbons include small hydrocarbons such as
methane and ethane. Since the oxidation reaction is exothermic, it is common to use water to
steam reform part of the hydrocarbon mix, as in eqn. 3.
CnH(2n+2) + (n/2)02 --> nCO + (n+1)H2 (1)
CnH(2n+2) + (m/2)02 --> mCO + C(n-m)H2(n-m) + H2 (2)
CnH(2n+2) + nH20 --> nCO + (2n+1)H2 (3)
To understand the fundamentals underlying hydrocarbon reforming technology, we are
employing both experimental measurements and chemical modeling of the systems. To
experimentally measure fuel reformation, we have developed partial oxidation (POx) reactors
with the supporting test equipment to test the feasibility of generating reformate. Carbon
formation for different operating conditions and fuel components was monitored by in situ laser
scattering. Mapping of the onset of carbon formation for different fuel components as a function
of operating conditions has been conducted with these techniques. Modeling of equilibrium
carbon formation has been used to predict the operating conditions for the onset of carbon
formation for various fuel blends. Modeling is conducted using commercial codes, such as
ASPEN to model equilibrium concentrations of expected outlet species of the fuel reformer.
Other codes have been developed, specifically to model different carbon species and the
formation thereof. In particular, modeling of equilibrium carbon formation has been used to
predict the operating conditions for the onset of carbon formation for various fuel blends
Carbon Formation Modeling Results
Solid carbon formation equilibrium was determined using thermodynamic data to determine the
equilibrium composition of reactions at various temperatures, pressures and feed composition.
Figure 1 displays the temperature at which carbon formation is no longer observed, for a specific
operating condition and fuel blend for various steam/carbon ratios. Depending upon the relative
oxygen content, the carbon disappearance temperature can vary by up to 150 *C. The relative
effect of increasing the steam content of the fuel mixture greatly reduces the tendency for carbon
formation. Ternary, or triangular diagrams have also been used to present the carbon formation
modeling with the each corner representing an atom fraction of 1.0 for each one of the atoms, C-
H-O, to define regions where carbon will form. The apex of the triangle represents a
composition of 100% carbon. The left bottom corner represents a composition of 100%
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Borup, R. L. (Rodney L.); Inbody, M. A. (Michael A.); Perry, W. L. (William Lee) & Parkinson, W. J. (William Jerry),. Fuels for fuel cells: Fuel and catalyst effects on carbon formation, article, January 1, 2002; United States. (https://digital.library.unt.edu/ark:/67531/metadc930816/m1/2/: accessed April 19, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.