Page 35 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

16 High Temperature Solid Oxide Fuel Cells: Fundamentals, Design and Applications

anode but requires ceria or other catalysts to provide suitable sites for direct
oxidation [ 3 61.
Fuel reforming can also take place on nickel at the anode. This occurs when
steam is added to the hydrocarbon fuel, typicalIy at a ratio of 3 parts steam to
1 part of fuel. The reaction of methane is then given by
CH4 + HzO +. CO + 3H2

The hydrogen and carbon monoxide released by this reaction can then react
individually with oxide ions emerging from the electrolyte. Usually the CO
conversion is sluggish so the shift reaction also occurs on the anode to produce
more hydrogen:

CO+HzO + COz +H2

It was demonstrated in the 1960s that hydrocarbons could be injected directly
into SOFCs if steam was supplied [37]. The steam can beneficially be obtained
from the spent fuel stream. The main problem with direct use of hydrocarbons is
that coke can form to block up and contaminate the anode. There are two
damaging reactions which can occur on the nickel:
2co +. coz + c

CH4 + 2H2 + C

When carbon formation was investigated in detail, by temperature-
programmed reaction, three different types of material were discovered on the
nickel, as indicated by the temperature required for oxidation [38]. The most
stable carbon could not be removed beIow 1100 I< and tended to form when
current was flowing through the cell.
The other damaging mechanism of SOFC failure stems from fuel impurities.
Sulphur is the most prevalent impurity and can be present up to l?h level in
marine diesel fuel. SOFCs cannot operate with this amount of sulphur. More
typically, natural gas often has ‘odorant’ sulphur compounds added to make
leaks more easily detectable. Even the lower levels of such additions, about
10 ppm, are damaging for SOFCnickel anodes, and the upper limits around 100 ppm
could cause failure in about 1 h of operation. There are two approaches to
solving this problem: adding a sulphur absorber to the fuel processing unit: and
using anode metals which are less affected by sulphur. Fortunately, the levels of
sulphur in gasoline and diesel fuel are now being reduced for environmental
reasons, with the best formulations containing less than 10 ppm.
The second difficulty is the number of additives in conventional fuels which
have been formulated for other technologies. For example, regular gasoline
contains more than 100 different molecules, some added as lubricants or
surfactants. Moreover, the mixture can change with time and place because the
standard is dictated by octane number and not composition. Consequently, it is
unlikely that SOFCs will be able to run directly on gasoline, although this has