Page 299 - Biofuels Refining and Performance

278 Chapter Nine

Hardware. In the SOFC, both CO and hydrogen are used as direct fuel.
Therefore, it is important that the fuel and air streams are kept sepa-
rate, and a thermal balance should be maintained to ensure that oper-
ating temperatures remain within an acceptable range. Several designs
of the SOFC (tubular and planer) have been developed to accommodate
these requirements. The SOFC is a solid-state device and shares certain
properties and fabrication techniques with semiconductor devices.
Individual cells in the stack are connected by interconnects, which
carry an electrical current between cells and can also act as a separa-
tor between the fuel and oxidant supplies. In high-temperature SOFCs,
the interconnects that are used are ceramic such as lanthanum chromite,
or if the temperature is limited to less than 1000 C, a refractory alloy
based on Y/Cr may be used. The interconnects constitute a major pro-
portion of the stack cost. Stack and other plant construction materials that
are used also need to be refractory to withstand the high-temperature
gas streams. Volatility of chromium-containing ceramics and alloys can
result in contamination of the stack components, and the presence of a
6
toxic material such as Cr requires special disposal procedures.
The high operating temperature (1000 C) of the SOFC requires a sig-
nificant start-up time. The cell performance is very sensitive to operat-
ing temperatures. A 10% drop in temperature results in an ~12% drop
in cell performance due to the increase in internal resistance to the flow
of oxygen ions. The high temperature also demands that the system
include significant thermal shielding to protect personnel and to retain
heat. Also, the materials required for such high-temperature operation,
particularly for interconnect and construction materials, are very expen-
sive. Operating the SOFC at temperatures lower than 700 C would be
very beneficial as low-cost metallic materials, such as ferritic stainless
steels, that can be used as interconnect and construction materials.
This will make both the stack and balance of a plant cheaper and more
robust. Using ferritic materials also significantly reduces the problems
associated with chromium. The other advantages of low/intermediate-
temperature operation are rapid start-up and shutdown and signifi-
cantly reduced corrosion rates.
However, to operate at reduced temperatures, several changes are
required in stack design, cell materials, reformer design and operation,
and operating conditions. With the reduction in operating temperature,
the ionic conductivity of the electrolyte decreases and the parasitic
losses due to the conductivity of the electrodes and interconnects
increase. This results in a rapid deterioration of the performance of the
SOFC. This can be overcome to some extent by reducing the thickness of
the electrolyte to compensate for its reduced ionic conductivity. The thick-
ness reduction that is required to accommodate, say a 200 C reduction
in the operating temperature, leads to impracticably thin membranes.