Page 295 - Biofuels Refining and Performance

274 Chapter Nine

Internal reforming of natural gas and partially cracked hydrocarbons
is possible in the inlet chamber of the MCFC, eliminating the separate
fuel processing of natural gas or other hydrogen-rich fuels. The require-
ment for CO makes the digester gas (sewage, animal waste, food pro-
2
cessing waste, etc.) an ideal fuel for the MCFC; other fuels such as
natural gas, landfill gas, propane, coal gas, and liquid fuels (diesel,
methanol, ethanol, LPG, etc.) can also be used in the MCFC system. The
elimination of the external fuel reformer also contributes to lower costs,
and high-temperature waste heat can be utilized to make additional elec-
tricity and cogeneration. MCFCs can reach overall thermal efficiencies
as high as 85%.
With the increase in operating temperature, the theoretical operat-
ing voltage for a fuel cell decreases, but increases the rate of the elec-
trochemical reaction and therefore the current that can be obtained at
a given voltage. This results in the MCFC having a higher operating volt-
age for the same current density and higher fuel efficiency than a PAFC
of the same electrode area. As size and cost scale roughly with the elec-
trode area, the MCFC is smaller and less expensive than a PAFC of com-
parable output. Another advantage of the MCFC is that the electrodes
can be made with cheaper nickel catalysts rather than the more expen-
sive platinum used in other low-temperature fuel cells. Endurance of the
cell stack is a critical issue in commercialization, and MCFC manufac-
turers report an average potential degradation of 2 mV/1000 h over a
cell stack lifetime of 40,000 h. The high temperature limits the use of
materials in the MCFC, and safety issues prevent their application for
home use. MCFC units require a few minutes of fuel burning at the start
up to heat up the cell to its operating temperature and therefore are not
very suitable for use in automobiles. However, they are very good for sta-
tionary power applications and units with up to 2 MW have been con-
structed, and designs for units with up to 100 MW exist [3, 23–25].

9.3.6 Solid oxide fuel cells (SOFCs)
The SOFC has the most desirable properties for generating electricity
from hydrocarbon fuels. The SOFC uses a solid electrolyte and is very effi-
cient. It can internally reform hydrocarbon fuels and is tolerant to impu-
rities. The SOFC operates at a very high temperature (700–1000 C) and
so does not require any cooling system for maintaining a fuel cell oper-
ating temperature. For small systems, insulation has to be provided to
maintain the cell temperature. In large SOFC systems, the operating
temperature is maintained internally by the reforming action of the
fuel and by the cool outside air (oxidant) that is drawn into the fuel cell.
At high operating temperatures, chemical reaction rates in the SOFC
are high and air compression is not required. This results in a simpler