Page 297 - Biofuels Refining and Performance

276 Chapter Nine

and carbon monoxide inside the cell, and the high-temperature waste
thermal energy can be recycled back for fuel reforming. During oper-
ation, the SOFC is at the same time a generator and a user of heat.
Heat is generated through exothermic chemical reactions and ohmic
losses, while it is absorbed by the reforming reaction. It is possible to
design the SOFC to be thermally balanced, thereby eliminating the
requirement for external insulation and heating. Small SOFC systems
are not thermally self-sustaining and may require an external heat
source to start and maintain operation. In large systems, the heat gen-
erated is not fully absorbed by fuel reforming, and the excess heat can
be used in gas turbines for generating electricity or for cogeneration.
Another advantage of the SOFC is that expensive catalysts are not
required. However, a few minutes of fuel burning is required to reach
the operating temperature of the SOFC at the start. This time delay
is a disadvantage for an automotive application, but for stationary
electric power plants, this is not a problem as they run continuously for
long periods of time.

Electrochemistry of SOFCs. Hydrogen or carbon monoxide in the fuel
2
stream reacts with oxide ions (O ) from the electrolyte to produce water
or CO and to deposit electrons into the anode. The electrons pass out-
2
side the fuel cell, through the load, and back to the cathode, where
oxygen from the air receives the electrons and is converted into oxide
ions, which are injected into the electrolyte. In the SOFC, oxygen ions
are formed at the cathode. The reaction at the cathode is

O 4e → 2O 2
2
At the operating temperature, the electrolyte offers high ionic con-
ductivity and low electrical conductivity; therefore, oxygen ions migrate
through the electrolyte to the anode. The overall reaction occurring at
the anode is as follows:
The hydrogen in the fuel reacts with the oxygen ions to produce water
and releases two electrons.

2
H O → H O 2e
2
2
Carbon monoxide present in the fuel causes a shift reaction to produce
additional fuel (H ).
2
CO H O ↔ H CO 2
2
2
The following internal reforming reaction for the hydrocarbon fuel takes
place on the anode side:
y
C H xH O → xCO (x )H 2
x
2
y
2