Page 293 - Biofuels Refining and Performance

272 Chapter Nine

is a well-developed fuel cell and is a commercially viable technology for
a stationary power plant, compared to other fuel cell types. A number
of MCFC prototype units in the power range of 200 kW to 1 MW and
higher are operating around the world. The cost and useful life issues
are the major challenges to overcome before the MCFC can compete with
the existing (thermal or other) electric power generation systems for
widespread use.

Electrochemistry of MCFC. The electrochemical reactions occurring in
the cell are:
Anode half reaction. At the anode, hydrogen reacts with carbonate ions to
produce water, carbon dioxide, and electrons. The electrons travel through
an external circuit—creating electricity—and return to the cathode.
H CO 3 2 → H O CO 2e
2
2
2
Cathode half reaction. At the cathode, oxygen from the air and carbon
dioxide recycled from the anode react with the electrons to form car-
bonate ions that replenish the electrolyte and transfer the current
through the fuel cell, completing the circuit.

1
2
O 1 CO 1 2e S CO 3 2
2
2
2
The overall cell reaction is
1
H 1 O 1 CO scathoded S H O 1 CO sanoded
2
2
2
2
2
2
If a fuel such as natural gas is used, it has to be reformed either exter-
nally or within the cell (internally) in the presence of a suitable cata-
lyst to form H and CO by the reaction:
2
CH H O → 3H CO
2
2
4
Although, CO is not directly used by the electrochemical oxidation, but
produces additional H by the water gas shift reaction:
2
CO H O → H CO 2
2
2
Typically, the CO generated at the anode is recycled to the cathode,
2
where it is consumed. This requires additional equipment to either trans-
fer CO from the anode exit gas to the cathode inlet gas or produce CO 2
2
by combustion of anode exhaust gas and mix with the cathode inlet gas.
Electrolyte. The MCFC uses a molten carbonate salt mixture as its
electrolyte. At operating temperatures of about 650 C, the salt mixture