Page 291 - Biofuels Refining and Performance

270 Chapter Nine

Temperature and humidity management. Temperature and humidity
management are essential for proper operation of a PAFC. The PAFC
system has to be heated up to 130 C before the cell can start working.
At lower temperatures, concentrated phosphoric acid does not get dis-
sociated, resulting in a low availability of protons. Also, due to lower vapor
pressure of the concentrated acid, the water generated will not come out
with the reactant stream and the moisture retention dilutes the acid.
This causes an increase in acid volume, which results in acid oozing out
through the electrode. With the start of normal cell operation, its tem-
perature increases and acid concentration gets back to its normal value
that causes acid volume to shrink, resulting in drying of the electrolyte
matrix pores if the acid is not replenished. Controlled stack heating at
start-up is achieved by using an insertable heater system. During oper-
ation, the temperature of the stack is maintained by controlling the air
flow in the oxidant channel. At high loading conditions, insertable coolers
may be used to remove excess heat from the stack. Large-power PAFC
systems use a water-cooling system.
Moisture generated at the cathode dilutes the acid on the cathode side
of the electrolyte matrix, causing higher vapor pressure. This results in
more moisture out with the oxidant stream. With the movement of pro-
tons from anode to cathode, moisture migration takes place at the cath-
ode side also. This water evaporation results in an acid concentration
gradient from anode to cathode, causing low availability of protons and
a lower potential of the cell. Therefore, water management is needed to
maintain humidity of the anode stream gas at a sufficient level so that
the vapor pressure matches the acid concentration level at the operat-
ing temperature.

Performance. For good performance, the normal operating tempera-
ture range of a PAFC is 180 C < T < 250 C; below 200 C, the decrease in
cell potential is significant. Although an increased temperature increases
performance, higher temperatures also result in increased catalyst sin-
tering, component corrosion, electrolyte degradation, and evaporation.
2
PAFCs operate in the current density range of 100–400 mA/cm at
600–800 mV/cell. Voltage and power limitations result from increased
corrosion of platinum and carbon components at cell potentials above
approximately 800 mV. Since the freezing point of phosphoric acid is
42 C, the PAFC must be kept above this temperature once commis-
sioned to avoid the thermal stresses due to freezing and thawing. Various
factors affect the PAFC life. Acid concentration management by proper
humidity control is very important to prevent acid loss and performance
degradation. A PAFC has a life of 10,000–50,000 h, commercially avail-
able (UTC Fuel Cells) PAFC systems operating at 207 C have shown a