Page 288 - Biofuels Refining and Performance

Fuel Cells 267

A higher concentration of KOH decreases the life of O electrodes when
2
operating with air containing CO . However, operation at higher tem-
2
peratures is beneficial because it increases the solubility of CO in the elec-
2
trolyte. The operational life of air electrodes polytetrafluoroethylene
[PTFE] bonded carbon electrodes on porous nickel substrates) at a current
2
density of 65 mA/cm in 9-N KOH at 65 C ranges from 4000 to 5500 h
with CO -free air, and their life decreases to 1600–3400 h when air
2
(350-ppm CO ) is used. For large-scale utility applications, operating
2
times >40,000 h are required, which is a very significant hurdle to com-
mercialization of AFC devices for stationary electric power generation.
Another problem with the AFC is that the electrodes and catalysts
degrade more on no-load or light-load operation than on a loaded condition,
because the high open-circuit voltage causes faster carbon oxidation
processes and catalyst changes. The AFC with immobilized KOH electrolyte
suffers much more from this as the electrolyte has to stay in the cells caus-
ing residual carbonate accumulation, separator deterioration, and gas cross
leakage during storage or unloaded periods if careful maintenance is not
carried out. In circulating an electrolyte-type AFC, the electrolyte is emp-
tied from the cell during nonoperating periods. Shutting off the H elec-
2
trodes from air establishes an inert atmosphere. This shutdown also
eliminates all parasitic currents and increases life expectancy. The
exchangeability of the KOH in a circulating electrolyte-type AFC offers the
possibility to operate on air without complete removal of the CO [20, 21].
2

9.3.4 Phosphoric acid fuel cells (PAFCs)
Phosphoric acid fuel cells (see Fig. 9.8) operate at intermediate tem-
peratures (~200 C) and are very well developed and commercially avail-
able today. Hundreds of PAFC systems are working around the world
in hospitals, hotels, offices, schools, utility power plants, landfills and
wastewater treatment plants, and so forth. Most of the PAFC plants are
in the 50- to 200-kW capacity ranges, but large plants of 1- and 5-MW
capacity have also been built; a demonstration unit has achieved 11 MW
of grid-quality ac power [3]. PAFCs generate electricity at more than 40%
efficiency and if the steam produced is used for cogeneration, efficien-
cies of nearly 85% can be achieved. PAFCs use liquid phosphoric acid
as the electrolyte. One of the main advantages to this type of fuel cell,
besides high efficiency, is that it does not require pure hydrogen as fuel
and can tolerate up to 1.5% CO concentration in fuel, which broadens
the choice of fuels that can be used. However, any sulfur compounds
present in the fuel have to be removed to a concentration of <0.1 ppmV.
Temperatures of about 200 C and acid concentrations of 100% H PO are
3
4
commonly used, while operating pressure in excess of 8 atm has been
used in an 11-MW electric utility demonstration plant [3, 22, 23].