Page 275 - Biofuels Refining and Performance

254 Chapter Nine

The anode reaction in hydrogen fuel cells is direct oxidation of hydro-
gen. For fuel cells using hydrocarbon fuels, the anodic half reaction con-
sists of indirect oxidation through a reforming step.
In most fuel cells, the cathode reaction is oxygen (air) reduction. The
overall reaction for hydrogen fuel cells is

1
H 1 O S H O with G 52237.2 kJ/mol
2
2
2
2
where, G is the change in Gibbs free energy of formation. The reaction
product is water released at the cathode or anode, depending on the type
of fuel cell.
For an ideal fuel cell, the theoretical voltage E under standard con-
0
ditions of 25 C and 1 atm pressure is 1.23 V, whereas typical operating
voltage for high-performance fuel cells is ~0.7 V. Stack voltage depends
on the number of cells in a series in a stack. Cell current depends on the
cross-sectional area (the size) of a cell.
Fuel cell systems are not limited by Carnot cycle efficiency. Therefore,
a fuel cell system with a combined cycle and/or cogeneration has very high
efficiency (55–85%) as compared to the efficiency of about 30–40% of cur-
rent power generation systems. In a distributed generation system, fuel
cells can reduce costly transmission line installation and transmission
losses. There are no moving parts in a fuel cell and very few moving parts
(compressors, fans, etc.) in a fuel cell system. Therefore, it has higher reli-
ability compared to an internal combustion or gas turbine power plant.
Fuel cell-based power plants have no emissions when pure hydrogen
and oxygen are used as fuel. However, if fossil fuels are used for gener-
emissions. Compared
ating hydrogen, fuel cell power plants produce CO 2
to a steam power plant, a fuel cell plant has very low water usage;
water/steam is a reaction product in a fuel cell. This clean water/steam
does not require any pretreatment and can be used for reactant humid-
ification and cogeneration. Another advantage of the fuel cell power
plant is that it does not produce any solid waste and its operation is very
silent as compared to a steam/gas turbine power plant. The noise gen-
erated in a fuel cell power plant is only from the fan/compressor used
for pumping/pressurizing the fuel and the air supply to the cathode.
A fuel cell power plant has good load-following capability (it can
quickly increase or decrease its output in response to load changes). The
modular construction of fuel cell plants provides good planning flexibility
(new units can be added to meet the growth in electric demand when
needed), and its performance is independent of the power plant size
(efficiency does not vary with variation in size from W to MW size).
The major technical challenges in fuel cell commercialization at pres-
ent are (1) high cost, (2) durability, and (3) hydrogen availability and