Environmental Engineering Reference
In-Depth Information
of gas turbine power to steam turbine power, and the fraction of the fuel heat that is removed in
the condenser of the steam plant.
Problem 3.6
In synthesizing hydrogen for use in a fuel cell, methane may be used in the overall reaction
CH
4
+
2H
2
O
→
CO
2
+
4H
2
which is endothermic. Using the data of Table 3.1, calculate the amount of heat that must be
supplied for this synthesis, per unit mass of methane consumed, if it proceeds at 25
◦
C and one
atmosphere of pressure. If this heat is supplied by burning additional methane in air, calculate the
total kilograms of methane consumed per kilogram of hydrogen produced.
Problem 3.7
The potential difference
of a hydrogen fuel cell, as a function of the cell current density
i
amp/cm
2
, is found to be
=
(
1
.
1 volt
)(
1
−
0
.
5
i
)
Calculate the maximum power output of the cell in W/m
2
, and calculate the values of
i
,
, and
the cell efficiency at maximum power.
Problem 3.8
A PEM fuel cell using methanol as fuel is supplied with a methanol-water mixture at the anode.
The anode reaction produces hydrogen ions in the electrolyte and anode electrons according to the
reaction
6H
+
{
el
}+
6e
−
{
a
}+
CH
4
O
+
H
2
O
→
CO
2
At the cathode, the cathodic electrons and electrolytic hydrogen ions combine with oxygen to form
water molecules,
6H
+
{
el
}+
6e
−
{
c
}+
3O
2
→
3H
2
O
The combination of these reactions is the oxidation of methanol while producing a current flow in
the external circuit across the electrode potential difference
a
−
c
,
6e
−
{
a
}−
6e
−
{
c
}
CH
4
O
+
3O
2
→
2H
2
O
+
CO
2
+
(a) Using the data of Table 3.1, calculate the maximum potential difference
a
−
c
that the fuel
cell can generate. (b) Calculate the ratio of the maximum electrical work per unit of fuel mass
(J/kg) to the fuel heating value.
