Page 532 - Advanced thermodynamics for engineers

21.8 PROBLEMS 525

For methane, the change in enthalpy for the above reaction is zero when y is equal to 1.115.
The primary benefits of the AR are:
1. its high thermal efficiency (as a result of combining reactions),
2. its compactness (as a result of avoiding a separate combustion stream to provide heat as with a
steam reformer), and
3. its rapid start up and quick load following (as a result of combining reactions).
The primary disadvantage is its lower hydrogen yield, only 42–48% hydrogen content in the output
gas stream (dry basis), as compared with SR which produces 75–80% hydrogen (dry basis). The low
hydrogen yield results from the intake of nitrogen, which dilutes the output gas.

21.7 CONCLUDING REMARKS
This chapter has described some of the various types of fuel cell under development. Some of the
practical losses and problems that occur in fuel cells have been discussed. The thermodynamic
equations for the processes that occur in fuel cells have been developed, and a new property, elec-
trochemical potential, has been introduced. While some analysis of fuel cell parameters can be made
using equilibrium thermodynamics, the theory has been developed to consider the irreversible ther-
modynamics that occurs when the fuel cell is delivering a power output.

21.8 PROBLEMS

Assume the value of the Faraday constant, F ¼ 96,487 kC/kmol

P21.1 An electric cell has the following chemical reaction

ZnðsÞþ 2AgClðsÞ¼ ZnCl 2 þ 2AgðsÞ
and produces an emf of 1.005 Vat 25 C and 1.015 V at 0 C, at a pressure of 1 bar. Estimate

the following parameters at 25 C and 1 bar:

1. change in enthalpy during the reaction,
2. amount of heat absorbed by the cell, per unit amount of zinc, to maintain the
temperature constant during reversible operation.
[ 216,937 kJ/kmol Zn; 23,002 kJ/kmol Zn]
P21.2 An electric cell is based on the reaction Pb þ Hg 2 Cl 2 /PbCl 2 þ 2Hg. If the enthalpy of
reaction for this reaction, Q p ,at 25 Cis 95,200 kJ/kg Pb, calculate the emf, and rate of

change of emf with temperature at constant pressure if the heat transfer to the cell is
8300 kJ/kg Pb.
[0.5363 V; 1.44 10 4 V/K]
P21.3 Calculate the emf of a hydrogen–oxygen fuel cell operating reversibly if the overall
reduction in Gibbs energy is 238 MJ/kg H 2 . If the cell operates at 75% of the reversible emf