Page 332 - Advanced thermodynamics for engineers

14.8 PROBLEMS 321

itself. Lean burn catalysts, based on zeolites, are currently under research and will have a major impact
on engine operation – both spark-ignition and diesel.
Another pollutant from diesel engines is the particulates. These are a significant problem and at-
tempts have been made to control them by traps. Cyclone traps are being investigated, as well as
gauze-based devices. A major problem is that after a short time the trap becomes full and the carbon
has to be removed. Attempts have been made to achieve this by burning the particulates, either by self-
heating or external heating. At present this area is still being investigated.
Engine clean-up methods are discussed in more detail in Stone (2012).

14.6.6 OTHER METHODS OF PRODUCING POWER FROM HYDROCARBON FUELS
All of the devices discussed above, i.e. reciprocating engines, gas turbines, steam turbines, etc.,
produce power from hydrocarbon fuels by using the energy released from the combustion process to
heat up a working fluid to be used in a ‘heat engine’. Hence, all of these devices are limited by the
Second Law efficiency of a heat engine, which is itself constrained by the maximum and minimum
temperatures of the working cycle. It was shown in Chapters 2 and 4 that the maximum work
obtainable from the combustion of a hydrocarbon fuel was equal to the change in Gibbs energy of the
fuel as it was transformed from reactants to products: if there was some way of releasing all of this
energy to produce power, then a Second Law efficiency of 100% would be achievable. A device which
can perform this conversion is known as a fuel cell: this obeys the laws of thermodynamics but,
because it is not a heat engine, it is not constrained by the Second Law efficiency of heat engines. Fuel
cells are discussed in Chapter 21.

14.7 CONCLUDING REMARKS
This is the first time non-equilibrium thermodynamics has been introduced. It has been shown that,
while most thermodynamics processes are extremely rapid, it is not always possible for them to reach
equilibrium in the time available.
The effect of rate kinetics on the combustion reactions is that some of the reactions do not achieve
equilibrium and this can be a major contributor to pollutants. The rate equations for nitric oxide have
been developed.
Finally, the pollutants caused by combustion have been introduced, and their effects have been
described.

14.8 PROBLEMS
P14.1 A reaction in which the pre-exponential term is independent of temperature is found to be a
100 times faster at 200 C than it is at 25 C. Calculate the activation energy of the reaction.

What will be the reaction rate at 1000 C?

[30,840 kJ/kmol; 13,814]
P14.2 A chemical reaction is found to be 15 times faster at 100 C than at 25 C. Measurements

show that the pre-exponential term contains temperature to the power of 0.7. Calculate the
activation energy of the reaction. What will be the reaction rate at 700 C?

[31,433 kJ/kmol; 15,204]