The CDQ system installed in China Steel Corporation
was principally composed of a cooling chamber,
waste heat boiler, steam turbine-generator set, air supply
system and balance of plant system. In such a process,
the heat transfer phenomena between coke, cooling gas,
energy transfer devices and the environment were involved.
In addition, the combustion reaction of the
coke volatile matter and the circulating gas also needed
to be considered. Therefore, three types of exergy
flow, comprising the work transfer, the heat transfer
and the energy transfer associated with mass transfer,
had to be considered in analyzing this system.
For exergy analysis, the work of a given form of
energy is equivalent to exergy.(2) The exergy of a heat
transfer at the control surface is determined from the
maximum work that could be obtained from it using the
environment as a reservoir of zero-grade thermal energy.
For the energy transfer associated with mass transfer,
three categories of the materials can be found in this
system, they are: (1) the stream of coke; (2) the streams
of various gases including the air stream, volatile matter
stream and the circulating gas stream; and (3) the water
and steam streams of the waste heat boiler. Furthermore,
the total exergy of the above-mentioned streams can be
separated into physical exergy and chemical exergy.
Physical exergy represents the maximum work obtainable
when the substance under consideration is brought
from its initial state to the environmental state, which
involves only thermal interaction with the environment.
Chemical exergy represents the maximum work obtainable
when the substance under consideration is
brought from the environmental state to the dead state
by processes involving heat transfer and the exchange
of substances with the environment. Table 1 summarizes
the characteristics of the exergy flows for the CDQ
system.