Geology Reference
In-Depth Information
9.5.3 The chemical exergy of fossil fuels
Fossil fuels are a specific type of minerals and therefore their chemical and concen-
tration exergies can be calculated using Eqs. 9.29 and 9.30. Liquid and gaseous
fuels have the particularity that their quality (grade) remains near constant with
extraction, which is not so for solid minerals (a mineral's concentration decreases
as the deposit is being extracted). For such cases, concentration exergy is not as
relevant as it is for other mineral resources. It is thus not taken into account for
the calculations presented in this topic. Furthermore, the value of fuels is closely
related to their chemical exergy content.
The chemical exergy of fuels is very di
cult to calculate with Eq. (9.29) due
to their complexity. That said, it has been largely demonstrated that it can be in
many cases, satisfactorily approximated to their HHV. More complex calculation
procedures (such as those of Rant (1961), Szargut and Styrylska (1964), Shieh
and Fan (1982), Lozano and Valero (1988), Stepanov (1995)), do not always lead
to better results. Both, the experimental error associated with the determination
of the HHV and that linked to the correlations of the more complete models are
comprised reasonably in an interval close to 2%.
Due to the level of detail required for the calculations in this topic, the authors
use the model developed by Lozano and Valero (1988). In this model, the reference
environment is considered to be an additional variable allowing for an assessment of
a given fossil fuel's exergy loss in function of changes to environmental conditions,
as can be expected in the event of climatic change brought about by their burning.
The detailed methodology is explained in Sec. D.3 on 563.
9.6 Exergy costs
As stated previously, fuel minerals are appreciated for their inherent chemical
exergy. Non-fuel minerals are in contrast not valued for their chemical exergy but
rather for their costs associated with extraction. The exergy of a very abundant and
concentrated non-fuel mineral in the crust, such as iron, will be high whereby its
extraction cost will be low. On the contrary, a very dispersed and scarce one such
as gold, will have a low exergy value but a very high extraction cost. Obviously,
such costs are a very important component of the final market price which is why
the scarcest minerals tend to be the most expensive.
The exergy replacement cost (B
), first defined by the authors in Valero (1998)
is the exergy required by a given available technology to return a resource from the
dispersed state of Thanatia, to the physical and chemical conditions in which it was
first delivered by an ecosystem(s). It describes something similar to the natural
bonus of minerals and can be interpreted as a hidden avoided cost that Man does
not need to spend.
So, if exergy is the minimum thermodynamic cost needed to perform a given ac-
tion, the exergy cost is a methodical measurement of the actual exergy expenditure
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