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magnitude, when compared to the chemical and concentration exergies and hence
can be practically omitted.
Subsequently, the special case of fossil fuels has been investigated. For these
minerals the concentration exergy is not so relevant and thus they are evaluated
solely in terms of their chemical exergy. Due to their complex chemical structure,
formulas used for non-fuel minerals are di cult to apply. Hence, special calculation
procedures need to be sort. Such calculations show that the chemical exergy of fossil
fuels can be, in many cases, approximated to their HHV. Nevertheless, the formulas
developed in Lozano and Valero (1988) are used in this topic because through them,
one can analyse fuel exergies relative to variations in environmental conditions.
Generally speaking, exergy (B) values are very small, if compared to the real
energy one would require to return natural resources to the state in which they were
found originally. This is because exergy per se considers processes to be reversible,
thereby providing only the theoretical minimum required to complete them. To
overcome this issue, such values need to be multiplied by factor k. The latter is
dimensionless and accounts for the ine ciencies of Man's technology. The resulting
exergy replacement cost (B ) thus represents the real exergy, using a given available
technology, that would be required to return a resource to the physical and chemical
conditions as first delivered by Nature. As opposed to exergy, exergy costs cannot
be considered as a resource property, since k introduces a degree of uncertainty into
the calculation. Nevertheless, they can be used as a suitable indicator for assessing
the value of non-fuel mineral resources, as they combine under one parameter, not
only composition, concentration and comminution but also the current state of
technology.
Finally this chapter has explained how exergy replacement costs can be used for
the analysis of mining and metallurgical processes through Thermoeconomics. This
novel discipline has been widely used for the optimisation of thermal systems, in
which mainly energy flows come into play. Nevertheless, when non-fuel minerals are
involved, the general rules used for conventional thermoeconomic analysis need to
be extended. As a result, mining and metallurgical systems can be analysed using
Thermoeconomics but only after taking into account two considerations. The first
is that external resources are accounted in terms of exergy replacement costs instead
of exergy alone, if these are non-fuel minerals. The second is that when bifurcations
of products occur, costs are also allocated in terms of exergy replacement costs (and
not exergy).
The following chapter explains in detail the reference baseline selected for the
exergy assessment of minerals, namely the Crepuscular Earth Model depicting Tha-
natia.
 
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