Chemical exergy reference environment

For the determination of a compound s chemical exergy value we need to define a reference environment. This reference environment is a reflection of our natural environment, the earth, and consists of components of the atmosphere, the oceans, and the earth s crust. If, at P0 and T0, the substances present in the atmosphere, the oceans, and the upper part of the crust of our earth are allowed to react with each other to the most stable state, the Gibbs energy of this whole system will have decreased to a minimum value. We can then define the value of the Gibbs energy for a subsystem, the "reference environment"—at sea level, at rest, and without other force fields present than the gravity field—to be zero as well as for each of the phases present under these conditions. It is a logical extension of these assumptions to... 

A chemical substance has its chemical energy in terms of the chemical potential and has its chemical exergy as well. Let us consider a chemical substance present at unit activity in the normal environment at temperature T0 and pressure p0 and examine its chemical exergy in relation with the exergy reference species in the atmospheric air, in seawater, and in lithospheric solids (Refs. 9 and 11). 

In calculating the numerical values of the standard molar exergy e°of chemical elements and compounds, we usually make clear the exergy reference species at zero level of exergy in our natural environment of the atmosphere, the hydrosphere and the lithosphere. 

The physical exergy Eph is equal to the maximum amount of work obtainable when a compound or mixture is brought from its temperature T and pressure P to environmental conditions, characterized by environmental temperature T and pressure Pq. The standard chemical exergy of a pure chemical compound Ech is equal to the maximum amount of work obtainable when a compound is brought from the environmental state, characterized by the environmental temperature To (298.15 K) and environmental pressure Po (1 atm), to the dead state, characterized by the same environmental conditions of temperature and pressure, but also by the concentration of reference substances in a standard environment. 

In the natural environment, however, there are components of states differing in their composition or thermal parameters from thermodynamic equilibrium state. These components can undergo thermal and chemical processes. Therefore, they are natural resources with positive exergy. Only for commonly appearing components can a zero value of exergy be accepted. A correct definition of the reference level is essential for the calculation of external exergy losses. The most probable chemical interaction between the waste products and the environment occurs with the common components of the environment. 

The most stable end products of reactions involving the resource under consideration are chosen as reference chemical species rs in the environment (air, ocean, or earth cmst). These reference species are not in thermodynamic equilibrium, as the biosphere is not a thermodynamic equilibrium system. Reference species are assigned a specific free energy (exergy) b s due to their molar fraction in the environment by means of the formula = —RT Inz s, where the reference species are assmned to form an ideal mixture or solution. 

Exergy is an extensive property and a thermodynamic potential. In contrast to energy, exergy is not conserved and decreases in irreversible processes. If we discharge the waste product of the process into the environment, external exergy loss occurs due to the deviation of thermal parameters and the chemical composition between the product and the components of the environment. The thermal state and chemical composition of the natural environment represent a reference level (dead state) for the calculation of exergy. 

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