Chemical exergy values

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... 

From these data, we can calculate the chemical, exergy values of these components in the pure state at P0 and T0. Air at these conditions can, to a good approximation, be considered as an ideal gas therefore, separation... 

The standard chemical exergy values for the main constituents of air as listed in Table 7.1 are given in Table 7.2. 

Exergy values for the elements in their stable modification at T0 = 298.15 K and P0 = 101.325 kPa are called standard chemical exergy values Ex. For the calculation of the chemical exergy value of all kinds of substances, the standard chemical exergy values of all elements are required. 

The following example for graphite illustrates how the chemical exergy value for all other elements can now be calculated (Table 7.3). For the calculation of... 

Standard Chemical Exergy Values at P0/ T0 of Various Components Present in Air... 

For the remaining elements, reference compounds have been chosen, as they occur in seawater or in the lithosphere, the earth s crust. An important aspect of this choice has been that the calculated exergy values of most compounds should be positive. Table 7.3 lists the standard chemical exergy values of the elements as presented in Szargut s well-known standard work [1]. Chapter 8 gives an example, the adiabatic combustion of H2, to illustrate the use of these exergy values in an interesting application. 

Table 7.3 is useful for the calculation of the standard chemical exergy values of compounds. We illustrate this for methane and start from its hypothetical formation reaction at standard conditions ... 

Standard Chemical Exergy Values of Selected Compounds... 

Exn2 and Exo2 are the chemical exergy values of the pure components of air. The exergy value of air at standard conditions is given by... 

In chemical thermodynamics the standard chemical potential ut of a compound i is defined as the molar free enthalpy Ag° for the formation of the compound from its constituent elements j in their stable molecular form in the standard state, and their chemical potential values are set zero in the standard state fit-Ag°f. In exergy engineering the standard molar exergy e° of a compound i is defined as consisting of the molar free enthalpy Ag°f for the formation of the compound in the standard state from its constituent elements and the stoichiometrical sum of the standard chemical exergy values e° of the constituent elements j in their stable state at the standard temperature T° and pressure p° ef- Ag°f + 2 vy e°. 

Standard chemical exergy values, in units of kJ/kmol, are based on reference conditions T0 and P(h such as 298.15 K (536.67 R) and 1 atm, respectively, and consist of a set of reference substances with standard concentrations of gaseous, liquid, and solid components. The standard chemical exeigy tables often simplify the application of exeigy principles. 

Table 6.2 lists exergy values for methane. It is clear from this table that methane carries an impressive amount of exergy as chemical exergy. Further, the table shows (1) the influence of increased pressure and temperature on the physical exergy and (2) that this latter contribution of exergy is nearly two orders smaller than the chemical contribution. Chemical exergy is the exclusive subject of Chapter 7. 

In our environment, there are many substances that, like oxygen in our atmosphere, cannot further diffuse and/or react toward more stable configurations and may be considered to be in equilibrium with the environment. Neither chemical nor nuclear reactions can transform these components into even more stable compounds. From these components, we cannot extract any useful work, and therefore an exergy value of OkJ/mol has been assigned to them. This has been done for the usual constituents of air N2,02/ C02/ H20, DzO, Ar, He, Ne, Kr, and Xe at T0 = 298.15 K and P0 = 99.31 kPa, the average atmospheric pressure [1]. Their partial pressures P in air are given in Table 7.1. 

Suppose we deal with a process in which iron, Fe, has to be used as a reactant, for example, in a reduction reaction. The standard chemical exergy of Fe is 376.4 kj/mol. If we wish to carry out a thermodynamic or exergy analysis of this process, this value is not appropriate. After all, to put the exergy cost of the product, for which Fe was needed as a reactant, in proper perspective, we need to consider all the exergetic costs incurred in order to produce this product all the way from the original natural resources— iron ore and fossil fuel in this example. The production of iron from, for example, the iron ore hematite and coal has a thermodynamic efficiency of about 30% [1], and therefore it is not 376.4 kj/mol Fe that we need to consider... 

The computation of the chemical exergy of the various streams is achieved by using documented values of the chemical exergy [2] of the chemical species... 

From the foregoing discussion, it follows that the standard exergy of one of the reactants can be estimated by use of the standard affinity of the reaction, provided that we know the values of the standard exergy of the other reactants and products. The numerical values of the molar exergy thus obtained of various chemical substances in the standard state (temperature T° = 298 K, pressure p° = 101.3 kPa, activity a° = 1) are tabulated as the standard chemical exergy of chemical substances in the literature on engineering thermodynamics [Ref. 9.]. 

The values of availability (exergy) and energy (enthalpy relative to the dead state) of all materials that are in complete, stable equilibrium with the dead state are zero. The datum level materials and their concentrations that age used in this work to compute the specific chemical enthalpy, 8, and the specific chemical exergy, e, are listed in Table I. 

The correct fuel cell characterisation (Barclay, 2002), it is reiterated, is via the fuel chemical exergy in watt seconds, and numerically a much larger quantity than the calorific value in joules. Fuel... 

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