Chemical exergy element

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... 

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. 

In general, we can calculate the standard chemical exergy of a component from the standard chemical exergy of its elements with the equation... 

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°. 

X ch,i, where Abf is the standard chemical exergy of formation of the resoiuce from elements, and (>ch,i is the standard chemical exergy of the /th element relative to its reference species. Tabulated values of free energy can be used for this purpose. 

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. 

It should be noted that Q is not the actual entropy production (the latter would also include terms due to diffusion of the gas components etc.), and Ql cannot be generally put equal to the lost exergy of the exit gas , thus to m(H -TQS ) by (6.1.6). The latter value will depend on the zero levels adopted for enthalpy and entropy in the whole set of production units for instance in the combustion chamber, one will take zero levels rather for chemical elements such as C (carbon), thus the final CO2 will be assigned negative enthalpy value at its standard state. 

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