Chemical route exergy

An efficient plant must be based on making an approach, as near as is practical and economic, to the perfect processes of an equilibrium diagram, such as Figure A.4, which was initially composed as a calculation route for methane chemical exergy, and then realised to have larger implications. Methane had to be consumed in an isothermal equilibrium reversible process. 

The equilibrium routes which essentially underlie any fuel chemical exergy calculation involve careful avoidance of all irreversibility, so that the routes can be rather unreal (e.g. Figures A.l, A.2 and A.4). These figures however, bring out the major point that equilibrium concentrations must be used in equilibrium diagrams. Any failure to accept that point results in omission of consideration of circulators, or erroneous sizing thereof. See, for example, Kotas (1995), Figure 2.14, p. 46. 

In other texts, the fuel chemical exergy is thought of as a value independent of temperature and pressure, like combustion enthalpy. Instead it has, above, a maximum at FgTg. The major difference in calculation routes is that the author uses equilibrium conditions dictated by the equilibrium constant within the isothermal enclosure of the fuel cell, or Faradaic reformer, whereas other writers put reactants in, and take products out, at standard conditions. 

A.3.10 Route 2 Fuel Chemical Exergy by Direct Methane Oxidation... 

When this appendix was in preparation, methane electrochemical oxidation had not been achieved. A reformer was essential That fact influenced the author s choice of initial calculation route for the methane chemical exergy, to be via oxidation via an equilibrium reformer. Meanwhile direct oxidation has been achieved in the laboratory, as mentioned in Section A.3.2 (route 1). 

Figures A.4 and A.5 were drawn initially in the pursuit of a calculation route for the chemical exergy of methane. For that purpose the two equilibrium diagrams each provide a reversible route to power production from methane. Each process gave a confidence-raising, similar answer for the chemical exergy of methane.

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