Thermodynamic analysis of chemically reacting systems; choice of a reference state for exergy

The environmental state of reference plays a substantial role in the calculation of exergy of chemically reacting systems and in general of advanced combined systems. However, the definition of the reference system is problematic, since the environment is not in equilibrium. If a nonequilibrium system is chosen, parts of the environment possess exergy in respect to one another. On the other hand, an environment that is in equilibrium precludes the presence of energy sources and deposit of raw materials, and thus prevents the occurrence of those processes whose effectiveness one is examining. No reference state exists, which satisfies the two requirements to the same extent. The aim of this paper is firstly an examination of the conditions that the classical definition of exergy mandates: the system must be in a state of internal unrestricted equilibrium and the environment must be in state of internal unrestricted equilibrium, which does not change during time. Then, the possible choices of an exergy reference state for chemically reacting systems on the basis of the recent literature are critically examined. Subsequently, some remarks are made on the most suitable formulation of exergy efficiency for these systems.