Second-Law Analysis of Energy Processes Part IV: The Exergy of Hydrocarbon Fuels

Abstract

Exergy is the maximum amount of work that can be extracted from a thermodynamic system in a given environment. This thermodynamic function, which depends both on the state of the system and on the conditions in the environment, is the basis of convenient engineering methods of Second-Law analysis of energy processes. The present paper deals with the exergy of hydrocarbon fuels, for which the reference environment is the atmosphere. In the case of pure hydrocarbons or simple mixtures whose composition is known exactly, it can be calculated precisely. It turns out that the exergy B is linearly related to the lower heating value LHV of these substances, and depends only weakly on local atmospheric conditions. The relationship approaches the simple proportionality B/LHV = 1,065 as the molecular weight of the fuels increases. That proportionality was first suggested by the correlations between B and LHV for various families of pure fuels. Subsequently, both the proportionality and the value of the constant were derived. It is conjectured that the same proportionality holds for heavy complex hydrocarbon fuels of unknown composition.