Evaluation of the effects of fuel and combustion-related processes on exergetic efficiency

The fuel used in combustion applications has significant influence on irreversibility generation and hence the exergetic efficiency of the system. This work discusses a method of estimating the availability destructions and exergetic efficiencies of combustion for different classes of fuels viz. hydrogen, hydrocarbons, alcohols and biodiesel surrogates. A ranking of these fuels is presented based on their exergetic efficiencies during isobaric and isochoric combustion. It is observed that availability destruction is greater for heavier hydrocarbon fuels and oxygenated fuels with higher oxygen fraction. Though unsaturated hydrocarbon fuels are associated with lower availability destruction, they result in poor exergetic efficiency as a significant fraction of the fuel availability is lost in the products. Hydrogen and acetylene are identified as the fuels with maximum and minimum exergetic efficiencies respectively. Optimum exergetic efficiency is obtained for reactant mixtures on the leaner side of fuel–air stoichiometry. Availability destruction increases with exhaust gas recirculation (EGR) and decreases with oxygen enrichment of the supplied air. However, oxygen enrichment entails significant chemical availability losses and lowers exergetic efficiency. Preheating the reactants is found to be effective in mitigating availability destruction.