Thermodynamic Analysis of a Regenerative Brayton Cycle Using H-2, CH4 and H-2/CH4 Blends as Fuel

Abstract

Considering a simple regenerative Brayton cycle, the impact of using different fuel blends containing a variable volumetric percentage of hydrogen in methane was analysed. Due to the potential of hydrogen combustion in gas turbines to reduce the overall CO<mml:semantics>2</mml:semantics> emissions and the dependency on natural gas, further research is needed to understand the impact on the overall thermodynamic cycle. For that purpose, a qualitative thermodynamic analysis was carried out to assess the exergetic and energetic efficiencies of the cycle as well as the irreversibilities associated to a subsystem. A single step reaction was considered in the hypothesis of complete combustion of a generic H<mml:semantics>2</mml:semantics>/CH<mml:semantics>4</mml:semantics> mixture, where the volumetric H<mml:semantics>2</mml:semantics> percentage was represented by <mml:semantics>fH2</mml:semantics>, which was varied from 0 to 1, defining the amount of hydrogen in the fuel mixture. Energy and entropy balances were solved through the Engineering Equation Solver (EES) code. Results showed that global exergetic and energetic efficiencies increased by 5\% and 2\%, respectively, varying <mml:semantics>fH2</mml:semantics> from 0 to 1. Higher hydrogen percentages resulted in lower exergy destruction in the chamber despite the higher air-excess levels. It was also observed that higher values of <mml:semantics>fH2</mml:semantics> led to lower fuel mass flow rates in the chamber, showing that hydrogen can still be competitive even though its cost per unit mass is twice that of natural gas.