Prediction of flammability limits of gas mixtures containing inert gases under variable temperature and pressure conditions

Gas turbines burn a large variety of gaseous fuels under elevated pressure and temperature conditions. During transient operations like maintenance, start-ups or fuel transfers, variable gas/air mixtures flow through the gas piping system and can cause damages in case of ignition. In order to properly control this risk of explosion and ensure safe operation, it is of the essence to have a good knowledge of the flammability limits of the gas mixtures involved, and to be in position to define safe inerting conditions every time it is required. While well-established methods are available in the engineering science to calculate flammability limits of fuel/air mixtures, no systematic methodology exists - to the authors' knowledge - for the prediction of the Lower and Upper Flammability Limits (UFL-LFL) of gaseous blends containing variable amounts of inert components and over a large temperature and pressure range. The purpose of this study was then the evaluation of the LFL and UFL of multi component fuels in air, in function of pressure, temperature and the concentrations of the most frequently used inerting gases, namely: nitrogen, carbon dioxide and steam. Different prediction criteria proposed in the literature were tested and eventually an original methodology based on the adiabatic flame temperature (Tad) was adopted as criterion and extended to gas mixtures and to high temperatures and pressures. Flame temperatures of different blends were calculated for different initial conditions assuming the access to the chemical equilibrium Minimum temperature criteria corresponding to the Tad values reached at the LFL and UFL equivalence ratios were deduced from experimental data for each individual combustible molecule. It has been then possible to evaluate the minimum Tad values which ensure flame propagation fuel blends on the lean and rich sides and to deduce the flammability limits. These calculations were repeated while adding various contents of the three selected inert gases. The methodology was validated by comparison against experimental data when available. The method proves to be simple, accurate, easy to use and applicable to large ranges of pressure, temperature and fuel compositions and to various diluents. The results confirm and quantify some well-known trends of flammability limits, e.g. their widening with increasing initial temperature and pressure, with a stronger effect on UFL than on LFL. The impact of the nature of the inerting gas was also successfully simulated for variable initial conditions and fuel compositions.