Effects of Hydrogen Enrichment on Confined Methane Flame Behavior

Many land based power generation units presently operate on natural gas, whose major constituent is methane, and many of them would need to tackle the challenges due to a fuel switch towards synthesis gas in the near future. Operating conditions and stability of a pre-mixed gas turbine combustor is quite sensitive to the changes in the fuel composition. Behavior of a premixed confined hydrogen enriched methane flame is studied with regard to thermo-acoustic instability induced flame flashback, emissions, flammability limits and acoustics over a wide range of operating conditions. However, most emphasis is put on lean combustion, which is an industry standard method used to lower pollutant emissions by reducing adiabatic flame temperatures. Hydrogen addition extends the flammability limits and enables lower nitric oxide emissions levels to be achieved at leaner equivalence ratios. On the other hand, increased root-mean-square pressure fluctuation levels, and higher susceptibility to flashback is observed with increasing hydrogen volume fraction inside the fuel mixture. This phenomenon is mostly attributed to much higher burning speeds of hydrogen in contrast to pure methane. A semi-analytical model has been utilized to capture the flame holding and thermo-acoustically induced flame flashback dynamics for a pre-mixed gas turbine combustor. A simple linearized acoustic model, derived from the basic conservation laws, and a front-tracking algorithm based on the Markstein’s G-equation are coupled together in order to track the flame initiation front, which in turn yields in an understanding of dynamic flame holding characteristics. A limit cycle behavior in the flame front movement is observed during simulations due to a non-linearity in the feedback term that relates acoustic velocity to heat release. Sets of experiments including flashback speed measurements have been performed at varying fuel composition. Phase locked CH radical imaging measurements have also been performed in order to track the flame initiation front in time with respect to the dominant instability cycle. Computer simulations are performed to study flashback and combustor acoustics together numerically and it is observed that these are in good qualitative agreement with the experiments.