SkeleCHy: A skeletal mechanism for low hydrocarbon content blends with hydrogen

Abstract

In order to achieve short-term reductions in carbon dioxide emissions, combustion systems may be retrofitted to operate with blends of methane and hydrogen. However, the volume fuel fraction of hydrogen must be at least 70% to achieve significant reductions in carbon dioxide emissions. Comprehensive chemical kinetic mechanisms can be used to simulate these high hydrogen content hydrogen/methane blends but such mechanisms are computationally demanding, especially when nitrogen chemistry is included. However, formation of nitrogen oxides (${\mathrm{NO}}_x$) is one of the major factors to consider for the transition towards hydrogen-fuelled carbon neutrality. This motivates development of a skeletal mechanism that can capture the key attributes of laminar combustion, which are the burning velocity, ignition delay time, distribution of major species and emissions, and the extinction strain rate. To identify the best available base mechanism for the chemical mechanism reduction process, an extensive experimental database of approximately 2000 data points is considered explicitly for volume fuel fractions of hydrogen of 70% or greater and the performances of several commonly employed mechanisms are reviewed. State-of-the-art chemiluminescence kinetics as well as nitrogen chemistry are added to the best available comprehensive mechanism. A skeletal mechanism, SkeleCHy, is derived with quasi-steady state assumptions, the directed relation graph with error propagation method and isomer lumping. The reduction is validated with approximately 2200 data points and the skeletal mechanism is further validated with a subset of approximately 330 experimental data points of high interest. It is demonstrated that the key attributes of laminar combustion for hydrogen/methane–air mixtures with high hydrogen content with unburnt mixture temperatures of $T_u=300$ to $700K$ and operating pressures of $p=1$ to $10\mathrm{bar}$ are captured while the computational cost of the SkeleCHy mechanism is comparable to the commonly used GRI 3.0 mechanism. Novelty and significance statement: The fuel volume fraction of hydrogen in hydrogen/methane fuel blends targeting decarbonisation is at least 70%. Emissions of NO are a concern in this blending regime and chemiluminescence is frequently used for validation of simulations. It is therefore of high interest to find the best available chemical kinetic mechanism for this fuel blending regime. This work is the first to assess chemical kinetic mechanisms explicitly in this fuel blending regime using an extensive database of experimental measurements. A skeletal mechanism is developed to be used specifically for this fuel blending regime, filling a hole in the current range of reduced mechanisms for combustion of hydrogen/methane blends. This is to achieve significantly lower computational cost but maintain accurate estimations of the key attributes of laminar combustion for reactant temperatures of 300 to 700 K, atmospheric pressures to 10 bar and hydrogen fuel volume fractions of at least 70% with methane. The skeletal mechanism is heavily validated using the measurements and the comprehensive mechanism.