A Comparative Study of Different Reaction Models for Turbulent Methane/Hydrogen/Air Combustion

Reaction modelling of methane/hydrogen combustion has two important aspects. First, such mixtures may be used in future in combustion devices like gas turbines and gas engines in the frame of the demand for efficient energy storage systems, where the amount of hydrogen in natural gas delivering systems may vary according to varying hydrogen production from renewable energies. Second, this can be an important aspect for safety, as such mixtures may occur in disastrous situations and calculations may allow the prediction of safety issues. Modelling of such mixed fuel combustion processes is non-trivial due to the involved preferential diffusion effects, coming from the different diffusivities of methane and hydrogen. In turbulent flame modelling, this topic is of special interest, as also thermo-diffusive instabilities and local influence of the local burning velocity near leading edges of the flame seem to be of importance even for highly turbulent flames. This numerical work deals therefore with a comparative study of five different turbulent combustion 1. INTRODUCTION Reaction modelling of turbulent methane/hydrogen combustion has two important aspects. First, such flex-fuel mixtures may be used broadly in future in combustion devices like gas turbines and gas engines. This holds especially with respect to the search for new large scale energy storage systems with respect to strongly varying energy production from renewable energies like wind and solar energy. Here, it is proposed that on peak sun or wind situations electrolytically produced hydrogen may be stored within the existing large scale natural gas delivering and storage system. This chemical energy storage option would allow the allocation of the huge energy capacity needed for the broad use of renewable energies. However, this option would require that the common combustion devices where natural gas is used, like gas turbines or gas engines, are able to operate under varying fuel conditions. For the calculation of such devices suitable reaction models are needed. Second, also safety aspects of such fuel mixtures are of significant importance. This holds for the energy storage scenario being described before, if such fuel mixtures are released uncontrolled. Even without that, hydrogen safety is a general issue for the chemical industry, for nuclear power plant failures or if the vision of hydrogen delivering systems is followed up. Though hydrogen is a potential energy carrier offering CO2 free emission during the combustion, this cannot be directly used for combustion due to its high diffusivity, reactivity and burning velocity. Instead, blending hydrogen into hydrocarbon could solve such difficulties. With that, safety issues may be important not only for pure hydrogen but also for hydrogen/methane fuel mixtures. Also here, relevant calculation methods are needed. An additional aspect is flame stability. The addition of hydrogen to natural gas or methane flames can increase the flame stability in very lean combustion modes. These are of interest for instance in stationary gas turbines due to the ultralow emission characteristics of NOx and soot. In the current study, therefore the extension of premixed turbulent reaction rate models for hydrogen/methane fuels is investigated in the frame of Reynolds averaged Navier-Stokes (RANS) simulation techniques. It is well known that hydrogen has a higher reactivity compared to other hydrocarbon fuels. Also the high diffusivity of hydrogen allows this fuel to diffuse faster into the reaction zone. Both effects together are included in increased laminar burning velocities for hydrogen/air flames as well as for hydrogen/hydrocarbon/air flames [1]. low swirl flames [5]. These flames operate for a broad range of conditions, as the stability limit is wide. Two approaches to models are followed. In the first group the mean turbulent reaction rate is modelled as a function of the laminar flame speed (which depends on the hydrogen content) and of turbulence parameters. It will be shown, that none of these models is sufficient to calculate the test cases with enhanced hydrogen content. In the second part of this work, a modified approach is followed therefore, taking into account additional effects from molecular diffusion. Here a rather simple modification of single fuel models with an effective Lewis number approach allows to calculate essential features of the whole set of experimental data