Stabilization of methane–hydrogen flames inside a divergent porous media reactor

Abstract

The energy transition process triggered by the threat of climate change has created the need for cleaner heat generating systems. Using blends of hydrocarbons with increasing presence of green fuels, such as green hydrogen, has been proposed as an initial step of this process. The present study aims to investigate methane–hydrogen flames stabilized inside a divergent inert porous media burner. Experimentally, four stable operating points were found for the considered burner with a hydrogen presence between 0% and 30% in the fuel mixture at a constant thermal output of 2 kW. Temperatures are presented for each condition, noticing a decreasing trend as hydrogen presence was increased, since the equivalence ratio was gradually reduced to achieve stabilization. CO and NO emissions were below 15 ppm for all studied cases. A 2-dimensional numerical model incorporating the GRI-Mech 3.0 mechanism was developed and validated against experimental data.The model revealed that the stabilized flame front is observed as a straight line that runs parallel to the radial coordinate. This line exhibits a slight curvature close to the burner wall, which is caused by heat losses and the influence of the divergent geometry. Solid surface temperature was not constant due to the reduced convective heat exchange with flue gases in the periphery of the burner. To reduce this issue a lower expansion angle is proposed as an alternative. In addition, changes in the chemical kinetics due to H addition were also evaluated. As results, a general trend towards reaction shifting was found with a 56% of the reactions being strongly promoted by H presence, which is associated with an increase in radical concentration in the flame front due to H decomposition.