Flameless combustion investigation of CH4/H2 in the laboratory-scaled furnace

Abstract

In the present study, mild flame or “flameless” combustion of methane, hydrogen and methane/hydrogen blended fuels in a furnace having internal recirculation of product gas have been numerically investigated with Gri-Mech 2.11 using Eddy Dissipation Concept model (EDC) under ANSYS Fluent 18.2. Detailed chemical kinetics mechanism utilizing EDC model performed with four turbulent models, namely Reynolds Stress model, Standard, Realizable and RNG k−ε models, to obtain better estimations of the temperature and emissions during the flameless combustion of simulated fuels. The reason for choosing RSM, Realizable, and RNG models is that they show significant progress over the standard k−ε model where the flow characteristics involve rotation, recirculation, and swirl. When the results obtained from the numerical anaylsis are compared with the experimental data published by Ayoub et al. at the entire operating range, it determines that the RNG is more capable than RSM, Standard, and Realizable k−ε models considering CO emissions and flue gas temperatures. As for the NOx emissions, the RSM model predictions are more consistent with experimental results, among others. The numerical results have revealed that not only the mean flame temperature but also flame length, flame diameter, HCN, and OH intermediate species also affect the NOx formation due to thermal NO, and prompt NO, especially at lower flame temperatures and reaction rates under mild flame mode. Furthermore, hydrogen, having the highest flame temperature compared to methane and hydrogen blending fuels, emits the lowest NOx since it does not contain C species and HCN intermediates. However, the addition of 20% hydrogen to methane contributes not only thermal NOx but also prompt NOx due to flame temperature, relatively high amounts of OH and HCN intermediate species. © 2020 Hydrogen Energy Publications LLC