Stabilization of premixed laminar flames of H2/CH4/air mixtures on a perforated plate
Résumé
In a European context of energy transition aiming for carbon neutrality by 2050, hydrogen (H2) is an alternative to fossil fuels, particularly as a decarbonized fuel. Most of the burners used in boilers are premixed, fueled by natural gas and have a cylindrical geometry, with a multi-perforated structure for flame development and stabilization. However, the combustion characteristics of H2 are very different from those of conventional hydrocarbons: higher flame temperature and burning velocity, reduction in ignition energy, extension of the flammability range, reduction of the quenching distance, modification of the emission spectrum. This can lead to potentially harmful phenomena for the burner like the appearance of hot spots and an increased risk of flashback, hence the need to develop a burner adapted to H2. At present, however, H2 burners are still at the prototype stage and are not on the market, while there are few experimental studies on hydrogen burners [1][2]. The global aim of this study is to understand the phenomena involved and the impact of modifying the boundary conditions to achieve a stable operating mode. The experimental setup, specifically developed for this study, consists of a vein for laminarization of the flow, a mixer in which the premix is formed, and a flat injection plate with multiple injection holes, acting as a burner head where the flames stabilize. The layout and size of the injection holes on the plate were chosen on the basis of literature and industrial patents. Few plates, with different shape, will be tested in order to assess the influence of the size and layout of the injection holes as the injection plate is the main parameter of the study. Stabilization tests are carried out with H2-CH4/air mixtures: at a given equivalence ratio, starting with pure methane, the %vol.H2 is gradually increased to 100% H2 where possible, or to the point where stabilization is not achieved. A stabilization point, at a given equivalence ratio and %vol. H2, is reached and validated when the flame stabilizes on the plate for at least 20 minutes without flashback. A stabilization diagram has been drawn up, showing the stabilization points obtained as a function of equivalence ratio and %H2. K-type thermocouples are placed in the flame to record the flame temperature and under the plate to measure the fresh gas temperature. In parallel, numerical simulations using the GRI Mech 3.0 reaction mechanism were carried out to estimate burning velocity at a given configuration of equivalence ratio, %vol. H2 and fresh gas temperature. At present, only one stabilization diagram has been drawn up for a fixed air flow rate of 20 NL/min for a single plate. Several stabilization points have been obtained at 100% hydrogen for 0.3 ≤ ϕ ≤ 0.45 but also several flashbacks have occurred under others conditions. Analysis of all these data will improve our understanding of the stabilization of premixed H2-air flames on this type of burner. From a practical point of view, this study will also provide elements to aid the design of multi-perforated burners to extend their range of use by integrating the increased risks of flashback associated with the use of H2.