The laminar burning velocity of hybrid iron-methane-air flames

Iron combustion has gained a lot of traction in the last decade since it was proposed as a CO₂-emission-free energy carrier. To make this novel technology feasible, better understanding of iron combustion process is needed. The laminar burning velocity S_L of iron-particle-laden flames is an important parameter that can reveal a lot about the governing properties of the combustion process of these iron particles. However, the amount of experimental data on the burning velocity of iron flames is limited. Recently, (Hulsbos et al., 2024) proposed the Heat Flux Method (HFM) as a way to measure the burning velocity of hybrid-iron-methane-air flames with a stoichiometric methane-air equivalence ratio as base as a first step in producing experimental data of S_L in iron-laden flames. This work advances on the work from Hulsbos et al. and extends the measurements to hybrid iron-methane-air flames with a lean methane-air flame as base. By comparison with SiC particles and simulations, it is found that at low iron particle concentrations the iron acts as a heat sink and also interferes chemically with the methane-air flame, significantly reducing the burning velocity of the hybrid flame. For high iron particle concentrations within the flame the iron becomes the dominant fuel and an asymptotic burning velocity is reached. This asymptotic burning velocity is shown to be independent of both the iron or methane content in the flame. Novelty and significance statement The novelty of this study is the extension of the recently presented burning velocities to hybrid flames by Hulsbos et al. (2024) to cases with a lean methane-air flame as a basis and a comparison with inert SiC particles. From this extensive range of experimental data, a hypothesis of iron-burning behaviour in a methane flame is produced. Two clear flame regimes are identified: One where the methane-air dominates the burning velocity, and one where the iron powder dominates the burning velocity. This work also shows at what conditions hybrid iron-methane-air flames transitions from one regime to the other, and elaborates on the consequences of this regime transition with respect to the burning velocity of the hybrid iron methane-air flames. The results from this can be used to validate models considering iron combustion and significantly increases the knowledge about the combustion behaviour of micron-sized iron particles.