Burning velocities of hybrid iron-methane-air flames

In the transition towards a more cyclic and sustainable energy society, storage and transport of energy are of paramount importance and proper energy carriers are needed. Micron-sized iron powder is a suitable candidate to act as an energy carrier. The energy can be stored through the reduction of iron oxide powder with green energy from the wind or solar. This powder can be transported efficiently and safely to wherever needed. CO2-free combustion of the iron powder
makes the energy utilization particularly suitable for high-temperature applications. The iron oxide combustion products can be captured after combustion and reused, creating a cyclic energy system. However, a better understanding of iron powder combustion is needed for an efficient and cyclic process. The burning velocity is considered a fundamental property of the flames, but is difficult to quantify since it is often influenced by fluid dynamics. Because the interference of fluid dynamics in iron-laden flames is still not well understood, the fundamental propagation mechanism without fluid dynamics interference is difficult to obtain. Therefore, laminar burning velocities of flat hybrid iron-methane-air flames are measured to map the transition from conventional, wellunderstood methane-air flames to iron-governed flames and investigate the effect iron has on a methane-air flame. For this research, the Heat Flux Method is used. This method uses a perforated plate-stabilized flame to measure burning velocities of quasi-1D flames where the fluid flow is particularly well defined. The method and setup are extensively described in our earlier work [1]. The results are presented in Figure 1, and show the burning velocity as a function of different iron concentrations added to methane-air flames with different equivalence ratios ranging from 0.7 to 1. Analysis of the estimated burning velocities of the hybrid flames shows that iron has a strong quenching effect on methane-air flames at relatively low concentrations, and it is expected that the presence of the iron interferes with the radical pool of the methane-air flame. For relatively high iron concentrations, the iron becomes the governing fuel over methane, while the critical concentration at which iron becomes the dominating fuel depends on the methane-air equivalence ratio. Flames governed by iron converge to a laminar burning velocity of 15 cm/s for iron concentrations over 250 g/m3. Here, the laminar burning velocity becomes independent of both the iron and methane concentration.