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It is demonstrated that in the (near) zero-gravity experiments conducted by Tang et al. (Combust. Flame; 2009, 2011) iron powder aerosols created using the finest powders are optically thick, implying that radiative heat transfer between particles should not be neglected. To test this concept, an iron particle oxidation model has been implemented in OpenFOAM, including a coupling with the P1-model for radiative heat transfer. For flame simulations in which radiation is not included, obtained flame propagation velocities deviate less than 8% with results obtained using Chem1D-Fe and also show a good correspondance with algebraic models for optically thin aerosols. No significant difference in predicted flame propagation velocity is observed between 1D and 3D simulations: contrary to what is seen in gaseous flames, including the curvature of the flame does not increase predicted flame speeds substantially. However, measured flame propagation velocity values exceed numerically obtained predictions excluding thermal radiation by a factor of three to four. To the authors’ knowledge, this discrepancy is exemplary for the difference between experimentally obtained values for flame propagation velocities, and predictions made using numerical simulation tools neglecting radiative heat transfer. Accounting for radiation increases predicted flame propagation velocities, in the absence of confining boundaries, by approximately a factor of 10 which is in line with algebraic models for optically thick aerosols. In 3D simulations for the two finest iron powders in the experiments, including radiation and accounting for the presence of the confining tube wall results in an error of 11% and 35% with respect to measured flame propagation velocities, significantly smaller than predictions obtained excluding thermal radiation. Although these flames are not purely radiation-driven, inclusion of particle-to-particle radiative heat transfer enhances flame propagation velocities in simu

In this work, we use a boundary layer resolved model to improve a Lagrangian point particle model to simulate the combustion of single iron particles. By resolving the full boundary layer, mass and heat transfer are accurately modeled, including Stefan flow. Therefore, the model is suitable to improve point particle models. This work focuses on the first stage of iron combustion, which lasts up to the maximum temperature. Temperature- and composition-dependent properties are used and phase transitions from solid to liquid and liquid to gas are taken into account. The Nusselt and Sherwood correlations are investigated in conditions typical for iron particle combustion. It is found that the 1/2-film temperature is the best film rule to use to model heat- and mass transfer for iron particle combustion. The boundary layer resolved model is used to validate the point particle models. Then, the model is systematically elaborated by including a temperature-dependent particle density, slip velocity and Stefan flow. The individual and combined effect of these phenomena on the burn duration are investigated. Including all these effects decreases the time to maximum temperature by around 25%. Furthermore, it is shown that if one neglects physical phenomena like slip and Stefan flow, but uses the 1/3-film rule instead of the 1/2-film rule, errors cancel and still reasonable agreement is obtained with experiments.

In the present study, a novel in situ particle sizing approach is proposed and used to measure the characteristic timescales of micron-sized iron particle combustion under low-oxygen (10–17 vol%) dilution conditions. The particle size is determined by probing the light emission intensity of a burning particle during melting, which is proportional to the cross-section area of the particle projected to the camera. Detailed descriptions of the calibration, validation, and characterization of the experimental method are elaborated. With systematic measurements, we obtain one-to-one correlations between combustion timescales and single particle diameters at various diluted oxygen concentrations. Furthermore, we formally derive a theoretical model for heterogeneous combustion of growing (iron) particles in the diffusion-limited regime. The model suggests that the diffusion-limited burn time scales with the initial particle diameter squared (i.e., a new, generalized d2-law). Owing to accounting for the particle growth, the newly derived model suggests a significantly (1.66 times) shorter combustion duration compared to the conventional d2-law for shrinking particle combustion. It turns out that the new model agrees well with the experiment. This agreement also suggests that under low-oxygen dilution conditions, the combustion regime of iron particles during the intensive burning stage (i.e., from ignition to the peak particle temperature) is limited by external oxygen diffusion.

Micrometric spherical particles of iron in two narrow size ranges of (38–45) µm and (45–53) µm were injected in a bench scale, transparent drop-tube furnace (DTF), electrically heated to 1400 K. Upon experiencing high heating rates (104–105 K/s) the iron particles ignited and burned. Their combustion behavior was monitored pyrometrically and cinematographically at three different oxygen mole fractions (21%, 50% and 100%) in nitrogen. The results revealed that iron particles ignited readily and exhibited a bright stage of combustion followed by a dimmer stage. There was evidence of formation of envelope micro-flames around iron particles (nanometric particle mantles) during the bright stage of combustion. As the burning iron particles fell by gravity in the DTF, contrails of these fine particles formed in their wakes. Peak temperatures of the envelope flames were in the range of 2500 K in air, climbing to 2800 K in either 50% or 100% O2. Total luminous combustion durations of particles, in the aforesaid size ranges, were in the range of 40–65 ms. Combustion products were bimodal in size distribution, consisting of micrometric black magnetite particles (Fe3O4), of sizes similar to the iron particle precursors, and reddish nanometric iron oxide particles consisting mostly of hematite (Fe2O3).

A theoretical model is considered to predict the minimum ambient gas temperature at which fine iron particles can undergo thermal runaway–the ignition temperature. The model accounts for Knudsen transition transport effects, which become significant when the particle size is comparable to, or smaller than, the molecular mean free path of the surrounding gas. Two kinetic models for the high-temperature solid-phase oxidation of iron are analyzed. The first model (parabolic kinetics) considers the inhibiting effect of the iron oxide layers at the particle surface on the kinetic rate of oxidation, and a kinetic rate independent of the gaseous oxidizer concentration. The ignition temperature is solved as a function of particle size and initial oxide layer thickness with an unsteady analysis considering the growth of the oxide layers. In the free-molecular limit (small particles), the thermal insulating effect of transition heat transport can lead to a decrease of ignition temperature with decreasing particle size; however, the presence of the oxide layer slows the reaction kinetics, and its increasing proportion in the small-particle limit can lead to an increase of ignition temperature with decreasing particle size. This effect is observed for sufficiently large initial oxide layer thicknesses. The continuum transport model is shown to predict the ignition temperature of iron particles exceeding an initial diameter of 30 µm to a difference of 3% or less (30 K or less) when compared to the prediction of the transition transport model. The second kinetic model (first-order kinetics) considers a porous, non-hindering oxide layer, and a linear dependence of the kinetic rate of oxidation on the gaseous oxidizer concentration. The ignition temperature is resolved as a function of particle size with the transition and continuum transport models, and the differences between the ignition characteristics predicted by the two kinetic models are identified and discussed.

Following the ignition and solid-to-liquid phase transition of a fine (on the order of 10–100 µm in diameter) iron particle, the self-sustained combustion of a liquid-phase droplet of iron and its oxides takes place. The objective of the current work is to develop an interpretive and explanatory model for the liquid-phase combustion of a single fine iron particle. A zero-dimensional physicochemical model is developed assuming fast internal processes, such that the combustion rate is limited by the rate of external oxygen (O 2) transport. The model considers a particle covered by a shell of liquid-phase FeO enclosing a core of liquid-phase Fe. Stefan flow and diffusion are considered for the gas-transport of O 2, while the gas-transport of gas-phase Fe and FeO are calculated via diffusion only. The outward gas-phase Fe and FeO consume inward-transported O 2 to stoichiometrically form hematite (Fe 2O 3), and the remaining oxygen that reaches the particle surface is entirely consumed to form liquid-phase FeO. The time history of simulated particle temperature shows consistent overprediction of the peak particle temperature when compared to experimental temperature measurements, indicating that the assumption of fast internal kinetics may be incorrect. The model is also unable to capture the apparent slow cooling rate observed in experiments. A further analysis is performed through a heuristic model with a calibrated reaction-rate law, where the internal diffusion of reactive Fe and O ions may become rate-limiting. The calibration of the pre-exponential factor in the Arrhenius term to match the experimental peak temperature yielded good agreement of time to peak temperature, as well as the slow cooling rate. The heuristic model considering internal diffusion predicts a plateau in peak temperature with increasing oxygen concentration. Possible uncertainties of the models, as well as future work, are discussed.

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Temporally, spatially, and thermally resolved measurements of in situ nanoparticle cloud formation during combustion of single micron-sized iron particles are carried out. The critical particle temperature for the cloud formation is identified using high-speed and high-magnification shadowgraphy, recording the evolution of a nanoparticle cloud, synchronized with two-color pyrometry probing the surface temperature of the burning micron particle. The finding could serve as an important criterion to minimize nanoparticle generation in the flame of iron-particle suspensions, making iron powders a cleaner and more recyclable fuel.

The combustion behavior of single laser-ignited iron particles is investigated. Transient particle radiant intensities at 850 nm and 950 nm are measured by post-processing recorded high-speed camera images using an in-house developed particle tracking program. Then, the time-resolved particle temperature is obtained based on two-color pyrometry. A plateau-like stage shortly after the ignition is repeatedly observed, and identified as iron particle melting by the measured temperature and the estimated melting time. Besides, an abrupt brightness jump near the end of combustion is observed for most burning particles, while a small portion of the particles (< 10%) show a second plateau-like stage instead. The particle temperature right after the brightness jump (1880±70 K) is almost identical to that during the second plateau-like stage. This temperature corresponds to two phase-change temperatures in the Fe-O phase diagram: i) L2 <=> Fe 3O 4 (s) at 1869 K (congruent melting) and ii) L2 <=> Fe 3O 4 (s) + O 2 (g) at 1855 K (eutectic reaction), where L2 represents a liquid iron oxide. Based on this, the presence of the brightness jump (spear point) is explained by a sudden solidification of supercooled iron oxide droplets with an atomic O/Fe ratio larger than or close to 4/3. Particles’ near-peak temperatures are also measured based on time-integrated spectra. The results indicate that the near-peak temperature increases first fast and then slowly with an increase of oxygen concentration. At higher oxygen concentrations, smaller particles have a slightly lower temperature. The effect of particle size on the near-peak temperature is negligible at lower oxygen concentrations due to weaker radiation. The morphology of combusted particles is examined by micrography. Some burned particles appear as hollow thin-shell spheres at all adopted oxygen concentrations. Additionally, nano oxides are found at 13–51

The combustion behavior of single laser-ignited iron particles is investigated. Transient particle radiant intensities at 850 nm and 950 nm are measured by post-processing recorded high-speed camera images using an in-house developed particle tracking program. Then, the time-resolved particle temperature is obtained based on two-color pyrometry. A plateau-like stage shortly after the ignition is repeatedly observed, and identified as iron particle melting by the measured temperature and the estimated melting time. Besides, an abrupt brightness jump near the end of combustion is observed for most burning particles, while a small portion of the particles (< 10%) show a second plateau-like stage instead. The particle temperature right after the brightness jump (1880±70 K) is almost identical to that during the second plateau-like stage. This temperature corresponds to two phase-change temperatures in the Fe-O phase diagram: i) L2 <=> Fe 3O 4 (s) at 1869 K (congruent melting) and ii) L2 <=> Fe 3O 4 (s) + O 2 (g) at 1855 K (eutectic reaction), where L2 represents a liquid iron oxide. Based on this, the presence of the brightness jump (spear point) is explained by a sudden solidification of supercooled iron oxide droplets with an atomic O/Fe ratio larger than or close to 4/3. Particles’ near-peak temperatures are also measured based on time-integrated spectra. The results indicate that the near-peak temperature increases first fast and then slowly with an increase of oxygen concentration. At higher oxygen concentrations, smaller particles have a slightly lower temperature. The effect of particle size on the near-peak temperature is negligible at lower oxygen concentrations due to weaker radiation. The morphology of combusted particles is examined by micrography. Some burned particles appear as hollow thin-shell spheres at all adopted oxygen concentrations. Additionally, nano oxides are found at 13–51

To successfully transition from fossil-fuel to sustainable carbon-free energy carriers, a safe, stable and high-density energy storage technology is required. The combustion of iron powders seems very promising in this regard. Yet, little is known about their in-process morphological and microstructural evolution, which are critical features for the circularity of the concept, especially the subsequent reduction of the combusted oxide powders back to iron. Here, we investigated two iron powder combustion pathways, one in air and one with the assistance of a propane pilot flame. Both processes resulted in spherical hollow particles composed of a complex microstructure of wüstite, magnetite and/or hematite. Partial evaporation is indicated by the observation of nanoparticles on the micro-sized combustion products. The associated gas production inside the liquid droplet could be the origin of the internal porosity and micro-explosion events. Cracking at the end of the combustion process results in mostly open porosity, which is favorable for the subsequent reduction process. With this study, we aim to open the perspective of iron metal fuel from macroscopic combustion analysis towards a better understanding of the underlying microscopic thermodynamic, kinetic, microstructural and thermomechanical mechanisms.

Temporally, spatially, and thermally resolved measurements of in situ nanoparticle cloud formation during combustion of single micron-sized iron particles are carried out. The critical particle temperature for the cloud formation is identified using high-speed and high-magnification shadowgraphy, recording the evolution of a nanoparticle cloud, synchronized with two-color pyrometry probing the surface temperature of the burning micron particle. The finding could serve as an important criterion to minimize nanoparticle generation in the flame of iron-particle suspensions, making iron powders a cleaner and more recyclable fuel.