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Powdered iron is being investigated for its potential use as a carbon-free fuel due to its ability to burn heterogeneously and produce oxide particles, which can be collected, reduced back to iron and burned again. However, high temperature oxidation of iron particles can induce partial vaporization/decomposition and evolution of nanometric iron oxide particles. To investigate the formation process of nanoparticles in iron combustion, iron powders (consisting of spheroidal 45–53 μm particles) were injected in an electrically-heated drop tube furnace, operated at a maximum gas temperature of 1375 K, where they experienced high heating rates (104 K/s). The particles reacted with oxygen at concentrations of 15, 21, 35, 50 and 100 % by volume in nitrogen diluent gas. Particles ignited and burned brightly, with peak temperatures reaching 2344–2884 K, depending on the oxygen concentration. The observed distribution of the combustion products of iron was bimodal in size and composition, containing (a) dark gray spherical micrometric particles bigger than their iron particle precursors composed of both magnetite and hematite, and (b) highly agglomerated orange-reddish nanometric particles composed of hematite. The mass fraction of nanometric particles accounted for up to 1.7–7.4 % of the collected products, increasing with the oxygen partial pressure. The nanometric particles were spherules, 30–100 nm in diameter. However, they were highly agglomerated with aggregate aerodynamic diameters peaking at 180–560 nm. The yield of nanoparticles increased with increasing oxygen concentration in the furnace. A heuristic model was used to investigate the impact and sensitivity of various strategies for modeling evaporation, aiming to identify key mechanisms that limit the evaporation rate. This study highlights that understanding the type of liquid at the particle surface is crucial, as evaporation can increase significantly with a homogeneous liquid Fe-O particle compared to a core–shell morphology. Additionally, the analysis suggests that evaporation likely occurs in an intermediate regime where gaseous Fe-containing species oxidize in the boundary layer. Understanding these boundary layer processes is essential for accurately modeling the evaporation rate while maintaining computational efficiency. 1. 

Abstract As the effects of climate change become a greater threat, the need for a viable source of sustainable energy grows daily. Iron powder has been proposed as a potential alternative to conventional fossil fuels. Pulverized iron can be burned similarly to coal. Unlike coal, however, iron combustion does not create CO2 as a by-product. It also produces a negligible amount of NOx. Iron is also abundant in the Earth's crust, has a low explosion range, possesses a competitive energy density to hydrocarbons, and reacts well with oxygen. Finally, the iron oxide produced during combustion can be collected and reduced back to iron, creating a fully sustainable process. In this analysis, different power generation cycles were analyzed to maximize the energy and exergy efficiencies as well as the work output per unit mass of iron. It was found that the power cycle that maximized both the energy and exergy efficiencies as well as the work output per unit mass of input iron was a combined power cycle, where the topping cycle was a gas turbine cycle with one-stage compression and expansion and the bottoming cycle was a steam turbine cycle with two-stage expansion and reheat. This brought the theoretical energy efficiency to 59.87%, the theoretical exergy efficiency to 65.37%, and the theoretical work output per unit mass of iron to 4422 kJ/kg. The energy efficiency decreased to 56.81% when auxiliary devices were considered. 

This manuscript reports on the combustion of powdered iron, for the purpose of utilizing it as an environmentally friendly circular energy carrier. The conducted research investigated the spectral emissivity and temperature of iron particles, burned either individually or in groups. Combustion experiments were conducted under high heating rates in an externally-heated drop tube furnace. The pressure was atmospheric and the axial temperature was nearly-constant at ~1350 K. The oxidizer gas contained 15-100% oxygen in nitrogen diluent. Iron particles were sieve-classified in the 44-53 µm range. Results showed that, depending on the oxygen concentration, and consequently the particle temperature, the average spectral emissivities of single burning particles varied between 0.18 and 0.46, in the 600-1000 nm wavelength range. Corresponding temperatures of single particles varied between 2300 K and 2800 K, increasing with increasing oxygen concentration in the gas. In the case of groups of iron particles burning in air at different particle number densities, average spectral emissivities were found to be in the range of 0.42-0.45, with the upper value associated with denser particle clouds. Corresponding peak temperatures of particle burning in groups were found to be in the range of 2160 K to 2100 K, with the lower value attributed to denser particle clouds. 

This research focused on the size and overall porosity (pore volume) of carbonaceous chars, originating from high-heating rates and high-temperature pyrolysis and/or combustion of biomass. Emphasis was given to torrefied biomass chars. First, the porosity of char residues of single biomass particles of known mass was determined, based on an assumed value of skeletal density and by comparing experimentally observed temperature-time histories with numerical predictions of their burnout times. The average char porosities (effective porosities) of several raw and torrefied biomass particles were calculated to be in the range of 92–95%. Thereafter, these deduced porosity values were input again to the model to calculate the size of chars of other biomass particle precursors, whose initial size and mass were not known. Such biomass particles were sieve-classified to different nominal size ranges. This time, besides the porosity, representative time-temperature profiles of biomass particles in the aforementioned size ranges were also input to the model. Biomass particles are highly irregular with large aspect ratios and, in many cases, they melt and spherodize under high heating rates and elevated temperatures. Knowledge of the initial size of the chars, upon extinction of the volatile flames, is needed for modeling their heterogeneous combustion phase. For this purpose, numerical predictions were in general agreement with measurements of char size obtained from both scanning electron microscopy of captured chars and real-time high-speed, high- magnification cinematographic observations of their combustion. Results showed that the generated chars of the examined biomass types were highly porous with large cavities. The average initial dimension of the chars, upon rapid pyrolysis, was in the range of 50–60% the mid-value of the mesh size of the sieves used to size-classify their highly irregular parent biomass particles. 

Abstract This work assesses the evolution of acid gases from raw and torrefied biomass (distiller’s dried grains with solubles and rice husk) combustion in conventional (air) and simulated oxy-combustion (oxygen/carbon dioxide) environments. Emphasis was placed on the latter, as oxy-combustion of renewable or waste biomass, coupled with carbon capture and utilization or sequestration, could be a benefit toward mitigating global warming. The oxy-combustion environments were set to 21%O2/79%CO2 and 30%O2/70%CO2. Results revealed that combustion of either raw or torrefied biomass generated CO2 emissions that were lower in 21%O2/79%CO2 than at 30%O2/70%CO2, whereas CO emissions exhibited the opposite trend. Emissions of CO from combustion in air were drastically lower than those in the two oxy-combustion environments and those in 21%O2/79%CO2 were the highest. Emissions of NO followed the same trend as those of CO2, while HCN emissions followed the same trend as those of CO. Emissions of NO were higher than those of HCN. The emissions of SO2 were lower in oxy-combustion than in air combustion. Moreover, combustion of torrefied biomass generated higher CO2 and NO, comparable CO and SO2, and lower HCN emissions than combustion of raw biomass. Out of the three conditions tested in this study, oxy-combustion of biomass, either in the raw and torrefied state, attained the highest combustion effectiveness and caused the lowest CO, HCN, and SO2 emissions when the gas composition was 30%O2/70%CO2.