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![Stabilized flames of different metal powders compared to a methane-air flame. The flame temperature of a stoichiometric iron-air flame is around 2300 K, similar to that of hydrocarbon flames, while aluminum burns with air at temperatures above 3000 K. Reprinted from [90] with permission of Elsevier.](publication/327372687/figure/fig15/AS:866937647153152@1583705450017/Stabilized-flames-of-different-metal-powders-compared-to-a-methane-air-flame-The-flame.png)
Stabilized flames of different metal powders compared to a methane-air flame. The flame temperature of a stoichiometric iron-air flame is around 2300 K, similar to that of hydrocarbon flames, while aluminum burns with air at temperatures above 3000 K. Reprinted from [90] with permission of Elsevier.
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Metal fuels, as recyclable carriers of clean energy, are promising alternatives to fossil fuels in a future low-carbon economy. Fossil fuels are a convenient and widely-available source of stored solar energy that have enabled our modern society; however, fossil-fuel production cannot perpetually keep up with increasing energy demand, while carbon...
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... Especially iron powder shows a great potential due to its high energy density, abundance, and circularity via hydrogen reduction. Bergthorson et al. (2015); Bergthorson (2018) have elaborated upon the viability of iron fuel compared to other sustainable energy solutions as well as the renewable energy cycle. ...
Iron powder, as a recyclable and carbon-free fuel, is a promising alternative to fossil fuels and other energy carriers. One challenge in developing a practical iron-powder combustor is the deposition of particles on the inner surfaces of the combustion chamber. The objective of this study is to understand the mechanism underlying the surface deposition of combustible iron particles in a size range between 20 µm and 70 µm. The effects of the temperature and different material properties of the surface, including melting point and thermal conductivity, on the deposition behavior are examined experimentally and further analyzed via numerical modeling. For the experimental study, a Jet-in-Hot-Coflow (JHC) burner is used to heat and inject iron particles onto a plate. Various surface materials, including carbon steel, stainless steel, aluminum, and brass, are analyzed. The experiments reveal that unignited solid iron particles cannot deposit on any surface. When particle ignition occurs, molten iron particles can deposit on all of the herein considered surface materials. The experimental measurements show that a higher temperature, lower melting point, and lower thermal conductivity of the surface material enhance deposition, suggesting that the key deposition mechanism (at least, for sufficiently smooth surfaces) is likely a local melting and fusion (or welding) process. An overall positive correlation between the experimentally measured deposition degree for different surface materials and the theoretically estimated local melting parameter (θm) supports the hypothesis of the deposition mechanism.
... [1][2][3][4] Another class of promising chemical energy carriers are metal fuels and in particular iron. [5,6] Beside the high gravimetric energy density of iron (16.1 kW h L À 1 [7] ) compared to liquefied hydrogen (2.4 kW h L À 1 [8,9] ) and ammonia (3.8 kW h L À 1 [10] ), the main advantages of iron as metal fuel are the non-toxicity and the high abundance in the earth crust. Micron-sized iron powder can be combusted in retro-fitted coal-fired power plants for energy release. ...
The carbon‐free chemical storage and release of renewable energy is an important task to drastically reduce CO2 emissions. The high specific energy density of iron and its recyclability makes it a promising storage material. Energy release by oxidation with air can be realized by the combustion of micron‐sized iron powders in retro‐fitted coal fired power plants and in fixed‐bed reactors under milder conditions. An experimental parameter study of iron powder oxidation with air was conducted based on thermogravimetric analysis in combination with wide‐angle X‐ray scattering and Mössbauer spectroscopy. In agreement with literature the oxidation was found to consist of a very fast initial oxidation of the outer particle layer followed by much slower oxidation due to diffusion of iron ions through the Fe2O3/Fe3O4 layer being the rate‐limiting step. Scanning electron microscopy analysis of the iron particle before and after oxidation reveal a strong particle morphology transformation. This impact on the reaction was studied by cyclization experiments. Up to 10 oxidation‐reduction cycles show that both, oxidation and reduction rates, increase strongly with cycling due to increased porosity.
... Metal powder combustion is one of the promising sustainable power generating alternatives to conventional sources of power such as fossil fuels. Metal powders are dense energy carriers that can be turned into a power source using two processes (Bergthorson, 2018). The first is known as the wet cycle in which the metal is reacted with water at high temperatures for heat and hydrogen production (Dirven et al., 2018). ...
... A key advantage of using iron as a power source is its overall abundance as a resource. Iron makes up around 5% of the earth's crust making it the 4 th most common element in the crust (Bergthorson, 2018). This means that there will be enough resource availability for this particular use alongside the conventional uses of iron in construction and other sectors. ...
... Using either coke (a high-carbon distillate of coal) or hydrogen it is possible to reduce the iron oxides back into iron powder making the energy generation process fully circular process, as shown in Figure 1. (Bergthorson, 2018) This has the main advantage that generally only one supply of iron powder is required for a specific application as this source can be infinitely reused through the reduction process. This is, of course, not possible with fossil fuels as their combustion products cannot be reduced back into the fuel. ...
This paper investigates the feasibility of iron powder energy generation systems on board a semi-submersible crane vessel. This is done using a design model that integrates design information and a simulated mission profile to determine a hybrid iron powder setup split. This setup is then placed within a set of vessel designs to calculate a base level feasibility looking at the draft, stability, and emissions decrease. For those concepts that were technically feasible, the new hybrid iron powder setup contributed to a reduction of CO2 up to 25-50% and a reduction of NOx emissions between 15-50%, depending on the mission profile.
... 7 Various metals, including aluminum, boron, magnesium, zinc, titanium, and lithium are known to be energetic materials. 8,9 In this regard, researchers have studied the potential of energetic materials by mixing them into hydrocarbon fuels, targeting the enhancement in combustion and lowering the ignition delay. 10,11 Among various energetic materials, boron has been receiving wide attention since 1939, when it was first used as an additive for fuel because of its high gravimetric calorific value (13,925 kcal kg −1 ) compared with others. ...
... However, the intermittent nature of these renewable resources cannot guarantee a stable and adjustable power supply to the power grid. The metal-fuel cycles proposed in recent studies [1][2][3] provide an ideal solution to this problem. In such a cycle, clean renewable energies are used to reduce metal oxides to metal fuels, which could then be transported over long distances and used for power generation by direct combustion. ...
Micron-sized iron particles are promising carbon-free fuels. Efforts have been made to understand the combustion process of pure iron particles. However, it is not well-established how important are the impurities and their influence on combustion. The ignition and combustion characteristics of iron particles with 6.18 wt.% silicon were studied both by online optical diagnostics and offline scanning electron microscope (SEM) and X-ray Diffraction (XRD) analysis in this work. It is found that the ignition delay time is proportional to with = 1.23 ∼ 1.56, indicating that the oxidation reaction is unlikely solely limited by internal ionic diffusion before ignition. Compared with pure iron particles, the silicon-containing iron particle has a lower peak temperature and less oxidized combustion products. The intensity-rising time in the first intensity peak stage scales (1∕X O 2) with = 0.80 or 1.38, which implies an external diffusion controlled combustion regime. Silicon in the oxide layer likely reduces the internal diffusion rate of oxygen, which becomes a rate-limiting step after the first intensity peak. Meanwhile, the surface tension also decreases due to the silicon content, which may facilitate the gas bubble nucleation and burst. The particle could partially change back to the external diffusion regime when the oxide layer is broken by the bubble expansion. This mechanism allows the reaction rate to increase and particle temperature to rise again. This explanation is verified by plenty of holes in the combustion products as well as the fact that the rising time in the large peak stage is proportional to (1∕X O 2) with = 0.37 or 0.32. With the increase in the temperature of the ambient gas flow, evaporation was observed. The high ambient temperature is hypothesized to be a key factor to trigger the evaporation of silica. Novelty and significance statement A unique combustion process including multiple intensity peaks is observed for the first time in this work and is found to be caused by a 6.18% silicon impurity in the iron particles. The ignition and combustion characteristics of micro-sized iron particles with high silicon impurities are studied. The relationship between particle diameter and the ignition delay time of iron particles with silicon impurity is determined by a two-camera system which could capture both the moment of the particle injection into the hot gas flow and the particle ignition. The radiation, temperature evolution and the combustion products of the iron particle with silicon impurity burning in hot gas flow with different oxygen concentrations and temperatures are studied. The possible combustion regime is discussed based on these experimental results.
... Jeong et al. developed a prototype composed of Fe/TiO 2 as a catalytic system for a chemical looping via stepwise ethane dehydrogenation, reduction T < 550 • C, with a CO 2 activation to CO during the oxidation, T > 700 • C, to gain a higher yield than the conventional dehydrogenation of ethane by co-feeding CO 2 , after five reduction/oxidation cycles [22]. Furthermore, iron oxides have been employed in a reduction/oxidation cycle as carbon-free carriers of renewable energy [23]. Specifically, De Biagi et al. reported on a reduction/oxidation process for energy conversion, where iron is used as a fuel [24] to sustain high-temperature oxidation. ...
The reduction kinetics of iron ore with H 2 and its re-oxidation with air are investigated by thermogravimetric analysis (TGA) over reiterated reduction/oxidation cycles. Using non-isothermal methods, three steps of reaction have been identified for reduction (activation energies of 133, 30 and 83 kJ/mol) and two for re-oxidation (30 and 70 kJ/mol). Isothermal analysis, instead, leads to wrong kinetic expressions. A remarkable "annealing" effect is observed: when the temperature of reduction is increased from 600 to 850 • , the pre-exponential factors of the re-oxidation step decrease by an order of magnitude. Upon repeated cycles, reactivity decreases in the first three cycles, but is restored after 4-5 cycles and even increases upon further cycles. X-ray Diffraction (XRD) analysis shows changes in the crystalline phase of hematite upon reduction and re-oxidation, suggesting a possible spin-flip effect. After 4-5 reiterated cycles the opening up of porosity may counterbalance negative changes at the nanoscale.
... Recently, metal powders have emerged as a solution for energy production with the aim of minimizing CO 2 production. Among the various applications of metal powders is their use as a substitute for coal with modifications to the ignition chamber; Therefore, it is crucial and important to develop an ignition chamber burner that can burn metal powders effectively [3]. ...
... Lately, iron powder has been receiving interest because it serves as a carbon free and recyclable energy carrier, with high energy density. Additionally, it is easy to transport and store while being nontoxic and cost effective to manufacture [3,13]. Reducing emissions from high energy density fuels can improve our understanding of dynamic optimization of combustion behavior and develop the application of using high density fuels in combustion chambers [14,15]. ...
... 2) to probe the sample interaction with the electrolyte, then at 0.0 V vs RHE, which is above the onset for the HER but below the Fe(II/III) redox transition (cond. 3), and operando at −0.8 V vs RHE, which is in the regime of HER and below typical Fe(II) → Fe(I) reduction potentials (cond. 4). ...
Nuclear forward scattering (NFS) is a synchrotron-based technique relying on the recoil-free nuclear resonance effect similar to Mössbauer spectroscopy. In this work, we introduce NFS for in situ and operando measurements during electrocatalytic reactions. The technique enables faster data acquisition and better discrimination of certain iron sites in comparison to Mössbauer spectroscopy. It is directly accessible at various synchrotrons to a broad community of researchers and is applicable to multiple metal isotopes. We demonstrate the power of this technique with the hydrogen evolution mechanism of an immobilized iron porphyrin supported on carbon. Such catalysts are often considered as model systems for iron–nitrogen-carbon (FeNC) catalysts. Using in situ and operando NFS in combination with theoretical predictions of spectroscopic data enables the identification of the intermediate that is formed prior to the rate-determining step. The conclusions on the reaction mechanism can be used for future optimization of immobilized molecular catalysts and metal–nitrogen–carbon (MNC) catalysts.
... After combustion, the generated iron oxides are captured and reduced back to iron, see Fig. 1. It is also possible to transport the iron over long distances, or store for the longterm with minimal loss [3]. Feasibility studies of the cycle have shown that replacing coal plants with iron is possible with adaptations [4,5]. ...
... According to Bergthorson et al. [3], the utilization of micron-sized fuel particles, undergoing heterogeneous combustion, simplifies particle collection because they are the product of the reaction. Metal oxides particles are collected through the adoption of cyclones, filtration, electrostatic and electromagnetic separators. ...
... However, almost all renewable energy resources are intermittent, necessitating solutions to high-density energy storage that are transportable, efficient, and sustainable. Recently, metals have been proposed as promising media for efficient and transportable energy storage, given their beneficial features such as high energy density, sustainability, carbon neutrality, safety, ease of storage, and transportability (Bergthorson et al., 2015;Julien and Bergthorson, 2017;Bergthorson, 2018;Dirven et al., 2018;Baumann et al., 2020). This is referred to as metal fuels or metal fuel cycle, which essentially incorporates the metal oxidation/combustion and reduction in a cyclic manner (circular) to store and release the energy. ...
... Among others, iron is particularly attractive owing to its abundancy (by mass) on the earth (Frey and Reed, 2012), a non-volatile combustion process in the air (Soo et al., 2017;Goroshin et al., 2022), reasonably low flame temperature of ~ 1957 • C (Bergthorson et al., 2015;Tóth et al., 2020), economically transportable and low market price (Beach et al., 2007;Bergthorson, 2018;Debiagi et al., 2022;Dirven et al., 2018). Iron powder production is the starting feedstock of iron fuel and the key to closing the cycle. ...
Low-temperature electrochemical reduction (electroreduction) of iron oxides is a promising alternative to the conventional methods for iron production due to its CO2-free operation and relatively low energy consumption. In this work, we demonstrate a novel approach for electrochemical iron production by promoting the formation of dendritic structures during iron electrodeposition, which facilitates the easy harvesting of deposits in powder form. Experiments were conducted using a single pair of parallel plate electrodes, immersed in a mixture of hematite (Fe2O3) powder and aqueous alkaline (NaOH) slurry. The effects of current density, Fe2O3 mass fraction, temperature, and powder size on current efficiency and deposit morphology are investigated. A large quantity of dendritic iron structures is observed when experiments are carried out without stirring and/or applying heat from a heating plate. This condition suggests temperature and (ion/species) concentration gradients in the system. The dendrites are mainly deposited on the cathode's sides, corners, and edges. Different deposits and dendritic structures (compact layer deposit, moss-like deposit, deposit with whisker-like dendrites, and deposit with crystal-like dendrites) are observed as operating conditions change. Overall, a cathodic deposition of metallic iron with a high Faradaic efficiency (≥90 %) is successfully accomplished. The present findings provide new insights into the production of electrolytic iron powder and its future use as a carbon neutral and sustainable fuel/energy carrier.
