CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This Application claims priority to US Provisional Application No. 63/362,650, filed April 7, 2022, which is incorporated herein by reference.

FIELD OF INVENTION

[0002] The present invention relates to iron powder for use as recyclable fuel, methods for converting the iron powder into heat to provide iron oxide particles for use in recyclable fuels, and systems associated with same.

SUMMARY

[0003] The market of iron powders for use in heat-intensive industry is huge if used to import and store our sustainable energy. A major part of the sustainable energy needed in densely populated and industrial areas needs to be imported from areas with an abundant amount of solar/wind energy in 2030.

[0004] Iron powders have been proposed as recyclable fuels for heating and power generation. Because of the very high boiling point of iron (around 3100 K) compared to the adiabatic mixture temperature (around 2240 K for stoichiometric fuel/air mixtures), it is speculated that iron-oxide nanoparticles are unlikely to form during combustion of iron powder in air. Therefore, the combustion products, iron-oxide particles, will be of similar size as the original iron particles, namely micron sized, so that they can be easily captured after combustion. By subsequent reduction, the regenerated iron powders can be burned again, and the combustion-reduction cycle can then be used repeatedly. Although combustion and reduction processes are under development, the cyclicity of the concept has so far been theoretical and practical techniques for enabling cyclic oxidation/recycling processes are still under development. Cyclic behavior during repetitive combustion and reduction of iron (oxide) powder is, however, needed to establish the repetitive use of iron powder as cyclic carrier of energy, without significant losses. This means that the processes on combustion and reduction have to be closely linked together, because poor combustion will lead to difficult and inefficient reduction and vice versa. [0005] The combined processes/techniques of combustion and reduction should be developed such that they together provide: 1) minimal loss of iron when operated over and over in the metal fuel cycle; 2) minimal unwanted dust and other emissions, including NOx; 3) minimal particle (dis)integration (such as micro-explosion, swelling, cracking, sintering, elutration etc.); 4) mean particle size changes should be minimized over multiple cycles; 5) well-controlled and efficient combustion (safe ignition and high conversion factor); 6) enabling close to full energy conversion during each step in the cycle; 7) easy-to-use powders (high dispersibility and flowability); and 8) safe to handle/store powder (low dust cloud tendency, low explosion risk, low health risk).

[0006] The present disclosure relates to characteristics of iron powder and optimal operational conditions during combustion for safeguarding the recyclability of iron powders as energy carriers.

[0007] In one aspect, a method of converting iron particles into heat in the form of hot exhaust gases and hot iron oxide particles for use in recyclable fuels includes the steps of: a) mixing a primary gas comprising an oxidizer with iron powder having an average particle diameter between about 10pm to 150 pm to form a primary mixture; and b) igniting and combusting the primary mixture to form the hot exhaust gases and hot iron oxide particles. In certain embodiments, the primary gas comprises air or a mixture of O2 and an inert gas. Nonlimiting examples of an inert gas include N2 and CO2. In certain embodiments, the iron powder comprises about 1% or less of impurities. In certain embodiments, the oxidized iron comprises about 1% or less of impurities. In some embodiments, the total percentage of C-based impurities and S-based impurities is less than 0.05%. In certain embodiments, these impurities comprise C-based impurities, S-based impurities, or a combination thereof. In certain embodiments, the combustion creates a peak particle temperature of about 2150K or less, which is about 70% of the boiling temperature of the iron powder. In certain embodiments, the equivalence ratio, (|) (also referred to as the mass ratio of iron powder to oxidizer divided by the stoichiometric iron to oxidizer mass ratio, which is calculated assuming FesC is the high- temperature combustion product) is between about 1 to 1.2. In certain embodiments, when the equivalence ratio is greater than one, the method includes the step of adding a secondary gas into the primary fuel/air mixture. In certain embodiments, the secondary gas comprises air or a mixture of O2 and an inert gas such as N2 or CO2. In certain embodiments, when the mass ratio is equal to 1 or close to 1, a concentration of oxygen in the primary gas is less than about [0008] In yet another aspect, iron particles having an average particle diameter of between about 10pm to 150 pm are burnt after igniting a gas comprising an oxidizer mixed with said iron particles, thereby forming the oxidized iron particles. In an embodiment, the oxidized iron particles are free from cavities. The feature of being cavity -free advantageously provides for cyclicity of oxide particles.

[0009] In still another aspect, a system for burning iron particles includes: a) burning chamber encapsulating a flame produced from the mixture of gas and iron particles; and b) at least one gas outlet for the hot iron oxide particles and the hot exhaust gases.

[0010] In another aspect, a system converts iron particles and releases energy in a flame to heat gases and the iron oxide particles, and includes: a) a combustion chamber; and b) a gas inlet for providing a gas comprising an oxidizer to the system.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] FIG. 1 shows an experimentally obtained nano particle cloud for 50pm iron particle burning in three different oxygen concentrations, where the particle peak temperature is around 2150K, 2300K, and 2400K, respectively.

[0012] FIG. 2 shows a numerically obtained nano particle cloud for a 50pm iron particle where the particle temperature is 2150K, 2300K, and 2450K, respectively.

[0013] FIG. 3 shows a diagram of a system described herein for combusting iron particles with a gas comprising an oxidizer. In this embodiment, there is one gas inlet for providing the primary gas to the system.

[0014] FIG. 4 shows a diagram of a system described herein for combusting iron particles with a gas comprising an oxidizer. In this embodiment, the system comprises a second inlet to provide a secondary gas to the system.

DETAILED DESCRIPTION

[0015] The present disclosure identifies certain characteristics of iron powders, and conditions during combustion of the iron powders to enhance recyclability of iron powders. Such iron powders may be used as recyclable fuel. The average particle diameter of the iron powder is in the range from 1pm to 150pm, preferably from 10pm to 150pm, more preferably from 10pm to 100pm. Particle diameters less than 10pm are more difficult to ignite and may lead to high emissions of nanoparticles and powder dispersion problems. Particle diameters greater than 150pm lead to slow combustion, i.e., have insufficient time to heat up, ignite, and burn completely. Particles with smaller diameters combust faster, but are more difficult to fluidize/disperse, and they sinter more. In some embodiments, the distribution of particles is bimodal, with a distribution of smaller particles, e.g., about 10pm, in combination with larger particles e.g., about 100pm, and in a range from 10pm to 150pm, preferably from 10pm to 100pm, thus smaller particles with larger particles are advantageous for this reason.

[0016] Normally, these iron powders have less than about 0.05% of volatile impurities. Non-limiting examples of impurities include C-impurities and S-impurities. Larger fractions of volatile impurities lead to swelling and micro-explosion of particles. Table 1 shows that during the combustion of reduced powders (using hydrogen-based gas), the occurrence of particle micro-explosion diminishes dramatically since volatile impurities such as C and S have been largely removed during a first-round of combustion. In the reduction process, it is therefore important to use low-carbon-based reducing media like H2, which will avoid the addition of carbon in iron particles.

[0017] Table 1 : Comparison between particle combustion behavior and peak temperature of original and reduced sample iron powders burning at atmospheric condition.

[0018] According to one embodiment, during combustion and independent of gas temperature, the individual particle temperatures are not larger than a value of 70% of the particle boiling temperature, or a temperature of about 2150K during combustion, to avoid excessive evaporation and nano-particle formation.

[0019] Iron-oxide nanoparticles are unwanted during combustion because they are hard to capture. FIGs. 1 and 2 show iron particles burning at different oxygen concentrations in O2/N2 mixtures at different temperatures and the effects on the formation of iron-oxide nanoparticles. The nanoparticle formation is very sensitive to the particle temperature. FIG. 1 shows an experimentally obtained nano particle cloud for 50pm iron particle burning in three different oxygen concentrations, where the particle peak temperature is around 2150K as in plot 110, 2300K as in plot 120, and 2400K as in plot 130, respectively. FIG. 2 shows a numerically obtained nano particle cloud for a 50pm iron particle where the particle temperature is 2150K as in plot 210, 2300K as in plot 220, and 2450K as in plot 230, respectively.

[0020] When the peak particle temperature does not rise above 2150 K, a nanoparticle cloud cannot be observed anymore, as shown in plot 110 of FIG. 1. This is in contrast to the nanoparticle clouds as observed in plots 120 and 130 of FIG. 1 for T _ma x 2300K and T _ma x 2400 K, respectively. This means that during combustion, the local particle peak temperature should be controlled below 2150 K to avoid evaporation of more than 0.5% of the iron mass. It is important to realize that particle peak temperatures can increase temporarily far above the adiabatic flame temperature (maximum around 2240K for stoichiometric fuel/air mixtures). Peak temperatures up to 2800 K are observed, which is close to the boiling temperature of iron. It is therefore very important to avoid local peak temperatures above 2150 K.

[0021] One way to control the flame temperature is to regulate the oxygen concentration in the initial gaseous mixture used to burn the iron powder. Table 2 shows the calculated maximum equilibrium temperate of Fe/O2/N2 flames as a function of O2 concentration in the gaseous mixture.

[0022] Table 2. Calculated maximum equilibrium temperature of Fe/O2/N2 flames, measured single particle combustion temperature, and the predicated mass loss by liquid iron evaporation for single particle combustion as a function of oxygen volume fraction in O2/N2 mixtures.

[0023] In embodiments, the primary fuel/oxidizer equivalence ratio, (|>, is in the range of 1 - 1.2. At this ratio, the gas and particles stay close to thermal local equilibrium; hence there is no temporal overshoot of the particle temperature. Smaller initial (]> values (initial fuel-lean mixture, values of equivalence ratio less than 1.0) lead to higher particle peak temperatures and larger values (greater than 1.2) lead to inefficient combustion.

[0024] In one embodiment, when 4> is initially larger than 1 (fuel-rich initial combustion), during the iron oxidation with primary gas, secondary gas is added behind the primary flame front (i.e., the maximum temperature region) to oxidize leftover iron and suboxides. This step first reduces the flame temperature and hence minimizes NOx and nanoparticle formation. Furthermore, it can yield a full conversion of iron and thus an efficient combustion process. When the mixture equivalence ratio 4> is 1 or close to 1, the oxygen concentration should be not higher than 18% in order to control the mass fraction of evaporated nanoparticles below 0.5%. When the mixture equivalence ratio is 1 or close to 1, a secondary air flow is preferably added at the downstream of the flame where particle temperature is below 1870K (i.e., melting point of FesCh) for enabling close to full combustion in the form of the following reaction: FesO^s) + 02(g) = Fe2O3(s). This step is performed because it is easier to reduce Fe2O3(s) to Fe compared to reducing Fe3O4(s) to Fe. In some embodiments, the mixture equivalence ratio is 1 +/- 0.1.

[0025] An initial overall equivalence ratio (calculated based on the total amount of oxidizer from both the primary and secondary gas supplies) close to unity may be used to avoid extra oxygen dissolving into liquid iron oxide during the second combustion phase (i.e., combustion from the secondary gas). The dissolved extra oxygen leads to oxygen gas (O2) release at a later stage during the solidification of oversaturated liquid iron oxide, L2 (i.e., L2

Fe3O4(s) + 02(g)), which in turn leads to swelling of burning particles due to enclosed oxygen bubbles. In the downstream region, where the flame cannot be observed anymore (i.e., where the flame has very low thermal radiation due to low particle temperatures below the melting point of Fe3O4, about 1870K), another stream of oxidizer gases (a tertiary gas) is preferably added for enabling close to full combustion in the form of the following reaction: Fe3O4(s) + 02(g) = Fe2O3(s). This step of oxidation only occurs in the solid state after solidification of the iron oxide particle. This step is performed because it is easier to reduce Fe2O3(s) to Fe compared to reducing Fe3O4(s) to Fe. The entrapped oxygen bubbles lead to hollow spheres and/or burst spheres, when the internal pressure of the particle increases to high values to crack the solid outer layer, which is undesired.

[0026] The oxidizer in the primary and/or secondary gas may be oxygen mixed with an inert gas. Non-limiting examples of an inert gas include N2 or CO2. The oxygen concentration in the initial primary gas can be reduced to below 18% to control the flame temperature. Inert gases, such as but not limited to, flue gases, may be admixed with the initial air as well to decrease the initial oxygen concentration compared to the value of pure air (21%). Alternatively, pure air can be used in an initial fuel rich blend of iron in air, namely a fuel/air equivalence ratio, (|>, larger than around 1.2 compared to stoichiometric conditions. Additional oxidizer gases should be admixed in the post-flame gases to enhance complete oxidation. Combinations of the mentioned approaches are also valid. [0027] FIGs. 3 and 4 show embodiments of systems for converting iron particles and releasing energy in the flame to heat the gases and iron oxide particles. FIG. 3 shows a diagram of a system 300 described herein for combusting iron particles 310 with a gas 320 comprising an oxidizer into inlet 330. Gas 320 may be a mixture of gases, e.g., oxygen and an inert gas or air. In this embodiment, there is one inlet 330 for providing the mixture of iron particles and the primary gas to the system for combusting upon contact with flame 370. System 300 includes outlet 360 for removing oxide particles/exhaust mixture 380.

[0028] FIG. 4 shows a diagram of a system 400 described herein for combusting iron particles 410 with a gas 420 comprising an oxidizer. In this embodiment, the system 400 comprises at least one additional gas inlet 490 to provide a secondary gas 440 to the system. Gases 420 and/or 440 may be a mixture of gases, e.g., oxygen and an inert gas or air.

[0029] Said systems include: a) a combustion chamber (350, 450); and b) an inlet (330, 430) for providing a gas (320, 420) comprising an oxidizer and/or iron particles (310, 410) to the system, c) a flame (370, 470), and d) an outlet (360, 460) for removing oxide particles and exhaust from the system. The gas inlets provide gas to the system for oxidizing the iron particles, and they allow for controlling the oxidizer concentration in the gas. When there is only one gas inlet 320 in the combustion system 300, as shown in FIG. 3, leading to a relatively simple oxidizer supply system and burner geometry, the oxidizer concentration in the initial mixture could be reduced below 18% in order to control the flame temperature under 2150 K. As shown in FIG. 4, a second gas 440 may be added in addition to the first gas 420 to the system 400 to ensure complete oxidation of the iron particles 410. In the embodiment as in FIG. 4, the second gas 440 is provided downstream of the flame 470.

[0030] Clause 1. A method of converting iron particles into heat in the form of hot exhaust gases and hot iron oxide particles for use in recyclable fuels, comprising: a) mixing a primary gas comprising an oxidizer with iron powder comprising iron particles having an average particle diameter between about 10pm to 150 pm to form a primary mixture; and b) igniting and combusting the primary mixture to form the hot exhaust gases and hot iron oxide particles.

[0031] Clause 2. A system for converting iron particles and releasing energy in the form of hot exhaust gases and hot iron oxide particles, the system comprising: a) a combustion chamber; and b) a gas inlet for providing a primary gas comprising an oxidizer to the system. [0032] Clause 3. The method of clause 1 or the system of claim 2, wherein the primary gas comprises air or a mixture of O2 and an inert gas.

[0033] Clause 4. The method of clause 2, wherein the inert gas comprises N2 or CO2.

[0034] Clause 5. The method of clause 1, wherein the iron powder comprises about 1% or less of volatile impurities, or about 0.05% or less of volatile impurities.

[0035] Clause 6. The method of clause 1, wherein after combusting the hot iron oxide particles comprise about 1% or less of volatile impurities.

[0036] Clause 7. The method of clause 1, wherein the hot iron oxide particles comprise about 0.05% or less of volatile impurities.

[0037] Clause 8. The method of clause 7, wherein the volatile impurities comprise C- based impurities, S-based impurities, or a combination thereof.

[0038] Clause 9. The method of clause 1, wherein during combusting a peak particle temperature reaches about 2150K, or 70% of the boiling temperature of iron, or less.

[0039] Clause 10. The method of clause 1, wherein an equivalence ratio, (|>, of iron powder to oxidizer is between about 1 to 1.2.

[0040] Clause 11. The method of clause 10, further comprising mixing a secondary gas comprising an oxidizer with the primary mixture when the equivalence ratio is greater than 1.

[0041] Clause 12. The method of clause 10, wherein a concentration of oxygen in the primary gas is less than about 18% when the equivalence ratio is equal to 1 or close to 1.

[0042] Clause 13. The method of clause 11, wherein the secondary gas comprises air or a mixture of 02 and an inert gas.

[0043] Clause 14. Oxidized iron particles with an average particle diameter of between about 10 pm to 100 pm made by the method of clause 1, or wherein a distribution of average particle sizes is bimodal.

[0044] Clause 15. The oxidized iron particles of clause 14, wherein the oxidized iron particles are free from cavities. * * *

[0045] The present invention is not to be limited in scope by the specific embodiments described herein. Indeed various modifications of the invention in addition to those described herein will be apparent to those skilled in the art from the foregoing description and figures. Such modifications are intended to fall within the scope of the appended claims.

[0046] It is further to be understood that all values are approximate and are provided for description. All references cited and discussed in this specification are incorporated herein by reference in their entirety and to the same extent as if each reference was individually incorporated by reference.

REFERENCES

[0047] 1. Daoguan Ning, Yuriy Shoshin, Jeroen A. van Oijen , Giulia Finotello,

Laurentius P.H. de Goey, “Critical temperature for nanoparticle cloud formation during combustion of single micron-sized iron particle”, Combustion and Flame 244 (2022) 112296; https://doi.Org/10.1016/j.combustflame.2022.112296.

[0048] 2. L.C. Thijs, C.E.A.G. van Gool, W.J.S. Ramaekers, J.A. van Oijen, L.P.H. de

Goey, “Resolved simulations of single iron particle combustion and the release of nanoparticles”, Proceedings of the Combustion Institute (2022), ISSN 1540-7489, http s : //doi . org/ 10.1016/j . proci .2022.07.044.