System and method for spontaneous combustion of iron particles - Patents.com

Abstract

A system for continuous combustion of iron particles is provided. The system includes a multi-annular combustion tube defining at least three distinct passages in cross section from its inlet to its outlet. A first innermost tube defines a first passage providing a primary air flow in which the iron particles are suspended. A second tube defines an inner annular space providing a secondary air flow, a pilot combustible flow, and an ignition point for a spark generator. A third tube defines a third passage, includes a swirl generator, and provides a tertiary air flow. The tubes are nested at predetermined locations within the multi-annular combustion tube. The system includes a diverging nozzle at an outlet of the multi-annular combustion tube; a combustion furnace in fluid communication with the diverging nozzle for generating and stabilizing a turbulent iron flame for combusting the iron particles to produce oxidized iron particles; and a cyclone.

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to U.S. Provisional Patent Application No. 63 / 239466, filed September 1, 2021, which is incorporated by reference in its entirety.

[0002] The present disclosure relates generally to the field of metal fuel combustion, and more specifically to a system and method for generating a self-sustaining turbulent iron flame using iron particles. [Background technology]

[0003] Fossil fuel supplies are limited and the transition to cleaner, more sustainable energy sources has already begun. The energy industry has demonstrated the success of solar, wind, hydro, and other forms of renewable energy. However, renewable solutions for energy storage and transportation remain a challenge. Metals as solid fuels have been investigated as potential alternatives to fossil fuels such as coal. Under the right conditions, metals can be burned and oxidized to release their energy. Indeed, metals generally have high energy density and are denser and heavier compared to coal. Unfortunately, current attempts to utilize metal particles (e.g., aluminum or iron particles) as solid fuels have stalled due to scale-up limitations and sustainability challenges. The sustainability challenges of metal particle combustion are due in part to the need for a continuous supply of hydrocarbon fuel to sustain the metal flame. This problem was addressed by Chinese Patent No. 111853762 (hereafter '762), which describes an aluminum flame ignited with a mixture of methane, air, and oxygen. '762 describes a burner with multiple micro-holes, 0.8 mm in diameter, that stabilizes an aluminum flame. '762 reports the use of a flat-flame burner to obtain a stable, laminar aluminum flame that burns without the continuous addition of methane. Thus, although a stable flame was obtained without feeding a hydrocarbon, the scalability of the flat-flame burner is limited. Additionally, alternative metals to aluminum, including iron, are described in '762, but only aluminum combustion is exemplified. Thus, improvements in the efficiency, sustainability, and scalability of metal-fueled furnaces are needed. Summary of the Invention [Problem to be solved by the invention]

[0004] [Means for solving the problem]

[0005] In one aspect, a system for continuous combustion of iron particles is provided comprising: 1. A multi-annular combustion tube having an inlet and an outlet, the multi-annular combustion tube defining in cross section at least three distinct passages from the inlet to the outlet: a first tube, innermost, defining a first passageway for providing a primary air flow having ferrous particles suspended therein; a second tube exterior to the first tube and defining a second passageway, the second tube being an inner annular space defined between the first tube and the second tube, the inner annular space providing a secondary air flow and a pilot combustible flow and further comprising an ignition point for the spark generator; and a third tube positioned outside the second tube and defining a third passageway, the third tube being an outer annular space defined between the second tube and the third tube, the outer annular space including a swirl generator and providing a tertiary air flow; a multi-annular combustion tube, the first tube, the second tube and the third tube being nested in position within the multi-annular combustion tube; a diverging nozzle at the outlet of the multi-annular combustion tube; a combustion furnace including a furnace outlet opposite the furnace inlet in fluid communication with and hydraulically connected to a diverging nozzle at the furnace inlet for generating and stabilizing a turbulent iron flame for combusting the iron particles to generate oxidized iron particles; a cyclone having a cyclone inlet, a gas outlet and a particle outlet, the cyclone inlet being in fluid communication with the furnace outlet; A system is provided comprising:

[0006] In some embodiments, the system further comprises an air gap defined between the cyclone inlet and the furnace outlet, the air gap providing a quaternary air flow to the cyclone inlet. In some embodiments, the system further comprises a quaternary flow provided into the combustion furnace by a pressurized air flow through an injection port in the combustion furnace. In a further embodiment, the multi-annular combustion tube is a triple concentric tube. In yet another embodiment, the system further comprises a filter downstream of the cyclone separator for capturing oxidized iron particles exiting the cyclone separator. In yet another embodiment, the system further comprises a magnetic separator downstream of or integrated into the cyclone separator. In yet another embodiment, the system further comprises a temperature control system coupled to the cyclone separator. In an additional embodiment, the system further comprises an energy generating device. In yet another additional embodiment, the energy generating device is selected from a heat engine, a Stirling engine, or a steam engine. In yet another additional embodiment, the inner annular space further comprises flame arrester beads. In yet another additional embodiment, the system further comprises a pressure valve in said inner annular space to release pressure in the event of an increase in pressure. In some embodiments, the system further comprises a metal fuel storage compartment comprising a metal fuel powder silo and a compressed air system coupled to the metal fuel powder silo that provides a primary air stream in which the iron particles are suspended. In further embodiments, the system further comprises a combustibles shut-off valve. In yet another embodiment, the system further comprises a radiation reflecting enclosure that contains the combustion furnace.

[0007] In a further aspect, a method of combusting iron particles, comprising: into the combustion furnace via a diverging nozzle, A primary air stream in which the ferrous particles are suspended; a secondary air flow that is physically separated from and surrounds the primary air flow; and A tertiary airflow that is a turbulent, swirling flow that is physically separated from and surrounds the secondary airflow providing a multi-annular flow comprising: providing a pilot combustible flow along with a secondary air flow and a spark for igniting a pilot flame; Ignition of a turbulent iron flame by a pilot flame; stabilizing a turbulent iron flame and burning iron particles in a reaction zone of a combustion furnace to generate an air stream containing oxidized iron particles, the combustion furnace having a recirculation zone surrounding the reaction zone and maintained by a tertiary air stream; Stopping pilot flammable logistics; Stabilizing an iron-air flame without a combustible pilot stream; recovering the oxidized iron particles from the air stream in a cyclone; A method is provided that includes:

[0008] In some embodiments, the method further includes providing a quaternary air stream upstream of the cyclone to control the temperature and further oxidize the iron particles. In further embodiments, the pilot combustible stream is provided for less than 1 minute. In yet another embodiment, the step of recovering the oxidized iron particles includes controlling the temperature of a wall of the cyclone. In yet another embodiment, the iron particles have a size between 1 and 100 μm. In an additional embodiment, the oxidized iron particles are at least 60% by weight magnetite (Fe3O4). In yet an additional embodiment, the oxidized iron particles include less than 1% particles having a size smaller than 8 μm.

[0009] Many additional features and combinations thereof relating to improvements of the present invention will become apparent to those of skill in the art upon reading this disclosure. [Brief description of the drawings]

[0010] [Figure 1] FIG. 2 is a schematic cross-sectional view of a combustion zone of a system according to one embodiment of the present disclosure. [Diagram 2] 2 is a schematic radial cross-section of a multi-annular tube according to the embodiment of FIG. 1 taken across line AA. [Diagram 3] FIG. 2 is a schematic cross-sectional view of a pre-combustion section of a system according to one embodiment of the present disclosure. [Figure 4A] FIG. 2 is a schematic perspective view of a bottom plate of a swirl generator according to one embodiment of the present disclosure. [Figure 4B] FIG. 2 is a schematic perspective view of a swirl generator top plate according to one embodiment of the present disclosure. [Figure 4C] 4B is a photograph of a swirl generator having the bottom plate of FIG. 4A and the top plate of FIG. 4B. [Figure 4D] 1 is a photograph of a swirl generator according to one embodiment of the present disclosure. [Diagram 5] FIG. 2 is a schematic perspective longitudinal cross-sectional view of a combustion zone of a system according to one embodiment of the present disclosure. [Figure 6A] FIG. 2 is a schematic cross-sectional view of a cyclone inlet zone of a system according to one embodiment of the present disclosure. [Figure 6B] FIG. 2 is a schematic cross-sectional view of a cyclone inlet zone of a system according to one embodiment of the present disclosure. [Figure 7] FIG. 2 is a schematic cross-sectional view of a combustion zone of a system according to one embodiment of the present disclosure. [Figure 8] FIG. 1 is a schematic cross-sectional view of a cyclone according to one embodiment of the present disclosure. [Figure 9] FIG. 1 is a graph of volume frequency (%) as a function of iron particle size (solid line = iron particles produced by Tata Steel Ltd, dashed line = produced oxidized iron particles). [Figure 10] FIG. 2 is a graph showing the volume frequency (%) as a function of iron particle size (solid line = iron particles from TLS Technik Spezialpulver Gmbh™, dashed line = produced oxidized iron particles). [Figure 11] Scanning electron microscope images of iron particles. Scanning electron microscope images of iron particles from Tata Steel Ltd before combustion (FIGS. 11A and B). Scanning electron microscope images of iron particles from Tata Steel Ltd after combustion, i.e. oxidized (FIGS. 11C and D). [Figure 12]Scanning electron microscope images of iron particles. Scanning electron microscope image of iron particles from TLS Technik Spezialpulver Gmbh™ before combustion (FIG. 12A). Scanning electron microscope image of iron particles from TLS Technik Spezialpulver Gmbh™ after combustion, i.e. oxidized (FIG. 12B). [Figure 13] 1 is a comparative scanning electron micrograph obtained by oxidizing iron particles from BASF SE in a laminar flame. [Figure 14] 1 is a graph showing the amount of heat (kW) gained by water from a turbulent iron flame as a function of hot water flow rate (gpm). [Figure 15] 1 is a graph showing turbulent iron flame temperature (in K) for a series of measurements (data set). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] A continuous combustion system fueled by iron particles is provided that can achieve a self-sustaining turbulent iron flame without the addition of combustibles (e.g., hydrocarbon fuels) other than to ignite the flame. The combustion of iron, like other fuels, produces heat that can be captured to generate electricity or hot water, or used as is. Iron as a fuel source is advantageous because it is non-toxic, non-explosive, and safe to transport and store. In contrast to fossil fuels and coal, where the emission of CO2 to the atmosphere is of major concern, the combustion of iron does not produce carbon pollutants such as CO and CO2. Thus, a sustainable, virtually carbon-free power system is achieved. Furthermore, unlike other fuel sources, the exhaust gases of an iron burner are free of iron oxide nanoparticles, CO2, and nitrogen oxides (NO2) in the resulting products. xThe iron oxide particles have a negligible content of NO and NO2 species, so that they can be released into the atmosphere without major environmental concerns. Although there is a limited amount of available iron, it is an abundant element that can be advantageously recycled and reused. Thus, the sustainability of the systems and methods of the present invention is further supported by capturing the oxidized iron particles so that they can be reduced (recycled) back to their original unoxidized form. High recovery or capture efficiencies of iron oxide particles are achieved, greater than 97%, and in some embodiments greater than 99%. This recovery is done by cyclones, which may eliminate the need for consumables such as filters, wet scrubbers, etc. Recycling the burned iron powder using clean primary energy can provide a continuous cycle of recyclable fuel that does not capture CO2 into the atmosphere, replacing other unsustainable fuels.

[0012] The term "iron particles" as used herein refers to micron-sized particles of iron, e.g., iron powder. In some embodiments, the iron particles have a diameter between 1 μm and 500 μm, between 1 μm and 200 μm, between 1 μm and 100 μm, between 10 μm and 100 μm, between 20 μm and 100 μm, between 10 μm and 50 μm, or between 20 μm and 50 μm. An advantage of the iron particles of the present disclosure is that they do not need to have a uniform particle distribution.

[0013] The terms "combustion," "combust," "combusted," and the like, in the context of the combustion of iron particles, refer to the oxidation reactions that iron undergoes in a turbulent iron flame. As used herein, the expression "turbulent iron flame" refers to a flame sustained by the combustion of iron particles in a turbulent flow profile. Thus, a turbulent iron flame does not have a fixed shape or a fixed size, in contrast to a laminar flame, which is generally characterized by a fixed conical shape.

[0014] The terms "stabilize," "stabilized," "sustained," "self-sustaining," and the like, in the context of a turbulent iron flame, mean that the turbulent iron flame is capable of burning and continuing to burn by burning iron particles without the addition of an external stimulus, such as a combustible, such as a hydrocarbon.

[0015] It is known that metals burning at flame temperatures above their boiling point produce lifted diffusion flames, which lead to the production of metal oxides with sizes on the order of nanometers. If the flame temperature is well below the boiling point, it is hypothesized that the combustion process can occur in a purely heterogeneous combustion mode, producing metal oxide combustion products with sizes larger than the fuel particles.

[0016] This disclosure has experimentally demonstrated heterogeneous combustion of iron with only a small amount of nanometer-scale iron oxide. Iron particles start to burn in the solid phase, producing solid iron oxide. Above about 1650 K, the combustion products can include liquid ferrous oxide (FeO), i.e., wustite. Above the melting point of iron (1811 K), iron particles burn as liquid droplets. Above the melting point of iron (III) or hematite, 1838 K, and above the melting point of iron (II, III) oxide or magnetite, 1870 K, iron droplets burn to produce liquid iron oxide. At higher combustion temperatures, specifically above the boiling point of iron, 3134 K, and above 2500 K, it has been observed that iron combustion occurs partially in the gas phase, producing nanometer-scale iron oxide (hematite, Fe2O3). This disclosure demonstrates efficient combustion of iron particles with limited production of iron oxide nanoparticles. In some embodiments, the present disclosure does not produce iron oxide nanoparticles from the combustion of iron.

[0017] The combustion reaction of iron in a turbulent iron flame 51 which produces magnetite (Fe3O4) as a product is detailed below: Reaction: 3Fe+2O2→Fe3O4 data: Fe: 55.85g / mol Fe3O4: 231.55g / mol

[0018]

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[0019]

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[0020]

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[0021] FIG. 1 shows a schematic longitudinal cross-sectional view of a combustion zone 1 of a continuous combustion system according to the present disclosure. The multi-annular combustion tube can comprise three or more combustion tubes. In one embodiment, as shown in FIG. 1, the multi-annular combustion tube is comprised of a first tube 10, a second tube 20, and a third tube 30. The first tube 10 is the innermost and defines a first passageway that provides a primary air flow 11. The primary air flow 11 has iron particles 12 suspended in air 13.

[0022] Solid iron particles are dense and will settle rapidly in a quiescent environment. A minimum level of laminar or turbulent air / gas flow is required to keep the iron particles suspended. Furthermore, the combustion of solid iron particles depends on their size distribution and morphology. The flow rate required to suspend large iron particles can exceed the laminar burning velocity of an iron flame. Thus, turbulence is essential to stabilize high load flows or flows of large particles. In general, turbulent flames are suitable for scalability (including large industrial scale), whereas laminar flames are not. Furthermore, the turbulence of a turbulent iron flame induces mixing, thereby enhancing combustion efficiency. This is in contrast to laminar flows, where only limited mixing occurs. The use of laminar flames is limited to laboratory or domestic scale.

[0023] To create a self-sustaining flame, the iron particles need to be dispersed with an appropriate iron mass flow rate. This can be achieved by sizing the central iron pipe with a powder dispersion device connected to the iron fuel storage compartment with iron particle silo, along with additional air flow provided from an adjustable compressed air system to form the primary air flow 11. The primary air flow 11 helps to control the velocity and concentration of the iron and air mixture. The appropriate iron mass flow rate depends on the scale of the system. In one embodiment, the appropriate iron mass flow rate is between 1 g / s and 2 g / s, resulting in approximately 10 kW heat output for the final reaction product, which is magnetite.

[0024] Figure 2 shows a radial cross section of the multi-annular tube of Figure 1 taken across line AA. In this embodiment, the multi-annular tube 2 is a triple concentric tube made up of a first tube 10, a second tube 20 and a third tube 30. Referring now to Figure 3, the pre-combustion section 3 is shown. The multi-annular tube may be supported by a bottom plate 14. The second tube 20 may comprise a combustible tube 20a optionally supported by a second tube holder 25 and optionally supported by a combustible tube holder 26.

[0025] The second passage is defined in an inner annular space between the first tube 10 and the second tube 20. The inner annular space is supplied with a secondary air flow 21, which includes combustibles as fuel, igniting air, and burning air. As used herein, the term "combustibles" refers to species that can be used for flame ignition by spark stimulation. Combustibles can be hydrocarbon fuels, hydrogen, or any suitable gas that can be ignited to obtain a flame, such as methane, ethane, propane, or butane. In some embodiments, as shown in FIG. 1, the secondary air flow 21 is a mixture of a combustible stream 21a and an air stream 21b. In further embodiments, the second tube 20 has a flame arrester mechanism 22, for example, flame arrester beads, such as ceramic beads, enclosed in a porous housing 23 (e.g., a mesh). The second tube 20 includes an ignition point for a spark generator to generate a pilot flame 24. The flame arrester mechanism 22 avoids flashback. Flashback is when a flame burns too fast compared to the incoming flow and flows back toward the source of the flow, leading to an explosion. The section of the second tube 20 where the combustibles and air mix may be equipped with a pressure relief valve 27 that opens automatically if pressure increases. The established pilot flame 24 is used to ignite the incoming iron particles 12 to create a turbulent iron flame 51. Once the turbulent iron flame 51 is ignited and stabilized, the pilot flame 24 is extinguished. The pilot flame 24 may be extinguished by closing the combustibles shutoff valve.

[0026] Stabilization of the turbulent iron flame 51 is achieved in part by a tertiary air flow 31. The tertiary air flow 31 is fed into an outer annular space defined between the second tube 20 and the third tube 30. The third tube includes a swirl generator 32 and a side air inlet 33. The air flow 31 passes through the swirl generator 32 and becomes a turbulent swirling flow. In some embodiments, as shown in FIG. 3, the swirl generator 32 includes a base 32a, a bottom plate 32b, and a top plate 32c. FIG. 4A shows an exemplary embodiment of the bottom plate 32b, and FIG. 4B shows an exemplary embodiment of the top plate 32c. Both the top plate 32b and the bottom plate 32c have an O-ring 34 and a prismatic projection 35. The prismatic projections 35 of the bottom plate 32b and the top plate 32c can be arranged to interlock to form radial channels, interlock to form tangential channels, or the projections 35 can be arranged so that they do not touch, forming a series of both radial and tangential channels. Figures 4C and 4D show an exemplary assembly of the bottom plate 32b and the top plate 32c to form the swirl generator 32. The various configurations possible by moving the plates of the swirl generator 32 relative to each other allow the formation of various optimizable gaseous swirls. Thus, this exemplary swirl generator 32 is suitable for swirl optimization at a laboratory scale, although other swirl generators 32, for example at an industrial scale, are also contemplated by the present disclosure. However, the present disclosure is not limited to the exemplary swirl device of Figures 4A-4D, but includes other swirl shapes such as fixed vane type swirl devices and variable vane type swirl devices.

[0027] FIG. 5 is a perspective cross-sectional view of the combustion zone 5. A diverging nozzle 40 is disposed at the exit of the multi-annular combustion tube. The diverging nozzle 40, also called a quarl, has a shape (e.g., a diverging diameter) that diverges downstream from the exit of the multi-annular combustion tube. In some embodiments, the diverging nozzle 40 is a frustum of a cone, as can be seen in FIG. 5. In a further embodiment, the diverging nozzle is a frustum of a right circular cone. The term "frustum" should be understood as known in the art and can be defined as the base of a solid cone or pyramid formed by truncating the top by a plane parallel to the bottom, or a solid portion intersecting between two substantially parallel planes. The diverging nozzle 40 helps to stabilize the flame and direct the flow into the combustion furnace 50.

[0028] The challenge overcome by the inventors of this disclosure was to ensure adequate flow velocity within the combustion furnace 50 so that the turbulent iron flame 51 is stable and does not blow away or flash back towards the inlet. The use of swirl generators 32 creates a central recirculation zone 53, creating a backflow near the diverging nozzle 40. This helps stabilize the flame, but the particles have too much inertia to remain in the recirculation zone 53, so even a jet with too much velocity can blow the flame away. Currently, there is limited data in the literature on the turbulent burning velocity of iron (or any metal), but the inventors have been successful in designing a system to operate a stable turbulent iron flame. The combustion furnace 50 helps contain, stabilize, and guide the turbulent iron flame 51 so that it can capture its heat. To transfer its radiation and capture heat, the walls 52 of the combustion furnace can be made of fused quartz or other suitable material. The combustion chamber is in fluid communication with the diverging nozzle 40 at the inlet and is fluidly connected. The combustion chamber has an outlet 54 opposite the inlet. In some embodiments, the turbulent iron flame 51 extends throughout the combustion chamber 50 such that the secondary oxidation zone of the turbulent iron flame 51 briefly reaches the outlet 54. Optionally, the quaternary air stream can be directed to the secondary oxidation zone 56 through an air gap 55 between the outlet 54 and a cyclone inlet 61. Alternatively, the quaternary air stream may be supplied by a pressurized air stream that enters the combustion chamber at various locations. Thus, in some embodiments, the secondary oxidation zone 56 extends to the cyclone inlet 61. As can be seen in FIG. 6A, in some embodiments, the cyclone inlet 61 may be elongated (e.g., straight tube, corrugated tube, serpentine tube, etc.) to extend to the secondary oxidation zone 56. FIG. 6A shows the quaternary air stream 57 entering the air gap 55. Due to the suction at the cyclone 60, the pressure at the cyclone inlet 61 is less than the pressure at the furnace outlet 54. Thus, the quaternary air stream 57 entering through the air gap is drawn into the cyclone inlet 61 and does not substantially disrupt the flow within the furnace 50. Without being bound by theory, the quaternary air stream 57 allows for temperature control which is important to minimize nanoparticle formation and prevent NOx formation. Alternatively, the quaternary air stream 57 may be supplied by the compressed air stream entering the combustion chamber 50 at various locations, as shown in FIG. 6B.In such an embodiment, the air gap may be eliminated or sealed and the air injection port 58 of the combustion chamber 50 may be used instead. Figure 6B shows a combustion chamber made from several steel sections with an optional sight glass 59a, air injection port 58, and optional air sampling port 59b. This combustion chamber can be mounted so that there is no air gap between the inlet of the cyclone duct and the end of the combustion chamber. Air injected into the quaternary injection port can be up to 1200 cm. 3 / sec and NOx measurements were below 3 ppm for all test conditions. The embodiments of Figures 6A and 6B provide similar flame stability and characteristics as well as NOx performance.

[0029] 7 and 8, downstream of the furnace outlet 54 is a cyclone 60 having a cyclone inlet 61, a gas outlet 62, and a particle outlet 63. As can be seen in FIG. 7 showing the combustion zone 7, the cyclone inlet 61 is in fluid communication with the combustion furnace 50. In some embodiments, the cyclone inlet 61 and the furnace outlet 54 are physically separated to provide an air gap through which additional airflow (relative to the burner flow rate) enters the cyclone inlet 61. This has the effect of lowering the temperature of the particles and preventing the generation of nanometer-sized particles. The cyclone receives hot, abrasive oxidized iron particles. Cyclones are typically designed to collect dry, cool, larger, non-abrasive particles. In general, the higher the mass flow rate (and velocity) of the fluid passing through the cyclone, the higher its collection efficiency. Thus, most prior art cyclones are designed to produce the highest possible flow rate and flow velocity for a given application. Commercially available cyclones use relatively high flow rates to remove more waste media and generally have low efficiency because the captured material is waste and not needed for another purpose. In contrast, the cyclones of the present disclosure are designed to keep flow rates to a minimum so as not to disturb the flame near the upstream of the cyclone and to keep the exhaust as hot as possible. In some embodiments, the cyclone suction velocity is less than 100 CFM, less than 80 CFM, or less than 60 CFM, less than 40 CFM, less than 20 CFM, or less than 10 CFM.

[0030] Another reason for the relatively low suction volume of the cyclone is to control the flow of an additional air stream, e.g., cold ambient air, and therefore the mixture temperature. In other words, the cyclone suctions air from the surroundings in addition to the burner exhaust and mixes the two. Without wishing to be bound by theory, it is believed that this significantly reduces, and in some embodiments eliminates, the production of NOx and nanoparticles in the combustion products by controlling (in this case lowering) the temperature at the right time. The temperature is maintained high enough to promote a high level of oxidation of the fuel, if not completely, which not only increases the combustion efficiency of the system but also results in high quality heat products for better heat extraction. In some embodiments, the temperature of the particles in the mixture in the secondary oxidation zone 56 is maintained between about 500 and about 1377°C.

[0031] In some embodiments, the combustion products of the present disclosure include at least 60%, at least 65%, at least 70%, or at least 75% magnetite by weight. The cyclone 60 can capture hot oxidized iron particles with an efficiency of over 99%. Typically, a filter is used to achieve such high efficiency. However, in the present system, a filter is not necessary or desirable because the filter may be damaged due to the capture of powder and the high temperature of the particles. In fact, this would result in additional unnecessary operating costs. However, in some embodiments, a particulate filter is added to the present system for the purpose of verifying the collection efficiency of the cyclone separator.

[0032] Because the products of combustion are solids, radiation is a more significant heat transfer mechanism than in conventional hydrocarbon flames. Radiation energy can be collected by the radiation heat transfer mechanism. Radiation is an electromagnetic wave, does not require a medium to travel through, and is not significantly absorbed by air. Radiation can be absorbed by solids, which increase in temperature. In some embodiments, the combustion furnace 50 can have a transparent fused quartz wall 52 that is connected to an enclosure that surrounds it and captures any incident flame radiation and focuses it toward the heat transfer mechanism. The enclosure wall can be made of stainless steel, for example, polished to a mirror finish, so that all incident radiation is reflected and not absorbed.

[0033] Thus, electricity can be generated from a turbulent iron flame by using a heat engine that uses a temperature gradient to drive a piston and generate electricity, a steam engine combined with a turbine, or a Stirling engine. A Stirling engine requires a water flow to cool the working fluid and produces hot water in addition to electricity. Only the coils of the engine are coated to absorb as much radiation as possible. This way, only the coils of the engine heat up, as the flame radiation does not heat the reflective / non-absorbing surfaces. The coils allow heat transfer completely isolated from the flow of iron particles, which means that the coils of the engine are kept clean, which is a requirement for the operation of a Stirling engine. Furthermore, the flow of the burning particles to the cyclone is advantageously unimpeded despite the heat recovery. Alternatively, electricity can be generated using a steam turbine engine in which the iron burner acts as a boiler. In this case, the heat generated from the turbulent iron flame is captured by a double-walled cast or tube-wound steel combustion chamber, as shown in Figure 7, in which water circulates to absorb heat from the flame. Water 55a flows through a flowmeter 55b and a thermocouple 55c to the coils 56. Water 55a is heated in the coil and becomes hot water / steam 55d which passes through a second thermocouple 55e to power the steam engine.

[0034] The secondary oxidation zone 56 and cyclone 60 are designed to maintain the hot gases coming from the combustion furnace 50 at the desired temperature to promote oxidation, inhibit the formation of nanoparticles and NOx, and improve the heat extraction quality. Multi-stage heat extraction can be achieved through a secondary fluid cooling cycle, for example, a water jacket 64 shown in FIG. 8. The fluid circulates through a water jacket integrated with the cyclone, or a copper heat exchanger in the cyclone body 65. The heat present in the exhaust and particles can be recovered and used for further energy generation, auxiliary heat, or preheating the burner system.

[0035] Without wishing to be bound by theory, it is believed that heat extraction in a cyclone reduces NO from the burning iron particles. x More precisely, temperature control throughout the system keeps the iron particles in combustion mode, preventing the production of NO. x and inhibits the formation of nanometer-sized iron oxides. Some heat is extracted from the hot stream, but at a low enough temperature to inhibit NOx formation, while remaining hot enough to promote complete combustion of the particles. Furthermore, the system advantageously avoids particle sintering throughout the system. Heat extraction in the cyclone is also believed to prevent particle sintering, thus improving collection efficiency. Sintering can also cause the size of the iron oxide product to be larger than the initial iron particles, which would lead to additional steps such as crushing and separation when recycling the iron oxide into iron fuel.

[0036] Achieved NO xSimilar to inhibition, temperature control throughout the system encourages high levels of iron particle combustion while inhibiting the formation of iron oxide nanoparticles. It has been shown previously that combustion of metal particles forms nanoparticles. This can be due to a combination of spontaneous explosion, the combustion regime in which the particles are placed, and other parameters. In the present system, the iron particles are initially burned at a temperature above the melting point of the iron and iron oxide. By controlling the temperature of the mixture, the particles maintain their newly formed spherical shape after melting, but cool rather quickly below their melting point. The temperature is controlled by heat extraction and a quaternary air stream (a relatively cool air stream). This process does not result in microexplosions or gas-phase combustion of the iron or its suboxides, which can lead to the formation of iron oxide nanoparticles. In some embodiments, adding a temperature regulation section (i.e., air gap 55) downstream of the combustion furnace 50 but before the entrance to the cyclone 60 can help control which iron oxides are formed. In some embodiments, the iron oxide particles may even increase in size (compared to the iron particles before combustion) due to their oxidation. Thus, the oxidized iron particles generally have a size similar to that of the iron particles in their pre-combustion state (e.g., 10 to 500 μm.) In some embodiments, the oxidized iron particles contain less than 3 wt.%, less than 2 wt.%, or less than 1 wt.% particles having a diameter smaller than 8 μm.

[0037] A method of combusting iron particles using the system described herein is also provided. The method includes feeding a multi-annular flow in a multi-annular tube to a diverging nozzle and then to a combustion furnace. The multi-annular flow is comprised of a primary air flow, a secondary air flow, and a tertiary air flow. The primary air flow includes suspended iron particles. The secondary air flow is physically separated from and surrounds the primary air flow, as described above. The tertiary air flow is also physically separated from and surrounds the secondary air flow. The tertiary air flow is a turbulent swirling flow generated by a swirl generator. A pilot combustible flow is provided to the secondary air flow along with a spark to ignite a pilot flame. The pilot flame ignites the turbulent iron flame and oxidizes the iron particles. The combustion furnace has a recirculation zone created and maintained by the tertiary air flow. The pilot combustible flow is then stopped. The pilot flame (e.g., methane flame) can be provided for less than 1 minute, less than 30 seconds, for example between 10 and 30 seconds, or between 10 and 20 seconds. The oxidized iron particles are collected using a cyclone. Optionally, an additional air flow is provided to the cyclone from an air gap between the furnace outlet and the cyclone inlet. The additional air flow controls the temperature of the oxidized iron particles, prevents the formation of nanoparticles, and reduces the formation of NOx. Thus, in some embodiments, the step of collecting the oxidized iron particles includes controlling the temperature of the walls of the cyclone. In further embodiments, at least a portion of the iron particles provided to the primary air flow are recycled oxidized iron particles produced by the method. Since the majority of the oxidized iron product is magnetite, which is magnetic, in some embodiments, magnetic separation by a magnetic separator can be performed to further improve the collection efficiency of the method.

[0038] The method advantageously uses ambient air (i.e., about 21 vol.% oxygen) and does not require the addition of oxygen to the air stream. Not requiring the addition of oxygen to the air reduces operational procedures, complexity, and costs. Indeed, it is an advantage of the method to achieve oxygen-starved combustion to reduce the formation of nitrogen oxide species and promote heterogeneous combustion of the iron. Oxygen-starved combustion can be achieved by optimizing the iron particle flow rate in the primary air stream and / or by optimizing one or more of the primary, secondary, and tertiary air streams. EXAMPLES

[0039] The primary air flow was generated using a powder feeder purchased from Powder & Surface Gmbh. Iron particles were obtained from Tata Steel Ltd and TLS Technik Spezialpulver Gmbh™. The primary air flow was characterized by 1.33 g / s iron and 415 cc / s air with a central equivalence ratio of approximately 4. The tertiary air flow was 2460 cc / s with an overall equivalence ratio of 0.6. The theoretical heat output of the generated flame was 8.9 kW. The methane ignition flame had a secondary air flow of 370 cc / s with an equivalence ratio of 1, i.e. 35 cc / s methane and 335 cc / s air with a heat output of 1.15 kW. The type of swirl device selected and its shape were designed with the aim of stabilizing the iron flame (Figure 4C and Figure 4D). The combustion chamber was also designed for this purpose and sized for a 10 kW iron flame. The disclosed design was able to produce a stable flame that remained self-sustaining for over 20 minutes without any other heat or fuel source (such as methane). The system had a stratified burner with a primary air stream rich in iron particles and leaner outer air streams (secondary and tertiary air streams). The key to this design was to reduce NO by starving the core of oxygen. x The goal was to achieve high combustion efficiency by burning the remaining iron particles leaner at lower temperatures in the surrounding flow while minimizing the formation of NO. xMeasurements of NO were made with sampling probes at various positions on the burner and in the cyclone duct (the burner in Figure 6B was used). x Little or no NO was observed (Table 1). x The measured values ​​are for the ultra-low NOx of the burner. x The oxygen concentration indicates how depleted the air stream is in oxygen, i.e. how much oxygen has been used for the combustion of the iron particles. The size distribution of the iron particles and oxidized iron particles is shown in Figure 9 and Table 2 for the iron particles of Tata Steel Ltd and in Figure 10 and Table 3 for the iron particles of TLS Technik Spezialpulver Gmbh™ (TLS Technik GmbH & Co.™ Spezialpulver KG™, now owned by ECKART TLS GmbH™).

[0040] [Table 1]

[0041] [Table 2]

[0042] [Table 3]

[0043] A conventional high efficiency particulate air (HEPA) filter was installed downstream of the cyclone to capture the oxidized iron particles not separated by the cyclone. This filter allowed the amount of oxidized iron particles leaving the cyclone to be quantified by weighing the filter, first when "empty" and then after cyclone operation. After separating more than 10 kg of oxidized iron particles, the filter's mass increased by less than 10 g, meaning that the cyclone retained more than 99% of the oxidized iron particles dispersed in the system.

[0044] X-ray diffraction, shown in Table 4, gives information about the specific elements / phases in the samples. Four different powders were tested using XRD analysis: unburned pure iron, pure iron that underwent slow oxidation in a thermogravimetric analyzer (TGA), iron oxide particles resulting from combustion in a turbulent burner, and finally, oxidized iron particles resulting from combustion in a turbulent burner and then underwent slow oxidation in a TGA. XRD results of the iron burner combustion products show that very little pure iron (2.7% Fe) is present, and the overall combustion efficiency of the burner was greater than 96%. In addition, the products were mostly magnetite (77.1% Fe3O4), with wustite (9.5% FeO) and hematite (10.7% Fe2O3) present.

[0045] [Table 4]

[0046] Examination of the combustion products of the iron flame showed that its composition was predominantly magnetite, with some hematite present, and a small portion present as wustite. By controlling the quaternary air flow, and therefore the mixture temperature, the composition of the products and the level of combustion (oxidation) of the fuel particles are controlled. TGA and XRD results are consistent with this theory, with both pure iron and magnetite being oxidized to hematite after slow low temperature oxidation in TGA and oxidation of the powder at 800°C. In addition, as shown in Figures 11C and 11D and 12B, scanning electron microscopy (SEM) of the burned powder showed that the morphology was mostly spherical, indicating that the iron particles rose above their melting point (1538°C) and became spherical during cooling. The measured temperature of the burned iron droplets was approximately 1805°C using spectroscopy (Figure 15). Particle size analysis showed that the particle size distribution after combustion was larger, indicating that micron-sized particles were formed rather than nanometer-sized oxides. This was independent of the initial iron particle, with both irregular sponge iron and gas-atomized spherical powders showing similar results. This was supported by scanning electron microscope (SEM) images, which showed no evidence of nanometer-sized oxides (Figures 11A-11D and 12A-12B). In contrast, comparative combustion in a laminar flame showed the presence of nano-oxides and particle explosion (Figure 13).

[0047] Various methods of heat extraction for the combustion chamber were tested, taking advantage of the radiative properties of the metal flame and without disturbing the turbulent iron flame. Experiments with steel pipe, copper tubing, and water demonstrated that heat could be extracted from the combustion chamber, as shown in Figure 7. Figure 14 demonstrates that, on average, 3 kW of heat was extracted from a turbulent iron flame of approximately 9 kW with a 12-inch cross section. It is important to note that the heat was extracted without changing the combustion characteristics of the flame and products. In addition, particle temperature measurements using spectroscopy were able to be made. Measured flame temperatures from over 21 experiments are shown in Figure 15, demonstrating the consistency of the burner's performance and the fact that the average iron combustion temperature (2080 K) is well above the melting points of iron (1811 K), wustite (FeO, 1650 K), magnetite (Fe3O4, 1870 K), and hematite (Fe2O3, 1838 K), but well below the boiling point of iron (3135 K).

Claims

1. A continuous combustion system for iron particles, comprising: A multi-annular combustion tube having an inlet and an outlet, and defining at least three different passages in cross-section from the inlet to the outlet: A first tube defining a first passage for supplying a primary air flow in which iron particles are suspended, located innermost; A second tube located outside the first tube, defining a second passage which is an inner annular space defined between the first tube and the second tube, the inner annular space supplying a secondary air flow and a pilot combustible flow, and further comprising a spark generator ignition point; and A third tube located outside the second tube, defining a third passage which is an outer annular space defined between the second tube and the third tube, the outer annular space comprising a swirl generator and supplying a tertiary air flow; The first tube, the second tube and the third tube are nested at predetermined positions within the multi-annular combustion tube, the multi-annular combustion tube; A diverging nozzle at the outlet of the multi-annular combustion tube; A combustion furnace including a furnace outlet which is in fluid communication with the diverging nozzle at the furnace inlet and is hydraulically connected on the opposite side of the furnace inlet for generating and stabilizing a turbulent iron flame for burning the iron particles to produce oxidized iron particles; A cyclone having a cyclone inlet, a gas outlet and a particle outlet, the cyclone inlet being in fluid communication with the furnace outlet A system comprising.

2. The system according to claim 1, wherein an air gap for supplying a quaternary air flow to the cyclone inlet is defined between the cyclone inlet and the furnace outlet.

3. The system according to claim 1, further comprising a quaternary flow supplied into the combustion furnace by a pressurized air flow through an injection port in the combustion furnace.

4. The system according to any one of claims 1 to 3, wherein the multi-annular combustion tube is a triple concentric tube.

5. The system according to claim 1, further comprising a filter for capturing the oxidized iron particles exiting from the cyclone separator downstream of the cyclone separator.

6. The system according to claim 1, further comprising a magnetic separator downstream of the cyclone separator or incorporated in the cyclone separator.

7. The system according to claim 1, further comprising a temperature control system connected to the cyclone separator.

8. The system according to claim 1, further comprising an energy generation device.

9. The system according to claim 8, wherein the energy generation device is selected from a heat engine, a Stirling engine, or a steam engine.

10. The system according to claim 1, wherein the inner annular space further includes frame arrest beads.

11. The system according to claim 1, further comprising a pressure valve in the inner annular space to release pressure when the pressure rises.

12. The system according to claim 1, further comprising a metal fuel storage section provided with a metal fuel powder silo and a compressed air system connected to the metal fuel powder silo for supplying the primary air flow in which the iron particles are suspended.

13. The system according to claim 1, further comprising a combustible shut-off valve.

14. The system according to claim 1, further comprising a housing that reflects radiation and houses the combustion furnace.

15. A method of burning iron particles, comprising: supplying a multi-annular flow including, through a diverging nozzle, to a combustion furnace, a primary air flow in which the iron particles are suspended, a secondary air flow physically separated from the primary air flow and surrounding the primary air flow, and a tertiary air flow physically separated from the secondary air flow and surrounding the secondary air flow, which is a turbulent swirl flow; supplying a pilot combustible flow together with a spark for igniting the secondary air flow and a pilot flame; igniting a turbulent iron flame with the pilot flame; stabilizing the turbulent iron flame, burning the iron particles within a reaction zone of the combustion furnace, and generating an air flow containing oxidized iron particles, wherein the combustion furnace has a recirculation zone surrounding the reaction zone that is generated and maintained by the tertiary air flow; stopping the pilot combustible flow; stabilizing the turbulent iron flame; recovering the oxidized iron particles from the air flow with a cyclone including the method.

16. The method according to claim 15, further comprising supplying a quaternary air flow upstream of the cyclone to control the temperature and further oxidize the iron particles.

17. The method according to claim 15 or 16, wherein the pilot combustible flow is supplied for less than 1 minute.

18. The method according to claim 15, wherein the step of recovering the oxidized iron particles includes controlling the temperature of the wall of the cyclone.

19. ​ The method according to claim 15, wherein the iron particles have a size between 1 and 100 μm.

20. The oxidized iron particles are at least 60% by weight of magnetite (Fe ３ O ４ ), the method according to claim 15.

21. The method according to claim 15, wherein the oxidized iron particles contain less than 1% of particles having a size smaller than 8 μm.

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