CROSS-REFERENCE TO RELATED APPLICATION

[1] The present application claims priority from U.S. provisional patent application 63/239466 filed on September 1 , 2021 and incorporated herein by reference in its entirety.

TECHNICAL FIELD

[2] This disclosure generally relates to the field of metal fuel combustion, more specifically to a system and method for producing a self-sustained turbulent iron flame with iron particles.

BACKGROUND OF THE ART

[3] The supply of fossil fuels is limited, and the transition towards 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, a renewable solution for the storage and transportation of energy remains a challenge. Metal as a solid fuel has been investigated as a potential replacement for fossil fuels, such as coal. Under the appropriate conditions it is possible to combust a metal, oxidizing it and releasing its energy. Indeed, metals generally have a high energy density, and compared to coal are denser and heavier. Unfortunately, the current efforts towards using metal particles (e.g. aluminum or iron particles) as a solid fuel have stalled because of scaling limitations and a sustainability challenge. The sustainability challenge for metal particle combustion stems in part from the necessity to continuously supply a hydrocarbon fuel to sustain the metal flame. This problem was tackled by CN111853762 (henceforth 762) which describes an aluminum flame igniting with methane and a mixture of air and oxygen. 762 describes a burner with a plurality of micro-holes having a diameter of 0.8 mm that stabilize the aluminum flame. 762 reports using a flat flame burner to obtain the stabilized laminar aluminum flame that burns without the continuous addition of methane. Accordingly, although a stabilized flame was obtained without a hydrocarbon feed, the scalability of a flat flame burner is limited. Furthermore alternative metals to aluminum including iron are mentioned in 762, however only aluminum combustion is exemplified. Accordingly, improvements in the efficiency, sustainability and scalability of metal fuel reactors are required.

SUMMARY [4] In one aspect, there is provided a continuous combustion system for iron particles comprising: 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; the multi-annular tube comprising: a first tube that is innermost, the first tube defining a first passage providing a primary air flow wherein the iron particles are suspended in the primary air flow; a second tube, outside the first tube defining a second passage that is an inner annular space defined between the first tube and the second tube, wherein the inner annular space provides a secondary air flow and a pilot combustible flow, the inner annular space further comprises an ignition point of a spark generator, and a third tube, the third tube positioned outside the second tube defining a third passage that is an outer annular space defined between the second tube and the third tube, wherein the outer annular space comprises a swirl generator and provides a tertiary air flow; the first tube, the second tube and the third tube nested in position within the multi-annular combustion tube; a divergent nozzle at the outlet of the multi-annular combustion tube; a combustion reactor, comprising a reactor outlet opposite a reactor inlet, in fluid communication and hydraulically connected with the divergent nozzle at the reactor inlet, for the generation and stabilization of a turbulent iron flame that burns the iron particles and produces oxidized iron particles; and a cyclone having a cyclonic inlet, a gas outlet and a particle outlet, wherein the cyclonic inlet is in fluid communication with the reactor outlet.

[5] In some embodiments, the system further comprises an air gap that provides a quaternary air flow into the cyclonic inlet is defined between the cyclonic inlet and the reactor outlet. In some embodiments, the system further comprises a quaternary flow provided in the combustion reactor by a pressurized air flow through injection ports in the combustion reactor. In further embodiments, the multi-annular combustion tube is a triple concentric tube. In yet further embodiments, the system further comprises a filter downstream of the cyclonic separator to capture the oxidized iron particles that escape the cyclonic separator. In still further embodiments, the system further comprises a magnetic separator downstream of or incorporated in the cyclonic separator. In yet still further embodiments, the system further comprises a temperature controlling system coupled to the cyclonic separator. In additional embodiments, the system further comprises an energy generator. In yet additional embodiments, the energy generator is selected from a heat engine, a Stirling engine or a steam engine. In still additional embodiments, the inner annular space further comprises flame arrestor beads. In yet still additional embodiments, the system further comprises pressure valves in the inner annular space to relieve the pressure in case of pressure build-up. 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 providing the primary air flow with the iron particles suspended. In further embodiments, the system further comprises a combustible shut-off valve. In yet further embodiments, the system further comprises an enclosure that reflects radiation, the enclosure housing the combustion reactor.

[6] In a further aspect, there is provided a method of burning iron particles, the method comprising: providing multi-annular flow to a combustion reactor through a divergent nozzle, the multiannular flow comprising: a primary air flow wherein the iron particles are suspended in the primary air flow, a secondary air flow physically separated from the primary air flow, wherein the primary air flow is enveloped by the secondary air flow, and a tertiary air flow physically separated from the secondary air, wherein the secondary air flow is enveloped by the tertiary air flow, and wherein the tertiary air flow is a turbulent swirling flow; providing a pilot combustible flow with the secondary air flow and a spark igniting a pilot flame; igniting a turbulent iron flame with the pilot flame; allowing the turbulent iron flame to stabilize and the iron particles to burn in a reaction zone of the combustion reactor producing an air flow comprising oxidized iron particles, wherein the combustion reactor has a recirculation zone surrounding the reaction zone generated and sustained by the tertiary air flow; stopping the pilot combustible flow; stabilizing an iron-air flame without any combustible pilot flow; and recovering the oxidized iron particles from the air flow with a cyclone.

[7] In some embodiments, the method further comprises providing a quaternary air flow upstream of the cyclone to control the temperature and further oxidize the iron particles. In further embodiments, the pilot combustible flow is provided for less than 1 minute. In yet further embodiments, the step of recovering the oxidized iron particles includes controlling the temperature of the walls of the cyclone. In still further embodiments, the iron particles have a size of between 1 and 100 pm. In additional embodiments, the oxidized iron particles are at least 60 % by weight magnetite (Fe ₃ O ₄ ). In yet additional embodiments, the oxidized iron particles comprise less than 1 % of particles having a size of less than 8 pm.

[8] Many further features and combinations thereof concerning the present improvements will appear to those skilled in the art following a reading of the instant disclosure.

DESCRIPTION OF THE DRAWINGS

[9] FIG. 1 is a schematic longitudinal cross section view of a combustion zone of a system according to an embodiment of the present disclosure;

[10] FIG. 2 is a schematic radial cross section view across line A-A of the multi-annular tube according to the embodiment of FIG. 1 ;

[11] FIG. 3 is a schematic longitudinal cross section view of a pre-combustion section of a system according to an embodiment of the present disclosure;

[12] FIG. 4A is a schematic perspective view of a bottom plate of a swirl generator according to an embodiment of the present disclosure; [13] FIG. 4B is a schematic perspective view of a top plate of a swirl generator according to an embodiment of the present disclosure;

[14] FIG. 4C is a photograph of a swirl generator with the bottom plate of FIG. 4A and the top plate of FIG. 4B;

[15] FIG. 4D is a photograph of a swirl generator according to an embodiment of the present disclosure;

[16] FIG. 5 is a schematic perspective longitudinal cross section view of a combustion zone of a system according to an embodiment of the present disclosure;

[17] FIG. 6 is a schematic cross section view of a cyclonic inlet zone of a system according to an embodiment of the present disclosure;

[18] FIG. 7 is a schematic longitudinal cross section view of a combustion zone of a system according to an embodiment of the present disclosure;

[19] FIG. 8 is a schematic longitudinal cross section view of a cyclone according to an embodiment of the present disclosure;

[20] FIG. 9 is a graph of volume frequency (%) as a function of the iron particle size (solid line = iron particles from Tata Steel Ltd and dashed line = oxidized iron particles produced);

[21] FIG. 10 is a graph showing the volume frequency (%) as a function of the particle size (solid line = iron particles from TLS Technik Spezialpulver Gmbh™ and dashed line = oxidized iron particles produced);

[22] FIGs. 11A-D are scanning electron microscopy images of iron particles from Tata Steel Ltd before combustion (11 A and 11B) and after combustion i.e. oxidized (11C and 11D);

[23] FIGs. 12A-B are scanning electron microscopy images of iron particles from TLS Technik Spezialpulver Gmbh™ before combustion (12A) and after combustion i.e. oxidized (12B);

[24] FIG. 13 is a comparative scanning electron microscopy obtained by oxidizing iron particles from BASF SE in a laminar flame; [25] FIG. 14 is a graph illustrating the heat gained by the water from the turbulent iron flame (in kW) as a function of the flow rate of heated water (gpm); and

[26] FIG. 15 is a graph illustrating the turbulent iron flame temperature (in K) for a series of measurements (data set).

DETAILED DESCRIPTION

[27] There is provided a continuous combustion system for iron particles as a fuel that achieves a sustained turbulent iron flame without the addition of a combustible (e.g. a hydrocarbon fuel) other than to ignite the flame. The combustion of iron, similarly to other fuels, will produce heat which can be captured to produce electricity or heated water, or used as is. Iron as a fuel source is advantageous because iron is non-toxic, non-explosive and safe to transport and store. The combustion of iron does not produce any carbon contaminants such as CO and CO2 in contrast with fossil fuels and coal where the emissions of CO2 are of great concern for the atmosphere. Accordingly, an essentially carbon free power system that is sustainable is achieved. Furthermore, unlike other fuel sources, the gas exhaust of an iron burner can be released into the atmosphere without any significant environmental concerns because the product obtained has a negligible content of iron oxide nanoparticles, CO2, and nitrogen oxide (NOx: NO and NO2) species. Although there is a finite amount of iron available, iron is an abundant element that can advantageously be recycled and reused. Thus, the sustainability of the present system and methods is further supported by capturing the oxidized iron particles so that they can be reduced (recycled) back to their original non-oxidized form. A high recovery or capture efficiency of oxidized iron particles of more than 97%, and in some embodiments more than 99% is achieved. The recovery is performed with a cyclone which can eliminate the need for consumables such as filters, wet scrubbers and the like. By using clean primary energy to recycle the burnt iron powder, a continuous cycle of recyclable fuel that does not introduce CO2 into the atmosphere can be implemented to replace other non-sustainable fuels.

[28] The term “iron particles” as used herein refers to micron size particles of iron, for example an iron powder. In some embodiments, the iron particles have a diameter of between 1 pm and 500 pm, between 1 pm and 200 pm, between 1 pm and 100 pm, between 10 pm and 100 pm, between 20 pm and 100 pm, between 10 pm and 50 pm or between 20 pm and 50 pm. An advantage of the iron particles of the present disclosure is that they do not need to have a uniform particle distribution. [29] The terms “burn”, “burning”, “burnt” and the like in the context of the combustion of iron particles refer to the oxidation reaction that iron undergoes in a turbulent iron flame. The expression “turbulent iron flame” as used herein refers to a flame sustained by the combustion of iron particles in a turbulent flow profile. The turbulent iron flame therefore does not have a constant shape nor a constant size, in contrast with a laminar flame which is generally characterized by a constant conical shape.

[30] The terms “stabilize”, “stabilization”, “sustained”, “self-sustained” and the like in the context of the turbulent iron flame mean that the turbulent iron flame can burn and continue burning by combusting the iron particles without the addition of an external stimuli such a combustible such as a hydrocarbon.

[31] For metals burning with flame temperatures above the boiling point, it is known that a lifted diffusion flame will form that leads to the production of metal oxides of small sizes, on the order of nanometers. If the flame temperature is sufficiently below the boiling point, it is hypothesized that the combustion process can occur in a purely heterogeneous combustion mode producing metal-oxide combustion products that are larger in size than the fuel particles.

[32] The present disclosure has demonstrated experimentally the heterogeneous combustion of iron with negligible production of nanometric iron oxides. The iron particles start to burn in the solid phase, producing solid iron oxides. Above approximately 1650 K, the combustion products can contain ferrous oxide (FeO), or wustite, in a liquid state. Above the iron melting temperature (181 1 K), the iron particles burn as liquid droplets. Above 1838 K, the melting point of iron(lll) or hematite and then above 1870 K, the melting point of iron (II, III) oxide, or magnetite, liquid iron droplets are burned to produce liquid iron oxides. At higher combustion temperatures, specifically above 3134 K which is the boiling point of iron as well as for temperatures above 2500 K, it has been observed that the combustion of iron can occur partially in the vapour phase producing nanometric iron oxides (hematite, Fe2Os). The present disclosure demonstrates efficient combustion of iron particles with a limited production of iron oxide nanoparticles. In some embodiment, the present disclosure does not produce any iron oxide nanoparticles from the combustion of iron.

[33] The combustion reaction of iron in a turbulent iron flame 51 producing magnetite (Fe ₃ O ₄ ) as the product is detailed below:

Reaction: Data:

Fe: 55.85 g/mol

Fe ₃ O ₄ : 231 .55 g/mol

Heat release of reaction:

Mass increase of reaction:

[34] Figure 1 illustrates a longitudinal cross section of a combustion zone 1 of the 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 Figure 1 , the multi-annular combustion tube is composed of a first tube 10, a second tube 20, and a third tube 30. The first tube 10 is innermost and defines a first passage providing a primary air flow 11 . The primary air flow 11 has iron particles 12 suspended in air 13.

[35] Solid iron particles are dense and rapidly settle in a quiescent environment. In order to keep them in suspension, a minimum level of either laminar orturbulent air/gas flow is required. In addition, the combustion of solid iron particles depends on their size distribution and morphology. The flow rate required to suspend larger iron particles can exceed the laminar burning velocity of the iron flame. Therefore, turbulent flow is essential to stabilizing heavily laden or large particle flows. Generally, turbulent flames are suited for scalability (including large industrial scale) while laminar flames are not. In addition, the turbulence in a turbulent iron flame causes mixing which in turn increases the efficiency of the combustion. This is in contrast with a laminar flow where limited mixing occurs. The use of laminar flames is limited to laboratory or domestic scale.

[36] In orderto create a self-sustained flame, a dispersion of iron particles at an appropriate iron mass flow rate is needed. This can be achieved with a powder dispersion device connected to an iron-fuel storage compartment comprising an iron particle silo and by sizing of the central iron pipe along with an additional air flow provided by a compressed air system that can be adjusted to form the primary air flow 11 . The primary air flow 11 helps control the velocity and concentration of the iron-air mixture. The appropriate iron mass flow rate will vary based on the scale of the system. In one embodiment, the appropriate mass flow rate of iron is between 1 g/s to 2 g/s, which yields a thermal power of roughly 10 kW for a final reaction product being magnetite.

[37] Figure 2 shows a radial cross section across line A-A the multi-annular tube of Figure 1. In this embodiment, the multi-annular tube 2 is a triple concentric tube that comprises the first tube 10, the second tube 20, and the third tube 30. Now referring also to Figure 3, the precombustion section 3 is shown. The multi-annulartube can be supported by a base plate 14. The second tube 20 is optionally supported by a secondary tube holder 25 and can comprise a combustible tube 20a that is optionally supported by a combustible tube holder 26.

[38] A second passage is defined in the inner annular space between the first tube 10 and the second tube 20. A secondary air flow 21 is provided in the inner annular space, the secondary air flow 21 comprises a combustible as a fuel and air during ignition and air during combustion. The term “combustible” as used herein refers to a species that can be used to a ignite flame with a spark stimulus. The combustible can be a hydrocarbon fuel, hydrogen or any suitable gas that ignites to obtain a flame, for example methane, ethane, propane or butane. In some embodiments, as illustrated in Figure 1 , the secondary air flow 21 is a mixture of a combustible flow 21 a and an air flow 21 b. In further embodiments, the second tube 20 has a flame arrestor mechanism 22, for example flame arrestor beads such as ceramic beads that are enclosed in a porous housing 23 (e.g. a mesh). The second tube 20 comprises an ignition point of a spark generator to generate a pilot flame 24. The flame arrestor mechanism 22 avoids flashback. Flashback is the phenomenon where a flame burns too quickly compared to the incoming flow and travels back to the source of the flow which can lead to explosion. The section of the second tube 20 where a combustible and air mix, can be equipped with a pressure relief valve 27, which automatically opens if pressure were to build up. The pilot flame 24 that is established is used to ignite the incoming iron particles 12 to produce a turbulent iron flame 51. Once the turbulent iron flame 51 has been ignited and has stabilized, the pilot flame 24 is extinguished. The pilot flame 24 may be extinguished by closing a combustible shut-off valve.

[39] The stabilization of the turbulent iron flame 51 is achieved in part thanks to the tertiary air flow 31. The tertiary air flow 31 is provided in an outer annular space defined between the second tube 20 and the third tube 30. The third tube comprises a swirl generator 32 and a lateral air inlet 33. The air flow 31 passes through the swirl generator 32 to become a turbulent swirling flow. In some embodiments, as shown in Figure 3, the swirl generator 32 comprises a mount 32a, a bottom plate 32b and a top plate 32c. Figure 4A illustrates an exemplary embodiment of a bottom plate 32b and Figure 4B illustrates an exemplary embodiment of a top plate 32c. Both the top plate 32b and the bottom plate 32c have an o-ring 34 and prism shaped protrusions 35. The prism shaped protrusions 35 of the bottom plate 32b and the top plate 32c can be arranged to interlock and form radial channels, can be arranged to interlock and form tangential channels, or can be positioned so as the protrusions 35 are not in contact and form 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 a swirl generator 32. The different configurations possible by moving the plates of the swirl generator 32 relative to each other allow the formation of various optimizable gaseous swirls. Accordingly, this exemplary swirl generator 32 is suitable for optimizing the swirl at a laboratory scale however other swirl generators 32 are contemplated by the present disclosure for example at industrial scale. However, the present disclosure is not limited to the exemplified swirl device of Figures 4A-4D and includes other swirl geometries such as fixed-vane swirlers and variable-vane swirlers.

[40] Figure 5 shows a perspective cross section of the combustion zone 5. A divergent nozzle 40 is positioned at the outlet of the multi-annular combustion tube. The divergent nozzle 40, also known as a quad, has a diverging geometry (for example a diverging diameter) downstream from the outlet of the multi-annular combustion tube. In some embodiments, the divergent nozzle 40 is a truncated cone as can be seen in Figure 5. In further embodiments, the divergent nozzle is a frustum of a right circular cone. The term “frustum” is to be understood as is known in the art, and can be defined as the basal part of a solid cone or pyramid formed by cutting off the top by a plane parallel to the base or the part of a solid intersected between two substantially parallel planes. The divergent nozzle 40 helps stabilize the flame and direct the flow into the combustion reactor 50. [41] A challenge that the inventors of the present disclosure overcame was ensuring appropriate flow velocity in the combustion reactor 50 so that the turbulent iron flame 51 would sit stably and not be blown off or flashback towards the inlet. The use of a swirl generator 32 forms a central recirculation zone 53, which creates a reverse flow near the divergent nozzle 40. This helps stabilize the flame, but a jet with too large a velocity can still blow off a flame as the particles have too much inertia to stay in the recirculation zone 53. There is currently limited data in the literature on turbulent burning velocities of iron (or any metal), however the present inventors successfully designed a system that operates a stable turbulent iron flame. The combustion reactor 50 helps to contain, stabilize and guide the turbulent iron flame 51 so that its heat can be captured. To transmit the radiation and capture the heat, the walls 52 of the combustion reactor can be made of fused quartz or other suitable materials. At the inlet, the combustion chamber is in fluid communication and hydraulically connected with the divergent nozzle 40. 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 reaches transiently the outlet 54. Optionally, a quaternary air flow can be introduced at the secondary oxidation zone 56 through an air gap 55 between the outlet 54 and the cyclonic inlet 61. Alternatively, the quaternary air flow can be provided by a pressurized air flow entering the combustion chamber at various locations. Accordingly in some embodiments, the secondary oxidation zone 56 extends into the cyclonic inlet 61 . As can be seen in Figure 6A, in some embodiments, the cyclonic inlet 61 may be elongated (e.g. linear tube, undulating tube, serpentine tube, etc.) to extend the secondary oxidation zone 56. Figure 6A illustrates the quaternary air flow 57 entering at the air gap 55. Because of the suction at the cyclone 60, the pressure at the cyclonic inlet 61 is smaller than that of the reactor outlet 54. Accordingly, the quaternary air flow 57 entering from the air gap is sucked into the cyclonic inlet 61 and does not substantially disrupt the flow in the reactor 50. Without being bound to theory, the quaternary air flow 57 allows for a temperature control that is critical to minimizing nanoparticle formation and preventing NOx formation. Alternatively, the quaternary air flow 57 is provided by a pressurized air flow entering the combustion chamber 50 at various locations as illustrated in Figure 6B. In such embodiments, the air gap can be eliminated or sealed and air injection ports 58 on the combustion chamber 50 can be used instead. Figure 6B shows a combustion chamber made out of several steel sections with optional viewing windows 59a, air injection ports 58 and optional air sampling ports 59b. This combustion chamber can be fitted such that there is no air gap between the entrance of the cyclone ducting and the end of the combustion chamber. The air injected at the quaternary injection ports was up to 1200 cm ³ /s, and the NOx measurements were below 3 ppm for all test conditions. The embodiments of Figure 6A and Figure 6B provide similar flame stability and characteristics as well as NOx performance.

[42] Turning to Figure 7 and Figure 8, downstream of the reactor outlet 54 is a cyclone 60 having a cyclonic inlet 61 , a gas outlet 62, and a particle outlet 63. As can be seen in Figure 7 which shows the combustion zone 7, the cyclonic inlet 61 is in fluid communication with the combustion reactor 50. In some embodiments, the cyclonic inlet 61 and the reactor outlet 54 are physically separated so as to provide an air gap from which an additional air stream (to the burner flow rates) will flow into the cyclonic inlet 61. This has the effect of reducing the temperature of the particles and impeding the creation of nanometrically sized particles. The cyclone receives hot, abrasive, oxidized iron particles. Cyclones are usually designed to collect, dry, cold, larger, non-abrasive particles. In general, the higher the fluid mass flow (and velocity) through a cyclone, the higher its collection efficiency. Thus most prior art cyclones are designed to develop flow rates and velocities as high as possible for a given application. Commercial cyclones use relatively high flow rates, to remove larger waste media and generally have lower efficiency since the captured material is waste and not needed for another purpose. In contrast, the cyclone of the present disclosure is designed to keep the flow rate at a minimum, so as to not disturb the flame closely upstream of the cyclone and to maintain the exhaust as hot as possible. In some embodiments, the cyclone suction rate 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.

[43] The other reason for the relatively low suction rate of the cyclone is to control the flow of the additional air stream, which is for example cold ambient air, thus controlling the mixture temperature. In other words the cyclone sucks air in from the surrounding, 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 creation of NOx and nanoparticles in the combustion products, by controlling their temperature (in this case reducing it), at the right moment. The temperature is maintained high enough to promote a high, if not complete, level of oxidation of the fuel, which increases the combustion efficiency of the system, as well as yielding high-quality heat products for better heat extraction. In some embodiments, the temperature of the particles in the mixture within the secondary oxidation zone 56 is maintained between about 500 to about 1377°C.

[44] In some embodiments, the products of the combustion of the present disclosure comprise at least 60 %, at least 65 %, at least 70 %, or at least 75% by weight of magnetite. The cyclone 60 can collect the hot oxidized iron particles with more than 99% efficiency. Generally a filter is used to achieve such high efficiency. However, in the present system a filter is not required nor is it desired due to its entrapment of the powder and the high temperature of the particles which could damage the filter. Indeed, this would incur additional unnecessary operational costs. However, in some embodiments a fine particle filter is added to the present system for the purpose of verifying the collection efficiency of the cyclonic separator.

[45] Since the products of combustion are solid bodies, radiation is a more important heat transfer mechanism than in a conventional hydrocarbon flame. The collection of radiation energy can be performed with radiation heat transfer mechanisms. Radiation is an electromagnetic wave that does not need a medium to travel in and is not absorbed by air by any appreciable amount. Radiation can be absorbed by solid bodies, which then increase in temperature. In some embodiments, the combustion reactor 50 can have walls 52 made of transparent fused quartz coupled with an enclosure that surrounds it and captures all the incident radiation of flame and focuses it towards the heat transfer mechanism. The walls of the enclosure can for example be made of stainless steel that is polished to a mirror finish so that all incoming radiation is reflected and not absorbed.

[46] Accordingly, electricity can be generated from the turbulent iron flame by using a heat engine which uses temperature gradients to drive a piston and create electricity, a steam engine coupled to a turbine, or a Stirling engine. The Stirling engine requires a flow of water to cool the working fluid which produces hot water in addition to electricity. Only the coils of the engine are coated such that they absorb the maximum amount of radiation possible. In this manner, only the engine coils will get hot as the flame radiation will not heat reflective/non absorbing surfaces. The coils allow for a heat transfer that is completely separated from the flow of the iron particles, which means that the engine coils will be kept clean. This is a requirement forthe operation of the Stirling engine. Furthermore, the flow of burnt particles into the cyclone is advantageously unimpeded despite the heat collection. Alternatively, power can be generated with a steam turbine engine where the iron burner would act as the boiler. In this case, heat produced from the turbulent iron flame is captured by a double walled, cast or tube wrapped steel combustion chamber within which water is circulated to absorb heat from the flame, as illustrated in Figure 7. Water 55a flows through a flow meter 55b and a thermocouple 55c and then into the coils 56. The water 55a will heat up in the coils and become hot water/steam 55d which then passes through a second thermocouple 55e to then power the steam engine. [47] The secondary oxidation zone 56 and the cyclone 60, are designed to maintain the incoming hot gases from the combustion reactor 50 at a desired temperature to promote oxidation, impede nanometric particles and NOx formation, and to improve heat extraction quality. Multi-stage heat extraction can be done for example through a secondary fluid cooling cycle, e.g. a water jacket 64 as illustrated in Figure 8. The fluid is circulated through a water jacket mated to the cyclone or through copper heat exchangers on the cyclone body 65. Heat present in the exhaust and the particles can be collected and used for further energy generation, auxiliary heat or preheat of the burner system.

[48] Without wishing to be bound by theory, it is believed that heat extraction at the cyclone impedes the creation of NOx from the burning iron particles. More precisely, the temperature control throughout the whole system keeps the iron particles in a burning mode that inhibits the creation of NOx and nanometric iron oxides. Some heat is extracted from the hot flow, but it remains hot enough to promote complete particle combustion while being cold enough to impede NOx formation. In addition, the present system advantageously avoids the sintering of particles throughout the system. Heat extraction at the cyclone is further believed to prevent the sintering of particles and therefore improve the efficiency of collection. Sintering can also lead to an increased size of the iron oxide products compared to the initial iron particles, which would lead to additional steps in recycling the iron oxide to iron fuel such as crushing and separating.

[49] Similarly to the NOx inhibition achieved, the temperature control throughout the system impedes the formation of iron oxide nanoparticles while promoting a high level of iron particle combustion. It has previously been shown that metal particle combustion forms nanoparticles. This is can be due to spontaneous explosions, the combustion regime that the particle is in, and other combinations of parameters. In the present system, the iron particles initially burn at temperatures higher than the iron and iron-oxide melting temperatures. By controlling the temperature of the mixture, the particles keep their newly formed spherical shape following the melting, but are cooled below the melting point rather quickly. The temperature is controlled by the means of heat extraction and the quaternary air stream (a relatively cold air flow). This process does not allow for micro explosions, or vapor-phase combustion of iron or its suboxides, which can lead to the formation of iron oxide nanoparticles. In some embodiments, the addition of a temperature regulation section (i.e. the airgap 55) downstream of the combustion reactor 50 but before the inlet to the cyclone 60, can help control which iron oxides are formed. In some embodiments, the iron oxide particles may even gain in size (compared to pre-burnt iron particles) due to their oxidation. Accordingly, the oxidized iron particles generally have a size similar to the iron particles pre-burned state (e.g. 10 to 500 pm). In some embodiments, the oxidized iron particles comprise less than 3%, less than 2%, or less than 1 % by weight of particles having a diameter smaller than 8 pm.

[50] There is also provided a method of burning iron particles with the system described herein. The method comprises providing the multi-annular flow in the multi-annular tube to the divergent nozzle and then to the combustion reactor. The multi-annular flow comprises the primary air flow, the secondary air flow and the tertiary air flow. The primary air flow comprises suspended iron particles. The secondary air flow is physically separated from the primary air flow and envelops the primary air flow as described above. The tertiary air flow is also physically separated from the secondary air flow and envelops the secondary air flow. The tertiary air flow is a turbulent swirling flow generated by the swirl generator. A pilot combustible flow is provided in the secondary air flow along with a spark to generate a pilot flame. The pilot flame ignites the turbulent iron flame that oxidizes the iron particles. The combustion reactor has a recirculation zone generated and sustained 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 recovered with the cyclone. Optionally, the additional air stream is provided to the cyclone from the airgap between the reactor outlet and the cyclonic inlet. The additional air stream controls the temperature of the oxidized iron particles, preventing nanoparticle formation and reducing NOx formation. Accordingly, in some embodiments, the step of recovering 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 in the primary air flow are recycled oxidized iron particles produced by the present method. Since the majority of the oxidized iron products are magnetite which is magnetic, in some embodiments, a magnetic separation with a magnetic separator can be performed to further improve the collection efficiency of the present methods.

[51] The method advantageously uses atmospheric air (i.e. about 21 vol. % oxygen) and does not require the addition of oxygen to the air stream. Eliminating the need to add oxygen to the air reduces the steps, complexity, and costs of the operation. In fact, it is an advantage of the present method to achieve an oxygen starved combustion in order to mitigate the formation of nitric oxide species and promote heterogeneous combustion of iron. The oxygen starved combustion can be achieved by optimizing the iron particles flow rate in the primary air flow and/or by optimizing the flow of air in one or more of the primary air flow, the secondary air flow and the tertiary air flow.

EXAMPLE

[52] The powder feeder purchased from Powder & Surface Gmbh was used to create the primary air flow. The iron particles were obtained from Tata Steel Ltd and TLS Technik Spezialpulver Gmbh™. The primary air flow was characterized by 1.33 g/s of iron with 415 cc/s of air, yielding a central equivalence ratio of roughly 4. The tertiary air flow rate was 2460 cc/s, yielding an overall equivalence ratio of 0.6. The theoretical thermal power of the produced flame was 8.9 kW. The methane ignition flame had a secondary air flow of 370 cc/s with an equivalence ratio of 1 , meaning 35 cc/s methane and 335 cc/s air with a thermal power of 1 .15 kW. The chosen type of swirl device and the geometry of it were designed for the purpose of stabilizing an iron flame (Figures 4C and 4D). The combustion chamber was also designed for this purpose and sized for a 10 kW iron flame. With the design according to the present disclosure, it was possible to create a stable flame without other heat or fuel sources (such as methane) that remained selfsustained for over 20 minutes. The system included a stratified burner, which had an iron particle rich primary air flow, and leaner outer flows (secondary air flow and tertiary air flow). The point of that design was to minimize NO _X formation by starving the core of oxygen, while still achieving high combustion efficiency by having the rest of the iron particles burn lean at lower temperatures in the surrounding flow. NO _X measurements were taken using a sampling probe at various locations in the burner and cyclone ducting (the burner of Figure 6B was used). Little to no NO _X was observed (Table 1). The NOx measurements below 4 ppm demonstrated the ultra-low NOx capabilities of the burner. The oxygen concentration indicated how depleted the air flow is of oxygen, i.e. the oxygen has been used in the combustion of the iron particles. The particle size distribution of the iron particles and oxidized iron particles is shown in Figure 9 and Table 2 for Tata Steel Ltd iron particles and in Figure 10 and Table 3 for TLS Technik Spezialpulver Gmbh™ (owned by TLS Technik GmbH & Co. ™ Spezialpulver KG™ now ECKART TLS GmbH™) iron particles.

Table 1 : NOx and O2 measurements taken from iron flame, iron particles were provided from TLS Technik Spezialpulver Gmbh™.

Table 2: Size distribution of the iron particles from Tata Steel Ltd and corresponding oxidized iron particles Table 3: Size distribution of the iron particles from TLS Technik Spezialpulver Gmbh™ and corresponding oxidized iron particles

[53] A conventional high efficiency particulate air (HEPA) filter was placed downstream of the cyclone to capture any oxidized iron particles that do not get separated by the cyclone. The filter allowed to quantify the amount of oxidized iron particles that would escape the cyclone by weighing the filter initially when “empty” and then after running the cyclone. After separating more than 10kg of oxidized iron particles, the filter gained less than 10 grams of mass, meaning the cyclone retained more than 99% of the oxidized iron particles dispersed in the system.

[54] The X-ray diffraction shown in Table 4 gives information about the specific elements/phases in a sample. Four different powders were tested with XRD analysis: unburnt pure iron, pure iron that had undergone slow oxidation in a thermogravimetric analyzer (TGA), oxidized iron particles from the combustion in the turbulent burner and lastly oxidized iron particles from the combustion in the turbulent burner that have subsequently undergone slow oxidation in the TGA. The XRD results for the combustion products of the iron burner indicated that there is very little presence of pure iron (2.7% Fe), with a total combustion efficiency of above 96% for the burner. In addition, the products were mostly magnetite (77.1 % FesO^, with the presence of wustite (9.5% FeO) and hematite (10.7% Fe2O3).

Table 4: Results of the XRD analysis in weight percent

[55] Testing of the combustion products of the iron flame indicated that the composition is mainly magnetite, with some presence of hematite and a small fraction present as wustite. By controlling the quaternary air flow and in turn the mixture temperature, the product composition and the combustion (oxidation) level of the fuel particles is controlled. TGA and XRD results are consistent with this theory, as both pure iron and magnetite oxidize to hematite after slow lower temperature oxidation in the TGA, which oxidizes the powders at 800 °C. In addition, as shown in figures 11 C&D and 12B, scanning electron microscopy (SEM) of the burnt powder shows that the morphology is mostly spherical, indicating that the iron particles have been elevated above their melting temperature (1538 °C) and during cooling formed a sphere. The measured temperature of the burning iron droplets is roughly 1805 °C using spectroscopy (Figure 15). Particle size analysis shows that the particle size distribution is larger after combustion, and indicates that micron sized particles are being formed, not nanometric oxides. This is independent of the initial iron particles, with both an irregular sponge iron and a gas-atomized spherical powder showing similar results. This is corroborated by the scanning electron microscopy (SEM) images, which do not show the presence of any nanometric oxides (Figures 11A-D and 12A-B). In contrast, a comparative combustion was performed with a laminar flame which yielded the presence of nano-oxides and particle explosions (Figure 13). [56] Different heat extraction methods for the combustion chamber were tested, taking advantage of the radiant properties of metal flames and without impeding the turbulent iron flame. An experiment with a 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, out of a roughly 9 kW turbulent iron flame in a 12 inch section. It is important to note that the heat was extracted without changing the combustion properties of the flame and products. In addition, it was possible to take particle temperature measurements using spectroscopy. The measured flame temperatures from over 21 experiments are shown in Figure 15, which demonstrates the consistency in the performance of the burner and the fact that the average iron burning temperature of (2080 K) is well above the melting point of iron (181 1 K), wustite (FeO, 1650 K), magnetite (Fe ₃ O ₄ , 1870 K) and hematite (Fe ₂ O ₃ , 1838 K) but well below the boiling point of iron (3135 K).