Method for injecting hot gas into a direct reduction shaft and installation thereof

Abstract

The invention relates to a method for injecting hot gas into a direct reduction shaft, wherein said method comprises the following steps: - introducing gas into at least one device for heating gas, said device for heating gas comprising: o a pipe allowing gas to flow inside of it, o an insert located inside said pipe, said insert being electrically conductive, o an inductor located in the vicinity of said pipe, - feeding energy into said insert using said inductor of said device for heating gas so that said energy fed into the insert is transferred to the gas introduced into said device for heating gas, to heat said gas to a temperature from 800°C to 1100°C, - releasing said hot gas into the direct reduction shaft.

Description

[0001] Method for injecting hot gas into a direct reduction shaft and installation thereof

[0002]

[0001] The present invention relates to a method for injecting hot gas into a direct reduction shaft and a direct reduction of iron installation. More precisely, the present invention relates to a method for injecting hot gas into a direct reduction shaft using induction heating.

[0003]

[0002] In the process of making steel using the direct reduction of iron process, gas needs to be injecting into the direct reduction shaft at temperatures around 1000°C. For this, it needs to be heated beforehand. In classical facilities, the heating of gas is usually done using fuel combustion.

[0004]

[0003] However, fuel combustion causes CO2 emissions and as CO2 emissions need to be reduced in the steel industry, there is a need for an alternative method for heating gas that allows CO2 emissions to be reduced or even suppressed.

[0005]

[0004] It is known from Chinese patent application CN113566044 that a gas can be heated to a temperature of around 1000°C with an electrical method, more precisely, using induction heating. The patent application describes a method for continuously heating a chlorine gas to a temperature of 700 to 1600°C consisting in heating a graphite pipeline using an induction heater and make the chlorine gas flow into the hot pipe.

[0006]

[0005] This method allows the heating of gas without the use of combustion thus reducing CO2 emissions of the process. However, for the usual flow rates of gas needed in the direct reduction of iron process, the implementation of this method is difficult as it would require a massive installation to reach similar temperatures.

[0007]

[0006] The aim of the present invention is therefore to remedy the drawbacks of the prior art by providing a method for injecting hot gas into a direct reduction shaft within a limited space while reducing CO2 emissions.

[0007] For this purpose, a first object of the present invention consists in a method for injecting hot gas into a direct reduction shaft, wherein said method comprises the following steps:

[0008] - introducing gas into at least one device for heating gas, said device for heating gas comprising: o a pipe allowing gas to flow inside of it, o an insert located inside said pipe, said insert being electrically conductive, o an inductor located in the vicinity of said pipe,

[0009] - feeding energy into said insert using said inductor of said device for heating gas so that said energy fed into the insert is transferred to the gas introduced into said device for heating gas, to heat said gas to a temperature from 800°C to 1100°C,

[0010] - releasing said hot gas into the direct reduction shaft.

[0011]

[0008] The method for injecting hot gas into a direct reduction shaft according to the invention may also have the optional features listed below, considered individually or in combination:

[0012] - the gas is heated to a temperature from 850°C to 1050°C,

[0013] - the flow of gas inside the pipe of the device for heating gas has a Reynolds number superior to 2800,

[0014] - a feeding pipe is connected to the exit of the at least one device for heating gas and opens into the direct reduction shaft, said feeding pipe allowing the hot gas exiting said at least one device for heating gas to be released into said direct reduction shaft,

[0015] - the exit of the at least one device for heating gas is open directly into the direct reduction shaft so that said hot gas is released directly into the direct reduction shaft,

[0016] - the heated gas is a reducing gas,

[0017] - the reducing gas is mainly composed of hydrogen,

[0018] - the reducing gas is mainly composed of natural gas and recycled top gas, the method comprises a preliminary step of reforming and so that the resulting reducing gas is heated after said reforming step using said device for heating gas.

[0019]

[0009] A second object of the invention consists in a direct reduction of iron (DRI) installation, wherein said DRI installation comprises:

[0020] - a direct reduction shaft,

[0021] - at least one device for heating gas, said device for heating gas comprising: o means for injecting gas into said device for heating gas, o a pipe allowing the gas injected to flow inside of it, o an insert located inside said pipe, said insert being electrically conductive, o an inductor located in the vicinity of said pipe and able to feed energy into said insert, so that said energy fed into said insert is transferred to the gas flowing into said pipe, wherein said device for heating gas has a power from 1 kW to 50MW,

[0022]

[0010] The DRI installation according to the invention may also have the optional features listed below, considered individually or in combination:

[0023] - the DRI installation further comprises a feeding pipe, said feeding pipe being connected to the exit of the at least one device for heating gas and opening into the direct reduction shaft,

[0024] - the exit of the at least one device for heating gas is open directly into the direct reduction shaft,

[0025] - the insert is designed to have an exchange surface with the gas to be heated that ranges from 0.5m2to 100m2by meter of pipe,

[0026] - the insert is made of one of the following materials: magnetic steel, graphite, tungsten, molybdenum,

[0027] - the inductor is a coil.

[0028]

[0011] Other characteristics and advantages of the invention will be described in greater detail in the following description.

[0012] The invention will be better understood by reading the following description, which is provided purely for purposes of explanation and is in no way intended to be restrictive, with reference to:

[0029]

[0013] Figure 1 , which is a general view of the heating device according to the invention,

[0030]

[0014] Figure 2, which is a cross-sectional view of the interior of the pipe of the heating device according to one embodiment,

[0031]

[0015] Figures 3a and 3b, which are representations of embodiments of the insert of the heating device according to the invention,

[0032]

[0016] Figure 4, which represents a direct reduction shaft,

[0033]

[0017] Figures 5a, 5b and 5c, which are representations of embodiments of the invention,

[0034]

[0018] Figure 6, which is a comparative graph representing the heating of a gas with a device according to the prior art and a device for heating gas according to the invention.

[0035]

[0019] Direct reduction methods consist in obtaining sponge iron from the direct reduction of iron oxide carriers. The direct reduction shaft 5, represented in Fig 4, comprises a reduction zone 7 at top, a transition zone 8 at the middle, and a cooling zone 9 at the cone shape bottom.

[0036]

[0020] Reduction of the iron oxides occurs in the upper section of the direct reduction shaft 5. Iron ores, in form of lump ores and / or pellets, containing around 30% by weight of oxygen are charged to the top of a direct reduction shaft 5 and are allowed to descend, by gravity, through a reducing gas. This reducing gas is entering the direct reduction shaft 5 from the bottom of reduction zone 7 and flows counter-current from the charged iron ores. Oxygen contained in iron ore is removed via the stepwise reduction of iron oxides by the gaseous reductants. Oxidant content of the gas phase is increasing while the gas is moving to the top of the furnace.

[0021] With reference to Figure 1 and 2, the method for injecting hot gas into a direct reduction shaft 5 according to the invention and the direct reduction of iron (DRI) installation according to the invention will be described in detail.

[0022] The method for injecting hot gas into a direct reduction shaft 5 according to the invention comprises first the step of introducing gas into at least one device for heating gas 1. The device for heating gas 1 first comprises a pipe 2 that allows the gas to be heated to flow inside of it. The device for heating gas 1 also comprises an insert 3 located inside the pipe 2 and that is electrically conductive as well as an inductor 4 located in the vicinity of the pipe 2. The second step consists in feeding energy to the insert 3. To achieve this, the inductor 4 creates a magnetic field that induces a current in the insert 3. This energy fed to the insert 3 results in the heating of the insert 3. This energy is then transferred from the hot insert 3 to the gas flowing into the pipe 2 that is thus heated to a temperature from 800°C to 1100°C. The third step consists in releasing the heated gas into the direct reduction shaft 5.

[0037]

[0023] A first embodiment of the invention is represented in Fig 5a. In this embodiment, the total flow rate of the gas injected into the direct reduction shaft 5 is heated thanks to a single device for heating gas 1. The total flow rate of the hot gas is retrieved in a feeding pipe 6 connected to the exit of the device for heating gas 1 and that opens into the direct reduction shaft 5 so that the hot gas is released into the direct reduction shaft 5.

[0038]

[0024] A second embodiment of the invention is represented in Fig 5b. In this embodiment, the total flow rate is divided into several devices for heating gas 1 that each will heat only a part of the total flow rate. The hot gas is then retrieved in a feeding pipe 6, like in the first embodiment, to be released into the direct reduction shaft 5. This embodiment allows an easier implementation of the invention when high flow rates of gas are needed as it allows to have smaller devices that each requires less energy.

[0039]

[0025] A third embodiment of the invention is represented in Fig 5c. In this embodiment, several devices for heating gas 1 are placed around the direct reduction shaft 5 and having their exit open directly into the direct reduction shaft 5. The total flow rate of gas is divided beforehand, and each divided flow is heated by the associated device for heating gas 1 just before being injected into the direct reduction shaft 5. The hot gas exiting the device for heating gas 1 is released directly into the direct reduction shaft 5. The injection to the shaft can preferably be done with a specific angle to facilitate gas penetration into the pellet bed and to avoid pellets flow in the gas pipe. This embodiment allows the gas to remain cold until just before the injection, thus facilitating the control and automation of the gas distribution. It also allows to have several small devices for heating gas 1 that are thus easier to implement.

[0040]

[0026] The invention is not limited to the embodiments represented in Fig 5 and other configurations can be considered.

[0041]

[0027] Preferably, the gas heated in the invention is a reducing gas. In a first embodiment, the reducing gas is mainly composed of hydrogen. In a second embodiment, the reducing gas is made from natural gas. In this embodiment, the natural gas is mixed with recycled top gas. The top gas is the gas retrieved at the top of the DRI. In this embodiment, the method optionally comprises a preliminary step of reforming and so that the resulting reducing gas is heated after the reforming step. The reforming step can be steam reforming or dry reforming. The reforming step consists in transforming methane into a reducing gas. Steam reforming transforms methane through the reaction: CPU + H2O — CO + 3H2. Dry reforming transforms methane through the reaction: CPU + CO2 — 2CO + 2H2.

[0042]

[0028] The insert 3 is a piece that is located inside the pipe 2. Its purpose is to transfer energy to the gas flowing into the pipe 2. This transfer occurs as the gas flows into the pipe 2 and enters in contact with the exchange surface of the insert 3. The gas is thus heated by forced convection and / or radiation.

[0043]

[0029] For the structure of the insert 3, increasing the exchange surface will improve the efficiency of the heating as the insert 3 can transfer more energy to the gas over the length of the pipe 2. The exchange surface corresponds to the surface of the insert 3 that the gas can enter in contact with. The higher the surface exchange of the insert 3 is, the shorter the pipe 2 length required to reach a given target temperature is as well.

[0044]

[0030] For each meter of length of pipe 2, the insert 3 preferably has an exchange surface from 0.5 m2to 100 m2For an exchange surface below 0.5 m2for each meter of length of pipe 2, the efficiency of the heating can be insufficient to reach the target temperature. For an exchange surface above 100 m2for each meter of pipe 2, the gas could reach the target temperature way before the end of the pipe 2 thus reducing the efficiency of the heating compared to the material used. Preferably, for each meter of length of pipe 2, the insert 3 has an exchange surface from 0.5 m2to 80 m2or from 0.5 m2to 60 m2or from 0.5 m2to 50 m2or from 0.5 m2to 40 m2or from 0.5 m2to 30 m2or from 0.5 to 25 m2or from 0.5 m2to 20 m2or from 0.5 m2to 15 m2or from 0.5 m2to 10 m2or from 0.5 m2to 7.5 m2or from 0.5 m2to 5 m2or from 0.5 to 2.5 m2or from 1 m2to 100 m2or from 1 m2to 80 m2or from 1 m2to 60 m2or from 1 m2to 50 m2or from 1 m2to 40 m2or from 1 m2to 30 m2or from 1 to 25 m2or from 1 m2to 20 m2or from 1 m2to 15 m2or from 1 m2to 10 m2or from 1 m2to 7.5 m2or from 1 m2to 5 m2or from 1 m2to 2.5 m2

[0045]

[0031] The efficiency of the heating can also be improved by imposing an agitation to the gas flow inside the pipe 2. This is provided by creating a turbulent flow inside the pipe 2.

[0046]

[0032] The flow patterns are usually characterized by the Reynolds number (Re). Usually, the Reynolds number is defined as: puL Re = - —

[0047] A p: density of the fluid (kg / m3) u: flow speed (m / s)

[0048] L: characteristic length (m) p: dynamic viscosity (kg / (m.s)) A turbulent flow is characterized by a Reynolds number Re > 2800.

[0049]

[0033] The shape of the insert 3 has an influence on the Reynolds number as it modifies the characteristic length L of the system. A shape designed to provide turbulent flow to the gas flowing into the pipe 2 can thus improve the efficiency of the heating of the gas.

[0050]

[0034] To further improve the efficiency, the insert 3 preferably extends from the entrance of the pipe 2 to the exit of the pipe 2. The insert 3 also preferably has a section that represents from 50% to 100% of the free section inside the pipe 2, that is to say, the section where the gas is flowing. Having an insert 3 with a section that is almost the same as the free section of the pipe 2 allows to avoid that a large fraction of the gas flows around the insert 3 without being heated by the insert 3. However, having a smaller section allows the insert 3 to expand more freely with thermal dilatation without damaging the device for heating gas 1 . An insert 3 with a section of 100% of the free section of the pipe 2 can be considered with a material such as Invar™ that has a thermal expansion rate sufficiently low so as not to damage the pipe 2. With a section lower than 50% of the free section of the pipe 2, the efficiency of the heating could be too low. Preferably, the section of the insert 3 comparatively to the free section of the pipe 2 is from 50% to 97.5% or from 50% to 95% or from 50% to 92.5% or from 50 to 90% or from 50% to 85% or from 50% to 80% or from 50% to 75% or from 60% to 97.5% or from 60% to 95% or from 60% to 92.5% or from 60 to 90% or from 60% to 85% or from 60% to 80% or from 60% to 75% or from 65% to 97.5% or from 65% to 95% or from 65% to 92.5% or from 65 to 90% or from 65% to 85% or from 65% to 80% or from 65% to 75% or from 70% to 97.5% or from 70% to 95% or from 70% to 92.5% or from 70 to 90% or from 70% to 85% or from 70% to 80% or from 70% to 75%.

[0051]

[0035] Examples for insert designs are represented in Fig 3a and Fig 3b. The insert represented in Fig 3a has a screw shape and the insert represented in Fig 3b has a perforated twisted tape shape. These designs provide a high exchange surface comparatively to the length of the pipe 2 and can also provide a turbulent flow for typical gas speeds injected in a direct reduction shaft.

[0052]

[0036] The designs of the insert 3 are not limited to the ones represented in Fig 3a and Fig 3b and other designs can be considered.

[0037] In order to be heated, the insert 3 has to be made of a material that is electrically conductive. The insert 3 is preferably made of magnetic steel or graphite but can also be made of any electrically conductive material. An electrically conductive material is any material that has an electrical conductivity value o > 10 000 (Q.m)-1.

[0053]

[0038] Magnetic steel can be used as a material for the insert 3 for heating non oxidizing gas to temperatures up to 1000°C, preferably up to 1100°C without material degradation. For higher temperatures, it is preferable to use graphite as a material for the insert 2 as graphite can reach temperatures higher than 3000°C. Tungsten and molybdenum can also be considered as material for the insert 3 for high temperatures as they can resist to temperatures higher than 3000°C. For heating oxidizing gases, stainless steel can be used as the induced material.

[0054]

[0039] The insert can be manufactured using additive manufacturing.

[0055]

[0040] The inductor 4 is preferably a coil and is located in the vicinity of the pipe 2. Preferably, the inductor 4 is located inside the pipe 2 close to its wall as represented in Fig 2. This configuration allows to avoid heating the pipe 2 alongside the insert 3 if the pipe 2 is made of a material that is also electrically conductive. In this configuration, a refractory material is placed between the insert 3 and the inductor 4 to serve as a thermal insulator. A magnetic yoke is also preferably installed inside the pipe 2, between the inductor 4 and the wall of the pipe 2. It can be a plate of electric steel. Its purpose is to confine the magnetic field inside the pipe 2 so as not to heat the pipe 2, to increase the energy efficiency. Alternatively, the inductor 4 can be placed outside the pipe 2, as in Fig 1 , if the pipe 2 is made of a material that is not electrically conductive.

[0056]

[0041] The pipe 2 preferably has a cylindrical shape. To reach a certain power, a minimal surface and / or mass of induced material is needed in the form of the insert 3. The shape of the insert 3 and the dimensions of the pipe 2 are arranged to provide optimal efficiency and compacity of the device for heating gas 1 depending on the power needed for the application it is used for. As the pipe 2 is not heated in the device for heating gas 1 according to the invention, the thickness of the pipe 2 does not have an influence on the efficiency of the heating contrary to the prior art where the pipe 2 itself is heated.

[0057]

[0042] The gas is heated to a temperature from 800°C to 1100°C. Preferably, the gas is heated to a temperature from 850°C to 1050°C.

[0058]

[0043] A step of cleaning the gas is preferably done before heating to remove most of the particles that can be present. This step of cleaning can be done with a specific equipment using venturi effect and scrubbers to remove particles with water. This step can also be done by mechanical separation or any other means to remove particles from a gas. Preferably, the amount of particles in the heated gas is not more than 5mg / Nm3. Nm3stands for Normal cubic meters and is a unit of measurement of the quantity of gas which corresponds to the content of a volume of one cubic meter, for a gas under normal temperature and pressure conditions (0°C and 1 atm.). Preferably, the size of the particles present in the heated gas is not more than 50pm.

[0059]

[0044] The flow rates for the gas that are considered for the invention are usual flow rates of gas for the injection into a direct reduction shaft 5. The person skilled in the art would know what flow rates of gas is needed for the application he wants to use the invention.

[0060]

[0045] In the applications of the method according to the invention, a given global gas flow rate of a given composition is heated to a target temperature. Depending on the value of this global gas flow rate and the target temperature, the required global heating power can range from 1 kW to 50MW. For low power values, for example in the order of 1 kW to 10kW, a single device for heating gas 1 would be sufficient to provide this level of power, like the first embodiment of the invention. For higher power range, for example in the order of 10MW to 50MW, depending on the technology available to the person skilled in the art, a single device for heating gas 1 can be insufficient to obtain this level of power. In this case, the global gas flow rate to be heated can be divided in several streams of lower flow rate that will each be heated by a single device for heating gas and require much lower power to reach the same target temperature, like the second or the third embodiment of the invention.

[0061]

[0046] Preferably, the power of the steelmaking vessel is from 1 kW to 10MW, more preferably from 1 kW to 1 MW, more preferably from 1 kW to 100kW, more preferably from 1 kW to 50kW, more preferably from 1 kW to 10kW.

[0062]

[0047] Examples of dimensioning for the device for heating gas 1 in the method according to the invention are given in Table 1 and Table 2 below.

[0063]

[0048] Table 1 shows examples of three reducing gases composition that can be heated and injected into a direct reduction shaft using the method according to the invention.

[0064] Table 1

[0065]

[0049] Case 1 represents the composition of a reducing gas after the preliminary step of steam reforming. Case 2 represents the composition of a reducing gas without the preliminary step of steam reforming or dry reforming. Case 3 represents the composition of a gas that is composed mainly of H2.

[0066]

[0050] Table 2 shows examples of a DRI installation according to the invention in a usage configuration.

[0067] Table 2

[0051] For each case, the composition of the heated gas is the same as represented in Table 1. For each case a production of DRI in tons per hour is represented, a target gas flow rate and a target gas temperature are set, and a number of devices for heating gas is defined. The unit of the flow rate is Nm3 / h with Nm3which stands for Normal cubic meters and which is a unit of measurement of the quantity of gas which corresponds to the content of a volume of one cubic meter, for a gas under normal temperature and pressure conditions (0°C and 1 atm.). For each case, the total power needed to heat the gas at the target flow rate to the target temperature is calculated using a simulation. The simulation can be done using the software Aspen Plus™, developed by AspenTech. The power needed per device represents the power that each device should be able to provide, based on the total power needed and the number of devices defined, to heat the gas at the target flow rate to the target temperature.

[0068]

[0052] Fig 6 represents a comparative heating of a gas heated according to the prior art and a gas heated according to the invention. The first curve represents the heating with a device for heating gas according to the prior art. A gas flows inside the pipe and the pipe is heated using induction heating. The second curve represents the heating with the heating device according to the invention. The pipe 2 is not heated and only the insert 3 is heated.

[0069]

[0053] The curves were calculated using a simulation with Fluent, a software published by Ansys, Inc. For both curves, the conditions of heating are the same. The pipe has a diameter of 300mm, a thickness of 5mm and a length of 2m. The gas heated is H2 with a mass flow of 68kg / h. For the curve of the heating according to the invention, the inductor 4 is located outside the pipe 2, the insert 3 is of screw type, like represented in Fig 3a, with a diameter equal to the diameter of the pipe 2. The total surface of the insert is 2.66 m2and the Reynolds number is 9300.

[0070]

[0054] The comparison shows that a more efficient heating over a shorter length can be obtained with the heating device according to the invention.

[0055] The method according to the invention allows to successfully inject hot gas into the direct reduction shaft using only a limited space while reducing the CO2 emissions of the process.

Claims

CLAIMS1 ) A method for injecting hot gas into a direct reduction shaft (5), wherein said method comprises the following steps:- introducing gas into at least one device for heating gas (1 ), said device for heating gas (1 ) comprising: o a pipe (2) allowing gas to flow inside of it, o an insert (3) located inside said pipe (2), said insert being electrically conductive, o an inductor (4) located in the vicinity of said pipe (2),- feeding energy into said insert (3) using said inductor (4) of said device for heating gas (1 ) so that said energy fed into the insert (3) is transferred to the gas introduced into said device for heating gas (1 ), to heat said gas to a temperature from 800°C to 1100°C,- releasing said hot gas into the direct reduction shaft (5).2) A method for injecting hot gas into a direct reduction shaft (5) according to claim 1 wherein said gas is heated to a temperature from 850°C to 1050°C.3) A method for injecting hot gas into a direct reduction shaft (5) according to any of the preceding claims wherein the flow of gas inside the pipe of the device for heating gas has a Reynolds number superior to 2800.4) A method for injecting hot gas into a direct reduction shaft (5) according to any of claim 1 to 3, wherein a feeding pipe (6) is connected to the exit of said at least one device for heating gas (1 ) and opens into said direct reduction shaft (5), said feeding pipe (6) allowing said hot gas exiting said at least one device for heating gas (1 ) to be released into said direct reduction shaft (5).5) A method for injecting hot gas into a direct reduction shaft according to any of claim 1 to 3, wherein the exit of said at least one device for heating gas (1 ) is open directly into the direct reduction shaft (5) so that said hot gas is released directly into the direct reduction shaft (5).6) A method for injecting hot gas into a direct reduction shaft (5) according to any of the preceding claims wherein said heated gas is a reducing gas.7) A method for injecting hot gas into a direct reduction shaft (5) according to claim 5 wherein said reducing gas is mainly composed of hydrogen.8) A method for injecting hot gas into a direct reduction shaft (5) according to claim 5 wherein said reducing gas is mainly composed of natural gas and recycled top gas.9) A method for injecting hot gas into a direct reduction shaft (5) according to claim 7 wherein said method comprises a preliminary step of reforming and so that the resulting reducing gas is heated after said reforming step using said device for heating gas (1 ).10) A direct reduction of iron (DRI) installation, wherein said DRI installation comprises:- a direct reduction shaft (5),- at least one device for heating gas (1 ), said device for heating gas (1 ) comprising: o means for injecting gas into said device for heating gas (1 ), o a pipe (2) allowing the gas injected to flow inside of it, o an insert (3) located inside said pipe (1 ), said insert (3) being electrically conductive, o an inductor (4) located in the vicinity of said pipe (2) and able to feed energy into said insert (3), so that said energy fed into said insert (3) is transferred to the gas flowing into said pipe (2), wherein said device for heating gas (1 ), has a power from 1 kW to 50MW.11 ) A DRI installation according to claim 10, wherein said DRI installation further comprises a feeding pipe (6), said feeding pipe (6) being connected to the exit of said at least one device for heating gas (1 ) and opening into said direct reduction shaft (5).12) A DRI installation according to claim 10, wherein the exit of said at least one device for heating gas (1 ) is open directly into the direct reduction shaft (5).13) A DRI installation according to any of claims 10 to 12, wherein said insert (3) is designed to have an exchange surface with the gas to be heated that ranges from 0.5m2to 100m2by meter of pipe (2).14) A DRI installation according to any of claims 10 to 13, wherein said insert (3) is made of one of the following materials: magnetic steel, graphite, tungsten, molybdenum.15) A DRI installation according to any of claim 10 to 14, wherein said inductor (4) is a coil.

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