A method for producing iron fuel from metal oxide containing charge materials via reducing the metal oxide containing charge materials

Abstract

The present invention relates to a method for producing iron fuel from metal oxide containing charge materials via reducing the metal oxide containing charge materials, the method comprising a step of feeding metal oxide containing charge materials to a reactor, a step of reducing the metal oxide containing charge materials by flowing a reduction gas through the reactor, a step of removing partially spent reduction gas from the reactor, and a step of removing a stream containing iron fuel from the reactor.

Description

The present invention relates to a method for producing iron fuel from metal oxide containing charge materials via reducing the metal oxide containing charge materials. A reduction process, such as an iron oxide powder reduction process, is known from WO 2023 / 121465 in the name of the present applicants. According to such method the metal oxide containing charge materials is fed to a fluidized bed unit, wherein the metal oxide containing charge materials is reduced by flowing a reduction gas through the fluidized bed unit, wherein the fluidized bed unit is operated under specific reduction conditions. Partially spent reduction gas is removed from the fluidized bed unit and admixed with fresh reduction gas and the mixture of partially spent reduction gas and fresh reduction gas is returned to the fluidized bed unit. A stream containing iron fuel is continuously removed from the fluidized bed unit. Such method for producing iron fuel further comprises pre-heating and / or drying the metal oxide containing charge materials before feeding the materials into the fluidized bed unit, wherein the pre-heating temperature is preferably in a range of 40 and 1000 °C. No details about the specific way of pre-heating or heat integration has been disclosed in WO 2023 / 121465. Other processes for reducing metal oxide containing charge materials are known from, inter alia, US 2016 / 348199, US 4,082,545, US 3,288,590, US 4,420,332, WO 00 / 01856, and US 3,303,017. GB 1 506 170 relates to a process for reducing a pulverized material containing iron oxides in a circulating fluidized bed containing the pulverized material mixed with coke, the pulverized material containing iron oxides, pulverized solid carbonaceous material and molecular oxygen containing gas being fed to an intermediate portion of the reaction zone; exhaust gas and entrained solid material being withdrawn from the upper portion of the reaction zone, solid material being separated from the gas and returned to the intermediate portion of the reaction zone; a portion of the separated exhaust gas, after removal of CO2 and H2O, being returned to the lower portion of the 2 reaction zone for fluidizing the bed and reducing material therein; and solid material containing completely or partially reduced iron oxide being withdrawn from the lower portion of the reaction zone. US 5 431 711 relates to a method for reducing iron oxide from a source of fines, comprising the steps of providing a closed system comprising a source of iron oxide fines communicating with at least one fluidizable bed which in turn communicates with a cyclone, the cyclone communicating with a cooler-scrubber, the cooler-scrubber communicating with a reformer, and the reformer communicating with the fluidizable bed. In the fluidizable bed iron oxide fines are reduced to metallized iron fines and a partially spent reducing gas is formed in the fluidizable bed. Large particles of metallized iron fines are separated from a partially spent reducing gas in the fluidizable bed and partially spent reducing gas is removed from the fluidizable bed into the cyclone along with small metallized iron particles for separating small particles of metallized iron fines from the partially spent reducing gas in the cyclone, wherein the removed partially spent reducing gas is cleaned and cooled and the cleaned and cooled partially spent gas is delivered to a reformer for reforming with natural gas, and the reduced metallized iron fines are collected. US 5 527 379 relates to a process for a direct reduction of iron oxide containing materials to produce sponge iron and for a carburization to form FesC in a first fluidized bed supplied with a circulating reducing gas, said process comprising the steps of in a first reducing stage charging the iron oxide containing materials into a fluidized bed reactor of a circulating fluidized bed system, supplying hot reducing gas as a fluidizing gas to the fluidized bed reactor, pre-reducing the iron oxides to form reduced iron-containing solids, treating a suspension discharged from the fluidized bed reactor in a recycle cyclone of the circulating fluidized bed system to remove substantially all reduced iron-containing solids, and recycling separated reduced iron-containing solids to the fluidized bed reactor, supplying reduced iron-containing solids from the first reducing stage in a second reducing stage to a conventional fluidized bed reactor that is not part of a circulating fluidized bed system, supplying hot reducing gas as a fluidizing gas to the conventional fluidized bed reactor, reacting the reduced iron-containing solids to remove remaining oxygen content and to convert the iron content thereof substantially completely to FesC, supplying an exhaust gas from the conventional fluidized bed reactor as a secondary gas to the first fluidized bed reactor 3 ,and withdrawing a product which contains FesC from the conventional fluidized bed reactor; cooling the exhaust gas from the recycle cyclone used in the first and condensing water from the exhaust gas; drawing off a partial stream of the exhaust gas; and replenishing the remaining partial stream by addition of reducing gas thereto and reheating the replenished partial stream and using same as a recycle gas, a part of which is supplied as a fluidizing gas to the circulating fluidized bed reactor of the first reducing stage and another part of which is supplied to the conventional fluidized bed reactor. Energy consumption in commercial plants plays a big role in the overall costs of operating a plant. In addition, the energy consumption should be reduced where possible, especially with regard to the emissions of harmful substances such as CO2 and NOX to the environment. An object of the present invention is to provide a method for producing iron fuel from metal oxide containing charge materials via reducing the metal oxide containing charge materials, in which method heat integration of several product streams takes place. Another object of the present invention is to provide a method for producing iron fuel from metal oxide containing charge materials via reducing the metal oxide containing charge materials, in which method measurements are applied for maintaining stable reactor conditions. The present invention thus relates to a method for producing iron fuel from metal oxide containing charge materials via reducing the metal oxide containing charge materials, the method comprising a step of feeding metal oxide containing charge materials to a reactor, a step of reducing the metal oxide containing charge materials by flowing a reduction gas through the reactor, a step of removing partially spent reduction gas from the reactor, and a step of removing a stream containing iron fuel from the reactor, wherein the stream containing iron fuel removed from the reactor is sent to a heat exchanger, wherein the heat extracted from the stream containing iron fuel is used to heat the metal oxide containing charge materials. The present inventors found that by such a method one or more of the objects are achieved. The principle of the present method is based on heat integration of the solids, i.e. metal oxide containing charge materials, in an iron fuel production plant 4 such that the energy efficiency of the plant is increased, and the product quality of the produced iron powder is improved. In an iron fuel production plant iron oxide powder reacts with a reduction gas, e.g. gaseous hydrogen, in a reactor such that iron powder is produced. This process takes place in a reactor at high temperature. This reduction reaction is an endothermic reaction which means that energy is needed for the reaction, thereby reducing the temperature in the reactor. When solids having a low temperature are introduced into the reactor, this has to be compensated for by increasing the gas inlet temperature, or alternatively by heating the reactor walls, such that the reactor remains at the operating temperature. The present inventors found that it is important to keep the temperature in the reactor as uniform and constant as possible. One of the reasons is that when the reactor temperature is too high this may result in agglomeration of the powder, potentially causing defluidization and blockage of the process. A uniform reactor temperature will produce powder by the same reaction mechanism, which will result in a final product having uniform final properties, for example particle porosity, pore size, e.g. specific surface area and composition. A more uniform distribution of the specific surface area means a more constant and predictable ignition temperature which can be beneficial for the combustion plant and for process safety. On the other hand, a reactor temperature that is too low may result in the formation of pyrophoric powder. Both particle agglomeration and the formation of pyrophoric material form a barrier for the use of iron in a specific powder form as a starting material in iron fuel combustion. The term “pyrophoricity” is to be understood as a property of a material. In more detail, a material is qualified as pyrophoric if it ignites spontaneously in air at or below 54 °C or within 5 minutes after coming into contact with air. It is caused by the high specific surface area of the material, yielding an extremely low ignition temperature for oxidation. When the ignition temperature is low enough, auto-ignition at atmospheric conditions can take place. Pyrophoricity of iron particles after reduction has been reported to be a serious issue in fluidized bed reduction, where a high surface area of the material is obtained. The tendency to reoxidation depends on the reduction temperature. Studies have shown that the surface area of fine metal powders generally decreases with higher reduction temperature. 5 In an example the metal oxide containing charge materials are first heated with the heat extracted from the stream containing iron fuel, and the thus preheated metal oxide containing charge materials are further heated by mixing the metal oxide containing charge materials with partially spent reduction gas removed from the reactor. In such an example the combination of two relatively hot process streams is used for heat integration, i.e. a stream of solids, namely the heat extracted from the stream containing iron fuel, and a stream of gas, namely the partially spent reduction gas removed from the reactor. In an example the step of feeding metal oxide containing charge materials to the reactor comprises adding a gaseous mixture containing the metal oxide containing charge materials to a separation unit, said separation unit having an inlet for solids and gas, an outlet for solids and an outlet for gas, wherein the outlet for solids is connected to the reactor. In an example of the present method the separation unit is a cyclone or a series of cyclones. A cyclone provides an intense interaction between solids and gas and is therefore very efficient in heat transfer processes. In addition, cyclones have simple designs, leading to lower maintenance requirements and are generally cost-effective due to their simplicity and effectiveness in heat transfer processes. In a cyclone there is a gas-solid interaction because the gas containing solid particles enters tangentially, creating a high-speed rotating airflow within the cyclone chamber. Another aspect of a cyclone is the centrifugal force, wherein due to the centrifugal force generated by the cyclone's rotation, solid particles move towards the outer wall, experiencing greater force than the gas molecules. In addition, two heat transfer mechanisms play a big role in the cyclone, i.e. convection and radiation. The high-speed gas flow around the solids leads to convective heat transfer. As the gas moves rapidly, it carries heat energy, which can be transferred to the solid particles. Depending on the temperature difference between the gas and solid particles, radiation heat transfer might occur. Hot gas can radiate heat energy to the cooler solid surfaces within the cyclone. In an example the gaseous mixture containing the metal oxide containing charge materials is obtained by mixing the metal oxide containing charge materials with partially spent reduction gas removed from the reactor, wherein the mixture thus obtained is added to the cyclone or the series of cyclones via the inlet for solids and gas. In such a situation an optimal heat integration is carried out in which the relatively 6 hot partially spent reduction gas is used as a heat transfer medium for the solids, i.e. the metal oxide containing charge materials. In addition, the relatively hot partially spent reduction gas removed from the reactor may contain solid material and this solid material will be returned to the reactor via the outlet for solids of the cyclone. According to the present method the stream containing iron fuel removed from the reactor is sent to a heat exchanger, wherein the heat extracted from the stream containing iron fuel is used to heat the metal oxide containing charge materials. In such an example another relatively hot stream, i.e. stream containing iron fuel removed from the reactor, is used as a source of energy for heating the metal oxide containing charge materials. In an example of the present invention the reactor is a fluidized bed unit, or a combination of fluidized bed units. In an example the outlet for solids of the cyclone or the series of cyclones is positioned in such a way that gas from the fluidized bed cannot flow back into the cyclone via that outlet, causing it to no longer work. This can be achieved by protruding the outlet for solids of the cyclone or the series of cyclones, also called standpipe, into the bed so that the powder column provides resistance, or by using a valve or nozzle in the standpipe. In an example the outlet for solids of the cyclone or the series of cyclones is positioned just below the fluidizing surface of the fluidized bed unit thereby enabling a smooth contact between the solids coming from the cyclone or the series of cyclones and the solids already present in a fluidisation condition in the fluidized bed unit. Such a contact minimizes a disturbance of the fluidisation conditions in the fluidized bed unit. To ensure a more uniform reactor temperature it is thus desired to preheat the powder before it interacts with the introduced hydrogen gas. This can be done by a bulk solid heat exchanger (BSHE) and / or by heating the powder by a gas flow. Using a bulk solid heat exchanger has its limitations in the maximum powder temperature which can be achieved since that temperature is highly dependent on the specific heat transfer medium. As the powder needs to be cooled after extraction from the reactor for safety reasons and handling reasons, a bulk solid heat exchanger can also be used here. The extracted heat from such a bulk solid heat exchanger can be used to heat the solids at the inlet stream from the reactor. The present inventors found that this 7 process of heat integration will however still not preheat the solids all the way up to the desired reactor temperature due to the difference in mass flow of the particles in and out and their specific heat capacity. Therefore a second preheating step is needed in which the solids are introduced into the outlet gas flow of the reactor after which the heat up even further. A drawing schematically illustrate an example of a method according to the invention according to the invention. The present method is not restricted to the specific example disclosed here. Figure 1 is an example of the method for producing iron fuel. A stream of metal oxide containing charge materials 12 is sent to a heat exchanger 11. A heated stream of metal oxide containing charge materials 10 is sent a mass flow device 9 and forwarded as stream 5 to a separation unit 3, e.g. a cyclone. Examples of such mass flow device are screw conveyors or feeders, pneumatic conveying systems, vibratory feeders, chutes, and hoppers, wherein screw conveyors or feeders are preferred. A partially spent reduction gas 4 from a reactor 7, such as a fluidized bed unit, is mixed with stream 5 and a mixed stream 6 is added to the separation unit 3. In separation unit 3 a stream of further heated metal oxide containing charge materials 2 is added to reactor 7, such as a fluidized bed unit. In a situation wherein the temperature of metal oxide containing charge materials 2 is not at the right temperature for being added to reactor 7, such as a fluidized bed unit, an additional heating of stream 2 may be needed. Such an additional heating step may for example take place via conduction, in which the solids are passed through a heated surface or using heated plates to transfer heat directly. Or such an additional heating step may for example take place via indirect heat transfer in which heat is transferred to the solids without direct contact through a medium such as a jacketed tube, wherein a solid material passes through a tube surrounded by a heating jacket through which a heat transfer fluid flows, or even via radiant heating, wherein a radiant heat source (infrared heaters) is used to heat solids as they pass through or beneath. Separation unit 3 also produces a gas stream 1, which can be used for further processing. A stream 15 containing iron fuel is removed from reactor 7, such as a fluidized bed unit, and forwarded to heat exchanger 16. Heat exchanger 16 is connected to heat exchanger 11 via a circuit of a heat transfer medium. A relatively hot stream 13 of heat transfer medium flows from heat exchanger 16 to heat exchanger 8 11, and a relatively cold stream 14 of heat transfer medium flows back from heat exchanger 11 to heat exchanger 16. According to such a circuit of heat transfer medium the heat extracted from stream 15 containing iron fuel is used to heat the metal oxide containing charge materials 12. 5          Although the Figure shows only one cyclone as separation unit, the present invention is not restricted to a single cyclone construction. A series of cyclones, placed in series or even parallel, or a combination thereof, can be used as well in the present method. The same applies to the fluidized bed unit which may consist of multiple fluidized bed units, placed in series or even parallel, or a combination thereof. A similar 10 construction may be present for the heat exchanger, mass flow device, i.e. multiple units placed in series or even parallel, or a combination thereof. In an example multiple reactors placed in series are connected to the same separation unit. In another example multiple reactors placed in parallel are connected to the same separation unit. In another example one or more separation units are 15 connected to multiple reactors placed in series.

Claims

1. A method for producing iron fuel from metal oxide containing charge materials via reducing the metal oxide containing charge materials, the method comprising a step of feeding metal oxide containing charge materials to a reactor, a step of reducing the metal oxide containing charge materials by flowing a reduction gas through the reactor, a step of removing partially spent reduction gas from the reactor, and a step of removing a stream containing iron fuel from the reactor,wherein the stream containing iron fuel removed from the reactor is sent to a heat exchanger, wherein the heat extracted from the stream containing iron fuel is used to heat the metal oxide containing charge materials.

2. A method according to claim 1, wherein the metal oxide containing charge materials are first heated with the heat extracted from the stream containing iron fuel, and the thus preheated metal oxide containing charge materials are further heated by mixing the metal oxide containing charge materials with partially spent reduction gas removed from the reactor.

3. A method according to any one or more of claims 1-2, wherein the step of feeding metal oxide containing charge materials to the reactor comprises adding a gaseous mixture containing the metal oxide containing charge materials to a separation unit, said separation unit having an inlet for solids and gas, an outlet for solids and an outlet for gas, wherein the outlet for solids is connected to the reactor.

4. A method according to claim 3, wherein the separation unit is a cyclone or a series of cyclones.

5. A method according to any one or more of claims 3-4, wherein the gaseous mixture containing the metal oxide containing charge materials is obtained by mixing the metal oxide containing charge materials with partially spent reduction gas removed from the reactor, wherein the mixture thus obtained is added to the cyclone or the series of cyclones via the inlet for solids and gas.

6. A method according to any one or more of the preceding claims, wherein the reactor is a fluidized bed unit.

7. A method according to claim 6, wherein the outlet for solids of the cyclone or the series of cyclones is positioned just below the fluidizing surface of the fluidized bed unit.

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