Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21B—MANUFACTURE OF IRON OR STEEL
  • C21B11/00—Making pig-iron other than in blast furnaces
  • C21B11/02—Making pig-iron other than in blast furnaces in low shaft furnaces or shaft furnaces
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21B—MANUFACTURE OF IRON OR STEEL
  • C21B13/00—Making spongy iron or liquid steel, by direct processes
  • C21B13/02—Making spongy iron or liquid steel, by direct processes in shaft furnaces
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22B—PRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
  • C22B5/00—General methods of reducing to metals
  • C22B5/02—Dry methods smelting of sulfides or formation of mattes
  • C22B5/12—Dry methods smelting of sulfides or formation of mattes by gases

Definitions

• the present invention relates to a method for operating a direct reduction furnace and a method for producing reduced iron.
• direct reduction ironmaking (sometimes called direct reduction or direct ironmaking) has been developed as a reduction process different from that used in blast furnaces.
• the direct reduction ironmaking process works as follows: A direct reduction furnace is charged with iron oxide raw material (hereinafter simply referred to as iron oxide), such as lump iron ore (lump ore) or pellets (powdered iron ore solidified into a spherical shape). Reducing gas is then injected into the direct reduction furnace to reduce the iron oxide and obtain reduced iron. The obtained reduced iron is then cooled in a region (cooling zone) below the reduction gas injection point of the direct reduction furnace. The reduced iron is then discharged from the bottom of the direct reduction furnace. The reduced iron discharged from the direct reduction furnace is then melted in an electric furnace.
• iron oxide raw material hereinafter simply referred to as iron oxide
• iron oxide such as lump iron ore (lump ore) or pellets (powdered iron ore solidified into a spherical shape).
• Reducing gas is then injected into the direct reduction furnace to reduce the iron oxide and obtain reduced iron.
• the obtained reduced iron is then cooled in a region (cooling zone)
• a shaft furnace is mainly used as the direct reduction furnace.
• natural gas such as Midrex (registered trademark) or Hyl (registered trademark)
• top gas exhaust gas
• Patent Document 1 describes: "In the operation method of a direct reduction furnace using a shaft furnace system to produce reduced iron using reducing gas mainly consisting of hydrogen, A method for operating a direct reduction furnace using preheated raw material, characterized in that preheated raw material iron oxide is charged into the direct reduction furnace. has been disclosed.
• Patent Document 2 states: "In the operation method of a direct reduction furnace using a shaft furnace system to produce reduced iron using reducing gas mainly consisting of hydrogen, A method for operating a direct reduction furnace with top gas circulation, characterized in that a portion of the gas discharged from the top of the furnace is blown into the middle of the furnace. has been disclosed.
• Patent Document 3 states: "A method for producing reduced iron by reducing iron oxide charged in a shaft furnace, A method for producing reduced iron, characterized in that a heated mixed gas containing a reducing gas containing 90% by volume or more of hydrogen gas and nitrogen gas is blown into the shaft furnace. has been disclosed.
• Patent Documents 1 to 3 increased pressure loss within the direct reduction furnace could lead to problems such as the raw material hanging and not descending properly, resulting in reduced productivity and operational shutdowns.
• the present invention has been developed to solve the above problems, and aims to provide a method for operating a direct reduction furnace that reduces CO2 emissions and enables stable operation while avoiding problems such as reduced productivity and shutdowns.
• Another aim of the present invention is to provide a method for producing reduced iron by using the above method for operating a reduction furnace.
• any numerical range expressed using "to” means a range that includes the numerical values before and after "to" as the lower and upper limits, respectively.
• iron oxide such as pellets may be pulverized during the reduction process (hereinafter also referred to as reduction pulverization).
• reduction pulverization is more pronounced than when natural gas is used as the reducing gas, which is likely to lead to an increase in pressure loss in the direct reduction furnace.
• poor descent such as hanging of the raw material, occurs, which can lead to problems such as reduced productivity and operational shutdowns.
• the reduction rate of the iron oxide at the height position y of the direct reduction furnace is referred to as the reduction progress rate A(y) of the iron oxide.
• the cracks occur remarkably when iron oxide remains at a height where the reduction progress rate A(y) is 30 to 60%, particularly at a height where the reduction progress rate A(y) is 30 to 40%.
• it is effective to increase the speed of the reduction reaction at the above height (hereinafter also referred to as the reduction reaction rate) and narrow the range of the above height, in other words, to shorten the residence time of iron oxide at the above height.
• Increasing the temperature at the height position is effective in increasing the reduction reaction rate at the height position.
• top gas hereinafter also referred to as circulating top gas
• circulating top gas a portion of the top gas that is discharged from the top of the direct reduction furnace.
• inject circulating top gas into the direct reduction furnace and set the vertical injection position at the height position.
• a method for operating a direct reduction furnace comprising: The method for operating the direct reduction furnace comprises: a charging step of charging iron oxide into the direct reduction furnace; a first blowing step of blowing a reducing gas containing hydrogen as a main component into the direct reduction furnace; a second blowing step of blowing circulating furnace top gas into the direct reduction furnace; a reduction step of reducing the iron oxide in the direct reduction furnace to obtain reduced iron; and the circulating furnace top gas is a part of the furnace top gas, and the furnace top gas is a gas discharged from the furnace top of the direct reduction furnace, a vertical injection position of the circulating furnace top gas in the second injection step at a height position at which a reduction progress rate A(y) is 30 to 60%.
• the reduction progress rate A(y) is the reduction rate of iron oxide at the height position y in the direct reduction furnace.
• a method for producing reduced iron comprising the steps of operating a direct reduction furnace as described in any one of 1 to 8 above.
• the present invention by using a reducing gas containing hydrogen as a main component, it is possible to reduce CO2 emissions while avoiding problems such as reduced productivity and shutdowns, and to perform stable operation of a direct reduction furnace. Furthermore, the method of operating a reducing furnace of the present invention is extremely advantageous in terms of improving reaction efficiency.
• FIG. 1 is a schematic diagram showing an example of the configuration of a direct reduction furnace used in a method for operating a direct reduction furnace according to an embodiment of the present invention.
• FIG. 1 is a schematic diagram showing an example of a conventional process for producing reduced iron.
• FIG. 1 is a schematic diagram illustrating an example of a method for producing reduced iron according to an embodiment of the present invention.
• FIG. 1 is a diagram showing an example of the relationship between the height position y of a direct reduction furnace and the reduction progress rate A(y).
• a method for operating a direct reduction furnace includes the steps of: a charging step of charging iron oxide into the direct reduction furnace; a first blowing step of blowing a reducing gas containing hydrogen as a main component into the direct reduction furnace; a second blowing step of blowing circulating furnace top gas into the direct reduction furnace; a reduction step of reducing the iron oxide in the direct reduction furnace to obtain reduced iron; and the circulating furnace top gas is a part of the furnace top gas, and the furnace top gas is a gas discharged from the furnace top of the direct reduction furnace,
• the vertical blowing position of the circulating furnace top gas is set to a height position at which the reduction progress rate A(y) is 30 to 60%.
• Figure 1 is a schematic diagram showing an example of the configuration of a direct reduction furnace (shaft furnace) used in a method of operating a direct reduction furnace according to one embodiment of the present invention.
• reference numeral 1 denotes a shaft furnace
• 1a denotes iron oxide
• 1b denotes reduced iron
• 1c denotes the furnace top
• 1d denotes the reduction zone
• 1e denotes the cooling zone
• 2 denotes the iron oxide charging port
• 3 denotes the furnace top gas outlet
• 4 denotes the circulating furnace top gas inlet
• 5 denotes the reducing gas inlet
• 6 denotes a temperature control device
• 7 denotes the cooling gas inlet
• 8 denotes the cooling gas suction port
• 9 denotes the reduced iron outlet.
• iron oxide is charged from the top of the furnace, particularly the iron oxide charging port, and gradually falls through the reduction zone.
• High-temperature reducing gas primarily composed of hydrogen
• the reduced iron is then discharged through the reduced iron outlet located at the bottom of the shaft furnace.
• Top gas is also discharged through the top gas outlet.
• Circulating top gas a portion of the top gas, is injected back into the shaft furnace through the circulating top gas inlet.
• the temperature of the circulating top gas is optionally controlled using a temperature control device. Note that reducing gas, primarily composed of hydrogen, is not injected into areas other than the reduction zone (for example, the cooling zone or the reduced iron outlet).
• the cooling zone located at the bottom of the shaft furnace is equipped with a cooling gas inlet and a cooling gas suction port. Cooling gas is blown into the cooling zone through the cooling gas inlet. The cooling gas suction port sucks the cooling gas so that it does not enter the furnace top.
• N2 can be used as the cooling gas.
• a reducing gas mainly composed of hydrogen is injected into the direct reduction furnace.
• “mainly composed of hydrogen” means that the H2 concentration is preferably 80% by volume or more, more preferably 90% by volume or more, and even more preferably 95% by volume or more.
• the upper limit of the H2 concentration of the reducing gas is not particularly limited, and the H2 concentration may be 100% by volume.
• the type of remaining gas other than H2 is not particularly limited. Examples of the remaining gas include N2 , H2O , CO, and CO2 , as well as gases produced as by-products in the steelmaking process (hereinafter also referred to as by-product gases).
• Examples of by-product gases include blast furnace gas (BFG) and coke oven gas (COG). However, if the remaining gas contains CO or CO2 , the furnace gas will contain CO2 . In this case, it is preferable to subject the CO2 contained in the furnace gas to chemical synthesis utilization (CCU) or carbon capture and storage (CCS), as described below.
• CCU chemical synthesis utilization
• CCS carbon capture and storage
• the temperature of the reducing gas blown in through the reducing gas inlet is, for example, 700°C to 1200°C. Furthermore, if the temperature of the reducing gas is below 800°C, the heat inside the furnace may be insufficient, slowing the reduction reaction. On the other hand, if the temperature of the reducing gas exceeds 1000°C, fusion of the reduced iron particles (called clustering) may progress, making it difficult to discharge the reduced iron. Therefore, the temperature of the reducing gas is preferably 800 to 1000°C.
• reducing disintegration can be effectively suppressed by increasing the reduction reaction rate at these heights and narrowing the range of these heights—in other words, by shortening the residence time of iron oxide at these heights.
• Increasing the temperature at these heights can effectively increase the reduction reaction rate at these heights.
• the vertical injection position of the circulating furnace top gas is set at a height position where the reduction progress rate A(y) is 30 to 60%, preferably a height position where the reduction progress rate A(y) is 30 to 40%.
• the reduction progress rate A(y) can be calculated by the following formula.
• [Reduction progress rate A (y) (unit: %)] ([oxygen content of iron oxide before reduction (unit: mass%)] - [oxygen content of iron oxide at height position y in direct reduction furnace (unit: mass%)]) / [oxygen content of iron oxide before reduction (unit: mass%)] ⁇ 100
• the oxygen amount (mass %) of iron oxide before reduction is the total oxygen amount (mass %) contained in FeO and Fe2O3 contained in the iron oxide before reduction.
• the oxygen amount contained in Fe2O3 can be calculated, for example, by assuming that the amount of Fe present as Fe2O3 is calculated by subtracting the amount of Fe in FeO (mass %) from the total iron content (total mass %) of iron atoms in the iron oxide before reduction. Furthermore, the oxygen amount (mass %) of iron oxide at height y in the direct reduction furnace (hereinafter also referred to as iron oxide at height y) is the total oxygen amount (mass %) contained in FeO and Fe3O4 contained in the iron oxide at height y. For example, the oxygen amount can be calculated by subtracting the amount of FeO and M.Fe from the total iron content (total mass %) of iron atoms in the iron oxide at height y. Assuming that the amount of Fe present as Fe 3 O 4 is present as Fe 3 O 4, the total amount of oxygen contained in Fe 3 O 4 (mass %) can be calculated by subtracting the amount of Fe (mass %) in Fe (Fe present as metal).
• the reduction progress rate A(y) based on the planned operating conditions of the direct reduction furnace (hereinafter also referred to as planned operating conditions).
• Methods for determining the reduction progress rate A(y) include, for example, chemical analysis of iron oxide, analysis of the gas concentration in the furnace gas, and methods using computational simulation. Of these, methods using computational simulation are preferred. Specific examples of each method are as follows:
• a direct reduction furnace is pre-operated according to the planned operating conditions, and during this operation, the discharge of reduced iron and the injection of reducing gas are suddenly stopped. Cooling N2 gas is then injected into the direct reduction furnace to quench the iron oxide in the furnace. The iron oxide is then gradually discharged from the bottom of the direct reduction furnace. Iron oxide that is thought to have accumulated at various vertical heights in the furnace based on the volume of the discharged iron is then sampled, and the reduction progress rate A(y) is determined by chemical analysis.
• the direct reduction furnace is pre-operated according to the planned operating conditions. Iron oxide samples during reduction are then collected through sampling ports installed at various vertical heights in the direct reduction furnace, and the reduction progress rate A(y) of the iron oxide is determined by chemical analysis.
• the direct reduction furnace is pre-operated according to the planned operating conditions. Then, the furnace gas is sampled from gas sampling pipes installed at various height positions in the vertical direction of the direct reduction furnace, and the gas concentration is analyzed by gas chromatography to determine the amount of hydrogen and other components reduced from the reducing gas before it was injected into the direct reduction furnace, and the reduction progress rate A(y) of iron oxide is calculated.
• Computational Simulation Computational simulation (for example, a two-dimensional DEM-CFD method that combines a one-dimensional mathematical model or a DEM (Discrete Element Method) that calculates the behavior of solid particles with a CFD (Computational Fluid Dynamics) method that performs gas fluid calculations) is performed in accordance with the planned operating conditions to calculate the reduction progress rate A(y) of iron oxide.
• the planned operating conditions include the shape of the direct reduction furnace, the production rate of reduced iron (amount of iron oxide charged), the composition, temperature, pressure, and injection amount (gas flow rate) of the reducing gas (under steady-state operation), and the type of iron oxide (particle size and component composition). Furthermore, when determining the reduction progress rate A(y), the circulation of the furnace gas (injection of circulating furnace gas) is not taken into consideration. The above-mentioned chemical analysis of the iron oxide and analysis of the gas concentration in the furnace gas may also be performed during actual operation of the direct reduction furnace (rather than during pre-operation) to determine the reduction progress rate A(y).
• the reduction progress rate A(y) may also be determined based on, for example, past operating data of a direct reduction furnace similar to the planned operating conditions, literature, etc.
• the average value of the reduction rates of iron oxide at the dimensionless radius of the direct reduction furnace of 0, 0.50, and 1.00 can be taken as the reduction rate of iron oxide at height position y of the direct reduction furnace.
• the dimensionless radius of the direct reduction furnace is the dimensionless radius in the furnace radial direction, with the center position of the direct reduction furnace being 0 and the furnace wall of the direct reduction furnace being 1.00.
• the vertical injection position of the circulating top gas can be adjusted, for example, by installing multiple circulating top gas injection ports in the height direction (vertical direction) of the direct reduction furnace and determining the circulating top gas injection port actually used depending on the reduction progress rate A(y) of the iron oxide.
• the vertical injection position of the circulating top gas can also be adjusted by providing the circulating top gas injection port with a lifting function (a function to change the position in the vertical direction) and a function to adjust the injection angle.
• the representative vertical injection position (vertical middle position) of the circulating furnace top gas is at a height where the reduction progress rate A(y) is 30 to 60%, preferably a height where the reduction progress rate A(y) is 30 to 40%.
• the entire vertical injection position of the circulating furnace top gas is included in this range of height positions.
• the temperature of the circulating top furnace gas may be controlled before being injected into the direct reduction furnace.
• increasing the temperature at the height described above is effective in suppressing reduction disintegration.
• it is effective to raise the temperature of the circulating top furnace gas above the temperature at the height described above, preferably to a temperature at least 50°C higher than the temperature at the height where the reduction progress rate A(y) is 60%, and more preferably to a temperature at least 100°C higher than the temperature at the height where the reduction progress rate A(y) is 60%.
• the upper limit of the circulating top furnace gas temperature is not particularly limited.
• the temperature of the circulating top furnace gas is preferably 1200°C or lower, more preferably 1000°C or lower.
• the temperature control method for the circulating top furnace gas is not particularly limited, and a general gas heating device may be used.
• the temperature of the top furnace gas is 300 to 400°C.
• the temperature at the height where the reduction progress rate A(y) is 60% may be measured, for example, using a thermometer installed in the direct reduction furnace, or may be determined by the above-mentioned calculation simulation.
• the amount of circulating top gas is preferably 10 to 50 volume % of the total amount of top gas (total amount of top gas discharged from the top gas outlet). If the amount of circulating top gas is less than 10 volume % of the total amount of top gas, it may be difficult to achieve the effect of increasing the reduction reaction rate as described above. On the other hand, if the amount of circulating top gas exceeds 50 volume % of the total amount of top gas, gas pressure loss increases around the circulating top gas injection section, which may cause problems with poor descent, such as iron oxide hanging.
• the amount of circulating top gas is preferably 10 to 50 volume % of the total amount of top gas.
• the amount of circulating top gas is more preferably 20% by volume or more of the total amount of top gas.
• the amount of circulating top gas is more preferably 40% by volume or less of the total amount of top gas.
• the amount of circulating top gas may be determined by performing the above-mentioned calculation simulation according to the planned operating conditions.
• [Reduction process] iron oxide is reduced using a reducing gas containing hydrogen as a main component, for example, according to the above formula (i) to obtain reduced iron.
• the reducing gas is subjected to a reduction reaction in the direct reduction furnace and then discharged as top gas from the top of the direct reduction furnace.
• the temperature of the top gas is, for example, 300 to 400°C.
• the method for operating a direct reduction furnace according to one embodiment of the present invention preferably further includes a heat supply step of supplying heat to the direct reduction furnace.
• the method for operating a direct reduction furnace according to one embodiment of the present invention uses a reducing gas containing hydrogen as a main component.
• the reduction reaction by CO is an exothermic reaction
• the reduction reaction by H2 is an endothermic reaction. Therefore, when a gas with an H2 concentration close to 100% by volume is used as the reducing gas, the temperature inside the direct reduction furnace may decrease. In this case, it is preferable to supply heat to the direct reduction furnace from the viewpoint of improving reaction efficiency by endothermic compensation.
• the heat source is not particularly limited.
• biomass combustion heat is suitable as a heat source.
• biomass combustion heat By utilizing the biomass combustion heat, it is possible to substantially eliminate not only the CO2 emitted from the direct reduction furnace but also the CO2 emitted from the entire production process.
• the biomass semi-carbonized biomass or tar obtained during the pyrolysis of biomass is preferred.
• Carbonized biomass and tar have a high calorific value per unit volume and are considered promising as alternative fuels to coal.
• Biomass combustion heat can be supplied through a reducing gas inlet or a circulating furnace top gas inlet.
• a reducing gas mainly composed of hydrogen the temperature tends to decrease from the upper to the middle of the direct reduction furnace. Therefore, supplying heat from the upper to the middle of the direct reduction furnace improves reaction efficiency. Therefore, it is preferable to supply heat through, for example, a circulating furnace top gas inlet.
• biomass is a general term for a certain amount of accumulated animal and plant resources and waste derived from these resources (excluding fossil resources).
• any biomass that produces charcoal through pyrolysis such as agricultural, forestry, livestock, fisheries, and waste, can be used.
• biomass with a high effective calorific value is preferred, such as woody biomass.
• woody biomass is forestry biomass.
• forestry biomass examples include: - Papermaking by-products such as pulp black liquor and chip dust, - Sawmill by-products such as bark and sawdust, branches, leaves, treetops, scraps and other forest residues, - Special forest products such as thinned timber from cedar, cypress, pine, etc., and waste logs for edible fungi, - Examples include firewood and charcoal forests such as chinquapin, oak, and pine, and short-rotation forestry materials such as willow, poplar, eucalyptus, and pine.
• waste-based biomass e.g. - General waste such as pruned branches from roadside trees in municipalities and garden trees in private homes, - Pruned branches of national and prefectural street trees, corporate garden trees, and industrial waste such as construction and building waste can also be suitably used as woody biomass.
• agricultural biomass e.g. - Waste and by-products
• energy crop sources such as rice bran, rapeseed, and soybeans can also be used favorably as woody biomass.
• Semi-carbonized biomass is biomass that is not completely carbonized but is partially carbonized, and preferably has a density of 700 to 850 kg/m 3.
• Semi-carbonized biomass can also be defined as biomass that has been subjected to a heat treatment on biomass, such as woody biomass, to reduce moisture and promote carbonization, thereby increasing density and strength.
• the method for producing semi-carbonized biomass is not particularly limited.
• raw biomass uncarbonized biomass
• woody biomass (raw biomass) with a density of 200 kg/ m3 is used as the raw material, it is possible to produce semi-carbonized biomass with a density of approximately 750 kg/ m3 .
• the composition of the reducing gas used e.g., a gas with a hydrogen concentration of 100% by volume
• a typical direct reduction ironmaking process e.g., a gas with a volume ratio of H 2
• the iron oxide used in the method for operating a direct reduction furnace according to one embodiment of the present invention is, for example, iron ore.
• Specific examples include lump iron ore (lump ore) and iron oxide pellets (spherical iron ore formed by solidifying powdered iron ore).
• the grade of the iron ore used as the iron oxide, i.e., the iron content is not particularly limited, but from the viewpoint of reduction in a shaft furnace, it is generally preferable that the iron content be 65% by mass or more.
• low-grade ore Fe content: 63% by mass or less
• from Australia which is an inexpensive and abundant resource, may also be used as needed.
• Fig. 2 is a schematic diagram showing an example of a conventional reduced iron production process.
• Fig. 3 is a schematic diagram showing an example of a reduced iron production method (production process) according to an embodiment of the present invention.
• reference numeral 1 denotes a shaft furnace
• 1a denotes iron oxide
• 1b denotes reduced iron
• 6 denotes a temperature control device
• 10 denotes a dust removal device
• 11 denotes a dehydration device
• 12 denotes a natural gas supply unit
• 13 denotes an air supply unit
• 14 denotes a heating reformer
• 15 denotes a reducing gas injection unit
• 16 denotes a CO2 separation unit
• 17 denotes a hydrogen supply unit
• 18 denotes a gas heater
• 19 denotes a methanol synthesis unit.
• furnace top gas containing mainly CO, CO 2 , H 2 , and H 2 O is discharged from the top of the shaft furnace.
• This furnace top gas is cleaned in a dust collector, and a portion of it is adjusted in moisture in a dehydrator to serve as raw material gas before being sent to a thermal reformer.
• a hydrocarbon-containing gas such as natural gas from a natural gas supply, is supplied to the thermal reformer together with the moisture-adjusted furnace top gas.
• the supplied gas is then heated in the thermal reformer.
• a reforming reaction then occurs, producing high-temperature reducing gas containing mainly CO and H 2 .
• This reducing gas is then directly injected into the reduction furnace through a reducing gas injection device.
• the remaining part of the furnace gas is dehydrated and then used as heating fuel in the combustion chamber of the thermal reformer.
• a thermal reforming device is not required.
• hydrogen gas gas having an H concentration of preferably 80 vol. % or more, more preferably 90 vol. % or more, and even more preferably 95 vol. % or more
• the heating temperature of the hydrogen gas is not particularly limited, but is preferably, for example, 900 to 1200°C.
• the heated hydrogen gas is injected as reducing gas into the shaft furnace by a reducing gas injection device.
• the reducing gas is subjected to a reduction reaction inside the shaft furnace, it is discharged from the shaft furnace as a furnace top gas containing mainly H and H O.
• a circulating furnace top gas which is a part of the furnace top gas, is injected again into the shaft furnace through the circulating furnace top gas injection port.
• the temperature of the circulating furnace gas may be optionally controlled by a temperature control device before being injected.
• the remaining furnace gas other than the circulating furnace gas is subjected to dust removal by a dust removal device, dehydration by a dehydration device, and then mixed with newly introduced H2 and injected into the shaft furnace as reducing gas.
• the top gas contains CO2 because the reducing gas contains CO and CH4 , it is preferable to separate and recover the CO2 contained in the remaining top gas other than the circulating top gas and use it for basic chemical synthesis (CCU) or storage (CCS).
• CCU basic chemical synthesis
• CCS storage
• the example in Figure 3 shows an example in which CCU is performed. That is, the example in Figure 3 shows an example in which CO2 is separated from the remaining top gas other than the circulating top gas using a CO2 separator, and then the separated CO2 is supplied to a methanol synthesis unit. This makes it possible to reduce CO2 emissions from the entire reduced iron production process to essentially zero.
• the heat source of the gas heater is not particularly limited.
• the heat from biomass combustion is suitable as the heat source.
• the biomass is preferably the semi-carbonized biomass described above or tar obtained in the process of pyrolysis of biomass.
• a method using a shaft furnace has been described in particular.
• the type of direct reduction furnace is not limited to this, and methods using a fluidized bed, rotary kiln, rotary hearth furnace (RHF), etc. are also possible.
• a shaft furnace is preferred as a direct reduction furnace due to its high production efficiency, availability, and operational stability.
• the majority of direct reduction furnaces operating worldwide are shaft furnace-type Midrex (registered trademark) and Hyl (registered trademark) models.
• a test for producing reduced iron was conducted by operating a direct reduction furnace using the reduced iron production process shown in Figure 3.
• the test conditions are as shown in Table 1 and below. Conditions not specified were those according to conventional methods or the general description in the specification.
• the vertical injection position of the circulating furnace top gas in Table 1 is a representative position (the middle position in the vertical direction). In some examples, the temperature of the circulating furnace top gas was controlled so that the temperature was in the range of 600 to 700°C.
• the height position at which the reduction progress rate A(y) was 30-60% was a position where the height y of the direct reduction furnace was approximately 2.5-6.0 m.
• the height position at which the reduction progress rate A(y) was 30-40% was a position where the height y of the direct reduction furnace was approximately 4.0-6.0 m.
• the height y of the direct reduction furnace was set to 0 m at the lowest point of the reduction zone of the direct reduction furnace (the boundary between the reduction zone and the cooling zone).
• biomass was combusted using a boiler, and the combustion heat was supplied directly to the reduction furnace.
• pressure loss The pressure drop (hereinafter also referred to as pressure loss) and reaction efficiency in the direct reduction furnace were evaluated using the following method.
• Evaluation 1 Evaluation of Pressure Drop in Direct Reduction Furnace (hereinafter also referred to as Evaluation 1)] As reduction disintegration of iron oxide progresses, the particle size of the iron oxide decreases. Therefore, the pressure drop in the direct reduction furnace generally tends to increase, for example, according to the Ergun equation. In Evaluation 1, the degree of reduction disintegration was evaluated based on the pressure drop in the direct reduction furnace.
• the pressure of the reducing gas (kPaG) at the reducing gas inlet and the pressure of the top gas (kPaG) at the top gas outlet were measured, and the difference between these pressures was calculated as the pressure loss in the direct reduction furnace.
• A Pressure loss in the direct reduction furnace is less than 50 kPa.
• B Pressure loss in the direct reduction furnace is 50 kPa or more and less than 70 kPa.
• C Pressure loss in the direct reduction furnace is 70 kPa or more and less than 100 kPa.
• D Pressure loss in the direct reduction furnace is 100 kPa or more.
• Evaluation 2 was performed based on the reduction rate of the reduced iron obtained as a product.
• the oxygen content (mass%) of iron oxide before reduction is the total oxygen content (mass%) contained in FeO and Fe2O3 contained in the iron oxide before reduction.
• the total oxygen content (mass%) contained in Fe2O3 was calculated on the assumption that the amount of Fe obtained by subtracting the amount of Fe in FeO (mass%) from the total iron content (mass%) of the iron oxide before reduction was present as Fe2O3 .
• the oxygen content (mass %) of reduced iron (obtained after reduction) is the total oxygen content (mass %) contained in FeO and Fe3O4 contained in the reduced iron.
• the total oxygen content (mass %) contained in Fe3O4 was calculated on the assumption that the amount of Fe obtained by subtracting the amount of Fe (mass %) in FeO and M.Fe from the total Fe (mass %) of the reduced iron is present as Fe3O4 .
• the reduced iron discharged from the direct reduction furnace is then melted in an electric furnace. Therefore, it is generally required that the reduction rate of the reduced iron obtained as the final product be 90% or higher. Therefore, cases A, B, and C were evaluated as having excellent reaction efficiency.
• Example No. 1 where the vertical injection position of the circulating top gas was adjusted to a height where the reduction progress rate A(y) was 30-40% and the amount of circulating top gas was adjusted to 10-50% by volume of the total top gas, the reduction disintegration was more effectively suppressed, further reducing pressure loss in the direct reduction furnace.
• the pressure loss in the direct reduction furnace was very small, and reduction disintegration was more effectively suppressed.
• the reaction efficiency was also extremely excellent.

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