Patent Application
20120279353
This invention relates generally to a system and method for producing metallic iron nodules (NRI) by thermally reducing iron oxide in a moving hearth furnace. Metallic iron nodules have been produced by reducing iron oxide such as iron ores, iron pellets and other iron sources. Various such methods have been proposed so far for directly producing metallic iron nodules from iron ores or iron oxide pellets by using reducing agents such as coal or other carbonaceous material.
Various types of hearth furnaces have been described and used for direct reduction of metallic iron nodules (NRI). One type of hearth furnace used to make NRI is a rotary hearth furnace (RHF). The rotary hearth furnace is partitioned annularly into a drying/preheating zone, a reduction zone, a fusion zone, and a cooling zone, between the supply location and the discharge location of the furnace. An annular hearth is supported rotationally in the furnace to move from zone to zone carrying reducible material the successive zones. In operation, the reducible material comprises a mixture of iron ore or other iron oxide source and reducing material such as carbonaceous material, which is charged onto the annular hearth and initially subject to the drying/preheat zone. After drying and preheating, the reducible material is moved by the rotating annular hearth to the reduction zone where the iron ore is reduced in the presence of the reducing material, and then to the fusion zone where the reduced reducible material is fused into metallic iron nodules, using one or more heating sources (e.g., natural gas burners). The reduced and fused NRI product, after completion of the reduction process, is cooled on the moving annular hearth in the cooling zone to prevent reoxidation and facilitate discharge from the furnace. Another type of furnace used for making NRI is the linear hearth furnace such as described in U.S. Pat. No. 7,413,592, where similarly prepared mixtures of reducible material are moved on moving hearth sections or cars through a drying/preheating zone, a reduction zone, a fusion zone, and a cooling zone, between the charging end and discharging end of a linear furnace while being heated above the melting point of iron.
A limitation of these methods and systems of making metallic iron nodules has been their energy efficiency. The iron oxide bearing material and associated carbonaceous material generally had to be heated in a reduction furnace to about 2500° F. (about 1370° C.), or higher, to reduce the iron oxide and produce metallic iron nodules. The reduction process has generally required natural gas methane or propane to be burned to produce the heat necessary to heat the iron oxide bearing material and associated carbonaceous material to the high temperatures to reduce the iron oxide and produce a metallic iron material. Furthermore, the reduction process involved production of volatiles in the furnace that had to be removed from the furnace and secondarily combusted to avoid an environmental hazard, which added to the energy needs to perform the iron reduction. See, e.g., U.S. Pat. No. 6,390,810.
In the past, furnace systems for production of iron nodules heated by oxy-fuel burners had reduced efficiency due to loss of heat through the exhaust stack. Recovery of heat through preheating of oxygen and fuel entering the oxy-fuel burners has not been possible as oxygen gas and fuel sources contain too little mass to efficiently transfer heat from one location in the furnace to another, and tend to be more volatile when heated. Additionally, the oxy-fuel burners have produced flame temperatures resulting in internal burner temperatures causing damage to the burner and the furnace refractory. We have found a method and reduction furnace system for making metallic iron nodules that reduces the energy consumption needed to reduce the iron oxide bearing material to produce metallic iron nodules more efficiently.
A method of production of metallic iron is disclosed comprising the steps of
The roof of the furnace may be higher at the entry end than the discharge end of the furnace. The roof of the furnace may be sloped, stepwise or seesaw to provide the desired flow of gases through the furnace.
In one alternative, at least one burner may be provided adjacent the discharge end directing a flow of gases toward the entry end. Combustible fuel delivered to the burners may be natural gas, methane, propane, syn-gas, coal or other combustible fuel. Alternatively or in addition, a stream of diluted oxygen gas may be provided to control flame temperature and flame stability and inhibit damage to the refractories in the furnace. The stream of diluted oxygen gas may be oxygen diluted with carbon dioxide, air or nitrogen, and/or process flue gas from the exhaust stack or some other source. The stream of diluted oxygen gas is such as to provide the desired flame temperature and flame stability and heat to the furnace, and to provide a gas flow of appropriate mass to conduct heat in and through the furnace. However, if a sequestration of carbon dioxide is desired from the exhaust stack gases or a portion thereof the dilution of the oxygen should be with carbon dioxide or flue gas relatively high in carbon dioxide. The combustible fuel may also be preheated where desired to deliver more heat to the furnace through the flow of gases into the furnace.
The method may further include delivering from the roof or side walls of the furnace diluted oxygen gas along the furnace from the exit end to the entry end. The oxygen gas may be diluted with carbon dioxide, air or nitrogen, process flue gas from the exhaust stack or some other source the same as the combustible fuel delivered to the burner, but optionally with a different percentage of dilution along the furnace to tailor temperature control in the furnace as needed in the different stages of the conversion and fusion process in reducing the reducible material to form metallic iron nodules. However, again, if a sequestration of carbon dioxide is desired from the exhaust stack gas or a portion thereof, the dilution of the oxygen should be with carbon dioxide or flue gas relatively high in carbon dioxide. Like the flow of gas delivered to at least the burner at the discharge end of the furnace, the diluted oxygen gas may also be preheated for delivery along the furnace where desired to deliver more heat to the furnace through the roof or side walls of the furnace.
The flow of diluted oxygen gas may include delivering oxygen gas and at least one of carbon dioxide, flue gas, air, and nitrogen at a plurality of locations along the furnace. The flow of diluted oxygen gas may include between about 10% and 40% oxygen gas by volume, and may be between about 15% and 35% oxygen gas by volume. Alternatively, the flow of oxygen gas and may be between 25% and 40% oxygen gas by volume.
The method may include the step of delivering a flow of fuel into the furnace above the reducible material. Delivering a flow of fuel may include delivering fuel above the reducible material at a plurality of locations along the furnace. The fuel may be one selected from the group consisting of syn-gas, methane, propane, natural gas, coal and a combination of two or more thereof.
Additionally, the method may include sensing the temperature of the furnace at a desired location, and delivering the flow of fuel above the reducible material responsive to the sensed temperature.
The method may include processing at least a portion of the flue gas in a gasifier to produce syn-gas, and delivering a flow of the syn-gas into the furnace above the reducible material. The syn-gas may be preheated by directing flue gas through a heat exchanger and preheating the syn-gas in the heat exchanger.
The flue gas may be processed to produce a gas stream having a composition of at least 90% carbon dioxide, and may be at least 95% carbon dioxide, by oxidizing carbon monoxide and hydrogen, treating the gas stream to remove at least one of sulfur-containing and halogen-containing compounds, and condensing water vapor from the gas stream. The flow of diluted oxygen gas delivered to the furnace may include carbon dioxide from the gas stream processed from the flue gas. The gas stream of carbon dioxide may be preheated by directing flue gas through a heat exchanger and preheating the carbon dioxide in the heat exchanger.
The method may include sensing the temperature of the furnace at a desired location, and delivering the flow of diluted oxygen gas responsive to the sensed temperature. Alternatively or in addition, the method may include sensing the oxygen concentration in the flue gas, and delivering the flow of diluted oxygen gas responsive to the sensed oxygen concentration. The flow of diluted oxygen gas may be delivered to the furnace through a plurality of gas injection lances and/or gas injection ports.
The step of providing a layer of reducible material may include discrete portions being pre-formed briquettes or balls, or compacts made in situ.
The present method permits metallic iron to be produced while recovering waste heat from the exhaust stack. In the method, the reducible material in the conversion zone may be heated to the temperature between about 1800 and 2350° F. (about 980 and about 1290° C.). Further, reducible material in the fusion zone may be heated to the temperature between about 2400 and 2550° F. (about 1310 and about 1400° C.). Additionally, the hearth furnace may have a drying zone and the drying zone may be heated to a temperature between about 300-600° F. (150-315° C.). The hearth furnace may also include a cooling zone and/or a cooling zone outside the furnace downstream of the hearth furnace.
The present method of making metallic iron nodules may include the additional step of providing an overlayer of coarse carbonaceous material over at least a portion of the layer of reducible material either before introduction into the furnace as described in PCT/US2007/074471, filed Jul. 26, 2007, or adjacent introduction of the heated reducible material to the fusion zone as described in Ser. No. 12/569,176, filed on Sep. 29, 2009, with this application. The coarse carbonaceous material is greater than 6 mesh in size and may have an average particle size greater than an average particle size of the hearth material layer carbonaceous material. The coarse carbonaceous material may be between 6 mesh and ½ inch in size.
If desired, the oxygen gas may be delivered to the conversion zone and the fusion zone through a plurality of gas injection lances and/or gas injection ports.
A stoichiometric amount of reducing material is the amount necessary for complete metallization and formation of metallic iron nodules from a predetermined quantity of reducible iron bearing material. At least a portion of the reducible material has a predetermined quantity of reducible iron bearing material and between about 80 percent and about 110 percent of the stoichiometric amount of reducing material necessary for complete iron reduction of the reducible iron bearing material, or metallization, where the iron bearing material includes waste material such as mill scale as described in International Patent Application PCT/US2010/021790, filed Jan. 22, 2010. Alternatively, at least a portion of the reducible material has a predetermined quantity of reducible iron bearing material and between about 70 percent and about 90 percent of the stoichiometric amount of reducing material necessary for complete iron reduction of the reducible iron bearing material where the iron bearing material is magnetite and/or hematite.
The reducible iron bearing material may contain at least a material selected from the group consisting of mill scale, magnetite, hematite, and combinations thereof in the proportions as described above. The reducing material may contain at least a material or mixture of materials selected from the group consisting of, anthracite coal, coke, char, sub-bituminous coal, and bituminous coal.
Also disclosed is a hearth furnace for producing metallic iron comprising
An alternative hearth furnace is disclosed for producing metallic iron comprising
In either furnace, a plurality of gas injection lances may be positioned along the furnace adapted to deliver the flow of diluted oxygen gas. The hearth furnace may further comprise a plurality of gas injection ports positioned along the furnace adapted to deliver a flow of fuel into the furnace above the reducible material.
The hearth furnace may have a sloped roof higher at the entry end and lower at the discharge end. Alternatively or in addition, the roof of the furnace may be sloped, stepwise or seesaw to provide the desired counter-flow of gases through the furnace.
A temperature sensor may be provided adapted to sensing the temperature of the furnace at a desired location. The hearth furnace may include a fuel metering valve adapted to delivering the flow of fuel above the reducible material responsive to the sensed temperature. Alternatively or in addition, a metering device may be provided adapted to delivering the flow of diluted oxygen gas to the furnace responsive to the sensed temperature.
Alternatively or in addition, the hearth furnace may include a gas analyzing sensor adapted to sensing the oxygen concentration in flue gas exhausted from the exhaust stack. The fuel metering valve may be adapted to delivering the flow of fuel above the reducible material responsive to the sensed oxygen concentration. Also, the metering device may be adapted to delivering the flow of diluted oxygen gas to the furnace responsive to the sensed oxygen concentration.
A gasifier may be provided adapted to processing at least a portion of flue gas from the exhaust stack to produce syn-gas. The hearth furnace may include a scrubber adapted to processing at least a portion of flue gas from the exhaust stack to produce a gas stream comprising at least 90% carbon dioxide. Alternatively, the gas stream may comprise at least 95% carbon dioxide.
The hearth furnace may include a heat exchanger connected to at least a portion of the flue gas adapted to preheat the carbon dioxide or other gas for diluting oxygen in the heat exchanger. Alternatively or in addition, a heat exchanger may be connected to at least a portion of the flue gas adapted to preheat the flow of fuel in the heat exchanger.
The above summary of the present invention is not intended to describe each embodiment or every implementation of the present invention. A more complete understanding of the invention and its advantages will become apparent by referring to the following detailed description and claims in conjunction with the accompanying drawings.
As shown in block 112 of
The refractory material lining the interior of the furnace may be, for example, refractory board, refractory brick, ceramic brick, or a castable refractory material. More than one refractory material may be used in different locations as desired. For example, a combination of refractory board and refractory brick may be selected to provide additional thermal protection for any underlying substructure. The hearth 20 may include a supporting substructure that moves the refractory material (e.g., a refractory lined hearth) forming hearth 20 through the furnace. The supporting substructure may be formed from one or more different materials, such as, for example, stainless steel, carbon steel, or other metals, alloys, or combinations thereof that have suitable high temperature characteristics for furnace operation.
The hearth furnace 10 is divided into at least a conversion zone 13 capable of providing a reducing atmosphere for reducible material, and a fusion zone 14 capable of providing an atmosphere to at least partially form metallic iron material. A drying/preheating zone 12 may be provided in or adjacent the furnace housing capable of providing a drying/preheating atmosphere for the reducible material. Additionally, a cooling zone 15 capable of providing a cooling atmosphere for reduced material containing metallic iron material may be provided in or adjacent the furnace housing immediately following the fusion zone 14. As noted, the cooling zone may be in the furnace housing 11, but as shown in
In any case, the conversion zone 13 is positioned between the drying/preheating zone 12 and the fusion zone 14 and is the zone in which volatiles from the reducible material, including carbonaceous material, is fluidized, as well as the zone in which at least the initial reduction of metallic iron material occurs. The entry end of the hearth furnace 10, at the drying/preheating zone 12, may be at least partially closed by a restricting baffle 19 that may inhibit fluid flow between the outside ambient atmosphere and the atmosphere of the drying/preheating zone 12, yet provides clearance so as not to inhibit the movement of reducible material into the furnace housing 11. Additionally, a baffle 60 may be positioned between the fusion zone 14 and the cooling zone 15. The discharge baffle 60 may extend to within a few inches of the reducible material positioned on the hearth 20 as reducible material moves through the furnace housing 11 to inhibit direct fluid communication between the atmosphere of the fusion zone 14 and the atmosphere of the cooling zone 15, yet provide clearance so as not to inhibit the movement of reducible material out of the furnace housing 11. The baffles 19, 60 may be made of suitable refractory material such as silicon carbide or a metal material if the temperatures are sufficiently low. The pressure of the atmosphere in the hearth furnace 10 is typically maintained at a positive pressure compared to the ambient atmosphere to further inhibit fluid flow from the ambient atmosphere to the hearth furnace. The method of producing metallic iron nodules may therefore include reducing the reducible material in the hearth furnace 10 to metallic iron nodules substantially free of air ingress from the surrounding environment.
The hearth 20 provided within the furnace housing 11 may comprise a series of movable hearth cars 21, which are positioned contiguously end to end as they move through the furnace housing 11. Hearth cars 21 may move on wheels 22 that typically engage rails 23. The upper portion of the hearth cars 21 are lined with a refractory material suitable to withstand the temperatures for reduction of the iron oxide bearing material into metallic iron nodules as explained herein. The hearth cars are positioned contiguously end to end to form hearth 20 and move through the furnace housing 11, so that the lower portions of the hearth cars are not damaged by the heat generated in the furnace as reduction of the iron oxide-bearing material into metallic iron nodules proceeds. Alternatively, the hearth 20 may be a moving belt or other suitable conveyance medium provided with refractory material for the temperatures of the furnace atmospheres.
The zones of the furnace 10 are generally characterized by the temperature reached in each zone and the processing of reducible material in each zone. In the drying/preheating zone, moisture is driven off from the reducible material and the reducible material is heated to a temperature short of substantial fluidization of volatiles in and associated with the reducible material positioned on the hearth cars 21. The design is to reach in the drying/preheating atmosphere a cut-off temperature in the reducible material just short of substantial volatilization of carbonaceous material in and associated with the reducible material. This temperature is generally in the range of about 200-400° F. (90-200° C.), and is selected usually depending in part on the particular composition of the reducible material and the particular composition of carbonaceous material. One or more preheating burners 26 may be provided in the drying/preheating zone, for example, in the side walls of the furnace housing 11. The preheating burners 26 may be oxy-fuel burners or air/natural gas fired burners as desired, depending on the desired disposition of the stack gas from the drying/preheating zone and further processing of that stack gas.
The conversion zone 13 is characterized by heating the reducible material to drive off remaining moisture and most of the remaining volatiles in the reducible material, and heating the reducible material to at least partially reduce the reducible material. The heating in the conversion zone 13 may initiate the reduction reaction in forming the reducible material into metallic iron nodules and slag. The conversion zone 13 is generally characterized by heating the reducible material to about 1800 to 2350° F. (about 980° C. to about 1290° C.), or higher, depending on the particular composition and form of reducible material of the particular embodiment.
Referring to block 120 of
Combustion gases and other furnace gases may be delivered as a flow of flue gas from the furnace through an exhaust stack 130.
In the configuration shown in
The burners 16 are directed counter-current to the direction of travel of the hearth. The burners 16 may produce a nozzle velocity between about 300 and 500 feet per second. The gases from the burner and the combustion gases therefrom form a primary flow of gases from the burners 16 toward the entry end of the furnace toward the exhaust stack 130. Additionally, the burners 16 provides an induced draft of gases near the reducible material drawing the gases into the flame to be burned, shown as D in
The primary flow of gases may be supplemented with oxygen and/or fuel for combustion. At least one gas injection port may be provided adjacent the discharge end of the furnace adapted to deliver a flow of gases selected from a group consisting of combustible fuel, oxygen and carbon dioxide, oxygen and flue gas, oxygen and air, or a combination thereof, into the hearth furnace to heat the furnace to a temperature sufficient to at least partially reduce the reducible material.
As shown in
Alternatively or additionally, the hearth furnace may comprise a plurality of gas injection ports positioned along the sides of the furnace adapted to deliver a flow of fuel into the furnace above the reducible material. We have found it beneficial to place the injection ports directed to reduce impingement of oxygen onto the materials on the hearth.
The burners 16 may be oxy-fuel burners. Alternatively or additionally, one or more burners may be fuel-air burners.
The primary flow of gases from the burners 16 toward the entry end of the furnace may be selected from a group consisting of combustible fuel, oxygen and carbon dioxide, oxygen and flue gas, oxygen and air, or a combination thereof delivered into the hearth furnace at least through the burner to heat the furnace to a temperature sufficient to at least partially reduce the reducible material. The flow of gases may include a flow of diluted oxygen gas delivered adjacent the discharge end of the furnace and/or gas injection ports along the furnace directing a flow of gases toward the entry end. The stream of diluted oxygen gas may have an oxygen concentration as desired for combustion of fuel and volatiles, to control flame temperature and flame stability, and inhibit damage to the refractories in the furnace. The stream of diluted oxygen gas may be oxygen diluted with carbon dioxide, air or nitrogen, and/or process flue gas from the exhaust stack or some other source. As used herein, a flow of diluted oxygen gas may have an oxygen concentration as desired in a range from air to nearly pure oxygen. The stream of diluted oxygen gas is such as to provide the desired flame temperature and flame stability and heat to the furnace, and to provide a gas flow of appropriate mass to conduct heat in and through the furnace. If a sequestration of carbon dioxide is desired from the exhaust stack gases, then the dilution of the oxygen should be with carbon dioxide or flue gas relatively high in carbon dioxide. The stream of diluted oxygen gas may be preheated to deliver more heat to the furnace through the flow of gases into the furnace.
We have found that the system performance improves when the axial velocity, or the velocity of the flow of gas in the longitudinal direction along the furnace, of the primary flow of gases through the conversion zone 13 and fusion zone 14 is greater than about 4 feet per second near the hearth. In one alternative, the axial velocity is between about 5 feet per second and 10 feet per second near the hearth. In yet another alternative, the axial velocity is between about 4 feet per second and 15 feet per second near the hearth. Higher axial velocities may be may be achieved with consideration of the materials on the hearth to reduce entrainment of solids from the hearth into the flow of gasses. Prior furnaces provided localized movement of gas at increased velocities, for example, near burner ports, but could not provide increased velocity along the length of the furnace.
The axial flow of gasses through the furnace may be increased by providing a roof restriction 50 decreasing the cross-sectional area in one or more locations along the furnace. In the configuration shown in
Each roof restriction 50 includes a leading transition portion 52 and a trailing transition portion 54 in the direction of gas flow. The leading transition portion 52 extends from the roof 17 to the roof restriction 50 having a slope to direct to the flow of gas toward the restriction, and may act as a nozzle. The trailing transition portion 54 extends from the roof restriction 50 to the roof 17. The height of the roof 17 may be greater on the trailing side of roof restriction 50 than on the leading side.
In one example shown in
Other furnace configurations are contemplated to provide axial velocity of the primary flow of gases through the reduction zone and fusion zones between about 4 feet per second and 15 feet per second near the hearth. The desired height of the roof restriction 50 may vary with the gas selected. For example, a flow of gas having a high concentration of oxygen may use a roof height generally lower than when using a gas flow of a dilute oxygen stream, such as oxygen enriched air.
As shown in
The gas may be injected into the furnace using traverse pipe injectors 58 as shown in
In an alternative such as shown in
Referring to block 114 of
The hearth material layer may comprise a mixture of finely divided coal and a material selected from the group of coke, char, and other carbonaceous material found to be beneficial to increase the efficiency of iron reduction. The coal particles may be a mixture of different coals such as non-coking coal, non-caking coal, sub-bituminous coal, or lignite. The hearth material layer may, for example, include Powder River Basin (“PRB”) coal and/or char. Additionally, although up to one hundred percent coal is contemplated for use as a hearth material layer, in some embodiments the finely divided coal may comprise up to twenty-five percent (25%) and may be mixed with coke, char, anthracite coal, or other low-volatile carbonaceous material, or mixtures thereof. In other embodiments, up to fifty percent (50%) of the hearth material layer may comprise coal, or up to seventy-five percent (75%) of the hearth material layer may comprise coal, with the remaining portion coke, char, other low-volatile carbonaceous materials, or mixtures thereof. The balance will usually be determined by the amount of volatiles desired in the reduction process and the furnace.
Using coal in the hearth material layer provides volatiles to the furnace to be combusted providing heat for the process. The volatiles can be directly burned near the location of their volatilization from the coal, or may be communicated to a different location in the furnace to be burned at a more desirable location. Regardless of the location in the hearth furnace, the volatiles can be consumed to at least partially heat the reducible material. The carbonaceous material in the hearth layer also may provide a reductant source for reducing the iron bearing material in the furnace while still protecting the hearth refractories.
The hearth material layer is of a thickness sufficient to prevent slag from penetrating the hearth material layer and contacting the refractory material of the hearth 20. For example, the carbonaceous material may be ground or pulverized to an extent such that it is fine enough to prevent the slag from such penetration, but typically not so fine as to create excess ash. As recognized by one skilled in the art, contact of slag with the hearth 20 during the reduction process may produce undesirable damage to the refractory material of hearth 20. A suitable particle size for the carbonaceous material of the hearth layer is less than 4 mesh and desirably between 4 and 100 mesh, with a reasonable hearth layer thickness about ½ inch or more effective protection for the hearth 20 from penetration of the slag and metallic iron during processing. Carbonaceous material less than 100 mesh may be avoided because it is generally high in ash, and resulting in entrained dust that is difficult to handle in commercial operations. The mesh sizes of the discrete particles are measured by Tyler Mesh Size for the measurements given herein.
As shown in block 116 of
The iron-bearing material may include any material capable of being formed into metallic iron nodules via method 110 for producing metallic iron nodules as described with reference to
In one alternative, the reducible material may contain mill scale containing more than 55% by weight FeO and FeO equivalent, such as disclosed in International Patent Application PCT/US2010/021790, filed Jan. 22, 2010, incorporated herein by reference.
The iron-bearing material may be finely-ground or otherwise physically reduced in particle size. The particle size of the mill scale or mixture of mill scale and similar metallurgical waste may be at least 80% less than 10 mesh. Alternatively, the iron-bearing metallurgical waste may be of a particle size of at least 80% less than 14 mesh. In one alternative, the iron-bearing material may be ground to less than 65 mesh (i.e., −65 mesh) or less than 100 mesh (i.e., −100 mesh) in size for processing according to the disclosed method of making metallic iron nodules. Larger size particles, however, of iron-bearing material may also be used. For example, pellet screened fines and pellet plant wastes are generally approximately 3 mesh (about 0.25 inches) in average size. Such material may be used directly, or may be reduced in particle size to increase surface contact of carbonaceous reductant with the iron bearing material during processing. A smaller particle size tends to reduce fusion time in the present method.
Various carbonaceous materials may be used in providing the reducible material of reducing material and reducible iron-bearing material. The reducing material may contain at least a material selected from the group consisting of, anthracite coal, coke, char, bituminous coal and sub-bituminous coal such as Jim Walter coal and Powdered River Basin coal, or combinations thereof. For example, eastern anthracite and bituminous non-caking coals may be used as the carbonaceous reductant in at least one embodiment. However, sub-bituminous non-caking coal may also be used, such as PRB coal. Sub-bituminous coal may be useful in some geographical regions, such as on the Iron Range in northern Minnesota, as such coals are more readily accessible with the rail transportation systems already in place and in some cases are lower in cost and lower in sulfur levels. As such, western sub-bituminous coals may be used in one or more embodiments of the present method as described herein. Alternatively, or in addition, the sub-bituminous coals may be carbonized, such as up to about 1650° F. (about 900° C.), prior to its use. Other coals may be provided, such as low sulfur bituminous coal from Elkhorn seams from eastern Kentucky, as described below. In any case, the carbonaceous material in the reducible material may contain an amount of sulfur in a range from about 0.2% to about 1.5%, and more typically, in the range of 0.5% to 0.8%.
The amount of reducing material in the mixture with iron bearing material to form the reducible material will depend on the stoichiometric quantity necessary for complete metallic reduction of the iron in the reducing reaction in the furnace. Such a quantity may vary depending upon the percentage of iron in the iron-bearing material, the reducing material and the furnace used, as well as the furnace atmosphere in which the reducing reaction takes place. In some embodiments, where the iron bearing material is hematite or magnetite or mixtures thereof, the carbonaceous material in the reducible material may be between 70 and 90% of the stoichiometric amount to complete reduction of the iron in the iron-bearing material. Where the iron bearing material in the reducible material is mill scale or the like with high levels of FeO, the reducible material may include an amount of carbonaceous material that is between 80 and 110% of the stoichiometric amount needed to reduce the iron-bearing material to metallic iron. In other alternative embodiments where mill scale or the like is used for the iron bearing material, the quantity of reducing material necessary to carry out the reduction of the iron-bearing material is between about 85 percent and 105 percent of the stoichiometric quantity of reducing material needed for carrying out the reduction to metallize the iron, and may be between 90 percent and 100 percent.
In an alternative embodiment of the present method, a layer containing coarse carbonaceous material may also be provided over at least a portion of the layer of reducible material. The coarse carbonaceous material of the overlayer may have an average particle size greater than an average particle size of the hearth layer carbonaceous material. In addition or alternatively, the overlayer of coarse carbonaceous material may include discrete particles having a size greater than about 4 mesh or about 6 mesh, and in some embodiments, the overlayer of coarse carbonaceous material may have discrete particles with a size between about 4 mesh or 6 mesh and about ½ inch (about 12.7 mm). There may be of course some particles in the coarse carbonaceous material less than 4 mesh or 6 mesh in size in commercially made products, but the substantial majority of the discrete particles will be greater than 4 mesh or 6 mesh where a coarse carbonaceous material of particle size greater than 4 mesh or 6 mesh is desired. Finer particles of carbonaceous material that may be present in some commercially available compositions may be included but less desired. The coarse carbonaceous material may be selected from the group consisting of anthracite coal, bituminous coal, sub-bituminous coal such as PRB coal, coke, char, and mixtures of two or more thereof.
The conversion zone and fusion zone may be heated to a temperature sufficient to reduce the reducible material by heat from the oxy-fuel burners 16 and the delivery of oxygen gas and carbon dioxide through the gas injection ports 29 and/or the oxy-fuel burners 16. The flow of oxygen gas and carbon dioxide may be delivered at a plurality of locations along the furnace to aid in the combustion of volatiles evolving from the carbonaceous materials as well as the carbonaceous material in the hearth furnace providing additional heating to the furnace. The oxygen gas may be pure oxygen, which for purposes of this disclosure, includes commercially available oxygen gas having a concentration of at least 95% oxygen. The flow of diluted oxygen gas through the gas injection ports 29 along the conversion zone 13 and fusion zone 14 may be between about 10% and 40% oxygen gas by volume, and may be between about 15% and 35% oxygen gas by volume. Alternatively, the flow of diluted oxygen gas may be between about 25% and 40% oxygen gas by volume. In yet another alternative, the flow of diluted oxygen gas may include an oxygen concentration of between about 35% and 50%, or greater by volume. The stream of diluted oxygen gas may be oxygen diluted with carbon dioxide, air or nitrogen, and/or process flue gas from the exhaust stack or some other source. The stream of diluted oxygen gas is such as to provide the desired flame temperature and flame stability and heat to the furnace, and to provide a gas flow of appropriate mass to conduct heat in and through the furnace. The stream of diluted oxygen gas may be varied by an amount of diluent such as carbon dioxide to regulate the oxygen concentration in the furnace.
As shown in
Referring to block 118 of
The flow of diluted oxygen gas into the furnace may be regulated along the length of the conversion zone 13 and fusion zone 14 of the furnace 10 according to the concentration of carbon monoxide and volatiles fluidized from the reducible materials to more efficiently oxidize the carbon monoxide and combust the volatiles. The fluidization of volatiles is dependent upon the composition of the carbonaceous materials charged into the furnace and the temperature profile of the furnace. A higher flow of oxygen gas may be directed to where higher levels of carbon monoxide are found along the length of the conversion zone and fusion zone, such as toward the beginning of the conversion zone. Less oxygen gas may then be directed to where lower levels of carbon dioxide are present within the furnace, such as the downstream end of the fusion zone. The amount of oxygen gas delivered to the furnace may be varied by increasing or decreasing the flow of diluted oxygen gas, or controlling the amount of oxygen in the stream of diluted oxygen gas, or a combination of both.
By increasing the amount of carbon monoxide and hydrogen gas oxidized in the furnace 10, the resultant flue gas from the exhaust stack 130 of the furnace has a reduced concentration of carbon monoxide and hydrogen and increased concentrations of carbon dioxide and water vapor, as compared to the flue gas generated when oxygen gas is delivered to the conversion zone 13 and fusion zone 14 of the furnace 10 in more even concentration along the furnace. Decreasing the carbon monoxide and hydrogen content in the flue gas results in reducing if not eliminating the need for a thermal-oxidizer in the flue gas stream to oxidize the flue gas, as described below with reference to
While the oxygen gas may be delivered into the conversion zone 13 and fusion zone 14 of the furnace 10 at a desired ratio of pounds of oxygen per pound of iron in the reducible iron bearing material in the conversion zone and fusion zone, the flow of diluted oxygen gas may be varied along the length of the furnace. The flow of oxygen gas to certain points along the furnace may cause the ratio of oxygen gas to iron in reducible iron bearing material to be higher than said ratio to the conversion zone 13 and fusion zone 14 overall. For example, the ratio of oxygen gas to iron in the reducible material delivered at the upstream end of the conversion zone 13 may be higher than said overall ratio of oxygen gas to iron in the reducible material delivered to the conversion zone 13 and fusion zone 14. Additionally or alternatively, the flow of oxygen gas may be higher in certain other parts of the furnace than the overall ratio of oxygen gas to iron in the reducible material, such as near the downstream end of the conversion zone 13 and the upstream end of the fusion zone 14, where higher concentrations of hydrogen gas and carbon monoxide are likely found. The flow of oxygen gas may be lower in certain other parts of the furnace, such as near the downstream end of the fusion zone 14, where excess oxygen may not be desired. Again, by this regulation of oxygen gas, the hydrogen gas and carbon monoxide are more likely oxidized in the furnace, thereby increasing the concentration of water vapor and carbon dioxide in the flue gas while decreasing the concentration of hydrogen and carbon monoxide in the flue gas.
The oxy-fuel burners 16 may also be fired with a fuel, for example natural gas, methane, propane, fuel oil, and coal, at the start of a campaign to heat each zone of the furnace to sufficient temperature, for example, at least about 2350° F. (about 1290° C.) in the conversion zone and at least about 2550° F. (1400° C.) in the fusion zone. Subsequently, the oxygen gas may be continuously delivered into the conversion and fusion zones through the oxygen ports 29 and/or through the oxy-fuel burners 16 at a rate sufficient to maintain the zones at the temperatures to reduce reducible material in the furnace and produce metallic iron nodules. Note the oxygen gas may also be delivered during start up to assist in heating the zones of the furnace to desired temperatures to reduce the reducible material in the furnace and produce metallic iron nodules. In some embodiments, once the rate of oxygen gas delivery is sufficient to maintain the desired temperature through combustion of the evolved volatiles, carbonaceous material from the furnace charge, and reductant gases delivered to the furnace, the delivery of the combustible fuels through the oxy-fuel burners may be substantially reduced and may be shut off to avoid fuel usage and more efficiently operate the furnace to produce metallic iron nodules in accordance with the present method.
In any case, the metallic iron nodules, slag and related material are cooled in cooling zone 15 from its formation temperature in the fusion zone 14 to a temperature at which the metallic iron nodules can be separated and the slag and related materials processed. This temperature is generally below 800° F. (425° C.) and may be below about 550° F. (290° C.). Alternatively, the temperature of the material on the moving hearth 30 after the cooling zone 15 may be between about 300 to 600° F. (150-315° C.). The cooling can be achieved by injection of nitrogen or carbon dioxide through nozzles 96 in the roofs and/or side walls of the furnace housing or external to the furnace housing. Alternatively or in addition, the cooling step may be accomplished or completed outside the furnace housing 11 by water spray 93 in the cooling zone 15, where provisions are made for water handling within the system. Alternatively or additionally, a system of coolant tubes 94 may be positioned over the moving hearth 20 as shown in
In the present method with an oxygen and carbon dioxide gas stream delivered to the furnace, stack emissions produced with the present method are sufficiently high in carbon dioxide that a thermal oxidizer may not be necessary in the flue gas stream. By reducing the moisture and further cleaning the flue gas stream exhausted through the stack 130, a gas stream can be produced having at least 90% carbon dioxide, and may be at least 95% carbon dioxide. Referring to block 218 of
With reference to block 222 of
The water vapor may be removed from the gas stream by cooling to a temperature at which any water vapor present in the gas stream would condense, for example at a temperature below about 212° F. (about 100° C.) at atmospheric pressure. The remaining gas stream contains a high concentration of CO2, and may exit the scrubber 140 between about 100° F. and 500° F. (between about 40° C. and 260° C.). Alternatively, the gas stream may be cooled to a temperature of about 80° F. (27° C.). A blower 142 may be provided to convey the CO2 stream cooled to a temperature suitable for the blower 142 as desired. The cooled carbon dioxide stream is shown as C in
As sulfur-containing and halogen-containing compounds are not desirable in the carbon dioxide gas stream, these compounds may also be removed from the gas stream in the scrubber. The gas stream may be treated using lime and/or limestone, which may react with sulfur dioxide present in the gas stream to form calcium sulfate dihydrate (CaSO4.2H2O), also known as gypsum.
It is to be understood that the gas stream may be cooled to condense the water vapor before or after the gas stream is treated with lime and/or limestone in order to remove sulfur-containing and/or halogen-containing compounds.
Once the gas stream has been treated and water has been condensed therefrom, a gas stream containing at least 90% or 95% carbon dioxide remains. This gas stream having a high carbon dioxide concentration is a salable product or may be subsequently processed. The cooled carbon dioxide stream, shown as C in
The CO2 stream may be utilized in the furnace 10 in producing iron nodules by the present methods. The CO2 stream may be heated and directed into the furnace housing 11 as desired. The flow of oxygen gas and carbon dioxide may include carbon dioxide from the gas stream processed from the flue gas. Additionally, the carbon dioxide may be preheated before delivery to the furnace. The CO2 may be directed through a heat exchanger 144. At least a portion of the flue gas may be directed through the heat exchanger 144 to transfer heat from the flue gas to the carbon dioxide to recover heat from the flue gas exiting the furnace 10. The heated CO2 stream, shown as B in
Lower flame temperatures may be used to decrease the wear of burner components exposed to excessive heat, increasing burner life and reducing maintenance. Flame temperatures are controlled by the concentration of oxygen in the stream. Flame temperature increases with increasing oxygen concentration. The adiabatic flame temperature for an oxy-fuel burner operating on pure oxygen and methane is approximately 5000° F. (2760° C.), while the adiabatic flame temperature for an oxy-fuel burner operating on a 30% oxygen/70% carbon dioxide mixture approaches that of an air/natural gas flame at about 3800° F. (about 2090° C.). Since flame temperature is dependent on the oxygen concentration, the delivery of oxygen to the oxy-fuel burner may be diluted with carbon dioxide to adjust the flame temperature as desired. Diluting the oxygen stream with carbon dioxide reduces the relative concentrations of fuel and oxidant thereby decreasing flame temperature. Additionally, dilution of oxygen with carbon dioxide enables recovery of a portion of the waste heat to the furnace, such as by direct transfer of gases, or using heat exchange with hot flue gases. As discussed above, the CO2 may be directed through the heat exchanger 144 before such mixing with the oxygen to recover heat from the flue gas exiting the furnace 10. The CO2 may be preheated to about 750° F. (about 400° C.) in the heat exchanger 144. Alternately, the CO2 may be preheated to between about 400° F. (about 200° C.) and 1500° F. (about 810° C.) in the heat exchanger 144.
Alternately or in addition, at least a portion of the flue gas may be directed into a gasifier 146. The gasifier 146 may be utilized to process carbon-containing materials such as by-products from the iron reduction process, including ash, char and coal powders, slag, and other waste materials. The flue gas may be processed in the gasifier 146 with injected oxygen and carbon-containing materials to produce a mixture of CO and H2, or syn-gas. The syn-gas stream, shown as A in
Ports 74 may be positioned capable of injecting fuel or volatiles or other gases into the furnace above the reducible material. A plurality of ports 74 may be positioned along the furnace for delivery of fuel above the reducible material at a plurality of locations along the furnace. The delivery of fuel or volatiles or other gases may provide heat to regions of the furnace beyond the reach of direct radiation from the burner flame. The fuel delivered through the ports 74 may be syn-gas, shown as A in
As noted, cooling may begin in the furnace housing 11. In one alternative, a fuel or reductant gas may be delivered over the reduced iron material in the fusion zone 14 through gas ports 74 adjacent the discharge end of the furnace. The flow of reductant over the reduced iron bearing material may begin cooling the reduced iron bearing material as it exits the fusion zone, and the reductant provides additional fuel to the fusion zone to maintain temperatures as desired.
As discussed above, the flow of oxygen gas may be regulated along the length of the furnace according to the concentration of carbon monoxide and volatiles fluidized from the reducible materials to more efficiently oxidize the carbon monoxide and combust the volatiles. A higher flow of oxygen gas may be directed to where higher levels of carbon monoxide are found along the length of the furnace, such as toward the beginning of the conversion zone. Less oxygen gas may then be directed to where lower levels of carbon monoxide are present within the furnace, such as the downstream end of the fusion zone. In any event, oxygen gas may be diluted with carbon dioxide to regulate flame temperatures and furnace temperatures as desired.
Oxygen gas and preheated carbon dioxide may be mixed and delivered into the furnace through the roof injection lances or gas injection ports 29 to maintain furnace temperatures as desired. The delivery of carbon dioxide and oxygen gas through the gas injection ports 29 may be regulated by controlling the flow of oxygen gas and carbon dioxide, the oxygen concentration, and the preheat temperature carbon dioxide.
A metering system may be provided capable of regulating the amount of oxygen and carbon dioxide delivered into the furnace. As shown in
Alternatively or in addition, the metering system and metering device 88 may be adapted to deliver air or nitrogen, flue gas, or other gas to dilute the oxygen gas as desired.
The metering device 88 may increase or decrease the flow of oxygen gas to the furnace responsive to furnace temperatures in the furnace at desired locations. A temperature sensor may be provided in the furnace at a desired location to sense the temperature of the furnace at the desired location. Then, the flow of oxygen gas may be increased or decreased as desired responsive to the sensed temperature.
As shown in
A gas analyzing sensor may be positioned capable of analyzing the flue gas from the exhaust stack 130. The gas analyzing sensor may be provided to determine the concentration of oxygen in the flue gas. The flow of oxygen gas into the conversion zone 13 and fusion zone 14 may be increased or decreased as desired responsive to the sensed oxygen concentration. Alternatively or in addition, the flow of fuel or volatiles or other gases into the furnace above the reducible material through ports 74 along the furnace may be increased or decreased as desired responsive to the sensed oxygen concentration. Alternatively or in addition, the gas analyzing sensor may be provided to determine the concentration of carbon monoxide in the flue gas. Then, the flow of oxygen gas and the flow of fuel may be increased or decreased as desired responsive to the sensed carbon monoxide concentration.
In an oxygen and carbon dioxide system, the metering devices 88 may comprise a CO2 gas valve controlling the flow of carbon dioxide, an oxygen gas valve controlling the flow of oxygen gas, and controller configured to operate the CO2 gas valve and oxygen gas valve as desired responsive to the sensed temperature, the oxygen concentration in the flue gas, the carbon monoxide concentration in the flue gas, or a combination thereof. Alternatively or in addition, the metering devices 88 may be configured to operate responsive to operator input.
The fuel metering valve 90 may comprise a gas valve controlling the flow of fuel, volatiles, or other gases, and controller configured to operate the gas valve as desired responsive to the sensed temperature, the oxygen concentration in the flue gas, the carbon monoxide concentration in the flue gas, or a combination thereof. Alternatively or in addition, the fuel metering valve 90 may be configured to operate responsive to operator input.
The flow of oxygen gas and carbon dioxide may be directed to enter the furnace at an angle θ to the furnace roof to flow down towards the bed in the direction of flue gas travel such as shown in
The combination of oxygen gas delivery through the gas injection ports 29 near the roof and fuel delivery through the ports 74 near the reducible material may result in a semi-stationary flame front 76 approximately midway between the roof and the reducible material, radiating energy back to the reducible material above the hearth.
Optionally, a horizontal baffle or hood 78 may be positioned between the reducible material and the burner flame 76 in at least a portion of the furnace near the discharge end positioned to draw a flow of furnace gases under the horizontal baffle. The horizontal baffle 78 may be positioned in the fusion zone 14 and/or at least a portion of the conversion zone 13 enhancing the fluid flow near the reducible material. A flow of furnace gases may be drawn under the horizontal baffle 78 to flow with the direction of movement of the hearth. In this location, the flow of gases under the hood will include mostly CO2 and water vapor if there is no syn-gas injection. Alternatively or in addition, a reducing material may be delivered adjacent an edge of the horizontal baffle 78 positioned to flow beneath the horizontal baffle with the flow of furnace gases. The reductant may be syn-gas delivered beneath the hood to maintain a high reducing potential under the hood. Alternatively or in addition, the syn-gas may be delivered through port 74′ positioned adjacent the edge of the hood such that the flow of furnace gases draws the syn-gas beneath the hood. Alternatively or in addition, carbonaceous material such as coal may be delivered above the hearth at a position adjacent the edge of the hood such that the flow of furnace gases draws the carbonaceous material under the hood providing a gasified product. This gas stream emerges at the discharge end of the hood and is drawn up into the flame where volatiles are consumed.
Referring now to
The gas stream ports 84 may be spaced around the annular gas port 83 with approximately equidistant spacing between at least a portion of the gas stream ports 84. As shown in
The oxygen gas may be delivered through the gas stream ports 84 having a nozzle velocity less than about 100 ft/s. Alternatively, the gas nozzle velocity may be between about 60 ft/s and about 200 ft/s. Optionally, the oxygen gas may be mixed with carbon dioxide, nitrogen or other inert gas to reduce the concentration of oxygen in the gas stream ports 84. The gas through the gas stream ports may have an oxygen concentration between about 75% and 95% oxygen by volume. Alternatively, the gas through the gas stream ports may have an oxygen concentration between about 90% and 100% oxygen by volume.
At least one supplemental oxygen port 85 may be provided positioned to direct a stream of oxygen to the fuel flow through the flow from the annular gas port 83. In the embodiment of
As discussed above, carbon dioxide may be delivered through the oxy-fuel burner 16. In the burners of
The flow of carbon dioxide may be used to recover waste heat from the system. For example, as shown in
The flow of carbon dioxide and oxygen through the burner block provides cooling of the burner, reducing thermal gradients and stresses. The delivery of carbon dioxide through the oxy-fuel burner 16 may be regulated to control the oxygen concentration in the oxy-fuel burner and the flame temperature. Flame temperature is dependent on the oxygen concentration through the burner.
While the invention has been illustrated and described in detail in the foregoing drawings and description, the same is to be considered as illustrative and not restrictive in character, it being understood that only illustrative embodiments thereof have been shown and described, and that all changes and modifications that come within the spirit of the invention described by the following claims are desired to be protected. Additional features of the invention will become apparent to those skilled in the art upon consideration of the description. Modifications may be made without departing from the spirit and scope of the invention.
This international patent application claims priority to and the benefit of U.S. patent application 61/246,787, filed Sep. 29, 2009.
The present invention was made with support by the Department of Energy, Sponsor Award DE-FG36-05GO15185. The United States government may have certain rights in the invention.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US10/50730 | 9/29/2010 | WO | 00 | 5/18/2012 |
| Number | Date | Country | |
|---|---|---|---|
| 61246787 | Sep 2009 | US |
