Patent Application
20250059572
The present invention relates generally to steel-making and more particularly to the production of direct reduced iron (DRI).
Most of the steel currently made in the U.S.A. is made in electric arc furnaces that employ steel scrap as a feed material. A problem that arises therefrom is the presence in the scrap of tramp copper, which has an adverse effect on the quality of steel made from that scrap, limiting the products that can be made from copper-containing steel.
Copper cannot be metallurgically removed from steel, but the adverse effect of copper can be offset by diluting the concentration of copper in the molten steel in the electric arc furnace by adding thereto a feed material containing pure iron. One such material is direct reduced iron (DRI), a material made by reacting iron ore, an iron oxide, with a reducing gas at an elevated temperature below the melting point of iron ore and iron.
A brochure entitled “Midrex NG” describes a natural gas process for producing DRI, a process developed by Midrex Technologies, Inc. of Charlotte, NC, a world leader in developing DRI technology (Idem, page 3). The Midrex process employs a vertical shaft furnace into the top of which is introduced iron ore as pellets or lumps, which descend by gravity through the furnace. A combination of reducing gases, CO+H2, is introduced into about the mid-point of the furnace and ascends through the descending iron ore, converting the iron ore to direct reduced iron, which is removed through the bottom of the furnace. The reducing gases are converted to CO2 and H2O, which are removed from the furnace as top gas.
The feed material for producing the reducing gases is natural gas, essentially methane (CH4), which is introduced into a heated Midrex processing apparatus called a reformer, which is maintained at an elevated temperate at which methane cracks in the presence of a catalyst, breaking down into carbon and gaseous hydrogen (CH4→C+2H2). The carbon is oxidized into carbon monoxide by carbon dioxide in the furnace top gas recycled into the reformer (C+CO2→2CO). This is all summarized in the brochure in a generalized statement, as the reaction of CH4 and CO2 to produce 2H2 and 2CO (Id. Page 6).
The reformer is heated by fuel gas, including scrubbed top gas containing unreacted CO and H2, combusted with air by burners at the reformer (Ibid. pp. 3, 5). The reducing gases are introduced into the vertical shaft furnace at an elevated temperature, which heats the iron ore and sustains the reduction reactions in the furnace but does not melt either the iron ore or the DRI.
Hot DRI discharged from the bottom of the vertical shaft furnace has the character of sponge iron. It is very porous and will reoxidize quickly. It must be cooled, and special precautions must be taken during storage and shipping. This problem can be overcome by subjecting the hot DRI to a briquetting procedure producing hot briquetted iron (HBI), which, when cooled, can be stored and shipped without special precautions. Hot DRI can be transferred directly from the vertical shaft furnace to an electric arc furnace (EAF). The mode of transfer depends upon the distance between the two furnaces. Hot DRI is introduced into the EAF at an elevated temperature, which reduces the power required to melt the DRI compared to DRI introduced at ambient temperature. This is an advantage not available with HBI, which has been stored and transported at ambient temperature after briquetting.
The Midrex NG process uses natural gas, a fossil fuel, as the basis of the reducing gas for producing DRI and as a fuel for process heat. Carbon dioxide is emitted and is not offset, unlike the carbon dioxide emitted in processing using biogas.
Midrex has acknowledged that there may be circumstances where natural gas should be replaced with alternative gases, most of which are fossil fuel-based and have the same disadvantages regarding carbon dioxide emission as natural gas. (Midrex brochure “DRI Products & Applications” (2018), page 12, penultimate paragraph; Midrex brochure “MXCOL” (2014), pages 2-5.)
Verbio Nevada, LLC produces, at a plant in central Iowa, a biogas having a composition essentially the same as pipeline natural gas (99% methane), using corn plant residue as the starting material. Also known as stover, corn plant residue is the corn plant material remaining in the field after the corn has been harvested and comprises stalks, cobs, leaves and husks and any part thereof. The stover is baled and transported on flat-bed semi-trailers to a processing location (a) next to an ethanol manufacturing facility that uses corn as a feed material and (b) very close to a natural gas pipeline.
At the processing location, the corn plant residue is chopped into small pieces, less than ¼ inch in size, at a hammermill, screened and mixed with water. This mixture, together with cattle manure, is introduced into a fermentation plant, where the mixture undergoes anaerobic fermentation for about 25 days. The cattle manure provides the bacteria that decomposes the corn plant residue and creates a biogas consisting essentially of methane and carbon dioxide (50-60% CH4 and 40-50% CO2).
The carbon dioxide is removed from the biogas by processing that produces a gas containing 99% methane, the major part of which is injected into the nearby (natural gas) pipeline for eventual consumption downstream as fuel at a power plant. A minor part of that gas is retained as fuel for boilers at the adjacent ethanol manufacturing facility.
A fermentation vat residue remaining in the fermentation vat is removed, subjected to phase separation into fermentation vat residue solid and liquid, and the solid, called humus, is spread on farm fields as organic fertilizer and soil amendment.
The Verbio biogas processing plant receives corn plant residue from corn fields within a radius of 45 miles or so from the Verbio plant. This is the distance that a semi-trailer, loaded with bales of corn plant residue and traveling at 45 miles per hour, can cover in one hour.
The Verbio plant and its operation is described in the following publications:
Many, if not most, steel-making mini-mills lack commercially viable access to natural gas (either from gas fields or gas pipelines); but many mini-mills do have access to cornfields and the corn plant residue provided by these fields. As used herein, the phrase “access to cornfields” refers to corn fields within a 45-mile radius of the mini-mill.
The present invention is directed to a method that provides a mini-mill with locally produced DRI using biogas produced at a processing location associated with a direct reduction facility at the mini-mill and using locally accessed feed materials that are annually renewable and sustainable.
In essence, the present invention uses (a) the first part of the above-described Verbio process for making biogas, with modification, in combination with (b) a modification of the above-described Midrex process for making DRI. All of the equipment and processing steps employed from the Verbio process are in commercial use and commercially proven. All of the equipment employed from the Midrex process is in commercial use and commercially proven.
More particularly, the present method employs the Verbio anaerobic fermentation step and the steps preceding the fermentation step to produce a biogas consisting essentially of methane and carbon dioxide (CH4+CO2). But unlike the Verbio process, which removes the carbon dioxide from the biogas (in order to obtain 99% methane for use as pipeline gas), the present method retains the carbon dioxide in the biogas along with the methane therein. The equipment and processing steps Verbio employs to remove the CO2 are eliminated along with the capital and operating expenses associated with the CO2 removal.
The biogas of the present invention (CH4+CO2) is used as a replacement for the natural gas (CH4 alone) employed in the Midrex process—as feed gas for the Midrex reformer. At the temperature conventionally maintained in the reformer, and in the presence of the reformer catalyst, the methane in the biogas is broken down into carbon and hydrogen gas (CH4→C+2H2), and importantly, the retained carbon dioxide in the biogas converts some of the carbon to carbon monoxide (CO2+C→2CO).
The Verbio biogas reportedly contains 50-60% CH4 and 40-50% CO2. From a molar ratio standpoint, there is not enough CO2 to convert all C resulting from the breakdown of CH4, and this deficit in CO2 is made up by recycling CO2 from the top gas of the vertical shaft furnace, as in the Midrex process.
A principal difference between (a) the Midrex process and (b) the method of the present invention arises from the use, in (a), of natural gas (methane alone) versus the use, in (b), of a fermentation-derived biogas consisting essentially of methane plus retained carbon dioxide (e.g., 50-60% CH4+40-50% CO2). The processing in the vertical shaft furnace and downstream from there can be essentially the same in (b) as in (a).
Another difference is that, unlike the Midrex process, the present invention is not entirely dependent upon recycled top gases to form the CO reducing gas. Moreover, the employment of corn plant residue in accordance with the method of the present invention enables one to use, in the production of DRI, a gas for which the source is annually renewable by planting in successive years, with corn, the farm fields from which the corn plant residue was collected, or planting an equivalent acreage that is similarly accessible to the processing plant.
A purpose of the present invention is two-fold:
In addition, there is the potential for carbon credits. Not only does the present invention replace fossil fuel gas with a renewably sourced biogas, but also any CO2 greenhouse gas emissions arising from the use of the biogas in the present invention are offset by the carbon dioxide absorbed from the atmosphere during the growth of the corn plant, the residue of which is the source of the biogas.
Referring initially to
In accordance with the present invention, there is a biomass processing facility 14 (typically processing corn stover into a biogas) that is associated with direct reduction facility 12, 13. A biomass processing facility such as 14 is associated with a direct reduction facility 12, 13 when facility 14 is connected to reformer 12 by a pipeline that transports, from facility 14 to reformer 12, a biogas consisting essentially of methane and carbon dioxide (CH4+CO2), each present in double-digit percentages in the same order of magnitude.
In a preferred embodiment, steel mill 10 and its components 11-13 are physically associated with a stover processing facility 14. Two entities are deemed to be physically associated when one of them is at or next to or adjacent to the other, or if both entities are physically associated with a third entity in the manner described in the preceding part of this sentence.
The physically associated entities described in the preceding paragraph (entities 10-14) have access to a plurality of corn fields 15 that supply stover that is collected and baled by a baler 16 that produces stover bales that are loaded on flat-bed semi-trailers and transported to stover processing facility 14.
A corn field is deemed to be accessible when a semi-trailer loaded with stover bales from that field, and moving at an average speed of 45 miles per hour, can travel from that field to processing facility 14 in one hour.
A farm field further than 45 miles from processing facility 14 is deemed to be accessible if the cost of transporting stover bales from the more distant field, or from a collection site, is reasonably competitive with the cost of transporting stover bales from the 45-mile field.
Referring initially to
Large proportions of the stover starting material are composed of cellulose, hemicellulose and lignin, which are relatively difficult to degrade and convert to biogas absent the application of external expedients to intensify degradation. Many such expedients are known to those skilled in the art, and some of them are discussed in Schwarz et al. [0005]-[0021]. One such expedient employed in accordance with an embodiment of the present invention is thermopressure hydrolysis, discussed later below.
At the end of the retention time, vat 20 contains a fermentation residue composed essentially of undegraded solid starting material, microorganisms and water. The residue is removed from vat 20 and subjected to phase separation at 21, e.g., by centrifugation or pressing, to produce a liquid phase containing (i) the water and the microorganisms and (ii) a solid phase. The solid phase represents approximately 20% to 30% of the volume of the fermentation residue from vat 20 and contains essentially the gas-forming potential still available.
The solid phase is subjected to thermopressure hydrolysis at 22. This procedure comprises heating the solid phase at a temperature in the range 170° C.-250° C. at a pressure in the range 1 MPa-4 MPa for a time in the range 10 minutes-120 minutes. Other details are contained in Schwarz et al. at [0029]-[0034] and [0045]. In accordance with the present invention, the source of heat energy for the thermopressure hydrolysis is excess, unrecycled top gas from vertical shaft furnace 13 (to be discussed below).
As a result of undergoing the thermopressure hydrolysis, the cellulose, hemicellulose and lignin, in the solid phase of the fermentation residue, are broken down and made accessible to further conversion to biogas in a subsequent anaerobic fermentation step.
The subsequent anaerobic fermentation step can be performed in a second fermentation vat shown at 23 in
Alternatively, the solid phase that has undergone thermopressure hydrolysis can be recycled back to first vat 20, in a batch process fermentation step, along with the liquid phase from the phase separation at 21. The second fermentation step occurs for 20 days, and the total degree of degradation of the organic material is 80% for the two fermentation steps.
After the second fermentation step at vat 23 (or vat 20), the fermentation residue is separated into (i) a liquid phase and (ii) a solid phase composed of spent material called digestate or hummus, which can be spread on farm fields as a fertilizer or a soil amendment.
The biogas generated at stover processing facility 14 consists essentially of 25%-50% carbon dioxide (CO2) and 50%-75% methane (CH4), e.g., 55% methane and 45% carbon dioxide. Unlike the biogas produced by Verbio Nevada, the carbon dioxide in the biogas of the present invention is not removed from the biogas. There is retained, in the biogas directed to the direct reduction facility 12, 13, at least a major part (and preferably all) of the carbon dioxide generated at stover processing facility 14. Also, unlike the Verbio Nevada biogas production process, there is no consumption of a major part of the biogas, at a location physically unassociated with steel mill 10, for a purpose unrelated to the direct reduction of iron. Instead, the totality of the biogas is directed to direct reduction facility 12, 13 without there having been removed any of the carbon dioxide generated at stover processing facility 14.
Referring to
Catalytic material 25 normally comprises (a) nickel (sometimes cobalt), as the active ingredient to speed up the reactions in reformer 12, and (b) an alumina or magnesia carrier that gives the catalytic material its strength and shape.
Referring now to
Reformer box 28 is heated to, e.g., about 1000° C. The contents of reformer box 28, including catalytic material 25, are heated by heating arrangement 33 to an elevated temperature, e.g., 900° C. In reformer 12, some of the methane in the biogas is reacted with the retained carbon dioxide in the biogas, at the elevated temperature in the reformer, in the presence of the nickel catalyst, to reform that methane and produce a reducing gas comprising carbon monoxide and gaseous hydrogen (CO+H2). The reforming of the methane is performed upstream of, and apart from, vertical shaft furnace 13.
The reducing gas is discharged from reformer 12 at outlet 31 (
The retained carbon dioxide in the biogas is less than the stoichiometric amount required to react with all the methane in the biogas. The methane in the biogas that is unreformed, by the carbon dioxide in the biogas, undergoes reforming with carbon dioxide from another source, as discussed more fully below.
Vertical shaft furnace 13 includes four zones. There is a reducing zone that extends upwardly from bustle 46, where the reducing gas has been introduced, and a heating zone extending downwardly from upper furnace end 26 toward the reducing zone. Extending downwardly from the reducing zone is a transition zone, and below the transition zone is a cooling zone extending upwardly from lower furnace end 27 toward the transition zone.
The reducing gas is introduced into furnace 13 at a temperature in the range, e.g., 950° C.-1000° C. and heats furnace 13 and its contents. Iron ore in pellet and/or lump form is introduced into furnace 13 at upper end 26 and descends through furnace 13 under the urging of gravity, while hot reducing gas introduced at bustle 46 rises through the descending iron ore, which has been converted by the reducing gas to direct reduced iron that is removed from furnace 13 at lower end 27. The temperature in furnace 13 is below the melting point of both iron ore and direct reduced iron.
The conversion reactions between the reducing gas (CO+H2) and the iron ore (Fe2O3) produce, as reaction product gases, carbon dioxide (CO2) and water vapor (H2O), which are removed from furnace 13 as top gas, which also includes some unreacted reducing gas (CO and H2).
Some of the top gas is directed through a recycle line 41 into reformer feed gas line 43, where it joins the biogas. The recycled top gas is then conducted through heat-recovery system 35, where the recycled top gas is preheated. The preheated, recycled top gas is then directed into reformer 12, where methane in the biogas, unreformed by the retained carbon dioxide in the biogas, is reformed by reacting with the carbon dioxide in the recycled top gas, at an elevated temperature, in the presence of a catalyst (the nickel in tubes 25 (
The carbon dioxide in the top gas recycled into reformer 12 is less than the totality of the carbon dioxide introduced into the reformer. The biogas introduced into the reformer provides the reformer with carbon dioxide independent of the carbon dioxide provided to the reformer by the recycled top gas. As previously noted, the biogas that is received in the reformer comprises at least a major part of the carbon dioxide produced in the biogas at stover processing facility 14 (
The top gas leaving furnace 13 is processed at a scrubber 40. Part of the scrubbed gas is employed as a fuel for reformer 12, to which the fuel is directed by fuel gas line 39. The fuel gas, together with preheated combustion air, is combusted at burner arrangement 33 of reformer 12 (
There are four options for delivering direct reduced iron (DRI) discharged from lower end 27 of shaft furnace 13. These options are: cold DRI, hot briquetted iron (HBI), hot DRI delivered directly to electric arc furnace 11 through a gravity feed system or through a hot transport conveyor or a hot DRI transport vessel. These options are illustrated diagrammatically in Satyendra (2017) in
DRI is typically discharged at lower end 24 of furnace 13 at a temperature of about 400° C., a temperature that reflects cooling conducted in the cooling zone of furnace 13 by the introduction of cooling gas at ambient temperature (e.g., unheated biogas). When DRI is to be delivered directly to the electric arc furnace (EAF), the higher the delivery temperature, the less electric power required to melt the DRI at the EAF.
As previously noted, some of the methane in the biogas introduced into reformer 12 is reformed there into reducing gas by the retained carbon dioxide in that biogas. The methane in the biogas that is unreformed by the CO2 in the biogas, and that undergoes reforming, is reformed by carbon dioxide in the recycled top gas. The amount (a) of recycled top gas required to complete the reforming of the methane in the reformer, when the methane is part of a biogas also containing retained carbon dioxide, is less than (b) the amount of recycled top gas required to reform the same amount of methane when the methane is unaccompanied by retained carbon dioxide.
The difference between (a) and (b) is (c) excess unrecycled top gas that has an elevated temperature up to 400° C., which is the temperature of the top gas as initially discharged from furnace 13.
As previously noted, the recycled top gas (line 41) and the top gas employed as fuel for reformer 12 (line 39) are scrubbed at scrubber 40. The excess, unrecycled top gas is unscrubbed and uncooled, and as such, it is available for use as a heating medium. To this end, the excess, unrecycled top gas is directed, via line 47 located upstream of scrubber 40, for use as process heat for the thermopressure hydrolysis 22 performed at stover processing facility 14 (see
Thus, a biogas containing both methane and carbon dioxide in accordance with the present invention has heat energy advantages. For example, and more particularly, assuming that the gasification of corn stover produces a biogas consisting essentially of 50%-60% methane (CH4) and 40%-50% carbon dioxide (CO2), the biogas itself will provide 24%-36% of the CO2 required to convert, to carbon monoxide, the carbon produced by the cracking of the methane in the reformer (CH4→C+2H2). The rest of the CO2 required to convert the C to CO, 64%-76% of the required CO2, is provided by the CO2 in the recycled top gas. Absent the CO2 in the biogas (24%-36% of the required CO2), the recycled top gas would have to provide an additional 24%-36% CO2. The presence of the CO2 in the biogas itself frees up top gas containing the same amount of CO2, at a temperature up to 400° C., for use as a heating medium in the direct reduction facility or elsewhere in the steel mill or at the biomass processing facility when that facility is physically associated with the direct reduction facility.
Optimum productivity for the direct reduction process is achieved by maximizing the temperature of the furnace burden (iron ore undergoing conversion to DRI) and maximizing the quality of the reducing gas introduced into the furnace. The temperature of the furnace burden is maximized by increasing the temperature of the reducing gas (currently above 900° C.-1000° C.) (See Lüngen, et al. (2022) page 5), and the quality of the reducing gas is increased by increasing the CO and H2 contents. Various procedures and expedients for doing both are described in Satyendra (2017), at pages 7-8, previously incorporated herein by reference. The composition of the various DRIs produced at furnace 13 are detailed in Satyendra (2017), 9th page. DRI is predominantly elemental iron (Fe) with up to about 4% carbon (C), and there is some oxygen present as iron oxide (FeO). The carbon in DRI can be combined with iron, as cementite (Fe3C), or the carbon can be present as solid, elemental, uncombined carbon. Metallized iron, the total of elemental iron and cementite, is preferably in the range 94%-96% of the DRI when the DRI is used as a feed material at an electric arc furnace (Satyendra, 2013, page 3).
About two-thirds of the carbon in DRI is present in combined form (Fe3C) (Ibid page 4), which is preferable to uncombined carbon. The carbon in DRI performs a function during steel-making in an EAF, reacting with the FeO in the molten steel to reduce the oxygen content of the steel. Fe3C is a stable compound, but elemental carbon can be lost to combustion in the EAF atmosphere (Idem.). In addition, Fe3C dissolves in the steel bath and dissociates therein in an exothermic reaction yielding energy in the process whereas the dissolution of uncombined carbon in the steel bath is endothermic. (Ibid, page 5).
The carbon content of DRI can be increased by injecting natural gas (methane (CH4)) into the transition zone of the vertical shaft furnace. The heat in the transition zone, together with the iron particles there (a catalyst), cracks the methane (CH4→C+2H2) and deposits uncombined carbon in the DRI. The H2 released by the cracking flows upwardly, providing additional reduction (Satyendra, 2017, pages 8, 18).
In the present invention, natural gas (CH4) is replaced by biogas (CH4+CO2), and the biogas reacts with the carbon from the cracked methane (CH4) to produce carbon monoxide (CO2+C→2CO), which reacts with the iron in the DRI to produce cementite (3Fe+2CO→Fe3C+CO2), an exothermic reaction. (Satyendra, 2017, page 4). The biogas is introduced into the transition zone via line 49 (
There is not enough CO2 in the biogas to react with all the carbon from the cracked methane of the biogas, so there is deposition of unreacted carbon in the DRI, as well as cementite, when biogas is injected in the transition zone. In contrast, the injection of natural gas (methane without carbon dioxide) deposits only unreacted carbon in the DRI (Satyendra, 2017, supra, page 8).
Midrex has developed a procedure for increasing the carbon content of the DRI up to 4% with 85%-90% of the carbon combined as cementite (Fe3C). (Lüngen, page 5) Carbon monoxide made in the Midrex reformer is added to the transition zone with the natural gas. The carbon monoxide reacts with iron in the DRI, and the heat from the resulting exothermic reaction is used to produce additional carbon (via methane cracking (CH4→C+2H2) without sacrificing temperature (Idem.) The carbon monoxide from the Midrex reformer is mixed with gaseous hydrogen (CO+H2) as the CO leaves the reformer, and the two gases have to be separated before the CO is injected into the transition zone.
The introduction, into the transition zone, of biogas containing retained carbon dioxide, in accordance with the present invention, enables one to provide the DRI with cementite using internally generated carbon monoxide. As previously noted, the carbon dioxide in the biogas reacts with the carbon from the cracked methane to produce carbon monoxide (CH4→C+H2; CO2+C→2CO).
About one-third of the carbon resulting from the cracking of the methane in the biogas is converted to carbon monoxide. This reduces the amount of externally generated, reformer-produced carbon monoxide required to provide a predetermined amount of cementite and decreases the amount of reducing gases (CO+H2) diverted from introduction into the reduction zone of the vertical shaft furnace.
In the Midrex process, as described in Satyendra (2017), unheated natural gas is introduced into the transition zone. This has a cooling effect, which limits the amount of natural gas that can be introduced there. Preheating the natural gas allows larger amounts of gas to be employed, which would produce more carbon in the direct reduced iron (either as Fe3C or as solid carbon) and a higher DRI production rate, all without the quenching effect of unheated natural gas. Preheating is being explored, per Satyendra (Idem., page 8).
In accordance with the present invention, the biogas introduced into the transition zone is preheated using, as a heating medium, the excess unrecycled gas, uncooled and unscrubbed, as removed from the vertical shaft furnace at a temperature of up to 400° C. Heat transfer is produced using, e.g., a conventional gas-to-gas heat exchanger (not shown).
DRI undergoes cooling when unheated gas is introduced into the transition and cooling zones, per conventional practice. In such cases, DRI is discharged from the lower end of the vertical shaft furnace at a temperature of about 400° C. When DRI is to be delivered directly to a physically associated electric arc furnace, the DRI is cooled less, if at all, and is desirably discharged at a temperature in the range of 600° C.-700° C. The higher the discharge temperature, the less electrical energy required to heat the DRI to a temperature above its melting point. Preheating the biogas introduced into the transition zone of the vertical shaft furnace contributes to a higher discharge temperature.
The present invention is directed to the use of biogas made from biomass. A preferred form of biomass is grain plant residue.
Corn is a grain. The corn plant residue (stover) is a grain plant residue. The present invention is applicable to grain plant residues, generally, as well as other forms of biomass. The discussion herein has been primarily in the context of corn plant residue because corn fields are the grain fields most readily accessible in the areas of the U.S. where most mini-mills are located and where most of the DRI is consumed (Midwest and South). Other relevant grains include rye, oats, barley, wheat and the like.
