Patent Publication Number: US-2022235426-A1

Title: Method and system for producing steel or molten-iron-containing materials with reduced emissions

Description:
BACKGROUND OF THE INVENTION 
     The present invention generally relates to methods and systems for producing steel or similar molten-iron-containing materials in melting or smelting furnaces utilizing pre-reduced iron ore, known also as direct reduced iron (DRI) or sponge iron, wherein the emission of CO 2  and other greenhouse gases is significantly low. 
     Production of steel contributes with an important proportion of the CO 2  industrial emissions mainly due to the use of coal as energy source and raw material in integrated steelmaking plants comprising blast furnaces and blast oxygen converters. Steel is also produced through an alternate route comprising direct reduction of iron ores 
     Although several proposals of methods and systems for recovery of heat from hot gases to produce steam and power may be found in the prior art, the present invention offers an integrated system for “green” steelmaking wherein the CO 2  footprint is considerably decreased since the reducing agent is hydrogen being transformed to water in an iron reduction facility and said water used to produce hydrogen by electrolysis with great economic advantages. Such an integrated system is the gist of the present invention. 
     Applicants have found U.S. Pat. No. 8,587,138 to Statler et al. as an example of some proposals to utilize the heat generated in the melting process of metals and smelting of ores to generate electricity. Statler however does not disclose or suggest the integration of a direct reduction plant with the metal melting plant to minimize the CO 2  emissions of the steelmaking process. 
     Some proposed methods for decreasing the CO 2  emissions in steelmaking refer to utilization of renewable energy sources, such as solar, wind and biomass energy to produce electricity which is in turn used to produce hydrogen by electrolysis, however these systems are still under development and the cost of such electricity is still high as compared to the grid power available from other sources. 
     Almost 70% of the energy losses in EAF (Electric Arc Furnace) steelmaking are associated with the off gas, through which about 15% of the energy input is lost as sensible heat. Un-combusted CO evolved during the melting and refining process carried out in the EAF is burnt with air in a post-combustion chamber for the off gas. It is estimated that more than 25% of the total energy input of the EAF (Electric Arc Furnace) can be recovered and utilized. This heat recovery of the EAF off gas however is not widely practiced due to the harsh environment of the fume system of the EAF and the discontinuity of the gases generation as the EAF process is a batch-process. 
     The heat of the EAF (Electric Arc Furnace) off gas can be recovered using high-pressure tubes designed to withstand the fume system conditions at pressures of 15 to 40 bar and produce high-pressure steam at 216° C. The temperature of the off gas after the heat recovery step is reduced to about 600° C. Using a steam accumulator, the high-pressure steam production can be utilized in a continuous manner irrespective of the EAF (Electric Arc Furnace) process cyclic nature. Steam production at an average rate of 20 t/h from 140 t/h EAF (Electric Arc Furnace) has been demonstrated by Tenova S.p.A. A second heat recovery stage can be added to use the heat content of the fume gases after the steam production, wherein the temperature of the off gas is lowered from about 600° C. to about 200° C. using a standard waste heat boiler. Utilizing the two heat recovery stages, about 75% to 80% of the total energy content of the EAF off gas can be recovered. This recovered energy amounts to about 24,000 megawatt hour (MWh/year). 
     The present invention utilizes heat energy produced in steelmaking processes that otherwise is wasted by an integration of a DRI melting furnace, a DRI production plant and an electrolysis unit to generate hydrogen thus decreasing the use of hydrocarbons to produce said DRI and consequently the CO 2  emissions to the atmosphere. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method for producing molten steel or molten-iron-containing materials with reduced emissions of carbon dioxide comprising producing DRI in a direct reduction furnace with a reducing gas comprising hydrogen; melting at least a portion of said DRI in a melting furnace and generating hot gases; producing steam and/or hot water using the heat contained in said hot gases. From said steam is produced hydrogen and/or hot water by electrolysis and at least a portion of said hydrogen may be fed to said direct reduction furnace as a component of said reducing gas to produce said DRI (Direct Reduced Iron). 
     The invention also provides a system for producing molten steel or molten-iron-containing materials wherein the CO 2  emissions are minimized by using hydrogen in a DRI production facility which is produced through electrolysis using energy from a DRI melting facility. 
     In an embodiment, the system of the invention comprises a direct reduction furnace to produce DRI; a DRI melting furnace (EAF) to melt said DRI generating hot gases; a heat recovery unit to produce steam and/or hot water using the heat contained in said hot gases; and an electrolysis unit to produce hydrogen from said steam and/or hot water, which hydrogen is fed to said direct reduction furnace to produce DRI. 
     In an embodiment, the system of the invention further comprises an electric power generator to produce electric energy utilizing steam from the heat recovery unit, which electricity is used in said electrolysis unit to produce hydrogen. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  shows a system for producing steel comprising a direct reduction plant to produce DRI, a DRI melting furnace generating gases at high temperature, a heat recovery unit wherein steam is produced to generate electricity which is used in an electrolysis unit to produce hydrogen utilized in said direct reduction plant. 
         FIG. 2  shows an embodiment of the steelmaking system of  FIG. 1  with some more details of the main components of the steel making system. 
     
    
    
     DETAILED DESCRIPTION OF SOME PREFERRED EMBODIMENTS 
     Referring to  FIG. 1 , numeral  10  generally designates a steelmaking system with reduced CO 2  emissions comprising a direct reduction facility  12  where iron oxides  14  are chemically reduced in solid form by means of a reducing agent such as hydrogen  16  and which are transformed to direct reduced iron (DRI)  18  in solid form containing metallic iron. The DRI  18  is further processed, alone or mixed with steel scrap, in a melt-shop  20  typically comprising electric-arc furnaces (EAF) and ladle furnaces for metallurgical processing to produce steel  22  or other molten iron containing products such as pig iron or ferroalloys. The EAF of the melt-shop  20  produces an important amount of gases  24  at high temperature, in the order of from 1000° C. to 1400° C. The heat of these hot gases  24  is recovered in a heat recovery unit  26  by means of suitable heat-exchangers to produce steam  28 , and/or hot liquid water at a temperature below about 100° C., from water feed  30  and the colder flue gases  32  are treated in a manner known in the art before being properly released to the atmosphere. 
     Steam  28 , and/or hot liquid water, produced utilizing the heat of the hot gases  24  is fed to an electrolysis unit  34  where water is split into a hydrogen stream  16  and an oxygen stream  36 . The hydrogen stream  16  is fed to a direct reduction facility  12  to produce DRI  18  from iron oxides bearing materials  14 . Iron oxides  14  are chemically reduced to metallic iron (DRI)  18  by hydrogen  16  whereby water  38  is produced by the reaction: Fe x O y +yH 2 →xFe+yH 2 O. The oxygen  36  can be used in the direct reduction plant  12  and/or in the DRI melting furnace  20  and for many other purposes in the steelmaking system  10 . 
     In a steelmaking system according to the invention, the water  38  produced by the reduction reaction of hydrogen with the iron oxides is cleaned, properly treated, heated and converted to steam  28  which is split again into hydrogen  16  which is re-used for the reduction of iron oxides thus forming a hydrogen recycling loop. This synergy of a direct reduction plant  12  with a heat recovery unit  26  and an electrolysis unit  34  significantly reduces the carbon footprint of the steelmaking process. 
     In another embodiment of the invention, all or a portion of the steam  40 , produced in the heat recovery unit  26 , is used to generate electricity  42  in an electric generator  44 , which electricity is then used in the electrolysis unit  34  together or in place of other sources of electricity  43 . The water condensed  45  after generation of electricity can also be utilized in the electrolysis unit  34  for hydrogen generation. 
     Referring to  FIG. 2 , the direct reduction plant  12  comprises a direct reduction shaft furnace  50  having a reduction zone  52  and a lower discharge zone  54  from which DRI  18  is discharged at a regulated rate by means of a suitable discharge mechanism  56 , for example a rotary star feeder, a vibrating feeder, a screw feeder and the like. Iron oxides in the form of pellets, lumps or mixtures thereof  14  are fed to the reduction furnace  50  and descend by gravity through the reduction zone  52  were DRI containing metallic iron is formed by reaction of said iron oxides with a reducing gas stream  6  at high temperature in the range between about 800° C. and about 1050° C. and mainly composed of hydrogen  16 , which can also comprise carbon monoxide, carbon dioxide, methane and nitrogen in those embodiments wherein a hydrocarbon such as natural gas or a syngas derived from coal is used as the source of the reducing gas  6 . 
     A stream of exhausted reducing gas is extracted from said reduction furnace  50  as top gas  58  containing non-reacted hydrogen due to the limitations of chemical equilibrium and kinetics of the reduction reactions, water produced as a by-product of said reduction reactions, and in some embodiments also carbon monoxide, carbon dioxide, methane and nitrogen in case a hydrocarbon such as natural gas or a syngas derived from coal is used as the source of the reducing gas  6  in the direct reduction furnace  50 . The top gas  58  exits the direct reduction furnace  50  at a temperature in the range from about 300° C. to 450° C. and is passed through a heat exchanger  60  where a suitable fluid  62  is heated, for example water, to produce steam  64 . The steam  64  can be utilized in an optional CO 2  removal unit or utilized to produce hydrogen in the electrolysis unit  34 , or alternatively the heat of top gas  58  can be utilized to preheat the reducing gas stream  6  fed to the reduction furnace. 
     The top gas  58 , after exiting heat exchanger  60  through the conduit  158  is cleaned and cooled down in a gas cooler  66  with water. The water vapour contained in the top gas  58  is condensed in the cooler  66  as water stream  68  and can be fed to the electrolysis unit  34  through the conduit  168  after a proper treatment in a manner known in the art. 
     A minor portion of the clean and cooled top gas  70 , added in  FIG. 2 , may be withdrawn from the reduction system as gas stream  72  to prevent accumulation of inert gases in the reducing gas recycle, if applicable. Valve  74  is used to regulate the amount of top gas purged from the reduction plant  12  and also to regulate the operating pressure of the reduction plant  12 . A major portion  76  of the clean top gas  70  is recycled through a compressor  78 , which elevates its pressure to recycle said top gas  70  to the reduction furnace  50 . 
     The recycled gas stream  80 , mainly composed of hydrogen is then passed through a gas heater  85  to elevate its temperature in the range from about 800° C. to about 1000° C. so that the reduction reactions with iron oxides take place inside the reduction furnace  50 . 
     Optionally, a stream of make-up hydrocarbon gas  82  is added to the recycled gas stream  80  from a suitable source  84  to generate hydrogen and carbon monoxide that may form part of the reducing gas  6  fed to the reduction furnace  50 . 
     In an embodiment, the hydrocarbon gas  82 , such as natural gas, is transformed to hydrogen and carbon monoxide by reaction with water and carbon dioxide contained in the recycled gas stream  80  in a catalytic reformer  85  thus forming the reducing gas stream  6 . 
     In a further embodiment, reformation of the hydrocarbon gas  82  to hydrogen and carbon monoxide is effected in the reduction furnace  50  along with the reduction reactions and in such case the combined stream  86  of recycled gas  80  and make-up hydrocarbon gas  82  is heated in heating unit  85  without a catalyst in contrast to the case where said unit  85  is a reformer. 
     In the invention embodiments wherein a hydrocarbon gas  82  is also utilized as a source of the reducing gas  6 , the recycled gas stream  80  is treated in a CO 2  removal unit  88  to separate CO 2    89  produced as a by-product of the reduction reactions of carbon monoxide with the iron oxides. The CO 2  removal unit may be of the type wherein CO 2  is selectively removed by action of a solvent such as a solution of amines or potassium carbonate or can be of the type where CO 2  is separated by physical adsorption on a PSA (pressure swing adsorption), VPSA (vacuum pressure swing adsorption) or gas membranes unit. 
     Optionally, the make-up hydrocarbon gas  82  is coke oven gas, natural gas, syngas from biomass or other methane-containing and/or H 2  or CO-containing gas. 
     Optionally, the carbon content in the DRI can be adjusted for its further processing in the melting furnace  90 , in a wide range from about 0.5% to about 6%, preferably between about 2.5% to 3.5%, by injecting a carburizing gas  46  from a suitable source  48 , which may be a hydrocarbon gas, coke oven gas, natural gas, syngas from biomass, or mixtures thereof, or other methane-containing and/or CO-containing syngas or any other carbon-containing gas that may deposit carbon in the DRI. 
     In an embodiment, the DRI is discharged cold from the reduction furnace  50  by circulating a cooling gas in the lower portion  54  of the reduction furnace  50  in a manner known in the art. In this case, the carbon content of the DRI can be effected by using as cooling gas a DRI carburizing gas, which may be a hydrocarbon gas, coke oven gas, natural gas, syngas from biomass, or mixtures thereof, or other methane-containing and/or CO-containing syngas or any other carbon-containing gas that may deposit carbon in the DRI. 
     Typically, DRI is discharged from said reduction furnace  50  at high temperature in the range between about 300° C. and about 750° C., preferably between about 600° C. and about 700° C. and charged hot to a melting furnace  90 , usually an electric arc furnace, having electrodes  92  and a gas extraction duct  94  to collect the hot gases that are produced during the charging, melting and refining of DRI and optionally also steel scrap. These hot gases exit the electric arc furnace  90  at high temperature in the range of 1000° C. to 1400° C. 
     Heat contained in hot gas  24  extracted from the melting furnace  90  through duct  94  is utilized to produce steam  108  in a heat exchanger  96  where water  98  is fed from a suitable source  100 . A steam drum  102  collects the steam and forms part of a heat recovery loop through which water is circulated by means of one or several pumping units  104  and pipes  106 ,  108  and  110 . 
     Optionally, hot water can also be extracted from steam drum  102 , through pipe  112  and pumping unit  114 , and fed to electrolysis unit  34 . 
     Water collected to the electrolysis unit  34  is used to produce hydrogen  16  which is fed through pipe  116  to the direct reduction furnace  50  after mixing with recycled gas stream  80  and hydrocarbon gas  82  in the combined stream  86  entering the gas heater or catalytic reformer indicated with numeral  85  according to the alternative embodiment discussed above. 
     In another embodiment, energy of steam  40  withdrawn from steam drum  102  is fed to a turbine  118  which drives an electricity generator  120  to produce electricity  122  which is used in the electrolysis unit  34  for the production of hydrogen  16 . 
     Optionally, hot water exiting from turbine  118  can be fed to the electrolysis unit  34  through the conduit  168  after a proper treatment in a manner known in the art. 
     In the electrolysis unit  34  a stream of oxygen  124  is produced and can be used optionally to raise the temperature of the reducing gas  6  by partial combustion feeding it through pipe  126  to pipe  116 , or in the DRI melting or refining process carried out in the electric arc furnace  90  or otherwise in the direct reduction plant  12  or the melting furnace facility  20 . 
     The electrolysis unit  34  may be of any type available for industrial use and can also be a co-electrolysis unit wherein both water is decomposed into hydrogen and oxygen and CO 2  is also split into carbon monoxide and oxygen. The electricity  122  produced by generator  120  can be used in the electrolysis unit  34  together or in place of other available sources of electricity  43 . 
     In another embodiment, the system of the invention comprises a polymer electrolyte membrane electrolyser (PEM), or an alkaline electrolyser where a liquid alkaline solution of sodium or potassium hydroxide is used as the electrolyte, or a solid oxide electrolyser (SOE) which uses a solid ceramic material as the electrolyte that selectively conducts negatively charged oxygen ions at elevated temperatures. 
     The invention thus provides a synergistic system to produce steel or a molten-iron containing material by integrating a direct reduction plant  12 , a DRI melting furnace  20 , a heat recovery unit  26 , a steam turbine-electricity generator  44  and an electrolysis unit  34  with lower CO 2  emissions than the currently used steelmaking systems. 
     It is of course to be understood that the above description of the invention has been written for illustrative purposes and that the scope of the invention is not limited to the embodiments herein described but is defined by the appended claims, and that a number of changes and modifications may be made to the embodiments of the invention comprised by the scope of these claims.