Abstract:
The present invention concerns a method and an apparatus for producing DRI (Direct Reduced Iron) utilizing a high-oxidation reducing gas containing carbon monoxide and hydrogen, derived directly or indirectly from the gasification of hydrocarbons or coal, with a high content of oxidants (H 2 O and CO 2 ). The invention provides a more efficient method and plant comprising a reactor in which particulate material of iron ore comes into contact with a high temperature reducing gas to produce DRI, with lower investment and operating costs, avoiding the need for a fired heater for the reducing gas fed into the reduction reactor. The reducing gas is heated to a temperature above 700° C. in two steps, a first step at a temperature below about 400° C. to prevent the phenomenon of metal dusting, by exchange of sensible heat supplied by the stream of hot spent gas removed from the reduction reactor; and a second step by means of partial or total combustion with oxygen, maintaining the temperature of the combustion gas below the limits established by the construction materials of the combustion chamber.

Description:
FIELD OF THE INVENTION 
     The present invention is referred to a method and an apparatus for producing direct reduced iron utilizing a source of reducing gas comprising hydrogen and carbon monoxide. 
     BACKGROUND OF THE INVENTION 
     In the recent years, the necessity of increasing the steelmaking process efficiency and productivity has become more urgent, due to rising production costs and also due to the restrictions imposed upon steel plants by environmental regulations. 
     One of the successful routes for steelmaking, which is being increasingly promoted, is the gas based Direct Reduction of Iron Ore to produce Direct Reduced Iron (DRI), also known in the steel industry as sponge iron, by circulating a reducing gas through a moving bed of particulate iron ore at temperature of the order of 700° C. to 1100° C. Oxygen is removed from the iron ore by chemical reduction for the production of highly metallized DRI. 
     Some of the advantages of direct reduction plants are the wide range of production capacity, that the metallic iron is produced in solid form with low sulfur and silicon content, and that the resulting DRI may be used as raw material for the electric furnace and may constitute the whole charge thereof. 
     Additionally, and as a peculiar advantage of the technology proposed, given that a part of the CO 2  produced as by product of the reduction reactions is selectively removed from the process, total CO 2  emitted in the atmosphere may be considerably reduced if compared with others routes for steel production. 
     The reducing agents utilized in the direct reduction plants are hydrogen and carbon monoxide, obtained by reformation of natural gas in an external catalytic reformer or “in situ” within the iron reduction system. Nevertheless, a direct reduction plant can be also designed for utilizing other sources of energy available in the form of gases from coke ovens, blast furnaces, coal or oil gasification, natural gas, exhaust gases containing hydrogen and carbon monoxide arriving from other chemical/metallurgical processes, etc. 
     A possible source of reducing gas is the excess gas produced in the combination of a plant for the production of pig iron based on the use of coal (for example a blast furnace or a plant known in the industry with the tradename Corex) and a direct reduction reactor. Corex plants or blast furnaces produce pig iron using gasified coal by partial combustion with an oxygen-containing gas. The exhausted reducing gas withdrawn from this process, still containing H 2  and CO, can be utilized for reduction, after removal of at least a portion of H 2 O and CO 2 . 
     U.S. Pat. No. 5,238,487 to Hauk et al. discloses a process comprising a melter-gasifier, a first reduction reactor and a second reduction reactor wherein DRI is produced using directly reducing gas effluent from said first reactor. As indicated in this patent, the effluent reducing gas, after being only cleaned, is mixed with dewatered spent reducing gas and treated in a CO 2  removal unit. The gas leaving the decarbonation station is then heated in a heat exchanger and finally subjected to a partial combustion to reach the right temperature required for the reduction reaction. Additionally, this patent teaches to use sulfur oxides and chlorine to inhibit carbon monoxide decomposition. All embodiments of this patent however utilize heat exchangers that consume a fuel for heating the reducing gas prior to the partial combustion heating stage. 
     U.S. Pat. No. 5,676,732 to Viramontes-Brown et al. discloses an improved method and an apparatus for utilizing in a direct reduction plant the excess exhausted gas from a first reduction reactor, which receives reducing gas from a melter-gasifier. Said method suggests to use a catalytic reactor, or shifter, for adjusting the composition of the gas stream effluent from said first reactor in order to avoid carbon deposition and corrosion in the gas heater required to heat fresh gas before feeding it into the reduction reactor. In order to get the maximum yield of H 2  product from the CO shift conversion, a special catalyst in a fixed bed reactor is used. For this reason, Syngas has to be further treated in order to remove substances that are poisonous for the catalyst. 
     Referring now to Syngas from a gasifier as alternative source of reducing gas, U.S. Pat. No. 6,149,859; and U.S. Pat. No. 6,033,456 to Jahnke et al. describe an integrated process for supplying high-pressure Syngas from a gasifier to a direct reduction plant. As in the prior art, this patent suggests to treat the Syngas in a shifter with the purpose of changing its composition in order to avoid carbon deposition when said gas is heated at a temperature higher than 400° C. (condition commonly achieved in a typical process gas heater of a Direct Reduction Plant). In this way, the conditioned gas stream, after being treated in a dedicated unit to remove CO 2  and being expanded to the pressure of the direct reduction circuit, is ready for being used as make up in the DRI process. 
     WO-A-2008/146112 discloses the additional possibility of having, in a process as described in U.S. Pat. No. 6,149,859 and U.S. Pat. No. 6,033,456, a single absorption unit wherein the acid-gas content is removed from a combined stream of both the Syngas produced in the gasifier and the recycle reducing gas from the reduction reactor. 
     U.S. Pat. No. 5,846,268 to Diehl et al. discloses a process for producing liquid pig iron or liquid steel pre-products and DRI from iron ore. The process shown in this patent is much similar to the process described by U.S. Pat. No. 5,238,487 to Hauk et al. where a reducing gas, derived from the gasification of coal is used for reducing iron ore in a first reduction shaft furnace and the exhausted reducing gas effluent from said first shaft furnace is utilized for producing more DRI in a second shaft furnace. This patent teaches several ways of using heat of the gas stream effluent from the second shaft furnace for preheating a portion of the same gas stream which is then utilized as fuel in a fired gas heater, but does not teach or suggests using said heat to preheat the stream of reducing gas fed to the reduction reactor. 
     None of the above patents teach or suggest the distinctive features of the present invention which overcome a number of disadvantages of the prior art and provide a more efficient method and apparatus for producing DRI utilizing gas derived from coal gasification in a gasifier or derived from a melter-gasifier, for example, using heat from the top gas effluent from the reduction reactor for heating the reducing gas to be fed to said reactor without consuming any additional fuel and within the practical limits of the degree of oxidation of the reducing gas for an efficient reduction of iron ore. 
     An additional advantage of the present invention is that the carbon dioxide emissions to the atmosphere can be decreased because there is no combustion in the heat exchanger for raising the temperature of the reducing gas prior to second heating stage of partial oxidation with oxygen. 
     OBJECTS OF THE INVENTION 
     It is therefore an object of the present invention to provide an improved process and apparatus for producing direct reduced iron (DRI) using a gas with high content of carbon monoxide possibly after being cleaned in order to remove dusts and TAR, and feeding this cleaned gas with high carbon monoxide content directly to the reduction circuit without additional treatment in a water gas shifter for changing its composition. This simplified process configuration has the further advantage that removal of compounds that are poisonous for the shifter catalyst is not required. 
     It is another object of the present invention to provide an improved method and apparatus for producing hot or cold DRI in which upgraded reducing gas, obtained treating a stream of Syngas previously mixed with dewatered and spent reducing gas in a CO 2  removal unit, is heated exclusively in a heat exchanger, without any additional fuel combustion and exploiting only sensible heat recovered from spent gas removed from the reactor; in this way the energy per ton of produced iron is decreased. Finally, said gas stream (heated CO 2  lean gas stream), available after heating at a temperature of less than 450° C., before being finally fed to the reactor, is subjected to a partial combustion in a combustion chamber with a stream of a molecular-oxygen-containing gas. Alternatively a portion of this gas stream is subjected to a total combustion and the combustion products are combined with the rest of said heated CO 2  lean gas stream. No additional gas heating means between said heat exchanger and said combustion chamber are included. 
     It is a further object of the invention to provide a method and apparatus for producing cold DRI utilizing a gas with a high content of carbon monoxide, and cooling said DRI by flowing through the conical discharge part of the reduction reactor a cooling gas with carburizing potential, that can be Coke Oven Gas. 
     SUMMARY OF THE INVENTION 
     The objects of the invention are generally achieved by providing a method and apparatus for producing DRI in a direct reduction system comprising a reduction reactor using a reducing gas with a high carbon monoxide content (for example a Syngas from any source which is cleaned of dust), wherein at least a part of the spent reducing gas removed from the reduction reactor is cleaned and cooled before being mixed with said cleaned Syngas to produce a combined gas stream which is subsequently fed to a CO 2  separation unit. Preferably, said CO 2  separation unit is of the adsorption type whereby a CO 2  laden gas stream and a CO 2  lean gas stream flow out of said CO 2  separation unit. The upgraded CO 2  lean reducing gas stream passes through a heat exchanger where only exchanging sensible heat recovered from said spent reducing gas removed from the reduction reactor and without any combustion, is heated at a temperature lower than 450° C. This heated CO 2  lean reducing gas stream is then partially combusted with a molecular-oxygen-containing gas in order to raise its temperature above 700° C. measured at the reactor inlet, thus dispensing the need of an additional heating in a conventional fired gas heater. 
     Another object of the invention is achieved by providing a method and apparatus for producing DRI as described above and having a desired amount of carbon, by cooling said DRI with a carburizing gas that is circulated in the lower part of said reduction reactor as for example Coke Oven Gas. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The detailed description of some preferred embodiments of the invention will be better understood with reference to the accompanying drawings wherein like numerals designate like elements for convenience of the reader. 
       The Figures disclose: 
         FIG. 1  shows a schematic process diagram of a direct reduction process incorporating one embodiment of the invention; 
         FIG. 2  shows a schematic process diagram of a direct reduction process incorporating a second embodiment of the invention; 
         FIG. 3  shows a schematic process diagram of a direct reduction process incorporating a third embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a Direct Reduction System where numeral  10  generally designates a vertical shaft, moving bed, iron ore gaseous reduction reactor, having a reduction zone  12 , to which iron ore  15  is fed through at least one inlet  16  in the form of lumps, pellets, or any blend thereof. This iron ore  15  descends by gravity through the reactor  10  in counter-current contact with a reducing gas at high temperature. 
     This reducing gas is introduced to the reactor through pipe  46  located in the lower part of the reduction zone  12 , and is mainly comprised of hydrogen and carbon monoxide which react with the iron ores to produce direct reduction iron (DRI)  18 , which is discharged from reactor  10  through its conical lower part  14 . 
     Spent reducing gas  44 , removed from the top of the reactor at a temperature ranging from 300° C. to 600° C., is treated to be upgraded in a recycle circuit and finally returned back to the reduction zone  12  through pipe  46 . In detail, this spent reducing gas stream  44 , with a partially depleted reducing capacity, passes through a heat exchanger  42 , where sensible heat removed from said gas stream  44  is recovered to preheat the upgraded portion of reducing gas  50  prior to being recycled back to the reactor  10 . 
     After passing through heat exchanger  42 , the partially-cooled spent gas is conducted through conduit  43  to a cleaning station  38 , where dust is removed by contact with a water stream  40  withdrawn as stream  36 , and the effluent clean gas  39  is then passed to a cooling station  30 , usually of the direct contact type, where the water by-product of the reduction reaction is condensed by contact with water  32  and then removed from the reducing gas as water stream  54 . 
     For maintaining a low N 2  concentration in the recycle circuit, a minor portion of the cleaned and dewatered spent gas is purged from the system through pipe  26  having a pressure control valve  28  (for pressure control). The purged gas stream  26  contains carbon monoxide, carbon dioxide, hydrogen and methane in quantities such that the gas can be used as fuel in standard combustion systems. The remaining main portion of this cleaned and dewatered reducing effluent gas is subsequently transferred, flowing through conduit  27 , to a compressor  24  wherein its pressure is raised to a level suitable for further treatment and use. 
     The compressed reducing gas stream  29  undergoes an additional cooling step in a heat exchanger or a quench tower packed vessel  22 , required to lower the gas temperature after compression; the stream of gas obtained  35  is mixed with make-up gas stream  23  containing carbon monoxide and hydrogen, for example Syngas derived from the gasification of coal or other hydrocarbon feedstock or export gas from a melter-gasifier system effluent from its associated reduction furnace. 
     This Syngas  23 , supplied from a suitable source  1 , is fed through pipe  2  to a gas cleaning system  6  where dust, tar and water are removed. The obtained stream  7  of clean Syngas, mainly composed of H 2 , CO, CO 2  and CH 4 , is first compressed in a Syngas compressor  20  and cooled in a dedicated equipment  21 , that can be a heat exchanger or a quench tower, before being added as make-up to the reduction circuit of reactor  10  as stream  23 . 
     After mixing the dewatered reducing gas stream  35  with the clean Syngas (make-up gas stream  23 ), the CO 2  contained in this resulting gas stream  31  is at least partially removed in the CO 2  removal unit  70 . Said unit is preferably of the type of PSA (Pressure Swing Adsorption) units or VPSA (Vacuum Pressure Swing Adsorption) whereby CO 2  is concentrated in a gas stream  33 , which is subsequently removed from the system as purge and eventually used as fuel; the gas stream  33  is adjusted by a pressure control valve  60 . The PSA unit, that utilizes adsorbent surfaces to block polar and less volatile molecules, removes from said stream  31  water and H 2 S molecules too. 
     According to a principle of the invention, the upgraded portion of the reduction gas  50 , with a low CO 2  concentration and an improved high reducing potential, leaves the CO 2  removal unit and is fed to the previously described heat exchanger  42  where it is heated at a temperature lower than 450° C. in order to prevent the onset of chemical corrosion reactions of the metal materials of the exchanger  42  (for example using the mechanism known as “metal dusting”). Since there is no combustion in exchanger  42  there is no additional emissions of carbon dioxide to the atmosphere. 
     The temperature of the resultant gas stream  45 , at a value below 450° C., is then increased up to the desired final value in a second stage by means of combustion of a portion of the preheated CO 2  lean gas stream. To this end, the preheated CO 2  lean gas stream  45  is divided in a first portion  132  which is directly sent to a combustion chamber  47 , where it is combusted with a stream of a molecular-oxygen-containing gas stream  48 , preferably oxygen of industrial purity, supplied from a suitable source  49 . The amount of oxygen is regulated by valve  52  in response to the level of temperature desired for the reducing gas flowing through pipe  46 . The amount of oxygen is also regulated so that the value of the ratio of reducing agents to oxidant agents (H 2 +CO)/(H 2 O+CO 2 ) in the heated gas stream  46  is at least 7; or that the reducing index calculated as: (H 2 +CO)/(H 2 +CO+H 2 O+CO 2 ) of gas stream  46  fed to the reduction reactor is at least 0.87. 
     The combustion may be carried out by means of a dedicated burner or by injection of oxygen through injection lances in a combustion zone  53  located inside the combustion chamber  47 . The remaining portion of reducing gas  45 , or gas stream  130 , is then fed to the mixing zone  55  of combustion chamber  47  so that the partially or totally combusted gas, mixed with the remaining reducing gas  130 , reaches a temperature higher than 700° C. at the reactor inlet. The gas stream  130  may also be fed to the combustion zone  53  of combustion chamber  47  in order to protect the materials of the combustion chamber from the high temperatures that may be reached due to the stream. Regulation of the amount of gas stream  132  is controlled by control valve  134  in response to the desired temperature for the reducing gas  46  to be introduced into the reduction zone  12  of the reactor  10  and in accordance with the maximum temperature allowed by the design and materials of the combustion chamber. In one example, the flow rate of gas stream  132 , which is partially or totally combusted in combustion chamber  47 , is in the range of 50% to 70% of the flow rate of gas stream  45 . The flow rate of reducing gas  132  and the quantity of oxygen  48  are controlled in accordance with the temperature desired for the reducing gas stream flowing through the pipe  46  by means of the valves, respectively  134  and  52 . 
     The gas stream  130  may also be combined with the partially or totally combusted gas stream outside of the combustion chamber  47  to adjust the temperature of the combined reducing gas stream until it reaches the suitable value for being introduced into the reduction zone of the reactor  10  through conduit  46  for reducing the iron ore contained therein. 
     The combustion chamber  47  is preferably preheated to temperatures above 600° C. for assuring that the mixture of reducing gas and oxygen is maintained under ignition in order to prevent the formation of any potential explosive mixtures. 
     Particulate solid iron ores  15  are contacted within the reduction zone  12  with said high-temperature upgraded reducing gas fed through pipe  46  into the reactor  10 . In this way the solid material, flowing counter-currently with this gas, reacts with hydrogen and carbon monoxide producing direct reduced iron (DRI). The DRI, flowing through the lower discharge zone  14 , is then discharged from said reactor  10  through the lower discharge zone  14 , hot or cold, depending on the type of subsequent utilization of the DRI. 
     When DRI is discharged at high temperature (as shown in  FIG. 1 ), on the order of 400° C. to 750° C., it can be subsequently briquetted for further storage and handling or pneumatically transported, or alternatively by means of tanks or inertized belts, directly to a steelmaking furnace in a manner known in the art. 
     If DRI has to be cold produced (as shown in  FIG. 2 , where identical components to those in  FIG. 1  have the same reference numbers and are therefore not described again here), the DRI is cooled down by passing counter-currently a cooling gas stream  122  at a relatively low temperature through the conical part  14  of the reactor  10 , whereby the cooling gas temperature is increased and the temperature of the DRI is lowered to a temperature usually below 100° C. Cooling gas make-up  80  is fed to the cooling gas circuit from a suitable source  81  that can be for example Coke Oven Gas if available, natural gas or other hydrocarbon-containing gas so that said hydrocarbons are cracked in contact with the hot DRI and in this way DRI with the desired amount of combined carbon or graphite is produced. 
     In another embodiment of the invention shown in  FIG. 3  (where identical components to those in  FIG. 1  have the same reference numbers and are therefore not described again here), Coke Oven Gas from a source  81  is used as carburizing and cooling gas stream  80  and fed directly to the reactor cone at a desired location where the temperature of DRI is high. In this way, a further advantage is obtained because the hydrocarbons typically contained in Coke Oven Gas are destroyed by cracking. 
     The hot and spent cooling gas stream  90  may be cooled down and recycled in a manner well known in the art. Briefly, the warmed up gas withdrawn from the top of the cooling zone, is further treated in a cleaning station  92  to remove dust by washing with water  93  which is withdrawn through pipe  95 ; the clean gas  94  is treated in a cooling station  96 , where it is completely de-watered and cooled by contact with water  97  which is withdrawn through pipe  99 . The gas obtained  98  is compressed by means of compressor  100  before being fed to the reactor through pipe  120 . 
     In a further embodiment of the invention, the DRI may be hot discharged from the reduction reactor at a temperature in the order of 400° C. to 750° C., and it may be cooled down to a temperature lower than 100° C., to avoid its re-oxidation by atmospheric oxygen and water, in a separate DRI cooling vessel (not shown) external to the reduction reactor  10 , with a cooling gas system similar to the cooling gas system previously described. With this configuration, the iron reduction plant, designed to produce hot DRI for its immediate melting, can provide also for an emergency discharge of DRI in safe conditions, with the material available at an adequate temperature for storage and later utilization. 
     An alternative design for a direct reduction plant with the capacity of producing both hot or cold DRI provides the reduction reactor with a cooling gas system designed to optionally enable or not the operation of the cooling system, whereby the same reactor may cool the DRI inside the discharge cone or discharge it at high temperature. 
     According to an exemplary embodiment of the invention, in which the second heating stage of the CO 2  lean gas stream is combusted with 76 Nm3/t of DRI with 95% pure oxygen, the relative amounts of some of the components of the reducing gas which is heated to 838° C. suitable for reduction of iron ores, are presented in Table 1 below: 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 HYDROGEN 
                 GAS 
                 REDUCING 
                 EFFLUENT 
               
               
                   
                   
                 AND CO 
                 EFFLUENT 
                 GAS FED 
                 GAS FROM 
               
               
                   
                   
                 CONTAINING 
                 FROM PSA 
                 TO 
                 REDUCTION 
               
               
                   
                   
                 GAS 
                 UNIT 
                 REACTOR 
                 ZONE 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 GAS STREAM 
                   
                 23 
                 50 
                 46 
                 44 
               
               
                 NO. 
                   
                   
                   
                   
                   
               
               
                 TEMPERATURE 
                 ° C. 
                 45 
                 48 
                 838 
                 519 
               
               
                 FLOW 
                 NCM/ 
                 1622 
                 1997 
                 1998 
                 2060 
               
               
                   
                 TON 
                   
                   
                   
                   
               
               
                   
                 DRI 
                   
                   
                   
                   
               
               
                 Composition 
                   
                   
                   
                   
                   
               
               
                 H2 
                 VOL 
                 18.070 
                 33.926 
                 26.354 
                 23.196 
               
               
                   
                 % 
                   
                   
                   
                   
               
               
                 CO 
                 VOL 
                 43.168 
                 48.418 
                 48.408 
                 29.503 
               
               
                   
                 % 
                   
                   
                   
                   
               
               
                 CO2 
                 VOL 
                 32.125 
                 2.000 
                 2.000 
                 19.669 
               
               
                   
                 % 
                   
                   
                   
                   
               
               
                 H2O 
                 VOL 
                 1.116 
                 0.001 
                 7.566 
                 12.627 
               
               
                   
                 % 
                   
                   
                   
                   
               
               
                 CH4 
                 VOL 
                 2.309 
                 3.326 
                 3.325 
                 2.930 
               
               
                   
                 % 
                   
                   
                   
                   
               
               
                 N2 and other 
                 VOL 
                 3.212 
                 12.333 
                 12.347 
                 11.975 
               
               
                 gases 
                 % 
                   
                   
                   
                   
               
               
                 REDUCING 
                   
                 0.65 
                 0.98 
                 0.89 
                 0.62 
               
               
                 INDEX 
                   
                   
                   
                   
                   
               
               
                   
               
             
          
         
       
     
     The reducing index is calculated as: (H 2 +CO)/(H 2 +CO+H 2 O+CO 2 ) and indicates the reducing power of each gas stream. 
     From table 1, it can be seen that the present invention provides an effective method and apparatus for producing DRI utilizing a gas containing H2 and CO with a low Reducing Index and an effective two-stages gas heating to the desired reduction temperature. 
     The present invention brings a number of advantages over the prior art, namely, a simpler iron ore reduction plant and process are possible because the fired heater, for preheating the reducing gas before raising its temperature to the reduction levels, is not needed. Therefore a direct reduction plant incorporating the invention has lower capital and operation costs because an important piece of equipment (the heater) requiring operation and maintenance materials and manpower is avoided. 
     It is of course to be understood that the above description of some embodiments of the invention has been made for purposes of illustration and not of limitation of the scope of the invention and that a number of changes may be made to the embodiments herein described as the application of the invention best fits a particular practical case without departing from the spirit and scope of the invention which is determined by the appended claims.