Abstract:
A blast furnace where coke is combusted with oxygen, instead of air, and where a top gas comprising CO, CO 2 , H 2 , and without excess nitrogen is withdrawn from the upper part of the blast furnace, cleaned of dust, the H 2 /CO volume ratio adjusted to between 1.5 to 4.0 in a water shift reactor, water and CO 2  are removed (increasing its reduction potential), heated to a temperature above 850° C. and fed back to the blast furnace above where iron starts melting (thereby increasing the amount of metallic iron reaching the dead-man zone and decreasing the amount of coke used for reduction). Also carbon deposit problems caused by heating the CO-containing recycled gas are minimized by on-line cleaning of the heater tubes with steam without significantly affecting the reduction potential of the recycled reducing gas.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to the field of the iron and steel industry, more particularly to a more efficient method and plant for production of liquid iron in blast furnaces, with reduced coke consumption by recycling upgraded top gas with an adjusted H 2 /CO ratio. 
       BACKGROUND OF THE INVENTION 
       [0002]    In a blast furnace producing pig iron, iron ore is charged together with coke and fluxes. A hot air blast is injected through tuyeres at the bottom of the furnace, thereby generating heat by the combustion of carbon in the coke which melts down the charge. Periodically liquid iron and slag are tapped from the furnace. The combustion gases flow up through the furnace and reduce the iron oxides, and exit the furnace as a stream of dust-laden hot gas which from heat is recovered for preheating the air blast, and which is then normally used as fuel in other areas of the steel plant. 
         [0003]    Metallurgical coke is needed in the charge of a blast furnace because this material (produced by pyrolysis of coal in coke ovens) provides the structural support of the charge of the furnace above the so-called “dead man” zone where the metallic iron starts melting and falling down to the bottom part of the furnace where molten iron and slag are collected, There is a minimum amount of coke which cannot be replaced by other fuels in order to assure the needed structural support of the charge. 
         [0004]    Coke also provides the heat for melting the iron charge by its combustion with an oxygen containing gas, typically preheated air, the combustion gases, mainly composed of CO and CO 2 , some H 2  and water flow upwardly through the shaft portion of the furnace and reduce the iron oxides to wustite (FeO). Final reduction of wustite to metallic iron at the conditions prevailing in the blast furnace is carried out by reaction with carbon (known in the art as “direct reduction”). 
         [0005]    Several proposals for recycling top gas in a blast furnace with the aim of reducing the coke rate, are found in the prior art. One of the problems to be solved is the nitrogen content of the top gas originating from the air blast. Another problem is the need for reheating the recycled gas prior to its being fed into the reduction area of the blast furnace, because the high content of CO causes carbon formation which may cause clogging and fouling of the reheating equipment and of the heating path equipment. 
         [0006]    Applicants have found the following patents and patent applications concerning top gas recycle to a blast furnace: 
         [0007]    U.S. Pat. No. 3,460,934 to Kelmar discloses a method of making iron or steel in a blast furnace where pure oxygen is fed to the tuyeres instead of air. The high temperature developed by combustion of coke with oxygen is moderated by injecting with oxygen recycled gas, iron ore, limestone, coke or flue dust. Using oxygen instead of air eliminates the nitrogen in the top gas allowing recycling of the top gas. 
         [0008]    U.S. Pat. No. 3,784,370 to Stephenson teaches a method of operating a blast furnace in which the top gas is recycled, The top gas is cleaned in a dust collector and a static filter. Thereafter, the clean gas is stripped of nitrogen utilizing molecular sieves and then is heated to a temperature of about 2000° F.=1,093° C. and recycled back to the tuyeres of the blast furnace, whereby the CO and H 2  contained in the recycled gas contribute to the reduction of iron ores lowering the coke consumption. 
         [0009]    U.S. Pat. No. 4,844,737 to Oono et al, discloses a blast furnace where pure oxygen instead of air is fed to the blast tuyeres along with pulverized coal, top gas is cleaned and cooled down and a portion thereof is heated and recycled to the middle portion of the shaft. No teaching is found of modifying the gas composition for decreasing the CO content and increasing the H 2  content in the recycled gas. 
         [0010]    U.S. Pat. No. 4,917,727 to Saito et al. describes a method of operating a blast furnace where pure oxygen instead of air is fed to the blast tuyeres along with pulverized coal, top gas, is cleaned and cooled down and a portion thereof is heated and recycled to the middle portion of the shaft. No teaching is found of modifying the gas composition for decreasing the CO content and increasing the H 2  content in the recycled gas. 
         [0011]    U.S. Pat. No. 4,363,654 to Frederick et al discloses a process for producing a reducing gas by partial combustion of oil and/or coal and/or coke for utilization in a direct reduction furnace or a blast furnace. This patent shows a blast furnace where air is used as blast for coke combustion and where in the off-gas of said blast furnace contains nitrogen. The top gas is cleaned and scrubbed and treated in a CO shifter for adjusting the CO content. CO 2  is removed from the shifted gas thus forming a Hydrogen-rich stream which is then heated to a temperature from 500° C. to 700° C. and the hot hydrogen is fed back to the blast furnace. This process has the drawback of needing a cryogenic plant for separating nitrogen contained in the top gas. Since the recycled stream is mainly hydrogen, the patent is mute regarding the coking, clogging and fouling problems in heaters when heating a gas containing CO. 
         [0012]    U.S. Pat. No. 5,234,490 to Kundrat describes a method of producing pig iron in a blast furnace where top gas is cleaned of dust and soot and then dehydrated and cooled down. A portion of the cooled and clean top gas is preheated to a temperature between 900° C. and 1000° C. before it is recycled to the blast furnace for reducing the iron ore. However, the process of this patent is limited in the amount. This patent is mute about the recycled gas preheater and about the carbon deposits, clogging or fouling of heaters when heating a CO-containing gas. 
         [0013]    U.S. Patent application No. 2010/0212457 A1 describes a blast furnace with air blast where a stream of hydrogen generated from the top gas is heated prior to being recycled. CO 2  is removed from the top gas and the CO is used for reducing a metal oxide which thereafter is used for generating hydrogen by oxidizing the reduced metal. The method of this patent does not have any problems relative to heating a CO-containing gas and the benefits of H 2 /CO adjustment. 
         [0014]    British Patent No. GB 1,218,912 discloses a blast furnace where preheated air blast is fed to the tuyeres so that the coke consumption is reduced while a reducing gas is generated by reaction of a hydrocarbon with off-gas of the bias, furnace which is cleaned, washed and cooled down, mixed with a hydrocarbon, such as methane or naphta and heated in a tubular heater to 950° C. and fed to the blast furnace for reducing the iron ores at a level above the blast tuyeres. This patent is mute regarding the elimination of nitrogen and about carbon deposit problems arising when a hydrocarbon is heated in tubular heaters. 
         [0015]    Japanese Patent Publication No. JP55113814 describes a blast furnace where the consumption of coke is reduced by supplying fuel and oxygen fed as a gas blast through the lower tuyeres into the blast furnace. Top gas is treated to remove CO 2  therefrom, is heated, and is recirculated to the blast furnace. No teaching is found of modifying the gas composition for decreasing the CO content and increasing the H 2  content in the recycled gas. 
         [0016]    None of the above patents or patent applications teach or suggest that modifying the composition of the recycled top gas using a CO shift reactor to drastically reduce the coking problems of the heaters for the recycled gas Solutions for this practical problem of the heaters and the possibility of utilizing tubular fired heaters for improving the operation of blast furnaces and allowing for a significant reduction of the overall CO 2  emissions are not envisioned in the prior art, while the present invention allows for the practical design and construction of new blast furnaces and revamping of existing blast furnaces with lower coke consumption. 
       OBJECTS OF THE INVENTION 
       [0017]    It is an object of the invention to provide a method and apparatus for improving the operation of a blast furnace by upgrading and recycling top gas. 
         [0018]    It is another object of the invention to provide a method and apparatus for improving the operation of a blast furnace by decreasing the coke consumption per ton of iron produced. 
         [0019]    It is a further object of the invention to provide a method and apparatus for improving the operation of a blast furnace by decreasing the carbon dioxide emissions per ton of iron produced. 
         [0020]    It is another object of the invention to provide a method for heating recycled top gas in tubular fired heaters minimizing the problems of carbon deposition, dogging or fouling of the heater and other equipment in the heating path of the CO-containing recycled gas. 
         [0021]    Other objects of the invention will be evident for those skilled in the art or will be pointed out in the description of the invention. 
       SUMMARY OF THE INVENTION 
       [0022]    The objects of the invention in its broader aspects can be achieved by providing a method of producing iron in a blast furnace where the combustion of the coke is carried out by feeding oxygen to the tuyeres instead of air thereby avoiding a large nitrogen content in the top gas; withdrawing the top gas stream comprising CO, CO 2  and H 2 ; cleaning the top gas stream of dust and adjusting the volume ratio of H 2 /CO in the top gas to the range between 1.5 to 4 by reaction with water; cooling said top gas stream for removing water therefrom; removing CO 2  from a major portion of said cooled top gas stream, resulting in an effective reducing gas stream; heating said reducing gas stream to a temperature above 850° C., and feeding said hot reducing gas stream as a recycled reducing gas to the blast furnace. Removal of CO 2  from the cooled top gas stream may be carried out by absorption using an amines solution or carbonates solution or by physical adsorption in a pressure swing adsorption (PSA) or vacuum pressure swing adsorption (VPSA) unit. The resulting upgraded top gas is then heated to a temperature above 800° C. and fed to the blast furnace, above the level where iron starts melting, for reducing the iron oxides charge to metallic iron. 
         [0023]    The objects of the invention in its broader aspects can be achieved by a further embodiment providing a blast furnace system for producing molten iron in a blast furnace to which iron ore, metallurgical coke and fluxes are charged at its upper part and molten iron and slag are tapped from its lower part, said blast furnace having a plurality of tuyeres in its lower part for introducing an oxygen-containing gas for generating heat and reducing gases by combustion of the coke within said furnace characterized by comprising means for feeding oxygen instead of air through the tuyeres of said blast furnace; outlet means for withdrawing a top gas stream comprising CO, CO2 and H2; means for cleaning the top gas stream of dust connected to said outlet means; means for adjusting the volume ratio of H2/Co to the range between 1.5 to 4 by reaction with water; means for cooling said top gas stream for removing water therefrom; means for removing CO2 from a portion of said cooled top gas stream forming a CO2 lean reducing gas stream, means heating said reducing gas stream to a temperature above 850° C., and corresponding piping means connecting the components of said blast furnace system to recycle said hot reducing gas stream to said blast furnace. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]      FIG. 1  is a schematic process diagram showing a preferred embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    Referring to  FIG. 1 , numeral  10  generally designates a blast furnace having a crucible section  12  where molten iron and slag are collected, a blast section  14  where the oxygen containing gases are introduced for carrying out the combustion of coke, and a shaft section  16  where iron ore particles in the form of sinter, pellets or lumps and mixtures thereof are charged along with coke, limestone and other fluxes  18 , and next the iron oxides are reduced to wustite and finally to metallic iron as is known in the art. Molten iron  19  and slag  21  are periodically tapped from the bottom zone  12  of blast furnace  10 . 
         [0026]    Oxygen from a source  26  of industrial purity, instead of air, is fed to mixing device  24  where a temperature moderating agent is fed from a source  28  for preventing the flame temperatures from reaching excessively high levels and therefore from damaging the blast nozzles in tuyeres  27 . The temperatures moderating agents  28  may be for example, steam, carbon dioxide, oil, pulverized coal, coke fines or other hydrocarbon that will undergo an endothermic reaction with the oxygen and lower the temperatures to levels of about 2000° C. to 2600° C. Also a portion of the top gas after treatment can be recycled to the tuyeres for moderating the high combustion temperature of oxygen with coke. Oxygen blast  26  combined with the moderating agent  28  are fed to header  23  and then through feeding pipes  25  to tuyeres  27 . 
         [0027]    The composition of top gas varies in a wide range depending on the characteristics of the materials charged to the blast furnace. A typical composition on a dry basis is 25% CO, 12% CO 2 ; 5% H 2  and 56% N 2  and traces of other gases. The top gas effluent from the top of the blast furnace  10  exits through pipe  30  and is fed to a de-dusting device  32 , where dusts from the charge and soot or other solid materials  34  are separated. The cleaned gas flows through pipe  36  to shift reactor  38  where the composition of the cleaned and cooled gas is adjusted to increase the hydrogen content so as to obtain a H 2 /CO ratio of 1.5 to 4, preferably between 2 and 3 (measured by % volume). Steam  40  is supplied as the reactant for the shift reaction through pipe  42 . The CO reacts with H 2 O to form H 2  according to the reaction: 
         [0000]      CO+H 2 O→H 2 +CO 2  
 
         [0028]    The reaction temperature is above about 300° C.; so, if necessary, the top gas stream may be heated by means known in the art as a heat exchanger before being fed to shift reactor  38 . The shifted gas is then passed through pipe  46  to a cooler/scrubber  48  with water  50  where the water content of the gas is condensed and extracted as water stream  52 . 
         [0029]    The de-watered gas then flows through pipe  49  from where a minor portion of the cleaned and dewatered gas  54  is purged from the recycle circuit through pipe having a pressure control valve  56  (for pressure control of, and for maintaining a N 2  concentration below 13% by volume in, the recycle circuit). A majority of the gas stream flows through pipe  58  to be recycled to the blast furnace  10 . The purged gas  54  may be advantageously utilized as fuel in burners  88  for the gas heater  70  and optionally, if needed, may also be supplemented with other fuel as for example coke oven gas or natural gas  86 . 
         [0030]    The cleaned and dewatered reducing effluent gas is then transferred to compressor  60  through pipe  58  wherein its pressure is raised to a level suitable for further treatment prior to its ultimate recycling to blast furnace  10 . In order to upgrade the reducing potential of the recycled reducing gas, the pressurized effluent gas flows through pipe  62  to an absorption tower  64  where CO 2    66  is removed, leaving a reducing gas mainly composed of CO and H 2 . The CO 2  lean gas is led through pipe  68  to heater  70  where its temperature is raised above 800° C. The resulting hot reducing gas is led through pipe  71  to header  72 , and this recycled reducing gas is introduced into the shaft part  16  of the blast furnace through peripheral pipes  74  and nozzles  76 . Oxygen from source  78  may be added to the hot reducing gas for further increasing the temperature of the reducing gas to between 1000° and 1100° C. A suitable fuel  86 , for example natural gas or coke oven gas, is used in burners  88  of heater  70 . 
         [0031]    It is known that under the thermodynamic equilibrium conditions of the gas composition derived from the combustion of coke (mainly composed of CO, CO 2  and H 2 ), the reduction of the iron oxides the progresses until wustite is formed. The continued reduction, of wustite to metallic iron, is carried out by the direct reduction reaction of carbon with FeO Therefore, the amount of coke needed for reduction may only be decreased if more metallic iron reaches the blast furnace&#39;s dead-man zone  15 , thus requiring less carbon for final reduction of wustite; in which case, since the heat for melting the charge may be obtained from fuels other than coke, then the coke rate can be effectively lowered. This approach of adjusting reducing gas composition of recycled gas as a mixture of hydrogen and CO has not been addressed in the prior art. Recycling of hydrogen only would not produce the same results, because hydrogen requires higher energy levels, while a proper combination of hydrogen and CO will efficiently reduce the wustite to metallic iron. 
         [0032]    Removal of CO 2  from the cooled gas stream being recycled may be carried out by absorption using an amines solution or carbonates solution or by physical adsorption in a pressure swing adsorption (PSA) or vacuum pressure swing adsorption (VPSA) unit. 
         [0033]    Upgraded recycled top gas with an improved H2/CO ratio and high reduction potential (measured as the ratio of H 2 +CO/CO 2 +H 2 O and having a value above 2), is heated in coils  80  of the heater  70  to a temperature above 800° C. for reducing the iron oxides in the shaft section  16 . The relatively high content of CO however, may produce carbon deposits by the reaction 2CO→C+CO 2  in the heating path of the recycled gas. Such carbon deposits may require that heater  70  be periodically shut down for cleaning such carbon residues to avoid clogging or fouling of coils  80  and other equipment where CO flows in the range where the thermodynamic conditions of the gas tend to produce elemental carbon. 
         [0034]    In order to avoid the above mentioned production losses caused by the heater carbon cleaning, such cleaning may be done by shutting down the heater  70  and passing steam  82 , or steam and an oxidizing agent  84 , which may be air or oxygen, through the heating tubes  80 , whereby the carbon deposits are gasified and eliminated. A preferred and efficient way of performing such carbon cleaning is done by injecting said steam  82  with or without an oxidizing agent  84  only to one heating tube, or a group of heating tubes, of the heater  70  so that the overall reduction potential of the reducing gas composition fed to the blast furnace is not significantly affected by the increase in the amount of oxidants in said tubes  80 . In this way, the carbon cleaning is done in-line without shutting down the heater  70  and avoiding production losses. 
         [0035]    Although a tubular fired heater  70  is preferred, it will evident to those skilled in the art that the upgraded recycled gas stream may be heated using other types of heaters like ceramic heaters, also called regenerative heaters, as pebble heater, stoves similar to those used for heating the air blast for blast furnaces. 
         [0036]    The present invention may be applied to either new of existing furnaces so as to provide the advantages of lower coke consumption per ton of molten iron and also making the blast furnace more environmentally friendly because the CO 2  removal from the recycled top gas allows for CO 2  utilization for other industrial purposes or for its sequestration. At the same time the lowered coke consumption decreases emission of CO 2  and other contaminants to the environment which coke ovens normally produce, thus decreasing the overall amount of CO 2  released to the atmosphere per ton of molten iron produced. 
         [0037]    It is of course to be understood that in this specification only some preferred embodiments of the invention have been described for illustration purposes and that the scope of the invention is not limited by such described embodiments but only by the scope of the appended claims.