Abstract:
An apparatus and method for adjusting the parameters of a reducing gas stream prior to introduction into a direct reduction furnace, such parameters including temperature of the gas stream, and amount of hydrocarbon, carbon monoxide, and hydrogen contained in the reducing gas. The apparatus is placed in-line with the reducing gas recycle loop of a direct reduction furnace, which has an enrichment section which introduces hydrocarbon components to the main stream, and an oxygen/fuel injection system, located downstream from the enrichment section, which injects a shrouded stream of oxygen and hydrocarbon gas into the reducing gas stream. Temperature, carbon monoxide content, and hydrogen content of the reducing gas are adjusted by controlling the flow of oxygen and the ratio of hydrocarbon to oxygen injected in the oxygen/fuel injection system. Hydrocarbon content of the reducing gas is adjusted primarily by controlling the flow rate of the enrichment section.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of U.S. Provisional application Ser. No. 60/191,680, filed on Mar. 23, 2000, and is a continuation in part of co-pending U.S. patent application Ser. No. 09/781,816, filed Feb. 12, 2001, which is a continuation in part of co-pending U.S. patent application Ser. No. 09,456,111, filed Dec. 7, 1999, which is a continuation in part of U.S. patent application Ser. No. 08/924,686, filed Sep. 5, 1997, now U.S. Pat. No. 5,997,5 96, which issued Dec. 7, 1999. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates an apparatus and method for the direct reduction of iron oxide, and more particularly to an apparatus and method for the reformation or modification of reducing gas for use with a direct reduction furnace.  
         BACKGROUND OF THE INVENTION  
         [0003]    The production of direct reduced iron in a vertical shaft reactor involves reduction of the ore in a reduction zone through which is passed a suitable hot reducing gas, known as bustle gas, largely composed of carbon monoxide and hydrogen at temperatures in the range of 700° C. to 1000° C. and cooling of the directly reduced iron in a cooling zone through which is passed a gaseous coolant at a temperature below about 200° C. The ore to be treated is charged to the top of the reactor and caused to flow downwardly through the reduction zone wherein it is reduced by heated reducing gas which flows upwardly through the reactor, after which the reduced ore flows into and downwardly through the cooling zone to be cooled and carburized by contact with a stream of suitable cooling gas. The cooled iron is then discharged through the bottom of the reactor. Typically, both the reducing gas and cooling gas are recirculated, optionally in closed loops, to which streams of fresh (i.e. “make-up”) reducing gas are added and from which streams of spent gas are removed.  
           [0004]    In various known direct reduction processes, the reducing gas required for reduction of the iron ore is generated in a catalytic reforming unit by conversion of natural gas in accordance with the following reactions: 
           (1) CH 4 +H 2 O=&gt;CO+3H 2   
           (2) CH 4 +CO 2 =&gt;2CO+2H 2   
           [0005]    In the reforming reactions the natural gas, comprised mainly of methane, is converted to hydrogen and carbon monoxide in the presence of an oxidizing agent. As a result, the reformed gas is substantially composed of hydrogen and carbon monoxide. In recent times, due to the ever decreasing availability and increasing cost of natural gas it has become extremely important and therefore desirable to develop a direct reduction process in which the required quantity of natural gas is minimized.  
           [0006]    The reducing gas being fed to the reduction zone of the reactor is typically at an elevated temperature and is caused to contact the downwardly moving iron ore to reduce the iron oxides therein according to the following basic reduction reactions: 
           (3) 3Fe 2 O 3 +H 2 /CO−&gt;2Fe 3 O 4 +H 2 O/CO 2   
           (4) Fe 3 O 4 +H 2 /CO=&gt;3FeO+H 2 O/CO 2   
           (5) 3FeO+H 2 /CO=&gt;3Fe+H 2 O/CO 2   
           [0007]    The spent reducing gas leaving the reactor is cooled to remove water produced by the reduction of the iron ore with hydrogen after which the cooled and de-watered effluent gas is recycled and then reintroduced to the reduction zone of the reactor. The recirculation, or recycling, of the effluent gas can be accomplished in various ways. For example, the gas may be recycled directly back into the reactor, the gas may be recycled through a reformer and a heater, or the gas may simply be recycled through the heater. In each case, however, fresh reducing gas must be added to the recycled effluent gas prior to injection into the reactor. Since the amount of carbon dioxide generated in the process by the reduction reactions occurring in the reactor is considerable, a portion of the spent gas must be vented or purged from the system in order to reduce the amount of carbon dioxide and to maintain a proper overall carbon balance within the reduction system.  
           [0008]    Efficiency in the direct reduction process is determined, in large part, by the quality and temperature of the reducing gas. “Quality” of a reducing gas is defined by the ratio of carbon monoxide and hydrogen (the reductants) to carbon dioxide and water (the oxidants), i.e., [CO and H 2 ]/[CO 2  and H 2 O]. Hydrogen and carbon monoxide are the preferred components of the reducing gas. Carbon dioxide is not desired because of its inability to reduce iron oxides. Excessive amounts of hydrocarbons are not desired because they react endothermically upon entering the reactor, which reduces the kinetics of the desired iron reduction reactions. And, of course, free oxygen is not desired in the reducing gas because of its tendency to re-oxidize reduced iron. The temperature of the reducing gas affects the reaction kinetics between the various components of the reducing gas with each other and with the iron and iron oxide.  
           [0009]    Reducing gas generators of the prior art, that use oxygen and/or fuel to generate reducing gas, can be classified in two categories; oxygen injection systems and oxy-fuel burners. Oxygen injection systems combust mostly reductant and minor amounts of hydrocarbons. This elevates the temperature of the reducing gas entering the reduction reactor. Excessive reducing gas temperatures will cause sintering of the metal and plugging of the reducing reactor.  
           [0010]    Oxy-fuel burners are designed specifically to maximize the amount of reductant formed by precisely adjusting the flows of oxygen and fuel to a burner. However, the use of oxy-fuel burners does not allow independent control of reducing gas temperature and reduction reactor temperature, because oxy-fuel burners must be operated within certain limits of oxygen and fuel throughput to obtain good conversion of fuel to reductant, to prevent soot formation, to prevent overheating of the burner, and to maintain flame stability. Also, the use of oxy-fuel burners is often prohibitively expensive.  
           [0011]    What is needed is an apparatus and method which generates reducing gas, adding to the recycled/reformed reducing gas, therefore increasing the quantity of hydrogen and carbon monoxide within the reducing gas, while effectively controlling the temperature of the resulting gas stream, and while controlling the amount of CH4 in the resulting gas stream, so that the reduction reactor temperature may be controlled effectively. Such an invention would greatly enhance the economics of the direct reduction process by: 1) controlling reduction reactor temperature for optimum utilization of hydrogen and carbon monoxide in the reduction reactor, thereby minimizing the amount of natural gas required for the direct reduction process; and 2) by minimizing the cost of the apparatus which generates the fresh reducing gas.  
         OBJECTS OF THE INVENTION  
         [0012]    The principal object of this invention is to provide means for generating reducing gas, particularly H 2  and CO, from the combustion of oxygen and fuel, particularly natural gas.  
           [0013]    Another object of the invention is to provide means for increasing the amount of reducing gas generated, particularly H 2  and CO, from the combustion of oxygen and fuel, and to produce a high quality reducing gas by combustion of oxygen and fuel, particularly natural gas.  
           [0014]    Another object of the invention is to provide means for increasing the amount of reducing gas generated, particularly H 2  and CO, from the combustion of oxygen and fuel, and to produce a high quality reducing gas by combustion of oxygen and fuel, particularly natural gas, while accurately controlling the temperature of the reducing gas stream.  
           [0015]    Another object of the invention is to generate reducing gas, particularly H 2  and CO, from the combustion of oxygen and fuel, particularly natural gas, while accurately controlling the concentration of methane/hydrocarbons in the reducing gas stream.  
           [0016]    Another object of the invention is to provide means for increasing the amount of reducing gas generated, particularly H 2  and CO, from the combustion of oxygen and fuel, and to produce a high quality reducing gas by combustion of oxygen and fuel, particularly natural gas, while accurately controlling the temperature of the reducing gas stream and simultaneously controlling the level of methane/hydrocarbons in the gas stream.  
           [0017]    Another object of the invention is to provide means for generating reducing gas, particularly H 2  and CO, from the combustion of oxygen and fuel, particularly natural gas, while simultaneously minimizing the temperature of the reducing gas stream, for a given oxygen to fuel ratio, and minimizing the cost of the apparatus that generates the reducing gas.  
         SUMMARY OF THE INVENTION  
         [0018]    The present invention is an apparatus and method for increasing the amount of reducing gas within the reducing gas stream of a direct reduction furnace, while also simultaneously controlling the temperature of the reducing gas stream, and the temperature of the reduction reactor. The method improves the efficiency of direct reduction, by removing and reforming spent reducing gas from a direct reduction furnace to form a high percentage of H 2  and CO therein for use as bustle gas, injecting hydrocarbon-containing enrichment gas into the bustle gas, injecting an oxygen-hydrocarbon gas mixture into the bustle gas and combusting the mixture, and introducing the hydrocarbon-enriched bustle gas into the direct reduction furnace. The apparatus is a refractory lined shaft through which reducing gas, also called bustle gas, passes before entering the direct reduction furnace. The improved apparatus has two main sections, an enrichment section and an oxygen/fuel injection section which communicate with the bustle of the shaft. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    [0019]FIG. 1 shows a block diagram of how the method proceeds.  
         [0020]    [0020]FIG. 2 shows a side cutaway view of the invented apparatus.  
         [0021]    [0021]FIG. 3 shows a front cutaway view of the invented apparatus. 
     
    
     DETAILED DESCRIPTION  
       [0022]    The present invention will be described more fully hereinafter with reference to the accompanying drawings, in which a preferred embodiment of the invention is shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiment set forth herein; rather, this embodiment is provided so that this disclosure will be, thorough and complete, and will convey the scope of the invention fully to those skilled in the art. Like numbers refer to like elements throughout.  
         [0023]    Referring now to the drawings, the present invention is a method and an apparatus for increasing the amount of reducing gas within the reducing gas stream, also referred to as “bustle gas stream” of a direct reduction furnace while simultaneously controlling the temperature and hydrocarbon content of the reducing gas stream. The apparatus is a refractory lined shaft  10  through which reducing gas, also called bustle gas, passes before entering the direct reduction furnace. The apparatus has two main sections, an enrichment section  12  and an oxygen/fuel injection section  14 .  
         [0024]    The invented apparatus is located between the reducing gas output of the direct reduction furnace and the reducing gas input of the direct reduction furnace. Upon entering the apparatus, concentration of simple hydrocarbons, typically CH 4 , is increased by the enrichment section  12  of the apparatus. As the reducing gas continues through the apparatus, the temperature of the gas is adjusted by combustion of an oxygen and fuel mixture injected by the oxygen/fuel injection system  14 .  
         [0025]    The invented apparatus is preferably operated in conjunction with a reducing gas reformer unit. In operation, the apparatus is placed between the output of the reformer and the reducing gas input of the direct reduction furnace. Spent reducing gas exits the furnace and enters the reforming unit, completing the recycling circuit of the reducing gas. The reformer, preferably a catalytic reformer, reacts the methane in the reducing gas stream to improve the quality of the reducing gas according to the equations: 
         (1) CH 4 +H 2 O=&gt;CO+3H 2   
         (2) CH 4 +CO 2 =&gt;2CO+2H 2   
         [0026]    The enrichment section  12  is positioned in the upstream region of shaft  10 . The enrichment section  12  introduces natural gas or other hydrocarbon fuels into the shaft  10  so that the main reducing gas stream is enriched with hydrocarbons as the gas stream passes through the shaft  10 . The controlled injection of natural gas or other hydrocarbon increases the methane (and other hydrocarbon) content of the reducing gas. Preferably, the natural gas injected by the enrichment section  12  is preheated, up to about 450° C., though the natural gas need not be preheated for effective use of the invention.  
         [0027]    The enrichment section  12  comprises a header  16 , external to the shaft  10 , which is connected to several separate nozzles  18  which protrude into the shaft  10  from the header  16 . In the preferred embodiment, there are  12  such nozzles  18  equally spaced about and protruding through the shaft  10 . The nozzles  18  may be constructed from any suitable material but are preferably constructed from stainless steel.  
         [0028]    The nozzles  18  project the natural gas in such manner that the gas enters the shaft  10  perpendicular to the main reducing gas stream. The flow of natural gas into the enrichment section  12  is measured by a methane sensor  20  located far downstream from the enrichment section  12  but located at a location upstream from the reducing gas input to the furnace. Through control means commonly known in the art, the flow rate of hydrocarbons introduced by the enriching section  12  is adjusted by control valve  22  based upon the measurements of methane sensor  20  so that the methane concentration of the reducing gas entering the furnace remains in the range of from about 1% to about 10%.  
         [0029]    The oxygen/fuel injection section  14  is positioned in the shaft  10  downstream from the enrichment section  12 . The oxygen/fuel injection section  14  has two sets of components, each set composed of a header and a series of injection ports. The first set of components, the oxygen injection components  24 , carry oxygen or an oxygen containing gas into the shaft  10 . The second set of components, the fuel injection components  26  for fuel injection, carry a hydrocarbon fuel which is preferably natural gas into the shaft  10 . Preferably, the natural gas injected by the oxygen/fuel injection section  14  is preheated, up to 450° C., though the natural gas need not be preheated for effective use of the invention.  
         [0030]    In the preferred embodiment, oxygen injection components  24  of the oxygen/fuel injection section  14  are shrouded by fuel injection components  26  of the oxygen/fuel injection section  14 . The key to the preferred embodiment is that the oxygen injection components  24  are coaxially disposed within the fuel injection components  26 . This arrangement of injection components in the oxygen/fuel injection section  14  promotes the immediate mixing of oxygen supplied by components  24  with the hydrocarbons supplied by components  26  upon entry into the main shaft. The mixing promotes the combustion of the natural gas injected in the oxygen/fuel injection section  14 , rather than allowing the injected oxygen to combust with the hydrogen and carbon monoxide already present in the reducing gas.  
         [0031]    Controlling the manner and extent to which the injected oxygen combusts the natural gas injected by components  26  increases the quantity of the reductant in the resulting reducing gas while, at the same time, regulating the temperature of the reducing gas. By properly controlling the ratio of hydrocarbons and O 2  injected by the oxygen/fuel injection system  26 , the mixture may be maintained as a rich mixture, resulting in the desired incomplete combustion which results in CO and H 2  as products of the combustion. At this stage, excess O 2  is undesirable for two reasons. First, any O 2  which is not combusted with the hydrocarbons provided by the fuel injection components  26  is free to further combust the CO and H 2  already present in the main reducing gas stream into the undesirable products of CO 2  and H 2 O. Second, limited O 2  combusted with the hydrocarbons provided by oxy-fuel injection system  26  results in incomplete combustion of the hydrocarbon stream, producing CO and H 2  as products rather than the CO 2  and H 2 O which would theoretically result from complete combustion.  
         [0032]    The unique arrangement of shrouding the oxygen with the hydrocarbons supplied by the oxy/fuel injection system  26  results in combustion of the injected oxygen with the shrouding hydrocarbon fuel which reduces the flame temperature within the main shaft. The partial combustion of the inputted oxygen with natural gas or other hydrocarbon, CH 4  for example, results in a low temperature flame, and also results in the production of CO and H 2 , both desired reductants. If the injected oxygen were not shrouded by the natural gas and allowed to combust with the H 2  or CO already present in the main reducing gas stream, higher temperatures would result and H 2 O and CO 2 , both undesired oxidants, would be generated.  
         [0033]    In operation, the temperature of the reducing gas is measured downstream  20  from the oxygen injection section  14 . The flow of oxygen to the oxygen injection section  14  is increased or decreased to maintain the desired reducing gas temperature. The flow of hydrocarbon fuel to the oxy/fuel injection section  26  is increased or decreased by valve  27  in order to remain in constant ratio with the oxygen flow. Adjustment of hydrocarbon fuel to the oxy/fuel injection section  26  is preferably made by measuring the flow of oxygen to the oxygen injection section  24  and using an electronic control device  36  to adjust a flow valve  38  in-line with the hydrocarbon supply of the oxy/fuel injection system  26  such that the hydrocarbon is supplied in a ratio or other predetermined relationship to the amount of oxygen supplied.  
         [0034]    Preferably, proper mixing of the oxygen and the injected hydrocarbons in the oxygen/fuel injection system  14  is accomplished by using dual external headers  42 ,  44  feeding twelve separate stainless steel nozzles, each having a plurality of oxygen injection ports  46  coaxially aligned with and disposed within a hydrocarbon injection port  48 . This arrangement allows the injected hydrocarbon to shroud the injected oxygen as the two gases are projected into the shaft  10 . The tips of the hydrocarbon injection ports  48  converge upon the tips of the oxygen injection ports  46  to promote better mixing between the oxygen and the injected hydrocarbons. The injection ports  46 , 48  are housed in nozzles  28 , which are angled downstream into the main bustle gas shaft  10 , unlike the perpendicular alignment of the nozzles  18  in the enrichment section  12 , to allow for longer flame lengths, avoiding damage to the refractory on the opposite side of the shaft  10 . Of course, more than or less than twelve injection ports  46 ,  48  may be used in the oxygen/fuel injection system  14  with favorable results. Also, a port ( 46 , 48 ) arrangement other than the disclosed shrouding arrangement may be utilized. For instance, a side by side or other port arrangement may be utilized as long as the oxygen and fuel of the oxygen/fuel injection system remain in intimate contact with one another as they are projected into the shaft  10 .  
         [0035]    The refractory inside the bustle gas shaft  10  is designed to allow the oxygen/hydrocarbon fuel mixture to burn outside of the turbulence of the main stream of reducing gas. A section of the refractory  50  protrudes slightly from the inner wall of the shaft  10 , beginning downstream of the enrichment section  12  and sloping gently inwards to a point just upstream of the oxygen/fuel injection system  14 . The oxygen/fuel injection ports  46 , 48  are positioned just downstream of the protruding refractory section  50 , and are angled downstream such that the flow of reducing gas and enriched gas flowing through the shaft  10  are directed to the center region of the shaft  10 , and such that the oxygen and hydrocarbons introduced in the oxygen/fuel injection system  14  enter the shaft  10  and are momentarily maintained outside of the turbulence of the main gas stream, thereby allowing the oxygen and fuel of the oxygen/fuel injection system  14  to mix well and bum with one another before entering the main gas stream.  
         [0036]    The oxygen header  42 , hydrocarbon fuel header  44 , and nozzles  28  have been designed so the oxygen nozzle can be easily removed for inspection, maintenance, etc. The oxygen injection nozzles  46  preferably are made of stainless steel and designed to promote mixing with the shrouding hydrocarbon fuel.  
         [0037]    The enrichment section  12  is located upstream from the oxygen/fuel injection section  14  so that the hydrocarbons injected in the enrichment section  12  can potentially bum with any unconsumed oxygen from the oxygen/fuel injection section  14  before the oxygen combusts with H 2  or CO in the main reducing gas stream.  
       EXAMPLE  
       [0038]    The following are examples of the calculated flow rates of oxygen and hydrocarbon fuel expected given the component properties and measured variables below:  
         [0039]    Reducing Gas from Reformer  
         [0040]    Temperature: 940° C.  
         [0041]    H 2 /CO is 1.55  
         [0042]    Composition is 1.6% CH 4 , 2.8% CO 2 , 6.0% H 2 O, 54.2% H2, 34.9% CO, 0.5% N2  
         [0043]    Flow is 300,00 Nm3/h  
         [0044]    Enrichment Gas (same as fuel for oxygen/fuel injector)  
         [0045]    Temperature: 400° C.  
         [0046]    Composition is 87.55% CH 4 , 8.86% C 2 H 6 , 2.45% CO, 0.5% CO 2 , 0.6% C 3 + 
         [0047]    Oxygen  
         [0048]    Temperature: 20° C.  
         [0049]    Composition: 99.5% O 2  
                                                                 Case 1   Case 2                                    Measured Bustle Gas Temp   1050° C.   1050° C.       Measured Reduction Shaft Temp   850° C.   900° C.       Measured Bustle Gas CH 4     4.72% wet   3.30% wet       Oxygen/Fuel Injection Flow (O 2 )   3766 Nm 3 /h   3141 Nm 3 /h       Oxygen/Fuel Injection Flow (Hydrocarbon)   6277 Nm 3 /h   5235 Nm 3 /h       Enrichment Flow (Hydrocarbon)   5923 Nm 3 /h   1490 Nm 3 /h                  
 
       SUMMARY OF THE ACHIEVEMENT OF THE OBJECTS OF THE INVENTION  
       [0050]    From the foregoing, it is readily apparent that we have invented an improved method and apparatus for optimizing the use of oxygen in the direct reduction of iron which may be utilized to increase the amount of reductant generated from the combustion of oxygen and fuel relative to the current oxygen injection systems; which is capable of producing a higher quality reducing gas than present oxygen injection systems; and which can accurately control the temperature of the gas stream and control the level of methane/hydrocarbons in the reducing gas stream.  
         [0051]    It is to be understood that the foregoing description and specific embodiments are merely illustrative of the best mode of the invention and the principles thereof, and that various modifications and additions may be made to the apparatus by those skilled in the art, without departing from the spirit and scope of this invention, which is therefore understood to be limited only by the scope of the appended claims.