Abstract:
A highly efficient method and apparatus for production of molten iron using coal by coal gasification in a molten iron gasifier-melter. The gasifier-melter is coupled to a direct reduction shaft furnace and utilizes both the gaseous and solid output of the shaft furnace. The process is especially efficient when using non-metallurgical coals.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to an improvement in molten bath gasifier-melters for producing molten iron with continuous melting of iron feed in a bath. The invented method is particularly applicable to the use of non-metallurgical coals. 
     It is known to feed pulverized coal and oxygen to the bottom of a melter-gasifier and to inject oxygen to burn some of the coal gas evolved from the molten bath in order to heat and melt the iron charge fed to the top of the vessel. Difficulties encountered are inefficient heating of the bath and localized combustion. 
     U.S. Pat. No. 4,317,677 of Weber, et al., teaches a fluidized bed gasifier wherein a fluidized bed of coal is formed above a hearth. Water or steam and/or hydrocarbon are introduced through the side of the gasifier and oxygen is also introduced through the side of the gasifier at a lower elevation. The fluidized bed gasifier is coupled directly to a direct reduction furnace above the gasifier, the gases from the gasifier being removed, cleaned, cooled, then introduced into the furnace wherein they act as reducing gases. The direct reduced iron drops directly from the discharge of the furnace through the fluidized bed into the molten metal bath. This differs substantially from the present invention wherein gas from the gasifier-melter is mixed with some recycled gas and introduced to a shaft furnace and the spent top gas is cleaned and cooled, then a portion is mixed with the gasifier gas and a second portion is introduced directly into the gasifier-melter. The present invention makes more efficient use of the gases than any previously known molten bath gasifier-melter. 
     Other prior art patents relating to molten bath gasifier-melters, including Hartwig U.S. Pat. No. 4,007,034, Morvay U.S. Pat. No. 2,750,278, Sanzenbacher, U.S. Pat. No. 4,238,226, and Halley U.S. Pat. No. 2,889,219, do not use a fluidized bed. 
     Another known process utilizes coal gas from a molten bath coal gasifier-melter to produce direct reduced iron. In this process it is undesirable to produce excess gas which requires export for usage outside of the battery limits of an associated direct reduction plant or the utility system which serves the direct reduction plant. The gas produced is employed to produce a highly metallized direct reduced iron. Direct reduction plants generally produce products having about 92% metallization. 
     SUMMARY OF THE INVENTION 
     A highly efficient method and apparatus for production of molten iron using non-metallurgical coal by coal gasification in a molten iron bath gasifier-melter, which is coupled to a direct reduction furnace and utilizes both the gaseous and solid output of the direct reduction furnace. 
     OBJECTS OF THE INVENTION 
     It is the principal object of this invention to provide a method for producing molten metal from direct reduced iron, while efficiently utilizing the energy values within the direct reduction system. 
     It is also an object of this invention to provide a method for producing molten iron utilizing the full chemical energy of partially spent reducing gas from the direct reduction process to provide the fuel required in the gasifier-melter. 
     It is another object of this invention to utilize, if required for a particular production site, the full chemical energy content of the partially spent discharge gas or top gas from the reduction furnace to provide the fuel needed in the gasifier-melter to melt the direct reduced iron produced and to provide for efficient re-utilization of the energy values in the direct reduction system. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects are better understood by referring to the following detailed description and the appended drawing, in which: 
     The single FIGURE is a schematic flow sheet of the method of the invention showing all of the equipment required to carry out the invented method. 
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawing, a molten iron gasifier-melter 10 is provided with one or more tuyeres 12 in its bottom wall, each tuyere being connected to a source 14 of oxygen and a source 16 of coal dust. The gasifier-melter 10 has a direct reduced iron introduction means 18, an oxyfuel burner 20 fueled by oxygen from source 21 and recycled gas from line 22. The gasifier-melter also has a product gas outlet 24, a flux injection means 26, a molten iron tap 28 and a slag tap 30. 
     A direct reduction furnace 40 is provided with an iron oxide feeding inlet 42 at its top, a reducing gas inlet system 44 intermediate the ends of the furnace, and a direct reduced iron product outlet 46 at the bottom of the furnace. A reacted gas outlet or offtake 48 is provided at the top of the furnace, which outlet communicates with a scrubber 52. Sludge is removed at 54 in the form of a slurry. The gas outlet from the scrubber, gas line 56, connects to three gas lines 58, 60, and 62. 
     Gas line 58 communicates with a steam plant 64 and provides combustible fuel for the steam plant. Gas line 60 is connected to a carbon dioxide removal system 66. Steam enters the carbon dioxide removal system at 68. Carbon dioxide is removed at 70 and CO 2  -lean gas is removed at 72. Export fuel gas can be removed from the system through gas removal pipes 62 or 76 if desired, or if required elsewhere in the process. 
     The CO 2  -lean gas is again divided, a portion moving through conduit 73 to a gas mixer 74 (temper mixer). A second portion passes through conduit 22 to oxyfuel burner 20 of the gasifier-melter 10. 
     In operation, molten iron gasifier-melter 10 receives a mixture of coal dust and oxygen through tuyeres 12 located in the bottom of the gasifier-melter. Direct reduced iron, preferably in the form of metallized iron fines, may be fed from a direct reduction furnace. Metallized iron fines are fed through the top of the gasifier-melter through introduction means 18 if in large particulate form, or they are fed through the bottom of the gasifier-melter from conduit 78 with the coal, if in finely divided form. A flux from source F, consisting of lime or calcined dolomite, along with other fluxing agents, is fed to the gasifier-melter through line 26 or through tuyere 12 in the bottom of the gasifier-melter, as desired. Calcium oxide in the flux serves to maintain fluidity of the slag and act as a sulfur acceptor to bulk desulfurize the gas produced in the gasifier-melter. To provide additional heat to melt all of the metallized iron fines fed to the gasifier-melter, a self-cooling oxy-fuel burner 20, fueled with recycle gas from conduit 22 which has been processed to produce a water and carbon dioxide lean gas, is provided in the roof of the gasifier-melter. 
     The product gas from the gasifier-melter is removed through outlet 24 and delivered to a mix station 74 where carbon dioxide lean top gas is heated to temper the product gas from the gasifier-melter system to between 760 and 900 C. This temper gas is introduced to direct reduction furnace 40 through inlet 44, wherein it is used as reducing gas to reduce iron oxide to a highly metallized form of direct reduced iron. Preferably, all of the direct reduced iron removed from the direct reduction furnace at product outlet 46 is fed to the gasifier-melter 10. However, some of the direct reduced iron may be removed at 80 to stockpile 82 for later use in the gasifier-melter, or removed at 83 for use elsewhere. Additional metallized iron fines from stockpile 82, either cold or preheated, may be combined with the direct reduced iron in conduit 84 to form the feed to the gasifier-melter. 
     As the gas reacts countercurrently with the iron oxide in the reduction furnace, an equilibrium barrier is approached in the utilization of all of the hydrogen and carbon monoxide caused by thermodynamic limits. Therefore, the top gas from the reduction system contains valuable hydrogen and carbon monoxide. It is cooled and scrubbed in cooler-scrubber 52 in preparation for reuse. During the cooling process it is possible to produce some of the steam required for the regeneration unit in the carbon dioxide removal system by indirect heat exchange. A small part of the cooled and scrubbed top gas is then diverted to a steam plant 64 as fuel to generate all or at least the remainder of the steam required. The remaining portion of the scrubbed top gas is compressed in a compressor 65 and delivered to the carbon dioxide removal system 66. The carbon dioxide lean recycle gas produced in the carbon dioxide removal system 66 provides the fuel for the oxy-fuel burner 20 of the gasifier-melter 10, and also provides the gas required to temper the product gas from the gasifier-melter in mixer 74 to the temperature required for reduction, as previously described. 
     The system is surprisingly well balanced, leading to an unexpectedly high efficiency and low coal requirements. If extra top gas is developed because of changes in coal composition, more of the recycle gas may be added to the oxy-fuel burner for reheat, and less coal/oxygen mixture need be fed to the gasifier-melter. If more recycle gas is needed to feed the substoichiometric oxy-fuel burner, the temperature of the bath can be lowered from 1500° C. to as low as 1400° C., releasing some recycle gas from its use as a temper gas for use as a burner fuel. If even more recycle gas is needed, steam can be produced from the hot coal gas also releasing recycle gas used for tempering and for use as a burner fuel. 
     PROCESS EXAMPLE 
     To illustrate the performance of the process, the performance of a molten bath gasifier-melter operating at 1500° C. was calculated. The process system selected includes indirect heat exchange to produce steam by recovering energy from the hot top gas (also known as spent reducing gas) discharged from the iron oxide reduction furnace. The carbon dioxide is removed by an energy efficient carbon dioxide removal system, such as one of the hot potassium carbonate processes. The remaining steam needed by the carbon dioxide removal unit is produced by burning scrubbed top gas. In this example, no steam is produced from the sensible heat of the product gas from the gasifier-melter. A dry powdered Saar coal is used in the gasifier-melter. Carbon dioxide lean gas is used in a substoichiometric burner to provide the extra heat required to melt all of the direct reduced iron fed to the gasifier-melter. No export gas is produced beyond the battery limit, although other process conditions could allow some export gas. The analysis of the Saar coal is given in Table I. 
     
                       TABLE I______________________________________Saar Coal DustAnalysis on Moisture-Free (M.F.) Basis______________________________________Carbon              75.2   wt. %Hydrogen            5.0Nitrogen            1.5Oxygen              9.0Sulfur              1.6Ash                 7.7______________________________________ Moisture in feed coal is 0.025 Kg/Kg M.F. Coal Lower heating value (M.F.) is 7230 Kcal/Kg 
    
     Important solids analysis and solids flow rates per tonne of molten iron and per tonne of direct reduced iron (DRI) are presented in Tables II and III. 
     
                       TABLE II______________________________________Solids Flow Rate per Tonne         t/t DRI                t/t Molten Iron______________________________________Iron Oxide      1.41     1.49Lime            0.059    0.062DRI (production/           1.0      1.055consumption)Coal (M.F.)     0.435    0.459______________________________________ 
    
     
                       TABLE III______________________________________Analysis of Iron Containing RawMaterial, Intermediate and Product  Wt %  Fe.sub.2 O.sub.3          FeO    Fe       Gangue C______________________________________Iron Oxide    97.0      --     --     3.0    --DRI      --        8.4    86.0   4.1    1.5Molten Iron*    --        --     96.8   --     3.0______________________________________ NOTE: *The molten iron does not add exactly to 100% because of contained silicon, etc. 
    
     Important gas analysis and flow rates are presented in Tables IV and V. 
     
                       TABLE IV______________________________________Gas Analysis of Important StreamsStream    CO     CO.sub.2                    H.sub.2                         H.sub.2 O                               CH.sub.4                                    Ar + N.sub.2______________________________________Oxygen    --     --      --   --    --   2Raw Gas   61.0   6.0     22.0 8.5   --   2.5To Reduction     55.3   5.5     29.4 2.9    .8  6.1FurnaceTop Gas   34.7   25.9    21.1 11.2  1.0  6.1Recycle, CO.sub.2     53.8   1.0     32.7 1.5   1.5  9.5Lean Gas______________________________________ 
    
     
                       TABLE V______________________________________Gas Flow Rate Per Tonne of DRIStream             Flow Rate______________________________________Oxygen (98%)       345 normal cubic metersRaw Gas            1012To Reduction Furnace              2059Top Gas            2039Recycle, CO.sub.2 Lean Gas              1189Recycle to Temper  1047Recycle to Gasifier-Melter              142CO.sub.2 Removed   465Steam Production   883 KgPercent Steam from Heat              34%Recovery from Top GasPercent of Top Gas to              9.7%Fuel Steam Plant______________________________________ 
    
     The temperature of the feed gas is 815° C. When operating under these conditions, the coal requirement is 3.44 Gcal (HHV) or 3.32 Gcal (LHV) per tonne of molten iron. 
     ALTERNATIVE EMBODIMENTS 
     The direct reduction furnace 40 can be a shaft furnace or a fluidized bed furnace. The lower portion 88 of the furnace can constitute a cooling zone to cool the reduced metallized iron product. 
     In order to totally utilize the energy available in the system, a stem generator 90 can be placed in the gasifier gas conduit 92, or in the top gas conduit 94. 
     It is clear from the foregoing specification that other modifications can be made and still be within the scope of the invention. Therefore, it is to be understood that the invention is limited only by the scope of the appended claims.