Abstract:
A process and system for producing reducing gases are disclosed, wherein volatile components derived from coal are transformed into reducing gases suitable for utilization as synthesis gas, as a reducing agent for the direct reduction of iron ores and/or as a clean fuel.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention refers to a process and system for producing reducing gases mainly composed of hydrogen and carbon monoxide from coke-making operations where coal is transformed into metallurgical coke, and more particularly to a process wherein the volatile components derived from coal are transformed into reducing gases suitable for chemical utilization as synthesis gas, as a reducing agent for direct reduction of iron ores and/or as a clean fuel.  
       BACKGROUND OF THE INVENTION  
       [0002]     It is known that in the process of producing metallurgical coke, coal is heated in order to eliminate most of the volatile components and preserving mostly the carbon structure. Coke is thereby provided with the physical and chemical properties which make it fit for providing energy and burden support in blast furnaces. Volatile matter of coal comprises a number of compounds which are distilled in coke ovens constituting what is known as coke oven gas. The volume and composition of coke oven gas depends on the characteristics of the coal utilized but typically untreated coke oven gas comprises about 44% of water, about 29% of hydrogen, about 3% of carbon monoxide, about 13% of methane and many impurities as ammonia, sulfur, benzene-type compounds, etc.  
         [0003]     Since coke oven gas has a high calorific value, it is utilized mostly for heating purposes in steel plants. Typically, coke oven gas is cooled, cleaned and treated in a number of chemical processes for separating valuable compounds such as ammonia and other petrochemicals and for removing sulfur before the gas is finally burned. Cleaning of coke oven gas require&#39;s a complex and costly chemical plant.  
         [0004]     A number of proposals are found in patents and other technical literature for utilizing coke oven gas after cleaning, alone or in combination with other gases, for iron reduction purposes or for steam and electricity generation.  
         [0005]     U.S. Pat. No. 4,178,266 describes a process for conveying hot crude coke oven gas generated in coke ovens to a position of utilization while preventing condensation of higher hydrocarbons. This patent teaches to increase the temperature of the coke oven gas by injecting an oxygen-containing gas into the coke oven gas stream so that it is partially combusted. Such injection of oxygen may increase the temperature of the mixture to about 950° C. to 1500° C. The purpose of the oxygen injection is to avoid condensation of impurities and of higher hydrocarbons, thereby preventing many problems in the gas conveying systems. This patent does not teach or suggest carrying out a partial combustion of coke oven gas for producing a high-quality reducing gas.  
         [0006]     British Patent Specification No. 1,566,970 describes a process for the treatment of coke oven gas. This patent recognizes the value of performing a partial combustion of coke oven gas in order to produce a reducing gas useful for direct reduction of iron ores. The partial combustion transforms coke oven gas into a cracked gas rich in carbon monoxide and hydrogen. The partial combustion process of this patent however has a number of disadvantages and does not teach or suggest a process and apparatus which integrates the energy in an improved way.  
         [0007]     U.S. Pat. No. 4,235,624 describes a method for processing coke oven gas almost identical to the method of the above-referred British patent 1,566,970. This patent does not teach or suggest the integration of thermal energy in the treatment and cleaning of coke oven gas comprising a partial combustion step as the present invention.  
         [0008]     U.S. Pat. No. 4,235,624 describes a method for processing coke oven gas almost identical to the method of the above-referred British patent 1,566,970. Although this patent teaches generally that the coke oven gas may be used as a reducing gas in an iron ore reduction shaft furnace, no detail is provided about a preferred manner of utilizing the integration of the thermal energy that the coke oven gas contains. As a matter of fact, the claims of this patent specify that the invention lies in the utilization of hot coke oven gas, that means that the coke oven gas is fed to the reactor without cooling it before its introduction into the shaft reactor. This process scheme has a number of disadvantages since all equipment involved in the handling and conduction of the coke oven gas from the coke ovens to the shaft reactor must be prepared for high-temperature operation.  
         [0009]     In contrast, the present invention provides a method and apparatus with a practical and economical way of using the coke oven gas by cooling it immediately after the partial combustion cracking and also utilizing the heat produced in said partial combustion for at least two specific purposes which are critical for the operation of the reduction reactor: e.g. for the CO2 removal unit of the recycled reducing gas to the shaft reactor, and also for the sulfur removal unit needed for cleaning the coke oven gas prior to its utilization.  
       OBJECTS OF THE INVENTION  
       [0010]     It is therefore an object of the present invention to provide a process and system for treating coke oven gas in order to produce a reducing gas comprising hydrogen and carbon monoxide, particularly useful for the direct reduction of iron ores.  
         [0011]     It is another object of the present invention to provide a process and system for treating coke oven gas and producing a synthesis gas useful as raw material in chemical processes or for producing heat, steam, electricity or power avoiding the complex and expensive cleaning processes normally utilized in coking plants.  
         [0012]     Other objects of the invention will be pointed out in this specification or will be evident to those skilled in the art.  
       SUMMARY OF THE INVENTION  
       [0013]     The present invention provides a method of treating hot crude coke oven gas produced in coke ovens comprising: mixing said hot crude coke oven gas with an oxygen-containing gas in order to carry out a partial combustion of said coke oven gas producing a reducing gas having a high content of hydrogen and carbon monoxide at a temperature higher than about 1000° C.; passing said hot reducing gas through a heat exchanger in order to produce high temperature steam; utilizing a first portion of said steam to produce electricity; removing sulfur compounds and other impurities of said reducing gas in a sulfur removing unit; and utilizing a second portion of said steam in said sulfur removing unit.  
         [0014]     In a preferred embodiment of the invention the method further includes an additional step of utilizing at least a portion of said clean reducing gas in a direct reduction plant comprising a direct reduction reactor, a CO2 removal unit and a gas heater.  
         [0015]     In another preferred embodiment of the invention the method further includes the additional step of utilizing a third portion of said exhausted steam in said CO2 removal unit of said direct reduction plant. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  is a schematic process diagram of a preferred embodiment of the invention.  
         [0017]      FIG. 2  illustrates an example of the process showing, in a schematic process diagram similar to  FIG. 1 , values of process parameters.  
         [0018]      FIG. 3  is a table supplementing the information of process streams shown in  FIG. 2 .  
         [0019]      FIG. 4  also illustrates another example of the process showing, in a schematic process diagram of a direct reduction plant, values of process parameters operating according to the coke oven gas cleaning method of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]     The invention will be described with reference to the accompanying drawings, it being understood that many changes and modifications may be made by those skilled in the art without departing from the spirit of the invention, which is defined by the appended claims.  
         [0021]     With reference to  FIG. 1 , numeral  10  designates a battery of coke ovens, to which coal  12  is charged and heated therein by combustion of gases  14  and air  16 . Coke  18  is discharged from ovens  10  in a manner known in the art and hot crude coke oven gas  20  is produced typically at a temperature of about 600° C. degrees to 700° C.  
         [0022]     Hot crude coke oven gas  20  is then mixed with an oxygen-containing gas  22  which may be produced in an air separation plant  24  from air  26 . The air separation plant  24  may be of the cryogenic or a PSA-type since the purity of oxygen is not critical for this application. The process may also operate with oxygen-enriched air, although the quality of the resulting reducing gas is affected by the amount of nitrogen from said air feed.  
         [0023]     Hot crude coke oven gas  20  at a temperature of about 650° C. is subject to a partial combustion with oxygen  22  in combustion chamber  28  reaching temperatures above about 1000° C. At this temperature all organic compounds are converted into a high temperature reducing gas  30  mainly composed of a mixture of hydrogen and carbon monoxide, carbon dioxide, methane and water.  
         [0024]     Reducing gas  30  is passed through a heat exchanger  32  where it heats steam  34  from steam drum  36  and super-heated steam  38  is utilized in turbine  40  to produce electricity in electric generator  42 . Electricity is fed through line  44  and is used in air separation plant  24  to produce oxygen for the partial combustion of coke oven gas.  
         [0025]     A first portion  50  of exhausted steam  46  from turbine  40  is used in sulfur separation unit  48  and a second portion  52  is used in the CO2 separation unit  54  of a direct reduction plant.  
         [0026]     After passing through heat exchanger  32 , the reducing gas is fed to a boiler  56  where steam  58  is produced from water  60  and is held in steam drum  36 . From boiler  56 , reducing gas  62  is finally quenched by direct contact with water in cooler  64 . Condensed water is withdrawn through pipe  66 . Cold reducing gas  68  is then fed to blower  70  and then led through pipe  72  to be treated in a sulfur removal unit  48  in a manner known in the art producing solid sulfur  74  and a stream of clean reducing gas  76 .  
         [0027]     A portion of the reducing gas  76  is used for heating the coke ovens through pipe  14  and another portion  78  is compressed in compressor  80  to be injected to the reduction gas loop of a direct reduction reactor  82 . Iron oxides bearing particles  84 , for example iron ore pellets, lumps or mixtures thereof, are fed to reduction reactor  82  and descend by gravity through said reactor where they are contacted with high temperature reducing gases  86 , for example above 900° C. whereby iron oxides are reduced to metallic iron, known as DRI or sponge iron  88 , and discharged from the lower part of the reduction reactor  82  to be utilized in steelmaking operations.  
         [0028]     Reacted reducing gas  90  is withdrawn from reactor  82  and cooled down in cooler  92  where water is removed from the gas by condensation a portion of the cooled gas  94  is recycled to the reduction reactor and another portion  96  may be vented from the reduction loop and used for example in gas heater  98 . The recycled reducing gas portion  94  is compressed in compressor  100  and fed to a carbon dioxide removal unit  54  where CO2 is removed from the system thus regenerating the reducing potential of the recycled gas by elimination of water  102  and CO2  104  which are the main products of the reduction reactions carried out in reactor  82 .  
       EXAMPLE  
       [0029]     A computer-model calculation of a plant incorporating the invention was made and the results are illustrated in  FIGS. 2, 3  and  4 . This example clearly shows the advantages of the invention regarding its application for obtaining a reducing gas for producing good-quality direct reduced iron.