Abstract:
The present invention provides a method for reducing iron oxide to metallic iron using coke oven gas, including: dividing coke oven gas from a coke oven gas source into a plurality of coke oven gas streams; providing a first coke oven gas stream to a hydrogen enrichment unit to form a hydrogen-rich product stream that is delivered to a reduction shaft furnace as part of a reducing gas stream; and providing a tail gas stream from the hydrogen enrichment unit to a reforming reactor to form a reformed gas stream that is delivered to a reduction shaft furnace as part of the reducing gas stream. Optionally, a spent top gas stream from the reduction shaft furnace is cleansed of CO 2  and recycled back to the reducing gas stream.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present patent application/patent claims the benefit of priority of co-pending U.S. Provisional Patent Application No. 62/053,864, filed on Sep. 23, 2014, and entitled “METHOD FOR REDUCING IRON OXIDE TO METALLIC IRON USING COKE OVEN GAS,” the contents of which are incorporated in full by reference herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to a method for reducing iron oxide to metallic iron. More specifically, the present invention relates to a method for reducing iron oxide to metallic iron using coke oven gas (COG). 
       BACKGROUND OF THE INVENTION 
       [0003]    For the direct reduction (DR) of iron oxide to sponge iron, the reducing agents H 2  and CO, or any combination of them, can be used. Normally, a hydrocarbon source, such as natural gas, is utilized to produce these reducing agents via a catalytic or non-catalytic reforming process. The most practiced reforming processes are catalytic steam/CO 2  reforming in a tubular reformer with an external heat supply, autothermal reforming (ATR) in a packed catalyst bed with internal heat supplied by the partial combustion of the hydrocarbon(s) with oxygen, and a non-catalytic partial oxidation (PDX) reforming process whereby the hydrocarbon source is partially oxidized and reformed to a mixture of H 2  and CO, while internally providing the required heat for the reactions. 
         [0004]    Besides natural gas, another widespread source of hydrocarbons in the iron and steel industry is COG, which typically contains 20-28% methane. Due to its considerable CH 4  content, COG can be reformed into H 2  and CO to produce reducing gas to reduce iron oxide to metallic iron, in the form of direct reduced iron (DRI), hot direct reduced iron (HDRI), or hot briquetted iron (HBI) in a DR plant. A typical COG stream coming from a COG treatment plant contains between 50.0-65.0% H 2 , 4.0-8.0% CO, up to 2.0% benzene-toluene-xylene (BTX), and up to 5.0% higher hydrocarbons, such as ethane, propane, and ethylene. The data available for different plants in China, Japan, and Germany indicate that the minimum concentration of H 2  in a typical COG stream is 52.0%. The presence of such high concentrations of H 2  in COG has adverse consequences for both catalytic and non-catalytic reforming processes, since it is the main product of reforming reactions; and, therefore, reduces the efficiency of the reforming reactions. In other words, since the rate of reforming reactions is slower in the presence of high concentrations of hydrogen, more energy is consumed to reform the hydrocarbons to H 2  and CO. 
         [0005]    More specifically, for ATR and PDX processes, the presence of high amounts of H 2  has another undesired outcome, i.e. this useful reducing agent is combusted to form H 2 O, an oxidizing agent. In other words, while the hydrocarbon source is being reformed by oxygen to produce H 2  (and CO), the already present H 2  consumes loads of available oxygen to produce H 2 O, which translates into wasting material and energy resources. 
         [0006]    Thus, the present invention provides an economic DR process in which the majority of the H 2  and CO is recovered from the COG before sending it to the PDX unit. Therefore, the overall efficiency of the plant improves considerably in terms of material and energy consumption, and capital and operating investments (i.e. CAPEX and OPEX). 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    The present invention makes use of an industrially well-practiced technology, i.e. a pressure swing adsorption (PSA) unit, by which the majority of the reducing agents (H 2  and CO) are recovered from the COG through adsorption of other components on proper adsorbents. The PSA unit delivers two streams: (1) a product stream that is rich in reducing agents, but can also contain some CH 4  and other components, and (2) a tail gas stream that is rich in CH 4  and all other hydrocarbons present in the COG. The latter stream flows to the reforming unit, more specifically to a PDX reactor, for producing more reducing agents, H 2  and CO. The PSA product stream then blends with the reducing gas generated in the reforming step, before injection into the DR shaft furnace. 
         [0008]    The present invention makes use of the available heat in the product gas coming off the PDX reactor, typically with a temperature of &gt;1200 degrees C., to increase the temperature of the hydrogen-rich stream recovered in the PSA unit to the proper level required for reduction reactions, typically between 900 degrees C. and 1000 degrees C. 
         [0009]    Thus, the present invention provides a simple way to increase the carbon content of DR iron by injecting the hydrocarbon-rich stream (i.e. tail gas) coming off the PSA unit into the reduction reactor, when COG is the only available hydrocarbon source to the DR plant. 
         [0010]    Thousands of PSA units have been installed all around the world by different vendors in oil refineries, petrochemical plants, and other chemical industries where the separation of H 2  or CO 2  from the process streams is vital for efficient operation of the plant. There are other industrially practiced methods for H 2  or CO 2  removal from gas mixtures as well, such as cryogenic separation, organic and inorganic membrane modules, vacuum pressure swing adsorption (VPSA), and temperature swing adsorption (TSA) offered by different vendors that can be used in conjunction with the present invention instead of a PSA, if they can meet the process requirements for COG applications. 
         [0011]    In various exemplary embodiments, the present invention provides a method for reducing iron oxide to metallic iron using coke oven gas, including: dividing coke oven gas from a coke oven gas source into a plurality of coke oven gas streams; providing a first coke oven gas stream to a hydrogen enrichment unit to form a hydrogen-rich product stream that is delivered to a reduction shaft furnace as part of a reducing gas stream; and providing a tail gas stream from the hydrogen enrichment unit to a reforming reactor to form a reformed gas stream that is delivered to a reduction shaft furnace as part of the reducing gas stream. The hydrogen enrichment unit includes one of a pressure swing adsorption unit, a vacuum pressure swing adsorption unit, a temperature swing adsorption unit, a cryogenic separator, and an organic or inorganic membrane module. The reforming reactor includes a partial oxidation reactor. The method also includes compressing the coke oven gas. The method further includes compressing and heating the tail gas stream. The method still further includes removing condensates and heavy hydrocarbons from the coke oven gas by passing the coke oven gas through a separation unit e.g. packed beds of charcoal or a coalescing knockout drum. The method still further includes providing a second coke oven gas stream to the reforming reactor to fire a burner of the reforming reactor. The method still further includes compressing the second coke oven gas stream. The method still further includes providing a third coke oven gas stream to the reduction shaft furnace as part of the reducing gas stream. The method still further includes heating the third coke oven gas stream. Optionally, the method still further includes providing a fourth coke oven gas stream to a transition zone of the reduction shaft furnace. The method still further includes heating the fourth coke oven gas stream. Optionally, the method still further includes providing a fifth coke oven gas stream to a cooling zone of the reduction shaft furnace. Optionally, the method still further includes recycling a spent top gas stream from the reduction shaft furnace back to the reducing gas stream. The method still further includes using the spent top gas stream to generate steam in a waste heat recovery steam generator. The method still further includes scrubbing and cooling the spent top gas stream in a scrubber/cooler. The method still further includes removing CO 2  from the spent top gas stream using a CO 2  removal unit. The method still further includes heating the spent top gas stream in a radiant heater. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    The present invention is illustrated and described herein with reference to the various drawings, in which like reference numbers are used to denote like method steps, as appropriate, and in which: 
           [0013]      FIG. 1  is a process flowsheet illustrating one exemplary embodiment of the method for reducing iron oxide to metallic iron using COG of the present invention, highlighting a once-through configuration; and 
           [0014]      FIG. 2  is a process flowsheet illustrating another exemplary embodiment of the method for reducing iron oxide to metallic iron using COG of the present invention, highlighting a recycle configuration. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    The object of the present invention is to develop an efficient and cost effective process by which commercially available COG streams can be used in a DR plant to produce metallic iron. The present invention provides a method to effectively use the H 2  and CO content of a COG stream, where it partially bypasses the reforming step, specifically the PDX step, and, therefore, can be directly used as a reducing agent, instead of being combusted or circulated within the reforming step. 
         [0016]    For a PDX-based DR plant that uses COG as the main source for the feed and fuel, the following advantages can be outlined for a PSA unit that separates reducing gases from COG:
       1. The oxygen consumption of the plant (i.e. OPEX) decreases tangibly owing to the fact that in the proposed design more than ca. 50% of the combustible gases (i.e. H 2  and CO) bypass the PDX reactor. In other words, theoretically, the oxygen consumption of the plant can decrease by ca. 50×0.5=25.0% (based on only the feed gas flow and composition—each mole of H 2  or CO needs half a mole of oxygen to combust).   2. Since the fuel consumption of a PDX reactor is typically proportional to oxygen consumption, it is expected that the required fuel flow for the PDX reactor (OPEX) also decreases.   3. In the case of using COG as the fuel in the PDX reactor burners, the above-mentioned saving in fuel consumption can be used to boost the plant production.   4. The above noticeable drop in the feed and fuel/oxygen flow rates to the PDX reactor (i.e. total throughput) cut the number and/or size of the required PDX reactor(s) and other related unit operations for a given job (i.e. CAPEX). If the PDX reactor is designed based on the total throughput, the number of required reactors will be at least half of the necessary units as compared to when no PSA unit is utilized. However, if the size of the PDX reactor is limited to the capacity of the combustion unit for each specific fuel, theoretically a minimum reduction of ca. 25.0% in the number of PDX reactors is predictable (i.e. CAPEX).   5. High-pressure PSA product gas can be used as the fuel in the PDX, and, thus, there is no need for any further compressor equipment.   6. Since only ca. 50.0% of the COG flows into the PDX reactors, in the case of a short-time reactor failure, the PSA unit can still supply enough reducing gas to the plant. In addition, this feature removes the necessity for standby PDX reactors for the plant.   7. The product stream rich in the reducing agents H 2  and CO coming off the PSA towers at ambient temperature can be directly used to cool down the PDX generated gas from &gt;1200° C. to ˜1100° C., offering the best energy efficiency possible for the plant (i.e. OPEX). In other words, the extra energy available in the PDX generated gas can be directly used to heat-up the cold hydrogen-rich gas, and, therefore, there is no need to do any kind of quenching and reheating unit operations (i.e. CAPEX).   8. Since only about 50.0% of the original COG is processed by the PDX reactor, the size of the COG feed compressor (i.e. CAPEX), and its energy consumption (i.e. OPEX), are noticeably less than the processes with no PSA unit.   9. Since only about 50.0% of the original COG is processed by the PDX reactor, the size of the COG feed heater (i.e. CAPEX), and its energy consumption (i.e. OPEX) are drastically less than the processes with no PSA unit.   10. Since all the sulfur compounds present in the COG are collected in the tail gas stream coming off the PSA unit, only 50.0% of the original COG is processed by the desulphurization unit; and, therefore, the size of the desulphurization unit (i.e. CAPEX) is significantly less than the processes with no PSA unit.   11. The resulting reducing gas (i.e. reformed PSA tail gas+PSA product stream+enrichment COG) is ready to use directly in the shaft furnace, with a quality of ˜10 (defined as [CO+H 2 ]/[CO 2 +H 2 O]) and a temperature of 970-1000 degrees C. Therefore, the whole process can be designed as a very efficient once-through process where the spent gas off the reduction furnace is not treated for further application in the DR facility.   12. In the case of a once-through flowsheet, there is no need to have supplemental unit operations, like waste heat boilers, heat exchangers, heaters, etc., which, in turn, results in considerable savings in capital cost (i.e. CAPEX). The lesser number of unit operations also makes running and controlling the process much easier, with less maintenance cost (i.e. OPEX).   13. Since the bustle gas has high enough temperature with a satisfactory quality, there is no need to use too much oxygen to boost the bustle gas temperature (i.e. OPEX).   14. The amount of oxygen required for the partial combustion of COG is set based on only the hydrocarbon content of the COG, and not based on other combustibles, like H 2  and CO. At the same time, the lower number of these combustibles in the reacting mixture results in more accessibility of the hydrocarbons to oxygen; and, therefore, the better and more uniform distribution of oxygen among hydrocarbons is predictable. This, in turn, diminishes the amount of soot and water vapor produced by side reactions during the partial combustion of the COG.   15. The hydrocarbon-rich tail gas off of the PSA towers can be used instead of COG to increase the carbon content of the DRI in the transition and cooling zones.   16. The H 2 -rich product gas off of the PSA unit can be used as the fuel in the PDX burners, producing more hot steam in the flame, which reduces the likelihood of soot formation within the reactor. This, in turn, can cut the amount of excess oxygen required to diminish soot formation inside the PDX reactor (i.e. OPEX).   17. Since the reduction furnace can handle CO, as well as H 2 , it is not necessary to utilize conventional high-pressure PSA units with high purity H 2  as the desired product in oil refineries or ammonia plants. In fact, the operation of a low-pressure PSA unit not only lowers the energy consumption required for the initial compression of COG (i.e. OPEX), but also preserves a notable amount of CO in the product stream coming off of the PSA unit.       
 
         [0034]    Referring now to  FIGS. 1 and 2 , in various exemplary embodiments, the method  100 ,  200  of the present invention starts with the compression of the COG from a COG source  102 ,  202  to ˜5.0-7.0 barg in a compressor  104 ,  204 . This pressure can change based on the requirements set by the vendor of the PSA unit  106 ,  206  or the desired recovery rate of the hydrogen as set by the project. The compressed gas passes through a coalescing knockout drum  105 ,  205  in order to avoid the entry of liquids into the PSA unit  106 ,  206 . The coalescer section in the knockout drum  105 ,  205  is important, due to the fact that heavier hydrocarbons, like BTX, naphthalene, and tar, occur as aerosols (i.e. very small droplets) that pass normal knockout drums. Alternative methods, such as TSA units, packed beds of charcoal or activated carbon, or cold traps, can also be used, instead of the coalescing knockout drum  105 ,  205 , as PSA guard units. 
         [0035]    The compressed COG is then distributed in the DR plant  150 ,  250  for different applications, i.e. feed for the reformer reactor  108 ,  208 , fuel for the reformer reactor  108 ,  208 , enriching the reducing gas, and a transition zone stream to increase the carbon content of the product DRI. The enrichment and transition zone streams can also be taken off before the coalescing knockout drum  105 ,  205 . For a PDX-based plant  150 ,  250 , depending on the design of the PDX reactor  108 ,  208 , it may be required to increase the pressure of the fuel stream for the PDX burner using a booster compressor  110 ,  210 . 
         [0036]    The main portion of the gas flows to the PSA unit  106 ,  206 , where the majority of its hydrogen content (typically more than 70.0%) separates from the COG. In addition, for a conventional H 2 -PSA system, ˜30.0-40.0% of the CO and up to ˜15.0% of the CH 4  come with the H 2  to this product stream. Since PSA units typically deliver a dry gas in the product stream, the quality of the hydrogen-rich gas, defined as (H 2 +CO)/(H 2 O+CO 2 ), is very high, but at a low temperature. 
         [0037]    The tail gas coming off the PSA towers, now rich in methane and other heavy hydrocarbons, does not have enough temperature and pressure and, thus, it is compressed in a compressor  112 ,  212  and preheated in a preheater  114 ,  214  to a proper temperature and pressure before flowing to the reforming section  108 ,  208 , where it is partially combusted and reformed by oxygen into a syngas stream rich in H 2  and CO. 
         [0038]    If it is required by process, the tail gas coming off the PSA towers flows into a series of treatment units (not illustrated) for sulfur removal and heavy hydrocarbon conversion. The latter step, converts or removes cyclic and olefin hydrocarbons into more saturated ones to protect the downstream equipment from coking. 
         [0039]    Although the quality of the syngas generated within the PDX reactor  108 ,  208  is not typically outstanding, it comes off the PDX reactor  108 ,  208  at an elevated temperature of more than about 1200 degrees C. Therefore, as an option, it can be blended with the PSA product stream and/or preheated enrichment COG to form a high quality gas with a moderate temperature of ca. 1000 degrees C. The resulting bustle gas flows into the shaft furnace  116 ,  216  for reducing iron oxides. 
         [0040]    In order to increase the carbon content of the DRI, transition zone COG, which can also be taken off after the PSA unit  106 ,  206  from the tail gas stream, is preheated and flows in to the transition zone of the shaft furnace  116 ,  216 . 
         [0041]    Referring now specifically to  FIG. 2 , in one exemplary embodiment, the spent gas coming off the reduction furnace  216  has a high enough energy content to be recovered in a waste heat recovery steam generator (HRSG)  218 . The generated steam can be used inside the plant  250  for different applications, like CO 2  stripping, or it can be exported to neighboring facilities, if demanded. 
         [0042]    The gas leaving the HRSG unit  218  then flows into a cleaning and cooling unit  220 , preferably a venturi scrubber equipped with packed bed coolers, where the majority of its water content is removed through direct contact with cooling water. Direct contact with cooling water also removes most of the solid particulates (i.e. fines and gangues) carried by the spent gas. 
         [0043]    The clean gas stream coming off the scrubbing unit afterwards runs into a conventional CO 2  removal unit  222 , i.e. a pressure/temperature swing adsorption, physical or chemical absorption, or membrane module, where the majority of its CO 2  content is captured to reduce the oxidant level of the gas. The gas, now rich in H 2  and CO, passes through a radiant heater  224  to reach the very high temperatures, e.g. about 950 degrees C., necessary for reduction reactions within the reduction shaft furnace  216 . The hot gas leaving the radiant heater  224  then blends with the syngas generated in the reformer reactor  208  before entering the reduction shaft furnace  216 . 
         [0044]    The two schematics of  FIGS. 1 and 2  include typical mass balance numbers. The once-through case ( FIG. 1 ) is suggested for the production of 123.0 tons/h of high quality DRI from COG using a PDX reactor, for example. The recycle case ( FIG. 2 ) is suggested for the production of 235.0 tons/h of high quality DRI from COG using a PDX reactor, for exmple. For the recycle-based design ( FIG. 2 ), the spent gas coming off the reduction furnace  216  passes through a series of treatment units before being reutilized in the process for boosting the production rate of the plant  250 . Both flowsheets are supported by the corresponding mass balance figures. 
         [0045]    Although the present invention is illustrated and described herein with reference to preferred embodiments and specific examples thereof, it will be readily apparent to those of ordinary skill in the art that other embodiments and examples may perform similar functions and/or achieve like results. All such equivalent embodiments and examples are within the spirit and scope of the present invention, are contemplated thereby, and are intended to be covered by the following non-limiting claims.