Patent Publication Number: US-8974571-B2

Title: Partially-reduced iron producing apparatus and partially-reduced iron producing method

Description:
TECHNICAL FIELD 
     The present invention relates to a partially-reduced iron producing apparatus and a partially-reduced iron producing method for producing a partially-reduced iron by reducing agglomerates containing a iron oxide. 
     BACKGROUND ART 
     For example, Patent Literature 1 listed below discloses a conventional technique of producing a partially-reduced iron by packing carbon composite pellets on a moving grate and thermally reducing the pellets, the carbon composite pellets not being coated with a carbon material for combustion. 
     However, the technique described in Patent Literature 1 has the following problems and a partially-reduced iron with a high degree of reduction cannot be obtained. 
     (1) After being dried, the carbon composite pellets are ignited with a gas torch and air is made to flow therethrough to combust and heat the carbon composite pellets. Accordingly, a portion of a packed bed of the carbon composite pellets on a side into which air flows keeps on combusting and reduction dose not proceed in this portion. Moreover, even if the reduction proceeds, the carbon composite pellets are reoxidized by air and thus the degree of reduction does not improve at all. Moreover, since a high temperature state is maintained, a molten slag is excessively generated and an operation may thereby become difficult in some cases. 
     (2) The pellets having moved out of a carbonization area is heated by a high-temperature inert gas whose oxygen concentration is equal to 5% or less and metallization proceeds by using a remaining portion of the carbonaceous material. However, the amount of remaining carbon is small and the degree of metallization is low. Moreover, until a lower portion of the packed bed reaches a high temperature, an upper portion of the packed bed is exposed to oxidant gases such as carbon dioxide and water vapor generated from the high-temperature carbonaceous material, causing reoxidation of the upper portion to proceed. 
     (3) A high-temperature gas in a metallization area where a large amount of heat is required is produced by combusting part of a flammable volatile component in the coal which is generated in the carbonization area and CO gas which is generated by the reduction reaction. However, since the amount of flammable components is small with respect to the amount of the entire exhaust gas, a supplementary fuel is additionally required. 
     In view of the problems above, for example, Patent Literatures 2 and 3 each disclose a conventional technique of producing partially-reduced iron in which pellets formed by mixing and pelletizing a reduction carbon material, a fine iron ore, and a slag-forming flux are added with a carbon material for combustion by coating the pellets with the carbon material for combustion, the carbon material for combustion is ignited, and then the pellets are subject to sintering with air being suctioned downward. 
     CITATION LIST 
     Patent Literatures 
     Patent Literature 1 Japanese Examined Patent Application Publication No. Sho 45-39331 
     Patent Literature 2 Japanese Examined Patent Application Publication No. Hei 8-9739 
     Patent Literature 3 Japanese Patent Application Publication No. 2005-97645 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, the conventional methods of producing partially-reduced iron which are described in Patent Literatures 2 and 3 have the following problems. First, since the added carbon material for combustion combusts first, carbon monoxide and the flammable volatile component in coal which are generated from the heated pellets hardly combust and are discharged from the packed bed without being effectively used. Accordingly, the basic unit of consumption of fuel becomes larger and CO 2  emissions thereby increase. Moreover, since the carbon material for combustion continues to combust until there is no carbon component left therein, the cooling speed of the pellets is slow and thus exhausted metal iron in the reduced pellets is in contact with air in a high temperature state for a long period. Hence, reoxidation proceeds and the degree of metallization is low. 
     In other words, in the conventional method, the raw-material pellets are ignited and combusted by use of the ignited combustion carbon material and the partially reduced iron is produced. Using the combustion carbon material in this 
     The present invention has been made to solve the problems described above and an object thereof is to provide a partially-reduced iron producing apparatus and a partially-reduced iron producing method which enable producing a partially-reduced iron without using a combustion carbon material. 
     Solution to Problem 
     A partially-reduced iron producing apparatus according to a first aspect of the present invention which solves the aforementioned problems includes: ignition raw-material pellet supply means for laying ignition raw-material pellets to a predetermined height on an endless grate, the ignition raw-material pellets made of a material that is the same as a material of raw-material pellets formed by mixing and pelletizing a reduction carbon material and a raw material containing iron oxides; heating means for heating the ignition raw-material pellets laid on the endless grate to a reduction temperature range; raw-material pellet supply means for laying the raw-material pellets on the ignition raw-material pellets heated by the heating means; and exhaust gas circulation means for supplying an oxygen-containing gas to the raw-material pellets heated by a heat of the ignition raw-material pellets, the oxygen-containing gas made by circulating part of an exhaust gas discharged from the raw-material pellets by use of a heat of the ignition raw-material pellets and mixing it with air. In the apparatus, a partially-reduced iron is produced by thermally reducing the whole of the raw-material pellets in a bed height direction thereof through a combustion region for the raw-material pellets and a heating region for the raw-material pellets, the combustion region formed on an upstream side in a travelling direction of the endless grate by supplying the oxygen-containing gas having a high oxygen concentration to the ignition raw-material pellets heated by the heating means, the heating region formed downstream of the combustion region for the raw-material pellets in the travelling direction of the endless grate by supplying the oxygen-containing gas having a low oxygen concentration to the raw-material pellets. 
     A partially-reduced iron producing apparatus according to a second aspect of the present invention which solves the aforementioned problems is the partially-reduced iron producing apparatus according to the first aspect. In the apparatus, the heating means is a heating furnace capable of controlling an interior temperature thereof, and the heating furnace has such a length that allows the heated ignition raw-material pellets to be maintained at a high temperature for a predetermined period. 
     A partially-reduced iron producing method according to a third aspect of the present invention which solves the aforementioned problems includes the steps of: laying ignition raw-material pellets to a predetermined height on an endless grate, the ignition raw-material pellets made of a material that is the same as a material of raw-material pellets formed by mixing and pelletizing a reduction carbon material and a raw material containing iron oxides; heating the ignition raw-material pellets laid on the endless grate to a reduction temperature range by heating means, and then packing the raw-material pellets on the ignition raw-material pellets; heating the raw-material pellets adjacent to the ignition raw-material pellets by use of a heat of the ignition raw-material pellets to generate and combust a flammable volatile component from the reduction carbon material in the raw-material pellets; causing a temperature of the raw-material pellets to further rise by use of a combustion heat of the flammable volatile component, so that a reduction reaction proceeds and a carbon monoxide gas is generated, while causing the raw-material pellets adjacent thereto to be heated by use of the combustion heat, so that a flammable volatile component is generated from the reduction carbon material in the adjacent portions of the raw-material pellets; increasing a concentration of the carbon monoxide gas near the raw-material pellets having the temperature further raised, to a combustion range of the carbon monoxide gas by supplying an oxygen-containing gas to the raw-material pellets, so that the carbon monoxide gas combusts and a combustion zone is formed, the oxygen-containing gas made by circulating a remaining portion of the flammable volatile component and the carbon monoxide gas and mixing the remaining portion and the gas with air; and moving the combustion zone sequentially in a bed height direction of a packed bed of the raw-material pellets in a period between the supplying of the raw-material pellets onto the ignition raw-material pellets and discharging thereof, so that the packed bed of the raw-material pellets is thermally reduced and a partially reduced iron is produced. 
     A partially-reduced iron producing method according to a fourth aspect of the present invention which solves the aforementioned problems is the partially-reduced iron producing method according to the third aspect, in which a bed height of part of the raw-material pellets is higher than 5 mm but is lower than 20 mm. 
     Advantageous Effects of Invention 
     In the present invention, the packed bed of raw-material pellets is heated by the combustion heat of the ignition raw-material pellets. The flammable volatile component is thus generated from the reduction carbon material in the raw-material pellets and combusts. By the combustion of the flammable volatile component, the temperature of the raw-material pellets further rises. Accordingly, a reduction reaction proceeds and a carbon monoxide gas is produced. Meanwhile, the raw-material pellets adjacent to the heated pellets are heated and the flammable volatile component is generated from the reduction carbon material in the adjacent raw-material pellets. An oxygen-containing gas made by circulating a remaining portion of the flammable volatile component and the carbon monoxide gas and mixing them with air is supplied to the raw-material pellets whose temperature has further risen, and the concentration of the carbon monoxide gas near the raw-material pellets is thereby increased to the combustion range of the carbon monoxide gas. Hence, the carbon monoxide gas combusts and the temperature increases. The combustion zone of a temperature required for the reduction of iron is thus formed. The combustion zone sequentially moves in a bed height direction of the packed bed of the raw-material pellets, in a period between the supplying of the raw-material pellets onto the ignition raw-material pellets and discharging thereof. Thus, the packed bed of the raw-material pellets is thermally reduced and the partially-reduced iron is produced. Accordingly, no coating of carbon material to be a heat source is required for the raw-material pellets. As a result, the amount of coal used in the entire partially-reduced iron producing process (apparatus) can be reduced. This reduces the carbon material consumption and the carbon dioxide emissions. Moreover, when the reduction ends, the generation of carbon monoxide gas stops and the concentration of carbon monoxide gas in the atmosphere falls abruptly. The combustion of the carbon monoxide gas stops as soon as the concentration of carbon monoxide falls below the combustion range of carbon monoxide, so that the raw-material pellets are cooled. Hence, the time in which the pellets are in contact with oxygen in a high temperature state is short, suppressing the reoxidation. Thus, a partially-reduced iron with high degree of metallization can be produced. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic diagram showing a main embodiment of a partially-reduced iron producing apparatus of the present invention. 
         FIGS. 2A and 2B  are explanatory diagrams of the main embodiment of the partially-reduced iron producing apparatus of the present invention.  FIG. 2A  shows a cross section of a reduction furnace included in the partially-reduced iron producing apparatus.  FIG. 2B  shows a relationship between an oxygen concentration in the reduction furnace and a bed height direction of a packed bed of raw-material pellets. 
         FIG. 3  is a graph showing an example of a temperature change from a bottom surface of the packed bed in a bed height direction thereof in the reduction furnace included in the main embodiment of the partially-reduced iron producing apparatus of the present invention, observed when the raw-material pellets are packed at the height of 200 mm in the reduction furnace and are heated while the mixed gas is vented upward. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Descriptions are given below of a mode for carrying out a partially-reduced iron producing method and a partially-reduced iron producing apparatus of the present invention. 
     {Main Embodiment} 
     A main embodiment of the partially-reduced iron producing method and the partially-reduced iron producing apparatus of the present invention is described based on  FIGS. 1 to 3 . In  FIG. 1 , the arrow A shows a travelling direction of a grate. 
     As shown in  FIGS. 1 ,  2 A, and  2 B, the partially-reduced iron producing apparatus of the embodiment includes a grate reduction furnace  100  of an upward suction type. The grate reduction furnace  100  includes an ignition raw-material pellet supplying device  10 , a heating furnace  20 , and a reduction furnace (partial reduction furnace)  30 . These components are arranged from upstream in the travelling direction of a grate (endless grate)  101  in the order of description. 
     The ignition raw-material pellet supplying device  10  is a device which supplies ignition raw-material pellets  1  onto the grate  101  and lays the ignition raw-material pellets  1  to a predetermined height. In other words, the ignition raw-material pellet supplying device  10  forms raw-material pellet supply means. The ignition raw-material pellets  1  are made of the same material as that of raw-material pellets  3  to be described later in detail and form part of the raw-material pellets  3 . The laying height of the ignition raw-material pellets  1  is such a height that the after-mentioned raw-material pellets  3  packed on an ignition raw-material pellet layer  2  can be ignited, and is, for example, higher than 5 mm and lower than 20 mm, preferably higher than 5 mm and 10 mm or less. When the laying height of the ignition raw-material pellet layer  2  is equal to or below 5 mm, the amount of heat generated by the combustion of the ignited ignition raw-material pellets  1  is so small as to be insufficient for generation of a flammable volatile component from a reduction carbon material in the raw-material pellets  3 . Meanwhile, when the laying height is 20 mm or greater, the pellets in a lowermost layer are poorly heated and some of the pellets are not reduced. 
     The heating furnace  20  includes a combustion burner  21  which heats the ignition raw-material pellet layer  2  (ignition raw-material pellets  1 ) supplied onto the grate  101  to a reduction temperature range. In other words, the heating furnace  20  forms heating means which is capable of controlling an interior temperature thereof. The heating furnace  20  has such a length that the heated ignition raw-material pellet layer  2  can be maintained at a high temperature for a predetermined period. The heating furnace  20  also includes a combustion gas exhaust pipe  22 . The combustion gas exhaust pipe  22  is provided with a valve V 1 . A front end opening portion  22   a  of the combustion gas exhaust pipe  22  is disposed at a position upstream of the combustion burner  21  in the travelling direction of the grate  101 . The combustion gas exhaust pipe  22  is connected to an exhaust manifold  24  and a rear end portion of the exhaust manifold  24  is connected to a dust collector  27 . Accordingly, a combustion gas generated when the ignition raw-material pellet layer  2  is heated by the combustion burner  21  is exhausted to the outside of a system through the combustion gas exhaust pipe  22 , the exhaust manifold  24 , and the dust collector  27 . 
     The reduction furnace  30  is a device which produces an agglomerate-like partially-reduced iron  5  by reducing the raw-material pellets  3  and has an annular shape as a whole. The reduction furnace  30  includes a raw-material pellet supplying device  31 , a reduction furnace main body  32 , and a partially-reduced iron discharging device  39  which are arranged in this order from upstream in the travelling direction of the grate  101 . The raw-material pellet supplying device  31  (feed hopper)  31  is a device which supplies the raw-material pellets  3  onto the ignition raw-material pellet layer  2 . The raw-material pellet supplying device  31  not only supplies the raw-material pellets  3  onto the ignition raw-material pellet layer  2 , but also adjusts the height of a packed bed  4  of the raw-material pellets, which is formed by packing the raw-material pellets  3 , to be a predetermined height. The raw-material pellets  3  are a raw material for the partially-reduced iron to be eventually produced and are formed by mixing and pelletizing a raw material containing iron oxides, the reduction carbon material, and a lime-based slag-forming flux and then coating the resultant object with an anti-oxidant. For example, the raw-material pellets  3  each contain coal by about 20% of its total amount and the amount of the flammable volatile component in the coal is 30% or more. 
     The reduction furnace main body  32  described above includes a wind box  33 , an annular hood  34 , and tracks  35 ,  35 . The wind box  33  is installed below the grate  101  and is a fixed structure. The hood  34  is installed above the wind box  33  with the grate  101  interposed therebetween and is a fixed structure. The tracks  35 ,  35  are laid in an annular shape on both sides of the wind box  33 . 
     The aforementioned wind box  33  includes multiple wind boxes depending on the diameter of the grate, such as a first wind box  33   a , a second wind box  33   b , a third wind box  33   c , a fourth wind box  33   d , and a fifth wind box  33   e  which are arranged in this order from a side close to the raw-material pellet supplying device  31  in the travelling direction of the grate  101 . 
     Two partition boards  38   a  and  38   b  are provided on a ceiling plate  34   a  of the aforementioned hood  34  and three regions  71   a ,  71   b , and  71   d  are thus defined in the travelling direction A of the grate  101 . The first partition board  38   a  is disposed at such a position as to define a space (ignition raw-material pellet combustion region  71   a  to be described later) above the first wind box  33   a  and a space (raw-material pellet heating region  71   b  to be described later) above the second wind box  33   b . The second partition board  38   b  is disposed at such a position as to define a space (raw-material pellet heating region  71   b  to be described later) above the fourth wind box  33   d  and a space (raw-material pellet cooling region  71   c  to be described later) above the fifth wind box  33   e . Temperature sensors  72   a ,  72   b , and  72   c  are provided respectively in the ignition raw-material pellet combustion region  71   a , the raw-material pellet heating region  71   b , and the raw-material pellet cooling region  71   c.    
     The grate  101  is porous and is configured such that a gaseous body can pass therethrough in a vertical direction but the ignition raw-material pellet  1  and the raw-material pellets  3  cannot. The grate  101  is divided into multiple units and the annular grate  101  is formed by arranging these units in a circumferential direction. Each of the divided units is tiltably attached to annular support portions  36 ,  36  provided respectively on both sides of the grate  101 . The support portions  36 ,  36  are provided with rollers  37 ,  37  travelling on the tracks  35 ,  35 . Causing the rollers  37 ,  37  to travel on the tracks  35 ,  35  allows the grate  101  to horizontally circulate in a space between the wind box  33  and the hood  34 . 
     Water seal boxes  41 ,  41  filled with water are annularly provided in upper portions of the support portions  36 ,  36  of the grate  101 , along the entire peripheries thereof. Seal plates  42 ,  42  extending downward are annularly provided in lower portions of the hood  34  on both sides, along the entire peripheries thereof. Lower end portions of the seal plates  42 ,  42  are submerged in a liquid in the water seal boxes  41 ,  41 . Hence, spaces between the support portions  36 ,  36  of the grate  101  and the lower portions of the hood  34  on both sides are sealed in an air-tight manner. In other words, the water seal boxes  41  and the seal plates  42  form a water seal device above the grate. 
     Meanwhile, water seal boxes  43 ,  43  filled with water are annularly provided in upper portions of the wind box  33  on both sides, along the entire peripheries thereof. Seal plates  44 ,  44  extending downward are annularly provided in lower portions of the support portions  36 ,  36  of the grate  101 , along the entire peripheries thereof. Lower end portions of the seal plates  44 ,  44  are submerged in a liquid in the water seal boxes  43 ,  43 . Hence, spaces between the support portions  36 ,  36  of the grate  101  and the upper portions of the wind box  33  on both sides are sealed in an air-tight manner. In other words, the water seal boxes  43  and the seal plates  44  form a water seal device below the grate. 
     A raw-material pellet cooling region gas exhaust pipe  82  is provided to communicate with the hood  34  forming the raw-material pellet cooling region  71   c . The raw-material pellet cooling region gas exhaust pipe  82  communicates with the aforementioned exhaust manifold  24 . A flow rate adjustment valve V 31  is provided in the raw-material pellet cooling region gas exhaust pipe  82  and thereby the discharge amount of gas in the raw-material pellet cooling region can be adjusted. 
     The aforementioned reduction furnace  30  further includes an exhaust gas circulation device (exhaust gas circulation means)  50  which circulates an exhaust gas  91  by discharging the exhaust gas  91  from the ignition raw-material pellet combustion region  71   a  and the raw-material pellet heating region  71   b  and then supplying the exhaust gas  91  to the wind boxes  33   a  to  33   e , the ignition raw-material pellet combustion region  71   a  surrounded by the grate  101 , the hood  34 , and the first partition board  38   a , the raw-material pellet heating region  71   b  surrounded by the grate  101 , the hood  34 , the first partition board  38   a , and the second partition board  38   b . The exhaust gas circulation device  50  includes a first exhaust pipe  51 , a second exhaust pipe  52 , a dust remover  53 , a dust-removed gas delivery pipe  54 , a gas cooler  55 , a flow rate adjustment valve V 11 , a pump  56 , a circulating gas delivery pipe  58 , and first to fifth branch circulating gas delivery pipes  59   a  to  59   e.    
     One end portion of the first exhaust pipe  51  communicates with the hood  34  forming the ignition raw-material pellet combustion region  71   a  and the other end portion thereof is connected to the dust remover  53 . A base end of the second exhaust pipe  52  communicates with the hood  34  forming the raw-material pellet heating region  71   b  and a front end thereof communicates with an intermediate portion of the first exhaust pipe  51 . With this configuration, the exhaust gas  91  in the ignition raw-material pellet combustion region  71   a  and the raw-material pellet heating region  71   b  is delivered to the dust remover  53  through the, first exhaust pipe  51  and the second exhaust pipe  52 , and solid contents such as dust in the exhaust gas  91  is removed by the dust remover  53 . One end portion of the dust-removed gas delivery pipe  54  is connected to the dust remover  53  and the other end portion thereof is connected to the pump  56 . The gas cooler  55  is provided in an intermediate portion of the dust-removed gas delivery pipe  54 . With this configuration, an exhaust gas  92  (dust-removed gas) from which dust is removed has its temperature adjusted to a predetermined temperature by the gas cooler  55  and the flow rate thereof adjusted by the flow rate adjustment valves V 21  to V 25 . An O 2  sensor  57  which measures the oxygen concentration in the dust-removed gas  92  is provided in the piping at a position downstream of the gas cooler  55 . One end portion of the circulating gas delivery pipe  58  is connected to the pump  56  and the other end portion thereof branches into the first to fifth branch circulating gas delivery pipes  59   a  to  59   e . The first to fifth branch circulating gas delivery pipes  59   a  to  59   e  communicate respectively with the first to fifth wind boxes  33   a  to  33   e . The first to fifth branch circulating gas delivery pipes  59   a  to  59   e  are respectively provided with the flow rate adjustment valves V 21  to V 25 . 
     The aforementioned reduction furnace main body  32  further includes an air supplying device  60  forming air supply means which is connected to the first to fifth branch circulating gas delivery pipes  59   a  to  59   e  of the aforementioned exhaust gas circulation device  50  and supplies air to the first to fifth branch circulating gas delivery pipes  59   a  to  59   e . The air supplying device  60  includes an air supplying source  61 , an air feed pipe  62 , a pump  64 , and an air delivery pipe  65 . One end portion of the air feed pipe  62  is connected to the air supplying source  61  and the other end portion thereof is connected to the pump  64 . One end portion of the air delivery pipe  65  is connected to the pump  64  and the other end portion thereof branches into first to fifth branch air delivery pipes  66   a  to  66   e communicating respectively with the first to fifth branch circulating gas delivery pipes  59   a  to  59   e . The first to fifth branch air delivery pipes  66   a  to  66   e  are provided respectively with flow rate adjustment valves V 41  to V 45  forming flow rate adjustment means for adjusting the flow rate of air. 
     With the above configuration, gases (oxygen-containing gases)  94   a  to  94   e  containing oxygen and carbon monoxide whose concentrations are adjusted to desired levels can be supplied to the wind boxes  33   a  to  33   e , respectively, by adjusting the opening degree of each of the flow rate valve V 11 , the flow rate adjustment valves V 21  to V 25 , and the flow rate adjustment valves V 41  to V 45  based on the oxygen concentration measured by the O 2  sensor  57  and the temperatures measured by the temperature sensors  72   a  to  72   c . In other words, the oxygen concentration can be adjusted to the desired level in each of the ignition raw-material pellet combustion region  71   a , the raw-material pellet heating region  71   b , and the raw-material pellet cooling region  71   c.    
     The partially-reduced iron discharging device  39  is a device which discharges, from the grate  101 , the partially-reduced iron  5  having been produced while passing through the regions  71   a  to  71   c  described above. 
     Descriptions are given of a procedure of producing the partially-reduced iron by using the partially-reduced-iron producing apparatus having the aforementioned configuration. 
     First, the ignition raw-material pellet supplying device  10  supplies the ignition raw-material pellets  1  onto the grate  101 . At this time, the height of the ignition raw-material pellet layer  2  is adjusted to be within a range of 5 mm to 10 mm, for example. Then, the grate  101  moves forward and the burner  21  heats the ignition raw-material pellet layer  2  to the reduction temperature range which is, for example, about 1200° C. Next, the grate  101  moves forward and the raw-material pellets  3  are supplied onto the ignition raw-material pellet layer  2  from the raw-material pellet supplying device  31 . The height of the raw-material pellet packed bed  4  made of the raw-material pellets  3  is adjusted to about 200 mm, for example. Subsequently, the grate  101  moves forward and mixed gases of the circulated gas and air are vented into the hood  34 . The mixed gas  94   a  whose oxygen concentration is adjusted to 15% is vented to the first wind box  33   a . This causes the raw-material pellets  3  adjacent to the heated ignition raw-material pellets  1  to be heated by the heated ignition raw-material pellets  1  in the ignition raw-material pellet combustion region  71   a . The flammable volatile components are thus generated from the heated raw-material pellets  3  and are combusted. The raw-material pellet packed bed  4  on the ignition raw-material pellet layer  2  is heated by the heat of this combustion. 
     The grate  101  further moves forward and the mixed gases  94   b  to  94   d  whose oxygen concentrations are adjusted to 11% are vented to the second to fourth wind boxes  33   b  to  33   d . Due to this, the following phenomena occur in the raw-material pellet packed bed  4 , which is heated by the ignition raw-material pellet layer  2 , in the raw-material pellet heating region  71   b  above the second to fourth wind boxes  33   b  to  33   d . The flammable volatile component is generated from the reduction carbon material in the raw-material pellets  3  and about 75% to 90% of the flammable volatile component is combusted. This combustion of the flammable volatile component further increases the temperature of the raw-material pellets  3  and the reductive reaction proceeds. Thus, a carbon monoxide gas is generated and a part of the generated gas is combusted. As a result, high concentration of carbon monoxide, which is about 8%, for example, is generated in a center portion of the inside of the hood  34  in the grate travelling direction. Meanwhile, this combustion heats the raw-material pellets  3  adjacent thereto and the flammable volatile component is generated from the reduction carbon material in the adjacent raw-material pellets  3 . The mixed gases  94   b  to  94   d  (oxygen containing gas), which are made by circulating remaining portion of the flammable volatile component and the carbon monoxide gas and mixing them with air, are supplied to the raw-material pellets  3  whose temperature has increased. As shown in  FIG. 2B , this causes the carbon monoxide gas in the mixed gases  94   b  to  94   d  to be added to the carbon monoxide gas generated due to the reduction. As a result, the concentration of the carbon monoxide gas near the raw-material pellets  3  is increased to a level within the combustion range (12% or more) of the carbon monoxide gas and about 50% to 60% of the entire carbon monoxide gas combusts, thereby increasing the temperature. This creates a combustion zone of a temperature required for the reduction of partially-reduced iron. In other words, the reduction proceeds by causing carbon in the reduction carbon material in the raw-material pellets  3  to turn into gas and generate carbon monoxide and then causing the thus-generated carbon monoxide to bond with oxygen in the raw material containing iron oxides. The gas  91  in the raw-material pellet heating region  71   b  such as carbon monoxide and the remaining portion of the flammable volatile component which have not used for the combustion flows through the second exhaust pipe  52  and the first exhaust pipe  51 , has solid objects such as dust therein removed by the dust remover  53 , cooled to the predetermined temperature by the gas cooler  55 , and is fed to the wind boxes  33   a  to  33   e  via the pump  56  and the first to fifth branch circulating gas delivery pipes  59   a  to  59   e . Note that the atmosphere temperature is adjusted to about 1300° C. in the raw-material pellet heating region  71   b.    
     With reference to  FIG. 3 , descriptions are given of an example of a temperature change in a bed height direction of the packed bed of raw-material pellets from a bottom surface of the packed bed in the partially-reduced iron producing apparatus having the configuration described above, observed when the raw-material pellets are packed at the height of 200 mm in the reduction furnace and are heated while the mixed gas of the circulated gas and air is vented upward from the wind boxes below the raw-material pellets. In  FIG. 3 , the solid line shows a temperature history at a position away from the bottom surface of the packed bed by 50 mm, the dotted line shows a temperature history at a position away from the bottom surface of the packed bed by 100 mm, and the dot-dashed line shows a temperature history at a position away from the bottom surface of the packed bed by 150 mm. Note that the oxygen concentration in the first wind box is adjusted to 15% and the oxygen concentration in each of the second to fifth wind boxes is adjusted to 11%. 
     As shown in  FIG. 3 , it is found that temperatures which are equal to or above 1200° C. and which are equal to or below 1400° C. are obtained at all of the positions away from the bottom surface of the packed bed respectively by 50 mm, 100 mm, and 150 mm, i.e. across the entire layer height of the packed bed of the raw-material pellets. A temperature equal to or above 1200° C. is required for the reduction of the raw-material pellets and a temperature equal to or below 1400° C. prevents excessive melting. 
     The temperatures at the positions away from the bottom surface of the packed bed by 50 mm, 100 mm, and 150 mm reach their peaks sequentially along with the elapse of time. Hence, it is found that the combustion zone moves in the bed height direction of the packed bed of raw-material pellets. The raw-material pellets after the gas combustion are quickly cooled in few minutes from the peak temperature to a temperature equal to or below 500° C. at which reoxidation is less likely to occur. 
     Accordingly, in the raw-material pellet heating region  71   b  described above, the heating of the raw-material pellets  3 , the generation and combustion of the flammable volatile component, the generation of carbon monoxide gas, the combustion of carbon monoxide gas by the circulation of the carbon monoxide gas and the remaining portion of the flammable volatile component, and the reduction reaction of iron oxides sequentially occur from the bottom surface of the raw-material pellet packed bed  4  to an upper layer thereof, while the grate  101  rotates between the position above the second wind box  33   b  and the position above the fourth wind box  33   d.    
     Next, the grate  101  moves forward and the mixed gas  94   e  whose oxygen concentration is adjusted to be 5% or lower is vented to the fifth wind box  33   e . This causes the raw-material pellet packed bed  4  whose reduction has proceeded to a predetermined degree to be cooled to about 100° C. to 800° C. in the raw-material pellet cooling region  71   c  above the fifth wind box  33   e  and the desired partially-reduced iron is produced. When the grate  101  further moves forward, the partially-reduced iron  5  is discharged from the partially-reduced iron discharging device  39 . 
     In the partially-reduced iron producing apparatus of the embodiment, the carbon monoxide gas produced by reduction, which has been conventionally discharged in an exhaust gas and then emitted into the atmosphere or which has been conventionally combusted outside the system by using a supplemental fuel to recover exhaust heat therefrom with a boiler, is circulated through the packed bed  4  of the raw-material pellets and then added to a carbon monoxide gas which is produced by the reduction. Thus, the carbon monoxide gas is combusted with the concentration thereof being increased, thereby improving the combustion rate. Moreover, the carbon monoxide gas is directly effectively used as a heat source in the packed bed  4  of the raw material pellets. Hence, no carbon material for combustion, with which the conventional raw-material pellets are coated, is required. As a result, it is possible to reduce the consumption of carbon materials and to reduce carbon dioxide emissions. Furthermore, since the raw-material pellets  3  are heated by the combustion of the gas generated by heating the raw-material pellets  3 , the amount of generated gas is small. Hence, the combustion of carbon monoxide gas ends as soon as the concentration of carbon monoxide gas in the combustion zone of the packed bed  4  of raw-material pellets falls below the combustion range of carbon monoxide, and the raw-material pellets  3  are thereby cooled. Thus, a time in which the raw-material pellets  3  are in contact with oxygen in a high temperature state is short, thereby reducing reoxidation. As a result, a partially-reduced iron with a high degree of metallization can be produced. 
     In the case of the conventional raw-material pellets coated with coal powder for combustion, the amount of coal in the coal powder for combustion is about 5of the total. Accordingly, using the raw-material pellets coated with no ignition coal can reduce the usage amount of coal compared to that with the conventional method of producing reduced iron. 
     The partially-reduced iron producing apparatus of the embodiment includes: the partition boards  38   a  and  38   b  which are provided in the hood  34 , which are surrounded by the hood  34  and the grate  101 , and which define the space in the center portion in a longitudinal direction of the grate (region  71   b ); the exhaust gas circulation device  50  which discharges the exhaust gas in the region  71   b  and supplies the exhaust gas to the wind boxes  33   b  to  33   d  disposed to face the region  71   b ; the air supplying device  60  which is connected to the exhaust gas circulation device  50  and which supplies air; and the flow rate adjustment valves V 42  to V 44  which are provided in the air supplying device  60  and which adjust the flow rate of air. This configuration makes it possible to effectively use the carbon monoxide gas with relatively high concentration which is generated in the region  71   b  and to thereby suppress carbon dioxide emissions. 
     The descriptions have been given above by using the partially-reduced iron producing apparatus including the grate reduction furnace  100  of the up-draft type. However, the partially-reduced iron producing apparatus may include a grate reduction furnace of a down-draft type in which the raw-material pellet supplying device and the heating furnace are arranged in this order from upstream in the travelling direction of the grate. 
     INDUSTRIAL APPLICATION 
     The partially-reduced iron producing apparatus and the partially-reduced iron producing method of the present invention enable producing a partially-reduced iron without using a combustion carbon material and reducing carbon dioxide emissions. Accordingly, the partially-reduced iron producing apparatus and the partially-reduced iron producing method can be used effectively in steel industry and the like. 
     REFERENCE SIGNS LIST 
       1  IGNITION RAW-MATERIAL PELLET 
       2  IGNITION RAW-MATERIAL PELLET LAYER 
       3  RAW-MATERIAL PELLET 
       4  PACKED BED OF RAW-MATERIAL PELLETS 
       5  PARTIALLY-REDUCED IRON 
       10  IGNITION RAW-MATERIAL PELLET SUPPLYING DEVICE 
       20  HEATING FURNACE 
       21  COMBUSTION BURNER 
       22  EXHAUST PIPE 
       30  REDUCTION FURNACE 
       31  RAW-MATERIAL PELLET SUPPLYING DEVICE (FEED HOPPER) 
       32  REDUCTION FURNACE MAIN BODY 
       33   a  TO  33   e  WIND BOX 
       34  HOOD 
       35  TRACK 
       36  SUPPORT PORTION 
       37  ROLLER 
       38   a ,  38   b  PARTITION BOARD 
       41 ,  43  WATER SEAL BOX 
       42 ,  44  SEAL PLATE 
       51  FIRST EXHAUST PIPE 
       52  SECOND EXHAUST PIPE 
       53  DUST REMOVER 
       54  DUST-REMOVED GAS DELIVERY PIPE 
       55  GAS COOLER 
       56  PUMP 
       57  O 2  SENSOR 
       58  CIRCULATING GAS DELIVERY PIPE 
       59   a  TO  59   e  FIRST TO FIFTH BRANCH CIRCULATING GAS DELIVERY PIPES 
       60  AIR SUPPLYING DEVICE 
       61  AIR SUPPLYING SOURCE 
       62  AIR FEED PIPE 
       63  FLOW RATE ADJUSTMENT VALVE 
       64  PUMP 
       65  AIR DELIVERY PIPE 
       66   a  TO  66   e  FIRST TO FIFTH BRANCH AIR DELIVERY PIPES 
       71   a  IGNITION RAW-MATERIAL PELLET COMBUSTION REGION 
       71   b  RAW-MATERIAL PELLET HEATING REGION 
       71   c  RAW-MATERIAL PELLET COOLING REGION 
       82  RAW-MATERIAL PELLET COOLING REGION GAS EXHAUST PIPE 
       100  GRATE REDUCTION FURNACE 
       101  ENDLESS GRATE