Abstract:
A combustion system capable of decreasing nitrogen oxide discharged from exhaust gas is provided. The combustion system includes: a combustion furnace ( 2 ) having a burner unit ( 2   a ) for supplying fuel and combustion oxygen to the inside of the furnace, a reduction zone formed on a downstream side of the burner unit ( 2   a ) for combusting the fuel, and a combustion oxygen supply port ( 2   b ) for supplying combustion oxygen ( 21 ) so that unburned fuel which has passed the reduction zone completely combusts; and a smoke removal device ( 9 ) for removing smoke in the exhaust gas discharged from the combustion furnace ( 2 ). Part of exhaust gas ( 22 ) diverging from between the combustion furnace ( 2 ) and the smoke removal device ( 9 ) is introduced to the burner unit ( 2   a ), while part of exhaust gas ( 23 ) diverging from a downstream side of the smoke removal device ( 9 ) is introduced to the combustion oxygen supply port ( 2   b ).

Description:
TECHNICAL FIELD  
       [0001]    The present invention relates to a combustion system, and more particularly relates to removal of nitrogen oxide in exhaust gas. 
       BACKGROUND ART   
       [0002]    Coal is generally used as fuel in thermal power plants and the like since the resource volume of coal is abundant. However, coal is high in carbon contents in fuel compared with oil and gas. When coal is combusted by an air combustion boiler system, the emission amount of carbon dioxide is increased. 
         [0003]    In order to increase concentrations of carbon dioxide for easy recovery, an oxygen combustion boiler system  101  as shown in  FIG. 5  is used. The oxygen combustion boiler system  101  includes a coal pulverizer  103  for pulverizing coal, an oxygen combustion boiler  102  for combusting the coal pulverized by the coal pulverizer  103  and discharging exhaust gas, a denitration device  104  for removing nitrogen oxide in the exhaust gas discharged from the oxygen combustion boiler  102 , a dust removal device  105  for removing dust and the like in the exhaust gas, a desulfurization device  106  for removing sulfur oxide in the exhaust gas, and a gas cooler  107  for cooling the exhaust gas to remove moisture in the exhaust gas. 
         [0004]    Carrier gas is introduced into the coal pulverizer  103  for drying the pulverized coal and conveying the pulverized coal from the coal pulverizer  103  to the oxygen combustion boiler  102 . As the carrier gas, exhaust gas (hereinafter referred to as “primary recirculation gas”) discharged from the desulfurization device  106  and traveling through the gas cooler  107  is used. The primary recirculation gas is heated by an air heater  108  provided between the denitration device  104  and the dust removal device  105  for drying the coal. The air heater  108  performs heat exchange between high-temperature exhaust gas discharged from the denitration device  104  and low-temperature exhaust gas which has passed the gas cooler  107 , and thereby heats the primary recirculation gas introduced to the coal pulverizer  103 . 
         [0005]    A boiler capable of conducting denitration inside a furnace (not shown) of the oxygen combustion boiler  102  by two-stage combustion (e.g., Patent Literature 1) is used as the oxygen combustion boiler  102 . The oxygen combustion boiler  102  includes a burner unit  102   a  for supplying oxygen introduced from a combustion oxygen supply system, later-described secondary recirculation gas, and coal as fuel into the oxygen combustion boiler  102 . The oxygen combustion boiler  102  also includes an additional air port (hereinafter referred to as “AA port”)  102   b  provided on the downstream side of the burner unit  102   a  for supplying oxygen introduced from the combustion oxygen supply system and later-described secondary recirculation gas into the oxygen combustion boiler  102 . 
         [0006]    The oxygen supplied into the oxygen combustion boiler  102  through the burner unit  102   a  and the AA port  102   b  contains part of the exhaust gas (hereinafter referred to as “secondary recirculation gas”) which has been introduced from the downstream of the gas cooler  107  and mixed therein as dilution gas. The secondary recirculation gas is heated by the air heater  108  and is mixed into the oxygen which is introduced to the burner unit  102   a  and the AA port  102   b.    
         [0007]    The oxygen supplied to the burner unit  102   a  is adjusted so that the amount of oxygen introduced from the combustion oxygen supply system is not larger than 1.0 times the theoretical combustion oxygen amount of the coal supplied from the coal pulverizer  103 . A remaining amount of the oxygen introduced from the combustion oxygen supply system is supplied to the AA port  102   b . Accordingly, a zone between the burner unit  102   a  and the AA port  102   b  in the oxygen combustion boiler  102  is in an oxygen-poor state. 
         [0008]    Since oxygen is poor in between the burner unit  102   a  and the AA port  102   b , the zone between the burner unit  102   a  and the AA port  102   b  is made to have a reducing atmosphere. The fuel charged into the oxygen combustion boiler  102  through the burner unit  102   a  combusts and generates exhaust gas. Nitrogen oxide (NOx) contained in the generated exhaust gas is partially reduced when passing the reducing atmosphere present between the burner unit  102   a  and the AA port  102   b . This makes it possible to decrease nitrogen oxide within the oxygen combustion boiler  102 . 
         [0009]    Patent Literature 2 and Patent Literature 3 disclose oxygen combustion boilers for receiving part of exhaust gas introduced as secondary recirculation gas which has passed a denitration device, an air heater, a dust removal device, and a desulfurization device. 
       CITATION LIST   
     Patent Literature  
     {PTL 1} 
       [0010]    The Publication of Japanese Patent No. 3068888 
       (PTL 2) 
       [0011]    Japanese Unexamined Patent Application, Publication No. Hei6-94212 
       (PTL 3) 
       [0012]    Japanese Unexamined Patent Application, Publication No. Sho59-195013 
       SUMMARY OF INVENTION   
     Technical Problem  
       [0013]    However, in the oxygen combustion boiler of the invention disclosed in Patent Literature 3 as well as in the oxygen combustion boiler system  101  shown in  FIG. 5 , the exhaust having a high nitrogen oxide concentration discharged from the oxygen combustion boiler  102  was recirculated as secondary recirculation gas and was recharged into the oxygen combustion boiler  102 . As a consequence, the denitration device  104  provided on the downstream side of the oxygen combustion boiler  102  had a heavy treatment burden. 
         [0014]    Moreover, since the concentration of nitrogen oxide treated by the denitration device  104  was high, the consumption of ammonia sprayed to the exhaust gas passing the denitration device  104  increased. 
         [0015]    In view of the above-stated problems, an object of the present invention is to provide a combustion system capable of decreasing nitrogen oxide discharged from exhaust gas. 
       Solution to Problem 
       [0016]    A combustion system of the present invention employs the following solutions to solve the foregoing problems. 
         [0017]    A combustion system according to the present invention includes: a combustion furnace having a burner unit for supplying fuel and combustion oxygen to an inside of the furnace, a reduction zone formed on a downstream side of the burner unit for combusting the fuel, and a combustion oxygen supply port for supplying combustion oxygen so that unburned fuel which has passed the reduction zone completely combusts; and a smoke removal device for removing smoke in the exhaust gas discharged from the combustion furnace, wherein part of exhaust gas diverging from between the combustion furnace and the smoke removal device is introduced to the burner unit, while part of exhaust gas diverging from a downstream side of the smoke removal device is introduced to the combustion oxygen supply port. 
         [0018]    In the combustion system according to the present invention, part of exhaust gas diverging from between the combustion furnace and the smoke removal device is resupplied to the inside of the combustion furnace through the burner unit. A zone under reducing atmosphere (reduction zone) is formed between the burner unit of the combustion furnace and the combustion oxygen supply port. Consequently, the exhaust gas diverging from between the combustion furnace and the smoke removal device can be reduced in the zone under reducing atmosphere formed in the combustion furnace and then be discharged. This makes it possible to decrease the flow rate of the exhaust gas introduced from the combustion furnace to the smoke removal device and to decrease the amount of smoke. Therefore, the capacity of the smoke removal device can be reduced. 
         [0019]    Moreover, part of exhaust gas diverging from the downstream side of the smoke removal device is the exhaust gas having the concentration of nitrogen oxide, which is smoke, decreased by the smoke removal device. The exhaust gas having a decreased nitrogen oxide concentration is introduced to the combustion oxygen supply port and is used for promoting complete combustion of the unburned fuel contained in the exhaust gas which has partially been reduced by passing the reducing atmosphere inside the combustion furnace. Therefore, the exhaust gas can be discharged out of the combustion furnace while the concentration of nitrogen oxide contained therein is maintained low. The exhaust gas having a decreased nitrogen oxide concentration is recirculated to the combustion furnace and the smoke removal device, so that increase in the concentration of nitrogen oxide in an outlet of the combustion furnace can be suppressed. 
         [0020]    Further, in the combustion system according to the present invention, the smoke removal device includes: a denitration unit for removing nitrogen oxide in the exhaust gas discharged from the combustion furnace; a heat exchange unit for conducting heat exchange between exhaust gas which has passed the denitration unit and exhaust gas which is introduced to the combustion oxygen supply port; a dust removal unit for removing dust in exhaust gas which has passed the heat exchange unit; a desulfurization unit for removing sulfur oxide in exhaust gas which has passed the dust removal unit; and a cooling unit for cooling exhaust gas which has passed the desulfurization unit, wherein part of exhaust gas diverging from between the desulfurization unit and the cooling unit is introduced to the combustion oxygen supply port. 
         [0021]    In the combustion system according to the present invention, part of the exhaust gas diverging from between the desulfurization unit and the cooling unit is introduced to the combustion oxygen supply port of the combustion furnace. This makes it possible to decrease the flow rate of nitrogen oxide in the exhaust gas introduced to the denitration unit as well as to decrease the flow rate of the exhaust gas introduced to the cooling unit. Therefore, it becomes possible to downsize the denitration unit and to reduce the capacity of the cooling unit. 
         [0022]    Further, in the combustion system according to the present invention, part of exhaust gas diverging from between the dust removal unit and the desulfurization unit is introduced to the combustion oxygen supply port. 
         [0023]    In the combustion system according to the present invention, part of the exhaust gas diverging from between the dust removal unit and the desulfurization unit is introduced to the combustion oxygen supply port. This makes it possible to decrease the flow rate of nitrogen oxide in the exhaust gas introduced to the denitration unit as well as to decrease the flow rate of the exhaust gas introduced to the desulfurization unit and the cooling unit. Therefore, it becomes possible to downsize the denitration unit and to reduce the capacity of the desulfurization unit and the cooling unit. 
         [0024]    Further, in the combustion system according to the present invention, part of exhaust gas diverging from between the denitration unit and the dust removal unit is introduced to the combustion oxygen supply port. 
         [0025]    In the combustion system according to the present invention, part of the exhaust gas diverging from between the denitration unit and the dust removal unit is introduced to the combustion oxygen supply port. This makes it possible to decrease the flow rate of nitrogen oxide in the exhaust gas introduced to the denitration unit as well as to decrease the flow rate of the exhaust gas introduced to the heat exchange unit, the dust removal unit, the desulfurization unit and the cooling unit. Therefore, it becomes possible to downsize the denitration unit and to reduce the capacity of the heat exchange unit, the dust removal unit, the desulfurization unit and the cooling unit. 
         [0026]    Further, in the combustion system according to the present invention, the denitration unit includes: an ammonia supply unit for supplying ammonia into exhaust gas; and a catalyst unit for allowing exhaust gas supplied by the ammonia supply unit to pass therethrough. 
         [0027]    In the combustion system according to the present invention, the exhaust gas having a decreased flow rate of nitrogen oxide is introduced to the denitration unit. Therefore, the amount of ammonia to be supplied can be reduced as compared with the case where the exhaust gas whose flow rate of nitrogen oxide is not decreased. 
       Advantageous Effects of Invention 
       [0028]    In the present invention, part of exhaust gas diverging from between the combustion furnace and the smoke removal device is resupplied to the inside of the combustion furnace through the burner unit. A zone under reducing atmosphere is formed between the burner unit of the combustion furnace and the combustion oxygen supply port. Consequently, the exhaust gas diverging from between the combustion furnace and the smoke removal device can be reduced in the zone under reducing atmosphere formed in the combustion furnace before being discharged. This makes it possible to decrease the flow rate of the exhaust gas introduced from the combustion furnace to the smoke removal device and to decrease the flow rate of smoke. Therefore, the capacity of the smoke removal device can be reduced. 
         [0029]    Moreover, part of exhaust gas diverging from the downstream side of the smoke removal device is the exhaust gas having the concentration of nitrogen oxide, which is smoke, decreased by the smoke removal device. The exhaust gas having a decreased nitrogen oxide concentration is introduced to the combustion oxygen supply port and is used for promoting complete combustion of the unburned fuel contained in the exhaust gas which has partially been reduced by passing the reducing atmosphere inside the combustion furnace. Therefore, the exhaust gas can be discharged out of the combustion furnace while the concentration of nitrogen oxide contained therein is maintained low. The exhaust gas having a decreased nitrogen oxide concentration is recirculated to the combustion furnace and the smoke removal device, so that increase in the concentration of nitrogen oxide in the outlet of the combustion furnace can be suppressed. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0030]      FIG. 1  is a schematic structure view of a combustion system according to a first embodiment of the present invention. 
           [0031]      FIG. 2  is a schematic structure view of a combustion system according to a second embodiment of the present invention. 
           [0032]      FIG. 3  is a schematic structure view of a combustion system according to a third embodiment of the present invention. 
           [0033]      FIG. 4  is a schematic structure view of a combustion system according to a fourth embodiment of the present invention. 
           [0034]      FIG. 5  is a schematic structure view of a conventional oxygen combustion boiler system. 
       
    
    
     DESCRIPTION OF EMBODIMENTS  
     First Embodiment 
       [0035]      FIG. 1  is a schematic structure view of a combustion system according to the first embodiment of the present invention. 
         [0036]    The combustion system  1  includes a coal fired boiler (combustion furnace)  2 , a coal pulverizer  3  for pulverizing coal supplied to the coal fired boiler  2 , and a smoke removal device  9 . 
         [0037]    The coal fired boiler  2  is an oxygen combustion boiler which can conduct denitration inside the furnace (not shown) by two-stage combustion. The coal fired boiler  2  includes a furnace inside for combusting fuel, a burner unit  2   a , and an additional air port (hereinafter referred to as “AA unit”)  2   b . Coal as fuel supplied from the coal pulverizer  3 , oxygen (combustion oxygen) introduced from a combustion oxygen supply system  21 , and later-described secondary recirculation gas for burner unit  22  are introduced to the burner unit  2   a . Remaining part of the oxygen introduced from the combustion oxygen supply system  21  to the burner unit  2   a  and later-described secondary recirculation gas for AA unit  23  are introduced to the AA unit (combustion oxygen supply port)  2   b.    
         [0038]    The coal pulverizer  3  is for pulverizing the coal, which is supplied to the coal fired boiler  2 , into fine powder of a size of several μm to hundreds of μm. Part of exhaust gas (hereinafter referred to as “primary recirculation gas”)  24  discharged from the smoke removal device  9  is introduced to the coal pulverizer  3  as high-temperature carrier gas for drying the pulverized coal and conveying the pulverized coal from the coal pulverizer  3  to the coal fired boiler  2 . 
         [0039]    The smoke removal device  9  includes a denitration device (denitration unit)  4 , a gas heater (heat exchange unit)  8 , a dust removal device (dust removal unit)  5 , a desulfurization device (desulfurization unit)  6 , and a gas cooler (cooling unit)  7 . 
         [0040]    The denitration device  4  includes an ammonia supply unit (not shown) for spraying ammonia to the exhaust gas, and a catalyst unit (not shown) for allowing the exhaust gas having ammonia sprayed thereto to pass therethrough. The denitration device  4  is for removing nitrogen oxide in the exhaust gas by spraying ammonia to the introduced exhaust gas and for having the exhaust gas pass the catalyst unit. 
         [0041]    The gas heater  8  is for conducting heat exchange of the high-temperature exhaust gas, which has come from the coal fired boiler  2  and passed the denitration device  4 , with the primary recirculation gas  24  and the secondary recirculation gas for AA unit  23 . As a consequence of the heat exchange, the primary recirculation gas  24  reaches a temperature suitable for drying the coal pulverized by the coal pulverizer  3 , while the secondary recirculation gas for AA unit  23  reaches a temperature suitable for being introduced to the inside of the furnace through the AA unit  2   b  of the coal fired boiler  2 . 
         [0042]    The dust removal device  5  is for removing dust in the exhaust gas, and the desulfurization device  6  is for removing sulfur oxide in the introduced exhaust gas. 
         [0043]    The gas cooler  7  is for cooling the introduced exhaust gas. 
         [0044]    Description is now given of the flow of exhaust gas in the present embodiment. 
         [0045]    Oxygen introduced from the combustion oxygen supply system  21 , part of exhaust gas (hereinafter referred to as “secondary recirculation gas for burner”)  22  introduced from between the coal fired boiler  2  and the denitration device  4 , and coal introduced from the coal pulverizer  3  are supplied to the burner unit  2   a  of the coal fired boiler  2 . The secondary recirculation gas for burner  22  is used as diluents for diluting the oxygen introduced from the combustion oxygen supply system  21 . 
         [0046]    Oxygen introduced from the combustion oxygen supply system  21  and part of exhaust gas (hereinafter referred to as “secondary recirculation gas for AA unit”)  23  purified by passing the smoke removal device  9  are supplied to the AA unit  2   b  of the coal fired boiler  2 . The secondary recirculation gas for AA unit  23  is used as dilution gas for diluting the oxygen introduced from the combustion oxygen supply system  21 . 
         [0047]    The amount of oxygen supplied from the combustion oxygen supply system  21  to the coal fired boiler  2  through the burner unit  2   a  and the AA unit  2   b  is set to be 1.15 times the theoretical combustion oxygen amount of the coal supplied to the inside of the furnace through the burner unit  2   a . The amount of oxygen supplied to the inside of the furnace of the coal fired boiler  2  is not larger than 1.0 times the theoretical combustion oxygen amount of the coal supplied to the inside of the furnace through the burner unit  2   a.    
         [0048]    A remaining amount of the oxygen, which is introduced from the combustion oxygen supply system  21  to the burner unit  2   a , is used as the oxygen supplied to the inside of the furnace through the AA unit  2   b . The amount of oxygen supplied through the AA unit  2   b  is up to about 40% of the amount of oxygen introduced from the combustion oxygen supply system  21  to the coal fired boiler  2 . 
         [0049]    The amount of oxygen charged to the inside of the furnace through the burner unit  2   a  is made not larger than 1.0 times the theoretical combustion oxygen amount of coal, and oxygen is also charged to the inside of the furnace through the AA unit  2   b . As a consequence, a zone between the burner unit  2   a  and the AA unit  2   b  becomes short of oxygen. Due to the shortage of oxygen in the zone between the burner unit  2   a  and the AA unit  2   b , the zone between the burner unit  2   a  and the AA unit  2   b  inside the furnace is put in the state of reducing atmosphere. 
         [0050]    Nitrogen oxide in the exhaust gas generated by combustion of coal and oxygen charged to the inside of the furnace through the burner unit  2   a  is reduced when passing the zone under reducing atmosphere through the burner unit  2   a . Consequently, nitrogen oxide in the exhaust gas generated within the coal fired boiler  2  is removed inside the furnace of the coal fired boiler  2 . 
         [0051]    The concentration of nitrogen oxide contained in the exhaust gas discharged from the coal fired boiler  2  is decreased by in-furnace NOx removal in the coal fired boiler  2 . The exhaust gas with a decreased nitrogen oxide concentration is introduced to the smoke removal device  9 . As the concentration of nitrogen oxide decreases, the amount of exhaust gas introduced from the coal fired boiler  2  to the smoke removal device  9  is decreased. 
         [0052]    The exhaust gas introduced to the smoke removal device  9  is introduced to the denitration device  4  which constitutes the smoke removal device  9 , where residual nitrogen oxide is removed. The exhaust gas with nitrogen oxide removed is introduced to the air heater  8 . The temperature of the exhaust gas introduced to the air heater  8  is high. In the air heater  8 , the high-temperature exhaust gas is subjected to heat exchange with the secondary recirculation gas for AA unit  23  and the primary recirculation gas  24 . After heat exchange in the air heater  8 , the high-temperature exhaust gas introduced from the denitration device  4  is cooled and then introduced to the dust removal device  5 . 
         [0053]    The exhaust gas introduced to the dust removal device  5  is subjected to removal of dust and the like before being discharged. The exhaust gas discharged from the dust removal device  5  is introduced to the desulfurization device  6 , where sulfuric compounds are removed. The exhaust gas purified through the denitration device  4 , the dust removal device  5 , and the desulfurization device  6  is mostly composed of carbon dioxide and steam. This purified exhaust gas is introduced to the gas cooler  7 , where the temperature thereof is lowered. The exhaust gas whose temperature is lowered by the gas cooler  7  is discharged from the smoke removal device  9 . 
         [0054]    Part of the exhaust gas discharged from the smoke removal device  9  is introduced to the air heater  8  as secondary recirculation gas for AA unit  23 , where heat exchange is conducted between the secondary recirculation gas for AA unit  23  and the high-temperature exhaust gas discharged from the denitration device  4 , so that the temperature of the secondary recirculation gas for AA unit  23  increases. The thus-obtained high-temperature secondary recirculation gas for AA unit  23  is then introduced to the AA unit  2   b  of the coal fired boiler  2 . Part of the exhaust gas discharged from the smoke removal device  9  is further introduced to the air heater  8  as primary recirculation gas  24 . The primary recirculation gas  24  introduced to the air heater  8  is heated by exchanging heat with the high-temperature exhaust gas discharged from the denitration device  4 . The thus-obtained high-temperature primary recirculation gas  24  is then introduced to the coal pulverizer  3 . The high-temperature primary recirculation gas  24  introduced to the coal pulverizer  3  is used as carrier gas for drying coal and for conveying pulverized coal to the coal fired boiler  3 . 
         [0055]    The exhaust gas purified by the smoke removal device  9  is mostly introduced to a system such as a carbon dioxide recovery system (not shown) where carbon dioxide in the exhaust gas is recovered. Thus, the exhaust gas with carbon dioxide, nitrogen oxide, and sulfur oxide removed therefrom is emitted to the outside of the combustion system  1 . 
         [0056]    The combustion system according to the present embodiment has following operation effects. 
         [0057]    In the combustion system according to the present embodiment, the secondary recirculation gas for burner (part of exhaust gas)  22  diverging from between the coal fired boiler (combustion furnace)  2  and the smoke removal device  9  is resupplied to the inside of the furnace (not shown) of the coal fired boiler  2  through the burner unit  2   a . Consequently, the exhaust gas containing high-concentration nitrogen oxide introduced to the smoke removal device  9  is decreased. Therefore, it becomes possible to reduce the load of the denitration device  4 . 
         [0058]    Moreover, the reducing atmosphere is formed between the burner unit  2   a  and the AA unit (combustion oxygen supply port)  2   b  of the coal fired boiler  2 . Consequently, the secondary recirculation gas for burner unit  22  diverging from between the coal fired boiler  2  and the smoke removal device  9  can be reduced in the zone under reducing atmosphere formed in the coal fired boiler  2  and then be discharged. This makes it possible to decrease the flow rate of the exhaust gas introduced from the coal fired boiler  2  to the smoke removal device  9  and to decrease the flow rate of nitrogen oxide in smoke. Therefore, the capacity of the smoke removal device  9  can be reduced. 
         [0059]    The secondary recirculation gas for AA unit  23  which is part of the exhaust gas discharged from the downstream side of the smoke removal device  9  is the exhaust gas having the concentration of nitrogen oxide decreased by the denitration device  4 . This exhaust gas is introduced to the coal fired boiler  2  through the AA unit  2   b  and is used for promoting complete combustion of unburned pulverized coal contained in the exhaust gas which is partially reduced by passing the reducing atmosphere inside the furnace of the coal fired boiler  2 . Therefore, the exhaust gas is discharged out of the furnace of the coal fired boiler  2  while the concentration of nitrogen oxide contained therein is maintained low. As a result, the exhaust gas having a decreased nitrogen oxide concentration is recirculated between the coal fired boiler  2  and the smoke removal device  9 , so that increase in the concentration of nitrogen oxide in the outlet of the furnace of the coal fired boiler  2  can be suppressed. 
         [0060]    The exhaust gas with a decreased flow rate of nitrogen oxide is introduced to the denitration device (denitration unit)  4 , so that the amount of ammonia sprayed from the ammonia supply unit (not shown) to the exhaust gas can be decreased as compared with the case where the exhaust gas whose flow rate of nitrogen oxide is not decreased is introduced to the denitration device  4 . Therefore, it becomes possible to downsize the denitration device  4 . 
       Second Embodiment 
       [0061]    Hereinafter, the second embodiment of the present invention will be described. A combustion system of the present embodiment is different from the first embodiment in the point that the secondary recirculation gas for AA unit is introduced from between the desulfurization device and the gas cooler. Other structural members are similar to those of the first embodiment. Therefore, like structural members and flows are designated by like reference signs to omit description. 
         [0062]      FIG. 2  is a schematic structure view of a combustion system according to the second embodiment of the present invention. 
         [0063]    Oxygen (combustion oxygen) introduced from a combustion oxygen supply system  21  and part of exhaust gas diverging from between a desulfurization device (desulfurization unit)  6  and a gas cooler (cooling unit)  7  which constitute a smoke removal device  9  are supplied to an AA unit (combustion oxygen supply port)  2   b  of a coal fired boiler (combustion furnace)  2  as secondary recirculation gas for AA unit  23 . 
         [0064]    The combustion system according to the present embodiment has following operation effects. 
         [0065]    In the combustion system according to the present embodiment, the secondary recirculation gas for AA unit (part of exhaust gas)  23  diverging from between the desulfurization device (desulfurization unit)  6  and the gas cooler (cooling unit)  7  is introduced to the AA unit (combustion oxygen supply port)  2   b  of the coal fired boiler (combustion furnace)  2 . Accordingly, the flow rate of the exhaust gas introduced to the gas cooler  7  can be decreased. This makes it possible to reduce the capacity of the gas cooler  7  and to decrease the flow rate of nitrogen oxide in the exhaust gas introduced to the denitration device (denitration unit)  4  for the same reason as that in the first embodiment. As a result, the denitration device  4  can be downsized. 
       Third Embodiment 
       [0066]    Hereinafter, the third embodiment of the present invention will be described. A combustion system of the present embodiment is different from the first embodiment in the point that the secondary recirculation gas for AA unit is introduced from between the dust removal device and the desulfurization device. Other structural members are similar to those of the first embodiment. Therefore, like structural members and flows are designated by like reference signs to omit description. 
         [0067]      FIG. 3  is a schematic structure view of a combustion system according to the third embodiment of the present invention. 
         [0068]    Oxygen (combustion oxygen) introduced from a combustion oxygen supply system  21  and part of exhaust gas diverging from between a dust removal device (dust removal unit)  5  and a desulfurization device (desulfurization unit)  6  which constitute a smoke removal device  9  are supplied to an AA unit (combustion oxygen supply port)  2   b  of a coal fired boiler (combustion furnace)  2  as secondary recirculation gas for AA unit  23 . 
         [0069]    The combustion system according to the present embodiment has following operation effects. 
         [0070]    In the combustion system according to the present embodiment, the secondary recirculation gas for AA unit (part of exhaust gas)  23  diverging from between the dust removal device (dust removal unit)  5  and the desulfurization device (desulfurization unit)  6  is introduced to the AA unit (combustion oxygen supply port)  2   b . Accordingly, the flow rate of the exhaust gas introduced to the desulfurization device  6  and a gas cooler  7  can be decreased. This makes it possible to reduce the capacity of the desulfurization device  6  and the gas cooler  7  and to decrease the flow rate of nitrogen oxide in the exhaust gas introduced to the denitration device (denitration unit)  4  for the same reason as that in the first embodiment. As a result, the denitration device  4  can be downsized. 
       Fourth Embodiment 
       [0071]    Hereinafter, the fourth embodiment of the present invention will be described. A combustion system of the present embodiment is different from the first embodiment in the point that the secondary recirculation gas for AA unit is introduced from between the denitration device and the air heater. Other structural members are similar to those of the first embodiment. Therefore, like structural members and flows are designated by like reference signs to omit description. 
         [0072]      FIG. 4  is a schematic structure view of a combustion system according to the fourth embodiment of the present invention. 
         [0073]    Oxygen (combustion oxygen) introduced from a combustion oxygen supply system  21  and part of exhaust gas diverging from between a denitration device (denitration unit)  4  and an air heater (heat exchange unit)  8  which constitute a smoke removal device  9  are supplied to an AA unit (combustion oxygen supply port)  2   b  of a coal fired boiler (combustion furnace)  2  as secondary recirculation gas for AA unit  23 . 
         [0074]    The combustion system according to the present embodiment has following operation effects. 
         [0075]    In the combustion system according to the present embodiment, the secondary recirculation gas for AA unit (part of exhaust gas)  23  diverging from between the denitration device (denitration unit)  4  and the air heater (heat exchange unit)  8  is introduced to the AA unit (combustion oxygen supply port)  2   b . Accordingly, the flow rate of the exhaust gas introduced to the air heater  8 , the dust removal device (dust removal unit)  5 , the desulfurization device (desulfurization unit)  6 , and the gas cooler (cooling unit)  7  can be decreased. This makes it possible to reduce the capacity of the air heater  8 , the dust removal device  5 , the desulfurization device  6  and the gas cooler  7  and to decrease the flow rate of nitrogen oxide in the exhaust gas introduced to the denitration device (denitration unit)  4  for the same reason as that in the first embodiment. As a result, the denitration device  4  can be downsized. 
       REFERENCE SIGNS LIST   
       [0000]    
       
           1  Combustion system 
           2  Coal fired boiler (combustion furnace) 
           2   a  Burner unit 
           2   b  AA unit (combustion oxygen supply port) 
           3  Coal pulverizer 
           4  Denitration device (denitration unit) 
           5  Dust removal device (dust removal unit) 
           6  Desulfurization device (desulfurization unit) 
           7  Gas cooler (cooling unit) 
           8  Air heater (heat exchange unit) 
           9  Smoke removal device 
           21  Combustion oxygen supply system (combustion oxygen) 
           22  Secondary recirculation gas for burner unit (part of exhaust gas) 
           23  Secondary recirculation gas for AA unit (part of exhaust gas) 
           24  Primary recirculation gas (part of exhaust gas)