Abstract:
The present invention relates to a method for drying high moisture, low calorific value lignite for a generating set and recovering water contained therein and an apparatus thereof, which mainly consisting of a rotary steam tube dryer, a washing cooling tower, a coal mill, a  1 st bag filter I, a condenser, a weighing surge bunker, a water ring vacuum pump and so on. In the present invention, a drying system is integrated with a milling system, every dryer and the corresponding coal mill are disposed and are directly connected via a surge bunker, thereby not only saving the heat lost during the transportation of pulverized coal, but also omitting a long-distance transportation from a conventional drying system to a conventional milling system, effectively avoiding such phenomena as dust pollution, waste and spontaneous combustion during transportation and transshipment, simplifying the coal preparation system employed in the front-end process of drying.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a method for drying high moisture, low calorific value lignite for a generating set and recovering water contained therein and an apparatus thereof. The apparatus is additionally provided with a system for drying lignite and recovering water, so that high moisture lignite has an increased calorific value to meet electricity generation requirements, and water in the lignite can be recovered and used as a supplementary source of water, thereby reducing water consumption and saving energy. 
       BACKGROUND 
       [0002]    For the milling system employed in a conventional coal-fired power plant, a coal mill (routinely there are two types of coal mills: medium speed coal mills and fan-type coal mills) is used for milling and drying coal, into which is introduced hot flue gas at 150 to 300° C. from a coal-fired boiler economizer, and then blowing the resulting powder into a boiler and burning it. However, this method has such shortcoming that the degree of drying is so low that it is impossible to directly make high moisture lignite meet electricity generation requirements and moisture in the lignite cannot be recovered. 
         [0003]    The drying system and the milling system employed in the above-mentioned conventional coal-fired power plant are two independent units, so the dried coal is transported to a coal bunker of the milling system via transportation equipment such as a scraper, a belt conveyor and a bucket elevator, etc. But there are problems as follows: 
         [0004]    (1) The temperature of the dried pulverized coal ranges from 60 to 80° C., thus a large amount of dust and steam are produced during transport. Due to the major amount of fine powder and lower moisture content, a large amount of dust is easy to produce when a belt conveyor, a scraper or the like is used for transport. As a consequence, the environment is contaminated and the operating environment is relatively bad; besides, fine pulverized coal would spontaneously ignite very easily, and even a serious accident like flash explosion would occur, thereby affecting the stability of transportation equipment. 
         [0005]    (2) The temperature of the hot coal, which has a temperature ranging from 60 to 80° C. after drying, is reduced to 30 to 60° C. through the step of transportation, and then the coal is fed into a coal mill and accordingly milled. The heat energy carried by the dried hot coal is wasted. According to calculation, the heat energy thus wasted accounts for 5 to 20% of the energy required by the milling system. 
         [0006]    (3) The dry exhaust produced by coal drying is dedusted and then directly discharged into atmosphere. The heat and water vapor contained in the exhaust are not recycled. 
       SUMMARY OF THE INVENTION 
       [0007]    To overcome the problems existing in the drying and milling of lignite in conventional coal-fired power plants, the present invention provides an apparatus for drying high moisture, low calorific value lignite for a generating set and recovering water contained therein, comprising a rotary steam tube dryer, a washing cooling tower, a coal mill, a 1st bag filter, a 2nd bag filter, a steam condenser, a weighing surge bunker, a water ring vacuum pump, a Venturi jet pump, a wet coal bunker, a nitrogen gas heater and a pulverized coal collecting tank; 
         [0008]    wherein the input end of the rotary steam tube dryer is connected with the wet coal bunker, the upper portion of the other end thereof is connected with the 1st bag filter and the lower portion is connected with the weighing surge bunker, the upper portion of the 1st bag filter is connected to the washing cooling tower, the weighing surge bunker is connected to the coal mill, one side of the coal mill is connected to the nitrogen gas heater, the top of the coal mill is connected to the 2nd bag filter, the upper portion of the 2nd bag filter is connected to the washing cooling tower and the other side thereof is connected to the inlet of the nitrogen gas heater, and the lower portion of the 2nd bag filter is connected to the pulverized coal collecting tank; the upper portion of the 1st bag filter is connected to the washing cooling tower, and the lower portion thereof is connected to the pulverized coal collecting tank; one side of the washing cooling tower is connected to the nitrogen gas heater. 
         [0009]    To solve the above problems, the present invention further provides a method for drying high moisture, low calorific value lignite for a generating set and recovering water contained therein, comprising the following steps: 
         [0010]    step 1: by means of Level 1 drying, wet coal is crushed, passed through a wet coal bunker, fed into a rotary steam tube dryer in sequence, and then heated and dried in heating steam and a dry carrier gas for a certain time to obtain coal particles B with a certain proportion of water and dry gas F at a certain temperature; the dry exhaust is fed into a 1st bag filter and dedusted therein to obtain exhaust D containing water vapor and pulverized coal T, the pulverized coal T is directly fed into a collecting tank, and at the same time the exhaust D is fed into a washing cooling tower; 
         [0011]    step 2: by means of Level 2 drying and milling, the coal particles B are fed into a surge bunker and then into a coal mill, and milled under preset conditions for a preset time to obtain the dried, pulverized coal C; after nitrogen gas P is heated by a nitrogen gas heater to a preset temperature, a portion of the nitrogen gas P is introduced into the coal mill to dry and mill the coal particles B so as to obtain pulverized coal C; the other portion of the nitrogen gas P is introduced into the coal mill through a sealing fan and sealed for later use; exhaust G discharged from the top of the coal mill is introduced into a 2nd bag filter and collected therein to obtain pulverized coal C and the dedusted exhaust H, wherein the pulverized coal C is directly fed into a pulverized coal collecting tank and collected therein, a portion of the exhaust H is sent back to the nitrogen gas heater and heated therein to a preset temperature, and then introduced into the coal mill, while the other portion of the exhaust H is fed into the washing cooling tower and treated therein; 
         [0012]    step 3: the exhaust D and the exhaust H are cooled and dehumidified in the washing cooling tower to form bottom liquid J in the bottom of the washing cooling tower, water vapour obtained by vacuum flash cooling of the bottom liquid J is dehumidified and then introduced into a steam condenser to be condensed, so as to form clean water M which is transported to a condensate water recovery tank and stored therein, and at the same time non-condensable gas that cannot be condensed is discharged into the air; 
         [0013]    step 4: a portion of nitrogen gas E discharged from the side of the washing cooling tower is introduced into a carrier gas heater and heated therein, and then introduced into the input end of a rotary steam tube dryer for use as a dry carrier gas; the other portion of the nitrogen gas E is introduced into the nitrogen gas heater and heated therein, and then introduced into the coal mill for use as a medium for drying and milling, so as to form a nitrogen gas closed loop system; 
         [0014]    step 5: the pulverized coal T and the pulverized coal C are mixed together in the pulverized coal collecting tank, then fed into the Venturi jet pump, mixed with air and burned in a boiler 25° C. 
         [0015]    The present invention has the following advantages over the prior art: 
         [0016]    1. The present invention makes it possible that after high moisture lignite is dried by a rotary steam tube dryer, the lignite with a moisture content as high as 61.3% generates electricity at full load, thereby meeting the operating requirements of coal-fired power plants and transforming low-quality coal which cannot be applied into steam coal which can generate electricity, so that the low-quality coal can be utilized effectively and the range of resource utilization is broadened. 
         [0017]    2. The present invention makes it possible that 95% of moisture in high moisture lignite is recovered and changed into clean water after washing, flashing, dehumidification and condensation, for use as make-up water recycling in power plants, thereby saving valuable water resources. 
         [0018]    3. All of the heat sources employed in the present invention come from steam extracted by a steam turbine. Utilization of the loss of cold sources in the system (i.e., utilization of latent heat of condensation after the work of high-pressure high-temperature steam) not only greatly reduces the energy consumption of a drying system but also can significantly reduce the power supply coal consumption of a generating set. 
         [0019]    4. In the present invention, a drying system is integrated with a milling system, every rotary steam tube dryer and the corresponding coal mill are disposed, i.e., the outlet of a rotary steam tube dryer is directly connected to a coal mill via a surge bunker, and the dried pulverized coal is milled in the coal mill after a buffer, thereby not only saving the heat lost during the transportation of pulverized coal, but also omitting a long-distance transportation from a conventional drying system to a conventional milling system, and shortening the process flow so as to effectively avoid such phenomena as dust pollution, waste and spontaneous combustion during transportation and transshipment. 
         [0020]    5. The present invention leaves out the intermediate, complex step of transportation, but only uses a surge bunker and a measuring rotary valve instead; meanwhile, the coal preparation system employed in the front-end process of drying is simplified, thereby saving the construction investment in coal bunkers of the original milling system and significantly reducing production costs. 
         [0021]    6. In the present invention, by virtue of the drying function of a conventional milling system, the drying system is divided into two levels: Level 1 drying using a steam rotary dryer and Level 2 drying using a milling system. Thus, the moisture content of high moisture lignite can be reduced to from 2 to 5%, thereby greatly increasing the calorific value of lignite. 
         [0022]    7. The coal drying system according to the present invention allows inert gases to be circulated and dried. A washing cooling tower recovers the water vapor contained in dry exhaust as clean water. After heating of the nitrogen gas discharged, it is returned to the Level 1 drying system, the Level 2 drying system and the milling system, and becomes a dry carrier gas and a heat source for Level 2 drying and milling, thereby achieving the closed circulation of the drying system and reducing energy consumption. Meanwhile, the oxygen content of the drying system is absolutely controllable; the dried exhaust (containing a large amount of water vapor) after dust collection is directly discharged into atmosphere, and the heat and water vapor contained in the exhaust are recycled. Dried exhaust will not be discharged into atmosphere any longer, thus coal drying systems in power plants will be safer and more environmentally friendly. 8. The present invention requires low investment, causes low energy consumption, has a high economic value, and is easy to carry out. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    The sole FIGURE is a process flow chart of the present invention, wherein within the dotted box is a conventional generating set. 
       
    
    
       [0024]    The following is a description of the drawing: 
         [0000]    
       
         
               
               
             
           
               
                   
               
             
             
               
                  1-Carrier gas heater 
                  2-3rd circulating fan III 
               
               
                  3-Wet coal bunker 
                  4-1st cleaning circulation pump 
               
               
                  5-2nd cleaning circulation pump 
                  6-Washing cooling tower 
               
               
                  7-Measuring belt scale 
                  8-Rotary seal valve 
               
               
                  9-Rotary steam tube dryer 
                 10-4th circulating fan IV 
               
               
                 11-Nitrogen gas heater 
                 12-Surge bunker 
               
               
                 13-1st measuring rotary valve I 
                 14-Coal mill 
               
               
                 15-Sealing fan 
                 16-5th circulating fan V 
               
               
                 17-Steam condenser 
                 18-Water ring vacuum pump 
               
               
                 19-Condensate water recovery tank 
                 20-1st circulating fan I 
               
               
                 21-1st bag filter I 
                 22-1st double-layer electric flap 
               
               
                   
                 valve I 
               
               
                 23-2nd bag filter II 
                 24-2nd circulating fan II 
               
               
                 25-Coal-fired boiler 
                 26-2nd double-layer electric flap 
               
               
                   
                 valve II 
               
               
                 27-Pulverized coal collecting tank 
                 28-2nd measuring rotary valve II 
               
               
                 29-Venturi jet pump 
                 30-Conveying fan 
               
               
                 31-Boiler drum 
                 32-Rotary joint 
               
               
                 33-Condensate storage tank 
                 34-Condensate pump 
               
               
                 35-Steam turbine 
                 36-Condenser 
               
               
                 37-Deaerator 
               
               
                   
               
             
          
         
       
     
       DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0025]    Hereinafter, the technical solution of the present invention is further described in detail with reference to the accompanying drawing. 
         [0026]    As shown in the sole FIGURE, an apparatus for drying high moisture, low calorific value lignite for a generating set and recovering water contained therein mainly comprises a rotary steam tube dryer  9 , a washing cooling tower  6 , a coal mill  14 , a 1st bag filter  21 , a 2nd bag filter  23 , a condenser  19 , a weighing surge bunker  12 , a water ring vacuum pump  18 , a Venturi jet pump  29 , a wet coal bunker  3 , a nitrogen gas heater  11 , circulating fans and a pulverized coal collecting tank  27 . The input end of the rotary steam tube dryer  9  is connected with the wet coal bunker  3  through a measuring belt scale  7  and a rotary seal valve  8 , the upper portion of the other end thereof is connected with the 1st bag filter  21  and the lower portion of the other end thereof is provided with the weighing surge bunker  12 , and the upper portion of the 1st bag filter  21  is connected to the washing cooling tower  6  via a 2nd circulating fan  20 ; the weighing surge bunker  12  is connected to the coal mill  14  via a 1st measuring rotary valve  13 ; one side of the coal mill  14  is connected to a 4th circulating fan  10  via the nitrogen gas heater  11  and the other side thereof is connected to a sealing fan  15 , and the top of the coal mill  14  is provided with the 2nd bag filter  23 ; one side of the upper portion of the 2nd bag filter  23  is connected to the washing cooling tower  6  via a 2nd circulating fan  24  and the other side is connected to the inlet of the nitrogen gas heater  11  via a 5th circulating fan  16 , and the lower portion is connected to the pulverized coal collecting tank  27  via a 2nd double-layer electric flap valve  26 ; the upper portion of the 1st bag filter  21  is connected to the washing cooling tower  6  via a 1st circulating fan  20 , and the lower portion is connected to the pulverized coal collecting tank  27  via a 1st double-layer electric flap valve  22 ; one side of the washing cooling tower  6  is connected to a carrier gas heater  1  via a 3rd circulating fan  2  and the other side is connected to the nitrogen gas heater  11  via a 4th circulating fan  10 , the bottom thereof is provided with a cleaning circulation pump  4  and a 2nd cleaning circulation pump  5 , and the top thereof is connected to the steam condenser  17 ; the top of the steam condenser  17  is connected to the water ring vacuum pump  18 , the middle portion of the steam condenser  17  is provided with a cool water inlet, the upper portion and the bottom thereof are each provided with a cool water backwater inlet, and the bottom is connected to a condensate water recovery tank  19 ; the carrier gas heater  1  is connected to the input end of the rotary steam tube dryer  9 ; the pulverized coal collecting tank  27  is connected to the Venturi jet pump  29  via a 2nd measuring rotary valve  28 ; one side of the Venturi jet pump  29  is connected to a conveying fan  30  which is connected to a coal-fired boiler  25 , and the other side thereof is connected to atmosphere; the upper portion of a rotary joint  32  at the tail end of the rotary steam tube dryer  9  is connected to the pump cylinder of a steam turbine  35 , and the lower portion thereof is connected to a condensate storage tank  33 ; the condensate storage tank  33  is connected to a condensate pump  34 , the condensate pump  34  is connected to a condenser  36 , the condenser  36  is connected to a deaerator  37 , and the deaerator  37  is connected to a boiler drum  31 . 
         [0027]    The carrier gas heater  1  plays a part in steaming new liquid and heating low-pressure steam, and transporting the steam to the rotary steam tube dryer  9 . Five circulating fans are used for compressing and transporting exhaust containing water vapor and nitrogen gas. The wet coal bunker  3  is provided with a hopper used for supplying the rotary steam tube dryer  9  with wet lignite. The two cleaning circulation pumps play a part in circulating the bottom liquid stored in the washing cooling tower  6  into a flash zone at the top of the washing cooling tower  6  for vacuum flashing under the action of the pumps; the washing cooling tower  6  plays a part in bringing the exhaust, which contains water vapor and nitrogen gas and which is introduced from the circulating fans, into vapor-liquid reverse contact with the overhead cool water cooled by vacuum flashing in the tower, for the purpose of cooling dehumidification. The measuring belt scale  7  plays a part in measuring wet coal in the wet coal bunker  3  and transporting the wet coal to the rotary steam tube dryer  9  via the rotary seal valve  8 . The rotary seal valve  8  plays a part in transporting the wet coal measured by the measuring belt scale  7  to the rotary steam tube dryer  9  relatively airtightly. The rotary steam tube dryer  9  is an inclined rotary cylinder, within which a number of tube arrays are arranged, wherein 2 to 7 layers of tube arrays are concentrically arranged, steam passes through the tubes and coal is passed through outside of tubes, the input end of the dryer  9  is provided with a coal particle inlet and a carrier gas inlet, and the output end of the dryer  9  is provided with a coal particle outlet, an exhaust outlet, a steam inlet and a condensate outlet. The nitrogen gas heater  11  is a device for heating low-pressure nitrogen gas to be transported to the washing cooling tower  6 . The surge bunker  12  plays a part in adjusting and balancing the coal particles dried by the rotary steam tube dryer  9 , and feeding them into the coal mill  14  via the measuring rotary valves. The two measuring rotary valves play a part in measuring the coal weighed and dried by the surge bunker  12 , and feeding them into the coal mill  14 . The coal mill  14  mills the coal particles dried by the rotary steam tube dryer  9  into fine pulverized coal, and the sealing fan  15  plays a part in filtering nitrogen gas and introducing it into the coal mill  14  for later use. The two bag filters filter pulverized coal and the dry exhaust containing water vapor, the pulverized coal is fed into the collecting tank, and the dry exhaust containing nitrogen gas, water vapor and a small amount of air is introduced by the circulating fans into the washing cooling tower  6 . The condenser  17  plays a part in performing vacuum flashing in the flash zone at the top of the washing cooling tower  6 ; water vapor K obtained by flash distillation is dehumidified and then fed into the condenser  17  by means of the pumping action of a water ring vacuum pump  18 , the water vapor K is condensed in the condenser  17  and accordingly transformed into clean water M at a temperature below 40° C., and the clean water M is directly fed into a condensate water recovery tank  19  and stored therein. The water ring vacuum pump  18  is used for pumping the water vapor obtained by flash distillation at the top of the washing cooling tower into condenser  17  for condensation. The condensate water recovery tank  19  is used for storing the condensed water produced from the condenser  17 . The two double-layer electric flap valves play a part in quickly discharging the pulverized coal filtered by the bag filters into a pulverized coal collecting tank  27  by electric control. The pulverized coal collecting tank  27  plays a part in collecting the water vapor, nitrogen gas and pulverized coal filtered by the 2nd bag filter  23 . The Venturi jet pump  29  plays a part in injecting a mixture of the pressurized air and the pulverized coal into a boiler via a nozzle for combustion. The conveying fan  30  plays a part in emitting the purified exhaust into the atmosphere. 
         [0028]    In order to meet the requirements of coal-fired power plants for utilization of lignite, the present invention puts forward Level 2 drying of lignite. That is to say, the rotary steam tube dryer  9  is responsible for Level 1 drying, the coal mill  14  is responsible for Level 2 drying, the heat source for Level 1 drying is 0.3 to 2.0 MPa and has a temperature of 120 to 360° C., the heat source for Level 2 drying is hot nitrogen gas at 150 to 200° C., the heat source for Level 1 drying is steam extracted by the steam turbine  35 , and after heat transfer, the steam is changed into a condensate which is then returned to the deaerator  37  to form a closed cycle of a heat source for drying; after drying, milling, dedusting, washing and other process steps, the heat source for Level 2 drying is returned to the nitrogen gas heater  11  via the 4th circulating fan  10  to form a closed cycle of a heat source for drying and milling. 
         [0029]    In order to control the flow of lignite, each of the coal mill  14 , the 1st circulating fan  20 , the 2nd circulating fan  24 , the circulating fan III  2 , the circulating fan IV  10 , the circulating fan V  16 , the sealing fan  15 , the vacuum pump  18 , the 1st cleaning circulation pump  4  and the 2nd cleaning circulation pump  5  according to the present invention is provided with a flowmeter  5 . In addition, all of the inlets of the rotary steam tube dryer  9 , the coal mill  14  and the Venturi jet pump  29  are provided with measuring and weighing devices. 
         [0030]    The carrier gas heater  1  and the nitrogen gas heater  11  are liquid-gas type finned tube heaters for heating using hot water or tubular heat exchangers. 
         [0031]    The washing cooling tower  6  is any one of a packed tower, a plate tower or a spray tower. 
         [0032]    The bag filter I  21  and the bag filter II  23  are efficient, offline nitrogen back blow filters. 
         [0033]    Hereinafter the steps of the method adopted for the drying milling and water recovery devices of a power generation system using a pulverized coal furnace, in which high moisture, low calorific value lignite burns, are described as follows: 
         [0034]    (1) After wet coal A with a moisture content of 25% to 62% is crushed such that it had a particle size ≦20 mm, it is fed into the wet coal bunker  3  and then into the rotary steam tube dryer  9  through the measuring belt scale  7  and the rotary seal valve  8 , and coal particles B with a moisture content ≦15% after drying and dry exhaust F at a temperature of 90 to 110° C. are obtained after drying for 30 to 60 min on the condition that the heating steam had a steam pressure of 0.3 to 2.0 MPa and a temperature of 120 to 360° C. and the dry carrier gas had a temperature below 120° C. (carrier gas flow: 15000 to 35000 Nm3/h), i.e., under the conditions of nitrogen gas N; the coal particles B are fed into the surge bunker  12 , and the dry exhaust F is fed into the 1st bag filter  21  and dedusted therein at a pressure of −200 to +500 Pa and a temperature of 90 to 110° C. to obtain exhaust D containing water vapor and pulverized coal T, the pulverized coal T is directly fed into the collecting tank  27 , and the exhaust D containing nitrogen gas, water vapor and a small amount of air is introduced by the 1st circulating fan  20  into the washing cooling tower  6  to complete the process of Level 1 drying. 
         [0035]    (2) After the coal particles B are fed into the surge bunker  12 , they are measured through a weighing device disposed in the middle portion of the surge bunker  12  and a measuring rotary valve  13  disposed in the lower portion of the surge bunker  12 , and then fed into the coal mill  14 . After milling at a hot-air pressure of 2000 to 6000 Pa and a temperature of 180 to 200° C. for 6 to 25 S, pulverized coal C with a moisture content ≦2 to 5% after drying are obtained; nitrogen gas P is blown into the nitrogen gas heater  11  through the 4th circulating fan  10  and then is heated thereby to 180 to 200° C., wherein 90% of the hot nitrogen gas P is fed into the coal mill  14  to dry and mill the coal particles B to obtain pulverized coal C, and 10% of the hot nitrogen gas P is introduced into the coal mill  14  through the sealing fan  15  and sealed for later use; exhaust G cooled to 90 to 110° C. after drying and milling is discharged from the top of the coal mill  14 , the exhaust G containing large amounts of pulverized coal C, water vapor and nitrogen gas is introduced into the 2nd bag filter  23  via the 2nd circulating fan  24  and collected in the 2nd bag filter  23  to obtain pulverized coal C and the dedusted exhaust H containing water vapor and nitrogen gas; the pulverized coal C is directly fed into the pulverized coal collecting tank  27 ; 15% of the exhaust H is directly sent back to the inlet of the nitrogen gas heater  11  via the 5th circulating fan  16  and then mixed with the nitrogen gas E sent back by the 4th circulating fan  10 , the resulting mixture is heated to 180 to 200° C. and then directly introduced into the coal mill  14  for later use, wherein after 85% of the exhaust H is fed into the washing cooling tower  6  via the 2nd circulating fan  24  and treated in the washing cooling tower  6 , the process of Level 2 drying and milling is completed. 
         [0036]    (3) After the exhaust D containing nitrogen gas, water vapor and a small amount of air and the exhaust H containing water vapor and nitrogen gas are introduced into the washing cooling tower  6 , they are brought into vapor-liquid reverse contact with the cool water cooled by flash distillation in the flash zone at the top of the washing cooling tower 6 to 40 to 60° C. in the washing cooling tower  6 , so that they are cooled and dehumidified. The large amount of water vapor in the exhaust D and the exhaust H is condensed and accumulated in the bottom of the washing cooling tower  6  to form bottom liquid J at a temperature ranging from 80 to 90° C.; the bottom liquid J at 80 to 90° C. stored in the bottom of the washing cooling tower  6  is fed into the flash zone at the top of the washing cooling tower  6  for vacuum flashing under the action of the 1st cleaning circulation pump  4  and the 2nd cleaning circulation pump  5 , water vapor K at 40 to 70° C. flashed off is dehumidified and then fed into the condenser  17  by means of the pumping action of the water ring vacuum pump  18 , the water vapor K is condensed in the condenser  17  and accordingly transformed into clean water M at a temperature below 40° C., the clean water M is directly fed into the condensate water recovery tank  19  and stored therein, and non-condensable gas that could not be condensed is evacuated from the water ring vacuum pump  18 ; the bottom liquid J at 40 to 60° C. obtained after flash cooling is fed into the distributor of the washing cooling tower  6  via a liquid level control system, and is brought into reverse contact with the exhaust G and the exhaust H again to be cooled and dehumidified so as to form a closed loop system; the cooling medium of the condenser  17  is circulating cool water; after the recovery of the water vapor in the exhaust G and the exhaust H, the remaining nitrogen gas E is recycled to Level 1 and Level 2 drying systems. 
         [0037]    (4) The temperature of the nitrogen gas E discharged from the side of the washing cooling tower  6  after dehumidification and cooling ranged from 45 to 65° C., wherein 35% of the nitrogen gas E is pressurized by the 3rd circulating fan  2  to form nitrogen gas N, which is blown into the carrier gas heater  1  and heated therein to 120° C. or less, and then introduced into the input end of the rotary steam tube dryer  9  for use as a dry carrier gas, 65% of the nitrogen gas E is pressurized by the 4th circulating fan  10  to form nitrogen gas P, which is blown into the nitrogen gas heater  11  and heated therein to 180 to 200° C., and then introduced into the coal mill  14  for use as a medium for drying and milling, so as to form a nitrogen gas closed loop system, and a loss of nitrogen cycle of 5% or less is supplemented by an external system. 
         [0038]    (5) After the pulverized coal T and the pulverized coal C discharged from the 1st bag filter  21  and the 2nd bag filter  23  into the pulverized coal collecting tank  27  are mixed together, the resulting mixture is discharged from the measuring rotary valve  28  and fed into the Venturi jet pump  29 . By virtue of the pressurization of the conveying fan  30 , air is introduced into the Venturi jet pump  29  and mixed with the pulverized coal, and then the resulting mixture is blown into the boiler  25  for combustion. 
         [0039]    (6) All of the heating media employed in the carrier gas preheater  1 , the steam rotary dryer  9  and the nitrogen gas heater  11  came from steam extracted by the steam turbine  35 , and they are pressurized by the condensate pump  34  and sent back to the deaerator  37  via the rotary joint  32  and the condensate storage tank  33 .