Method for producing pulverized coal

Abstract

Method for producing pulverized coal, the method comprising the steps of heating a drying gas, preferably an inert gas, in a hot gas generator ( 26 ) to a predefined temperature; feeding the heated drying gas into a pulverizer ( 20 ); introducing raw coal into the pulverizer ( 20 ), the pulverizer ( 20 ) grinding the raw coal into pulverized coal; collecting a mixture of drying gas and pulverized coal from the pulverizer ( 20 ) and feeding the mixture to a filter ( 34 ), the filter ( 34 ) separating the dried pulverized coal from the drying gas; collecting the dried pulverized coal for further use and feeding the drying gas from the filter ( 34 ) to a recirculation line ( 38 ) for returning at least part of the drying gas to the hot gas generator ( 26 ); and determining an oxygen level in the drying gas, preferably in the recirculation line ( 38 ), and comparing the determined oxygen level to a predetermined oxygen level threshold. According to a preferred embodiment of the invention, if the determined oxygen level is higher than a predetermined oxygen threshold, water is injected into the heated drying gas before it is fed into the pulverizer ( 20 ), the volume of water injected being calculated so as to reduce the oxygen level below the predetermined oxygen level threshold.

1 . Method for producing pulverized coal, the method comprising the steps of:
 heating a drying gas in a hot gas generator to a predefined temperature;   feeding the heated drying gas into a pulverizer;   introducing raw coal into the pulverizer, the pulverizer turning the raw coal into pulverized coal;   collecting a mixture of drying gas and pulverized coal from the pulverizer and feeding the mixture to a filter, the filter separating the dried pulverized coal from the drying gas;   collecting the dried pulverized coal for further use and feeding the drying gas from the filter to a recirculation line for returning at least part of the drying gas to the hot gas generator   determining an oxygen level in the drying gas and comparing the determined oxygen level to a predetermined oxygen level threshold   wherein the oxygen level in the drying gas is determined during a grinding cycle, in which heated drying gas is fed through the pulverizer and raw coal is introduced into the pulverizer, and   if, during the grinding cycle, the determined oxygen level is higher than the predetermined oxygen level threshold, a volume of water is injected into the heated drying gas before it is fed into the pulverizer, the volume of water injected being calculated so as to reduce the oxygen level below the predetermined oxygen level threshold.   
     
     
         2 . Method according to  claim 1 ,
 wherein, in the recirculation line, a volume of fresh air is injected into the drying gas, and   wherein, if the determined oxygen level is higher than the predetermined oxygen level threshold, the volume of fresh air injected into the drying gas is reduced.   
     
     
         3 . Method according to  claim 2 , wherein,
 if the volume of fresh air injected reaches zero and the oxygen level is still higher than the predetermined oxygen threshold, a volume of water is injected into the heated drying gas before it is fed into the pulverizer, the volume of water injected being calculated so as to reduce the oxygen level below the predetermined oxygen level threshold.   
     
     
         4 . Method according to  claim 1 , wherein the predetermined oxygen threshold is chosen to be between 0 and 14 volume %. 
     
     
         5 . Method according to  claim 4 , wherein the predetermined oxygen threshold is chosen to be between 5 and 12 volume %. 
     
     
         6 . Method according to  claim 1 , comprising:
 determining an exit temperature of the mixture of drying gas and pulverized coal exiting the pulverizer; and   controlling the exit temperature by controlling a volume of water injected into the heated drying gas before feeding it into the pulverizer, the volume of water injected being calculated so as to bring the exit temperature to a preferred working temperature.   
     
     
         7 . Method according to  claim 6 , wherein the method comprises:
 a startup cycle wherein heated drying gas is fed through the pulverizer without introducing raw coal, the exit temperature being kept below a first temperature threshold, and   a grinding cycle wherein heated drying gas is fed through the pulverizer and raw coal is introduced into the pulverizer, the exit temperature being kept at a preferred working temperature,   wherein
 during the startup cycle, said drying gas is heated to a temperature above the first temperature threshold and a volume of water is injected into the heated drying gas, the volume of water being calculated so as to reduce the temperature of the heated drying gas to obtain an exit temperature below the first temperature threshold; and 
 at the beginning of the grinding cycle, the volume of water injected into the heated drying gas is reduced so as to compensate for a drop in exit temperature. 
   
     
     
         8 . Method according to  claim 6 , wherein the volume of water injected into the heated drying gas is reduced at a rate determined by the exit temperature. 
     
     
         9 . Method according to  claim 1 , wherein the volume of water injected into the heated drying gas is reduced at a rate determined by a pressure drop measured across the pulverizer. 
     
     
         10 . Method according to  claim 7 , wherein, during the grinding cycle and after compensation for the drop in exit temperature, the method comprises the steps of:
 reducing the heating of the drying gas; and   reducing the volume of water injected into the heated drying gas to maintain the preferred exit temperature.   
     
     
         11 . Method according to  claim 1 , wherein, in the recirculation line, at least part of the drying gas is extracted as exhaust gas. 
     
     
         12 . Method according to  claim 1 , wherein, in the recirculation line, fresh air and/or hot gas is injected into the drying gas. 
     
     
         13 . Method according to  claim 1 , comprising:
 continuous monitoring of the exit temperature and comparing the measured exit temperature to a maximum temperature; and   if the measured exit temperature exceeds the maximum temperature, increasing the volume of water injected into the heated drying gas.   
     
     
         14 . Method according to  claim 1 , wherein the drying gas is heated in a hot gas generator powered by a lance burner. 
     
     
         15 . Method according to  claim 1 , wherein water is injected into the heated drying gas by means of a water injection device arranged between the hot gas generator and the pulverizer. 
     
     
         16 . Method according to  claim 7 , wherein the volume of water injected into the heated drying gas is reduced at a rate determined by the exit temperature.