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    Coal Grinding Technology (3rd Part)            
                       
    9    Drying problems            
      9.1 Preliminary considerations:            
        - As mentioned in the first part of this presentation, coal has 2 types of moisture (inherent and free) and coal has always a   
          moisture content at mill plant inlet.            
        - Almost all grinding coal systems incorporate moisture removal from the coal as part of the circuit.    
        - VRM have a better drying efficiency than ball mills. VRM can dry material with a moisture higher than 20%.    
          At the contrary, ball mills can dry up to 12-15% maximum.          
        - The drying portion of the system requires serious attention in design since the possibility of fires and explosions  
          increases as the coal becomes finer, dryer and hotter.            
        - High moisture coals require more air for drying, or the mill must be operated at a high inlet temperature.    
          This second alternative is dangerous and must be avoided.          
        - Indirect firing system is more beneficial for plants using very wet coal.        
      9.2 Moisture content limit:            
        - It i s recommended that coal dust moisture be no less than 1-1,5% at any point in the "grinding-drying-transport-firing" circuit.
          This 1-1,5% level does not apply to subbituminous and lignite coals with high inherent moisture content.    
          These are often dried with 4-8% retained moisture.            
        - Some figures of moisture content limit:            
         
     
               
      9.3 Temperature at mill inlet:            
        - The drying air should not be too hot to cause premature ignition of coal fines (Reactive coal ignites at a lower temperature). 
        - It is further recommended that the drying gas inlet temperature be no higher than 250-300°C if the drying gas oxygen  
          content is 21% by volume.            
        - Some figures of inlet gas temperatures:            
          
     
               
        - Systems operating at low dew points (0°C to 10°C above ambient) can avoid condensation by operating at 10°C above   
          dew points.            
        - Systems operating at higher dew points (greater than 10°C above ambient) should be operated at 15-20°C above dew point.
      9.4 Temperature at mill outlet:            
        - It is recommended that the temperature of dust coal/gas mixture be limited to 65°C for indirect systems and 80°C for 
          direct firing systems. These temperatures do not apply for inert situations (see later for inert systems).    
        - The outlet temperature is also depending of the type of coal. See the diagram here below:      
         
     
         
      9.5 Calculation of the drying capacities:            
        - These calculations must take into account the limits described here above.        
        - Methods presented on the following link can be applied for coal drying process:        
          http://www.thecementgrindingoffice.com/dryingcapacitiescalculations.html      
        - A summary of coal drying calculations is presented on the next page in form of graphics.      
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    10    Fineness of the dust coal            
        - Fineness depends of the type of coal and the area where coal has to be burned.        
        - A bad coal fineness can bring problems:            
          * Formation of CO            
          * Increase of the volatility of sulfur            
        - Residues on 90µ (#170Mesh) and 200µ (#70Mesh) are generally considered for getting the fineness target.    
        - Here below, a diagram of the fineness in function of the coal type (% of volatiles):        
         
     
           
        - An other sinple way to define the fineness in function of the coal type is presented in the following table:    
         
     
     
        - The residue on 200µ must be as small as possible because coarser particles will ignite later.      
        - Particle size has an inverse relationship to spontaneous heating of coal. The smaller the coal particle the greater is   
          the exposed surface area and the greater is the tendency toward spontaneous heating.      
          This is very critical for anthracite and petcoke grinding.            
          It is the reason that coal grinding plants must be designed with high safety levels.        
                       
    11    Safety considerations            
      11.1 Introduction:            
        - There is always a potential risk of fires and explosions when operating with coal.        
        - Some reminders:            
          The fire triangle            
         
     
             
          The explosion pentagon            
         
     
             
        - In addition to traditional safety equipments, inert operating systems are a good solution to diminish the problem of fires
          and explosions.            
      11.2 Inert systems:            
        - Inert systems can avoid dust explosions and smouldering fires in silos, coal mills and filter equipment by creating an inert  
          atmosphere.            
        - It is well known that explosions occur in case of oxygen content higher than 12-14%.      
        - The oxygen concentration below which a deflagration can occur is the limiting oxygen concentration (LOC).    
        - A system is inert if the oxygen concentration < LOC.            
        - LOC for coal dust is 12% according NFPA (USA).            
          Gas quality for inert conditions should be low moisture, contain sufficient heat to dry the coal and source should be   
          reasonably close.            
        - Preheater and precalciner cement kiln gas (300-350°C and 4% O2) are good quality gases.      
        - In an inert system, If the oxygen content exceeds 10%, an oxygen analyzer causes an acoustic alarm.    
          At 12% O₂, the grinding plant will stop operation automatically.          
          Before the mill start, it is necessary to decrease the oxygen content after the filter to less than 10%.    
        - In the closed loop system, the gas from the filter is partly re-circulated to the inlet of the mill. This creates an inert  
          condition which minimizes the oxygen content in the system and reduces the chances of coal dust explosions.  
        - Even with inert operating conditions, the cyclone and dust collector should be equipped with pressure relief valves.  
      11.3 Non-inert systems:            
        - Oxygen content of mill sweep gas is usually above 12%            
        - In this case, oxygen analyzers not needed            
        - Non-inert systems allow using excess air from the clinker cooler or from the hot gas generator.      
                       
    12    Dust collectors            
        - The combining phenomena of comminution, handling and processing of the coal leads to the generation of large volumes 
          of dust loaded gases.            
        - To prevent the dust from escaping into the environment two types of dust collectors are used in cement plants:  
          * Bag filters            
          * Electrostatic Precipitators            
          Bag filter is often selected for coal grinding plants due to the following reasons:        
          * Lower initial investment             
          * Lower operating costs            
          * Less space requirement            
          * Good for gas temperature and dust collection efficiency          
                       
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