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    Coal Grinding Technology (2nd Part)            
                       
    5    Reasons for grinding coal            
        - The ground coal allows more efficient combustion, because in the process one will achieve two important purposes on coal:
          * The dehydration since moisture reduction has a positive effect on the rate of heat transfer      
          * The increase of specific surface area, i.e. the carbon surface exposed to combustion per unit volume    
        - The maximum fresh size of coal is usually between 20 and 75 mm. Up to a size of 75mm, the ability of some mills is not affected
          while others show sensitivity all the time.            
        - Finer humid coal with a size of 3,35 mm or less can be the main cause of blockages, particularly when its content exceeds  
          25% to 30%H2O by mass.            
        - The particle size distribution varies a lot depending of the type of coal, extraction method, coal cleaning, crushing equipment
          and the degree of handling and manipulation.            
        - In the coal grinding process, coal is reduced up to 80% passing on 75µ and this powder is then taken out of the mill through a 
          pneumatic transport system, in a mixture of coal dust air.          
        - During the milling process and transportation, coal is dried to optimize its fuel properties.      
        - Two types of mills are used in the coal grinding process: Ball mills and Vertical roller mills.      
                       
    6    Ball mills            
      6.1 Ball mill description:            
        - Ball mills used for coal grinding process are always airswept mills. It means that all the material at mill discharge is transported
          to the separator by pneumatic system (No bucket elevator at mill outlet).        
        - There are two compartments, 1 drying chamber with lifters to allow a good drying efficiency and a grinding chamber, longer.
        - Ratio Length/Diameter only with grinding chamber: 1,2 to 2          
          Ratio Length/Diameter with grinding and drying chambers: 2 to 3          
        - Speed of the mill: from 72-74% of the critical speed for bigger diameters to 76-78% of the critical speed for smaller diameters.
        - Filling degree: from 24 to 28%            
        - Even if the VRM is nowadays the preferred solution, new ball mills plants still emerge (we saw one new installation in  
          erection in July of 2013).            
        - Sketch of an airswept mill:            
         
     
           
        - Generally, the lining of the grinding chamber is a classifying lining. But in some extreme cases, the first part of the chamber
          can have a lifting lining in order to allow bigger balls and a better crushing efficiency (big granulometry and/or high   
          hardness of coal).            
        - The ball charge gradation is normally calculated with balls of diameter 60-50-40-30-25-20-17-15mm.     
          In some case like explained here above, bigger balls are used.          
        - The efficiency of the ball mill for coal grinding is  proportional to the coal grindability (HGI). The following chart gives an idea
          of the specific power consumption (kWh/t) of the mill itself in function of the HGI for different residues on 90µ.   
          These values are only indicative!            
         
     
         
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      6.2 Ball mill circuit:            
        - It is a closed circuit with a static separator. The rejects of the separator go back to the mill inlet and are mixed with the fresh 
          wet feed. Fines will go to cyclones. Depending if the system is direct or indirect firing, the product of cyclones goes directly
          to the firing point or is stocked in a silo.            
      6.3 Direct firing system:            
        - Fuel is ground and blown directly into the burner (Mill sweep is primary air - Non-inert system).      
        - There is not pulverized fuel storage.            
        - Advantage of simplicity for some systems.            
        - This system can be less efficient due to higher primary air used.          
        - The mill must operate at all times when burning coal.            
        - The kiln must be stopped during mill downtimes.            
        - This system is considered safer than other arrangements.          
        - Only few coal grinding plants in the cement industry are arranged for direct firing.        
        - Flow-sheet of a typical direct firing system:            
         
     
       
      6.4 Semi-Direct firing system:            
        - Same as direct firing system except,            
        - There is a silo between the cyclone product and the kiln.            
        - The kiln is less dependent on the coal grinding circuit.            
        - Flow-sheet of a typical semi-direct firing system:            
         
     
       
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      6.5 Indirect firing system:            
        - Indirect firing systems are today the most common in cement plants.          
        - A product silo and a filter for dedusting the mill air are included in the circuit.        
        - Pulverized fuel is then metered to the burner as required.          
        - The system then requires dust collection which can become a fire/explosion risk.        
        - There is a need for high safety precaution.            
        - High capital cost of the equipment.            
        - Better control of flame shape due to independent primary air control.          
        - The coal mill can be stopped for maintenance independent of firing the burner.        
        - The coal grinding system can feed various kilns.            
        - Flow-sheet of a typical indirect firing system:            
         
     
     
                       
    7    Vertical Roller mills (VRM)            
      7.1 Vertical Roller mill description:            
        - Within the cement industry, Vertical Roller mills (VRM) are the predominant equipment used for rawmeal process  
          for two decades.            
        - Now, this predominance also exists for grinding coal units.          
        - In two words, VRM is a very compact unit which allows 4 functions: drying, grinding, separation and transport in one piece.
        - Sketch of a VRM:            
         
     
             
        - As the ball mill, the efficiency of the VRM for coal grinding is  proportional to the coal grindability (HGI). The following chart 
          gives an idea of the specific power consumption (kWh/t) of the mill itself in function of the HGI for different residues on 90µ. 
          These values are only indicative!            
         
     
         
      7.2 VRM direct firing system:            
        - Fuel is ground and blown directly into the burner (Mill sweep is primary air - Non-inert system).      
        - There is not pulverized fuel storage.            
        - Advantage of simplicity for some systems.            
        - This system can be less efficient due to higher primary air used (up to 30%).        
        - The mill must operate at all times when burning coal.            
        - The kiln must be stopped during mill downtimes.            
        - Sampling for fineness control is difficult.            
        - This system is considered safer than other arrangements.          
        - Only few coal grinding plants in the cement industry are arranged for direct firing.        
        - Flow-sheet of a typical direct firing system:            
         
     
           
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      7.3 VRM semi-direct firing system:            
        - A modified version of the direct firing system.            
        - Flow-sheet of a semi-direct firing system:            
         
     
         
      7.4 VRM indirect firing system:            
        - Indirect firing systems are today the most common in cement plants.          
        - A product silo and a filter for dedusting the mill air are included in the circuit.        
        - Pulverized fuel is then metered to the burner as required.          
        - Water vapour from coal drying is not introduced into the kiln.          
        - Simple flame control.            
        - Low primary air ratio.            
        - The system then requires dust collection which can become a fire/explosion risk.        
        - There is a need for high safety precaution.            
        - High capital cost of the equipment.            
        - Better control of flame shape due to independent primary air control.          
        - Sampling for fineness control is easy.            
        - The coal mill can be stopped for maintenance independent of firing the burner.        
        - The coal grinding system can feed various kilns.            
        - Flow-sheet of a typical indirect firing system:            
         
     
       
      7.5 VRM semi-indirect firing system:            
        - A modified version of the indirect firing system.            
        - Flow-sheet of a semi-indirect firing system:            
         
     
       
                       
    8    Comparison Ball mills versus Vertical mills for coal grinding        
      8.1 Advantages of the VR mill in comparison with a ball mill:     Ball mills:    
        - VRM can handle materials with sizes greater than 85mm     up to 25mm    
        - VRM mill can remove more than 20% moisture.     up to 12-15%    
        - Vertical roller mills plants use less space            
        - Grinding plants VR mills have lower specific energy consumption (15-40%)        
        - VRM are more flexible, the capacity can easily be varied from 40 to 100%        
          of the nominal capacity            
        - Lower noise level for VRM grinding plants            
      8.2 Disadvantages of the VR mill in comparison with a ball mill:            
        - Initial investment of VRM is higher            
        - Possibility of expansion is not possible with VRM circuits            
        - Higher maintenance costs for VRM            
        - VRM are very sensitive to variations in feed rate and feed quality          
        - A roller mill requires more air to carry the pulverized material than a ball mill        
        - VRM is less interesting in case of high coal abrasiveness because increase         
          of wear and downtimes            
        - VRM is less interesting in case of hard coal (petcoke with HGI < 30)          
                       
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