Please fill all required fields
    All rights reserved © 2012-2015 The Cement Grinding Office
    Coal Grinding Technology (1st Part)            
    1    Introduction            
        - Noble fuels are coal, fuel oil and natural gas.            
        - Due to the increase of the oil prices in the last decades, coal usage as primary fuel source in the cement industry has grown
          in large proportions.            
        - For safety reasons, coarse coal is always supplied on site.           
        - Then, a grinding/drying plant is necessary to convert the coal into valuable fuel.        
        - Before choosing and sizing the coal grinding circuit, we have to take into consideration the following factors:    
          * Coal quality            
          * Hot air sources for coal drying            
          * Number of fuel consumption points            
          * Type of kiln and available space, especially when installing a new plant to an existing kiln      
          * Industrial safety systems            
        - Concerning the machinery and the system, a choice must be made between:        
          * Ball mill and Vertical Roller mill            
          * Direct and indirect firing system            
          * Operation of the system in inert or non-inert condition            
          * Bag filter or electrostatic precipitator for dust removal purposes          
        - These points will be analysed in the following chapters.            
    2    Types of coals            
      2.1 Introduction:            
        - Coal is a readily combustible rock containing more than 50 percent by weight of carbonaceous material formed   
          from compaction and indurations of variously altered plants and similar to those in peat.      
        - See the classification of coal after.            
      2.2 Classification of coal:            
        - Coal can be classified in four categories: Peat, Lignite, Bituminous and Anthracite.        
      2.3 Peat:            
        - Peat is the first stage in the conversion of vegetable matter to coal.          
        - Peat is an organic fuel consisting of spongy material formed by the partial decomposition of organic matter, primarily  
          plant material,             
          in wetlands such as swamps, muskegs, bogs, fens, and moors (from Encyclopædia Britannica).      
        - Over time, the formation of peat is often the first step in the geological formation of other fossil fuels such as coal,   
          particularly low grade coal such as lignite.            
        - With color ranges from yellowish brown to black, peat has a low fixed carbon content of between 50 and 55%,  
          a lower calorific 2000 kCal/kg and a high humidity, so it is very bad fuel.        
      2.4 Lignite:            
        - This variety of coal intermediate quality between peat and bituminous coal has a higher carbon content than peat,   
          between 55 and 65%.            
        - It usually black brown colored with a fibrous and woody structure.           
        - It has a lower heat capacity than the common coal, 4100 kCal/kg.          
        - The high volatile matter content (19%) can cause disintegration of the lignite exposed to air.      
      2.5 Sub-bituminous coal:            
        - Sub-bituminous coal is between lignite and bituminous coal and are used primarily as fuel for steam-electric power generation.
        - This coal is from dark brown to black.            
        - Its heat content is between 4600 and 6400 kCal/kg.            
      2.6 Bituminous coal (hard coal):            
        - Bituminous coal is the most common coal.            
        - It has an even higher carbon content of between 60 and 80%, so that their heat capacity is also higher.     
        - Bituminous coal ignites easily and burns long with a relatively long smoky yellow flame.      
        - It differs from lignite by its higher calorific value between 7200 and 8600 kCal/kg.        
        - Bituminous coal can be divide in two sub-groups:            
          * Thermal coal because it is used to fire power plants that produce steam for electricity and industrial uses.    
          * Metallurgical coal because it is used in the process of creating coke necessary for iron and steel-making.    
      2.7 Anthracite:            
        - It is the best of the coals, with a very low pollution and high calorific value of at least 8600 kCal/kg.    
        - Anthracite is very pure and contains 75 to 95% carbon. After that, it is graphite which is almost pure carbon and which   
          is incombustible.            
        - Anthracite ignites with difficulty and burns with a short, blue, and smokeless flame.      
    3    Coal properties            
      3.1 Introduction:            
        - The degree of metamorphism or carbonization of the coal, from the state of peat up to anthracite, has an important   
          relationship with its properties, and is known as the class or rank of coal.        
        - The low-rank coals, such as lignite and sub​​-bituminous coals are typically softer, friable, opaque with appearance of earth, 
          are characterized by high humidity and low carbon content, and therefore, low energy.      
        - High rank coals are typically harder and stronger.            
        - The increase in the range is accompanied by an increase in carbon content of the coal and energy, as well as a decrease  
          in the moisture level.            
        - The most important properties used for evaluation of coal are treated below.        
      3.2 Heat of combustion:            
        - Is the amount of heat released per unit mass or unit volume of a substance when the substance is completely burned.  
        - Can influence significantly on the requirements of handling and spraying equipments and storage.    
      3.3 Specific heat:            
        - The specific heat is the amount of heat per unit mass of coal required to raise the temperature by one degree Celsius.  
        - This depends on the water content and ash.            
      3.4 Moisture:            
        - The moisture content of coal is due to 2 types of moistures: inherent and free.        
        - Inherent moisture is the one retained in the pores of the coal and is a characteristic of the coal type.    
        - Free moisture is the one on the carbon surface and in the interstices between the particles and is the result    
          of the waters of the mine, the water for washing and the rain.          
      3.5 Granulometry:            
        - For safety reasons (spontaneous combustion and explosion), coal arriving in the cement plants are coarse.    
        - It is then very important to know the granulometry of the coal and its rank for dimensioning the machinery required to grind it.
        - The particle size distribution of milled coal depends partly on the rank of the coal which determines its brittleness.  
      3.6 Size stability:            
        - It is the ability of coal to resist to breakage during handling or transportation.        
        - This stability is determined by a drop-shatter test which consists in a method involving a dropping predetermined size coal 
          onto a steel plate from a specified height.            
      3.7 Volatiles Content:            
        - This factor affects the combustibility of coal powder and the required degree of grinding because the fineness required   
          increases if the content of volatile is decreasing.            
        - The coals with low volatile content have a less easy ignition and need more time for combustion.    
      3.8 Grindability:            
        - This factor is determined by the Hardgrove Grindability Index (see link here below).      
      3.9 Abrasivity:            
        - It is necessary to know the abrasiveness of the coal in order to predict the wear of the different parts in the grinding  
        - This abrasiveness is obtained using a laboratory test according to British Standard BS1016, part 19.    
        - The index unit is milligrams of metal blades that eroded per kilogram of coal samples used (mg/kg).    
        - Some figures:            
      3.10 Density:            
        - Coal density varies with the size, the moisture content and the coal settlement ability when it is stacked.    
        - Most ranks of coal are low in density (about 700 to 1800 kg/m3) in comparison with other rocks.      
      3.11 Ash content:            
        - The ash content of coal is an important measure for the coal quality.          
        - The ash is defined as the non-combustible part of the coal.          
      3.12 Summary table of coals:            
        - This table gives the following characteristics of the different coals.          
    4    Petcoke            
        - Petroleum coke is a solid substance composed mainly of carbon and is a by-product of oil refining.    
          Petroleum coke is used as a source of energy as well as coal.          
        - One particularly interesting property of petcoke is its high heat of combustion (about 8800 kCal/kg).    
          Its ash content is also very low compared to coal            
        - Different petcokes forms available are:            
          * Green coke usually refers to calcineable coke, i.e, potential anode quality coke that still contains moisture and volatile   
          material, i.e, before calcining.             
          * Calcinated coke usually refers to petroleum coke or green coke that has been processed in a calciner.    
          * Fluid coke usually refers to a product derived from the process of a fluidized bed consisting of a reactor and a burner vessel.
          * Flexi coke usually refers to a product like fluid coke which is gasified into a weak fuel gas.      
          * Shot coke is characterized by small round spheres of coke produced by delaying coking process.    
        - HGI of petcoke can vary from 30 to 90. Some petcokes are very difficult to grind.]        
        - Summary table:            
    All rights reserved © 2012-2015 The Cement Grinding Office