Abstract:
A classifier [ 100 ] is disclosed that employs adjustable vanes [ 140 ] that may be interactively adjusted to change the distribution of fuel particle sizes that are passed on to a furnace. The classifier employs a frame [ 133 ] having a plurality of windows [ 131 ] each having an adjustable vane [ 140 ]. A control ring [ 160 ] is rotated with respect to the frame [ 133 ] to simultaneously move links [ 150 ] connected between the control ring [ 160 ] and the vanes [ 140 ]. This causes the vanes [ 140 ] to open or close, changing the air flow path and changing the size distribution of particles passing through the classifier [ 100 ] to the furnace. The system may include an adjustment system [ 260 ] that can automatically sense particle size to optimize several physical parameters related to particle size.

Description:
BACKGROUND 
     The present disclosure generally relates to an adjustable classifier that can adjust the size of particles separated in a solid fuel mill. 
     Power plants employ solid fuel furnaces in boilers for various purposes, such as for generating steam to create electric power. 
     The solid fuel, typically coal, is pulverized into a powder that is blown into the furnace to be burned. Mills (or pulverizers) are used to pulverize coal into powder. The mills typically create a distribution of particles sizes. However, for combustion, particles above a given size do not completely burn and therefore, fuel is wasted. 
     However, if all particles are pulverized to a very fine size, energy required to pulverize the particles is wasted. Also, the throughput of mill particles drops significantly when all particles are required to be very small. This would then require additional mills, which can become very expensive. 
     Therefore, there is a tradeoff of particle size in which balances the amount of coal that will be unburned vs. the throughput requires to efficiently run the boiler. 
     The particle size chosen is determined on how long it takes to burn the particle and how much unburned fuel is acceptable. 
     The particles are blown through the furnace and based upon their speed have a limited time in the furnace to burn. The rate of burning is related to the mass of the fuel to be burned, the surface area of the particles, the energy of the furnace flames, the water content and the type of fuel used. If all of these factors are fixed and the classifier is designed to separate particles with a size corresponding to these factors, the system runs well. However, if one or more of these factors changes necessitating different sized particles to be used, conventional classifiers are not easily modified to separate different sized particles. 
     Currently, there is a need for an adjustable classifier that can adjust the particles size distribution that is allowed to exit the mill and is fed to the boiler. 
     A classifier system  100  for separating coarser particles from finer particles entrained in an upward air stream is described having: 
     a housing  110  having a general circular cross section; 
     a truncated cone  120  inside of the housing  110  having a larger section at its top and a small cone outlet  230  at its bottom, the cone  120  defining an inner chamber  125 ; 
     an outer chamber  190  between the housing and the cone  120  adapted to receive coarser and finer particles entrained in the upward air stream; 
     a classifier ring  130  at the top of the cone  120  having a frame  133  with a plurality of windows  131  with vanes  140  hinged adjacent to each window  131 ; wherein the vanes  140  are adjustable to partially or fully close the windows  131  thereby affecting the size of particles allowed through them and into the inner chamber  125 ; 
     a fuel tube outlet  240  above the classifier ring  130  adapted to allow the air stream to exit the classifier system  100 . 
     The invention may also include: 
     an adjustment system  260  having: 
     at least one pressure sensor  263  upstream of the classifier ring  130  to measure air pressure entering the classifier ring  130 ; 
     at least one pressure sensor  261  downstream of the classifier ring  130  to measure air pressure exiting the classifier ring  130 ; 
     a coarseness sensing device  269  adapted to sense particle size exiting the fuel tube outlet  240 ; and 
     a control unit  265  adapted to receive signals from the sensors and calculate a vane  140  setting. 
     The disclosure may be understood more readily by reference to the following detailed description of the various features of the disclosure and the examples included therein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring now to the figures wherein the like elements are numbered alike: 
         FIG. 1  is an elevational view of one embodiment of a classifier according to the present invention, as it appears in a mill. 
         FIG. 2  is a partially cut-away, perspective view of the classifier of the present invention, as it appears in a mill. 
         FIG. 3  is a perspective view of the classifier ring of  FIG. 2 ; an 
         FIG. 4  is a perspective view from inside of the classifier ring of  FIG. 2 , showing two classifier vanes according to the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Theory 
     The force on a particle by flowing air is proportional to its drag coefficient in the direction of the flow. Gravity also applies a force to the particles in a downward direction. Since the particles are entrained in a stream of air and are moving at a speed in a direction they have momentum. 
     For example, If the stream changes direction, there is a force, proportional to the drag coefficient directing the particle in the new direction. If two particles having similar drag coefficients but significantly different masses are in entrained in the same stream of air, and the stream changes directions, there is a similar force exerted on both particles. Assuming that the mass of a first particle was small enough to have a small momentum that was easily diverted by the force and its velocity was redirected into the new direction of the stream. However, assuming the second particle has more mass and more momentum and therefore, the force only partially diverts the velocity of the second particle. 
     If the stream changed its direction to go around a solid barrier, it is possible that the second particle was not redirected enough to avoid the barrier, and impacted the barrier. In this case, it imparts most of its velocity energy to the barrier and either slows or bounces. In either case, it is probably outside of the airstream and therefore, gravity will pull it downward to the pulverizer. 
     If the heavier second particle, was diverted enough to miss the barrier, but directed to an outer portion of the airstream, it will then fall out of the airstream. Typically, the periphery of airstreams have slower moving air. Since drag force that entrains particles is a velocity-dependent force, there may not be enough force to keep the particle entrained and, again the second particle falls downward out of the stream. 
     It was initially assumed that the drag force of both particles was similar. Even though heavier particles are typically larger, the drag force does not increase in the same proportions as the mass. Therefore this assumption is valid. 
     As the radius of curvature of the airstream having entrained particles becomes smaller, the average size of particles remaining entrained is also smaller. 
     Mill product classification is achieved by exposing the air/coal flow to radial acceleration as it passes through the vanes of the classifier. Larger particles possessing greater momentum are unable to pass through the contorted flow path and are returned to the table for further grinding while fine particles exit the classifier entrained with the primary air. 
     If a classifier is designed to reject all particles except those of a very small size, the larger particles are blown up to the classifier, are rejected and fall back to the pulverizer. This may happen many times, increasing the energy required to produce a required amount of fuel for a furnace. 
     However, if the particles provided to a furnace are too large, they do not fully burn and result in unburned carbon in the ash, making it unsuitable for the manufacture of concrete. 
     Finer particles yields improvements in combustion efficiency and reduces the amount of unburned carbon. This indirectly results in a reduction of NOx emissions. 
     Therefore, there should be a tradeoff of these constraints to determine the particle size used. 
     Therefore the ability to adjust the classifier blades while the mill is in service allows for its performance to be optimized. 
     This present invention relates to certain new and useful improvements in a classifier, more particularly a classifier of the cyclone type adapted to be used in direct communication with a mill or pulverizer to divide the finer sufficiently pulverized material from the coarser material which is returned to the mill for further grinding. 
     DETAILED DESCRIPTION 
     Referring now to  FIGS. 1 and 2 , coal is provided to a mill (not shown) where the coal is ground, through a feed pipe  210 . The classifier  100  is designed to receive a mix of coarse and fine particles entrained in an upward air stream from a mill below (not shown). The particles and air stream, indicated by arrows “A” are blown upward in an outer chamber  190  formed between an outer housing  110  and an inner cone  120 . 
     The air stream and entrained particles enters a classifier ring  130  by blowing past vanes  130 , past a flow diverter  250  and into an inner chamber  125 , inside of cone  120 . 
     Due to the turns of the air stream, heavier particles drop out of the stream and slide down the inside of cone  120  to cone outlet  127  and back to the grinding table of the mill to be re-ground. 
     Lighter particles follow the airstream flow out of the top of the housing  110  and out the fuel tube  240 . 
       FIG. 3  is a perspective view of the classifier ring of  FIG. 2 . 
     The classifier ring  130  provides the tortuous path for the air stream and particles that causes particles to drop out of the air stream. As indicated above, the smaller the radius of curvature of an air stream, the small the particles that remain entrained in the air stream. Therefore, by adjusting the shape of the air stream, the particle distribution that passes through the classifier  100  changes. 
     The frame  133  has a plurality of windows  131  each having a vane  140 . A ring adjustment device  170  actuates a control ring  160  to move a plurality of links  150 , each connected to one side of a vane  140 . The control ring  160  is inside of housing  110 . This allows it to be protected and less likely to become damaged or clogged with material. 
       FIG. 4  is a perspective view from inside of the classifier ring of  FIG. 2 , showing two classifier vanes according to the present invention. 
     Now with respect to  FIGS. 3 and 4 , the other side of each vane  140  has a pivot  141  attached to the frame  133 . Links  150  have a vane attachment pivotally attached to the vane  140 , and the other side pivotally attached to the control ring  160 . 
     A handle  173  of the ring adjustment device  170  may be used to manually move pin  171  to a new hole  177  in fixed plate  175 . This manually moves the control ring  160  relative to the frame  133  to cause links  150  to either further open or close vanes  140 . By changing the position of the vanes  140  relative to the windows  131  of frame  133 , causes different air stream patterns, and hence a different distribution of particles will pass out of the classifier to the furnace. 
       FIG. 4  also shows the curved aerodynamic shape of the vanes  140 . The prior art designs have flat angled plates that functioned as vanes. The air stream that passed into the windows  131  would impinge upon the prior art vane and pass around the vane. This would cause significant turbulence inside of the cone ( 120  of  FIGS. 1 and 2 ) and inside of the inner chamber ( 125  of  FIG. 1 ). Since turbulence causes increased entrainment of particles, this extends the time in which the coarser particles are separated out of the airstream. 
     The curved vanes  140 , which also may have an airfoil cross section, allow the airstream to pass over the vanes with less turbulence. This allows faster separation and less recirculation. 
     The embodiment of the present invention as described above, can be adjusted to provide finer particles when required. The finer particles improves combustion performance, and reduces the amount of fuel that is wasted as carbon in the ash. Lower concentrations of carbon in the ash allows the ash to be sold for making concrete and minimizes the amount that has to be disposed of by other means, usually land fill. Similarly, low concentrations of carbon in fly ash allows the gypsum created in the FGD (Flue Gas De-sulfurization) systems to be sold creating revenue instead of incurring costs for its disposal. 
     Adjustment of the vanes also allows the system to be optimized to reduce NOx emissions and reduce air pressure drop through the pulverizer. These both result in additional cost savings. 
     Alternative Embodiments 
     In an alternative embodiment of the system, an adjustment circuit  260  is employed. It has an air pressure sensor  261  located at the exit of the classifier near the fuel tube outlet  240 . There is also a coarseness sensing device  269  at the fuel tube outlet  240 . This determines the relative coarseness of the output particles. 
     Another pressure sensor  263  measures the air pressure before the air stream enters the classifier. In this embodiment it is in the outer chamber  190 . 
     The sensed information from the pressure sensors  261 ,  263  and the coarseness sensing device  269  are provided to a control unit  265 . It then makes calculations and actuates a motor  267  to adjust the position of the vanes  140 . Since this may be done iteratively, the adjustment system can try many different settings, while monitoring this information and determine an optimum particle coarseness and pressure drop. Control unit  265  may include conventional user interface to allow a user to select various combinations of vane settings, pressure drop and particle coarseness. 
     In another alternative embodiment, NOx sensors are added to the adjustment system  260  and positioned in the flue gases exiting a furnace that receives the air/particle stream from the fuel tube outlets  240 . Now the control unit can also monitor the NOx emissions from the furnace. Taking into account the time lag for the particles to leave the fuel pipes  240 , be burned in the furnace and create NOx in the flue gas, the adjustment system  260  may now track how vane  140  positions can affect NOx emissions. Again, they system can iteratively select various vane  140  positions and monitor the results. The NOx emission will be minimized at some setting. In reality, the setting chosen may not be the NOx minimum, but a tradeoff between NOx emission and pressure drop. 
     In still another embodiment, other physical parameters may be measured, such as temperature, humidity, etc. and provided to control unit  265  to make intelligent decisions on the best settings for the vanes  140 . 
     Advantageously, the present invention overcomes the problems noted in the prior art. 
     Unless otherwise specified, all ranges disclosed herein are inclusive and combinable at the end points and all intermediate points therein. The team “first,” “second,” and the like, herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another. The terms “a” and “an” herein do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item. All numerals modified by “about” are inclusive of the precise numeric value unless otherwise specified. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to make and use the invention. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.