Abstract:
A system for controlling coal flow in a coal-fired boiler including: a coal flow damper in operable communication with a burner pipe; a coal flow sensor in operable communication with the burner pipe which generates a coal flow signal; and a fuel trim controller in operable communication with the coal flow damper, the fuel trim controller controlling the coal flow damper responsive to the coal flow signal.

Description:
BACKGROUND  
       [0001]     The present disclosure relates generally to a method and system for controlling coal flow, and more particularly to controlling the coal flow to the burners in a coal fired boiler in order to optimize boiler operations.  
         [0002]     Coal fired boilers utilize pulverizers to grind coal to a desired fineness so that it may be used as fuel for the boilers. Typically, raw coal is fed through a central coal inlet at a top of the pulverizer and falls by gravity to a grinding area. Once pulverized, the coal is transported upwards, using air as the transport medium. The pulverized coal passes through classifier vanes within the pulverizer. These classifier vanes may vary in structure, but are intended to establish a swirling flow within a “rejects” cone to prevent coarse coal particles from flowing into a discharge turret of the pulverizer. The centrifugal force set up in the “rejects” cone forces coarse coal particles to drop back down into the grinding area until the desired fineness is met. Once the coal is ground finely enough to pass through the classifier, it enters the discharge turret. From the discharge turret the pulverized coal is distributed among multiple pulverized coal outlet pipes and into respective fuel conduits where it is carried to the burners. Each coal pulverizer is an independent system and delivers the pulverized coal to a group of burners.  
         [0003]     Poor balance of pulverized coal distribution between burner pipes is commonly experienced in boilers. This imbalance results from, inter alia, system resistance of each individual fuel conduit, physical differences inside the pulverizer, and coal fineness. Unbalanced and generally random varying distribution of coal among the burner pipes adversely affects unit performance and leads to decreased combustion efficiency, increased unburned carbon in fly ash, increased potential for fuel line plugging and burner damage, increased potential for furnace slagging, and irregular heat release within the combustion chamber. In addition, it is critical for low NO x . (Nitric Oxides) firing systems to precisely control air-to-fuel ratios in the burner zones to achieve low levels of NO x  formation.  
         [0004]     For an understanding of the degree of imbalance and accompanying issues commonly experienced reference is made to  FIG. 1 .  FIG. 1  illustrates an example of the coal flow distribution under typical, or baseline, conditions for a plant comprising five mills with five burner pipes each.  FIG. 1  shows significant coal flow imbalances between the burner pipes. Data from a number of plants show even worse coal flow balance with coal flow deviations of more than thirty percent.  
       BRIEF DESCRIPTION  
       [0005]     Disclosed herein is a system for controlling coal flow in a coal-fired boiler including: a coal flow damper in operable communication with a burner pipe; a coal flow sensor in operable communication with the burner, pipe which generates a coal flow signal; and a fuel trim controller in operable communication with the coal flow damper, the fuel trim controller controlling the coal flow damper responsive to the coal flow signal.  
         [0006]     Also disclosed herein is a method for controlling coal flow in a coal-fired boiler including: sensing a coal flow in a burner pipe; generating a coal flow signal indicative of the coal flow in the burner pipe; and adjusting the coal flow in the burner pipe responsive to the coal flow signal.  
         [0007]     Further disclosed herein is a system for controlling coal flow in a coal-fired boiler comprising: means for sensing a coal flow in a burner pipe and means for adjusting the coal flow in a burner pipe responsive to the coal flow in the burner pipe.  
         [0008]     Other systems, methods, and/or computer program products according to exemplary embodiments will be or become apparent to one with skill in the art upon review of the following drawings and detailed description. It is intended that all such additional systems, methods, and/or computer program products be included within this description, be within the scope of the present invention, and be protected by the accompanying claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     Referring to the exemplary drawings wherein like elements are numbered alike in the accompanying Figures:  
         [0010]      FIG. 1  illustrates an example of the coal flow distribution under baseline conditions for a plant including 5 mills each with five burner pipes;  
         [0011]      FIG. 2  illustrates an exemplary embodiment of a coal flow control system;  
         [0012]      FIG. 3  illustrates an exemplary embodiment of a coal flow damper;  
         [0013]      FIG. 4  illustrates another exemplary embodiment of a coal flow control system;  
         [0014]      FIG. 5  illustrates an example of the coal flow distribution for one mill at a plant operating under baseline conditions, under manual coal flow balancing, and under automatic coal flow balancing;  
         [0015]      FIG. 6  illustrates an example of targeted coal flow biasing;  
         [0016]      FIG. 7  illustrates another example of targeted coal flow biasing;  
         [0017]      FIG. 8  illustrates the impact of the coal flow biasing capability on FEGT (Furnace Exit Gas Temperature); and  
         [0018]      FIG. 9  illustrates the impact of the coal flow biasing capability on NOx emissions. 
     
    
     DETAILED DESCRIPTION  
       [0019]     In coal fired boilers, balanced coal flow among burners is traditionally an important factor for combustion optimization. In order to effectively optimize furnace combustion performance, coal flow to each individual burner must be closely controlled. Coal flow control can be accomplished automatically and continuously through the use of a coal flow control system. Referring to  FIG. 2 , an exemplary embodiment of the coal flow control system is shown generally at  10 . A coal flow sensor  14  measures the coal flow in a burner pipe and communicates a coal flow signal, indicative of coal flow in the burner pipe, to a fuel trim controller  16 . A coal flow damper  12  controls the coal flow in the burner pipe and receives command signals from the fuel trim controller  16 . The coal flow damper  12  is discussed in more detail hereinafter with reference to  FIG. 3 . The fuel trim controller  16  receives the coal flow signal from the coal flow sensor  14  and input signals from a controller, indicative of a desired coal flow in a burner pipe. The fuel trim controller  16  calculates a desired change in coal flow in the burner pipe responsive to the coal flow signal and the input signal. The desired change in coal flow is communicated to the coal flow damper  12 , which responsively alters the coal flow in the burner pipe.  
         [0020]     Continuing with  FIG. 2 , the coal flow sensor  14  is used to measure the mass flow of coal in the burner pipe, which in this embodiment is determined from the concentration of coal and the velocity of coal. In an exemplary embodiment, attenuation of low-power, low-frequency microwaves between a transmitting and receiving sensor are used to quantify coal concentration. Additionally, in this embodiment the velocity of the coal flow is measured by a cross correlation method, which consists of measuring the time for a coal density signal to travel from one sensor to another. It is preferable to install the coal flow sensors  14  in vertical up-flow pipe runs because horizontal pipe runs may impact the measurement reference conditions if “coal layout”, i.e. accumulation of coal particles in the pipe, occurs. Additionally, the coal flow sensors  14  should be installed in sufficient straight pipe runs to minimize flow disturbances. For example, for pipe sizes up to eighteen inches, the total straight, undisturbed coal flow distance should be a minimum of seven pipe diameters.  
         [0021]     Turning now to  FIG. 3 , an exemplary embodiment of a coal flow damper  12  is illustrated. The coal flow damper  12  includes a box, which encloses a flow restrictor  20 . The flow restrictor  20  forms an adjustable orifice  22 , which controls the coal flow through the burner pipe. The flow restrictor  20  may have a ceramic coating, such as a highly abrasion resistant alumina-ceramic coating, to prevent corrosion and increase the useful life of the flow restrictor  20 . The flow restrictor  20  may be a manually adjusted flow restrictor  24  or an actuator  30  may control the position of the flow restrictor  20 . The position of the flow restrictor  20  may be adjusted as needed during operation to maintain proper coal flow through the burner pipe. As the flow restrictor  20  reduces the area of the adjustable orifice  22  a pressure drop is induced, which changes the coal flow in the burner pipe. The actuator can be controlled manually or automatically when coupled with the fuel trim controller  16 . The damper  12  also includes an air purge line (e.g., side of the flow restrictor  20 ) to prevent coal deposits in a flow restrictor guides. In an exemplary embodiment, a threaded adjusting rod  24  allows the flow restrictor  20  to be manually positioned. In an exemplary embodiment, there are two adjustable flow restrictors  20  in the coal flow damper  12 . One flow restrictor  20  may be actuated for continuous trimming adjustment while another flow restrictor  20  may be used for extending control range and approximately centering the orifice.  
         [0022]     Turning now to  FIG. 4 , an alternative exemplary embodiment of the coal flow control system  10  is depicted. The coal flow control system  10  includes three coal flow dampers  12 , three coal flow sensors  14 , the fuel trim controller  16 , a coal flow measurement system  26 , and a controller HMI (Human Machine Interface)  28 . The coal flow measurement system  26  receives sensor signals, indicative of the coal flow in the burner pipe, from the coal flow sensors  14  and calculates the coal flow in each of the burner pipes. The coal flow measurement system  26  communicates the coal flow in each burner pipe to the fuel trim controller responsive to the sensor signals. The fuel trim controller  16  uses algorithms and diagnostics to determine adjustments to the coal flow damper  12  responsive to the command signals from the coal flow measurement system  26  and input signals received from the controller HMI  28 . The fuel trim controller  16  may include design features and alarm functions that enable a remote tuning option. Additionally, the fuel trim controller  16  may include several unique capabilities such as an automatic balancing mode, a prescribed coal flow biasing mode, a redundant coal flow damper  12  constraint system, or an operational and safety alarm. The coal flow control system  10  provides improved coal flow distribution to a coal fired burner and is able to maintain the improved coal flow distribution over a wide range of mill operating loads. Additionally, the coal flow control system  10  can provide rapid and continuous coal flow adjustments for tighter control of boiler combustion performance.  
         [0023]     The fuel trim controller  16  may be operated in either a manual or automatic mode. The manual operating mode of the fuel trim controller  16  allows a controller to set and hold the position of the coal flow damper  12 . The automatic operating mode, allows the fuel trim controller  16  to adjust the position of the coal flow dampers  12  automatically to maintain desired coal flow distribution. To maintain reliable damper operation, the fuel trim controller  16  includes an adjustable purge system to prevent coal deposits in the damper flow restrictor guides. The purge system is operated at least once before adjusting the coal flow damper  12  and intermittently (operator adjustable). The fuel trim controller  16  may have an automatic balancing mode that can be used to balance coal flow distribution. The fuel trim controller  16  may also have a prescribed fuel biasing mode capability that allows lateral coal flow biasing between burners of a mill which in turn allows the operator to use the controller HMI  28  to set prescribed coal flow biases for each pipe to overcome flow field anomalies, local fouling and slagging, local corrosion, local flame impingement, off-centered fireballs, spatial combustion performance (reducing CO and carbon in ash) and economizer O2 stratification. The stand-alone operating mode of the fuel trim controller  16  can maintain a set coal flow or it may be readily integrated with a boiler optimizer control system to automatically adjust coal flow distribution targets as a function of boiler operating conditions. The -fuel trim controller  16  operational and safety alarms detect low coal flow velocities in the pipes and improper positioning of the coal flow damper  12 .  
         [0024]     Additionally, the fuel trim controller  16  may include a physics-based mill circuit coal flow model to predict burner coal flow velocity as a safety constraint to coal flow damper  12  adjustments. This velocity prediction ensures that the coal flow damper  12  adjustments do not cause coal flow velocity to fall below a configurable minimum velocity and above a configurable maximum velocity. Adjustment of one of the coal flow dampers  12  can affect coal flow in other burners within that mill. The physics-based mill circuit coal flow model evaluates planned adjustments to the coal flow damper  12  to ensure that the minimum and maximum predicted coal flow velocities are not violated.  
         [0025]     Continuing with  FIG. 4 , the coal flow control system  10  includes methods to assess the measurement capability of the coal flow sensors  14 . Additionally, a targeted performance band can be set by the coal flow control system  10  to minimize coal flow damper  12  adjustments in response to small fluctuations in sensor signals transmitted by the coal flow sensor  14  that may result from sensor noise or coal flow pulsations. This technique involves operating the mill at constant load for a period of time and recording data from the coal flow sensors  14 . After an acceptable probability of making a false adjustment is determined, i.e. 95% or two standard deviations, the coal flow measurement average and upper and lower limits that fall within the acceptable probability may be calculated. These values are entered into the fuel trim controller  16  to provide the pass band, such that when coal flow is between the target flow minus the lower limit and the target flow plus the upper limits, no adjustments are made to the coal flow damper  12 . The fuel trim controller  16  may also include the ability to adapt by continuously evaluating the upper and lower limit and adjusting these values. This adaptive feature can be used to account for changes in mill performance or coal flow sensor  14  health over time. Another method that may be used to reduce the probability of making an unnecessary adjustment to the coal flow damper  12 , is to use a rolling average of the coal flow sensor  14  sensor signals.  
         [0026]      FIG. 5  illustrates the coal flow for one mill at a plant for typical coal distribution (e.g., as-found), manual coal flow balancing, and automatic coal flow balancing. The typical coal distribution performance illustrates the initial coal flow bias for this boiler with one pipe having a −25% bias. The manual operating mode of the coal flow control flow system  10  was effective at achieving +/−10% coal flow deviation. The automatic operating mode of the coal flow control system  10  was able to further improve coal flow balance consistently to within +/−5%. The automatic operating mode of the coal flow control system  10  is also capable of achieving these balanced conditions at various mill load conditions, which is not possible with manual tuning.  
         [0027]     In terms of prescribed coal flow biasing,  FIGS. 6 and 7  illustrate the ability of the coal flow control system  10  to achieve targeted coal flow biases; the targeted bias and the actual bias are illustrated for two different prescribed bias conditions.  FIG. 6  illustrates a bias condition of redistributing an additional 10% of the coal flow from the left side to the right side of the furnace.  FIG. 7  illustrates another bias condition where an additional 10% of the coal flow is directed to the center burner and 5% to the two burners next to the center burner. In both examples a successful operation of the control system is shown, in which the system was able to consistently achieve the targeted coal flows within +/−5% deviation.  
         [0028]     Turning now to  FIG. 8 , the impact of the coal flow biasing capability on the FEGT (Furnace Exit Gas Temperature) distribution is illustrated. The baseline coal distribution condition corresponds to a typical (e.g. as-found) coal flow distribution. The balanced coal distribution condition consists of distributing the coal flow evenly between the pipes. The optimized coal distribution condition refers to a prescribed bias of coal flow. The results showed that the FEGT on the west side of the furnace was reduced from 2520° F. for the baseline (e.g. as found) coal distribution condition to 2460° F. for the optimized coal distribution condition; this is a 60° F. reduction in FEGT. In addition the west to east FEGT difference was reduced from 110° F. baseline coal distribution to just 30° F. for the optimized coal distribution condition and average FEGT was reduced 20° F.  FIG. 9  illustrates the impact of the coal flow biasing capability on the NO x  emissions. The results showed that the NO x  was reduced by 8% when operating at the optimized coal distribution. Additionally, the coal flow biasing may result in a reduction in a unburned carbon on fly ash and improved spatial combustion.  
         [0029]     While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best or only mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims. Moreover, the use of the terms first, second, etc. do not denote any order or importance, but rather the terms first, second, etc. are used to distinguish one element from another. Furthermore, the use of the terms a, an, etc. do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item.