Abstract:
A method and system for controlling a pulverized coal fired boiler wherein the flow of a coal/air mixture flowing to each burner is monitored and transmitted to a distributed control system. The distributed control system also monitors and controls the position of dampers in a splitter that feeds the coal/air mixture to the burners. The dampers are controlled in a closed loop fashion to achieve a optimal boiler performance.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a national stage entry of PCT/US08/85671, which claims the priority to U.S. provisional patent application Ser. No. 61/012,089 filed on Jul. 12, 2007 entitled “ A System and Method for Full Combustion Optimization for Pulverized Coal-Fired Steam Boilers,” the contents of which are relied upon and incorporated herein by reference in their entirety. 
     
    
     DESCRIPTION OF THE PRIOR ART 
       [0002]    There are a number of advanced control techniques for the optimization of combustion within a pulverized coal-fired boiler (PCFB). These methods typically involve the use of advanced model-predictive control, and/or neural net-based controls, to monitor, balance and control the admittance of fuel and air to various stages of the boiler, including primary, secondary, overfire, and underfire air controls. Other variables, such as burner tilts and attemperator spray flows may be controlled as well, in order to optimize the combustion process. As is well known, attemperators reduce the steam temperature by bringing superheated steam into direct contact with water. The steam is cooled through the evaporation of the water. 
         [0003]    The combustion process should be controlled and optimized to obtain the “best possible” performance thereby meeting in an economically and/or environmentally optimized fashion the competing goals of NOx reduction, CO and unburned fuel reduction, and heat rate improvement. However, this optimization is in large part limited by physical process parameters that the system used to optimize combustion often does not have the ability to control. 
         [0004]    One example of a system  1  using such prior art control techniques without coal flow management is shown in the air and fuel flow diagram of  FIG. 1 . As is well known in the art, the system  1  of  FIG. 1  usually includes a distributed control system (DCS) to control the process such as the DCS  14  shown in  FIG. 2  and may also include a combustion control and optimization system (COS) such as the COS  12  shown in that figure. 
         [0005]    As shown in  FIG. 1 , ambient air enters the system  1  on the left hand side of the diagram. Most of this air becomes primary air whose main function is to carry the pulverized fuel out of the one or more coal pulverizers  2 . 
         [0006]    The air and pulverized fuel must be in a stoichiometric ratio at the burners  4  and that mix is obtained by adding secondary ambient air as shown on the right hand side of the diagram. 
         [0007]      FIG. 1  also shows several dampers  6   a ,  6   b ,  6   c  and  6   d  that are associated with the flow of air. Damper  6   a , known as the hot air damper, is associated with the flow of heated ambient air that is in the primary air duct  3 . Damper  6   b , known as the cold air damper, is associated with unheated ambient air in the tempering air duct  5  to temper the hot primary air. Damper  6   c , known as the primary air damper, provides the mixture of the primary air and the tempering air to the pulverizers  2  and the burner lines  7  associated with the pulverizers and the burners  4  and provides the tempered hot primary air to the burners  4 . Damper,  6   d , known as the control damper, provides secondary heated air in the secondary air duct  8  to the burners  4 . As is well known to those in this art, the major adjustment to these dampers  6   a ,  6   b ,  6   c  and  6   d  are load related and the signals to make that adjustment come from a distributed control system such as DCS  14  of  FIG. 2 . 
         [0008]    One element which is missing from the prior art is the ability to provide a closed-loop controllable flow of a homogeneous and balanced air-fuel mixture to the burner systems of the PCFB. Past technologies and implementations have used methods and apparatus such as riffle boxes to homogenize the air-fuel mixture. Riffle boxes have been associated with high pressure drops which may lead to rapid-wear. Manual set-and-forget balancing techniques have also been used, which are configured at one load condition for the PCFB, typically with fixed orifices to balance the admittance of primary combustion air with the stream of fuel from the pulverizer  2 . 
         [0009]    The present invention provides an improved combustion optimization system that is designed to monitor, modify and control the combustion process, including the load-varying air-fuel mixing and homogenization processes. 
       SUMMARY OF THE INVENTION 
       [0010]    According to one aspect of the present invention, a system is provided for controlling a pulverized coal-fired boiler having at least one pulverizer for pulverizing coal and forming an air and coal mixture, a plurality of burners, each said burner fed said air and coal mixture by a burner line. The system includes a combustion optimization system having a combustion model of the pulverized coal-fired boiler. A distributed control system is in communication with the combustion optimization system and receives control commands from the combustion optimization system. A coal flow sensor is positioned to monitor the velocity of the air and coal mixture fed into each burner. An air flow homogenizer is positioned downstream of the pulverizer and includes a splitter for separating the air and coal mixture into the burner lines. The splitter has a plurality of dampers to control the flow of the air and coal mixture flowing to the burners. The distributed control system controls the position of the dampers in a closed loop fashion using a signal indicative of the present position of the dampers in combination with signals from the coal flow sensors. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a partially schematic view of a prior art pulverized coal-fired boiler; 
           [0012]      FIG. 2  is a schematic view of a COS and DCS control system for a pulverized coal-fired boiler; 
           [0013]      FIG. 3  is a coal flow monitoring sensor; 
           [0014]      FIG. 4  is a partially schematic view of a pulverized coal-fired boiler control system according to the present invention; and 
           [0015]      FIG. 5  is a process flow chart for the pulverized coal-fired boiler control system. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    Referring now to  FIG. 2 , there is shown a block diagram for one embodiment of the system  10  of the present invention. System  10  includes an advanced Combustion Control and Optimization System (COS)  12 . COS  12 , models the multivariable nonlinear relationships of the combustion process. The relationships between signals/parameters are identified by analyzing their historical data. COS  12  is based on advanced model predictive control techniques and uses the combustion model and a cost function that describes the weighted customer optimization targets to provide setpoint and setpoint bias values  18  to the distributed control system (DCS)  14  of system  10 . DCS  14  includes operator setpoints and provides process values  20  to COS  12 . COS  12  has a model of the process and has as inputs the constraint variable limits  22 , the controlled variable targets  24  and the manipulated variable targets and limits  26 . One example of COS  12  is the Optimax Combustion Optimizer System, available from ABB. 
         [0017]    The DCS  14  is connected to the boiler and final control elements  16  of system  10 . The DCS  14  provides the multiple boiler control values  28 , the air damper position  30  and the coal/air gate position  32  to the boiler and final control elements  16 . 
         [0018]    The boiler process, with instrumentation and final control elements  16 , also includes various instruments that provide the process values  34  to the DCS  14 . In turn, the DCS  14  controls the process by sending control signals to the final control elements. The instruments may for example include flame detectors such as those that detect the presence or absence of flame and also measure the quality of the flame. This flame quality measurement can be used to ensure that the combustion process is operating efficiently. One example of such a flame detector is the Uvisor™ SF810 i  system available from ABB that provides in a single housing both flame detection and a measurement of the quality of the flame. Associated with the flame detector is a suitable solution for monitoring the quality of the flame such as the Flame Explorer which is also available from ABB. 
         [0019]    The instruments may also include a system that has sensors to measure the velocity of the pulverized coal feeds into the boiler, the concentration of coal therein and optionally temperature. This system uses the input from the sensors to provide closed loop combustion optimization of boilers fired with pulverized coal. One example of such a system is the PfMaster system available from ABB that with one signal processing unit can measure up to 24 pulverized fuel (pf) burner feeds. One example of such a sensor is the ABB coal flow monitoring sensor shown in  FIG. 3 . 
         [0020]    An air and fuel flow diagram for system  10  is shown in  FIG. 4 . As shown therein, system  10  includes everything shown in  FIG. 1  and also has the following elements that are not in the prior art diagram of  FIG. 1 : (a) An air-fuel flow homogenizer  40  that has a fuel flow splitter with dampers (identified in  FIG. 4  as control-gate dampers  42 ) in the burner lines  7  from the pulverizer  2  to control the flow of the homogenized air-fuel mixture of pulverized coal to two or more of the burners  4  of the boiler. (b) A flame scanner  46  with a combustion index which may for example be the flame scanner described above. (c) Coal flow sensors  48  which monitor each of the burner lines. 
         [0021]    One example of the sensors  48  and associated coal flow monitoring system is the Pf Master system described above. Sensors  48  may measure velocity, coal concentration and temperature with a single sensor. 
         [0022]    As with the prior art air and fuel flow diagram of  FIG. 1 , the air dampers  6   a ,  6   b ,  6   c  and  6   d  shown in  FIG. 4  are controlled by the DCS  14 . In the prior art, the dampers of the splitter  42  are manually configured at one load condition. In accordance with the present invention, the position setting of the dampers of the splitter  42  are controlled by the DCS  14 . DCS  14  provides closed loop control of the dampers for splitters  42  by using a signal indicative of their present position in combination with signals from the coal flow monitoring system. Positioner and actuator devices such as those available from ABB provide the signal indicative of the damper position and to move the associated damper to the setpoint from DCS  14 . 
         [0023]    The controlled diversion of the homogenized air-fuel mixture results in a balanced delivery of air and fuel to individual burners  4  within the burner array with appropriate stoichiometric ratios. Additionally, the COS  12  can modify the overall air-fuel delivery profile to the burner array such that the best burner input flows amongst the burners  4  in the array may be achieved for a given load. 
         [0024]    One example of an air-fuel flow homogenizer  40  is the variable area rope breaker system PF diffusing system available from Greenbank Terotech Ltd. One example of a fuel flow splitter  42  with dampers is the coal flow control gate splitter also available from Greenbank. As is described above, the coal flow control gate dampers in splitter  42  are controlled by COS  12  of system  10  through the DCS  14 . 
         [0025]    As can be appreciated the conversion of the fuel flow splitter  42  to closed-loop controlled operation, provides for the initial balancing of the air-fuel mixture to the burners  4  fed by its piping. This achieves the capability to dynamically balance the air-fuel flow to individual burners of the PCFB under varying load conditions. These varying load conditions affect the incoming two-phase distribution of air and fuel and create the need for a dynamic response over the desired load range. 
         [0026]    As can be further appreciated, the coupling of the local closed-loop controls of the fuel flow splitter  42 , to the COS  12  creates the following additional benefits which are beyond what any one of the separate elements can provide alone: (a) Complete monitoring and control of the combustion process, from the initial mixing of fuel with air in a homogenized and ratio-balanced fashion, through the required distribution to various burners within the PCFB, and finally the controlled ignition and optimized combustion of the air-fuel stream within the confines of the boiler interior. (b) The ability to dynamically create, monitor and control relative air-fuel flows between the multiple-burners of a PCFB, such that load-induced effects from the pulverization, air-induction, and flame creation processes can be manipulated and optimized to obtain true “best possible” performance, such that the competing goals of NOx reduction, CO and unburned fuel reduction, and heat rate improvement, are met in an economically and/or environmentally optimized fashion. (c) The capabilities as described above can be achieved in an automated fashion, where the operators of the PCFB have a substantially reduced need to manually balance and control the multitude of individual air and fuel flows of the typical pulverized coal fired boiler combustion process. 
         [0027]    A flow chart of system  10  is shown in  FIG. 5 . As shown therein, the COS  12  provides, in response to the external load demand and process values, states and control modes from DCS  14  both real-time optimization and advanced process control to DCS  14 . DCS  14  controls the actuators that are used to position the dampers shown in  FIG. 4  and sensors provide process related values such as coal flow and flame detection and quality. 
         [0028]    As can be also appreciated, the monitoring of flame status and quality insures that individual burners are performing as expected, with the MPC model from COS  12  tracking the correlation of combustion index with individual burner load and performance. 
         [0029]    As can be appreciated from the above description, the present invention provides over the prior art, substantially improved combustion efficiencies and unit heat rate, and the reduction and control of emissions to acceptable levels. Additional benefits may include the mitigation of costly fan-limited operation, due to the overall lowering of resistance in the air-fuel paths between pulverizers and burners. 
         [0030]    The advantages provided by the system of the present invention include, reductions in LOI (Loss on ignition—i.e. unburned fuel and wastage), reduced or eliminated use of auxiliary (co-firing) fuels during low loads, reduced waterwall wastage due to CO rich “dark zones”, and reduced emissions (CO 2 , CO and NOx). Further PCFB operational improvements which can result from the use of the present invention include, improved unit heat rate (thermal efficiency), improved unit ramp rate, improved flame and fireball stability over a much wider load range, elimination of some/all riffle boxes for fuel distribution, with improved draft fan efficiency results, and controllable variations in the air/fuel ratio to adapt to boiler load conditions. 
         [0031]    It should be appreciated that while the embodiment for the system of the present invention shown in  FIG. 2  and its associated air and fuel flow diagram shown in  FIG. 4  can as described above include a flame scanner with a combustion index, the system of the present invention will provide an improvement over the systems of the prior art even if the flame scanner used in the system does not have a combustion index.