Patent Publication Number: US-2011056416-A1

Title: System for combustion optimization using quantum cascade lasers

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates generally to a method and apparatus for controlling operation of a boiler that generates steam to drive a turbine or provide process steam or heating, and more specifically to method and apparatus for monitoring and controlling combustion within a boiler by sensing a plurality of operating conditions at a common location with a common sensor. 
     2. Description of Related Art 
     One method of generating electricity includes driving a turbine generator with steam. A boiler for generating the steam commonly includes a furnace with an array of individual burners for burning the hydrocarbon fuel in the presence of oxygen to raise the temperature of water and produce the steam to be delivered to the turbine. The combustion performance of an individual burner provided to the furnace can affect the combustion performance locally within the furnace, and thereby affect the overall performance of the boiler as a whole. 
     If one or more of the burners is not operating in an optimal manner, a condition referred to as a combustion anomaly, the boiler can emit unsatisfactory levels of by products such as oxides of nitrogen (“NO X ”), carbon monoxide (“CO”), mercury (“Hg”), and possibly other byproducts such as unburned carbon (commonly expressed as loss-on-ignition or “LOI”). The combustion anomaly can also result in fuel-rich gases and high local gas temperatures that can contribute to the formation of difficult to remove ash deposits called slag, or can cause boiler tube-wall wastage through corrosion and thermal fatigue. In such circumstances the offending burner(s) must be singled out from the array of burners, and then adjusted to optimize performance of the boiler. Once the offending burner(s) is identified, the performance of that burner can be optimized by means of combustion control, which can include varying the flow rate at which the hydrocarbon fuel is introduced to the burner, the flow rate at which air is introduced to the burner, the rotational velocity component, i.e. spin, of the feed, the angle of injection, an additive level or other suitable variable that can rectify the combustion anomaly. 
     Traditional boiler control systems have relied upon the monitoring of the exhaust from the furnace as a whole (i.e., the collective exhaust resulting from operation of all burners operating simultaneously) to detect combustion anomalies. In response to the detection of a combustion anomaly based on a measured quantity from this collective exhaust the supply of fuel and/or air to the entire array of burners could be adjusted in an attempt to optimize operation of the boiler. Such control methods fail to consider the local effects each burner has on the boiler, and fails to attribute the individual contribution of each burner to the combustion anomaly. 
     More recently, attempts have been made to trace a combustion anomaly back to one or more offending burners, from among the entire array of burners that is/are the primary cause of the combustion anomaly. Determining the presence of a combustion anomaly and identifying the offending burner(s), however, is typically not determined as a function of a single operating condition, such as a measured temperature, at a particular location within the boiler. Instead, to identify, or at least narrow down the location of the offending burner(s), such control methods rely on a plurality of measured operating conditions sensed at various different locations within the boiler. 
     Arrays of individual sensors are disposed at various different locations throughout the boiler to monitor different operating conditions at each of those different locations. For example, the concentration of carbon monoxide (“CO”) has been monitored by an array of sensors disposed at an exhaust port of the boiler downstream of the furnace exit. Further, an array of temperature sensors has been disposed adjacent to a nose of the furnace provided to the boiler to monitor the temperatures near the burners. The temperatures near the burners to which the temperature sensors are exposed are typically too high for the CO concentration sensors to be co-located with the temperature sensors. Thus, the array of CO concentration sensors is spatially located away from the temperature sensors at another, distant location of the boiler where they are subjected to much lower temperatures that will not damage the CO concentration sensors. But in order to consider both the measured CO concentration and the measured temperature at a common location in the boiler to evaluate a combustion anomaly, one of these measured operating conditions was required to be mapped to correspond to an equivalent value at the location of the other operating condition. In other words, the CO concentration measured by each CO concentration sensor adjacent the exhaust port, for example, was adjusted to correspond to the value of the CO concentration that could be expected to be measured at the location of each respective temperature sensor. Thus, the measured temperature and the equivalent CO concentration at a common location in the boiler could be used to determine whether a combustion anomaly has occurred, and if so, which of the burners is contributing to the combustion anomaly. 
     Such attempts have improved boiler control over the traditional methods of controlling the boiler solely on the CO concentration at the exhaust port. But the mapping of a sensed operating condition from one location to another location within the boiler introduces a degree of error in evaluating a combustion anomaly, limiting the ability to effectively identify the offending burner(s). Further, the mapping requires the use of many mathematical models and robust control equipment, making boiler control expensive and complex. 
     Accordingly, there is a need in the art for a method and apparatus for controlling operation of a boiler to optimize performance thereof. The method and apparatus can allow for sensing a plurality of operating conditions at a common location within the boiler to detect a combustion anomaly and allow for optimization of boiler operation. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The following summary presents a simplified summary in order to provide a basic understanding of some aspects of the systems and/or methods discussed herein. This summary is not an extensive overview of the systems and/or methods discussed herein. It is not intended to identify key/critical elements or to delineate the scope of such systems and/or methods. Its sole purpose is to present some concepts in a simplified form as a prelude to the more detailed description that is presented later. 
     According to one aspect, the invention provides a method of controlling operation of a system that includes a boiler with a plurality of burners. The method includes sensing a plurality of operating conditions at a first common location along the boiler. At least one of the plurality of operating conditions sensed at the first common location is indicative of a combustion anomaly occurring during operation of the boiler. The method includes tracing the combustion anomaly back to an offending burner that is at least partially responsible for the combustion anomaly based on a model that takes into consideration at least two of the plurality of operating conditions sensed at the first common location. The method includes adjusting at least one of a process input and a boiler configuration to establish a desired value of the operating conditions at the first common location. 
     According to another aspect, the invention provides a system that includes a steam-driven turbine and a boiler. The boiler includes a plurality of burners arranged in an array to burn a hydrocarbon fuel. The system includes a plurality of sensors, each adapted to sense a plurality of operating conditions at a common location within the boiler and to transmit a signal indicative of a combustion anomaly when one or more of the operating conditions falls outside of a predetermined range of suitable values indicative of desired combustion. The system includes an actuator for controlling at least one of a process input and a boiler configuration to affect operation of at least one of the burners. The system includes a controller in communication with the plurality of sensors to receive the signals indicative of the combustion anomaly, wherein, responsive to receiving the signals the controller traces the one or more operating conditions outside of the predetermined range of suitable values to identify an offending burner contributing to the combustion anomaly and controls the actuator to adjust the at least one of the process input and the boiler configuration to bring the one or more operating conditions into the predetermined range of suitable values. 
     According to yet another aspect, the invention provides a system including a steam-driven turbine and a boiler. The boiler includes a plurality of burners arranged in an array to burn a hydrocarbon fuel. The system includes a plurality of non-invasive sensors each adapted to remotely sense a plurality of operating conditions at a common location within the boiler and to transmit a signal indicative of a combustion anomaly when at least one of the operating conditions falls outside of a predetermined range of suitable values indicative of desired combustion. The system includes a controller in communication with the plurality of non-invasive sensors to receive the signals indicative of the combustion anomaly. The controller includes a computer-accessible memory storing a model that relates the operating conditions falling outside of the predetermined range of suitable values from one or more of the sensors to at least one offending burner contributing to the combustion anomaly. The system includes an actuator to be controlled by the controller for controlling at least one of a flow rate of the hydrocarbon fuel introduced to the offending burner, a flow rate of air introduced to the offending burner, a flow rate of an additive introduced to the boiler through an injector, and an angle of the injector for introducing the additive into the boiler to bring the operating conditions into the predetermined range of suitable values. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may take physical form in certain parts and arrangement of parts, embodiments of which will be described in detail in this specification and illustrated in the accompanying drawings which form a part hereof and wherein: 
         FIG. 1  is a schematic illustration of a power generating system that includes a coal-fired boiler; 
         FIG. 2  is a schematic illustration of a furnace provided to the boiler shown in  FIG. 1 , wherein a portion of the furnace is cutaway; 
         FIG. 3  is a flow diagram illustrating an embodiment of a method for controlling combustion within a coal-fired boiler; 
         FIGS. 4A-4D  are cross sectional views of an exhaust port of a boiler divided into zones across which combustion gradients are to be minimized according to a method of controlling operation of the boiler; and 
         FIG. 5  is a schematic view of a control system for controlling combustion within a boiler to minimize combustion gradients. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Example embodiments that incorporate one or more aspects of the invention are described and illustrated in the drawings. These illustrated examples are not intended to be a limitation on the invention. For example, one or more aspects of the invention can be utilized in other embodiments and even other types of devices. Moreover, certain terminology is used herein for convenience only and is not to be taken as a limitation on the invention. Still further, in the drawings, the same reference numerals are employed for designating the same elements. 
     A method of optimizing operation of a fuel fired boiler is described below in detail. The method includes the use of a plurality of different sensors at different spatial locations within a fuel fired boiler furnace to track in-furnace combustion conditions and the relative differences between the performance of individual burners. Each of the sensors can be used to sense a plurality of operating conditions at the different spatial locations to make adjustments to individual burners and yield an optimized boiler performance. The optimized operating burner conditions can vary from one burner to another. This means that one or both of the air flow and fuel flow, for example, can vary from burner to burner and that the air to fuel ratio to individual burners is not predetermined. Rather, each burner can be individually biased and adjusted to meet boiler performance objectives as indicated by the in-furnace sensors as described in detail below. Optimized performance includes, for example, reduced NO x  emissions, reduced LOI emissions, increased efficiency, increased power output, improved superheat temperature profile, reduced slagging, reduce waterwall wastage, and/or reduced opacity relative to un-optimized operation of the boiler. Corrective actions such as burner adjustments, boiler configuration adjustments, or both can include, for example, fuel flow, air flow, fuel to air ratio, burner register settings, overfire airflows, the orientation of injectors  55  for introducing an additive (including air or fuel) into the furnace, and other furnace input settings. 
     Referring to the drawings,  FIG. 1  is a schematic view of a power generating system  10  that includes, in an exemplary embodiment, a boiler  12  coupled to a steam-turbine generator  14 . Steam is produced in the boiler  12  and subsequently flows through a steam pipe  16  to the generator  14 , which is driven by the steam to produce electric power. The boiler  12  burns a fossil fuel such as coal, or other suitable hydrocarbon fuel source, for example, in a furnace  18  to produce the heat required to convert water into steam for driving the generator  14 . Of course, in other embodiments the fossil fuel burned in the furnace  18  can include oil, natural gas or any other suitably combustible material. However, for the sake of brevity the description that follows will refer to coal as the fuel. Crushed coal, for example, is stored in a silo  20  and is ground or pulverized into fine particulates by a pulverizer or mill  22 . A coal feeder  24  adjusts the flow of coal from the coal silo  20  into the mill  22 . A forced air source such as a fan  26 , for example, is used to create an airflow including entrained particulate coal from the mill  22  to convey the coal particles to furnace  18  where the coal is burned by burners  28 . The air from the fan  26  used to convey the coal particles from the mill  22  to the burners  28  is referred to as primary air. 
     A second fan  30  supplies secondary air to the burners  28  through an air conduit  32  and a windbox  33 . The secondary air is heated before being introduced into the furnace  18  upon passing through a regenerative heat exchanger  34 , transferring heat from a boiler exhaust line  36  to the air conduit  32 . Secondary air can optionally be introduced into the furnace  18  in addition to the primary air when there is insufficient oxygen present within the furnace  18  to allow complete combustion of the fuel being burned, a condition referred to herein as an oxygen deficiency. 
     The boiler  12  also includes a network of actuators that are operable to control at least one of a process input and a boiler configuration to affect the combustion occurring within the furnace  18 . The actuators can be adjusted to regulate the process inputs such as a flow rate of fuel and/or air into the furnace  18  to bring the operating conditions sensed by an array of sensors  38  ( FIG. 2 ) provided to the furnace  18  within a predetermined range of suitable values indicative of substantially-balanced combustion as described below. For instance, valves  41  ( FIG. 1 ) between the fan  26  and the furnace  18  can be adjusted to regulate the supply of fuel to the burners  28 , individually and/or collectively. Similarly, a damper  52  can be adjusted to regulate the flow of primary air, secondary air, or both primary and secondary air into the furnace  18 . Operation of the fans  26 ,  30 , coal feeder  24 , and mill  22 , alone or in any combination, can optionally be adjusted and controlled to act as the actuators and bring the operating conditions into the predetermined range of suitable values. 
     According to alternate embodiments, the configuration of the boiler  12  itself can be adjusted instead of, or in addition to the actuators in an attempt to bring the values of the operating conditions to within the predetermined range of suitable values. For example, the furnace  18  can optionally be provided with an additive injector  55  that penetrates a wall of the furnace  18 , thereby extending into the furnace  18  for injecting a desired additive from a reservoir  57  into the furnace  18 , and optionally into the primary combustion zone. A myriad of additives (such as a combustion additive, or magnesium oxide for slag) could be used, and any specifics about additives should not be considered to be a limitation upon the invention. The additive can be injected into the furnace  18 . The angle at which the additive injector  55  introduces the additive into the furnace  18  can be adjusted to affect the operating conditions within the furnace  18 . 
     The process input(s) associated with each individual burner  28  can optionally be adjusted independent of the process input(s) of other burners  28  to affect the combustion performance of the individual burners  28 . Likewise, the boiler configuration, such as the injection angle of a first additive injector  55  can be adjusted independently of another additive injector (not shown). This independent adjustment of the boiler configuration can primarily affect the combustion performance of a burner  28  adjacent to the first additive injector  55  without significantly affecting the combustion performance of another burner  28  spatially separated from the first additive injector  55 . Thus, the combustion performance of each of the burners  28  can be adjusted and corrected individually to promote substantially-balanced combustion. 
     A flue gas including gaseous combustion products such as fully combusted fuel in the form of CO 2 , in addition to undesirable byproducts such as NO x , and CO compositions, for example, travels in a substantially vertical direction upward within the furnace  18 . The flue gas travels upward beyond a nose  35  that protrudes into an interior chamber defined by the furnace  18 , and then generally vertically downward through an exhaust port  37  leading to the exhaust line  36 . The exhaust port  37  is said to be “downstream” of the burners  28  as the flue gas travels from a region adjacent to the burners  28  and then to the exhaust port  37  in a direction generally indicated by arrow  39  shown in  FIG. 2 . Similarly, the burners  28  are said to be “upstream” along the flue gas path indicated by the arrow  39  relative to the exhaust port  37 . 
     Substantially-balanced combustion is achieved when a flue gas has substantially-uniform operating conditions across the cross-section of the exhaust port  37  of the furnace  18  as described below with reference to  FIGS. 4A-4D . The operating conditions can be any property within the furnace  18  indicative of the completeness of combustion of the hydrocarbon fuel attributable to one or more burners  28  within the furnace  18 . Examples of such operating conditions according to one embodiment can include a temperature of the flue gas. According to other embodiments, the operating condition can include a component composition of the flue gas, wherein the component can be one or more of CO X  (where X=1 or 2), NO X  (representing any binary compound of oxygen and nitrogen, or to a mixture of such compounds, such as when X=1 or 2), O 2 , N 2 , total hydrocarbons (“THC”), volatile organic compounds (“VOC”), SO 2 , SO 3 , H 2 O, OH radicals, LOI, and any particulate matter, for example. 
     Referring also to  FIG. 2 , the furnace  18  includes a plurality of non-invasive sensors  38  arranged a regular, grid formation and located downstream from a flame envelope  42  formed by burning coal in burners  28  in a primary combustion zone within the furnace  18 . The grid locations of the sensors  38  can optionally correspond to the locations of the burners  28 , which can also be arranged in a regular, grid arrangement. For example, one of the sensors  38  can be substantially vertically aligned in a column  48  with one of the burners  28 . The furnace  18  can also include a plurality of overfire air jets  47  and a plurality of reburn fuel jets  49  disposed downstream from the burners  28 . The reburn fuel jets  49  introduce fuel into a secondary combustion zone  44  downstream from the primary combustion zone. The fuel from the reburn fuel jets  49  is mixed with combustion products from the primary combustion zone in the presence of oxygen from air introduced into the furnace  18  downstream from the reburn fuel jets  49  by the overfire air jets  47 . The combination of the fuel from the reburn fuel jets  49 , the oxygen from the overfire air and the combustion gasses from the primary combustion zone within the furnace establishes a balanced stoichiometry that encourages complete combustion of the fuel and minimizes the formation and emission of unwanted combustion byproducts such as CO and NO X , for example. 
     Each sensor  38  can optionally be any non-invasive sensor capable of sensing a plurality of operating conditions at a common location within the furnace  18  without physically protruding into the interior of the furnace, and without physically contacting or consuming combustion products to sense the operating conditions. Thus, the non-invasive embodiment of the sensor can measure the plurality of operating conditions at the common location within the furnace from a remote spatial location. Each sensor  38  can sense a qualitative or quantitative value of two or more operating conditions at substantially the same location within the furnace  18 , which can optionally be a location where the sensor would be damaged when exposed to the operating conditions if physically located at that location. For example, if the operating conditions to be sensed at the common location include a temperature and a quantity of carbon monoxide, the temperature sensed at the common location is greater than a maximum temperature that a carbon monoxide sensor can withstand. 
     The sensors  38  can also transmit signals indicative of a combustion anomaly when one or more of the sensed operating conditions falls outside of a predetermined range of suitable values indicative of a desired, balanced combustion of the fuel-air mixture. The sensed value of the operating condition can be obtained from absolute measurement, relative measurement, and drawing from analysis of fluctuations in combustion quality. Examples of suitable non-invasive sensors  38  for sensing the operating conditions include, but are not limited to, a quantum cascade laser (“QCL”) paired with an optical detector  45  for receiving laser light  51  from the QCL, tunable diode laser or other optical sensor, a radiation sensor, and any other sensor that can measure operating conditions at a common location remotely located from the sensor itself. 
     Although the sensors  38  are described in detail below as including a combination of QCL and optical detector  45 , other embodiments can include any suitable sensor that can withstand the conditions at the common location where the plurality of operating conditions is to be sensed. Further, two sensors could optionally be co-located according to alternate embodiments to sense their respective operating condition at the common location. Examples of such alternate embodiments of sensors  38  include, but are not limited to LOI sensors, temperature sensors, CO sensors, CO 2  sensors, NO x  sensors, O 2  sensors, THC sensors, volatile organic compounds (“VOC”) sensors, sulfur dioxide (SO 2 ) sensors, heat flux sensors, radiance sensors, opacity sensors, emissivity sensors, moisture sensors, hydroxyl radicals (OH) sensors, sulfur trioxide (SO 3 ) sensors, particulate matter sensors, and emission spectrum sensors. 
       FIG. 5  shows an illustrative embodiment of a control system  82  in communication with the plurality of sensors  38  ( FIG. 2 ) to receive the signals indicative of the combustion anomaly to control the actuators and execute the control method disclosed herein. As shown, the control system  82  includes a central processor  84  in communication with a computer-accessible memory  86 . A data bus  88  establishes a communication channel to facilitate the transmission of signals as part of the method disclosed herein. The computer-accessible memory  86  stores computer-executable instructions that, when executed by the central processor  84  instruct the central processor  84  to respond to signals from the sensors  38  to initiate control of the actuators as needed to promote substantially-balanced combustion within the furnace  18 . More specifically, responsive to receiving the signals the control system  82  traces one or more operating conditions sensed by the sensors  38  that fall outside of the predetermined range of suitable values to identify an offending burner contributing to a combustion anomaly. The central processor  84 , executing the computer-executable instructions from the computer-accessible memory  86  controls one or more of the actuators to adjust the at least one of the process input and the boiler configuration to bring the operating conditions into the predetermined range of suitable values as described in detail below. 
     A method of controlling operation of the system  10  specific to the boiler  12 , which includes a plurality of burners  28 , in accordance with an embodiment can be understood with reference to  FIG. 3 . The method illustrated in  FIG. 3  will be described with reference to a boiler  12  including a plurality of QCL embodiments of the sensors  38 , each for non-invasively measuring a plurality of operating conditions along the laser light between the QCL and its respective optical detector  45 . According to such an embodiment, the method includes sensing a plurality of operating conditions at a first common location, such as along the laser light  51  ( FIG. 2 ) for example, within the furnace  18  at  100  ( FIG. 3 ). The first common location where the plurality of operating conditions are sensed in the present embodiment can be thought of as an intersection of the laser light  51  ( FIG. 2 ) and a plane normal to the path of the laser light  51  within the furnace  18 . The sensed operating conditions at this intersection can represent an average value of the operating conditions sensed by the QCL embodiment of the sensors  38  between each QCL and their respective optical detector  45 . For the present embodiment, the plurality of operating conditions sensed include both the temperature of the flue gas and an amount of CO in the flue gas at the first common location. 
     The value of the operating conditions as determined by the QCL and its respective optical detector  45  can be recorded in a computer-accessible memory at  105 . Recording the value of the operating conditions preserves the sensed value of the operating conditions for comparison with those sensed values during a subsequent iteration of the present method to determine whether the combustion anomaly has been improved. 
     At least one of the plurality of operating conditions (both operating conditions in the present example) sensed at the first common location can be compared at step  107  to a range of predetermined acceptable values for those operating conditions. If the sensed value of each operating conditions falls within the respective predetermined ranges of acceptable values, the boiler  12  is operating properly and substantially-balanced combustion is achieved. Combustion is maintained at step  109  and the method returns to step  100  to continue monitoring of the operating conditions at the first common location. 
     If, however, it is determined at step  107  that one or more of the operating conditions falls outside the predetermined range of acceptable values for that operating condition, such a condition is indicative of a combustion anomaly occurring during operation of the boiler  12 . During the combustion anomaly the flue gas exiting the exhaust port  37  of the furnace  18  of the boiler  12  does not exhibit substantially-balanced combustion. 
     At step  110  in  FIG. 3 , the combustion anomaly indicated by one or more of the plurality of operating conditions sensed at the first common location is traced back to an offending burner that is at least partially responsible for the combustion anomaly. Tracing the combustion anomaly is based on a mathematical model that takes into consideration at least two of the plurality of operating conditions sensed at the first common location. Since the plurality of operating conditions are sensed at approximately the same location within the furnace  18 , these sensed operating conditions can be traced back to the offending burner without mapping one of the sensed operating conditions from a different spatial location within the furnace  18  to another, different spatial location where another operating condition was sensed. In other words, both the temperature and amount of CO sensed in the present example at the first common location within the furnace  18  can be traced from that same first common location back to one or more offending burners. Both are sensed at the first common location within the furnace  18 , and thus, one does not first have to be mapped to an equivalent value at another location where the other operating condition is sensed as a precursor to tracing the operating conditions back to the offending burner. Instead, each of the operating conditions can be considered as sensed at the first common location in identifying one or more offending burners responsible for the combustion anomaly indicated by one or both of the operating conditions falling outside of a predetermined range of acceptable values for those operating conditions. 
     The value of each of the sensed operating conditions can optionally be used in tracking the combustion anomaly back to the offending burner(s)  28 , and can optionally be used to identify the one or more burners  28  providing the most significant contributions to the combustion anomaly. For instance, a burner  28  vertically aligned with a QCL embodiment of a sensor  38  and a respective optical detector  45  may contribute more significantly to an under-temperature condition at the common location where the temperature is measured than another burner  28  horizontally offset from the QCL and respectively optical detector  45 . Further, if an amount of CO detected by the sensor  38  at the first common location where the temperature was also sensed exceeds a maximum allowable value, it can be determined that an oxygen deficiency exists within the furnace  18 . Based on the fluid dynamics within the particular furnace configuration this oxygen deficiency can be traced back to one or more burners  28  that are operating without sufficient levels of oxygen. 
     In response to tracing the combustion anomaly back to an offending burner at step  110  in  FIG. 3 , the method continues to include adjusting at least one of a process input and a boiler configuration affecting combustion of the offending burner at step  115 . For instance, if the amount of CO detected by the sensor  38  at the first common location where the temperature was also sensed exceeds a maximum allowable value, it can be determined that an oxygen deficiency exists within the furnace  18 . One or more actuators such as a damper  52  can be adjusted to introduce more oxygen into the furnace than the amount of oxygen being introduced in the environment of the offending burner  28  when the combustion anomaly was detected. The surplus of oxygen can establish a stoichiometry in the furnace  18  that promotes complete combustion of the hydrocarbon fuel to produce CO 2  instead of CO. According to alternate embodiments, a boiler configuration such as the angle at which an additive is injected into the furnace  18  can be adjusted at step  115  to bring the operating conditions sensed at the common location within the furnace  18  within the predetermined range of acceptable values. 
     The adjustment made at step  115  can be recorded at  117  to develop a real-time data model for correlating future deviations of the operating conditions sensed at the first common location within the furnace  18  to particular adjustments of process inputs and/or boiler configurations. The mathematical model can be updated in response to each adjustment to reflect the cause and effect of such adjustments on the operating conditions sensed at the first common location in the furnace  18  for subsequent iterations to correct future combustion anomalies. 
     To illustrate substantially-uniform operating conditions across the cross-section of the exhaust port  37  during substantially-balanced combustion, the exhaust port  37  in  FIG. 2  can be divided by two-dimensional grid lines  59  ( FIG. 4A ) for purposes of the present method. Dividing the cross-section of the exhaust port  37  into zones by the grid is conceptual for monitoring and controlling the combustion performance of the furnace  18 , and not a physical division of the exhaust port  37 . The grid dividing the exhaust port  37  into a plurality of zones is illustrated in  FIGS. 4A-4D , which is a cross section of the exhaust port  37  taken along line  4 - 4  in  FIG. 2 . 
       FIG. 4A  represents a cross-section of the flue gas exiting the furnace  18  during a combustion anomaly, wherein the flue gas exhibits non uniform operating conditions across the cross-section of the exhaust port  37 . The flue gas in  FIG. 4A  includes many temperature, CO, combustion or other suitable operating condition gradients, which are indicated in  FIG. 4A  by broken lines designated generally as  60 . Each broken line  60  indicates a combustion gradient, separating regions of the cross section exhibiting different degrees of combustion. For example, the region  62  enclosed by broken line  64  can include a greater amount of CO in the flue gas than the region  66  immediately outside of the broken line  64 . For alternate embodiments, the region  62  can represent a region of the flue gas that has a higher temperature than the region  66  immediately outside of the broken line  64 . In general, the broken lines  60  separate regions where combustion has progressed to different stages of completeness. 
     The operating conditions of the flue gas exiting via each zone are affected differently by the combustion of each burner  28  at different spatial locations within the furnace  18 . Thus, the adjustments to the process input and/or boiler configuration brought about by the method described above with reference to  FIG. 3  can be specific to those burners  28  contributing to the combustion anomalies within the various zones of the exhaust port  37 . For example, the region  62  having an unacceptably high CO concentration defined by broken line  64  in  FIG. 4A  can be rectified by adjusting the process input and/or boiler configuration affecting combustion of the offending burners  28  primarily contributing to the CO concentration at zones a, b, c and d defined by the grid. Adjusting the combustion performance of the offending burners  28  without altering the combustion performance of non-offending burners minimizes the number of combustion gradients  60  across the cross section of the exhaust port  37 . 
     The adjustment of the process input and/or boiler configuration as described with reference to  FIG. 3  minimizes combustion gradients  60  at the exhaust port  37  ( FIG. 2 ) of the furnace  18 . The cross-sectional view of the flue gas leaving the exhaust port  37  in  FIG. 4B  following a first iteration of the method resulting in an adjustment of at least one of the process input and the boiler configuration. Although the region  62  indicative of the combustion anomaly remains, fewer combustion gradients indicated by broken lines  60  exist than before the first adjustment of the process input and/or boiler configuration. Further, the degree of the combustion gradients may also be less than the degree of the combustion gradients appearing in  FIG. 4A . For instance, the region  62  in  FIG. 4B  may represent a CO concentration of the flue gas that is less than the CO concentration in the region  62  in  FIG. 4A . But just as for  FIG. 4A , the zones a, b, c and d in  FIG. 4B  where the region  62  is primarily located correspond to the same offending burners  28  contributing to the region  62  in  FIG. 4A , so further adjustment of the process input and/or boiler configuration for those offending burners is appropriate to promote substantially-balanced combustion. 
       FIG. 4C  illustrates another cross-sectional view of the combustion gradients for the flue gas exiting the exhaust port  37  of the furnace  18  following another iteration of the method appearing in  FIG. 3 . The cross-sectional view in  FIG. 4C  is approaching substantially-balanced combustion, and includes a primary combustion anomaly region  70  defined by the broken line  72  representing a combustion gradient. The primary combustion anomaly region  70  is present in a greater number of zones (outlined in bold lines and designated generally a-h) than the region  62  in  FIGS. 4A and 4B . In other words, there are fewer combustion gradients and more uniform combustion across the cross section of the exhaust port  37  in  FIG. 4C  than in  FIGS. 4A and 4B , which is indicative of substantially-balanced combustion. Further adjustment of the process input and/or boiler configuration will be specific to the offending burners  28  that are primary contributors to the combustion anomaly appearing across zones a-h in  FIG. 4C . 
     Finally, following yet another iteration of the method described with reference to  FIG. 3 , substantially-balanced combustion is achieved and the combustion gradients indicated generally by broken line  80  in  FIG. 4D  are minimized. As shown, the combustion is substantially uniform across a majority of the cross section of the exhaust port  37 . Substantially-balanced combustion does not necessarily require a complete absence of combustion gradients, but only that the combustion gradients are minimized over most of the cross section of the exhaust port  37 . 
     The invention has been described with reference to the example embodiments described above. Modifications and alterations will occur to others upon a reading and understanding of this specification. Example embodiments incorporating one or more aspects of the invention are intended to include all such modifications and alterations insofar as they come within the scope of the appended claims.