Abstract:
A combustion system is operated with reference to compliant values of a governmentally regulated exhaust emission parameter. If an alarm condition is detected during an ordinary mode of operation, the combustion system is shifted into an assured compliance mode of operation. The shift to the assured compliance mode is made while continuing to operate the combustion system without a shut-down interruption.

Description:
TECHNICAL FIELD 
     This technology relates to combustion systems for industrial heating plants. 
     BACKGROUND 
     Industrial heating plants such as boilers, steam generators, dryers and process heaters have combustion systems that produce exhaust emissions. The exhaust emissions may be subject to governmental regulations. Such regulations may require the operator of the heating plant to shut down a heating process upon discovering that an exhaust emission parameter has a non-compliant value above a maximum permitted value. The operator must then take corrective action before resuming the heating process. 
     SUMMARY 
     The claimed invention provides a method of operating a combustion system with reference to compliant values of a governmentally regulated exhaust emission parameter. 
     In its fullest extent the method includes the initial step of specifying a low compliant value below the maximum compliant value. Exhaust is then generated in a preliminary mode of operation by operating a reference combustion system with reactant flow rates that produce available heat. The available heat and the regulated emission parameter are measured in the preliminary mode of operation. Those measurements are used to identify a low range of available heat at which the regulated emission parameter of the exhaust is not greater than the low compliant value. 
     An ordinary mode of operation follows the preliminary mode. Exhaust is generated in the ordinary mode of operation by operating a user combustion system with reactant flow rates that produce available heat in a target range above the low range. If an alarm condition is detected during the ordinary mode of operation, the user combustion system is shifted into an assured compliance mode of operation. This is done while continuing to operate the user combustion system without a shut-down interruption. The shift into the assured compliance mode is accomplished by reducing the available heat from within the target range to within the low range. 
     Preferably, while the user combustion system continues to be operated without a shut-down interruption, the alarm condition is corrected and the user combustion system is subsequently shifted back from the assured compliance mode of operation to an ordinary mode of operation by increasing the available heat from within the low range to within the target range. 
     The reference combustion system and the user combustion system are preferably the same system, but could be different systems. The claimed invention further provides a method of retrofitting a combustion system by rendering it operative to perform as claimed. It follows that the invention further includes a retrofitted apparatus as well as an originally constructed apparatus that is operative as recited in the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view showing parts of an industrial heating apparatus having a combustion chamber, a combustion system including a burner that fires into the combustion chamber, and a flue that discharges exhaust from the combustion chamber to the atmosphere. 
         FIG. 2  is a schematic view showing additional parts of the apparatus of  FIG. 1 . 
         FIGS. 3 and 4  are similar to  FIGS. 1 and 2 , but show a reference apparatus. 
     
    
    
     DETAILED DESCRIPTION 
     The apparatus shown schematically in the drawings can be operated in steps that are examples of the elements recited in the method claims, and has parts that are examples of the elements recited in the apparatus claims. The following description thus includes examples of how a person of ordinary skill in the art can make and use the claimed invention. It is presented here to meet the requirements of enablement and best mode without imposing limitations that are not recited in the claims. The various parts, as shown, described, and claimed, may be of either original or retrofitted construction as required to accomplish any particular implementation of the invention. 
     The parts that are shown schematically in  FIG. 1  include a burner  10  in an industrial heating plant  12  for which the exhaust emissions are subject to governmental regulation. Such plants include, for example, boilers, steam generators, dryers, and process heaters. The burner  10  is part of a combustion system  14  that is operative to fire into a combustion chamber  15  containing the load (not shown) to be heated. A flue  17  discharges the exhaust from the combustion chamber  15  to the atmosphere. 
     A fuel source  20 , which is preferably a supply of natural gas, and an oxidant source  24 , which is preferably an air blower, provide the burner  10  with streams of those reactants. The burner  10  communicates with the fuel source  20  through a fuel control valve  30 . The blower  24  communicates with the ambient atmosphere through an oxidant control valve  32 . 
     Other parts of the combustion system  14  include a controller  40  that is operatively associated with the valves  30  and the  32 , and a sensor  50  that is operatively associated with the flue  17 . The controller  40  has hardware and/or software that is configured for operation of the burner  10 . The controller  40  may thus comprise any suitable programmable logic controller or other control device, or combination of control devices, that is programmed or otherwise configured to perform as recited in the claims. As the controller  40  carries out those instructions, it actuates the valves  30  and  32  to initiate, regulate, and terminate flows of reactant streams that cause the burner  10  to fire into the combustion chamber  15 . 
     The controller  40  provides the burner  10  with reactant streams at controlled flow rates. In an ordinary mode of operation, the flow rates of the reactant streams are controlled to be appropriate for the amount of available heat needed for the industrial heating process to be performed in the chamber  15 , and also to have a fuel-to-oxidant ratio within a target range. Available heat is defined as the gross quantity of heat released within a combustion chamber minus both the dry flue gas loss and the moisture loss. It represents the quantity of heat remaining for useful purposes and to balance losses to walls, openings, or conveyors, etc. The target range of fuel-to-oxidant ratios is determined with reference to predetermined flow rate data indicating that the exhaust produced by combustion of the reactant streams will contain oxides of nitrogen (NOx) at levels that are likewise within a target range. The target range of NOx levels is below a threshold alarm level which, in turn, is below a maximum level permitted in compliance with an applicable governmental regulation. 
     The sensor  50  is responsive to the composition of the exhaust in the flue  17 . The sensor  50  could sense and indicate NOx content directly. Alternatively, the sensor  50  could sense a different component of the exhaust, such as oxygen, and thus indicate NOx content by implication. In the latter case, the controller  40  would be configured to infer the NOx content of the exhaust based on a known relationship of NOx to total oxygen. In either case, the controller  40  measures the NOx content of the exhaust based on input from the sensor  50 , and compares the measured NOx content with the target range of NOx levels that are expected to result from the target range of fuel-to-oxidant ratios at the burner  10 . If the measured NOx level is above the target range of NOx levels, the controller  40  generates an alarm signal. 
     In addition to the alarm signal that indicates NOx above the target range, the controller  40  is further operative to generate alarm signals that indicate other alarm conditions. As shown schematically in  FIG. 2 , the controller  40  is operatively associated with other sensors in addition to the sensor  50  as described above. That sensor  50  provides the controller  40  with input that directly or indirectly indicates the amount of NOx in the exhaust in the flue  17 . At least one other sensor  60  similarly provides the controller  40  with input that directly or indirectly indicates the amount of a different exhaust component such as, for example, CO. The controller  40  measures the CO content of the exhaust based on input from the other sensor  60 , and compares the measured CO content with a target range of CO levels that are expected to result from the target range of fuel-to-oxidant ratios at the burner  10 . If the measured CO level is above the target range of CO levels, the controller  40  generates an alarm signal. 
     The additional sensors that are shown as examples in  FIG. 2  include a fuel composition sensor  70 , a fuel flow rate sensor  80 , and an oxidant flow rate sensor  90 . Also shown in  FIG. 2  is an exhaust temperature sensor  100 . These sensors  70 ,  80 ,  90 , and  100 , as well as the sensors  50  and  60  described above, are known devices. The exhaust sensors  50 ,  60  and  100  are operatively associated with the flue  17 , as indicted by the sensor  50  shown in  FIG. 1 . The other sensors  70 ,  80  and  90  can be installed in the apparatus of  FIG. 1  at any suitable locations known in the art. In accordance with the claimed invention, the controller  40  can generate an alarm signal if input from the fuel composition sensor  70  indicates an unacceptable change in fuel composition. The controller  40  can also generate an alarm signal indicating an unacceptable change in a reactant flow rate through a control valve  30  or  32 , as indicated by input from the corresponding flow rate sensor  80  or  90 . Input from the exhaust temperature sensor  100  might also prompt an alarm signal from the controller  40 . Moreover, the controller  40  can generate an alarm signal in response to input from any of the sensors  50 - 100 , or from any other device with which the controller  40  is operatively associated in the combustion system  14 , if that input indicates a malfunction of the respective device. 
     The alarm signal generated by the controller  40  might indicate that the detected alarm condition is relatively minor. For example, an alarm signal responding to input from the exhaust content sensor  50  might indicate that the measured level of NOx in the exhaust exceeds a specified threshold alarm level that is higher than the target range but complies with the governmental regulation. If so, the controller  40  continues to fire the burner  10  in the ordinary operating mode by providing the burner  10  with reactant streams having fuel-to-oxidant ratios within the target range. However, the alarm signal generated by the controller  40  might indicate that the measured level of NOx in the exhaust approaches or exceeds the maximum compliant level. If so, the burner  10  continues to be fired without a shutdown interruption, but the controller  40  shifts from the ordinary operating mode to an assured compliance mode in which the exhaust generated by the burner  10  contains NOx at levels below the specified threshold alarm level. 
     The controller  40  can shift the combustion system  14  into an assured compliance mode by reducing the available heat in the chamber  15  such that NOx production will be reduced to a level below the level at which the alarm signal was generated. The controller  40  can measure available heat in a known manner, and the reduction can be accomplished by any operating technique or condition that is known to reduce available heat. For example, the controller  40  can calculate available heat from input received from the exhaust oxygen sensor  50 , the fuel flow rate sensor  80 , and the exhaust temperature sensor  100 . Additional input from the other sensors  60 ,  70 , and  90  also could be used in the calculation of available heat in a known manner. Techniques for reducing the available heat include delivering steam or recirculated flue gas to the burner  10  as a diluent. This could be accomplished by the use of a flue gas recirculation (FGR) line  102  with an FGR valve  104 . In a preferred implementation of the claimed invention, the reduction in available heat is accomplished by actuating the fuel and oxidant control valves  30  and  32  to reduce the fuel-to-oxidant ratio at the burner  10 . That ratio is reduced to a value below the target range sufficiently to reduce the NOx content of the exhaust to a compliant level that is below the specified threshold alarm level, and is thus well below the maximum compliant level. For example, the reduced level of NOx content could be about 75% of the maximum compliant level. 
     Shifting into an assured compliance mode enables the burner  10  to continue firing into the combustion chamber  15  without a shut-down interruption that otherwise would be necessary to avoid NOx emissions above the maximum compliant level. The industrial heating process performed by the plant  12  can then be continued without a shut-down interruption while the operator examines the plant  12  to identify a cause for the non-compliant level of NOx. If the cause can be corrected without shutting down the combustion system  14 , the assured compliance mode enables the heating process to be continued still further without a shut-down interruption while the operator makes a correction and, in this example, subsequently actuates the controller  40  to shift the combustion system  14  back from the assured compliance mode to an ordinary operating mode by increasing the fuel-to-oxidant ratio at the burner  10  to a value within the target range. 
     The foregoing example proposes that an assured compliance mode can be reached by reducing the available heat in the chamber  15  to a value at which the exhaust NOx is well below the maximum compliant level. In order to shift into an assured compliance mode in this manner, the controller  40  would require a predetermined reference value of low available heat to which the measured value of available heat must be reduced. The reference value of low available heat would preferably be established empirically. In the given example it could be established by first specifying a low compliant level of NOx to be produced in the assured compliance mode. The burner  10  would then be fired in a preliminary operating mode in which a low range of available heat is identified. Specifically, the burner  10  would be fired with reactant flow rates that produce NOx at or below the specified low compliant level, as measured by the controller  40  with input from the sensor  50  at the flue  17 . The available heat in the chamber  15  would be measured at those flow rates to identify a low range of available heat at which NOx production will not exceed the specified low compliant level. Accordingly, that low range of available heat could then serve as the reference to which the measured available heat can be reduced to accomplish a shift into the assured compliance mode. 
     In the foregoing example, the preliminary mode of operation that establishes the reference range of available heat is performed by the same combustion system  14  that later uses the reference range in an assured compliance mode. However, it is not considered necessary to use a single combustion system as both the reference system and the user system. A suitable reference system could be operated in the preliminary mode to establish a low range of available heat for an assured compliance mode in one or more other user systems. Such a reference system  14 ′ is shown in  FIGS. 3 and 4 . 
     The heating plant  12  can be constructed such that the combustion system  14  is originally configured to operate with an assured compliance mode. Alternatively, a heating plant with an existing combustion system that is not configured to operate with an assured compliance mode can be retrofitted to do so. This could be accomplished by operating either the existing combustion system or a different reference system in a preliminary operating mode to establish an assured compliance mode in the manner described above. The existing combustion system could then be configured to operate in an ordinary operating mode with the capability of shifting into the assured compliance mode that was established in the preliminary operating mode. 
     The patentable scope of the invention is defined by the claims, and may include other examples of how the invention can be made and used. In this regard, the schematic illustration of  FIG. 1  shows a burner system in which two reactant streams are delivered to a single burner  10 , but the claimed invention can be practiced with a burner system that includes a plurality of burners, and the fuel-to-oxidant ratio could be defined in whole or in part by staged reactants. Moreover, a particular heating process may include operating modes in which some or all of a plurality of burners are cycled on and off in accordance with predetermined conditions of time, temperature and/or other heating process parameters. The assured compliance mode for any particular heating process can include any such burner firing interruptions without including a shut-down interruption of the burner system that would terminate the heating process. Such other examples, which may be available either before or after the application filing date, are intended to be within the scope of the claims if they have structural or method elements that do not differ from the literal language of the claims, or if they have equivalent structural or method elements with insubstantial differences from the literal language of the claims.