Patent Publication Number: US-2007100502-A1

Title: Systems and methods to control a multiple-fuel steam production system

Description:
FIELD OF THE DISCLOSURE  
      The present disclosure relates generally to processor control systems and, more particularly, to process control systems and methods to control a multiple-fuel steam production system.  
     BACKGROUND  
      Process systems, like those used in paper production or other manufacturing processes, often use steam production processes to generate steam for powering various sub-systems and for on-site generation of electricity. To produce steam, a steam production process system is provided with an energy source such as a combustible fuel. Single fuel steam production systems are typically powered using refined oil or natural gas. To reduce the costs associated with fuel, companies have implemented multiple-fuel powered steam production systems. A multiple-fuel (i.e., multi-fuel) steam production system provides cost effective steam production by burning fossil fuels (e.g., gas, oil, coal, etc.) in addition to alternative, lower-cost fuels such as, for example, waste wood and shredded tires. By balancing the supply rate, feed rate, flow rate, etc. of a fossil fuel and a lower-cost fuel, a multi-fuel steam production system can operate at a relatively lower cost by relying relatively more on the lower-cost fuel than the more expensive fossil fuel whenever possible while maintaining required steam output.  
      Maintaining the appropriate balance between fuels in a multi-fuel steam production system often poses a challenge due to the varying energy content, concentrations, or output associated with each fuel type. For example, while refined fossil fuels typically provide a constant energy content per volume (e.g., an amount of energy per volume measured in, for example, joules or British Thermal Units (BTU&#39;s)), the amount of energy per volume in lower-cost fuels such as waste wood or shredded tires (or other waste material) varies within each batch and from batch to batch as the alternative lower-cost fuel is supplied or fed to the steam production system.  
      Steam output can become non-compliant or fall out of a desired or required operating range when energy content per volume changes within a batch or between batches of alternative fuel. For instance, settings (e.g., fuel ratios) of a steam production system may be set according to a particular energy content of a previously supplied alternative fuel (e.g., a previous batch of waste wood) when a subsequent supply of alternative fuel having a different energy content is supplied. In this case, if the change in energy content per volume in the alternative fuel decreases, the amount of produced steam decreases, thus requiring an increase in the amount of refined fossil fuel needed to compensate for the decreased energy content in the alternative fuel.  
      To compensate for the varying levels of energy content per volume of alternative, lower-cost fuels, some traditional steam production systems require a skilled operator to monitor various aspects of the steam production process to ensure that an appropriate balance is maintained between the amount of refined fossil fuel supplied and the amount of alternative fuel supplied to maintain steam production within an acceptable operating range. These traditional systems require that an operator constantly observe measurement gauges and alarms and make adjustments to fuel supply ratios in response to non-compliant gauge readings or alarms indicative of an inappropriate balance between the fossil fuel supply and the alternative fuel supply. Traditional steam production systems controlled manually by a skilled operator are often inefficient due to the limited knowledge or skill of the operators, the response times of operators, and the operators&#39;interpretations of measurement gauges and alarms. Further, the efficiencies and fuel consumption of these traditional systems are typically non-deterministic because operator responses can vary over time and between operators.  
      To automate the procedure of maintaining balance (e.g., fuel ratios) between fossil fuels and alternative fuels, other traditional steam production systems use one or more proportional-integral-derivative (PID) controllers that monitor steam production quantities to dynamically determine steam output and automatically adjust fuel ratio settings. However, these traditional systems use measurements of present operation to adjust fuel ratios in a reactive or lagging manner such that inefficiencies result between the time at which steam production output is recognized as being non-compliant (e.g., outside of a target operating range), the time at which the PID controller detects the non compliancy of the steam production, and the time at which the PID controller adjusts of the fuel ratio to correct the non-compliant steam production output.  
      The inefficiencies associated with known manually controlled and PID-controlled steam production systems can lead to higher operating costs because excessive amounts of relatively higher-cost fossil fuels are used. These known systems can also result in lower manufacturing product yields (e.g., paper production yields) when steam production outputs fall below minimal threshold levels due to inappropriate fuel ratios.  
     SUMMARY  
      Example systems and methods to control a multiple-fuel steam production system are disclosed. An example method involves obtaining a plurality of input values associated with producing steam and using a model predictive controller to determine a first value associated with predicting an amount of first fuel and a second value associated with predicting an amount of second fuel to produce an amount of steam. Fuel feed rates of the first and second fuels are then controlled based on the first and second values.  
      In accordance with another example, an example system includes a model predictive controller to determine a first value associated with predicting an amount of a first fuel and a second value associated with predicting an amount of a second fuel to produce an amount of steam. The example system also includes first and second fuel feeder controls to control fuel feed rates of the first and second fuels based on the first and second values.  
      In accordance with another example, an example machine accessible medium includes instructions stored thereon that, when executed, cause a machine to determine a first value associated with predicting an amount of a first fuel and a second value associated with predicting an amount of a second fuel to produce an amount of steam. Additionally, the instructions cause the machine to control fuel feed rates of the first and second fuels based on the first and second values. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  depicts an example multi-fuel steam production process system.  
       FIG. 2  is a detailed block diagram of the example control system of  FIG. 1  that may be used to implement the example systems and methods described herein.  
       FIG. 3  is a flow diagram that depicts an example method that may be used to control the example steam production system of  FIG. 1 .  
       FIG. 4  is a flow diagram depicting an example method that may be used to determine predicted trajectory adjustment output values associated with fuel supply rates to a boiler furnace.  
       FIG. 5  is a flow diagram of an example method that may be used to determine energy compensation values associated with adjusting fuel feed rates in the example steam production system of  FIG. 1  in response to varying fuel energy content.  
       FIG. 6  is a flow diagram of an example method that may be used to determine required amounts of fuel to operate the example steam production system of  FIG. 1  within specified operating conditions.  
       FIG. 7  is a flow diagram of an example method that may be used to determine and control the required airflows of the example steam production system of  FIG. 1 .  
       FIG. 8  is a block diagram of an example processor system that may be used to implement the example systems, methods, and articles of manufacture described herein. 
    
    
     DETAILED DESCRIPTION  
      Although the following discloses example systems including, among other components, software and/or firmware executed on hardware, it should be noted that such systems are merely illustrative and should not be considered as limiting. For example, it is contemplated that any or all of these hardware, software, and firmware components could be embodied exclusively in hardware, exclusively in software, or in any combination of hardware and software. Accordingly, while the following describes example systems, persons of ordinary skill in the art will readily appreciate that the examples provided are not the only way to implement such systems.  
      In contrast to some known multiple-fuel powered steam production systems that use proportional-integral-derivative (PID) controllers to automatically control the fuel ratios used to supply or feed different fuels to steam boilers using reaction-driven techniques, the example systems, methods, and articles of manufacture described herein may be used to automatically control fuel ratios using predictive analyses and control. In some known multiple-fuel powered steam production systems, automatic process controllers analyze measurement of current operating conditions and react to those operating conditions by, for example, adjusting fuel ratios only when the operating conditions have approached or exceeded a compliant operating condition. Known systems typically use PID feedback loops that can only react to a present or current state of operation. As a result, these known systems often fall into non-compliant operating states before any automatic correction adjustment is made or effective. Thus, known steam production systems often operate inefficiently due to the delay between the time at which the system starts operating in a non-compliant condition and the time at which a process controller detects the condition and reacts by making corrective adjustments.  
      In contrast to the above-noted known systems, the example systems and methods described herein use predictive techniques to determine the manner in which a steam production system should be controlled to substantially reduce or prevent instances in which (or the time during which) the steam production system operates outside of certain operating thresholds or ranges, thus increasing the efficiency of the steam production system. An example implementation uses a model predictive controller and fuzzy logic to monitor and process various measurement data (e.g., energy content of fuel(s), fuel feed rate(s), steam flow, steam pressure, fuel cost(s), etc.) associated with the steam production system to determine forward-looking or predicted control parameters that should be used to configure the steam production system to maintain efficient and compliant operation.  
      In some example implementations, efficient operation involves maintaining a desired steam production output by relying more on alternative lower-cost fuels (e.g., waste wood, shredded tires, etc.) than on relatively more expensive fossil fuels (e.g., coal, gas, oil, etc.). Multi-fuel steam production systems can become inefficient when a fossil fuel-to-alternative fuel ratio is higher than necessary. Compliant operation typically involves outputting an amount of steam that is within a desired or a required operating range (e.g., an amount of steam output required to run other manufacturing subsystems or to generate a desired amount of electricity via steam-powered turbines) so that other sub-systems of a manufacturing site can receive the required steam power (or electric power) to operate or operate efficiently.  
      As described in greater detail below, the example systems and methods use model predictive controllers and fuzzy logic in multi-fuel steam production systems to determine fuel supply ratios associated with alternative, lower-cost fuels having varying energy content per volume and fossil fuels to produce a desired or required amount of steam production output, while maintaining relatively low operating costs. Because the price of oil fluctuates continuously, the price of oil is a factor that the example methods and systems described herein can use to determine fuel supply ratios such that a steam production system produces a desired or required amount of steam, yet operates within prescribed budgetary constraints.  
       FIG. 1  is a diagram representative of an example steam production process system  100 . The example steam production system  100  is a multi-fuel system that may be implemented at a manufacturing site (e.g., a paper mill) to produce steam used for operating various manufacturing sub-systems, and/or to produce on-site electricity (e.g., via steam turbines), and/or for any other purpose. The example systems and methods are described herein as being advantageously applicable to controlling a steam production system (e.g., the example steam production system  100 ) that burns different fuel types, at least one of which may be associated with a varying energy content characteristic (e.g., varying BTU per volume). In particular, the example steam production system  100  is described below as using a fossil fuel and an alternative lower-cost fuel. However, in alternative implementations, the example systems and methods described herein may be used to control steam production systems that use any other combination of two or more fuel types. For example, the example systems and methods described herein may be used to control a steam production system that uses a first fuel type having particular characteristics (e.g., cost characteristics, energy content characteristics, byproduct characteristics, etc.) and a second fuel type having different characteristics (e.g., a different cost, a different energy content, different byproducts, etc.) than the first fuel type.  
      As shown in  FIG. 1 , the example steam production system  100  includes a steam boiler  102  that receives water from a water supply  104 . The steam boiler  102  includes a furnace  106  that burns multiple fuel types to produce steam. In particular, the furnace  106  receives a fossil fuel (e.g., a first fuel) from a fossil fuel supply reservoir  108  (e.g., a first fuel type supply reservoir) and an alternative fuel (e.g., a second fuel) from an alternative fuel supply reservoir  110  (e.g., a second fuel type supply reservoir). The fossil fuel may be, for example, coal, oil, gas, etc., and the alternative fuel may be a lower-cost fuel such as, for example, wood waste, shredded tires, etc.  
      The example steam production system  100  also includes an example control system  112  to acquire and monitor various operating conditions (e.g., energy content of fuel, fuel costs, steam flow, steam pressure, etc.) of the steam production system  100  to determine configuration settings (e.g., fuel supply ratios) that should be used to maintain the steam production output within a predetermined, required or desired operating range (e.g., output a particular amount of steam), while maintaining other operating characteristics (e.g., fuel consumption costs, emissions, steam pressure, etc.) within predetermined, required or desired operating ranges. As described in greater detail below in connection with  FIG. 2 , the example control system  112  uses model predictive controllers and fuzzy logic to predict configuration settings to substantially reduce or eliminate instances (or the time) during which the example steam production system  100  operates in a non-compliant (and potentially inefficient) condition. In particular, the control system  112  uses measurements of current and/or previous operating conditions to perform analyses to predict how the steam production system  100  may operate in the near or distant future and, based on those analyses, generates configuration settings that are forward-looking to prevent the steam production system  112  from operating outside the predetermined, required or desired operating range(s).  
      As shown in  FIG. 1 , the example control system  112  communicates with a water supply valve  114  to control the supply rate or feed rate of water to the boiler  102 , a fossil fuel supply valve  116  to control the supply or feed rate of fossil fuel supply to the furnace  106 , an alternative fuel supply valve  118  to control the supply or feed rate of alternative fuel to the furnace  106 , and an air supply valve  120  to control the supply or feed rate of air to the furnace  106  via an air intake  121 . To measure the feed rates or flow rates of each of the supplies (e.g., fuel, water, or air), the control system  112  may be communicatively coupled to a plurality of sensors  122 ,  124 ,  126 , and  128 .  
      Although the illustrated example of  FIG. 1  depicts using the fossil fuel supply valve  116  and the alternative fuel supply valve  118  to control the feed rate of each of the fuels, in other example implementations, the feed rates of either one or both of the fossil fuel and the alternative fuel may be controlled using a conveyor and a conveyor speed control. For example, if the fossil fuel is coal, the coal may be delivered from the fossil fuel reservoir  108  to the furnace  106  using a conveyor system, and the speed of the conveyor system may be controlled using a conveyor speed control to increase or decrease the fossil fuel feed rate. Additionally, if the alternative fuel is waste wood (e.g., tree bark), the waste wood may be delivered from the alternative fuel reservoir  110  to the furnace  106  using a conveyor system and conveyor speed control.  
      The control system  112  is also communicatively coupled to a steam flow sensor  130  to measure the flow rate of steam supplied by the boiler  102 . Of course, in alternative implementations, the steam flow sensor  130  may be placed at any other location such as, for example, a steam supply pipe coupled to a steam header.  
      The control system  112  is also communicatively coupled to a pressure sensor  132  to measure the steam pressure in the boiler  102 . Those of ordinary skill in the art will readily appreciate that in alternative implementations the pressure sensor  132  may be placed at any other location throughout a steam production system other than that shown in  FIG. 1  such as, for example, at a steam header or a steam supply pipe.  
      To measure exhaust emissions produced by the furnace  106 , the control system  112  is communicatively coupled to an emission sensor  134  located at an emission exhaust fan  136 .  
      Although not shown, the control system  112  may be communicatively coupled to other sensors (e.g., temperature sensors, flow/feed sensors, pressure sensors, etc.) located throughout the example steam production system  100  to obtain measured values for use in implementing the example systems and methods described herein.  
       FIG. 2  is a detailed block diagram of the example control system  112  of  FIG. 1 . The control system  112  may use predictive control techniques to control operation of the example steam production system  100  by determining forward-looking or predicted configuration settings based on present-time monitored conditions. In this manner, the control system  112  can proactively respond to the monitored conditions by changing or adjusting any necessary configuration settings to substantially reduce or prevent the steam production system  100  from operating out of predetermined, desired or required operating conditions (e.g., a steam flow, a steam pressure, fuel consumption cost, etc.). The control system  112  is configured to operate in a steam flow monitoring mode and a steam pressure monitoring mode. For example, the control system  112  may control operation of the steam production system  100  based on monitoring the steam flow measured via, for example, the steam flow sensor  130  ( FIG. 1 ). Alternatively, for example, the control system  112  may control operation of the steam production system  100  based on monitoring the steam pressure measured via, for example, the steam pressure sensor  132  ( FIG. 1 ). Whether the control system  112  operates in a steam flow monitoring mode or a steam pressure monitoring mode may be controlled manually by an operator or automatically based on, for example, a schedule and/or any other criteria.  
      The example structures shown in  FIG. 2  depicting the example control system  112  may be implemented using any desired combination of hardware and/or software. For example, one or more integrated circuits, discrete semiconductor components, or passive electronic components may be used. Additionally or alternatively, some or all, or parts thereof, of the example structures of  FIG. 2  may be implemented using instructions, code, or other software and/or firmware, etc. stored on a computer-readable medium that, when executed by, for example, a processor system (e.g., the processor system  810  of  FIG. 8 ), perform the methods described herein.  
      To operate in a steam flow mode, the control system  112  includes a steam flow model predictive controller (MPC) master  202 . In an example implementation, the steam flow MPC master  202  may be implemented using an MPC available in the Delta V control system designed and sold by Emerson Process Management, Austin, Tex. The steam flow MPC master  202  is configured to control an amount of steam flow in response to, among other inputs or parameters, steam flow measurements and/or changes in steam flow requirements provided by, for example, an operator. The steam flow MPC master  202  determines two separate outputs associated with setpoints for the alternative fuel feed rate (e.g., the rate at which fossil fuel is supplied to the furnace  106  from the fossil fuel reservoir  108  of  FIG. 1 ) and the fossil fuel feed rate (e.g., the rate at which alternative fuel is supplied to the furnace  106  from the alternative fuel reservoir  110  of  FIG. 1 ).  
      The steam flow MPC master  202  uses measured steam flow values and a plurality of other input values to determine a predicted trajectory adjustment output value  204  to achieve a specified fossil fuel feed rate and a predicted trajectory adjustment output value  206  to achieve a specified alternative fuel feed rate. The fossil fuel adjustment output value  204  is indicative of a required change (e.g., an increase or decrease) in fossil fuel demand to achieve a particular level of energy (e.g., BTU&#39;s) to increase or decrease steam flow. The alternative fuel adjustment output value  206  is indicative of a required change in the alternative fuel supply rate to achieve a particular level of energy. The steam flow MPC master  202  may determine an example predicted trajectory output value based on analyses of historical system condition data and response data. Alternatively or additionally, the example predicted trajectory output value may also be determined using curve-fit techniques or data interpolation techniques. In any case, the example predicted trajectory output values  204  and  206  represent forward looking settings associated with fuel feed rates (e.g., alternative and/or fossil fuel feed rates) that can keep the steam production system  100  operating for a particular or minimum amount of time in the future based on current operating condition values and/or other values obtained by the steam flow MPC master  202 .  
      In the illustrated example, the adjustment output values  204  and  206  work in combination to provide a suitable fossil fuel-to-alternative fuel supply ratio that enables the steam production system  100  to operate within specified operating conditions (e.g., to produce a specified steam flow, operate within steam pressure constraints, operate within cost limits, etc.). When operating in a steam flow control mode, the adjustment output values  204  and  206  are provided to (e.g., cascade) to respective inputs of a fossil fuel total energy module  240  and an alternative fuel total energy module  250  described in greater detail below. Specifically, the fossil fuel adjustment output value  204  is a setpoint value for the fossil fuel total energy module  240  and the alternative fuel adjustment output value  206  is a setpoint value for the alternative fuel total energy module  250 .  
      The steam flow MPC master  202  may be configured to use relatively more alternative lower-cost fuel than fossil fuel to meet economic or budgetary operating conditions. To tune or adjust the fuel type preference (e.g., using relatively more of one type of fuel than another) the steam flow MPC master  202  is provided with a fuel costs input  208  and fuel use preference settings (not shown). In this manner, the steam flow MPC master  202  can adjust use of fuel types as needed in response to changes in fuel prices and based on the fuel use preference settings. An operator may provide particular fuel use preference settings to the steam flow MPC master  202  to configure the steam flow MPC master  202  to use relatively more (e.g., maximize use) or relatively less (e.g., minimize use) of a particular fuel (e.g., one of the fossil fuel or alternative fuel) based on, for example, the fuel costs input  208 . For example, the fuel use preference settings may include minimum and/or maximum fuel cost thresholds for each of the fossil fuel and alternative fuel that configure the steam flow MPC master  202  to use relatively more or less of the fuel types when respective fuel costs exceed (e.g., become less than or greater than) respective minimum or maximum fuel cost thresholds. For instance, under some operating conditions, if the cost of fossil fuel provided via the fuel costs input  208  becomes greater than a maximum fossil fuel cost threshold (provided via the fuel use preference settings), the steam flow MPC master  202  can reduce the supply rate of fossil fuel as much as possible whenever possible and increase the supply rate of alternative fuel (e.g., optimize fuel use) until, for example, the cost of fossil fuel falls below the maximum and/or minimum fossil fuel cost threshold.  
      As shown in  FIG. 2 , the steam flow MPC master  202  receives steam flow measured values from the steam flow sensor  130  and a steam flow setpoint input value  212  (i.e., a specified or predetermined, desired or required steam flow value). In some example implementations, to substantially reduce or eliminate the effects of pressure and temperature on the steam flow measured values, the steam flow MPC master  202 , or some other device or module, may obtain pressure and temperature measurements associated with the boiler  102  to generate temperature and pressure-compensated steam flow values based on the steam flow measured values received from the steam flow sensor  130 . The steam flow setpoint input value  212  may be provided by an operator and may be based on the amount of steam required to operate steam-operated sub-systems of, for example, a manufacturing site.  
      The steam flow MPC master  202  determines the adjustment output values  204  and  206  by determining an error or a deviation between the steam flow measured values and the steam flow setpoint input value  212  and determining the required change in fuel demand (e.g., alternative fuel and/or fossil fuel) to substantially reduce or eliminate the error or deviation. To maintain the steam flow measured values substantially equal to the steam flow setpoint input value, the steam flow MPC master  202  generates the adjustment output values  204  and  206  to cause an increase or decrease in the fuel feed rates. For example, if the energy content of the alternative fuel decreases over time due to, for example, a change in waste wood quality, the furnace  106  may not create sufficient heat to create the required steam flow. In this case, one or both of the adjustment output values  204  and  206  may be increased to increase the amount of fuel delivered to the furnace to cause the boiler  102  to increase the steam flow rate. The steam flow MPC master  202  generates the adjustment output values  204  and  206  in accordance with a fossil fuel-to-alternative fuel feed rate ratio that complies with, for example, the fuel costs input  208 , fuel use preference settings (e.g., maximize, minimize, or otherwise optimize use of the fossil fuel or the alternative fuel), and the required energy to produce the required steam flow.  
      In some example implementations, the steam flow MPC master  202  may be provided with maximum feed rate limits for one or both of the fuel types. For example, as shown in  FIG. 2 , the steam flow MPC master  202  is provided with an alternative fuel setpoint  213  that indicates the maximum amount or feed rate for the alternative fuel. Under some conditions, the maximum feed rate limits may prevent the steam flow MPC master  202  from maintaining a fossil fuel-to-alternative fuel feed rate ratio that complies with the fuel costs input  208  and fuel use preference settings. For instance, if the energy content of the alternative fuel is not high enough to create the required steam flow even when the alternative fuel feed rate has been set or increased to the maximum limit (i.e., equal to the setpoint  213 ), the steam flow MPC master  202  will increase the fossil fuel adjustment output value  204  to provide the required energy, regardless of the resulting fossil fuel-to-alternative fuel feed rate ratio.  
      The steam flow MPC master  202  determines the adjustment output values  204  and  206  at periodic or aperiodic time intervals. In particular, after the steam flow MPC master  202  analyzes its plurality of input values and determines suitable adjustment output values  204  and  206 , the steam flow MPC master  202  determines when it should subsequently analyze the input values to determine whether different adjustment output values  204  and  206  should be generated. Specifically, because the control system  112  controls the steam production system in a proactive, predictive, forward-looking manner, the output values or control values (e.g., the adjustment output values  204  and  206 ) provided by the control system  112  are generated so that the steam production system  100  operates within specified operating conditions for at least a particular or minimum amount of time (t f ) in the future. The steam flow MPC master  202  can specify a time that is prior to the expiration of the future time tf at which to again analyze the steam flow measurement.  
      To prevent operating the steam production system  100  in unstable or undesirable conditions, the steam flow MPC master  202  is also provided with a plurality of constraint values  214 . The constraint values  214  are measured variables associated with specified threshold limits that may be provided by, for example, an operator. As the constraint values  214  approach their respective threshold limits, the steam flow MPC master  202  determines adjustment values (e.g., the adjustment output values  204  and  206 ) to relieve (e.g., increase or decrease) the constraint values  214 .  
      As shown in  FIG. 2 , the constraint values  214  include an alternative fuel reservoir level, an induced-draft (ID) damper position, an ID fan amperage rating, a boiler drum water level, a measured steam pressure (e.g., a boiler header pressure), an emissions output level, and an oxygen intake. The alternative fuel reservoir level indicates the amount of alternative fuel remaining in the alternative fuel reservoir  110  ( FIG. 1 ). The measured steam pressure may be obtained from the steam pressure sensor  132  ( FIG. 1 ). The emissions output level may be obtained from the emission sensor  134  ( FIG. 1 ). The oxygen intake may be obtained from the air flow sensor  128  ( FIG. 1 ).  
      Each of the constraint values  214  is associated with one of a plurality of constraint priorities  216 . An operator may provide the constraint priorities  216  to prioritize each of the plurality of constraint values  214 . Prioritizing the constraint values  214  specifies the order in which the steam flow MPC master  202  considers (or complies) with each of the constraint values  214 . For example, an operator may assign first priority (e.g., a highest priority) to the boiler drum water level constraint value to ensure that the steam flow MPC master  202  determines values for the adjustment output values  204  and  206  that will not cause the boiler drum water level to exceed a boiler drum water level constraint threshold. In some cases, to ensure that higher priority constraint values (e.g., the boiler drum water level constraint value) do not violate respective constraint thresholds, the steam flow MPC master  202  may determine values for the adjustment output values  204  and  206  that incidentally or purposely cause lower priority constraint values to violate respective constraint thresholds.  
      To monitor the effect on steam amounts or quantities demanded by steam-powered machines or sub-systems of a process system that is at least in part powered by the steam produced by the steam production system  100  ( FIG. 1 ), the steam flow MPC master  202  is provided with a plurality of disturbance values  218 . The disturbance values  218  may be provided by field devices, field sensor, or field monitors that monitor the operation of sub-systems or machines that use steam produced by the steam production system  100 . In this manner, when any sub-system or machine that demands a particular amount of steam shuts down, starts operation, slows operation, increases operation, etc., the steam flow MPC master  202  can predict an increase or decrease in steam demand and determine the adjustment output values  204  and  206  accordingly to ensure that the steam production system  100  increases or decreases steam production to proactively account for any subsequent increase or decrease in steam demand caused by the change in operation of any steam-demanding sub-system or machine. Instead of waiting for steam demand changes to substantially affect operating conditions (e.g., steam pressure) of the steam production system, proactively determining (e.g., predicting) the adjustment output values  204  and  206  to account for any subsequent changes in steam demand based on the disturbance values  218  ensures that the steam demand changes will not substantially affect (e.g., adversely affect) operating conditions of the steam production system  100 .  
      To operate in a steam pressure mode, the control system  112  includes a steam pressure MPC master  222 . In an example implementation, the steam pressure MPC master  222  may be implemented using an MPC available in the Delta V control system designed and sold by Emerson Process Management, Austin, Tex. The steam pressure MPC master  222  is configured to control the amount of steam pressure in response to, among other inputs or parameters, steam pressure measurements and/or changes in steam pressure requirements provided by, for example, an operator. The steam pressure MPC master  222  operates to control the amount of steam pressure generated by the boiler  102  ( FIG. 1 ) as the steam flow MPC master  202  operates to control an amount of steam flow as described above. For instance, the steam pressure MPC master  222  determines two separate outputs associated with setpoints for the alternative fuel feed rate and the fossil fuel feed rate. Specifically, the steam pressure MPC master  222  uses measured steam pressure values and a plurality of other input values to determine a predicted trajectory adjustment output value  224  to achieve a specified fossil fuel feed rate and a predicted trajectory adjustment output value  226  to achieve a specified alternative fuel feed rate. The adjustment output values  224  and  226  work in combination to provide a suitable fossil fuel-to-alternative fuel supply ratio that enables the steam production system  100  to operate within specified operating conditions. The control system  112  uses the adjustment output values  224  and  226  in substantially the same manner as described above in connection with the adjustment output values  204  and  206 .  
      A difference between the steam pressure MPC master  222  and the steam flow MPC master  202  described above is that the steam pressure MPC master  222  determines the adjustment output values  224  and  226  by determining an error or deviation between steam pressure measured values received from the pressure sensor  132  ( FIGS. 1 and 2 ) and a steam pressure setpoint input value  228  provided by, for example, an operator.  
      To prevent operating the steam production process in unstable or undesirable conditions the steam pressure MPC master  222  is also provided with a plurality of constraint values  230 , which may be substantially similar or identical to the plurality of constraint values  214  described above in connection with the steam flow MPC master  202 . However, because the steam pressure MPC master  222  receives steam pressure measured values from the pressure sensor  132 , the steam pressure MPC master  222  is not provided with a separate measured steam pressure constraint value as is the steam flow MPC master  202 , but is instead provided with a measured steam flow constraint value as part of the plurality of constraint values  230 .  
      The steam pressure MPC master  222  is also provided with a plurality of constraint priorities  232  that the steam pressure MPC master  222  uses in a manner that is substantially similar or identical to the manner in which the steam flow MPC master  202  uses the plurality of constraint priorities  216  described above. Additionally, the steam pressure MPC master  222  is provided with a plurality of disturbance values  233  that are substantially similar or identical to the plurality of disturbance values  218  described above.  
      During operation, the control system  112  may be configured to operate in a steam flow mode, a steam pressure mode, or a manual mode. The manual mode may involve an operator controlling fuel feed rates based on steam flow and/or steam pressure. In any case, to enable a bumpless seamless switching between modes, the control system  112  may be configured to track or follow the adjustment output values  204 ,  206 ,  224 , and  226  with one another and/or with a manual mode fuel feed rate control. For example, to prevent any abrupt changes in operation when, for example, an operator configures the control system  112  to switch from the steam flow mode to the steam pressure mode, each of the adjustment output values  224  and  226  of the steam pressure MPC master  222  is, at least for a period of time at switchover set to track (e.g., is continuously set equal to) a respective one of the adjustment output values  204  and  206  determined by the steam flow MPC master  202 .  
      To prevent abrupt operation changes when the operator switches the control system  112  from steam flow mode to manual mode, manual control fuel feed rate values track (at least for a period of time at the switch over) the adjustment output values  204  and  206  determined by the steam flow MPC master  202 . In either case, by tracking the adjustment output values  204  and  206  the control system  112  is substantially prevented from causing any abrupt changes in operation because the fuel feed rates remain the same when the mode changes are made. Similarly, when operating in the steam pressure mode, the adjustment output values  204  and  206  and manual control fuel feed rate values track the adjustment output values  224  and  226 . Also, when operating in a manual mode, the adjustment output values  204 ,  206 ,  224 , and  226  follow respective manual control fuel feed rate values.  
      To determine energy content variances in, for example, the alternative fuel, the control system  112  is provided with an energy compensator  234  that provides energy compensation values to the fossil fuel control  240  and the alternative fuel control  250  based on calculated variances in energy content of the alternative fuel. The energy compensator  234  may be implemented using a PID controller that responds with reverse control action to a calculated deviation in alternative fuel energy content. The energy compensator  234  performs a relative energy calculation as the quality of the alternative fuel (e.g., the energy content per volume of fuel) changes over time. Specifically, the relative energy calculation determines the energy content of a current batch or supply of alternative fuel relative to a previously monitored or analyzed batch of alternative fuel based on a measured oxygen consumption and a measured air consumption. If the relative energy content of a current batch or supply of alternative fuel is relatively less, then the energy compensation values indicate a required increase in an amount of alternative fuel and/or fossil fuel to maintain delivery or a relatively constant amount of energy to the furnace  106 . The energy compensator  234  may increase or decrease the energy compensation values based on, for example, historical data of variances in fuel quality, a fuel-to-energy function curve, and/or a required alternative-to-fossil fuel ratio.  
      The energy compensator  234  is configured to ensure that the alternative fuel and fossil fuel feed rates are sufficient to maintain a consumed air index of, for example, 100%, regardless of changes in fuel quality (e.g., the energy content per volume of fuel). Maintaining a consumed air index of 100% ensures that 100% of the air drawn or forced into the furnace  106  is combusted by the fuels for a given boiler load (i.e., a given steam production requirement). In this manner, the same amount of energy is burned regardless of changes in fuel quality, thus providing the boiler  102  the required energy (e.g., heat) to produce a required amount of steam (e.g., boiler load). The energy compensator  234  outputs or provides the energy compensation values to the fossil fuel total energy module  240  and the alternative fuel total energy module  250 .  
      To determine the total amount of fossil fuel required to achieve a desired operating condition (e.g., a particular steam pressure, a particular steam flow, a fuel consumption cost, a fuel ratio, etc.), the control system  112  is provided with the fossil fuel total energy module  240 . The fossil fuel total energy module  240  receives the fossil fuel adjustment output value  204  (when operating in a steam flow mode) from the steam flow MPC master  202  or the fossil fuel adjustment output value  224  (when operating in a steam pressure mode) from the steam pressure MPC master  222 . The fossil fuel total energy module  240  also receives an energy compensation value from the energy compensator  234  and based on the energy compensation value and one of the fossil fuel adjustment output values  204  or  224  determines the total amount of fossil fuel required to produce a required amount of steam flow or steam pressure.  
      To control the fossil fuel feed rate, the control system  112  is provided with a fossil fuel feed controller  242 . The fossil fuel feed controller  242  receives a required fossil fuel amount value from the fossil fuel total energy module  240  and controls, for example, the fossil fuel supply valve  116  to provide fuel at the required feed rate to supply the furnace  106  where the required amount of fossil fuel is determined by the fossil fuel total energy module  240 .  
      To determine the total amount of alternative fuel required to achieve a desired operating condition (e.g., a particular steam pressure, a particular steam flow, a fuel consumption cost, a fuel ratio, etc.), the control system  112  is provided with the alternative fuel total energy module  250 . The alternative fuel total energy module  250  receives the alternative fuel adjustment output value  206  (when operating in a steam flow mode) from the steam flow MPC master  202  or the fossil fuel adjustment output value  226  (when operating in a steam pressure mode) from the steam pressure MPC master  222 . The alternative fuel total energy module  250  also receives an energy compensation value from the energy compensator  234  and based on the energy compensation value and one of the alternative fuel adjustment output values  206  or  226  determines the total amount of fossil fuel required to produce a required amount of steam flow or steam pressure.  
      To control the alternative fuel feed rate, the control system  112  is provided with an alternative fuel feed controller  252 . The alternative fuel feed controller  252  receives a required alternative fuel amount value from the alternative fuel total energy module  250  and controls, for example, the alternative fuel supply valve  118  to provide the required feed rate to supply the furnace  106  with the required amount of alternative fuel as determined by the alternative fuel total energy module  250 .  
      To control the amount of combustion air provided to the boiler  102  for the alternative and fossil fuels, the control system  112  is provided with an air system that splits the supplied air into undergrate air (i.e., air provided under a fuel-carrying grate) and overfire air (i.e., air provided over the combusting fuel). The air system is configured to determine a total air demand for the alternative fuel and a total air demand for the fossil fuel based on the adjustment output values  204  and  206  from the steam flow MPC master  202  or the adjustment output values  224  and  226  from the steam pressure MPC master  222 .  
      The air system includes a total air demand module  260  that receives the alternative fuel adjustment output value  206  (when operating in a steam flow mode) from the steam flow MPC master  202  or the fossil fuel adjustment output value  226  (when operating in a steam pressure mode) from the steam pressure MPC master  222  and determines the total amount of required boiler airflow, which is the sum of undergrate airflow and overfire airflow. The total air demand module  260  may be implemented using a PID controller to respond to any deviations between measured airflow supply and airflow requirements using reverse control action.  
      The output of the total air demand module  260  is provided to a forced-draft (FD) fan control  262  and an air ratio function module  264 . The forced-draft (FD) fan control  262  controls a FD fan damper to provide the required undergrate airflow to the furnace  106  ( FIG. 1 ) based on the output of the total air demand module  260 . The air ratio function module  264  may be implemented using an undergrate air-to-overfire air function curve to determine the required amount of overfire airflow based on the required undergrate airflow. The output of the air ratio function  264  is provided to an overfire fan control  266 , which controls an overtire fan damper to supply the required amount of overfire airflow to the furnace  106 .  
      To ensure that supplied overfire airflow supplied by the overfire fan control  266  is sufficient as fuel quality (e.g. energy content per volume of fuel) changes or varies over time, the control system  112  is provided with a fuzzy heat release control  272 . The fuzzy heat release control  272  may be implemented using a multivariable fuzzy logic engine including a 5×5 fuzzy matrix that references a steam flow-to-total feeder speed ratio value associated with the alternative fuel (i.e., a steam-feeder ratio), a consumed air ratio (overfire air-to-undergrate air ratio), and a relative-energy controller response (i.e., the output of the energy compensator  234 ).  
      The steam-feeder ratio and the consumed air ratio should track one another and, thus, can be used as a check and balance for the fuzzy logic calculation. The fuzzy heat release control  272  monitors the consumed air ratio and the rate of change of the consumed air ratio over, for example, one minute, and generates an overfire air bias value  274  to change the consumed air ratio as necessary. The fuzzy heat release control  272  provides the overfire air bias value  274  to the overfire fan control  266  to change the undergrate air-to-overfire air ratio or split. In addition, the fuzzy heat release control  272  compares the output of the energy compensator  234  with the overtire air bias value  274  to determine whether there are incremental increases in fuel quality (e.g., energy content) without an incremental decrease in overfire air or to determine whether there are incremental decreases in fuel quality without incremental increases in overfire air. If an imbalance exists between the fuel quality and the overfire air, the fuzzy heat release control  272  adjusts the overfire air bias value  274 . In this manner, by monitoring the consumed air ratio and the rate of change in the consumed air ratio, and by comparing the output of the energy compensator  234  with the overfire air bias value  274 , the fuzzy heat release control  272  can continuously and incrementally adjust the overfire bias value  274  as fuel quality changes over time.  
      In addition to adjusting the overfire air bias value  274 , the fuzzy heat release control  272  can also adjust an oxygen setpoint bias value  276 , which causes an increase or decrease in total air delivered to the furnace  106 . Typically, the fuzzy heat release control  272  adjusts the oxygen setpoint bias value  276  only when changing the overfire air bias value  274  does not provide a correct undergrate air-to-overfire air ratio for a current fuel quality.  
      To prevent supplying too much air to the furnace  106  when fuel is no longer entering the furnace  106 , the fuzzy heat release control  272  is provided with an enable/disable constraint value (not shown) indicative of the amount of fuel entering the furnace  106 .  
      Each portion (e.g., the steam flow MPC master  202 , the steam pressure MPC master  222 , the energy compensator  234 , the fossil fuel total energy module  240 , the fossil fuel feed control  242 , the alternative fuel total energy module  250 , the alternative fuel feed control  252 , the total air demand module  260 , the forced-draft fan control  262 , the air ratio function  264 , the overfire fan control  266 , and the fuzzy heat release control  272 ) of the system controller  112  described above can be operated in an automatic mode or a manual mode. In some example implementations, each of the portions of the system controller  112  can be independently selectable for operation in an automatic or a manual mode.  
      To enable bumpless or seamless transitioning between automatic and manual operating modes so that the steam production system  100  does not experience abrupt changes in operating conditions, each of the outputs of the portions of the system controller  112  can be tracked between manual mode controls and automatic mode controls. In this manner, when transitioning between each mode, the outputs remain the same until they are changed by an automatic control or an operator via a manual control. For example, when operating in automatic mode, the outputs of each portion of the system controller  112  are tracked or followed by (e.g., set equal to) respective manual mode control values so that any subsequent transition between automatic and manual mode will not cause any abrupt changes in operation of the steam production system  100 .  
       FIGS. 3 through 7  are flow diagrams that depict example methods that may be used to implement the example systems and methods described herein. The example methods depicted in the flow diagrams of  FIGS. 3 through 7  may be implemented in software, hardware, and/or any combination thereof. For example, the example methods may be implemented in software that is executed via the control system  112  of  FIGS. 1 and 2  and/or the example processor system  810  of  FIG. 8 . Although, the example methods are described below as a particular sequence of operations, one or more operations may be rearranged, added, and/or eliminated to achieve the same or similar results.  
       FIG. 3  is a flow diagram that depicts an example method that may be used to control the example steam production system  100  of  FIG. 1 . The example method of  FIG. 3  is described below by way of example as being implemented using the control system  112  described above in connection with  FIG. 2 . Although the example method of  FIG. 3  may be implemented by the control system  112  in an automatic or manual steam flow mode or steam pressure mode, for purposes of clarity, the example method is described with respect to an automatic steam flow mode.  
      Initially, the steam flow MPC master  202  determines if a specified operating time limit is expired (block  302 ). The specified operating time limit is specified by the steam flow MPC master  202  after each time that it generates the predicted trajectory adjustment output values  204  and  206  and is associated with the amount of time that the steam production system  100  can operate within operating constraints (e.g., a required amount of steam flow) without requiring updates to the predicted trajectory adjustment output values  204  and  206  to maintain operation within the operating constraints. The operating time limit may be based on a timer or a time of day (e.g., a real-time clock).  
      If the steam flow MPC master  202  determines that the operating time limit has not expired, the steam flow MPC master  202  continues to check if the operating time limit has expired (block  302 ) until the time limit expires or until the control system  112  receives an interrupt or an instruction to do otherwise. If the steam flow MPC master  202  determines at block  302  that the operating time limit has expired, the steam flow MPC master  202  determines the predicted trajectory adjustment output values  204  and  206  (block  304 ) for the fossil fuel and alternative fuel as described in detail below in connection with the flow diagram of  FIG. 4 .  
      The energy compensator  234  then determines energy compensation values (block  306 ) associated with the amount of energy (e.g., energy content of the fuel) being delivered to the furnace  106  ( FIG. 1 ) as described in detail below in connection with  FIG. 5 . The alternative fuel total energy module  250  and the fossil fuel total energy module  260  then determine the required amounts of fuels (block  308 ) based on the predicted trajectory adjustment output values  204  and  206  received from the steam flow MPC master  202  and the energy compensation values received from the energy compensator  234  as described in detail below in connection with  FIG. 6 .  
      The fossil fuel feed controller  242  and the alternative fuel feed controller  252  then control the feed rate of the fossil fuel and the alternative fuel, respectively (block  310 ). For example, the alternative fuel feed controller  252  may receive an alternative fuel requirement value from the alternative fuel total energy module  250  and generate a fuel feed rate that will cause delivery of the required amount of alternative fuel to the furnace  106  ( FIG. 1 ). The alternative fuel feed controller  252  can then adjust or control the alternative fuel supply valve  118  ( FIG. 1 ) (which may be implemented using a conveyor speed control to control the speed of a waste wood conveyor) based on the generated fuel feed rate value.  
      The control system  112  then determines and delivers a required amount of airflow (e.g., undergrate airflow and overfire airflow) (block  312 ) as described in detail below in connection with  FIG. 7 .  
      The control system  112  then determines whether to end the control process (block  314 ). For example, if an operator or some other control system (e.g., a safety control system) provides the control system  112  with a stop request, the control system  112 , in response to the stop request, ends the control process and/or returns control to a calling process or function such as, for example, a shutdown process, an idle process, etc. Otherwise, if the control system  112  determines that it should not end the control process, control is passed back to block  302 .  
       FIG. 4  is a flow diagram depicting an example method that may be used to implement the operation of block  304  of  FIG. 3  to determine the predicted trajectory adjustment output values  204  and  206  ( FIG. 2 ). Initially, the steam flow MPC master  202  obtains setpoint values (block  402 ) associated with determining amounts of required fuels. For example, as shown in  FIG. 2 , the steam flow MPC master  202  receives the steam flow setpoint  212  and the alternative fuel setpoint  213 . The steam flow setpoint  212  specifies a required amount of steam and the alternative fuel setpoint  213  specifies the maximum alternative fuel amount or feed rate.  
      The steam flow MPC master  202  then obtains one or more of the fuel costs  208  ( FIG. 2 ) (block  404 ) and fuel use preference settings (e.g., maximize or minimize use of particular fuel based on fuel costs  208 ) (block  406 ). For example, the fuel costs  208  may include the cost of the alternative fuel and/or the fossil fuel. The steam flow MPC master  202  uses the costs of the alternative fuel and/or the fossil fuel in combination with the fuel priority to determine a suitable fuel ratio.  
      The steam flow MPC master  202  then obtains one or more constraint value(s) (block  408 ) such as, for example, the constraint values  214  ( FIG. 2 ). The steam flow MPC master  202  then uses a model predictive control algorithm to determine the predicted trajectory adjustment output values  204  and  206  for the alternative and fossil fuel supplies (block  410 ). For example, the steam flow MPC master may use the values obtained at blocks  402 ,  404 ,  406 , and  408  to determine changes in amounts of fossil and/or alternative fuels to keep the steam production system operating within specified operating conditions to maintain the amount of steam flow indicated by the steam flow setpoint  212 . To determine the predicted trajectory adjustment output values  204  and  206 , the steam flow MPC master  202  may use one or more of the model prediction algorithms in an MPC available in the Delta V control system designed and sold by Emerson Process Management, Austin, Tex.  
      In an example implementation, at block  410  the steam flow MPC master  202  may use the fuel costs  208  and fuel use preference settings to determine an economic-based alternative-to-fossil fuel ratio that will keep the steam production system  100  operating within specified operating conditions based on some or all of the values obtained at blocks  402  and  408  (e.g., the steam flow setpoint  212 , the alternative fuel setpoint  213 , and the constraints  214 ). In some implementations, the steam flow MPC master  202  may determine the fuel ratio and the predicted trajectory adjustment output values  204  and  206  based on historical data indicating previous similar conditions and corresponding adjustment output values. After the steam flow MPC master  202  determines the predicted trajectory adjustment output values  204  and  206 , control is returned to, for example, a calling function or process such as the process of the example method of  FIG. 3 .  
       FIG. 5  is a flow diagram of an example method that may be used to implement the operation of block  306  of  FIG. 3  to determine energy compensation values (via the energy compensator  234  ( FIG. 2 )) associated with adjusting fuel feed rates in response to varying energy contents in the alternative fuel. Initially, the energy compensator  234  obtains a total measured airflow (block  502 ) indicative of the total air intake into the furnace  106  ( FIG. 1 ). The energy compensator  234  then obtains a total air demand (block  504 ). The total air demand (or total air requirement) may be provided by an operator or by the total air demand module  260  and is associated with the total amount of air that should be provided to the furnace  160 .  
      The energy compensator  234  then obtains a measured oxygen value (block  506 ). For example, the energy compensator  234  may receive the measured oxygen value from an oxygen sensor (not shown) that may be located at the air intake  121  ( FIG. 1 ). The energy compensator  234  then obtains an oxygen setpoint ( 506 ) provided by, for example, an operator, or the constraint values  214  and/or from an oxygen setpoint bias provided by the fuzzy heat release control  272  ( FIG. 2 ).  
      The energy compensator  234  then determines a percentage of total target excess air (block  510 ) by, for example, subtracting the oxygen setpoint value obtained at block  508  from the total air demand value obtained at block  504 . The total target excess air is the target amount of air that will not be combusted in the furnace  106 . The energy compensator  234  then determines a total actual excess air (block  512 ) by, for example, subtracting the measured oxygen value obtained at block  506  from the total measured air value obtained at block  502 .  
      The energy compensator  234  then determines a relative energy of fuel value (block  514 ) using, for example, Equation 1 shown below.  
               ENERGY   ⁢           ⁢     (   BTU   )       =       (       TAF   TAD     ×   100     )     +     (     TEA   -   AEA     )               Equation   ⁢           ⁢   1             
 
      The energy compensator  234  uses Equation 1 above to determine a relative energy of fuel in BTU&#39;s (e.g., ENERGY(BTU)). As shown in Equation 1, the energy compensator  234  determines the relative energy by dividing total measured airflow (TAF) obtained at block  502  by the total air flow demand (TAD) obtained at block  504  to produce a quotient (TAF/TAD). The total measured airflow (TAF) and the total air flow demand (TAD) may be provided as values measured in kilopounds per hour (kpph). The energy compensator  234  then multiplies the quotient (TAF/TAD) by a unit conversion “100” to generate a product  
         (       TAF   TAD     ×   100     )     .       
 
 The energy compensator  234  then subtracts the actual excess air (AEA) determined at block  512  from the target excess air (TEA) determined at block  510  to generate the subtraction result (TEA−AEA). The actual excess air (AEA) and the target excess air (TEA) may be provided as excess air percentage values. The energy compensator  234  then determines the relative energy of the fuel by adding the product  
       (       TAF   TAD     ×   100     )       
 
 and the subtracting result (TEA−AEA). 
 
      After the energy compensator  234  determines a relative energy value at block  514 , the energy compensator  234  determines energy compensation values (block  516 ) based on the relative energy value. For instance, the relative energy value determined at block  514  is indicative of fuel quality changes in, for example, the alternative fuel over time. If the alternative fuel feed rate remains relatively constant over time, but the energy content of the alternative fuel decreases, the relative energy value will be indicative of the energy content decrease. Accordingly, the energy compensator  234  may generate energy compensation values based on the energy content decrease as indicated by the relative energy value to cause the alternative fuel total energy module  250  and/or the fossil fuel energy module  240  to increase respective fuel feed rates to compensate for the decrease in alternative fuel quality. In some example implementations, the energy compensator  234  may generate energy compensation values that cause the total fuel energy modules  240  and  250  to increase or decrease respective fuel amounts differently based on specified fuel ratios determined by an operator or the steam flow MPC master  202  according to the fuel costs  208  and fuel use preference settings. The energy compensator  234  may also generate the energy compensation values based on the alternative fuel setpoint  213  ( FIG. 2 ), which defines the maximum allowable amount of alternative fuel. After the energy compensator  234  determines the energy compensation values at block  516 , control is returned to, for example, a calling process or function such as the example process described above in connection with  FIG. 3 .  
       FIG. 6  is a flow diagram of an example method that may be used to implement the operation of block  308  of  FIG. 3  to determine required amounts of fuel to operate the example steam production system  100  of  FIG. 1  within specified operating conditions. Although the example method of  FIG. 6  is described by way of example in connection with the alternative fuel total energy module  250  ( FIG. 2 ) the example method of  FIG. 6  may also be used in connection with the fossil fuel total energy module  240  in a substantially similar or identical manner to that described below. Initially, the alternative fuel total energy module  250  obtains the predicted trajectory adjustment output value  206  from the steam flow MPC master  202  ( FIG. 2 ) (block  602 ) and an energy compensation value from the energy compensator  234  ( FIG. 2 ) (block  604 ). For example, the alternative fuel total energy module  250  may obtain the predicted trajectory adjustment output value  206  determined at block  410  of  FIG. 4  and an energy compensation value determined at block  516  of  FIG. 5 .  
      The alternative fuel total energy module  250  then determines a compensated fuel requirement set point (block  606 ) based on the predicted trajectory adjustment output value  206  obtained at block  602  and the energy compensation value obtained at block  604 . For example, if the energy compensation value indicates that the fuel quality (e.g., energy content) of the alternative fuel has decreased, then the alternative fuel total energy module  250  determines a compensated fuel requirement setpoint to increase the alternative fuel feed rate to compensate for the decreased fuel quality.  
      The alternative fuel total energy module  250  then determines a deviation between the compensated fuel requirement setpoint and a current fuel feed rate (block  608 ). The alternative fuel feed controller  252  then adjusts the current fuel feed rate to substantially eliminate the deviation (block  610 ) determined at block  608  by the alternative fuel total energy module  250 . In some example implementations, the alternative fuel feed controller  252  can incrementally or gradually increase or decrease the fuel feed rate over time until the energy compensated values, which may be continually generated by the energy compensator  234 , indicate a zero change or no change in fuel quality. After adjusting the fuel feed rates, control is returned to, for example, a calling function or process such as the example process of  FIG. 3 .  
       FIG. 7  is a flow diagram of an example method that may be used to implement the operation of block  312  of  FIG. 3  to determine and control the required airflows of the example steam production system  100  of  FIG. 1 . Initially the total air demand module  260  determines the alternative fuel air demand (or requirement) (block  702 ) using, for example, Equations 2 and 3 below.  
               AAD   =     SADa   ×     (     1   +     TEA   100       )         ,     
     ⁢   where           Equation   ⁢           ⁢   2               SADa   =     AFD   ×     A   Fa               Equation   ⁢           ⁢   3               
      As shown above in Equation 2, the total air demand module  260  determines an alternative fuel air demand (AAD) by dividing the target excess air (TEA) determined at block  510  of  FIG. 5  by one hundred (100) to produce a quotient value (TEA/100). The target excess air (TEA) may be provided as a percentage of target excess air. The total air demand module  260  then adds one to the quotient value (TEA/100) to produce a sum value  
         (     1   +     TEA   100       )     .       
 
 The total air demand module  260  then multiplies the sum value  
       (     1   +     TEA   100       )       
 
 by stoichiometric air demand for the alternative fuel (SADa) to determine the alternative fuel air demand (AAD). The stoichiometric air demand for the alternative fuel (SADa) may be provided in kpph units and may be determined according to Equation 3. 
 
      As shown above in Equation 3, the total air demand module  260  determines the stoichiometric air demand for the alternative fuel (SADa) by multiplying an alternative fuel feed demand (AFD) by an air-to-fuel ratio for the alternative fuel (A/Fa). The alternative fuel feed demand (AFD) may be provided as a percentage value indicative of percentage of alternative fuel demand (e.g., the required amount of alternative fuel as determined by the alternative fuel total energy module  250  of  FIG. 2 ) included in a total fuel demand including alternative fuel and fossil fuel demands. The air-to-fuel ratio for the alternative fuel (A/Fa) may be provided as a percentage of required air per unit of demanded alternative fuel.  
      The total air demand module  260  then determines a fossil fuel air demand (FAD) (block  704 ) using, for example, Equations 4 and 5 below.  
               FAD   =     SADf   ×     (     1   +     TEA   100       )         ,     
     ⁢   where           Equation   ⁢           ⁢   4               SADf   =     FFD   ×     A   Ff               Equation   ⁢           ⁢   5             
 
      As shown above in Equations 4 and 5, the total air demand module  260  determines the fossil fuel air demand (FAD) in a manner substantially similar to that described above in connection with Equations 2 and 3. One difference between Equations 4 and 2 is that the fossil fuel air demand (FAD) is determined based on the stoichiometric air demand for fossil fuel (SADf) instead of the stoichiometric air demand for alternative fuel (SADa). Notable differences between Equations 5 and 3 are that the stoichiometric air demand for fossil fuel (SADf) is determined based on a fossil fuel feed demand (FFD) instead of the alternative fuel feed demand (AFD) and based on an air-to-fuel ratio for the fossil fuel (A/Ff) instead of the air-to-fuel ratio for the alternative fuel (A/Fa).  
      The total air demand module  260  then determines the total air demand (block  706 ) by, for example, adding the alternative fuel air demand (AAD) determined at block  702  to the fossil fuel air demand (FAD) determined at block  704 . The total air demand module  206  then determines the current total air supply (block  708 ). For example, the total air demand module  260  may determine the current total air supply by receiving airflow measurements from a furnace air intake airflow sensor (e.g., the airflow sensor  128  of  FIG. 1 ). Alternatively, the total air demand module  260  may obtain and sum a measured overtire airflow value and a measured undergrate airflow value. In some example implementations, the total air demand module  260  may perform calculations based on the received measured overfire and undergrate airflow values to generate an air temperature and/or air pressure compensated current total air supply value.  
      The total air demand module  260  then compares the total air demand determined at block  706  to the current total air supply determined at block  708  (block  710 ) and causes the forced-draft fan control  262  and the overfire fan control  266  to adjust the current total air supply based on the comparison (block  712 ). For example, the total air demand module  260  may adjust the current total air supply using reverse control action in connection with proportional and integral tuning constants to substantially minimize or eliminate the deviation between the total air demand determined at block  706  and the current total air supply determined at block  708 . Also, in the illustrated example of  FIG. 2 , the amount of overfire airflow is based on the amount of undergrate airflow. Specifically, the amount of undergrate airflow determined by the total air demand module  260  is communicated to the forced-draft fan control  262  to control the amount of undergrate airflow and to an air ratio function  264  to determine an amount of overfire airflow. The air ratio function  264  may be a curve or function that causes the overfire airflow to be determined based on the undergrate airflow. In some example implementations, the air ratio function  264  is substantially fixed during operation of the example steam production system  100  ( FIG. 1 ).  
      In some example implementations, because of the changing or varying fuel quality (e.g., energy content per volume of fuel) of, for example, the alternative fuel, the furnace  106  ( FIG. 1 ) may require adjustments to the amount of overfire airflow supply. That is, the air ratio function  264  may generate relatively less efficient overfire airflow supply values as fuel quality varies over time. To adjust the overfire airflow to ensure that the steam production system  100  operates within specified operating conditions (e.g., steam flow conditions, steam pressure conditions, economical conditions, etc.), the fuzzy heat release control  272  determines incremental adjustments to the overfire airflow supply (block  714 ). For example, the fuzzy heat release control  272  may determine the incremental adjustments as described above in connection with  FIG. 2  using, for example, a 5×5 fuzzy matrix that references a steam flow-to-total feeder speed ratio value associated with the alternative fuel (i.e., a steam-feeder ratio), a consumed air ratio (overfire air-to-undergrate air ratio), and the energy compensation values generated by the energy compensator  234 .  
      The overfire fan control  266  then controls the overfire fan based on the overfire airflow supply values determined by the air ratio function  264  and the incremental adjustment values determined by the fuzzy heat release control  272  (block  716 ). Control is then returned to, for example, a calling function or process such as the example process of  FIG. 3 .  
       FIG. 8  is a block diagram of an example processor system that may be used to implement the example apparatus, methods, and articles of manufacture described herein. As shown in  FIG. 8 , the processor system  810  includes a processor  812  that is coupled to an interconnection bus  814 . The processor  812  includes a register set or register space  816 , which is depicted in  FIG. 8  as being entirely on-chip, but which could alternatively be located entirely or partially off-chip and directly coupled to the processor  812  via dedicated electrical connections and/or via the interconnection bus  814 . The processor  812  may be any suitable processor, processing unit or microprocessor. Although not shown in  FIG. 8 , the system  810  may be a multi-processor system and, thus, may include one or more additional processors that are identical or similar to the processor  812  and that are communicatively coupled to the interconnection bus  814 .  
      The processor  812  of  FIG. 8  is coupled to a chipset  818 , which includes a memory controller  820  and an input/output (I/O) controller  822 . As is well known, a chipset typically provides I/O and memory management functions as well as a plurality of general purpose and/or special purpose registers, timers, etc. that are accessible or used by one or more processors coupled to the chipset  818 . The memory controller  820  performs functions that enable the processor  812  (or processors if there are multiple processors) to access a system memory  824  and a mass storage memory  825 .  
      The system memory  824  may include any desired type of volatile and/or non-volatile memory such as, for example, static random access memory (SRAM), dynamic random access memory (DRAM), flash memory, read-only memory (ROM), etc. The mass storage memory  825  may include any desired type of mass storage device including hard disk drives, optical drives, tape storage devices, etc.  
      The I/O controller  822  performs functions that enable the processor  812  to communicate with peripheral input/output (I/O) devices  826  and  828  and a network interface  830  via an I/O bus  832 . The I/O devices  826  and  828  may be any desired type of I/O device such as, for example, a keyboard, a video display or monitor, a mouse, etc. The network interface  830  may be, for example, an Ethernet device, an asynchronous transfer mode (ATM) device, an 802.11 device, a DSL modem, a cable modem, a cellular modem, etc. that enables the processor system  810  to communicate with another processor system.  
      While the memory controller  820  and the I/O controller  822  are depicted in  FIG. 8  as separate functional blocks within the chipset  818 , the functions performed by these blocks may be integrated within a single semiconductor circuit or may be implemented using two or more separate integrated circuits.  
      Although certain systems, methods, and articles of manufacture have been described herein, the scope of coverage of this patent is not limited thereto. To the contrary, this patent covers all systems, methods, and articles of manufacture fairly falling within the scope of the appended claims either literally or under the doctrine of equivalents.