FIELD OF THE INVENTION
The present invention relates generally to combustion control for use in fired equipment and deals more particularly with a metering combustion control for fired equipment.
BACKGROUND OF THE INVENTION
Combustion control strategies applied to fired equipment, both commercial and industrial, generally are one of three general category types or some subtle variation to one or the other of them. The control strategies as known to a person skilled in the art are: 1) single point positioning control also known as jackshaft positioning; 2) parallel positioning control; and 3) metered cross-limited control.
Each of these are fuel/air ratio combustion control strategies wherein a firing rate demand signal generated as a result of an attempt to maintain a selected “process variable” (PV) equal to a desired “set-point” (SP) is simultaneously directed to a fuel flow regulating element and a combustion air flow regulating element.
The currently known and implemented combustion control strategies are not entirely satisfactory. To applicant's knowledge, none of the known combustion control strategies meet Underwood Laboratories (UL) approval as a parameter based combustion control instrument capable of carrying out a metering fuel/air ratio combustion control strategy.
The currently known and implemented metered cross-limited combustion control strategies are not entirely satisfactory. Current implementations utilize two, or more, PID (Proportional-Integral-Derivative) control logic blocks, one for fuel and one for air. Cross-limiting logic must be applied to coordinate the two independent proportional integral derivative logics. This combination requires considerable skill to tune and calibrate, and results in a slow firing rate demand response time.
Accordingly what is needed is a parameter based combustion control instrument capable of choosing via parameter selection a selected one of a single point positioning control strategy, a parallel positioning control strategy and a metering fuel/air ratio combustion control strategy.
SUMMARY OF THE INVENTION
In accordance with a broad aspect of some embodiments of the invention, combustion is controlled in a fired equipment by metering both the fuel flow rate and the combustion air flow rate in a desired ratio corresponding to a master firing rate demand, and by trimming the master firing rate demand directed to the combustion air regulating element in response to an error based correction adjustment determined from the respective values of the fuel flow meter and combustion air flow meter input signals to drive the ratio between the fuel flow rate and the combustion air flow rate toward the desired ratio for controlling the combustion in accordance with the master firing rate demand
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a function schematic representation of an example of a parallel positioning combustion control system.
FIG. 2 is a functional schematic representation of an example of a parallel positioning combustion control system with oxygen trim.
FIG. 3 is a functional schematic representation of an example of a full metering combustion control system with fuel flow and combustion air flow cross limiting.
FIG. 4 is a functional schematic representation of an example of a metering combustion control system with oxygen trim according to some embodiments of the present invention.
FIG. 5 shows a flowchart of the basic steps of the method according to some embodiments of the invention.
FIG. 6 shows a combustion controller enabled device according to some embodiments of the invention for providing combustion control in a fired equipment.
FIG. 7 is a functional block diagram of an example of a signal processor for carrying out the invention.
FIG. 8 is a functional block diagram of an example of a combustion controller for carrying out the steps of the method according to some embodiments of the invention.
FIG. 9 shows a combustion controller chipset according to some embodiments of the invention for providing combustion control in a fired equipment.
WRITTEN DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION
The basic purpose and intent of a combustion control strategy in a fired equipment is to maintain as stated above a process variable equal to a desired set-point by directing a firing rate demand signal to a fuel flow regulating element and a combustion air flow regulating element in the fired equipment. Such fired equipment may be for example, a steam generator, a hot water heater, a boiler, a chemical process heater, a heated manufacturing process, or other boiler combustion fired equipment although the invention is not limited to such fired equipment. For purposes of explanation and by way of example only, consider a steam generator. In this case the process variable is the steam pressure. Under the combustion control strategy, a reduction in the steam pressure relative to the set-point of the desired pressure results in an increase in the master firing rate demand signal with a coincidental call for an increase in the fuel input and combustion air input to the burner to increase the firing rate to produce more steam to drive the pressure upward toward the desired pressure. Likewise, an increase in the steam pressure relative to the set-point results in a decrease in the master firing rate demand signal with a coincidental call for a decrease in the fuel input and combustion air input to decrease the firing rate to produce less steam to drive the pressure downward toward the desired pressure.
For further purposes of explanation and by way of a further example, consider a hot water heater. In this case the process variable is hot water temperature. Under the combustion control strategy, a reduction in water temperature relative to the set-point of the desired water temperature results in an increase in the master firing rate demand signal with a coincidental call for an increase in the fuel input and combustion air input to the burner to increase the firing rate to produce hotter water to drive the water temperature upward toward the desired water temperature. Likewise, an increase in the water temperature relative to the set-point results in a decrease in the master firing rate demand signal with a coincidental call for a decrease in the fuel input and combustion air input to decrease the firing rate to lessen the heat input allowing the water temperature to decrease thus driving the water temperature toward the desired water temperature.
In both examples, a reduction in the process variable relative to the set-point results in an increase in the master firing rate demand signal (MFDS) with a coincidental call for an increase in fuel and combustion air inputs to the burner of the fired equipment while an increase in the process variable relative to the set-point results in a decrease in the MFDS and the fuel and combustion air inputs.
FIG. 1 shows a schematic representation of a combustion controller generally designated 10 in which a parallel positioning control strategy is utilized. Two firing rate demand signals are used in parallel positioning control, with one signal going to the fuel flow regulating element and the other going to the combustion air regulating element, hence the term parallel positioning. For example, a reduction in the process variable relative to the set-point results in an increase in the master firing rate demand signal with a coincidental call for an increase in the fuel input and combustion air input to the fired equipment, whereas an increase in the process variable relative to the set-point results in a decrease in the master firing rate demand signal and as a result a decrease in the fuel input and the combustion air input. As shown in FIG. 1, the master firing rate demand signals are generated or retrieved from a suitably configured module 10a. The generated output (0 to 100%) is the result of a comparison of the design operating set-point versus the actual process state. The fuel firing rate demand signal is conditioned by a fuel function generator module 10b and directed to the fuel flow regulating element 10c. The fuel function generator 10b characterizes the opening of the fuel flow regulating element to produce a near linear fuel flow as a function of the master firing rate demand signal. The fuel flow regulating element 10c may be for example, a flow control valve or a metering pump, and is responsive to the fuel firing rate demand signal from the fuel function generator module 10b to increase or decrease fuel flow. The combustion air firing rate demand signal is conditioned by a combustion air function generator 10d and directed to the air flow regulating element 10e. The combustion air function generator 10d characterizes the opening and/or speed of the air flow regulating element 10e to produce the desired fuel/air ratio as a function of the master firing rate demand signal. The fuel/air ratio is not a constant and varies due to the need to maintain ideal fuel/air mixing velocity ratios throughout the burner firing rate range. The air flow regulating element 10e may be for example, a burner or forced draft fan damper and/or a forced draft fan variable frequency drive or a turbine.
Another combustion control strategy known as “single point positioning” or “jackshaft positioning” control is a variation of the parallel positioning control strategy in which the flow regulating elements are of a design that is arranged to regulate their respective flows via the action of one or more linkage rods, each of which are connected to a common “jackshaft”. That jackshaft is in turn mechanically linked to a single positioning actuator or servo-motor, which receives the master firing rate demand signal input. In this way only one master firing rate demand signal is directed to the fired equipment and the relative flow regulating characteristics of each flow regulating element, i.e., the fuel flow regulating element and the combustion air flow regulating element, is accomplished by mechanical means, for example, linkage adjustments, or adjustable cam/roller assemblies or both or in other ways well known and understood by those skilled in the art.
FIG. 2 shows a schematic representation of a combustion controller generally designated 12 in which a parallel positioning control strategy with oxygen trim is utilized. As explained in connection with the discussion of FIG. 1, two firing rate demand signals are used with the fuel firing rate demand signal going to the fuel flow regulating element and the combustion air firing rate demand signal being trimmed prior to going to the combustion air regulating element. As shown in FIG. 2, the master firing rate demand signals are generated or retrieved from a suitably configured module 12a. The generated output (0 to 100%) is a result of a comparison of the design operating set-point versus the actual process state. The fuel firing rate demand signal is conditioned by a fuel function generator module 12b and directed to the fuel flow regulating element 12c. The fuel function generator 12b characterizes the opening of the fuel flow regulating element to produce a near linear fuel flow as a function of the master firing rate demand signal. The fuel flow regulating element 12c may be for example, a flow control valve or a metering pump, and is responsive to the fuel firing rate demand signal from the fuel function generator module 12b to increase or decrease fuel flow. A flue gas oxygen analyzer module 12d determines the actual oxygen in the flue gas. An air demand trim computer module 12e compares the actual oxygen content in the flue gas to a master firing rate demand-based flue gas excess oxygen set-point and adjusts either the master firing rate demand combustion air flow signal input to the controller or the master firing rate demand signal combustion air flow signal directed to the air flow regulating element. Generally, the oxygen trim computation will include limits on the amount of variation that can be enacted to change the master firing rate demand combustion air flow signal because of the concern for possible failure of the flue gas oxygen analyzer. The master firing rate demand combustion air flow signal is conditioned by a combustion air function generator 12f and directed to the air flow regulating element 12g. The combustion air function generator 12f characterizes the opening and/or speed of the air flow regulating element 12g to produce the desired fuel/air ratio as a function of the master firing rate demand signal. The air flow regulating element 12g may be for example, a burner or forced draft fan damper and/or a forced draft fan variable frequency drive or a turbine. The fuel/air ratio is not a constant and varies due to the need to maintain ideal fuel/air mixing velocity ratios throughout the burner firing rate range.
FIG. 3 shows a schematic representation of a combustion controller generally designated 14 in which a full metering control strategy with fuel flow and combustion air flow cross limiting is utilized. In a traditional full metering combustion control system, the values of the respective input of the actual fuel flow and the combustion air flow are compared to their respective flow set-point in a proportional integral derivative controller. The actual firing rate demand signal for the fuel flow input and the combustion air flow input is then the error correction based output signal from the respective controller. The set-point of each of the fuel flow controller and the combustion air flow controller was originally the master firing rate demand signal. The traditional full metering control strategy differs from the ‘basic’ jackshaft and parallel positioning control strategies (without oxygen trim) in that neither of those strategies has any form of “feedback” pertaining to the actual affect on flow rates resulting from a change in the master firing rate demand signal. The addition of “cross limiting” adds low and high signal selectors and logic to assure that on increases in the firing rate, the air demand would increase before the fuel demand and on decreases in firing rate, the reverse action would be assured.
Still referring to FIG. 3, the master firing rate demand signals are generated or retrieved from a suitably configured module 14a. The generated output (0 to 100%) is a result of a comparison of the design operating set-point versus the actual process state. The fuel flow master firing rate demand signal is input to a low value selector module 14d along with an actual combustion air flow signal from a combustion air flow transmitter module 14e to provide a fuel set-point signal that is input to the fuel flow proportional integral derivative controller 14c along with an actual value of the fuel flow signal from a fuel flow transmitter module 14b to provide a fuel firing rate demand signal. The fuel firing rate demand signal is conditioned by a fuel function generator module 14f and is directed to the fuel flow regulating element 14g. The fuel function generator 14f characterizes the opening of the fuel flow regulating element to produce a near linear fuel flow as a function of the master firing rate demand signal. The fuel flow regulating element 14g may be for example, a flow control valve or a metering pump, and is responsive to the fuel firing rate demand signal from the fuel function generator module 14f to increase or decrease fuel flow. The combustion air flow master firing rate demand signal is input to a high value selector module 14h along with an actual fuel flow signal from the fuel flow transmitter module 14b to provide an air flow set-point that is input to the air flow proportional integral derivative controller 14i along with an actual combustion air flow signal from the combustion air flow transmitter module 14e to provide a combustion air firing rate demand signal. The combustion air firing rate demand signal is conditioned by a combustion air function generator 14j and directed to the air flow regulating element 14k. The combustion air function generator 14j characterizes the opening and/or speed of the air flow regulating element 14k to produce the desired fuel/air ratio as a function of the master firing rate demand signal. The air flow regulating element 14k may be for example, a burner or forced draft fan damper and/or a forced draft fan variable frequency drive or a turbine. The fuel/air ratio is not a constant and varies due to the need to maintain ideal fuel/air mixing velocity ratios throughout the burner firing rate range.
Turning now to FIG. 4, a functional schematic representation of an example of a metering combustion control system with oxygen trim according to some embodiments of the present invention is shown therein and generally designated 16. Now in contrast to the combustion control system strategies discussed above, the metering combustion control system strategy with oxygen trim embodying the present invention blends the benefits of all of the above described approaches by using the respective flow meter input signals in combination with the master firing rate demand signals in a proportional integral controller to “trim” the basic parallel positioning master firing rate demand signal being directed to the air flow regulating element. Further, limits are applied to the allowable level of error correction based adjustment so that even if there is a flow meter failure, continued operation in the basic parallel positioning format (with or without oxygen trim) is possible. Ideally the fuel input varies linearly with the master firing rate demand whereas for reasons associated with maintaining optimum mixing influenced velocities, the combustion air level is relatively higher at lower firing rates (i.e. fuel/air ratio is not a constant throughout the range).
As shown in FIG. 4, the master firing rate demand signals are generated or retrieved from a suitably configured module 16a. The generated output (0 to 100%) is a result of a comparison of the design operating set-point versus the actual process state. The firing rate demand signal is applied simultaneously to a fuel function generator 16b and an air flow demand summing module 16k. The fuel function generator 16b characterizes the opening of the fuel flow regulating element 16c to produce a near linear fuel flow as a function of the master firing rate demand signal. The air flow demand summing module 16k adds the firing rate demand signal to the air flow proportional integral derivative controller 16g trim signal and applies the resultant signal to the combustion air function generator 16i. The combustion air function generator 16i characterizes the opening and/or speed of the air flow regulating element 16j to produce the desired fuel/air ratio as a function of the master firing rate demand signal. The fuel/air ratio is not a constant and varies due to the need to maintain ideal fuel/air mixing velocity ratios throughout the burner firing rate range.
The fuel flow regulating element 16c may be for example, a flow control valve or a metering pump, and is responsive to the fuel firing rate demand signal from the fuel function generator module 16b to increase or decrease fuel flow. The air flow regulating element 16j may be for example, a burner or forced draft fan damper and/or a forced draft fan variable frequency drive or a turbine.
A flue gas oxygen analyzer module 16d determines the actual oxygen in the flue gas. An air flow trim computer module 16e receives a signal representative of the value of the actual oxygen in the flue gas along with a signal representative of the combustion air flow from a combustion air flow transmitter module 16f to provide an adjusted combustion air flow signal in accordance with the required oxygen content in the flue gas. The output signal from the air flow trim computer module 16e is input to an air flow proportional integral derivative controller module 16g along with the value of the actual fuel flow from a fuel flow transmitter module 16h for determining the combustion air flow trim signal to be directed to the air flow demand summing module 16k.
It should be recognized that the turndown capability (i.e. ability to operate at reduced rates) of a burner governed by a traditional metering control strategy is tied to the flow meter's limited turndown capabilities that is, flow measurement accuracy at reduced rates. In contrast according to some embodiments of the present invention, the turndown capabilities are equivalent to that of parallel positioning due to the lack of the absolute dependence on the fuel flow and combustion air flow signals.
It should be recognized that according to some embodiments of the present invention, the fuel and air regulating elements (16c and 16j) respond instantly to changes in the firing rate demand 16a. In contrast, traditional metered control strategy low selector 14d, high selector 14h and the differing response rates of fuel proportional integral derivative controller 14c and air flow proportional integral derivative controller 14i all combine to delay the response of the respective fuel and air regulating elements (16c and 16j) to a change in the firing rate demand 16a.
FIG. 5 shows a flowchart of the basic steps of the method for metering combustion control in fired equipment according to some embodiments of the present invention. The basic method is shown in the flowchart generally designated 20 and includes the steps of controlling combustion in a fired equipment, for example a steam boiler or hot water heater by metering both the fuel flow rate and the combustion air flow rate in a desired ratio corresponding to a master firing rate demand (step 20a), and trimming the master firing rate demand signal directed to the combustion air regulating element in response to an error based correction adjustment determined from the respective values of the fuel flow meter and combustion air flow meter input signals to drive the ratio between the fuel flow rate and the combustion air flow rate toward the desired ratio for controlling the combustion in accordance with the master firing rate demand (step 20b).
FIG. 6 shows by way of example a metering combustion control enabled device 22 according to some embodiments of the invention for use in a fired equipment 24 such as described above. The metering combustion control enabled device 22 includes one or more modules 22a configured for controlling combustion in a fired equipment, for example a boiler or hot water heater, according to a master firing rate demand, one or more modules 22b configured for metering the fuel flow rate and the combustion air flow rate in a desired ratio to correspond to the master firing rate demand, one or more modules 22c configured for providing an error based correction adjustment based on the value of the fuel flow meter input signal and the value of the combustion air flow meter input signal, and one or more modules 22d configured for trimming the master firing rate demand signal value directed to the combustion air flow regulating element in response to the error based correction adjustment to drive the ratio between the fuel flow rate and the combustion air flow rate toward the desired ratio for controlling the combustion in accordance with the master firing rate demand. Consistent with that described above, the metering combustion control enabled device may include other modules 22e that do not necessarily form part of the underlying invention and are not described in detail herein.
By way of example, and consistent with that described above, the functionality of the modules 22, 22a, 22b 22c, 22d and/or 22e may be implemented using hardware, software, firmware, or a combination thereof, although the scope of the invention is not intended to be limited to any particular embodiment thereof. In a typical software implementation, the modules 22a, 22b, 22c and 22d would be one or more microprocessors-based architectures having a microprocessor, a random access memory (RAM), a read only memory (ROM), input/output devices, memory, flow meter control, and control, data and address buses connecting the same such as shown in FIG. 7. A person skilled in the art would be able to program such a microprocessor-based implementation to perform the functionality described herein without undue experimentation. The scope of the invention is not intended to be limited to any particular implementation using technology now known or later developed in the future. Moreover, the scope of the invention is intended to include the modules 22a, 22b, 22c and 22d being a standalone module, as shown, or in the combination with other circuitry for implementing another module. Moreover, the real-time part may be implemented in hardware, while the non-real-time part may be done in software.
According to some embodiments the present invention may be implemented as a computer program product comprising a computer readable structure embodying computer program code therein for execution by a computer processor instructions for performing a method comprising controlling combustion in a fired equipment according to a master firing rate demand; metering the fuel flow rate and the combustion air flow rate in a desired ratio to correspond to the master firing rate demand; providing an error based correction adjustment based on the value of the fuel flow meter input signal and the value of the combustion air flow meter input signal, and trimming the master firing rate demand signal value directed to the combustion air flow regulating element in response to the error based correction adjustment to drive the ratio between the fuel flow rate and the combustion air flow rate toward the desired ratio for controlling the combustion in accordance with the master firing rate demand.
Turning now to FIG. 8, a schematic functional block diagram of an example of a metering combustion control is illustrated therein showing the major operational functional components which may be required to carry out the intended functions of the combustion controller and implement the steps of the method according to some embodiments of the invention and is generally designated 24. A processor such as the signal processor of FIG. 7 carries out the computational and operational control of the metering combustion control in accordance with one or more sets of instructions stored in a memory. A user interface may be used to provide alphanumeric input and control signals or other program steps and set-points by a user and is configured in accordance with the intended function to be carried out. A display sends and receives signals from the controller that controls the graphic and text representations shown on a screen of the display in accordance with the function being carried out. The controller controls a fuel flow meter and an air combustion flow meter that operate in a manner well known to those skilled in the art. The functional logical elements for carrying out the metering combustion control operational functions are suitably interconnected with the controller to carry out the metering combustion control as contemplated in accordance with some embodiments of the invention. An electrical power source such as a battery is suitably interconnected within the combustion controller to carry out the functions described above. It will be recognized by those skilled in the art that the metering combustion control according to some embodiments of the invention may be implemented in other ways other than that shown and described, including using pneumatic control elements and other mechanical and electrical devices. It will also be recognized by those skilled in the art that the metering combustion control strategy for fired equipment according to some embodiments of the invention can be implemented using other suitably configured and arranged devices including but not limited to pneumatic, electronic, microprocessor, computer, signal processor, logic devices, wired logic, firmware, computational and memory components, software instruction sets, and other devices and components now known or future developed.
Consistent with that discussed above, the metering combustion control according to some embodiments of the invention may be implemented as a chipset for use in a combustion control enabled fired equipment generally designated 26 for example as illustrated in FIG. 9. The metering combustion control chipset generally designated 26a is suitably configured for controlling combustion in a fired equipment by metering both the fuel flow rate and the combustion air flow rate in a desired ratio corresponding to a master firing rate demand, and for trimming the master firing rate demand directed to the combustion air regulating element in response to an error based correction adjustment determined from the respective values of the fuel flow meter and combustion air flow meter input signals to drive the ratio between the fuel flow rate and the combustion air flow rate toward the desired ratio for controlling the combustion in accordance with the master firing rate demand. Consistent with that described above, the metering combustion control chipset may include other metering combustion chipsets 26b that do not necessarily form part of the underlying invention and are not described in detail herein.
Patent Application
20090142717
