Metering combustion control
Metering combustion control in a fired equipment is disclosed in which both the fuel flow rate and the combustion air flow rate are metered in a desired ratio corresponding to a master firing rate demand, and the master firing rate demand combustion air flow directed to the combustion air regulating element is trimmed 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.
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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 INVENTIONCombustion 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 INVENTIONIn 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
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.
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.
Still referring to
Turning now to
As shown in
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.
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
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
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
Claims
1. 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.
2. The method according to claim 1 further comprising the fuel flow input signal and the combustion air flow input signal being input to a proportional integral derivative controller for determining the value of the error based correction adjustment.
3. The method according to claim 1 further comprising limiting in response to the failure of a fuel flow meter and/or an air combustion flow meter, the value of the error correction based adjustment to a predetermined allowable level to insure continued combustion.
4. The method according to claim 1 further comprising providing a turndown capability without dependence on flow meter flow signals.
5. The method according to claim 4 wherein the turndown capability is equivalent to a parallel positioning combustion control operation.
6. The method according to claim 1 further comprising providing a reduced response time capability without dependence on low selectors, high selectors or differences in independent fuel and air flow PID tunings.
7. The method according to claim 1 further comprising controlling the fuel British Thermal Unit (BTU) flow rate and controlling the combustion air oxygen mass flow rate.
8. The method according to claim 1 further comprising analyzing the oxygen level in the flue gas for adjusting the combustion air flow meter input signal.
9. The method according to claim 1 further comprising characterizing the opening of the fuel flow regulating element to produce a near linear fuel flow as a function of the trimmed master firing rate demand signal directed to the fuel flow regulating element.
10. The method according to claim 1 further comprising characterizing the opening/speed of the air flow regulating element to produce the desired fuel flow rate/combustion air flow rate ratio as a function of the trimmed master firing rate demand signal directed to the combustion air flow regulating element.
11. A controller, comprising:
- one or more modules configured for controlling combustion in a fired equipment according to a master firing rate demand;
- one or more modules 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 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 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.
12. The controller according to claim 11 further comprising one or more modules configured as a proportional integral derivative controller for determining the value of the error based correction adjustment based on the respective values of the fuel flow input signal and the combustion air flow input signal.
13. The controller according to claim 11 wherein said fired equipment is a boiler configured and arranged for generating steam.
14. The controller according to claim 11 wherein said fired equipment is a hot water heater.
15. The controller according to claim 11 wherein said fired equipment is at least one of a steam generator, a boiler, a chemical process heater, a heated manufacturing process, a boiler combustion fired equipment.
16. A computer program product comprising a computer readable structure embodying computer program code therein for execution by a computer processor, said computer program further comprising 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.
17. A method according to claim 1 wherein the method further comprises implementing the steps of the method via a computer program running in a processor, controller or other suitable module located in or interfaced with the fired equipment.
18. A chipset, comprising:
- a first chipset module 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
- a second chipset module configured 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.
19. The chipset according to claim 18 further comprising a proportional integral derivative controller configured for determining the value of the error based correction adjustment based on the respective values of the fuel flow input signal and the combustion air flow input signal.
Type: Application
Filed: Dec 4, 2007
Publication Date: Jun 4, 2009
Applicant:
Inventor: Peter Lavelle (Danbury, CT)
Application Number: 11/999,103
International Classification: F23N 1/02 (20060101);