Advanced control system for enhanced operation of oscillating combustion in combustors

Abstract

Methods for optimizing emission levels from combustion operations, which include a system and process for optimizing levels of NOx and CO during fuel combustion including supplying flows of fuel (which is predetermined) and main oxidant to a burner. Oscillating combustion is generated by oscillating the fuel flow with an oscillating valve and combusting the oscillating fuel with the main oxidant adjacent the burner to produce combustion products. A post-combustion oxidant is injected into the combustion products where it is combusted with the combustion products. A controller is operatively associated with control units for controlling the main oxidant and post-combustion oxidant flow rates and the oscillating valve.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit under 35 U.S.C. § 119(e) to provisional application No. 60/635,737, filed Dec. 13, 2004, the entire contents of which are incorporated herein by reference.

RELATED ART

The concept of oscillating combustion is a recognized technology for the reduction of NOx in industrial furnaces. The principle and the various methods of implementation are broadly described in U.S. Pat. No. 4,846,665, U.S. Pat. No. 5,302,111, and U.S. Pat. No. 5,522,721. The main idea is to pulse the flow of fuel, or air being supplied to at least one burner of the furnace, to generate successive fuel-rich and fuel-lean zones in a flame; thereby, reducing NOx emissions.

The inventors are only aware of oscillating combustion's commercial implementation in large multi-fired industrial furnaces, such as glass and steel reheating furnaces. In these processes, integration of oscillating combustion to the furnace is relatively easy since the furnace is often operated continuously, at fixed or almost steady load, and in conditions which don't require additional injection of oxidant in the furnace in order to eliminate excessive formation of unburned hydrocarbons, or carbon monoxide (CO). In such furnaces, a stand-alone automated logic control device, as proposed in U.S. Pat. No. 6,398,547, may be used to promote safe and efficient operation of the oscillating combustion system.

Some have proposed to implement the same concept of oscillating combustion in processes presenting more constraints, especially in units with smaller combustion chamber, and operating one burner or a limited number of burners. Typical examples for these units are industrial boilers and process heaters. Some technical solutions have been provided for these processes, some of which include optimized injections of additional oxidant and specific mixing devices located in certain position of the process in order to eliminate excessive formation of unburned hydrocarbons, or carbon monoxide (CO), while maintaining optimum NOx reductions. These so-called post-combustion solutions are described in U.S. Pat. No. 6,398,547, U.S. Pat. No. 6,913,457, and published U.S. Patent Application No. 2003/0134241. In most cases, these proposed processes are running at varying loads without human/operator intervention, and are subject to frequent shut-downs and start-ups. This complicates the operation on top of controlling the additional oxidant flow. In these conditions, some experimental testing of the oscillating combustion has been performed in such processes, yielding attractive performances, such as significant NOx reduction, but only under a tight supervision and command by a team of operators.

Therefore, there is a need for an improved control system and method of the oscillating combustion technology allowing a safe and efficient integration to industrial fired processes such as, but not limited to, industrial boilers and process heaters, in which so-called post-combustion is implemented.

Many problems arise from the implementation of the oscillating combustion technology in these types of processes. The air/fuel stoichiometry at the burner level has to be reduced in order to compensate the injection of additional oxidant downstream of the burner. The oscillating and post-combustion parameters have to be adjusted according to firing rates of the process. The total amount of oxidant injected (both at burner and downstream, in post-combustion step) has to be closely controlled in order to supply enough oxygen for maintaining CO below regulated levels, but not too much in order to keep an optimum boiler efficiency. Additional amounts of oxidant have to be supplied during transient phases in order to maintain the overall excess air above a safe level. The oscillating combustion and post-combustion have to be operated in such a way that the boiler can be started up safely after a shut-down. For this purpose, the valve oscillations have to be interrupted before the boiler starts up again in order to allow a safe establishment of the flame. Finally, the oscillating combustion and post-combustion systems have to allow a safe operation in a non-oscillating mode. This is needed in order to keep the process operating in case of a failure or when maintenance of the oscillating equipment is required.

SUMMARY OF THE INVENTION

A process for optimizing levels of NOx and CO during fuel combustion is performed and includes the steps of a flow of a fuel to a burner is supplied, and flow of a main oxidant is supplied to a burner. The flow rate of the main oxidant is controlled by a main oxidant flow rate control unit. Oscillating combustion is generated by oscillating the fuel flow with an oscillating valve and combusting the oscillating fuel with the main oxidant adjacent the burner to produce combustion products. A post-combustion oxidant is injected into the combustion products. The injection rate of the post-combustion oxidant injection being is controlled by a post-combustion oxidant flow rate control unit. The combustion products and the injected post-combustion oxidant are combusted. A rate of the fuel flow is predetermined. A controller is provided that is operatively associated with the main oxidant flow rate control unit, the oscillating valve, and the post-combustion oxidant flow rate control unit. A value or values associated with one or more combustion parameters (including a rate of flow of the main oxidant, a rate of flow of the post-combustion oxidant, a frequency of the oscillating fuel flow, an amplitude of the oscillating fuel flow, a duty cycle of the oscillating fuel flow) is determined. The combustion parameter associated with the determined value or values is adjusted. The determined value or values is based upon the predetermined fuel flow rate. The determining step is performed by the controller.

A system for improved operation of a combustion process utilizing oscillating combustion includes:

• • a supply of fuel;
  • a supply of a main oxidant;
  • a supply of a post-combustion oxidant;
  • a burner for receiving the fuel and main oxidant and initiating combustion thereof to produce combustion products;
  • an oscillating valve operatively associated with the supply of fuel for achieving an oscillating flow of said fuel at the burner;
  • walls defining a combustion chamber operatively associated with the burner and enclosing an oscillating combustion space and a post-combustion space;
  • a main oxidant flow rate control unit operatively associated with the supply of main oxidant and the burner adapted and configured to control a flow rate of the main oxidant;
  • a post-combustion oxidant flow rate control unit operatively associated with the supply of post-combustion oxidant and the post-combustion space;
  • a post-combustion injection element operatively associated with the post-combustion oxidant flow rate control unit; and
  • a controller operatively associated with the main oxidant flow rate control unit, post-combustion oxidant flow rate control unit, and oscillating valve.

The oscillating combustion space is upstream of the post-combustion space. The main oxidant flow rate control unit is adapted and configured to control a flow rate of the oxidant. The post-combustion oxidant flow rate control unit is adapted and configured to control a flow rate of the post-combustion oxidant. The post-combustion injection element is adapted and configured to inject the post-combustion oxidant into the post-combustion space to achieve combustion of the combustion products and the post-combustion oxidant.

BRIEF DESCRIPTION OF THE DRAWINGS

For a further understanding of the nature and objects of the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers and wherein:

FIG. 1 is a schematic of one embodiment of the system according to the invention;

FIG. 2 is a flowchart showing the startup sequence of one embodiment of the invention; and

FIG. 3 is a flowchart showing the shutdown sequence for one embodiment of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

In order to solve the above mentioned problems, the present invention proposes an innovative system and method for operating and controlling the oscillating combustion technology in combustors subject to demanding operating constraints, such as industrial boilers and process heaters. An improvement of the present invention is directed to performing safe startups and shutdowns with a controller. One purpose is to integrate an innovative oscillating combustion control scheme to the existing process in order to allow a safe operation of this process at any time and/or to adjust the various control parameters in order to maintain optimal or satisfactory NOx (and optionally CO) reduction and process efficiency.

The proposed invention can be applied to any industrial application involving the use of at least one burner utilizing oscillating combustion, but is more especially suited for industrial boilers and process heaters utilizing oscillating combustion. Suitable oscillating combustion industrial boiler processes to which this invention may be applied include those described in published U.S. Patent application 2003/0134241 and U.S. Pat. No. 6,913,457. Suitable types of industrial boilers include both firetube, and watertube. Such firetube and watertube boilers are well known to those skilled in the art and need not be described herein.

As best illustrated in FIG. 1, an embodiment of the invention is a system 50 for performing oscillating combustion. Fuel flows from fuel supply 23 flows first to fuel flow rate control unit 27, then to oscillating valve 33 where the flow is pulsed to achieve an oscillating flow, and then to burner 1. The main oxidant flows from main oxidant supply 25 to main oxidant flow rate control unit 29 for controlling the flow rate of the main oxidant, then to burner 1, where combustion of the fuel and main oxidant is initiated. Oscillating combustion of the fuel and main oxidant continues at flame 3 within oscillating combustion space 2 enclosed by inner wall 5.

Fuel flow rate control unit 27 is typically a valve that is operable manually and/or automatically. Fuel flow rate control unit 27 may also be one of any number of well-known to one of ordinary skill in the art for controlling a fuel flow rate in a combustion process, especially for boilers.

The main oxidant flow rate is controlled by a main oxidant flow rate control unit 29. The main oxidant flow rate control unit 29 may be any one of a number of devices well known to one of ordinary skill in the art for combustion processes, especially for boilers. Two typical devices include dampers and variable speed fans.

Post-combustion oxidant from post-combustion oxidant supply 37 is injected through post-combustion oxidant flow rate control unit 39 into post-combustion space 4 enclosed by inner wall 5. In post-combustion space 4, combustion products from the combustion of the fuel and main oxidant at flame 3 within oscillating combustion space 2 are themselves combusted with the post-combustion oxidant to produce flue gas 11. Preferably, staggered baffles or a swirler are disposed within inner walls to enhance mixing of the post-combustion oxidant and the combustion products from the combustion of the fuel and main oxidant. Details regarding these baffles and swirler may be found in U.S. Pat. No. 6,913,457.

Flue gas 11 exits stack 9. At stack 9, the oxygen sensor 13 senses an oxygen concentration in the flue gas 11 and communicates it to the controller 17 by communication line 15. While communication line 15 is preferred, the oxygen concentration may also be communicated wirelessly

The oxygen concentration sensed by oxygen sensor 13 is communicated to the controller 16 by communication line 15. While a communication line 15 is preferred, the oxygen concentration may also be communicated wirelessly.

A value associated with the fuel flow rate is either input into the controller 17 by an operator or it is sensed at fuel flow rate control unit 27 and communicated to controller 16 via communication line 19. The value may be the actual flow-rate itself or a derivation thereof. Typically, an operator adjusts the fuel flow rate in response to the desired degree of combustion heat, such as a desired load for a boiler. However, the fuel flow rate may be adjusted by controller 17 if desired.

A value associated with the main oxidant flow rate is either input into the controller 17 by an operator or it is sensed at main oxidant flow rate control unit 29 or interpreted from fuel flow measurement by 27 and communicated to controller 17 via communication line 21. The value may be the actual flow-rate itself or a mathematical derivation thereof.

A value associated with the post-combustion oxidant flow rate is either output from the controller 17 or by an operator by sensing the main oxidant flow rate by control unit 29 and communicated to controller 17 via communication line 21. The value may be the actual flow-rate itself or a derivation thereof.

The post-combustion oxidant flow rate control unit 39 may be any one of a number of devices well known to one of ordinary skill in the art for combustion processes, especially for boilers. Two devices include dampers and variable speed fans.

Three oscillation parameters of fuel flow oscillation from oscillating valve 33 may be adjusted: duty cycle, frequency, and amplitude. Typically, the controller 17 adjusts the duty cycle, frequency, and amplitude in accordance with the later detailed description of controller 17. However, one, two, or all three parameters may be adjusted by an operator, again in accordance with that later description.

The main oxidant may be air or oxygen-enriched air. Similarly, the post-combustion oxidant may be air or oxygen-enriched air. Preferably, each of these oxidants is air. If oxygen-enriched air is selected as the main oxidant or post-combustion oxidant, the oxygen concentration is preferably between 21% and 35% by volume, but may range up to 100%.

To optimize the post-combustion of CO without re-creation of NOx, through the implementation of means to lower and control the temperature in the post-combustion region, additional inert fluids can be injected into the post-combustion space along with the oxidant so as to create heat sinks that can absorb the heat released during the combustion of CO and unburned HC. These inert fluids include nitrogen, recirculated flue gas from the exhaust duct, carbon dioxide, water or steam. It is preferred to use fluids with high heat capacities, so water and steam are preferred heat sinks. Water is even more preferred, since on top of its high heat capacity, its heat of vaporization when transformed into steam inside the combustion chamber constitutes and additional heat sink. Injection of inert fluids as heat sinks is particularly indicated when oxygen-enriched air or pure oxygen is used as post-combustion oxidant.

Typical fuels include, but are not limited to, natural gas, fuel oil, and crude oil residuals. One of ordinary skill in the art will understand that, crude oil residuals include coke, asphalt, tar, waxes (and other starting material for making other products), that are obtained from refining crude oil by distillation. Generally, they are solids, multiple-ringed compounds with 70 or more carbon atoms, and having a boiling range at atmospheric pressure of greater than 600° C. Preferably, the fuel is natural gas.

The controller 17 includes a PLC having an algorithm for determining desirable operating conditions for the oscillating combustion process in order to achieve desirable NOx (and optionally CO) levels in the flue gas. The inventors varied the fuel flow rate, main oxidant flow rate, post-combustion oxidant flow rate, oscillating frequency, oscillating amplitude, and oscillating duty cycle during operation of an industrial boiler. For each permutation of these conditions, the inventors recorded the oxygen, NOx and CO levels in the flue gas. Based upon the recorded data, the algorithm was created as described later in this specification.

During operation, the information needed by the algorithm is the fuel flow rate and one of either the main oxidant flow rate or the post-combustion oxidant flow rate. Based upon these data, the algorithm will then determine a value associated with the oxidant flow rate not known. This value is also indicative of operating conditions that yield desirably low levels of NOx (and optionally CO). A value associated with the unknown flow rate means that the value may be the actual flow rate itself or a mathematical derivation thereof. The unknown flow rate may then be adjusted in accordance with the value, i.e., the flow rate is adjusted to a level such that the desirably low levels of NOx (and optionally CO) will be achieved. Optionally, the algorithm may also determine a value(s) associated with one, two, or all three of the oscillating parameters of frequency, amplitude, and duty cycle. A value associated with an oscillating parameter means that the value may be the actual frequency, amplitude, or duty cycle itself, or it may be a mathematical derivation thereof.

In a first example, the fuel flow rate is adjusted to a level desired for whichever process it is being used for, such as the load of a boiler. Based upon the fuel flow rate, the algorithm determines values associated with the main and post-combustion oxidant flow rates and the oscillating frequency, amplitude, and duty cycle that will yield desirably low levels of NOx (and optionally CO). The controller 17 sends signals via communication lines 21, 35, and 31 to the main oxidant flow rate control unit 29, post-combustion oxidant flow rate control unit 39, and oscillating valve 33, respectively, that in turn, automatically adjust the main and post-combustion oxidant flow rates, and oscillating frequency, amplitude, and duty cycle, respectively in accordance with the associated determined values. It is understood that if any of the determined value is the same as the immediately preceeding value, the controller 17 need not send a signal for adjustment of the associated combustion parameter.

In a second example, the fuel flow rate is adjusted to a level desired for whichever process it is being used for, such as the load of a boiler. Also, one, two or three of the oscillating parameters of frequency, amplitude, and duty cycle are set to desirable levels, examples of which are disclosed in U.S. Pat. Nos. 5,302,111, 5,522,721, and 4,846,665, and published U.S. Patent application 2003/0134241, all of the contents of which are incorporated by reference.

Based upon the fuel flow rate, the algorithm determines values associated with the main and post-combustion oxidant flow rates and any non-selected oscillating parameters of frequency, amplitude, and duty cycle that will yield desirably low levels of NOx (and optionally CO). The controller 17 sends signals via communication lines 21, 35, and 31 to the main oxidant flow rate control unit 29, post-combustion oxidant flow rate control unit 39, and oscillating valve 33 (if applicable), respectively, that in turn automatically adjust the main and post-combustion oxidant flow rates and oscillating frequency (if applicable), amplitude (if applicable), and duty cycle (if applicable), respectively in accordance with the associated determined values. It is understood that if any of the determined value is the same as the immediately preceeding value, the controller 17 need not send a signal for adjustment of the associated combustion parameter.

In a third example, the fuel flow rate is adjusted to a level desired for whichever process it is being used for, such as the load of a boiler. The flow rate of the main oxidant is then selected such that the stoichiometric amount of oxygen is about 0.7 and 1.0 of the amount necessary for complete combustion of the fuel. Based upon the fuel and main oxidant flow rates, the algorithm determines values associated with the post-combustion oxidant flow rate and the oscillating frequency, amplitude, and duty cycle that will yield desirably low levels of NOx (and optionally CO). The controller 17 sends signals, via communication lines 35 and 31, to the post-combustion oxidant flow rate control unit 39 and oscillating valve 33, respectively, that in turn, automatically adjusts the main and post-combustion oxidant flow rates, and oscillating frequency, amplitude, and duty cycle, respectively in accordance with the associated determined values. It is understood that if any of the determined value is the same as the immediately preceeding value, the controller 17 need not send a signal for adjustment of the associated combustion parameter.

In a fourth example, the fuel flow rate is adjusted to a level desired for whichever process it is being used for, such as the load of a boiler. The flow rate of the main oxidant is then selected such that the stoichiometric amount of oxygen is about 0.7 and 1.0 of the amount necessary for complete combustion of the fuel. Also, one, two or three of the oscillating parameters of frequency, amplitude, and duty cycle are set to desirable levels, examples of which are disclosed in U.S. Pat. Nos. 5,302,111, 5,522,721, and 4,846,665, and published U.S. Patent application 2003/0134241, all of the contents of which are incorporated by reference. Preferably, all three of the oscillating parameters are set to desirable levels.

In this fourth example, based upon the fuel and main oxidant flow rates, the algorithm determines values associated with the post-combustion oxidant flow rate and any of the non-selected oscillating parameters that will yield desirably low levels of NOx (and optionally CO). Preferably, all three oscillating parameters have already been set to desirable levels, thus the algorithm does not determine values associated with such oscillating parameters. The controller 17 then sends signals via communication lines 35 and 31 to the post-combustion oxidant flow rate control unit 39 and oscillating valve 33 (if applicable), respectively, that in turn automatically adjust the main and post-combustion oxidant flow rates and oscillating frequency (if applicable), amplitude (if applicable), and duty cycle (if applicable), respectively in accordance with the associated determined values. It is understood that if any of the determined value is the same as the immediately preceeding value, the controller 17 need not send a signal for adjustment of the associated combustion parameter.

In a fifth example, the fuel flow rate is adjusted to levels desired for whichever process it is being used for, such as the load of a boiler. As taught in U.S. Pat. No. 6,913,457 (the entire contents of which are incorporated by reference), low NOx combustion techniques typically are run above stoichiometric conditions, i.e., with excess air-there is always some oxygen available in the combustion products, even if some CO and unburned HC are present. So, generally, the stoichiometric amount of oxygen contained in the main oxidant should be maintained between about 0.7 and 1.0 of the amount necessary for complete combustion. While the amount of main oxidant to be combusted is not selected by an operator, based upon the stoichiometry selected and the amount of fuel being combusted, the stoichiometric amount of oxygen amount may be generally determined. And, based upon the generally determined amount of main oxidant and the desired degree of flue gas oxygen leaving the stack, the post-combustion oxidant rate is then selected to a level in accordance with the above explanation.

In this fifth example, based upon the fuel and post-combustion oxidant flow rates, the algorithm determines values associated with the flow rate of the main oxidant and the oscillating parameters of frequency, amplitude, and duty cycle that will yield desirably low levels of NOx (and optionally CO). The controller 17 sends signals via communication lines 21 and 31 to main oxidant flow rate control unit 29 and oscillating valve 33, respectively, that in turn, automatically adjusts the main oxidant flow rate, and oscillating frequency, amplitude, and duty cycle, respectively in accordance with the associated determined values. It is understood that if any of the determined value is the same as the immediately preceeding value, the controller 17 need not send a signal for adjustment of the associated combustion parameter.

In a sixth example, the fuel flow rate is adjusted to levels desired for whichever process it is being used for, such as the load of a boiler. As taught in U.S. Pat. No. 6,913,457 (the entire contents of which are incorporated by reference), low NOx combustion techniques typically are run above stoichiometric conditions, i.e., with excess air-there is always some oxygen available in the combustion products, even if some CO and unburned HC are present. So, generally, the stoichiometric amount of oxygen contained in the main oxidant should be maintained between about 0.7 and 1.0 of the amount necessary for complete combustion. While the amount of main oxidant to be combusted is not selected by an operator, based upon the stoichiometry selected and the amount of fuel being combusted, the stoichiometric amount of oxygen amount may be generally determined. And, based upon the generally determined amount of main oxidant and the desired degree of flue gas oxygen leaving the stack, the post-combustion oxidant rate is then selected to a level in accordance with the above explanation. Moreover, one, two or three of the oscillating parameters of frequency, amplitude, and duty cycle are set to desirable levels, examples of which are disclosed in U.S. Pat. Nos. 5,302,111, 5,522,721, and 4,846,665, and published U.S. Patent application 2003/0134241, all of the contents of which are incorporated by reference.

In this sixth example, based upon the fuel and post-combustion oxidant flow rates, the algorithm determines values associated with the flow rate of the main oxidant and any non-selected oscillating parameters that will yield desirably low levels of NOx (and optionally CO). The controller 17 sends signals via communication lines 21 and 31 to main oxidant flow rate control unit 29 and oscillating valve 33 (if applicable), respectively, that in turn automatically adjust the main oxidant flow rate and oscillating frequency (if applicable), amplitude (if applicable), and duty cycle (if applicable), respectively in accordance with the associated determined values. It is understood that if any of the determined value is the same as the immediately preceeding value, the controller 17 need not send a signal for adjustment of the associated combustion parameter.

Apart from maintaining the satisfactory NOx levels (optionally CO), the controller 17 of FIG. 1 can continuously monitor the safety of the process by monitoring the oscillating valve 33. In case of any problem detected in the valve 33 by the controller 16 or 17, the process/boiler may be shutdown or only the oscillations may be stopped to ensure safety and integrity of the process. The feedback of O₂ measurement by sensor 13 can also be used in monitoring the safety of the process. In addition to the safety, the oscillations can also be synchronized with the boiler operation. For instance, when the boiler is starting up, the oscillations are paused until a stable main flame is established as it is not safe to start a boiler with pulsing gas flow. When the boiler is not operating or shutdown, the oscillations are also stopped and reconvened in next startup sequence.

The invention optionally includes an O₂ trim system. It is a control device that may be implemented in boilers to adjust (through close-loop control) the air/fuel stoichiometric ratio (at the burner) in order to achieve a pre-set O₂ concentration in the flue gas. For this purpose, the O₂ concentration in the flue gas is measured with oxygen sensor 13.

As best shown in FIG. 2, the controller 17 allows for a safe startup sequence commencing at 100, at which time the boiler is fired under non-oscillating conditions with no post-combustion oxidant injection. The controller 17 determines whether or not a boiler alarm exists 105, i.e., whether or not the boiler is operating in a safe operating mode. Typically, this consists in sensing that a normally open relay input is not energized. Most commercial burner management systems are equipped with relay outputs for this very purpose. If the boiler alarm is detected, then the startup sequence is aborted 110 and returned to commencement 100.

If no boiler alarm is detected, the controller 17 next determines whether or not the oscillating valve alarm exists 115, i.e., whether or not the oscillating valve 33 is operating in a safe alarm-free mode. This information is important during system start-up. In practice, the controller 17 will verify that the relay alarm output of the oscillating valve 33 is not energized. If the oscillating valve alarm is detected, then the startup sequence is paused 120.

If no oscillating valve alarm is detected, then the controller 17 starts a timer 125. The controller 17 determines whether or not the time is up 130, and when it is, the system starts oscillation 135 of the fuel at the oscillating valve 33.

The controller 17 next determines whether or not the post-combustion alarm exists 140, i.e., whether or not the post-combustion oxidant flow rate control unit 39 is operating in a safe alarm-free mode. If the post-combustion alarm is detected, then the startup sequence is paused 145 and the controller again determines whether or not the post-combustion alarm exists 140. If no such post-combustion alarm is detected, then the controller 17 starts a timer 150. The controller 17 determines whether or not the time is up 155, and when it is, it starts 160 the post-combustion flow rate control unit so that the post-combustion oxidant may be injected into post-combustion space 4. Feedback signal from the post-combustion flow rate control unit 39 will be systematically compared to the signal output to the actuator by the controller 17 to verify the two are in agreement. Any significant deviation will result in an alarm signifying this control loop is an error mode.

The controller 17 next determines whether or not the O₂ trim system alarm exists 165, i.e., whether or not the O₂ trim system is operating in a safe alarm-free mode. If the O₂ trim system alarm is detected, then the startup sequence is paused 170 and the controller again determines whether or not the O₂ trim system alarm exists 165. If no such O₂ trim system alarm is detected, then the controller 17 starts a timer 175. The controller 17 determines whether or not the time is up 180, and when it is, the O₂ trim system is started 185 and the startup sequence completed 190.

The errors associated with the operation of the oscillating valve 33 are monitored all the time and corresponding actions are taken for the safety of the process. The controller 17 senses “communication errors” that occurs due to failure of communication between the controller and oscillating valve 33, and “motion errors” that is caused due to abnormal or lack of motion of the valve components. To ensure the safety in these situations, routines of the algorithms are made such a way that the oscillations are stopped in case of “communication error” and the combustion process is shut down in case of “motion error”. The ability to stop the oscillations and/or shut down the combustor (often by communicating with a burner management system) whenever required greatly establishes the safe operation of the oscillating combustion technology.

As best shown in FIG. 3, the controller 17 allows for a safe startup sequence commencing at 200. The controller 17 determines if an emergency stop situation is indicated 205. If it is, then the controller 17 stops oscillations 220 at oscillating valve 33. If no emergency stop situation is indicated, then the controller 17 starts a shutdown timer 210. The controller 17 then determines whether the timer is up 215. Once the timer is up, the controller 17 stops oscillations 220 at the oscillating valve 33.

After oscillations are stopped 220, the controller 17 again determines whether an emergency stop is required 225. If so, it proceeds to stop 240 the post-combustion oxidant from being injected into post-combustion space 4. If no emergency stop situation is indicated, then the controller 17 starts a shutdown timer 230. The controller 17 then determines whether the timer is up 235. Once the timer is up, the controller 17 stops 240 the post-combustion oxidant from being injected into post-combustion space 4.

After post-combustion oxidant injection is stopped 240, the controller 17 again determines whether an emergency stop is required 245. If so, it proceeds to stop the O₂ trim system 260. If no emergency stop situation is indicated then the controller 17 starts a shutdown timer 250. The controller 17 then determines whether the timer is up 255. Once the timer is up, the controller 17 stops the O₂ trim system 260 whereat the shutdown sequence ends 265.

The PLC is used as a data concentrator. All recorded data (real time inputs and outputs) can be utilized to calculate the state space representation, as the system was deemed continuous and linear from experimental observations:
{dot over (x)}(t)=[A(t)]x(t)+[B(t)]u(t)  [1]
y(t)=[C(t)]x(t)+[D(t)]u(t)  [2]

The first model is known as the control law [1], while the second is the observer equation [2], wherein x is the state vector, y the controlled variable or process output, and the u the manipulated variable or control action. Matrix A describes the influence of the current state and matrix B the effect of the control action on the states rate of change or time variance respectively. There is also a dynamic relationship between the process output and the states and the control action respectively represented by matrices C and D.

The states used in [1] are essentially a set of internal process variables-measured or unmeasured-the knowledge of which is sufficient to characterize the combustion process in terms of quality and performance. The column vector x is comprised of temperature T, % CO and % NOx in the off-gas. For practical reasons, % CO and % NOx are not always available on a real time basis for control. The controlled variable is about % O2 (often available through existing oxygen trimming systems), and the manipulated variables are the oscillating combustion parameters (Amplitude, Duty Cycle, and Frequency), and post-combustion airflow rate.

The proposed algorithm in this invention is based on the state observer's theory. Specifically, the knowledge of control manipulation u (post-combustion airflow rate and oscillating frequency, amplitude and duty-cycle) and controlled variable y (measured % O2) over a time interval [t₁,t₂] allows calculating the state of the system at time t₁. The application of the Laplace transform to the control law [1] and resolution of the differential equation leads to: $\begin{array}{cc}x\left(t\right)=e^{\left[A\right]t}x\left(0\right)+{\int }_0^t{e}^{\left[A\right]\left(t-\tau \right)}\left(B\right)e\left(\tau \right)d\tau ,& \left[3\right]\end{array}$
where $\begin{array}{cc}{e}^{\left[A\right]t}=(I+\left[A\right]t+\frac{{\left[A\right]}^2}{2!}{t}^2+\dots +\frac{{\left[A\right]}^n}{n!}{t}^n+\dots )& \left[4\right]\end{array}$
is the transition matrix.

The transition matrix can also be calculated in many different ways, one of which is the Cayley-Hamilton method, by solving the characteristic equation of matrix A: det(λI−[A])=λⁿ+a_n-1λ^n-1+ . . . +a₁=0.

This equation can be utilized at each time interval during which the system is observable to estimate or predict x. The later estimate may be then injected into control law to come up with the appropriate control action. As a result this control algorithm, NOx, and CO levels in the stacks can be inferred, better yet maintained at desirably low levels.

All the algorithms and control laws are embedded in a master PLC. In the case of a retrofit, the PLC communicates with the existing standalone oscillating combustion. Regardless, the PLC communicates with whichever control techology is used to manipulate the oscillating valve 33 using the MODBUS RTU industrial protocol over RS232 (serial) or in its digital form using MODBUS TCP. The communications include, but are not limited to, oscillating combustion parameters (frequency, amplitude, duty cycle) and safety parameters, such as errors generated by OCT valve.

In embodiments where the fuel and main oxidant flow rates are known and the post-combustion oxidant flow rate is determined, the real-time fuel flow, (which is generally directly proportional to the firing rate) is transferred into post-combustion airflow using a semi-empirical transfer function. The coefficients in the transfer function are determined during the commissioning of the combustor and fine-tuned during start-up. Residence times and other boiler dynamic variables are convoluted in the derivation of said transfer function.

Again, in embodiments where the fuel and main oxidant flow rates are known and the post-combustion oxidant flow rate is determined, the flow rate of the post-combustion air is corrected by filtering out natural dynamic behavior of the natural gas valve from the firing rate signal. The Fourier transform (in practice a Fast Fourier Transform or FFT) of the resulting signal is computed. The later derivation allows it to decompose the signal into its phase and amplitude in the frequency domain. As per the frequency convolution theorem, the oscillatory behavior due to the oscillating combustion mechanism is separated from the actual firing rate signal. A transfer function is then applied to each resulting system to come up with a performing correction.

It will be understood that many additional changes in the details, materials, steps, and arrangement of parts, which have been herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above and/or the attached drawings.

Claims

1. A process for optimizing levels of NOx and CO during fuel combustion, said process comprising the steps of:

a) supplying a flow of a fuel to a burner;

b) supplying a flow of a main oxidant to a burner whose rate is controlled by a main oxidant flow rate control unit;

c) generating oscillating combustion by oscillating the fuel flow with an oscillating valve and combusting the oscillating fuel with the main oxidant adjacent the burner to produce combustion products;

d) injecting a post-combustion oxidant into the combustion products, a rate of the post-combustion oxidant injection being controlled by a post-combustion oxidant flow rate control unit;

e) combusting the combustion products and the injected post-combustion oxidant;

f) predetermining a rate of the fuel flow;

g) providing a controller operatively associated with the main oxidant flow rate control unit, the oscillating valve, and the post-combustion oxidant flow rate control unit;

h) determining a value or values associated with a combustion parameter selected from the group consisting of a rate of flow of the main oxidant, a rate of flow of the post-combustion oxidant, a frequency of the oscillating fuel flow, an amplitude of the oscillating fuel flow, a duty cycle of the oscillating fuel flow, and two or more combinations thereof; and

i) adjusting the combustion parameter associated with the determined value or values, wherein: i) the determined value or values is based upon the predetermined fuel flow rate; and ii) said determining step is performed by the controller.

2. The process of claim 1, further comprising the step of measuring an oxygen concentration of the flue gas, wherein the determined value or values is based upon both the predetermined fuel flow rate and the measured oxygen concentration.

3. The process of claim 1, wherein the combustion parameter is the flow rate of the main oxidant.

4. The process of claim 1, wherein the combustion parameter is the flow rate of the post-combustion oxidant.

5. The process of claim 1, wherein the combustion parameter is the oscillating frequency.

6. The process of claim 1, wherein the combustion parameter is the oscillating amplitude.

7. The process of claim 1, wherein the combustion parameter is the oscillating duty cycle.

8. The process of claim 1, wherein said combusting step is performed in a boiler enclosing an oscillating combustion space where the oscillating combustion occurs and a post-combustion space where the combustion products and the post-combustion oxidant are combusted.

9. The process of claim 8, further comprising the step of recirculating a portion of flue gas created from combustion of the combustion products and the post-combustion oxidant into one of the oscillating combustion space and the post-combustion space.

10. The process of claim 1, wherein said combusting step is performed in an industrial boiler having a single burner, the boiler enclosing an oscillating combustion space where the oscillating combustion occurs and a post-combustion space where the combustion products and the post-combustion oxidant are combusted.

11. The process of claim 10, wherein the combustion parameter is the flow rate of the post-combustion oxidant.

12. The process of claim 11, wherein the fuel is natural gas.

13. The process of claim 12, further comprising the step of predetermining the oscillating frequency, amplitude, and duty cycle.

14. The process of claim 1, further comprising the step of:

a) before said steps of generating oscillating combustion, injecting a post-combustion oxidant, combusting the combustion products and the injected post-combustion oxidant, and adjusting the combustion parameter are performed, determining with the controller whether said steps of generating oscillating combustion, injecting a post-combustion oxidant, combusting the combustion products and the injected post-combustion oxidant, and adjusting the combustion parameter should be performed.

15. The process of claim 1, further comprising the steps of determining with the controller, whether one or more of said steps of generating oscillating combustion, injecting a post-combustion oxidant, combusting the combustion products and the injected post-combustion oxidant, and adjusting the combustion parameter should be discontinued.

16. A system for improved operation of a combustion process utilizing oscillating combustion, comprising:

a) a supply of fuel;

b) a supply of a main oxidant;

c) a supply of a post-combustion oxidant;

d) a burner for receiving said fuel and main oxidant and initiating combustion thereof to produce combustion products;

e) an oscillating valve operatively associated with said supply of fuel for achieving an oscillating flow of said fuel at said burner;

f) walls defining a combustion chamber operatively associated with said burner, and enclosing an oscillating combustion space, and a post-combustion space, said oscillating combustion space being upstream of said post-combustion space;

g) a main oxidant flow rate control unit operatively associated with said supply of main oxidant and said burner adapted and configured to control a flow rate of said main oxidant;

h) a post-combustion oxidant flow rate control unit operatively associated with said supply of post-combustion oxidant and said post-combustion space adapted and configured to control a flow rate of said post-combustion oxidant;

i) a post-combustion injection element operatively associated with said post-combustion oxidant flow rate control unit adapted and configured to inject said post-combustion oxidant into said post-combustion space to achieve combustion of the combustion products and said post-combustion oxidant; and

j) a controller operatively associated with said main oxidant flow rate control unit, post-combustion oxidant flow rate control unit, and oscillating valve.

17. The system of claim 16, wherein said controller has an algorithm designed to determine a value or values associated with a combustion parameter selected from the group consisting of:

a) a rate of flow of the main oxidant;

b) a rate of flow of the post-combustion oxidant;

c) a frequency of the oscillating fuel flow;

d) an amplitude of the oscillating fuel;

e) a duty cycle of the oscillating flow; and

f) two or more combinations thereof, wherein the determined value or values is based upon a predetermined fuel flow rate.

18. A process for optimizing levels of NOx and CO during fuel combustion in an industrial boiler, said process comprising the steps of:

a) providing flows of a fuel and a main oxidant to a burner;

b) generating oscillating combustion of the fuel and the main oxidant within a boiler to produce combustion products;

c) injecting a post-combustion oxidant into the combustion products;

d) combusting the combustion products and the post-combustion oxidant;

e) predetermining a rate of the fuel flow;

f) providing a controller adapted and configured to control a flow rate of the post-combustion oxidant;

g) determining a value associated with the flow rate of the post-combustion oxidant with a controller, the determined value being based upon the predetermined fuel flow rate; and

h) adjusting the flow rate of the post-combustion oxidant at the command of the controller using the determined value.