U.S. patent application number 11/301905 was filed with the patent office on 2006-08-10 for advanced control system for enhanced operation of oscillating combustion in combustors.
Invention is credited to Omar Germouni, Erwin Penfornis, Rajani K. Varagani.
Application Number | 20060177785 11/301905 |
Document ID | / |
Family ID | 36780381 |
Filed Date | 2006-08-10 |
United States Patent
Application |
20060177785 |
Kind Code |
A1 |
Varagani; Rajani K. ; et
al. |
August 10, 2006 |
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.
Inventors: |
Varagani; Rajani K.;
(Lombard, IL) ; Penfornis; Erwin;
(Levallois-Perret, FR) ; Germouni; Omar; (Chicago,
IL) |
Correspondence
Address: |
AIR LIQUIDE
2700 POST OAK BOULEVARD, SUITE 1800
HOUSTON
TX
77056
US
|
Family ID: |
36780381 |
Appl. No.: |
11/301905 |
Filed: |
December 13, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60635737 |
Dec 13, 2004 |
|
|
|
Current U.S.
Class: |
431/12 ;
431/181 |
Current CPC
Class: |
F23N 5/006 20130101;
F23N 2227/02 20200101; F23N 1/022 20130101; F23K 2900/05003
20130101; F23C 9/08 20130101 |
Class at
Publication: |
431/012 ;
431/181 |
International
Class: |
F23N 1/02 20060101
F23N001/02 |
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.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims benefit under 35 U.S.C. .sctn.
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
[0002] 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.
[0003] 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.
[0004] 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.
[0005] 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.
[0006] 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
[0007] 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.
[0008] A system for improved operation of a combustion process
utilizing oscillating combustion includes: [0009] a supply of fuel;
[0010] a supply of a main oxidant; [0011] a supply of a
post-combustion oxidant; [0012] a burner for receiving the fuel and
main oxidant and initiating combustion thereof to produce
combustion products; [0013] an oscillating valve operatively
associated with the supply of fuel for achieving an oscillating
flow of said fuel at the burner; [0014] walls defining a combustion
chamber operatively associated with the burner and enclosing an
oscillating combustion space and a post-combustion space; [0015] 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; [0016] a post-combustion
oxidant flow rate control unit operatively associated with the
supply of post-combustion oxidant and the post-combustion space;
[0017] a post-combustion injection element operatively associated
with the post-combustion oxidant flow rate control unit; and [0018]
a controller operatively associated with the main oxidant flow rate
control unit, post-combustion oxidant flow rate control unit, and
oscillating valve.
[0019] 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
[0020] 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:
[0021] FIG. 1 is a schematic of one embodiment of the system
according to the invention;
[0022] FIG. 2 is a flowchart showing the startup sequence of one
embodiment of the invention; and
[0023] FIG. 3 is a flowchart showing the shutdown sequence for one
embodiment of the invention.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0024] 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.
[0025] 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.
[0026] 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.
[0027] 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.
[0028] 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.
[0029] 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.
[0030] 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
[0031] 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.
[0032] 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.
[0033] 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.
[0034] 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.
[0035] 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.
[0036] 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.
[0037] 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%.
[0038] 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.
[0039] 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.degree. C. Preferably, the fuel is
natural gas.
[0040] 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.
[0041] 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.
[0042] 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.
[0043] 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.
[0044] 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.
[0045] 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.
[0046] 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.
[0047] 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.
[0048] 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.
[0049] 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.
[0050] 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.
[0051] 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.
[0052] 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 O.sub.2 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.
[0053] The invention optionally includes an O.sub.2 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 O.sub.2 concentration in
the flue gas. For this purpose, the O.sub.2 concentration in the
flue gas is measured with oxygen sensor 13.
[0054] 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.
[0055] 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.
[0056] 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.
[0057] 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.
[0058] The controller 17 next determines whether or not the O.sub.2
trim system alarm exists 165, i.e., whether or not the O.sub.2 trim
system is operating in a safe alarm-free mode. If the O.sub.2 trim
system alarm is detected, then the startup sequence is paused 170
and the controller again determines whether or not the O.sub.2 trim
system alarm exists 165. If no such O.sub.2 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 O.sub.2 trim system is started 185 and the startup sequence
completed 190.
[0059] 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.
[0060] 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.
[0061] 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.
[0062] 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 O.sub.2 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 O.sub.2 trim system 260 whereat the
shutdown sequence ends 265.
[0063] 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]
[0064] 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.
[0065] 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.
[0066] 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 [t.sub.1,t.sub.2] allows
calculating the state of the system at time t.sub.1. The
application of the Laplace transform to the control law [1] and
resolution of the differential equation leads to: x .function. ( t
) = e [ A ] .times. t .times. x .function. ( 0 ) + .intg. 0 t
.times. e [ A ] .times. ( t - .tau. ) .function. ( B ) .times. e
.function. ( .tau. ) .times. d .tau. , [ 3 ] ##EQU1## where e [ A ]
.times. t = ( I + [ A ] .times. t + [ A ] 2 2 ! .times. t 2 + + [ A
] n n ! .times. t n + .times. ) [ 4 ] ##EQU2## is the transition
matrix.
[0067] 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(.lamda.I-[A])=.lamda..sup.n+a.sub.n-1.lamda..sup.n-1+ . . .
+a.sub.1=0.
[0068] 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.
[0069] 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.
[0070] 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.
[0071] 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.
[0072] 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.
* * * * *