U.S. patent application number 10/580523 was filed with the patent office on 2008-05-15 for burner control sensor configuration.
This patent application is currently assigned to NUVERA FUEL CELLS, INC.. Invention is credited to Jonathan R. Leehey, Vincent G. Rizzo, Joey Dewayne Veasey.
Application Number | 20080113306 10/580523 |
Document ID | / |
Family ID | 34632945 |
Filed Date | 2008-05-15 |
United States Patent
Application |
20080113306 |
Kind Code |
A1 |
Veasey; Joey Dewayne ; et
al. |
May 15, 2008 |
Burner Control Sensor Configuration
Abstract
In domestic installations of any type of furnace or equivalent,
proof of lean combustion is necessary to satisfy requirements of
certification agencies. Exhibiting proof of lean combustion during
the operation of an integrated fuel reformer and fuel cell system
can be problematic because the point of combustion and its nature
may shift during operation. In addition, the preferred ratio of
fuel to air in these fuel-reforming systems is often near a
stoichiometry of one. In the present invention, the combination of
a flame-detecting device, a temperature sensing device and an
oxygen or hydrocarbon sensor is used to verify the occurrence of
combustion, and show proof of lean combustion.
Inventors: |
Veasey; Joey Dewayne; (Wills
Point, TX) ; Leehey; Jonathan R.; (Wayland, MA)
; Rizzo; Vincent G.; (Norfolk, MA) |
Correspondence
Address: |
FINNEGAN, HENDERSON, FARABOW, GARRETT & DUNNER;LLP
901 NEW YORK AVENUE, NW
WASHINGTON
DC
20001-4413
US
|
Assignee: |
NUVERA FUEL CELLS, INC.
Cambridge
MA
|
Family ID: |
34632945 |
Appl. No.: |
10/580523 |
Filed: |
November 22, 2004 |
PCT Filed: |
November 22, 2004 |
PCT NO: |
PCT/US04/39361 |
371 Date: |
September 28, 2007 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
60524986 |
Nov 25, 2003 |
|
|
|
Current U.S.
Class: |
431/6 ;
431/75 |
Current CPC
Class: |
F23N 2229/16 20200101;
F23N 5/006 20130101; F23N 1/022 20130101; F23N 2227/16 20200101;
F23N 5/102 20130101; F23N 2237/12 20200101; F23C 2900/03002
20130101; F23N 5/242 20130101; F23N 2241/14 20200101 |
Class at
Publication: |
431/6 ;
431/75 |
International
Class: |
F23N 5/10 20060101
F23N005/10; F23N 5/00 20060101 F23N005/00 |
Claims
1. A method for verifying combustion in a burner of a fuel reformer
during warm up and steady operation, the method comprising the
steps of: detecting for flame within a burner of a fuel reformer
during an initial warm-up stage of operation of a fuel reformer;
proceeding with operation of the fuel reformer if a flame is
detected within the burner; monitoring a temperature of a catalyst
within the burner to determine the occurrence of flameless
catalytic combustion; proceeding with operation of the fuel
reformer if a predetermined temperature is achieved by the catalyst
within the burner; and producing a burner exhaust.
2. The method of claim 1, wherein the step of detecting for flame
is accomplished by at least one flame detector.
3. The method of claim 2, wherein the at least one flame detector
is selected from the group consisting of a flame ionization
detector, an ionization/rectification flame detector, a light-based
flame detector, an ultraviolet light detector, a photoelectric eye,
a visible light detector, an infrared detector, or a combination
thereof.
4. The method of claim 1, wherein the step of monitoring a
temperature is accomplished by at least one temperature sensor.
5. The method of claim 4, wherein the at least one temperature
sensor is selected from the group consisting of thermocouples,
thermistors, resistive temperature devices, thermometers, infrared
detectors, and a combination thereof.
6. The method of claim 1, further comprising the step of
determining the completeness of combustion after the step of
producing a burner exhaust.
7. The method of claim 6, wherein the step of determining the
completeness of combustion comprises the step of sensing for oxygen
in the burner exhaust to produce a reading, wherein a positive
reading for oxygen indicates complete combustion.
8. The method of claim 6, wherein the step of determining the
completeness of combustion comprises the step of sensing for
hydrocarbon fuel in the burner exhaust to produce a reading,
wherein a negative reading for hydrocarbon fuel indicates complete
combustion.
9. The method of claim 7, wherein the step of sensing for oxygen is
accomplished by an oxygen sensor.
10. The method of claim 9, wherein the oxygen sensor comprises an
automotive-type oxygen sensor.
11. The method of claim 8, wherein the step of sensing for
hydrocarbon fuel is accomplished by a hydrocarbon sensor.
12. The method of claim 1, wherein the predetermined temperature of
the catalyst within the burner is above the temperature at which
the catalyst operates as a flameless oxidation catalyst.
13. The method of claim 12, wherein the step of monitoring a
temperature comprises the steps of: providing at least two
temperature sensors within the burner; comparing an output of each
temperature sensor; and registering a "system error" if the
difference between any two outputs exceeds a predetermined
value.
14. The method of claim 6, further comprising the step of
controlling a burner input based on the completeness of
combustion.
15. The method of claim 7, further comprising the step of
controlling a burner input based on the reading for oxygen in the
burner exhaust.
16. The method of claim 8, further comprising the step of
controlling a burner input based on the reading for hydrocarbon
fuel in the burner exhaust.
17. A burner assembly associated with a fuel reformer designed to
combust fuel in a manner in which lean combustion of the fuel can
be verified, the burner assembly comprising: an outer shell housing
a combustion chamber comprising a burner; a catalyst bed situated
within the combustion chamber; a mixing zone in fluid communication
with the combustion chamber; an air inlet and a fuel inlet in
communication with the mixing zone, wherein a supply of air through
the air inlet and a supply of fuel through the fuel inlet are mixed
within the mixing zone; an exhaust outlet in fluid communication
with the combustion chamber, wherein an exhaust stream is
discharged from the combustion chamber through the exhaust outlet;
a flame detector positioned such that it is capable of detecting
the existence of flame in the combustion chamber; a temperature
sensor positioned such that it is capable of monitoring temperature
of the catalyst bed; and an exhaust detector positioned downstream
of the catalyst bed and capable of detecting at least one of either
oxygen or hydrocarbon in the exhaust stream.
18. The burner assembly of claim 17, further comprising a
controller for controlling at least one of either the supply of
fuel and the supply of air admitted to the mixing zone.
19. The burner assembly of claim 17, wherein the flame detector is
selected from the group consisting of a flame ionization detector,
an ionization/rectification flame detector, a light-based flame
detector, an ultraviolet light detector, a photoelectric eye, a
visible light detector, an infrared detector, or a combination
thereof.
20. The burner assembly of claim 17, wherein the temperature sensor
is selected from the group consisting of thermocouples,
thermistors, resistive temperature devices, thermometers, infrared
detectors, and a combination thereof.
21. The burner assembly of claim 17, wherein the exhaust sensor
comprises a hydrocarbon sensor for sensing hydrocarbons in the
exhaust stream to produce a reading, wherein a negative reading for
hydrocarbon indicates complete combustion.
22. The burner assembly of claim 17, wherein the exhaust sensor
comprises an oxygen sensor for sensing oxygen in the exhaust stream
to produce a reading, wherein a positive reading for oxygen
indicates complete combustion.
23. The burner assembly of claim 22, wherein the oxygen sensor
comprises an automotive-type oxygen sensor.
Description
RELATED APPLICATION
[0001] The present application is related to and claims priority of
U.S. Provisional Application Ser. No. 60/524,986 filed Nov. 25,
2003.
FIELD OF INVENTION
[0002] The field of invention pertains to combustion detection
within a burner or tail gas combustor that is part of a fuel
processing system comprising a fuel reformer and a fuel cell.
BACKGROUND
[0003] Hydrogen, which is used in fuel cells, is most commonly
created by steam reforming. The steam reforming reaction in a fuel
processing system is endothermic, meaning heat is absorbed by the
reaction. Therefore, heat needs to be supplied to the system to
drive the reaction. In "pure" steam reforming, heat is supplied
from an outside source to a catalyst bed. In "partial oxidation"
and "autothermal reforming," heat is created within the catalyst
bed by oxidation of some fuel with air.
[0004] The need to transport heat from an outside source to a
catalyst bed can be an obstacle to rapid startup or rapid change of
operating parameters in a steam reformer. This can be avoided by
using autothermal reforming. In autothermal reforming, fuel, steam
and also a controlled amount of air are mixed and injected into the
reactor. The oxygen in the air then reacts with some of the fuel,
usually in the presence of a catalyst, thereby producing heat. The
heat is absorbed by the reforming reaction, as described above,
which is occurring at the same time in the same catalyst bed. Once
the system is at operating temperature, the amount of heat required
can be generated in a controlled way by controlling the ratio of
the inlet air to the amount of fuel being reformed. Likewise, in
steam reforming the amount of external heating by a burner or other
heat source may be varied.
[0005] However, fuel processing systems usually must be initially
heated so that the catalyst reaches an effective operating
temperature. Preheating by hot gas, or electricity, or by
pre-combustion or local ignition is required to start up a cold
steam reformer or ATR reactor. While this is not difficult in a
large, fixed chemical plant, it is more demanding in a mobile
reformer, for example in a vehicle, or in a small reformer at a
non-industrial site, such as in a distributed electric power
generating system. Such small reformers may need to undergo
frequent cold startup.
[0006] A key problem is how to start the reforming reaction in the
first place. Clearly heat must be supplied from some other source,
such as combustion external to or within the reforming bed, or via
electrical heating of the catalyst, to raise the temperature enough
to start the autothermal reforming or steam reforming reactions.
When combustion is used to supply heat, the combustion reaction
typically occurs in a burner. In a burner, air and fuel are mixed
and burned to generate heat and the heat is used to heat up a
reformer, for example, to bring the catalyst to the operation
temperature, and/or to continually supply heat to sustain the
reforming reaction.
[0007] For example, when the reformer is a "pure" steam reformer, a
dedicated burner is used to supply the heat required for reforming.
Such burners typically start up using a spark to ignite a flame.
The flame heats a catalytic combustor up to operating temperature,
and subsequently catalytic combustion may occur on the combustion
catalyst in the burner. The heat released in the burner is used to
heat up the steam reforming catalyst to its operating temperature.
Once the stream reforming starts, the burner continues to run to
supply heat to sustain the endothermic steam reforming
reaction.
[0008] In an autothermal reformer (ATR), an auxiliary burner may be
used at startup to supply heat to bring ATR catalyst to a
predetermined operating temperature. The auxiliary burner likewise
starts up with a spark and flame. The hot combustion gas then heats
up the catalyst in the burner until the temperature of the catalyst
reaches a point that catalytic oxidation of the fuel occurs on the
surface of the catalyst, which is referred to as the flameless
catalytic combustion. Moreover, in an integrated fuel reformer/fuel
cell system (a "fuel processor"), hydrogen or reformate that is not
consumed in the fuel cell is consumed in the same burner or in a
different auxiliary burner, sometimes called a tail gas combustor
or TGC. The heat so generated may be used to assist in reforming
fuel, or used for other purposes such as space heating.
[0009] When a combustion source is to be used in a home or a
building, "proof of lean combustion" is required by certification
agencies for combustion equipment. Historically, proof of
combustion has been provided by supplying a suitable detector for
an open flame, such as an ionization type detector. However, in a
fuel processor, it can be difficult to establish combustion by such
a simple method because, as described above, the point of
combustion and/or the type of combustion may change during
operation. In particular, during startup the combustion may change
from a flame driven by a spark, to a flameless oxidation on the
combustion catalyst.
[0010] Lean combustion is also desirable, and generally required,
for safety reasons as well as for regulatory purposes. Lean
combustion means that more than stoichiometric amount of air is
present in the combustion reaction, i.e., a fuel/air stoichiometry
of less than 1. "Fuel" may include hydrocarbon fuels such as
hydrocarbons (e.g., natural gas and gasoline) as well as alcohols,
and unreacted hydrogen, such as those in anode exhaust gas from the
fuel cell. If combustion is incomplete due to lack of sufficient
air, undesirable byproducts such as carbon monoxide may be formed
and unreacted fuel may be present. These are considered safety and
environmental hazards. Furthermore, incomplete combustion lowers
the system efficiency since all the fuel is not reacted.
Accordingly, lean combustion is preferred to operate efficiently,
since if incomplete combustion takes place fuel is wasted.
[0011] Described below is an innovative way to evidence lean
combustion in a fuel reformer or fuel processor. The method uses a
flame detector, one or more temperature sensing devices, and
optionally an oxygen or hydrocarbon sensor, to provide signals to a
controller that can be used to monitor and control combustion.
SUMMARY OF THE INVENTION
[0012] The present invention is generally directed to a method for
verifying combustion in a burner of a fuel reformer during warm up
and steady operation comprising first detecting for flame within a
burner of a fuel reformer during an initial warm-up stage of
operation of a fuel reformer, then proceeding with operation of the
fuel reformer if a flame is detected within the burner. The
operation of the reformer can be discretionary continued or halted
absent the positive detection of a flame in the burner. Further,
the method may include monitoring a temperature of a catalyst
within the burner to determine the occurrence of flameless
catalytic combustion, then proceeding with operation of the fuel
reformer if a predetermined temperature is achieved by the catalyst
within the burner. The predetermined temperature of the catalyst
within the burner is preferably above the temperature at which the
catalyst operates as a flameless oxidation catalyst Again, the
operation of the reformer may be discretionally continued or halted
absent the achievement of the predetermined temperature. Finally,
the method comprises detecting for flame within the burner to
indicate continued combustion during subsequent stages of operation
of the reformer, and then producing a burner exhaust.
[0013] The method may further comprise the step of determining the
completeness of combustion after producing a burner exhaust. In one
embodiment of the invention, determining the completeness of
combustion comprises sensing for oxygen in the burner exhaust to
produce a reading, wherein a positive reading for oxygen indicates
complete combustion. Alternatively, in another embodiment,
determining the completeness of combustion comprises sensing for
hydrocarbon fuel in the burner exhaust to produce a reading,
wherein a negative reading for hydrocarbon fuel indicates complete
combustion.
[0014] It is also an aspect of an embodiment of the invention that
the step of monitoring a temperature comprises providing at least
two temperature sensors within the burner, and comparing an output
of each temperature sensor. If the difference between any two
outputs of different sensors exceeds a predetermined value, then a
"system error" is registered.
[0015] In another embodiment of the invention, a burner input can
be controlled based on the completeness of combustion. Control of
burner input can alternatively be based on the reading for oxygen
in the burner exhaust or on the reading for hydrocarbon fuel in the
burner exhaust.
[0016] In still another embodiment of the invention, the method for
verifying combustion in a burner of a fuel reformer during warm up
and in steady operation comprises the steps of providing at least
one flame detector to detect combustion during an initial warm up
stage, providing at least one temperature-sensing device to monitor
a temperature of a burner catalyst to evidence that the catalyst
reaches a predetermined temperature, thereby indicating flameless
catalytic combustion, and then providing a system controller to
select a flame detection device to validate combustion at each
stage of operation.
[0017] An embodiment of this method may further comprise the step
of providing at least one of either an oxygen sensor and a
hydrocarbon sensor monitoring a burner exhaust to demonstrate lean
combustion. Again, the method may control burner input based on a
reading from the at least one of either an oxygen senor and a
hydrocarbon sensor monitoring the burner exhaust.
[0018] In still another embodiment, the present invention may be
used for proving lean combustion in a catalytic burner associated
with a fuel reformer. The method may include the steps of providing
at least one flame detector capable of sensing the presence a
flame, and providing at least one temperature sensor capable of
determining a temperature of a catalyst in the catalytic burner. In
an embodiment of this method, the at least one flame detector
verifies combustion when the catalyst in the burner is below a
predetermined operating temperature, and the at least one
temperature sensor verifies combustion when the catalyst
temperature is above a predetermined operating temperature. A
response in the temperature of the catalyst due to a variation in a
supply of air is used to verify lean operation of burner.
[0019] With respect to the apparatus of the present invention used
to practice the various embodiments of the disclosed methods,
generally a burner assembly associated with a fuel reformer
designed to combust fuel in a manner in which combustion of the
fuel can be verified is disclosed. The burner assembly preferably
comprises an outer shell housing a combustion chamber comprising a
burner, a catalyst bed situated within the combustion chamber, a
mixing zone in fluid communication with the combustion chamber, an
air inlet and a fuel inlet in communication with the mixing zone,
wherein a supply of air through the air inlet and a supply of fuel
through the fuel inlet are mixed within the mixing zone, an exhaust
outlet in fluid communication with the combustion chamber, wherein
an exhaust stream is discharged from the combustion chamber through
the exhaust outlet, a flame detector positioned such that it is
capable of detecting the existence of flame in the combustion
chamber, a temperature sensor positioned such that it is capable of
monitoring temperature of the catalyst bed, and an exhaust detector
positioned downstream of the catalyst bed and capable of detecting
at least one of either oxygen or hydrocarbon in the exhaust
stream.
[0020] In an additional embodiment, the burner assembly may further
comprise a controller for controlling at least one of either the
supply of fuel and the supply of air admitted to the mixing
zone.
[0021] In still another embodiment of the burner assembly, lean
combustion of the fuel can be verified comprising an outer shell
housing a combustion chamber comprising a burner and a catalyst
bed, a mixing zone in fluid communication with the combustion
chamber, an air inlet and a fuel inlet for directing a supply of
air and fuel, respectively, into the mixing zone, an exhaust outlet
for discharging exhaust from the combustion chamber, a flame
detector comprising a window for detecting flame in the chamber, a
temperature sensor for sensing a temperature of the catalyst bed,
and a controller programmed to verify, after initial warm-up, lean
combustion by varying the input of at least one of either a supply
of air and a supply of fuel and determining if the temperature in
at least one of either the combustion chamber and the catalyst bed
respond in an expected manner.
[0022] These an other embodiments of the method and apparatus of
the present invention will be understood from the following
description in conjunction with the drawing figures appended
hereto.
FIGURES
[0023] FIG. 1 is a schematic illustration of one embodiment of the
present invention.
[0024] FIG. 2 is a schematic illustration of another embodiment of
the present invention.
DESCRIPTION OF THE INVENTION
[0025] In this application, the term "burner" and the phrase "tail
gas combustor" are used interchangeably to describe a vessel where
the combustion of fuel and air takes place to generate heat. The
term "fuel" includes any hydrocarbon, alcohol, reformate stream,
unreacted hydrogen from a fuel cell stack, and reformate from a
fuel cell stack. "Air" includes any oxygen containing gas suitable
for use in a burner. Likewise, "hydrogen" includes any hydrogen
containing gas suitable for use in a burner, and in particular pure
hydrogen and reformate. "Reformer" includes any catalytic vessel
responsible for the production of hydrogen by a steam reforming
reaction.
[0026] The present invention describes methods and apparatus for
demonstrating proof of combustion within a burner or tail gas
combustor of a fuel processing system in a fuel cell power plant.
This process may be generally referred to in the following
description as verifying (-ication), validating, demonstrating,
proving, and evidencing combustion within a burner. Such references
are used interchangeably without distinction. However, a
distinction is made herein as to verifying combustion and verifying
lean combustion--the latter being a subset of the former.
[0027] The disclosed methods and apparatus may comprise the use of
flame detection devices, one or more temperature sensing devices,
and an oxygen or hydrocarbon fuel-sensing device to produce an
output demonstrating combustion, and more particularly, in certain
embodiments, demonstrating lean combustion.
Flame Detectors
[0028] The flame-detecting device shows proof of flame, i.e. that
the air and fuel have been ignited within the burner. Any type of
flame detector is potentially suitable for use in the present
invention. Commonly used types of flame detectors that may be used
for proof of combustion include, without limitation, flame
ionization detectors (FID), particularly ionization/rectification
flame detectors; and light-based flame detectors, including
ultraviolet light detectors, photoelectric eyes (visible light
detectors), and infrared detectors.
[0029] An ionization/rectification flame detector (FID), often
referred to as a flame rod, is a well known device and is
preferred. Ions are released in an intermediate phase of
combustion. In a typical FID, two electrodes are placed within the
flame. A potential difference is placed across the electrodes,
which produces an electric current between the electrodes when an
ionizing flame is present.
[0030] In a FID, it is possible to form a carbon bridge between the
two electrodes which conducts a current and causes a false positive
detection of flame. To avoid this, it is common practice for one
electrode to have a greater surface area than the other electrode,
and the potential difference across them is alternated. The
resulting current will be rectified if conducted by ions produced
in a flame and not rectified if conducted by a carbon bridge.
[0031] Ultraviolet (UV) light detectors, infrared (IR) light
detectors, and photoelectric (visible) eyes may also serve the same
function as the FID, i.e., to prove whether or not there is a
flame. Each of these type of detectors uses a window to select the
wavelength of a light emitting from a flame. The light then falls
on a detector such as a photodiode or other electronic detection
device. If the intensity of light exceeds a threshold, the presence
of flame is validated.
Temperature Sensors
[0032] Once the combustion catalyst in a catalytic burner is at its
operation temperature, catalytic oxidation may occur on the
catalyst surface. The catalytic oxidation usually does not have a
visible flame and therefore a flame detector may not be effective
in this operation mode. Instead, temperature sensors may be used to
monitor the catalyst temperature and ensure that combustion
continues. Such monitoring may be a perceptible display of a
temperature, or merely a simple signal to indicate achievement of a
threshold temperature.
[0033] In one embodiment, the temperature sensing devices are used
to prove combustion by verifying that the temperature of the burner
catalyst exceeds the auto combustion temperature of the fuel being
placed in the burner catalyst bed. Two or more temperature sensing
devices are often used to monitor combustion in various locations
in the burner. The types of temperature sensing devices which may
be used in this invention include, without limitation, any
conventional device capable of detecting and reporting high
temperatures, for example in the range of about 200 to about 700
deg. C. Many such devices are known in the art; examples include
type K thermocouples, type N thermocouples, thermistors, resistive
temperature devices, thermometers, and infrared detectors. Type K
thermocouples are presently preferred.
Oxygen/Fuel Sensors
[0034] A flame detector combined with at least one temperature
sensor may verify combustion. However, this combination of elements
may not be able to differentiate between a lean and a rich
combustion mixture. Accordingly, an additional sensor may be needed
to determine completeness of combustion.
[0035] In a specific embodiment, an oxygen sensor for detecting
oxygen levels in the burner exhaust is used. The results from the
oxygen sensor, in way of a reading, display, perceptible signal, or
the like, may be used to determine whether or not complete
combustion has taken place. To ensure lean combustion, a burner
controller monitors the mixture of the flame with the oxygen
sensor. For example, if the oxygen sensor detects oxygen in the
combustion exhaust, it typically indicates complete combustion
(i.e., a rich combustion mixture). However, if oxygen is not
detected in the exhaust, this may be indicative of incomplete
combustion. In that case, the oxygen sensor could send a signal and
the burner controller could close off the fuel supply for the
burner. Also, the oxygen sensor may indicate whether or not
combustion has taken place, since the level of oxygen in the flow
is decreased by combustion. That is, if the oxygen level in the
exhaust shows a decrease when measured against the oxygen input to
the system, combustion within the system can be presumed.
[0036] While a variety of oxygen sensor types are available, a
preferred type for the present embodiments is an automotive-type
oxygen sensor. Other oxygen sensor types may also find suitable
uses with alternative embodiments.
[0037] Instead of or in addition to the oxygen sensor, a
hydrocarbon or fuel sensor may also be used to verify combustion,
operating in a similar fashion to the oxygen sensor. If, after
combustion, the hydrocarbon sensor detects the presence of
hydrocarbons, this will indicate that combustion is incomplete.
However, if the hydrocarbon sensor fails to detect any
hydrocarbons, this may verify that complete combustion has taken
place.
[0038] In a current embodiment, an oxygen sensor is used to verify
that combustion is complete. The oxygen sensor may be preferred in
some cases because it is lower in cost and more durable than
current hydrocarbon detectors.
[0039] Since a small or trace amount of oxygen or hydrocarbon may
exist even in the case of complete combustion, the intensity of
signals which triggers controller actions shall be determined
experimentally, as described later in the specification.
[0040] Finally, it is preferable to test the sensor or sensors
periodically during operation to ascertain signal validity. For
temperature measurement, the preferred method, in one embodiment,
is to provide two or more temperature sensors, preferably three or
more, and to compare their signal levels by digital or analog
methods. In addition, the reading can be compared under "cold"
conditions to maintain calibration. For an oxygen sensor, the
signal obtained under cold conditions (i.e., the air is, or can be
arranged to be, at atmospheric levels) can be compared with preset
values. It is also possible to validate the oxygen sensor by
starting the flame under known rich conditions, and increasing air
flow to known lean conditions and observing the sensor output.
Other strategies are also possible.
[0041] FIGS. 1 and 2 show two embodiments of the present invention.
In FIG. 1, a combination of an ionization/rectification flame
detector and thermocouples is used to show proof of combustion, and
an oxygen sensor is used to demonstrate leanness of the combustion.
The burner/tail gas combustor, shown generally at 10, has inlets
for air 12 and fuel 13 which are mixed in a mixing zone 14. At
startup, the fuel/air mixture from mixer 14 is ignited by ignitor
15 (for example, a spark plug) and enters through an opening in the
burner wall 16 of a chamber 17. The flame 18 created by the spark
is initially found in chamber 17. A flame ionization detector 19
detects ions from the flame 18. The hot exhaust gas from the flame
18 then enter a catalyst bed 20. As the catalyst warms up, its
temperature is monitored by thermocouples 21, type K in the present
embodiment. Two thermocouples 21 are illustrated, but three or more
may actually be used. The burner wall 16 is typically grounded by a
ground connection 22 to improve sensor stability. After passing
through catalyst bed 20, the combusted gases enter an exhaust 23.
There, the oxygen concentration would be detected by oxygen sensor
24. If complete combustion has taken place, the oxygen sensor 24
will detect excess oxygen. If a hydrocarbon or hydrogen detector
were used, it could also be located at or near the exit; it would
signal lean combustion by failing to detect hydrocarbons.
[0042] FIG. 2 is essentially identical to FIG. 1 except that the
FID is replaced by a transparent window 25 behind which is a
photodetector 26--these devices may physically be combined in a
single device. As noted above, the window 25 is typically
transparent in a selected wavelength range, most typically in the
ultraviolet and/or visible range, but may also or instead be in the
near infrared range.
[0043] In operation, the igniter 15 and the detectors 19 or 26, 21
and 24 are connected to a microprocessor or other controller (not
shown), which controls the burner during startup, steady operation,
and shutdown. The controller can also shut off the supplies of air
12 and fuel 13. The decision routines are programmed into the
controller by standard procedures. At startup, the controller
admits air and fuel into the mixer 14, and the igniter 15 is
energized. If a flame 18 is not detected by flame detector 19 or
photocell 26 within a set period, the system can be shut down. If a
flame 18 is detected, then the controller monitors the oxygen
detector 24 and the thermocouples 21. The controller expects to see
a reduction in oxygen concentration (below atmospheric) after
ignition. A zero reading from the oxygen sensor 24 is permissible
shortly after ignition, but after a set time the oxygen sensor 24
is expected to read above a certain level to prove that the
combustion is lean. If the criterion is not met, the system can be
shutdown by the controller.
[0044] The controller is then capable of monitoring the rate of
increase of the temperature in the catalyst bed 20 by reading the
output from the thermocouples 21. The temperature may be required
to reach a preset level within a fixed time. Failure to achieve
this requirement might indicate that a problem exists and the
system could be shut down. In normal operation, the catalyst bed
would reach a temperature known to be sufficient for flameless
combustion of the fuel/air mixture ("light-off" temperature). When
this temperature has been exceeded by a predetermined minimum
amount for a set time, the controller would preferably deactivate
the igniter 15 and, optionally, the flame detector 19 or 26. If a
reading of the oxygen sensor 24 did not change within certain
limits, and provided the temperature of the catalyst bed 20 stays
above a critical temperature, then the system could continue to
operate in a steady-state mode. The controller can be optionally
programmed to perform tests periodically during prolonged operation
to validate readings of the various sensors, particularly the
thermocouples 21 and the oxygen sensor 24. One method of achieving
this might be to slightly change the air/fuel ratio and observe
expected changes in the bed temperature and in the oxygen
concentration in the exhaust.
[0045] Shut-down can be very simple; for example, the fuel supply
could be turned off, and after a sufficient degree of cooling the
air supply could also be turned off. The shutdown process provides
another opportunity to calibrate the sensors.
[0046] The above description of certain embodiments is intended to
enable a skilled person to understand the invention, as set forth
in the appended claims. The limits of the invention are found in
the claims.
* * * * *