U.S. patent application number 15/199753 was filed with the patent office on 2016-12-15 for method and apparatus for extending flammability and stability limits in a combustion reaction.
The applicant listed for this patent is ClearSign Combustion Corporation. Invention is credited to JOSEPH COLANNINO, JAMES K. DANSIE, JESSE C. DUMAS.
Application Number | 20160363315 15/199753 |
Document ID | / |
Family ID | 53494035 |
Filed Date | 2016-12-15 |
United States Patent
Application |
20160363315 |
Kind Code |
A1 |
COLANNINO; JOSEPH ; et
al. |
December 15, 2016 |
METHOD AND APPARATUS FOR EXTENDING FLAMMABILITY AND STABILITY
LIMITS IN A COMBUSTION REACTION
Abstract
A method and apparatus for controlling a combustion reaction
includes steps and structures for applying an electric field across
a combustion reaction. Application of the electric field results in
broadening the flammability and stability limits of the fuel.
Inventors: |
COLANNINO; JOSEPH;
(BELLEVUE, WA) ; DANSIE; JAMES K.; (Renton,
WA) ; DUMAS; JESSE C.; (SEATTLE, WA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ClearSign Combustion Corporation |
Seattle |
WA |
US |
|
|
Family ID: |
53494035 |
Appl. No.: |
15/199753 |
Filed: |
June 30, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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PCT/US2014/073086 |
Dec 31, 2014 |
|
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15199753 |
|
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61922430 |
Dec 31, 2013 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F23C 99/001 20130101;
F23N 5/003 20130101; F23N 1/022 20130101; F23N 5/00 20130101 |
International
Class: |
F23C 99/00 20060101
F23C099/00; F23N 5/00 20060101 F23N005/00; F23N 1/02 20060101
F23N001/02 |
Claims
1. A method, comprising: introducing fuel and air into a combustion
volume in a first ratio that is within a first range defined by an
upper stability limit and a lower stability limit of the fuel;
igniting the fuel to start a combustion reaction; producing a
modified range defined by a modified upper stability limit higher
than the upper stability limit and a modified lower stability limit
lower than the lower stability limit of the fuel by applying an
electric field across the combustion reaction; and after igniting
the fuel and after producing the modified range, adjusting a flow
rate of the fuel or air to produce a second ratio within the
modified range and outside the first range.
2. The method of claim 1, further comprising: prior to producing
the modified range, monitoring one or more characteristics of the
combustion reaction; and detecting values of the one or more
monitored characteristics that indicate a requirement that the
ratio of fuel and air be adjusted from the first ratio to the
second ratio, wherein producing the modified range is performed in
response to detecting values of the one or more monitored
characteristics.
3. The method of claim 1, wherein applying the electric field
modifies an upper flammability limit and a lower flammability limit
of the fuel.
4. A combustion system, comprising: a burner configured to support
a combustion reaction by emitting fuel and oxidizer; first and
second electrodes positioned to apply an electric field across the
combustion reaction; and a voltage supply operatively coupled to
the first and second electrodes, and configured to supply voltage
signals to the first and second electrodes sufficient to cause the
electric field to produce a modified upper stability limit higher
than a normal upper stability limit of the fuel and a modified
lower stability limit lower than a normal lower stability limit of
the fuel.
5. The system of claim 4, wherein the first electrode comprises a
fuel nozzle portion of the burner.
6. The system of claim 4, wherein the second electrode comprises a
mesh electrode positioned above the burner.
7. The system of claim 4, further comprising: a controller
configured to detect operation of the burner and to control the
voltage supply to supply voltage signals to the first and second
electrodes sufficient to produce the modified upper stability limit
and the modified lower stability limit of the fuel.
8. A method for controlling a combustion reaction, comprising:
receiving, via a data interface, a command to establish a
particular fuel stability limit; reading data corresponding to a
fuel parameter; determining, as a function of the data, a
particular voltage to be applied to a physical electrode
operatively coupled to a combustion reaction; applying the
particular voltage to the physical electrode; and causing the fuel
to combust at a mixture within the particular stability limit
responsive to an electric field generated by application of the
particular voltage to the physical electrode.
9. The method for controlling a combustion reaction of claim 8,
wherein determining the particular voltage includes determining
second data corresponding to the particular voltage.
10. The method for controlling a combustion reaction of claim 9
wherein determining the particular voltage includes: converting the
second data into a signal; driving a voltage amplifier with the
signal; and outputting the particular voltage from the voltage
amplifier to the electrode.
11. The method for controlling a combustion reaction of claim 9,
wherein determining the second data includes algorithmically
calculating the second data.
12. The method for controlling a combustion reaction of claim 9,
wherein determining the second data includes looking up the second
data.
13. The method for controlling a combustion reaction of claim 8,
wherein the fuel parameter includes ambient pressure.
14. The method for controlling a combustion reaction of claim 8,
wherein the fuel parameter includes ignition temperature.
15. A low NOx burner, comprising: a physical flame holder
configured to receive a fuel and oxidant mixture at a particular
condition; and a first and second electrode configured to apply an
electric field to the fuel and oxidant mixture, wherein the fuel
and oxidant are characterized by a leaner mixture than would stably
combust at the particular condition without being exposed to the
electric field.
16. The burner of claim 15, wherein the particular condition
includes a temperature proximate to the physical flame holder.
17. The burner of claim 15, wherein the particular condition
includes atmospheric pressure proximate to the physical flame
holder.
18. A method, comprising: introducing fuel and air into a
combustion volume in a first ratio that is outside a range defined
by an upper stability limit and a lower stability limit of the
fuel; producing a modified range defined by a modified upper
stability limit and a modified lower stability limit of the fuel by
applying an electric field across the fuel and air, the first ratio
falling within the modified range; and after producing the modified
range, igniting the fuel.
19. The method of claim 18, further comprising: prior to producing
the modified range, monitoring one or more characteristics of the
fuel and air; prior to producing the modified range, detecting
values of the one or more monitored characteristics that indicate
that the first ratio of the fuel and air is at or above the upper
stability limit or the lower stability limit; and producing the
modified range after the monitoring and the detecting.
20. The method of claim 18, wherein producing a modified range
includes: determining a magnitude of the electric field to be
applied, based at least in part on a value of the first ratio and a
value of one of the upper stability limit or the lower stability
limit; and applying the electric field at the determined
magnitude.
21. The method of claim 19, wherein the producing a modified range
includes: applying the electric field at a preselected magnitude;
repeating the monitoring and the detecting; and increasing the
magnitude of the applied electric field by a preselected increment.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] The present application is a U.S. Continuation-in-Part
Application which claims priority benefit under 35 U.S.C. .sctn.120
of co-pending International Patent Application No.
PCT/US2014/073086 entitled "METHOD AND APPARATUS FOR EXTENDING
FLAMMABILITY LIMITS IN A COMBUSTION REACTION," filed Dec. 31, 2014
(docket number 2651-203-04), co-pending herewith; which application
claims priority benefit from U.S. Provisional Patent Application
No. 61/922,430, entitled "METHOD AND APPARATUS FOR EXTENDING
FLAMMABILITY LIMITS IN COMBUSTION REACTION," filed Dec. 31, 2013
(docket number 2651-203-02); each of which, to the extent not
inconsistent with the disclosure herein, is incorporated herein by
reference.
BACKGROUND
[0002] Combustion requires fuel, such as hydrogen or a hydrocarbon
gas; an oxidant such as oxygen carried in air, and an ignition
source. For any given fuel, there is a range of fuel to oxygen
ratios within which combustion can occur or be sustained.
Flammability limits convey information about the minimum and
maximum concentrations of the fuel that will sustain combustion
under standard conditions. The flammability limits of the
particular fuel, i.e., the lowest and highest ratios of fuel:
oxygen at which the fuel is flammable, are determined under
standard conditions (unless specifically defined otherwise).
Typically, the standard conditions used for measuring the
flammability limits for many fuels are 25.degree. C. and one bar of
pressure, absolute (100 kPa), with the oxidizer specified to be
oxygen at nominal atmospheric concentration in air.
[0003] It is generally considered true that flammability limits are
an intrinsic property of a fuel or fuel mixture. Flammability
limits are expressed as a percentage of fuel within a volume of
air. For example, the lower flammability limit of gasoline (100
octane) is 1.4%, i.e., a mixture containing 1.4% gasoline and 98.6%
air. This is the lowest, or leanest concentration of gasoline that
is combustible. At the other end of the range, the upper
flammability limit of gasoline is 7.6%, representing the richest
fuel concentration that is combustible.
[0004] Stability limits are analogous to flammability limits.
Flammability limits are considered properties of the fuel--that is,
a flammability limit is device-independent. Stability limits, by
comparison, are the actual combustible limits realizable by a given
device such as an actual burner or combustor. In industrial
burners, stability limits often govern the safe operation of the
burner. The lower stability limit is the most fuel-lean composition
whose combustion a given burner can support, while the upper
stability limit is the most fuel-rich composition whose combustion
a given burner can support. In practical combustion equipment, the
upper and lower stability limits define the stable combustion
operating range for a given burner or combustion device.
SUMMARY
[0005] According to an embodiment, a method includes introducing
fuel and air into a combustion volume in a first ratio that is
outside a range of fuel concentrations between an upper stability
limit and a lower stability limit, igniting the fuel, and producing
a modified range of fuel concentrations defined by a modified upper
stability limit and a modified lower stability limit of the fuel by
applying an electric field across a flame supported by the fuel and
air, wherein the first ratio is within the modified range.
[0006] According to an embodiment, a combustion system includes a
burner configured to support a combustion reaction, first and
second electrodes positioned and configured to apply an electric
field across the combustion reaction supported by the burner, and a
voltage supply, operatively coupled to the first and second
electrodes, and configured to supply voltage signals to the first
and second electrodes. In some embodiments, controller is
configured to detect the combustion reaction, and to control the
voltage supply to supply voltage signals to the first and second
electrodes sufficient to produce a modified upper stability limit
and/or a modified lower stability limit of the fuel to extend
stability of the combustion reaction.
[0007] According to an embodiment, a method for controlling a
combustion reaction includes receiving, via a data interface, a
command to establish a particular fuel stability limit, reading
data corresponding to a fuel parameter (such as pressure and/or
temperature, for example), and determining, as a function of the
data, (e.g., algorithmically calculating or looking up) second data
corresponding to a signal selected to cause a particular voltage to
be applied to a physical electrode operatively coupled to the
combustion reaction. The second data is converted into the signal
used to drive a voltage amplifier. The method further includes
outputting from the voltage amplifier and transmitting to the
electrode, electric current at the particular voltage. The applied
electric field causes the fuel to combust at the particular
stability limit responsive to exposure to the energized physical
electrode.
[0008] According to an embodiment, a low NOx burner includes a
physical flame holder configured to receive a particular fuel and
oxidant mixture at a particular condition and an electrode
configured to apply an electric field to the fuel and oxidant
mixture, the electric field being selected to cause the fuel and
oxidant to undergo a combustion reaction at the particular mixture.
The fuel and oxidant are characterized by a leaner mixture than
would react in combustion at the particular condition without being
exposed to the electric field.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 is a diagram of a combustion system, according to an
embodiment.
[0010] FIG. 2 is a flow chart depicting a method of operation of a
combustion system, according to an embodiment.
[0011] FIG. 3 is a diagram showing elements of a test system
employed by the inventors to investigate and prove principles
described and claimed herein, according to an embodiment.
[0012] FIG. 4 is a ternary mixture diagram depicting three fuels in
experimental mixture space, according to an embodiment.
[0013] FIGS. 5A-5B are ternary diagrams illustrating the widening
of flammability limits in the ternary hydrogen-methane-propane
(H.sub.2--CH.sub.4--C.sub.3H.sub.8) mixture space in the presence
of an electric field, according to an embodiment.
[0014] FIGS. 6A-6C are ternary diagrams showing the contours for
flammability limit changes determined by experiment, according to
an embodiment.
[0015] FIG. 7 is a graph showing enhancement of rich flammability
limits, according to an embodiment.
[0016] FIGS. 8A-8B are ternary diagrams showing percent change
contours for lean and rich flammability limits per equations,
according to an embodiment.
DETAILED DESCRIPTION
[0017] In the following detailed description, reference is made to
the accompanying drawings, which form a part hereof. In the
drawings, similar symbols typically identify similar components,
unless context dictates otherwise. Other embodiments may be used
and/or other changes may be made without departing from the spirit
or scope of the disclosure.
[0018] As used herein, the term lower flammability limit (LFL)
refers to the leanest concentration of a given fuel (such as a fuel
mixture, for example) in air that is combustible under standard
measurement conditions. The term lower stability limit (LSL) refers
to the leanest concentration of the given fuel that is combustible
under actual operating conditions for a particular burner. Upper
flammability limit (UFL) refers to the richest concentration of a
given fuel in air that is combustible under standard measurement
conditions. Upper stability limit (USL) refers to the richest
concentration of the given fuel that is combustible under actual
operating conditions of a particular burner.
[0019] The terms modified lower flammability limit (MLFL) and
modified upper flammability limit (MUFL) (which may be collectively
referred to using the term modified flammability limit (MFL)) are
used to refer to the respective flammability limits as modified
using the structures and methods described hereafter. The terms
modified lower stability limit (MLSL) and modified upper stability
limit (MUSL) (or, collectively, modified stability limit (MSL)) are
used to refer to the respective stability limits as modified using
the structures and/or methods disclosed hereafter. To avoid
confusion and for economy of language, the terms stability limit(s)
and flammability limit(s) may be used interchangeably.
[0020] The terms combustion, combustion reaction, and flame may be
used interchangeably herein, and shall be regarded as equivalent
unless context dictates otherwise.
[0021] As used herein, the term fuel shall be understood to include
a fuel mixture (i.e., a mixture including two or more pure fuels)
unless context indicates otherwise.
[0022] Other terms that are related to, or synonymous with terms
defined above may also be used hereafter, the meanings of which
will be clear in view of the context.
[0023] It has been long understood in the art that flammability
limits of any given fuel are substantially immutable. Tables
setting forth the flammability limits of selected or common fuels
can be found in many combustion engineering texts and general
references. Such tables are relied upon by designers of combustion
systems when calculating the parameters of individual system
components to ensure that systems perform as intended.
[0024] The inventors have discovered that parameters describing
combustion stability limits in a furnace system can be driven to
diverge from corresponding flammability limits of many fuels. In
particular, combustion can be made more stable by applying an
electric field to the combustion reaction. The inventors discovered
that the stability limits of combustion equipment were expanded by
applying an electric field to the flame.
[0025] When an electric field is present across a flame, the LFL
and UFL of the particular fuel or mix of fuels no longer represent
the flammability limits of the particular fuel or mix of fuels. The
application of an electric field creates a MLFL that is lower
(i.e., leaner) than the LFL, and a MUFL that is higher (i.e.,
richer) than the UFL. The degree to which the stability limits are
modified varies according to the particular fuel and the strength
of the electric field.
[0026] As previously noted, flammability limits are generally
expressed as values corresponding to specific standard temperatures
and pressures. As the temperature and/or pressure changes, the
stability limits diverge from published flammability limits at
standard conditions. Many combustion systems are configured to
operate at pressures and/or temperatures that are far removed from
the standard conditions associated with the standard tables.
Furthermore, many combustion systems are not configured to operate
using a mix of air and fuel, but instead employ other oxidizers, or
may further dilute the oxygen by introducing recirculated flue gas,
etc. Nevertheless, the stability limits of a given fuel can be
calculated for any reasonable combination of temperature, pressure,
and oxygen concentration. Thus, the applicable standard
flammability limits are considered to apply, as revised, to
accommodate the varied conditions of individual burners.
[0027] The inventors found that experimental results related to
upper and lower flammability limits (UFL, LFL) and modified upper
and lower flammability limits (MUFL, MLFL) at standard conditions
are predictive of upper and lower stability limits (USL, LSL) and
modified upper and lower stability limits (MUSL, MLSL) in given
burners.
[0028] FIG. 1 is a diagram showing a combustion system 100, which
is configured to modify the stability limits of fuels or mixes of
fuels, according to an embodiment. The combustion system 100
includes a burner 102 and a combustion control system 104. The
burner 102 is configured to support a combustion reaction 106, and
includes a fuel nozzle 108, an oxidizer conduit 110, a fuel source
112, and an oxidizer source 114. The fuel source 112 and oxidizer
source 114 are coupled to the fuel nozzle 108 and oxidizer conduit
110, respectively, via corresponding transmission conduits 116. The
fuel source 112 and oxidizer source 114 are each configured to be
controllable to regulate the volume or flow rate of fuel and
oxidizer, respectively. Optionally, the oxidizer source 114 may
include a forced draft (blower) system.
[0029] The fuel source 112 and oxidizer source 114 can be arranged
in any of a large number of configurations, depending upon factors
such as, for example, the size, capacity, intended purpose,
complexity, and projected duty cycle of the combustion system 100.
Additionally, in many cases, the combustion system may be subject
to governmental regulations relating to emissions, which further
affect the design of the system. According to an embodiment, the
oxidizer source 114 can be configured to control oxygen input
and/or air temperature by introducing recirculated flue gas.
According to an embodiment, the burner 102 is configured to premix
fuel and air, then to emit the mixture from the nozzle 108.
According to another embodiment, the fuel is ejected with some
force from the nozzle 108 in a form that entrains air from the
oxidizer conduit 110. According to an alternate embodiment, the
oxidizer is forced into the combustion volume with a blower.
According to some embodiments, only one of the fuel or oxidizer is
regulated, the other being supplied at a substantially constant
rate.
[0030] According to an alternative embodiment, the fuel and
oxidizer are premixed to a selected mixture, and the combustion
reaction 106 is supported by the premixed fuel and oxidizer. In an
embodiment, fuel and oxidizer are premixed to a selected mixture
that falls outside the normal stability limits of the fuel. As will
be appreciated, a mixture of fuel and oxidizer that is outside the
stability limits is nominally non-combustible. Such a mixture can
be considered a safe mixture because a flame will not propagate
into a mixer containing the mixture. According to an embodiment,
the mixture combusts only where and when the mixture is exposed to
an appropriate electric field.
[0031] According to an embodiment, a mixture of fuel and oxidizer
is ejected from the burner 102 at a ratio that is below the LSL of
the fuel, i.e., too lean to maintain combustion. An electric field
is applied as described in more detail below. Application of the
electric field produces a MLSL that is below the current ratio,
rendering the mixture combustible, but only while the electric
field is present. Removal of the field again renders the mixture
non-flammable. As the mixture flows from the burner 102, it can
only become leaner, and thus cannot become flammable absent the
electric field. In an embodiment, a failsafe is thus provided,
significantly reducing the danger of accidental combustion.
[0032] The combustion control system 104 includes first and second
electrodes 118, 120 and a voltage source 124. In some embodiments,
the combustion control system 104 includes a control unit 128 and
one or more sensors 122 operatively coupled to the control unit.
The first electrode 118 is positioned downstream from the burner
102 adjacent to the combustion reaction 106. A surface of the
burner nozzle 108 can be operatively coupled to the voltage source
for operation as the second electrode 120. Connectors 130 couple
the voltage source 124 to the first and second electrodes 118, 120.
In some embodiments, an electrode 120 may carry chassis and/or
earth ground, and a connector 130 may be replaced by conductive
structures of the burner 102. Various interfaces including physical
connector(s) and wireless interface(s) can be used to couple the
control unit 128 to the sensor 122. Connectors (or wireless
interfaces) may optionally couple control signals from the control
unit 128 to the voltage source 124, the fuel source 112, and the
oxidizer source 114.
[0033] The voltage source 124 is configured to apply a voltage
difference across the combustion reaction 106 via the first and
second electrodes 118, 120. The sensor 122 represents one or more
individual sensors, each configured to measure one or more
characteristics of the combustion reaction 106 and to supply
corresponding signals to the control unit 128. For example, the
sensor 122 can be configured to measure characteristics such as
temperature, oxygen concentration, luminosity, combustion
byproducts, electrical charge, etc.
[0034] Structures configured to electrically connect components or
assemblies shown in the drawings are depicted generically as
connectors 130, inasmuch as electrical connectors and corresponding
structures are very well known in the art, and equivalent
connections can be made using any of a very wide range of different
types of structures. The connectors 130 can be configured to carry
high-voltage signals, data, control logic, etc., and can include
single conductors or multiple separately-insulated conductors.
Additionally, where a voltage potential, control signal, feedback
signal, etc., is transmitted via intervening circuits or
structures, such as, for example, for the purpose of amplification,
detection, modification, filtration, rectification, etc., such
intervening structures are considered to be incorporated as part of
the respective connector.
[0035] In an embodiment, fuel is supplied and regulated to the fuel
nozzle 108 by the fuel source 112, while the oxidizer is similarly
supplied and regulated by the oxidizer source 114. The control unit
128 monitors selected parameters or characteristics of the
combustion reaction 106, and may control the fuel source 112 and
oxidizer source 114 to keep the selected characteristics within
defined limits. Additionally or alternatively, various aspects of
the combustion reaction 106 can be controlled by application of
electrical energy via the first and/or second electrodes 118, 120.
When the control unit 128 receives sensor signals indicating that
the combustion reaction 106 is operating near its upper or lower
stability limit, the control unit 128 is configured to control
other elements of the combustion system 100 to bring operation of
the combustion reaction to a point farther removed from the
stability limits, or to modify the stability limits to be farther
from the current point of operation.
[0036] For example, the control unit 128 may detect that the
combustion reaction 106 is unstable, or that it repeatedly blows
out, requiring periodic re-ignition; or the control unit 128 may
determine that a leaner fuel/air mixture is required in order to
obtain a selected emissions value, which results in producing an
unstable flame, etc.
[0037] In an embodiment, upon detection of operation at a fuel/air
ratio that approaches the LSL or USL (or previously applied MLSL or
MUSL) of the combustion reaction 106, the control unit 128 is
configured to control the voltage source 124 to apply or modify a
voltage difference across the combustion reaction 106, via the
first and second electrodes 118, 120, in order to establish an
electric field across the flame. The magnitude of the voltage
difference to be applied can be determined by reference to a lookup
table or by calculation based on the fuel type, degree of desired
modification of the stability limits, and/or on other predetermined
factors. Additionally or alternatively, the control unit 128 can be
configured to control the voltage source 110 to adjust the
magnitude upward until signals from the sensor 122 indicate that
the combustion reaction 106 is in a stable operation state.
[0038] FIG. 2 is a flow chart depicting a method of operation 200
for controlling a combustion reaction, according to an embodiment.
At 202, a fuel/air mixture is supplied to a combustion reaction.
This will typically be in the form of a fuel and air mixture, but
can include any appropriate oxidizer, and can include other known
components that affect the fuel to oxygen ratio, including, for
example, recirculated flue gas.
[0039] At 204, a determination is made whether the fuel/air ratio
of the mixture is outside the range defined by the stability
limits. If the determination is made that the ratio is within the
range (the NO path), the process returns to step 202, at which the
fuel/air mixture continues to be supplied to the combustion
reaction.
[0040] If, at step 204, it is determined that the fuel/air ratio is
outside the stability limit range (the YES path), the process
proceeds to step 206, where the stability limits are modified by
application of an electrical field across at least a portion of the
combustion reaction. Following step 206, the process returns to
step 202 and repeats.
[0041] According to an embodiment, step 206 can include selecting
an adequate magnitude of the electric field to modify the stability
limits to a degree sufficient to encompass the current fuel/air
ratio. According to an alternative embodiment, the magnitude of the
electric field is increased incrementally with each repetition of
the cycle, so that multiple cycles of the process may be required
before the stability limits have been sufficiently modified.
[0042] According to a further embodiment, the process can include a
step in which a previously applied electric field is reduced
incrementally each time the NO path is taken from step 204, or,
alternatively, after a predetermined number of times that the NO
path is taken. In this way, the strength or magnitude of the
electric field is maintained near the minimum value necessary for
proper operation, and the electric field is removed when no longer
necessary.
Examples
[0043] FIG. 3 is a diagram showing elements of a test system 300
used by the inventors in experiments performed to demonstrate the
principles described with reference to embodiments described
herein. The test system 300 included a burner 302, a fuel/air
control system 304, and a combustion control system 306.
[0044] The burner 302 included a sintered bronze plate 308, a
cooling coil 310, and a plenum chamber 312 defined by a plenum
chamber wall 313. The bronze plate 308 was porous, configured to
permit fuel and air to pass from the plenum chamber 312 below the
plate 308 to an upper side of the plate 308. A quartz cylinder 314
was positioned above and surrounding the bronze plate 308. The
cooling coil 310 included a coolant inlet 316 and a coolant outlet
318. During operation of the test system, water was pumped through
the cooling coil 310 to control the temperature of the bronze plate
308, and to prevent transmission of heat from a combustion reaction
320 above the plate 308 to the plenum chamber 312, below the plate
308. A fuel inlet 322 was provided to permit introduction of a
fuel/air mixture into the plenum chamber 312.
[0045] The fuel/air control system 304 included fuel sources A, B,
and C, each configured to provide a respective fuel. An air source
324 was also provided. A respective valve/flow meter 326a-d was
associated with each of the fuel sources A, B, and C, and the air
source 324. The air source 324 and each of the fuel sources A, B,
and C were coupled, via their respective valve/flow meter 326a-d,
to a mixer 328, which was in turn coupled via a master valve/flow
meter 326e to the fuel inlet 322.
[0046] The combustion control system 306 included a voltage supply
330 operatively coupled to a stainless steel mesh electrode 332
positioned above the quartz cylinder 314. The voltage supply 330
was also coupled to the bronze plate 308, and was configured to
apply a voltage difference between the electrode 332 and the bronze
plate 308.
[0047] During operation of the tests, the inventors controlled the
valve/flow meters 326a-d to regulate the type and mixture of fuel,
and the ratio of fuel to air. The fuel and air were mixed in the
mixer 328, and the total volume of fuel and air mixture 334
introduced into the plenum chamber 312 was controlled by the master
valve/flow meter 326e. The mixture 334 was ignited as it passed
through the bronze plate 308, and the tests were conducted as
described in detail below. The inventors observed that a modest
electric field significantly widened stability limits for
hydrogen-methane-propane (H.sub.2--CH.sub.4--C.sub.3H.sub.8) fuel
blends simulating refinery fuel gas. Observed stability limits were
mathematically modified to standard conditions to calculate
flammability limits. The lean flammability limit was decreased by
between 2.7% and 5.9%, depending on the fuel blend. The rich
flammability limit was increased by between 5.6% and 14.1%,
depending on the fuel blend. Overall widening of the flammability
limits was 8.5% to 20.3%, depending on the fuel blend.
[0048] The upper flammability limit correlated well with the square
root of the molar hydrogen to carbon ratio H/C in the fuel
(r.sup.2=99.0%). Modification of the upper flammability limit may
accrue from better transport of the oxidizing species from
combustion air under the influence of the electric field. One
possible explanation for this may be an attraction of hydronium
ions (H.sub.3O.sup.+) from the air to the grounded nozzle, causing
an increase in hydroxide ion concentration [OH-] delivered to the
flame. This, in turn, would enhance carbon monoxide (CO) oxidation:
CO+OH=CO.sub.2+H, and thus the overall reaction rate of the
hydrocarbons.
[0049] The inventors observed a change in the lower flammability
limit (to the MLFL) of about half of that of the modified upper
flammability limit. Possibly, this is due to the presence of CO in
much lower concentration in lean flames; therefore, the enhancement
of CO oxidation is presumably less beneficial. The change in the
lower flammability limit also correlates negatively with the
C.sub.3H.sub.8 concentration (r.sup.2=99.0%) and to a much lesser
extent with the addition of H.sub.2 concentration (or positively to
CH.sub.4 addition) to the model (r.sup.2=99.98%).
[0050] As to propane's effect, one possibility is that
C.sub.3H.sub.3.sup.+ impeded oxidation at the lower flammability
limit (see Section 5, Conclusions). Since C.sub.3H.sub.3.sup.+
correlates with C.sub.3H.sub.8 concentration, this effect would be
more pronounced with propane-rich fuels. The reason that an
increase in the hydrogen concentration slightly reduced the
relative widening of the lean limit is less clear. One possibility
is that at the lean limit H.sub.2 scavenges OH generated by
H.sub.3O.sup.+. Another possibility is that CH.sub.4 is actually
the important moiety (and as C.sub.3H.sub.8 increases, H.sub.2
must, from mathematical necessity, decrease in a ternary
mixture).
[0051] Flammability and stability limits were found to be
significantly widened in an electric field.
[0052] Apparatus
[0053] The apparatus included a 51 mm diameter sintered bronze disk
through which a premixed fuel and air mixture flowed. An added
quartz tube atop the burner isolated the flame from the
surroundings. The quartz tube had an inner diameter of 56 mm. A
circular stainless steel screen electrode was positioned 8 mm above
the quartz tube. An electric potential between the screen electrode
and the flame was maintained at 10 kV, generating an electric field
of 1.2 kV/cm. Cooling water was used to stabilize the flame.
[0054] A circular stainless steel screen electrode was placed above
the quartz tube resting on an electrically grounded burner.
Hydrogen, methane, propane, and air were individually fed through
four OMEGA.TM. thermal flow meters (not shown, available from Omega
Engineering, Inc., Stamford Conn., USA) to generate the desired
fuel flow rates. Each meter was electrically floated with an
inverter and a battery so as to keep the meters from being shocked
by high voltage. The gases were then blended and controlled by
valves downstream of the fuel blend but upstream of the burner (not
shown). The valve panel was grounded for safety. The fuels were fed
from a tank farm; air was fed from an air compressor. Ambient
humidity was neglected in the calculations (and constituted about
1% of the air by volume).
[0055] Procedure
[0056] To investigate the LFL and UFL (and MLFL and MUFL), the fuel
feed was held constant while increasing or decreasing the airflow
until reaching blowout. In the case of the UFL and MUFL, the
airflow was decreased so as to make the mixture more fuel rich. In
the case of the LFL and MLFL, the airflow was increased so as to
make the mixture more fuel lean. Blowout was defined as the
condition where the flame blew completely out of the quartz tube.
The blowout limit was determined without electric field under
constant fuel flow before testing with electric field application.
While the burner could withstand higher flows, the airflow meter
had a maximum 50 SLPM capacity. Thus, the airflow was set to 80% of
maximum (to give some margin for leaner limits under an electric
field) and then decreased the fuel to give the maximum lean
condition. The airflow was then reduced, the burner was relit, and
an electric field was applied. Airflow was increased until blowout
occurred. In all cases, a reduction in LFL and LSL was achieved
when an electric field was applied. For the upper flammability
(fuel-rich) limit, the airflow was reduced little by little until
the flame blew out. This procedure was then repeated under the
influence of an electric field. In all cases, the application of an
electric field yielded higher UFL and USL.
[0057] Data and Results
[0058] Table 1 shows properties of the tested fuels and mixtures.
The tested fuel mixtures had a maximum of 50% hydrogen
concentration. In Table 1, H/C is the molar hydrogen to carbon
ratio in the fuel, LHV is the lower heating value in BTU/scf, AFT
is the adiabatic flame temperature, .PHI. is the stoichiometric
fuel/air mixture ratio, .tau. is the theoretical fuel/air mixture
ratio (defined at stoichiometric ratio .PHI.=1), %. Fuel is the
volume fuel concentration in the fuel/air mixture, LL is the
(fuel-lean) lower flammability or stability limit in terms of the
percent of fuel in the fuel/air mixture, and RL is the (fuel-rich)
upper flammability or stability limit expressed on the same basis.
In Table 1, no electric field has been applied and the UFL and LFL
represent native fuel properties as measured by the apparatus. Such
limits show good general agreement with literature values for
flammability limits despite literature values being measured in
more exacting apparatuses.
TABLE-US-00001 TABLE 1 Fuel and Mixture properties Fuel Fuel/Air
Mixture Properties Composition Properties .PHI. = 1 Flammability
Code H.sub.2 CH.sub.4 C.sub.3H.sub.8 H/C LHV AFT .tau. % Fuel LL, %
RL, % 0-100-0 0 100 0 4.00 912 2223 9.52 9.50 5.2 14.4 0-0-100 0 0
100 2.67 2385 2265 23.81 4.03 2.4 8.7 0-50-50 0 50 50 3.00 1649
2244 16.67 5.66 3.1 10.9 50-50-0 50 50 0 6.00 593 2346 5.95 14.38
4.8 19.9 50-0-50 50 0 50 3.33 1330 2369 13.10 7.09 2.8 13.1
25-50-25 25 50 25 3.60 1121 2294 11.31 8.12 3.7 13.9
[0059] FIG. 4 is a ternary mixture diagram depicting the fuels in
experimental mixture space, according to an embodiment. The circles
show the experimental blends that were investigated. Hydrogen
blends above 50% were not investigated. Table 2 shows fuel
flammability limits (LFL, UFL) and modified flammability limits
(MLFL, MUFL) respectively without and with the presence of an
electric field, E, of 1.2 kV/cm field strength.
TABLE-US-00002 TABLE 2 Flammability Limits With and Without an
Electric Field Composition E = 0 kV/cm E = 1.2 kV/cm Change, .chi.
Code H.sub.2 CH.sub.4 C.sub.3H.sub.8 LFL, % RFL, % MLFL, % MUFL, %
Lean % Rich % Total, % 0-100-0 0 100 0 5.1 14.4 4.8 15.8 -5.9 9.9
18.5 0-0-100 0 0 100 2.3 8.7 2.2 9.2 -2.7 5.6 8.5 0-50-50 0 50 50
3.1 10.9 3.0 11.6 -3.5 7.3 11.7 50-50-0 50 50 0 4.8 19.9 4.6 22.7
-5.6 14.1 20.3 50-0-50 50 0 50 2.8 13.1 2.7 14.3 -3.2 8.9 12.1
25-50-25 25 50 25 3.7 13.9 3.6 15.2 -4.3 9.4 14.5
[0060] From inspection, one sees that the LFL/MLFL and UFL/MUFL
differ for the electric field off (0 kV/cm) and on (1.2 kV/cm); for
example, the lean flammability limit for methane (first row)
decreased from 5.1% methane to 4.8%, and the rich flammability
limit increased from 14.4 to 15.8%. In general, the effect of the
electric field was to widen the flammability limits for all fuels
and blends with the lean limit becoming leaner and the rich limit
becoming richer. In order to compare lean and rich limits, the
inventors defined a change parameter (.chi..sub.r, .chi..sub.l),
per Equation 1.
r = .lamda. r , e .lamda. r - 1 = .lamda. r , e - .lamda. r .lamda.
r ; l = .lamda. l , e .lamda. l - 1 = .lamda. l , e - .lamda. l
.lamda. l , ( 1 ) ##EQU00001##
where .chi..sub.r, .chi..sub.l are the fractional change of the
rich and lean limits, respectively, .lamda..sub.r,e,
.lamda..sub.l,e are the modified flammability limit (rich or lean,
respectively) expressed as the fuel fraction in the presence of the
electric field, and .lamda..sub.r, .lamda..sub.l are the
flammability limits with no electric field.
[0061] Under this definition, widening of the lean limit is
negative while widening of the rich limit is positive. To calculate
a total widening over the entire range, the inventors modified
Equation (1) as follows.
T = .lamda. r , e - .lamda. l , e .lamda. r - .lamda. l - 1 = (
.lamda. r , e - .lamda. r ) + ( .lamda. l - .lamda. l , e ) .lamda.
r - .lamda. l , ( 2 ) ##EQU00002##
where .chi..sub.T is the total fraction of change in the
flammability or stability range (rich minus lean) with and without
electrical charge, .lamda..sub.r,e is the MUFL (modified rich
limit) in the presence of the electric field, .lamda..sub.r is the
UFL (rich limit) in the absence of the electric field,
.lamda..sub.l,e is the MLFL (modified lean limit) in the presence
of the electric field, and .lamda..sub.l is the LFL (lean limit) in
the absence of the electric field.
[0062] FIGS. 5A-B depict two ternary diagrams illustrating the
widening of flammability limits in the ternary
H.sub.2--CH.sub.4--C3H.sub.8 mixture space in the presence of an
electric field, according to an embodiment. A modest electric field
significantly widened flammability limits. The ternary diagram in
FIG. 5A shows the normal (no electric field) flammability limits
that were measured at each point in the mixture space. The ternary
diagram in FIG. 5B shows wider flammability limits in the presence
of an electric field with the MLFL (lean flammability limit) being
leaner than the LFL and the MUFL (rich flammability limit) being
richer than the UFL.
[0063] Analysis
[0064] Contour lines for six fuel blends can be fit exactly with a
mixture model of the form
y = k = 1 3 c k z k + j < k 2 k = 1 3 c jk z j z k , ( 3 )
##EQU00003##
where y is the response of interest (e.g., lower or upper
flammability limit), j,k are indexes for the three fuel components
(1.ltoreq.j<k.ltoreq.2; 1.ltoreq.k.ltoreq.3), z.sub.jz.sub.k are
the fuel components i.e., z.sub.1=H.sub.2, z.sub.2=CH.sub.4,
z.sub.3=C.sub.3H.sub.8.
k = 1 3 z k = 1 , ##EQU00004##
because the fuel components must sum to 100%, c.sub.k, c.sub.jk are
the associated coefficients for the pure components and blends
(c.sub.1, c.sub.2, c.sub.3, and c.sub.12, c.sub.13, c.sub.23,
respectively).
[0065] Note that Equation (3) contains no error term, nor is it
possible to deduce one, as six mixture points will determine six
coefficients with certainty.
[0066] FIGS. 6A-C depict three ternary diagrams showing the
contours for flammability limit changes deduced in this way,
according to an embodiment. The series shows percent changes in
flammability or stability limits in ternary mixture coordinates.
The diamonds indicate the data points, with actual percent changes
indicated above each point. In FIG. 6A, the lines and negative
values indicate the percent change in lean limits. In FIG. 6B, the
lines and positive values indicate the percent change in rich
limits. FIG. 6C gives the contours for total percent change over
the entire range. Overall, a 1.2 kV/cm electric field widened
limits between 8.5 and 20.3%, depending on the fuel. Because of the
way these contours were derived they are purely empirical and agree
exactly with the values of the data set.
[0067] In order to perform statistical tests the number of
coefficients must be less than the number of fuel blends tested;
that is, fewer than six and preferably only two or three. Equations
(4) and (5) satisfy the rich and lean change fractions,
respectively.
.chi..sub.r.sup.2=a.sub.0+a.sub.1x+.epsilon. (4)
.chi..sub.l=b.sub.0+b.sub.1z.sub.1+b.sub.3z.sub.3+b.sub.33z.sub.3.sup.2+-
.epsilon. (5)
where .chi..sub.r, .chi..sub.l are the fractional change in the
rich and lean limits, respectively, x is the H/C ratio of the fuel,
z.sub.1 is the fraction of H.sub.2 in the fuel, z.sub.3 is the
fraction of C.sub.3H.sub.8 in the fuel, a.sub.0-2, b.sub.0-33 are
the respective coefficients, and .epsilon. is the error term.
[0068] The rich limits were found to be a function of H/C ratio
alone. The model has the following statistics.
TABLE-US-00003 TABLE 3 ANOVA for Equation (4) Source DF SS MS F p
Model 1 16,373 16373 381.3 <.0001 Residual 4 172 42.9 r.sup.2
0.990 Total 5 16,545 361.4 r.sub.p.sup.2 0.978
[0069] Table 3 shows the analysis of variance (ANOVA) for equation
(4). The model contains 1 degree of freedom (DF), leaving 4 DF to
estimate the error, and thus comprising a total of 5 degrees of
freedom. In actuality, the model may be said to contain 2 degrees
of freedom: a.sub.0, and a.sub.1; however, if the model were not
significant--termed the null hypothesis--then all data points are
replicates and would be best expressed by a mean value. Since the
null hypothesis contains 1 degree of freedom (the mean) it is
subtracted from the degrees of freedom of the model to give a net 2
degrees of freedom, which is what is reported in the analysis of
variance table. Furthermore, it may be said that there are actually
6 DF for the model corresponding to the six data points collected.
However, if the model was not significant, one would average all
six values with a single mean. Since the mean represents 1 degree
of freedom, the net amount of degrees of freedom is actually 5.
[0070] Table 3 entries have the following meanings: the sum of
squares (SS) column shows the variance proportional to each
respective source of variance. If the model were to fit the data
exactly, it would be equal to the Total SS with zero Residual SS.
The mean square (MS) column is derived by dividing the SS column by
the DF column, excluding the bottom number, which will be discussed
later. The ratio of Model MS with the Residual MS gives an F
ratio--in this case of 381.3. If the model were no better at
explaining the variance than chance, F would be .about.1. Since
381.3>>1, the model is statistically different from chance
deviation. The probability, p, that the F ratio is significant is
given by the value p<0.0001. Thus, the F ratio of 381.3 is
estimated to occur by chance less than 1 time in 10,000. In other
words, the model is statistically significant with >99.99%
certainty (1-0.0001=0.9999). Generally, a model is considered
statistically significant if p<0.05, which is the case here.
r.sup.2 is the ratio of the Model SS/Total SS. If the model fits
the data exactly the r.sup.2=1, and the model explains 100% of the
data variation. In the present case, r.sup.2=0.990; that is, the
model explained 99.0% of the total variation. The bottom number is
the PRESS statistic (predicted sum of squares). It is not derived
from the ANOVA, but it may be used to make an inference about the
predictive power of the model (as opposed to the correlative power
of the model given by r.sup.2). A predictive estimate,
r.sub.p.sup.2, was calculated by subtracting from 1 the ratio of
PRESS/Total SS. In this case r.sub.p.sup.2=0.978 and infers that
model is likely to have good predictive power. With knowledge that
the model is statistically significant, the data was examined
further to derive coefficient estimates for each of the model terms
(Table 4).
TABLE-US-00004 TABLE 4 Statistics for Equation (4) Term Est Std Err
t ratio p a.sub.0 -90.28 9.67 -9.34 0.0007 a.sub.1 48.178 2.47
19.53 <.0001
[0071] The Table 4 entries have the following meaning: The Term
shows the respective coefficients for Equation (4). The estimate
column (Est) gives the least squares value for each coefficient.
The standard error (Std Err) gives the uncertainty of the
associated coefficient. For example, a.sub.0 is estimated to be
-90.28.+-.9.67. Thus the estimate was many times larger than the
standard error and was likely to be statistically significant. The
t ratio column is the estimate divided by the standard error. One
prefers to see |t|>>1 as is the case here. The p value gives
the probability that a particular t ratio may occur by chance. For
a.sub.0, the p value is 0.0007, meaning there is merely a 0.07%
probability that a t ratio of -9.34 may occur by chance. In
general, the inventors rejected the null hypothesis if p<0.05,
as is the case here.
[0072] FIG. 7 is a graph showing that the enhancement of the rich
limits correlated well with square root of the H/C ratio (Equation
4), according to an embodiment.
[0073] Tables 5 and 6 give the associated statistics for Equation
5.
TABLE-US-00005 TABLE 5 ANOVA for Equation (5) Source DF SS MS F p
Model 3 8.7047 2.9016 3243.4 0.0003 Residual 2 0.0018 0.0009
r.sup.2 0.9998 Total 5 8.7065 0.1553 r.sub.p.sup.2 0.9822
TABLE-US-00006 TABLE 6 Statistics for Equation (5) Term Est Std Err
t ratio p VIF b.sub.0 -5.449 0.0264 -206.69 <.0001 b.sub.1 0.596
0.0597 9.97 0.0099 1.20 b.sub.3 4.013 0.0416 96.4 0.0001 1.39
b.sub.33 -3.120 0.1099 -28.4 0.0012 1.44
[0074] All of the model coefficients are statistically significant
at p<0.05, both the correlation coefficient and the predictive
coefficient are very close to 1. The variance inflation factor
(VIF) characterizes the correlation among factor variables. If
there is no correlation (desired) then VIF=1. The relationship
between VIF and r.sup.2 is r.sup.2=1-1/VIF. Thus, VIFs of 1.20,
1.39, and 1.44 correspond respectively to r.sup.2s of 0.167, 0.281,
and 0.306--all quite meager, meaning that the factor disposition is
well dispersed in mixture space with little collinearity. Equation
(5) generates the contours shown in FIGS. 8A-B.
[0075] FIGS. 8A-B depict two ternary diagrams showing the percent
change contours for lean and rich flammability limits per equations
(5) and (4), according to an embodiment. FIG. 8A shows the percent
decrease in the lean limits as a function of mixture fraction based
on Equation (5). FIG. 8B shows the increase in rich limits
according to Equation (4). In general, the contours compare well to
the exact contours (FIG. 6).
CONCLUSIONS
[0076] Flammability limits can be defined as "the state at which
steady propagation of the planar premixed flame . . . fails to be
possible." It has been long known that increased temperature widens
flammability limits. Increased pressure also widens flammability
limits because it increases the fuel and oxygen concentrations.
Increases in oxygen concentration widen flammability limits,
particularly on the rich side because the additional oxygen can
react under rich conditions, whereas on the lean side oxygen is not
the limiting reagent. If an electric field enhances the fuel
concentration at the lean side or the oxidant concentration on the
rich side, then it would likewise widen the overall flammability
limit.
[0077] Two facts are now apparent. The rich limit widens in direct
proportion to the square root of the H/C molar ratio. The lean
limit widens in opposition to the C.sub.3H.sub.8 concentration and
to a lesser extent with the hydrogen concentration.
[0078] While various aspects and embodiments have been disclosed
herein, other aspects and embodiments are contemplated. The various
aspects and embodiments disclosed herein are for purposes of
illustration and are not intended to be limiting, with the true
scope and spirit being indicated by the following claims.
* * * * *