Note: Descriptions are shown in the official language in which they were submitted.
CA 02487146 2004-11-08
TITLE OF THE INVENTION:
IMPROVED FUEL STAGING PROCESS FOR LOW
NOx OPERATIONS
BACKGROUND OF THE INVENTION
[0001] The present invention relates to fuel staging processes and systems for
reducing
nitrogen oxide (NOx) emissions, and in particular to such processes and
systems using fuel
dilution tips in low NOx burners.
[0002] One of the challenges confronting the Chemical Process Industry (CPI)
is the
combustion of waste fuels for economic reasons and at the same time meeting
low NOx and
CO emissions requirements. The waste fueis contain a cocktail of higher C/H
ratio gases
which combust with very luminous flames due to carbon oxidation and also
produce soot
particles or carbon depending on the combustion process. Typical refinery fuel
composition
contains varying amounts of fuels and inert gases (e.g., Cl, C2, C3.... Cn,
olefins, hydrogen,
nitrogen, C02, water vapor). If carbon or soot particles are formed on the
fuel tips, the soot
structure generally grows under favorable pressure and temperature conditions
existing near
the tip exit. This could result in fuel jet blockage, fuel jet deflection, and
overheating of tips
and furnace parts, such as process tubes and refractory walls, and the
potential shutdown of
the burners and furnace operation. The shutdown of a furnace could result in
significant
financial penalties, including liability arising from downstream process
interruption.
[0003] Dirty refinery fuels consisting of higher carbon and containing gases
such as
acetylene, ethane, propane, butane and olefins (e.g., ethylene and propylene)
generally
produce soot particles if fuel tips are subjected to:
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^ inadequate mixing in the furnace (depending on the number of jets, jet
geometry,
injection angles and injection velocities not being optimum) (generally
classified as a
burner design issue);
= lack of combustion air or oxidant availability in the vicinity of fuel jets
(generally
classified as a burner flow configuration issue);
^ inadequate cooling of fuel tips (exposure to furnace radiation on regular
basis)
(generally classified as a fuel tip configuration and burner design issue);
^ interruption of fuel flows (upstream fuel equipment reliability) (generally
classified as
a process issue);
= lower firing operation (lower fuel flow rates due to process turndown)
(generally
classified as a process issue); or
= fluctuation of refinery fuel composition in terms of carbon containing
species
(generally classified as a process requirement issue).
[0004] The burner or tip design significantly affect tip overheating, soot
production, tip
plugging, and resulting frequent maintenance forthe burner equipment. These
problems are
compounded by changing process conditions, such as low end of process turndown
and/or
interruption of fuel flows, which affect required cooling needed on the fuel
tips. Changing
process conditions and fuel composition changes are common in refinery
operation.
[0005] Another challenge confronting the CPI is the requirement of low NOx
emissions to
meet emission regulations. There are various areas in the United States where
NOx
regulations (under the 1990 Clean Air Act) require less than 10 ppm NOx
emissions from
process heaters, boilers, gas turbines, and other stationary combustion
equipment. The
most common or BACT (Best Available Control Technology) solution in the CPI is
to use a
SCR (Selective Catalytic Reactor) for post cleanup of flue gas for reduction
of NOx
contained in the flue stream (by converting NOx into N2) using ammonia
injection inside a
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large catalytic reactor. This process is very capital intensive and requires
significant
quantities of ammonia, hot air, and electricity for ID fan operation.
[0006] Most refineries would like to avoid SCR installation and instead use
low NOx burners
to meet their NOx compliance requirements. However, low NOx burners have not
consistently produced less than 10 ppm NOx in various process heating
applications, such
as steam methane reformers (SMR), crude heaters, ethylene crackers, or
boilers. For this
reason, the use of low NOx burners has not been certified by regulating
agencies as the
BACT. In other words, SCR currently is the only commercially viable solution
for meeting
stringent NOx levels in ozone attainment regions where ground level ozone
concentration
exceeds legal limits.
[0007] Typically, operators in the CPI utilize clean natural gas or an optimum
blend of natural
gas and dirty refinery fuels to reduce penalties on maintenance issues.
However, due to
natural gas shortages and the high cost of fuels, it is not always possible
for process
industries to utilize clean natural gas for combustion. The refineries that
can combust waste
fuels typically have higher productivity and a relatively favorable
competitive status compared
to other refineries which are under utilizing the waste fuel potential.
[0008] With regard to NOx reduction, the common NOx control methods include
utilization of
low NOx burners equipped with higher levels of fuel staging and dilution of
air/fuel with flue
gas recirculation (FGR). By injecting non-reactive or inert chemical species
in the fuel/oxidant
mixture, the average flame temperature is reduced and thus, NOx emissions are
reduced.
However, these methods require additional piping and energy costs associated
with the
transport of flue gas. In addition, there is an energy penalty due to required
heating of the
gases from ambient temperature to the process temperature. In addition, the
field data
published in the literature do not indicate that these methods achieve less
than 10 ppm NOx
performance.
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[0009] Various devices and methods using fuel staging have been developed with
the goal
of reducing NOx emissions. Several of these are discussed below.
[0010] U.S. Patent Application No. 2003/0148236 (Joshi, et al.) discloses an
ultra low NOx
burner using staged fuel nozzles. The burner has eight fuel staging lances
located around
the main burner body. The center part of the burner is used for supplying 100%
of the
combustion air and a very small amount of fuel (- 10%) is injected for overall
flame stability.
The rest of the fuel (- 90%) is injected using multiple fuel staging lances.
The fuel staging
lances have special fuel nozzle tips with two circular holes. As shown in
Figures 1A-1C,
these lances have axial and radial divergence angles for delayed mixing with
the combustion
air and entraining furnace gases due to a relatively high jet velocity (500 to
1,000 feet/sec or
5 to 15 psig fuel supply pressure depending on the firing rate).
[0011] U.S. Pat. No. 6,383,462 (Lang) discloses a method and an apparatus
which has a
mixing chamber outside of the "bumer and fumace for mixing flue gases from
the furnace
with the fuel gas, as shown in Figure 2. A converging diverging venturi mixer
is utilized to
further dilute the fuel gas with additional flow motivating gas. The resulting
mixture (diluted
fuel with flue gas) is then sent to the burner wherein the mixture is combined
with the
combustion air and bumed in the furnace. Depending on the flue gas dilution
level, a NOx
emission reduction from 26 ppm to 14 ppm may be obtained. This apparatus and
method do
not reduce NOx emissions below 10 ppm and the results are not comparable to
those
typically achieved with SCR technology. .
[0012] U.S. Pat. No. 6,481,209 (Johnson, et al.) discloses a fuel staging
system suitable for
gas turbine engines. Efficient combustion with air is achieved with lower NOx
and CO
emissions by splitting fuel injection in two stages: 1) injectors installed in
swirl mixers, and 2)
injectors installed in the trapped vortex region of the combustor. However,
this injection
scheme is not suitable for large furnaces where trapped vortex zones are not
possible due to
furnace and load geometry.
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[0013] U.S. Pat. No. 6,558,154 (Eroglu, etal.) discloses a control based fuel
staging strategy
for an aero engine in which two separate instrumented fuel staging nozzles are
used. A set
of emission and pulsation sensors are installed downstream of each staging
zone. These
sensors measure the quality of combustion products issued from each staging
zone and
then a control unit varies relative amounts of fuels injected in each zone
depending on
changing operating and environmental conditions.
[0014] U.S. Pat. No. 5,601,424 (Bernstein, et al.) discloses a method for
reducing NOx using
atomizing steam injection control. The NOx levels are lowered by adding to the
burner flame
atomizing steam, which is available for fuel oil atomization. For 30% NOx
reduction,
approximately 0.5 lb steam/lb of fuel flow is necessary. A large amount of
steam is
necessary to reduce flame temperature and obtain a required NOx reduction. In
addition, if a
large amount of steam is used for flame quenching, there is a possibility of
flame instability
and sputtering. Thus, there is an upper limit for steam injection on flame
stability grounds.
[0015] The gas turbine industry also uses a similar steam injection technique
for NOx
control. However, due to an inefficient steam injection mode, a large economic
penalty is
paid in order to reduce NOx emissions. The steam consumption is very large,
and the
technique is relatively inefficient and not cost effective for NOx control.
[0016] It is desired to have a cost effective, retrofit apparatus and method
for NOx emission
reduction, which provide the ability to combust refinery waste gases without
excessive NOx
emissions.
[0017] It is further desired to have an apparatus and method which reduce
equipment
maintenance due to problems such as plugging of burner tips and over-heating
of process
tubes, and which will provide additional benefits of improved fuel efficiency
and furnace
productivity.
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[0018] It is still further desired to have an apparatus and method which will
allow current low
NOx burners to meet SCR level NOx performance and allow refiners to comply
with NOx
regulations without using the capital-intensive SCR technology.
[0019] It is still further desired to have an apparatus and method which will
enable process
industries to consume cheaper waste fuel without incurring penalties on
maintenance issues
such as tips plugging, equipment overheating, process interruptions, etc.,
while at the same
time meeting NOx regulations by producing less than 10 ppm NOx emissions.
[0020] It is also desired to have an apparatus and method for combusting a
fuel which afford
better performance than the prior art, and which also overcome many of the
difficulties and
disadvantages of the prior art to provide better and more advantageous
results.
BRIEF SUMMARY OF THE INVENTION
[0021] The present invention is a method and a system for diluting a fuel to
reduce nitrogen
oxide emissions through fuel staging. The invention also includes a fuel
dilution device that
may be used in the method or the system.
[0022] There are multiple steps in a first embodiment of the method for
diluting a fuel to
reduce nitrogen oxide emissions through fuel staging. The first step is to
provide a fuel
dilution device, which includes: a first conduit having an inlet and an outlet
spaced apart
from the inlet, the first conduit adapted to transmit a stream of the fuel
entering the inlet and
exiting the outlet at a first thermodynamic state and a first fuel index; and
a second conduit
having an intake and an outtake spaced apart from the intake, the second
conduit adapted to
transmit a stream of a fluid entering the intake and exiting the outtake at a
second
thermodynamic state and a second fuel index, the second fuel index being
different from the
first fuel index by at least about 0.1 and the second thermodynamic state
being different from
the first thermodynamic state, whereby a potential for mixing exists between
the stream of
the fuel exiting the outlet of the first conduit and the stream of the fluid
exiting the outtake of
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the second conduit. The second step is to feed the stream of the fuel to the
inlet of the first
conduit, said stream of the fuel exiting the outlet of the first conduit at
the first thermodynamic
state and the first fuel index. The third step is to feed the stream of the
fluid to the intake of
the second conduit, said stream of the fluid exiting the outtake of the second
conduit at the
second thermodynamic state and the second fuel index, whereby at least a
portion of the
stream of the fuel exiting the outlet of the first conduit mixes with at least
a portion of the
stream of the fluid exiting the outtake of the second conduit at a location
proximate both the
outlet and the outtake, thereby generating at least one diluted fuel stream
having an
intermediate fuel index between the first fuel index and the second fuel
index. The fourth
step is to provide a source of an oxidant. The fifth step is to combust a
portion of the oxidant
with at least a portion of at least one of the stream of the fuel, or the
stream of the fluid, or
the diluted fuel stream, thereby generating a gas containing a reduced amount
of nitrogen
oxide, said reduced amount of nitrogen oxide being less than a higher amount
of nitrogen
oxide that would be generated by combusting the fuel using a means other than
the fuel
dilution device.
[0023] There are many variations of the first embodiment of the method. In one
variation,
the fluid is a fuel. In another variation, the fluid is selected from a group
consisting of steam,
flue gas, carbon dioxide, nitrogen, argon, helium, xenon, krypton, other inert
fluids, and
mixtures or combinations thereof.
[0024] In another variation of the first embodiment of the method, the first
conduit is adjacent
the second conduit. In yet another variation, at least a substantial portion
of the second
conduit is disposed in the first conduit. In still yet another variation, the
second conduit has
an equivalent diameter (D,) and the outtake of the second conduit is located
at a distance
behind the outlet of the first conduit, said distance being in a range of
about (2 D,) to about
(20 Dc).
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[0025] A second embodiment of the method for diluting a fuel to reduce
nitrogen oxide
emissions through fuel staging is similar to the first embodiment but includes
two additional
steps. The first additional step is to provide a swirler disposed in the
second conduit. The
second additional step is to transmit at least a portion of the stream of the
fluid through the
swirler, thereby swirling at least a portion of the fluid exiting the second
conduit.
[0026] A third embodiment of the method is similar to the first embodiment,
but includes two
additional steps. The first additional step is to provide a zipper nozzle in
fluid communication
with the outlet of the first conduit. The second additional step is to
transmit through the
zipper nozzle at least a portion of a diluted fuel stream.
[0027] A fourth embodiment of the method is similar to the first embodiment
but includes the
additional step of placing the fuel dilution device in fluid communication
with a furnace
containing a quantity of a furnace gas, whereby at least a portion of the
quantity of the
furnace gas mixes with at least a portion of the diluted fuel stream.
[0028] Another embodiment of a method for diluting a fuel to reduce nitrogen
oxide
emissions through fuel staging includes multiple steps. The first step is to
provide a fuel
dilution device, which includes: a first conduit having an inlet and an outlet
spaced apart
from the inlet, the first conduit adapted to transmit a stream of the fuel
entering the inlet and
exiting the outlet at a first pressure, a first velocity, and a first fuel
index; and a second
conduit having an intake and an outtake spaced apart from the intake, the
second conduit
adapted to transmit a stream of a fluid entering the intake and exiting the
outtake at a second
pressure, a second velocity, and a second fuel index, the second fuel index
being different
from the first fuel index by at least about 0.1 and at least one of the second
pressure and the
second velocity being different from at least one of the first pressure and
the first velocity,
whereby a potential for mixing exists between the stream of the fuel exiting
the outlet of the
first conduit and the stream of the fluid exiting the outtake of the second
conduit. The
second step is to feed the stream of the fuel to the inlet of the first
conduit, said stream of the
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fuel exiting the outlet of the first conduit at the first pressure, the first
velocity, and the first
fuel index. The third step is to feed the stream of the fliiid to the intake
of the second
conduit, said stream of the fluid exiting the outtake of the second conduit at
the second
pressure, the second velocity, and the second fuel index, whereby at least a
portion of the
stream of the fuel exiting the outlet of the first conduit mixes with at least
a portion of the
stream of the fluid exiting the outtake of the second conduit at a location
proximate both the
outlet and the outtake, thereby generating at least one diluted fuel stream
having an
intermediate fuel index between the first fuel index and the second fuel
index. The fourth
step is to provide a source of an oxidant. The fifth step is to combust a
portion of the oxidant
with at least a portion of at least one of the stream of the fuel, or the
stream of the fluid, or
the diluted fuel stream, thereby generating a gas containing a reduced amount
of nitrogen
oxide, said reduced amount of nitrogen oxide being less than a higher amount
of nitrogen
oxide that would be generated by combusting the fuel using a means other than
the fuel
dilution device.
[0029] There are multiple elements in a first embodiment of a fuel dilution
device for diluting
a fuel to reduce nitrogen oxide emissions through fuel staging. The first
element is a first
conduit having an inlet and an outlet spaced apart from the inlet, the first
conduit adapted to
transmit a stream of a fuel entering the inlet and exiting the outlet at a
first thermodynamic
state and a first fuel index. The second element is a second conduit having an
intake and an
outtake spaced apart from the intake, the second conduit adapted to transmit a
stream of a
fluid entering the intake and exiting the outtake at a second thermodynamic
state and a
second fuel index, the second fuel index being different from the first fuel
index by at least
about 0.1 and the second thermodynamic state being different from the first
thermodynamic
state, whereby a potential for mixing exists between the stream of the fuel
exiting the outlet
of the first conduit and the stream of the fluid exiting the outtake of the
second conduit,
whereby at least a portion of the stream of the fuel exiting the outlet of the
first conduit mixes
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with at least a portion of the stream of the fluid exiting the outtake of the
second conduit at a
location proximate both the outlet and the outtake, thereby generating at
least one diluted
fuel stream having an intermediate fuel index between the first fuel index and
the second fuel
index. The third element is a source of an oxidant. The fourth element is a
means for
combusting a portion of the oxidant with at least a portion of at least one of
the stream of the
fuel, or the stream of the fluid, or the diluted fuel stream, thereby
generating a gas containing
a reduced amount of nitrogen oxide, said reduced amount of nitrogen oxide
being less than
a higher amount of nitrogen oxide that would be generated by combusting the
fuel using a
means other than the fuel dilution device.
[0030] There are many variations of the first embodiment of the fuel dilution
device. In one
variation, the fluid is a fuel. In another variation, the fluid is selected
from a group consisting
of steam, flue gas, carbon dioxide, nitrogen, argon, helium, xenon, krypton,
other inert fluids,
and mixtures or combinations thereof.
[0031] In another variation, the first conduit is adjacent the second conduit.
In yet another
variation, at least a substantial portion of the second conduit is disposed in
the first conduit.
In still yet another variation, the second conduit has an equivalent diameter
(DJ and the
outtake of the second conduit is located at a distance behind the outlet of
the first conduit,
said distance being in a range of about (2 x DJ to about (20 x DJ.
[0032] In another variation of the first embodiment, the fuel dilution device
is in fluid
communication with a furnace containing a quantity of a furnace gas, whereby
at least a
portion of the quantity of the furnace gas mixes with at least a portion of
the diluted fuel
stream.
[0033] A second embodiment of the fuel dilution device is similar to the first
embodiment but
includes a swirier disposed in the second conduit. A third embodiment of the
fuel dilution
device is similar to the first embodiment, but includes a zipper nozzle in
fluid communication
with the outlet of the first conduit.
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[0034] Another embodiment of the fuel dilution device for diluting a fuel to
reduce nitrogen
oxide emissions through fuel staging includes multiple elements. The first
element is a first
conduit having an inlet and an outlet spaced apart from the inlet, the first
conduit adapted to
transmit a stream of a fuel entering the inlet and exiting the outlet at a
first pressure, a first
velocity, and a first fuel index. The second element is a second conduit
having an intake and
an outtake spaced apart from the intake, the second conduit adapted to
transmit a stream of
a fluid entering the intake and exiting the outtake at a second pressure, a
second velocity,
and a second fuel index, the second fuel index being different from the first
fuel,index by at
least about 0.1 and at least one of the second pressure and the second
velocity being
different from at least one of the first pressure and the first velocity,
whereby a potential for
mixing exists between the stream of the fuel exiting the outlet of the first
conduit and the
stream of the fluid exiting the outtake of the second conduit, whereby at
least a portion of the
stream of the fuel exiting the outlet of the first conduit mixes with at least
a portion of the
stream of the fluid exiting the outtake of the second conduit at a location
proximate both the
outlet and the outtake, thereby generating at least one diluted fuel stream
having an
intermediate fuel index between the first fuel index and the second fuel
index. The third
element is a source of an oxidant. The fourth element is a means for
combusting a portion
of the oxidant with at least a portion of at least one of the stream of the
fuel, or the stream of
the fluid, or the diluted fuel stream, thereby generating a gas containing a
reduced amount of
nitrogen oxide, said reduced amount of nitrogen oxide being less than a higher
amount of
nitrogen oxide that would be generated by combusting the fuel using a means
otherthan the
fuel dilution device.
[0035] Another aspect of the invention is a system for diluting a fuel to
reduce nitrogen oxide
emissions through fuel staging. The system includes multiple elements. The
first element is
a fuel dilution device, which includes: a first conduit having an inlet and an
outlet spaced
apart from the inlet, the first conduit adapted to transmit a stream of the
fuel entering the inlet
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and exiting the outlet at a first thermodynamic state and a first fuel index;
and a second
conduit having an intake and an outtake spaced apart from the intake, the
second conduit
adapted to transmit a stream of a fluid entering the intake and exiting the
outtake at a second
thermodynamic state and a second fuel index, the second fuel index being
different from the
first fuel index by at least about 0.1 and the second thermodynamic state
being different from
the first thermodynamic state, whereby a potential for mixing exists between
the stream of
the fuel exiting the outlet of the first conduit and the stream of the fluid
exiting the outtake of
the second conduit. The second element is a means for feeding the stream of
the fuel to the
inlet of the first conduit, said stream of the fuel exiting the outlet of the
first conduit at the first
thermodynamic state and the first fuel index. The third element is a means for
feeding the
stream of the fluid to the intake of the second conduit, said stream of the
fluid exiting the
outtake of the second conduit at the second thermodynamic state and the second
fuel index,
whereby at least a portion of the stream of the fuel exiting the outlet of the
first conduit mixes
with at least a portion of the stream of the fluid exiting the outtake of the
second conduit at a
location proximate both the outlet and the outtake, thereby generating at
least one diluted
fuel stream having an intermediate fuel index between the first fuel index and
the second fuel
index. The fourth element is a source of an oxidant. The fifth element is a
means for
combusting a portion of the oxidant with at least a portion of at least one of
the stream of the
fuel, or the stream of the fluid, or the diluted fuel stream, thereby
generating a gas containing
a reduced amount of nitrogen oxide, said reduced amount of nitrogen oxide
being less than
the high amount of nitrogen oxide that would be generated by combusting the
fuel using a
means other than the fuel dilution device.
[0036] Another embodiment of the system for diluting a fuel to reduce nitrogen
oxide
emissions through fuel staging includes multiple elements. The first element
is a fuel dilution
device, which includes: a first conduit having an inlet and an outlet spaced
apart from the
inlet, the first conduit adapted to transmit a stream of the fuel entering the
inlet and exiting
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the outlet at a first pressure, a first velocity, and a first fuel index; and
a second conduit
having an intake and an outtake spaced apart from the intake, the second
conduit adapted to
transmit a stream of a fluid entering the intake and exiting the outtake at a
second pressure,
a second velocity, and a second fuel index, the second fuel index being
different from the
first fuel index by at least about 0.1 and at least one of the second pressure
and the second
velocity being different from at least one of the first pressure and the first
velocity, whereby a
potential for mixing exists between the stream of the fuel exiting the outlet
of the first conduit
and the stream of the fluid exiting the outtake of the second conduit. The
second element is
a means for feeding the stream of the fuel to the inlet of the first conduit,
said stream of the
fuel exiting the outlet of the first conduit at the first pressure, the first
velocity, and the first
fuel index. The third element is a means for feeding the stream of the fluid
to the intake of
the second conduit, said stream of the fluid exiting the outtake of the second
conduit at the
second pressure, the second velocity, and the second fuel index, whereby at
least a portion
of the stream of the fuel exiting the outlet of the first conduit mixes with
at least a portion of
the stream of the fluid exiting the outtake of the second conduit at a
location proximate both
the outlet and the outtake, thereby generating at least one diluted fuel
stream having an
intermediate fuel index between the first fuel index and the second fuel
index. The fourth
element is a source of an oxidant. The fifth element is a means for combusting
a portion of
the oxidant with at least a portion of at least one of the stream of the fuel,
or the stream of
the fluid, or the diluted fuel stream, thereby generating a gas containing a
reduced amount of
nitrogen oxide, said reduced amount of nitrogen oxide being less than a higher
amount of
nitrogen oxide that would generated by combusting the fuel using a means other
than the
fuel dilution device.
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BRIEF DESCRIPTION OF THE DRAWINGS
[0037] The invention will be described by way of example with reference to the
accompanying drawings, in which:
[0038] Figure 1 A is a cross-sectional plan view of a prior art fuel staging
nozzle used in an
ultra low-NOx burner;
[0039] Figure 1 B is a cross-sectional elevation view of the prior art fuel
staging nozzle of
Figure 1A;
[0040] Figure 1 C is a side view of the prior art fuel staging nozzle of
Figure 1 B;
[0041] Figure 2 is a cross-sectional elevation view of a prior art mixing
chamber for mixing
flue gases from a furnace and a flow motivating gas with a fuel gas;
[0042] Figure 3 is a schematic diagram illustrating a cross-sectional view of
one embodiment
of the present invention;
[0043] Figure 4 is a schematic diagram illustrating a cross-sectional view of
another
embodiment of the invention;
[0044] Figure 5A is a schematic diagram illustrating another embodiment of the
invention
which uses strong jet-weak jet entrainment;
[0045] Figure 5B is a schematic diagram illustrating a cross-sectional view of
another
embodiment of the invention which uses a swirl induced entrainment;
[0046] Figure 6 is a schematic diagram illustrating a cross-sectional view of
another
embodiment of the invention;
[0047] Figure 7 is a schematic diagram illustrating a cross-sectional view of
another
embodiment of the invention which includes a zipper tip or nozzle;
[0048] Figure 8A is a schematic diagram illustrating a front view of a zipper
tip or nozzle;
[0049] Figure 8B is a schematic diagram illustrating a side view of a zipper
tip or nozzle
attached to a lance, such as that shown in Figure 7;
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[0050] Figure 8C is a schematic diagram illustrating a plan view of a zipper
tip or
nozzle;
[0051] Figure 8D is a schematic diagram illustrating a portion of the front
view of the
zipper tip or nozzle in Figure 8A in detail for dimensioning; and
[0052] Figure 9 is a schematic diagram illustrating a cross-sectional view of
another
embodiment of the invention which includes a zipper tip or nozzle.
DETAILED DESCRIPTION OF THE INVENTION
[0053] The present invention addresses a number of issues encountered in
combustion equipment design, such as burners used for heating reformers,
process
heaters, boilers, ethylene crackers, or other high temperature furnaces. The
invention
relates to an improved fuel staging process. In particular, two general
approaches that
provide for rapid dilution and mixing, depending on the required process
objectives,
are:
I. Staging Fuel with Another Fuel (F-F): High-pressure refinery waste fuel,
atomized liquid fuel, etc. are injected in the vicinity of a relatively clean
and low-
pressure gaseous fuel for clean, maintenance free, low NOx operation; and
II. Staging Fuel with Inert Gas (F-I): High-pressure inert fluids such as
steam,
nitrogen, COz, etc. are injected in the vicinity of a low-pressure gaseous
fuel for NOx
reduction.
[0054] As used herein, the term "fuel index" (FI) is defined as the weighted
sum of the
fuel carbon atom number where molecular H2 is assigned a carbon number 1.3,
the
weights being the component mole fractions: Fl =T_ C;x;/ T_ x;, where C; and
x; are
the number of carbon atoms and the mole fraction of component I, respectively.
The fuel indices of a number of fuels and inerts are listed in Table I.
Generally,
a fuel with a higher fuel index cracks more easily and produces more NOx
through the prompt NOx mechanism. H2 is a special case in this definition.
Although H2 does not have any carbon atoms, it is well known that H2 addition
in natural gas increases NOx emissions. The literature suggests that about a
30% higher NOx emission occurs for pure H2 flames as compared to methane
flames. The increased NOx emission from H2 flames is attributable to higher
flame temperatures via the thermal NOx mechanism. Since the fuel index is
used as an indicator for NOx emissions herein, a value of 1.3 is assigned to
H2
to be consistent with its NOx emission potential.
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CA 02487146 2008-01-18
Table I: Fuel Indices for Selected Fuels and Inerts
Fuels or Inerts Fuel Index
H2 1.3
H20 0
C02 0
CO 1
N2 0
CH4 1
C3H$ 3
ROG 1) 1.434
PSA offgas (2) 0.57
Natural gas (3) 1.08
Natural gas (4) 1.14
(1) ROG: H2 18%, CH4 44%, C2H2 38%.
(2) PSA offgas: H2 30%, CH4 18%, C02 52%
(3) Natural gas: CH4 91 %, C2H6 4%, C3H$ 3%, N2 1%, C02 1%.
(4) Natural gas: CH4 84%, C2H6 12%, C3H$ 2%, N2 2%.
[0055] As discussed herein, the term "thermodynamic state" is defined as a
state of existence for a matter. This definition is based on the generally
known
concept of thermodynamics, but with an extensions to include not only the
usual temperature and pressure but also velocity, concentration, composition,
volume fraction, flow rate, electric potential, etc., to completely
characterize a
stream. This definition is used to precisely define mixing as the result of a
difference in the thermodynamic state between two streams.
[0056] The two approaches are discussed in detail below.
I. Staging Fuel with Another Fuel (F-F):
[0057] This approach may be used to combust refinery waste fuels at a high
supply pressure that contain a blend of hydrogen and higher C/H fuels (ethane,
propane, butane,
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CA 02487146 2004-11-08
olefins, etc.) with a second relatively cleaner, low-pressure fuel gas.
Maintenance problems
arise with such refinery waste fuel due to thermal cracking of the high C/H
fuels and
subsequent soot build-up in the burner fuel tips. In addition, combustion of
such fuels results
in higher than normal NOx emissions.
[0058] To improve combustion of high C/H refinery waste fuels, the dirty fuel
is diluted with a
relatively cleaner (secondary) fuel stream (e.g., hydrogen, syngas, natural
gas, or a low BTU
fuel blend). In one embodiment shown in Figure 3, a high-pressure refinery
fuel gas
(containing high C/H ratio fuel gases) is injected through a center lance 32
and a relatively
clean, low-pressure fuel gas, such as natural gas, syngas, process gas, PSA
off gas
(recycled fuel gas after removing product hydrogen from PSA adsorbent beds),
etc is
injected through an annular region 33 between the center lance 32 and an outer
lance 34.
As shown in Figure 3, the exit 36 of the center lance is recessed a preferred
distance from
the exit 38 of the outer lance. This distance preferabiy is 2 to 20 times the
equivalent
diameter (Dc) of the center lance. Depending on the fuel split between the
high-pressure
refineryfuel gas and the cleaner low-pressure fuel gas, the distance
preferably is about 1/16"
to 1 ".
[0059] Persons skilled in the art will recognize that the reference to "high
pressure" in
Figures 3-7 and 9 also could state "high velocity" or "high pressure or high
velocity."
Similarly, the reference to "low pressure" in those figures could state "low
velocity" or "low
pressure or low velocity."
[0060] The arrangement shown in Figure 3 allows the dirty high-pressure
refinery fuel gas to
mix with the cleaner low-pressure fuel gas due to turbulent jet interaction.
The velocity of the
high-pressure refinery fuel gas through the center lance 32 preferably is
about 900 to 1400
feet/sec (preferably sonic or choked velocity). The velocity of the low-
pressure fuel gas
through the annular region 33 between the center lance 32 and the outer lance
34 preferably
is about 100 to 900 feet/sec, depending on the available supply pressure of
the low-pressure
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CA 02487146 2004-11-08
gas. The higher velocity gas stream exiting the exit 36 of the center lance
entrains the lower
velocity gas stream approaching the exit 38 of the outer lance and provides
"first stage"
mixing before the streams exit through an orifice(s) 40. The outer lance
orifice geometry,
angles, etc. are designed for optimum "second stage" mixing in the furnace
atmosphere. A
very large amount of furnace gas 42 is entrained for second stage dilution,
thereby lowering
the peak flame temperatures and subsequent reduction in NOx emissions.
[0061] Figure 4 illustrates an arrangement for liquid fuel (F-F) staging. In
this embodiment, a
high-pressure (and high C/H ratio) liquid fuel (e.g., fuel oil, diesel, bunker
C, waste liquid fuel,
etc.) is diluted using a low-pressure fuel gas before being injected into a
furnace atmosphere
for further dilution. For example, heavy fuel oil can be atomized with an
atomizing fluid, such
as steam, and then diluted with a low-pressure fuel gas for soot free (clean)
combustion
inside the furnace. This embodiment also decreases NOx emissions due to lower
peak
flame temperatures.
[0062] In Figure 4, X is the distance from the exit of the center lance 32 to
the back face of
the exit for the outer lance 34. Dr is the flow area-equivalent diameter of
the exit of the
center lance, that is, the total flow areas of the exit of the center lance is
the same as a circle
of diameter D, De is the flow area-equivalent diameter of the outer lance,
that is, the total
flow area of the exit of the lance is the same as a circle of diameter De.
[0063] Two other embodiments of (F-F) staging are shown in Figures 5A and 5B.
In Figure
5A, a strong jet - weak jet interaction takes place between the high-pressure
refinery fuel
gas and the low-pressure fuel gas. The high-pressure refinery fuel gas is
injected in a high-
pressure lance 52 at a high velocity (about 900 to 1400 feet/sec) in a
preferred direction, and
a low-pressure fuel gas, which is injected in a low-pressure lance 54, is
entrained by the
high-pressure refinery fuel gas.
[0064] In Figure 5B, the high-pressure refinery fuel gas is swirled in a
center lance 32 using
a fuel swirler 56, and the low-pressure fuel gas is entrained in the collapsed
region (central
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CA 02487146 2004-11-08
region) of the high velocity swirl. This allows good mixing of the high-
pressure refinery fuel
gas and the low-pressure fuel gas before they exit the outer lance 34 and
enter the furnace
(not shown), where additional dilution takes place with the furnace gases 42.
This approach
is beneficial for applications requiring a short flame profile or a smaller
combustion space.
[0065] An application for (F-F) staging is found in steam methane reformers
(SMR) where
the high-pressure fuel gas is generally a supply of natural gas or a refinery
off-gas which is
generally classified as a trim fuel. Referring to Figure 6, the high-pressure
fuel gas is
injected in the center lance 32. The low-pressure fuel gas injected in the
annular region 33
between the center lance 32 and the outer lance 34 is generally PSA (pressure
swing
adsorption) off-gas or clean vent stream from PSA that contains C02 (-45%),
hydrogen
(-30%), methane (-15%), and CO (-10%) with a fuel index of about 0.64. The PSA
off-gas
is permeate out of the adsorption bed after hydrogen product is separated. The
high-
pressure trim fuel accounts for between 10% to 30% of a total energy for
typical reformers
having PSA for hydrogen separation.
[0066] A secondary advantage of this staging application is to improve PSA
recovery by
increasing the range of PSA pressure cycle, particularly at the low end.
Referring to Figure 7,
this is achieved by creating a low-pressure region inside the outer lance 34.
The high
velocity central jet 72 shown in Figure 7 creates a low-pressure region around
the jet body
where the slower moving low-pressure fuel gas is entrained by the faster
moving central jet.
Due to an active entrainment process, the supply pressure for the low-pressure
fuel gas is
reduced for the same fuel flow rate.
[0067] In one laboratory firing experiment, the supply pressure of low-
pressure PSA off-gas
was reduced from 2 psig to 1.6 psig (20%. reduction). This was achieved by
injecting the
high- pressure fuel gas at 25 psig (1300 feet/sec velocity). The combustion
energy split
between the high-pressure fuel gas and the low-pressure fuel gas was 30:70
respectively.
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CA 02487146 2004-11-08
[0068] To further quantify details of the (F-F) staging process, laboratory
test results were
considered using a low NOx burner. The burner had 10 fuel lances distributed
around a
circle of 18" diameter. Of the 10 fuel lances, two lances were reserved for
the (F-F) type
staging configuration. The lances had special fuel tips and multiple diverging
slots (zipper
tips 74) to improve passive mixing. A schematic diagram of the (F-F) fuel
staging
configuration using zipper tips 74 is shown in Figure 7. The burner was rated
at 8 MM Btu/hr
firing rate utilizing 644 F air preheat and it was designed to utilize two
types of fuels. The
details of the two fuels are provided below:
^ High-pressure refinery fuel gas: H2 (18%), natural gas (44%) and
ethylene (38%). This fuel has a fuel index of 1.43 and accounts for
30% of the total energy input.
^ Low-pressure fuel gas: C02 (52%), natural gas (18%) and H2 (30). It
has a fuel index of 0.57, and accounts for about 70% of the total
energy input.
[0069] Referring to the arrangement illustrated in Figure 7, the high-pressure
fuel gas was
injected in a center lance 32 made of standard tubing having a 3/8" diameter x
0.035" wall
thickness, which was placed concentrically in an outer lance 34 made of pipe
of'/" sch 40.
A zipper tip 74 was attached to the end of the pipe. The zipper tip was sized
for 0.51"
equivalent diameter and, as shown in Figures 8A-8D, had four vertical slots
and one
horizontal slot. The divergence angles (a1 and (x2) for the vertical slots
were 18 and 6
respectively for the axial zipper nozzle tip geometry as follows: 1) a series
of vertical
structures at intersecting planes between adjacent primary shapes; 2) flow
induced
downstream.instabilities; and 3) a high level of molecular (small-scale)
mixing between the
first fluid (fuels) and the second fluid (furnace gases). The above mixing
also was achieved
in the shortest axial distance. The low NOx burner laboratory experiments
conducted with
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CA 02487146 2004-11-08
the lance-in-lance configuration of Figure 7 (including zipper tips), indicate
a rapid axial
mixing, higher furnace gas entrainment with the divergence angle (3 at 7 .
[0070] The overall fluid processes according to the arrangement of Figure 7
resulted in more
uniform heat transfer to the load and ultra low (< 15 ppmv) NOx and CO
emissions at a fuel
pressure less than 2 psig. It was also noticed that without the lance-in-lance
process, the
combustion of high pressure and high C/H ratio fuel produces a visible soot
rich flame. Also,
the NOx emissions were as high as 25 to 30 ppm. This experiment demonstrated
that the
F-F staging process could lower NOx emissions dramatically. The F-I staging
process could
reduce the emissions even more with inerts.
[0071] The visual proof of enhanced mixing was observed in a furnace in a
laboratory
whenever the lance-in-lance (F-F) fuel staging configuration was used for
refinery fuels
consisting of butane (C4H1 0) as high as 50%. The individual flames were found
to mix much
more quickly with furnace gases and created a spacious or flameless
combustion. On the
other hand, simple lances with cylindrical nozzle lances created a rather
visible (bluish) and
relatively longer flame, indicating less furnace gas dilution and mixing, and
at the same time
produced relatively higher NOx and CO emission levels at given fuel supply
pressure.
[0072] Table II provides a preferred firing range, dimensions, dimensionless
ratios and
injection angles for a proposed lance-in-lance configuration. Simple
circulartubing was used
for high-pressure refinery fuel whereas a zipper tip was used for the low-
pressure PSA off-
gas fuel. These lances are critical components of a low NOx burner because the
reliability of
burner performance directly affects steam methane reformer on stream
performance.
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CA 02487146 2004-11-08
Table II: Dimensional parameters for Lance-in-Lance fuel staging tips
Low pressure zipper tip High Prs. Cyl. Tip
(H) (W) (Ro/Rl) (H/Ra) (0, a2) ((3) L/De Dc X/D,
Burner Slot Slot Slot end Slot Axial Radial Zipper tip Tube Dist.
Firing Height Width radius to height div. div. thickness Dia back
Capacity (In) (In) center to Angle Angle to equiv. (inch) zipper
(MM radius corner ( ) ( ) diameter tip inlet
Btu/Hr) ratio radius ratio
ratio
(1/32 -1) (1/4 -2) 1.6 3.7 15 7 0.625 0.305 4
8 (1-3) (2-6) (0 - 30) (0-30) (0.05-3) (1/16 - 2) (2-20)
5.2 (1/32 - 1) (1/4 -2) 1.6 3.7 15 7 0.625 0.277 4
(1 - 3) (2-6) (0 - 30) (0-30) (0.05-3) (1/16 - 2) (2 -10)
[0073] The above dimensional ranges are valid for a variety of fuels, such as
natural gas,
propane, refinery off gases, low BTU fuels, etc. The nozzles are optimally
sized depending
on fuel composition, flow rate (or firing rate) and supply pressure available
at the burner inlet.
In Table II, the dimensions, ratios and ranges are estimated for a 2 to 10 MM
Btu/Hr burner
firing rate. However, these dimensions and ranges can be scaled up for higher
firing rate
burners (> 10 MM Btu/Hr) using standard engineering practice of keeping
similar flow
velocity ranges.
II. Staging Fuel with Inert Gas (F-1):
[0074] The improved fuel staging with high-pressure inert gases, such as steam
(dry or
saturated, C02, flue gas, nitrogen, or other inert gases, is performed with
low-pressure fuel
gases to reduce NOx emissions. The staging fuels that may be used include but
are not
limited to natural gas; low BTU process gas (consisting of hydrogen and other
refinery fuels);
and PSA off-gas. The injection tip configurations are similar to those shown
in Figures 3-7.
The main objective is to further reduce NOx emissions. A preferred embodiment
is illustrated
in Figure 9.
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CA 02487146 2004-11-08
[0075] Referring to Figure 9, a high pressure (30 to 100 psig) saturated or
dry steam is sent
through the center lance 32 at about 900 to 1400 feet/sec and low-pressure
fuel gas is sent
through the annular region 33 between the center lance 32 and the outer lance
34. A high
velocity steam jet 92 entrains the fuel gas for first stage dilution (and
mixing) inside the
annular region. The resulting mixture then exits through a zipper tip 74 at a
high velocity
(about 600 to 1400 feet/sec) for second stage dilution in the furnace (not
shown) using
furnace gases (not shown). The second stage dilution is very effective due to
high steam
velocities and entrainment loops set up by individual flames formed by the
zipper tip. Due to
the zipper tip geometry and steam-assist, improved fuel dilution is obtained.
The peak flame
temperatures are further reduced and ultra low NOx emissions are obtained.
Table III
provides estimated steam consumption numbers for a large steam methane
reformer
furnace.
Table III: Steam Consumption Economics with Proposed (F-I) Staging Process
steam injection rate lb stm/Ib fuel 0.02 0.05
firing rate mmbtu/hr LHV 850 850
fuel heating value btu/scf, LHV 1000 1000
fuel cost $/mmbtu, LHV 6 6
fuel molecular weight 18 18
steam needed Ib/hr 806 2,016
mmscfd 0.408 1.02
energy required to generate steam at
100 psia and 400 F from water at 60 F btu/scf 57.1 57.1
btu/Ib 1203.2 1203.2
steam cost $/day 140 349
$/ ear 50,992 127,480
[0076] As shown in Table 111, due to the unique method of fuel staging with an
inert gas such
as steam, the amount of steam required for fuel dilution is extremely low. The
amount of
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CA 02487146 2004-11-08
steam needed for (F-I) staging is from about 2% to 10% on a lb per lb basis
when compared
to the low-pressure fuel. The high velocity of steam is used for a two-stage
dilution process:
1) inside the lance tube using steam and low-pressure fuel gas, and 2) in the
furnace space
using high velocity fuel-steam mixture and furnace gases.
[0077] The laboratory experiments using an inert gas, such as nitrogen, have
shown that
NOx reductions of about 30% to 40% are possible based on a comparison between
the
simple prior art lance configuration (zipper or circular tips alone without
lance-in-lance
arrangement) and the lance-in-lance configuration of Figure 9. For example,
using a low
NOx burner, at 5 MM btu/Hr firing rate, using ambient combustion air, a
furnace operating at
an average temperature of 1600 F, exhaust gas at 2000 F, using a nitrogen flow
rate of 10%
on a weight basis, the NOx emission is reduced from about 10 ppm (corrected at
3% 02) for
no inert gas in the center to about 7 ppm (corrected at 3% 02) with nitrogen
gas in the
center.
[0078] In each of the embodiments discussed above, the favorable results
achieved by the
present invention are driven by two differences in the streams exiting the two
conduits. The
first difference is a difference in the thermodynamic states of the respective
streams, and the
second difference is a difference in the fuel indices of the respective
streams. Specifically, in
order for there to be a potential for mixing between the two streams exiting
the two conduits,
there must be a difference in the thermodynamic states of the two streams, and
a difference
of at least 0.1, and preferably at least 0.2, between the fuel indices of the
two streams must
exist for meaningful NOx reduction.
[0079] In the embodiments illustrated in the figures and discussed above, the
difference
between the thermodynamic states of the two streams is expressed in terms of
the pressure
differential (i.e., a "high pressure" fluid in one conduit, and a "low
pressure" fluid in the other
conduit). However, persons skilled in the art will recognize that the
differential in
thermodynamic states may also be expressed in terms of, and achieved as a
result of,
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CA 02487146 2004-11-08
differences in velocity, temperature, concentration, composition, volume
fraction, flow rate,
electric potential, etc.
[0080] Therefore, the present invention includes many other embodiments and
variations
thereof which are not illustrated in the figures or discussed in the Detailed
Description of The
Invention. Those embodiments and variations, however, do fall within the scope
of the
appended claims and equivalents thereof.
[0081] Those skilled in the art also will recognize that the embodiments and
variations
illustrated in the drawings and discussed in the Detailed Description of The
Invention do not
disclose all of the possible arrangements of the present invention, and that
other
arrangements are possible. Accordingly, all such other arrangements are
contemplated by
the present invention and are within the scope of the present invention. For
example, in
each of the embodiments illustrated in Figures 3-7 and 9, the arrangement of
the low
pressure and high pressure streams may be reversed (i.e., the low pressure
lance may be
the inner lance, and the high pressure lance may be the outer lance).
[0082] In addition to reduced NOx emissions, there are other advantages and
benefits of the
present invention, some of which are discussed below:
^ The proposed fuel staging method enables active tip cooling due to
either (F-F) staging or (F-I) staging. For fuel tips having relatively
large tip exit area, the nozzle tips are actively cooled by exiting high
velocity fuel gas or inert stream. This is a sigriificant improvement
over conventional circular nozzles.
^ Due to relatively poor entrainment efficiency and higher operating
temperature, conventional tips have serious maintenance issues and
soot plugging problems using high C/H fuels. In comparison the
present invention has the following advantages:
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CA 02487146 2004-11-08
- reduced tendency to coke while using higher carbon content
fuels
- ability to use smaller flow rates or higher heating value fuels
- ability to use cheaper fuel nozzle material (Stainless steel 304
or 310 is adequate)
[0083] Thermal cracking is a main concern for many refinery furnaces where
fuel
compositions contain hydrocarbons ranging from Cl to C4. The cracked carbon is
found to
plug burner nozzles and create over heating of burner parts, reduced
productivity and poor
thermal efficiency. Thus, having maintenance free operation (using F-F or F-I
staging) is a
critical advantage for the refinery operator.
[0084] Although illustrated and described herein with reference to certain
specific
embodiments, the present invention is nevertheless not intended to be limited
to the details
shown. Rather, various modifications may be made in the details within the
scope and range
of equivalents of the claims and without departing from the spirit of the
invention.
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