U.S. patent application number 10/195764 was filed with the patent office on 2003-01-16 for oxygen enhanced low nox combustion.
This patent application is currently assigned to Praxair Technology, Inc.. Invention is credited to Bool, Lawrence E. III, Kobayashi, Hisashi.
Application Number | 20030009932 10/195764 |
Document ID | / |
Family ID | 25048514 |
Filed Date | 2003-01-16 |
United States Patent
Application |
20030009932 |
Kind Code |
A1 |
Kobayashi, Hisashi ; et
al. |
January 16, 2003 |
Oxygen enhanced low NOx combustion
Abstract
Fuel such as coal is combusted in a staged combustion device in
a method comprising feeding into a first combustion stage of said
furnace said fuel and gaseous oxidant containing more than 21 vol.
% oxygen, and preferably 21.8 to 29 vol. % oxygen, at a
stoichiometric ratio below that which, if the stage were operated
with air as the only oxidant, would produce the same amount of NOx,
and combusting said fuel with said gaseous oxidant in said
combustion stage to produce combustion products and unburned
fuel.
Inventors: |
Kobayashi, Hisashi; (Putnam
Valley, NY) ; Bool, Lawrence E. III; (Hopewell
Junction, NY) |
Correspondence
Address: |
PRAXAIR, INC.
LAW DEPARTMENT - M1 557
39 OLD RIDGEBURY ROAD
DANBURY
CT
06810-5113
US
|
Assignee: |
Praxair Technology, Inc.
|
Family ID: |
25048514 |
Appl. No.: |
10/195764 |
Filed: |
July 15, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10195764 |
Jul 15, 2002 |
|
|
|
09757611 |
Jan 11, 2001 |
|
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Current U.S.
Class: |
44/620 |
Current CPC
Class: |
Y02E 20/34 20130101;
F23C 6/045 20130101; F23L 2900/07007 20130101; F23L 7/007 20130101;
F23C 2900/06041 20130101 |
Class at
Publication: |
44/620 |
International
Class: |
C10L 005/00 |
Claims
What is claimed is:
1. A method for combusting hydrocarbon fuel, comprising: feeding
into a first combustion stage said fuel and gaseous oxidant
containing more than 21 vol. % oxygen, at a stoichiometric ratio
below that which, if the stage were operated with air as the only
oxidant, would produce the same amount of NOx, and combusting said
fuel with said gaseous oxidant in said combustion stage to produce
combustion products and unburned fuel.
2. A method according to claim 1 wherein the average oxygen
concentration of the oxidant fed to the first combustion stage is
21.8 vol. % to 29 vol. % oxygen.
3. A method according to claim 1 further comprising heating the
oxidant before it is fed to said first combustion stage.
4. A method according to claim 1 further comprising combusting said
unburned fuel in a second combustion stage with additional gaseous
oxidant comprised such that the average oxygen content of the
oxidant fed to said first and second stages is in the range of
20.9-27.4 vol. % oxygen while removing sufficient heat from the
combustion products and unburned fuel from the first stage to reach
a temperature low enough to minimize additional formation of NOx in
said combustion in said second stage.
5. A method according to claim 1 wherein the stoichiometric ratio
in said first stage is below that which, if the stage were operated
with air as the only oxidant, would produce the same amount of NOx,
but is at least the lower stoichiometric ratio at which the amount
of NOx formed by combustion of said fuel with said oxidant under
otherwise identical conditions is said same amount.
6. A method according to claim 1 wherein said fuel is coal.
7. A method according to claim 6 wherein the average oxygen
concentration of the oxidant fed to the first combustion stage is
21.8 vol. % to 29 vol. % oxygen.
8. A method according to claim 6 further comprising heating the
oxidant before it is fed to said first combustion stage.
9. A method according to claim 6 further comprising combusting said
unburned fuel in a second combustion stage with additional gaseous
oxidant comprised such that the average oxygen content of the
oxidant fed to said first and second stages is in the range of
20.9-27.4 vol. % oxygen while removing sufficient heat from the
combustion products and unburned fuel from the first stage to reach
a temperature low enough to minimize additional formation of NOx in
said combustion in said second stage.
10. A method according to claim 6 wherein the stoichiometric ratio
in said first stage is below that which, if the stage were operated
with air as the only oxidant, would produce the same amount of NOx,
but is at least the lower stoichiometric ratio at which the amount
of NOx formed by combustion of said fuel with said oxidant under
otherwise identical conditions is said same amount.
11. A method for operating a furnace in which a hydrocarbon fuel is
combusted, so as to reduce the amount of NOx formed by the furnace
compared to the amount of NOx formed by combustion of said fuel in
said furnace with air as the only oxidant, comprising: feeding into
a first combustion stage of said furnace said fuel and gaseous
oxidant containing more than 21 vol. % oxygen, at a stoichiometric
ratio below that which, if the stage were operated with air as the
only oxidant, would produce the same amount of NOx, and combusting
said fuel with said gaseous oxidant in said combustion stage to
produce combustion products and unburned fuel.
12. A method according to claim 11 wherein the average oxygen
concentration of the oxidant fed to the first combustion stage is
21.8 vol. % to 29 vol. % oxygen.
13. A method according to claim 11 further comprising heating the
oxidant before it is fed to said first combustion stage.
14. A method according to claim 11 further comprising combusting
said unburned fuel in a second combustion stage with additional
gaseous oxidant comprised such that the average oxygen content of
the oxidant fed to said first and second stages is in the range of
20.9-27.4 vol. % oxygen while removing sufficient heat from the
combustion products and unburned fuel from the first stage to reach
a temperature low enough to minimize additional formation of NOx in
said combustion in said second stage.
15. A method according to claim 11 wherein the stoichiometric ratio
in said first stage is below that which, if the stage were operated
with air as the only oxidant, would produce the same amount of NOx,
but is at least the lower stoichiometric ratio at which the amount
of NOx formed by combustion of said fuel with said oxidant under
otherwise identical conditions is said same amount.
16. A method according to claim 11 wherein said fuel is coal.
17. A method according to claim 16 wherein the average oxygen
concentration of the oxidant fed to the first combustion stage is
21.8 vol. % to 29 vol. % oxygen.
18. A method according to claim 16 further comprising heating the
oxidant before it is fed to said first combustion stage.
19. A method according to claim 16 further comprising combusting
said unburned fuel in a second combustion stage with additional
gaseous oxidant comprised such that the average oxygen content of
the oxidant fed to said first and second stages is in the range of
20.9-27.4 vol. % oxygen while removing sufficient heat from the
combustion products and unburned fuel from the first stage to reach
a temperature low enough to minimize additional formation of NOx in
said combustion in said second stage.
20. A method according to claim 16 wherein the stoichiometric ratio
in said first stage is below that which, if the stage were operated
with air as the only oxidant, would produce the same amount of NOx,
but is at least the lower stoichiometric ratio at which the amount
of NOx formed by combustion of said fuel with said oxidant under
otherwise identical conditions is said same amount.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to combustion of hydrocarbon
fuels, particularly of coal.
BACKGROUND OF THE INVENTION
[0002] Environmental awareness is growing in the U.S. and around
the world leading to increasing public and regulatory pressures to
reduce pollutant emissions from boilers, incinerators, and
furnaces. One pollutant of particular concern is NOx (by which is
meant individual oxides of nitrogen such as but not limited to NO,
NO.sub.2, N.sub.2O, N.sub.2O.sub.4, and mixtures thereof), which
has been implicated in acid rain, ground level ozone, and fine
particulate formation.
[0003] A number of technologies are available to reduce NOx
emissions. These technologies can be divided into two major
classes, primary and secondary. Primary technologies minimize or
prevent NOx formation in the combustion zone by controlling the
combustion process. Secondary technologies use chemicals to reduce
NOx formed in the combustion zone to molecular nitrogen. The
current invention is a primary control technology.
[0004] In primary control technologies, commonly called staged
combustion, mixing between the combustion air and fuel is carefully
controlled to minimize NOx formation. The formation of NOx from
fuel nitrogen is based on a competition between the formation of
NOx and the formation of N.sub.2 from the nitrogenous species in
the fuel volatiles and char nitrogen. Oxygen rich conditions drive
the competition towards NOx formation. Fuel rich conditions drive
the reactions to form N.sub.2. Primary control strategies take
advantage of this phenomenon by carefully controlling the mixing of
air and fuel to form a fuel rich region to prevent NOx formation.
To reduce NOx emissions, the fuel rich region must be hot enough to
drive the NOx reduction kinetics. However, sufficient heat has to
be transferred from the fuel rich first stage to the furnace heat
load in order to prevent thermal NOx formation in the second
stage.
[0005] By far the most common type of primary control device is the
low NOx burner (LNB). In this device the air is typically
aerodynamically staged to form a fuel rich zone followed by a
burnout zone. A conventional low NOx burner includes a first zone,
near the feed orifice, which is controlled by primary air and fuel,
and which is very fuel rich. In a second zone, the remainder of the
secondary air and any tertiary air then allow the fuel nitrogen to
continue to be chemically processed to form N.sub.2 provided that
the local stoichiometrics are rigidly controlled. In this region
the hydrocarbons and the char are burned out. Although the LNB is a
fairly inexpensive way to reduce NOx, currently available versions
are not yet capable to reach the emissions limits in pending
regulations. Other issues are increased carbon in the ash and
reduced flame stability.
[0006] Low NOx burners represent a fairly mature technology and as
such are discussed widely throughout the patent and archival
literature. Many ideas have been proposed to enhance the
effectiveness of LNB's while minimizing detrimental impacts such as
poor flame stability and increased carbon in the ash. Of these
ideas two are particularly relevant: preheating the air to the
first stage, and converting the combustor to oxy-fuel firing.
[0007] Both air preheat and oxy-fuel combustion enhance the
effectiveness of staged combustion by increasing the temperature in
the primary zone without increasing the stoichiometric ratio.
Oxy-fuel combustion offers the additional advantage of longer
residence times in the fuel rich region, due to lower gas flows,
which has been shown to reduce NOx emissions. As discussed above,
staged combustion uses a fuel rich stage to promote the formation
of N.sub.2 rather than NOx. Since the reactions to form N.sub.2 are
kinetically controlled, both the temperature and the hydrocarbon
radical concentration are critical to reducing NOx formation. For
example, if the temperature is high and the radical concentration
is low, such as under unstaged or mildly staged conditions, NOx
formation is increased. When the radical concentration is high but
the temperature is low, such as under deeply staged conditions, the
conversion of intermediate species such as HCN to N.sub.2 is
retarded. When air is added to complete burnout, the intermediates
oxidize to form NOx, therefore the net NOx formation is increased.
Sarofim at al. "Strategies for Controlling Nitrogen Oxide Emissions
During Combustion of Nitrogen bearing fuels", 69.sup.th Annual
Meeting of the AIChE, Chicago, Ill., November 1976, and others have
suggested that the first stage kinetics can be enhanced by
preheating the combustion air to fairly high temperatures.
Alternately Kobayashi et al. ("NOx Emission Characteristics of
Industrial Burners and Control Methods Under Oxygen Enriched
Combustion Conditions", International Flame Research Foundation
9.sup.th Members' Conference, Noordwijkerhout, May 1989), suggested
that using oxygen in place of air for combustion would also
increase the kinetics. In both cases the net result is that the gas
temperature in the first stage is increased while the radical
concentration stays the same, resulting in reduced NOx formation.
Further, using both air preheat and oxy-fuel firing allows the
first stage to be more deeply staged without degrading the flame
stability. This allows even further reductions in NOx
formation.
[0008] Oxy-fuel firing offers a further advantage for LNB's.
Timothy et al ("Characteristics of Single Particle Coal
Combustion", 19.sup.th Symposium (international) on Combustion, The
Combustion Institute, 1983) showed that devolatilization times are
significantly reduced, and the volatile yield is increased, when
coal is burned in oxygen enriched conditions. These tests were
single particle combustion tests performed under highly fuel lean
conditions, which does not provide information on how much oxygen
is needed to accomplish this under more realistic combustion
conditions. The higher volatile yield means that the combustibles
in the gas phase increase as compared to the baseline--leading to a
more fuel rich gas phase which inhibits NOx formation from the
volatile nitrogen species. In addition, the fuel volatiles ignite
rapidly and anchor the flame to the burner, which has been shown to
lower NOx formation. The enhanced volatile yield also leads to
shorter burnout times since less char is remaining.
[0009] Although the prior art describes several elegant
enhancements for staged combustion and LNB's, several practical
problems have limited their application. First, preheating the
combustion air to the levels required to enhance the kinetics
requires several modifications to both the system and the air
piping. The air heater and economizer sections must be modified to
allow the incoming air to be heated to higher temperatures, which
may require modifications to the rest of the steam cycle
components. The ductwork and windbox, as well as the burner itself,
must also be modified to handle the hot air. All of the
modifications can be costly and can have a negative impact on the
operation of the boiler.
[0010] The primary barrier to the use of oxy-fuel firing in boilers
has been the cost of oxygen. In order for the use of oxygen to be
economic the fuel savings achieved by increasing the process
efficiency must be greater than the cost of the supplied oxygen.
For high temperature operations, such as furnaces without
significant heat recovery, this is easily achieved. However, for
more efficient operations, such as boilers, the fuel savings
attainable by using oxy-fuel firing is typically much lower than
the cost of oxygen. For example, if a typical coal-fired utility
boiler were converted from air firing to oxygen firing,
approximately 15 to 20% of the power output from that boiler would
be required to produce the necessary oxygen. Clearly, this is
uneconomic for most boilers.
[0011] Thus there remains a need for a method for achieving reduced
NOx emissions in combustion of fuel (particularly coal) containing
one or more nitrogenous compounds and especially for a method which
can be carried out in existing furnaces without requiring extensive
structural modifications.
BRIEF SUMMARY OF THE INVENTION
[0012] An aspect of the present invention is a method for
combusting fuel which contains one or more nitrogenous compounds,
comprising:
[0013] feeding into a first combustion stage said fuel and gaseous
oxidant containing more than 21 vol. % oxygen, and preferably 21.8
to 29 vol. % oxygen, at a stoichiometric ratio below that which, if
the stage were operated with air as the only oxidant, would produce
the same amount of NOx, and combusting said fuel in said combustion
stage to produce combustion products and unburned fuel.
[0014] Preferably, said unburned fuel is combusted in a second
combustion stage using additional oxidant stream(s) comprised such
that the average oxygen content of all oxidant streams, including
those fed to the first stage, is in the range of 20.9-27.4 vol. %
oxygen while removing sufficient heat from the combustion products
and unburned fuel from the first stage, such as through heat
exchange with steam producing tubes, to reach a temperature low
enough to minimize additional formation of NOx in said combustion
in said second stage.
[0015] Another aspect of the invention is that it enables ready
adaptation ("retrofitting") of existing furnaces, in which a
hydrocarbon fuel is combusted with air as the only oxidant, to
reduce the amount of NOx formed by the furnace. This embodiment is
a method for operating a furnace in which a hydrocarbon fuel is
combusted, so as to reduce the amount of NOx formed by the furnace
compared to the amount of NOx formed by combustion of said fuel in
said furnace with air as the only oxidant, the method
comprising:
[0016] feeding into a first combustion stage of said furnace said
fuel and gaseous oxidant containing more than 21 vol. % oxygen, at
a stoichiometric ratio below that which, if the stage were operated
with air as the only oxidant, would produce the same amount of NOx,
and combusting said fuel with said gaseous oxidant in said
combustion stage to produce combustion products and unburned
fuel.
[0017] In either of the foregoing embodiments, the oxygen can be
fed as either a single stream of pure oxygen or of substantially
oxygen-enriched air, or as a plurality of streams of pure oxygen
and/or substantially oxygen-enriched air.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] The FIGURE is a graph of NOx formation plotted against the
stoichiometric ratio in the first stage of a staged furnace.
DETAILED DESCRIPTION OF THE INVENTION
[0019] The current invention overcomes the aforementioned hurdles
while enhancing the effectiveness of staged combustion. It is also
useful in single stage burners. This invention is applicable to
combustion of hydrocarbon fuels such as coal, fuel oil (including
heavy oil), and bitumen. Such fuels generally contain a minor
amount of naturally occurring nitrogenous hydrocarbon compounds,
typically heterocyclics.
[0020] In the following description, it should be understood that
the oxygen content of the oxidant fed to a stage of a combustion
device represents the overall average oxygen content taking the
stage as a whole, even though within the stage the oxygen content
can vary at different given points.
[0021] The current invention takes advantage of the discovery that
within certain ranges and ratios of oxygen and fuel, using a
surprisingly small amount of oxygen leads to a significant
reduction of the formation of NOx, thus eliminating the need for
extensive boiler modifications or the cost of pure oxy-fuel firing
as modes of reducing NOx formation.
[0022] More specifically, it has been determined that, as expected
from the relevant teachings of the prior art, at stoichiometric
ratios conventionally observed for the first stage of staged
combustion in air, raising the oxygen content of the air increases
the formation of NOx. As used herein, "stoichiometric ratio" is the
ratio of oxygen fed, to the total amount of oxygen that would be
necessary to convert fully all carbon, sulfur and hydrogen present
in the substances comprising the feed to carbon dioxide and sulfur
dioxide, and water.
[0023] However, and quite surprisingly, it has been discovered that
there are lower stoichiometric ratios having the property that
combustion at such lower stoichiometric ratios accompanied by a
relatively slight increase in the overall oxygen content of the
oxidant gas results in a significant decrease in the formation of
NOx.
[0024] At a certain point representing a certain value of the
stoichiometric ratio for a given set of combustion conditions, and
for a given overall oxygen content, somewhat higher than that of
air, in the oxidant gas, the NOx formation as expressed in mass per
unit fuel input will be the same whether combustion is carried out
in air or in that oxidant gas. This point will be referred to
herein as the "inflection point"; this term is chosen to help
promote understanding of the description herein of the invention
and no additional implication should be attached to the particular
word "inflection". The particular value of the stoichiometric ratio
at the inflection point can be expected to vary from case to case
depending e.g. on the fuel composition and on the overall oxygen
content of the oxidant. The present invention carries out
combustion in the first stage (or in the fuel rich portion of a
staged combustor) at stoichiometric ratios below the stoichiometric
ratio at that point.
[0025] As an example, the impact of oxygen addition on NOx
formation is shown schematically in the FIGURE. This FIGURE,
derived through the use of chemical kinetics calculations where the
volume and heat removal from the primary zone were kept constant,
shows two curves that depict the NOx formation as a function of the
first stage stoichiometric ratio when the oxidant was air, and when
10% of the oxygen required for complete combustion of the fuel was
supplied by pure oxygen and this oxygen was fed into the first
(fuel rich) stage. The fuel used for these calculations was a
typical bituminous coal with a 34% volatile matter content. The
FIGURE shows a point "A" at which the two curves intersect, which
is the point at which NOx formation is the same when combustion is
carried out in air or in gaseous oxidant formed wherein some
portion (in this example 10%) of the oxygen required for complete
combustion of the fuel is supplied by pure oxygen, and the balance
supplied by air. Point "A" is the "inflection point" as defined
hereinabove. At point "A" in this example, the stoichiometric ratio
is about 0.585. For this example it was assumed that approximately
52 wt. % of the coal was in the vapor phase and participating in
the reactions. Thus although the overall stoichiometric ratio is
much less than 1, the gas phase may only be slightly fuel rich at
this inflection point.
[0026] When the gas phase becomes fuel lean, in this example at a
primary stage stoichiometric ratio greater than about 0.585, the
effect of adding oxygen is to significantly increase NOx formation.
However, it has now been discovered that there are lower
stoichiometric ratios (below about 0.585 in this example, being the
stoichiometric ratio at the point at which the two curves
intersect), at which the effect of modestly increasing the overall
oxygen content of the oxidant in the first stage (e.g. by addition
of relatively modest amounts of pure or substantially enriched
oxygen) is to dramatically decrease NOx formation. The present
invention carries out combustion in the first stage (or in the fuel
rich portion of a staged combustor) at stoichiometric ratios below
the stoichiometric ratio at that point.
[0027] The preferred mode for practicing this invention is based on
a combination of the requirements to facilitate staged combustion
and materials or economic limitations. As noted, a principal
objective of this invention is reduction of NOx formation, but
another objective is of course remaining able to initiate and
maintain combustion. If the stoichiometric ratio is too low, e.g.
below approximately 0.4 in the example represented in the FIGURE,
ignition and combustion in the first stage will be difficult. This
lower bound is strongly dependent on the fuel characteristics, such
as the amount of volatiles released in the first stage, and the
oxidant characteristics. In the previously discussed example, the
optimal range was approximately stoichiometric ratio of 0.4-0.585
based on the whole coal. This corresponds to a range of 0.575-0.85
based on the assumed fuel in the gas phase. As another example,
feeding a significantly preheated stream of pure, or substantially
enriched, oxygen will allow combustion at much lower stoichiometric
ratios than an equivalent oxygen stream at lower temperatures.
However, in any case it is noted that in the region where the
stoichiometric ratio is below some critical value, about 0.4 in the
previously discussed example, the NOx formation exceeds that which
is achieved even without the oxygen addition and stoichiometric
ratio control in accordance with the present invention.
[0028] In view of this, to be surer of achieving reduced NOx
formation under a given set of combustion conditions (including a
given oxidant stream(s) having an overall oxygen content somewhat
greater than that of air), it is preferred to operate at a
stoichiometric ratio which is below the stoichiometric ratio at the
aforementioned inflection point at which combustion in air or in
the given oxidant gas produce the same amount of NOx, but at least
the lower stoichiometric ratio at which the NOx formation (obtained
with the given oxidant stream(s)) has again risen and reached the
value of the NOx formation at the aforementioned inflection
point.
[0029] In other words, referring to the FIGURE, at point "A" (i.e.
at the inflection point) the NOx formation is the same for
combustion in air or in a gaseous oxidant wherein 10% of the oxygen
required for combustion is supplied by pure oxygen and the balance
from air, and at point "B" the NOx formation is the same as the NOx
formation at point "A"; and it is preferred to carry out combustion
at a stoichiometric ratio which is below the stoichiometric ratio
at point "A" and at least the stoichiometric ratio at point
"B".
[0030] The optimal stoichiometric ratios for operation depend
strongly on the fuel characteristics, type of combustion device,
fraction of the oxygen required for combustion that is supplied by
pure, or substantially enriched oxygen, and average oxidant
temperature. Several methods are available to determine the optimal
operating regime. These include kinetic calculations as illustrated
above, which give rise to important information on the kinetic
limitations. These calculations must pay careful attention to the
amount of fuel in the vapor phase under the fuel rich conditions to
adequately describe fuel to oxygen ratio, and therefore NOx
formation, in the vapor phase. Computational fluid dynamic (CFD)
calculations can be used to take into account the impact of
aerodynamic staging in a combustion device where some portion of
the combustion air has been replaced with oxygen. Finally,
experimentation can be used to verify the modeling results before
installation of the device.
[0031] As can be seen, the effects discovered and described herein
are based on increases in the overall oxygen content of the
oxidant. The increases can be provided by literally replacing air
with oxygen, or by other means such as adding oxygen-enriched air,
replacing air with oxygen-enriched air, adding pure or nearly pure
oxygen, or replacing air with pure or nearly pure oxygen. For
convenience herein the increased overall oxygen content of the
oxidant is most often referred to in terms of replacement of air
with pure oxygen, meaning an oxidant that is the equivalent of air
having been replaced in part with pure oxygen so as to maintain the
same amount of oxygen. Given that air is understood to comprise
about 20.9 vol. % oxygen, replacement of various given percentages
of the air with oxygen produces oxidant with a higher overall
oxygen content in accordance with the following table:
1 Replacement of this produces oxidant having vol. % of air with
oxygen: this vol. % of oxygen: 0 20.9 5 21.8 10 22.7 15 23.7 20
24.8 25 26.1 30 27.4 35 28.9 40 30.6
[0032] The practical overall oxygen content of the oxidant gas,
whether effected by replacement of air with pure oxygen or
otherwise, is based on lower limits where oxygen will not have
enough impact to warrant its use and upper limits where cost is
prohibitive or maintaining the boiler or furnace balance will be
problematic. While pure oxygen, or substantially oxygen enriched
oxidant streams, can be used to supply 25% or more, or even 30% or
more, of the stoichiometric oxygen requirement for combustion,
calculations based on the current cost of oxygen and the kinetics
of NOx control suggest as an optimal range using gaseous oxidant
containing 21.8 vol. % to 24.8 vol. % oxygen, i.e. corresponding to
replacing between 5-20% of the total combustion air with oxygen (or
any of the values between 5% and 20% such as appear in the
foregoing table). When all of the oxygen is used in the first stage
combustion zone and no oxygen is used in the second stage
combustion zone, the optimum range of replacing the first stage
combustion air with oxygen becomes much higher than the above
range, which depends on the stoichiometric ratio of the first stage
combustion zone.
[0033] Combustion air and combustion oxygen and oxygen-enriched air
can be supplied as one stream or as more than one stream. The
optimal method for delivering the oxygen, or substantially enriched
oxidant stream, is based on maximizing NOx reduction and minimizing
retrofit and system complexity. Consistent with these objectives,
oxygen can be delivered to the first combustion zone by feeding it
through a lance extending through the burner into that stage, or by
feeding it through the walls adjacent to the burner. This method
provides the highest effect of the increased oxygen concentration
in the first combustion zone and allows a simple lance
configuration to be installed. In addition, with this method the
local oxygen concentration can be as high as the oxygen purity used
for the process, which will enhance devolatilization of the fuel
particles or droplets even further and help anchor the flame. This
method would also allow preheated oxygen to be injected without the
concern of premature ignition or softening of the fuel.
[0034] Other aspects of the practice of the present invention can
be carried out in conventional manner which is familiar and readily
ascertainable to those of ordinary skill in this art. Coal to be
combusted is first pulverized to a fine particle size permitting it
to be fed under gaseous pressure, through the feed orifice of a
burner head for such purpose, into a furnace or like combustion
device. Burner heads, techniques for feeding the pulverized coal,
and furnaces and other combustion devices useful for combusting
coal, suitable for use in this invention, are conventional. The
stoichiometric ratio, and the oxygen content of the gaseous oxidant
fed to the combustion zones, are adjusted by control means familiar
to those with experience in this field. Combustion of fuel in
accordance with the present invention is useful for recovering heat
for power generation or for heating purposes.
[0035] A simple way to practice this invention is to inject oxygen
into the windbox of a low NOx burner to provide oxidizer gas having
the desired increased oxygen content, so that the resulting oxidant
gas is fed to the entire furnace, including to the first combustion
stage and making the first combustion stage more fuel rich by
adjusting the air or fuel flow to the first stage. This would be a
useful approach for staged combustion where the entire low nitrogen
burner is operated fuel rich and overfire air is added further
downstream in the boiler to complete fuel burnout. Another approach
is to feed the majority of the oxygen to the primary, or fuel rich,
stage to enhance the reactions forming N.sub.2. The remaining
oxygen is fed to either subsequent stages of the low nitrogen
burner, or to overfire air, to promote burnout. The most preferred
configuration is to feed all the oxygen to the first combustion
stage through a lance and to reduce the flow rate of the first
stage combustion air by an appropriate amount.
[0036] Oxygen enrichment can be achieved in a number of ways. One
is to simply install a sparger in the boiler windbox so that the
desired amount of oxygen mixes with all the combustion air before
it enters the burner. Although this approach is the simplest, the
NOx reduction efficiency will be reduced as compared to direct
injection into the first combustion zone. Another method to deliver
the somewhat oxygen enriched air to the burner is to pipe a
premixed (air-oxygen) mixture directly into the first combustion
zone. Although this would lead to better NOx reduction than simple
mixing in the windbox, the additional piping and windbox
modifications required may be less attractive than the optimal
case.
[0037] The degree of oxygen enrichment can also be varied according
to site-specific requirements. While it has been determined that
increasing the oxygen replacement above 15% of the stoichiometric
oxygen further enhances NOx reduction, the current cost of oxygen
may make replacement of more than 40% of the air uneconomic
compared to other methods of NOx control. Further, when the
invention is used in retrofits to existing boilers and furnaces, or
installed in new furnaces with conventional designs, there is an
upper limit to the amount of oxygen that can be provided in place
of air before boiler balance is detrimentally impacted. This limit
is fuel and site specific, but is commonly 20 to 30% (corresponding
to overall oxygen contents of 24.8% to 27.4% based on the mixture
of the total combustion air and oxygen).
[0038] Another useful aspect of the present invention is to preheat
the incoming oxygen, or substantially enriched oxidant. The
preheated oxidant, heated to a temperature of up to 1800.degree. F.
or even to a temperature of up to 3000.degree. F., will accelerate
ignition of the fuel, enhance combustion in this zone, and increase
volatile yield. Material issues for process piping will limit the
upper temperature.
[0039] The invention can also be used to reduce boiler NOx by
selectively enriching just those burners that have been shown to
produce most of the NOx and unburned carbon in a given boiler.
[0040] The invention can also be used to regain boiler capacity
that has been lost due to boiler balancing problems, such as when a
boiler has switched from one fuel to a lower heating value fuel.
For example, when a boiler switches from a bituminous coal to a
subbituminous coal, the higher flue gas volume associated with a
subbituminous coal typically causes problems with too much heat
passing through the radiant section and being absorbed in the
convective section. This often results in a derate of the boiler.
However, when as little as 5% of the total combustion air is
replaced with oxygen as part of the invention the flue gas volume
becomes the same as that fired with a bituminous coal, thereby
regaining lost boiler capacity.
[0041] When the present invention is carried out as the first stage
of a staged combustion device having a second stage, the combustion
products from the first stage (including unburned fuel, and flue
gas) proceed to a second combustion stage. Additional air or oxygen
is fed to this stage, and unburned fuel from the first stage is
combusted. The combustion in this stage should be carried out so as
to suppress NOx formation, and preferably to minimize NOx
formation. Preferably, sufficient air or oxygen should be provided
to achieve combustion of the unburned fuel to the maximum possible
extent consistent with suppressed or minimized formation of NOx in
this stage.
[0042] Another advantage of this invention is that combustion under
the conditions described herein in the first combustion stage of a
staged combustion device (or in the fuel rich region of a staged
combustor) provides increased devolatilization of volatile matter
from the fuel, so that the amount of char resulting under these
conditions is expected to be dramatically lower, resulting in much
better burnout than in conventional staged devices.
[0043] Yet another advantage of the present invention is that the
flame in the first (or single) combustion stage is better attached
to the burner orifice. This feature is advantageous because it
corresponds to reduced NOx formation compared to situations in
which the flame is detached from the burner, i.e. in which the base
of the flame is some distance from the burner orifice. Furthermore,
the replacement of a portion of combustion air with oxygen and more
fuel rich operation of the first combustion stage result in a
longer residence time in this stage which facilitates further
reduction of NOx formation.
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