Note: Descriptions are shown in the official language in which they were submitted.
CA 02631898 2008-06-02
WO 2007/063386 PCT/IB2006/003366
1
METHODS AND SYSTEMS FOR REDUCED NOx COMBUSTION OF COAL
WITH INJECTION OF HEATED NITROGEN-CONTAINING GAS
Background
NOX generally refers to nitrogen monoxide NO and nitrogen dioxide NO2.
10. Both are major contributors to acid rain and smog (ground level ozone)
issues.
The NOX partition in the flue gases of pulverized coal boilers is typically
more
than 95% NO and the remainder NO2 (Mitchell S.C., NO, in Pulverized Coal
Combustion, IEA Clean Coal Center Report CCC/05, 1998). During coal-
combustion, the NOX production originates from three different mechanisms:
= Fuel-NOX mechanism,
= Thermal-NOX mechanism, and
= Prompt-NOx mechanism.
In pulverized coal boilers, 70% to 80% of NOx is formed from the fuel-
bound nitrogen species (fuel-N) via the fuel-NOX mechanism, and the remaining
NOX is formed from atmospheric nitrogen (N2), via the thermal-NOx mechanism
(5-25%) and via the prompt-NOx mechanism (less than 5%) (Wu Z., NO, control
for pulverized coal-fired power stations, IEA Clean Coal Center Report CCC/69,
2002). Understanding and limiting the NOX formation in pulverized coal
combustion is therefore strongly related to the fuel-N conversion mechanism. A
complex series of reactions explains the transformation of coal bound fuel-
nitrogen into NOX or N2, including more than 50 intermediate species and
hundreds of reactions.
The two main parameters affecting the fuel-NOx formation process are the
volatile matter content of the fuel and the stoichiometry (air/fuel ratio).
Coal
nitrogen content (bound nitrogen only), also strongly impacts NOX emission
levels. Coal typically contains 0.5% to 3% nitrogen by weight on a dry basis.
For
comparison, natural gas also contains some nitrogen (0.5 to 20%); however it
is
molecular nitrogen N2, and thus is not affected by the fuel-NOX mechanism.
CONFIRMATION COPY
CA 02631898 2008-06-02
WO 2007/063386 PCT/IB2006/003366
2 1
Figure 1 summarizes the main reactions affecting fuel-nitrogen in the
combustion process (Zevenhoven R., Kilpinen P., Control of pollutants in flue
gases and fuel gases, Picaset Oy, Espoo, ISBN 951-22-5527-8, 2001). Four
main steps can be identified:
1- Devolatilization releasing coal nitrogen compounds (coal-N) in a
gaseous phase (Volatile-N), mainly as HCN, some as NH;. The remaining
coal-nitrogen compounds stay in the solid phase (char), and are referred
to as char-N.
2- HCN evolution to NHI species.
10' 3- NH; oxidation to NO or reduction to N2 depending on local conditions.
4- Reburning, as some NO is recirculated back to the hot reducing zone of
the flame and converted back to N2 while contacting CH; radicals
Both volatile-N and char-N can be evolved as NO or as N2. Fuel-NOX
formation is minimized by implementing specific conditions leading to N2
rather
than NO (see Van Der Lans R. P., Glarborg P. and Dam-Johansen K., Influence
of process parameters on nitrogen oxide formation in pulverized coal burners,
Prog. Energy Combust. Sci. Vol. 23, p. 349-377, 1997; Bowman C.T., Kinetics of
Pollutant Formation and Destruction on Combustion, Prog Energy Combust Sci 1
33-45, 1975; and Proceedings of the 6th International Conference on
Technologies and Combustion for a Cleaner Environment, Oporto, Portugal,
2001)
For a given coal and particle size, three main conditions will independently
or
in combination promote fuel-bound nitrogen conversion into molecular nitrogen
N2 rather than NO:
- Fuel rich (reducing) conditions at the burner level: by arranging
fuel-rich "zones" in the furnace during the devolatilization stage, the
nitrogen species in gas phase (volatiles) are more likely to be reduced to
molecular nitrogen (N2) rather than oxidized to NO.
- High temperature in the early stages of combustion increases the
volatiles yield. As volatiles burn close to the burner exit, controlling the
volatile-N (gas) to N2 conversion is much easier than the char-N (solid) to
N2 conversion. High temperature at the burner exit also increases both the
reburning rate of recirculated NO and the conversion rate of volatile-N into
CA 02631898 2008-06-02
WO 2007/063386 PCT/IB2006/003366
3
N2 (see Sarofim A. F., Pohl J. H., Taylor B.R., Strategies for Controlling
Nitrogen Oxide Emissions during Combustion of Nitrogen-bearing fuels,
69th Annual Meeting of the AIChe, Chicago, IL, 1976; and Bose A.C.,
Dannecker K.M. and Wendt J.O.L., Energ. Fuel, Vol. 2, p.301, 1988. )
- Long residence times in the high temperature and reducing zones
in the boiler lead to higher fuel-N to N2 and NO to N2 conversion.
Prior research indicates that oxygen was used alone to decrease the NOx
formation. Due to safety and other concerns, oxygen was injected at a
relatively
low temperature and also in the burner just before the combustion.
Summary
A method is provided and a system for performing the method. A stream
of nitrogen-containing gas is heated and injected into a stream of coal and
conveying gas to produce a stream of mixed nitrogen-containing gas, coal, and
conveying gas. The mixed nitrogen-containing gas, coal, and conveying gas are
combusted with oxygen in a combustion chamber.
Brief Description of the Drawings
For a further understanding of the nature and objects of the present
system and method, reference should be made to the following detailed
description, taken in conjunction with the accompanying drawings, in which
like
elements are given the same or analogous reference numbers and wherein:
Figure 1 is a schematic summarizing the main reactions affecting fuel-
nitrogen in the combustion process;
Figure 2 is a schematic view of the system with oxygen injection upstream
of the burner.
Figure 3 is a schematic view of the system with oxygen injection at the
burner.
Figure 4 is a perspective view of a tubular injection element having
rectangular apertures;
Figure 5A is a schematic of a circular aperture for use in a tubular injection
element;
CA 02631898 2008-06-02
WO 2007/063386 PCT/IB2006/003366
4
Figure 5B is a schematic of a rectangular aperture for use in a tubular
injection element;
Figure 5C is a schematic of a triangular aperture for use in a tubular
injection element;
Figure 5D is a schematic of an elliptical aperture for use in a tubular
injection element;
Figure 6 is a perspective view of a tubular injection element having three
sets of rectangular apertures;
Figure 7 is a perspective view of a tubular injection element having three
sets of decreasingly shorter rectangular apertures;
Figure 8 is a perspective view of a tubular injection element having
rectangular apertures arranged in a staggered pattern;
Figure 9 is a perspective view of a tubular injection element having a
vertically non-uniform distribution of rectangular apertures;
Figure 10 is a perspective view of a tubular injection element having an
aerodynamic pointed tip with rectangular apertures;
Figure 11 is a perspective view of a tubular injection element having an
aerodynamic rounded tip with rectangular apertures;
Figure 12 is a perspective view of a tubular injection element having an
aerodynamic rounded tip with elliptical apertures;
Figure 13 is a perspective view of a tubular injection element having an
aerodynamic pointed tip with elliptical apertures;
Figure 14 is a cross-sectional view of two concentric injections with swirier-
type injection elements;
Figure 15A is a perspective view of two injections with a swirler disposed
on the nitrogen lance and a tangentially injecting injection element disposed
on
an inner wall of the fuel duct wherein the swirl and tangential injections are
generally in the same direction;
Figure 15B is a perspective view of two injections with a swirler disposed
on the nitrogen lance and a tangentially injecting injection element disposed
on
an inner wall of the fuel duct wherein the swirl and tangential injections are
generally in the opposite direction;
Figure 16 is a side elevation view of a swirier showing opening and wall
widths;
CA 02631898 2008-06-02
WO 2007/063386 PCT/IB2006/003366
Figure 17 is a perspective view (with the aerodynamic tip not illustrated) of
four injection elements radially spaced from one another having a leg with at
least
one aperture at an end thereof;
Figure 18 is a side elevation view (with the aerodynamic tip illustrated) of
5 the injection element configuration of Figure 17;
Figure 19 is a front elevation view (with the aerodynamic tip illustrated) of
the injection element configuration of Figure 17;
Figure 20 is a front elevation view of a two-injection element configuration
having a fin configuration;
Figure 21 is a side elevation view of the two-injection element
configuration of Figure 20;
Figure 22A is a side elevation view of an axial injection element with a
vertically oriented, elliptical cross-sectional shape;
Figure 22B is a side elevation view of an axial injection element with a
horizontally oriented, elliptical cross-sectional shape;
Figure 23 is a perspective view of a tubular injection element having three
radially spaced apertures at an end, thereof for injecting oxygen at an angle
to
the axis;
Figure 24A is a side elevation view of a tubular injection element with
apertures configured as circles arranged in a circle with one aperture in the
middle;
Figure 24B is a side elevation view of a tubular injection element with a
saw tooth-shape pattern of apertures at a peripheral portion thereof;
Figure 24C is a side elevation view of a tubular injection element with a
four-wedge type pattern of apertures;
Figure 24D is a side elevation view of a tubular injection element with a
star-shaped aperture;
Figure 24E is a side elevation view of a tubular injection element with a
curved, cross-shaped aperture disposed at a center thereof;
Figure 24F is a side elevation view of a tubular injection element with a
curved, cross-shaped aperture similar to that of Figure 24E but having a
greater
thickness and extending to a peripheral portion thereof; and
CA 02631898 2008-06-02
WO 2007/063386 PCT/IB2006/003366
6
Description of Preferred Embodiments
Increasing the temperature of the coal to safely release volatiles in an
oxygen deficit environment prior to combustion can hinder the NOx formation
mechanism. This is achieved in the proposed method and system by injecting
high temperature nitrogen-containing gas upstream of the burner and optionally
by injecting oxygen-containing gas in the burner or just upstream of the
burner.
Injection of hot nitrogen-containing gas in an 02-deficit environment causes
devolatilization of the coal to release volatiles and can also decompose them
to
N2. The fuel-bound nitrogen in char can also be decomposed to N2. This
process also increases the residence time of the volatiles-N and char-N in the
main combustion zone thereby favoring decomposition to N2. Further NOx
reductions are obtained by the optional injection of oxygen-containing gas in
the
burner or just upstream of the burner to compensate the injected nitrogen and
to
increase the temperature of the flame in fuel rich condition. Both the
processes
promote formation of N2 instead of NOx.
The system for burning coal with reduced NOx emissions includes the
following: a source of a mixture of coal and conveying gas; a source of oxygen-
containing gas; a source of nitrogen-containing gas; a heating device adapted
and configured to heat nitrogen from the nitrogen source; a combustion
chamber;
a burner disposed at a wall of the combustion chamber; a burner operatively
associated with a combustion chamber; a fuel duct in fluid communication with
the source of a mixture of coal and conveying gas, the fuel duct extending
towards the burner; and a nitrogen-containing gas injection element in fluid
communication with the heating device and the fuel duct, the nitrogen
injection
element being adapted and configured to inject heated nitrogen-containing gas
from the heating device into a stream of a mixture of coal and conveying gas
and
mix therewith inside the fuel duct.
A method of combusting coal with reduced NOx emissions includes the
following steps. A stream of nitrogen-containing gas is heated. The heated
stream of nitrogen-containing gas is injected into a stream of coal and
conveying
gas to produce a stream of mixed nitrogen-containing gas, coal, and conveying
gas. The mixed nitrogen-containing gas, coal, and conveying gas are introduced
at a burner disposed at a wall of a combustion chamber. The coal is combusted
with oxygen in the combustion chamber.
CA 02631898 2008-06-02
WO 2007/063386 PCT/IB2006/003366
7
The system or method can include any one or more of the following
aspects:
- an oxygen-containing gas injection element is in fluid communication with
the source of oxygen and the fuel duct, the oxygen-containing gas injection
element fluidly communicating with the fuel duct downstream of where the
nitrogen-containing gas injection element fluidly communicates with the fuel
duct
and upstream of or at the burner, the oxygen-containing gas injection element
being adapted and configured to inject oxygen-containing gas from the oxygen-
containing gas source into the stream of mixed heated nitrogen-containing gas,
coal, and conveying gas
- the source of oxygen-containing gas and the source of nitrogen-
containing gas comprise an Air Separation Unit (ASU)
- the conveying gas comprises flue gas from the combustion chamber
mixed with oxygen-containing gas from the oxygen-containing gas source
- the heating device is adapted and configured to directly impart heat to
nitrogen from the nitrogen-containing gas source from a flame
- the heating device is a heat exchanger adapted and configured to
exchange heat between nitrogen-containing gas from the nitrogen-containing gas
source and heat from combustion of the coal and oxygen in the combustion
chamber
- the conveying gas is flue gas from the combustion chamber mixed with
oxygen
- the heating device is a heat exchanger adapted and configured to
exchange heat between nitrogen-containing gas from the nitrogen-containing gas
source and heat from combustion of the coal and oxygen in the combustion
chamber - the heating device is a heat exchanger adapted and configured to
exchange heat between nitrogen-containing gas from the nitrogen-containing gas
source and heat from combustion of the coal and oxygen in the combustion
chamber
- the conveying gas is air
- the oxygen-containing gas injection element fluidly communicates with
the fuel duct at the burner
CA 02631898 2008-06-02
WO 2007/063386 PCT/IB2006/003366
8
- the nitrogen-containing gas injection element fluidly communicates with
the fuel duct at a peripheral portion of the fuel duct
- the nitrogen-containing gas injection element fluidly communicates with
the fuel duct along a central axis of the fuel duct
- the oxygen-containing gas injection element fluidly communicates with
the fuel duct at a peripheral portion of the fuel duct
- the oxygen-containing gas injection element fluidly communicates with
the fuel duct along a central axis of the fuel duct
- the nitrogen-containing gas and oxygen are obtained from an ASU
- the step of heating a nitrogen-containing gas stream comprises directly
imparting heat from a flame to the nitrogen stream
- the step of heating a nitrogen-containing gas stream comprises indirectly
imparting heat from a flame to the nitrogen stream via a heat exchanger.
- the step of heating a nitrogen-containing gas stream comprises indirectly
imparting heat from the step of combusting to the nitrogen stream via a heat
exchanger.
- the stream of nitrogen-containing gas is heated to a temperature such
that a desired level of devolatilization occurs
- the stream of nitrogen-containing gas is heated to a temperature in the
range of from about 1,000 F to about 1,800 F.
- injection of the heated stream of nitrogen-containing gas causes
devolatilization of most of a volatile species content in the coal.
- collecting at least some of any flue gas produced from the step of
combusting, injecting oxygen-containing gas into the collected flue gas to mix
therewith, and introducing the mixed oxygen and flue gas to the burner
- oxygen-containing gas is injected into the collected flue gas in an amount
such that an oxygen concentration in the mixed oxygen-containing gas and flue
gas is from about 3% to about 20%.
- the step of heating a nitrogen-containing gas stream comprises indirectly
imparting heat from the step of combusting to the nitrogen stream via a heat
exchanger.
The proposed method and system also reduces the fuel-NOx formation in
a coal combustion process. As described in the above section, the fuel bound N
can be transformed into either molecular N (N2) or NO depending on the local
CA 02631898 2008-06-02
WO 2007/063386 PCT/IB2006/003366
9
conditions where the devolatization took place. Injecting hot nitrogen-
containing
gas into the coal stream releases volatiles and fuel-bound N compounds in a
reducing environment. The reducing environment drives the coal derived N
compounds to convert to N2.
The temperature and quantity of nitrogen-containing gas to be injected
depends on the type of coal and the NOx reduction targets. The temperature of
the nitrogen-containing gas is chosen to be above the devolatilization
temperature of the volatile species in the coal. The volatilization
characteristics
of various general types of coals are well known. In the case of a specific
type of
coal, the volatilization characteristics may be determined experimentally in a
known manner. Generally speaking, the temperature should be selected such
that a desired degree of devolatilization occurs for the particular type of
coal
being combusted. A suitable temperature is in the range of from about 1,000 F
to about 1,800 F. The location of nitrogen-containing gas injection should be
strategically placed so that just enough residence time is available for the
devolatilization and the conversion to nitrogen-containing gas to occur.
Injecting
hot nitrogen-containing gas more than this distance can pose safety issues as
volatiles are very flammable and unfavorable combustion could occur.
The nitrogen-containing gas need not be pure nitrogen. Indeed, gaseous
mixtures having a majority of nitrogen with minor amounts of other gases are
suitable for use with the process and system. Such minor constituents include
02
and inert gases such as Ar and CO2. A preferred source for both the nitrogen-
containing gas to be heated and the 02 is from an air separation unit (ASU).
Suitable ASU's include those operated via pressure swing adsorption (PSA),
vacuum swing adsorption (VSA), cryogenic distillation, and membrane
permeation. Typical N2 and 02 concentrations in nitrogen-enriched and oxygen-
enriched streams from these types of ASU's are well known and need not be
repeated here. Other sources of the nitrogen can include a gaseous mixture
comprising nitrogen and flue gas
The oxygen-containing gas to be optionally injected into the mixed
nitrogen-containing gas, conveying gas, and coal also need not be pure.
Suitable
gases include those having an oxygen concentration greater than that of air.
up
to 100% pure oxygen.
CA 02631898 2008-06-02
WO 2007/063386 PCT/IB2006/003366
In the case of N2 from an ASU, the nitrogen-containing gas can be heated
in "direct fired mode" or "indirect fired mode". In a direct fired mode the
incoming
nitrogen-containing gas is heated by direct contact with a small flame. In
indirect
fired mode, the nitrogen-containing gas is heated at a heat exchanger taking
heat
5 from a small flame or from a combustion process.
In fuel-rich flame conditions, injection of oxygen in the main combustion
zone increases the temperature. This higher temperature reducing environment
promotes formation of N2 from the remaining volatiles released from the coal.
If
optional oxygen-containing gas injection is selected, the oxygen-containing
gas is
10 injected at a location which achieves both safety goals and good mixing
with the
stream of coal/conveying gas/nitrogen-containing gas. The location is
desirably
upstream of the burner throat in order to reduce the risk of incurring partial
combustion of coal particles in local pockets that are oxygen-enriched. At the
same time, the location is not so close to the burner that little mixing of
the
oxygen-containing gas and coal/conveying gas/nitrogen-containing gas is
achieved.
The conveying gas comprises any gas to convey fuel particles from a
particle storage or generation location, e.g., mills, to the burner level and
the
combustion chamber. For example, this gas can comprise the primary air used to
convey pulverized or micronized coal in a coal-fired boiler. Preferred
conveying
gases are air and mixtures of recirculated flue gas and oxygen. Typically,
these
mixtures of recirculated flue gas and oxygen include about 60-90% CO2, 5-20%
N2,
and 3-20% 02. An especially preferred mixture of recirculated flue gas and
oxygen
contains about 80% CO2 and about 20% 02.
Several different types of injection elements may be employed. It should
be noted that each of the nitrogen-containing gas and oxygen-containing gas
injection elements may be the same as one another or different. Several
examples of injection elements follow.
A system for performing the method is best illustrated in Figures 2-9. As
best illustrated in Figure 2, in one embodiment a stream of coal and conveying
gas 1 enters fuel duct 8. A heated stream of nitrogen-containing gas from
first
injection element 3 is mixed with the coal and conveying gas downstream of
element 3. Oxygen-containing gas is optionally injected into the mixed coal,
conveying gas, and nitrogen-containing gas by injection element 4 upstream of
CA 02631898 2008-06-02
WO 2007/063386 PCT/IB2006/003366
11
burner 9. The mixed nitrogen-containing gas, oxygen-containing gas (if
optionally
injected), coal, and conveying gas is introduced to combustion chamber 6 via a
burner where combustion 7 takes place.
As best illustrated in Figure 3, in another embodiment a stream of coal and
conveying gas I enters fuel duct 8. A heated stream of nitrogen-containing gas
from first injection element 3 is mixed with the coal and conveying gas
downstream of element 3. Oxygen-containing gases optionally injected into the
mixed coal, conveying gas, and nitrogen-containing gas by injection element 5
at
burner 9. The mixed nitrogen-containing gas, oxygen-containing gas (if
optionally
injected), coal, and conveying gas is introduced to combustion chamber 6 via
burner 9 where combustion 7 takes place.
It should be noted that injection elements 3, 5 need not be disposed
centrally along an axis of the fuel duct 8. Rather, they may be disposed along
a
peripheral portion of the fuel duct 8. Some of these various configurations
are
best illustrated in some of the following injection element designs.
Radially Injecting Injection Elements Designs:
As illustrated in Figure 4, one injection element 10 is a tube having a
closed end 16 and plurality of rectangular apertures 13. This design provides
radial injection from the circumferential face of the injection element 10.
The length, DI, and width, D2, of these apertures, as well as the
circumferential arc distance, Do, between two adjacent apertures may be varied
to control the momentum ratio J (ratio of the oxygen-containing gas or
nitrogen-
containing gas jet momentum to the momentum of the stream of non-gaseous
fuel/conveying gas). DI, D2, and Do also control the penetration of the
injection
gas into the primary stream or primary stream mixed with nitrogen-containing
gas
as appropriate. A small D2/D1 ratio (streamlined rectangular apertures) will
minimize the perturbation to solid fuel particles, such as coal. A big D2/D1
ratio
(bluff-body slots) will have a greater influence on the solid phase and will
push
solid fuel particles, such as pulverized coal, away from the centerline of the
burner primary air duct. Those two different aspect ratios will lead to
different
distribution of particles and nitrogen or oxygen at the duct outlet.
CA 02631898 2008-06-02
WO 2007/063386 PCT/IB2006/003366
12
Those three parameters, Sl, Dl, and D2, in turn, control the penetration of
the injection gas into the primary stream or primary stream mixed with
nitrogen-
containing gas as appropriate. A small D2/D1 ratio (streamlined slots) will
minimize the perturbation to the solid phase. A big D2/D1 ratio (bluff-body
slots)
will have a greater influence on the solid phase and will push the coal
particles
away from the centerline of the burner primary air duct. Those two different
aspect ratios will lead to different distribution of particles and nitrogen or
oxygen
at the duct outlet. As shown in Figures 5A-5D, the slot shape itself could be
circular, rectangular, triangular, or elliptical, respectively.
As depicted in Figure 6, the injection element 20 includes apertures 23
arranged in axially extending rows along the axis of the injection element 20.
This pattern performs a better mixing if the axial distance D3 between two
adjacent apertures 23 in a same row is sufficiently large. The dimension D3
between the apertures 23 could be the same or could vary in the axial
direction
towards the closed end 26.
As best illustrated in Figure 7, the length dimensions DI, D4, and D5 of the
apertures 33 in injection element 30 may vary from short to long going in the
direction of the closed end 36. Alternatively, these length dimensions could
vary
in any order from short to long, long to short, long to short and then back to
long,
short to long and then back to short, and other permutations. In addition, the
dimensions D, or D2 could also vary in the azimuthal (radial) direction. This
offers more precise control over the penetration of the injection gas into the
primary stream. Finally, D3 can be tailored to the conditions of each process
to
optimize mixing and minimal redistributions of particles.
As shown in Figure 8, the apertures 43 in injection element 40 need not
extend in the axial direction. Rather, they may be staggeredly disposed at
different angles O with respect to one another. O can vary from less than 1800
(streamlined slots/axial slots) to 90 (bluff-body slots/radial slots).
As depicted in Figure 9, the injection element 50 need not have a uniform
distribution of apertures 53 in the azimuthal direction. As discussed
previously, in
coal-fired boilers, the coal particle loading is not always uniform throughout
the
cross-section (sometimes due to the so-called "roping phenomenon"). In the
case of coal, the particle concentration in the stream of coal/conveying gas
56 (or
CA 02631898 2008-06-02
WO 2007/063386 PCT/IB2006/003366
13
coal/conveying gas/ nitrogen-containing gas) at the bottom of the injection
element 50 may be higher than the same in the stream of coal/conveying gas (or
coal/conveying gas/ nitrogen-containing gas) 57 at the top of the injection
element 50. In this figure, the thickness of arrows represents the loading of
particles in the gas stream. The advantage offered by this is that more
nitrogen-
containing gas or oxygen-containing gas could be introduced in the locations
where particle loading is higher 58 than locations where particle loading is
lower
59. This will reduce the likelihood of creating local pockets with less
devolatilization potential (in the case of nitrogen-containing gas injection)
or local
pockets that are fuel-lean (in the case of oxygen-containing gas injection)
each of
which could lead to higher levels of NOx. With respect to this problem and
solution, the particle loading distribution could easily be determined by
experimental or modeling studies.
Similar to the injection element designs 10, 20, 30, 40, the apertures 53
may be staggered and vary in size in the axial and azimuthal directions. The
distance between apertures 53, the number of rows of apertures 53, or the
surface area of apertures 53 could also be varied.
This injection element 50 has a particularly beneficial application to coal-
fired boilers whose burner geometry include coal concentrators or splitters
(identified technique in the prior art for reducing NOX emissions from
pulverized
coal burners). Varying levels of nitrogen-containing gas or oxygen-containing
gas injection may be located to achieve higher concentration of N2 or 02 in
coal
richer zones. As a result, the equivalence ratio between coal and N2 (in the
case
of nitrogen-containing gas injection) coal and 02 (in the case of oxygen-
containing gas injection) can be controlled in the coal richer zone
(concentrated
zone) as well as in the coal leaner zones.
Aerodynamic Injection Element Designs:
As depicted in Figures 10-14, the injection element 100, 110, 120, 130,
140 may have an aerodynamic closed end 106, 116, 126, 136, 146. An
aerodynamic shape tends to reduce re-circulation of the stream of coal/
conveying gas (in the case of nitrogen-containing gas injection) or of the
stream
of coal/conveying gas/nitrogen-containing gas (in the case of oxygen-
containing
CA 02631898 2008-06-02
WO 2007/063386 PCT/IB2006/003366
14
gas injection), and creation of a particle deficient and low/reverse velocity
zone in
the wake of the injection element 100, 110, 120, 130, 140.
Referring to the injection element 100 of Figure 10, rectangular apertures
103 could be added to closed end 106 in all the permutations described in
Figures 1-7. The closed end 106 could be pointed, and terminate at point Pi.
The distances D8 and D9 and the angle a defined by lines L, and L2 could be
varied in order to optimize the mixing in a shortest distance and to cause
least
disturbance to the non-gaseous fuel.
Referring to the injection element 110 of Figure 11, rectangular apertures
113 could be added to closed end 116 in all the permutations described in
Figures 1-7. The closed end 116 could be rounded, instead of extending to
point
P2 at the intersection of lines L4 and L5. The distances Djo and Dll, and the
angle
6 defined by lines L4 and L6 could be varied in order to optimize the mixing
in a
shortest distance and to cause least disturbance to the non-gaseous fuel.
As illustrated in Figure 12, elliptical (or circular) apertures 123A, 123B,
123C may be present on injection element 120. The injection element 120
extends to a rounded tip 126. Each of apertures 123A, 123B, and 123C is
configured to inject streams of nitrogen-containing gas or oxygen-containing
gas
PA, PB, Pc into the mixed stream of coal/conveying gas (in the case of
nitrogen-
containing gas injection) or coal/conveying gas/nitrogen-containing gas (in
the
case of oxygen-containing gas injection) at an angle to the axis of the lance.
As shown in Figure 13, elliptical (or circular) apertures 133A, 133B, 133C
may be present on injection element 130. The injection element 130 extends to
a
pointed tip 136. Each of apertures 133A, 133B, and 133C is configured to
inject
a stream of nitrogen-containing gas or oxygen-containing gas Po, PE, PF into
the
mixed stream of coal/conveying gas (in the case of nitrogen-containing gas
injection) or coal/conveying gas/nitrogen-containing gas (in the case of
oxygen-
containing gas injection) at an angle to the axis of the oxygen lance.
Swirl-Type Injection Element Designs:
The designs presented in this section are based upon the patented
Oxynator (US 5,356,213) concept. It is designed to minimize mixing distance
and to prevent high nitrogen or oxygen concentrations near the pipe walls.
CA 02631898 2008-06-02
WO 2007/063386 PCT/IB2006/003366
With respect to the first configuration and as illustrated in Figure 14, the
arrangement of the fuel duct 231 with respect to the conduit 239 defined by
walls
232A, 232B is a tube within a tube. Nitrogen-containing gas is fed to the
central
injection element 235 from oxygen lance 236. It is injected with a swirl S2.
5 Oxygen-containing gasis fed from conduit 239 to the single peripheral
injection
element 234, which is disposed flush with the inner wall of fuel duct 231.
Oxygen-containing gasis injected from the inner wall of fuel duct 231 with a
swirl
S, by injection element 234. The directions of swirls SI, S2 may the same or
different. The flow passage leading to and from the peripheral injection
element
10 234 could be aerodynamically (like a venturi) designed to cause minimum
disturbance to the flow. In other words, shoulders before and after the
injection
element 234 could be used. It should also be understood that fuel duct 238
need
not extend beyond injection element 231A, 231B.
With respect to the second configuration, the conduit 239 may actually be
15 a plurality of conduits surrounding the fuel duct 231, any or all of which
feeds
injection element 234.
As shown in Figure 15A, another Oxynator -based design includes fuel
duct 241 surrounded by a conduit 249 (known by those ordinarily skilled in the
art
as a secondary or transition stream zone) defined by walls 242A, 242B.
Disposed in a central axis of fuel duct 241 is nitrogen-containing gas lance
244 at
the end of which is an injection element 244 (based upon Oxynator . Disposed
along the inner wall of the fuel duct 241 is a plurality of tangentially
injecting
injection elements 245A, 245B, 245C, 245D. In operation, nitrogen-containing
gas fed by lance 244 to injection element 244 is injected into fuel duct 241
with a
swirl S3. Oxygen-containing gasfed by conduit 249 to injection elements 245A,
245B, 245C, 245D is tangentially injected with respect to fuel duct 241 into
fuel
duct 241 with a swirl S4 that is in the same direction as swirl S3.
As shown in Figure 15B, another Oxynator -based design includes fuel
duct 251 surrounded by a conduit 259 (known by those ordinarily skilled in the
art
as a secondary or transition stream zone) defined by walls 252A, 252B.
Disposed in a central axis of fuel duct 251 is nitrogen-containing gas lance
254 at
the end of which is an injection element 254 (based upon Oxynator . Disposed
along the inner wall of the fuel duct 251 is a plurality of tangentially
injecting
injection elements 255A, 255B, 255C, 255D. In operation, nitrogen-containing
CA 02631898 2008-06-02
WO 2007/063386 PCT/IB2006/003366
16
gas fed by lance 254 to injection element 254 is injected into fuel duct 251
with a
swirl S5. Oxygen-containing gas fed by conduit 259 to injection elements 255A,
255B, 255C, 255D is tangentially injected with respect to fuel duct 251 into
fuel
duct 251 with a swirl S6 whose direction is opposite that of swirl S5.
All of the Oxynator -based designs of Figures 14, 15A, and 15B may be
varied as follows. As depicted in Figure 16, injection element Arc 222 along
the
circumferential border of open space 221 between two adjacent vanes 223 has a
dimension A,. On the other hand, the circumferential edge of vane 223 has a
dimension A2. The number of vanes 223 and the dimensions Al, and Al may be
varied in order to optimize the mixing and particle loading. The ratio of
dimensions A,, A2 may be chosen to optimize the injection velocity and thus
the
penetration of the jet. A small ratio A2/A1 is preferred to minimize the
disturbance
to the solid phase.
Bluff Body Injection Element Designs:
Oxygen-containing gas may be injected at several locations at roughly a
single axial position by several different injection elements.
As shown by Figures 17-19, extending from a lance portion 301 is an
injection element comprising a leg member having first and second portions
302A, 303A and at least one aperture 304A at the end of second portion 303A.
Other injection elements similarly comprise a leg member having first and
second
portions (302B, 303B; 302C, 303C, 302D, 303D) and at least one aperture 304B,
304C, 304D at the end of the second portions 303B, 303C, 303D. While not
depicted in Figure 17 for clarity's sake, an aerodynamic tip 306 is included
at the
end of lance portion 301 just after the junction between lance portion 301 and
the
first portions 302A, 302B, 302C, 302D.
As illustrated by Figure 19, each injection element has height and length
dimensions D13, D14. The injection elements inject nitrogen-containing gas or
oxygen-containing gas into the fuel duct at an angle P with respect to an axis
of
the fuel duct and defined by lines Llo, and Ll I. By strategically placing the
injection elements of at various locations, mixing of the oxygen and the
coal/conveying gas is enhanced by controlling the jet momentum. The cumulative
projection area of all these injection elements perpendicular to the flow area
is
much smaller than the flow area of the primary stream. Thus, these injection
CA 02631898 2008-06-02
WO 2007/063386 PCT/IB2006/003366
17
elements do not offer any significant obstruction to the flow of the particle-
laden
stream. In this design, the dimensions D13, and D14, injection angle P, and a
diameter of each aperture could be independently adjusted to precisely control
the nitrogen-containing gas penetration or oxygen-containing gas penetration
and
local mixing.
As depicted in Figures 20-21, the first and second portions are replaced
with shapes that are more streamlined. Extending from a lance portion 401 are
radially spaced fins 402. The side elevation of Figure 19 depicts a plurality
of
apertures 403 on surfaces of at least two fins that face in a direction
perpendicular to that of the flow of the coal/conveying gas. However, this
type of
surface, an opposed surface on the other side of the fin or a surface of the
fin
facing downstream could have apertures 403 to introduce injection gas with
precise control over the jet momentum and local penetration of the injection
gas.
, The lance 402 portion terminates in an aerodynamic body 405 having an
aerodynamic tip 406. Each of the fins 402 is aerodynamically streamlined in
shape. The apertures 403 are configured as circular holes, slots, slits, and
other
shaped openings such as those depicted in Figures 3A-3D.
In all the bluff body designs of Figures 16-21, the shape of any tip at the
end of the lance has an aerodynamic design with or without one or more
openings. The openings on the tip could be of any design previously described
above.
Axially Injecting Injection Element Designs:
Another type of injection element is configured to inject nitrogen-containing
gas or oxygen-containing gas axially into the flow of coal/conveying gas from
a
surface that faces downstream. This surface could have any number of
apertures of any shape. Some exemplary shapes 701A-F are best shown in
Figures 24A-F. The number of apertures, size, shape and angle of injection
could be adjusted in order to optimize mixing and solid fuel loading.
Baffles arranged near the outlet end can facilitate a uniform mixing of
nitrogen-containing gas and/or oxygen-containing gas (the use of baffles is an
improvement over prior art designs as it accomplishes more efficient mixing by
increasing the turbulence at the outlet end). Various baffles number, shape
and
size may be utilized. As the velocity control of the jet outgoing from the
pipe is a
CA 02631898 2008-06-02
WO 2007/063386 PCT/IB2006/003366
18
crucial parameter governing burner aerodynamics, the cross-sectional area of
those baffles will be chosen carefully.
Similar types of axially injecting injection elements have a modified cross-
section. As gravity has an influence on motion of the particles, a vertical
elliptical
cross-section, for example, will cause fewer disturbances to the particle
trajectories and at the same time could provide improved mixing. Modifications
of
the cross-section of the pipe allow decreasing or increasing the velocity of
the
axial nitrogen-containing gas or oxygen-containing gas jet. As best
illustrated in
Figure 22A, nitrogen or oxygen lance 503 terminates in a horizontally oriented
elliptical end 502. Similarly, Figure 22B depicts a vertically oriented
elliptical end
505.
As depicted in Figure 23, another axial injecting-type of injection element
includes member 601 having radially spaced apertures 602A, 602B, 602C on a
downstream surface. Each of apertures 602A, 602B, 602C is configured to inject
flows of nitrogen-containing gas or oxygen-containing gas F4, F5, F6 at an
angle
with respect to an axis of the fuel duct.
Preferred processes and apparatus for practicing the present invention
have been described. It will be understood and readily apparent to the skilled
artisan that many changes and modifications may be made to the above-
described embodiments without departing from the spirit and the scope of the
invention. The foregoing is illustrative only and that other embodiments of
the
method and system may be employed without departing from the true scope of
the invention whose aspects are described in the following claims.
