U.S. patent application number 10/391825 was filed with the patent office on 2004-09-23 for mixing process for combustion furnaces.
Invention is credited to Moberg, Goran.
Application Number | 20040185401 10/391825 |
Document ID | / |
Family ID | 32987767 |
Filed Date | 2004-09-23 |
United States Patent
Application |
20040185401 |
Kind Code |
A1 |
Moberg, Goran |
September 23, 2004 |
Mixing process for combustion furnaces
Abstract
A system and method for increasing combustion furnace
efficiency, including the steps of providing a furnace with a
plurality of secondary air injection ducts, asymmetrically
positioned in an tangentially reinforcing manner; injecting fuel
with substoichiometric primary air through the burners; injecting
secondary air through the plurality of secondary air injection
ducts; wherein the velocity of the injected air is such that the
ratio of the advected air velocity to the furnace width is between
about 2 sec.sup.-1 to about 150 sec.sup.-1; thereby increasing
combustion efficiency and reactor efficiency via mixing and
rotation of the combustion space.
Inventors: |
Moberg, Goran; (Cary,
NC) |
Correspondence
Address: |
JINAN GLASGOW
P O BOX 28539
RALEIGH
NC
276118539
|
Family ID: |
32987767 |
Appl. No.: |
10/391825 |
Filed: |
March 19, 2003 |
Current U.S.
Class: |
431/9 ; 431/10;
431/173 |
Current CPC
Class: |
F23L 9/02 20130101; F23C
5/32 20130101; F23C 2201/101 20130101; F23C 6/045 20130101; F23J
7/00 20130101 |
Class at
Publication: |
431/009 ;
431/010; 431/173 |
International
Class: |
F23C 005/32 |
Claims
We claim:
1. A method for increasing combustion furnace efficiency,
comprising: providing a furnace with a plurality of secondary air
injection ducts, asymmetrically positioned in an opposing manner;
injecting fuel with primary air through the burners prior to
injection of secondary air; injecting secondary air through the
plurality of secondary air injection ducts; wherein the velocity of
the injected air is such that the ratio of the velocity to the
furnace width is between about 2 sec.sup.-1 to about 150
sec.sup.-1; thereby increasing combustion efficiency and furnace
efficiency via swirl, peripheral turbulence, and rotation-induce
turbulence of the combustion space.
2. The method of claim 1, wherein the temperature of the injected
air is between about 40 and about 460 degrees centigrade (10-50%
redirected combustion air).
3. The method of claim 2, wherein the temperature of the injected
air is between about 76 and about 340 degrees centigrade (20-40%
redirected combustion air).
4. The method of claim 1, wherein the system has at least two
levels of secondary air ducts for injection of secondary air.
5. The method of claim 4, wherein the system has at least three
levels of secondary air ducts for injection of secondary air.
6. The method of claim 1, wherein the velocity of the injected air
is such that the ratio of the velocity to the furnace width is
between about 3 sec.sup.-1 to about 60 sec.sup.-1.
7. A method for increasing combustion furnace efficiency,
comprising: providing a furnace with a plurality of secondary air
injection ducts, asymmetrically positioned in an opposing manner;
injecting fuel with primary air through the burners prior to
injection secondary air; injecting secondary air through the
plurality of secondary air injection ducts; wherein the velocity of
the injected air is such that the penetration of the injected air
is greater than the furnace width by at least about 1.5 widths;
thereby increasing furnace efficiency via mixing and rotation of
the combustion space.
8. The method of claim 7, wherein the temperature of the injected
air is between about 40 and about 460 degrees centigrade (10-50%
redirected combustion air).
9. The method of claim 8, wherein the temperature of the injected
air is between about 76 and about 340 degrees centigrade (20-40%
redirected combustion air).
10. The method of claim 7, wherein the system has at least two
levels of secondary air ducts for injection of secondary air.
11. The method of claim 10, wherein the system has at least three
levels of secondary air ducts for injection of secondary air.
12. A method for increasing combustion furnace efficiency,
comprising: providing a furnace with a plurality of secondary air
injection ducts, asymmetrically positioned in an opposing manner;
injecting fuel with primary air through the burners prior to
injection secondary air; injecting secondary air through the
plurality of secondary air injection ducts; wherein the velocity of
the injected air is such that the combustion zone rotates at least
one half revolution prior to exiting the furnace; thereby
increasing furnace efficiency via mixing and rotation of the
combustion space.
13. The method of claim 12, wherein the temperature of the injected
air is between about 40 and about 460 degrees centigrade (10-50%
redirected combustion air).
14. The method of claim 13, wherein the temperature of the injected
air is between about 76 and about 340 degrees centigrade (20-40%
redirected combustion air).
15. The method of claim 12, wherein the system has at least two
levels of secondary air ducts for injection of secondary air.
16. The method of claim 15, wherein the system has at least three
levels of secondary air ducts for injection of secondary air.
Description
BACKGROUND OF THE INVENTION
[0001] (1) Field of the Invention
[0002] The present invention relates generally to a system and
method for improving the efficiency of combustion reactions, and,
more particularly, to a system and method for improving the
efficiency of furnaces.
[0003] (2) Description of the Prior Art
[0004] Increases in fuel costs have required power generation
plants seek increases in furnace efficiencies in order to reduce
power generation costs. However, NOx formation must also be
prevented to comply with environmental regulations. NOx formation
is reduced in furnaces by the process of stage combustion, which
includes administering an initial substoichiometric or suboptimal
ratio of oxygen to fuel to maintain combustion gas temperatures
below the peak NOx-producing temperature, about 2,800 degrees F.
(approximately 1540 degrees C.), followed by the addition of
secondary air, or over-fire-air (OFA), to finish the combustion
reaction. Proper mixing of secondary air and combustion gases
inside a furnace is thus important to achieve optimum combustion
and has been improved by the use of rotating over-fire-air (ROFA).
However, these existing NOx reduction systems do not optimize
combustion efficiency or furnace heat exchange efficiency.
[0005] Therefore, a need exists to improve furnace energy
efficiency of ROFA systems without affecting NOx reduction.
SUMMARY
[0006] The present invention is directed to a mixing process and
system for increased combustion efficiency.
[0007] The present invention is further directed to a system and
method for increased furnace efficiency through increased retention
time in the furnace. In a preferred embodiment, the process employs
systems and methods to improve the reaction homogeneity and
combustion zone swirling, resulting in combustion efficiency gains
and thermal flux gains with corresponding gains in reactor
efficiency.
[0008] The present invention is directed toward increasing furnace
energy efficiency via increased combustion efficiency and increased
furnace thermal flux.
[0009] It is one aspect of the present invention to increase
combustion efficiency by the induction of turbulence in the gas
column. Another aspect of the present invention is to increase
thermal flux in a furnace by increasing the residence time of
combustion gases in furnace and decreasing the laminar flow at heat
exchange surface. In the present invention, these parameters are
increased by the induction of turbulence in the combustion gases
and at the combustion gas/furnace interface.
[0010] Furthermore, the present invention increases the combustion
efficiency through the rapid, thorough mixing of the injected
secondary air with the combustion gases via increased turbulence.
This rapid, thorough mixing effects a more complete burning of the
fuel while reducing the secondary air requirements.
[0011] These and other aspects of the present invention will become
apparent to those skilled in the art after a reading of the
following description of the preferred embodiment when considered
with the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 is a side view of a combustion furnace operated
according to the present invention.
[0013] FIG. 2 is a cross-sectional view of Zone A of the furnace of
FIG. 1 showing the gas swirl and deflection turbulence induced by
operation according to the present invention.
[0014] FIG. 3 is a cross-sectional view of Zone A of the furnace of
FIG. 1 showing the gas rotation induced by operation according to
the present invention.
[0015] FIG. 4 is a cross-sectional view of Zone B of the furnace
showing the turbulence induced by rotation in a non-circular
furnace.
[0016] FIG. 5 is a cross-sectional view of Zone C of the furnace
showing the swirl, deflection, and rotation-induced turbulence
induced by operation according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0017] In the following description, like reference characters
designate like or corresponding parts throughout the several views.
Also in the following description, it is to be understood that such
terms as "forward," "rearward," "front," "back," "right," "left,"
"upwardly," "downwardly," and the like are words of convenience and
are not to be construed as limiting terms.
[0018] Referring now to the drawings in general, the illustrations
are for the purpose of describing a preferred embodiment of the
invention and are not intended to limit the invention thereto.
Shown in FIG. 1 is a side view of a combustion furnace, generally
described as 12, equipped with an air injection system composed of
injection ports 14. As best seen in FIGS. 2 and 3, the present
invention provides for an air injection system that creates swirl
20, peripheral turbulence 24, and air column rotation 30 through
the tangential injection of secondary air into the furnace. The
present invention thus creates turbulence and improves mixing of
the overfire air with the combustion gases.
[0019] According to the present invention, the overfire air is
injected into the combustion gases at a velocity and orientation
such that the swirl and high turbulence generated in the combustion
gases achieve a rapid and thorough mixing of the advected gases and
the combustion gases.
[0020] As shown in FIG. 2, injection of the overfire air into the
combustion gases is effected in a manner such that the advected air
travels across the column of combustion gases and is deflected by
the opposing wall. This forceful injection induces turbulent mixing
of the advected air and combustion gases in at least three ways: 1)
by the generation of swirl 20 in the gas column, 2) the generation
of turbulence in proximity of the opposing wall after deflection of
the advected air by the wall 24, and 3) by the turbulence caused by
the rotation of the column of combustion gases in a non-circular
furnace, shown as 26 in FIG. 4. Swirl 20 is also generated by the
rotation of the gas column, as shown in FIG. 4.
[0021] The rotation, shown as 30 in FIG. 3, is produced through the
tangential injection into the furnace of the advected ROFA air,
i.e. there is an injection port on each side of the furnace. The
injection port on the right may be, for example, toward the rear of
the furnace while the injection port on the left side may be toward
the front side of the furnace. This placement of ports results in a
"swirl" being created in the furnace much like the injection of
water in a whirlpool can create a swirl, resulting in mixing, such
as described in U.S. Pat. No. 5,809,910 issued Sep. 22, 1998 to
Svendssen. This system provides for the asymmetrical injection of
overfire air (OFA) in order to create turbulence in the furnace,
thus more thoroughly mixing the secondary air and the combustion
gases.
[0022] Turbulence generated in proximity of the opposing wall is
achieved when the advected air strikes the opposing wall before
being completely mixed into the combustion gases. That is, the
penetration of the injected secondary air is greater than the width
of the furnace and the secondary air deflects off the opposing wall
and generates turbulent flow. To achieve penetration and,
therefore, turbulence, the advected gas must have sufficient linear
momentum to penetrate the primary gas, strike the deflecting
surface, and rotate. This linear momentum is described as mass flow
for a continuous gas stream. The mass flow (m) of a fluid is
defined as follows:
m=density of fluid.times.Area.times.average fluid velocity normal
to Area
[0023] The mass flow of the advected gas must be sufficient to
traverse the column of flue gas, strike the deflecting surface, and
create turbulence. The distance from injection to deflection,
represented by the width of the flue gas chamber, dictates the
necessary mass flow required to achieve turbulence. However, since
the desired rate of added gas mass is limited, it is often
desirable to increase the velocity of the advected gas, thereby
increasing the mass flow. Thus, greater mass flow of the advected
air can be attained by increasing the velocity of the gas.
[0024] Rotation of combustion gas column in a furnace with a
non-circular cross-section causes additional turbulence formation
due to the non-circular cross-section. The rotation is achieved, as
previously described, by the use of opposing, coordinated,
tangential injection of secondary air into the combustion gas
column. Thus, rotation of the gas column in a non-circular
cross-section furnace produces rotation-induced turbulence,
especially at the furnace/gas interface.
[0025] In a system according to the present invention, a series of
gas ducts with nozzles advecting gas into a moving column of flue
gas are positioned in a predetermined manner to create rotational
flow of the combustion zone, as described in U.S. Pat. No.
5,809,910, incorporated herein by reference in its entirety. The
injection ducts are preferably arranged to act at mutually separate
levels on the mutually opposing walls of the furnace, as shown in
FIGS. 1 and 2 of an incineration unit and/or are displaced
laterally in pairs in relation to one another. Additionally, the
nozzles are preferably positioned at successively increasing
distances along the axis of flow of the furnace away from the
furnace, as shown in FIG. 1, such that rotation is maintained by
the coordinated, reinforcing, tangential injection of high-velocity
secondary air into the combustion gas column, generally described
as 50 in FIG. 5.
[0026] A fourth means of producing turbulence is through the
advection of overfire air or gases that are cooler than the
combustion gases. This cooler air produces additional turbulence
from the thermal expansion it undergoes upon mixing with the
combustion gases. That is, the advected gas expands as it is warmed
to the combustion gas temperature by the combustion gas, thus
displacing and further mixing the surrounding combustion gas.
However, in the case of combustion power plants, the advected air
should not be so cold as to reduce the temperature of the exiting
combustion gases and thus reduce heat exchange efficiency. In these
furnaces, ambient air between -20 and 100 degrees centigrade (-4 to
212 degrees F.) can be used in the advected gas. Preheated gas,
such as from redirected combustion air, may also be used in the
advected gas. The redirected combustion air is preferably between
100 and 500 degrees centigrade (200 and 930 degrees F.) and is
preferably mixed, if needed, with the ambient air at between 10 to
50% of the total advected gas, to provide an advection gas with
temperature of between about 40 and 460 degrees centigrade. More
preferably, the redirected combustion air is mixed at 20-40% of the
total advected gas, if needed to provide an advection gas with
temperature of between about 76 and 340 degrees centigrade. This
gas mixture is therefore warm enough not to reduce the combustion
gas temperature significantly and can also readily participate in
the combustion reaction upon mixing with the combustion gas.
[0027] These turbulences can thus be further augmented by using
high-velocity secondary air. During testing of the system,
secondary air was injected into furnaces of various sizes at
velocities ranging from 60-300 m/s using booster fans. The velocity
necessary to provide sufficient mixing is dependent upon the size
of the furnace, the vertical velocity of the combustion gasses and
the configuration of the furnace.
[0028] Surprisingly, the turbulence generated was sufficient that
the entire furnace began operating as a single burner. The
increased turbulence, mixing swirl, and rotation in the furnace
resulted in improved combustion, increased efficiency of the fuel
combustion, reduction in secondary air requirements with
consequential increased retention time of the combustion gases in
the furnace, lower furnace exit gas temperatures due to better heat
exchange in the furnace, increased boiler efficiency and lower
pollutant emissions.
[0029] From the tests it was determined that the ratio of the
advected air velocity to the furnace width (v/w) needs to be
between about 2 to about 150 sec.sup.-1, preferably between about 3
and 60 sec.sup.-1.
[0030] Furthermore, it was determined that the velocity of the
advected air should result in the combustion gas column rotating at
least one half-turn prior to exiting the furnace, more preferably
at least 1 turn prior to exiting the furnace. To achieve this
rotation, at least two levels of injection are required. More
preferably at least three levels of injection are used.
[0031] Alternatively, the velocity of the injected air needs to be
such that the penetration of the injected air is greater than the
furnace width by at least about 1.5 furnace widths, more preferably
by at least 2 furnace widths.
[0032] The reduction in the secondary air results in a decrease in
combustion gas volume, which results in an increased residence time
of the combustion gases in the furnace and thus more time for
thermal flux to occur into the furnace water/steam conduits.
[0033] Additionally, the rotation of furnace gas column in a
non-circular furnace generates turbulence at the gas/furnace
surface interface. This turbulence reduces the laminar flow of the
combustion gases at the interface and therefore improves the heat
transfer across the interface. The turbulence generated by the
rotation also further mixes the combustion gases and reduces
laminar or parallel flow up the furnace. Combustion reactions in
prior art non-circular furnaces tend to demonstrate sidedness, that
is the reactions are on a particular side or zone of the furnace
versus other sides, resulting in non-uniform combustion. Thus, the
present invention advantageously utilizes the non-circular nature
of the furnace to eliminate the sidedness of the furnace. The
rotation that overcomes this sidedness is achieved by the
co-ordinated, reinforcing, tangential injection of high-velocity
secondary air into the combustion gas column.
[0034] Similarly, the vigorous mixing in the combustion area
produced by the present invention also prevents the laminar flow
and consequential lower residence time of higher inertia particles
in the furnace, such as combustible particulate, thereby allowing
them more time to burn in the furnace and further increasing the
combustion efficiency and thermal flux efficiency of the
furnace.
[0035] Thus, the present invention utilizes the co-ordinated,
reinforcing, tangential injection of high-velocity secondary air to
improve the combustion efficiency and thermal flux efficiency of
furnaces.
[0036] A method according to the present invention for increasing
combustion furnace efficiency includes providing a furnace with a
plurality of secondary air injection ducts, asymmetrically
positioned in an opposing manner; injecting fuel with primary air
through the burners prior to the injection of secondary air;
injecting secondary air through the plurality of secondary air
injection ducts at a velocity such that the ratio of the velocity
to the furnace width is between about 2 sec.sup.-1 to about 150
sec.sup.-1, preferably between about 3 and about 60 sec.sup.-1;
thereby increasing combustion efficiency and reactor efficiency via
mixing and rotation of the combustion space.
[0037] Alternatively or additionally, the velocity of the injected
air is such that the penetration of the injected air is greater
than the furnace width by at least about 1.5 widths and/or the
combustion zone rotates at least one half revolution prior to
exiting the furnace.
[0038] Certain modifications and improvements will occur to those
skilled in the art upon a reading of the foregoing description. All
modifications and improvements have been deleted herein for the
sake of conciseness and readability but are properly within the
scope of the following claims.
* * * * *