U.S. patent application number 11/817145 was filed with the patent office on 2008-10-16 for combustion method and system.
This patent application is currently assigned to Clean Combustion Technologies, LLC. Invention is credited to Xiaoguang Jia, Zhen Jia, Xing Li, Taofang Zeng.
Application Number | 20080250990 11/817145 |
Document ID | / |
Family ID | 36141244 |
Filed Date | 2008-10-16 |
United States Patent
Application |
20080250990 |
Kind Code |
A1 |
Jia; Zhen ; et al. |
October 16, 2008 |
Combustion Method and System
Abstract
A method of combustion for pulverized hydrocarbonaceous fuel
includes the steps of injecting an air/fuel stream into a burner,
causing a low-pressure zone; directing a flow of a high-temperature
combustion gas from a combustion chamber into the low-pressure zone
in the burner; mixing the high-temperature combustion gas with the
injected air/fuel stream to heat the injected air/fuel stream, and
injecting the heated air/fuel stream from the burner to the
combustion chamber, wherein the air/fuel stream is rapidly
devolatilized and combusted in a flame that has a high temperature;
sensing a combustion parameter; and based on the sensed combustion
parameter, controlling combustion to achieve at least one of a
desired NOx reduction and a desired distance from the burner to a
flame front.
Inventors: |
Jia; Zhen; (Beijing, CN)
; Zeng; Taofang; (Raleigh, NC) ; Jia;
Xiaoguang; (Beijing, CN) ; Li; Xing; (Beijing,
CN) |
Correspondence
Address: |
SQUIRE, SANDERS & DEMPSEY L.L.P.
1 MARITIME PLAZA, SUITE 300
SAN FRANCISCO
CA
94111
US
|
Assignee: |
Clean Combustion Technologies,
LLC
Raleigh
NC
|
Family ID: |
36141244 |
Appl. No.: |
11/817145 |
Filed: |
February 27, 2006 |
PCT Filed: |
February 27, 2006 |
PCT NO: |
PCT/US06/07025 |
371 Date: |
June 24, 2008 |
Current U.S.
Class: |
110/347 ;
110/189; 431/12 |
Current CPC
Class: |
F23D 2201/20 20130101;
F23D 1/00 20130101; F23N 1/022 20130101; F23N 2239/02 20200101;
F23C 2900/99004 20130101; F23K 1/04 20130101; F23N 2223/36
20200101 |
Class at
Publication: |
110/347 ;
110/189; 431/12 |
International
Class: |
F23C 1/12 20060101
F23C001/12; F23C 6/04 20060101 F23C006/04 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 25, 2005 |
CN |
200520005019.1 |
Nov 22, 2005 |
US |
11283930 |
Claims
1-48. (canceled)
49. A method of combustion for pulverized hydrocarbonaceous fuel,
the method comprising: injecting an air/fuel stream into a burner,
causing a low-pressure zone; directing a flow of a high-temperature
combustion gas from a combustion chamber into the low-pressure zone
in the burner; mixing the high-temperature combustion gas with the
injected air/fuel stream to heat the injected air/fuel stream, and
injecting the heated air/fuel stream from the burner to the
combustion chamber, wherein the air/fuel stream is rapidly
devolatilized and combusted in a flame that has a high temperature;
sensing a combustion parameter; and based on the sensed combustion
parameter, controlling combustion to achieve at least one of a
desired NOx reduction and a desired distance from the burner to a
flame front.
50. A method according to claim 49, wherein the step of controlling
the combustion includes controlling the pressure of the
low-pressure zone.
51. A method according to claim 50, therein the step of controlling
the pressure of the low-pressure zone includes controlling a
tertiary air fed into the low pressure zone to control the pressure
of the low-pressure zone.
52. A method according to claim 51, wherein a feeding pipe for
feeding the tertiary air is located in the first two-third of the
burner measured from its entrance for the air/fuel stream.
53. A method according to claim 49, wherein the step of controlling
the combustion includes controlling the flow rate of the
high-temperature combustion gas from the combustion chamber into
the low-pressure zone in the burner.
54. A method according to claim 49, wherein the step of controlling
the combustion includes controlling at least one of the flow rate
and air/fuel ratio of the injected air/fuel stream.
55. A method according to claim 49, wherein the air/fuel stream is
a concentrated air/fuel stream.
56. A method according to claim 55, wherein the concentrated stream
has a weight ratio of air to fuel in the range of 0.4 to 2.2.
57. A method according to claim 55, wherein the concentrated stream
is heated to a temperature of 700.degree. C. to 1200.degree. C. in
a distance between 250 mm and 1950 mm as measured from the entrance
of the burner for the high-temperature gas.
58. A method according to claim 55, wherein the concentrated stream
has a weight ratio of air to fuel in the range of 0.7 to 1.8.
59. A method according to claim 55, wherein the concentrated stream
is injected into the burner at a speed from 10 to 60 m/s.
60. A method according to claim 55, wherein the concentrated stream
is injected into the burner at a speed from 15 to 50 m/s.
61. A method according to claim 49, wherein a cross-sectional area
of the injected air/fuel stream at the entrance to the burner is a
fraction of a cross-sectional area of the burner.
62. A method according to claim 61, wherein the cross-sectional
area of the injected air/fuel stream at the entrance to the burner
is less than 50% of the cross-sectional area of the burner.
63. A method according to claim 49, wherein the fuel is at least
one of coal and oil coke.
64. A method according to claim 55, further comprising separating a
primary air/fuel stream into the concentrated air/fuel stream and a
diluted air/fuel stream, and feeding the diluted stream into the
combustion chamber.
65. A method according to claim 64, wherein the step of controlling
the combustion includes controlling the feeding of the diluted
stream into the combustion chamber.
66. A method according to claim 64, wherein the separating of the
primary air/fuel stream into the concentrated stream and the
diluted stream is performed by a bent pipe.
67. A method according to claim 66, wherein the winding angle of
the bent pipe is between 60.degree. and 120.degree..
68. A method according to claim 64, wherein the primary stream
contains 10% to 35% of stoichiometric air.
69. A method according to claim 49, wherein the combustion
parameter includes at least one of a pressure sensor, a temperature
sensor, and a chemical sensor for sensing the content of a gas.
70. A method according to claim 49, wherein the sensing step is
performed by a sensor that is placed in the burner or combustion
chamber or embedded in a wall of the burner or combustion
chamber.
71. A method according to claim 49, further comprising injecting at
least one additional air and fuel stream.
72. A method according to claim 71, further comprising heating one
of the at least one additional air and fuel stream by an additional
reflow of a high-temperature combustion gas from the combustion
chamber.
73. A method according to claim 71, wherein one of the at least one
additional air and fuel stream is an over-fire air, wherein the
over-fire air is 0 to 30% of the total air fed to the combustion
chamber.
74. A method according to claim 73, wherein the step of controlling
the combustion includes controlling the feeding of the over-fire
air.
75. A method according to claim 71, wherein one of the at least one
additional air and fuel stream is a secondary diluted air and fuel
stream.
76. A method according to claim 75, further comprising feeding the
secondary stream to the combustion chamber adjacent to the
periphery of the exit of the burner for the first air/fuel
stream.
77. A method according to claim 75, wherein the step of controlling
the combustion includes controlling the feeding of the secondary
stream.
78. A method according to claim 75, wherein the secondary stream is
one of a straight flow or a swirling flow.
79. A method according to claim 78, further comprising dividing the
swirling secondary stream into an inner secondary stream and an
outer secondary stream.
80. A method according to claim 79, wherein the swirling strength
is between 0.1 and 2.0.
81. A method according to claim 71, wherein the first air/fuel
stream is a first concentrated air/fuel stream, and wherein one of
the at least one additional air and fuel stream is a second
concentrated air and fuel stream.
82. A method according to claim 81, wherein the second concentrated
air/fuel stream is heated.
83. A method according to claim 49, wherein the step of controlling
combustion includes controlling combustion to maximize NOx
reduction without impermissible slagging.
84. A combustion system for pulverized hydrocarbonaceous fuel, the
device comprising: a burner that is designed to receive an air/fuel
stream; a combustion chamber that is connected to the burner to
send to the burner a flow of a high-temperature combustion gas to
heat the air/fuel stream, and to receive the heated air/fuel stream
from the burner for combustion; a sensor for sensing a combustion
parameter; and a controller for controlling combustion based on the
sensed combustion parameter to achieve at least one of a desired
NOx reduction and a desired distance from the burner to a flame
front.
85. A system according to claim 84, wherein the controller controls
the pressure of the low-pressure zone.
86. A system according to claim 85, therein the controller controls
a tertiary air fed into the low pressure zone to control the
pressure of the low-pressure zone.
87. A system according to claim 84, wherein the controller controls
the flow rate of the high-temperature combustion gas from the
combustion chamber into the low-pressure zone in the burner.
88. A system according to claim 84, wherein the controller controls
at least one of the flow rate and air/fuel ratio of the injected
air/fuel stream.
89. A system according to claim 84, wherein the air/fuel stream is
a concentrated air/fuel stream.
90. A system according to claim 84, wherein the combustion
parameter includes at least one of a pressure sensor, a temperature
sensor, and a chemical sensor for sensing the content of a gas.
91. A system according to claim 84, wherein at least one additional
air and fuel stream is injected into the combustion chamber.
92. A system according to claim 91, wherein one of the at least one
additional air and fuel stream is heated by an additional reflow of
a high-temperature combustion gas from the combustion chamber.
93. A system according to claim 91, wherein one of the at least one
additional air and fuel stream is a secondary diluted air and fuel
stream.
94. A system according to claim 91, wherein the first air/fuel
stream is a first concentrated air/fuel stream, and wherein one of
the at least one additional air and fuel stream is a second
concentrated air and fuel stream.
95. A system according to claim 94, wherein the second concentrated
air/fuel stream is heated.
96. A system according to claim 84, wherein the controller controls
combustion to maximize NOx reduction without impermissible
slagging.
Description
TECHNICAL FIELD OF THE INVENTION
[0001] The present invention relates to a combustion method, and a
combustion system, for solid hydrocarbonaceous fuel.
BACKGROUND OF THE INVENTION
[0002] Solid fossil fuel, such as coal, is an important energy
source, particularly for power generation. Pollutants emitted from
coal combustion, however, are a major source of air pollution. Of
the pollutants from coal combustion, nitrogen oxides (NOx) have
attracted extensive attention.
[0003] There are two primary sources of NOx generated during
combustion: fuel NOx and thermal NOx. Fuel NOx is NOx formed due to
the conversion of chemically bound nitrogen (fuel nitrogen) during
combustion. Fuel nitrogen (or char-N) is released in several
complex combustion processes. The primary initial product of
combustion is either HCN or NH3. HCN is then either oxidized to NO
or reduced to N.sub.2. If the gases are oxidant or the fuel is
lean, NO will be the dominant product of fuel nitrogen. If it is
fuel rich, HCN is reduced to N.sub.2 by CO or C (char) on the coal
char surface.
[0004] Thermal NOx refers to NOx formed from high temperature
oxidation of atmospheric nitrogen. Thermal NOx formation is an
exponential function of temperature and a square root function of
oxygen concentration. A lower combustion temperature or a lower
oxygen concentration yields lower NOx. Therefore, the production of
thermal NOx can be controlled by controlling the reaction
temperature or the oxygen concentration. However, a lower
combustion temperature or a lower oxygen concentration leads to an
inefficient burning of coal, i.e., a slow burning rate. A slow
burning rate may result in an incomplete burning of coal and a
prolonged burning of coal.
[0005] Various technologies have been developed to reduce NOx
emission. These technologies either reduce the combustion
temperature or manipulate the oxygen concentration. The first is
called "dilution based combustion control technique," and the
latter is referred to as "stoichiometry based combustion control
technique." The dilution based combustion technique introduces
inert gases such as water or flue gases to reduce the flame peak
temperature. The stoichiometry based combustion technique involves
lowering the oxygen concentration in the flame zone and generating
a reducing atmosphere, thus allowing NOx to be reduced. Examples
are low-NOx staged burners and OS combustion, e.g., over-fire-air
and burner-out-of-service. These techniques control NOx generation
by providing air and/or fuel staging to create fuel-rich zones
(partial combustion zones) followed by air-rich zones to complete
the combustion process. These low-NOx burners can reduce the NOx
emission to 0.65 to 0.25 pounds per million BTUs. Another type of
NOx control technology is gas reburning. The reburning technology
can lower the NOx emission to 0.45 to 0.18 pounds per million
BTUs.
[0006] However, these NOx reduction techniques are less than
adequate. For example, they cannot meet the emission requirements
(less than 0.15 pounds per million BTUs) under the U.S. Clean Air
Act. Additionally, in almost all low-NOx combustion techniques, the
combustion time has to be increased significantly. As a result, the
boiler size must be increased to accommodate the long combustion
time so that coal combustion can be completed at an economically
acceptable level. Consequently, almost all the NOx control
technologies require significant capital investment, and the cost
of operation is high.
[0007] Recent studies have shown that feeding coal with
high-temperature gas could significantly reduce NOx emission and
unburned carbon in fly ash. In the combustion process with
high-temperature gas, the fuel nitrogen is devolatilized rapidly,
and reduced to nitrogen during devolatilization and combustion in a
fuel rich zone.
SUMMARY OF THE INVENTION
[0008] The present invention is based on the inventors' recognition
of several problems associated with the prior art. One of the
problems is that although the prior art technologies for reducing
NOx are based on solid theories, the devices based on the
technologies often do not achieve optimum NOx reduction. The reason
is that those devices do not, or cannot quickly, adjust operating
parameters to adapt to changing operating conditions for optimum
NOx reduction. For example, when the quality or type of coal
changes or when the load is changed, the prior art devices do not,
or cannot quickly, recognize the change and adjust the operating
parameters to adapt to the change. As a result, an optimum NOx
reduction cannot be achieved for the coal being used. At the same
time, unburned carbon in fly ash also increases.
[0009] Another problem associated with the prior art is that, in
the case of the technology involving feeding high-temperature gas
to coal, which produces high combustion temperature, the failure to
adjust operating parameters to adapt to changing operating
conditions may result in the flame front becoming too close to the
wall of the burner and/or the wall of the combustion chamber. As a
result, slagging takes place on the wall of the burner and/or the
wall of the combustion chamber. For example, the inventors'
experiment shows that when the operating parameters are set for
anthracite coal (with volatile of 7.36%) but bituminous coal (with
volatile of 17.22%) is used, slagging takes place on the wall of
the burner due to over-heating and can cause a shout-down of the
combustion system.
[0010] The present invention is directed to a method of combustion
that has one or more advantages of low NOx emission, low unburned
carbon, automatic adaptability to any types of fossil fuel, and
reduced slagging. The combustion method may include injecting a
air/fuel stream into a burner to cause a low-pressure zone;
directing a flow of a high-temperature combustion gas from a
combustion chamber into the low-pressure zone in the burner; mixing
the high-temperature combustion gas with the injected air/fuel
stream to heat the injected air/fuel stream, and injecting the
heated air/fuel stream from the burner to the combustion chamber,
wherein the air/fuel stream is rapidly devolatilized and combusted
in a flame; sensing a combustion parameter; and based on the sensed
combustion parameter, controlling the combustion to achieve at
least one of a desired NOx reduction and a desired distance from
the burner to a front of the flame. In a preferred embodiment, the
combustion is controlled to maximize NOx reduction without
impermissible slagging. What constitutes "impermissible slagging"
cannot be determined in the abstract and must be determined on a
case-by-case basis from the design requirements for a given
combustion system. Such a determination can be made by a person
with ordinary skill in the art.
[0011] The present invention is directed also to a combustion
system for pulverized hydrocarbonaceous fuel. A combustion system
may include a burner that is designed to receive a air/fuel stream;
a combustion chamber that is connected to the burner to send to the
burner a flow of a high-temperature combustion gas to heat the
air/fuel stream, and to receive the heated air/fuel stream form the
burner for combustion; a sensor for sensing a combustion parameter;
and a controller for controlling the combustion based on the sensed
combustion parameter to achieve at least one of a desired NOx
reduction and a desired distance from the burner to a flame front.
In a preferred embodiment, the combustion is controlled to maximize
NOx reduction without impermissible slagging.
[0012] In a preferred embodiment, the velocity of the injected
air/fuel stream in the burner is 10 to 60 m/sec, more preferably 15
to 50 m/sec. The velocity can be designed so as to feed the
air/fuel stream without blocking the feed pipe, and to introduce a
pressure inside the burner that is lower than that in the
combustion chamber. The cross-sectional area of the injection at
the entrance of the burner may be a fraction of the cross-sectional
area of the burner, preferably 20% to 60%. The desirable ratio of
the two cross-sectional areas allows a certain amount of
high-temperature combustion gas to flow back into the burner from
the combustion chamber.
[0013] In another preferred embodiment, the air/fuel stream is a
concentrated air/fuel stream, i.e., a air/fuel stream having a low
air to fuel ratio. Preferably, the ratio of air to fuel solids in
the concentrated stream is 0.4 to 2.2 kg air/1 kg fuel, more
preferably 0.7 to 1.8 kg air/1 kg fuel. This represents only 8% to
25% of the stoichiometric ratio for fuels such as anthracite and
bituminous coals.
[0014] There are several reasons for the use of a concentrated
air/fuel stream. First, the concentrated stream allows the
maintenance of a highly fuel-rich flame inside the burner and
combustion chambers, which can significantly reduce the NOx.
Secondly, the concentrated stream can be heated up using a
relatively small amount of heat. Thus the concentrated stream can
be quickly heated up in a short distance. Third, the heated
concentrated stream releases a large amount of volatiles in the
fast heating. (Partial combustion also may take place during the
heating of the concentrated stream.) The released volatiles enhance
the ignition and combustion of the coal particles, reducing the
unburned carbon in fly ash. Additionally, a fast release of
volatiles including fuel-bound nitrogen in the fuel rich atmosphere
allows transformation of the fuel-bound nitrogen into N.sub.2
rather than NOx. The overall effects of the concentrated air/fuel
stream and the designed burner allow combustion to be performed and
maintained at a high temperature and in an atmosphere of reduced
gases, which is conductible to ultra-low NOx emission and low
unburned carbon in fly ash.
[0015] The air/fuel stream in the burner can be a swirling flow or
a straight flow. Some typical setups of the burner are wall fired,
opposite fired, tangential fired, and down-fired. The burner
preferably is arranged at the same vertical elevation as that of
the combustion chamber.
[0016] In still another preferred embodiment of the present
invention, the combustion system may include a separating device
that is designed to separate a air/fuel stream from a pulverizing
system into the concentrated air/fuel stream and a diluted air/fuel
stream. The separating device is connected to the burner to supply
the concentrated air/fuel stream to the burner. The ratio of air to
fuel solids for the concentrated stream is lower than that for the
air/fuel stream from the pulverizing system. Typically, the ratio
of air to the fuel solids in the air/fuel stream from the
pulverizing system may be 1.25 to 4.0 kg air/1 kg fuel. The ratio
of air to fuel solids in the concentrated air/fuel stream
preferably is 0.4 to 2.2 kg air/1 kg fuel, more preferably 0.7 to
1.8 kg air/1 kg fuel.
[0017] In general, an embodiment of the present invention may
include two or more air/fuel streams that are injected into a
combustion chamber. Each of these air/fuel streams may be a
concentrated air/fuel stream, which may have a ratio of air to fuel
solids between 0.4 to 2.2 kg air/1 kg fuel, more preferably between
0.7 to 1.8 kg air/1 kg fuel. Alternatively, each of these air/fuel
streams may be a diluted air/fuel stream, which may have a ratio of
air to fuel that is greater than that of a concentrated air/fuel
stream. Each of the air/fuel streams may be heated, as described
above, or unheated, before it is injected into the combustion
chamber.
[0018] For example, a preferred embodiment of the present invention
may include a primary air/fuel stream that is concentrated and
heated, and a secondary air/fuel stream that is diluted and may or
may not be heated. Preferably, the primary air/fuel stream is first
injected into the combustion chamber, and then the secondary
air/fuel stream is injected into the combustion chamber to complete
the combustion. The secondary air/fuel stream may contain
sufficient oxygen that the total amount of oxygen fed into the
combustion chamber makes up at least the stoichiometric amount
needed for a complete combustion of fuel. Preferably, the secondary
air/fuel stream is fed into the combustion chamber adjacent to the
exit of the burner for the primary stream. A typical secondary air
and fuel stream contains about 3.5 to 8.0 kg of air for 1 kg of
fuel, which represents about 65 to 90% of the stoichiometric
combustion air required for a complete combustion of anthracite
coal, bituminous coal, and oil coke.
[0019] In this example, an additional diluted air/fuel stream, such
as a so-called "over-fire air," is injected into the combustion
chamber. This additional diluted air/fuel stream may or may not be
heated. In some embodiments, the additional diluted air/fuel stream
contains sufficient oxygen such that the total amount of oxygen fed
into the combustion chamber is at least the stoichiometric amount
for a complete combustion of fuel.
[0020] For another example, a preferred embodiment of the present
invention may include two or more concentrated air/fuel streams
that may or may not be heated, and each of the concentrated
air/fuel stream may be followed by one or more diluted air/fuel
streams that may or may not be heated.
[0021] The controlling of combustion to optimize at least one of
NOx reduction and the distance from the burner to a flame front may
be carried out in several ways. For example, it may include
controlling one or more of the following control parameters: the
pressure in the low-pressure zone in a burner, at least one of the
flow rate and air/fuel ratio of a concentrated air/fuel stream, and
at least one of the flow rate and air/fuel ratio of a diluted
air/fuel stream.
[0022] Combustion control can be achieved by controlling the
pressure in the low-pressure zone, because the pressure in the
low-pressure zone affects the flow rate of the high-temperature
combustion gas from the combustion chamber into the low-pressure
zone in the burner and, thus, the heating of the air/fuel stream.
The pressure in the low-pressure zone can be controlled by
introducing a gas into the low pressure reflow zone. Preferably,
the gas is air (tertiary air). When the quantity of tertiary air is
increased, the pressure in the low-pressure zone is also increased,
resulting in a decreased flow of the high-temperature combustion
gas from the combustion chamber into the low-pressure zone. As a
result, the heating of the air/fuel stream is reduced, and
combustion temperature may be reduced. The amount of tertiary air
affects also the air/fuel weight ratio of the air/fuel stream,
which can also be used for combustion control.
[0023] Combustion control may also be achieved by controlling the
flow rate and air/fuel ratio of a air/fuel stream injected into the
burner, because the flow rate and/or concentration of the air/fuel
stream affect the pressure in the low-pressure zone and the
devolatilization and combustion of the air/fuel stream.
[0024] The combustion control of the present invention can be based
on one or more combustion parameters. Representative parameters may
be combustion temperature, pressure, and the concentration of one
or more selected gases such as carbon dioxide, carbon monoxide,
oxygen and nitrogen. Preferably, the temperature is used as the
combustion parameter. The control may be realized by sensing the
value of the combustion parameter inside the burner and/or the
combustion chamber, and comparing the sensed value with a preset
value. Based on the difference between the sensed value and preset
value, the controller, such as a close-loop controller or a
distributed control system, adjusts one or more of the
above-discussed control parameters to reduce the difference. When
the difference is reduced, the NOx emission is reduced, and/or a
desired distance from the burner to a flame front is maintained to
reduce slagging. This automatic control enables a burner to be used
with almost all kinds of fuel without changing the structure of the
combustion system.
[0025] Herein, the term "reflow" means a flow of the
high-temperature combustion gases from the combustion chamber back
to the burner. The flow of the combustion gases is in the opposite
direction of the fuel stream. Other terms for such types of flow
are "reflux" and "recirculation." The reflow is caused by the
pressure reduction resulted from the injection of the air/fuel
stream into the burner.
[0026] Herein, the term "heating" means heating of the air/fuel
stream in the burner. The heating source is from the reflow of the
high-temperature combustion gases. The heating may be conducted by
mixing and thermal radiation. In the case of the concentrated
air/fuel stream, the temperature of the air/fuel stream may reach
700.degree. C. to 1200.degree. C. in a distance ranging between 250
mm and 1950 mm measured from the exit of the feeding pipe for the
concentrated fuel stream to the burner.
[0027] Herein, the term "NOx" means oxides of nitrogen, including
NO, NO.sub.2, NO.sub.3, N.sub.2O, N.sub.2O.sub.3, N.sub.2O.sub.4,
N.sub.3O.sub.4, and their mixtures.
[0028] Herein, the term "bound nitrogen" means nitrogen that is a
composition of a molecule that composes of carbon and hydrogen and
possibly oxygen.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] FIG. 1 shows a cross section of a preferred embodiment of
the invention for creating a concentrated fuel stream and
performing heating in the burner and combustion in a combustion
chamber.
[0030] FIG. 2 shows the flow pattern for reflow and heating of the
air/fuel stream.
[0031] FIGS. 3 and 4 show cross section of a burner of the
embodiment shown in FIG. 1
[0032] FIGS. 5 and 6 show cross-sectional representations of
devices used in the present invention for feeding a concentrated
fuel stream to the combustion chamber, for creating reflow of
high-temperature combustion gases back into the burner, and for
controlling the re-flow of high-temperature combustion gases back
into the burner.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0033] The preferred embodiments of the present invention described
below are discussed sometimes in terms of coal combustion, and in
terms of air being the gaseous carrier and oxidant. The techniques
described are applicable to any other pulverized solid fuel and any
other gaseous carrier. The invention will be described with the aid
of the Figures, yet a description that refers to the Figures is not
used to limit the scope of the invention.
[0034] FIG. 1 to 4 show a preferred embodiment of a swirling burner
according to the present invention. Some embodiments of the burner
are described in more detail in FIGS. 4 and 5. The invention also
encompasses straight-flow burners where the secondary stream or/and
the other streams is (are) fed into the combustion chamber in a
straight flow.
[0035] FIG. 1 shows a combustion system includes a burner 3 and a
combustion device 1 having a chamber 2. The combustion device of
the present invention can be any apparatus within which combustion
takes place. Typical combustion devices include furnaces and
boilers. A burner 3 is mounted on a sidewall or at a wall corner of
the combustion device 1 and feeds fuel solids and air from sources
outside the combustion device 1 into the combustion chamber 2 of
the combustion device 1. Typical fuels include pulverized
hydrocarbon solids, an example of which is pulverized coal or
petroleum coke.
[0036] In the illustrated embodiment, fuel and air are supplied to
the combustion system as a main air/fuel stream A, and a secondary
diluted air/fuel stream for an aerodynamic control of the mixing
between the fuel and the air. In the main air/fuel stream A, the
air may be supplied with a stoichiometric ratio less than 1. The
air used to complete the combustion of the fuel may be supplied to
the combustion device 1 as the secondary stream B
(=B.sub.1+B.sub.2) and/or as an over-fire air as shown in FIGS. 1
to 4.
[0037] As shown in FIGS. 1 and 3 to 6, the burner 3 is comprised of
an injector 8, 16 for a primary concentrated air/fuel stream
a.sub.1, a secondary stream injector 13, 19, and an automatic
control unit 30. Preferably, a solid-gas separator 4 is placed in
front of the injector 8 for the primary concentrated air/fuel
stream a.sub.1 to separate the main air/fuel stream A into a
concentrated stream a.sub.1 and a diluted fuel stream a.sub.2. The
separator 4 is preferred to be a bent three-way separator but
should not be limited to a bend separator. The bent three-way
separator 4 includes a main-stream inlet pipe 5, a bent pipe 6, a
feeding pipe 7 for a diluted stream a.sub.2, and a feeding pipe 8
for the primary concentrated fuel stream a.sub.1. Preferably, the
winding angle of the bent pipe 6 is between 60.degree. and
120.degree.. The ratio of the inner radius of the pipe 8 for the
concentrated air/fuel stream to the inner radius of the pipe 7 for
the diluted fuel stream is between 0.5 and 2.0.
[0038] The main air/fuel stream A from a pulverizing system (not
shown in the figure) may be fed from the inlet pipe 5 through the
bent 3-way separator 4 at a velocity. Fuel powders can be
concentrated on the outer bend of the separator 4 by the design of
the separator 4 with a specified radius and a winding angle to
match the flow velocity. This separates the main stream A into the
primary concentrated stream a.sub.1 in the outer region of the bend
and a diluted stream a.sub.2 in the inner region of the bend. The
concentrated stream a.sub.1 is fed to the burner 3 through a
feeding pipe 8. Through a feeding pipe 7, the diluted stream
a.sub.2 is fed through a port 20 into the combustion device 1 at a
location close to the burner 3. The angle in the exit direction of
the separator 4 can be adjusted. A typical main stream A contains
about 1.25 to 4.0 kg of air for 1 kg of fuel solids, which
represents about 10 to 35% of the stoichiometric combustion air
required for a complete combustion of the fuel.
[0039] The flow rate and concentration of the concentrated stream
a.sub.1 or diluted stream a.sub.2 can be controlled by adjusting a
flap valve 27 disposed between the feeding pipe 8 for the
concentrated stream a.sub.2 and the feeding pipe 7 for the diluted
stream a.sub.2. Alternatively, some other arrangement may be made
to control the flow rate and concentration of the concentrated
stream a.sub.1 or diluted stream a.sub.2.
[0040] The secondary stream is from the secondary stream windbox 11
(FIG. 1). Preferably, the secondary stream is fed using two
passages: an inner secondary stream passage B.sub.1 and an outer
secondary stream passage B.sub.2. The inner secondary stream
passage B.sub.1 includes a throttle 9 for the straight-flow
secondary stream, a throttle 10 for the swirling-flow secondary
stream, an air deflector 12, and a secondary stream spurt pipe 13.
The outer secondary stream passage B.sub.2 includes a throttle 14
for the straight-flow secondary stream, a throttle 15 for the
swirling-flow secondary stream, an air deflector 18, and a
secondary stream spurt pipe 19. Those components are placed
concentrically along the axis of the fed line 16 of the
concentrated stream a.sub.1 if the components are in a circular or
cylindrical shape.
[0041] Fed from the windbox 11, the inner secondary stream B.sub.1
is then separated into two streams by adjusting the throttles 9 and
10. Of them, the first stream b.sub.11 is a straight-flow air, the
second stream b.sub.12 is a swirling flow air produced by the axial
air deflector 12. Adjusting the throttles 9 and 10 allows a
desirable swirling strength. Fed from the windbox 11, the outer
secondary stream B.sub.2 is then separated into two streams by
adjusting throttles 14 and 15. Of them, the first stream b.sub.21
is a straight-flow air, the second stream b.sub.22 is a swirling
flow produced by the axial air deflector 18. Adjusting the
throttles 14 and 15 allows a desirable swirling strength. A typical
secondary stream B contains about 3.5 to 8.0 kg of air for 1 kg of
fuel, which represents about 65 to 90% of the stoichiometric
combustion air required for a complete combustion of anthracite,
bituminous coals and oil coke. The swirl strength is controlled by
adjusting throttles 9 and 10 and 14 and 15. Preferably, a swirl
number, as defined in "Combustion Aerodynamics", J. M. Beer and N.
A. Chigier, Robert E. Krieger Publishing Company, Inc., 1983, is
0.1 to 2.0.
[0042] Preferably, an over-fire air is fed through an over-fire-air
port 21 into the combustion device 1 to make the entire combustion
zone inside the combustion device 1 fuel-rich and supplies more
oxygen to help a complete combustion of the fuel. The volume
percentage of the over-fire-air may be between 0 and 30% of the
total air sent to the combustion device 1 that is required for a
complete combustion of the fuel.
[0043] In a preferred embodiment, the concentrated stream enters
the burner chamber 40 and forms a fuel-rich zone C.sub.1 where the
stoichiometric ratio is between 0.08 and 0.25. A reflow of
high-temperature gas is introduced into the burner 3 from the
combustion chamber 2 to heat rapidly the concentrated stream to
devolatilize volatiles and bound nitrogen. And combustion takes
place between the fuel solids and the combustion air sequentially,
producing a flame C.sub.2. The secondary stream and sometimes the
over-fire air are injected into the combustion chamber 2 to
complete combustion. The reflow is caused by the relatively lower
pressure caused by the injection of the concentrated stream a.sub.1
at a relatively high velocity compared to the velocity of gases
inside the combustion device 1.
[0044] The rapid heating of the concentrated fuel stream in the
fuel-rich zone C.sub.1 generates a volatile fuel-rich zone. This
significantly increases the combustibility of the fuel stream. Thus
ignition is maintained and completed in a short time and range. And
fuel combustion can be maintained at a high temperature. Rapid
heating and devolatilization combined with high-temperature
combustion under an atmosphere of reducing gases generate nitrogen.
These exactly same combustion conditions also help the combustion
of fuel particles and thus reduce the unburned carbon in the fly
ash.
[0045] When the fuel concentration is higher or the ratio of
air/fuel is smaller, the ignition time will be shorter; the
combustion temperature will be higher; and the flame front is
closer to the burner. When the flame front is too close to the
mouth of the burner, for example, slagging may occur. This is
especially important when the fuel type changes from a low grade
fuel with a low content of volatiles such as anthracite coal to a
fuel with a high content of volatiles such as the bituminous coal.
In this case, the ratio of air/fuel should be increased to prevent
slagging.
[0046] The invention uses a sensor 22 to monitor the change of at
least one parameter in the burner 3 or in the combustion chamber 2.
Representative parameters include temperature, pressure, and the
content of a selected gas. The selected gas can be one or more of
O.sub.2, CO, CO.sub.2, NOx, N.sub.2, and HC. The sensor can be
placed in the burner 3 or in the combustion chamber 2, or in an
area where the burner 3 and the combustion device 1 intersect. For
example, the temperature sensor may be placed at or near a location
where slagging is likely to take place. The temperature signal is
sent to a closed-loop controller 23.
[0047] A typical controllers may be a PID
(proportional-integral-differential) controller or a DCS
(distributed control system) controller. The signal is compared to
a pre-set value. If the detected temperature signal is larger than
the pre-set value, meaning that the combustion temperature is too
high or that the flame front is closer than the desired distance
from the burner, the controller sends a command to the servo-motor
24, which then varies the opening of the valve 25 to reduce
combustion temperature. Specifically, the controller may allow more
tertiary air T (directly from the atmosphere or from a supplying
source) into the burner 3. The additional tertiary air dilutes the
fuel stream and reduces combustion gas reflow, increasing the
distance between the burner 3 and the flame front. The control
process automatically continues until the sensed temperature is the
same or sufficiently close to the desired value. The automatic
control allows the combustion system to be adaptable to different
types of fuel and to reduce NOx emissions.
[0048] Preferably, the total amount of air fed to the combustion
device 1, i.e., the sum of the air in the main air A
(=a.sub.1+a.sub.2), the secondary stream B (=B.sub.1+B.sub.2), and
the tertiary air T, is between 90 to 125% of the stoichiometric air
required for complete the combustion. Preferably, the air through
the over-fire-air port 21 is about 0 to 30% of the total air sent
to the combustion device 1. The amount of over-fire air can be
controlled by adjusting the opening of the over-fire air valve
26.
[0049] Preferably, the tertiary air T is controlled such that the
flame front is at a location between 100 mm and 1400 mm from the
burner. In some cases, when the flame front is closer to the burner
than this preferred range, slagging tends to occur.
[0050] The amount of air fed to the burner 3 and the arrangement of
the aerodynamics of the air preferably is used to establish a
stoichiometric ratio in the fuel-rich zone of the flame C.sub.2
that is less than 0.75. The amount of air in the concentrated
stream a.sub.1 is preferably less than 30% of the stoichiometric
amount required for the complete combustion of the solid fuel. More
preferably, the amount should be less than 20% of the
stoichiometric amount.
[0051] Both the NOx emission and the unburned carbon in the ash
depend on the stoichiometric ratio in the fuel-rich zone C.sub.1
and the fuel-rich flame zone C.sub.2 and on the heating rate or the
temperature rising rate of the fuel-rich zone C.sub.1. For example,
if the main stream A is directly sent to the burner 3, the heat
required to heat the stream to the ignition temperature is about or
more than two times of that required to heat the concentrated
stream a.sub.1. As a result, the ignition of the fuel stream will
be delayed, and the combustion may not be completed in the
combustion system. At the same time, NOx emission is increased
dramatically when the stoichiometric ratio is larger than 1.0.
[0052] In a preferred embodiment, the present invention creates and
maintains a controlled fuel rich flame by: concentrating the
conventional primary stream; then fast heating the concentrated
stream using reflowed combustion gases inside the burn 3 (the
reflow is caused by the negative pressure induced by the relatively
high-speed concentrated fuel stream itself); and controlling the
reflow using a control system. The flame of the highly concentrated
fuel stream is preferably maintained by the controlled reflow,
allowing a stoichiometric ratio well below the original primary air
values.
[0053] Fuel injectors in burners generally have a circular cross
section, an annual cross section (formed by two concentric pipes),
or a square or rectangular cross-section (for example, injectors in
tangentially fired boiler). These designs or layouts fulfill two
functions for the present invention: feeding fuel streams into the
combustion device, and generating the reflow of high-temperature
gases back into the burner that is used to heat the concentrated
stream. FIGS. 5 and 6 show some representative designs that perform
such functions. The present invention, nonetheless, includes all
designs or layouts that feed the fuel and generate re-flow of
high-temperature gases from the combustion device 1. These designs
can be used in wall-fired boilers, the tangentially fired boiler,
and the down-fired boilers.
[0054] FIG. 5 shows some fuel injectors that are without a tertiary
air inlet. It should be pointed out that while some embodiments of
the present invention use the tertiary air to control the pressure
in the low pressure reflow zone, other embodiments of the present
invention also include a burner that does not use the tertiary air.
In FIG. 5a, the feeding pipe 8 for a concentrated fuel stream is at
the centerline of a burner pipe 16. In FIG. 5b, the feeding pipe 8
is located off the centerline of the burner pipe 16. In FIG. 5c,
the feeding pipe 8 is arranged around the burner pipe 16. In FIG.
5d to 5g, the feeding pipe 8 is composed of two parts: a straight
section and a concentric section, and inside the burner pipe 16,
there could include a solid. When the tertiary air is not used to
control the pressure of the low-pressure zone in the burner 3, the
amount and/or content of the concentrated fuel stream flowing into
the burner may be controlled to adjust the pressure inside the
burner and/or to adjust the heating and the weight ratio of
fuel/air in the burner 3.
[0055] FIG. 6 shows some fuel injectors that have a tertiary air
inlet. In FIG. 6a, the tertiary air inlet is located on a side wall
of the burner pipe 16. Preferably, a tertiary-air pipe 17 is
located in the first two thirds of the burner pipe 16 (from the
fuel-stream entrance). In FIG. 6b, the tertiary air inlet 17 is
located on the front surface (herein the front is the entrance of
the fuel stream) of the burner pipe 16.
[0056] The burner pipe 16 and the tertiary-air pipe 17 can be of
any shape. Representative shapes are cylindrical, cubic, prismatic,
cone-shaped, elliptic, and frustum-shaped of pyramid. Additionally,
all feeding pipes 8 and burner pipes 16 shown in FIG. 5 can be used
as fuel injector with tertiary air. The preferable shapes are
cylindrical, cuboid, and prismatic. There can be any number of
feeding pipes for the concentrated fuel stream and tertiary-air
pipes. The tertiary pipe 17 can be at any angle with respect to the
burner centerline.
* * * * *