Note: Descriptions are shown in the official language in which they were submitted.
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COMBUSTION METHOD AND SYSTEM
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 N2. 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 N2 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.
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[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.
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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
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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
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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 N2
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.
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[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
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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
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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
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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 7000C to 12000C 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,
NO2, NOs7 N20, N203, N204, N304, 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
[00291 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. FIG. 2 shows the flow
pattern for reflow and heating of the air/fuel stream.
[0030] FIG. 3 and 4 show cross section of a burner of the embodiment
shown in FIG.1
[0031] 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.
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DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0032] 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.
[0033] 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.
[0034] 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.
[0035] In the illustrated embodiment, fuel and 'air are supplied to the
combustion systein 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(=B1 + B2) and/or as an over-fire air as shown in Figs 1 to 4.
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[0036] 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 al, 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 al to separate the main air/fuel stream A into a
concentrated stream al and a diluted fuel stream a2. 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 a2, and a feeding pipe
8
for the primary concentrated fuel stream al. Preferably, the winding angle of
the bent pipe 6 is between 600 and 1200. 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 2Ø
[0037] 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 al in the outer region of the bend and a
diluted stream a2 in the inner region of the bend. The concentrated stream al
is fed to the burner 3 through a feeding pipe 8. Through a feeding pipe 7, the
diluted stream a2 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 inain 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.
[0038] The flow rate and concentration of the concentrated stream al or
diluted stream a2 can be controlled by adjusting a flap valve 27 disposed
between the feeding pipe 8 for the concentrated stream a2 and the feeding
pipe 7 for the diluted stream a2. Alternatively, some other arrangement may
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be made to control the flow rate and concentration of the concentrated stream
al or diluted stream a2.
[0039] 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 B1 and an outer secondary stream passage
B2. The inner secondary stream passage Bi 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 B2 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 al if the components are in a circular or cylindrical shape.
[0040] Fed from the windbox 11, the inner secondary stream Bi is then
separated into two streams by adjusting the throttles 9 and 10. Of them, the
first stream bll is a straight-flow air, the second stream b12 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 B2 is then separated into two streams by adjusting throttles
14 and 15. Of them, the first stream b21 is a straight-flow air, the second
stream b22 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 2Ø
[0041] Preferably, an over-fire air is fed through an over-fire-air port 21
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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.
[0042] In a preferred embodiment, the concentrated stream enters the
burner chamber 40 and forms a fuel-rich zone C1 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 C2. 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 al at a relatively high velocity
compared to the velocity of gases inside the combustion device 1.
[0043] The rapid heating of the concentrated fuel stream in the fuel-rich
zone Ci 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.
[0044] 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
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volatiles such as the bituminous coal. In this case, the ratio of air/fuel
should
be increased to prevent slagging.
[0045] 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 02, CO, C02, NOx, N2, 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.
[0046] 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.
[0047] Preferably, the total amount of air fed to the combustion device 1,
i.e., the sum of the air in the main air A(=a1 + a2), the secondary stream B(=
B1 + B2), and the tertiary air T, is between 90 to 125% of the stoichiometric
air required for complete the combustion. Preferably, the air through the
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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.
[0048] 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.
[0049] 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 C2 that is less than 0.75. The amount of
air
in the concentrated stream al 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.
[0050] Both the NOx emission and the unburned carbon in the ash depend
on the stoichiometric ratio in the fuel-rich zone C1 and the fuel-rich flame
zone C2 and on the heating rate or the temperature rising rate of the fuel-
rich
zone Ci. 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 ai. As
a result, the ignition of the fuel streain 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 1Ø
[0051] 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);
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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.
[0052] 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.
[0053] 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.
[0054] FIG. 6 shows some fuel injectors that have a tertiary air inlet. In
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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.
[0055] 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 FIGURE 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.
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