Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BIOMASS CENTER AIR JET BURNER
FIELD OF THE INVENTION
[0001] The present invention relates generally to the field of industrial
burner
apparatuses for performing combustion functions for power generation.
[0002] As used herein the term "biomass" describes a wide range of organic
matter derived from diverse living, or recently-living organisms, such as
grasses and
wood products. Sources of biomass include trees, shrubs, bushes, residual
vegetation
from harvesting grains and vegetables. Biomass is commonly plant matter
harvested to
generate electricity or produce heat. Biomass may also include biodegradable
wastes of
organic origin that can be burned as fuel.
[0003] Biomass differs from fossil fuels, which are hydrocarbons found within
the
top layer of the Earth's crust. Common examples of fossil fuels include coal
and oil.
Unlike fossil fuels, biomass fuels are generally considered CO2 neutral and
renewable
resources, since CO2 generated from biomass combustion can be removed from the
atmosphere by the plants that provide the biomass.
[0004] As the physical properties and chemical composition of biomass differ
greatly from that of coal, biomass fuels for power generation have
historically been
utilized as a primary or auxiliary fuel in stoker and fluid bed style boilers.
Such boilers
do not rely on burners thereby enabling significantly higher furnace residence
time for
combustion and consequently have less stringent fuel preparation requirements.
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[0005] Global warming concerns relating to greenhouse gas emissions has
increased the interest in developing new technologies to enable widespread use
of
renewable resources for power generation. One area of such interest is the use
of
biomass fuels in suspension firing, wherein short furnace residence times
require fine
particles for efficient combustion.
[0006] Pulverized coal firing is the primary means of suspension firing in the
power generation industry. In a first step coal is mechanically pulverized
into fine
particles. The particles are then subsequently conveyed via suspension in a
primary air
stream to a burner, wherein the burner ejects the air/fuel mixture in a
furnace for
combustion. Residence times are nominally 1 - 2 seconds, which is normally
sufficient
for complete pulverized coal combustion with proper particle sizing.
[0007] Biomass firing in pulverized coal-fired boilers is becoming more
widespread as a strategy for reducing greenhouse gases. To enable this
strategy a
need exists to develop a burner capable of effectively utilizing biomass fuels
in
suspension firing.
[0008] Firing biomass fuels faces many technical challenges. As compared to
bituminous coal, biomass fuels have significantly lower heating values and a
higher
concentration of volatile matter. Heating value is inversely proportional to
moisture
content, such that it amounts to 25% to 75% that of a typical bituminous coal.
Biomass
moisture will often be reduced prior to firing for material handling reasons
and to
improve process efficiency and capacity. Nevertheless, firing biomass in place
of coal
requires considerably more fuel mass to achieve a comparable heat output.
Further,
while the highly-volatile nature of biomass makes the fuel inherently easy to
burn, the
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high moisture content can delay ignition. Delayed ignition is especially
undesirable in
suspension firing,
[0009] Another concern with biomass fuels if that biomass is not processed to
the
same particle size as pulverized coal. Experience indicates successful
suspension
firing can be achieved with wood particles sized 0.0625 in. compared to the
top size for
pulverized coal of 0.012 in. Particle volume varies by the diameter cubed,
thus wood
particles have approximately 150 times the volume of larger coal particles
used for
suspension firing. The larger volume of the biomass thus requires quick
ignition and
rapid combustion to enable use of biomass in furnaces designed for pulverized
coal
firing.
[0010] One known technique of utilizing biomass in suspension firing is
biomass
co-firing. In this technique biomass particulate is combined with pulverized
coal and
primary air in a single stream. The combined stream is then introduced into
the furnace.
This technique is however limited in practicality due to the resulting burner
nozzle
velocity necessary to maintain both types of particles in suspension.
Excessive burner
nozzle velocity results in flame instability, delayed ignition, and poor
combustion
performance.
[0011] Thus, there remains a need to develop a means for an efficient and
effective alternative to combusting coal for power generation and a means for
enabling
the widespread combustion of a carbon-neutral fuel for power generation
applications.
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t a
SUMMARY OF THE INVENTION
[0012] The embodiments of the present invention provide a novel combustion
apparatus. More specifically, embodiments of the present invention provide a
combustion apparatus capable of firing biomass fuel and alternating between
biomass
and coal firing, as needed, and/or combusting a combination of coal and
biomass fuels
concurrently.
[0013] Embodiments of the present invention extend the capability of prior art
burners. U.S. Patent 7,430,970 to LaRue et al. ('970 patent),. is hereby
incorporated
into the by reference in the entirety.
[0014] The present invention is an improvement upon prior art burners by
providing a novel device for combusting renewable fuels, including, but not
limited to,
biomass.
[0015] Embodiments of the present invention provide a superior method and
apparatus for co-firing biomass in combination with pulverized coal.
[0016] A combustion apparatus capable of firing biomass fuel including a
burner
assembly which includes a biomass nozzle concentrically surrounded by a core
air zone
and extending axially along the length of the core air zone, the burner
assembly residing
within a windbox, the windbox being attached to a furnace of a boiler, and the
burner
assembly being connected to the furnace by a burner throat, through which air
and fuel
supplied to the burner assembly are emitted into the furnace.
[0017] In embodiments of the present invention, the apparatus includes a
forced
draft fan providing a first supply of air to the windbox, a core air duct,
enclosing the core
air zone, for receiving a core portion of the first supply of air, the core
air duct having a
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h
core damper for regulating the core portion entering the core air duct, a core
nozzle for
receiving the core portion from said core air duct, the core nozzle delivering
said core
portion to said burner throat, a burner elbow for receiving pulverized coal
and a second
supply of air, the pulverized coal and said second supply of air continuing
through a coal
nozzle in an annulus formed between the core nozzle and the coal nozzle, the
core
portion serving to accelerate ignition of pulverized coal by contacting an
inner cylinder of
a coal jet leaving the coal nozzle, the core portion also serving to
accelerate
combustion.
[0018] The various features of novelty which characterize the invention are
pointed out with particularity in the claims annexed to and forming a part of
this
disclosure. For a better understanding of the invention, its operating
advantages and
specific objects attained by its uses, reference is made to the accompanying
drawing
and descriptive matter in which a preferred embodiment of the invention is
illustrated.
BRIEF DESCRIPTION OF THE DRAWING
[0019] In the drawings:
[0020] Fig 1. is a schematic side elevation view of an embodiment of the
present
invention.
[0021] Fig 2. is a schematic side elevation view of an alternative embodiment
of
the present invention.
[0022] Fig 3. is a schematic side elevation view of an alternative embodiment
of
the present invention.
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[0023] Fig 4. is a schematic cross sectional view of an embodiment of the
present
invention which identifies the concentric zones of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0024] Referring now to figures, wherein like references designate the same or
functionally similar elements throughout the several drawings, Fig. 1 shows a
burner
assembly 1 residing within windbox 2, which is attached to the furnace 3 of a
boiler (not
shown). Secondary air 22 is provided to windbox 2 by a forced draft fan (not
shown)
and heated by an air preheater (not shown). The burner assembly 1 is connected
to
furnace 3 by burner throat 4, through which air and fuel supplied to the
burner assembly
1 are emitted into the furnace 3. A portion of the secondary air 22
constitutes core air 5.
Core air 5 enters core air duct 6 and is regulated by core air damper 7. Core
air 5
continues through the burner assembly 1 through core nozzle 8, exiting through
the
burner throat 4.
[0025] Secondary air 22 is also supplied to the burner assembly (designated as
secondary air to the burner assembly 9). Secondary air 22 enters the burner
assembly
1 and travels through parallel flow paths of the inner air zone 10 and outer
air zone 11.
Swirl vanes in these zones serve to swirl secondary air 22 to facilitate
ignition and
combustion of secondary air 22 contacting the pulverized coal stream. An air
separation vane 12 at the exit of outer zone 11 acts to increase the size of
an internal
recirculation zone (IRZ) formed by resultant aerodynamics. Pulverized coal and
primary
air 13 enter burner elbow 14 and continue through coal nozzle 15, in the
annulus
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formed between core nozzle 8 and coal nozzle 15. The core air 5 serves to
accelerate
ignition of pulverized coal by contacting the inner cylinder of the coal jet
(not shown)
leaving the coal nozzle 15; and serves to accelerate combustion by a "bellows
effect"
supplying air to the center of the flame. LaRue `970 provides a detailed
discussion on
the accelerated ignition relating to core air.
[0026] The burner assembly 1 according to embodiments of the present invention
may be operated in combination with an over-fire-air ("OFA") system (not
shown). A
portion of the secondary air 22 supplied to the furnace for combustion is
supplied to the
OFA system, such that the total amount of air supplied to the burner assembly
1 is less
than theoretical air requirements. This produces a reducing environment in the
furnace
before OFA is supplied. The accelerated combustion, higher temperature flame,
and
larger IRZ all serve to more effectively reduce NOX under reducing conditions.
[0027] In embodiments of the present invention, biomass may be prepared for
suspension firing using shredders, hammer mills and the like (not shown),
collected and
regulated in feed rate by a screw feeder or equivalent device (not shown) and
pneumatically conveyed to the burner assembly 1 through an appropriate
conduit. The
conduit supplies biomass and transport air 16 through an elbow 14 whose outlet
is
situated at the axis of the burner 1.
[0028] In some embodiments, a reducer 17 may be used to reduce the cross-
sectional area of biomass nozzle 18 as the nozzle transverses the burner elbow
14 and
continues past the core air duct 16. A reducer 17 serves to lessen the flow
obstruction
as the biomass nozzle 18 extends through the length of the burner assembly 1.
Near
the furnace end of the burner assembly 1, the biomass nozzle tip 19 diameter
can be
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expanded as shown (Fig 1.) to reduce the biomass exit velocity to the optimum
value for
combustion. In certain embodiments, this exit velocity is between about 2500
ft/min and
about 5000 ft/min, and more preferably between about 3000 ft/min and 4000
ft/min.
[0029] In further embodiments, core air 5 surrounding the biomass nozzle tip
19
serves to accelerate ignition of the biomass as it enters the burner throat 4,
and
supplies air to feed combustion as the biomass continues into the furnace. The
hot
secondary core air that surrounds the biomass nozzle provides heat to enable
additional
moisture removal from the biomass fuel while supplying the fuel with an
oxidant to
facilitate ignition and combustion. This solves the problems related to
delayed ignition
and combustion associated with firing biomass in prior art burners. Core air
damper 7 is
adjusted to supply core air 5 in such quantity so as to minimize NO),
emissions when
firing biomass in combination with pulverized coal. For times when biomass is
not being
fired, the biomass supply system (not shown) serving the burner assembly 1 is
shut
down and valve 23 is closed. Valve 21 is then opened and adjusted in
combination with
core damper 7 to supply the optimum amount of core air 5 necessary for
minimizing
NO,, when firing the particular coal. When the biomass is to be fired, valve
21 is shut
and valve 23 is opened to admit biomass and transport air 16.
[0030] Referring now to Fig.4, a schematic cross section of the burner
assembly
1 of the present invention is shown wherein the five distinct zones of the
burner
assembly 1 are identified. A biomass zone 32 defined by biomass nozzle 18 is
concentrically surround by a core air zone 44 defined the area between biomass
nozzle
18 and core nozzle 8. A coal nozzle 15 concentrically surrounds core nozzle 8
defining
a first annular zone 47 wherein pulverized coal and primary air (PC/PA) 13
flows. A
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i
barrel 42 concentrically surrounds coal nozzle 15 and defines the inner air
zone 10
internal to barrel 42 and an outer air zone 11 external to barrel 42.
[0031] While a preferred embodiment has been shown, alternative embodiments
may also be achieved without departing from the scope of the present
invention.
[0032] One alternative embodiments includes a straight pipe without reducer 17
(Fig 2), and/or without expansion at the furnace end of biomass nozzle 18. In
this
embodiment the alternative of a shorter or recessed biomass nozzle 18 is also
shown
wherein the biomass nozzle tip 19 terminates within the core nozzle 8 near the
core air
duct 6. This embodiment provides the additional benefit of preheating and
premixing
the biomass with the core air, thereby further enabling additional moisture
removal from
the biomass fuel.'
[0033] A reducing taper may be used at the exit of the biomass nozzle 18 (Fig
3)
to accelerate the biomass fuel as it enters the furnace 3 to prevent flashback
into the
biomass nozzle 18. While the biomass nozzle 18 is illustrated as an open-ended
nozzle
in the figure, it may be readily fitted with deflectors or swirlers near the
exit to increase
mixing rate of biomass with core air.
[0034] In other embodiments, adjustment means may be included to facilitate
minor fore/aft adjustments in the end position .of the biomass nozzle 18
relative to the
core pipe to enable further optimization of combustion. While the biomass
nozzle 18 is
shown flush with the end of the core pipe in Figure 1, it may also be
positioned slightly
further back or further forward. In certain embodiments, valve 21 may be used
to admit
a small amount of air, either hot secondary air or unheated air, to add air to
the center of
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the flame while firing biomass. The purpose of this is to augment center
stoichiometry
for lowest NO, (as alternative to increasing transport air quantity).
[0035] Embodiments of the present invention provide a number of advantages.
The biomass co-fired air jet burner according to embodiments of the present
invention
provides a novel, superior structure and enables a superior method for firing
biomass
fuels.
[0036] The large core zone accommodates a biomass nozzle without changing
burner size, saving the engineering and manufacturing costs normally
associated with
building burners of different sizes to accommodate biomass firing.
[0037] The large biomass nozzle enables firing larger quantities of biomass in
selected burners, such that fewer burners need be supplied to fire biomass.
Biomass
firing rates up to 40% of rated burner input enable boiler biomass firing
rates of 20%
while using only half the burners.
[0038] The biomass fuel availability often varies with the seasons such that
biomass firing may not be conducted continuously. In an alternative embodiment
the
biomass nozzle can be supplied with secondary air when not firing biomass such
that
both the biomass nozzle 18 and core nozzle 8 provide a combined core air jet
for the
combustion of-pulverized coal.
[0039] Also, the transport air with biomass contributes to the preferred
center
stoichiometry of the burner when firing biomass in combination with coal. In
such case,
the coal flow is reduced such that a higher PA/PC ratio is supplied to the
burner. This is
augmented with transport air from biomass to provide a center stoichiometry
conducive
to very low NO, emissions.
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[0040] Further, the locating of the biomass nozzle in the core zone provides a
source of hot secondary air for igniting and feeding combustion of the biomass
fuel,
preventing the delayed ignition experienced in prior art as well as feed
combustion of
the co-fired biomass fuel.
[0041] While a specific embodiment of the invention has been shown and
described in detail to illustrate the application of the principles of the
invention, it will be
understood that the invention may be embodied otherwise without departing from
such
principles.
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