Note: Descriptions are shown in the official language in which they were submitted.
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LOW NOx BURNER
TECHNICAL FIELD
[0001] The present invention relates generally to burning and combusting of
hydrocarbons and, in particular, to burners or combustors for heaters.
BACKGROUND
[0002] Burning hydrocarbons in air produces NOx (mono-nitrogen oxides such
as
nitrogen oxide and nitrogen dioxide). NOx emissions are known to be
deleterious
for the ozone in the stratosphere. Atmospheric NOx also forms nitric acid,
which
contributes to acid rain. Because of these significant environmental concerns,
it is
highly desirable to reduce NOx emissions in the burning or combustion of
hydrocarbon fuels. Furthermore, in some jurisdictions, environmental
regulations
limit the amount of NOx emissions that a burner may emit.
SUMMARY
[0003] In general, the present invention provides a low NOx burner, i.e. a
burner
that emits reduced NOx.
[0004] Accordingly, one aspect of the present invention is a burner having
a
housing that includes a burner head defining a gas manifold and a primary
flame
zone downstream of the burner head, a gas inlet in the burner head for
connecting
to and receiving gas from a gas line, a plurality of flow-through air vents
disposed
around a center of the burner head and extending through the burner head
thereby
enabling cold core air to flow from an annular core space upstream of the
burner
head to the primary flame zone downstream of the burner head, a plurality of
premix
air vents in fluid communication with the manifold for premixing air and gas
within
the manifold and for emitting premixed air and gas into the primary flame
zone, a
plurality of staging pipes extending from the manifold into the primary flame
zone for
conveying gas into the primary flame zone, and an ignition device extending
into the
primary flame zone.
[0005] Another aspect of the present invention is a heater including a
burner that
has a blower and a burner. The burner has a housing that includes a burner
head
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defining a gas manifold and a primary flame zone downstream of the burner
head, a
gas inlet in the burner head for connecting to and receiving gas from a gas
line, a
plurality of flow-through air vents disposed around a center of the burner
head and
extending through the burner head thereby enabling cold core air to flow from
an
annular core space upstream of the burner head to the primary flame zone
downstream of the burner head, a plurality of premix air vents in fluid
communication
with the manifold for premixing air and gas within the manifold and for
emitting
premixed air and gas into the primary flame zone, a plurality of staging pipes
extending from the manifold into the primary flame zone for conveying gas into
the
primary flame zone, and an ignition device extending into the primary flame
zone.
[0006] Yet another aspect of the present invention is a method of burning a
combustible hydrocarbon gas in air while minimizing the emission of NOx. The
method entails supplying the combustible hydrocarbon gas through a gas inlet
to a
burner head defining a gas manifold through which the gas flows into a primary
flame zone downstream of the burner head through a plurality of staging pipes
extending from the manifold into the primary flame zone, flowing air through a
plurality of flow-through air vents disposed in the burner head around a
center of the
burner head enabling cold core air to flow from an annular core space upstream
of
the burner head to the primary flame zone downstream of the burner head,
premixing a portion of the gas entering the manifold with air and emitting
premixed
air and gas into the primary flame zone, and igniting the gas and air in the
primary
flame zone.
[0007] The details and particulars of these aspects of the invention will
now be
described below, by way of example, with reference to the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] Further features and advantages of the present technology will
become
apparent from the following detailed description, taken in combination with
the
appended drawings, in which:
[0009] FIG. 1 is an isometric cutaway view of a burner in accordance with
an
embodiment of the present invention;
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[0010] FIG. 2 is bottom view of a burner head for the burner of FIG. 1;
[0011] FIG. 3 is a vertical cross-sectional view of the burner head of FIG.
2;
[0012] FIG. 4 is a bottom view of a burner head assembly for the burner of
FIG.
1;
[0013] FIG. 5 is a sectional view (Section A-A) of the burner head assembly
taken through section A-A in FIG. 4;
[0014] FIG. 6 is a detailed bottom view (Detail B) of the burner head
assembly of
FIG. 4;
[0016] FIG. 7 is a detailed cross-sectional view (Detail C) of a portion of
the
burner head assembly shown in FIG. 5;
[0016] FIG. 8 is a cross-sectional view of the burner of FIG. 1 connected
to a
blower;
[0017] FIG. 9 is a detailed cross-sectional view (Detail D) of the burner
in FIG. 8;
and
[0018] FIG. 10 is a fuel concentration diagram showing the flame zones of
varying air/fuel mixtures.
[0019] It will be noted that throughout the appended drawings, like
features are
identified by like reference numerals.
DETAILED DESCRIPTION
[0020] A burner, generally designated by reference numeral 10, is
illustrated by
way of example in FIG. 1. The burner 10 has a housing 12 that includes a
burner
head 14 defining a gas manifold 16 and a primary flame zone 18 downstream of
the
burner head. The burner 10 includes a gas inlet 20 in the burner head for
connecting to and receiving a combustible hydrocarbon gas from a gas line 22
(not
shown in FIG. 1 but visible in FIGS. 5, 8 and 9). As depicted in the
illustrated
embodiment, the gas inlet 20 defines a tubular passage extending
longitudinally
along a central axis 21 of the housing. The gas inlet may be a central gas
inlet as
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shown by way of example in the drawings although the gas inlet need not be
central
to the burner head.
[0021] For the purposes of this specification, the expression "combustible
hydrocarbon gas" (herein referred to simply as "gas") is meant to include any
flammable vapour state hydrocarbon gas or combustible petroleum product such
as
propane, natural gas, butane, methane, or any other petroleum gas, or alcohol-
based fuels such as ethanol, etc.
[0022] FIG. 2 depicts a bottom view of a burner head 14 in accordance with
one
embodiment of the present invention. As depicted in FIG. 1 and FIG. 2, the
burner
head 14 includes a plurality of flow-through air vents 24 disposed around a
center of
the burner and extending through the burner head thereby enabling cold core
air
(primary air) to flow from an annular core space 26 upstream of the burner
head to
the primary flame zone 18 downstream of the burner head 14. The flow-through
air
vents 24 are continuous passageways or conduits through the manifold between
the
annular core space 26 and the primary flame zone 18. The burner includes a
plurality of premix air vents 28 in fluid communication with the manifold 16
for
premixing air and gas within the manifold and for emitting premixed air and
gas into
the primary flame zone 18. The premix air vents 28 are discontinuous
passageways
or conduits having openings or apertures into which gas is drawn from the
manifold.
The burner includes a plurality of staging pipes 30 extending from the
manifold into
the primary flame zone 18 for conveying gas into the primary flame zone 18.
Secondary air flows through an annular air passageway 38. The burner includes
a
secondary air deflector 40 (or collar) for deflecting the secondary air into
the primary
flame zone 18 and into close proximity with the gas from the staging pipes 30.
The
burner includes an ignition device 32 (shown in FIG. 5) extending into the
primary
flame zone. In the illustrated embodiment, the ignition device 32 is a flame
rod
although any other suitable igniter may be employed.
[0023] In the embodiment illustrated by way of example in FIGS. 1-3, the
housing
12 is a cylindrical (or tubular) housing and the burner head 14 may be in the
form of
a circular head plate for mounting within the cylindrical housing. In this
illustrated
embodiment, the premix air vents 28 are disposed radially outwardly of the
flow-
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through air vents 24 and the staging pipes 30 are disposed radially outwardly
of
some of the premix air vents 28.
[0024] As further illustrated by way of example in FIGS. 1-3, the flow-
through air
vents 24, premix air vents 28 and staging pipes 30 are disposed in concentric
circular arrangements around the gas inlet 20. In other words, the flow-
through air
vents 24 define a first circle of a first radius R1, some of the premix air
vents 28
define a second circle of a second radius R2 (where R2> R1) and the staging
pipes
30 define a third circle of a third radius R3 (where R3> R2). Additional
premix air
vents 28 may be included in the third circle as shown in the embodiment
depicted in
FIG. 2. The radial spacing RS1 between the first circle of radius R1 and the
second
circle of radius R2 (RS1 = R2 - R1) may be equal to the radial spacing RS2
between
the second circle of radius R2 and the third circle of radius R3 (RS2 = R3 ¨
R2). In
other embodiments, the radial spacing RS1 may be greater than RS2. In yet
other
embodiments, RS2 may be greater than RS1. In other embodiments, there may be
more than three rings of vents and staging pipes. Also the spacing between the
vents and staging pipes can vary, i.e. the radii R1, R2 and R3 may be varied.
In
most implementations, there are three zones ¨ a fresh air zone at radius R1, a
secondary fresh air zone in the annular air passage 38 (beyond the edge of the
burner head), and a premix zone between the other two zones. The figures show
one ring of vents in the central fresh air zone, and two rings in the premix
zone, but
there could be more in each of these zones. The staging pipes start in the
premix
zone and extend through the primary flame zone up to the secondary air
deflector/collar 40.
[0025] Although FIGS. 1-3 show one specific arrangement having ten flow-
through air vents, fifteen premix air vents, and five staging pipes, the
number of
flow-through air vents, premix air vents and staging pipes may be varied.
Similarly,
in the embodiment of FIGS. 1-3, there is a 1:1.5 ratio between flow-through
air vents
and premix air vents and a 3:1 ratio between premix air vents and staging
pipes. In
other words, the number of flow-through air vents is less than the number of
premix
air vents whereas the number of premix air vents is triple the number of
staging
pipes. However, in other embodiments, the ratios may be different. In the
illustrated
embodiment of FIGS. 1-3, the flow-through air vents and premix air vents are
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equally angularly spaced at successive angles of 36 degrees. As illustrated,
the
flow-through air vents are radially offset relative to the premix air vents.
As shown in
the embodiment of FIGS. 1-3, the staging pipes are evenly distributed with a
successive angular spacing of 72 degrees although this may be varied in other
embodiments. However, in other embodiments, the angular spacing and radial
alignments may be varied. The diameters of the flow-through air vents, premix
air
vents and staging pipes may be substantially equal or unequal. The gas inlet
20
may have a larger diameter than the diameter of any of the individual gas
outlets or
staging pipes, as shown in the figures. As shown in FIGS. 1-3, the burner head
may
include five staging pipes 30.
[0026] To promote turbulent mixing, outlets 31 of each respective staging
pipe 30
may be bevelled at an angle 13 of 30-90 degrees from a transverse plane
through the
pipe. As well, the bevelled outlets are oriented at an angle e of 25-60
degrees
relative to a radial line extending from the respective staging pipe to the
center of
the burner head. The outlets 31 are so oriented in order to swirl the gas
emitted by
the staging pipes 30. In the specific embodiment illustrated in FIGS. 1-9, the
outlets
31 are at an angle 0 of 30 degrees relative to the radial line. The angle 6 is
depicted in FIG. 3 and the angle e is depicted in FIG. 2.
[0027] In one embodiment, the outlets 31 are bevelled at an angle 6 of 40-
50
degrees as illustrated in FIG. 1. In the specific embodiment illustrated in
FIG. 3 and
FIG. 9, the outlets 31 are bevelled at an angle 13 of 45 degrees.
[0028] FIGS. 4-7 depict the burner head assembly which includes the burner
head 14, gas line assembly (including gas line 22), flame sensor 42 and flame
rod
32. As illustrated by way of example in these figures, the burner may include
the
circumferential or annular air passage 38 for secondary air to flow at the
circumference around the burner. The primary (core) air and secondary air may
be
cold or pre-heated air or it may be air mixed with exhaust. It may be possible
to
recirculate exhaust gases back into the fresh air flow. When mixing exhaust
with the
primary or secondary air, the exhaust may be taken from a combustion chamber
or
any point downstream from there.
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[0029] This burner may operate vertically, horizontally or in any other
spatial
orientation.
[0030] The burner may be constructed of stainless steel or any other
equivalent
or suitable material. The surfaces of the burner exposed to extreme heat (i.e.
the
primary flame zone and bottom face of the burner head) may furthermore be
coated
with a heat-resistant coating to prolong service life.
[0031] The burner may be used, or adapted for use, in a heater equipped
with a
blower 50 such as the one depicted by way of example in FIG. 8. The heater may
be a direct heater (in which combustion gases are exhausted directly into a
space to
be heated) or an indirect heater (in which a heat exchanger is used to
transfer heat
from the combustion gases to the space to be heated). For example, the burner
may be used in a construction heater or space heater. The burner may be used
as
part of a boiler, steam generator, oven (e.g. curing oven), paint booth, etc.
The
burner may be used in a variety of other applications such as in steel
treating or
other processes where heat is required.
[0032] FIGS. 8 and 9 show how the blower 50 may be interfaced with the
novel
burner 10. The blower can be belt-driven, direct-drive, or other drive
mechanism.
The heater may further include a flame sensor 42 disposed on the gas line 22
as
shown in FIG. 5. The flame sensor may be an ultraviolet (UV) flame sensor
capable
of detecting UV radiation emitted at the instant of ignition although other
types of
flame sensors may be used or adapted for use.
[0033] The burner also enables a method of burning a combustible
hydrocarbon
gas in air while producing reduced NOx emissions. The method entails supplying
the combustible hydrocarbon gas through a gas inlet to a burner head defining
a gas
manifold through which the gas flows into a primary flame zone downstream of
the
burner head through a plurality of staging pipes extending from the manifold
into the
primary flame zone, flowing air through a plurality of flow-through air vents
disposed
in the burner head around a center of the burner head enabling cold core air
to flow
from an annular core space upstream of the burner head to the primary flame
zone
downstream of the burner head, premixing a portion of the gas entering the
manifold
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with air and emitting premixed air and gas into the primary flame zone, and
igniting
the gas and air in the primary flame zone.
[0034] In one embodiment of the method, supplying the gas involves swirling
the
gas exiting the outlets of each respective staging pipe by orienting the
outlets at 25-
60 degrees, more particularly 25-35 degrees relative to a radial line
extending from
the respective staging pipe to a center of the burner head. Supplying the gas
may
also entail directing the gas exiting the outlets of each respective staging
pipe in the
burner at an angle by beveling the outlets at 30-90 degrees or more
particularly at
40-50 degrees. In one specific embodiment, the method may be performed by
swirling the gas using staging pipes that have outlets oriented at 30 degrees
to the
radial line and being beveled at 45 degrees. Premixing may be performed, in
one
embodiment, by using premix air vents that are disposed radially outwardly of
the
flow-through air vents so that the supplying of the gas is performed by the
staging
pipes that are disposed radially outwardly of the premix air vents.
[0036] The burner produces low NOx emissions in a compact design. The
shorter length of the burner provides for a more compact heater with a
superior
power density relative to axially staged burners. The injection of air via the
flow-
through air vents 24 has the effect of cooling the core of the flame, thereby
reducing
the amount of NOx being produced in the primary flame zone. Tests have
demonstrated emissions in the range of 9-19 ppm NOx for a burner embodying the
invention whereas a conventional comparable burner would produce approximately
100 ppm NOx.
[0036] FIG. 10 is a fuel concentration diagram showing the approximate
flame
zones of varying air/fuel mixtures. In this diagram, there are three discrete
flame
zones illustrated, a first zone 60, a second zone 62 and a third zone 64 with
the
combustion chamber 66. The first zone receives flow-through air and is
confined
within the second zone. The first zone is almost entirely air, with some gas
drawn in
from the second zone through recirculation. The second zone 62 is confined
within
the third zone and receives an extremely rich mixture of gas and air from the
premix
air vents 28. The combustion in this zone produces a high amount of CO because
there is insufficient oxygen in this zone to achieve complete combustion.
Because
of lack of oxygen, the temperature of the second zone is reduced and the
volume of
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the second zone enlarged. This allows the cracking of the fuel to form CO and
CH
radicals with minimal formation of NOx. The third zone 64 receives gas through
the
staging pipes and air from the secondary air, deflected by the secondary air
deflector 40. Although the overall ratio of gas and air in this zone is lean,
the mixture
near the outlet of each staging pipe is rich. This results in micro zones of
high CO
and low NOx at the outlet of each staging pipe. The third zone completes the
combustion of second zone and the micro zones of the staging pipes over a
shortened period of time and at a lower temperature to substantially prevent,
or at
least minimize, the formation of NOx while keeping the CO levels low.
[0037] This
invention has been described in terms of specific examples,
embodiments, implementations and configurations which are intended to be
exemplary only. Persons of ordinary skill in the art will appreciate that
obvious
variations, modifications and refinements may be made without departing from
the
scope of the present invention. The scope of the exclusive right sought by the
Applicant is therefore intended to be limited solely by the appended claims.
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