U.S. patent application number 15/552150 was filed with the patent office on 2018-03-15 for downshot burner.
This patent application is currently assigned to DOOSAN BABCOCK LIMITED. The applicant listed for this patent is DOOSAN BABCOCK LIMITED. Invention is credited to Gerard John HESSELMANN.
Application Number | 20180073728 15/552150 |
Document ID | / |
Family ID | 52821948 |
Filed Date | 2018-03-15 |
United States Patent
Application |
20180073728 |
Kind Code |
A1 |
HESSELMANN; Gerard John |
March 15, 2018 |
DOWNSHOT BURNER
Abstract
A burner outlet set for a downshot firing burner is described
comprising a first outlet array having at least one primary outlet,
and at least one vent air outlet disposed either side of the
primary outlet in an array direction of the first outlet array;
second and third outlet arrays each comprising an array of
secondary air outlets, respectively disposed either side of the a
first outlet array. A burner system with a plurality of such burner
outlet sets, a burner arch configured for downshot firing and
having one or more such burner sets, and a combustion furnace with
one or more such arches are also described.
Inventors: |
HESSELMANN; Gerard John;
(Crawley, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
DOOSAN BABCOCK LIMITED |
Crawley, Sussex |
|
GB |
|
|
Assignee: |
DOOSAN BABCOCK LIMITED
Crawley, Sussex
GB
|
Family ID: |
52821948 |
Appl. No.: |
15/552150 |
Filed: |
February 22, 2016 |
PCT Filed: |
February 22, 2016 |
PCT NO: |
PCT/GB2016/050438 |
371 Date: |
August 18, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F23D 1/00 20130101; F23C
7/00 20130101; F23C 5/24 20130101; F23D 2201/10 20130101; F23D
1/005 20130101; F23C 5/08 20130101; F23L 1/00 20130101 |
International
Class: |
F23D 1/00 20060101
F23D001/00; F23C 5/08 20060101 F23C005/08; F23C 7/00 20060101
F23C007/00; F23L 1/00 20060101 F23L001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 20, 2015 |
GB |
1502891.3 |
Claims
1. A burner outlet set for a downshot firing burner comprising: a
first outlet array comprising at least one primary outlet, and at
least one vent air outlet disposed either side of the primary
outlet in an array direction of the first outlet array; second and
third outlet arrays each comprising an array of secondary air
outlets, respectively disposed laterally either side of the a first
outlet array,
2. A burner outlet set thus comprises a first outlet array disposed
in an elongate first array direction and comprising at least one
primary outlet with at least one vent air outlet disposed either
side of the primary outlet, and paired second and third outlet
arrays each comprising a plurality of secondary air outlets
respectively disposed in elongate second and third array directions
that are laterally spaced either side of the first outlet array.
Typically the second and third outlet arrays are even laterally
spaced either side of the first outlet array. Typically the second
and third outlet arrays and arrayed in elongate array directions
generally parallel to the first array direction. Typically each of
the first, second and third outlet arrays extend in elongate array
directions to the same longitudinal extent.
3. A burner outlet set in accordance with claim 1 wherein each
primary outlet is a square or rectangular outlet disposed with a
median line direction parallel to and for example coincident with
the array direction of the first outlet array.
4. A burner outlet set in accordance with claim 1 or claim 2
wherein each vent air outlet is a square or rectangular outlet
disposed with a median line direction parallel to and for example
coincident with the array direction of the first outlet array.
5. A burner outlet set in accordance with any preceding claim
wherein each secondary air outlet is a square or rectangular outlet
disposed with a median line direction parallel to and laterally
spaced from the array direction of the first outlet array.
6. A burner outlet set in accordance with any preceding claim
wherein each primary outlet is associated with exactly one
secondary outlet in the second array and exactly one secondary
outlet in the third array.
7. A burner outlet set in accordance with claim 6 wherein each vent
air outlet is associated with exactly one secondary outlet in the
second array and exactly one secondary outlet in the third
array.
8. A burner outlet set in accordance with claim 7 wherein each
outlet in the first array is transversely aligned with exactly one
secondary outlet in the second array and exactly one secondary
outlet in the third array.
9. A burner outlet set in accordance with any preceding claim
wherein a first outlet array comprises at least one primary/vent
outlet module consisting exactly of a single primary outlet and a
single vent air outlet disposed either side of the primary
outlet.
10. A burner outlet set in accordance with any preceding claim
wherein a first outlet array comprises at least one primary/vent
outlet module consisting exactly of a pair of primary outlets and a
single vent air outlet disposed either side of the primary
outlet.
11. A burner outlet set in accordance with claim 9 or 10 wherein a
first outlet array comprises exactly one said primary/vent outlet
module.
12. A burlier outlet set in accordance with claim 9 or 10 wherein a
first outlet array comprises exactly two of the said primary/vent
outlet modules disposed adjacently.
13. A burner outlet set in accordance with any preceding claim
wherein each primary outlet is provided with a wedge shaped
bluff-body stabilizer.
14. A burner outlet set in accordance with claim 13 wherein the
bluff body stabilizer is slitted and located in a position
withdrawn into its outlet.
15. A burner outlet set in accordance with any preceding claim
wherein each primary outlet is a square or rectangular slot
provided with four ignition teeth located at the corners of the
square or rectangular slot.
16. A burner system comprising a plurality of burner outlet sets in
accordance with any preceding claim.
17. A fuel combustion system comprising: a pulverous solid fuel
supply source; a transport gas supply source to supply comburant
gas as a transport gas; a mixing module configured to entrain fuel
from the fuel supply within the transport gas; a transport conduit
to transport the fuel entrained in the transport gas; a separator
configured to separate the mixture of fuel and transport gas into a
relatively fuel rich primary stream and a relatively fuel depleted
vent air stream; a primary conduit to convey the primary stream to
primary outlets of a burner outlet set in accordance with any
preceding claim; a vent air conduit to convey vent air to vent air
outlets of a burner outlet set in accordance with any preceding
claim; a secondary air supply system including a secondary conduit
to convey secondary air to the secondary air outlets of a burner
set of a burner outlet set in accordance with any preceding
claim.
18. A burner arch configured for downshot firing of fuel in a
combustion chamber of a combustion apparatus having at least one
burner outlet set in accordance with one of claims 1 to 15 disposed
on the burner arch whereby combustion of the fuel is supported in
the vicinity of the burner set during use,
19. A combustion apparatus comprising: a combustion chamber; at
least one burner arch configured for downshot firing of fuel in the
combustion chamber; at least one burner outlet set in accordance
with one of claims 1 to 15 disposed on the burner arch whereby
combustion of the fuel is supported in the vicinity of the burner
set during use.
20. A combustion apparatus in accordance with claim 19 further
configured for the supply of overfire air to the combustion chamber
in that each burner set is provided in association with at least
one outlet or series of outlets for overfire air.
21. A combustion apparatus in accordance with claim 20 wherein each
burner set is provided in association with at least one outlet or
series of outlets for overfire air respectively provided at each of
first and second levels respectively below and above a flame zone
within the combustion chamber.
22. A combustion apparatus in accordance with claim 21 wherein a
first overfire air outlet or series of outlets is provided above
and closely adjacent to a hopper knuckle of the combustion chamber,
and wherein a second overfire air outlet or series of outlets is
above and closely adjacent to the burner arch.
23. A combustion apparatus in accordance with claims 19 to 22
further comprising: a pulverous solid fuel supply source; a
transport gas supply source to supply comburant gas as a transport
gas configured to entrain fuel from the fuel supply within the
transport gas; a transport conduit to transport the fuel entrained
in the transport gas; a separator configured to separate the
mixture of fuel and transport gas into a relatively fuel rich
primary stream and a relatively fuel depleted vent air stream; a
primary conduit to convey the primary stream to the primary
outlets; a vent air conduit to convey vent air to the vent air
outlets; a secondary air supply system including a secondary
conduit to convey secondary air to the secondary air outlets.
Description
[0001] The present invention relates to burners adapted for
downshot firing and further relates to combustion systems for
furnaces for example for steam generators for example for utility
power generation including such arrangements of burners. In
particular, but not exclusively, the invention relates to
burners/furnaces capable of utilising low volatile fuels, and
especially low volatile carbonaceous solid fuels such as low
volatile coals.
[0002] Low volatile coals such as anthracites and lean coals
present particular requirements for effective combustion so as to
overcome the inherent difficulties of achieving stable and
efficient combustion which arise from the lack of volatile material
in the coal to aid in the ignition, and the low reactivity of the
remaining char. Downshot or arch firing is the established
technology for low volatile coal combustion in utility power
generation applications. Downwardly directed burners are mounted on
one or more arches in an upper part of the combustion chamber to
give a typically U-shaped or W-shaped downwardly directed flame
with a long residence time for more effective combustion of low
volatile fuels. Burners may be mounted on a single arch (U-shaped
flame) or mounted on a pair of opposing arches (W-shaped flame).
Generally single arch firing is used in lower thermal capacity
(smaller) boiler and double arch firing (W-firing) is preferred for
higher capacity larger boilers.
[0003] An example prior art downshot firing arrangement is shown in
FIGS. 1 (furnace) and 2 (burner).
[0004] The firing arrangement of FIG. 1 is shown in a furnace of a
thermal electric utility power plant. FIG. 1 illustrates the
general furnace arrangement and principles which apply both to the
prior art and to burner sets in accordance with the invention.
[0005] The fuel source is envisaged to be a low volatile
carbonaceous fuel in pulverous form such as a low volatile coal in
pulverous form, for example anthracite or lean coal: Coal entrained
in a transport gas comprising transport air or other comburant gas
is taken from the pulverizing plant (in the case of direct firing)
or from a pulverised coal silo (indirect firing). In a typical
system for downshot firing, the fuel/transport air mix is then
separated into two streams in a separator which may be a cyclonic
separator known routinely in the art as a relatively fuel rich
"primary" stream containing most of the fuel (for example taken
from the lower part of the cyclone) and a relatively fuel lean
"vent" air stream (for example taken from the upper part of the
cyclone). Following separation, the coal and air are introduced
into the furnace via an arrangement of slots in the firing arch
which additionally include "secondary" (windbox) air slots and
possible further air slots. A possible arrangement is shown in FIG.
2.
[0006] FIGS. 1 and 2 are discussed in greater detail below.
[0007] The invention is directed to systems which have burner sets
comprising "primary", "vent", and "secondary" nozzles as they would
be understood by the skilled person in accordance with the above,
and the description here should be read accordingly.
[0008] In particular references to primary air will in familiar
manner generally be understood to refer to air or other comburant
gas mixtures admitted to the system with the fuel upstream of the
combustion zone to support combustion at the combustion zone in a
burner or furnace and in the context of this invention therefore
references to primary air will be understood to refer to the air or
other comburant gas forming the relatively fuel rich stream after
the separation process above described.
[0009] References to vent air will correspondingly be understood to
refer to the air or other comburant gas forming the relatively fuel
lean stream after the separation process above described.
[0010] References to secondary air will in familiar manner be
understood to refer to air or other comburant gas mixtures admitted
to the system separately from the fuel/primary mixture to support
combustion at the combustion zone in a burner or furnace. For
example secondary air is supplied via the windbox.
[0011] More generally, as is usual in the art, all references to
"air" include air, simulated air, and other comburant gas mixtures
capable of supporting combustion of the fuel. In a downshot firing
arrangement such as that illustrated, fuel and combustion air is
introduced into the furnace via burners situated on the firing
arches. The inlet flows are directed downwards to the hopper. In
the idealised situation a symmetrical "W" shaped flame pattern is
developed, although in practice there is often significant
asymmetry.
[0012] An increasing environmental pressure surrounds the control
of production of nitrogen oxides (NOx) during combustion. The use
of furnace air staging for NOx control is known for the firing of
higher volatile coals, though the details of its implementation
differ between designs. It has not been widely adopted for the
firing of higher volatile coals in downshot fired systems. Air
staging involves the removal of a proportion of the combustion air
from the burner and this can have a detrimental effect on flame
stability. Flame instability and asymmetry can be detrimental to
NOx control, particularly in relation to furnace air staging
methods.
[0013] There is a general desire to develop improved burner designs
for the more effective downshot firing of low volatile carbonaceous
fuels such as low volatile coals and in particular to develop
improved downshot burner designs that are susceptible to effective
NOx control. There is a particular desire to develop burner designs
that produce more stable ignition and/or flame symmetry.
[0014] Thus in accordance with the invention in a first aspect, a
burner outlet set for a downshot firing burner comprises:
[0015] a first outlet array comprising at least one primary outlet,
and at least one vent air outlet disposed either side of the
primary outlet in an array direction of the first outlet array;
[0016] second and third outlet arrays each comprising an array of
secondary air outlets, respectively disposed either side of the
first outlet array.
[0017] As the skilled person will appreciate, references to
"primary", "vent", and "secondary" outlets will be understood in
the context of the provision of comparable burner sets in the prior
and the description here should be read accordingly.
[0018] References to primary outlets will be understood to refer to
outlets for the air or other comburant gas forming the relatively
fuel rich stream after the separation process above described.
References to vent air outlets will correspondingly be understood
to refer to outlets for the air or other comburant gas forming the
relatively fuel lean stream after the separation process above
described. References to secondary air outlets will in familiar
manner be understood to refer to outlets for air or other comburant
gas mixtures admitted to the system separately from the
fuel/primary mixture to support combustion at the combustion zone
in a burner or furnace. For example secondary air is supplied via a
windbox.
[0019] A burner outlet set thus comprises a first outlet array
disposed in an elongate first array direction and comprising at
least one primary outlet with at least one vent air outlet disposed
either side of the primary outlet, and paired second and third
outlet arrays each comprising a plurality of secondary air outlets
respectively disposed in elongate second and third array directions
that are laterally spaced either side of the first outlet array.
Typically the second and third outlet arrays are even laterally
spaced either side of the first outlet array. Typically the second
and third outlet arrays are arrayed in elongate array directions
generally parallel to the first array direction. Typically each of
the first, second and third outlet arrays extend in elongate array
directions to the same longitudinal extent.
[0020] Each primary outlet is preferably a square or rectangular
outlet disposed with a median line direction parallel to and for
example coincident with the first array direction,
[0021] Each vent air outlet is preferably a square or rectangular
outlet disposed with a median line direction parallel to and for
example coincident with the first array direction.
[0022] Each secondary air outlet is preferably a square or
rectangular outlet disposed with a median line direction parallel
to a respective second or third array direction.
[0023] In a preferred embodiment, each primary outlet is associated
with exactly one secondary outlet in the second array and exactly
one secondary outlet in the third array. More preferably each
primary outlet is aligned in a direction transverse to the array
direction of the first outlet array with exactly one secondary
outlet in the second array and exactly one secondary outlet in the
third array. For example the outlets are so aligned in that their
respective transverse midlines line on a single common transverse
line. Thus, in such a case, each primary outlet is sandwiched in a
transverse direction between a pair of secondary outlets.
Optionally the secondary air may be biased towards one or other
outlet. In such an arrangement, each primary outlet is thus
completely surrounded by clean air or vent air.
[0024] In a preferred embodiment, each outlet in the first array is
associated with exactly one secondary outlet in the second array
and exactly one secondary outlet in the third array. More
preferably each primary outlet is aligned in a direction transverse
to the array direction of the first outlet array with exactly one
secondary outlet in the second array and exactly one secondary
outlet in the third array. For example the outlets are so aligned
in that their respective transverse midlines line on a single
common transverse line. Thus, in such a case, each outlet in the
first array is sandwiched in a transverse direction between a pair
of secondary outlets. Optionally the secondary air may be biased
towards one or other outlet.
[0025] Although the invention is not limited by particular theory,
it is generally suggested that efficient combustion of low volatile
coats and in particular effective operation of furnace air staging
as a method of NOx control requires gross flow patterns and flame
paths in the lower furnace to be as symmetrical as possible. Whist
flame asymmetry has a number of negative impacts on plant
performance, it is of particular concern when considering
in-furnace NOx reduction processes such as furnace air staging. It
is postulated that the inconsistent flame stand off identified in
prior art systems such as those shown in FIGS. 1 and 2 may be one
of the factors contributing to the variable asymmetry that is found
to be detrimental to performance.
[0026] From the above it follows that it is considered important to
NOx control in downshot fired systems that the gross flow patterns
and flame paths are broadly symmetrical, and that the flame paths
and stand-offs are steady and repeatable. The key requirements of
any improved burner design addressing these problems can be seen to
include:
[0027] a long flame that penetrates into the lower furnace to
utilize the furnace volume more fully and efficiently;
[0028] a flame whose ignition point is clearly defined in a
robustly repeatable manner; a symmetrical flame path.
[0029] Burners comprising at least one burner outlet set embodying
the principles of the invention are found to deliver these
requirements in admirable manner.
[0030] In accordance with the invention a first outlet array
comprises at least one primary/vent outlet module made up at least
one primary outlet and at least one vent air outlet disposed either
side of the primary outlet.
[0031] Such a primary/vent outlet module may comprise or consist of
exactly a single primary outlet and a single vent air outlet
disposed either side of the primary outlet, or a plurality of
adjacent primary outlets with a single vent air outlet disposed
either side of the primary outlet, or a single primary outlet with
a plural set of vent air outlets disposed either side of the
primary outlets or a plurality of adjacent primary outlets with a
plural set of vent air outlets disposed either side of the primary
outlets. In all cases the outlets making up such a vent outlet
module are disposed in an elongate first array direction.
Preferably, a vent outlet module comprises outlets configured and
disposed such as to give mirror symmetry about a transverse midline
of the module.
[0032] A first outlet array may comprise exactly one primary/vent
outlet module as herein defined.
[0033] Alternatively a first outlet array may comprise more than
one primary/vent outlet module arrayed adjacently. In such an
embodiment a first outlet array thus comprises at least one first
primary outlet with at least one vent air outlet disposed either
side of the primary outlet, and at least one further primary outlet
with at least one vent air outlet disposed either side of the
primary outlet adjacent thereto in an array direction of the first
outlet array. For example a first outlet array may comprise exactly
one pair of adjacent primary/vent outlet modules as herein
defined.
[0034] Each outlet is preferably a square or rectangular outlet
disposed with a median line direction parallel to its elongate
array direction. A rectangular outlet may be elongate in a
direction parallel to its elongate array direction or in a
direction transverse to its elongate direction.
[0035] The individual outlets making up the first outlet array may
be identically or differently sized and/or shaped to each other.
For example at least the primary outlets and the vent outlets
making up the first outlet array may be differently sized and/or
shaped from each other. In a preferred case a primary outlet may be
larger than its corresponding vent outlets.
[0036] The outlets making up the first outlet array are preferably
arrayed in aligned manner such that their respective median lines
are aligned with each other along an elongate array direction.
[0037] The individual outlets making up the second outlet array may
be identically or differently sized and/or shaped to each other.
The individual outlets making up the third outlet array may be
identically or differently sized and/or shaped to each other. In a
preferred case, regardless of whether the individual outlets making
up the respective outlet arrays are differently shaped, the second
outlet array and the third outlet array are identically configured.
In a preferred case each outlet in the second outlet array is
aligned with exactly one identically sized and shaped outlet in the
third outlet array.
[0038] The outlets making up the second outlet array are preferably
arrayed in aligned manner such that their respective outermost
edges are aligned with each other along an elongate array
direction. The outlets making up the third outlet array are
preferably arrayed in aligned manner such that their respective
outermost edges are aligned with each other along an elongate array
direction.
[0039] In a preferred embodiment, each outlet in the first array is
aligned with exactly one outlet in the second array and exactly one
outlet in the third array in that their median lines are aligned
with each other in a transverse direction. Thus, in such a case,
each outlet in the first array is sandwiched in a transverse
direction between a pair of secondary outlets. Optionally the
secondary air may be biased towards one or other outlet. Optionally
the secondary outlets may be of identical or different size and/or
shape to each other and of identical or different size and/or shape
to the outlet in the first array. In the event that the outlets are
differently shaped they are preferably still of the same
longitudinal extent in an elongate array direction.
[0040] The definitions of the outlets as primary, vent air, and
secondary air outlets will readily be understood by the skilled
person with reference to prior art systems. The invention subsists
not in a redefinition of these terms but in the arrangement on
configuration of slots. The invention is not considered limited to
particular definitions, but such definitions are advanced to guide
the skilled person as to preferred embodiments of the
invention.
[0041] Transport gas comprising a comburant gas comprising air or
other comburant gas mixture entrains and transports fuel from a
fuel supply zone. In the preferred case for example, where fuel is
pulverised coal, transport gas entrains and transports coal from
the coal pulverisers directly (direct firing) or from a pulverised
coal silo (indirect firing).
[0042] Transport gas comprising a comburant gas and fuel is split
into primary and "vent air" streams via a suitable separator which
may include a cyclone system. A "vent air" stream may contain a
majority of the air so separated for example 51 to 70% of the air
and a small minority of the fuel for example below 20% of the coal.
A "primary" stream may contain a minority of the air so separated
for example 30 to 49% of the air and a substantial majority of the
fuel for example at least 80%. A more complete combustion system in
accordance with the principles of the invention accordingly
comprises a separator adapted to effect such separation.
[0043] Separation of the fuel and transport air into a relatively
fuel lean and a relatively fuel rich stream aims to further improve
ignition and flame stability. Removal of air reduces the total mass
of fuel stream, and therefore reduces the time for this primary
stream to be heated to ignition temperature. Furthermore the higher
concentration of fuel in the primary stream creates a mixture that
is more conductive to combustion.
[0044] A more complete system in accordance with the principles of
the invention accordingly further comprises one or more of:
[0045] a fuel supply source which is for example a supply of
pulverous solid fuel, and for example includes a fuel pulveriser or
a pulverised fuel store;
[0046] a transport gas supply source to supply comburant gas as a
transport gas configured to entrain fuel from the fuel supply
within the transport gas;
[0047] a transport conduit to transport the fuel entrained in the
transport gas;
[0048] a separator configured to separate the mixture of fuel and
transport gas into a relatively fuel rich primary stream and a
relatively fuel depleted vent air stream;
[0049] a primary conduit to convey the primary stream to the
primary outlets of a burner set in accordance with the principles
of the invention;
[0050] a vent air conduit to convey vent air to the vent air
outlets of a burner set in accordance with the principles of the
invention;
[0051] a secondary air supply system including a secondary conduit
to convey secondary air to the secondary air outlets of a burner
set in accordance with the principles of the invention, and for
example including a windbox.
[0052] A burner system may be provided comprising a plurality of
burner outlet sets in accordance with the first aspect of the
invention.
[0053] In a possible embodiment a primary outlet may be associated
with a wedge shaped bluff-body stabilizer. Normally this will be
slitted and withdrawn into the nozzle (i.e. will be a cavity
bluff-body), but variants to the bluff-body arrangement may be
considered. The wedge may have a notional nozzle blockage ratio of
40%. The slit preferably comprises no more than 10% of the width of
the wedge. The orientation of the bluff-body will be such that the
induced recirculation aims to draw in flue gas from the combusting
vent air. The "V" of the wedge may be aligned so that it is
perpendicular to the furnace front/rear walls. In an alternative
embodiment the V" of the wedge may be aligned so that it is
parallel with the front or rear wall, such that the induced
recirculation draws in secondary air.
[0054] Optionally a primary outlet may contain four ignition teeth
located at the corners of a square or rectangular slot.
[0055] The primary outlet and vent air outlets are typically be
located in triples adjacent to each other; the vent air outlets
will be on the edges of the group while the primary outlet will be
in the middle.
[0056] In the preferred design each primary outlet and vent air
outlet on a burner will be sandwiched by a pair of secondary air
outlets (inner and outer). In this way the primary outlet will be
completely surrounded by clean air or vent air. Optionally the
secondary air will be biased towards the inner or outer air
outlets. In this context the inner side is the side nearest the
centre of the furnace and the outer side is the side next to the
front or real wall.
[0057] The secondary air outlets in a burner will preferably be
square or rectangular in shape; they will preferably be aligned
with the primary and vent air outlets.
[0058] The secondary air flow will for example deliver an overall
stoichiometry at the burner of .about.0.8, when including the sum
of primary, vent, secondary, and thermal biasing air streams.
[0059] An oil light-up burner may be located adjacent to the
primary and vent air nozzles; notionally there will typically be
one light-up burner for each pair of primary nozzles. Flame
scanning may follow conventional current practice.
[0060] In a more complete aspect of the invention, one or more
burner sets are disposed on a burner arch configured for downshot
firing of fuel in a combustion chamber of a combustion apparatus. A
burner set is disposed on a burner arch configured for downshot
firing with its respective outlet array directions aligned in a
general elongate arch direction. Thus the first outlet array
comprises in such an arch assembly a central array disposed in an
elongate arch direction along a burner arch configured for downshot
firing of a furnace. The respective second and third arrays
comprises respective inner and outer arrays disposed in an elongate
arch direction along a burner arch configured for downshot firing
of a furnace respectively towards an inner and an outer edge of the
arch.
[0061] The invention in accordance with this more complete further
aspect of the invention by analogy comprises a burner arch
configured for downshot firing of fuel in a combustion chamber of a
combustion apparatus having at least one burner set in accordance
with the first aspect of the invention disposed on the burner arch
whereby combustion of the fuel is supported in the vicinity of the
burner set during use.
[0062] Burners may be mounted on a single arch (for example to
produce a U-shaped flame) or mounted on a pair of opposing arches
(for example to produce a W-shaped flame).
[0063] According to the invention in a more complete aspect there
is provided a combustion apparatus comprising:
[0064] a combustion chamber defined by one or more combustion
chamber walls; at least one burner arch as above described
configured for downshot firing of fuel in the combustion
chamber;
[0065] at least one burner set as above described disposed on the
burner arch configured for downshot firing whereby combustion of
the fuel is supported in the vicinity of the burner set during
use.
[0066] The general principles of such a combustion apparatus
configured for downshot firing are established. The combustion
apparatus of the invention is distinctly characterised by the
provision of at least one burner set embodying the principles of
the first aspect in the invention.
[0067] Preferably, the combustion apparatus is further
characterised by the provision of air staging through the supply of
overtire air to the combustion chamber. Accordingly a burner set is
preferably provided in association with at least one outlet or
series of outlets for overfire air (OFA). Each supply of overfire
air may be directed into the combustion chamber at an angle to the
horizontal of between 0 degrees and 45 degrees downwardly.
[0068] In a particular preferred case a burner set is provided with
overfire air directed at first and second levels respectively below
and above a flame zone within the combustion chamber. Thus a burner
set is preferably provided in association with at least one outlet
or series of outlets for overfire air respectively provided at each
of first and second levels respectively below and above a flame
zone within the combustion chamber.
[0069] For example a first overfire air outlet or series of outlets
may be provided on a combustion chamber wall, and for example on
the front and rear combustion chamber walls of a rectangular
chamber, in the vicinity of and for example above and closely
adjacent to a hopper knuckle of the combustion chamber.
[0070] For example a second overfire air outlet or series of
outlets may be provided on a combustion chamber wall, and for
example on the front and rear combustion chamber walls of a
rectangular chamber, in the vicinity of and for example above and
closely adjacent to the burner arch.
[0071] As has been noted above, the invention is distinctly
characterised by the provision of at least one burner outlet set
embodying the principles of the first aspect in the invention, and
in particular with the specific outlet configuration and geometries
set out hereinabove, by means of which improved and more
symmetrical gross flow patterns can be achieved during downshot
firing, and a flame with a stable and repeatable ignition point can
more readily be created. It is intended that plural such burner
sets may be provided into a suitably configured downshot firing
system, and for example provided within a suitably configured
burner arch in generally familiar manner and applying generally
well established principles for downshot firing with which the
person skilled in the art will be familiar.
[0072] For example, a burner arch may comprise multiple laterally
spaced burner sets in accordance with the principles of the
invention. A burner arch may comprise other outlets in familiar
manner, for example to deliver wall air into the vicinity of the
combustion zone, to deliver thermal biasing air into the centre of
the combustion zone etc. A suitable example of a system for the
introduction of thermal biasing air may be found in WO9801701 A1
incorporated herein by reference.
[0073] Plural burner sets on a burner arch may be identical and
evenly spaced. Where plural burner sets are provided on a burner
arch, the burner arch may be configured such as to exhibit at least
mirror symmetry about a transverse midline. Conveniently, plural
burner sets in accordance with the principles of the invention may
be provided either side of a burner midline, with a central supply
of thermal biasing air. The burner sets may be arranged on an arch
in two discrete groups ("A" and "B" side) with a gap in the furnace
centre to provide a measure of thermal biasing.
[0074] Thermal bias air slots may be installed between the burner
groups to ensure separation, to enhance the cooling effect at the
centre of the furnace, and to provide an additional measure of air
staging. The individual thermal biasing air slots may notionally
have the same air flow as the equivalent secondary air slots, and
may be fed from the same windbox supply.
[0075] In familiar manner, symmetrically paired arches may be
provided to produce a stable W-shaped flame in the combustion
chamber during use.
[0076] Other features of these more complete aspects will be
understood by analogy.
[0077] The combustion apparatus is in the preferred case a furnace
for a steam generator in a thermal power plant which is fired by a
plurality of burner sets in accordance with the first aspect of the
invention.
[0078] Thus, in accordance with the most complete aspect of the
invention, there is also provided a thermal power plant comprising
at least one steam generator provided by thermal energy from a
furnace fired by burner sets as above described, with suitable fuel
supply means, and in fluid communication with suitable means to
generate electrical power from the steam produced by the steam
generator.
[0079] The invention will now be described by way of example only
with reference to FIGS. 1 to 8 of the accompanying drawings in
which:
[0080] FIG. 1 is a cut away side elevation of a prior art downshot
fired furnace,/steam generator arrangement for a thermal power
plant;
[0081] FIG. 2 is a plan view of a prior art burner outlet set of a
burner of the downshot fired furnace/steam generator arrangement of
FIG. 1;
[0082] FIG. 3 shows a possible arrangement for overfire air which
may be applied generally not only to the prior art furnace
illustrated in FIG. 1 but to a furnace modified to include burner
sets in accordance with the invention such as might be exemplified
in FIGS. 4-8;
[0083] FIGS. 4-8 show alternative example arrangements of burner
sets on half burner arches embodying the principles of the first
aspect of the invention.
[0084] An example prior art downshot firing arrangement is shown in
FIGS. 1 (furnace) and 2 (burner).
[0085] In a downshot firing arrangement for a steam generator 1
such as that illustrated, fuel and combustion air is introduced
into the furnace via burners 2 situated on the firing arches 3. The
inlet flows are directed generally downwards to the hopper 4.
Paired firing arches with equivalent burner arrangements are
provided so that in the idealised situation a symmetrical "W"
shaped flame pattern is developed. The flame geometry serves to
give a flame with a long residence time for more effective
combustion of low volatile fuels.
[0086] Because of the low volatile content of the coals typically
fired in downshot plant, flame stabilization cannot rely upon the
rapid release from volatiles combustion in the early part of the
flame (as practised in conventional swirl stabilized circular
burners for bituminous and low rank coals). Instead stabilization
is achieved by the recycling of heat from the hot products of
combustion through the following mechanisms:
[0087] radiation from the hot upward flowing combustion products
into the cold downward flowing inlet coal and air (6);
[0088] radiation from hot refractory surfaces in the lower furnace
into the cold inlet stream (7); refractory is typically installed
on the firing arches and the front/rear walls below the arches;
[0089] entrainment of hot products of combustion from the upward
flowing gases into the cold inlet stream (8).
[0090] The firing arrangement of FIG. 1 is shown in a furnace of a
thermal electric utility power plant. The thermal energy is used to
heat a process fluid and thereby drive a series of turbines in a
generator in familiar manner, and the skilled person will readily
infer the features of such a more complete thermal electric utility
power plant system. The invention is applicable to the general
furnace principles illustrated by FIG. 1, and is embodied in the
detail of the burner set arrangements such as illustrated by
example in FIGS. 4 to 8.
[0091] The fuel source is a low volatile carbonaceous fuel in
pulverous form such as a low volatile coal in pulverous form, for
example anthracite or lean coal.
[0092] Coal and transport air from the pulverizing plant is
separated into two streams in a cyclone (10); a "vent air" stream
11 containing typically 60% of the air and 10% of the coal, and a
"primary" stream 12 containing typically 40% of the air and 90% of
the coal. Following separation, the coal and air are introduced
into the furnace via an arrangement of slots in the firing arch and
described in more detail with reference to FIG. 2, a series of
slots each primary air/coal slot being located between a pair of
longer secondary (windbox) air slots.
[0093] Separation of the coal and transport air into a fuel lean
and a fuel rich stream aims to further improve ignition and flame
stability. Removal of air from the coal reduces the total mass of
fuel stream, and therefore reduces the time for this stream to be
heated to ignition temperature. Furthermore the higher
concentration of the coal creates a mixture that is more conducive
to combustion. It is generally considered that for low volatile
coals a particle concentration in the range 1.0.about.1.5
kg/m.sup.3 gives the highest propagation velocities (i.e. best
combustibility) (M Tarniguchi et al; "Comparison of flame
propagation properties of petroleum coke ad coals of different
rank"; Fuel 88 (2009), 1478-1484). In the described downshot system
the concentration of coal in the primary stream is typically 1.3
kg/m.sup.3.
[0094] A tertiary air stream 14 is introduced on the front and rear
walls just above the hopper knuckle.
[0095] Wall air, supplied from the secondary air windboxes, is
introduced at the outside edge of the furnace arch via slots
created by the removal of the membrane strip,
[0096] The various streams are introduced via a series of slots, as
shown in FIG. 2. Following separation, the coal and air are
introduced into the furnace via a series of slots, each primary
air/coal slot being located between a pair of longer secondary
(windbox) air slots. Vent air slots are additionally provided.
[0097] The slot dimensions are chosen to deliver the required inlet
velocities; in particular the velocity differential between the
primary and secondary streams facilitates the mixing between
them.
[0098] In the particular arrangement of FIG. 2, part of a burner
arch 20 is shown in simple schematic view carrying three burner
sets 21 per half arch, and a central outlet 22 for thermal biasing
air. This simple schematic is shown to the right of the figure. An
inlet showing greater detail of the slotted burner set 21 is shown
to the left of the figure.
[0099] In the illustrated burner set, four identical sets of
primary/vent/secondary slots are shown, each comprising an elongate
primary slot 25 and adjacent vent slot 26 with secondary slots 27
longer than and disposed either side of the primary slot 25. Wall
air is supplied through the wall air slots 28. This is an example
arrangement only. Similar arrangements with the primary and vent
slots swapped can be considered so that either the vent slots or
the primary slots may be next to the wall.
[0100] Although the general desire is to produce a symmetrical "W"
shaped flame pattern in practice there is often significant
asymmetry.
[0101] Typically the flames from one of the firing arches will
extend down to the hopper region before turning up (the "long"
flame path), while the flames on the opposite firing arch will turn
up almost immediately and bypass the lower furnace (the "short"
flame path). This behaviour will have an adverse effect on
combustion efficiency as the short flame has a reduced residence
time for burnout. It also raises concerns with in-furnace NOx
reduction technology based on staging of the combustion air, where
the reducing zone residence time is not well defined, and NOx
reduction performance is likely to be compromised for the short
flames. Finally the asymmetric flame pattern leads to significant
imbalances in the pattern of heat absorption in the furnace and
pendants, causing operational problems.
[0102] Some or all of these problems may be mitigated by
arrangements of primary/vent/secondary outlets in accordance with
the principles of the invention such as exemplified by the
illustrated embodiments of FIGS. 4 to 8. In particular in the
preferred case some or all of these problems may be mitigated by
provision of a system in which such burner sets are further
provided with overfire air (OFA), most preferably directed at first
and second levels respectively below and above a flame zone within
the furnace. The arrangement of level 1 OFA and level 2 OFA
described with reference to FIG. 3 also aims to mitigate flame
asymmetry.
[0103] FIG. 3 shows a possible arrangement for overfire air which
is applicable in general principle to burner sets in accordance
with the invention and may be viewed in the context of the general
furnace design of FIG. 1. Overfire air nozzles are provided at two
levels respectively below and above the flame.
[0104] Overfire air outlets (level 1) to produce the stream shown
will be located across the front and rear wall width below the
flame, for example just above the hopper knuckle. The air is angled
downwards (in the embodiment notionally 30.degree..about.45.degree.
below horizontal, although overfire air directed at any angle from
horizontal to 45.degree. below horizontal might be considered), and
turning vanes may be installed in the approach duct to reinforce
the downward direction. The air is typically introduced via a
series of nozzles. An example notional 15% of the air may be
supplied as level 1 OFA (equivalent to a stoichiometry of
.about.0.2). As well as air staging, the intention of the level 1
OFA is to draw the flames down towards the hopper, and to turn the
flames before they impinge in the hopper slope.
[0105] Overfire air outlets (level 2) to produce the stream shown
will be located across the front and rear wall width just above the
firing arch. The air is angled downwards (notionally
30.degree..about.45' below horizontal), and turning vanes may be
installed in the approach duct to reinforce the downward direction.
The air will be introduced via a series of nozzles. Notionally 15%
of the air will be supplied as level 2 OFA (equivalent to a
stoichiometry of .about.0.2). As well as air staging, the intention
of level 2 OFA ports is to prevent the premature turning of the
flames to reinforce the downward direction.
[0106] Air staging using conventional overfire air has not been
widely adopted for the firing of lower volatile coals in downshot
fired systems as it has been seen to have a detrimental effect on
flame stability. However, in preferred embodiments of the
invention, these overfire principles are effectively applicable to
burner sets in accordance with the principles of the invention such
as are exemplified in FIGS. 4 to 8.
[0107] All the embodiments exemplified in FIGS. 4 to 8 show a half
burner arch 40 with three burner nozzle sets 41 each embodying the
principles of the invention. Each example burner nozzle set has in
common a central array of primary 45 and vent 46 air slots with
inner 47 and outer 48 arrays of secondary air slots. The slots are
square or rectangular, but need not be identically configured,
although a burner set typically at least exhibits mirror symmetry
about both a longitudinal and a transverse mid line as regards the
disposition of these primary, vent air and secondary air slots. A
central thermal biasing air outlet 42 is optionally provided.
[0108] Other outlets (not shown) for example to deliver wall air,
may optionally be included.
[0109] The system comprises in familiar manner (not expressly
shown) a supply of pulverised low volatile coal which is for
example an on site pulverising plant and a supply of transport air.
The transport air entrains the pulverised coal in familiar manner
and transports the coal from the pulverising plant or other
supply.
[0110] The supply of transport air and coal is then split into
primary and vent air streams in accordance with suitable principles
such as will for example be familiar from the prior art. For
example the split between a primary stream and a vent air stream is
effected in a cyclonic separator.
[0111] Coal and transport air from the pulverizing plant is
separated into two streams in the cyclonic separator; a "vent air"
stream containing typically 60% of the air and 10% of the coal, and
a "primary" stream containing typically 40% of the air and 90% of
the coal. Following separation, the coat and air are introduced
into the furnace via an arrangement of slots in the firing arch via
primary air (PA), vent air (VA) and secondary air (SA) slots in
generally familiar manner, the invention being characterised by the
particular arrangement/configuration of slots in accordance with
the invention.
[0112] The invention is particularly characterised by the provision
of burner sets in accordance with the principles of the first
aspect of the invention, and such as might be exemplified by the
embodiments of burner set illustrated in FIGS. 4-8. Other aspects
of a more complete combustion system or furnace embodying the
principles of the invention not specifically shown in the drawings
or described herein will readily be inferred by the skilled person
from general knowledge of prior art downshot fired systems, whether
as generally exemplified by the system illustrated in figure or
otherwise. The invention encompasses all downshot fired systems
provided with burner shots embodying the principles of the first
aspect of the invention such as are illustrated in the following
figures.
[0113] In the embodiments shown a nozzle set provides a primary
outlet via a single primary air (PA) slot 45 or a pair of primary
slots 45 as the case may be. Each such primary slot or pair might
for example be referred to as a burner outlet, Each such primary
slot or pair (or burner outlet) has at least one vent air (VA) slot
46 at either side. Each primary or vent air slot 47, 48 in the
central row is aligned with exactly one secondary air (SA) slot in
a row transversely spaced on either side, not necessarily the same
shape as the central slot but in all the example cases of the same
width. Each burner outlet is thus completely surrounded by clean
air or vent air.
[0114] The following short description applies to each proposed
burner set configuration given by way of example.
[0115] The burner sets may first be categorised as grouped into one
of two options as follows.
[0116] Option A. Two PA slots per burner.
[0117] Option B: Single PA slot per burner. In the suggested
configurations of the embodiments such a PA slot can be made wider
than the VA slots. SA slots are made with the same widths as the
respective PA/VA slots to which they are aligned.
[0118] Each of the above options can then be sub-categorised based
on the manner in which the burner sets are defined. One possibility
is to assume each burner set has only one burner (ie only one
primary slot or adjacent pair or other adjacent plural series of
primary slots) while the other is to assume each burner set is
composed of two burners (two separate primary slots, or pairs or
series, in the context of the invention defined by and separated by
vent air slots). Dual burner sets are the current established
practice. Embodiments of the invention comprising two burners in a
set (and hence in accordance with the principles of the invention
two separate single primary slots, or adjacent pairs other adjacent
plural series of primary slots, separated by vent air slots) are
likely to be preferred.
[0119] Another important consideration for arranging burners within
the firing arch is to ensure they are better distributed as it is
beneficial for the NOx performance (plant operational experience
indicates that uniform distribution of burners within the firing
arch, hence uniform heat release, gives lower NOx emissions than
grouped and separated burner sets).
[0120] FIGS. 4 to 8 illustrate examples only of burner sets which
embody these principles.
[0121] In FIG. 4, a burner set comprising dual burners is shown.
Each burner comprises a pair of primary slots 45 disposed (with
reference to a direction longitudinal to the arch) between a pair
of vent air slots 46. Both the primary and the vent air slots are
rectangular and elongate in a direction transverse to the arch,
with the primary slots elongate to a greater extent. The primary
and vent air slots form a central array defining an array direction
coincident with the respective median lines of the rectangles.
[0122] Each primary and vent air slot is paired on either side in a
transverse direction by respective inner 47 and outer 48 secondary
slots. The secondary slots make up respectively the second and
third rows of slots so that each slot in the central row is
disposed between secondary slots on either side in a transverse
direction. The secondary slots are also rectangular, identical to
each other, and identical in longitudinal extent along the arch to
the longitudinal extent of the respective slots within the central
row with which they are paired.
[0123] The arrangement produces a situation in which each burner
outlet, in this case made up of a pair of adjacent primary slots,
is entirely surrounded by vent air or secondary slots. Two such
burners are provided in the illustrated embodiment.
[0124] Having regard to operational requirements and plant
geometrical constraints, such an arrangement might be too crowded
for many operational scenarios, and accordingly alternative
embodiments such as those exemplified in FIGS. 5 to 8 might be
explored instead.
[0125] In FIG. 5, a burner set comprising a single burner is shown.
The burner comprises a single primary slots 45 disposed between a
pair of vent air slots 46. Both the primary and the vent air slots
are rectangular and elongate in a transverse direction, with the
primary slots elongate to a greater extent. The primary and vent
air slots form a central array defining an array direction
coincident with the respective median lines of the rectangles.
[0126] Each primary and vent air slot is paired on either side in a
transverse direction by respective inner 47 and outer 48 secondary
slots. The secondary slots make up respectively the second and
third rows of slots so that each slot in the central row is
disposed between secondary slots on either side in a transverse
direction. The secondary slots are also rectangular, and identical
in longitudinal extent along the arch to the longitudinal extent of
the respective slots within the central row with which they are
paired, but in this case are of different lengths as shown. The
arrangement still maintains the principle however that the primary
slot is entirely surrounded by vent air or secondary slots.
[0127] In FIG. 6, a burner set comprising dual burners is shown.
Each burner comprises a single primary slot 45 disposed between a
pair of vent air slots 46. Each primary and vent air slot is again
paired on either side in a transverse direction by respective inner
47 and outer 48 secondary slots.
[0128] In FIG. 7, a burner set comprising a single burner is shown.
The burner comprises a single primary slot 45 disposed between a
pair of vent air slots 46. Each primary and vent air slot is again
paired on either side in a transverse direction by respective inner
47 and outer 48 secondary slots. It differs from the FIG. 5
embodiment in that the outlet slots have a different aspect
ratio.
[0129] In FIG. 8, a burner set comprising dual burners is shown.
Each burner comprises a single primary slot 45 disposed between a
pair of vent air slots 46. Each primary and vent air slot is again
paired on either side in a transverse direction by respective inner
47 and outer 48 secondary slots. It differs from the FIG. 6
embodiment in that the outlet slots have a different aspect
ratio.
[0130] All these alternative arrangements maintain the principle
that the primary slot is entirely surrounded by vent air or
secondary slots. Although the invention is not limited by
particular theory, it is generally suggested that such an
arrangement can assist in supporting a flame whose ignition point
is clearly defined in a robustly repeatable manner and encouraging
a symmetrical flame path.
* * * * *