U.S. patent application number 13/731648 was filed with the patent office on 2014-07-03 for biomass combustion.
This patent application is currently assigned to Air Liquide, Societe Anonyme pour Etude et Exploitation des Procedes Gorges Claude. The applicant listed for this patent is Air Liquide, Societe Anonyme pour Etude et Exploitation des Procedes Georges Claude. Invention is credited to Sebastian DE LIMON, Remi Pierre TSIAVA, Yuan XUE.
Application Number | 20140182491 13/731648 |
Document ID | / |
Family ID | 49943586 |
Filed Date | 2014-07-03 |
United States Patent
Application |
20140182491 |
Kind Code |
A1 |
XUE; Yuan ; et al. |
July 3, 2014 |
BIOMASS COMBUSTION
Abstract
Improved combustion of biomass is achieved by injected first and
second streams of biomass from a burner where the first stream of
biomass has a median particle size larger than the biomass of the
second stream and oxygen is injected with the first stream to
provide an oxygen-enriched environment around the larger median
sized particles. The oxygen-enriched environment is achieved either
by injecting the oxygen directly into the first stream or by
premixing the oxygen with the conveying air of the first
stream.
Inventors: |
XUE; Yuan; (Wilmington,
DE) ; TSIAVA; Remi Pierre; (Houston, TX) ; DE
LIMON; Sebastian; (Houston, TX) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Exploitation des Procedes Georges Claude; Air Liquide, Societe
Anonyme pour Etude et |
|
|
US |
|
|
Assignee: |
Air Liquide, Societe Anonyme pour
Etude et Exploitation des Procedes Gorges Claude
Paris
FR
|
Family ID: |
49943586 |
Appl. No.: |
13/731648 |
Filed: |
December 31, 2012 |
Current U.S.
Class: |
110/346 ;
110/105; 110/220; 110/255; 110/297; 431/181 |
Current CPC
Class: |
Y02E 20/34 20130101;
F23L 7/007 20130101; F23G 2209/26 20130101; F23D 1/00 20130101;
F23L 2900/07005 20130101; F23G 2201/602 20130101; F23G 7/10
20130101; F23G 5/02 20130101; Y02E 20/344 20130101; F23G 2205/14
20130101; F23G 5/14 20130101 |
Class at
Publication: |
110/346 ;
110/220; 110/255; 110/297; 110/105; 431/181 |
International
Class: |
F23L 7/00 20060101
F23L007/00; F23G 5/02 20060101 F23G005/02; F23G 5/14 20060101
F23G005/14; F23G 7/10 20060101 F23G007/10 |
Claims
1. A biomass combustion method, comprising the steps of: injecting
a first stream of fuel comprising particulate biomass, air, and
oxygen from a burner into a combustion chamber; injecting a second
stream of fuel comprising particulate biomass and air from the
burner into the combustion chamber, the second stream not including
any oxygen apart from the air present in the second stream;
combusting the biomass, air and oxygen of the first stream in the
combustion chamber; and combusting the biomass and air of the
second stream in the combustion chamber, wherein the particulate
biomass in the first stream has a median particle size larger than
that of the particulate biomass in the second stream.
2. The biomass combustion method of claim 1, wherein the biomass is
selected from the group consisting of wood pellets, straw, hog
fuel, crushed olive stones, dried sewage sludge, wood dust, and
combinations thereof.
3. The biomass combustion method of claim 1, wherein the median
particle size of the biomass of the first stream is less than 300
microns and the median particle size of the biomass of the second
stream is greater than 400 microns.
4. The biomass combustion method of claim 1, wherein the injected
oxygen is no greater than 8% vol/vol of the total amount of oxidant
injected from the burner.
5. The biomass combustion method of claim 1, wherein the injected
oxygen is supplied by an oxygen source selected from the group
consisting of an air separation unit, a vapor swing adsorption
unit, a vaporizer fed with liquefied oxygen, an oxygen pipeline,
and combinations thereof.
6. The biomass combustion method of claim 1, further comprising the
step of separating a biomass feedstock into a first flow of biomass
having a relatively larger median particle size and a second flow
of biomass having a relatively smaller median particle size,
wherein the first stream is derived from the first flow and the
second stream is derived from the second flow.
7. The biomass combustion method of claim 6, wherein: the first
flow is fed to a first hopper; the second flow is fed to a second
hopper; the first stream is drawn from the first hopper; and the
second stream is drawn from the second hopper.
8. The biomass combustion method of claim 6, wherein the oxygen is
premixed with the air of the first stream of biomass.
9. A biomass combustion system, comprising a biomass burner, a
biomass particle size separator, first and second biomass hoppers,
first and second blowers, first and second fuel conduits, and a
source of oxygen, wherein: the biomass particle size separator is
adapted and configured to separate a biomass feed stock into first
and second flows of biomass, the biomass in the first flow having a
median particle size larger than that of the biomass in the second
flow; the first and second biomass hoppers receive the first and
second flows of biomass, respectively; the first blower is adapted
and configured to direct a first stream of biomass from the first
biomass hopper, conveyed with air from the first blower, to the
biomass burner; the second blower is adapted and configured to
direct a second stream of biomass from the second biomass hopper,
conveyed with air from the second blower, to the biomass burner;
the burner comprises a first injector receiving the first stream of
biomass and a second injector receiving the second stream of
biomass; the first fuel injector receives the first stream of
biomass and is adapted and configured to inject it from the burner
into a combustion chamber; the second fuel injector receives the
second stream of biomass and is adapted and configured to inject it
from the burner into a combustion chamber; the burner receives
oxygen from the oxygen source and is adapted and configured to
inject it with the first stream of biomass injected from the burner
by the first fuel injector either premixed with the air of the
first stream of biomass or not premixed with the air of the first
stream of biomass.
10. The biomass combustion system of claim 9, wherein said source
of oxygen is selected from group consisting of a vacuum swing
adsorption system, an oxygen pipeline, a cryogenic air separation
unit, and a vaporizer connected to a tank of liquid oxygen.
11. The biomass combustion system of claim 9, wherein: the oxygen
is injected with the first stream of biomass through injection of
the oxygen by an oxygen injector disposed concentrically within the
first fuel injector; the first and second fuel injectors are
annular; and the second fuel injector is disposed concentrically
around the first fuel injector.
12. The biomass combustion system of claim 9, wherein: the oxygen
is injected with the first stream of biomass through injection of
the oxygen by a plurality of oxygen injectors radially distributed
within the first fuel injector; and the first and second fuel
injectors are annular.
13. The biomass combustion system of claim 9, wherein: the oxygen
is injected with the first stream of biomass through injection of
the oxygen by an oxygen injector disposed within the first fuel
injector; and the first fuel injector is disposed parallel and
adjacent to the second fuel injector.
14. A biomass-fired boiler installation, comprising the biomass
combustion system of claim 9 and a boiler, wherein the burner is
oriented to inject the oxygen and first and second streams of
biomass into a combustion chamber in an interior of the boiler.
15. The biomass-fired boiler installation of claim 14, wherein
there are a plurality of the burners.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] None.
BACKGROUND
[0002] 1. Field of the Invention
[0003] The present invention relates to biomass burners and methods
of combusting biomass.
[0004] 2. Related Art
[0005] The emission of carbon dioxide (CO.sub.2) as a cause of
global warming is of current concern to the power industry. The
potential role of biomass energy acquired a new dimension when it
was suggested that planting large areas of new forest could slow
the increase in atmospheric carbon dioxide by removing carbon
dioxide from the atmosphere. Therefore, the electric power industry
uses biomass in order to significantly reduce CO.sub.2
emissions.
[0006] In a typical staged combustion burner firing biomass, the
biomass fuel is combusted with primary combustion air in a first
combustion zone. Any non-combusted fuel is more completely
combusted in a second combustion zone downstream of the first
combustion zone with secondary combustion air injected around the
biomass. If tertiary combustion air is utilized, combustion is
completed in a third combustion zone downstream of the second
combustion zone with tertiary combustion air injected around the
secondary combustion air.
[0007] The primary combustion air must have a velocity that is
sufficiently high enough to convey the biomass particles from the
burner and into the combustion space. Too low of a primary
combustion air velocity will cause the particles to settle within
the burner. Conversely, too high of a primary combustion air
velocity will result in a residence time of the biomass particles
that is too short to allow satisfactory burnout of the particles
along the path line through the furnace. This might be no problem
for relatively small biomass particles since they generally require
shorter residence times for satisfactory burnout. However, this can
be a problem for relatively larger biomass particles due to the
higher residence times necessary for their satisfactory burnout
along the path line.
[0008] In order to overcome the above-discussed problem presented
by larger biomass particle sizes, the larger biomass particles
produced by comminuting process can be separated from the smaller
particles and then subjected to the comminuting process again. This
milling and re-milling of the particles is complex and costly. It
is complex due to the need for size-separation equipment. Moreover,
the milling equipment must either be over-sized in order to
thoroughly process all the biomass feed stock into adequately sized
particles or it must necessarily result in a lower throughput in
terms of adequately sized particles due to the presence of a side
stream of larger particles needing to be recycled back to the
milling device. The cost is in part due to the relatively poor
grindability of biomass. While both biomass and coal can be ground
to a desired size distribution, the grindability of biomass is
significantly higher than that of coal due to its fibrous nature.
Thus, for a given reduction from an initial particle size
distribution (such as d.sub.50<1000 m.mu.) to a final particle
size (such as d.sub.50<200 m.mu.), far more energy is consumed
in achieving the particle size reduction in biomass as compared to
coal. Additionally, the amount of energy necessary for size
reduction increases exponentially as the final particle size
decreases.
[0009] Even if the above-described problem associated with larger
biomass particles (i.e., the burnout time exceeds the residence
time along the path line through the furnace) is not experienced
when operating a burner at its nominal rated power, it can appear
when the burner is operated at higher powers. The nature of the
problem experienced at higher powers is described below.
[0010] In comparison to the relatively lower primary combustion air
velocities when the burner is operated at lower power, the
relatively higher primary combustion air velocities at higher
burner powers changes the pattern of heat transfer from the
combusting particles to the furnace. More particularly and in
comparison to lower burner powers, relatively less heat is
transferred to portions of the furnace closer to the burners and
relatively more heat is transferred to portions of the furnace
relatively distant from the burners. This shift in the amount of
heat transferred to portions of the furnace adjacent the
superheater can result in damage to that portion of the furnace
because it is not designed for excessive radiative heat
transfer.
[0011] A related disadvantage is realized for biomass furnaces that
were originally commissioned as coal-fired furnaces but which have
been retrofitted for biomass combustion. Coal-fired furnaces are
designed to be heated by a large number of burners. Together, those
burners provide a nominal power at which the furnace is designed to
operate. The nominal power is related to the heat flux from
combustion of the coal to water or stream in the boiler steam tubes
and which is realized in the form of mechanical or electrical
power. If the furnace is retrofitted with conventional biomass
burners, at relatively high biomass fuel firing rates the burners
may fall well short of the nominal power due to unsatisfactory
burnout of the biomass particles. Primarily, this is many biomass
fuels have median particle sizes of 200-500 .mu.m. This may be
contrasted with pulverized coal particles which typically have an
average size of around 60 .mu.m and a d.sub.90 of <100 .mu.m.
Although the furnace may have been designed to achieve the nominal
power with the more quickly combusting coal particles, the more
slowly combusting biomass particles shifts the pattern of heat
transfer from the combusting particles to the furnace. In
particular, less heat is transferred to portions of the furnace
adjacent to upstream portions of the path line and more heat is
transferred to portions of the furnace adjacent to downstream
portions of the path line. Typical furnaces are not designed for
such a modified heat transfer pattern where much of the heat
transfer is shifted downstream along the path line. So, as the flow
rate of the biomass fuel from the burner is increased in an attempt
to increase the power, the apparent power of the burner soon
reaches a limit beyond which it is difficult to increase by
increasing the flow rate of the biomass fuel. As a result of the
foregoing, when a coal-fired boiler is retrofitted to combust
biomass, more than 20 to 30% reductions in productivity can be
experienced.
[0012] There have been several biomass combustion processes
proposed in the patent literature.
[0013] U.S. patent application Ser. No. 13/479,877, filed May 24,
2012, and entitled "Biomass Combustion". Application '877 discloses
the splitting of an inner flow of biomass (conveyed with air) into
a central flow and an annular flow with a splitter in a burner.
Oxygen is injected either inside the central flow or in an annulus
surrounding the central flow and secondary and optionally tertiary
air is injected annularly around central and annular flows of
biomass. Due to the difference in velocities between the oxygen
flow and the central flow of biomass, enhanced mixing between these
two flows results in relatively higher oxygen concentrations
surrounding the biomass particles in the central flow. This tends
to allow earlier flame ignition and increases the rate at which the
biomass particles combust--all without requiring the biomass to be
fired with a high cost, supplementary biofuel such as rapeseed oil.
Application '877 does not disclose the use of separate flows of
biomass fuel having different particle sizes.
[0014] U.S. Pat. No. 5,107,777 describes combustion of a low BTU
high moisture biomass such as wood (known as Hog fuel). Biomass is
injected into the boiler 15-20 ft above the floor. The combustion
air, which is supplied from the bottom of the furnace, is enriched
with oxygen to a level of between 0.1 to 7%. Additional oxygen is
also injected from the side. Oil burners are fired from the top. It
claims that a higher flame temperature is achieved with injection
of oxygen.
[0015] US 2008/0261161 A1 describes a burner or furnace for the
combustion of biomass using two or more fuel injection ports
located at non-radial injection angles. The biomass is mixed with
oxidizer and then injected into the furnace via a cyclonic
combustion vortex.
[0016] U.S. Pat. No. 6,699,029 B2 describes a boiler system where a
low rank fuel is burned to achieve energy generation rate similar
to that achieved with conventional fuels such as coal. It proposes
certain oxygen injection methods for reducing the formation of
nitrogen oxides (NOx). Operations with typical US-origin coals are
described.
[0017] Thus, it is an object to provide biomass combustion systems
and methods that allow complete burnout of relatively larger
biomass particles. It is another object to provide biomass
combustion systems and methods combusting relatively larger biomass
particles that do not damage portions of the furnace that are not
designed for excessive radiative heat transfer. It is yet another
object to provide biomass combustion systems and methods combusting
relatively larger biomass particles that do not limit the apparent
power of the burner as the fuel flow rate is increased.
SUMMARY
[0018] There is disclosed a biomass combustion method comprising
the following steps. A first stream of fuel comprising particulate
biomass, air, and oxygen is injected from a burner into a
combustion chamber. A second stream of fuel comprising particulate
biomass and air from the burner is injected into the combustion
chamber, the second stream not including any oxygen apart from the
air present in the second stream. The biomass, air and oxygen of
the first stream are combusted in the combustion chamber. The
biomass and air of the second stream are combusted in the
combustion chamber, wherein the particulate biomass in the first
stream has a median particle size larger than that of the
particulate biomass in the second stream.
[0019] There is also disclosed a biomass combustion system,
comprising a biomass burner, a biomass particle size separator,
first and second biomass hoppers, first and second blowers, first
and second fuel conduits, and a source of oxygen. The biomass
particle size separator is adapted and configured to separate a
biomass feed stock into first and second flows of biomass, the
biomass in the first flow having a median particle size larger than
that of the biomass in the second flow. The first and second
biomass hoppers receive the first and second flows of biomass,
respectively. The first blower is adapted and configured to direct
a first stream of biomass from the first biomass hopper, conveyed
with air from the first blower, to the biomass burner. The second
blower is adapted and configured to direct a second stream of
biomass from the second biomass hopper, conveyed with air from the
second blower, to the biomass burner. The burner comprises a first
injector receiving the first stream of biomass and a second
injector receiving the second stream of biomass. The first fuel
injector receives the first stream of biomass and is adapted and
configured to inject it from the burner into a combustion chamber.
The second fuel injector receives the second stream of biomass and
is adapted and configured to inject it from the burner into a
combustion chamber. The burner receives oxygen from the oxygen
source and is adapted and configured to inject it with the first
stream of biomass injected from the burner by the first fuel
injector either premixed with the air of the first stream of
biomass or not premixed with the air of the first stream of
biomass.
[0020] There is also disclosed a biomass-fired boiler installation,
comprising the above-disclosed biomass combustion system and a
boiler, wherein the burner is oriented to inject the oxygen and
first and second streams of biomass into a combustion chamber in an
interior of the boiler.
[0021] The above-disclosed method, system and/or installation can
include one or more of the following aspects: [0022] the biomass is
selected from the group consisting of wood pellets, straw, hog
fuel, crushed olive stones, dried sewage sludge, wood dust, and
combinations thereof. [0023] the median particle size of the
biomass of the first stream is less than 300 microns and the median
particle size of the biomass of the second stream is greater than
400 microns. [0024] the injected oxygen is no greater than 8%
vol/vol of the total amount of oxidant injected from the burner.
[0025] the injected oxygen is supplied by an oxygen source selected
from the group consisting of an air separation unit, a vapor swing
adsorption unit, a vaporizer fed with liquefied oxygen, an oxygen
pipeline, and combinations thereof. [0026] a biomass feedstock is
separated into a first flow of biomass having a relatively larger
median particle size and a second flow of biomass having a
relatively smaller median particle size, wherein the first stream
is derived from the first flow and the second stream is derived
from the second flow. [0027] the first flow is fed to a first
hopper, the second flow is fed to a second hopper, the first stream
is drawn from the first hopper, and the second stream is drawn from
the second hopper. [0028] the oxygen is premixed with the air of
the first stream of biomass. [0029] said source of oxygen is
selected from group consisting of a vacuum swing adsorption system,
an oxygen pipeline, a cryogenic air separation unit, and a
vaporizer connected to a tank of liquid oxygen. [0030] the oxygen
is injected with the first stream of biomass through injection of
the oxygen by an oxygen injector disposed concentrically within the
first fuel injector, wherein the first and second fuel injectors
are annular, and wherein the second fuel injector is disposed
concentrically around the first fuel injector. [0031] the oxygen is
injected with the first stream of biomass through injection of the
oxygen by a plurality of oxygen injectors radially distributed
within the first fuel injector, wherein the first and second fuel
injectors are annular. [0032] the oxygen is injected with the first
stream of biomass through injection of the oxygen by an oxygen
injector disposed within the first fuel injector, wherein the first
fuel injector is disposed parallel and adjacent to the second fuel
injector. [0033] there are a plurality of the burners.
BRIEF DESCRIPTION OF THE DRAWINGS
[0034] For a further understanding of the nature and objects of the
present invention, reference should be made to the following
detailed description, taken in conjunction with the accompanying
drawings, in which like elements are given the same or analogous
reference numbers and wherein:
[0035] FIG. 1 is a schematic, cross-sectional view of an embodiment
of a burner for use with the invention.
[0036] FIG. 2 is a schematic, cross-sectional view of another
embodiment of a burner for use with the invention.
[0037] FIG. 3 is a schematic, cross-sectional view of another
embodiment of a burner for use with the invention.
[0038] FIG. 4A is an elevation, cross-sectional view of a boiler
including side-fired burners for use with the invention.
[0039] FIG. 4B is a top, cross-sectional view of the boiler of FIG.
3A.
[0040] FIG. 4C is an elevation, front-face view of an embodiment of
one of the side-fired burners of FIGS. 3A and 3B.
[0041] FIG. 4D is an elevation, front-face view of another
embodiment of one of the side-fired burners of FIGS. 3A and 3B.
[0042] FIG. 4E is an elevation, front-face view of yet another
embodiment of one of the side-fired burners of FIGS. 3A and 3B.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0043] By injecting oxygen with a stream of relatively larger sized
biomass particles conveyed with air (i.e., the first stream), the
resultant oxygen-enriched air surrounding the particles allows them
to be ignited sooner. Satisfactory burnout is also achieved. A
separate stream of relatively smaller sized biomass particles
(i.e., the second stream) need only be combusted with air (no
oxygen need be injected with the stream) because their smaller size
already allows earlier ignition and shorter burnout times. Because
the combustion air is not globally enriched with oxygen, the burner
consumes far less oxygen. Because satisfactory burnout of the
larger size biomass particles may be achieved, it is not necessary
to expend significant amounts of additional energy in reducing the
size of the larger size biomass particles in order to obtain the
small size particles that enjoy earlier ignition and shorter
burnout times. The oxygen may be injected with the relatively
larger biomass particles by premixing the oxygen with the air
conveying the relatively larger biomass particles or the oxygen may
be injected into the stream of larger biomass particles from an
oxygen injector inside the burner.
[0044] A stable flame rooted near the face of the burner is
achieved because the locally high concentration of oxygen in the
stream of larger biomass particles allows those particles to be
ignited more easily and at an earlier point than they would
otherwise without such a locally high concentration of oxygen.
Thus, a combustion reaction between oxygen (from the oxygen and the
air) and the larger biomass particles is commenced at an upstream
zone adjacent the burner face. The greater surface area to volume
or mass ratio of the relatively smaller biomass particles allows
them to be combusted with only air (and not oxygen). Thus, there is
no need to globally enrich the air injected from the burner from
all sources of combustion air.
[0045] The type of burner is not critical to the invention.
[0046] A pipe-in-pipe burner may be utilized where one of the two
streams of biomass is injected from the inner pipe while the other
of the two streams of biomass is injected from the outer pipe. In
this case, the oxygen is injected in one of a variety of ways. It
may be injected in a premixed state with the air of the stream of
larger biomass particles. It may instead be injected from an oxygen
injector disposed within the inner pipe in which case the stream of
larger biomass particles is injected from the inner pipe. It may
instead be injected from a plurality of oxygen injectors radially
distributed within the outer pipe in which case the stream of
larger biomass particles is injected from the outer pipe.
[0047] The burner may instead be of the side wall fired type
including two fuel ports (one for each stream of biomass) and an
overfire air port. The oxygen may be injected in a premixed state
with the air of the stream of larger biomass particles. It may
instead by injected from an oxygen injector disposed within the
fuel port from which the stream of larger biomass particles is
injected. In either case, the fuel ports and/or oxygen injector may
be of any configuration.
[0048] The combustion air may be swirled or not. The combustion air
may be two streams each one of which conveys one of the streams of
biomass or additional combustion air streams may be utilized
including a secondary combustion air stream surrounding or adjacent
to or spaced from the biomass streams and even a tertiary
combustion air stream surrounding or adjacent to or spaced from the
secondary combustion air stream. Either or both of the secondary
and tertiary combustion air streams may be swirled.
[0049] While the biomass fuel may be any biomass fuel known in the
art, typically it is wood pellets, straw, or so-called hog
fuel.
[0050] The oxygen is industrially pure oxygen. The specific purity
of the industrially pure oxygen depends upon the method of
production and whether or not the produced oxygen is further
purified. For example, the industrially pure oxygen may be gaseous
oxygen from an air separation unit that cryogenically separates air
gases into predominantly oxygen and nitrogen streams in which case
the gaseous oxygen has a concentration exceeding 99% vol/vol. The
industrially pure oxygen may be produced through vaporization of
liquid oxygen (which was liquefied from oxygen from an air
separation unit, in which case it, too, has a purity exceeding 99%
vol/vol. The industrially pure oxygen may be also be produced by a
vacuum swing adsorption (VSA) unit in which case it typically has a
purity of about 92-93% vol/vol. The industrially pure oxygen may be
sourced from any other type of oxygen production technology used in
the industrial gas business.
[0051] As best illustrated in FIG. 1, a first type of pipe-in-pipe
burner includes an inner first fuel injector 1, an outer second
fuel injector 12, and a secondary combustion air pipe 14 disposed
within a burner block B which is mounted on a wall W of the
combustion space C. The burner injects a first stream 8 of larger
particle size biomass conveyed with air from the fuel injector 1.
Oxygen is injected into stream 8 from a single oxygen injector 2
which is disposed within the first fuel injector 1. A second stream
6 of smaller particle size biomass conveyed with air is injected
from the second fuel injector 12. Finally, a stream 5 of secondary
combustion air is injected from the secondary air pipe 14 through
swirler 10. The biomass from the first stream 8 combusts with the
oxygen and conveying air in an inner core of the flame while the
biomass from the second stream 6 combusts with conveying air in an
outer portion of the flame.
[0052] As best shown in FIG. 2, a second type of pipe-in-pipe
burner again includes an inner first fuel injector 1, an outer
second fuel injector 12, and a secondary combustion air pipe 14
disposed within a burner block B which is mounted on a wall W of
the combustion space C. The burner injects a first stream 8 of
smaller particle size biomass conveyed with air from the fuel
injector 1. A second stream 6 of smaller particle size biomass
conveyed with air is injected from the second fuel injector 12.
Instead of injecting oxygen into one of the streams 8, 6, the
oxygen is already premixed with the conveying air of one of the
streams 8, 6, so there are no oxygen injectors per se within the
burner. Finally, a stream 5 of secondary combustion air is injected
from the secondary air pipe 14 through swirler 10. If the larger
size biomass is injected as stream 8, it combusts with
oxygen-enriched conveying air in an inner core of the flame while
the biomass from the second stream 6 combusts with the conveying
air in an outer portion of the flame. If the larger size biomass is
injected as stream 6, it combusts with oxygen-enriched conveying
air in an outer portion of the flame while the biomass from the
first stream 8 combusts with the conveying air in the inner core of
the flame.
[0053] In a variation of the burner of FIG. 1 and as best
illustrated in FIG. 3, a third type of pipe-in-pipe burner includes
an inner first fuel injector 1, an outer second fuel injector 12,
and a secondary combustion air pipe 14 disposed within a burner
block B which is mounted on a wall W of the combustion space C. The
burner injects a first stream 8 of smaller particle size biomass
conveyed with air from the fuel injector 1. A second stream 6 of
smaller particle size biomass conveyed with air is injected from
the second fuel injector 12. Oxygen is injected into stream 8 from
a plurality of oxygen injectors 2 which is radially distributed
within the second fuel injector 12. There are at least three oxygen
injectors 2 in the burner of FIG. 3 with an upper number only being
limited by the available space and the pressure drop in the stream
6 created by the injectors 2. Finally, a stream 5 of secondary
combustion air is injected from the secondary air pipe 14 through
swirler 10. The biomass from the first stream 8 combusts with
conveying air in an inner core of the flame while the biomass from
the second stream 6 combusts with the oxygen and conveying air in
an outer portion of the flame.
[0054] The oxygen injector 2 may be configured in any one of
several ways known in the field of oxy-combustion. Thus, the oxygen
lance 2 may be a straight tube with a constant diameter.
Alternatively, the oxygen lance 2 may diverge at its outlet end
where the oxygen mixes with the one of streams 6, 8.
[0055] As best shown in FIGS. 4A-E, the burner may instead be
configured as a side wall-fired burner 32 mounted on one of the
walls of the interior of a boiler 31.
[0056] FIG. 4C illustrates a first variation of the burner 32 where
the first stream of larger size biomass particles is injected from
a first fuel port 42 and the second stream of smaller size biomass
particles is injected from a second fuel port 41. Secondary
combustion air is injected from air ports 44 above and below the
fuel ports 41, 42. The oxygen is injected from an oxygen channel
(i.e., oxygen injector) 43 disposed vertically within the first
fuel port 42. FIG. 4D illustrates a second variation of the burner
32 similar to that of FIG. 4E where the oxygen channel (i.e.,
oxygen injector) 43 is disposed horizontally within the first fuel
port 42. Finally, FIG. 4E illustrates a third variation of the
burner 32 similar to those of FIGS. 4D-E where the oxygen channel
(i.e., oxygen injector) 43 has a cross-shape. In each of the
burners of FIGS. 4C-E, the surface area of the oxygen injector
typically does not take over 30% of the surface area of the first
fuel port 42 from which the larger size biomass stream is injected.
Instead of the oxygen channel (i.e., oxygen injector) 43
configurations of FIGS. 4C-E, the oxygen injector could comprise a
plurality of pipes adjacent to one another instead of a channel
(not shown).
[0057] Regardless of whether the oxygen is injected according to
the burner configurations of FIG. 1, 2, 3, or 4C-E, the presence of
oxygen in a stream of larger size biomass (whether the oxygen is
pre-mixed with the combustion air or injected into such stream)
helps increase burnout of that fuel in comparison to a conventional
biomass burner where no such oxygen injection is employed. Burnout
is increased because the local oxygen concentration surrounding the
larger biomass particles is increased. An oxygen-enriched
atmosphere at this region not only starts combustion of volatile
components in the biomass particles earlier but also starts
combustion of char earlier. As a result, satisfactory burnout of
the biomass particles is completed in the path line of the biomass
particles inside the furnace at a point earlier in comparison to
biomass particles from biomass burners where no such oxygen
injection is performed. There is no need to inject oxygen into the
stream of smaller biomass particles or enrich its conveying air
because the smaller size of those particles allows them to be
satisfactory burned out more quickly.
[0058] Faster burnout of the biomass particles is advantageous for
allowing satisfactory operation of the biomass burner at higher
apparent powers. This will be clearly evident when compared to
operation of a conventional biomass burner in which no oxygen
injection or premixing of the conveying air is performed. When
conventional biomass burners are operated at lower powers, the flow
rate of primary combustion air necessary for satisfactory
conveyance of the biomass particles has a velocity sufficiently low
that satisfactory burnout of the biomass particles may be achieved
over the path line traveled by the particles through the furnace.
At higher burner powers, the flow rate of primary combustion air
that is necessary for satisfactory conveyance of the biomass
particles must be increased because the total mass of solid biomass
particles is increased. As the flow rate of the primary combustion
air is increased, it will soon reach a velocity that is too high to
allow satisfactory burnout of the solid biomass particles along the
path line through the furnace and enter the superheater. In other
words, the residence time of a combusting biomass particle is
decreased when higher velocity combustion air is used (such as at
higher burner powers). Such a situation creates several
disadvantages.
[0059] One disadvantage is related to wear to the furnace. In
comparison to the relatively lower combustion air velocities when
the burner is operated at lower power, the relatively higher
combustion air velocities at higher burner powers changes the
pattern of heat transfer from the combusting particles to the
furnace. More particularly and in comparison to lower burner
powers, relatively less heat is transferred to portions of the
furnace closer to the burners and relatively more heat is
transferred to portions of the furnace relatively distant from the
burners. This shift in the amount of heat transferred to portions
of the furnace adjacent the superheater can result in damage to
that portion of the furnace because it is not designed for
excessive radiative heat transfer.
[0060] The second disadvantage is realized for biomass furnaces
that were originally commissioned as coal-fired furnaces but which
have been retrofitted for biomass combustion. Coal-fired furnaces
are designed to be heated by a large number of burners. Together,
those burners provide a nominal power at which the furnace is
designed to operate. The nominal power is related to the heat flux
from combustion of the coal to water or stream in the boiler steam
tubes and which is realized in the form of mechanical or electrical
power. If the furnace is retrofitted with conventional biomass
burners, at relatively high biomass fuel firing rates the burners
may fall well short of the nominal power due to unsatisfactory
burnout of the larger biomass particles. Primarily, this is because
the larger biomass particles (with a median size of around 200-500
.mu.m) combust more slowly than typical pulverized coal particles
(with an average size of around 60 .mu.m). Although the furnace may
have been designed to achieve the nominal power with the more
quickly combusting coal particles, the more slowly combusting
larger biomass particles shifts the pattern of heat transfer from
the combusting particles to the furnace. In particular, less heat
is transferred to portions of the furnace adjacent to upstream
portions of the path line and more heat is transferred to portions
of the furnace adjacent to downstream portions of the path line.
Typical furnaces are not designed for such a modified heat transfer
pattern where much of the heat transfer is shifted downstream along
the path line. So, as the flow rate of the biomass fuel from the
burner is increased in an attempt to increase the power, the
apparent power of the burner soon reaches a limit beyond which it
is difficult to increase by increasing the flow rate of the biomass
fuel.
[0061] When conventional biomass burners are instead fed only with
relatively smaller size biomass particles in an effort to avoid the
above two disadvantages, such an avoidance still results in a
higher cost associated with more grinding/milling of all of the
biomass and/or wasting of the larger biomass particles after
separation from the smaller biomass particles. The energy consumed
in milling/grinding biomass is a major component of its cost as a
fuel.
[0062] In contrast, by enriching the larger biomass particles with
oxygen (either through injection or premixing with the conveying of
that stream), the above disadvantages may be avoided without having
to increase the energy consumption and cost associated with
conventional biomass burner operation. The higher oxygen
concentrations surrounding the larger biomass particles tends to
ignite the flame earlier and increases the rate at which the larger
biomass particles combust. As a result, the impact of the
downstream shift in heat transfer that would otherwise be
experienced in furnaces fired with conventional biomass burners
(fired with larger size biomass particles or a mixture of larger
and smaller biomass particles) is reduced or nullified by the
increase in the rate of combustion of the biomass particles
afforded by the localized oxygen-enriched environment. Because the
distribution of heat transfer from the combusting particles to the
furnace more closely matches the distribution of heat transfer that
the furnace was originally designed for when it was commissioned as
a coal-fired furnace, the apparent power of the burner may still be
increased through an increase in the flow rate of the biomass fuel
from the burner. Also, the above-described increase in furnace wear
caused by conventional biomass burners is either decreased or
avoided.
[0063] Thus, the invention provides multiple benefits. The
invention can improve the overall system efficiency with minimum
modifications on the current boiler combustion system. It can
reduce a power plant's CO.sub.2 foot print. Oxygen enrichment will
reduce the flue gas volume. Oxygen enrichment of only the stream of
larger biomass particles is less costly than global enrichment of
the burner's combustion air. The avoidance of, or reduction in use
of, a higher heating value auxiliary fossil fuel or biomass fuel
reduces the operational cost. Finally, the apparent burner power
may be increased beyond levels achievable with conventional biomass
burners. Excess furnace wear may be reduced or avoided.
[0064] Preferred processes and apparatus for practicing the present
invention have been described. It will be understood and readily
apparent to the skilled artisan that many changes and modifications
may be made to the above-described embodiments without departing
from the spirit and the scope of the present invention. The
foregoing is illustrative only and that other embodiments of the
integrated processes and apparatus may be employed without
departing from the true scope of the invention defined in the
following claims.
* * * * *