U.S. patent application number 14/448824 was filed with the patent office on 2014-11-20 for oxygen enhanced combustion of biomass.
The applicant listed for this patent is Lawrence E. Bool, III, Hisashi Kobayashi. Invention is credited to Lawrence E. Bool, III, Hisashi Kobayashi.
Application Number | 20140338577 14/448824 |
Document ID | / |
Family ID | 44947251 |
Filed Date | 2014-11-20 |
United States Patent
Application |
20140338577 |
Kind Code |
A1 |
Kobayashi; Hisashi ; et
al. |
November 20, 2014 |
OXYGEN ENHANCED COMBUSTION OF BIOMASS
Abstract
The energy output of a power plant combustion chamber that
combusts fuel comprising biomass as all or part of the fuel can be
increased by feeding oxygen into the combustion chamber so that
said fuel is in contact with gaseous oxidant whose oxygen content
exceeds that of air by up to 5 vol. % above that of air.
Inventors: |
Kobayashi; Hisashi;
(Bedford, NY) ; Bool, III; Lawrence E.; (East
Aurora, NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Kobayashi; Hisashi
Bool, III; Lawrence E. |
Bedford
East Aurora |
NY
NY |
US
US |
|
|
Family ID: |
44947251 |
Appl. No.: |
14/448824 |
Filed: |
July 31, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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13285654 |
Oct 31, 2011 |
|
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14448824 |
|
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61412119 |
Nov 10, 2010 |
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Current U.S.
Class: |
110/346 |
Current CPC
Class: |
Y02E 50/10 20130101;
F22B 1/18 20130101; F23G 2900/7003 20130101; F23G 5/00 20130101;
F23G 7/10 20130101; Y02E 50/12 20130101; F23G 2209/26 20130101;
F23G 2900/7012 20130101; F23L 7/007 20130101; F23G 5/442 20130101;
F23G 5/44 20130101; F23G 2206/203 20130101; Y02E 20/344 20130101;
Y02E 20/34 20130101; F23G 2207/30 20130101; F23L 2900/07007
20130101; Y02E 20/12 20130101 |
Class at
Publication: |
110/346 |
International
Class: |
F23G 5/44 20060101
F23G005/44; F23L 7/00 20060101 F23L007/00 |
Claims
1. A method of combustion, comprising (A) providing apparatus that
includes a combustion chamber in which fuel having a given moisture
content and a given specific energy content fed into the combustion
chamber at a given mass feed rate can be combusted in air to
produce heat energy at a given rate, (B) feeding into said
combustion chamber fuel that contains biomass and that has a
specific energy content lower than said given specific energy
content, so that combustion in air of said fuel fed at said given
mass fed rate in said combustion chamber in air produces heat
energy at a rate lower than said given rate, while feeding oxygen
into said combustion chamber so that said fuel is in contact with
gaseous oxidant whose oxygen content exceeds that of air by up to 5
vol. % above that of air, and (C) combusting the fuel with said
gaseous oxidant in said combustion chamber.
2. The method of claim 1 wherein the fuel has an energy content of
less than 7500 BTU/lb.
3. The method of claim 1 wherein in step (B) oxygen is fed into
said combustion chamber so that said fuel is in contact with
gaseous oxidant whose oxygen content exceeds that of air by up to 1
vol. % above that of air.
4. A method of increasing fuel combustion rate in a combustion
chamber with a convective heat transfer zone in which fuel that
contains biomass is combusted with combustion air in said
combustion chamber to produce flue gas containing a specific oxygen
concentration between 3 vol. % and 8 vol. % at a given maximum fuel
feed rate limited by the capacity of an FD fan if present for
feeding said combustion air, the capacity of an ID fan if present
to evacuate flue gas from said combustion chamber, the flue gas
velocity in said convective heat transfer zone, or the carbon
monoxide concentration in said flue gas, feeding into said
combustion chamber additional fuel containing biomass and
additional oxidant containing at least 50 vol. % O.sub.2, reducing
said combustion air flow rate by the amount that reduces said
oxygen concentration in said flue gas by 0.1 to 5.0 vol. % and
combusting said additional fuel without exceeding said FD fan
capacity, said ID fan capacity, said flue gas velocity, nor said
carbon monoxide concentration.
5. The method of claim 4 wherein said oxygen concentration in said
flue gas is reduced by 0.1 to 1.0 vol. %.
6. The method of claim 4 wherein said additional oxidant contains
at least 90% vol. O.sub.2.
7. The method of claim 4 wherein the ratio of the oxygen contained
in said additional oxidant to said additional fuel is less than
2,000 SCF/MMBtu.
8. The method of claim 4 wherein the ratio of the oxygen contained
in said additional oxidant to said additional fuel is less than
1,500 SCF/MMBtu.
9. The method of claim 4 wherein said fuel combustion rate in said
combustion chamber is increased by 3% to 30% in Btu content.
10. The method of claim 4 wherein said additional oxidant is
injected to one or more oxygen deficient areas in said combustion
chamber.
11. The method of claim 4 wherein said additional oxidant contains
at least 90% vol. O2.
12. A method of increasing fuel combustion rate in a combustion
chamber with a grate for combustion of fuel with a convective heat
transfer zone in which fuel that contains biomass is combusted with
combustion air in said combustion chamber to produce flue gas
containing a specific oxygen concentration between 3 vol. % and 8
vol. % at a given maximum fuel feed rate limited by the carbon
monoxide concentration in said flue gas, feeding into said
combustion chamber additional fuel containing biomass and
additional oxidant containing at least 50 vol. % O.sub.2 to one or
more oxygen deficient areas on said grate to maintain or reduce
said carbon monoxide concentration.
13. The method of claim 12 wherein said oxygen concentration in
said flue gas is reduced by 0.2 to 1.0 vol. %.
14. The method of claim 12 wherein said additional oxidant contains
at least 90% vol. O2.
15. The method of claim 12 wherein said additional oxidant is
injected from above said grate, from below said grate, or from both
above and below said grate.
Description
RELATED APPLICATION
[0001] This application is a continuation application of and claims
priority from U.S. patent application Ser. Number 13/285,654 filed
Oct. 31, 2011, which claims priority from U.S. Provisional
Application Ser. No. 61/412,119 filed Nov. 10, 2010.
FIELD OF THE INVENTION
[0002] The present invention relates to combustion of biomass,
especially in power plants that generate steam.
BACKGROUND OF THE INVENTION
[0003] Growing demand for electrical power obtained from fuel-fired
power plants, combined with growing interest in using biomass as
fuel for such plants, has increased interest in finding efficient
methods for combusting biomass in power plants. The moisture
content of biomass is typically very high. For example green wood
typically contains 40 to 60% moisture. This increased moisture
content, and its low energy density, are among the primary issues
with firing biomass in boilers and especially boilers that were
designed for other fuels such as coal. For example, converting a
coal-fired boiler to fire biomass typically cause the boiler to be
derated by 30-50%.
[0004] Many boilers are `flue gas limited` and can only handle up
to a specific amount of flue gas. This flue gas limitation may be
due to the capacity of fans if present for impelling flow of flue
gas, or may be based on design limits. For example, boiler design
considerations, such as the maximum allowable velocity in the
convective section, can limit flue gas volume. Since the flue gas
volume per unit heat input, or "specific flue gas volume",
increases dramatically when a fuel such as coal is replaced with
biomass, it causes a large impact on the distribution of heat
absorption in the furnace. A boiler is typically designed for a
relatively narrow range of specific flue gas volume. Within this
range the boiler is designed for a specific amount of heat
absorption in the furnace, or radiant section, and the convective
section. When the specific flue gas volume is increased more heat
is `pushed` from the radiant section into the convective section.
This increase in heat transfer in the convective section often
requires the use of water sprays into the steam flow to maintain
the desired steam temperature, which may decrease overall
efficiency. This shifting of heat transfer from the radiative
furnace section to the convective section of the boiler further
reduces or derates the boiler capacity.
[0005] Conversion of an existing boiler to biomass firing can also
significantly degrade the combustion performance of the unit. The
reduction in combustion performance is due to both changes in the
fuel characteristics and the firing system. The high moisture
content the fuel makes it more difficult to ignite and burn. This
problem is compounded by the fact that grate firing systems often
suffer from uneven fuel distribution over the grate and non-uniform
mixing between the air and the fuel--leading to incomplete
combustion on parts of the grate and high CO emissions in flue gas.
To overcome both of these problems boiler operators typically
operate the boiler at increased stoichiometric ratios (defined as
the ratio of air supplied to that required to burn the fuel). The
stoichiometric ratio is often measured as the amount of oxygen left
in the flue gas at the end of the combustion process. For example,
a typical coal-fired boiler will operate with 3% "excess oxygen".
This means the flue gas contains 3% oxygen (by volume, wet basis).
In contrast the flue gas from a biomass-fired boiler typically
contains at least 4.5% O.sub.2 (vol, wet basis) to control CO
emissions within regulatory limits.
[0006] The extra air further increases the flue gas volume and
impacts both the thermal efficiency of the boiler, and the
auxiliary power required for the boiler. In the first case the
extra air volume carries heat out the stack, increasing the
sensible heat loss. The extra air also increases the power required
by both the blower that pushes combustion air into the boiler
(typically called the forced draft, or FD, fan), and the blower
used to draw the flue gas from the boiler (typically called the
induced draft, or ID, fan). Therefore the overall effect of the
excess air is to increase the specific flue gas volume, which is
the gas volume per units of energy output (further limiting the
amount of fuel that can be fired), reduce the thermal efficiency
(allowing less of the fuel that is fired to be used to raise
steam), and increase the auxiliary power (reducing the net power
available
[0007] The present invention provides an improved method for
combustion of biomass in boilers.
BRIEF SUMMARY OF THE INVENTION
[0008] One aspect of the present invention is a method of
combustion, comprising
[0009] (A) providing apparatus that includes a combustion chamber
in which fuel having a given moisture content and a given specific
energy content fed into the combustion chamber at a given mass feed
rate can be combusted in air to produce heat energy at a given
rate,
[0010] (B) feeding into said combustion chamber fuel that contains
biomass and that has a specific energy content lower than said
given specific energy content, so that combustion in air of said
fuel fed at said given mass fed rate in said combustion chamber in
air produces heat energy at a rate lower than said given rate,
while feeding oxygen into said combustion chamber so that said fuel
is in contact with gaseous oxidant whose oxygen content exceeds
that of air by up to 5 vol. % above that of air, and
[0011] (C) combusting the fuel with said gaseous oxidant in said
combustion chamber.
[0012] Another aspect of the invention is a method of increasing
fuel combustion rate in a combustion chamber with a convective heat
transfer zone in which fuel that contains biomass is combusted with
combustion air in said combustion chamber to produce flue gas
containing a specific oxygen concentration between 3 vol. % and 8
vol. % at a given maximum fuel feed rate limited by the capacity of
an FD fan if present for feeding said combustion air, the capacity
of an ID fan if present to evacuate flue gas from said combustion
chamber, the flue gas velocity in said convective heat transfer
zone, or the carbon monoxide concentration in said flue gas,
feeding into said combustion chamber additional fuel containing
biomass and additional oxidant containing at least 50 vol. %
O.sub.2, reducing said combustion air flow rate by the amount that
reduces said oxygen concentration in said flue gas by 0.1 to 5.0
vol. % and combusting said additional fuel without exceeding said
FD fan capacity, said ID fan capacity, said flue gas velocity, nor
said carbon monoxide concentration.
[0013] Yet another aspect of the invention is a method of
increasing fuel combustion rate in a combustion chamber with a
grate for combustion of fuel with a convective heat transfer zone
in which fuel that contains biomass is combusted with combustion
air in said combustion chamber to produce flue gas containing a
specific oxygen concentration between 3 vol. % and 8 vol. % at a
given maximum fuel feed rate limited by the carbon monoxide
concentration in said flue gas, feeding into said combustion
chamber additional fuel containing biomass and additional oxidant
containing at least 50 vol. % O.sub.2 to one or more oxygen
deficient areas on said grate to maintain or reduce said carbon
monoxide.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 is a cross-sectional view of one embodiment of
combustion apparatus in which the present invention can be
practiced.
DETAILED DESCRIPTION OF THE INVENTION
[0015] The present invention is an improvement in the combustion of
fuel comprising biomass in a combustion chamber. "Biomass," for the
purposes of the present invention, means any material not derived
from fossil resources and comprising at least carbon, hydrogen, and
oxygen. Biomass includes, for example, plant and plant-derived
material, vegetation, agricultural waste, forestry waste, wood,
wood waste, paper waste, animal-derived waste, poultry-derived
waste, and municipal solid waste. Other exemplary feedstocks
include cellulose, hydrocarbons, carbohydrates or derivates
thereof, and charcoal. Typically biomass can include one or more
materials selected from: timber harvesting residues, softwood
chips, hardwood chips, tree branches, tree stumps, leaves, bark,
sawdust, off-spec paper pulp, corn, corn stover, wheat straw, rice
straw, sugarcane bagasse, switchgrass, miscanthus, animal manure,
municipal garbage, municipal sewage, commercial waste, grape
pumice, almond shells, pecan shells, coconut shells, coffee
grounds, grass pellets, hay pellets, wood pellets, cardboard,
paper, plastic, and cloth. The present invention can also be used
for fuels that also comprise carbon-containing feedstocks other
than biomass, such as a fossil fuel (e.g., coal or petroleum coke),
i.e. mixtures of biomass and fossil fuels.
[0016] The present invention is especially applicable to combustion
of biomass in a combustion chamber that is part of a system that
includes, in addition to a combustion chamber, heat exchangers that
absorb heat of combustion into, for instance, water. Preferred
systems include power generation boilers, especially in which heat
exchange to boiler feed water is achieved by radiant heat transfer
and by convective heat transfer. The heat exchange produces steam,
superheated steam, and/or supercritical steam, which can be used to
generate electric power.
[0017] The present invention is especially applicable to combustion
of biomass in a combustion chamber including a grate on which fuel
rests as it is being combusted. However, the present invention can
be practiced in systems wherein the fuel is combusted in the
combustion chamber by grate firing, suspension firing, or a
combination of grate firing and suspension firing, or by firing in
a bubbling fluidized bed or in a circulating fluidized bed.
[0018] The following description refers to FIG. 1, and illustrates
practice of the invention in one embodiment in which grate firing
is employed.
[0019] Combustion chamber 1 includes grate 2 on which fuel can rest
after the fuel is fed into combustion chamber 1, for instance as
fuel stream 3. Grate 2 is solid and includes a plurality of
openings through which gas can flow, including primary air which is
fed as primary air stream 4. Optionally, overfire air stream 5 can
also be fed into the combustion chamber 1.
[0020] Combustion of fuel in combustion chamber 1 produces heat of
combustion, and flue gas which exits combustion chamber 1 as stream
7. The heat of combustion can be transferred to feed water flowing
through boiler tubes in the walls of combustion chamber 1, to heat
the feed water. Heat of combustion can also be transferred from
flue gas by indirect heat exchange to feed water, or to steam, in
heat section 6 which generally includes a region (the "radiant
section") in which heat transfer occurs predominantly by radiative
heat transfer, and a region (the "convective section") in which
heat transfer occurs predominantly by convective heat transfer.
[0021] In the present invention, oxygen is fed in small amounts
into the region below grate 2, into the region of the fuel on grate
2, or into both regions. Sufficient oxygen is fed so that the
gaseous atmosphere in contact with the fuel has an oxygen content
higher than that of air, i.e. at least 21 vol. %, up to 5 vol. %
higher than that of air and preferably not more than 1 vol. %
higher than that of air.
[0022] The oxygen can be fed into the region below grate 2 in any
of numerous ways, such as by mixing it with primary air that is fed
as stream 4, or inserting a lance 8 into the region below grate 2
and feeding the oxygen through the lance into the region below
grate 2 where it then can mix with primary air.
[0023] The oxygen can be fed into the region above grate 2 in any
of numerous ways, such as by inserting a lance 9 into the region
above grate 2 so that oxygen emerging from the lance 9 can contact
fuel present on the grate, and feeding oxygen through the lance
9.
[0024] The oxygen that is fed below or above the grate 2 is
preferably fed as a stream comprising at least 50 vol. % oxygen
preferably 90 vol. % oxygen. Streams having such oxygen content are
readily available from commercial sources. Alternatively, streams
having such oxygen content can be formed in apparatus located near
the combustion chamber such as VPSA units that separate oxygen from
air.
[0025] The practice of the present invention provides numerous
advantages in its own right, and especially compared to prior
practice relating to combustion of biomass.
[0026] The moisture content of fuel comprising biomass is typically
very high. This increased moisture content, and its low energy
density, are among the primary issues with firing biomass in
boilers and especially boilers that were designed for other fuels.
For example, converting a 50MW.sub.net coal-fired boiler (heat rate
of 11,500 Btu/kWh.sub.net) to fire biomass would be expected to
cause the boiler to be derated by 20-45% just to account for the
moisture in the fuel. The shift in boiler heat transfer balance and
the increased excess air requirement increase the required derate
to 30-50% for many boilers. The present invention permits efficient
combustion of biomass fuels, even in boilers that were designed for
combustion of fuels having lower water contents, and/or higher
energy density, than biomass. The invention is useful when the fuel
containing the biomass has a water content of at least 25 wt. %, or
when the fuel containing the biomass has an energy content less
than 7500 BTU/lb or even less than 5000 BTU/lb.
[0027] In the present invention, the addition of only a small
amount of oxygen enhances and controls combustion both on and above
the grate as a means to recover lost generating capacity. The
enhanced combustion, in turn, enhances flame stability and ensures
more complete burnout. Oxygen injection over the grate can also
stabilize and improve the combustion process. In general, by using
oxygen in the combustion environment according to the present
invention, it is possible to reduce the excess air flow, and
thereby reduce the specific flue gas volume. The lower specific
flue gas volume allows the boiler operator to increase the firing
rate to regain some of the generating capacity lost when the boiler
was converted to biomass firing. Even small reductions in excess
air can allow boiler capacity lost during the conversion to biomass
to be recovered (reducing the required boiler derate).
[0028] Another operational benefit of oxygen injection according to
the present invention is that less heat will be `pushed` into the
convective section due to both the reduced specific flue gas volume
and the increased temperature near the fuel bed on the grate. Both
of these effects lead to increased heat absorption in the radiative
part of the boiler--reducing the need to spray in cooling water to
control superheat and reheat temperatures in the convective
section.
[0029] In the present invention oxygen could be added by
combination of being directly injected or mixed with combustion air
(enrichment). For example, one might enrich the undergrate air to
ensure there are no "hot spots" nor "cold spots" on the grate,
while using high momentum lances to inject oxygen above the grate
to promote good mixing and volatiles/CO burnout. The over-bed
oxygen lances can also be used to move heat (by influencing mixing)
into different parts of the grate. For example, some of the heat
from the volatile combustion zone of the grate can be moved into
the drying portion of the grate to facilitate drying. Overfire air
5 (air supplied through ports located at one or more elevation from
the grate) can also be enriched to enhance volatile combustion.
Alternatively oxygen enrichment under the grate may be increased
through the use of a lance to target areas where the grate is known
to be `cold`, or combustion is poor. The amount of oxygen required
to recover capacity by enabling reduced excess oxygen operation is
much less than that estimated for a simple direct replacement of
combustion air. For example, the stoichiometric oxygen requirement
for a typical dry ash-free wood is about 2,000 SCF (123 lb) per
1,000,000 Btu and produces about 3,200 SCF of flue gas. Conversely,
1 lb of oxygen can combust about 8130 Btu of fuel and produces 26
SCF of flue gas. In order to maintain the original flue gas volume
and burn additional fuel a portion of the original combustion air
volume must be reduced and replaced with additional oxygen. The
oxygen requirement to increase the capacity (or fuel firing rate)
by 10% under the condition of constant flue gas volume flow rate
was calculated for both dry and wet wood with 45% moisture content
at two different excess oxygen levels (3 and 4.5% by volume in wet
flue gas) and summarized in Table 1. The amount of oxygen required
ranges from 2850 to 3410 SCF per MMBtu of additional fuel input at
the constant excess O.sub.2 in flue gas. By reducing the excess
oxygen level by 1 vol. %, the amount of oxygen required is reduced
to less than half, in a range from 1140 to 1510 SCF per MMBtu of
additional fuel input.
TABLE-US-00001 TABLE 1 Oxygen required (SCF/MMBtu): Constant 1%
reduction Biomass excess O2 in excess O2 Dry wood, 3% Excess O2
2850 1260 Dry wood, 4.5% Excess O2 2870 1140 Wet wood, 3% Excess O2
3410 1510 Wet wood, 4.5% Excess O2 3410 1330
[0030] The current invention has several additional advantages.
First, by using only enough oxygen enrichment to achieve flame
stability on and above the grate, gross changes to furnace
operation can be avoided. For example, many furnaces are designed
for a specific heat absorption pattern. In a steam boiler for power
generation the balance between heat transfer in the radiant
(furnace) section is often carefully balanced with that in the
convective section by the boiler designer. Variations in heat
transfer pattern from the design point can cause significant upsets
in boiler operation. When high oxygen enrichment levels, such as
those presented in the prior art (>25%) are used, the heat
transfer to the radiant section is often dramatically increased.
For a utility boiler this means the steaming rate (rate of steam
production) is increased, but there is insufficient heat available
to superheat the steam to the desired turbine inlet temperature. In
the current invention the transition to a high moisture fuel often
leads to off-design furnace operation where heat transfer to the
radiant section is reduced compared to the design case. By using a
small amount of oxygen enrichment and thereby reducing the excess
air requirement the radiative/convective heat transfer balance can
be restored, at least in part, without increasing the radiative
heat transfer past the design limits.
[0031] The present invention also does not require exhaust gas
recirculation for over-grate mixing. This leads to a much lower
capital requirement (EGR fans, ducts, and the like) and reduced
operating cost.
[0032] Additionally, by using the oxygen addition of the present
invention only to support combustion and thereby reduce the
specific flue gas volume through excess air reduction, the volume
reduction compared to oxygen use is much higher than in the prior
art. This enhanced effectiveness of oxygen addition for flue gas
reduction leads to much lower oxygen requirements.
[0033] A significant advantage of the current invention over the
prior art is related to the use of oxygen enrichment only to
support combustion and thereby reduce the specific flue gas volume
through excess air reduction, the flue gas volume reduction
compared to the simple replacement of a portion of combustion air
with oxygen is much higher than in the prior art. This enhanced
effectiveness of oxygen addition for flue gas reduction leads to
much lower oxygen requirements. An example for converting a 20
MW.sub.net coal-fired boiler to fire biomass is shown in Table 2.
For these calculations the flue gas volume was held constant,
consistent with a flue gas limited boiler. The baseline generating
capacity was defined as that after the boiler was converted to
biomass firing (using a 32% moisture fuel) and was 14.7 MWnet in
this example. The increased generating capacity was first estimated
assuming the oxygen concentration in the flue gas was held constant
at 4.5% (vol, wet) and combustion air was replaced with increasing
levels of oxygen. This condition is the conventional `volume
reduction` strategy where the nitrogen in the combustion air is
simply removed by using oxygen in place of a portion of the air. As
can be seen in Table 2, the generating capacity can be increased
significantly, but the oxygen requirements are high enough that
oxygen use may not be economically justified. In the case of the
current invention, kinetic data was used to estimate the increase
in firing rate from oxygen enrichment. The air injection rate was
reduced by the amount of oxygen injected and the firing rate
increased--resulting in a lower oxygen concentration in the flue.
With injection targeted to particular locations in the combustion
chamber, such as described below, the oxygen consumption may be
even lower. The data in Table 2 show that the oxygen use is
dramatically lower for a given increase in capacity for the current
invention. Using oxygen in this way can be economically viable.
TABLE-US-00002 TABLE 2 Increase in generation Oxygen required
(SCF/MW baseline): (% of baseline) Volume reduction Present
invention 2% 710 70 10% 3500 340 22% 7420 1020
"Volume reduction" means operating such that the reduction in
specific flue gas volume is attained only by the replacement of air
with an equal amount of oxygen. "Present invention" means operating
such that the reduction in specific flue gas volume is attained in
part by reduction in the amount of excess air. The optimal
embodiment of the current invention uses small amounts of oxygen to
support the various stages of biomass combustion. These stages
include: [0034] Preheating/drying, [0035] Volatile release, [0036]
Volatile combustion, [0037] Char combustion.
[0038] In a grate fired-combustor, such as that shown in FIG. 1,
these steps can occur in-flight or on the grate, depending on the
fuel characteristics (size) and fuel spreader/boiler design. For
example, fine particulate are likely suspended as they are `thrown`
into the furnace. Therefore for the fine materials the entire
combustion process occurs in flight. For the largest particles they
may dry slightly as they exit the fuel spreader but land on the
grate before drying is complete. Therefore, for these particles the
combustion process occurs primarily on the grate. Combustion
problems can occur when the fuel and air distribution are not
matched across the grate and overfire air. For example, if too much
fuel is deposited on a specific portion of the grate the combustion
air may be insufficient to burn the material. Although optimal
overfire air designs promote good mixing above the grate, there may
still be regions where the oxygen levels are too low to complete
combustion (and other areas where the excess air is much higher
than required for combustion). Further, the heat release pattern
from the volatile combustion may not match that required to promote
drying/devolatilization of materials that have landed on the
grate--causing material on portions of the grate to `smolder`
instead of burn.
[0039] It is known that high levels of oxygen enrichment can
enhance combustion and overcome problems associated with air/fuel
distribution and heat release mismatches. However, the objective of
the current invention is to use the least amount of oxygen to
enable the excess air to be reduced (and thereby enable an increase
in boiler firing rate). Therefore the optimal embodiment is to use
a lance, or lances, above the grate to inject oxygen into
oxygen-deficient areas above the grate. Often the oxygen deficient
area looks darker than the rest of the grate as the local
temperature is colder. Such area can be detected by in-furnace
video camera, by an optical pyrometer or by visual observation.
Other methods of detecting the oxygen deficient area include gas
analysis using a gas sampling probe and by an optical gas species
measurement device. With careful lance design mixing can be
controlled between the injected oxygen and the oxygen deficient
(and likely high CO) flue gas. Further, by targeting the injected
oxygen jet tragectory high oxygen containing flue gas `pockets` in
the furnace atmosphere can be drawn into the oxygen deficient area.
The combination of aerodynamic effects from the lance design and
the kinetic effect of high oxygen concentrations enhance volatiles
and CO combustion. The over-grate lances can also be used to `move`
volatile combustion to add heat to cooler portions of the grate to
support the combustion process on the grate.
[0040] In addition to the over-grate lances the optimal embodiment
can also use directed oxygen enrichment under the grate to enhance
combustion on specific regions of the grate. For example, if the
windbox under the grate has partitions to divide the airflow to
different parts of the grate, different levels of oxygen enrichment
could be used in the different partitioned areas (through use of
oxygen distributors in the air supply duct for each partition).
Alternately a carefully designed oxygen injection lance could be
installed either below the grate or immediately above the grate to
enrich the combustion air in the immediate vicinity of a known
`cold spot`, or oxygen deficient areas.
* * * * *