U.S. patent application number 13/033374 was filed with the patent office on 2011-08-25 for methods, apparatus and systems for improving the operation of cyclone boilers.
This patent application is currently assigned to Fuel Tech Inc.. Invention is credited to Kent W. Schulz, Christopher R. Smyrniotis.
Application Number | 20110203498 13/033374 |
Document ID | / |
Family ID | 44475392 |
Filed Date | 2011-08-25 |
United States Patent
Application |
20110203498 |
Kind Code |
A1 |
Smyrniotis; Christopher R. ;
et al. |
August 25, 2011 |
Methods, Apparatus and Systems for Improving the Operation of
Cyclone Boilers
Abstract
A process that uses targeted in-furnace Injection to feed a
fluxing agent of the chemical family of compositions containing
boron and/or alkali hydrates to either decrease heat transfer on
waterwalls of utility furnaces burning solid fuels to improve steam
generation, maintain steam temperature, and/or allow a protective
layer of slag to form inside the barrels of cyclones on cyclone
boilers burning fuels high in calcium so that the boiler can
operate at a wider variety of power settings while allowing proper
flow and drainage of slag from the cyclone barrels.
Inventors: |
Smyrniotis; Christopher R.;
(St. Charles, IL) ; Schulz; Kent W.; (Geneva,
IL) |
Assignee: |
Fuel Tech Inc.
Warrenville
IL
|
Family ID: |
44475392 |
Appl. No.: |
13/033374 |
Filed: |
February 23, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61307228 |
Feb 23, 2010 |
|
|
|
Current U.S.
Class: |
110/342 ;
110/234; 110/264; 110/265; 110/347 |
Current CPC
Class: |
F23C 3/008 20130101;
F23J 7/00 20130101 |
Class at
Publication: |
110/342 ;
110/347; 110/264; 110/265; 110/234 |
International
Class: |
F23J 7/00 20060101
F23J007/00; F23D 1/02 20060101 F23D001/02 |
Claims
1. A method for controlling slag properties in a cyclone burner of
the type having a barrel fed by a tangential supply of air and coal
and a layer of slag protecting the inside barrel and flowing out of
the barrel, which comprises: providing a boron composition in a
liquid vehicle; utilizing computational fluid dynamics to determine
the location, droplet size, and dosage amount for injection of a
boron composition in a liquid vehicle into a duct for providing air
to the cyclone burner, wherein the conditions of introduction of
the boron composition and liquid vehicle are determined to assure
first contact with the duct wall occur no sooner than after the
boron composition has been dried to a fine powder form, without
significantly wetting the wall with the liquid vehicle; injecting
the boron composition in a liquid vehicle into the duct to form a
dispersion of boron composition as a dry powder in air; and
directing flow of the dry powder of boron composition into the
cyclone burner in a tangential flow of air, whereby the fine powder
of boron composition uniformly contacts slag in the cyclone
burner.
2. A method according to claim 1, wherein the boron composition
comprises borate, boric acid, borax or blend of two or more of
these.
3. A method according to claim 1, wherein the boron composition
material also includes an alkali hydrate.
4. A method according to claim 1, wherein feed of the boron
composition is stopped and feed of alkali hydrates is started.
5. A method according to claim 1, wherein the boron composition is
introduced into a secondary air duct carrying heated combustion air
to the cyclone burner.
6. A method according to claim 1, wherein the boron composition is
introduced into the cyclone barrel with tangentially supplied
secondary air.
7. A method for controlling slag properties in a cyclone burner,
which comprises: utilizing computational fluid dynamics to
determine the location, droplet size, and dosage amount of a boron
composition in a liquid vehicle necessary for slag modification to
assure flow of slag from the cyclone burner while maintaining a
protective layer of slag in the burner; and providing precise
dosing as to location and amount of a boron composition in a
suitable vehicle by controlled feeding through injection ports on
each cyclone barrel such that the boron composition lowers the melt
point of the slag while making slag thicker, causing a protective
layer of slag to form on the cyclone barrel inside wall, but still
able to flow to drains.
8. A method for controlling slag properties in a cyclone burner of
the type having a barrel fed by a tangential supply of air and coal
and a layer of slag protecting the inside barrel and flowing out of
the barrel, which comprises: providing a boron composition in a
liquid vehicle; injecting the boron composition in a liquid vehicle
into a duct carrying hot air to the cyclone burner to dray and
transport the boron composition as a dry powder without
significantly wetting the wall with the liquid vehicle; and
directing a flow of the dry powder of boron composition into the
cyclone burner in a tangential flow of air, whereby the fine powder
of boron composition uniformly contacts slag in the cyclone
burner.
9. A method according to claim 8, wherein the boron composition is
introduced into the cyclone barrel with tangentially supplied
secondary air.
10. A method according to claim 8, wherein the boron composition
comprises borate, boric acid, borax or blend of two or more of
these.
11. A method according to claim 8, wherein the boron composition
material also includes an alkali hydrate.
12. A method according to claim 8, wherein feed of the boron
composition is stopped and feed of alkali hydrates is started.
13. A method according to claim 8, wherein the boron composition is
introduced into a secondary air duct carrying heated combustion air
to the cyclone burner.
14. A method according to claim 8, wherein computational fluid
dynamics is utilized to determine the location, droplet size, and
dosage amount for injection of a boron composition in a liquid
vehicle into a duct for providing air to the cyclone burner,
wherein the conditions of introduction of the boron composition and
liquid vehicle are determined to assure first contact with the duct
wall occur no sooner than after the boron composition has been
dried to a fine powder form, without significantly wetting the wall
with the liquid vehicle.
15. A system and/or apparatus comprising control means, sensors and
feed devices for effecting the process of claim 1.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to pending US Provisional
Patent Application Ser. No. 61/307,228, filed Feb. 23, 2010, which
is hereby incorporated by reference in its entirety.
FIELD OF THE INVENTION
[0002] The invention relates generally to improving the operation
of cyclone burners and the combustors or boilers they fire by
assuring adequate slag properties to modulate slag plasticity to a
new effective level, enabling formation and maintenance of a
protective layer of slag on inside wall of a cyclone barrel while
assuring a viscosity that permits the constantly forming slag to
flow to drains.
[0003] In particular, the invention provides a system, apparatus
and method for improving the operation of cyclone boilers by
increasing operating flexibility and efficiency of a cyclone
furnace while burning Powder River Basin (PRB) or similar coal by
using a specifically effective fluxing composition in a highly
efficient and effective manner.
BACKGROUND OF THE INVENTION
[0004] As originally developed, cyclone burners were characterized
as including a horizontally disposed cylindrical barrel attached
through the side of a boiler furnace. The original design achieved
a commercial following in part because they could take advantage of
a variety of coal grades, including some not suitable for
pulverized coal combustion. Cyclone furnaces will typically
spirally feed coal into a combustion chamber to achieve maximum
combustion efficiency. FIG. 1 is a schematic drawing of a prior art
cyclone burner. In addition to providing flexibility of coal type,
they also reduced fuel preparation time and costs, were smaller and
more compact than other furnaces and produced less fly ash and
convective pass slagging.
[0005] These furnaces require a protective layer of slag to be
formed and maintained on the inside of the cyclone barrel wall (12
in FIG. 1) to provide a degree of insulation for the wall
materials. This slag layer is constantly renewed and drained, and
it is essential that the slag viscosity always permits the slag to
flow to drains. The slag must have a consistency sufficient to
maintain the insulating layer, but not be so thick that it can cool
and stop flow to or through drains. The slag further functions to
hold larger coal particles as they continue to burn as the slag
empties from the combustor.
[0006] In these furnaces, the cyclone barrel is typically
constructed with water-cooled, tangential-oriented, tube
construction and the burners include a water-cooled horizontal
cylinder in which fuel (coal, gas, or oil) is fired and heat is
released at extremely high rates. When firing coal, the crushed
coal is introduced tangentially into the burner, usually with
primary air. The cyclone barrel extends into the furnace where it
opens to supply burning hot gases and slag to the furnace interior.
Typically during combustion of coal, volatile components are
released from the coal and burn well. However, the fuel carbon
results in "char" particles, which are less volatile and heavier.
The char requires higher temperatures and benefit from the swirling
supply of oxygen in a cyclone furnace, which provides thorough
mixing of coal particles and air with sufficient turbulence to
constantly renew fresh air to coal particle surfaces. The cyclonic
fuel swirling in these burners is increased and maintained by
tangentially-introduced, high-velocity secondary air.
[0007] The cyclone barrel is water cooled and cools the slag while
the slag insulates the barrel material as it cools. The cyclone is
designed to operate at high temperatures to maintain the slag in a
molten state and allow removal through the trap. A layer of molten
slag coats the burner and flows through traps at the bottom of the
burners. Because the slag is formed largely within the burners, the
amount of slag that would otherwise form on the boiler tubes in the
boiler or other combustor is reduced. While low volatile bituminous
coals, lignite coal, mineral rich anthracitic coal, wood chips,
petroleum coke, and old tires can and have all been used in
cyclones, certain subbituminous coals high in alkaline earth
metals, especially calcium, like Powder River Basin (PRB) coals,
tend to produce slags that suffer from inconsistent properties.
[0008] PRB and like subbituminous coals tend to produce slags that
exhibit a surprisingly sharp drop in viscosity over short
temperature spans. Drops of over 10,000 centipoises can occur in
the temperature range of from 2250.degree. to 2350.degree. F., to a
value below the normal operating temperature of the cyclone boiler.
Because it is generally agreed that stable operation of a slagging
cyclone combustor requires the ash layer to remain molten. The slag
viscosity must be low enough to permit continuous drainage as is
illustrated in FIG. 1. A typical viscosity for steady drainage has
been shown to be about 250 centipoises, and the art refers to the
temperature corresponding to this viscosity level as T250. Stable
operation mandates a slag temperature of greater than or equal to
T250. Unless the PRB coal is burned to achieve slag with the
correct viscosity-temperature relationship, the furnace cannot
operate efficiently at any temperature or will be restricted to
only higher loads. The slag can freeze and cause shut down at the
low end of a narrow temperature range or it might run too freely
and not provide the optimum temperature differential at the surface
of the water cooled-cyclone barrel at the high end of the
temperature range. If it is desired to reduce the load, a secondary
fuel may be required to just keep the slag hot enough.
[0009] PRB coals are desired because of their low sulfur contents
and economy, but have proved a challenge to cyclone burner
operators, and efforts have been made to correct the difficulties
experienced. Current remediating technology typically involves
feeding iron oxides, e.g., as made from scale that came from steel
plant hot strip rolling plants, into the furnace on a weight basis
with the amount of coal used. This material will typically arrive
dry in rail car quantities and is fed to the fuel using front end
loaders to the fuel hopper. Large quantities of the iron oxide will
be mixed the PRB coal. This method seems to provide poor mixing,
uses excessive quantities of material and is very imprecise.
Furthermore, too much material use also allows the fluxing agent to
escape the cyclone barrels into the greater furnace, causing
unwanted and undesired slagging of heat exchangers.
[0010] As exemplary of iron oxide treatment, Johnson in U.S. Pat.
Nos. 6,729,248, 6,773,471 and 7,332,002 describes introduction of
iron containing compounds to act as fluxing agents. The disclosures
are directed to additives for coal-fired furnaces, particularly
furnaces using a layer of slag to capture coal particles for
combustion. The additives include iron, mineralizers, handling
aids, flow aids, and/or abrasive materials. The iron and
mineralizers are said to lower the melting temperature of ash in
low-iron, high-alkali coals, leading to improved furnace
performance; but control is difficult. We have assessed the
problems and believe they are caused adding the treatment chemical
to the furnace as a whole, either as part of the fuel or with the
combustion air.
[0011] A different example of using iron agents is found in U.S.
Pat. No. 6,613,110, wherein Sanyal employs them to improve heat
transfer on the water-walls of highly-reflective ash-containing
boiler furnaces, and does not mention cyclone furnaces. The
disclosed method is said to inhibit accumulation of light-colored
ash on the walls of a furnace in which coal containing high levels
of (coal-bound) calcium is burned. The light color on the ash
surface reflects heat that is then not efficiently utilized and
exits the boiler stack. To correct this, an iron compound is added
to the coal prior to burning the coal, which when burned produces a
dark calcium ferrite that darkens the ash. Other chemicals besides
iron compounds have also been suggested for ash color control in
such a context, and Sanyal cites U.S. Pat. No. 5,819,672 to Radway,
which asserts that boron and metal oxides can act as darkening
agents on furnace water-walls. The disclosed method involves
exposing the walls to a darkening agent, or a combination of a
darkening agent and a fluxing agent. A preferred embodiment
involves direct application of the darkening agent to the water
wall. Again, cyclone boilers are not described, and slag flow from
them is not addressed.
[0012] Representative of early efforts for controlling slag in slag
tap furnaces burning higher grade coals, is U.S. Pat. No. 4,057,398
to Bennett. The patent asserts that boron additives can be
introduced into the furnace box of the boiler as an intimate
mixture of pulverized or crushed coal. Also, U.S. Pat. No.
4,377,118 to Sadowski and U.S. Pat. No. 5,207,164 to Breen suggest
the addition of any of a number of fluxing agents for slag benefit.
Sadowski is concerned with decreasing slag viscosity at the walls
of a furnace and employs various slag viscosity adjuvants formed as
particles of significant size and density, noting that pellets
would be sufficiently large whereas a dust would not be. Bennett is
concerned with decreasing the fusion point of coal ash. Breen
utilizes iron, rust or slag for high calcium ash, which is recycled
to the furnace to soften it so that it collects via gravity in a
bottom ash pit. None of these enable increasing operating
flexibility and efficiency of a cyclone furnace while burning
Powder River Basin or similar coal by using a specifically
effective fluxing compound in an efficient and effective manner to
increase the flow temperature of the slag with a minimal use of
additive.
[0013] The problem of slag control in cyclone furnaces remains
serious but has not been effectively resolved by the prior art
despite years of effort directed at the problem even with a good
understanding of slag properties, such as might be seen from the
text "Influence of Coal Quality and Boiler Operating Conditions on
Slagging of Utility Boilers", Rod Hatt, Coal Combustion, Inc.,
Versailles, Ky. This work contains a wealth of reference material
and a discussion on the types of slags produced by various coals,
causes of deposits and procedures for reducing and removing
deposits caused by coal ash. There is, however, no clear direction
on chemical addition control for cyclone furnaces burning PRB coal
that would be absolutely essential for economic and furnace
maintenance reasons.
[0014] There is a present need for a system, apparatus and method
for improving the operation of cyclone boilers by increasing
operating flexibility and efficiency of a cyclone furnace while
burning Powder River Basin or similar coal by using a specifically
effective fluxing compound in an efficient and effective
manner.
SUMMARY OF THE INVENTION
[0015] It is an object of the present invention to provide a
system, apparatus and method for improving the operation of cyclone
boilers by increasing operating flexibility and efficiency of a
cyclone furnace while burning Powder River Basin or similar coal by
using a specifically effective fluxing compound in an efficient and
effective manner.
[0016] In one aspect, the invention achieves uniform contact of the
burning coal with a precise amount of a boron composition fluxing
agent to improve the flow properties of the slag to assure proper
cyclone furnace operation with coals having low iron and high
calcium contents.
[0017] In one aspect, the invention allows dosing of the slag layer
formed on cyclone barrel walls with the precise amount of a boron
composition fluxing agent required to achieve the goal, complete
coverage of trouble spots and no more. The dosing will be guided by
problems as observed and/or calculated and can be prophylactic or
remedial. The net effect is that furnace downtime due to slagging
problems will be held to a minimum while chemical usage will also
be greatly reduced from conventional applications.
[0018] In another aspect, the present invention provides precise
dosing as to location and amount of a boron composition in a
suitable vehicle, e.g., slurry, particulate solid or solution form,
by controlled feeding through injection ports on each cyclone
barrel such that the boron composition, e.g., comprising borate,
borax, boric acid or a blend of two or more of these, lowers the
melt point of the slag while making slag thicker (more plastic),
causing a protective layer of slag to form on the cyclone barrel
inside wall, but still able to flow to drains.
[0019] In addition to treating the slag in the cyclone barrel at
the cyclone barrel walls, the boron composition material can be
advantageously applied in a targeted fashion to the water-walls of
the boiler furnace to react with slag thereon to either insulate
heat transfer and allow steam temperature to be maintained in the
superheaters of boilers where superheat temperature is too low due
to high furnace water-wall area, or in a blend with other metal
oxides, to improve heat transfer on the water-walls by decreasing
reflectivity of high alkali earth oxide containing boiler ash.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] The invention will be better understood and its advantages
will become more apparent from the following detailed description,
especially when taken with the accompanying drawings, wherein
[0021] FIG. 1 is a schematic view of a cyclone furnace combustor of
the prior art, showing air and slag flow to help explain material
movements in a burner of this type;
[0022] FIG. 2 is a schematic view of a cyclone furnace combustor,
as viewed from the portion that would extend into the furnace, and
showing one embodiment of flux addition according to the
invention;
[0023] FIG. 3 is a schematic perspective view of an exemplary
equipment layout for cyclone burner equipped furnace according to
the invention;
[0024] FIG. 4 is a schematic perspective view showing some of the
detail of ductwork for the embodiment shown in FIG. 3; and
[0025] FIG. 5 is a schematic view of an embodiment of the invention
with representative control and material feed arrangements.
DETAILED DESCRIPTION
[0026] Since the invention provides strong advantages in the
context of cyclone furnaces, the following description will refer
to such for clarity and consistency. It will be understood by those
skilled in the art, however, that the principals that make the
invention so effective in that setting will also make it effective
in others. FIG. 1, is a schematic view of a prior art cyclone
burner 10, showing air and slag flow to help explain material
movements in a burner of this type. FIG. 2 shows one embodiment of
the invention wherein a boron composition is added directly to the
cyclone burner 10 separate from the coal feed. Reference to FIG. 3
will help understand the arrangement of the of individual cyclone
furnace burners as part of the larger combustor or boiler, and FIG.
4 will help to better understand the role and importance of
computational fluid dynamics to assure efficient distribution of
boron composition into the cyclone burner.
[0027] The invention will also be described with specific reference
to certain subbituminous coals, like Powder River Basin (PRB)
coals, which tend to produce slags that have a short transition in
viscosity as the temperature varies and otherwise suffer from
inconsistent properties. The burning of these coals in a furnace
requiring slag flow, might result in the slag freezing and causing
shut down of the furnace if the operator wants to operate at less
than full load. In other cases, the slag might run too freely and
not provide the optimum temperature differential at the surface of
the water cooled-cyclone barrel. The PRB coals can produce slags
that have a surprisingly sharp drop in viscosity over short
temperature spans. Drops of over 10,000 centipoises can occur in
the temperature range of from 2250.degree. to 2350.degree. F., to a
value below the normal operating temperature of the cyclone
boiler.
[0028] It is generally agreed that stable operation of a slagging
cyclone combustor requires the ash layer to be molten, but not too
low in viscosity and not too high. It needs to have a viscosity low
enough to permit continuous drainage as is illustrated in FIG. 1,
but not so low that it runs out of the cyclone burner without
retaining the necessary protective layer of slag. PRB coals often
tend to run too quickly at desirable operating conditions. A
critical viscosity for steady drainage is has been shown to be
about 250 centipoises. The temperature corresponding to this
viscosity level is referred to in the art as T250, and stable
operation mandates a slag temperature of greater than or equal to
T250. The invention enables achieving the correct
viscosity-temperature relationship with very low additive levels by
appreciation of the unique operating aspects of cyclone burners and
tailoring a treatment regimen to them, preferably through the use
of computational fluid dynamic modeling.
[0029] While not limited to PRB coals, and generally useful for all
operations with coal-fired cyclone burners, the following provides
an idea of approximate values (dry, weight basis) for coal
compositions that can be successfully burned in a cyclone burner
utilizing the present invention. The coal is preferably fed,
crushed as particulates wherein about 95% passes through a 4 mesh
screen. Using crushed coal as opposed to coal pulverized to a
greater degree, mitigates the escape of fines from the barrel.
Other high-calcium and/or low iron fuels can also be effectively
treated according to the invention.
TABLE-US-00001 Total Ash 2-15% of the coal Si02 20-35% of the ash
Al203 13-20% of the ash Fe203 3-10% of the ash CaO 18-35% of the
ash MgO 3-10% of the ash Na20 0-3% of the ash K20 0-1% of the ash
S03/other 6-20% of the ash
[0030] The invention provides a method for controlling slag
properties in a cyclone combustor, which comprises: determining the
need, location, dosage amount and targeting information of a boron
composition necessary for slag modification for proper viscosity;
and providing precise dosing as to location and amount of a boron
composition in a suitable vehicle, e.g., slurry, particulate solid
or solution form, by controlled feeding through injection ports on
each cyclone barrel such that the boron composition, e.g., borate,
borax or blend, lowers the melt point of the slag while making slag
thicker (more plastic), causing a protective layer of slag to form
on the cyclone barrel inside wall, but still able to flow to
drains.
[0031] In one aspect, the invention achieves uniform contact of the
burning coal with a precise amount of a boron composition fluxing
agent to improve the flow properties of the slag to assure proper
cyclone furnace operation with coals having low iron and high
calcium contents.
[0032] Preferably, as will be explained in greater detail below
with specific regard to FIGS. 3 to 5, direct observation or
computational fluid dynamics (CFD) or other computer or cold flow
modeling will be employed to determine the location of injection
ports on each cyclone barrel such that the boron composition
modifies the melt point of the slag while making slag suitably
viscous (e.g., plastic), causing a protective layer of slag to form
on the cyclone barrel inside wall, but still able to flow to
drains. By virtue of the correct calculation and the correct
selection of boron compositions, the present invention can provide
precise dosing as to location and amount of a boron composition.
The disclosures of U.S. Pat. No. 5,740,745, U.S. Pat. No. 5,894,806
and U.S. Pat. No. 7,162,960, all to the inventor herein with
others, are incorporated by reference herein for their descriptions
of suitable computational fluid dynamics and other modeling
techniques.
[0033] The boron composition used according to the invention can be
a member selected from the group consisting of borax, borates,
boric and blends of two or more of these. In particular, borax or
boric acid, and sodium borate can be employed. They can be
effective alone or with a carbonate or sulfate boron salt, or the
like, e.g., as a stabilized boric acid blend, and will be employed
in a suitable physical form, e.g., particulate solid or solution,
in a suitable, preferably liquid, vehicle such as water as a
slurry, dispersed solid or solution, or in air, controlled feeding
through injection ports on each cyclone barrel such that the boron
composition, e.g., borate, borax or blend, lowers the melt point of
the slag while making slag thicker (more plastic), causing a
protective layer of slag to form on the cyclone barrel inside wall,
but still able to flow to drains. The composition can be a boron
composition and may also include alkali hydrates. In some cases,
the boron composition can be stopped and the alkali hydrates can be
started or continued. Exemplary of the alkali hydrates are soda
ash, coal ash, sodium salts with alkalinity, phosphorous compounds,
and the like, which like the boron composition will be employed in
a suitable physical form. The boron composition mixes with the
swirling gas in the cyclone burner. The swirling air flow in the
burner 10 can be best seen from the flow lines in FIG. 1. The boron
composition is exposed to sodium in the flux and is believed to
convert to sodium borate that does the fluxing in the cyclone
barrel.
[0034] The technology of the invention allows precise targeting of
the cyclone barrel walls with the precise amount of boron
composition required to achieve the goal, 100% coverage and no
more. While mixing at the slag surface will be problematical when a
fluxing agent is simply added with the fuel or air fed to a
combustor, the ability of the invention to provide precise
targeting enhances the mixing. An advantage of this approach is
that the invention is highly effective as a remedial measure when
slagging anomalies are identified. Doses may be increased for a
time period as necessary to achieve mixing and slag
modification.
[0035] It will be seen that the doses of the boron compositions
according to the present invention can be reduced from what might
otherwise be necessary without precise dosing. Typically, the boron
composition will be introduced in amounts as low as about 0.1
pounds per ton of PRB or like coal. Preferred dosings in many cases
will be less than 1.0 pounds per ton of PRB coal, e.g., from 0.2 to
about 0.5 pounds per ton of PRB coal.
[0036] In addition to treating the slag in the cyclone barrel at
the cyclone barrel walls, the boron composition material can be
advantageously applied in a targeted fashion to the water-walls of
the boiler furnace to react with slag thereon to either insulate
heat transfer and allow steam temperature to be maintained in the
superheaters of boilers where superheat temperature is too low due
to high furnace water-wall area, or in a blend with other metal
oxides, to improve heat transfer on the water-walls by decreasing
reflectivity of high alkaline earth oxide containing boiler
ash.
[0037] To best understand the invention, we first refer to FIGS. 1
and 2, which are schematic representations of a cyclone furnace
combustor 10. The first, FIG. 1 shows the prior art with material
flows of air, coal and slag, while FIG. 2 shows one embodiment of
the invention wherein the boron composition is added to the cyclone
burner through separate injectors 20, and FIG. 3 shows the
introduction of the boron composition with secondary air 15. This
type of introduction is greatly benefited by the use of
computational fluid dynamics that will enable complete coverage of
exposed slag surfaces in the cyclone, as will be explained in
greater detail below.
[0038] The combustor includes a barrel 12, a re-entrant throat 14,
secondary air inlet 15, and slag tap opening 16 through which slag
17 flows out. In operation of the apparatus as shown, crushed coal
is fed, preferably with the primary air, through feed opening 18.
Coal is desirably fed in particulate form and particles are thrown
outward as the flow spins through the barrel as shown by the arrows
(the flame 13, generally shown in FIG. 3, will swirl like the
arrows). This flow of air and fuel is caused by the tangential flow
imparted by the manner of introducing the air and coal. This flow
creates a region of high heat release adjacent the refractory
lining of the barrel wall. The high temperature in this region
causes the ash contained within the coal to melt. The molten slag
17 acts as a trap for the carbon-rich coal particles, retaining the
particles for a period of time enabling a high degree of carbon
burnout. The molten slag 17 eventually migrates forward along the
wall of the barrel, exits at the slag spout opening 16, and
continuously drains through a slag tap opening 16 located below the
re-entrant throat 14. Tertiary air is shown to be fed to the burner
at the tertiary air inlet 19, and secondary air (the main
combustion air) enters the cyclone combustor at the secondary air
inlet 15. Reference to FIGS. 1 and 3 shows the slag emptying from
the furnace 30 via a slag tap 32.
[0039] The flow of slag 17 from the cyclone burner at the proper
rate with PRB coal is assured by the invention, which introduces
the boron composition in a manner that it treats the upper surface
of the flow of slag 17. The boron composition is provided as a fine
powder, preferably from drying a solution or suspension by hot air
in an air duct such as 52 in FIGS. 3 and 4. While not wishing to be
bound to any particular theory of operation, the treatment at the
surface of the slag with the finely-divided boron composition as
opposed to the whole mass of it helps explain how the invention can
operate effectively with very low consumption of born composition.
In the arrangement shown in FIG. 2, a borate or other boron
composition is introduced via feed 20. In preferred embodiments,
such as illustrated in FIGS. 3 to 5, the borate or other boron
composition will be fed as part of the secondary air 15.
[0040] Reference to FIG. 3 will show the general orientation of the
combustor 10 in the furnace 30, which is partially cut away at the
bottom to show flame 13. In actual practice, the barrel 12 will be
on the outside of the furnace 30 and the reentrant throat 14 and
the slag tap opening 16 will be on the inside.
[0041] The control system illustrated in FIG. 5 is representative
of those that can be employed and preferably includes sensors
indicated in the drawings by a symbol which comprises the letter
"X" in a box (very small), and electrical connectors shown as
dotted lines. The connectors can be hard-wired or wireless. The
control system includes a controller 40 as shown with a monitor or
other reporting device, which will receive signals from the various
sensors, calculate the appropriate control response by feed forward
and feedback logic and send control signals to the various pumps
shown in the drawings as a triangle within a circle. The controller
40 will control both the coal feed 18 and a borate or other boron
composition feed 21 as necessary to provide precise targeting of
the borate compound onto the surface of the slag within the barrel
12 in a manner that enhances the contact of the boron composition
with the slag layer 17. It is an advantage of the invention that
the dosing can also be highly effective as a remedial measure when
slagging anomalies are identified. As noted, doses may be increased
for a time period as necessary to achieve mixing and slag
modification.
[0042] The boron composition will be fed from injectors 20 in FIG.
2 or 54 in FIG. 4, in a suitable vehicle, e.g., slurry, particulate
solid or solution form, by controlled feeding through injection
ports on each cyclone barrel 12 such that the boron composition,
e.g., borate, borax or blend, lowers the melt point of the slag
while making slag thicker (more plastic), causing a protective
layer of slag to form on the cyclone barrel 12 inside wall, but
still able to flow to drains 16. The boron composition is provided
as a fine powder, preferably from drying a solution or suspension
by hot air in an air duct such as 52 in FIGS. 3 and 4. The gas flow
makes a swirling pattern as it moves from the entrance end of the
burner 10 and the coal source 18 upstream within an annular region,
exiting from the barrel through the reentrant throat 14 into the
furnace 30.
[0043] Preferably, the boron composition is introduced as
determined by computational fluid dynamics (CFD), which can be used
to determine the location of injection ports within ducts 52 as
shown in FIGS. 3 and 4. To get the best distribution, each cyclone
barrel is individually modeled to help determine the concentration
of the boron composition solution or suspension, the size of
droplets sprayed from nozzles 54, the direction and velocity of the
spray, for given gas flow and temperature measurements within duct
52. Nozzles 54 will be capable of forming fine sprays, and their
final selection will depend on the results of the CFD modeling. The
particles of the boron composition after spray and drying will be
very fine, on the order of 0.1.mu. on a weight average basis. In
preferred embodiments the boron compositions will be sprayed as
soluble compositions to form dry salts in finely divided form.
[0044] The use of modeling can assure that the boron composition is
properly administered in accord with the disclosures of U.S. Pat.
No. 5,740,745, U.S. Pat. No. 5,894,806 and U.S. Pat. No. 7,162,960,
which are incorporated by reference herein for their descriptions
of suitable computational fluid dynamic and other modeling
techniques. Once a model for a given combustor is made, further
similar units can take advantage of that as following that
iteration of the inventive process. FIG. 4 illustrates a single
secondary air feed duct 52 having a transition 53 to a secondary
air duct 15, which is oriented to direct the air tangentially into
cyclone burner 10. The secondary air feed duct 52 contains within
its interior a nozzle 54 for spraying a solution or suspension of
boron composition. The hot, preheated air from preheater 34 in FIG.
3 moves rapidly through the duct as the spray pattern 56 is seen to
enlarge radially without contacting the interior surfaces of the
duct 52 while it is wet. If contact were to occur, deposits of
boron composition would form and ultimately cause problems.
[0045] Process operating variations and physical combustor designs
will cause the temperature of the hot air in duct 52 to vary.
However, it is desired to use a temperature of above about
300.degree. F., and preferably within the range of from 400.degree.
to 700.degree. F. If needed, supplemental heaters can be employed.
It is attempted in the drawing by means of shading to show that at
a point approximately at 58, the spray will be dry and covering
about 80 to 99%, e.g., 90%, of the area of the duct 52. The
modeling should determine that first contact of the injected
materials with the wall should occur no sooner than where drying
finally occurs and the boron composition is in a fine powder form,
the wall have not been significantly wetted with the solution or
suspension. The modeling preferably takes into account the whole
length of duct 52 from the point of introduction to at least the
transition 53, and preferably into secondary air duct 15 and most
preferably to final introduction into cyclone burner 10.
[0046] As described in patents cited just above, a suitable
computational fluid dynamics (CFD) modeling technique can establish
a three-dimensional temperature profile. For applications involving
future construction or where direct measurements are impractical,
CFD modeling alone can sufficiently predict furnace conditions.
[0047] A computational fluid dynamics software package called
"PHOENICS" (Cham. LTD.), has been found effective. This program and
others can solve a set of conservation equations in order to
predict fluid flow patterns, temperature distributions, and
chemical concentrations within cells representing the geometry of
the physical unit. It has been found helpful to also run, in
addition to the standard program features, a set of subroutines to
describe flue gas properties and injector characteristics which for
utilization in the solution of the equations.
[0048] Typical sprays produce droplets with a range of sizes
traveling at different velocities and directions. These drops
interact with the flue gas and evaporate at a rate dependent on
their size and trajectory and the temperatures along the
trajectory. Improper spray patterns and improper location are
typical of prior art slag reducing procedures and result in less
than adequate chemical distributions and lessen the opportunity for
effective treatment.
[0049] A frequently used spray model is the PSI-Cell model for
droplet evaporation and motion, which is convenient for iterative
CFD solutions of steady state processes. The PSI-Cell method uses
the gas properties from the fluid dynamics calculations to predict
droplet trajectories and evaporation rates from mass, momentum, and
energy balances. The momentum, heat, and mass changes of the
droplets are then included as source terms for the next iteration
of the fluid dynamics calculations, hence after enough iterations
both the fluid properties and the droplet trajectories converge to
a steady solution. Sprays are treated as a series of individual
droplets having different initial velocities and droplet sizes
emanating from a central point. Correlations between droplet
trajectory angle and the size or mass flow distribution are
included, and the droplet frequency is determined from the droplet
size and mass flow rate at each angle.
[0050] The correlations for droplet size, spray angle, mass flow
droplet size distributions, and droplet velocities are found from
laboratory measurements using laser light scattering and the
Doppler techniques. Characteristics for many types of nozzles under
various operating conditions have been determined and are used to
prescribe parameters for the CFD model calculations.
[0051] The above description is for the purpose of teaching the
person of ordinary skill in the art how to practice the invention.
It is not intended to detail all of those obvious modifications and
variations, which will become apparent to the skilled worker upon
reading the description. It is intended, however, that all such
obvious modifications and variations be included within the scope
of the invention which is defined by the following claims. The
claims are meant to cover the claimed components and steps in any
sequence which is effective to meet the objectives there intended,
unless the context specifically indicates the contrary.
* * * * *