U.S. patent application number 12/501590 was filed with the patent office on 2010-01-14 for targeted reagent injection for slag control from combustion of coals high in iron and/or calcium.
This patent application is currently assigned to FUEL TECH, INC.. Invention is credited to Emelito P. Rivera, Kent W. Schulz, Christopher R. Smyrniotis.
Application Number | 20100006014 12/501590 |
Document ID | / |
Family ID | 41507460 |
Filed Date | 2010-01-14 |
United States Patent
Application |
20100006014 |
Kind Code |
A1 |
Smyrniotis; Christopher R. ;
et al. |
January 14, 2010 |
TARGETED REAGENT INJECTION FOR SLAG CONTROL FROM COMBUSTION OF
COALS HIGH IN IRON AND/OR CALCIUM
Abstract
Disclosed is a process that increases the output of a combustor
fired with coal having high iron and/or calcium content, by
reducing the tendency of slag to form on heat exchange surfaces and
changing the nature of the slag to make it easier to remove. The
process includes combusting a slag-forming coal, having high iron
and/or calcium content, with an overall excess of oxygen; moving
the resulting combustion gases though heat exchange equipment under
conditions which cause cooling of slag formed by burning the fuel;
and prior to contact with said heat exchange equipment, introducing
aqueous aluminum trihydroxide in amounts and with droplet sizes and
concentrations effective to decrease the rate of fouling, and
preferably, increase the friability of the resulting slag.
Desirably, the aluminum trihydroxide reagent is introduced in the
form of an aqueous liquid and computational fluid dynamics is
employed to determine flow rates and select reagent introduction
rates, reagent introduction location(s), reagent concentration,
reagent droplet size and/or reagent momentum. In a preferred
aspect, the feed rate will up to about 6 pounds ATH per ton and
preferably with up to about 2 pounds Mg(OH).sub.2 per ton of coal.
A process is also provided for cleaning and maintaining cleanliness
of a combustor.
Inventors: |
Smyrniotis; Christopher R.;
(St. Charles, IL) ; Schulz; Kent W.; (Geneva,
IL) ; Rivera; Emelito P.; (Inverness, IL) |
Correspondence
Address: |
THADDIUS J. CARVIS
102 NORTH KING STREET
LEESBURG
VA
20176
US
|
Assignee: |
FUEL TECH, INC.
Warrenville
IL
|
Family ID: |
41507460 |
Appl. No.: |
12/501590 |
Filed: |
July 13, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61080004 |
Jul 11, 2008 |
|
|
|
12501590 |
|
|
|
|
Current U.S.
Class: |
110/343 ;
110/344 |
Current CPC
Class: |
C10L 10/04 20130101;
C10L 2290/141 20130101; C10L 2200/0218 20130101; C10L 2200/0254
20130101; C10L 2290/18 20130101; F23J 7/00 20130101; C10L 10/06
20130101; C10L 2200/0213 20130101 |
Class at
Publication: |
110/343 ;
110/344 |
International
Class: |
F23J 7/00 20060101
F23J007/00; F23J 3/00 20060101 F23J003/00; F23J 15/06 20060101
F23J015/06 |
Claims
1. A process for reducing slag cohesiveness and/or adhesiveness in
a combustor, thereby decreasing the rate of fouling, comprising:
combusting a slag-forming coal, having high iron and/or calcium
content, with an overall excess of oxygen; moving the resulting
combustion gases though heat exchange equipment under conditions
which cause cooling of slag formed by burning the fuel; and prior
to contact with said heat exchange equipment, introducing aqueous
aluminum trihydroxide in amounts and with droplet sizes and
concentrations effective to decrease the rate of fouling by
slag.
2. A process according to claim 1, wherein the treatment is
effective to increase the friability of resulting slag.
3. A process according to claim 1, wherein, the aluminum
trihydroxide reagent is introduced in the form of an aqueous liquid
and computational fluid dynamics is employed to determine initial
flow rates and select reagent introduction rates, reagent
introduction location(s), reagent concentration, reagent droplet
size and/or reagent momentum.
4. A process according to claim 1, wherein the treatment further
includes introducing magnesium hydroxide in amounts and with
droplet sizes and concentrations effective to decrease the rate of
fouling by slag.
5. A process according to claim 1, wherein the treatment comprises
introducing up to about 6 pounds of aluminum trihydroxide slurry
per ton of coal and up to about 2 pounds of Mg(OH).sub.2 per ton of
coal.
6. A process for removing slag deposits in a combustor burning
coal, comprising: introducing into hot combustion gases in the
combustor, aqueous aluminum trihydroxide in amounts and with
droplet sizes and concentrations effective to remove slag
deposits.
7. A process according to claim 6, wherein the treatment further
includes introducing magnesium hydroxide in amounts and with
droplet sizes and concentrations effective to decrease the rate of
fouling by slag.
8. A process according to claim 6, wherein the treatment comprises
introducing up to about 6 pounds of aluminum trihydroxide slurry
per ton of coal and up to about 2 pounds of Mg(OH).sub.2 per ton of
coal.
9. A process a cleaning and maintenance of a boiler comprising a
regimen of initial dosing of from about 3 to 6 pounds of ATH per
ton of coal and about 1 to 2 pounds of Mg(OH).sub.2 per ton of coal
for a time sufficient to reduce slag, followed by a reduced
reducing the dosing of from about 10 to about 50% of the initial
values for maintaining the combustor clean and operating
efficiently.
10. A process according to claim 9, wherein the treatment further
includes introducing magnesium hydroxide in amounts and with
droplet sizes and concentrations effective to decrease the rate of
fouling by slag.
Description
PRIORITY CLAIM
[0001] This application claims priority to U.S. Provisional Patent
Application No. 61/080,004, filed Jul. 11, 2008, the disclosure of
which is incorporated herein in its entirety.
BACKGROUND OF THE INVENTION
[0002] The invention relates to a process that increases the output
of a combustor fired with coal having high iron and/or calcium
content, by reducing the tendency of slag to form on heat exchange
surfaces, changing the nature of the slag to make it easier to
remove and actually removing slag.
[0003] Combustion of coal, like other fossil fuels, is invariably
less efficient than desired and can be a source of pollution.
Maintaining combustor operation at high efficiency and controlling
the quality of the emissions is essential for maintaining the
energy needed to power our economy while preserving the quality of
the air we require for survival. Because efficiency and emissions
are interrelated and some technological solutions have been shown
to be competitive with each other, it has been difficult to achieve
both. Economic operation of power plants and incinerators is in the
public interest, and new technologies are essential to this
effort.
[0004] Fuel selection plays an important role in mitigating some
pollution problems, but it cannot eliminate them. Some coals, such
as certain Appalachian and Illinois Basin bituminous coals, are
important in many plants designed for coal where economics limits
other options. The tendency to form slag and the properties of the
slag for such high iron content coals have been a major concern of
combustion engineers and plant operators for decades. There are a
number of factors that impact the physical and chemical properties
of slag. See, for example, Combustion Fossil Power, 1991, Joseph G.
Singer, P. E., editor, Chapter 3, Combustion Engineering. However,
as the industry stands today, there is a compromise between
selection of low-cost coal and the actual economics of energy
production where slagging becomes a problem. Slag accumulation is a
problem that causes decreased heat transfer and often leads to long
periods of downtime for cleaning.
[0005] An interrelated problem with coal is that large amounts of
ash and fine particulates are formed that must be captured and
disposed of. The art has used additives to control slag formation
and properties, but the additives can stress the solids recovery
systems employed in terms of sheer volume. Accordingly, optimum
slag control has often been compromised because the solids recovery
system could not effectively remove all of the solids necessary.
This is especially a problem with older plants where increasing the
solids collection capacity is not an option.
[0006] Making the problem more complex is the fact that coals react
differently to additives as a function of their composition. As a
general rule, there are no known formulae that make it possible to
address all different coal compositions with suitable additives at
effective levels that can be adequately handled by solids recovery
equipment. The discovery of individual coal composition and
additive regimens are highly sought after to assure that economical
power can be supplied while generating sufficient revenues for
effective pollution control.
[0007] There is a need for an improved process that more
effectively controls slagging, especially with problem fuels, such
as coals with sulfur contents that cause them to play an increased
role in slagging and also those having high iron and/or calcium
contents, to improve boiler efficiency and economics.
DISCLOSURE OF INVENTION
[0008] It is an object of the invention to provide an improved
technology for slag control in combustors utilizing fuels tending
toward the production of slag.
[0009] It is another object to provide a process to control slag
from the combustion of coal with high iron and/or calcium contents
while reducing chemical utilization.
[0010] It is another object to provide a process to remove slag
from boiler heat exchange surfaces due to the combustion of coal
with high iron and/or calcium contents while reducing chemical
utilization.
[0011] A yet further but more specific object is to provide a
process to more effectively control slag by decreasing the amount
of downtime associated with slag removal.
[0012] It is a more specific object of some aspects of the
invention to achieve the above objects while at the same time
improving combustor efficiency.
[0013] These and other objects are achieved by the present
invention in at least its preferred aspects which provides an
improved process for slag control in combustors burning
slag-forming coal with high iron and/or calcium content.
[0014] In one aspect, the invention provides a process for reducing
slag cohesiveness and/or adhesiveness in a combustor, thereby
decreasing the rate of fouling, comprising: combusting a
slag-forming coal, having high iron and/or calcium content, with an
overall excess of oxygen; moving the resulting combustion gases
though heat exchange equipment under conditions which cause cooling
of slag formed by burning the coal; and prior to contact with said
heat exchange equipment, introducing aqueous aluminum trihydroxide
in amounts and with droplet sizes and concentrations effective to
decrease the rate of fouling, and preferably, increase the
friability of the resulting slag.
[0015] In one preferred aspect, the aluminum trihydroxide reagent
is introduced in the form of an aqueous liquid and computational
fluid dynamics is employed to determine flow rates and select
reagent introduction rates, reagent introduction location(s),
reagent concentration, reagent droplet size and/or reagent
momentum.
[0016] In another preferred aspect, magnesium hydroxide is
introduced as an aqueous slurry along with the slurry of aluminum
trihydroxide.
[0017] In another aspect the invention provides a process for
cleaning furnace surfaces having a slag buildup, by introducing
aqueous aluminum trihydroxide in amounts and with droplet sizes and
concentrations effective to contact for fine particulates resulting
from drying the slurry to contact existing slag deposits.
[0018] In another aspect, the invention provides a process a
cleaning and maintenance of a combustor comprising a regimen of
initial dosing of from about 3 to 6 pounds of ATH per ton of coal
and about 1 to 2 pounds of Mg(OH).sub.2 per ton of coal for a time
sufficient to reduce slag, followed by a reduced reducing the
dosing of from about 10 to about 50% of the initial values for
maintaining the combustor clean and operating efficiently.
[0019] Other preferred aspects and their advantages are set out in
the description which follows.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] The invention will be better understood and its advantages
will become more apparent when the following detailed description
is read in conjunction with the accompanying drawings, in
which:
[0021] FIG. 1 is a schematic view of one embodiment of the
invention.
[0022] FIG. 2 is a photograph of a slag sample obtained after
operation for 24 hours of aluminum trihydroxide into a combustor
operated on a high iron content coal as set out in Example 2
below.
DETAILED DESCRIPTION OF THE INVENTION
[0023] Reference will first be made to FIG. 1, which is a schematic
view of one embodiment of the invention. FIG. 1 shows a large
combustor 10 of the type used for producing steam for electrical
power generation, process steam, heating or incineration. Coal is
fed by burners 20 and 20a and burned with air in a combustion zone
21. It is an advantage of the invention that coal that is high in
iron (e.g., iron contents of greater than about 15%, e.g., from
about 20 to 35%, based on the weight of the ash and expressed as
Fe.sub.2O.sub.3) and/or calcium content (e.g., calcium contents of
greater than 5%, e.g., from about 10 to 25%, based on the weight of
the ash and expressed as CaO). It is also an advantage of the
invention that slag can be effectively controlled even for coals
having significant sulfur contents, e.g., above about 1% and in the
range of from about 3 to about 5%. Here, and throughout this
description, all parts and percentages are by weight.
[0024] Air for combustion, supplied by fan 22 and ductwork 24, is
preferably preheated by a gas-to-gas heat exchangers (not shown)
which transfer heat from ductwork (not shown) at the exit end of
the combustor. Hot combustion gases rise and flow past heat
exchangers 26, which transfer heat from the combustion gases to
water for the generation of steam. Other heat exchangers, including
an economizer (downstream and not shown) may also be provided
according to the design of the particular boiler. Slag left
untreated would tend to form on these heat exchanger surfaces,
which are positioned within specific combustors based on design
considerations important to individual locations. It is an
advantage of the present invention that modeling techniques, such
as computational fluid dynamics, are employed to initially direct
treatment chemicals (especially, those identified as effective for
particular types of coal according to the invention) to the optimum
locations for reducing and/or controlling slag buildup and
maintaining efficient operation of the boiler.
[0025] A series of suitable, preferably air assisted atomizing,
nozzles in each of nozzle banks 30 and 30a are provided for
introducing aluminum trihydroxide alone or with magnesium hydroxide
slurry from vessels 40 and 40a respectively. Both the ATH and the
magnesium hydroxide are preferably aqueous, as slurries and/or
solutions as appropriate. Supply lines (e.g., 41) are shown as
double lines in the drawing. Valves (e.g., 42) are represented by
the common symbol and temperature sensors (e.g., 44) are
represented by the common symbol Both valves 42 and temperature
sensors 44 are connected to controller 46 via electrical leads
(e.g., 48) shown in dotted lines. These valves, temperature sensors
and leads are illustrative only, and the skilled worker using the
principles outlined herein will place them strategically to provide
appropriate control signals and responses. The controller 46 can be
a general purpose digital computer programmed in accord with a
predetermined control regimen with both feed forward and feedback
features.
[0026] Aluminum trihydroxide (Al(OH).sub.3), which has been found
effective according to the invention for greatly lessening the
deposition of slag or cleaning deposited slag from troublesome coal
types, is also known under other names such as ATH, aluminum
hydroxide and hydrated alumina. Regardless of the form of aluminum
trihydroxide raw material, it is preferred that it is mixed with
water for introduction from tank 40 through associated lines 41,
with or without chemical stabilizers, to concentrations suitable
for storage and handling, e.g., at least about 25%, and preferably
at least about 65%, solids by weight.
[0027] As will be described, the concentration and flow rates will
be initially determined by modeling to assure that the proper
amount of chemical is supplied to the correct location in the
combustor in the correct physical form to achieve the desired
results of reduced slagging and ease of clean up. For use in the
process, it is diluted as determined, e.g., by computational fluid
dynamics (CFD) to within the range of from about 0.1 to about 10%,
more narrowly from about 1 to about 5%. When the aqueous aluminum
trihydroxide contacts the hot gases in the combustor, it is
believed to be reduced to very small particles, e.g., nano-sized
particles, e.g., under 200 nanometers and preferably below about
100 nanometers. Median particle sizes of from 50 to about 150
nanometers are useful ranges for the process of the invention. To
approach this size, it is important that the ATH be introduced with
water. The small particles are believed to disrupt the normal
crystalline or glass that forms the slag. Regardless of the
mechanism involved it is a distinct advantage of the invention that
the slag that does form is highly friable and breaks easily with
brushing and can be crushed by hand.
[0028] It is a significant advantage of the invention that the
friability of slag that is formed is increased, making it easier to
remove. The invention also slows or eliminates the buildup of slag.
Advantageously, at high doses, the invention can actually remove
slag that has already formed. By the term "increase the friability
of the slag" it is meant that the slag after treatment requires
less force per unit area to crush than slag formed under the same
conditions without the treatment. By the term "remove slag" it is
meant that the weight of the slag adhering to boiler, particularly
heat exchange, surfaces is reduced from initial values by the
treatment of the invention. There are several additional and
attendant advantages of the invention, including the reduction of
SO.sub.3 for high sulfur coals, the reduction of the pressure drop
across heat exchange apparatus, the ability to use lower cost coal,
lower CO generation, lower CO.sub.2 generation due to increased
fuel consumption, better heat transfer, less down time, higher
throughput, cleaning on line, cleaner heat exchange surfaces,
ability to clean the whole combustor, and the ability to run at all
loads with greater efficiency.
[0029] The process for most coals works best with a combination of
ATH and magnesium hydroxide. While some coals, e.g., with low
silicate compositions can be burned with reduced problems
attributed to slag, the use of magnesium hydroxide, at least
initially, is preferred. The magnesium hydroxide reagent can
preferably be prepared from brines containing calcium and other
salts, usually from underground brine pools or seawater. Dolomitic
lime is mixed with these brines to form calcium chloride solution
and magnesium hydroxide which is precipitated and filtered out of
the solution. This form of magnesium hydroxide can be mixed with
water, with or without stabilizers, to concentrations suitable for
storage and handling, e.g., from 25 to 65% solids by weight. For
use in the process, it is diluted as determined by computational
fluid dynamics (CFD) to within the range of from 0.1 to 10%, more
narrowly from 1 to 5%. When it contacts the effluent in combustor,
it is believed reduced to nano-sized particles, e.g., under 200
nanometers and preferably below about 100 nanometers. Median
particle sizes of from 50 to about 150 nanometers are useful ranges
for the process of the invention. Other forms of MgO can also be
employed where necessary or desired, e.g., "light burn" or
"caustic" can be employed where it is available in the desired
particle size range.
[0030] To best achieve these effects, the invention will preferably
take advantage of CFD to project initial flow rates and select
initial reagent introduction rates, reagent introduction
location(s), reagent concentration, reagent droplet size and
reagent momentum. CFD is a well understood science, and it is
utilized with full benefit in this case, where it is desired to
supply a minimum amount of chemical for maximum effect.
[0031] It is noted as highly significant that the amount of
chemical will be substoichiometric in terms of affecting the fusion
point of the slag--often considered to be the controlling factor in
slag control. According to the present invention, there is good
evidence besides the relatively small amount of reagent employed
that the results of the invention are due to a physical disruption
of slag formation with possible boundary chemical and kinetic
effects not explained by the literature.
[0032] Testing has shown that initial feed rates determined by CFD
can be utilized with good effect and then adjusted based on
observed results. As a guide to feed rates, the initial feed rate
for the best economics for combustors operating similar to the one
exemplified below can be up to about 6 pounds of ATH(as dry active
ATH) or 8 pounds (as a 65-70% slurry) per ton of coal. For example,
when added as a preferred 70% slurry, amounts of from about 1 to
about 6 pounds of slurry will be effective (more narrowly, e.g.,
about 2 to about 3 pounds of slurry). It is preferred to also use
up to about 2 pounds of Mg(OH).sub.2 slurry (at about 50-60%
solids) per ton of coal. For example, when added as a preferred 60%
slurry, amounts of from about 0.5 to about 2 pounds of Mg(OH).sub.2
slurry per ton of coal, e.g., from about 0.7 to about 1 pounds of
Mg(OH).sub.2 slurry per ton of coal can be utilized. The slurries
are diluted as necessary, typically to a solids concentration of
from about 5% for smaller applications to about 35% or more.
[0033] The weight of the slag adhering to a combustor, particularly
heat exchange, surfaces is effectively reduced from initial values
by the treatment of the invention, especially when the ATH and
Mg(OH).sub.2 are used at high concentrations within the above
ranges, e.g., from about 3 to 6 pounds of ATH per ton of coal and
about 1 to 2 pounds of Mg(OH).sub.2 per ton of coal. This ability
to remove slag provides the ability to provide a cleaning and
maintenance regimen wherein the initial dosing is as just mentioned
for removing slag, with the dosing then reduced to from about 10 to
about 50% of the initial values for maintaining the combustor clean
and operating efficiently.
[0034] It is essential for optimum slag remediation according to
the invention, that the correct initial concentrations, rates and
introduction rates be calculated and employed for the effective
physical form of aluminum trihydroxide, and preferably, optionally
magnesium hydroxide, to be introduced into the hot combustion gases
in chamber 20 to enable the chemical to be added with the desired
effect. The implementation of CFD to the invention can be
accomplished as set out in U.S. Pat. No. 7,162,960 to Smyrniotis,
et al. Particulate removal equipment (not shown) can be employed to
remove particulates prior to passing the effluent up the stack.
[0035] In another alternate form of the invention, combustion
catalysts and or effluent treatment chemicals can be added to the
fuel, combustion zone or otherwise as described, for example in
U.S. Pat. No. 7,162,960 to Smyrniotis, et al.
[0036] The following examples are presented to further explain and
illustrate the invention and are not to be taken as limiting in any
regard. Unless otherwise indicated, all parts and percentages are
by weight.
EXAMPLE 1
[0037] This example illustrates introduction of aluminum
trihydroxide into a furnace burning 540 tons of coal per day. The
coal is a blend of Illinois basin and Appalachian bituminous coals,
giving the following analysis as combined:
TABLE-US-00001 Sample 1 2 3 Moisture, % 11.28 10.85 10.19 Ash, %
14.91 13.63 13.91 Volatile Matter, % 36.03 35.04 Fixed Carbon, %
39.49 40.86 Total, % 100 100 Sulfur, % 3.95 4.44 HHV, BTU/lb 10,742
10,730
[0038] For the test Al(OH).sub.3 (aluminum trihydroxide slurry or
ATH for short) is fed as a 70% by weight aqueous slurry at a rate
of 5 pounds slurry per ton of coal consumed from two banks of three
air-cooled nozzles positioned on the wall opposite of two banks of
pulverized coal burners--one bank at an elevation between the two
burners and one bank at an elevation above the uppermost coal
burners. The slurry is diluted to a concentration of 35 weight %
ATH. The density of the ATH slurry before dilution is about 14
pounds/gallon, meaning that the feed rate is about 193 gallons per
day (about 5 pounds per ton of coal) for ATH slurry.
[0039] Based on this test, it is estimated that an effective feed
rate for this particular combustor will be from about 1 to about 6
pounds of ATH slurry per ton of coal, e.g., about 2 to about 3
pounds per ton.
EXAMPLE 2
[0040] This example illustrates the effect of introducing
Mg(OH).sub.2 (magnesium hydroxide) into a furnace burning 540 tons
of coal per day in addition to the aluminum trihydroxide fed in
Example 1. The coal was a blend of Illinois basin and Appalachian
bituminous coals, as illustrated in Example 1.
[0041] The magnesium hydroxide was fed as a slurry at 2 lbs of 50
to 60 weight % slurry per ton of coal consumed. Density of the
magnesium hydroxide slurry was approximately 12 lbs/gallon.
Therefore, the feed rate was about 90 gallons per day for the
Mg(OH).sub.2 slurry. As before, we fed the aluminum trihydroxide
slurry at about 5 pounds of slurry per ton of coal consumed. The
density of the ATH was about 14 pounds/gallon, making the feed rate
about 193 gallons per day for ATH.
[0042] Based on this test, we estimate optimal feed rate for the
best economics for the this particular combustor to be about 0.5 to
about 2 pounds Mg(OH).sub.2 slurry per ton of coal (e.g., about 1
pound per ton) plus from about 1 to about 6 pounds ATH slurry per
ton (e.g., about 2 to about 3 pounds per ton). FIG. 2 is a
photograph of a slag sample obtained after operation for 24 hours
of ATH feed only. The slag was unexpectedly friable.
[0043] 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 that is effective to meet the objectives there intended,
unless the context specifically indicates the contrary.
* * * * *