U.S. patent application number 10/278806 was filed with the patent office on 2003-03-06 for method for treatment of coal ash, and method for desulfurization.
This patent application is currently assigned to Idemitsu Kosan Co., Ltd.. Invention is credited to Furuya, Osamu.
Application Number | 20030044337 10/278806 |
Document ID | / |
Family ID | 18049153 |
Filed Date | 2003-03-06 |
United States Patent
Application |
20030044337 |
Kind Code |
A1 |
Furuya, Osamu |
March 6, 2003 |
Method for treatment of coal ash, and method for
desulfurization
Abstract
Provided are a method of treating coal ash by mixing it with
water, in which the coal ash and water undergo a temperature
difference therebetween while they are mixed; and a method of
desulfurization in a coal combustion boiler system, which comprises
mixing coal ash that has been separated from a ash collector in the
system, with water on the condition that the two undergo a
temperature difference therebetween while they are mixed, and
circulating the resulting mixture that serves as a desulfurizing
agent into the coal combustor in the system. The desulfurization
capability of the desulfurizing agent used in the desulfurization
method is higher than that of the desulfurizing agent obtained
through hydration of coal ash with water or steam.
Inventors: |
Furuya, Osamu;
(Ichihara-shi, JP) |
Correspondence
Address: |
OBLON SPIVAK MCCLELLAND MAIER & NEUSTADT PC
FOURTH FLOOR
1755 JEFFERSON DAVIS HIGHWAY
ARLINGTON
VA
22202
US
|
Assignee: |
Idemitsu Kosan Co., Ltd.
Tokyo
JP
|
Family ID: |
18049153 |
Appl. No.: |
10/278806 |
Filed: |
October 24, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10278806 |
Oct 24, 2002 |
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09869156 |
Jun 25, 2001 |
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09869156 |
Jun 25, 2001 |
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PCT/JP00/07663 |
Oct 31, 2000 |
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Current U.S.
Class: |
423/244.08 ;
423/460 |
Current CPC
Class: |
F23J 15/006 20130101;
Y02W 30/91 20150501; Y02W 30/92 20150501; F23G 2201/701 20130101;
F23J 2215/20 20130101; C04B 18/06 20130101; F23G 2209/30 20130101;
B09B 3/00 20130101; F23G 2201/601 20130101; C04B 18/06 20130101;
C04B 20/02 20130101 |
Class at
Publication: |
423/244.08 ;
423/460 |
International
Class: |
B01D 053/50; C01B
007/00 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 4, 1999 |
JP |
11-314092 |
Claims
1. A method of treating coal ash that contains a limestone-derived
component, by mixing it with water, which comprises mixing the coal
ash with water on a condition that the coal ash and water undergo a
temperature difference therebetween while they are mixed.
2. The method of treating coal ash as claimed in claim 1, wherein
the temperature difference is at least 30.degree. C.
3. The method of treating coal ash as claimed in claim 1 or 2,
wherein coal ash at a temperature falling between 80 and
150.degree. C. is mixed with water at a temperature falling between
2 and 50.degree. C.
4. The method of treating- coal ash as claimed in claim 1 or 2,
wherein coal ash at room temperature is mixed with hot water at a
temperature falling between 60 and 98.degree. C.
5. The method of treating coal ash as claimed in any of claims 1 to
4, wherein 100 parts by weight of coal ash is mixed with from 20 to
200 parts by weight of water.
6. The method of treating coal ash as claimed in any of claims 1 to
5, wherein the mixture of coal ash and water has a mean grain size
of from 0.1 to 20 mm.
7. A desulfurizing agent comprising the mixture of coal ash and
water obtained in the treating method of any of claims 1 to 6.
8. A soil improver comprising the mixture of coal ash and water
obtained in the treating method of any of claims 1 to 6.
9. A method of desulfurization in a coal combustion boiler system,
which comprises mixing coal ash that has been separated from a ash
collector in the system and contains a limestone-derived component,
with water on the condition that the two undergo a temperature
difference therebetween while they are mixed, and feeding the
resulting mixture that serves as a desulfurizing agent into the
coal combustor in the system.
10. A method of desulfurization in a coal combustion boiler system,
which comprises mixing coal ash that has been separated from a ash
collector in the system and contains a limestone-derived component,
with water on the condition that the coal ash at a temperature
falling between 80 and 150.degree. C. is mixed with water at a
temperature falling between 2 and 50.degree. C., and circulating
the resulting mixture that serves as a desulfurizing agent into the
coal combustor in the system.
11. A method of desulfurization in a coal combustion boiler system,
which comprises mixing coal ash that has been separated from a ash
collector in the system and contains a limestone-derived component,
with water on the condition that the coal ash at room temperature
is mixed with hot water at a temperature falling between 60 and
80.degree. C., and circulating the resulting mixture that serves as
a desulfurizing agent into the coal combustor in the system.
12. The desulfurization method as claimed in any of claims 9 to 11,
wherein 100 parts by weight of coal ash is mixed with from 20 to
200 parts by weight of water.
13. The desulfurization method as claimed in any of claims 9 to 12,
wherein the mixture of coal ash and water having been so mixed that
the temperature difference therebetween is at least 30.degree. C.
and having a mean grain size of from 0.1 to 20 mm serves as a
desulfurizing agent.
Description
TECHNICAL FIELD
[0001] The present invention relates to a method for treatment of
coal ash (combustion ash of coal), precisely to production of a
desulfurizing agent and a soil improver through mixing treatment of
coal ash with water, and to a method of desulfurization with the
desulfurizing agent in a coal combustor.
BACKGROUND ART
[0002] Exhaust gas from a coal combustion boiler generally contains
from 100 to 2,000 ppm (by volume) of harmful substances such as
sulfur oxides. As they cause acid rain and photochemical smog, it
is desired to effectively treat them. Heretofore, a dry process
such as an activated charcoal process, and a wet process such as a
lime-gypsum process have been developed. However, the dry process
is problematic in that the ratio of removal of harmful substances
could not be increased therein. In the wet process, the ratio of
removal of harmful substances is high, but this is problematic in
that the treatment of waste water is difficult and the equipment
costs and the running costs are high.
[0003] To solve the problems, desired is a desulfurization process
in which the ratio of removal of harmful substances is high and
which can be run at low costs, not producing waste water.
Therefore, for desulfurization in coal combustion boilers,
<1> a semi-dry process of spraying slaked lime or its slurry
into exhaust gas, <2> a dry process of atomizing limestone
into high-temperature gas in a gas duct, and <3> a dry
process of directly feeding limestone into a combustor have been
proposed, apart from the processes mentioned above. When compared
with the activated charcoal process and the lime-gypsum process,
these processes are good as the equipment costs and the running
costs for them are low, but are still problematic in that the ratio
of removal of harmful substances therein is not always high.
[0004] In particular, the processes <1> and <2> are
problematic in that the time for contact of exhaust gas with the
desulfurizing component could not be prolonged therein. Therefore,
the process <3> of feeding limestone into a combustor,
especially into a fluidized-layer or fluidized-bed combustor is now
considered good. Specifically, for boiler systems capable of
efficiently burning coal therein, for example, circulation-type,
coal combustion boilers equipped with a fluidized-bed combustor
have been put into practical use, in which the fluidized bed is
formed of relatively large-size grains.
[0005] The coal combustion boiler of the type equipped with a
fluidized-bed combustor is described with reference to a conceptual
view showing it. FIG. 1 is a conceptual explanatory view showing a
fluidized bed-type, coal combustion boiler for one embodiment of
the desulfurization method of the present invention. The fluidized
bed-type, coal combustion boiler comprises a fluidized-bed
combustor 1 having a fluidized bed 2 in its part; a cyclone 3 for
separating particulates from the waste gas from the combustor 1; an
external heat exchanger 4 for utilizing the heat of the
particulates separated in the cyclone 3; and a convection-type
heat-transfer unit 5.
[0006] In the combustor 1, the fluidized bed 2 is formed of, for
example, gravel of from 10 to 20 mm or so in size; and a
desulfurizing agent such as limestone is fed along with coal into
the fluidized bed via a feed port 7. Below the fluidized bed 2, air
is fed into the combustor via a primary air introduction port 8,
with which the coal in the combustor is burned up. Burnt ash and
carbon and other small grains of limestone and ground gravel are
moved to the cyclone 3. The small grains are separated in the
cyclone 3, and drop down into the external heat exchanger 4
disposed below the cyclone 3.
[0007] The small grains, of which the heat is recovered in the heat
exchanger 4, are circulated back to the fluidized bed 2. The
high-temperature gas having been separated from the small grains in
the cyclone 3 is led into the convention-type heat-transfer unit 5,
in which the gas heats water in the heat-transfer tube 10 to
produce steam, and is thereby cooled. Then, the gas is led into a
bag filter 6, in which fine particulates in the gas are trapped,
and the gas with no particulates is discharged out of the
system.
[0008] The coal ash thus trapped in the bag filter (ash collector)
is referred to as "bag ash", and this fine particulates having a
size of a few microns and scattering. In general, therefore, water
is added thereto. Thus wetted, this is discarded or utilized for
land reclamation, or a part of it is utilized for cement. However,
the location of coal combustion boiler plants is not always near to
the location for land reclamation or to the location of cement
factories, and utilizing the coal ash for land reclamation or for
cement is problematic in that the costs for transporting it are
high. On the other hand, utilizing the coal ash for other building
materials, for example, for cement aggregate and roadbed materials
is investigated. However, since the composition of the coal ash is
not always constant, the quality control thereof for such building
materials is difficult. At present, therefore, the practical use of
the coal ash is limited.
[0009] In coal combustion boilers, limestone is added to the
fluidized bed in the combustor along with coal thereto, for
absorbing sulfur dioxide (SO.sub.2) from the burned coal.
Therefore, the coal ash contains about 10 to 40% by weight of the
ash component derived from the limestone. Of the limestone-derived
ash component, calcium having trapped sulfur dioxide therein is
generally about 30% by weight. In the other part of the ash
component, the limestone has absorbed sulfur dioxide only in its
surface and has been solidified to be gypsum (CaSO.sub.4), but
inside the gypsum, the limestone (CaCO.sub.3) is heated and
converted into quick lime (CaO), or that is, this is kept
unreacted.
[0010] Therefore, it is undesirable to use the unreacted
lime-containing, coal ash for land reclamation as it is, from the
viewpoint of natural resources saving and cost reduction. In
addition, it is well known that, when water is added thereto, the
unreacted quick lime is hydrated to form calcium hydroxide
(Ca(OH).sub.2) with suddenly generating great heat and much
expanding. Therefore, taking advantage of this knowledge, various
methods have been proposed for recycling the unreacted
lime-containing, coal ash.
[0011] For example, for the fluidized bed-type, coal combustion
boiler as in FIG. 1, <1> Japanese Patent Laid-Open No.
166110/1996 has proposed a method of recycling the combustion ash
from the fluidized-bed combustor, which comprises a step of
kneading and solidifying the ash having been collected after the
cyclone, with water or with water and a cement-type solidifier, a
step of grinding the resulting solid into grains, and a step of
circulating the grains into the combustor; and <2> Japanese
Patent Laid-Open No. 42614/1997 has proposed a method of recycling
the combustion ash from the fluidized-bed combustor, which
comprises a first step of wetting the combustion ash having been
collected in a bag filter, with mixing and stirring it for
hydration to thereby convert it into re-activated ash, a second
step of drying the re-activated ash, and a third step of
circulating the dried, re-activated ash into the combustor.
[0012] On the other hand, <3> Japanese Patent Laid-Open No.
35827/1986 has proposed a different method of purifying exhaust gas
according to a dry lime process, which comprises leading the
particulates having been separated from combustion ash in a bag
filter into a classifier where the particulates are classified into
a group of coarse, large-size particulates containing fly ash, and
a group of fine, small-size particulates containing lime particles
of which the surface has been compounded with a harmful acid
substance to form a shell to cover each particle, then hydrating
the lime particles in the group of fine particulates, with steam to
break and remove their shells owing to their volume expansion
through hydration, to thereby make them have non-reacted lime
exposed out of their surface to form recycled lime particles, and
circulating the recycled lime particles into exhaust gas.
[0013] In these methods, the gypsum layer formed around the lime
particles through the reaction of sulfur oxides with lime in the
coal ash is broken by hydration of lime, or that is, the hydration
of lime is efficiently utilized therein. Accordingly, in these
methods, the final desulfurizing agent suitable to the site where
it is fed into the coal combustion boiler system and to the unit
via which it is fed thereinto is produced and circulated in the
system thereby to ensure efficient desulfurization therein and to
reduce the coal ash from the system.
[0014] However, having investigated and evaluated the capabilities
of the desulfurizing agent formed through hydration of coal ash
with water or steam, I, the present inventor has found that the
degree of desulfurization with the desulfurizing agent is not high.
Therefore, it is believed that the desulfurization method for the
coal combustion boiler system of using the desulfurizing agent
formed through hydration of coal ash with water or steam could not
be put into practical use. Accordingly, the present invention is
essentially for providing a method of treating coal ash that makes
it possible to recycle (circulate) the treated ash as a
desulfurizing agent in a coal combustion boiler system, and
providing a desulfurization method for a coal combustion boiler
system.
DISCLOSURE OF THE INVENTION
[0015] To solve the above-mentioned problems in the related art, I,
the present inventor has assiduously studied the process of
treating the coal ash that contains a limestone-derived component,
for making the treated ash recyclable as a desulfurizing agent in
the process, and, as a result, has found that the condition for
hydration in the process has a significant influence on the
capability of the treated ash for desulfurization. On the basis of
this finding, the inventor has completed the present invention.
[0016] Specifically, the invention includes the following:
[0017] 1. A method of treating coal ash that contains a
limestone-derived component, by mixing it with water, which
comprises mixing the coal ash with water on a condition that the
coal ash and water undergo a temperature difference therebetween
while they are mixed.
[0018] 2. The method of treating coal ash of above 1, wherein the
temperature difference is at least 30.degree. C.
[0019] 3. The method of treating coal ash of above 1 or 2, wherein
coal ash at a temperature falling between 80 and 150.degree. C. is
mixed with water at a temperature falling between 2 and 50.degree.
C.
[0020] 4. The method of treating coal ash of above 1 or 2, wherein
coal ash at room temperature is mixed with hot water at a
temperature falling between 60 and 98.degree. C.
[0021] 5. The method of treating coal ash of any of above 1 to 4,
wherein 100 parts by weight of coal ash is mixed with from 20 to
200 parts by weight of water.
[0022] 6. The method of treating coal ash of any of above 1 to 5,
wherein the mixture of coal ash and water has a mean grain size of
from 0.1 to 20 mm.
[0023] 7. A desulfurizing agent comprising the mixture of coal ash
and water obtained in the treating method of any of above 1 to
6.
[0024] 8. A soil improver comprising the mixture of coal ash and
water obtained in the treating method of any of above 1 to 6.
[0025] 9. A method of desulfurization in a coal combustion boiler
system, which comprises mixing coal ash that has been separated
from a ash collector in the system and contains a limestone-derived
component, with water on the condition that the two undergo a
temperature difference therebetween while they are mixed, and
feeding the resulting mixture that serves as a desulfurizing agent
into the coal combustor in the system.
[0026] 10. A method of desulfurization in a coal combustion boiler
system, which comprises mixing coal ash that has been separated
from a ash collector in the system and contains a limestone-derived
component, with water on the condition that the coal ash at a
temperature falling between 80 and 150.degree. C. is mixed with
water at a temperature falling between 2 and 50.degree. C., and
circulating the resulting mixture that serves as a desulfurizing
agent into the coal combustor in the system.
[0027] 11. A method of desulfurization in a coal combustion boiler
system, which comprises mixing coal ash that has been separated
from a ash collector in the system and contains a limestone-derived
component, with water on the condition that the coal ash at room
temperature is mixed with hot water at a temperature falling
between 60 and 80.degree. C., and circulating the resulting mixture
that serves as a desulfurizing agent into the coal combustor in the
system.
[0028] 12. The desulfurization method of any of above 9 to 11,
wherein 100 parts by weight of coal ash is mixed with from 20 to
200 parts by weight of water.
[0029] 13. The desulfurization method of any of above 9 to 12,
wherein the mixture of coal ash and water having been so mixed that
the temperature difference therebetween is at least 30.degree. C.
and having a mean grain size of from 0.1 to 20 mm serves as a
desulfurizing agent.
BRIEF DESCRIPTION OF THE DRAWING
[0030] FIG. 1 is a conceptual explanatory view showing a fluidized
bed-type, coal combustion boiler for one embodiment of the
desulfurization method of the invention. The reference numerals and
signs are as follows:
[0031] 1: Fluidized-bed combustor
[0032] 2: Fluidized bed
[0033] 3: Cyclone
[0034] 4: External heat exchanger
[0035] 5: Convection-type heat-transfer unit
[0036] 6: Bag filter
[0037] 7: Coal feed port
[0038] 8: Primary air introduction port
[0039] 9: Pipe line
[0040] 10: Heat-transfer duct
[0041] A: Steam
[0042] B: Water
[0043] C: Coal ash
BEST MODES OF CARRYING OUT THE INVENTION
[0044] The invention is described in detail hereinunder.
[0045] The method of treating coal ash of the invention comprises
mixing coal ash that contains a limestone-derived component, with
water, in which the coal ash and water undergo a temperature
difference therebetween while they are mixed. The coal ash is one
derived from coal, limestone serving as a desulfurizing agent and
regenerated combustion ash, by burning them in the combustor in a
coal combustion boiler system, and this is after the cyclone,
especially after the bag filter in the system.
[0046] As so mentioned hereinabove, the coal ash contains in
addition to the ash of combustion coal, about 30% by weight of the
limestone-derived component and a small amount of carbon. The
temperature difference between the coal ash and water while the two
are mixed is preferably at least 30.degree. C., more preferably at
least 50.degree. C., even more preferably at least 60.degree. C.
The temperature difference is preferably larger, as the capability
of the resulting mixture for desulfurization is higher. For the
temperature difference between the two, the temperature of any one
of the coal ash and water is kept higher. Preferably, the
temperature of the coal ash is kept higher, for example, falling
between 80 and 150.degree. C., more preferably between 100 and
130.degree. C., and the temperature of water is kept lower than it
at least by 30.degree. C., more preferably at least by 50.degree.
C., for example, falling between 2 and 50.degree. C. For example,
the high-temperature, coal ash having been taken out of the bag
filter (ash collector) in a coal combustion boiler system may be
directly mixed with room-temperature water. In this method, the
energy loss can be reduced, and the working efficiency can be
increased, and, in addition, the desulfurization capability of the
resulting mixture can be enhanced. On the other hand, it is also
desirable to use hot water heated at a high temperature, in view of
the working efficiency, the temperature control and the
handlability in the method. Preferably, the temperature of hot
water is higher than that of cold ash by at least 30.degree. C.,
more preferably it falls between 60 and 98.degree. C., even more
preferably between 70 and 98.degree. C.
[0047] In the invention, the temperature difference is in the
initial stage of mixing the two, ash and water, and the temperature
difference between the two in the initial mixing stage is a matter
of importance. The amount of water to be mixed with the coal ash is
not specifically defined, and it is not smaller than the amount of
water enough for hydration of quick lime in the coal ash to convert
it into calcium hydroxide. However, the combustion ash contains, in
addition to the limestone-derived component, a large amount of
coal-derived, combustion ash component. Therefore, in consideration
of the water absorption of the coal-derived ash component, the
amount of water to be mixed with the combustion ash preferably
falls between 20 and 200 parts by weight, more preferably between
30 and 150 parts by weight, relative to 100 parts by weight of the
combustion ash. The amount of water may be determined in
consideration of the miscibility of the ash with water, the ability
of quick lime in the ash to be hydrated with water, the grain size
of the treated mixture and the post treatment of the mixture.
[0048] In case where the treated ash is used as a desulfurizing
agent as in the embodiment to be mentioned hereinafter, it may be
desirable that the treated ash is controlled to be grains having a
grain size of from a few mm to a few cm. For this, if the lime
content of the coal ash is small, the treated ash could not well
cure into grains. If so, from 1 to 10 parts by weight of a
cement-type solidifier may be added to the treated ash to cure it
into grains having suitable mechanical strength.
[0049] In the method of treating coal ash with water of the
invention, the ash and water undergo a temperature difference
between them while they are mixed. The resulting mixture can be
used as a desulfurizing agent, and its capability for
desulfurization is high. Though not clear, the capability of the
mixture for desulfurization may be increased because of the
following reasons: Breaking the gypsum layer formed around the lime
particles of coal ash may be accelerated by the thermal shock on
the surfaces of the particles; the thermal expansion of the
particles may be accelerated by the accelerated lime hydration
speed; and the two effects may be combined.
[0050] The method of treating coal ash with water of the invention
is characterized in that the ash and water undergo a temperature
difference between them while they are mixed. The specific
treatment characterized by the temperature difference significantly
enhances the desulfurization capability of the resulting mixture of
ash and water, which, however, could not be realized in ordinary
treatment of coal ash with water with no temperature control in the
two and in ordinary treatment of coal ash with steam. This will be
because the surfaces of the lime particles in the ash are fully
broken and the particles are fully hydrated through the
temperature-controlled treatment in the method of the invention.
The treated mixture of the invention may be used not only for a
desulfurizing agent but also for a soil improver having the
advantage of stable capabilities.
[0051] Regarding its morphology after hydration, the treated
mixture of coal ash with water of the invention may be controlled
in any desired manner to vary from fine particulates having a size
of a few microns to large solid grains having a size of a few cm,
in consideration of the object and the mode of using it. Therefore,
the treated mixture of coal ash with water may be heated with
stirring to thereby remove water from it; or the wet mixture may be
solidified into blocks, or may be granulated into granules of a
suitable size; or, if desired, the solid blocks or granules may be
ground into small grains of a desired size. In addition, the
blocks, the granules and the small grains may be cured to increase
their mechanical strength. Having been thus cured, they may be
handled more conveniently. If further desired, they may be dried to
reduce their water content. In that manner, the treated mixture may
be grains having a mean grain size of from 0.1 to 20 mm, more
preferably from 0.2 to 5 mm.
[0052] The treated mixture obtained in the method of treating coal
ash of the invention can be used as a desulfurizing agent in a coal
combustion boiler system. In case where the treated mixture is
ground into fine powder, it may be sprayed into the exhaust gas in
a coal combustion boiler system for the purpose of desulfurization
therein. However, the mixture obtained in the treating method of
the invention is preferably used for desulfurization in the method
mentioned below.
[0053] Specifically, in a coal combustion boiler system, the coal
ash that has been separated in a bag filter and contains a
limestone-derived component is mixed with water on the condition
that the two undergo a temperature difference therebetween while
they are mixed, and the resulting mixture that serves as a
desulfurizing agent is fed into the coal combustor in the system.
Accordingly, the invention further provides the desulfurization
method with the mixture of coal ash with water. In the method, the
desulfurizing agent of the mixture is preferably circulated into
the coal combustor in the coal combustion boiler system. The
temperature difference between the coal ash and water while they
are mixed is preferably at least 30.degree. C., more preferably at
least 50.degree. C., even more preferably at least 60.degree. C.
The temperature difference is preferably larger, as the capability
of the desulfurizing agent of the mixture is higher. For the
temperature difference between the two, the temperature of any one
of the coal ash and water is kept higher. Preferably, the
temperature of the coal ash is kept higher, for example, falling
between 80 and 150.degree. C., more preferably between 100 and
130.degree. C., and the temperature of water is kept lower than it
at least by 30.degree. C., more preferably at least by 50.degree.
C., for example, falling between 2 and 50.degree. C. For example,
the high-temperature, coal ash having been taken out of the bag
filter in the coal combustion boiler system may be directly mixed
with room-temperature water. In this method, the energy loss can be
reduced, and the working efficiency can be increased, and, in
addition, the desulfurization capability of the resulting mixture
can be enhanced. On the other hand, it is also desirable to use hot
water heated at a high temperature, in view of the working
efficiency, the temperature control and the handlability in the
method. Preferably, the temperature of hot water is higher than
that of cold ash by at least 30.degree. C., more preferably it
falls between 60 and 98.degree. C., even more preferably between 70
and 98.degree. C.
[0054] The temperature condition as above is preferred in view of
the thermal shock owing to the temperature difference between the
coal ash and water, and of the hydration speed of lime in the ash.
In the coal combustion boiler system, the coal combustor may have a
fixed layer or a fixed bed. The method of the invention is
applicable to any type of coal combustors.
[0055] Preferably, the desulfurizing agent is dried to be slaked
lime having a mean grain size of from 0.1 to 20 mm or so. Its grain
size is so selected that the dried desulfurizing agent well fits
with the power and the function of the coal combustor into which it
is fed, in order that the desulfurizing agent can well stay in the
fluidized layer or bed of the combustor to ensure its residence
time therein. The desulfurizing agent may be dried in combustors,
as it is heated therein. However, in consideration of the easiness
in its storage, transportation, and supply to combustors, it is
desirable that the desulfurizing agent is previously dried.
[0056] In the desulfurization method of the invention, the
high-temperature (80 to 150.degree. C.) coal ash having been
collected essentially from a bag filter is mixed with
room-temperature (about 2 to 50.degree. C.) water by stirring them.
Alternatively, the coal ash having been collected essentially from
a bag filter is cooled, and the cold (room-temperature) ash is
mixed with hot water at about 60 to 98.degree. C. by stirring them.
Through the treatment, the shell of ash gypsum formed around lime
particles in the mixture is broken, whereby the inner quick lime is
rapidly hydrated to expand, while generating much heat. The mixture
is further stirred, and the lime particles therein are hydrated
further, and their hydration will finish in around 20 to 90
minutes. In this treatment, the pre-treatment of breaking the
gypsum shell around the lime particles in the mixture may be
effected almost simultaneously with the hydration of the lime
particles. The hydration is essentially for breaking the gypsum
(calcium sulfate) shell formed around the lime particles through
reaction of lime with sulfur dioxide in coal ash, to thereby expose
the inner active quick lime outside the thus-broken lime particles,
and not merely for ordinary hydration only. Accordingly, the
mixture of coal ash with hot water expands and generates much heat
through hydration, and after having been thus hydrated, it is
readily solidified. In general, the thus-solidified mixture is
cured so that its mechanical strength could be in some degree. The
solid may be ground in a mill into grains having a mean grain size
of at most 20 mm. The grains may have a mean grain size of from 0.3
to 10 mm or so.
[0057] The desulfurizing agent thus obtained in the manner as above
is fed into the combustor in a coal combustion boiler system as in
FIG. 1, via the coal feed port 7, generally along with coal
conveyed thereinto on a conveyor. The coal and the desulfurizing
agent thus fed into the combustor are heated at around 800.degree.
C. in the fluidized layer of the combustor. In the desulfurization
method of the invention, the desulfurizing agent of the regenerated
coal ash may be used alone, but it may be combined with fresh
limestone. The amount of the desulfurizing agent to be used may be
determined, based on the sulfur oxide content of exhaust gas and on
the active lime content of the desulfurizing agent. For example,
the molar ratio Ca/S may fall between 1.5 and 5 or so, preferably
between 2 and 4 or so. The desulfurizing agent is solid, and all of
it could not act for desulfurization. Therefore, some excess amount
of the desulfurizing agent must be used.
[0058] The mixture obtained by treating coal ash with water in the
method of the invention is favorably used as a desulfurizing agent
in coal combustion boilers, as so mentioned hereinabove. The coal
ash treated according to the method of the invention is well
hydrated, or that is, the lime component of the ash is well
converted into calcium hydroxide, and the quality of the hydrated
ash is stable. Accordingly, the treated mixture can be used as a
soil improvers for neutralizing, thickening and solidifying soil in
treatment of sludge, treatment of acidic soil, and treatment of
desert soil. In addition, the treated mixture can be used in other
various fields of, for example, construction materials and building
materials.
[0059] The invention is described concretely with reference to
Examples, Comparative Examples and Reference Examples, which,
however, are not intended to restrict the scope of the
invention.
EXAMPLE 1
Comparative Examples 1 and 2
Reference Examples 1 to 3
Experiment I
Treatment of Coal Ash
[0060] Combustion ash (having a calcium compound content of about
23% by weight) was collected in a bag filter in a fluidized
bed-type coal combustion boiler. This was mixed with water in the
manner mentioned below.
[0061] <1> 4.6 kg of hot water at 95.degree. C. was added to
5 kg of the coal ash at room temperature, and mixed and kneaded for
10 minutes. The resulting mixture was dried at room temperature
(30.degree. C.) and solidified. The solid was ground in a mill into
grains not larger than 2 mm in size (having a mean grain size of
0.5 mm). The treated mixture is used as a desulfurizing agent.
[0062] <2>4.6 kg of water at 25.degree. C. was added to 5 kg
of the coal ash at room temperature, and mixed and kneaded for 10
minutes. The resulting mixture was dried at room temperature
(30.degree. C.) and solidified. The solid was ground in a mill into
grains not larger than 2 mm in size (having a mean grain size of
0.5 mm). The treated mixture of Comparative Example 1 is used as a
desulfurizing agent.
[0063] <3> 10 kg of the coal ash at room temperature was kept
contacted with steam at 150.degree. C. for 6 hours. The
thus-treated ash was dried at room temperature (30.degree. C.), and
ground into grains having a mean grain size of 0.5 mm. This is a
desulfurizing agent of Comparative Example 2.
[0064] <4> This is non-treated, coal ash (having a particle
size of from 10 to 20 .mu.m). This serves as a desulfurizing
agent.
[0065] <5> This is a basic desulfurizing agent, limestone
grains of not larger than 1 mm in size (having a mean grain size of
0.5 mm).
[0066] <6> No desulfurizing agent is used.
Desulfurization Method; Evaluation of Desulfurization
Capability
[0067] In a fluidized bed-type coal combustion boiler system as in
FIG. 1, coal powder of at most 2 mm in size (having a mean grain
size of 0.5 mm) was fed into the small-sized coal combustor having
a height of 5 m, at a coal feeding rate of 4 kg/hr, and was burned
therein. Prior to being fed into the combustor, the coal was mixed
with any of the desulfurizing agents as above, and then fed into
the combustor. The sulfur oxide concentration in the exhaust gas
from the system was measured, from which was obtained the degree of
desulfurization in the system. The amount of the desulfurizing
agent added to coal is shown in Table 1, in terms of the ratio of
Ca/S that indicates the amount of Cao in the desulfurizing agent
relative to the sulfur content (S) of coal.
[0068] The test results are shown in Table 1.
[0069] The degree of desulfurization of each desulfurizing agent
tested is compared with that of limestone, and is shown in Table 1
relative to the control data of limestone. It is obvious that the
desulfurization capability of the regenerated desulfurizing agent
of Example 1 is better than that of the comparative desulfurizing
agents of Comparative Examples 1 and 2.
EXAMPLE 2
Reference Examples 4 and 5
Experiment II
Treatment of Coal Ash
[0070] Coal ash was collected in a bag filter, but this differs
from that tested in Experiment I. The ash was mixed with water in
the manner mentioned below.
[0071] <7> 5 kg of cold water at 28.degree. C. was added to 5
kg of the coal ash at 120.degree. C., and mixed and kneaded for 10
minutes. The resulting mixture was dried at room temperature
(30.degree. C.) and solidified. The solid was ground in a mill into
grains not larger than 2 mm in size (having a mean grain size of
0.5 mm). This serves as a desulfurizing agent.
[0072] <8> This is a basic desulfurizing agent, limestone
grains of not larger than 1 mm in size (having a mean grain size of
0.5 mm). This is the same as in the above <5>.
[0073] <9> No desulfurizing agent is used.
Desulfurization Method; Evaluation of Desulfurization
Capability
[0074] The desulfurizing agents were tested in the same manner as
in Experiment I for their desulfurization capability. However, the
coal used in this experiment differs from that used in Experiment
I.
[0075] The test results are given in Table 2.
[0076] It is understood that the desulfurization capability of the
regenerated desulfurizing agent of Example 2 is high, and the
degree of desulfurization with the regenerated desulfurizing agent
of Example 2 is on the same level as that of desulfurization with
limestone.
1 TABLE 1 Amount of Treated Mixture Desulfurizing Sulfur Oxide
Degree of Desulfurization (desulfurizing Agent Added, Content of
Desulfurization Capability (based agent) Ca/S (by mol) Exhaust Gas
ppm % on limestone) % Example 1 hydration with 1.7 246 40 53 hot
water Comp. Ex. 1 hydration with 1.7 320 22 29 room-temperature
water Comp. Ex. 2 hydration with 1.7 275 33 44 steam Ref. Ex. 1
non-treated, 1.8 390 5 7 combustion ash Ref. Ex. 2 limestone 2.0
103 75 100 Ref. Ex. 3 none 0 410 -- --
[0077]
2 TABLE 2 Amount of Treated Mixture Desulfurizing Sulfur Oxide
Degree of Desulfurization (desulfurizing Agent Added, Content of
Desulfurization Capability (based agent) Ca/S (by mol) Exhaust Gas
ppm % on limestone) % Example 2 hydration of 3 9 78 100
high-temperature ash with room- temperature water Ref. Ex. 4
limestone 3 9 78 100 Ref. Ex. 5 none 0 41 -- --
Industrial Applicability
[0078] It is obvious that the desulfurization capability of the
desulfurizing agent obtained in the invention by treating coal ash
with water, especially with hot water is higher than that of the
desulfurizing agent obtained in conventional hydration of coal ash
with water or with steam. The reason why the coal ash treated with
hot water in the invention is better than that treated with steam
that is hotter than hot water will be because of the following
reasons. The calcium sulfate shell that covers each non-reacted
calcium oxide particle can be readily broken owing to the thermal
shock imparted thereto in the same manner in the two cases, but the
subsequent hydration of the ash differs between the two cases. In
the case of treating the ash with steam, the treatment time will be
long and the treatment pressure will be high. Therefore, from the
viewpoint of the equipment costs, the method of the invention is
better than the conventional, steam treatment method.
[0079] The effect and the mechanism of the desulfurizing agent
obtained through treatment of high-temperature coal ash with cold
water are the same as above. In this case, the temperature
difference between the coal ash and water can be made larger, and
the desulfurization capability of the treated mixture that serves
as a desulfurizing agent can be higher.
[0080] Therefore, the invention has made it possible to recycle
(circulate) the combustion ash in coal combustion boiler systems,
and contributes toward reducing the amount of coal ash wastes to be
discarded and toward reducing the amount of limestone to be used.
According to the desulfurization method and the method of producing
a desulfurizing agent of the invention, the substance that covers
the non-burned carbon grains in the combustion ash is broken, and,
as a result, the non-burned carbon is exposed outside. Accordingly,
the combustibility of the non-burned carbon in the coal ash is
promoted, and the combustion efficiency in combustors is increased.
In addition, even when the treated mixture is discarded, its
quality is stable. Therefore, the treated mixture of the invention
is useful for various soil improvers.
* * * * *