Note: Descriptions are shown in the official language in which they were submitted.
CA 02280586 1999-08-19
MCCARTHY TETRAULT FILE: 139201-257627
APPLICANT: MIURA CO. , LTD.
INVENTORS: JUNICHI NAKAJIMA
MASAZUMI YAMASHITA
KENICHI KIMURA
TITLE: METHOD FOR REMOVING
SCALE OF BOILER
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Method for Removing Scale of Boiler
BACKGROUND OF THE INVENTION
The present invention relates to a method for
removing scale of a boiler and, more particularly, to a method
for removing scale while the boiler is running.
During the operation of a boiler, it can occur that
hard, water-insoluble crystalloid solids, which are so called
scale, gradually deposit to the inside of water tubes or heat
transfer walls. For example, hardness such as calcium
carbonate, CaC03, and calcium sulfate, CaS09, will not increase
in solubility to water and, in some cases, becomes insoluble,
as the water temperature is increased, so that those components
precipitate by the heating and concentration of water at heat
transfer walls, thus depositing on the heat transfer walls as
scale. It can also occur that a relatively water-soluble salt
like calcium bicarbonate, Ca(HC03)2, is converted by heating
into a water-insoluble salt, that is, calcium carbonate, CaC03,
thus resulting in scale.
Causative substances for the scale are exemplified
by silica, SiOz, in addition to hardness such as calcium salts
and magnesium salts. Silica, Si02, which is contained in most
of surface water, will combine with hardness and other metal
ions such as zinc, aluminum and iron, resulting in scale. For
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example, silica, Si02, and a hardness, when combined together,
produce a substance having a low solubility in high temperature
water, which precipitate by heating and concentration at heat
transfer walls, thus resulting in scale. Further, iron is
brought into the system in the form of hydroxides or oxides,
producing floating particles or colloid, and these particles
or colloid deposits on the heat transfer walls and is baked and
solidified by repetitive drying and wetting or other processes,
thus resulting in scale.
Scale, once having deposited on the heat transfer
walls, causes various problems. First, because scale is as low
in thermal conductivity as one several hundredth that of the
material of water tubes (mild steel), the scale deposited on
the heat transfer walls reduces heat transfer from combustion
flame to the water within the water tubes, i.e. boiler water,
so that the thermal efficiency lowers to a large extent, making
a cause of an increase in fuel consumption. Besides, once heat
transfer from flame to boiler water reduces, the temperature
of the heat transfer walls elevates and the metal strength of
the heat transfer wall lowers. Then, because the inside of the
water tubes are normally of high pressure, the metal strength
of the heat transfer walls cannot withstand the internal
pressure of the water tubes, which causes tube swelling,
cleavage, ruptures or other accidents.
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This being the case, in order to prevent the
deposition of scale, there have been adopted such methods as
softening feedwater by a water softener or other apparatus,
maintaining the concentration of scale-forming components
within their solubilities by blowdown control, or making
hardness into sludge with precipitating agents. These methods
are explained in more detail. First, the method of softening
the boiler feedwater by a water softener or other apparatus is
a method in which hardness in the boiler feedwater that forms
scale, such as calcium ions and magnesium ions, are exchanged
for sodium ions and thereby removed by cation exchange resins
in the water softener, by which the deposition of scale is
prevented.
Next, the method of maintaining the concentrations
of scale-forming components within their solubilities by
blowdown control is a method in which the control of blowing
down (discharging) the boiler water out of the system
periodically is exercised more appropriately so that the
concentrations of scale-forming components in the boiler water
are maintained within their solubilities, by which the
deposition of scale is prevented. However, excessive blowdown
would cause some problems that the boiler body becomes more
corrosion-prone, and that heat loss is excessive.
Next, the method of making hardness components into
sludge with precipitating agents is a method in which, before
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the formation of scale from hardness, a chemical is used to
change the hardness into sludge lower in solubility to water
than scale at places far from the inner surfaces of the water
tubes where scale is likely to deposit, and then the produced
sludge is discharged and removed out of the system at each
blowdown, thus suppressing the occurrence of scale. As the
chemical .for this method, phosphate based chemicals are
commonly used, but phosphorus has been put into growingly
stricter effluent regulation in recent years, thus the method
having become harder to apply.
As shown above, various methods for preventing the
deposition of scale have been adopted. However, it is difficult
to fully prevent the deposition of scale even by these methods,
and there is a need for the work of removing the deposited scale.
Scale is too hard to mechanically remove, and therefore is
normally removed by acid cleaning. For one type of scale that
is composed singly of hardness, it is practiced to take a method
of dissolving and removing deposited scale by adding
hydrochloric acid and heating. For another type of scale that
is produced in combination of hardness and silica, Si02, because
the scale cannot be removed only by hydrochloric acid, the scale
is dissolved and removed by using hydrochloric acid and a
fluoride in combination. Also, for iron based scale produced
from iron compounds, it is practiced to apply a method of
dissolving and removing the scale by using citric acid.
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However, when the acid cleaning is performed, the
boiler must be halted during the work and moreover some problems
occur such as the corrosion of iron, which is the boiler
material, by the action of acid. Also, when hydrochloric acid
or, in particular, a fluoride based chemical is used, there is
a need of paying severe care to the handling of these chemicals
because of their great hazardousness.
Whereas chemicals composed mainly of phosphonic acid
have been used to remove the scale under the operation of the
boiler, these chemicals are acidic and, if excessively added
into the boiler water, would cause corrosion of the boiler body,
as a problem. Accordingly, there has been a desire for a method
of effectively removing scale with a chemical of neutral or
alkaline, which is the water quality of normal boiler water,
without causing the aforementioned corrosion problem.
In view of these and other problems, an object of the
present invention is therefore to provide a method for removing
scale under the operation of the boiler by adding a chemical
into boiler water.
In order to achieve this object, according to the
present invention, there is provided a method for removing scale
of a boiler characterized in that a total concentration of
chelating agent and dispersant in boiler water is regulated to
a specified concentration under a condition that the boiler water
is neutral or alkaline.
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In an embodiment of the present invention, the
chelating agent is any one compound or a combination of two ( 2 )
or more compounds selected from a group consisting of:
a) ethylenediaminetetraacetic acid (EDTA) and/or its
salts;
b) polyalkylene polyamine and/or its salts;
c) bipyridine and/or its salts;
d) glycine and/or its salts;
e) acetylacetone;
f) nitrilotriacetic acid (NTA) and/or its salts;
g) aminotri(methylphosphonic acid) and/or its salts; and
h) 1-hydroxyethylidene-1,1-diphosphonic acid and/or its
salts.
In an embodiment of the invention, the dispersant is
any one compound or a combination of two ( 2 ) or more compounds
selected from a group consisting of:
a) polymaleic acid and/or its salts;
b) 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTC)
and/or its salts;
c) bis(poly-2-carboxyethyl) phosphinic acid and/or its
salts;
d) phosphinocarboxylic acid copolymer and/or its salts;
e) acrylamide acrylate copolymer and/or its salts; and
f) polyacrylic acid and/or its salts.
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In an embodiment of the invention, the total
concentration of the chelating agent and the dispersant in
boiler water is within a range of 500 - 5000 ppm.
Further, in an embodiment of the invention, a weight
ratio of the chelating agent to the dispersant is within a range
of 3 . 97 - 97 . 3.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a graph showing scale removal effects in
a first embodiment of the present invention; and
Fig. 2 is a graph showing scale removal effects in
a second embodiment of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Embodiments of the present invention are described
below. In the invention, while the boiler is running, a scale
remover containing a chelating agent and a dispersant is added
into the boiler water at a time. More specifically, with a
chemical feeder provided in the boiler feedwater line, the scale
remover is stored in a chemical tank, and the pump of this
chemical feeder is operated to inject the scale remover to the
boiler feedwater. At the time of this addition, other chemicals
such as oxygen scavengers may be added concurrently.
The present invention has been achieved by obtaining
the following findings as a result of hard studies. The
findings are as follows. First, in the method of removing scale
under the operation of the boiler, too high a rate of scale
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removal would cause the removed scale of large piece accumulate
at narrowed portions of water tubes and the like, which may cause
blockage of the water tubes. Conversely, too low a rate of scale
removal may cause deterioration of the thermal efficiency as
well as tube swelling, cleavage, ruptures or the like of the
water tubes. Therefore, the rate of scale removal should be
such an appropriate level as can avoid the above-described
problems . Also, the chelating agent indeed has a function of
dissolving scale, but is liable to cause re-deposition of the
dissolved scale, which in turn may cause blockage of the water
tubes. Further, the dispersant also has a function of
dispersing and removing scale but is low in scale removal rate.
From these findings, the inventors have invented a method for
removing scale by adding a combination of a particular chelating
agent and a particular dispersant to the boiler water.
For the present invention, the chelating agent refers
to a compound which has two or more bonding sites capable of
forming coordinate bonds with metal ions and which is
water-soluble. This chelating agent is exemplified by
ethylenediaminetetraacetic acid (EDTA), such polyalkylene
polyamines as ethylenediamine, propylenediamine,
diethylenetriamine, and triethylenetetramine, bipyridine,
glycine, acetylacetone, nitrilotriacetic acid (NTA),
aminotri(methylphosphonic acid), 1-hydroxyethylidene-1,1-
diphosphonic acid, and the like as well as salts of these
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compounds (except acetylacetone). Among others,
ethylenediaminetetraacetic acid (EDTA) and nitrilotriacetic
acid (NTA) as well as their salts are preferable.
Then, the dispersant is exemplified by polymaleic
acid, 2-phosphonobutane-1,2,4-tricarboxylic acid (PBTC),
bis(poly-2-carboxyethyl) phosphinic acid,
phosphinocarboxylic acid copolymers, acrylamide acrylate
copolymers, sodium polyacrylate, and the like as well as their
salts.
Among these, for the removal of scale that is composed
of hardness with carbonate or hydroxide and scale that is
composed of hardness and silica, Si02, in combination
(hereinafter, referred to as hardness/silica based scale),
polymaleic acid, 2-phosphonobutane-1,2,4-tricarboxylic acid
(PBTC), bis(poly-2-carboxyethyl) phosphinic acid, 1-
hydroxyethylidene-1,1-diphosphonic acid, acrylamide acrylate
copolymers, sodium polyacrylate, and the like as well as their
salts are preferable, and polymaleic acid or its salts are
particularly preferable. Also, for the removal of iron based
scale that is composed of iron compounds, bis(poly-2-
carboxyethyl) phosphinic acid and phosphinocarboxylic acid
copolymers as well as their salts are preferable, and
bis(poly-2-carboxyethyl) phosphinic acid or its salts are
particularly preferable.
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The ratio of chelating agent to dispersant in the
scale remover is 3 : 97 - 97 : 3, preferably 15 : 85 - 85 : 15,
and more preferably 25 . 75 - 75 . 25.
The scale remover is generally dissolved in water,
manufactured as an aqueous solution and preserved for use.
Concentrations of the chelating agent and the dispersant in the
aqueous solution are generally about 1 - 40 wt o, respectively,
but not particularly limited. The scale remover, which can be
obtained by mixing the chelating agent and the dispersant, is
generally obtained by adding and dissolving specified
quantities of the chelating agent and dispersant into water,
another aqueous solvent or the like. The chelating agent and
the dispersant may be dissolved into water after mixed together.
It is also possible to add, to the scale remover, a
corrosion inhibitor such as dimethyl-p-substituted
benzylsulfoniumchloride within such a scope as will not impair
the object of the present invention.
It is still also possible to add, to the scale
remover, other components within such a scope as will not impair
the object of the present invention. These other components
are exemplified by pH regulators such as sodium hydroxide, NaOH,
potassium hydroxide, KOH, sodium carbonate, Na2C03, and
potassium carbonate, KZC03. Because some of chelating agents
and dispersants mentioned above show acidity, these pH
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regulators are added to neutralize the acidity and thereby make
the scale remover neutral.
The scale remover is added into the boiler water so
that its concentration in the boiler water is maintained at 500
- 5000 ppm, preferably 2000 - 3000 ppm. An example of the
addition process is that, with a blowdown rate of 10 0, the scale
remover is added continuously so that the quantity of the scale
remover becomes 200 - 300 ppm relative to the total quantity
of the boiler feedwater. Another example is that the scale
remover is added collectively so that the quantity of the scale
remover becomes 2000 - 3000 ppm relative to the total quantity
of the boiler water, where the boiler water is fully blown down
periodically.
Examples:
Concrete examples of the present invention are
described in detail below. However, the present invention is
not limited to these examples. In the description of the
examples, the term "parts" represents parts by weight.
First, four types of scale removers as described
below were prepared. As a first chemical, 15 parts of disodium
ethylenediaminetetraacetate (EDTA), 7.5 parts of polymaleic
acid, 6 parts of sodium hydroxide and 71.5 parts of water were
mixed and dissolved together, by which a scale remover
(hereinafter, referred to as chemical A) was prepared. Also,
15 parts of disodium ethylenediaminetetraacetate (EDTA), 6
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parts of bis(poly-2-carboxyethyl) phosphinic acid, 4.1 parts
of sodium hydroxide and 74.9 parts of water were mixed and
dissolved together, by which a scale remover (hereinafter,
referred to as chemical B) was prepared. Also, 15 parts of
disodium ethylenediaminetetraacetate (EDTA), 9 parts of 1-
hydroxyethylidene-1,1-diphosphonate, 3 parts of potassium
hydroxide and 73 parts of water were mixed and dissolved
together, by which a scale remover (hereinafter, referred to
as chemical C) was prepared. Further, 15 parts of disodium
ethylenediaminetetraacetate (EDTA), 9 parts of acrylamide
acrylate copolymer, 1 part of polyacrylate and 75 parts of water
were mixed and dissolved together, by which a scale remover
(hereinafter, referred to as chemical D) was prepared.
Next, a first example of the present invention is
described. A boiler (experimental boiler) having a holding
water quantity of about 0.15 L', to which scale composed of
hardness and carbonate and scale composed of hardness and
silica, SiOz, in combination had been deposited, was fed with
soft water with a hardness of 0.4 ppm and operated under
atmospheric pressure. During this operation, the chemical A
was added and the boiler was fully blown down every two hours
while the concentration in the boiler water was maintained at
2000 - 3000 ppm. The pH value of the boiler water in this case
was about 11.5. Results of measuring the removal Pffect after
an eight-hour operation are shown in Fig. 1. Also, the same
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operation was performed with the chemical B or the chemical C
instead of the chemical A, and besides, for comparison, the same
operation was performed without adding any chemical. Results
of those cases are shown also in Fig. 1 together. As apparent
from Fig. l, excellent scale removal effects can be obtained by
the chemicals according to the present invention. In
particular, the chemical A, which uses polymaleic acid as the
dispersant, is superior in performance. In this connection,
the case in which no chemicals were added resulted in zero
removal amounts.
Next, a second example of the present invention is
described. A boiler (experimental boiler) having a holding
water quantity of about 0.15 ~, to which iron based scale
composed of iron compounds had been deposited, was fed with soft
water with a hardness of 0.4 ppm and operated under atmospheric
pressure. During this operation, the chemical B was added and
the boiler was fully blown down every two hours while the
concentration in the boiler water is maintained at 2000 - 3000
ppm. The pH value of the boiler water in this case was about
11.5. Results of measuring the removal effect after an
eight-hour operation are shown in Fig. 2. Also, the same
operation was performed with the chemical A or the chemical D
instead of the chemical B, and besides, for comparison, the same
operation was performed without adding any chemical. Results
of those cases are shown also in Fig. 2 together. As apparent
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from Fig. 2, an excellent scale removal effect can be obtained
by the chemical B, which is a chemical according to the present
invention and which uses bis(poly-2-carboxyethyl) phosphinic
acid as the dispersant . In this example also, the case in which
no chemicals were added resulted in zero removal amounts.
As described above, by maintaining certain
concentration of the scale remover composed of chelating agent
and dispersant during the operation of the boiler, the scale
remover of the present invention can attain excellent
performance in scale removal.
Chelating agents we mentioned here have some bonding
sites where they coordinate to metal ions. These bonding ~i tP~
are oxygen atoms in carboxylate, nitrogen atoms, oxygen atoms
in phosphate, and the like. Bonding energies of coordinations
are basically electric, common to all the coordinations.
Therefore, the same function is achieved in both cases where a
single chelating agent is used and where a combination of two
(2) or more chelating agents are used.
Also, it is generally known that when we use dispersing
agent to prevent scale, the dispersant acts to inhibit the scale
nucleus from combining with another. The reason why the
combination of chelating agent and dispersant achieves an
excellent function of scale removal is thought that the scale
removed by the chelating agent is maintained as fine particles
in the boiler water by the dispersant. Therefore, the same
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function is achieved in both cases where a single dispersant is
used and where a combination of two (2) or more dispersants are
used.
As shown above, according to the present invention,
there can be provided a method which allows scale of a boiler
to be removed under normal operation and without halting the
boiler, by adding a chemical into the boiler water. Also, the
scale removal method according to the present invention is free
from hazards as would be involved in acid cleaning, hence high
safety, and moreover is carried out under a condition that the
boiler water is neutral or alkaline, thus never causing the
boiler to corrode. Further, the scale removal method according
to the present invention has an appropriate scale removal rate,
thus well suppressing a problem that the removed scale deposits
on narrowed portions of the water tubes so as to make the water
tubes blocked, as well as such problems of reduction in thermal
efficiency and swelling, cleavage, ruptures or the like of the
water tubes.
