U.S. patent application number 09/263709 was filed with the patent office on 2001-08-09 for methods for the regenertion of a denitration catalyst.
Invention is credited to IIDA, KOZO, NOJIMA, SHIGERU.
Application Number | 20010012817 09/263709 |
Document ID | / |
Family ID | 26517435 |
Filed Date | 2001-08-09 |
United States Patent
Application |
20010012817 |
Kind Code |
A1 |
NOJIMA, SHIGERU ; et
al. |
August 9, 2001 |
METHODS FOR THE REGENERTION OF A DENITRATION CATALYST
Abstract
This invention provides a method for the regeneration of a
denitration catalyst which comprises cleaning a denitration
catalyst having reduced denitration power with an aqueous alkaline
solution to remove the substances deposited thereon, and subjecting
the catalyst to an activation treatment with an aqueous acid
solution; a method for the regeneration of a denitration catalyst
which comprises cleaning a denitration catalyst having reduced
denitration power with a cleaning fluid comprising an aqueous
solution containing sulfuric acid or ammonia at a concentration of
0.05 to 20% by weight and maintained at a temperature of 10 to
90.degree.C.; and a method for the regeneration of a denitration
catalyst which comprises cleaning a denitration catalyst having
reduced denitration power under the above-described conditions, and
impregnating the denitration catalyst with a catalytically active
component so as to support it on the catalyst.
Inventors: |
NOJIMA, SHIGERU;
(HIROSHIMA-SHI, JP) ; IIDA, KOZO; (HIROSHIMA-SHI,
JP) |
Correspondence
Address: |
MYERS BIGEL SIBLEY & SAJOVEC
PO BOX 37428
RALEIGH
NC
27627
US
|
Family ID: |
26517435 |
Appl. No.: |
09/263709 |
Filed: |
March 5, 1999 |
Current U.S.
Class: |
502/26 ;
502/27 |
Current CPC
Class: |
B01J 38/66 20130101;
Y02C 20/30 20130101; B01D 53/96 20130101; B01D 53/8625 20130101;
B01J 38/62 20130101; B01J 23/92 20130101; B01J 38/60 20130101; B01J
38/68 20130101; B01J 38/64 20130101 |
Class at
Publication: |
502/26 ;
502/27 |
International
Class: |
B01J 023/90; B01J
038/66 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 24, 1998 |
JP |
209417/1998 |
Jul 24, 1998 |
JP |
209418/1998 |
Claims
1. A method for the regeneration of a denitration catalyst which
comprises cleaning a denitration catalyst having reduced
denitration power with a cleaning fluid comprising an aqueous
solution containing sulfuric acid or ammonia at a concentration of
0.05 to 20% by weight and maintained at a temperature of 10 to
90.degree. C.
2. A method for the regeneration of a denitration catalyst as
claimed in claim 1 wherein the temperature of said cleaning fluid
is in the range of 20 to 80.degree. C.
3. A method for the regeneration of a denitration catalyst which
comprises cleaning a denitration catalyst having reduced
denitration power with an aqueous alkaline solution to remove the
substances deposited thereon, and subjecting the catalyst to an
activation treatment with an aqueous acid solution.
4. A method for the regeneration of a denitration catalyst as
claimed in claim 3 wherein said aqueous alkaline solution is an
aqueous solution of NaOH, KOH, Na.sub.2CO.sub.3, NaHCO.sub.3 or
K.sub.2CO.sub.3 and said aqueous acid solution is an aqueous
solution of HCl, HNO.sub.3, HF or H.sub.2SO.sub.4.
5. A method for the regeneration of a denitration catalyst which
comprises cleaning a denitration catalyst having reduced
denitration power under the conditions described in any of claims 1
to 4, and impregnating the denitration catalyst with a
catalytically active component so as to support it on the catalyst.
Description
FIELD OF THE INVENTION AND RELATED ART STATEMENT
[0001] This invention relates to methods for the regeneration of a
denitration catalyst. More particularly, it relates to methods for
the regeneration of a denitration catalyst which makes it possible
to regenerate a denitration catalyst having reduced denitration
powder and considered to be hard to regenerate, and thereby utilize
it again effectively.
[0002] Recently, in order to remove nitrogen oxides (hereinafter
referred to as NO.sub.x) produced in boilers and various combustion
furnaces for the purpose of preventing air pollution, a catalytic
ammonia reduction process wherein ammonia is used as a reducing
agent and nitrogen oxides are decomposed to nitrogen and water by
contact with a catalyst is being widely employed. Most of the
NO.sub.x removal catalysts currently used for practical purposes
are honeycomb-shaped catalysts which have through-holes of square
cross section in order to prevent clogging with dust present in
exhaust gas and increase the gas contact area.
[0003] With respect to catalyst components, titanium oxide is
highly suitable for use as a principal component, and vanadium,
tungsten and the like are commonly used as active components. Thus,
TiO.sub.2-WO.sub.3 or TiO.sub.2-MoO.sub.3 binary catalysts and
TiO.sub.2-V.sub.2O.sub.5-WO.sub.- 3 or
TiO.sub.2-V.sub.2O.sub.5-MoO.sub.3 ternary catalysts are being
popularly used. The catalytic power of these denitration catalysts
tends to be gradually reduced with service time, and the cause for
the reduction in catalytic power varies according to the type of
the fuel used in the source of exhaust gas (e.g., boiler).
[0004] For example, in the case of exhaust gas from an oil-fired
boiler, sodium contained in the dust present in exhaust gas is
chiefly deposited on the catalyst and causes a reduction in
catalytic power. In the case of exhaust gas from a coal-fired
boiler, calcium contained in the dust present in exhaust gas is
chiefly deposited on the catalyst surfaces and reacts with sulfuric
anhydride present in the exhaust gas to form calcium sulfate. This
calcium sulfate covers the catalyst surfaces and hinders NO and
NH.sub.3 gases from diffusing into the interior of the catalyst,
resulting in reduced catalytic power.
[0005] It has conventionally been known that catalysts having
reduced catalytic power attributable to these causes of
deterioration can be effectively regenerated by cleaning them with
water or an aqueous solution of hydrochloric acid.
[0006] In the course of experiments on the regeneration of
catalysts having been used for exhaust gas from coal-fired boilers,
the present inventors have recognized that the conventional
cleaning method using water or an aqueous solution of hydrochloric
acid exhibits is scarcely effective in regenerating the catalytic
power of some catalysts. Upon examination of the cause therefor, it
has been found that a high concentration of arsenic
compound(As.sub.2O.sub.5) is present on the surfaces of the
catalysts for which cleaning with water or an aqueous solution of
hydrochloric acid fails to exhibit a regenerative effect.
[0007] Generally, when a denitration catalyst is applied to exhaust
gas produced by the combustion of a gaseous fuel, little reduction
in catalytic power is observed.
[0008] However, for catalysts used in exhaust gas from coal-fired
boilers in which coal of poor quality tends to be increasingly used
in recent years, a marked reduction in catalytic power is observed
in some cases. Examination of these deteriorated catalysts has
revealed that a high concentration of arsenic is present on the
catalyst surfaces as described above, and the conventional cleaning
method using water or an aqueous solution of hydrochloric acid
exhibits little regenerative effect on them. Moreover, in order to
clarify the cause for the deposition of arsenic on the surfaces of
a catalyst used for a coal-fired boiler, an investigation was made
on the fuel used in the source of exhaust gas. As a result, it has
been found that a high concentration of arsenic compounds are
present in such coal. These arsenic compounds are converted into
diarsenic trioxide (As.sub.2O.sub.3), which is carried by
combustion gas and becomes adsorbed on the catalyst. Then, this
diarsenic trioxide is oxidized on the catalyst according to the
following reaction formula (1) and fixed to the catalyst in the
form of stable diarsenic pentoxide (As.sub.2O.sub.5).
As.sub.2O.sub.3+O.sub.2.fwdarw.AS.sub.2O.sub.5 (1)
[0009] For this reason, there has been a problem in that, when the
substances responsible for the deterioration of the catalyst are
arsenic compounds deposited on the catalyst surfaces, the
conventional cleaning method using water or an aqueous solution of
hydrochloric acid exhibits little regenerative effect on the
catalyst.
OBJECTS AND SUMMARY OF THE INVENTION
[0010] In view of the above-described problem, the present
inventors made intensive investigations in order to develop a
method for the regeneration of a denitration catalyst which not
only can regenerate a denitration catalyst having reduced catalytic
power as a result of its long-time use, while avoiding the
conventionally known reduction in catalytic power due to the
deposition of sodium or calcium, but also can regenerate a
denitration catalyst that could not be effectively regenerated by
cleaning with water or an aqueous solution of hydrochloric acid
because of the presence of arsenic on the catalyst surfaces.
[0011] As a result, the present inventors have now found that the
above-described problem can be solved by treating a spent
denitration catalyst according to a method which comprises an
alkali treatment step for removing the arsenic compounds deposited
on the catalyst surfaces, and a subsequent activation treatment
step.
[0012] Moreover, the present inventors also have found that the
above-described problem can be solved by cleaning a spent
denitration catalyst with an aqueous solution of sulfuric acid or
ammonia to convert the arsenic compounds deposited on the catalyst
surfaces into water-soluble compounds and thereby remove them from
the catalyst surfaces.
[0013] The present invention has been completed from this point of
view.
[0014] According to a first embodiment of the present invention,
there is provided a method for the regeneration of a denitration
catalyst which comprises cleaning a denitration catalyst having
reduced denitration power with an aqueous alkaline solution to
remove the substances deposited thereon, and subjecting the
catalyst to an activation treatment with an aqueous acid solution.
In a preferred embodiment, the aforesaid aqueous alkaline solution
is an aqueous solution of NaOH, KOH, Na.sub.2CO.sub.3, NaHCO.sub.3
or K.sub.2CO.sub.3 and the aforesaid aqueous acid solution is an
aqueous solution of HCl, HNO.sub.3, HF or H.sub.2SO.sub.4.
[0015] According to a second embodiment of the present invention,
there is provided a method for the regeneration of a denitration
catalyst which comprises cleaning a denitration catalyst having
reduced denitration power with a cleaning fluid comprising an
aqueous solution containing sulfuric acid or ammonia at a
concentration of 0.05 to 20% by weight and maintained at a
temperature of 10 to 90.degree. C. In this method, the hardly
soluble arsenic compounds deposited on the catalyst surfaces can be
more effectively removed by maintaining the temperature of the
cleaning fluid in the range of 20 to 80.degree. C.
[0016] According to a third embodiment of the present invention,
there is provided a method for the regeneration of a denitration
catalyst which comprises cleaning a denitration catalyst having
reduced denitration power under any of the conditions described
above, and impregnating the denitration catalyst with a
catalytically active component so as to support it on the catalyst.
In this method, the catalytically active component with which the
catalyst is impregnated comprises, for example, vanadium or
tungsten that is liable to be dissolved out.
[0017] Conventionally, catalysts having arsenic compounds deposited
thereon have been incapable of regeneration and hence disposed of.
However, the regeneration methods of the present invention make it
possible to regenerate such catalysts and utilize them effectively
again as denitration catalysts.
BRIEF DESCRIPTION OF THE DRAWING
[0018] FIG. 1 is a perspective view of a honeycomb-shaped
denitration catalyst used in the examples of the present invention
which will be given later.
[0019] The reference characters shown in FIG. 1 are defined as
follows: 1, honeycomb-shaped denitration catalyst; L, length; and
P, pitch.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0020] Embodiment 1
[0021] The present invention relates to the regeneration of a
denitration catalyst which has been used for the removal of
nitrogen oxides present in combustion exhaust gas and has reduced
catalytic power due to the deposition of arsenic (As) compounds on
the catalyst surfaces. According to the first embodiment thereof,
the catalyst is regenerated by dissolving the arsenic compounds
(principally As.sub.2O.sub.5) deposited on the catalyst surfaces.
The denitration catalysts which can be regenerated according to the
present invention are ones comprising titanium oxide as a principal
component and containing vanadium, tungsten, molybdenum or the like
as an active component. Specific examples thereof include
TiO.sub.2-WO.sub.3 or TiO.sub.2-MoO.sub.3 binary catalysts, and
TiO.sub.2-V.sub.2O.sub.5-WO.sub.3 or
TiO.sub.2-V.sub.2O.sub.5-MoO.sub.3 ternary catalysts.
[0022] More specifically, the regeneration method of this
embodiment comprises an alkali treatment step and a subsequent
activation treatment step. If necessary, this regeneration method
may further include a step for impregnating the denitration
catalyst with a catalytically active component so as to support it
on the catalyst.
[0023] First of all, in the alkali treatment step, a denitration
catalyst having reduced catalytic power due to the deposition of
arsenic compounds is cleaned with an aqueous alkaline solution to
remove the arsenic compounds from the denitration catalyst. No
particular limitation is placed on the cleaning method, for
example, the cleaning method is carried out bringing the
denitration catalyst into contact with a cleaning fluid comprising
an aqueous solution of sulfuric acid or ammonia. Specific examples
thereof include a method in which the denitration catalyst is
soaked in an aqueous alkaline solution, a method in which the
denitration catalyst is allowed to stand in an aqueous solution of
sulfuric acid or ammonia, and a method in which, after the
denitration catalyst is placed in an aqueous alkaline solution, air
is bubbled through the solution or forced convection currents are
produced in the solution to promote the renewal thereof.
[0024] In this alkali treatment step, an aqueous solution of a
strongly basic compound is used as the aqueous alkaline solution.
For this purpose, it is preferable to use a basic compound which
can remove arsenic by forming a sodium or potassium compound
thereof. More specifically, the aqueous alkaline solution used in
the present invention may comprise, for example, an aqueous
solution of NaOH, KOH, Na.sub.2CO.sub.3, NaHCO.sub.3 or
K.sub.2CO.sub.3.
[0025] When the aqueous alkaline solution comprises an aqueous
solution of NaOH, KOH, Na.sub.2CO.sub.3, NaHCO.sub.3 or
K.sub.2CO.sub.3 as described above, it is usually effective that
the alkali concentration in the aqueous alkaline solution is in the
range of 0.05 to 20% by weight and the temperature of the aqueous
alkaline solution serving as a cleaning fluid is in the range of 10
to 90.degree. C. If the concentration of the aqueous alkaline
solution is less than 0.05% by weight or the temperature of the
cleaning fluid is lower than 10.degree. C., a sufficient cleaning
effect will not be obtained. On the other hand, if the
concentration of the aqueous alkaline solution is greater than 20%
by weight or the temperature of the cleaning fluid is higher than
90.degree. C., the cost of the treating equipment may be
considerably raised.
[0026] In the subsequent activation treatment step, the denitration
catalyst having undergone the alkali treatment is subjected to an
activation treatment with an aqueous acid solution.
[0027] Specifically, although the arsenic compounds can be removed
by cleaning the denitration catalyst in the above-described alkali
treatment step, the alkaline component remains on the catalyst and
acts as a poison thereto. Since the alkali metal itself is a
substance responsible for the deterioration of the denitration
catalyst, this denitration catalyst, unless properly treated, may
be deteriorated by the alkali metal, in spite of the fact that a
reduction in catalytic power due to the deposition of arsenic
compounds can be avoided.
[0028] Accordingly, in the present invention, the catalyst having
undergone the alkali cleaning is subjected to an activation
treatment with an aqueous acid solution so as to remove the alkali
remaining on the catalyst. Thus, the denitration catalyst is freed
of any catalyst poison.
[0029] In this activation treatment step, it is conceivable that an
aqueous solution of an organic acid or an inorganic acid may be
used as the aqueous acid solution. However, with consideration for
the cost required for after-treatment and the like, it is
preferable to use an aqueous acid solution prepared from an
inorganic acid. Any of various inorganic acids capable of ion
exchange with sodium or potassium may be used, whether they are
strong acids or weak acids. More specifically, the aqueous acid
solution used in the present invention may comprise, for example,
an aqueous solution of HCl, HNO.sub.3, HF or H.sub.2SO.sub.4.
[0030] When the aqueous acid solution comprises an aqueous solution
of HCl, HNO.sub.3, HF or H.sub.2SO.sub.4 as described above, it is
usually effective that the concentration of the aqueous acid
solution is in the range of 0.1 to 25% by weight and the
temperature of the aqueous acid solution is in the range of 10 to
90.degree. C. If the concentration of the aqueous acid solution is
less than 0.1% by weight or the temperature of the aqueous acid
solution is lower than 10.degree. C., a sufficient degree of ion
exchange may not be effected. On the other hand, if the
concentration of the aqueous acid solution is greater than 25% by
weight or the temperature of the aqueous acid solution is higher
than 90.degree. C., the cost of the treating equipment may be
considerably raised.
[0031] In the present invention, if necessary, the denitration
catalyst having undergone the above-described alkali treatment step
and activation treatment step may further be regenerated by
subjecting it to the following step for impregnating the
denitration catalyst with a catalytically active component so as to
support it on the catalyst.
[0032] When the catalyst is subjected to the above-described alkali
cleaning and activation treatment with an acid, vanadium or
tungsten forming a catalytically active component may be dissolved
out from the catalyst, thus causing a reduction in denitration
power due to a decreased active component concentration in the
catalyst. Consequently, according to the present invention, after
the catalyst is cleaned to remove arsenic compounds therefrom,
washed with water and dried, the catalyst may be impregnated with
vanadium or tungsten so that the active component is supported on
the catalyst and the active component concentration in the catalyst
is thereby adjusted to its level before regeneration.
[0033] In order to impregnate the catalyst with vanadium, the
catalyst may be soaked in an aqueous solution prepared by
dissolving a vanadium compound (e.g., vanadium pentoxide, ammonium
metavanadate or vanadyl sulfate) in water, an organic acid, or an
amine solution.
[0034] In order to impregnate the catalyst with tungsten, the
catalyst may be soaked in an aqueous solution prepared by
dissolving a tungsten compound (e.g., ammonium paratungstate,
tungsten trioxide or tungsten chloride) in water, hydrochloric
acid, an amine solution or an organic acid.
[0035] According to the above-described regeneration method of this
embodiment, a spent catalyst is first subjected to an alkali
treatment step for cleaning it with an aqueous alkaline solution,
so that the arsenic compounds [principally diarsenic pentoxide
(As.sub.2O.sub.5)] deposited on the catalyst are converted into
easily soluble Na.sub.3AsO.sub.4 according to the following
reaction formula (2). Thus, the arsenic compounds deposited on the
catalyst surfaces can be removed. The following reaction formula
represents the reaction taking place when NaOH is used for the
aqueous alkaline solution.
As.sub.2O.sub.5+6NaOH.fwdarw.2Na.sub.3AsO.sub.4+3H.sub.2O (2)
[0036] However, after this alkali treatment step, Na.sup.+ ion
remains on the catalyst.
[0037] Accordingly, in an activation treatment step subsequent to
the above-described alkali treatment step, the Na+ ion remaining on
the catalyst and acting as a catalyst poison is removed by ion
exchange using an aqueous solution of an acid such as HCl, so that
the Na.sup.+ ion is replaced by H.sup.+ ion. This makes it possible
to remove Na.sup.+ ion from the catalyst and thereby restore the
activity of the denitration catalyst.
[0038] As described above, the cleaning effect for removing arsenic
compounds is enhanced by the above-described alkali treatment and
activation treatment with an acid, but an increased amount of
vanadium or other element forming a catalytically active component
may be dissolved out, resulting in a reduction in the active
component concentration remaining in the catalyst. Thus, although
arsenic compounds responsible for the reduced denitration power
have been removed, it is apparently impossible to restore the
denitration power. Accordingly, when a considerable amount of the
active component is dissolved out from the catalyst under certain
cleaning conditions, it is effective to restore the catalytic power
suitably by impregnating the catalyst with vanadium or the like so
as to support it on the catalyst.
[0039] Embodiment 2
[0040] The present invention relates to the regeneration of a
denitration catalyst which has been used for the removal of
nitrogen oxides present in combustion exhaust gas and has reduced
catalytic power due to the deposition of As compounds on the
catalyst surfaces. According to the second embodiment thereof, the
catalyst is regenerated by cleaning the catalyst with an aqueous
solution of sulfuric acid (H.sub.2SO.sub.4) or ammonia (NH.sub.3)
and thereby dissolving As.sub.2O.sub.5 deposited on the catalyst
surfaces. The denitration catalysts which can be regenerated
according to the present invention are ones comprising titanium
oxide as a principal component and containing vanadium, tungsten,
molybdenum or the like as an active component. Specific examples
thereof include TiO.sub.2-WO.sub.3 or TiO.sub.2-MoO.sub.3 binary
catalysts, and TiO.sub.2-V.sub.2O.sub.5-WO.sub.3 or
TiO.sub.2-V.sub.2O.sub.5-MoO.sub.3 ternary catalysts.
[0041] In this embodiment, a denitration catalyst having reduced
catalytic power is cleaned with a cleaning fluid comprising an
aqueous solution containing sulfuric acid or ammonia at a
concentration of 0.05 to 20% by weight and maintained at a
temperature of 10 to 90.degree. C. No particular limitation is
placed on the cleaning method, and the purpose of cleaning is
accomplished by bringing the denitration catalyst into contact with
a cleaning fluid comprising an aqueous solution of sulfuric acid or
ammonia. Specific examples thereof include a method in which the
denitration catalyst is soaked in an aqueous alkaline solution, a
method in which the denitration catalyst is allowed to stand in an
aqueous solution of sulfuric acid or ammonia, and a method in
which, after the denitration catalyst is placed in an aqueous
alkaline solution, air is bubbled through the solution or forced
convection currents are produced in the solution to promote the
renewal thereof.
[0042] If the concentration of the aqueous solution of sulfuric
acid or ammonia used for this cleaning purpose is unduly low, a
sufficient regenerative effect will not be obtained. On the other
hand, if its concentration is unduly high, a satisfactory
regenerative effect is achieved, but part of the silica contained
in the clay (e.g., acid clay or diatomaceous earth) and glass
fibers (consisting chiefly of silica) which are added to the
catalyst in an amount of several to ten-odd percent during its
fabrication for the purpose of maintaining the strength of the
catalyst is also dissolved. As a result, the strength of the
catalyst may be reduced to a level lower than that required for use
in actual plants. Accordingly, in order to obtain a regenerative
effect while maintaining the strength of the catalyst, it is
necessary to clean the catalyst with an aqueous solution containing
sulfuric acid or ammonia at a concentration of 0.05 to 20% by
weight.
[0043] Moreover, when the arsenic compounds deposited on the
catalyst surfaces exist in hardly soluble form, a sufficient
regenerative effect may not be obtained by using an aqueous
solution of sulfuric acid or ammonia having a low temperature. In
such a case, the hardly soluble arsenic compounds deposited on the
catalyst surfaces can be removed by raising the temperature of the
cleaning fluid (i.e., the aqueous solution of sulfuric acid or
ammonia) to 10-90.degree. C. and preferably 20-80.degree. C.
[0044] However, when the temperature of the cleaning fluid (i.e.,
the aqueous solution of sulfuric acid or ammonia) becomes higher,
vanadium or tungsten forming a catalytically active component may
be dissolved out from the catalyst, thus causing a reduction in
denitration power due to a decreased active component concentration
in the catalyst. Consequently, according to the present invention,
after the catalyst is cleaned to remove arsenic compounds
therefrom, washed with water and dried, the catalyst may be
impregnated with vanadium or tungsten, if necessary, so that the
active component is supported on the catalyst and the active
component concentration in the catalyst is thereby adjusted to its
level before regeneration.
[0045] In order to impregnate the catalyst with vanadium, the
catalyst may be soaked in an aqueous solution prepared by
dissolving a vanadium compound (e.g., vanadium pentoxide, ammonium
metavanadate or vanadyl sulfate) in water, an organic acid, or an
amine solution.
[0046] In order to impregnate the catalyst with tungsten, the
catalyst may be soaked in an aqueous solution prepared by
dissolving a tungsten compound (e.g., ammonium paratungstate,
tungsten trioxide or tungsten chloride) in water, hydrochloric
acid, an amine solution or an organic acid.
[0047] According to the above-described regeneration method of this
embodiment, the arsenic compounds [principally diarsenic pentoxide
(As.sub.2O.sub.5)] deposited on a catalyst can be removed by
cleaning.
[0048] Specifically, when the catalyst is cleaned with an aqueous
solution of sulfuric acid, the arsenic compounds are converted into
arsenic acid (Na.sub.3AsO.sub.4) according to the following
reaction formula (3), so that the dissolution thereof is promoted.
Thus, the arsenic compounds deposited on the catalyst surfaces can
be removed.
As.sub.2O.sub.5+3H.sub.2O.fwdarw.2H.sub.3AsO.sub.4 (3)
[0049] On the other hand, when the catalyst is cleaned with an
aqueous solution of ammonia, the arsenic compounds are converted
into water-soluble ammonium arsenate [(NH.sub.4).sub.3AsO.sub.4]
according to the following reaction formula (4). Thus, the arsenic
compounds deposited on the catalyst surfaces can be removed
easily.
As.sub.2O.sub.5+6NH.sub.3+6H.sub.2O.fwdarw.2(NH.sub.4).sub.3AsO.sub.4.mult-
idot.3H.sub.2O (4)
[0050] Moreover, when hardly soluble arsenic compounds are
deposited on the catalyst surfaces, it is effective to raise the
temperature of the cleaning fluid and thereby enhance its cleaning
effect. However, when the temperature of the cleaning fluid becomes
higher, its cleaning effect is enhanced, but an increased amount of
vanadium or other element forming a catalytically active component
may be dissolved out, resulting in a reduction in the active
component concentration remaining in the catalyst. Thus, although
arsenic compounds responsible for the reduced denitration power
have been removed, it is apparently impossible to restore the
denitration power. Accordingly, when a considerable amount of the
active component is dissolved out from the catalyst under certain
cleaning conditions, it is effective to restore the catalytic power
suitably by impregnating the catalyst with vanadium or the like so
as to support it on the catalyst.
[0051] Conventionally, catalysts having arsenic compounds deposited
thereon have been incapable of regeneration and hence disposed of.
However, the above-described regeneration methods of the present
invention make it possible to regenerate such catalysts and utilize
them effectively again as denitration catalysts. Moreover, by
regenerating and reusing such catalysts, the regeneration methods
of the present invention contribute to a decrease in the amount of
industrial waste, and hence have an important industrial
significance from the viewpoint of environmental protection.
[0052] The present invention is more specifically explained with
reference to the following examples. However, these examples are
not to be construed to limit the scope of the invention.
Example 1
[0053] Denitration catalysts (composed of 89.2 wt. % of TiO.sub.2,
10.2 wt. % of WO.sub.3, and 0.6 wt. % of V.sub.2O.sub.5) having a
honeycomb configuration with a pitch of 7.4 mm as shown in FIG. 1
were used in exhaust gas from a coal-fired boiler plant A for about
29,000 hours.
[0054] In order to regenerate five denitration catalysts having
reduced denitration power as a result of the aforesaid use, each of
them was soaked in a cleaning fluid comprising a 1 wt. % aqueous
solution of NaOH, KOH, Na.sub.2CO.sub.3, NaHCO.sub.3 or
K.sub.2CO.sub.3 so that the volume ratio of the cleaning fluid to
the denitration catalyst was 4.0, allowed to stand at 40.degree. C.
for 4 hours, washed with water, and then dried.
[0055] After the above-described alkali cleaning, the catalyst was
soaked in an activating fluid comprising a 1% aqueous solution of
H.sub.2SO.sub.4 so that the volume ratio of the activating fluid to
the catalyst was 4.0, allowed to stand at 40.degree. C. for 1 hour,
washed with water, and then dried. As shown in Table 2 below, the
regenerated catalysts thus obtained are referred to as "Catalysts
1-5".
[0056] Three other catalysts were treated in the same manner as
described above for Catalyst 1, except that, in place of a 1%
aqueous solution of H.sub.2SO.sub.4, a 1% aqueous solution of HCl,
HNO.sub.3 or HF was used as the activating fluid. Specifically,
each of them was soaked in the activating fluid so that the volume
ratio of the activating fluid to the catalyst was 4.0, allowed to
stand at 40.degree. C. for 1 hour, washed with water, and then
dried. As shown in Table 2 below, the regenerated catalysts thus
obtained are referred to as "Catalysts 6-8".
Comparative Example 1
[0057] In order to regenerate two denitration catalysts having been
used in the same manner as in Example 1, each of them was soaked in
a cleaning fluid comprising water or a 1% aqueous solution of HCl
so that the volume ratio of the cleaning fluid to the catalyst was
4.0, allowed to stand at 20.degree. C. for 4 hours, washed with
water, and then dried. The catalyst treated with water is referred
to as "Catalyst 51" and the catalyst treated with an aqueous
solution of HCl as "Catalyst 61".
[0058] Moreover, another denitration catalyst was treated in the
same manner as described for Catalyst 1 in Example 1, except that
the activation treatment with an aqueous solution of
H.sub.2SO.sub.4 was omitted. Specifically, the catalyst was cleaned
with a cleaning fluid comprising an aqueous solution of NaOH,
washed directly with water, and then dried to obtain "Catalyst
71".
Example 2
[0059] Denitration catalysts (composed of 89.2 wt. % of TiO.sub.2,
10.2 wt. % of WO.sub.3, and 0.6 wt. % of V.sub.2O.sub.5) having a
honeycomb configuration with a pitch of 7.4 mm were used in a
coal-fired boiler plant B for about 55,000 hours.
[0060] In order to regenerate twelve denitration catalysts having
reduced denitration power as a result of the aforesaid use, each of
them was soaked in a cleaning fluid comprising a 5% aqueous
solution of NaOH, KOH or Na.sub.2CO.sub.3 so that the volume ratio
of the cleaning fluid to the denitration catalyst was 4.0, allowed
to stand at 60.degree. C. for 4 hours, washed with water, and then
dried.
[0061] After the above-described alkali cleaning, the catalyst was
soaked in an activating fluid comprising a 5 wt. % aqueous solution
of HCl, HNO.sub.3, H.sub.2SO.sub.4 or HF so that the volume ratio
of the activating fluid to the catalyst was 4.0, allowed to stand
at 40.degree. C. for 30 minutes, washed with water, and then dried.
As shown in Table 3 below, the regenerated catalysts thus obtained
are referred to as "Catalysts 9-20".
[0062] Moreover, these Catalysts 9-20 s were soaked in a solution
prepared by dissolving vanadium pentoxide in an aqueous solution of
oxalic acid, so that the vanadium concentration in the catalysts
was adjusted to its level before cleaning. The regenerated
catalysts thus obtained are referred to as "Catalysts 21-32".
Comparative Example 2
[0063] In order to regenerate two denitration catalysts having been
used in the same manner as in Example 2, each of them was soaked in
a cleaning fluid comprising water or a 1% aqueous solution of HCl
so that the volume ratio of the cleaning fluid to the denitration
catalyst was 4.0, allowed to stand at 20.degree. C. for 4 hours,
washed with water, and then dried. The catalyst treated with water
is referred to as "Catalyst 52" and the catalyst treated with an
aqueous solution of HCl as "Catalyst 62".
[0064] Moreover, this Catalyst 62 was soaked in a solution prepared
by dissolving vanadium pentoxide in an aqueous solution of oxalic
acid, so that the vanadium concentration in the catalyst was
adjusted to its level before cleaning. The regenerated catalyst
thus obtained is referred to as "Catalyst 63".
[0065] Moreover, another denitration catalyst was treated in the
same manner as described for Catalyst 9 in Example 2, except that
the activation treatment with an aqueous solution of HCl was
omitted. Specifically, the catalyst was cleaned with a cleaning
fluid comprising an aqueous solution of NaOH, washed directly with
water, and then dried to obtain "Catalyst 72". Furthermore, this
Catalyst 72 was soaked in a solution prepared by dissolving
vanadium pentoxide in an aqueous solution of oxalic acid, so that
the vanadium concentration in the catalyst was adjusted to its
level before cleaning. The catalyst thus obtained is referred to as
"Catalyst 73".
Example 3
[0066] The unused catalysts and spent catalysts for coal-fired
boiler plants A and B, the regenerated catalysts obtained in
Examples 1 and 2, and the regenerated catalysts obtained in
Comparative Examples 1 and 2 were comparatively tested for
denitration power under the conditions shown in Table 1.
[0067] Moreover, with respect to each of the regenerated catalysts,
its average arsenic content and its compressive strength were also
measured.
[0068] The results thus obtained are shown in Tables 2 and 3. In
Tables 2 and 3, the degree of denitration (%) and the compressive
strength ratio are defined as follows.
[0069] Degree of denitration (%)={[(Inlet NO.sub.x content)
-(Outlet NO.sub.x content)]/(Inlet NO.sub.x content)}.times.100
[0070] Compressive strength ratio=(Compressive strength of test
sample)/(Compressive strength of unused catalyst)
1 TABLE 1 Test sample Item Catalyst for coal-fired boilers Shape of
catalyst 46 mm .times. 53 mm .times. 800 mm(L) Flow rate of gas
20.2 Nm.sup.3/m.sup.2 .multidot. hr SV 10,400 h.sup.-1 NH.sub.3/Nox
1.0 Temperature of gas 380.degree. C. Composition of gas NOx = 150
ppm NH.sub.3 150 ppm SOx = 800 ppm O.sub.2 = 4% CO.sub.2 = 12%
H.sub.2O = 1.1% N.sub.2 = Balance (Notes) SV: Superficial velocity
(hr.sup.-1), i.e., the ratio of the flow rate of gas to the amount
of catalyst NH.sub.3/Nox: Molar ratio
[0071]
2TABLE 2 Degree of Amount of Amount of Example and Comparative
Example denitration As.sub.2O.sub.5 Na.sub.2O or K.sub.2O Plant
Cleaning fluid Activating fluid Catalyst (%) (wt %) (wt %) A
(coal-fired) Example 1 NaOH H.sub.2SO.sub.4 1 79.1 0.3 <0.1 KOH
H.sub.2SO.sub.4 2 78.3 0.4 <0.1 Na.sub.2CO.sub.3 H.sub.2SO.sub.4
3 77.6 0.2 <0.1 NaHCO.sub.3 H.sub.2SO.sub.4 4 79.3 0.3 <0.1
K.sub.2CO.sub.3 H.sub.2SO.sub.4 5 80.0 0.5 <0.1 NaOH HCl 6 80.1
0.3 <0.1 NaOH HNO.sub.3 7 79.6 0.3 <0.1 NaOH HF 8 77.4 0.2
<0.1 Comparative Water -- 51 53.3 2.8 0 Example 1 HCl -- 61 54.2
2.7 0 NaOH -- 71 46.0 0.3 2.4 Reference Example 3 Spent catalyst
51.0 3.2 0 Reference Example 4 Unused catalyst 80.7 0 0
[0072]
3TABLE 3 Degree of Amount of Amount of Example and Comparative
Example denitration V.sub.2O.sub.5 As.sub.2O.sub.5 Na.sub.2O or
K.sub.2O Plant Cleaning fluid Activating fluid Catalyst (%) (wt. %)
(wt %) (wt %) B (coal-fired) Example 2 5% NaOH 5% HCl 9 73.2 0.35
0.1 <0.1 21 79.8 0.6 5% HNO.sub.3 10 72.8 0.32 0.1 <0.1 22
80.2 0.6 5% H.sub.2SO.sub.4 11 74.1 0.30 0.1 <0.1 23 80.4 0.6 5%
HF 12 73.8 0.25 0.15 <0.1 24 80.6 0.6 5% KOH 5% HCl 13 74.1 0.35
0.1 <0.1 25 80.2 0.6 5% HNO.sub.3 14 72.4 0.30 0.15 <0.1 26
79.7 0.6 5% H.sub.2SO.sub.4 15 71.5 0.25 0.1 <0.1 27 79.4 0.6 5%
HF 16 73.2 0.35 0.15 <0.1 28 78.6 0.6 5% Na.sub.2CO.sub.3 5% HCl
17 70.2 0.25 0.1 <0.1 29 79.1 0.6 5% HNO.sub.3 18 71.2 0.25 0.15
<0.1 30 80.1 0.6 5% H.sub.2SO.sub.4 19 70.4 0.30 0.1 <0.1 31
80.3 0.6 5% HF 20 73.2 0.30 0.15 <0.1 32 80.5 0.6 Comparative
Water -- 52 50.1 0.6 3.6 <0.1 Example 2 5% HCl -- 62 53.1 0.5
3.3 <0.1 63 56.1 0.6 5% NaOH -- 72 41.2 0.4 0.1 2.2 73 45.8 0.0
Reference Example 1 Spent catalyst 48 0.6 4.2 0 Reference Example 2
Unused catalyst 80.7 0.6 0 0
[0073] It has been confirmed by these results that, when a catalyst
having reduced denitration power due to the deposition of arsenic
compounds on the catalyst surfaces is regenerated by cleaning it
with an aqueous alkaline solution and then subjecting it to an
activation treatment with an aqueous acid solution, most of the
arsenic compounds and alkaline substances acting as catalyst
poisons can be removed and, therefore, the catalyst can be
regenerated to the fullest extent.
[0074] Moreover, as shown in Example 2, it may happen that vanadium
forming a catalytically active component is dissolved out during
alkali cleaning and activation treatment, thus causing a reduction
in catalytic power. However, it has been found that, in such a
case, the catalytic power can be fully restored (or regenerated) by
dissolving and removing arsenic compounds from the catalyst and
then impregnating the catalyst with vanadium so as to make up for
the loss.
Example 4
[0075] Denitration catalysts (composed of 89.2 wt. % of TiO.sub.2,
10.2 wt. % of WO.sub.3, and 0.6 wt. % of V.sub.2O.sub.5) having a
honeycomb configuration with a pitch of 7.4 mm as shown in FIG. 1
were used in exhaust gas from a coal-fired boiler plant A for about
23,000 hours.
[0076] In order to regenerate six denitration catalysts having
reduced denitration power as a result of the aforesaid use, each of
them was soaked in a cleaning fluid comprising an aqueous solution
containing H.sub.2SO.sub.4 at a concentration of 0.03%, 0.05%,
0.3%, 1%, 20% or 30% so that the volume ratio of the cleaning fluid
to the denitration catalyst was 4.0, allowed to stand at 20.degree.
C. for 4 hours, washed with water, and then dried.
[0077] The regenerated catalysts thus obtained are referred to as
"Catalysts 101-106" in order of increasing sulfuric acid
concentration.
[0078] Moreover, in order to regenerate five other denitration
catalysts having reduced denitration power as a result of the
aforesaid use, each of them was soaked in a cleaning fluid
comprising an aqueous solution containing NH.sub.3 at a
concentration of 0.03%, 0.05%, 1%, 20% or 30% so that the volume
ratio of the cleaning fluid to the denitration catalyst was 4.0,
allowed to stand at 20.degree. C. for 4 hours, washed with water,
and then dried.
[0079] The regenerated catalysts thus obtained are referred to as
"Catalysts 107-111" in order of increasing ammonia
concentration.
Comparative Example 3
[0080] In order to regenerate two denitration catalysts having been
used in the same manner as in Example 4, each of them was soaked in
a cleaning fluid comprising water or a 1% aqueous solution of HCl
so that the volume ratio of the cleaning fluid to the catalyst was
4.0, allowed to stand at 20.degree. C. for 4 hours, washed with
water, and then dried.
[0081] The catalyst treated with water is referred to as "Catalyst
151" and the catalyst treated with an aqueous solution of HCl as
"Catalyst 161".
Example 5
[0082] Denitration catalysts (composed of 89.2 wt. % of TiO.sub.2,
10.2 wt. % of WO.sub.3, and 0.6 wt. % of V.sub.2O.sub.5) having a
honeycomb configuration with a pitch of 7.4 mm were used in a
coal-fired boiler plant B for about 45,000 hours.
[0083] In order to regenerate twelve denitration catalysts having
reduced denitration power as a result of the aforesaid use, each of
them was soaked in a cleaning fluid comprising an aqueous solution
containing H.sub.2SO.sub.4 at a concentration of 0.3%, 1% or 20% so
that the volume ratio of the cleaning fluid to the denitration
catalyst was 4.0, allowed to stand for 4 hours while maintaining
the temperature of the cleaning fluid at 10, 20, 80 or 90.degree.
C., washed with water, and then dried. As shown in Table 5 below,
the regenerated catalysts thus obtained are referred to as
"Catalysts 112-123".
[0084] Moreover, these Catalysts 112-123 were soaked in a solution
prepared by dissolving vanadium pentoxide in an aqueous solution of
oxalic acid, so that the vanadium concentration in the catalysts
was adjusted to its level before cleaning. As shown in Table 5
below, the regenerated catalysts thus obtained are referred to as
"Catalysts 124-135".
[0085] On the other hand, in order to regenerate four denitration
catalysts having reduced denitration power as a result of the
aforesaid use, each of them was soaked in a cleaning fluid
comprising an aqueous solution containing HNO.sub.3 at a
concentration of 1.0% so that the volume ratio of the cleaning
fluid to the denitration catalyst was 4.0, allowed to stand for 4
hours while maintaining the temperature of the cleaning fluid at
10, 20, 80 or 90.degree. C., washed with water, and then dried. The
regenerated catalysts thus obtained are referred to as "Catalysts
136-139" in order of increasing temperature of the cleaning
fluid.
[0086] Moreover, these Catalysts 136-139 were soaked in a solution
prepared by dissolving vanadium pentoxide in an aqueous solution of
oxalic acid, so that the vanadium concentration in the catalysts
was adjusted to its level before cleaning. The regenerated
catalysts thus obtained are referred to as "Catalysts 140-144".
Comparative Example 4
[0087] In order to regenerate two denitration catalysts having been
used in the same manner as in Example 5, each of them was soaked in
a cleaning fluid comprising water or a 1% aqueous solution of HCl
so that the volume ratio of the cleaning fluid to the denitration
catalyst was 4.0, allowed to stand at 20.degree. C. for 4 hours,
washed with water, and then dried. The catalyst treated with water
is referred to as "Catalyst 152" and the catalyst treated with an
aqueous solution of HCl as "Catalyst 162".
[0088] Moreover, this Catalyst 162 was soaked in a solution
prepared by dissolving vanadium pentoxide in an aqueous solution of
oxalic acid, so that the vanadium concentration in the catalyst was
adjusted to its level before cleaning. The regenerated catalyst
thus obtained is referred to as "Catalyst 163".
Example 6
[0089] The unused catalysts and spent catalysts for coal-fired
boiler plants A and B, the regenerated catalysts obtained in
Examples 4 and 5, and the regenerated catalysts obtained in
Comparative Examples 3 and 4 were comparatively tested for
denitration power under the conditions shown in the above Table
1.
[0090] Moreover, with respect to each of the regenerated catalysts
obtained in Examples 4 and 5, its average arsenic content and its
compressive strength were also measured.
[0091] The results thus obtained are shown in Tables 4 and 5. In
Tables 4 and 5, the degree of denitration (%) and the compressive
strength ratio are defined as follows.
[0092] Degree of denitration (%)={[(Inlet NO.sub.x content)
-(Outlet NO.sub.x content)]/(Inlet NO.sub.x content)}.times.100
Compressive strength ratio=(Compressive strength of test
sample)/(Compressive strength of unused catalyst)
4TABLE 4 Example and Degree of Compressive Amount of Comparative
denitration strength As.sub.2O.sub.5 Plant Example Catalyst (%)
ratio (wt. %) A (coal- Example 4 fired) 0.03% H.sub.2SO.sub.4 101
71.6 1.02 1.8 0.05% H.sub.2SO.sub.4 102 79.5 1.04 1.0 0.3%
H.sub.2SO.sub.4 103 80.7 1.00 0.4 1% H.sub.2SO.sub.4 104 80.8 0.95
0.3 20% H.sub.2SO.sub.4 105 80.9 0.93 0.2 30% H.sub.2SO.sub.4 106
80.7 0.60 0.1 0.03% NH.sub.3 107 71.3 1.01 2.0 0.05% NH.sub.3 108
78.3 1.04 0.9 1% NH.sub.3 109 79.8 1.00 0.3 20% NH.sub.3 110 80.3
0.96 0.2 30% NH.sub.3 111 80.5 0.65 0.1 Comparative 151 68.5 -- 2.7
Example 3 161 69.0 -- 2.4 Reference Spent 62.0 1.05 3.0 Example 5
catalyst Reference Unused 80.7 1.00 0 Example 6 catalyst
[0093]
5TABLE 5 Example and Degree of Amount of Comparative Cleaning
conditions denitration Compressive As.sub.2O.sub.5 Plant Example
Cleaning fluid Temperature (.degree. C.) Catalyst (%) strength
ratio (wt. %) B (coal-fired) Example 5 0.3% H.sub.2SO.sub.4 10 112
66.8 -- 2.0 124 73.8 1.00 20 113 70.3 -- 1.2 125 80.5 0.98 80 114
70.9 -- 0.8 126 80.8 0.97 90 115 71.2 -- 0.6 127 81.1 0.92 1.0%
H.sub.2SO.sub.4 10 116 67.1 -- 1.0 128 76.5 0.98 20 117 69.0 -- 0.6
129 81.5 0.96 80 118 68.7 -- 0.4 130 81.0 0.97 90 119 66.3 -- 0.3
131 81.2 0.85 20% H.sub.2SO.sub.4 10 120 67.4 -- 0.8 132 77.1 0.95
20 121 69.0 -- 0.5 133 81.3 0.95 80 122 69.5 -- 0.3 134 81.5 0.94
90 123 64.3 -- 0.2 135 81.0 0.8 1.0% H.sub.2SO.sub.4 10 136 68.0 --
1.0 140 76.5 0.99 20 137 70.5 -- 0.7 141 77.5 0.96 80 138 71.5 --
0.5 142 79.0 0.93 90 139 72.1 -- 0.2 143 80.0 0.80 Comparative
Water 20 152 66.4 -- 4.6 Example 4 1.0% HCl 20 162 67.6 -- 4.4 20
163 70.4 -- 4.2 Reference Example 7 Spent catalyst 66.8 1.09 5.0
Reference Example 8 Unused catalyst 80.7 1.00 0
[0094] It can be seen from these results that, when a catalyst
having reduced denitration power due to the deposition of arsenic
compounds on the catalyst surfaces is regenerated with the aid of a
cleaning fluid, its arsenic-removing effect is insufficient if the
sulfuric acid or ammonia concentration in the cleaning fluid is
less than 0.03% by weight. On the other hand, the denitration power
is restored at a sulfuric acid or ammonia concentration of 30% by
weight or greater, but part of the silica contained in the clay and
glass fibers added to the catalyst during its fabrication for the
purpose of maintaining the strength of the catalyst is also
dissolved to cause a reduction in strength.
[0095] Accordingly, the sulfuric acid or ammonia concentration of
the cleaning fluid must be greater than 0.03% by weight and less
than 30% by weight.
[0096] Moreover, it can be seen from the results of Example 5 that,
when the arsenic compounds deposited on the catalyst surfaces are
hardly soluble ones, they are not easily dissolved if the cleaning
fluid has a temperature of 10.degree. C. or so, and a sufficient
regenerative effect cannot be obtained. In such a case, it is
preferable to heat the cleaning fluid to 20.degree. C. or above.
However, if the temperature of the cleaning fluid reaches
90.degree. C., the strength of the honeycomb-shaped catalyst is
reduced. Accordingly, the temperature of the cleaning fluid should
desirably be in the range of 20 to 80.degree. C.
[0097] Furthermore, when the cleaning fluid has a temperature of
20.degree. C. or above, it may happen that vanadium forming a
catalytically active component is dissolved out during cleaning,
thus causing a reduction in catalytic power. However, it has been
found that, in such a case, the catalytic power can be fully
restored (or regenerated) by dissolving and removing arsenic
compounds from the catalyst and then impregnating the catalyst with
vanadium so as to make up for the loss.
* * * * *