Note: Descriptions are shown in the official language in which they were submitted.
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SPECIFICATION
TITLE OF THE INVENTION
FILTER MEDIUM FOR TREATING AN EXHAUST GAS
B~CKGROUND OF THE INVENTION
Field of the Invention and Related APP1 ication
This invention relates to the treatment of exhaust
gases and more particularly, to a filter medium for
effectively treating gases which are discharged in large
amounts from municipal refuse incineration plants and which
contain fly ashes from incinerators, hydrogen chloride,
nitrogen oxides, sulfur oxides and the like.
The present inventors already developed and proposed a
technique for forming a solid layer of slaked lime and
clacium chloride on a woven filter fabric in order to
efficiently eliminate hydrosen chloride from exhaust gases.
The filter fabric used in this technique is made of a glass
wool material which has a relatively poor heat resistance.
Accordingly, it is necessary that a gas passed through
the fabric be kept at a temperature lower than 250C.
Known heat-resistant filter materials include woven
filter fabrics of carbon fibers and stainless steel wires.
However, these fabrics have the disadvantage in that the
carbon fiber-woven filter fabric is not resistant to
abrasion and the stainless steel woven filter fabric is very
expensive.
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Although the present invention contemplates to
eliminate not only hydrogen chloride, but also nitrogen
oxides (i.e. denitration), it is difficult to deposit a
catalyst for denitration on the stainless steel woven filter
fabric in order to impart the denitration function. More
particularly, for the coating of the denitration catalyst on
the fabric surface, it is necessary that the coating
material (denitration catalyst) and the fabric surface be
wetted sufficiently, but the stainless steel filter fabric
has poor wettability, so that it is difficult to form a
uniform coating layer on the fabric.
OBJECT AND SUMMARY ûF THE INVENTION
It is accordingly an object of the invention to provide
an exhaust gas treating filter medium or material which
overcomes the above prior art problems by improving the heat
resistance of a filter material at low costs and which has
the function of eliminating both hydrogen chloride and
nitrogen oxides.
In order to solve the problems involved in the prior
art, the present inventors made intensive studies. As a
result, it was found when porous ceramics were used as a
filter material, the heat resistance was improved with good
affinity for catalysts. In addition, the ceramics had the
capability of dust collection and could be cleansed by
application of back-flow washing air. Thus, a favorable
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exhaust gas-treating medium coulcl be obtained using the ceramic
filter. The present invention is accomplished based on the
above finding.
According to the present invention, there is provided
a filter medium for treating an exhaust gas, which comprises:
a porous ceramic substrate having a first and
second sides,
a pre-coat layer of a material which is positioned
on the said first side of the porous ceramic substrate and
prevents clogging of the porous substrate,
a solid layer of a mixture of slaked lime or calcium
carbonate and calcium chloride formed on the said pre-coat
layer, and
a layer of a denitrating catalyst for eliminating
nitrogen oxides on the said second side of the said ceramic
substrate.
In a particular embodiment, the porous ceramic
substrate may be in a form of a double-cylinder body having
innner and outer hollow cylinders each closed at one end.
The catalyst layer for eliminating nitrogen oxides may be
formed on at least one side of the inner cylinder. The pre-
coat layer of a specific type of reaction aid and the layer
of a mixture of slaked lime or calcium carbonate and calcium
chloride may be formed, in this order, on at least one surface
of the outer cylinder.
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The present invention also provides a method of
eliminating hydrogen chloride and nitrogen oxides present
in an exhaust gas, which method comprises passing the gas
through the filter medium as defined above, from the said
solid mixture layer side to the catalyst layer side.
BRIEF DESCRIPTION OF THE DRAWINGS
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Fig. 1 is a ~chematic sectional view of an exhaust gas-
treating medium according to one embodiment of the invention;
Fig. 2 is a schematic enlarged view of a part of the
medium of Fig. 1;
Fig. 3 is a block diagram illustrating fabrication of
the medium of Fig. 1;
Fig. 4 is a schematic sectional view of a filter medium
according to another embodiment of the invention;
Fig. 5 is a schematic enlarged view of a part of the
medium of Fig. 4; and
Fig. 6 is a block diagram illustrating fabrication of
the medium.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The first embodiment of the in~ention is described with
reference to the accompanying drawings and particularly, to
Figs. 1 through 3.
As is particularly shown in Figs. 1 and 2, a filter
medium M of the present invention has a cylindrical
construction of a porous ceramic such as, for example,
mullite or highly pure alumina. The medium M has a substrate
1 and a layer 4 of a denitration catalyst, such as, for
example, a mixture of vanadium pentaoxide and titanium
oxide, on one side or an outlet side from which a gas is
flown out or discharged. The layer 4 is formed, for
example, after a firing operation on the surface of the
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substrate 1. On the other or inlet side of the substrate 1 from
which the gas is passed in, there is formed a pre-coat layer
2 of a specific type of reaction aid in order to prevent
clogging of the filter medium. A layer 3 of a mixture of
slaked lime or calcium carbonate (hereinafter referred to
generically as slaked lime) and clacium chloride is formed
on the pre-coat layer 2. In the figures, the arrows indicate
the directions of flow of a gas.
The specific type of reaction aid used as the pre-coat
layer 2 of the present invention should preferably have ~1)
a Qmall bulk density, (2) good air permeability, ~3) non-
adherence on absorption of moisture in air, and (4) a
particle size of from 1 tO 100 micrometers, preferably from
10 to 50 micrometers. Examples of the aid include powders
of zeolite, alumina, diatomaceous earth, pearlite, activated
clay, kaolin, feldspar, quartz and the like. These powders
may be used sinsly or in combination of two or more.
Of these materials, zeolite, diatomaceous earth and pearlite
are preferred.
The filter medium M of the invention shown in Figs. 1
and 2 has a cylindrical form and is constituted, for
example, by the following manner.
First, the substrate 1 on which the denitration
catalyst layer 4 has been formed is placed in a duct 10 for
an exhaust gas as shown in Fig. 3. Subsequently, a specific
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type of reaction aid as indicated abo~e is charged from a
feeder 11 of Fig. 3 to form the pre-coat layer 2 on one
surface of the substrate 1 opposite to the side on which the
catalyst layer 4 is formed.
When slaked lime is blown into the exhaust gas, it
reacts with hydrogen chloride in the exhaust gas and is
converted into calcium chloride. Accordingly, when the
exhaust gas into which slaked lime is incorporated is
introduced from line 12, calcium chloride and unreacted
lG slaked lime reach the surface of the substrate 1 in the duct
10, thereby forming the solid material layer 3 on the
pre-coat layer 2.
It will be noted that calcium chloride is deliquescent
in nature and may convert into a sticky solid substance
by condensation of moisture in the exhaust gas. If the
sticky solid substance is formed, the medium may be
clogged or the powder layer may be excessively compacted,
makin~ it difficult to keep the filter medium in a normal
condition.
To avoid this, the pre-coat layer of a specific type of
reaction aid is formed on the medium surface in the practice
of the invention.
When the exhaust gas entraining slaked lime as
described before i5 passed after the formation of the pre-
coat layer 2, the solid substances in the gas form the layer
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(powder layer) 3 on the pre-coat layer 2. At the same time,
as shown in Fig. 3, the reaction aid is invariably iniected
from the feeder 11 into the exhaust gas duct 10 so that the
compactness of the powder layer 3 can be suppressed.
In this connection, however, these layers 2 and 3 grow
with time, resulting in an increasing pressure loss of the
medium. The pcwder layer 3 has to be scraped off at ^ertain
intervals of time and a fresh powder layer should be formed
in the same manner as described above.
When an exhaust gas is passed through the medium of the
present invention, unreacted hydrogen chloride in the
exhaust gas reacts with slaked lime of the powder layer 3,
thereby eliminating the hydrogen chloride in a high
efficiency.
At the same time, dust in the exhaust gas is also
eliminated by the filter medium and thus, dust collection
can also be expected.
Moreover, if an alkaline substance is used as the
reaction aid, sulfur oxide can be removed from the exhaust
gas.
Since the medium of the invention has the denitration
catalyst layer 4, passage of an exhaust gas in which ammonia
gas is incorporated permits nitrogen oxides to be eliminated
on contact with or passage through the layer 4.
Another embodiment of the invention is described.
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As particularly described in Figs. 4 and 5, a filter
medium M according to another embodiment of the invention
has a double-cylinder construction of a porous ceramic such
as, for example, mullite, highly pure alumina or the like.
The construction is composed of an outer cylinder 21 and an
inner cylinder 22, which are hollow and are closed at one
end as shown. The inner cylinder 22 has at least one
layer 25 of a denitration catalyst such as, for example, a
mixture of vanadium pentaoxide and titanium oxide. The
layer 25 is formed, for example, after a firing operation on
the surface of the filter medium. In the figures, the
denitration catalyst layers 25 are formed on both sides of
the inner cylindsr 22, i.e. gas inlet and outlet sides, and
also on an outlet side of the outer cylinder 21. A pre-coat
layer 23 of a specific type of reaction aid is provided on
an inlet side of the outer cylinder 21 from which an exhaust
gas is passed. On the pre-coat layer 25 is formed a
layer 24 of a solid material comprising a mixture of slaked
lime or calcium carbonate and calcium chloride. In these
figures, the arrows indicate the directions of flow of the
exhaust gas.
The filter medium M of the invention is formed as
follows.
The inner and outer cylinders 22, 21 which have,
re~pectively, the denitration layers 25 as desired are placed
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in a duct 30 for an exhaust gas as shown in Fig. 6.
Subsequently, a specific type of reaction aid as defined
with regard to the first embodiment of the invention is
introduced from a feeder 31 into the duct 30, thereby
forming the pre-coat layer 23 on the outer side of the outer
cylinder 21.
On the other hand, when slaked lime in the form of
powder is brown into the exhaust gas, it reacts with
hydrogen chloride in the exhaust gas to form calcium
chloride. When the exhaust gas into which slaked lime has
been charged is introduced from line 32, calcium chloride
and unreacted slaked lime reach the surface of the outer
cylinder 21 on which the solid material layer 24 is formed.
The subsequent treatment of the pre-coat layer 23 and
the solid material layer or powder layer 24 is made in the
same manner as in the first embodiment described before.
This type of filter medium has similar effects as the
filter medium of the first embodiment. In addition, when
the denitration catalyst layers 25 are formed on the gas-
outlet side of the outer cylinder 21 and on opposite sidesof the inner cylinder 22, nitrogen oxides can be more
effectively removed on contact, with the layers 25, of the
exhaust gas in which ammonia gas has been incorporated.
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