Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Sulfur or sulfur containing component
Reexamination Certificate
2000-08-21
2004-11-09
Hendrickson, Stuart (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Sulfur or sulfur containing component
C423S244010
Reexamination Certificate
active
06814948
ABSTRACT:
TECHNICAL FIELD
This invention relates to exhaust gas treating systems for the removal of nitrogen oxides (NO
x
) and sulfur oxides (SO
x
) present in exhaust gas discharged from boilers, gas turbines, engines and combustion furnaces for burning various types of fuel.
This invention can also be suitably used for the removal of nitrogen oxides present in tunnels and for the removal of nitrogen oxides present in exhaust gas from nitric acid production plants.
BACKGROUND ART
An example of exhaust gas treatment by means of a conventional exhaust gas treating system is explained with reference to FIG.
1
.
In
FIG. 1
, reference numeral
1
designates a boiler;
2
, a denitrator;
3
, an air preheater;
4
, a dust collector;
5
, a gas-gas heater;
6
, a desulfurizer; and
7
, a stack.
As shown in
FIG. 1
, a denitrator
2
using a catalyst is installed at the outlet of a boiler
1
or the like, and an air preheater
3
is installed at the outlet of denitrator
2
so as to lower the temperature of the exhaust gas to about 130° C.
The exhaust gas having passed through the aforesaid air preheater
3
is dedusted in a dust precipitator
4
, passed through a gas-gas heater
5
and then introduced into a desulfurizer
6
where sulfur oxides (SO
x
) are removed therefrom. Thereafter, the exhaust gas is discharged into the atmosphere through a stack
7
.
In order to remove sulfur oxides (SO
x
) from exhaust in the aforesaid desulfurizer
6
, there has conventionally been employed the so-called lime-gypsum method in which the aforesaid sulfur oxides (SO
x
) are absorbed with the aid of calcium carbonate used as absorbing agent and recovered in the form of gypsum. In this method, attempts have been made to reduce the outlet concentration of sulfur oxides (SO
x
) by varying the gas-fluid ratio, the residence time and the like.
Usually, the concentration of sulfur oxides (SO
x
) in exhaust gas from boilers is in the range of 400 to 800 ppm, and it is intended in the aforesaid lime-gypsum method to reduce the outlet concentration thereof to 50-100 ppm.
However, recent environmental standards demand that the concentration of sulfur oxides (SO
x
) in exhaust gas should be reduced to a level of 5 ppm or less which is commonly known as a high-degree desulfurization level. In order to remove sulfur oxides (SO
x
) to a level of 50 to 100 ppm according to the aforesaid conventional lime-gypsum method, a marked increase in cost due to an increased size of equipment and the like is unavoidable, even though the conditions are optimized. Moreover, it is desired from the viewpoint of environmental problems to improve the efficiency of removal of sulfur oxides (SO
x
).
Furthermore, the aforesaid desulfurizer
6
employs the so-called lime-gypsum method in which sulfur oxides (SO
x
) present in exhaust gas are absorbed with the aid of calcium carbonate used as absorbing agent.
among dry processes, only an absorption process using active carbon has been put to practical use. However, this adsorption process uses water washing for the purpose of desorption and hence requires a large volume of water. Moreover, this process also involves problems concerning disposal of the resulting dilute sulfuric acid, drying of the adsorbant, and the like.
As described above, in the current practical process for the removal of nitrogen oxides present in exhaust gas from boilers, there is used a denitrator
2
based on the selective catalytic reduction (SCR) method in which nitrogen oxides are decomposed to nitrogen and water vapor by using a catalyst comprising V
2
O
5
supported on TiO
2
and a reducing agent comprising NH
3
. However this process involves the following problems. First, a reaction temperature of 300 to 400° C. is required because of the performance of the catalyst. Secondly, NH
3
is required for use as reducing agent. Thirdly, since the current leak level of NO
x
is from 5 to 40 ppm, an excess of NH
3
needs to be injected for the purpose of reducing the leak level of NO
x
to zero.
Moreover, recent environmental standards demand that the concentration of nitrogen oxides (NO
x
) in exhaust gas should be reduced to a level of 1 ppm or less which is commonly known as a high-degree denitration level. In the aforesaid conventional denitration treatment based on the selective catalytic reduction (SCR) method, a marked increase in cost due to an increased size of equipment and the like is unavoidable, even though the conditions are optimized. On the other hand, it is desired from the viewpoint of environmental problems to improve the efficiency of removal of nitrogen oxides (NO
x
).
In view of the above-described problems, an object of the present invention is to provide an exhaust gas treating system which can treat exhaust gas at low temperatures without requiring any heating means and, moreover, can treat exhaust gas efficiently without using a large amount of absorbing agent.
DISCLOSURE OF THE INVENTION
In view of the above-described problems of the prior art, the present inventors have made intensive investigations and have now found that an active carbon having been subjected to a specific heat treatment functions as an effective catalyst for desulfurization or denitration reactions. The present invention has been completed on the basis of this finding.
Accordingly, the present invention relates to a heat-treated active carbon for use in desulfurization or denitration reactions and a desulfurization or denitration process using it.
First of all, the present invention is described below in terms of desulfurization.
The present invention provides a heat-treated active carbon for use in desulfurization reactions which has been obtained by heat-treating a starting active carbon in a non-oxidizing atmosphere.
The present invention also provides a desulfurization process which comprises bringing a gas containing SO
2
, water and oxygen into contact with such a heat-treated active carbon for use in desulfurization reactions.
No particular limitation is placed on the type of the starting active carbon. For example, an active carbon fiber or an particulate active carbon is used. Active carbon fibers include those derived from pitch, polyacrylonitrile, phenol, cellulose and the like may be used. Commercial products may also be used. Among others, active carbon having highly hydrophobic surfaces are especially preferred. Specific examples thereof include pitch-based and polyacrylonitrile-based starting active carbon fibers.
The above-described starting active carbon is heat-treated in a non-oxidizing atmosphere. The term “non-oxidizing atmosphere” as used herein comprehends inert gases and reducing atmospheres. No particular limitation is placed on the type of the non-oxidizing atmosphere, so long as the starting active carbon is not oxidized thereby. In particular, inert gases such as nitrogen gas, argon gas and helium gas are preferred. Among them, nitrogen gas is especially preferred because it is readily available.
The heat-treating temperature may be any temperature that renders the surfaces of the starting active carbon hydrophobic. Although the heat-treating temperature may be suitably determined according to the type of the starting active carbon and the like, it is usually in the range of about 600 to 1,200° C. The heat-treating time may be suitably determined according to the heat-treating temperature and the like. This heat treatment makes it possible to obtain the heat-treated active carbon for use in desulfurization reactions according to the present invention. In the heat-treated active carbon for use in desulfurization reactions according to the present invention, all or part of the hydrophilic oxygen-containing functional groups have been removed in the form of CO, CO
2
and the like as a result of the heat treatment, so that its surfaces are highly hydrophobic as compared with those before heat treatment. Consequently, the adsorption of SO
2
to SO
2
oxidation sites occurs easily and, moreover, the discharge of the resulting sulfuric acid proceeds rapidly. Thus, it can perform a catalytic functio
Kasuh Takahiro
Kobayashi Norihisa
Mochida Isao
Setoguchi Toshihiko
Yasutake Akinori
Foley & Lardner LLP
Hendrickson Stuart
Mitsubishi Jukogyo Kabushiki Kaisha
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