Compositions – Co – s – negative element – or acid – bindant containing
Reexamination Certificate
1999-11-24
2002-03-05
Anthony, Joseph D. (Department: 1714)
Compositions
Co, s, negative element, or acid, bindant containing
C423S421000, C423S422000
Reexamination Certificate
active
06352653
ABSTRACT:
The present invention relates to an acid component-removing agent for a gas, a method for producing it and method for removing acid components from a gas.
It has been known to use slaked lime as an acid component-removing agent, to absorb and remove hydrogen chloride or a sulfur oxide in an exhaust gas discharged from e.g. a refuse incinerator. In such a case, slaked lime is dispersed in an exhaust gas discharge path from the incinerator at a region having temperatures of from 150 to 300° C., followed by collection by a bag filter or an electrical dust collector, whereby acid components are removed.
However, it is necessary to use slaked lime in an excessive amount of from 3 to 4 equivalent amount to reaction equivalent, whereby the amount of dust to be wasted will increase. Further, the dust is solidified by means of concrete, and the solidified dust is accounting for the dumping site for final disposal. The reaction product of hydrogen chloride and slaked lime is a calcium chloride which is water-soluble, whereby the reaction product can be removed by dissolving in water. However, some of slaked lime excessively added will form quick lime which is water-insoluble, and accordingly only a small amount of effects of reducing dust by water will be obtainable. Further, calcium scales will form during treatment of an aqueous calcium chloride solution at the final disposal site, thus causing troubles.
Further, it has been known to use sodium hydrogencarbonate as the acid component-removing agent instead of slaked lime. In such a case, unreacted sodium hydrogencarbonate will form sodium carbonate which is water-soluble, and accordingly it is effective to reduce dust. For example, EP0740577 discloses an acid component-removing agent comprising a composition containing sodium hydrogencarbonate in an amount exceeding 98 wt % and sodium carbonate in an amount of less than 2 wt %. It is disclosed that the mean particle diameter of the composition is at most 50 &mgr;m, preferably from 10 to 30 &mgr;m.
However, sodium hydrogencarbonate is expensive as compared with slaked lime, and it has thereby been desired to supply sodium hydrogencarbonate which has a high reactivity and presents effects with a small amount, with an industrial scale at a low cost. Further, it is desirable that the acid component-removing agent can be used without installation of a dust arrestor, it can be easily and stably injected to a gas to be treated, it can be well dispersed in the gas, the reaction rate is high, and a long-term preservation in a storage tank or a stock room at the operation site is possible.
Further, sulfur trioxide contained in an exhaust gas produced e.g. during operation of a boiler with fuel containing a sulfur content, will react with water vapor contained in the exhaust gas to form a sulfuric acid mist, which will form white smoke, violet smoke or white fume when discharged in the air, and will cause air pollution. Accordingly, in order to remove the sulfur trioxide component, it has been employed to preliminarily add a slurry having e.g. an oxide or hydroxide of calcium or magnesium dispersed in an organic solvent to the fuel, for prevention of sulfur trioxide formation, or to neutralize sulfur trioxide after combustion. However, with such methods, the additions are likely to deposit on a heat exchanger in the boiler, and when a large amount of the additives deposit thereon, the operation of the boiler will be hindered, and accordingly it is hard to use a large amount of the additives.
Further, to actively remove sulfur trioxide in a flue, it has been employed to inject a powder of the above-mentioned oxide or hydroxide or a slurry thereof, to the flue after the exhaust gas has passed through an air preheater. However, in the case where the powder itself is injected by this method, as a fine powder having a poor fluidity is discharged from the storage tank, and the powder is injected by means of e.g. a screw feeder as a positive-displacement powder transportation apparatus, quantitativeness will be poor, and no stable effect is likely to be obtainable. Further, in the case of injecting the slurry, the powder contained in the slurry is likely to deposit on and clog the transportation line for injecting the slurry, whereby the method can hardly be carried out stably.
As the acid component-removing agent for e.g. a refuse incineration plant, slaked lime has conventionally been used. Although slaked lime can be available at a low cost, there are problems in view of waste disposal, such as increase in water-insoluble dust to be wasted, or generation of calcium scales in the seepage water at the final disposal site. Further, in the case of using sodium hydrogencarbonate, as it is expensive, it is necessary to increase the reaction efficiency and to reduce the amount to be used. Further, when the amount to be used can be reduced, an equipped apparatus can be downsized, and the dust to be treated can be reduced.
On the other hand, in the case of using e.g. magnesium oxide to remove acid components in a boiler exhaust gas, such as sulfur trioxide or a sulfuric acid mist based on sulfur trioxide (hereinafter they will generically be referred to as SO
3
component), since magnesium oxide also has a low reaction efficiency, it is necessary to add an excessive amount of magnesium oxide. In such a case, unreacted magnesium oxide will remain in the flue, and may cause problems in post-treatment, since magnesium oxide has a low solubility in water. Further, it is difficult to quantify the injection amount of the powder, and accordingly it has been desired to develop an acid component-removing agent which can be sprayed on the flue stably and quantitatively.
Under these circumstances, it is an object of the present invention to industrially provide an acid component-removing agent which can effectively remove acid components such as hydrogen chloride, sulfur oxide such as SO
3
, nitrogen oxide or hydrogen fluoride from an exhaust gas, with which waste disposal can easily be carried out, and which can reduce the amount of waste.
The present invention provides an acid component-removing agent which comprises sodium hydrogencarbonate having a volume-based mean particle diameter of from 1 to 9 &mgr;m as measured by a laser diffraction and scattering method, and a method for producing it.
Of the acid component-removing agent of the present invention, the mean particle diameter of the sodium hydrogencarbonate is represented by the value of the volume-based mean particle diameter as measured by using a laser diffraction and scattering particle size analyzer. Hereinafter the mean particle diameter will be represented by the value as measured by this method, unless otherwise specified. In the present specification, the mean particle diameter is measured by using Microtrack FRA9220 manufactured by NIKKISO CO., LTD. For a supplementary examination, a volume-based mean particle diameter means the same as a mass-based mean particle diameter, because sodium hydrogencarbonate has a homogeneous density.
The acid component-removing agent of the present invention reacts with acid components present in a gas, and removes the acid components from said gas. The acid components to be removed are not particularly limited, and the acid component-removing agent of the present invention can be applied to various acid components such as hydrogen chloride, sulfur dioxide, sulfur trioxide, hydrogen fluoride and nitrogen oxides. Particularly when the acid component-removing agent of the present invention is applied to a compound containing chlorine such as hydrogen chloride, or to a SO
3
component, the reaction efficiency is high, and it is easy to handle it, as compared with a conventional one. Now, the present invention will be explained with reference to hydrogen chloride. However, the same applies to the other acid components.
By using sodium hydrogencarbonate, when sodium hydrogencarbonate particles are converted to sodium carbonate by calcination, the particles become porous, and effectively react with hydrogen chloride, whereb
Hirano Hachiro
Kusaka Makoto
Noda Hiroaki
Sakurai Shigeru
Tanaka Masaharu
Anthony Joseph D.
Asahi Glass Company Ltd.
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