Hazardous or toxic waste destruction or containment – Containment – Solidification – vitrification – or cementation
Reexamination Certificate
2002-08-13
2004-07-06
Silverman, Stanley S. (Department: 1754)
Hazardous or toxic waste destruction or containment
Containment
Solidification, vitrification, or cementation
C423S24000R, C588S253000
Reexamination Certificate
active
06759565
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method for decomposing a chlorine containing organic compound contained in an exhaust gas and a catalyst used for the method. More specifically, the present invention relates to a method for decomposing a chlorine containing organic compound contained in an exhaust gas in which method a chlorine containing organic compound, which is typified by dioxin contained in an exhaust gas from a refuse incinerator, can efficiently be decomposed even at a low temperature, and relates to a catalyst used for the method.
BACKGROUND ART
In recent years, a minute quantity of chlorine containing organic compounds possessing an extremely strong toxicity, for example, dioxins such as polychlorinated dibenzodioxins and polychlorinated dizenzofurans, and copulaner PCB (polychlorinated biphenyls) (hereinafter the compounds are sometimes referred to as DXNS), in addition to poisonous substances such as nitrogen oxides (NOx), sulfur oxides (SOx), and hydrogen chloride (HCl) are contained in exhaust gases from incinerators burning municipal refuses or industrial wastes. Thus, it is desired to establish technology for removing those compounds, as environmental contaminants. Further, it has lately come to be known that DXNs act as endocrine disruptors (the so-called environmental hormones) and that the DXNs are accumulated in breast milks up to a high concentration and adversely affect to newborn children. Thus, emission control for DXNs is being further strengthened. Accordingly, the importance of the technology for reducing the contents of DXNs in exhaust gases is more increased and various researches and developments are being carried out in many fields.
As the technology for decomposing DXNs contained in exhaust gases, thermal decomposition methods in the presence of a catalyst, oxidative decomposition methods with oxygen, and a method in which the activity for decomposing DXNs is promoted by adding ozone or hydrogen peroxide (Laid-open Japanese Patent Publication No. Hei 7-75720) are known. Among them, catalytic decomposition methods such as the thermal decomposition methods conducted in the presence of a catalyst and oxidative decomposition methods with oxygen are becoming mainstreams. Especially, commercialization of the oxydative decomposition methods with oxygen are widely being planned since the oxidative decomposition methods are not only high in the performance of decomposing DXNs but also have a capability of denitrating DXNs in addition.
Decomposition reaction of DXNs contained in exhaust gases by the thermal decomposition or oxidative decomposition with oxygen described above proceeds according to the following equation (1) or (2), respectively:
Thermal decomposition reaction R—Cl (chlorine containing organic compound)→
m
H
2
+n
C+
p
HCl+R′—Cl (1)
Oxidative decomposition reaction with oxygen R—Cl (chlorine containing organic compound)+
k
O
2
→m
CO
2
+n
H
2
O+
p
HCl (2)
wherein m, n, p, and k are integers, and R and R′ represent skeletons of hydrocarbons.
DISCLOSURE OF THE INVENTION
As a result of the investigations conducted by the present inventors, it has first been found that there exists such a problem as the rate of the thermal decomposition reaction of the equation (1) described above is slow and thus a large quantity of a catalyst and a high temperature are required in order to obtain a performance necessary for commercially adopting the reaction. Also, it has been found that a rate of the oxidative decomposition reaction commercially adoptable can not be obtained whereas the rate of the oxidative decomposition reaction of the equation (2) with oxygen is higher than that of the thermal decomposition described above, and that a catalyst is deteriorated when SOx are contained in an exhaust gas.
An object of the present invention is to provide a method for decomposing a chlorine containing organic compound contained in an exhaust in which method a high decomposition ratio of dioxins can be obtained even at a low temperature, the amount of a catalyst to obtain a performance or capability necessary for commercially adopting the decomposition method can be reduced, and the effects by the SOx can be suppressed down to a minimum, and to provide a catalyst used for the method.
Another object of the present invention is to provide a method for treating a catalyst which was used for decomposing chlorine containing organic compounds (DXNs) (hereinafter a catalyst already used for decomposing DXNs contained in an exhaust gas to purify the gas is sometimes referred to as a used catalyst). More specifically, another object of the present invention is to provide a method in which the DXNs adhered to a used catalyst can efficiently be decomposed and removed, and after which method was completed, safe working conditions at the time of conducting routine checkups of or taking out a used catalyst can be secured and a used catalyst can safely and readily be dumped or recycled.
In order to achieve the objects described above, various studies were conducted by the present inventors on thermal decomposition methods of DXNs in the presence of a conventional catalyst and on oxidative decomposition methods of DXNs with oxygen to find that the thermal or oxidative decomposition methods have the following problems:
That is, first, a high reaction rate of decomposing dioxins can not be obtained unless a temperature at which the catalytic reactions are initiated is high and the reactions are conducted at a high temperature. Especially, the thermal decomposition reaction is slow in reaction rate, and a reaction rate commercially adoptable can not be obtained unless the reaction temperature is as high as 300° C. or higher. Whereas a decomposition ratio of DXNs can be increased if the reaction temperature was raised, a re-synthesis reaction of dioxins from hydrocarbons, carbon monoxide (CO), chlorine compounds and others contained in an exhaust gas, and a polychlorination (isomerization) reaction having a higher toxicity proceed, leading to the generation of dioxins as opposed to the intention of reducing an amount of dioxins.
Second, the use of a large quantity of a catalyst is necessary since both the thermal decomposition reaction and oxidative decomposition reaction with oxygen are slow in reaction rate. It becomes a heavy burden, to small and medium sized municipalities operating refuse incinerators, to use a large quantity of an expensive catalyst. Moreover, when the amount of a catalyst is increased, a risk of generating dioxins tends to increase. That is when a catalyst exists in an exhaust gas, the increase in the amount of the catalyst apparently causes an increase of decomposition ratio of dioxins, since the thermal decomposition or oxidative decomposition reaction with oxygen, and such a re-synthesis reaction of DXNs as described above occur at the same time, the rate of the decomposition reaction is higher than that of the re-synthesis reaction, and thus the differential rate between both reactions becomes the amount of dioxins to be reduced. On the other hand, however, a risk that dioxins are re-synthesized increases, and there exists a risk that large quantities of dioxins are generated when the catalyst was deteriorated.
Third, a problem that decomposition ratio of dioxins is likely to be affected by sulfur oxides (SOx) contained in an exhaust gas can be mentioned. That is, generation of SOx at the time of burning refuses or industrial wastes is inevitable. Especially, a catalyst tends lately to be used at a lower temperature in order to avoid the re-synthesis of dioxins described above, deterioration of the catalyst by SOx becomes more remarkable at such a low temperature region, and thus it is not easy to obtain a high dioxins decomposition ratio according to conventional technology in which sufficient countermeasures against SOx are not taken.
Accordingly, diligent investigations were further carried out by the present inventors on conditions for efficiently decompose DXNs
Fujisawa Masatoshi
Kato Yasuyoshi
Babcock-Hitachi Kabushiki Kaisha
Fay Sharpe Fagan Minnich & McKee LLP
Johnson Edward M.
Silverman Stanley S.
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