Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Inorganic carbon containing
Reexamination Certificate
2000-07-07
2002-08-20
Langel, Wayne A. (Department: 1754)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Inorganic carbon containing
C423S24000R, C423S245300, C588S253000, C502S338000
Reexamination Certificate
active
06436868
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a process for treating a dioxin-containing exhaust gas and a composite catalyst composition for inhibiting the generation of dioxin, and more particularly, to a process for treating a dioxin-containing exhaust gas discharged from intermittent operation-type solid waste incineration facilities such as mechanical batch incinerators or semi-continuous incinerators as well as continuous operation-type solid waste incineration facilities, and a composite catalyst composition for inhibiting the generation of dioxin. Still more particularly, the present invention relates to a process for treating a dioxin-containing exhaust gas which can inhibit the generation of dioxin occurring either in the course of cooling of the exhaust gas between a downstream portion subsequent to a combustion chamber and a dust collector or due to a memory effect upon low-temperature combustion in flues or dust collector at the start-up or shut-down of intermittent operation-type solid waste incineration facilities, without large-scale incineration facilities or plant renovation and equipment investment, and a composite catalyst composition for inhibiting the generation of dioxin.
Upon waste disposal, various wastes such as municipal solid wastes or the like have been divided into reusable ones and the others. After the reusable ones are recovered as effective resources, remaining combustible wastes have been usually incinerated. Incinerators used for waste disposal are classified into four types according to its operation time a day (i.e., into mechanical batch incinerators, fixed batch incinerators, semi-continuous incinerators and full-continuous incinerators). In Japan, about 24% of these incinerators are of a large-scale continuous operation type. Whereas the waste incineration has been still conducted using intermittent operation-type incineration facilities such as mechanical batch incinerators or semi-continuous incinerators.
At present, dioxin contained in exhaust gas discharged from waste incineration facilities causes significant social problems. It is suggested that the dioxin is generated due to incomplete combustion in a combustion chamber or in the course of cooling between a downstream portion subsequent to the combustion chamber and a dust collector. The dioxin is extremely toxic and is considered to promote a carcinogenesis. Also, the dioxin is undecomposable in natural environment, and therefore, when being taken into human bodies through water or foods, the dioxin is accumulated within the human bodies. For these reasons, it is necessary to reduce the amount of dioxin generated to as small a level as possible.
In the continuous operation-type solid waste incineration facilities, wastes can be continuously incinerated in a combustion chamber thereof at an elevated temperature and, therefore, are readily subjected to complete combustion, so that the amount of dioxin generated in the combustion chamber can be limited to a low level. However, it has been reported that dioxin is generated in the course of cooling of exhaust gas between a downstream portion subsequent to the combustion chamber and a dust collector due to a so-called “de novo” synthesis (H. Huang et al., “Chemosphere”, 31, 4099(1995)). Even the continuous operation-type solid waste incineration facilities still fail to prevent the generation of dioxin in the course of cooling of the exhaust gas between a downstream portion subsequent to the combustion chamber and the dust collector to a sufficient extent.
Also, in the intermittent operation-type incineration facilities, dioxin tends to be generated in the course of cooling of the exhaust gas between a downstream portion subsequent to the combustion chamber and the dust collector like the continuous operation-type ones.
In particular, the intermittent operation-type incineration facilities are operated at predetermined time intervals. Accordingly, it takes several hours from the start-up until the incinerator can be operated in a steady state. At every start-up operation, there is caused a low-temperature combustion condition in which the dioxin tends to be generated. Also, when the incinerator is stopped, a part of the solid wastes is continued to smolder while causing incomplete combustion thereof. In such a case, when the incinerator is started up again, unburned substances caused due to incomplete combustion upon the shut-down of the incinerator on the previous day and upon the start-up thereof on the next day, still remain inside flues or dust collector of the incinerator. In consequence, there has been pointed out a disadvantage that such a so-called memory phenomenon that dioxin is re-synthesized and then discharged into an exhaust gas even though the temperature of the exhaust gas is as low as not more than 200° C., is caused and continued for several hours after the start-up of the incinerator (refer to Kawakami, Mtsuzawa and Tanaka, “Proceeding of the 5th Annual Conference of Japan Society of Waste Management Experts”, p. 264 (1994)). Thus, it has been reported that the intermittent operation-type incineration facilities generate a larger amount of dioxin as compared to that of continuous operation-type ones.
The amount of dioxin generated from waste incinerators has been legally strictly limited. The Ministry of Health and Welfare has planned to replace these intermittent operation-type incineration facilities with continuous operation-type ones hereafter. Further, according to “the Air Pollution Control Act” enforced on Dec. 1, 1997, the upper limit of amount of dioxin discharged from incinerators has been regulated. Unless the upper limit (new incinerators: 0.1 to 5 ngTEQ/Nm
3
, existing incinerators: 1 to 10 ngTEQ/Nm
3
, according to scales of incineration facilities) is reached within 5 years henceforth, the deficient incinerators must be scrapped.
However, due to the financial problems of local governments, etc., it is difficult to scrap the existing incinerators and construct news incinerators instead. For this reason, there has been studied a method of overcoming the regulation while continuously using the existing incinerators.
Therefore, it has been strongly required to provide a process for treating an exhaust gas so as to prevent the generation of dioxin without a large-scale incineration facilities or plant renovation and equipment investment.
Hitherto, as the prior arts relating to absorption or decomposition of dioxin generated upon incineration of the wastes, various methods have been reported. For example, there are known a method of decomposing poly-halogenated aromatic compounds having at least five carbon atoms by heating at a temperature of 200 to 550° C. in the presence of a catalyst such as iron oxide (Japanese Patent Publication (KOKOKU) No. 6-38863(1994)); a method of removing halogenated aromatic compounds or the like from an exhaust gas or reducing amounts thereof by heat-treating at a temperature of 300 to 700° C. in the presence of a catalyst containing iron oxide (Japanese Patent Application Laid-Open (KOAKI) No. 2-280816(1990)); a method of conducting the incineration of wastes under the coexistence of calcium carbonate particles and iron oxide particles in an incinerator (Japanese Patent Application Laid-Open (KOAKI) No. 8-82411(1996)); a method of adding a mixture of calcium hydroxide powder and activated carbon powder into exhaust gas flues in an incinerator (Japanese Patent Application Laid-Open (KOAKI) No. 11-63467(1999)); a method of introducing a blowing agent composed of calcium hydroxide and a porous inorganic oxide material (at least one material selected from the group consisting of clay minerals such as silicic acid, aluminum silicate, magnesium silicate, synthetic silicic acid, synthetic aluminum silicate, synthetic magnesium silicate, acid clay, activated clay, kaolin, bentonite, allophane and diatomaceous earth, and substances obtained by treating these clay minerals with acids to remove impurities such as aluminum and magnesium therefrom) into an exhaust gas pass
Fujii Yasuhiko
Hatakeyama Satoshi
Imai Tomoyuki
Inoue Hiroshi
Ishihara Masaki
Langel Wayne A.
Medina Maribel
Nixon & Vanderhye P.C.
Toda Kogyo Corporation
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