Gas separation: processes – Solid sorption – Moving sorbent
Reexamination Certificate
2001-02-06
2003-01-28
Smith, Duane S. (Department: 1724)
Gas separation: processes
Solid sorption
Moving sorbent
C095S133000, C096S150000
Reexamination Certificate
active
06511527
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method for treating an exhaust gas, in particular a dioxin-containing exhaust gas emitted from a waste incinerator. Dioxins are mainly generated from combustion of organic compounds in waste incinerators, such as municipal waste, medical waste, hazardous waste, and army stockpile (chemical agents).
FIG. 1
is a diagram illustrating a typical method for removing particulate matter (e.g., flyash) from exhaust gas produced by a waste incinerator. Burning waste (e.g., municipal waste) in an incinerator creates byproducts of (i) ash and (ii) exhaust gas and flyash, the former residing in the incinerator itself and the latter passing through the stack of the incinerator. It is standard operating procedure to flow the exhaust gas and flyash through a boiler to quench the exhaust gas and reduce the temperature thereof to a sufficiently low level so that a bag filter can be used to remove the flyash from the exhaust gas. The resultant exhaust gas is then passed through a scrubber and emitted to the environment through a stack.
It is well known that the incineration of municipal waste materials creates large volumes of organic compounds and hydrocarbons. These materials serve as precursors for various compounds, some of which are highly toxic. For example, aromatic compounds such as phenol or benzene, or chlorinated aromatic compounds such as chlorophenol or chlorobenzene, react in the presence of flyash to form dioxin, which is highly toxic. They are formed downstream of the combustion zone and decompose at temperatures only above 1200° C. The typical concentrations in the effluents from incinerators are in the range of 10-500 ng/Nm
3
.
Current regulations on dioxin emissions are complex, depending on the toxic equivalency of the actual compounds and O
2
concentration, and vary in different countries. Nonetheless, removal to well below 1 ng/Nm
3
is generally required. Since 1991, activated carbon adsorption has been widely adopted for dioxin removal from municipal and other waste incinerators in Europe and Japan.
It is believed that formation of dioxin in the presence of flyash is the result of a catalytic reaction wherein flyash is the catalyst. It is also believed that the catalytic reaction occurs when the temperature of the exhaust gas drops below 400° C., which typically occurs at a location between the boiler and the bag filter.
While it would seem logical to simply remove the flyash from the exhaust gas before the temperature of the exhaust gas drops below 400° C., and thus prevent the formation of dioxin in the first instance, there is no industrially practical method or apparatus for accomplishing such a goal. Accordingly, the industry has adopted various methods by which dioxin is removed from incinerator exhaust gas prior to being emitted to the environment through the stack of the incinerator.
The use of sorbent materials is the most common method for removing dioxin from incinerator exhaust gas. Sorbents are materials that adsorb or absorb dioxin or dioxin precursors, and examples of such sorbents include certain cements (JP 97-2678543 B), activated carbon and activated white clay (JP 92-87624 A and JP 96-243341 A), activated coke (JP 97-29046 A), silicates (JP 97-75719 A and JP 97-75667 A), and zeolites (JP 97-248425 A).
While it is most common to add such sorbents to the exhaust gas at an exhaust gas temperature of less than 400° C., to thereby sorb dioxin per se, another known method (EP 0 764 457) discloses adding sorbents to the exhaust gas at an exhaust temperature of greater than 400° C. to remove dioxin precursors from the exhaust gas. Subsequent to the sorption, the sorbent may be removed from the exhaust stream and heated to decompose the dioxin. Typically, this occurs above 600° C.
While all of the above-described methods are effective to remove dioxin from the exhaust gas to some degree, there are problems associated with each method. The main problem with using activated carbon-based sorbents is that they release dioxin at lower temperatures and thus have the potential to desorb prematurely and be emitted out of the incinerator stack. The dioxin-containing atmosphere resulting from the use of activated carbon thus needs to be managed to a level of precision too great to be of practical use in a large scale manufacturing process.
The problem with using other sorbents such as silicates and zeolites, for example, is that the desorption temperature of those materials is too close to the vaporization temperature of dioxin itself. Specifically, the vaporization temperature of dioxin is about 220° C., whereas the temperature at which dioxin desorbs from materials such as silicates and zeolites ranges from about 220° C. to 260° C. Sorption of dioxin is most effective when the dioxin is in a gaseous state, and the sorption efficiency of a sorbent depends largely upon how close the dioxin desorption temperature of the material is to the vaporization temperature of dioxin. Accordingly, the sorption efficiency of materials such as silicates and zeolites is relatively poor, because the desorption temperature of those materials is too close to the vaporization temperature of dioxin.
It would be desirable to provide a method for removing dioxin from incinerator exhaust gases without the problems of post-sorbtion treatment (associated with activated carbon) and of sorption inefficiency (associated with materials such as silicates and zeolites). Desirable is a sorbent which sorbs dioxin more strongly, and desorbs dioxin at a higher temperature (near or greater than the decomposition temperature of dioxin). To date, however, the industry has not provided any such method or sorbent.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method for removing dioxin from an exhaust gas that overcomes the above-discussed problems associated with the prior art methods.
In accordance with one embodiment of the present invention, carbon nanotubes are used as a sorbent to remove dioxin from an exhaust gas.
In accordance with another embodiment of the present invention, a method of removing dioxin from an exhaust gas includes the steps of introducing carbon nanotubes into a stream of the dioxin-containing exhaust gas, and sorbing dioxin on the carbon nanotubes.
The inventors discovered that carbon nanotubes sorb dioxin more strongly than activated carbon, and, thus, can be used effectively as a sorbent of dioxin contained in an exhaust gas. Additionally, since dioxin desorbs at a higher temperature from carbon nanotubes than from activated carbon, it is easier and more effective to deompose the dioxin during the post-sorption treating step to remove the dioxin form the sorbent material.
The inventors have discovered that carbon nanotubes are better sorbents for dioxin than activated carbon insofar as the nanotubes desorb dioxin at a higher temperature than the activated carbon. This provides for easier handling of the dioxin-containing exhaust and provides for more efficient post-sorption treatment of the dioxin-sorbed sorbent material. The desorption temperatures of carbon nanotubes and activated carbon, using the well-known temperature-programmed desorption (TPD) technique, are shown in Table I.
TABLE I
Heating rate (° C./min)
Sorbent
2
5
10
20
Desorption temperature
588
609
620
634
from carbon nanotubes (C.)
Desorption temperature
481
517
543
from activated carbon (C.)
The principal problem associated with the use of activated carbon is that of desorption of the dioxin prior to its decompostion. Table I demonstrates that, regardless of the heating rate, carbon nanotubes desorb dioxin at a higher temperature than that of activated carbon. Thus, when attempting to decompose the dioxin, carbon nanotubes do not require strict atmospheric control, or inordinately steep heating rates. The use of carbon nanotubes therefore is easier and less expensive in treating exhaust gas.
The carbon nanotubes used in the present invention may be either of the single-wall or multi-wall variety. The walls are typicall
Long Ruiqiang
Takahashi Tomonori
Yang Ralph T.
Burr & Brown
NGK Insulators Ltd.
Smith Duane S.
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