Chemistry: electrical and wave energy – Processes and products – Electrostatic field or electrical discharge
Reexamination Certificate
2000-09-26
2004-07-27
Mayekar, Kishor (Department: 1753)
Chemistry: electrical and wave energy
Processes and products
Electrostatic field or electrical discharge
C204S174000, C204S177000
Reexamination Certificate
active
06767434
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of treating substances by applying discharge plasma to a fluid containing the substances to be treated. The present invention also relates to an apparatus for carrying out such a substance treating method. Particularly, the present invention relates to a technique for decomposing, by discharge plasma, harmful or poisonous substances such as dioxins contained in waste gases emitted from burning systems into harmless substances or easily collectable substances.
2. Description of the Related Art
Various kinds of harmful substances are contained in waste gases discharged from city type incinerators and large scale industrial waste treating plants. Recently, not only NOx and SOx, but also dioxins have been classified as harmful substances. It is important to emit the waste gas into the atmosphere only after reducing the concentrations of these harmful substances below the allowable safety level.
Heretofore, many treating methods have been developed for reducing concentrations of harmful substances in waste gases. However, in the known methods, large scale treating equipment is required, a treating efficiency is relatively low, running costs are rather high, and maintenance is complicated. Therefore, the known treating methods could not be effectively used. For instance, in many burning systems, electric precipitation devices are used. However, it has been made clear that the electric precipitation itself might be a source generating harmful dioxins. Therefore, a bag filter has been utilized instead of the electric precipitation device. However, the bag filter has a short life time and its maintenance is rather complicated.
In order to remove or mitigate the above-mentioned demerits, there has been proposed a substance treating method in which harmful substances are reacted with electrons generated by corona discharge or dielectric barrier discharge and are transformed or converted into harmless substances or substances that could be collected easily.
FIG. 1
shows a known waste gas treating apparatus. An electrically conductive pipe
1
serving as a coaxial tubular reaction vessel is provided, a wire electrode
2
is arranged along a longitudinal axis of the pipe
1
and a pulse supply source
3
is connected across the pipe and the wire electrode to generate corona discharge within the pipe. Then, a waste gas is flowed through the pipe
1
and dioxins, NO
x
and SO
x
are reacted with radicals and accelerated electrons generated by the corona discharge and are decomposed into harmless substances.
FIG. 2
shows another known waste gas treating apparatus using the above-mentioned pulse discharge plasma. In this apparatus, an inner pipe
4
made of a dielectric material is arranged on an inner wall of an electrically conductive outer pipe
1
and a wire electrode
2
is arranged within the inner pipe
4
along its longitudinal axis. In this apparatus, an AC voltage supply source
5
is connected across the electrically conductive outer pipe
1
and the wire electrode
2
to generate the barrier discharge.
FIG. 3
is a perspective view depicting another known waste gas treating apparatus disclosed in Japanese Institute of Electrical Engineers Theses, Hiroyuki YASUI, “Waste Gas Treating Technique with Pulse Corona Discharge,” Vol. 119, No. 5, 1997. A plurality of plate electrodes
6
are arranged in parallel with each other, wire electrodes
7
are arranged between adjacent plate electrodes, and a pulse supply source
3
is connected across the plate electrodes
6
and the wire electrodes
7
. A waste gas containing substances to be treated is flowed through spaces between successive plate electrodes
6
.
In the known substance treating method using the discharge plasma, the wire electrode is arranged along the longitudinal axis of a gas flow passage having a relatively large cross sectional area, and therefore, the discharge plasma could not be generated uniformly along the space through which the waste gas flows. For instance, in the known apparatus shown in
FIG. 1
, the discharge plasma is locally produced around the wire electrode
2
as depicted in
FIGS. 4 and 5
, and the plasma is scarcely generated near the periphery of the gas flow passage. When the discharge plasma is localized, a possibility of the reaction of given substances contained in the waste gas with electrons generated by the plasma might be decreased, and the treating efficiency becomes low. This problem also occurs in the known apparatus shown in
FIG. 3
using the plate electrodes
6
and wire electrodes
7
.
In the above explained known substance treating apparatuses, the pulse supply source or AC voltage supply source is connected across the two kinds of electrodes. In order to decompose dioxins contained in a waste gas, dioxins have to be reacted with electrons having substantially high energy. However, the known waste gas treating apparatuses could not efficiently generate electrons having high energy. That is to say, although an ordinary AC voltage supply source is connected across the electrodes, it is impossible to generate efficiently electrons having desired high energy.
When the AC voltage supply source is utilized, electrons having energy of about 1 eV are predominantly generated as shown by a curve A (HF plasma) in
FIG. 6
, but a density of electrons having energy higher than 5 eV becomes very low. In order to decompose dioxins efficiently, electrons having energy of about 3-10 eV are effective, but a density of such high energy electrons is low. Therefore, the known apparatus using the high frequency voltage supply source could not decompose dioxins efficiently.
Moreover, in case of using the pulse supply source, in order to decompose dioxins efficiently, it is necessary to generate electrons having energy of about 3-10 eV. To this end, a voltage pulse applied across the discharge electrodes must have a sharp or steep raising edge as well as a short pulse duration. To this end, one may consider to utilize a pulse supply source including a thyratron as an active element. The pulse supply source using the thyratron has sharp raising edge, short pulse duration and large discharge current as shown in FIG.
7
. However, the thyratron has several drawbacks such as large size, low power efficiency, high cost, short life time, large secular variation and cumbersome maintenance. Particularly, in the waste treating system, the power consumption is liable to be large due to the cathode heater and the cost for exchanging the thyratron having a short life time becomes high.
In order to overcome the above mentioned problems, it is desired to utilize a pulse supply source having a semiconductor element as a switching element, because the semiconductor element has a high power efficiency and a very long life time. As the semiconductor switching element, use may be made of a GTO (Gate Turn-off Thyristor) and an IGBT (Insulated Gate Bipolar Transistor). However, a GTO has a very slow raising edge and a long duration as illustrated in FIG.
8
. Furthermore, electrons having a desired energy level could not be generated efficiently unless a plurality of large scale circuits including magnetic compression circuits are connected in cascade. Although an IGBT can generates a pulse having a steeper raising edge and a shorter duration than a GTO, as shown in
FIG. 9
, its raising edge is not sufficiently sharp to generate electrons having high energy of about 3-10 eV with a high density, such electrons being necessary for decomposing dioxins in an efficient manner.
There has been also proposed to treat harmful substances contained in waste gases using materials having catalysis. However, the treating efficiency is not sufficiently high, and waste gases emitted from city type incinerators and industrial waste treating plants could not be purged effectively, because such waste gases contain harmful substances with very high densities.
Recently it has been proposed to treat harmful substances using materials such as TiO
2
having
Imanishi Yuichiro
Ishii Shozo
Shimizu Naohiro
Burr & Brown
Mayekar Kishor
NGK Insulators Ltd.
LandOfFree
Method of treating substance and apparatus for carrying out... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method of treating substance and apparatus for carrying out..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method of treating substance and apparatus for carrying out... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3219293