Furnaces – Process – Incinerating refuse
Reexamination Certificate
2000-02-10
2001-09-11
Ferensic, Denise L. (Department: 3749)
Furnaces
Process
Incinerating refuse
C110S208000, C110S210000, C110S245000, C110S295000, C110S345000, C110S213000, C075S414000
Reexamination Certificate
active
06286443
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method for treating combustibles by slagging combustion in which combustible wastes such as municipal wastes, refuse-derived fuel, solid-water mixture, plastic wastes, FRP wastes, sewage sludges, biomass wastes, automobile wastes, low-grade coal, or waste oil are combusted by a slagging combustion furnace or a combination of a gasification furnace and a slagging combustion furnace without generating dioxins, and at the same time ash content in the combustible wastes is recovered as glassy slag from which heavy metals are not eluted out.
Among the combustible wastes, the refuse-derived fuel (RDF) is produced by crushing and classifying municipal wastes, adding quicklime to the classified municipal wastes, and compacting them to shape. The solid water mixture (SWM) is produced by crushing municipal wastes, converting them into a slurry by adding water, and converting the slurry under a high pressure into an oily fuel by hydrothermal reaction.
BACKGROUND ART
For reducing a volume of sewage sludges, a technology in which the sludges are dried and then combusted at a high temperature in a slagging combustion furnace has been already put to practical use. Further, a technology in which combustible wastes are combusted without emission of toxic matter by a combination of a gasification furnace and a slagging combustion furnace is about to be put to practical use. The purpose of this gasification and slagging combustion system is to prolong landfill sites by converting ashes into slag, utilize slag which has been converted from ashes to pavement materials or the like, decompose harmful substances such as dioxins completely, and establish a combustion technology which is suitable for environmental conservation, has a simple structure and low plant cost, yet has the above-mentioned functions.
FIG. 6
shows an example of a conventional gasification and slagging combustion system. As shown in
FIG. 6
, the gasification and slagging combustion system comprises a constant feeder
1
, a fluidized-bed gasification furnace
2
and a swirling-type slagging combustion furnace
3
. The fluidized-bed gasification furnace
2
has an air chamber
5
at a lower portion thereof and the air chamber
5
has an air diffusion plate
4
at an upper portion thereof. A fluidized-bed
6
of silica sand is formed over the air diffusion plate
4
. A freeboard
7
is provided above the fluidized-bed
6
for preventing silica sand from being carried over and suppressing pressure fluctuations. On the other hand, the swirling-type slagging combustion furnace
3
has a primary combustion chamber
8
, a secondary combustion chamber
9
and a slag separation chamber
10
therein.
Silica sand is located over the air diffusion plate
4
in the fluidized-bed gasification furnace
2
, and air “b” supplied into the air chamber
5
is ejected upwardly from the air diffusion plate
4
to thus form the fluidized-bed
6
of silica sand over the air diffusion plate
4
. The silica sand comprises river sand having a diameter of about 0.5 mm.
Combustible wastes “a” supplied into the fluidized-bed gasification furnace
2
by the screw-type constant feeder
1
fall into the fluidized-bed
6
which is kept at a temperature ranging from 450 to 850° C., and are contacted with the heated silica sand and quickly pyrolyzed, thus generating gas, tar and fixed carbon. Then, these pyrolyzed substances are gasified by being contacted with oxygen in air “b”. In the meanwhile, the fixed carbon is gradually pulverized by oxidization and a stirring action of the fluidized-bed.
Air “b” is blown into the freeboard
7
of the fluidized-bed gasification furnace
2
, if necessary, and hydrocarbon, tar and fixed carbon are partially combusted at a temperature ranging from 650 to 850° C. Large-sized incombustibles “d” are discharged together with silica sand from the bottom of the fluidized-bed gasification furnace
2
. The discharged incombustibles “d” contain metals such as iron, copper or aluminum. As the inside of the furnace is in a reducing atmosphere, metals can be recovered in a non-oxidized and clean condition. The discharged incombustibles and silica sand are separated from each other by a separating device (not shown), and the large-sized incombustibles are discharged to the outside of the separating device and the small-sized silica sand is returned to the fluidized-bed gasification furnace
2
.
The generated gas “c” discharged together with fixed carbon from the fluidized-bed gasification furnace
2
is supplied to the swirling-type slagging combustion furnace
3
, and they are mixed with preheated air “b” in a swirling flow and rapidly combusted at a high temperature ranging from 1200 to 1600° C. in the vertical primary combustion chamber
8
, and the secondary combustion chamber
9
inclined slightly with respect to the horizontal. The combustion reaction is completed in the secondary combustion chamber
9
. Because of the high temperature combustion, ash content in the fixed carbon is converted into slag mist which is mostly trapped by molten slag phase on an inner wall of the combustion chamber due to the centrifugal forces of the swirling flow. The molten slag “f” flows down on the inner wall and is discharged from the bottom of the slag separation chamber
10
. Thereafter, the molten slag “f” is cooled indirectly or directly, and is then discharged as granulated slag to the outside of the furnace.
On the other hand, the exhaust gas “e” discharged from the top of the slag separation chamber
10
passes through a series of heat recovery equipment or dust removing equipment (not shown), and is then discharged to the atmosphere. In this manner, 90% of ash content is discharged as the molten slag “f” and the remaining 10% of ash content is mostly collected as fly ash by a bag filter.
In the conventional system shown in
FIG. 6
, after the combustion reaction is completed in the secondary combustion chamber, the molten slag is discharged from the furnace, and hence the primary combustion chamber is in a reducing atmosphere and the secondary combustion chamber is in an oxidizing atmosphere. Since slag produced in the secondary combustion chamber is exposed to the oxidizing atmosphere, vaporization of heavy metals having a low boiling point from the slag is not sufficiently performed.
To be more specific, municipal wastes and plastic wastes which are typical combustible wastes contain a trace of heavy metals having a low boiling point, such as Hg, Cd, Pb, Zn, or As, and the inclusion of such heavy metals having a low boiling point into the obtained slag is inevitable in the conventional gasification and slagging combustion system shown in FIG.
6
. However, such heavy metals having a low boiling point entrapped in the slag are eluted out in an acid solution, and hence it is impossible to enclose the heavy metals having a low boiling point completely in the slag.
Further, in the complete combustion process in the slagging acombustion furnace, if wastes do not have a lower heating value of 2,000 kcal/kg or more, then auxiliary fuel is required. Therefore, there has been a need for lowering the heating value of wastes capable of being combusted independently. That is, there has been a need for such technology in which the lower limit of the heating value capable of operating the furnace without an auxiliary fuel can be lowered.
It is therefore an object of the present invention to provide a method for treating combustibles by slagging combustion which can obtain harmless molten slag whose content of heavy metals having a low boiling point is reduced to a level as low as possible, and can treat wastes without any auxiliary fuel even if the wastes have a low heating value.
According to a first aspect of the present invention, there is provided a method for treating combustibles by slagging combustion, characterized in that: combustibles and oxygen-containing gas are supplied to a slagging combustion furnace and the combustibles are partially oxidized in a reducing atmosphere to obtain combustibl
Fujinami Shosaku
Hirose Tetsuhisa
Oshita Takahiro
Ebara Corporation
Ferensic Denise L.
Rinehard K. B.
Wenderoth , Lind & Ponack, L.L.P.
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