Method of removing dioxins in a waste incineration plant

In a method of removing dioxins in a waste incineration plant for cooling combustion gases from an incinerator by means of a first and/or second heat exchangers, and for exhausting the gases into the atmosphere through a dust collector of a bag-filter. The method comprises the steps of controlling a temperature of the gases right after the first heat exchanger within a range of 200.degree. C..about.140.degree. C., by use of the first heat exchanger, and also controlling a pressure loss of the gases right after the first heat exchanger within a range of -20 mmH.sub.2 O.about.-60 mmH.sub.2 O.

BACKGROUND OF THE INVENTION 
(1) Field of the Invention 
The present invention relates to a method of dioxins in a waste 
incineration plant which treats by way of incineration a large amount of 
waste such as general garbage typified by municipal waste. 
(2) Prior Art 
Change of life style due to economical growth or concentration of 
population in cities, general garbage typified by municipal waste has 
increased greatly. For treating the waste, a waste incineration plant is 
built and designed not only for incinerating the waste, but also for 
utilizing waste incineration heat for producing steam in order to use the 
steam directly or for an electric generator. 
However, environmental pollution caused by harmful materials included in 
soot and smoke, particularly dioxins, has brought on a problem. To solve 
the problem, a dust collector is provided for all of the waste 
incineration plants. As the dust collector, a bag-filter type, for 
example, which is superior in an efficiency of dust collection, is 
becoming used more frequesntly. 
The waste incineration plant as mentioned above, is constructed as follows. 
Namely, the combustion gases in the incinerator is first cooled down to a 
temperature of about 400.degree. C. in a manner of water spray in a 
cooling chamber which is located at an exit of the incinerator, and 
further secondly cooled to a temperature of about 300.degree. C. by means 
of a first heat exchanger for exchanging the heat of the gases with the 
air for combustion for the incinerator by utilizing the exhaust gases. 
Furthermore, the gases are introduced to the dust collector after the 
gases are cooled at a temperature of about 200.degree. 
C..about.220.degree. C., considering heat resistance of the bag-filter of 
the dust collector, by means of a second heat exchanger, which is provided 
these days for preventing production of white smoke. Also, it is 
substantially the same manner that a boiler is used as the first heat 
exchanger. 
Before the dust collector, slaked lime is sometimes sprayed into the gases 
as one of the means for removing harmful gases, in order to have dioxins 
absorbed or adhered to the reacted compounds such as SOX, hydrogen 
chloride, etc., so that they may be filtered by the dust collector. 
It is known from a number of cases and experiments that dioxins are 
recompounded in the dust collector when the temperature of the gases at 
the entrance of the dust collector becomes about 300.degree. C., so that 
emission of dioxins is clearly observed even after the dust collector. 
From the fact, it is understood that it is preferable to control the 
temperature of the gases at the entrance of the dust collector to be about 
at 200.degree. C..about.220.degree. C., in order to prevent formation of 
dioxins. 
It becomes possible to control exhaust of dioxins to a large extent by 
provision of such a bag-filter type of dust collector, but it is the 
present situation that the guide lines at the end of 1990 of the Ministry 
of Health and Welfare, providing the exhaust value of dioxins to be less 
than 0.5 ng/m.sup.3, are not met. It is further expected these days to 
control the exhaust value to be less than 0.1 ng/m.sup.3, as in Europe, 
and therefore it is required to take severe measures of preventing 
production of dioxins. 
Research is ongoing in order to meet the above-mentioned guide lines, and 
such research is roughly divided into two fields. 
1 A research for increasing efficiency of removing dioxins by utilizing a 
bag-filter, namely a research on how to filter and remove produced dioxins 
efficiently. It is known that the bag-filter is the best as one of the 
means for removing dust and harmful materials. 
2 A research on formation itself of dioxins, coming to grips with the 
mechanism of formation or of recomposition of dioxins, in order to find 
how to control formation of dioxins. 
In the above-mentioned field of item 1 above, the research is directed to a 
capacity of bag-filter and in view of its limit the research is also 
directed to chemical materials such as active carbon or some chemical 
agents used in combination with active carbon, which materials utilize 
chemical reactions or adsorption. 
Further, in the above-mentioned field of item 2 above, this includes a 
fundamental problem of solving the mechanism of production of dioxins, but 
it is in a half way of solving the mechanism. Therefore, it is common 
knowledge in this field of the art that it still necessitates a lot of 
time to control production of dioxins in the direction of the field of 
item 2 above. 
In view of these circumstances, it is the actual state that the research is 
directed to the way of increasing the removing efficiency in order to 
immediately achieve the required removing efficiency in the 
above-mentioned guide lines. 
The inventor first recognized, while studying solutions along the 
above-mentioned two directions on the basis of the data of the actual 
waste incineration plants, that a dust collector of a bag-filter type was 
the best as a dust collector, considering some conditions such as an 
efficiency of combustion of a whole of the incineration plant and an 
efficiency of dust collection. Therefore, the inventor reached a 
conclusion that the research of removing dioxins should be continued on 
the basis of using the bag-filter type of dust collector which was already 
practiced in this field. 
Then, the inventor made an effort in continuing the research for using 
active carbon or chemical agents with the active carbon, utilizing 
adsorption or chemical reaction, but was not able to find the good result 
that met the above-mentioned guide lines for removing dioxins. 
In view of the circumstances, the inventor has taken a step in another view 
in order to elucidate the difficult mechanism of formation or recompound 
of dioxins, as referred to in the item 2 above, but not directly face the 
difficult theme. Namely, the inventor thought that he may find a way of 
breaking the impasse of the research so far by progressing the research in 
a viewpoint that under what kinds of conditions dioxins are formed. 
Fortunately, it is possible to collect the data and the experience from 
the several waste incineration plants which the inventor belongs to, and 
so the inventor has been engrossed in the the research on this basis. 
SUMMARY OF THE INVENTION 
An object of the present invention is to control discharge of dioxins to be 
minimum so as to meet the level of discharge required by the 
above-mentioned guide lines, without addition of chemical agents or 
provision of any special implements. 
To achieve the above object, the present invention has taken the following 
means. 
Namely, in a method of removing dioxins in a waste incineration plant for 
cooling combustion gases from an incinerator by means of a first heat 
exchanger for heating air for combustion, and for exhausting the gases 
into the atmosphere through a dust collector of a bag-filter type, the 
said method comprises the steps of controlling a temperature of the gases 
right after the first heat exchanger within a range of 200.degree. 
C..about.140.degree. C., by means of the first heat exchanger, and 
controlling a pressure loss of the gases right after the first heat 
exchanger within a range of -20 mmH.sub.2 O.about.-60 mmH.sub.2 O. 
In the present invention, it is preferable that the temperature of the 
gases right after the first heat exchanger is controlled within a range of 
170.degree. C..about.150.degree. C., by means of the first heat exchanger 
and also controlling a pressure loss of the gases right after the first 
heat exchanger within a range of -20 mmH.sub.2 O.about.-40 mmH.sub.2 O. 
In the present invention, where the combusion gases are cooled by a first 
heat exchanger for heating air for combustion and a second heat exchanger 
for heating air for preventing generation of white smoke, the said method 
comprises the steps of controlling a temperature of the gases right after 
the first heat exchanger within a range of 350.degree. 
C..about.300.degree. C., by means of the first heat exchanger, controlling 
a temperature of the gases right after the second heat exchanger within a 
range of 200.degree. C..about.140.degree. C., by means of the second heat 
exchanger, and controlling a pressure loss of the gases right after the 
first heat exchanger within a range of -20 mmH.sub.2 O.about.-60 mmH.sub.2 
O. 
In this case, it is preferable to control the pressure loss of the gases 
right after the first heat exchanger within a range of -20 mmH.sub.2 
O.about.-40 mmH.sub.2 O. 
Further, where the combustion gases from the incinerator is cooled by the 
first heat exchanger as a steam boiler, the method comprises the same 
steps of controlling a temperature of the gases, which gases go through 
the first heat exchanger within a range of 200.degree. 
C..about.140.degree. C., and also controlling a pressure loss of the gases 
right after the first heat exchanger within a range of -20 mmH.sub.2 
O.about.-60 mmH.sub.2 O. 
As the means for minimizing the pressure loss in comparison with the 
conventional method, the present invention may take a manner which 
minimizes a load of dust pile by using conventional means such as a 
shaking type, reverse air type or pulse jet type, and may construct the 
first heat exchanger to have a small pressure loss. 
According to the present invention, it has obtained a very high efficiency 
of removing dioxins by the dust collector of a bag-filter type by 
controlling the vapor phase of dioxins to be lowered in such a manner that 
the pressure loss and the temperature of the combustion gases right after 
the first (second) heat exchanger are set to be small values that are 
remote far away from those in the conventional art. 
Other advantages of the present invention will be apparent from the 
description of the embodiments with reference to the drawings. 
BRIEF DESCRIPTION OF THE DRAWINGS 
The drawings show embodiments of a method of removing dioxins in a waste 
incineration plant according to the present invention in which: 
FIG. 1 is a graph for explaining the method of removing dioxins in the 
waste incineration plant according to the present invention, 
FIG. 2 is another graph for explaining the method of removing dioxins in 
the waste incineration plant according to the present invention, 
FIG. 3 is still another graph for explaining the method of removing dioxins 
in the waste incineration plant according to the present invention, 
FIG. 4 is still another graph for explaining the method of removing dioxins 
in the waste incineration plant according to the present invention, and 
FIG. 5 is still another graph for explaining the method of removing dioxins 
in the waste incineration plant according to the present invention. 
FIG. 6 is a brief flow chart of an incineration plant according to the 
present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS 
The embodiments of the present invention will be described in detail with 
reference to the drawings. 
Considering an efficiency of removing dioxins by a dust collector of a 
bag-filter type, the inventor analyzed the data of a relationship between 
vapor pressure and temperature which have a great influence on the vapor 
phase of dioxins, since it is a very important element as to whether or 
not dioxins are existent in vapor phase. 
Analysis is made particularly on the data of the relationship between the 
vapor pressure and the temperature of the dioxins of tetra chloro diobenzo 
-p- dioxin, and the result is shown in FIG. 2. In expressing the result in 
FIG. 2, well known methods used in chemistry, both Calingart-Davis and 
Antonie methods are used , but only the Calingart-Davis method is adopted 
in this embodiment. 
Next, analysis is made on the data of an amount of dioxins removed from the 
conventional waste incineration plant in which the gas temperature at the 
entrance of the dust collector is set at 220.degree. C..about.225.degree. 
C., but its pressure loss of the dust collector is varied within a 
predetermined range. The result shows, as seen with the curve in FIG. 3, 
that the amount of removal is 100.about.200 ng/m.sup.3 N, and as seen with 
the curve at the right side in FIG. 1, that the pressure loss is -50 
mmH.sub.2 O.about.-60 mmH.sub.2 O. 
Now, the removal efficiency of dioxins measured in each of the three dust 
incineration plants A.about.C, which are actually under operation, is 
shown in the following tables. 
In this table, the "DX" stands for dioxin and dibenzofuran, and its 
concentration is shown in total. B.F. means the "bag-filter" of the dust 
collector and the "S.P. bef. B.F." has substantiall the same meaning of 
the static pressure before the bag-filter or the pressure loss at the 
entrance of the dust collector. The "Con." stands for concentration. 
The definition of sort of the vapor phase and the solid phase is that the 
solid phase is those which are caught by a cylindrical paper filter which 
may catch 99.9 percents of the dust having a diameter of more than 0.3 
.mu.m, and that the vapor phase is those which pass the filter. 
These expression and abbreviation are also cited in each of the following 
tables. 
Plant-A 
TABLE 1 
______________________________________ 
Con. of 
S.P. bef. Temp.bef. 
Con. of DX 
DX Per. of 
B.F. B.F. bef. B.F. after B.F. 
Removal 
______________________________________ 
DX in -30 mmAq .apprxeq.215.degree. C. 
143 1.6 98.9% 
solid 
DX in -30 mmAq .apprxeq.215.degree. C. 
130 117 10.0% 
gases 
______________________________________ 
Plant-A is provided with a single heat exchanger. 
Plant-B 
TABLE 2 
______________________________________ 
Con. of 
S.P. bef. Temp.bef. 
Con. of DX 
DX Per. of 
B.F. B.F. bef. B.F. after B.F. 
Removal 
______________________________________ 
DX in -40 mmAq 
solid -50 mmAq .apprxeq.150.degree. C. 
221 3.33 98.4% 
DX in -40 mmAq 
gases -50 mmAq .apprxeq.150.degree. C. 
880 85 90.0% 
______________________________________ 
Plant-C 
TABLE 3 
______________________________________ 
Con. of 
S.P. bef. Temp.bef. 
Con. of DX 
DX Per. of 
B.F. B.F. bef. B.F. after B.F. 
Removal 
______________________________________ 
DX in -50 mmAq 
solid -60 mmAq .apprxeq.147.degree. C. 
57 0.7 98.8% 
DX in -40 mmAq 
gases -60 mmAq .apprxeq.147.degree. C. 
250 58 76.8% 
______________________________________ 
Thus, it becomes possible to grasp the relationship among the predetermined 
temperature, the pressure loss of the bag-filter of the dust collector and 
the amount of removal of dioxins. 
Next, analysis is made on the relationship between the amount of removal of 
dioxins and a ratio of vapor phase to solid phase, which is very important 
in removing dioxins of vapor phase. From the standpoint that the removal 
efficiency of dioxins should be more than at least 90% calculation is made 
to find the level of the ratio of the vapor phase of dioxins at which the 
removal efficienct of 90% can be achieved. The relationship between an 
amount of removal of dioxins and a ratio of vapor phase to solid phase, is 
shown in the graph of FIG. 5. In short, where the ratio of vapor phase is 
within a range of 20%.about.80% (suitable removal range), the removal 
effficiency reaches about 90%. 
FIG. 5 shows the relationship between vapor pressure (mmAq) and a ratio of 
vapor phase in respective temperatures and static pressures. 
Then, the inventor studied and alnalyzed the results, and presumes that the 
removal efficiency of dioxins is very closely connected to the temperature 
of the gases at the entrance of the dust collector and the pressure loss. 
On the basis of the presumption, the inventor has tried to vary the 
construction of the first heat exchanger (as a heater for combustion air) 
to make it large-scaled, so as to gradually change the pressure loss of 
the conventional waste incineration plant to -20 mmH.sub.2 O.about.-60 
mmH.sub.2 O, and to increase a cooling efficiency of the gases to change 
the temperature thereof at the entrance of the dust collector to 
200.degree. C..about.140.degree. C. The test data are collected and shown 
by the curves in the left side of each of FIGS. 1 and 3. The result shows 
that it becomes possible to obtain a very high removal efficiency of 
dioxins. 
As shown in these test data, it is made clear that a very good removal 
efficiency can be obtained when the pressure loss at the entrance of the 
dust collector is set in a range of -20 mmH.sub.2 O.about.-40 mmH.sub.2 O, 
and the temperature of the gases is set in a range of 170.degree. 
C..about.150.degree. C. 
FIG. 6 is a brief flow chart of the incineration plant. The numeral 1 is an 
incinerator in which waste is burnt. The numeral 2 shows exhaust gases, 
which is generated in the incinerator 1. The exhause gases 2 are cooled by 
a first heat exchanger 3, which heats an air 9 for combustion in the 
incinerator in advance. The exhaust gases 2 after the first exchanger is 
introduced to a dust collector of a bag-filter type 6 and then discharged 
from a chimney 7. 
As shown with imaginary lines (two dotted lines), where a second heat 
exchanger 5 is provided for heating an air 10 in order to prevent 
generation of white smoke by using the heated air 10, the exhaust gases 2 
is utilized again after the first heat exchanger. In such a plant, a 
pre-heater may somtimes be provided before the second heat exchanger 5 for 
pre-heating the air by using the heated air 9. 
The numeral 8a and 8b show the points where the present invention picked up 
the data. 
The present invention will be modified and worked in accordance with the 
essentail idea of taking the values of the pressure loss and temperature 
which are referred to in the above mentioned embodiments. 
The foregoing relates to a preferred exemplary embodiment of the invention, 
it being understood that other variants and embodiments thereof are 
possible within the spirit and scope of the invention, the latter being 
defined by the appended claims.