Method of removing dioxins in waste incineration plant

Abstract: A method of removing dioxins in a waste incineration plant for cooling combustion gases from an incinerator (1) by means of a first (3) and/or second (5) heat exchangers, and for exhausting the gases into the atmosphere (7) through a dust collector of a bag-filiter type (6). The method comprises the steps of controlling a temperature of the gases right after the first heat exchanger within a range of 200° C ∼ 140° C, by means of the first heat exchanger (3), and also controlling a pressure loss of the gases right after the first heat exchanger within a range of - 20 mmH 2 O∼ - 60 mmH 2 O.

Description: The present invention relates to a method of removing 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. Change of life style due to economy growth or concentration of population in cities, general garbage typified by municipal waste are 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 plant. As the dust collector, a bag-filter type, for example, which is superior in an efficiency of dust collection, is becoming used more frequently. 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 °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 ° 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° C ∼ 220° 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° 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° C ∼ 220° C, in order to prevent formation of dioxins. It becomes possible to control exhaust of dioxins to 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 3 , are not met. It is further expected these days to control the exhaust value to be less than 0.1 ng/m 3 , as in Europe, and therefore it is required to take severe measures of preventing production of dioxins. Researches are ongoing in order to meet the above-mentioned guide lines, and such researches is roughtly 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 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 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 took 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 thinks 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. 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. According to one aspect of the invention therefore there is provided a method of removing dioxins from waste gases produced by a waste incineration plant comprising passing said gases through a heat exchanger for heating air for combustion in an incinerator thereby cooling the waste combustion gases and thereafter exhausting said gases to the atmosphere through a bag filter to remove dioxins therefrom; characterised by passing said waste gases through heat exchange means to reduce the temperature of said waste gases to between 200-140°C; and controlling the pressure loss of said waste gases across the heat exchange means to within a range of -196.4 ∼ -589.2 N.m -2 (-20 ∼ -60 mmH 2 O). The waste gases may be cooled to a temperature in the range 200-140°C by a heat exchanger for heating air for combustion in the incinerator. Alternatively the waste gases may be cooled to a temperature in the range 350-300°C by a first heat exchanger for heating air for combustion in the incinerator, and are thereafter passed through a second heat exchanger for heating air for preventing the generation of white smoke thereby further reducing the temperature of said waste gases to a temperature within the range 200-140°C. Preferably, the waste gases are cooled to a temperature in the range 170-150°C Preferably, the pressure loss of the gases across the heat exchange means is controlled to within a range of -196.4 ∼ -392.8 N.m 2 (-20 ∼ -40 mmH 2 O). The heat exchanger for heating air for combustion may comprise a steam boiler. The dioxin removed from the waste gases may be 2. 3. 7. 8-TCDD. According to another aspect of the invention there is provided apparatus for removing dioxins from waste gases produced by a waste incineration plant; comprising heat exchange means for reducing the temperature of waste combustion gases to a temperature within the range 200-140°C, which heat exchange means comprise a heat exchanger for heating air for combustion in the incinerator, pressure loss controlling means for minimising pressure loss of said waste gases across said heat exchange means to within a range of -196.4 ∼ -589.2 N.m -2 (-20 ∼ -60 mmH 2 O), and a bag filter for removing dioxins from said waste gases. Said heat exchange means of the apparatus may comprise a first heat exchanger for heating air for combustion in the incinerator and reducing the temperature of waste combustion gases to a temperature within the range 350-300°C, and a second heat exchanger for heating air for preventing the generation of white smoke and further reducing the temperature of said waste gases to a temperature within the range 200-140°C. 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 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 combusion 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. 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 anotehr 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. 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 relationship between vapor pressure and temperature which have 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 2. 3. 7. 8 - TCDD, and the result is shown in Fig. 2. In expressing the result in Fig. 2, 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 ° C ∼ 225°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 ∼ 200 ng/m 3 N, and as seen with the curve at the right side in Fig. 1, that the pressure loss is -491 N.m -2 ∼ - 589.2 N.m -2 (50mmH 2 O∼ - 60mmH 2 O.) Now, the removal efficiency of dioxins measured in each of the three dust incineration plants A ∼ 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 substantially 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 percent of the dust having a diameter of more than 0.3µm, 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 S.P. bef. B.F. Temp.bef. B.F. Conc. of DX bef. B.F. Con. of DX after B.F. Per. of Removal DX in solid -294.6 N.m -2 (- 30mmAq) ≒ 215° C 143 1.6 98.9% DX gases -294.6 N.M -2 - 30mmAq ≒ 215° C 130 117 10.0% Plant-A is provided wtih a single heat exchanger. Plant-B S.P. bef. B.F. Temp.bef. B.F. Con. of DX bef. B.F. Con. of DX after B.F. Per. of Removal DX in solid -392.8 N.m -2 (- 40mmAq) (- 50mmAq) -491 N.m -2 ≒150° C 221 3.33 98.4% DX in gases -392.8 Nm -2 (- 40mmAq) (- 50mmAq) -491 N.m -2 ≒ 150° C 880 85 90.0% Plant-C S.P. bef. B.F. Temp.bef. B.F. Con. of DX bef. B.F. Con. of DX after B.F. Per. of Removal DX in solid -491 N-m -2 (- 50mmAq) (- 60mmAq) -589.2 N.m -2 ≒ 147° C 57 0.7 98.8% DX in gases -392.8N.m -2 (- 40mmAq) (- 60mmAq) -589.2 N.m -2 ≒ 147' 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.4. In short where the ratio of vapor phase is within a range of 20% ∼ 80% (suitable removal range), the removal efficiency 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 analyzed 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 combusion air) to make it large-scaled, so as to gradually change the pressure loss of the conventional waste incineration plant to -196.4 N.m -2 ∼ 589.2 N.m -2 (- 20 mmH 2 O∼ - 60 mmH 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 ° C ∼ 140° 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 efficinecy can be obtained when the pressure loss at the entrance of the dust collector is set in a range of -196.4 N.m -2 ∼ 392.8 N.m -2 (- 20 mmH 2 O ∼ - 40 mmH 2 O), and the temperature of the gases is set in a range of 170 °C ∼ 150°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 imaginal 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.

Claims: A method of removing dioxins from waste gases (2) produced by a waste incineration plant comprising passing said gases through a heat exchanger (3) for heating air for combustion in an incinerator (1) thereby cooling the waste combustion gases (2) and thereafter exhausting said gases to the atmosphere through a bag filter (6) to remove dioxins therefrom; characterised by passing said waste gases (2) through heat exchange means (3; 3, 5) to reduce the temperature of said waste gases to between 200-140°C; and controlling the pressure loss of said waste gases (2) across the heat exchange means (3; 3, 5) to within a range of -196.4 ∼ -589.2 N.m -2 (-20 ∼ -60 mmH 2 O). A method as claimed in claim 1, characterised in that said waste gases (2) are cooled to a temperature in the range 200-140°C by a heat exchanger (3) for heating air (9) for combustion in the incinerator (1). A method as claimed in claim 1, characterised in that said gases (2) are cooled to a temperature in the range 350-300°C by a first heat exchanger (3) for heating air (9) for combustion in the incinerator (1), and are thereafter passed through a second heat exchanger (5) for heating air (10) for preventing the generation of white smoke thereby further reducing the temperature of said waste gases (2) to a temperature within the range 200-140°C. A method as claimed in claim 2, characterised by cooling said waste gases (2) to a temperature in the range 170-150°C by means of said heat exchanger (3). A method as claimed in claim 1, claim 2 or claim 3, characterised by controlling the pressure loss of said gases (2) across said heat exchange means (3; 3, 5) to within a range of -196.4 ∼ -392.8 N.m -2 (-20 ∼ -40 mmH 2 O). A method as claimed in any preceding claim, wherein said heat exchanger (3) for heating air (9) for combustion comprises a steam boiler. A method as claimed in any preceding claim, wherein the dioxin is 2. 3. 7. 8-TCDD. Apparatus for removing dioxins from waste gases produced by a waste incineration plant; comprising heat exchange means (3; 3, 5) for reducing the temperature of waste combustion gases (2) to a temperature within the range 200-140°C, which heat exchange means comprise a heat exchanger (3) for heating air (9) for combustion in the incinerator (1), pressure loss controlling means for minimising pressure loss of said waste gases (2) across said heat exchange means to within a range of -196.4 ∼ -589.2 N.m -2 (-20 ∼ -60 mmH 2 O), and a bag filter (6) for removing dioxins from said waste gases (2). Apparatus as claimed in claim 7, wherein said heat exchange means comprise; a first heat exchanger (3) for heating air (9) for combustion in the incinerator (1) and reducing the temperature of waste combustion gases (2) to a temperature within the range 350-300°C, and a second heat exchanger (5) for heating air (10) for preventing the generation of white smoke and further reducing the temperature of said waste gases (2) to a temperature within the range 200-140°C.

Inventors: YOSHINO EIICHI, YOSHINO, EIICHI
Applicants: KAWASAKI GIKEN KK, KABUSHIKI KAISHA KAWASAKI GIKEN
Application year: 1997
Main classification: B01D53
IPC codes: B01D53