DE4432316A1 - Waste gas denitrification and removal of halogenated dibenzodioxine - Google Patents

Abstract

Purificn. of waste gas (I), esp. to remove NOx and/or halogenated dibenzodioxine and dibenzofuran cpds. (II) from (I) at under 300 deg.C, comprises passing the (I) stream into thermal regenerative oxidn. plant. This consists of at least two regenerators, each with layers of heat store material (III), which can be switched between heat-up, cool-down and purging phases. In each, at least one layer of commercially available (III) is replaced by dioxin decomposition catalyst (IV), which is catalytically bifunctional and also heat transferring, i.e. trifunctional. The plant used and (IV) are also claimed.

Description

The present invention relates to a method for purifying exhaust gases, especially for the removal of nitrogen oxides and / or halogenated dibenzodioxins and dibenzofurans from less than 300 ° C hot exhaust gases.

In many combustion processes, substituted dioxins, in particular 2,3,7,8-tetrachlorodibenzodioxin (TCDD) and halogenated dibenzofurans are formed in extremely small but toxicologically relevant amounts. Typical examples of this are combustion gases from waste, special waste, impregnated waste wood and waste paper as well as cable insulation. In fly ash from waste incineration plants, for. B. 2,3,7,8-tetrachlorodibenzodioxin found in a concentration of 10 ^-9 to 10 ^-10 grams per gram of fly ash.

The purification of halogenated exhaust gases containing dibenzodioxin and dibenzofuran among the legally permitted maximum value of 0.1 nanogram toxicity equivalents per dioxin Standard cubic meters encounter considerable procedural difficulties.

It is known to exhaust gases with adsorbents such as activated coke, activated carbon, Treat calcium hydroxide etc. However, the problem is not resolved solved, but only shifts the dioxin from the gas phase to the solid phase. The The result is contaminated adsorbents that are difficult to dispose of.

Also the attempt of thermal post-oxidation in post-combustion plants or Even regenerative afterburning plants do not lead to the required extent Success because a slight hatching and the formation of dioxins is not absolute be excluded.

PCT application WO 90/14560 describes a process for the thermal oxidation of organic carbon compounds in a thermally regenerative Afterburning plant known. The exhaust gas or exhaust air flow becomes here  alternately in at least two heat storage masses having regenerators warmed up and cooled. The heat storage masses consist of prismatic, in Layers of heat storage elements placed one above the other. In one above the Combustion chamber located regenerators, the compounds are oxidized. The Regenerators can choose between a heating, a cooling and a rinsing phase can be switched. Furthermore, the possibility is described by the use to remove nitrogen oxides (NOx) from the exhaust gas from reduction catalysts. A disadvantage However, the method described is that dibenzodioxins and dibenzofuran are not or can be removed from the exhaust gas only inadequately.

For the purification of halogen-dibenzodioxin and dibenzofuran-containing exhaust gases a temperature of over 300 ° C, have special, for the dioxin destruction optimized bifunctional catalysts proven.

These were specially developed and activated for the dioxin decomposition, from various Metal oxides - predominantly titanium dioxide - existing catalysts are bifunctional, that means that they are fully functional both as oxidation catalysts (e.g. for dioxin Oxidation and decomposition) as well as in the presence of reducing agents (e.g. NH₃) Reduction or DeNOx catalysts (e.g. for NOx reduction) can be used can.

Of the similarly composed, but different-acting DeNOx- Catalysts differ primarily from the bifunctional dioxin catalysts the additional pronounced ability of the dioxin decomposition, through its Composition and the significantly higher temperature level of 300-400 ° C, which for full effectiveness is required. In contrast to the usual DeNOx The bifunctional dioxin decomposition catalysts can also serve as catalysts can be used to remove NOx without adding ammonia.  

The subsequent connection of such special dioxin oxidation and decomposition catalysts for cleaning exhaust gases of 300 ° C and above is an effective and economic process.

With exhaust gas flows, the temperature of which is below 300 ° C, this method suffers considerably economic efficiency, since the exhaust gases then require additional energy required temperature level of at least 300 ° C must be raised. For large amounts of exhaust gas with low temperatures represent a huge cost factor Now, however, a large number of dioxin-containing gases occur at temperatures below 300 ° C.

Typical examples of this are all exhaust gases, which e.g. B. to remove Catalyst poisons in a scrubber before passing through the dioxin decomposition catalyst need to be treated. A replacement of these washers with pre-cleaning filters Activated carbon - as is done with conventional DeNOx catalysts - is the case of the bifunctional dioxin decomposition catalysts because of their higher Operating temperature not possible. DeNOx catalysts are already at gas temperatures effective of 160 ° C, i.e. a temperature with which exhaust gases the activated carbon Leave the pre-cleaning filter. However, dioxin decomposition catalysts show at 160 ° C still no noteworthy effectiveness regarding the decomposition of dioxins.

The present invention has as its object exhaust gases with a temperature accumulate below 300 ° C, economically from nitrogen oxides and / or halogenated Clean dibenzodioxins and dibenzofurans.

This object is achieved in that the exhaust gas flow in one known thermal regenerative oxidation system, which consists of at least two There are regenerators, each consisting of layers of heat storage masses have and switchable between a heating, a cooling and a rinsing phase are, by at least one instead of a layer of conventional heat storage Material built-in layer of catalytically bifunctional dioxin  Decomposition catalysts, which also have a heat storage function and are therefore trifunctional.

According to the method according to the invention, this layer is which reach operating temperatures between 300 and 400 ° C when the system is in operation, placed between the heat storage elements. So the gas passes through the layer with that Temperature at which the effect of the dioxin decomposition catalysts is optimal. In order to but also take over the otherwise catalytically bifunctional catalysts an additional function as a heat storage or heat transfer element, so that they appear tri-functional in a new way. Because with thermally regenerative Oxidation plants with the help of the heat storage and heat transfer elements of the Most of the heating energy (depending on the construction 95-98%) is recovered, the heating of the exhaust gases regardless of their Entry temperature can be carried out with a minimum of energy. Also far below 300 ° C hot exhaust gases can be extremely economical in the temperature range optimal effectiveness of the trifunctional dioxin decomposition catalyst and above to be brought.

After a special execution of the method according to the invention, the exhaust gases in the regenerators furthermore each via at least one layer of catalytic performed monofunctional DeNOx catalysts.

This layer is used to carry out the method according to the invention required system attached so that the exhaust gas flow the layer at the optimal Operating temperature of the DeNOx catalysts, which is between 169 and 290 ° C, happens. As a result, the DeNOx catalysts also act as heat stores or -transmitter and are therefore bifunctional.  

The exhaust gas stream advantageously passes through the layer of DeNOx catalytic converters Heating up in front of the layer of trifunctional dioxin decomposition catalysts and upon cooling after the layer of dioxin decomposition catalysts. Through this selected order, the effectiveness of the procedure can be increased.

In a further embodiment of the invention, the exhaust gas flow in the Cooling regenerator ammonia (NH₃) sprayed as a reducing agent. By spraying ammonia in the combustion chamber of the regenerative Oxidation plant generated nitrogen oxides (NOx) can be reduced as well as in one catalytically bifunctional dioxin decomposition catalyst or in one catalytically monofunctional DeNOx catalyst can be removed.

For this measure to be fully effective, it is necessary that the injection of Ammonia is carried out before the exhaust gas stream is used for cooling Regenerator the layer of trifunctional dioxin decomposition catalysts or has passed the layer of bifunctional DeNOx catalysts.

The method according to the invention thus finds a particularly effective removal halogenated dibenzodioxins originally present or newly formed and Dibenzofurans take place in three different areas of the facility: The first area is the Position (layer) of the trifunctional dioxin decomposition catalysts in the Heating regenerator tower, the second area is the combustion chamber and the third area the position (layer) of the trifunctional dioxin Decomposition catalysts in the cooling tower used for cooling.

This removal of the halogenated dibenzodioxins and dibenzofurans is dependent of possible NH₃ addition. With NH₃ addition is also a possibly existing NOx content of the raw gas is reduced.

A system for cleaning exhaust gases also serves to solve the problem, especially for the removal of nitrogen oxides and / or halogenated dibenzodioxins  and Dibenzofuranen from less than 300 ° C hot exhaust gases, which in a known manner as Thermally regenerative oxidation plant with at least three, between heating, cooling and Rinsing phase switchable regenerators and one above the regenerators located combustion chamber is designed and is characterized in that each at least one of the layers forming the heat storage masses of the regenerators tri-functional dioxin decomposition catalysts is built.

This at least one layer is advantageously placed in those plant areas, the temperatures between 200 and 450 ° C, preferably during operation of the system between 300 and 400 ° C. This measure ensures that the Exhaust gas flow the layer consisting of dioxin decomposition catalysts with that Temperature happens at which the catalysts unfold their optimal effectiveness.

Advantageously, 0.2-2 m³ per 1000 Nm³ / h exhaust gases to be cleaned, preferably 0.6 m³ of dioxin decomposition catalyst used. These The amount of catalyst is necessary in order to remove the pollutants mentioned as completely as possible to remove the exhaust gas.

In a further embodiment of the system according to the invention, the at least two regenerators one of the layers forming the heat storage mass replaced by a layer consisting of bifunctional DeNOx catalysts.

This at least one layer is preferably placed in those areas of the plant that are located at Operation of the system temperatures between 150 and 350 ° C, preferably between 160 and 290 ° C. This measure ensures that the exhaust gas flow layer consisting of DeNOx catalysts passes at the temperature at which the Catalysts develop their optimal effectiveness.

In particular, the at least one layer of dioxin is preferably Decomposition catalysts in the regenerators are higher than the at least one layer DeNOx catalysts attached.  

In a further embodiment of the system according to the invention, between the parts of the heat storage masses located in the upper part of the regenerators and the at least one made of dioxin in the lower part of the regenerators Decomposition catalysts existing layer, a free reaction space formed in which ammonia can be injected.

In particular, ammonia can be injected into the cooling regenerator before passing the layer of dioxin decomposition catalysts, so that it is ensured that the nitrogen oxides originating from the combustion chamber through the Ammonia can be reduced and made in the downstream layer trifunctional dioxin decomposition catalysts mostly from the exhaust gas stream can be removed.

In a further embodiment of the system according to the invention, between the at least one layer consisting of dioxin decomposition catalysts, and the at least one layer consisting of DeNOx catalysts, a free reaction space trained in which ammonia can be injected.

This device relates to those systems according to the invention that mention after preferred embodiment both with dioxin decomposition catalysts and with DeNOx catalysts are provided. After that coming out of the combustion chamber Exhaust gas in the regenerator for cooling the layer of dioxin Has passed decomposition catalysts, it becomes ammonia through this device loaded and in the downstream layer of DeNOx catalysts from the exhaust gas away.

The system according to the invention thus makes it possible in a new way originally existing and newly formed halogenated dibenzodioxins and Dibenzofurans are particularly effective in three different areas from the exhaust gas flow to remove; the first area is the position (layer) of the trifunctional dioxin  Decomposition catalysts in the regenerator tower used for heating, the second Area is the combustion chamber and the third area is the location (layer) of the trifunctional acting dioxin decomposition catalysts in the cooling Regenerator tower.

A catalyst for the decomposition of halogenated is also used to solve the problem Dibenzodioxins and dibenzofurans as well as of nitrogen oxides, which is characterized is that in addition to the installation in a thermally regenerative oxidation plant chemical effect has a heat-storing function.

This novel, trifunctional catalyst is particularly suitable effective way to remove halogenated dibenzodioxins and Dibenzofurans from exhaust gases.

This catalyst according to the invention advantageously consists of metal oxides, preferably titanium dioxide.

Three exemplary embodiments are described with reference to the drawings, in which 1 denotes a regenerator tower in the cooling phase, 2 a regenerator tower in the rinsing phase, 3 a regenerator tower in the heating phase, 4 a combustion chamber and 5 an additional burner.

Example 1 ( Fig. 1)

Each of the three regenerator towers 1 , 2 , 3 of a thermal-regenerative oxidation system is constructed in the following way:

On the floor grid 10 , 20 , 30 there are two layers 11 , 21 , 31 , each with 100 pieces of ceramic storage bodies with a honeycomb structure. Each storage body has a base area of 150 mm × 150 mm and a height of 300 mm. On these two layers, a layer 12 , 22 , 32 of 100 pieces of trifunctional-acting dioxin decomposition catalysts from BASF - totaling 0.675 m³ - of the same element size is layered. Above this catalyst layer there is a free space 13 , 23 , 33 with the dimensions 1500 × 1500 × 400 (height) mm, in which there is an ammonia injection device 14 , 24 , 34 . Above this free space, a second grate 15 , 25 , 35 made of a heat-resistant metal alloy is attached, on which three layers 16 , 26 , 36 , each with 100 pieces of ceramic storage bodies, are layered, which correspond in size and shape to those of the first two layers on the floor grate .

As is usual with thermal-regenerative oxidation systems, the three regenerator towers are connected by a common combustion chamber 4 . During operation, one regenerator tower is in the cooling phase (ie gas flowing through it is heated), one in the heating phase (ie gas flowing through it is cooled) and one in the rinsing phase.

1000 Nm³ / h raw gas at 20 ° C, which is contaminated with 1.2 g / Nm³ ethanol, 250 mg / Nm³ NOx and 0.8 nanogram toxicity equivalents dioxin / Nm³, is fed to the regenerator tower 1 which is in the cooling phase from below. The raw gas heats up as it passes through the preheated heat storage elements 11 , reaches the dioxin decomposition catalysts 12 , where the first dioxin decomposition takes place in a temperature range between 300 and 420 ° C.

The gas then continues to heat up in the three preheated layers 16 of heat storage elements, reaches the combustion chamber 4 and reaches a maximum temperature of 850 ° C., the majority of the organic compounds being oxidized. The pre-cleaned hot gas from the combustion chamber flows from top to bottom through the regenerator tower 3 which is in the heating phase and, after passing through the three-layer heat storage elements 36 , passes into the approximately 420 ° C. free space 33 between the upper grate 35 and the lower regenerator filling. Here ammonia gas is injected and distributed evenly. The gas loaded in this way then passes in descending order through the layer of the trifunctional dioxin decomposition catalyst 32 , where the majority of the dioxin and NOx is destroyed or converted, and, after passing through the last two layers 31, of ceramic storage bodies as pure gas at 42 ° C. the attachment.

The organic carbon content of the clean gas is 7 mg / Nm³, the NOx content 50 mg / Nm³ and the dioxin content 0.05 nanogram toxicity equivalents of dioxin. The third regenerator tower 2 is flushed with 80 Nm 3 / h of clean gas during this process in order to prevent untreated raw gas fractions from entering the clean gas stream in the switchover phase.

Example 2 ( Fig. 2)

As described in Example 1, the system is operated in a system which differs from that in Example 1 by the layer structure of the elements in the regenerators. On the floor grate 10 , 20 , 30 there is only one layer 11 , 21 , 31 of 100 pieces of ceramic storage body, on which a second layer 12 , 22 , 32 to 100 pieces of commercially available DeNOx catalyst elements, their effectiveness in a temperature range between 160 and 290 ° C, is piled up. Above this catalytically monofunctional and at the same time heat-transferring layer there is a free space 13 , 23 , 33 with a cross-sectional area of 1500 × 1500 mm and a height of 500 mm, in which the ammonia injection and distribution device 14 , 24 , 34 is installed. In addition, the second grate 15 , 25 , 35 is arranged on a heat-resistant metal alloy, on which a layer 16 , 26 , 36 of the tri-functional dioxin decomposition catalysts is placed. Three layers 17 , 27 , 37 (monofunctional) ceramic storage bodies are stacked on this layer.

The regenerator tower 1 , which is in the cooling phase, is supplied with 1000 Nm³ / h of raw gas at a temperature of 22 ° C and the following impurities: 0.7 g / Nm³ of benzene, 0.6 g / Nm³ of butyl acetate, 300 mg / Nm³ of NOx and 0 , 9 nanograms of toxicity equivalents dioxin / Nm³. The raw gas heats up to about 300 ° C. when it passes the first two heat storage element layers 11 , 12 and is evenly loaded with ammonia gas in the free space 13 between the DeNOx catalyst 12 and the upper grate 15 . Then it passes between 300 and 420 ° C, the layer 16 of the trifunctional dioxin decomposition catalysts, where the first dioxin decomposition and denitrification takes place.

The pre-cleaned gas heats up to about 820 ° C. as it rises through the last three layers 17 of the heat storage elements, passes through the combustion chamber 4 and flows through the regenerator tower 3 which is in the heating phase from top to bottom. After cooling in the first three layers of heat storage elements 37 , it reaches the layer of dioxin decomposition catalysts 36 at about 420 ° C., where the further dioxin destruction takes place. Then the gas flow passes the free space 33 between the upper grate 35 and simple DeNOx catalysts 32 , is loaded with NH₃ in this again, whereupon the remaining denitrification takes place in a temperature range of 170-310 ° C when passing through the DeNOx catalysts. After passing the last layer of ceramic storage bodies 31, the cleaned exhaust gas leaves the system at a temperature of 44 ° C.

The organic carbon content of the clean gas is 5 mg / Nm³, the NOx content is 25 mg / Nm³ and the dioxin content 0.03 nanograms of toxicity equivalents of dioxin.

Example 3 ( Fig. 1)

The procedure is as described in Example 1, but 950 Nm 3 / h of raw gas at 25 ° C. and the following pollutant content are fed to the regenerator tower 1 from below: 0.2 g / Nm 3 of isopropyl alcohol, 0.05 g / Nm 3 of benzene and 0.9 nanograms Toxicity equivalents dioxin / Nm³.

The clean gas leaves the system at 45 ° C and contains an organic carbon Proportion of 4 mg / Nm³ and 0.04 nanograms toxicity equivalents dioxin.

Claims (19)

1. Process for the purification of exhaust gases, in particular for the removal of nitrogen oxides and / or halogenated dibenzodioxins and dibenzofurans from less than 300 ° C hot exhaust gases, characterized in that the exhaust gas stream in a thermally regenerative oxidation system, which consists of at least two regenerators, each have heat storage measurements made up of layers and can be switched between a heating-up, a cooling-down and a rinsing phase, through at least one layer of catalytic bifunctional and at the same time heat-transferring, thus trifunctional-acting, dioxin decomposition catalysts installed instead of a layer of conventional heat-storing material. 2. The method according to claim 1, characterized in that the exhaust gas flow at Passing through at least one of trifunctional dioxins Decomposition catalysts existing layer a temperature of 200 to 450 ° C, preferably has 300 to 400 ° C. 3. The method according to claim 1 and 2, characterized in that the exhaust gas stream additionally in the two regenerators by at least one each from catalytic monofunctional and at the same time heat transferring, thus bifunctional, DeNOx catalysts existing layer is performed. 4. The method according to claim 1, characterized in that the exhaust gas stream at Pass the at least one of bifunctional DeNOx catalysts existing layer a temperature of 150 to 350 ° C, preferably 160 to 290 ° C, having. 5. The method according to claim 3 and 4, characterized in that the exhaust gas flow in the regenerator used for warming up first of all the bifunctional ones DeNOx catalysts consist of at least one layer and then that of trifunctional acting at least one layer passes through dioxin decomposition catalysts  as well as in the regenerator used for cooling, that of dioxin Decomposition catalysts consist of at least one layer and then that of DeNOx Catalysts happening at least one layer happens. 6. The method according to any one of claims 1 to 5, characterized in that in the Exhaust gas flow in at least one of the regenerators ammonia as a reducing agent is injected. 7. The method according to claim 6, characterized in that the injection of Ammonia in the cooling regenerator is carried out before the Exhaust gas flow the at least one layer of dioxin decomposition catalysts or the has passed at least one layer of DeNOx catalysts. 8. The method according to any one of claims 1 to 7, characterized in that halogenated dibenzodioxins and dibenzofurans located in the exhaust gas stream in three different areas, namely

• when passing through the layer of the trifunctional dioxin decomposition catalysts in the heating regenerator,
• - when passing the combustion chamber and
• - When passing through the layer of trifunctional dioxin decomposition catalysts in the cooling regenerator are removed from the exhaust gas stream.

9. Plant for the purification of exhaust gases, in particular for the removal of nitrogen oxides and / or halogenated dibenzodioxins and dibenzofurans from less than 300 ° C. hot exhaust gases, the exhaust gas stream being warmed up and cooled down alternately in at least two regenerators, each of which has layers of heat storage masses is characterized in that at least one of the layers ( 11 , 12 , 21 , 22 , 31 , 32 ) of the heat storage masses consists of catalytically bifunctional and at the same time heat-transmitting, thus trifunctional, dioxin decomposition catalysts ( 12 , 22 , 32 ). ( Fig. 1). 10. Plant according to claim 9, characterized in that the at least one layer of trifunctionally acting dioxin decomposition catalysts ( 12 , 22 , 32 ) is placed in those areas of the plant which operate temperatures of between 200 and 450 ° C, the thermally regenerative oxidation plant, preferably have 300 to 400 ° C. 11. Plant according to claim 9 and 10, characterized in that each regenerator ( 1 , 2 , 3 ) per 1000 Nm³ / h exhaust gas to be cleaned 0.2-2m³, preferably 0.6 m³ dioxin decomposition catalyst are present. 12. Plant according to one of claims 9, 10 or 11, characterized in that in addition in the at least two regenerators ( 1 , 2 , 3 ) each have at least one of the layers ( 11 , 12 , 21 , 22 , 31 , 32 ) of the heat storage masses consists of catalytically monofunctional and at the same time heat-transferring, thus bifunctional, DeNOx catalysts ( 12 , 22 , 32 ). ( Fig. 2). 13. Plant according to claim 12, characterized in that the at least one layer of DeNOx catalysts ( 12 , 22 , 32 ) is placed in those areas of the plant which temperatures during operation of the thermally regenerative oxidation plant between 150 and 350 ° C, preferably 160 up to 290 ° C. 14. Plant according to claim 13, characterized in that in particular the at least one layer of dioxin decomposition catalysts ( 16 , 26 , 36 ) in the regenerator is mounted higher than the at least one layer of DeNOx catalysts ( 12 , 22 , 32 ) . ( Fig. 2). 15. Plant according to one of claims 9, 10 or 11, characterized in that between the parts located in the upper part of the regenerators ( 15 , 16 , 17 , 25 , 26 , 27 , 35 , 36 , 37 ) of the heat storage masses and each in the lower part of the regenerators, at least one layer consisting of dioxin decomposition catalysts ( 12 , 22 , 32 ), a free reaction space ( 13 , 23 , 33 ) is formed, into which ammonia can be injected. 16. Plant according to one of claims 12 to 14, characterized in that in each case between the at least one layer consisting of dioxin decomposition catalysts ( 16 , 26 , 36 ), and the at least one layer consisting of DeNOx catalysts ( 12 , 22nd , 32 ), a free reaction space is formed in which ammonia can be injected. ( Fig. 2). 17. Plant according to one of claims 9 to 16, characterized in that a removal of originally existing or newly formed halogenated dibenzodioxins and dibenzofurans in three different areas, namely

• - In the layer of the trifunctional dioxin decomposition catalysts ( 12 , 22 , 32 ) in the heating regenerator ( 1 , 2 , 3 )
• - in the combustion chamber and
• - In the layer of trifunctional dioxin decomposition catalysts ( 12 , 22 , 32 ) is provided in the cooling regenerator ( 1 , 2 , 3 ). ( Fig. 1).

18. Catalyst for the decomposition of halogenated dibenzodioxins and dibenzofurans as well as nitrogen oxides, characterized in that it is built into a thermal regenerative oxidation system in addition to the chemical effect a heat-storing Function. 19. Catalyst according to claim 18, characterized in that it consists of metal oxides, preferably titanium dioxide.