WO2001066233A1

WO2001066233A1 - Method and device for catalytically treating exhaust gas containing dust and oxygen

Info

Publication number: WO2001066233A1
Application number: PCT/EP2001/002612
Authority: WO
Inventors: Gurudas Samant; Gerd Sauter
Original assignee: Gurudas Samant; Gerd Sauter
Priority date: 2000-03-10
Filing date: 2001-03-08
Publication date: 2001-09-13
Also published as: CA2391710C; EP1244511A1; AU5217001A; CA2391710A1

Abstract

The invention relates to a method and a device for catalytically treating exhaust gas containing sulphur oxide and nitrogen oxides, dust and oxygen and emanating from chemical processes or the combustion of fossil secondary fuels or fossil fuels from power plants, such as waste or slurry, or from glass and cement works. The exhaust gas is freed from sulphur oxides and nitrogen oxides in a reactor having a solid catalyst and at a temperature ranging from 200 °C to 500 °C and in the presence of and/or by adding one or more mediums selected from free oxides, carbonates and hydroxides of calcium, magnesium, sodium and potassium.

Description

METHOD AND DEVICE FOR THE CATALYTIC TREATMENT OF EXHAUST GASES CONTAINING SULFUR AND NITROGEN

description

In chemical processes or in the combustion of fossil z. For example, power plants or secondary fuels - such as gauze or sewage sludge or in glass and cement production - give off gases that contain sulfur and nitrogen oxides in addition to other pollutants. In the professional world, the sulfur oxides (S0 ₂ and S0 ₃ ) are referred to as SO _x and the nitrogen oxides (NO, N0 ₂ and N ₂ 0) as NO _x . Sulfur and nitrogen oxides are gaseous pollutants that act as environmental toxins and therefore from the exhaust gases must be removed before they are released into the atmosphere In recent years, considerable efforts have been made to reduce sulfur and nitrogen oxide emissions. Several processes are used in connection with the denitrification of exhaust gases. The currently most frequently used process is the SCR process (Selective Catalytic Reduction). Ammonia or compounds containing ammonium are introduced into the reaction chamber containing the catalyst and the nitrogen oxides in the flue gas are converted into nitrogen and water vapor , In connection with the SCR process, it is reported that sulfuric acid and ammonium hydrogen sulfate are formed in the case of SO ₂ -containing exhaust gases. The formation of sulfuric acid and ammonium hydrogen sulfate is undesirable since they cause considerable corrosion problems in the system parts which are connected downstream of the reactor usually with S0 ₂ -containing exhaust gases, separate desulfurization plants switched on before the SCR process, which works on the principle of Dry or wet flue gas desulphurization plant (REA) work. In the wet process, the exhaust gas is cooled and reheated for the subsequent SCR process, which is the case in most power plants and waste incineration plants. Such processes are associated with high costs and the formation of CaSO _{3 in} accordance with the reaction CaO + SO ₂ → CaSO ₃ cannot be avoided. The presence of CaSO ₃ in landfill materials is environmentally hazardous.

EP-A-0 671 201 describes a method for separating sulfur trioxide and for denitrification - in particular in waste incineration plants - whereby ammonia or ammonium-containing compounds in the flue gas stream before a heat exchanger package, preferably before the last heat exchanger package, or before the flue gas purification into the flue gas be introduced so that the catalytic denitrification of the dedusted flue gases is then operated in the low temperature range, in particular between 100 ° C. and 280 ° C. The aim is to reduce the SO ₃ concentration upstream of the SCR reactor by forming ammonium sulfate. The disadvantage of this process is that not only are ammonium sulfate aerosols formed, but also ammonium hydrogen sulfate, which is later deposited on the catalysts. The ammonium sulfate aerosols are difficult to dedust in downstream filter devices, so that the environment is considerably polluted. In addition, a separate scrubber is required for SO ₂ removal. The flue gas has to be heated up again after the scrubber, which cannot be done by heat exchange alone. Thus an additional firing, e.g. B. surface burner with natural gas required. Disadvantages are high investment and operating costs.

The production of cement includes the extraction and preparation of the raw materials, the burning of the raw material mixture consisting essentially of limestone, clay and silica to cement clinker in an oven and joint grinding of the clinker with ground materials, e.g. B. CaSO ₄ , to cement. In the Federal Republic of Germany, approx. 97% of cement clinker is burned in rotary kilns that are heated with natural gas, oil, coal dust or secondary fuels. In the flame area of the furnace with gas temperatures of 1,800 ° C to 2,000 ° C, firing material temperatures of 1,350 ° C to 1,500 ° C are reached, which are necessary for the formation of cement clinker. The raw meal is preheated either in the rotary kiln itself or in a separate preheater, which preferably consists of several cyclones arranged one behind the other. The hot exhaust gases from the rotary kiln flow through the cyclones from bottom to top. The raw meal is added to the exhaust gases before the top cyclone. The dust is separated from the gas in the individual cyclones and in again before the next cyclone suspended the gas flow. In this way, the raw meal is heated. After leaving the top cyclone, the exhaust gas still has temperatures of 300 ° C to 400 ° C and has a dust content of 50 - 100 g / m ³ _N , _r . With an exhaust gas quantity of, for example, approx. 100,000 m ³ _{N tr} / h and approx. 100 t / h raw meal feed, there are 5 t / h to 10 t / h dust in the exhaust gas, ie the dust separation in the cyclones is between 80% and 95%. The remaining heat content of the dust-containing exhaust gases is used in the grinding plant for raw meal drying. The dust from the filter system of the grinding plant is called raw meal and is the starting material for the clinker process. If the grinding system fails or comes to a standstill, the exhaust gas after the last cyclone is not passed over the grinding system, but cooled down via an evaporative cooler and then dedusted in a separate filter system. This means direct operation without utilizing the heat content in the grinding system.

At the high temperatures of the flame, sulfur and nitrogen oxides are formed in a considerable amount in the rotary kiln, which must be removed from the exhaust gases, since they act as environmental toxins. In addition, the exhaust gases can also contain carbon monoxide, hydrocarbons, halogen compounds, which must also be removed.

DE-A-196 35 385 provides a process for the denitrification of waste gases resulting from the production of cement, in which the waste gases, which have dust contents of more than 5 g / m ³ _{N f} , in a titanium oxide-containing catalyst with a hydraulic Diameters from 6.8 mm to 30 mm are used and these with an exhaust gas velocity of more than 3 m / s i. B. flow through. This is a denitrification of dust-containing exhaust gases from the cement industry using a titanium-containing catalyst. Such a method is also described in DE-A-296 15 192. In the proposal according to DE-A-196 35 385, the V ₂ O ₅ content in the catalytic converter is limited due to the formation of ammonium bisulfate if the SO ₂ content in the exhaust gas is high. The application EP-AI-0 534 087 A1 describes a process for removing nitrogen oxides from exhaust gases, in which alcohol is added to the exhaust gas before it enters the catalyst in order to reduce the conversion from SO ₂ to SO ₃ . The aforementioned prior art does not include the simultaneous catalytic decomposition of sulfur and nitrogen oxides contained in exhaust gases, but only the denitrification of exhaust gases with a low sulfur oxide content.

From WO-97/091 12 a system for cleaning the flue gases of a furnace, in particular a rotary cement kiln, is known, a reduction catalyst with injection-side injection of reactant immediately after the furnace, that is, before Dedusting, is arranged. The reduction catalytic converter is arranged shortly after the furnace, ie in front of the cyclone heat exchanger, since the cyclone heat exchanger also acts as a dedusting apparatus in addition to being a heat exchanger. However, it can be seen from the drawing that the reduction reactor is not arranged immediately after the furnace, but after the cyclone heat exchanger, ie after the partial dedusting in the cyclones. In addition, the ammonia or urea metering takes place directly in front of the reduction reactor. Here, an even distribution of the ammonia is not guaranteed. A high ammonia slip is to be expected due to the streak formation of the ammonia in the reduction catalyst. In addition, this device is not suitable for the simultaneous separation of sulfur and nitrogen oxides.

The invention has for its object to develop a method for simultaneous desulfurization and denitrification without the formation of ammonium sulfate or ammonium bisulfate, in which the degradation of NO _x to N ₂ and H ₂ O and SO ₂ to CaSO ₄ in one and the same Reactor takes place.

This object is achieved in that the treatment of the exhaust gases containing sulfur and nitrogen oxides in the presence and / or with the addition of one or more substances selected from the group of free oxides, carbonates, hydroxides of calcium, magnesium, sodium and potassium in a reactor is carried out with solid catalysts and during the treatment the operating conditions of the gas flow in the free reaction space in accordance with Froude's key figures in the area

1 <3/4 - - ^ - <100 with - ^ - = Fr ²

9 - d _k ρ _k - ρ _g g - d _k

lie.

It means:

μ the relative gas velocity in m / s

For the Froude number p _g the density of the gas in kg / m ³ p _k the density of the solid particle in kg / m ³ d _k the diameter of the spherical dust particle in mg the gravitational constant in m / s ² Surprisingly, it has been shown that, despite the approximately stoichiometric procedure of the NHs / NOx ratios, a degree of denitrification of 95% to 98% and a degree of desulfurization of 80% to 90% can be achieved, with the formation of ammonium sulfate, ammonium hydrogen sulfate and Sulfuric acid is avoided. This advantage is based on the fact that the catalytic treatment not only converts NO _x to nitrogen and water vapor, but also converts SO ₂ to SO ₃ and incorporates it in the presence of free oxides, carbonates, hydroxides of calcium, magnesium, sodium and potassium becomes. The formation of ammonium sulfates, ammonium bisulfates or sulfuric acid is suppressed. These incorporated sulfates of calcium, magnesium, sodium and potassium can be very easily in a downstream filter system, e.g. B. a hose or electrostatic precipitator can be separated and used.

A preferred embodiment of the invention is the use of honeycomb and or plate catalysts which, in addition to titanium dioxide and tungsten, contain more than 0.5% by weight of vanadium pentoxide. This increases the catalytic conversions of NO _x SO _x . A particularly preferred embodiment of the invention is that the catalysts preferably contain 2% to 8% vanadium pentoxide. With this mode of operation, levels of denitrification and desulfurization of over 95% are achieved.

A further preferred embodiment is the treatment in the presence and / or with the addition of one or more substances selected from the group of free oxides, carbonates, hydroxides of calcium, magnesium, sodium and potassium, with an average grain size d ₅₀ between 5 μm and 100 microns. The sulfur oxides are removed very quickly with little consumption of additives.

Furthermore, the costs are preferably minimized by the treatment in the presence and / or with the addition of one or more substances selected from the group of free oxides, carbonates, hydroxides of calcium, since calcium compounds are more economical compared to alkali compounds. In the cement industry, due to the process, the calcium compounds are present in large quantities before dedusting before dedusting. So an arrangement of the reactor before dedusting d. H. between heat exchanger tower and grinding plant advantageous.

NH ₃ -releasing compounds, such as (NH ₄ ) ₂ SO ₄ , (NH ₄ ) ₂ CO ₃ , (NH ₄ ) HCO ₃ , (COONH ₃ ) ₂ H ₂ O, HCOONH ₄ , NH ₃ , NH ₄ , are used as reducing agents OH, H ₂ O-CO-NH ₂ , NH ₂ CN, Ca (CN) ₂ , NaOCN, C ₂ H ₄ N ₄ , C ₃ H ₆ N ₆ and NH ₃ -containing wastewater from photochemical plants, individually or in groups , before entering the catalytic reactor in gaseous, liquid or solid state at several points in the flue gas stream in the temperature range between 200 ° C and 1000 ° C.

A preferred embodiment consists in that NH ₃ -releasing compounds in the form of dilute aqueous solutions, preferably in the temperature range between 300 ° C. and 550 ° C., are introduced into the flue gas stream as the reducing agent. Here, the water vapor partial pressure in the reaction chamber is increased and the sulfur incorporation is thus improved.

A particularly preferred embodiment of the invention is the presence or addition of one or more substances, selected from the group of free oxides, carbonates, hydroxides of calcium, magnesium, sodium and potassium, in the flue gas stream before the addition of NH ₃ -releasing compounds. The formation of ammonium bisulfate, ammonium sulfate and sulfuric acid is completely suppressed.

The flow can flow from the top or from the bottom. A particularly preferred embodiment of the invention is that the reactor equipped with the catalysts can be flowed in alternately from above and from below. Due to this alternating flow, the reactor can easily be kept clean of dust-containing waste gases and the blockage of the channels by dust can be avoided. Furthermore, the lifetime of the catalysts can be increased by changing the flow of the reactor.

A further preferred embodiment of the invention is that the reactor equipped with a catalyst can be used in addition to the breakdown of sulfur and nitrogen oxides for the breakdown of halogen compounds, halogenated organic compounds, hydrocarbons and CO.

A preferred embodiment of the invention is that the reactor equipped with a catalyst for the decomposition of sulfur and nitrogen oxides in dust-containing exhaust gases in the chemical and metallurgical as well as in the cement and lime industry, in power plants and in waste incineration plants in the process flow in the temperature range between 200 ° C and 500 ° C can be used without additional preheating of the exhaust gas.

A further embodiment according to the invention is the device for the treatment of exhaust gas containing dust and oxygen. The drawings show examples of the device for carrying out the method, which are explained in more detail below. They show schematically: ^ Fig. 1 shows an arrangement of the device in the cement industry

^<• Fig. 2 shows a device for the cement industry

3 shows an arrangement of the device for power plants

^{® °} Fig. 4 A device for power plants

Fig. 1 shows the arrangement of the device according to the invention in a cement plant with a rotary kiln for clinker production. The SCR reactor with catalyst modules and dust blowers is arranged in the flow direction after the suspended gas cyclone heat exchanger, consisting of the interconnected cyclones Z21 to Z24. For the dosing of NH ₃ -releasing compounds, several points A, B, C, D and E are provided in the temperature range from 300 ° C to 1000 ° C. Dosing points A, B and C are preferred for dosing ammonia, ammonia solution or urea solution. When dosing NH ₃ -containing wastewater from phototechnical systems and other compounds of NH ₃ , the dosing points D and E are preferred. The calcium-containing raw meal is fed in between the cyclones Z1 and Z2. After the treatment in the SCR reactor, the exhaust gas is either passed to the chimney in the case of combined operation via the raw material mill and the dedusting device, or passed to the chimney in the case of direct operation via the evaporative cooler and the dedusting device.

Fig. 2 shows a device with gas flow from bottom to top, from top to bottom and alternately from below and from above.

For an alternate gas flow from below and from above during operation, some additional lines and flaps are provided, which can be seen in FIG. 2. When the exhaust gases are switched alternately from bottom to top, calcium compounds and exhaust gas containing NH ₃ are introduced from below via line A in the reactor and drawn off via line 5. Flaps No. M1, M4, M6 and M8 remain closed and flaps M2, M3, M5 and M7 are open. Thereafter, the exhaust gas is conducted via the WT blower in the case of combined operation via line C to the raw material mill and dedusting device to the chimney or in direct operation via line D to the evaporative cooler and dedusting device to the chimney. The flaps M9 and M10 interact with each other to shut off the combined or direct operation. If the gas flow is switched alternately from above, calcium compounds and exhaust gas containing NH ₃ are introduced into the SCR reactor from above via lines E and B via the lines E and B and discharged from below via lines A and F to the WT blower. Here remain the Flaps M1, M8, M5, M3, M4 and M6 open and flaps M2 and M7 closed. Thereafter, exhaust gas is conducted via the WT blower in the case of combined operation via line C to the raw material mill and dedusting device to the chimney, or in direct operation via line D to the evaporative cooler and to the dedusting device to the chimney.

In the event of a malfunction or the SCR reactor is shut off, the addition of NH ₃ -releasing compounds is stopped and discharged either to the raw material mill or to the evaporative cooler via a bypass, ie via line F via the WT blower. Flaps M2, M4, M6 remain open and flaps M3, M1, M8, M7 and M5 are closed. The cold air damper M9 is intended to control the exhaust gas temperature upstream of the SCR reactor.

In the case of a design with gas routing only from below, the line E and the flaps M1, M8 and M7 are superfluous and the device is therefore only provided with an SCR reactor, bypass line F and the flaps M3, M4, M5, M6. In connection with space and cost savings, two individual flaps can be equipped with a switch flap. In addition, the WT blower can be installed shortly after the cyclone heat exchanger, depending on space requirements and design.

The SCR reactor is provided, for example, with five catalyst layers with modules for SO _x and NO _x degradation and one catalyst layer with modules for hydrocarbon and carbon monoxide degradation. The number of catalyst layers can be changed depending on the proportion of SO _x , NO _x , hydrocarbons and carbon monoxide. The catalytic converter elements or catalytic converter modules are provided with wear protection or wear grids made of hard metal or ceramic on the gas side against erosion of dust-containing exhaust gases. With alternating gas flow from above and below, wear protection of approx. 5 - 20 mm is attached on both sides.

Furthermore, for the cleaning of the catalyst surface, dust blowers are provided for each catalyst layer on the gas side. With alternating gas flow during operation from above and below, the dust blowers are provided on both sides. The air for the dust blower is heated to approx. 250 ° C before entering the reactor.

Fig. 3 shows the arrangement of the device according to the invention for power plants between the boiler and air preheater. Additives, e.g. B. Ca (OH) ₂ , are added after the boiler and before the NHOH metering.

Fig. 4 shows the gas flow from below or from above or alternately from below and above analogous to the description in Fig. 2 for cement plants. Compared to In cement plants, the exhaust gas in power plants is led to the chimney after the SCR reactor via air preheaters and dedusting devices.

The method according to the invention is explained below using exemplary embodiments.

A system according to FIG. 2 is installed in a cement plant with an exhaust gas volume of 100,000 m ³ _{N tr} / h. Experiments with partial gas flows of 3,000 - 10,000 m ³ _{N tr are} carried out. Before being fed into the reactor, the raw gas has the following composition:

NO _x content (calculated as NO ₂ ) = 1 500 mg m ³ _{N tr}

SO ₂ content = 500 mg / m ³ _N , _r

Dust content = 8 000 mg / m ³ _N , _r

O ₂ content = 3.2 vol. %

Temperature in the reactor = 320 ° C

The density of the gas is calculated based on the gas composition. The dust content before entering the reactor (mainly CaO and Ca (OH) ₂ ) is 8,000 mg / m ³ _{N tr} . The determined grain density of the dust is 3.1 kg / m ³ . Based on these data and operating conditions, a gas velocity of 6.5 m / s is determined in accordance with Froude's key figures.

Honeycomb catalysts with different proportions of active components and the following specifications are used in the tests:

free opening area = 85%

Pitch = 11 mm

clear width of the channels = 10 mm

Wall thickness = 1 mm.

The content of active component (e.g. V ₂ O ₅ ) in the catalysts is 0.1%, 0.3%, 1%, 3% and 5%. Gaseous NH ₃ is added as a reducing agent before entering the reactor with a stoichiometer, ie a molar ratio NH ^ NO _x of 0.85.

For the tests, a steel grille made of stainless steel is attached to the module of the catalyst as wear protection before the dust-containing exhaust gas enters.

The gas components NO _x , SO _x , NH ₃ , CO, CO ₂ and H ₂ O are continuously measured before and after the reactor using a MCS-100 multicomponent analyzer.

The breakdown of the most important components depending on the content of active component V ₂ O ₅ is shown in the table below:

The results show that with the method according to the invention NO _x and SO _x are broken down when suitable operating conditions are set - gas flow and selection of the active components.

In the tests with 5% V ₂ O ₅ , the NH ₃ content in the exhaust gas is <1 mg / m ³ _{N. t} . _, , The analyzes of the dust after the catalyst show no formation of ammonium sulfate, ammonium hydrogen sulfate or CaS0 ₃ . The SO _x content is bound as CaSO. About that In addition, no dust deposits are found in the reactor or in the catalyst channels in these experiments.

In a further series of experiments, the operating conditions of the gas flow in the free reaction space are changed with the same gas composition, the same dust content and the same catalysts outside of the key figures according to the invention from Froude. It is found that the NO _x degradation decreased sharply at gas velocities of less than 4 ms and the pressure difference at the reactor increased. The result is a total blockage of the catalyst channels with dust.

Claims

claims

1. Process for the treatment of dust and oxygen-containing exhaust gases, which contain sulfur and nitrogen oxides, in a temperature range from 200 ° C to 500 ° C by means of reducing agents in a reactor equipped with a solid catalyst with flow channels, in which the free opening area of the catalyst is more than 50% and in which the channels of the catalyst have a hydraulic diameter of more than 2 mm, characterized in that

a) the treatment in the reactor is carried out in the presence and / or with the addition of one or more substances selected from free oxides, carbonates, hydroxides of calcium, magnesium, sodium and potassium,

b) during the treatment, the operating conditions of the gas flow in the free reaction space according to Froude's key figures in the area

1 <3/4 - - ^ - <100

9 - d _k A - A,

With

can be set.

2. The method according to claim 1, characterized in that honeycomb and / or plate catalysts are used in the reactor which, in addition to titanium dioxide and tungsten, contain more than 0.5% by weight, preferably 2-8%, of vanadium pentoxide.

3. The method according to any one of claims 1 and 2, characterized in that the treatment in the presence and or with the addition of one or more substances selected from free oxides, carbonates, hydroxides of calcium, magnesium, sodium and potassium, with an average grain size d ₅₀ between 5 μm and 100 μm is carried out.

4. The method according to any one of claims 1 to 3, characterized in that the treatment of the exhaust gas is preferably carried out in the presence and or with the addition of one or more substances selected from free oxides, carbonates, hydroxides of calcium.

5. The method according to any one of claims 1 to 4, characterized in that as reducing agent NH ₃ -releasing compounds (NH ₄ ) SO ₄ , (NH ₄ ) ₂ CO ₃ , (NH ₄ ) HCO ₃ , (COONH ₃ ) ₂ H ₂ O, HCOONH ₄ , NH ₃ , NH ₄ OH, H ₂ O-CO-NH ₂ , NH ₂ CN, Ca (CN) ₂ , NaOCN, C ₂ H ₄ N ₄ , C ₃ H ₆ N ₆ or NH ₃ -containing wastewater from photochemical plants, individually or in groups.

6. The method according to claim 5, characterized in that the NH ₃ -releasing compounds are introduced into the flue gas stream in the temperature range between 200 ° C and 1000 ° C in the gaseous, liquid or solid state before the exhaust gases enter the reactor.

7. The method according to any one of claims 5 and 6, characterized in that the NH ₃ - releasing compounds in the form of dilute aqueous solutions in the temperature range between 300 ° C and 550 ° C are introduced into the flue gas stream.

8. The method according to any one of claims 1 to 7, characterized in that the presence or addition of one or more substances selected from free oxides, carbonates, hydroxides of calcium, magnesium, sodium and potassium in the flue gas stream preferably before the use of NH ₃ - releasing compounds takes place.

9. The method according to any one of claims 1 to 8, characterized in that the reactor equipped with the catalyst is flowed from above or from below.

10. The method according to any one of claims 1 to 9, characterized in that the reactor equipped with the catalyst is alternately flowed from above and below.

11. The method according to any one of claims 1 to 10, characterized in that the reactor equipped with the catalyst is used in addition to the breakdown of sulfur and nitrogen oxides for the breakdown of the halogen compounds, halogenated organic compounds, hydrocarbons and CO.

12. The method according to any one of claims 1 to 1 1, characterized in that the reactor equipped with the catalyst for the decomposition of sulfur and nitrogen oxides in dust-containing exhaust gases in the chemical and metallurgical as well as in the cement and lime industry, in power plants and in Waste incineration plants are used in the process flow in the temperature range between 200 ° C and 500 ° C without additional preheating of the exhaust gas.

13. Device according to FIGS. 1 and 2 for the treatment of dust and oxygen-containing exhaust gases from a cement plant, which contain sulfur and nitrogen oxides, characterized in that the reactor equipped with catalyst after the cyclone heat exchanger in the exhaust gas stream (and before the raw material mill and before bypass to the evaporative cooler).

14. Device according to claim 13 corresponding to FIG. 2, characterized in that the addition of NH ₃ -releasing compounds preferably takes place in the area of the raw meal feed and / or shortly after the raw meal feed, preferably before the last cyclone in the heat exchanger.

15. The device according to FIG. 3 for the treatment of dust and oxygen-containing exhaust gases from a power plant, which contain sulfur and nitrogen oxides and halogen compounds, halogenated organic compounds, hydrocarbons and CO, according to one of claims 1 to 12, characterized in that the Catalyst-equipped reactor is arranged after the boiler in the exhaust gas stream and before the air preheater.