WO1997040265A1

WO1997040265A1 - Process and device for treating waste gas

Info

Publication number: WO1997040265A1
Application number: PCT/EP1997/002070
Authority: WO
Inventors: Klaus Pochner; Willi Neff
Original assignee: Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V.
Priority date: 1996-04-23
Filing date: 1997-04-23
Publication date: 1997-10-30
Also published as: DE19616197C2; DE19616197A1

Abstract

A process is disclosed for treating waste gas by means of a dielectrically hindered gas discharge (barrier discharge) which burns across flat electrodes which form a reactor and between which the waste gas to be treated flows in the longitudinal direction. The invention is characterised in that in certain areas of the reactor a higher concentration of components from the waste gas is generated than in the remaining area of the reactor. A device for carrying out the process has at least one pair of mutually opposite flat electrodes (11, 13) which form an intermediate discharge space (70). The electrodes are connected to a voltage source (50), and so an electric field is generated in the discharge space. Moreover, in each pair of electrodes at least one electrode is coated with a dielectric (20) on its side that faces the discharge space. The waste gas to be treated flows through the discharge space. The device is characterised in that means (12) are provided for cooling one or several electrodes.

Description

METHOD AND DEVICE FOR TREATING EXHAUST GAS

DESCRIPTION

Technical field

The invention relates to a method for treating exhaust gas according to the preamble of claim 1 and a device for treating exhaust gas, in particular for carrying out the method according to the preamble of claim 1 1. The invention is used in all areas of technology in which flowing Exhaust gases to reduce pollutants have to be subjected to post-treatment, for example in automotive technology, in power plants or waste incineration plants.

State of the art

It is known that the chemical reactions in barrier discharges at atmospheric pressure do not take place homogeneously in the entire volume, but preferably take place in thin discharge channels which are statistically distributed over the electrode surface. In addition, an increasing efficiency of the reactions plentiful in the transverse direction to the electrodes in a loading _¬ of typically 100 microns thick before the dielectrics. There is a narrow area in front of the cathode dielectric in which the highest electric field strengths prevail and the highest average electron energy is thus achieved (R. Kling, R. Schruft, O. Wolf, M. Neiger: "Parametric studies and development more practical Simula _¬ tion models for excimer barrier discharges "BMFT final report 13N5990, 1994, M. Neiger: Abstract to" Incoherent plasma radiation sources "; Ratio DPG (VI 31 (1996) 701, M. Neiger." Incoherent plasma radiation sources, "lecture. , DPG spring conference Plasma physics, Rostock, March 21, 1996.). Further positive effects are expected from the interaction of molecules or radicals from the discharge with water or oil mist (cluster) and with particles in the exhaust gas (A. Czernichowski in: Workshop on Plasma Based Environmental Technologies; Berlin, December 7th) 1995.). Due to the flow profile and temperature profile as well as diffusion, however, a smaller concentration of these reactive centers is rather present in the narrow, particularly reactive area.

In the case of a corona discharge, it is known from DE 40 17 120 A1 to use an oil film running down on one of the electrodes. Due to the action of the pulsed discharge, the oil film is broken up into a fine mist which, in order not to increase the pollutant emission, then has to be laboriously removed from the exhaust gas stream again. In addition, degradation of the circulated oil by soot particles and exhaust gas condensate occurs after a short time, which is countered by drying the exhaust gas before the treatment, but which nevertheless requires frequent replacement of the oil. Finally, the type of discharge used for a corona discharge with a sharp-edged inner electrode, long distances and a pulsed power supply is significantly less efficient than the barrier discharge proposed here because of the strongly inhomogeneous electric field and the switching losses in the supply device.

DeNOx catalysts for the selective reduction of nitrogen oxides in oxygen-containing exhaust gas are also under development (G. Lepperhoff: "NOx reduction in DeNOx catalysts due to hydrocarbons in the exhaust gas from gasoline engines" in: Technische Aka¬ demie; Esslingen, 8. / December 9, 1994.), in which the addition of a chemical reducing agent to the exhaust gas is required in order to achieve good degrees of conversion. Ammonia, urea or fuel, ie unburned hydrocarbons, are proposed as reducing agents. In principle, these additives could also be used in combination with barrier discharges to increase efficiency. However, the expense of maintaining an exact dosage and the possible drastic increase in emissions in the case of imprecise regulation in transient operation are disadvantageous. Finally, the additives required - 3 -

even highly toxic substances, the release of which, including the release of a slight excess of starting material, must be avoided at all costs.

A device for plasma chemical conversion, in which specific configurations for cooling the discharge medium are used, can be found in WO 94/23185 A1. The temperature range of the equivalent of 65-150 ° C appears to be in a thermodynamically sensible, but hardly maintainable (full load!) Range. However, the type of discharge used for an arc discharge raises doubts about its effectiveness. The arc discharge as a thermal gas discharge heats electrons, ions and neutral particles equally to several thousand degrees, which already makes the proposed good heat dissipation through the wall necessary to prevent premature thermal destruction of the reactor. A possible improvement in efficiency due to the lower operating temperature becomes an academic question in view of the wasted energy expenditure for heating the gas. The specific setting of transverse temperature gradients in the discharge area is not apparent from the application.

Presentation of the invention

The invention is based on the object of specifying a method and a device for the aftertreatment of exhaust gas with which the pollutant conversion is improved and the energy efficiency is increased.

A solution for the method with which the exhaust gas is to be treated is specified with the features of patent claim 1. Advantageous further developments are specified in subclaims 2 to 10. A solution for the device for treating exhaust gas is specified with the features of patent claim 11. Advantageous refinements of the device according to the invention are specified in subclaims 12 to 23. For exhaust gas aftertreatment with electrical discharge, a barrier discharge is generated directly in the exhaust gas stream. This type of discharge also enables the generation of a thermal non-equilibrium state in the pressure range of approximately one atmosphere, which is important for the efficiency of the plasma chemical conversion. The discharge vessel, hereinafter called the reactor, consists of a narrow gap in which the discharge is ignited and two flat electrodes which delimit the gap. The gap can be closed off at the side, or the electrodes are rolled up to form a coaxial vessel. It is important that at least one of the electrodes on the side facing the discharge gas is covered by a dielectric barrier which gives the discharge its name. Every time a high-frequency high voltage rises, an electric field is created in the discharge space, which leads to the formation of streamer discharges. Due to the statistical nature of the ignition process, the discharge does not start at all points at the same time. The charge flowing through the streamer channels cannot flow to the electrodes via the insulating dielectric, and a surface charge builds up there, which shields the field applied from the outside in an environment around the streamer. In this area, ignition is no longer possible in the same voltage half-wave, so that the discharge consists of a mosaic of small channels. If the distance between the voltage half-waves is sufficient, the channels in the next half-wave ignite again statistically distributed over the surface and the discharge appears homogeneous on average over time.

Since in the described plasma chemical reactor the exhaust gas flows across a gap that is only a few millimeters wide, a flow profile is formed which has the highest speeds in the middle. As soon as the gas flow is in the turbulent regime, the velocity becomes constant in a wide central area, which is surrounded by a thin fluidized bed in front of both electrodes. At the same time, the gas heats up due to losses in the electrical discharge in the region of the channels, which in turn influences the course of the flow. Overall, however, the gas exchange in the boundary layer will take place more slowly than in the gas volume. If at least one of the electrodes is cooled, a temperature boundary layer is formed in addition to the flow boundary layer. This leads to an accumulation of reaction nuclei, electron attachment centers and reducing hydrocarbons, especially in the area in which the highest electron energy and thus the strongest stimulation of plasma chemical reactions prevail due to the field strength.

This shows a significant increase in pollutant conversion and energy efficiency, which is not solely due to the avoidance of corona losses. With a slight further increase in the cooling capacity at the electrode, the condensation of hydrocarbons begins. The optimum operating point is reached when the concentration of water, particles and hydrocarbons is locally increased by the temperature gradient in the discharge region, but without influencing the homogeneity.

The coaxial reactor described here is only an example of further possible configurations, where plate stacks or tube bundles, among other things, are used in the same way. The invention relates to all thermal or flow mechanical measures by means of which higher concentrations of certain substances are achieved in individual areas of the plasma reactor. Without increasing the raw concentration of these substances in the exhaust gas by changing the engine setting or by subsequent injection, a reductive area in otherwise oxidative exhaust gas can nevertheless be created locally, for example.

In this way, gas dynamic and thermal effects are used to set a more favorable local distribution of the reaction partners already contained in the exhaust gas. This means that the system does not require any potentially harmful additives and can nevertheless achieve a similar improvement in sales rate with high energy efficiency as known systems. Brief description of the drawings

The invention is described in more detail below in various exemplary embodiments and with reference to FIGS. 1-5c. Show it:

Fig. 1: Principle of dielectric discharge (barrier discharge)

Fig. 2: first embodiment of the device according to the invention

Fig. 3a: Velocity profile of a laminar flow

3b: Velocity profile of a turbulent flow

Fig. 3c: temperature profile

Fig. 3d: Profile of the maximum electric field strength

Fig. 4a: coaxial reactor according to the prior art in cross section and in

Fig. 5a in perspective

Fig. 4b: Coaxial electrode arrangement with a cooled outer electrode in

Cross section and in Fig. 5c in perspective

Fig. 4c: Coaxial electrode arrangement with a cooled inner electrode in

Cross section and in Fig. 5 b in perspective

Fig. 4d: Coaxial electrode arrangement with cooling fins on the outer electrode in cross section and in Fig. 5b in perspective Fig. 4e: System of several parallel reactors in a common

Housing in cross section and in Fig. 5e in perspective

embodiments

1 shows the basic structure for generating a barrier discharge in flowing gas. The reactor consists of a high-voltage electrode 10 and a ground electrode 30, at least one of which is electrically isolated from the gas space by a dielectric 20. The high-voltage electrode 10 is connected to an AC voltage source 50, not shown here; the gas to be treated flows through the gas space parallel to the electrodes (61, 62). When a sufficiently high voltage is applied, a gas discharge occurs which is formed at atmospheric pressure in the form of individual, homogeneously distributed filaments 71. These are spaced from each other by about the radius of their base 72 and are statistically generated on the entire electrode surface.

2 shows a first embodiment of the device according to the invention. A further wall 21 is applied tightly connected to the dielectric 20. The intermediate space 12 is filled with a cooling liquid, for example water, which is contacted via an electrode 11. Because of the conductivity of undistilled water and the high dielectric constant, it is sufficient if the electrode 11 lies loosely on the dielectric 20. Furthermore, the electrode 1 1 may have interruptions in the centimeter range without the homogeneity of the discharge 70 being disturbed. A wire mesh or (for round electrodes) a wire spiral meet these requirements. At the same time, the disturbing corona discharges 79 which occur in air in the case of high-voltage electrodes are prevented by the housing 21. The water filling 12 in the electrode can be replaced by means of lines (not shown here) and, if necessary, returned via heat exchangers. In the following FIGS. 3a and 3b, speed profiles for a laminar (FIG. 3a) and turbulent (FIG. 3b) flow between two plates (19, 39) are shown. It is noteworthy that the slightest gas exchange takes place at the edge. The turbulent flow has an approximately rectangular profile with an edge layer 65 which is exaggeratedly large here. The importance of the edge area is further emphasized by a temperature profile (FIG. 3c) of a reactor cooled on one side but heated homogeneously in volume 60 and by a profile of the maximum electric field strength (FIG. 3d). This reaches its maximum in a range of approximately 100 μm in front of the dielectric 20.

FIGS. 4a-4e (cross section) and 5a-5e (perspective) describe examples of reactors in coaxial geometry. A reactor according to the prior art (FIGS. 4a, 5a) is shown in section, wherein the ground electrode 30 can be used as shown as the outermost or as the innermost tube. Nonetheless, the dielectric 20 does not necessarily have to be attached on the inside and on the ground electrode. A filamented gas discharge 70 is again generated in the gap, which for simplification is not shown here and in the following drawings. The cooling of an electrode according to the invention by means of a water jacket can take place on the outside (FIGS. 4b, 5b) or on the inside (FIGS. 4c, 5c) or by a combination of both designs. In addition to a cooling liquid, passive cooling devices, such as, for example, ribs 31, can also be used on the outer ground electrode (FIGS. 4d, 5d). Modifications for the internal electrode and / or the high-voltage electrode, for example a heat pipe, were nevertheless tested. If several reactors 69 connected in parallel are required for greater gas throughput, they can also use a common water jacket 32 in an outer housing 35 (FIGS. 4e, 5e). Reference list

10 first or high voltage electrode

1 1 power electrode

12 water filling

15 cooled wall

19 any first wall

20 first dielectric

21 electrode housing

30 second or ground electrode

31 ground electrode with cooling fins

32 water filling

35 casing tube

39 any second wall 0 second dielectric, if present

50 high voltage generator

60 interior of the reactor

61 inflow side

62 outflow side

65 boundary layer of the flow 9 entire reactor (simplified)

70 Gas discharge area 1 filament 2 base 9 corona discharge Expectations

1. A method for treating exhaust gas by means of a dielectric barrier gas discharge (barrier discharge), which burns transversely between flat electrodes forming a reactor, between which the exhaust gas to be treated flows longitudinally, characterized in that a higher concentration of in individual spatial areas of the reactor Components from the exhaust gas is generated than in the rest of the reactor.

2. The method according to claim 1, characterized in that the higher concentration is generated by thermal measures.

3. The method according to claim 2, characterized in that in the region of the reactor where the highest average electron energy is reached, a lower temperature is set than in the rest of the reactor.

4. The method according to claim 2 or 3, characterized in that at least one of the electrodes is cooled by means of a cooling liquid and / or an air flow.

5. The method according to claim 4, characterized in that it is the exhaust gas of a vehicle engine and the air flow is caused by the headwind attacking a suitably shaped muffler. 6. The method according to claim 1, characterized in that the higher concentration is generated by fluid mechanical measures.

7. The method according to claim 6, characterized in that a Zyklonströmuπg is generated in a coaxial electrode system, which causes an enrichment heavy exhaust gas components in the area in front of the outer electrode.

8. The method according to claim 6, characterized in that the exhaust gas flows through one or more nozzles when entering the reactor.

9. The method according to claim 8, characterized in that the nozzles are set such that the exhaust gas stream is directed to cooled areas of the reactor, for example one of the electrodes.

10. The method according to claim 8, characterized in that the nozzles are adjusted such that the exhaust gas flow is directed in the direction of the area with the electrical discharge.

1 1. Device for the treatment of exhaust gas, in particular for performing the method according to one of claims 1 to 10, with at least one pair of flat electrodes which are arranged opposite one another so that they form a discharge space between them, which is connected to a voltage source are so that an electric field is generated in the discharge space, with at least one electrode on each pair of electrodes the side facing the discharge space is coated with a dielectric, and wherein the exhaust gas flows through the discharge space, characterized in that means (12, 31, 32) for cooling are provided for one or more electrodes (10, 1 1, 30) .

12. The apparatus according to claim 1 1, characterized in that a cooling liquid, for example water (12, 32) is provided for cooling, with which the electrodes (10, 1 1, 30) are acted upon.

13. The apparatus of claim 1 1 or 12, characterized in that one or more electrodes (10, 1 1, 30) are provided on the side facing away from the discharge space with cooling fins (31).

14. Device according to one of claims 1 1 to 13, characterized in that the electrodes are designed as plane-parallel plate electrodes or as a stack of a plurality of plate electrodes arranged one above the other.

15. The device according to one of claims 1 1 to 13, characterized in that the electrode pairs (10, 30) consist of a plate-shaped first electrode (30) and an opposite plate-shaped dielectric (20) that an electrically conductive structure as the second electrode ( 1 1) on the side of the dielectric (20) facing away from the discharge space, it is provided that this structure (1 1) is covered with a housing (21) and that the housing (21) is filled with coolant (12). 16. The apparatus according to claim 15, characterized in that it is in the electrically conductive structure (1 1) is a wire network.

17. The device according to any one of claims 1 1 to 13, characterized in that the pairs of electrodes consist of an inner tubular electrode (10) and an outer tubular electrode (30) arranged coaxially therewith.

18. The apparatus according to claim 17, characterized in that the inner tubular electrode (10) contains cooling liquid.

19. The apparatus according to claim 17 or 18, characterized in that the outer electrode (30) on the side facing away from the discharge space is provided with a housing (21) for receiving a cooling liquid.

20. Device according to one of claims 17 to 19, characterized in that the outer electrode (30) on the side facing the discharge space has a dielectric (40), the dielectric (40) being spaced from this electrode (30) and the intermediate space is filled with a conductive cooling liquid (32), for example undistilled water.

21. Device according to one of claims 17 to 20, characterized in that the outer electrode (30) is provided with cooling fins (31). 22. Device according to one of claims 17 to 21, characterized in that a plurality of electrode pairs (10, 30) are arranged parallel to one another in a common, preferably cylindrical, housing (35) and that the housing (35) with the cooling element liquid (32) is filled.

23. Device according to one of claims 1 1 to 22, characterized in that supply and discharge lines are provided for an exchange of the cooling liquid.