DE19510804A1 - Reduction of nitrogen oxide(s) in vehicle exhaust gas - Google Patents

Abstract

NOx present in the exhaust gases from combustion machines is reduced by introducing a selective gaseous reducing agent and bringing the mixture into contact with a catalyst. The reducing agent is converted to high pressure plasma with a marked formation of radicals before it comes into contact with the catalyst so that the contact reaction is accelerated. Also claimed is a device, in which the above process can be carried out, including a microwave source, a waveguide coupler and a cavity resonator.

Description

The invention relates to a method and a device for nitrogen oxide mining tion in exhaust gases from internal combustion engines, in particular for use in a motor vehicle.

The pro comes with post-engine nitrogen oxide reduction for diesel engines blemished that the well-known - regulated or unregulated - with Otto engines Three-way catalyst not due to the high air (oxygen) excess can be used.

The power plant denitrification (stationary application) is the selective one Catalytic reduction of nitrogen oxides with ammonia on special contacts ten (NH₃-SCR) known for mobile matters such. B. in a variant in which Ammonia is obtained from the hydrolysis of amines (e.g. urea) is portable. These amines also have an lower risk potential advantages.

In addition, methods are known in which environmentally friendly Reduk  tion means, e.g. B. hydrocarbons, by means of suitable contacts for selective Nitrogen oxide reduction can be used (KW-SCR).

Metal-doped zeolites, in particular, are used as catalysts their copper-containing materials and mixed oxides of types Pe rowskite, spinels and illmenite. The doping with copper also seems here to be of some importance.

The basic efficacy of the methods could be tested in laboratory tests be demonstrated, but neither is the more typical in diesel exhaust exhaust gas temperatures available for sufficient conversions give specific load change conditions so that they can be covered. The problem of contact deactivation is also not solved.

More recently, attempts have been made to make the insufficient Activity in KW-SCR processes through the use of hydrogen to improve as a selective reducing agent (H₂-SCR). However, it does Hydrogen also in the NH₃-SCR and KW-SCR (in the form of surfaces adsorbed radicals) play an important role, which is explained in more detail below is set out.

The reaction steps are not in any of the above SCR processes known in detail. However, it is quite likely that surfaces adsorbed radicals play an essential role.

a) The NH₃-SCR reactions

Ammonia is adsorbed at a suitable contact dissozia active in amine radicals and hydrogen atoms:

NH₃ → ^⚫ NH₂ + ^⚫ H (1)

Amine radicals react with NO molecules also adsorbed on the contact (not dissociative) after:

NH₂ + NO → N₂ + H₂O (2)

The chemical driving force for (2) is the formation of the stable N₂ molecule its triple bond.

This reaction is selective. It is thermodynamically favored and kinetic feasible in a simple elementary step.

Due to the thermodynamic / kinetic boundary conditions for this Re action is to be assumed that subsequent reactions with H atoms here only one under orderly role, since they are formulated much more complex have to:

NO + H ^⚫ → N ^⚫ + ^⚫ OH (3)

NO + N ^⚫ → N₂ + O ^⚫ (4)

NO + O ^⚫ → N ^⚫ + O₂ (5)

NO + ^⚫ OH → O₂ + ^⚫ NH (6)

O₂ + N ^⚫ + H ^⚫ (7)

In contrast to reaction (2), at least one additional step (3) is required before the possibility of forming the leads stable N₂ molecule. The NH₃-SCR reaction is technically in the Power plant denitrification, especially mixed oxide catalysts (WO₃, Cr₂O₃, V₂O₅) are used on a TiO₂ support. It is possible the reaction  to lead also on metal-doped zeolite catalysts.

b) The KW-SCR reactions

Hydrocarbons can be either radical-dis sociologically adsorbed, or there may be dehydration under olefinbil and surface-adsorbed hydrogen:

RH → R ^⚫ (8)

R-C₂H₅ → R-C₂H₃ + H₂ (9)

H₂ → 2 H ^⚫ (10)

Reactions (3) to (7) apply to subsequent reactions with NO and H atoms.

Possible subsequent reactions of NO with alkyl radicals (R ^⚫ ) and olefins (from alkane dehydrogenation (9) are complex and not necessarily NO-selective.

In addition to the desired oxidative degradation of the hydrocarbons by NO (KW + NO → CO₂ + H₂O + N₂) is, especially with a relatively low response temperatures, with the formation of organic nitrogen compounds (e.g. nitro and nitroso compounds) and aldehydes, ketones and carboxylic acids due to thermodynamic stability criteria and reaction mechanistic To calculate the circumstances, which is also confirmed in practice.

For example, noble metal-doped zeolites are used as catalysts set and conventional supported catalysts based on noble metal / Al₂O₃.

c) The H₂-SCR reactions

The dissociative adsorption of hydrogen is preferably carried out on noble metal  contacts. The possible radical follow-up reactions with NO are can be formulated analogously (3) to (7).

As already noted, the H₂-SCR method represents chemical-physical Nothing fundamentally new, because the relevant hydrogen atom-borne Reactions to both the NH₃-SCR and the KW-SCR method to step.

Because a supply of hydrogen from pressurized gas containers for the mobile Deployment can hardly be considered, this must be done on site compounds containing hydrogen are generated. In addition to water electrolysis reforming and dehydration reactions are also possible (e.g. KW / H₂O reforming, alkane dehydrogenation, ammonia decomposition).

task

In contact-induced gas reactions, such as. B. the above SCR-Re actions, the reaction is activated via the solid phase. More common the processes for reaction activation on solid surfaces are wise thermal nature. Sufficiently fast response times therefore require Temperatures that are not normally found in exhaust gases from diesel engines be available. Since a Re Actuator extension is only possible to a limited extent, the conversion rate also remains low.

It is therefore an object of the invention to provide a method for reducing nitrogen oxide in exhaust gases from internal combustion engines, especially in a motor vehicle create with which the nitrogen oxide reduction accelerates on the contact surface  can be nigt, so that even at moderate contact temperature higher Conversion rates can be achieved.

This object is achieved with the method according to claim 1. Beneficial Refinements of the method and a device for carrying it out the procedure are the subject of further claims.

In the process according to the invention, the reducing agent is added before enters into contact with the catalyst in the high-pressure plasma state leads. The plasma process activates the molecules in the gas phase reached. The plasma-induced formation of Ra is important here dical, less excited by ions / electrons or vibronic / electronic molecules.

Radicals exist longer in the gas phase than any other plasma spar zies, since their recombination is inhibited in the gas phase compared to the Ion / electron recombination (elementary process under photon emission) or for deactivating vibronically / electronically excited molecules (elementary process by photon emission or shock deactivation).

There is thus the possibility of radicals with appropriate plasma excitation of the type to generate, as otherwise specified by the above Adsorptive-dissociative processes arise at the contact.

The result is an acceleration of the nitrogen oxide reduction at the contact surface and thus higher conversion rates even with moderate contact temperature.  

The inventive method is for both KW-SCR reactions also suitable for NH₃-SR reactions and KW-SCR reactions.

The method according to the invention requires plasma generation at least least atmospheric pressure and only insignificant heating of the process gas ses (non-equilibrium plasmas). For this, the methods of the so-called silent discharges (Corona Discharge, Dielectric Barrier Discharge) such as special microwave-coupled plasmas (slot / hole coupling) is suitable.

With regard to the achievement of maximum radical production, these are microwave-coupled plasmas are particularly advantageous. However, this poses is not an exclusion criterion for the other plasma processes. The frequency The range of microwaves is between 0.95 GHz and 25 GHz, preferably the frequency 2.45 GHz. The particular advantage in plasma generation by high frequency electromagnetic radiation is the fact that here plasma generation can be achieved without the use of electrodes can. This can cause characteristic disadvantages when using electrical equipment that, especially electrode erosion and short downtimes, are avoided will.

The inventive method as a combination of plasma processes with Heterogeneous contacts can be in compliance with the following conditions be carried out particularly effectively:

• - The plasma process is essentially non-thermal in nature (not equilibrium plasmas) and preferably radical-forming.  
• - The respective selective reducing agents (ammonia or urea, Hydrocarbons / hydrogen) become gaseous in the plasma guided.
• - The plasma is generated immediately before entering the Ka talysator (avoidance of excessive radical recombination).
• - The catalyst consists of Wa coated with active material ben structures, the catalyst support being both ceramic and can also be metallic in nature.

The plasma / catalyst system can be in an advantageous embodiment be controllable with respect to different operating (load) states, both down visibly the amount of reducing agent currently required as well the currently required plasma power. A simple che detection of the volumetric nitrogen oxide concentration, e.g. B. via Senso ren is not sufficient. The current one must also be recorded Mass gas flow. In the case of diesel engines, this includes simultaneous detection of the current speed and the currently injected fuel quantity required, for example via an electronic map control. There at the current operating point of the machine is recorded and the optimum Operating conditions then approximately set.

In the case of silent discharges (AC / DC Corona Dielectric Barrier), the Power regulation made via the current intensity of the discharge.

With microwave-coupled plasmas, the power control can be done directly via the RF radiation by appropriate regulation of the cathode emit ter current of a MW magnetron or by detuning the resonant circuit  through electromechanical stub tuners in a waveguide (capacitive Measure).

Nitrogen or nitrogen compound such as e.g. B. ammonia, hydrazine or cyanuric acid is used. But it can also aliphatic or olefinic hydrocarbons or hydrogen used will.

Preferred catalyst materials are:

• - with elements of the platinum group, the copper group or the iron group doped zeolites
• - Oxides of aluminum, titanium or doped with elements of the platinum group the lanthanides or mixtures thereof
• - Mixed oxides of tungsten, chromium or vanadium.

The invention is explained in more detail with reference to figures. Show it

Fig. 1 shows a device for carrying out the procedural invention Rens,

Fig. 2 shows a device for generating a high pressure plasma by the method AC / DC Corona,

Fig. 3 shows a device for generating a high pressure plasma by the method AC Dielectric Barrier,

Fig. 4 shows a device for generating a high pressure plasma by micro waves.

Fig. 1 shows the basic structure of an apparatus for performing the method according to the invention. The exhaust gases leaving the engine are led via the exhaust line to the catalytic converter block, which contains the selective reduction contacts. The reducing agent in gaseous state is introduced into this exhaust line via a nozzle. Immediately from the inlet into the catalyst block, within the exhaust pipe, the plasma zone is arranged, in which the reducing agent is converted into the plasma state.

The device for generating the plasma is schematic in this Drawing of the power section only. Both the control of the Plasma power as well as the mass flow of the reducing agent is about controlled the engine map.

Fig. 2 shows an apparatus for generating a high pressure plasma by the AC / DC corona method. A central electrode 22 is arranged within the exhaust pipe, the wall 20 of which serves as an electrode. The power section (power supply) is connected to the electrically conductive wall 20 of the exhaust gas line and to the central electrode 22 . At the central electrode 22 , spray tips 24 are arranged to increase the electric field strength on the electrode surface in order to facilitate the removal of the electrons from the central electrode 22 .

The plasma forms in the annular gap between the central electrode 22 and the wall 20 of the exhaust pipe.

For AC corona operation and DC corona operation, exemplary process parameters are given below:
DC corona operation:
DC power supply
negative polarity of the central electrode
positive polarity of the wall
Voltage: 6 kV to 10 kV
Current: a few milliamps
AC corona operation:
high frequency AC voltage
Frequency: 10 to 100 kHz
Rising edge: a few nano-seconds.

Fig. 3 shows an apparatus for generating a high pressure plasma according to the Dielectric Barrier AC method. The power section delivers a high-frequency AC voltage similar to the AC-Corona method. However, the wall 30 of the exhaust pipe surrounding the central electrode 32 is here made of a dielectric material, e.g. B. glass, quartz, ceramics. The outer surface of the wall is surrounded by a metallic conductor 34 , for. B. in the manner of a network or as an additional coating (not shown), which forms the counter electrode to the central electrode 32 . Due to the dielectric wall 30 between the two electrodes 32, 34 , a plasma breakthrough is excluded, which has the advantage that higher voltages can also be applied. Otherwise, the plasma is generated analogously to the AC corona method. Likewise, 32 spray tips 36 are present on the central electrode.

Fig. 4 shows a device for generating a high pressure plasma by microwaves. An essential element of the device is a cylinder resonator 10 , within which the plasma is also generated. A central circular aperture 6, 8 is arranged on each of the two end faces of the cylindrical resonator 10 . Through these opposing openings 6, 8 , the exhaust pipe 14 extends, in which the engine exhaust gases and the reducing agent who led the. It is made of a dielectric material.

A waveguide 12 , into which the microwaves generated by a magnetron are coupled, runs parallel to the end face of the cylindrical resonator 10 .

The cylinder resonator 10 is excited by this microwave radiation, the coupling here via the two circular hole diaphragms 6, 8 at the front sides of the cylinder resonator 10 . This circular aperture 6, 8 the NEN here both as coupling holes for the excitation of the Zylinderresona gate 10 as well as exhaust gas and reducing agent guidance. The plasma itself arises within the exhaust pipe 14 in the area of the cylinder resonator 10 . After the plasma has been ignited due to the microwave energy absorbed by the cavity 10 , the cavity resonator 10 does not absorb any further energy from the waveguide 12 . The microwave radiation in the waveguide 12 is now coupled directly into the plasma gas.

The plasma zone within the reaction tube 14 is then no longer in the region of the cylindrical resonator 10 , but in the region of the hollow conductor 12 .

Electromechanical stub tuners (tuning pins) can be used for fine adjustment defined distances are arranged in the waveguide (not shown).

Advantageous parameters when carrying out the tests are:
Vibration mode cylinder resonator: E₀₁₀
Vibration mode waveguide: H₁₀
Waveguide: Rectangular waveguide R-26
Frequency: 2.45 GHz

Claims (20)

1. A process for nitrogen oxide reduction in exhaust gases from combustion machines with an excess of oxygen, the exhaust gases being brought into contact with a catalyst by supplying a selectively acting gaseous reducing agent, characterized in that the reducing agent predominantly before entering into contact with the catalyst in the high-pressure plasma state Radical formation is performed so that an acceleration of the contact reaction is achieved. 2. The method according to claim 1, characterized in that the High pressure plasma through silent high pressure discharges of the type AC / DC corona or AC dielectric barrier is generated. 3. The method according to any one of the preceding claims, characterized ge indicates that the high-pressure plasma by high-frequency elec tromagnetic fields is generated. 4. The method according to claim 3, characterized in that the high-frequency electromagnetic fields microwaves in a fre frequency range between 0.95 GHz and 24 GHz, preferably 2.45 GHz are. 5. The method according to any one of the preceding claims, characterized in that the plasma is generated with a cavity resonator ( 10 ), the cavity resonator ( 10 ) via hole or slot coupling with microwaves from a waveguide ( 12 ) is excited. 6. The method according to any one of the preceding claims, characterized ge indicates that the reducing agent is nitrogen or a stick material connection such as B. is ammonia, hydrazine or cyanuric acid. 7. The method according to any one of the preceding claims, characterized ge indicates that the reducing agent is an aliphatic or olefi is hydrocarbon or hydrogen. 8. The method according to any one of the preceding claims, characterized ge indicates that the catalyst is on a ceramic or metal is arranged honeycomb structure. 9. The method according to any one of the preceding claims, characterized ge indicates that the catalyst is made with elements of platinum pe, the copper group or the iron group doped zeolites stands. 10. The method according to any one of the preceding claims, characterized ge indicates that the catalyst is made with elements of platinum pe doped oxides of aluminum, titanium or lanthanides or Mixtures of these consist. 11. The method according to any one of the preceding claims, characterized characterized in that the catalyst from mixed oxides of tungsten,  Chromium or vanadium. 12. The method according to any one of the preceding claims, characterized ge indicates that the process is map-controlled. 13. Device for reducing nitrogen oxides in exhaust gases from combustion ma machines with excess oxygen containing a catalyst with which the exhaust gases with the supply of a selectively acting gaseous Reducing agent are brought into contact, characterized records that there is a device with which the reduction medium before entering into contact with the catalyst in high pressure plasma state is transferred with predominantly radical formation, so that an acceleration of the contact reaction is achieved. 14. The apparatus according to claim 13, characterized in that the Device for generating the high pressure plasma a device for the generation of silent high-pressure discharges of the type AC / DC-Corona or AC Dielectric Barrier. 15. Device according to one of the preceding claims, characterized in that the device for generating the high pressure plasma comprises the following elements:

• - a microwave source
• - A microwave waveguide ( 12 ) into which the microwaves generated by the microwave sources are coupled
• - A cavity resonator ( 10 ) which is excited via hole or slot coupling from the guided in the microwave waveguide ( 12 ) microwave radiation.

16. The apparatus according to claim 15, characterized in that the cavity resonator is a cylindrical resonator ( 10 ), with openings ( 6 , 8 ) being arranged on the end faces, which serve as coupling holes for the resonator excitation and through which the reducing agent and the exhaust gases are led. 17. The method according to any one of the preceding claims, characterized ge indicates that the catalyst is on a ceramic or metal is arranged honeycomb structure. 18. Device according to one of the preceding claims, characterized ge indicates that the catalyst is made with elements of platinum pe, the copper group or the iron group doped zeolites stands. 19. Device according to one of the preceding claims, characterized characterized in that the catalyst is made with elements of platinum group-doped oxides of aluminum, titanium or lanthanides or mixtures thereof. 20. Device according to one of the preceding claims, characterized characterized in that the catalyst from mixed oxides of tungsten, Chromium or vanadium.