DE19910553C1 - Method for continual removal of nitrogen dioxide in exhaust gases - Google Patents

Abstract

The method comprises production of a microwave induced high pressure plasma at above or equal to 1 bar. A nozzle is brought into the exhaust stream and microwave induced high pressure plasma is created through pulsed microwave radiation, which is guide via a reduced hollow conductor (HL) and saved in a hollow resonator (HR) via a hole coupling.

Description

The invention relates to a process for the continuous removal of nitrogen oxides in Exhaust gases from over-stoichiometric combustion engines, in particular for use in motor vehicles.

A large number of technical combustion processes are carried out with stoichiometry operated oxygen content (lean combustion). Examples of this are Heating burners or turbine combustion chambers. Beyond that Superstoichiometric burns in piston engines with internal mixture formation for use (diesel engines, direct-injection gasoline engines). In these processes can the downstream connected with stoichiometrically operated gasoline engines Nitrogen oxide reduction using regulated three-way catalysts made of green which are not used because the nitrogen oxide conversion is only minimal Exhaust gas conditions above λ = 1 drop drastically and under the conditions the typical superstoichiometric combustion is no longer present.

Instead, with overstoichiometric burns with regard to downstream nitrogen oxide reduction the selective catalytic reduction of Nitrogen oxides are used (Selective Catalytic Reduction, SCR). In doing so the nitrogen oxides using suitable reducing agents Catalysts implemented. A particularly effective reducing agent is included Ammonia (NH3). Hydrocarbons are less effective than selective reductants onsmittel.

The NH ₃ -SCR has so far been technically established for the denitrification of power plant emissions. The stationary operating conditions are of major advantage for the adaptation of the procedure. The catalytic converters can be operated at the optimum operating point (temperature adjustment by means of a heat exchanger) and the NH ₃ metering / control is comparatively simple. Furthermore, the storage of NH ₃ (cryo / pressure tanks or NH ₃ water) does not involve any infrastructural problems.

The use of NH ₃ -SCR technology in road vehicles with diesel engines requires considerable process changes compared to power plant operation. Since carrying NH ₃ by means of a cryo / pressure tank or in the form of NH ₃ water is not possible for safety reasons, NH ₃ -bearing unproblematic connections such. B. urea, from which NH _{3 is} generated by hydrolysis in situ. Since the catalysts cannot be operated in a stationary manner in this application (transient operation), considerable expenditures with regard to metering / control are required.

An alternative to heterogeneous catalytic processes has been available for several Years with increasing intensity investigated plasma chemical processes, using both high temperature and low temperature processes come. With regard to an effective nitrogen oxide reduction, is too in both cases demand that in particular nitrogen monoxide (NO) to molecular nitrogen and Oxygen is converted (NO represents about 95% in combustion processes predominant amount of the emitted nitrogen oxides).

It is known that activated nitrogen (a mixture of nitrogen atoms N and vibronically / electronically excited nitrogen molecules N2 *) can convert NO to the desired reaction products in a highly selective and fast gas reaction:

NO + N ⇒ N ₂ + O (1)

NO + O ⇒ O ₂ + N (2)

The gross reaction follows from the addition of both reaction equations to: 2NO ⇒ N ₂ + O ₂ , which corresponds to the NO decay reaction.

NO + N ₂ * ⇒ N ₂ + N + O (3)

NO + N ⇒ N ₂ + O (1)

NO + O ⇒ N ₂ + N (2)

From a reaction kinetic point of view, it can be stated that the reaction with N atoms leads to a conditionally self-supporting chain reaction, while the reaction with N ₂ * leads to a branched chain reaction. It can be seen from this that high proportions of N ₂ * in the activated nitrogen favor the NO conversion. Another advantage of the NO reaction with N ₂ * can be seen from the molecular-physical comparison of the potential course of NO and N ₂ * with reference to FIG. 1, which also shows the nature of the species N ₂ *.

The reaction-effective state of N ₂ * is the basic state of the Vegard-Kaplan system within the electronic excitation states of the molecular nitrogen. This is the lowest triplet state of nitrogen with an excitation energy of approx. 6 electron volts (eV). This state is characterized by an extremely long life of approx. 3 seconds with regard to the emission of electrical dipole radiation into the singlet ground state of nitrogen. The long service life is a result of the inter-combination ban (spin-prohibited transition). This state of excitation can therefore only be deactivated via radiation-free processes.

The NO + N ₂ * ⇒ N ₂ + O + N reaction is such a process. It can be seen from FIG. 1 that the excitation energy of the N ₂ * state which can be transmitted in a collision process between an NO molecule and an N ₂ * molecule corresponds almost exactly to the binding energy of the NO molecule. It can therefore be assumed that the effective cross-section of the reaction between NO and N ₂ * will be significantly larger than the gas-kinetic cross-section due to a quantum mechanical resonance effect.

The efficient generation of activated nitrogen requires the use of Plasma process. If thermal plasmas are used, the Process be carried out such that the production of activated nitrogen in one Plasma bypass is realized and the activated nitrogen generated in this way in NO containing exhaust gas is introduced in the form of a high-temperature plasma jet. On is such a method using microwave induced thermal plasmas described in DE 195 13 250 A1. It is a microwave Continuous wave method (cw method). The process works particularly effectively, if pure nitrogen is used as the plasma gas. Unless the plasma gas Contains oxygen or, in extreme cases, consists of air, additional measures must be taken in terms of avoiding thermal NO production in the expanding Plasmajet can be taken. That is only within certain limits. B. by a suitable nete gas-dynamic design of the jet feed into the exhaust gas, possible.

Another disadvantage of using thermal plasmas to generate activated nitrogen is that, due to the specific excitation conditions (excitation via thermal collisions between molecules / atoms), high N ₂ dissociation rates (N atom production) can be achieved, but an increase of the N ₂ * portion is only possible to a limited extent. This is due to the fact that in a thermal plasma (depending on the respective temperature) high degrees of ionization (and thus high concentrations of free electrons) can be achieved according to the Saha equation applicable to plasmas in full or local thermodynamic equilibrium (VTG / LTG) , however, the mean electron energy is in the range below approx. 1 eV. If one assumes a Maxwell-Boltzmann velocity distribution (or energy distribution) of the free electrons, then the proportion of electrons with energies above 6 eV in the Maxwell spur of the distribution function is comparatively low. Thus, the excitation of the N ₂ * state at about 6 eV thermal energy by electron impact in a thermal plasma is not a particularly effective process.

It is therefore the object of the invention to improve the continuous wave process described in DE 195 13 250 A1 in such a way that thermal NO production in the expanding plasma jet is prevented and at the same time an electron energy distribution function is implemented, the maximum of which is closer to the 6 eV Excitation energy of the metastable N ₂ * state comes to rest. The latter requirement is necessary for the effective excitation of the metastable N ₂ * state by electrons.

The object is achieved by the subject matter of patent claim 1 solved. Advantageous embodiments of the invention are the subject of dependent claims chen.

With the method according to the invention, the plasma generation is not the same weight conditions performed (non-equilibrium plasma NGP). NG plasmas are often referred to as low-temperature plasmas, d. that is, the gas kineti temperature (defined by the degrees of freedom of translation) remains clear below that temperature, which is determined by the increased occupied molecules Ren excitation states from a Boltzmann analysis. NG plasmas are always obtained when the plasma excitation mainly due to electron interference is controlled and not by gas kinetic collisions with atoms or molecules.

For microwave-induced plasmas with high power density at working pressures of 1 bar and above, as is particularly the case in motor vehicles is required, an NG plasma can only be generated by pulsed plasma excitation will be realized. The gas kinetic temperatures remain in this case in particular below a temperature required for possible NO formation (thermody namic and kinetic) of approx. 1500 K if the pulse widths are below approx. 100 µsec stay.  

In an advantageous embodiment of the method according to the invention Use microwave frequencies between 0.95 and 24 GHz, especially 2.46 GHz det. Here the electromagnetic waves are advantageous via waveguides guided. Due to their precisely defined geometry, these only leave whole certain types of waves. The radio frequency is generated either by Magnetron systems or through traveling wave tubes.

The pulse repetition frequency is preferably ≧ 0.5 kHz and ≦ 10 kHz.

In microwave pulse plasma operation there is a safe plasma ignition with every pulse of vital importance. Thereby in a pulse plasma system in this regard, the requirements are far higher than with a cw Plasma system that only requires one ignition. cw plasmas can e.g. B. ignited by means of electrical charge carriers from auxiliary discharges or flames become. Such ignition aids would either have to be constant with pulsed plasmas be available (auxiliary flame) or triggered according to the pulse repetition frequency systems (e.g. spark plug-like systems). It is possible to use such auxiliary ignition technically implement systems, but this is with a considerable Additional effort, especially when the lowest possible Susceptibility to failure should be realized. It is more advantageous to ignite the pulsed To realize plasmas through microwave technology measures. These will explained in more detail below.

The physically crucial parameter for plasma ignition in a gas is the respective electrical breakdown field strength. Such breakthroughs occur in air field strengths between 10-25 kV / cm, depending on e.g. B. from the current water vapor content, with a high water vapor content a lower breakthrough field strength required. According to an advantageous embodiment of the invention, the is for safe Pulse plasma ignition required high electrical field strength by using a tapered waveguide in which the microwave energy is applied to a cavity sonator is conducted. The feeding of the microwave energy into the cavity nator takes place in particular by hole coupling.  

In addition to this constructive measure can be used for a safe pulse plasma ignition the microwave pulse in terms of its energy distribution are designed such that the pulse increase in the nanosecond range, especially with a certain energy excess energy with respect to the average energy of the residual pulse. Alternatively however, a rectangular pulse shape is also possible, the rising edge is also in the nanosecond range.

The pulse rise times in the nanosecond range are due to primary ionization high local electrical field gradients required. The time limit of the Total pulse to below 100 µsec then leads to after plasma ignition Formation of a primarily electron impact controlled non-equilibrium plasma.

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

Fig. 1 * as explained the potential curve of NO and N ₂ in the introduction;

Fig. 2 is a particularly suitable for use in the inventive method, microwave pulse shape;

Fig. 3 shows an apparatus for performing the method according to the invention.

A particularly suitable micro wave pulse shape for use in the method according to the invention is shown in Fig. 2. One recognizes the steep rising edge with a duration in the range of a few nanoseconds, in the case shown about 1 ns. The pulse width is preferably less than 10 microseconds. At the pulse increase there is a certain increase relative to the average energy of the residual pulse.

Fig. 3 shows an apparatus for performing the method according to the invention. The exhaust gas to be cleaned is fed into the exhaust pipe AR via the opening A. Within a cavity resonator HR, a dielectric tube DR, z. B. made of quartz glass. Inside is the nitrogenous plasma gas. It is introduced into the exhaust gas stream in the form of a plasma jet via the nozzle D. In an embodiment not shown here, several nozzles can also be present. The microwaves are guided thereon by means of a waveguide HL, which tapers in the direction of the cavity resonator HR, and are fed into the latter by means of hole coupling.

Claims (6)

1. A process for the continuous removal of nitrogen oxides in exhaust gases from over-stoichiometrically operated internal combustion engines, wherein by generating a microwave-induced high-pressure plasma ≧ 1 bar of active nitrogen is generated, which is introduced into the exhaust gas stream via a nozzle, characterized in that the microwave-induced high-pressure plasma is pulsed Microwave radiation is generated. 2. The method according to claim 1, characterized in that the pulsed Microwave radiation conducted over a tapered waveguide (HL) and is fed into a cavity resonator (HR) via a hole coupling, wherein the plasma gas by means of a dielectric tube section (DR) through the hollow room resonator (HR) is guided. 3. The method according to claim 1 or 2, characterized in that the increase flank of the microwave pulses is in the nanosecond range, in particular ≧ 1 nm and ≦ is 10 nm. 4. The method according to any one of the preceding claims, characterized net that the pulse increase with an increase in the mean energy of the residual pulse. 5. The method according to any one of the preceding claims, characterized net that the pulse width is ≦ 100 microseconds. 6. The method according to any one of the preceding claims, characterized net that the pulse repetition frequency is ≧ 0.5 kHz and ≦ 10 kHz.