Method for generating a highly reactive plasma for exhaust gas aftertreatment and enhanced catalyst reactivity

A method for non-thermal plasma aftertreatment of exhaust gases the method comprising the steps of providing short risetime (about 40 ps), high frequency (about 5G hz), high power bursts of low-duty factor microwaves sufficient to generate a dielectric barrier discharge and passing a gas to treated through the discharge so as to cause dissociative reduction of the exhaust gases. The invention also includes a reactor for generating the non-thermal plasma.

FIELD OF THE INVENTION
 The present invention relates to exhaust gas remediation and in particular,
 a method and apparatus for enhancing the reactivity of the material
 catalysts found within catalytic converters of cars, trucks and power
 stations.
 BACKGROUND OF THE INVENTION
 Plasma aftertreatment of exhaust gases has previously been identified as a
 possible remediation technique since non-thermal plasma can induce a host
 of new chemical reactions due to electron excitation thereby causing the
 production of an abundant amount of radicals and excited state molecules.
 Plasma aftertreatments rely upon the generation of high local electric
 fields which directly produce energetic electrons. These energetic
 electrons can influence the chemistry, even the collision dominated
 regime, because they do not lose much energy in elastic collision due to
 their small mass, but instead bounce around and transfer most of their
 energy to molecules; either dissociating, ionizing, or otherwise exciting
 them. This excitation and radical production can cause extensive changes
 in reaction rates; and in some cases, by as much as a hundred
 thousand-fold increase.
 While the efficiency of non-thermal plasma may be limited by any of several
 factors; two limitations that especially inhibit efficient utilization of
 non-thermal plasma discharge in connection with aftertreatment of exhaust
 gases are low electrical efficiency and an undesirable reaction pathway.
 See B. M. Penetrante et al., "Comparison of Electrical Discharge
 Techniques for Nonthermal Plasma Processing of NO in N2," pp. 679-687 in
 IEEE Transactions in Plasma Science, Vol. 23, No. 4, 1995, the relevant
 portions of which are incorporated herein by reference.
 In the excited state chemistry of non-thermal plasma, it is desirable to
 produce as high an electric field as possible. This would at first seem to
 simply entail applying high enough voltages to a suitably arranged
 configuration of electrodes. However, for non-thermal plasma densities,
 there occurs considerable plasma shielding of the applied fields, even in
 the collision dominated regime and field limits. See J. H. Whealton and R.
 L. Graves, "Exhaust Remediation Using Non-Thermal (Plasma)
 Aftertreatments: A Review," Proceedings of the 1995 Diesel Engine
 Emissions Reductions Workshop, Vol. 23, LaJolla, calif., the relevant
 portions of which are incorporated herein by reference. This space charge
 shielding is due to the charge imbalances arising within the plasma
 because of the higher mobility of electrons as compared to positive ions.
 Accordingly, the electric fields are generally limited by high voltage
 breakdowns which in turn leads to low impedance discharge. Further, plasma
 shielding effects take place for fields at frequencies below the electron
 plasma frequency. As a result of this shielding, the volume of plasma
 where the more favorable high field enhanced reduction reactions occur is
 reduced to either within a few Debye lengths or an electron energy
 relaxation distance of the plasma edge thereby diminishing a high field
 reaction volume by orders of magnitude.
 As noted above, problems also arise with respect to the chemistry path.
 While both oxidation and reduction reaction pathways are possible avenues
 for the dissociation of NO.sub.x, their respective chemistries differ. The
 oxidation reaction pathway will result in the production of compounds that
 include N.sub.2 O and nitric acid. Since nitric acid is toxic, it's
 generation is to be avoided, especially in automobiles, trucks or other
 mobile applications. On the other hand, the dissociative attachment
 occurring in the reduction reaction pathway is more favorable since it
 lead to the formation of benign N.sub.2. Unfortunately, the above noted
 difficulty in reaching high E/N in prior art discharges has prevented
 development of a plasma aftertreatment device that will achieve a high
 fraction reduction of NO.sub.x as opposed to the less desirable oxidation.
 In view of the above problems and deficiencies of plasma aftertreatments of
 exhaust gases, the present invention was developed.
 OBJECTS AND SUMMARY OF THE INVENTION
 Accordingly, it is an object of the present invention to provide a method
 of non-thermal plasma aftertreatment of exhaust gas whereby the electric
 fields will no longer be limited by high voltage breakdowns and associated
 low impedance discharge.
 It is another object of the present invention to provide a method of
 non-thermal plasma aftertreatment of exhaust gas whereby significant
 increases in the arc transition electric field are achieved by
 incorporation of high frequency power (5 Ghz) implying a 40 ps risetime.
 It is a further object of the present invention to provide a method of
 non-thermal plasma aftertreatment of exhaust gas whereby any plasma
 shielding effects taken place for fields at frequencies below the electron
 plasma frequency are significantly reduced through the use of high
 frequency power.
 It is yet another object of the present invention to provide a method of
 non-thermal plasma aftertreatment of exhaust gas whereby efficient
 coupling of high frequency power to produce high electric fields is
 achieved through application of resonance cavities.
 It is an additional object of the invention to provide a method of
 non-thermal plasma aftertreatment of exhaust gas whereby any surface
 charging of dielectrics, to the extent it does occur, will enhance the
 fields in the next (reverse field) half cycle.
 It is another object of the present invention to provide a method of
 non-thermal plasma aftertreatment of exhaust gas whereby the electric
 fields ramp up so quickly (about 40 ps) that the probability of the
 discharge initiating near threshold is substantially reduced thereby
 resulting in a tilt of the reaction pathway toward the more desirable
 reduction pathway as opposed to the oxidation pathway.
 It is further object of the present invention to provide a method and
 apparatus of non-thermal plasma aftertreatment of exhaust gas for use with
 existing smart solid catalytic converters to synergistically improve the
 reactivity and efficiency of the catalyst and the exhaust remediation.
 It is yet another object of the present invention to provide a method of
 non-thermal plasma aftertreatment of exhaust gas that will enable much
 higher E/N, and a continuous production of atomic Nitrogen by employing
 bursts of high frequency AC electric field having fast risetimes and many
 cycles per burst; specifically, high-power microsecond bursts of about 6
 Ghz and microwaves having a risetime of about .about.40 ps to provide
 pulse risetimes approximately 100 times shorter than that of the prior art
 whereby thousands of electric field oscillations per burst of microwave
 power are achieved.
 It is an additional object of the invention to provide a method of
 non-thermal plasma aftertreatment of exhaust gas whereby the catalyst is
 heated to increase reactivity only very locally in places where the
 reactions are taking place for purposes of providing controlled and highly
 localized enhanced catalyst reactivity.
 It is another object of the present invention to provide a new plasma
 generator for the non-thermal plasma aftertreatment of exhaust gases
 whereby plasma shielding is reduced and a higher E/N in the bulk of the
 plasma is achieved, reducing the N.sub.2 vibrational excitation risk and
 permitting high electron temperatures.
 It is an additional object of the invention to provide a method of
 non-thermal plasma aftertreatment of exhaust gas whereby field reduction
 caused by plasma shielding is prevented, Atomic nitrogen is produced and
 made available during substantially the entire treatment process, any
 surface charging of the dielectrics that may occur will enhance fields in
 the next half cycle, field limits due to arc breakdowns are significantly
 increased and higher fields are achieved.
 The present invention is further directed to a method of non-thermal plasma
 aftertreatment of exhaust gases having reduced plasma electric field
 shielding, increased availability of atomic nitrogen, exploitation of
 surface charging of dielectrics, avoidance of (low field) threshold
 initiated discharges, and achievement of a higher high-energy tail on the
 electron distribution function.
 In summary, the present invention provides a method for non-thermal plasma
 aftertreatment of exhaust gases the method comprising the steps of
 providing short risetime(about 40 ps), high frequency (about 5 Ghz), high
 power bursts of low-duty factor microwaves sufficient to generate a
 dielectric barrier discharge and passing a gas to treated through the
 discharge so as to cause dissociative reduction of the exhaust gases.
 The present invention is also directed to a waveguide reactor for
 generating non-thermal plasma for aftertreatment of exhaust gases, the
 reactor comprising a pulsed microwave source for generating a DBD having
 electric field rise times of less than about 50 ps and a frequency of
 about 5 Ghz, a waveguide including resonance cavity, the waveguide for
 receiving pulsed microwaves from the microwave source and generating a
 dielectric barrier discharge therein whereby exhaust gases subjected to
 the generated dielectric barrier discharge are caused to be dissociated by
 a high fraction reduction reaction.
 The present invention is further directed to a waveguide reactor as set
 forth above and operatively associated with a material catalyst whereby
 the short burst, high-power microwave fields generated by the reactor are
 caused to increase the reactivity of the catalyst surface for purposes of
 exhaust gas remediation.
 These and other objects of the present invention will become apparent from
 the following detailed description.

DETAILED DESCRIPTION OF THE INVENTION
 Turning to FIG. 1, a waveguide reactor 2 for microwave generated
 non-thermal plasma according to the present invention is illustrated. The
 reactor 2 is shown to include a microwave source; namely, a pulsed
 magnetron tube 4 capable of generating a DBD having electric field rise
 times of less than about 50 ps. In the preferred embodiment, the pulsed
 magnetron tube 4 and power supply has a frequency output tunable from
 about 5.3 GHz to about 5.8 GHz, a peak output power of about 250kW and an
 average power. Output of about 250 W with a duty factor of 0.001. The
 output pulse widths can be 0.5, 1.5 and 2.25 .mu.s with a duty factor of
 0.001.
 The microwave energy generated by the pulsed magnetron tube 4 is conveyed
 to the waveguide reactor 2 by a WR-187 waveguide 6, the waveguide 6
 including a flow reactor 8. The flow reactor 8 is best shown in FIG. 2 and
 comprises a 43 cm length of the waveguide 6 provided with a 16 mm outer
 diameter and 14 mm inner diameter quartz tube 10 penetrating through the
 waveguide 6 at a 15 degree angle. The microwave electric field is
 generated between the top and bottom walls of the waveguide portion of
 flow reactor 8 and thus passes through the quartz tube 10 in the region
 where the quartz tube 10 is in the waveguide 6. The quartz tube 10 is
 angled to provide a slow change in the dielectric constant in the
 waveguide 6. Tube couplings 12 are provided to secure the quartz tube 10
 to a copper sheath tube 14 secured to the outer diameter of the quartz
 tube 10 and extending exterior of the waveguide 6. The copper sheath tube
 14 is shown to comprise a 15 cm length having a 16.25 mm inner diameter.
 In the alternative, the flow reactor may comprise a 2.5 cm outer diameter
 quartz tube having a 0.62 cm diameter copper rod down the center and
 including a perforated brass screen around the outside. This embodiment
 will create a coaxial line in which the microwaves can propagate with a
 radial electric field through which the gases to be treated will flow. The
 microwave energy is coupled into the coaxial line of the flow reactor by
 provision of a waveguide-to-coaxial adapter (not shown). Tuning elements
 (not shown) at the inlet and outlet of this coaxial line permit matching
 of the waveguide to the coaxial line. The characteristic impedance of this
 coaxial line is approximately 80 ohms.
 Returning to FIG. 1, the waveguide reactor 2 is shown to further include an
 E-plane bend 15 provided in the waveguide 6 at the outlet of the magnetron
 4. A flexible waveguide 16 is also provided together with an isolator 18
 and a signal sampling coupler 20 operably associated with coaxial
 detectors for measuring the applied and reflected power levels at the
 reactor 2. A shorting switch 22 is included at one end of the flow reactor
 8 and the outlet end 24 of the waveguide 6 is shorted with a plate 26 to
 reflect any power not absorbed in the first pass back into the reaction
 volume of the flow reactor 8.
 The waveguide reactor 2 is pressurized with sulfur-hexaflouride to prevent
 breakdown around the outside of the quartz tube 10, rather than the inside
 of the tube. The quartz tube 10 is filled with 4 mm dielectric beads to
 provide electric field enhancement and electrical breakdown and is further
 adapted to receive the gas to be treated. A DBD is generated when an arc
 is initiated between the dielectric surfaces (the inner and outer
 conductors) hits the quartz tube 10. The electrons at the leading edge of
 the arc are trapped on the surface of the dielectric, locally canceling
 out the electric field and extinguishing the arc. A new arc is then
 immediately formed, but intersecting the dielectric surface at a different
 location.
 The method and apparatus according to the invention employs bursts of
 high-frequency AC electric fields with fast risetimes and many cycles per
 burst. Specifically, high-power microsecond bursts of about 5 Ghz
 microwaves are used that have a risetime of around 40 ps to provide
 thousands of electric field oscillations per burst of microwave power.
 The above method and apparatus provides several advantages. For 40 ps
 risetime, the fields turn on with a time scale short compared with the
 time an electron drifts toward the edge of the plasma; strict space charge
 neutrality is thereby achieved in the plasma preventing field reduction
 due to plasma shielding. Atomic nitrogen is produced and available during
 the entire treatment since the time between microwave bursts is small
 compared to the recombination time thereby achieving a higher fraction of
 reduction of NOx as opposed to less desirable oxidation. Surface charging
 of dielectrics, to the extent it occurs at all, will enhance the fields in
 the next (reverse field) half cycle. Field limits due to arc breakdowns
 (e.g. Paschen limit) are significantly increased due to the high
 frequency. Higher fields caused by having the electric fields ramp up so
 fast substantially reduces the probability of the discharge initiating
 near threshold.
 In a preferred embodiment of the present invention, the waveguide reactor
 is used in conjunction with a solid catalyst surfaces (not shown) in
 proximity to the plasma generated. When applied in conjunction with
 so-called "smart" catalysts, a significant increase in reactivity on the
 catalyst surface results. The short burst, high power microwave fields
 cause enhanced surface effect on the catalyst due to the polarization
 catastrophe created. Polarizability catastrophe will occur in the
 transition between vibrational excitation (small displacement and,
 therefore, small field absorption) and rotational excitation (large
 displacement, and therefore, large field absorption). When the surface of
 the material gets hot enough, lattice excursions become sufficiently
 frequent so that there is occasionally room for a rotational excitation.
 When this happens, the heating rate increases a hundred fold and produces
 more excursions producing yet more rotational excitation and so on
 resulting in a nonlinear polarization catastrophe. On the edge of the
 dielectric, rotational excitation will occur, even at low temperatures
 since there is no lattice to get in the way. With the present invention,
 this heating becomes even more preferential since this is exactly the
 place where the electric field is strongest. Accordingly, the waveguide
 reactor of the present invention will allow controlled and highly
 localized heating of the catalyst material and therefore enhanced
 reactivity.
 The method of the present invention will be illustrated from the following
 representative example which incorporates herein by reference relevant
 portions of applicant's publication entitled "Non-Thermal Plasma Exhaust
 Aftertreatment: Are All Plasmas the Same?" Jul. 28-31, 1997, Diesel Engine
 Emission Reduction Workshop, UCSD, J. H. Whealton et al.
 EXAMPLE
 The plasma generator of the present invention was applied for conversion of
 NO.sub.x in N.sub.2. The gas concentrations of NO.sub.x were determined by
 chemiluminesnce (Rosemount model 951) operated in both NO and NO.sub.x
 mode. NO.sub.2 was calculated from the difference between the two numbers.
 O.sub.2 was measured with a fuel cell type analyzer (Illinois Instruments
 Model 2560).
 The example was conducted with the above-described waveguide reactor for
 treatment of a mixture of NO in N.sub.2. Initial gas concentration into
 the reactor was set to 612 ppm with a flow rate of 3.4 lpm. This resulted
 in a residence time of 3 s in the entire tube, or a space velocity of 1200
 h-1 and represented a lower limit of space velocity. In most cases, the
 discharge is active over a small portion of the tube. Because no H.sub.2 O
 was present in the gas mixture, the outlet of the reactor was fed directly
 into the analyzers.
 The microwave energy was coupled into the coaxial line of the flow reactor
 by means of the earlier noted waveguide-to-coaxial adapters. Tuning
 elements at the inlet and outlet of this coaxial line permit matching of
 the waveguide to the coaxial line. The characteristic impedance of the
 coaxial line is approximately 80 ohms. For each experimental run, the
 system was tuned to maximize the volume containing electrical discharge
 and the NO.sub.x conversion. The total active volume was typically 60
 cm.sup.+3. The tuning maximized the amplitude of the standing waves in the
 waveguide reactor by maximizing the power coupled into the coaxial line
 and minimizing the power leaving the line through either the inlet or
 outlet waveguide-to-coaxial adapters.
 Several different peak RF power levels and duty factors were applied. The
 waveguide reactor was capable of providing 0.5, 1.5, and 2.25 .mu.s pulse
 widths at repetition rates ranging from &lt;200 Hz to 2 kHz. The peak
 transmitted powers ranged from 100-250 kW with average (duty-factor
 corrected) transmitted powers of 50-315 W. Transmitted and reflected
 powers were monitored using a calibrated waveguide signal sampler having
 coaxial detectors. The absorbed power was taken as the difference of the
 transmitted and reflected powers. This is an upper bound for absorbed
 power because other losses are neglected.
 FIG. 3 illustrates a plot of NO conversion versus the absorber power for
 three different pulse widths at maximum peak powers, but with varying
 pulse repetition rates. The 1.5 .mu.s pulse width provided the best
 performance. This appears to be due to maximization of electron generation
 and heating before plasma shielding of the electric fields occurs.
 Because of the favorable results of the 1.5.mu.s pulses, a series of powers
 and frequencies were investigated. The best results were obtained with
 high power at repetition rates of 200-500 Hz. For comparative purposes,
 the input energy densities disclosed in the prior art of the Penetrante et
 al. publication, the relevant portions of which have been incorporated
 herein by reference, are shown in FIG. 4 for destruction of a 100 ppm NO
 stream. The prior art pulsed corona and dielectric barrier discharge are
 identified as Livermore 1 and 2, Karlsruhe 1, 2 and 3 and Siemens. The
 present invention is identified as ORNL 1, 2 and 3. The prior art data is
 also incorporated into FIG. 3. The resulting energy per molecule NO
 converted in the prior art is 238 eV/molecule and a line corresponding to
 that energy has been added to FIG. 3 for purposes of comparison. As can be
 seen, the microwave-generated plasma according to the present invention is
 converting at a significantly lower energy per molecule(74 eV/molecule).
 The electron mean energy, as a function of the reduced electric field, can
 also be seen. The prior art Livermore, Karlsruhe and Siemens data having
 an electron mean energy of about 4.2 eV whereas the present invention
 providing an electron mean energy of 6.6 eV.
 The high-peak power, low-repetition rate data indicates high efficiency of
 conversion with low-energy densities for the present invention. This
 appears to result from the efficiency at which microwave energy can
 penetrate a plasma and heat the electrons, provided the microwave
 frequency remains higher than the characteristic electron plasma
 frequency. In other words, cutoff is avoided. For the 5.5 Ghz typically
 used in the present example, the corresponding electron cutoff density is
 approximately 3.5.times.10-11 cm-3.
 The above demonstrates improvement of the present invention for removal of
 NO) in N.sub.2 over that provided in the prior art. As best shown in FIG.
 3, instead of the 238 eV per removed NO obtained with the slower rise time
 plasmas of the prior art, a 70 percent reduction (75 eV per removed NO)
 was obtained by the present invention. Further, the 6.5 eV electron
 temperature of the present invention is substantially greater than the 4
 eV provided by the prior art. The fast risetime (40 ps) discharge is
 comparable to electron transport times in the plasma. This is obtained
 from a 5.5 GHz microwave generator coupled to a suitable resonance cavity.
 Plasma shielding is thereby reduced and higher E/N in the bulk of the
 plasma achieved This appears to reduce the N.sub.2 vibrational excitation
 risk and provides for high electron temperatures as compared to that
 provided in the prior art. Instead of the 238 eV per removed NO obtained
 with the slower rise time prior art plasmas, a 25 percent reduction (167
 eV per removed NO) was obtained by the present invention. When the present
 invention is combined with smart catalytic converters, the higher electron
 temperature and concomitant different reaction kinetics i.e. enhanced
 excited state molecular production, of the present invention provides a
 synergistic and enhanced improvement of exhaust remediation. This is in
 addition to the. enhanced reactivity of the catalyst material itself due
 to the microwave induced rotational excitation at the catalyst surface
 edges.
 While this invention has been described as having preferred design, it is
 understood that it is capable of further modification, uses and/or
 adaptations following in general the principle of the invention and
 including such departures from the present disclosure as come within known
 or customary practice in the art to which the invention pertains, and as
 may be applied to the essential features set forth, and fall within the
 scope of the invention or the limits of the appended claims.