Chemistry: electrical and wave energy – Processes and products – Processes of treating materials by wave energy
Reexamination Certificate
1999-07-23
2001-02-20
Mayekar, Kishor (Department: 1741)
Chemistry: electrical and wave energy
Processes and products
Processes of treating materials by wave energy
C422S186000
Reexamination Certificate
active
06190507
ABSTRACT:
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
x
, their respective chemistries differ. The oxidation reaction pathway will result in the production of compounds that include N
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
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
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 ~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
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 (lo
Armfield Jeffrey S.
Bigelow Timothy S.
Graves Ronald L.
Hanson Gregory R.
Raridon Richard J.
Gottlieb Paul A.
Mayekar Kishor
Moser William R.
Schneider Emily G.
The United States of America as represented by the Department of
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