Non-thermal plasma reactor for lower power consumption

Details Non-thermal plasma reactor for lower power consumption Non-thermal plasma reactor for lower power consumption

2002-05-16

2004-12-28

Mayekar, Kishor (Department: 1753)

Chemical apparatus and process disinfecting, deodorizing, preser

Chemical reactor

With means applying electromagnetic wave energy or...

C060S275000, C204S177000

Reexamination Certificate

active

06835358

ABSTRACT:

TECHNICAL FIELD
The present invention relates to non-thermal plasma reactors and more particularly relates to non-thermal plasma reactors and methods for decomposing hazardous compounds in liquids or gases, such as particulate emissions in diesel engine exhaust streams, with reduced power consumption.
BACKGROUND OF THE INVENTION
Certain compounds in the exhaust stream of a combustion process, such as the exhaust stream from an internal combustion engine, are undesirable in that they must be controlled in order to meet government emissions regulations. Among the regulated compounds are hydrocarbons, soot particulates, and nitrogen oxide compounds (NOx). There are a wide variety of combustion processes producing these emissions, for instance, coal- or oil-fired furnaces, reciprocating internal combustion engines (including gasoline spark ignition and diesel engines), gas turbine engines, and so on. In each of these combustion processes, control measures to prevent or diminish atmospheric emissions of these emissions are needed.
Industry has devoted considerable effort to reducing regulated emissions from the exhaust streams of combustion processes. In particular, it is now usual in the automotive industry to place a catalytic converter in the exhaust system of gasoline spark ignition engines to remove undesirable emissions from the exhaust by chemical treatment. Typically, a “three-way” catalyst system of platinum, palladium, and rhodium metals dispersed on an oxide support is used to oxidize carbon monoxide and hydrocarbons to water and carbon dioxide and to reduce nitrogen oxides to nitrogen. The catalyst system is applied to a ceramic substrate such as beads, pellets, or a monolith. When used, beads are usually porous, ceramic spheres having the catalyst metals impregnated in an outer shell. The beads or pellets are of a suitable size and number in the catalytic converter in order to place an aggregate surface area in contact with the exhaust stream that is sufficient to treat the compounds of interest. When a monolith is used, it is usually a cordierite honeycomb monolith and may be pre-coated with gamma-alumina and other specialty oxide materials to provide a durable, high surface area support phase for catalyst deposition. The honeycomb shape, used with the parallel channels running in the direction of the flow of the exhaust stream, both increases the surface area exposed to the exhaust stream and allows the exhaust stream to pass through the catalytic converter without creating undue back pressure that would interfere with operation of the engine.
When a spark ignition engine is operating under stoichiometric conditions or nearly stoichiometric conditions with respect to the fuel-air ratio (just enough oxygen to completely combust the fuel, or perhaps up to 0.3% excess oxygen), a “three-way” catalyst has proven satisfactory for reducing emissions. Unburned fuel (hydrocarbons) and oxygen are consumed in the catalytic converter, and the relatively small amount of excess oxygen does not interfere with the intended operation of the conventional catalyst system.
However, it is desirable to operate the engine at times under lean burn conditions, with excess air, in order to improve fuel economy. Under lean bum conditions, conventional catalytic devices are not very effective for treating the NOx in the resulting oxygen-rich exhaust stream.
The exhaust stream from a diesel engine also has a substantial oxygen content, from perhaps about 2-18% oxygen, and, in addition, contains a significant amount of particulate emissions. The particulate emissions, or soot, are thought to be primarily carbonaceous particles and volatile organic compounds (VOC). It is also believed that other combustion processes result in emissions that are difficult or expensive to control because of, for instance, dilute concentrations of the compounds to be removed from the effluent stream or poor conversion of the compounds using conventional means.
In spite of efforts over the last decade to develop an effective means for reducing NOx to nitrogen under oxidizing conditions in a spark ignition gasoline engine or in a diesel engine, the need for improved conversion effectiveness has remained unsatisfied. Moreover, there is a continuing need for improved effectiveness in treating emissions from any combustion process, particularly for treating the soot particulate emissions from diesel engines.
Particulate filters have been shown to be an effective means of controlling diesel particulate emissions. The principle disadvantage of particulate filters is the need to periodically regenerate the filter to remove the trapped particulate mater. Regeneration removes particulate matter from the filter by oxidizing the carbon and volatile organic compounds (VOCs) to carbon dioxide and water. Thermal regeneration requires exhaust gas temperatures of about 600° C. The regeneration process is exothermic and is very difficult to adequately control to prevent melting or excessive thermal stresses in the filter substrate as the carbon and VOCs burn. An effective method of regeneration at lower temperatures is needed to enable the wide spread use of particulate filters.
An alternative way to treat the hydrocarbon, particulate, or NOx emissions in an exhaust or effluent stream is to destroy such emissions using a non-thermal plasma reactor. Plasma is regarded as the fourth state of matter (ionized state of matter). Unlike thermal plasmas, non-thermal plasmas (NTPs) are in gaseous media at near-ambient temperature and pressure but have electron mean energies considerably higher than other gaseous species in the ambient environment. NTP species include electrically neutral gas molecules, charged particles in the form of positive ions, negative ions, free radicals and electrons, and quanta of electromagnetic radiation (photons). These NTP species are highly reactive. In contrast to thermal processes (such as thermal plasma), an NTP process directs electrical energy to induce favorable gas chemical reactions, rather than using the energy to heat the gas. Therefore, NTP is much more energy-efficient than thermal plasma.
NTPs can be generated by electric discharge in the gas or injection of electrons into the gas by an electron beam. Among the various types of electric discharge reactors, pulse corona and dielectric barrier (silent) discharge reactors are very popular for their effectiveness and efficiency. However, pulse corona reactors have the major disadvantage of requiring special pulsed power supplies to initiate and terminate the pulsed corona. Consequently, dielectric barrier discharge has become a fast growing technology for pollution control.
Cylindrical and planar reactors are two common configurations for dielectric barrier discharge reactors. Both of these configurations are characterized by the presence of one or more insulating layers in a current path between two metal electrodes, in addition to the discharge space. Other dielectric barrier discharge reactors include packed-bed discharge reactors, glow discharge reactors, and surface discharge reactors.
Dielectric barrier discharge NTP reactor designs based upon the use of one or more dielectric ceramic pieces coated with a conductive material arranged to form the dielectric barrier-conductor-dielectric barrier configurations are known. A stacked shape non-thermal plasma reactor and element for use with diesel engines and the like is disclosed in commonly assigned U.S. patent application Ser. No. 09/511,590 filed Feb. 23, 2000 entitled “Design and Method of Manufacturing A Plasma Reactor For Treating Auto Emissions—Stacked Shapes”, which is hereby incorporated by reference herein in its entirety. Disclosed therein is a non-thermal plasma reactor prepared from a formed shape of dielectric material used as a building block for creating the region of the non-thermal plasma reactor wherein plasma is generated. The formed shape defines an internal cell in the plasma reactor having an exhaust passage for flowing exhaust gas to be treated there through. A conductive print

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