Abstract:
A device ( 10 ) for purifying a gas stream ( 30 ) made up of a plurality of DBD cells ( 12   a,    12   b,    12   c ) in series and, for each of the DBD cells ( 12   a,    12   b,    12   c ), a power supply ( 24   a,    24   b,    24   e ) for providing alternating current to each DBD cell ( 12   a,    12   b,    12   e ).

Description:
FIELD AND BACKGROUND OF THE INVENTION 
     The present invention relates to pollution abatement and, more particularly, to a device for destroying gaseous pollutants by passing the pollutants through a plasma and possibly additional gas. 
     Gases that are hazardous or otherwise undesirable are produced by many commercial and industrial processes. Notable examples include oxides of nitrogen and sulfur, emitted, for example, from internal combustion engines and from power plants; chemical and biological agents such as sarin and tabun; and fluorine-containing greenhouse gases (perfluorocarbons) such as CF 4 , CHF 3 , C 2 HF 5 , C 2 H 2 F 4  and SF 6  that are used in the fabrication of semiconductor devices. There are three general method to control emissions of these gases: 
     1. Control the processes that generate or use such gases to minimize their production or use. 
     2. In the case of gases deliberately introduced to industrial processes such as semiconductor device fabrication, collect and recycle the emitted gases. 
     3. Convert the gases to environmentally safer compounds. 
     The present invention addresses the third general method. Traditionally, the semiconductor industry has incinerated effluent gases. The burners used tend to be large, inefficient and expensive. Recently, it has been proposed to use plasmas, such as are used for generating ozone from oxygen, to destroy unwanted gaseous species. The high energy electrons of a plasma deliver their energy efficiently to atoms and molecules without heating the device which creates the plasma. The modification of the gas molecules is done by direct interaction with the electrons through electron attachment, dissociation or ionization, or through interaction with free radicals generated by the electrons. 
     There are two types of plasmas that may be used for pollution abatement: thermal plasmas and non-thermal plasmas. A thermal plasma is one that is in thermal equilibrium. Such plasmas may be generated by, for example, continuous RF or microwave energy. The particle energy in the plasma is a function of the plasma temperature, on the order of kT, where k is Boltzmann&#39;s constant and T is the plasma temperature. For typical thermal plasmas, the particle energy is on the order of electron volts. Non-thermal plasmas generate much higher electron energies, and therefore are characterized by more efficient energy transfer than thermal plasmas. The disadvantage of non-thermal plasmas is that they are more difficult to control and to keep uniform than are thermal plasmas. 
     Two types of non-thermal plasmas have been considered for pollution abatement: pulsed corona discharge and dielectric barrier discharge (DBD). In pulsed corona discharge, the plasma is generated between two electrodes by a pulse of high voltage across the electrodes, which creates a discharge in the gas between the electrodes. To prevent the creation of a single arc discharge which would carry the entire current and create a non-uniform plasma, the voltage pulse is kept short, on the order of tens of nanoseconds, and is repeated at a rate on the order of hundreds of times per second. The plasma discharge channels thus created do not have enough time to turn into an arc, so many discharge channels are created during the short lifetime of the pulse. Nevertheless, it is difficult to create a very uniform corona discharge. A representative U.S. patent describing a pulsed corona reactor is U.S. Pat. No. 5,490,973, to Grothaus et al. 
     In a DBD device, one or both of the electrodes are covered with an insulator so that the energy for the discharge is supplied capacitatively through the insulator. This limits the amount of energy that each discharge channel can receive. It therefore is possible to generate more channels and obtain a more uniform discharge. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided a device for purifying a gas stream, including: (a) a plurality of DBD cells in series; (b) for each of the DBD cells, a power supply for providing alternating current to the each DBD cell. 
     According to the present invention there is provided a method for purifying a gas stream, including the steps of: (a) providing a plurality of DBD cells in series; and (b) causing the gas stream to flow through said DBD cells. 
     The basic structure of the present invention is a plurality of DBD cells in series. By “series” is meant, not that the cells are electrically in series, for indeed each cell is part of an independent electrical circuit, but that the cells are arranged geometrically so that the gas stream to be purified passes sequentially from one cell to the next. Each cell is provided with its own independent high frequency power supply. In conformity with common usage, these power supplies are referred to herein as supplying “alternating current” to the DBD cells, although the parameter of the power supplies that actually is controlled is the voltage, with the supplied currents then depending on the impedances of the DBD cells according to Ohm&#39;s law. Preferably, the power supplies are switching mode resonant power supplies. 
     The use of several small DBD cells instead of one large DBD cell has the following advantages: 
     1. The smaller capacitance of a small cell makes it easier to drive at high frequencies. At higher frequencies, more discharge channels are created, so the plasma is more uniform. The smaller power supplies used with the smaller cells are simpler and more efficient than the large power supply that would be needed for a single large cell. 
     2. A plurality of cells is easier to control than a single cell. It is easier and more efficient to control the concentrations of chemical species inside a plurality of small cells than inside a single large cell. According to the present invention, sensors are provided to measure the concentrations of gaseous species emerging from each cell and plasma conditions inside each cell. Power supply parameters such as frequency and voltage are adjusted adaptively, in accordance with the results of the measurements, to enhance the destruction of the unwanted species. 
     3. A reactor made of a plurality of cells is modular. If one cell must be taken off line for maintenance, the reactor can continue to function. 
     The scope of the present invention also includes the injection of an additive gas, such as nitrogen or oxygen, into the gas stream, at the inlet to one or more of the cells, to enhance the destruction of the unwanted gaseous species and their conversion to safe gases. As in the case of the power supply parameters, the rate of injection of the additive gas is controlled in accordance with the measured concentrations and plasma conditions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein: 
     FIG. 1 is a schematic illustration of a basic device of the present invention; 
     FIGS. 2A through 2F are cross sections of alternative constructions of a DBD cell; 
     FIG. 3 is a schematic axial cross section of another DBD cell; 
     FIG. 4 is a schematic diagram of a power supply; 
     FIG. 5 is a schematic illustration of a preferred device of the present invention; 
     FIG. 6 is an axial cross section of an improved embodiment of the DBD cell of FIG.  2 E. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention is of a modular DBD reactor which can be used to destroy pollutant species in a gas stream, converting the pollutant species to environment-friendly gases. Specifically, the present invention can be controlled adaptively to optimize the destruction of the unwanted species. 
     The principles and operation of a modular DBD reactor according to the present invention may be better understood with reference to the drawings and the accompanying description. 
     Referring now to the drawings, FIG. 1 is a schematic illustration of a basic device  10  of the present invention. Device  10  includes three DBD cells  12   a ,  12   b  and  12   c . Each DBD cell includes two electrodes: cell  12   a  includes electrodes  14   a  and  16   a , cell  12   b  includes electrodes  14   b  and  16   b , and cell  12   c  includes electrodes  14   c  and  16   c . Electrode  14   a  is covered by a dielectric layer  18   a . Electrode  14   b  is covered by a dielectric layer  18   b . Electrode  14   c  is covered by a dielectric layer  18   c . Electrode  16   a  is covered by a dielectric layer  20   a . Electrode  16   b  is covered by a dielectric layer  20   b . Electrode  16   c  is covered by a dielectric layer  20   c . Dielectric layers  18   a  and  20   a  define between them a gap  22   a . Dielectric layers  18   b  and  20   b  define between them a gap  22   b . Dielectric layers  18   c  and  20   c  define between them a gap  22   c . Electrodes  16   a ,  16   b  and  16   c  are grounded. Electrodes  14   a ,  14   b  and  14   c  are connected to high frequency power supplies  24   a ,  24   b  and  24   c  respectively. 
     Each DBD cell has an input end, into which a gas stream  30  to be purified enters, and an output end, from which gas stream  30  exits after treatment in that cell: cell  12   a  has an input end  26   a  and an output end  28   a , cell  12   b  has an input end  26   b  and an output end  28   b , and cell  12   c  has an input end  26   c  and an output end  28   c . The cells are arranged in series, so that gas stream  30 , after exiting cell  12   a  via output end  28   a , immediately enters cell  12   b  via input end  26   b , and after exiting cell  12   b  via output end  28   b , immediately enters cell  12   c  via input end  26   c.    
     Electrodes  14   a ,  14   b ,  14   c ,  16   a ,  16   b , and  16   c  are made of an electrically conductive material, preferably a metal, most preferably copper, aluminum or stainless steel. Dielectric layers  18   a ,  18   b ,  18   c ,  20   a ,  20   b  and  20   c  are made of an electrical insulator, preferably a ceramic such as alumina or quartz. For simplicity, only three DBD cells are shown in FIG.  1 . Typically, device  10  includes 10 cells, but the scope of the present invention includes any convenient number of cells in device  10  greater than or equal to 2. If more than 5 cells are used, one cell may be taken off line for maintenance without disabling the entire device. 
     FIGS. 2A,  2 B,  2 C,  2 D,  2 E and  2 F show alternative constructions of DBD cells. FIG. 2A is an axial cross section of a cell  12   d  in which only one electrode  14   d  is covered by a dielectric layer  18   d ; electrode  16   d  is bare. Electrode  16   d  and dielectric layer  18   d  define between them a gap  22   d  through which gas stream  30  flows. FIGS. 2B,  2 C,  2 D,  2 E and  2 F are transverse cross sections of cylindrical DBD cells. FIG. 2B shows a cell  12   e  that includes two electrodes  14   e  and  16   e , in the form of cylindrical sections, on opposite sides of a dielectric tube  18   e . In a device  10  including cells such as cell  12   e , gas stream  30  flows through interior  22   e  of dielectric tube  18   e . FIG. 2C shows a cell  12   f  in which one electrode is an electrically conductive cylinder  14   f  and the other electrode is an electrically conductive wire  16   f  concentric with cylinder  14   f . The inner surface of cylinder  14   f  is coated with a cylindrical dielectric layer  18   f . In a device  10  including cells such as cell  12   f , gas stream  30  flows through interior  22   f  of cylinder  18   f . FIG. 2D shows a cell  12   g  in which the electrodes are concentric, electrically conductive cylinders  14   g  and  16   g . The inner surface of cylinder  14   g  is coated with a cylindrical dielectric layer  18   g . The outer surface of cylinder  16   g  is coated with a cylindrical dielectric layer  20   g . In a device  10  including cells such as cell  12   g , gas stream  30  flows through annulus  22   g  defined by dielectric cylinders  18   g  and  20   g . FIG. 2E shows a cell  12   h  in which the electrodes are concentric, electrically conductive cylinders  14   h  and  16   h , cylinder  16   h  being solid rather than hollow. Cylinder  14   h  is bare. The surface of cylinder  16   h  is coated with a cylindrical dielectric layer  20   h . In a device  10  including cells such as cell  12   h , gas stream  30  flows through interior  22   h  of cylinder  14   h . FIG. 2F shows a cell  12   i  in which there are three concentric electrodes: hollow, electrically conductive cylinders  14   i  and  15 , and solid, electrically conductive cylinder  16   i . The inner surface of cylinder  14   i  is coated with a cylindrical dielectric layer  18   i . The surface of cylinder  16   i  is coated with a cylindrical dielectric layer  20   i . In a device  10  including cells such as cell  12   i , gas stream  30  flows through both an annulus  23  defined by cylinders  15  and  20   i  and an annulus  23 ’ defined by cylinders  18   i  and  15 . In the operation of a cell such as cell  12   i , cylinder  15  is connected to a power supply such as power supply  24   a ,  24   b  or  24   c , and both cylinders  14   i  and  16   i  are grounded. 
     FIG. 6 is an axial cross section of an improved embodiment 112 of cell  12   h . Cell  112  includes a cylindrical outer electrode  114  and an inner electrode  116  having an axially varying transverse width w. In particular, w varies sinusoidally with a decreasing amplitude from input end  126  to output end  128 . Inner electrode  116  is coated with a dielectric layer  120  whose transverse width also varies axially. Inner surface  117  of outer electrode  114  is coated with a layer  118  of a catalyst such as black platinum or titanium for catalyzing the destruction of the pollutant species in gas stream  30 . Cell  112  also has, at output end  128 , an exit aperture  130  that limits the velocity of gas stream  30 , thereby increasing the pressure of the gas in interior  122  of cell  112 . Alternatively, catalyst layer  118  is on inner electrode  116  and dielectric layer  120  is on inner surface  117  of outer electrode  114 . 
     Typically, the lengths of DBD cells of the present invention are on the order of several centimeters, as are the diameters of cylindrical DBD cells and the widths of planar cells. The thicknesses of the dielectric layers and the widths of the gaps between dielectric layers, or between a dielectric layer and an opposite bare electrode, typically are on the order of several millimeters. 
     FIG. 3 is a schematic axial cross section of a DBD cell  12   j  that is geometrically similar to cells  12   a ,  12   b  and  12   c , having two electrodes  14   j  and  16   j  whose facing surfaces are coated with dielectric layers  18   j  and  20   j , dielectric layers  18   j  and  20   j  defining between them a gap  22   j . Cell  12   j  is provided with a mechanism for changing width  21  of gap  22   j . Specifically, cell  12   j  is mounted within an insulating housing that consists of an upper part  32  rigidly attached to electrode  14   j  and a lower part  34  rigidly attached to electrode  16   j . Parts  32  and  34  have matching threaded holes through which are inserted threaded rods  36 . Threaded rods  36  are extensions of the shafts of stepping motors  38 . Stepping motors  38  are activated as described below to rotate rods  36  to change width  23  during the operation of a device  10  that includes a cell such as cell  12   j . The mechanism illustrated in FIG. 3 is only illustrative: the scope of the present invention includes all suitable mechanisms for adjusting the interior geometries of the DBD cells. 
     Preferably, power supplies  24   a ,  24   b  and  24   c  are switching mode resonant power supplies, which are simple, efficient and inexpensive. FIG. 4 is a schematic diagram of a representative such power supply  24 . Power supply  24  includes a DC power source  40  in series with a switch  44 , a variable inductance  46 , and the primary winding of a transformer  48 ; and in parallel with a capacitor  42 . The secondary winding of transformer  48  is shown supplying the output AC current of power supply  24  to a DBD cell  12  represented by an equivalent circuit that includes a capacitance  50  in parallel with a resistance  52 . Power source  40  supplies a DC voltage on the order of several tens to hundreds of volts. Capacitor  42  is of low equivalent series resistance, to enable high peak currents through the primary coil of transformer  48 . Transformer  48  isolates power supply  24  from cell  12  and matches the load voltage and impedance of cell  12 . Typically, the peak voltage supplied by the secondary winding of transformer  48  to cell  12  is on the order of about 300 volts to about 100 kilovolts. Variable inductance  46  is used for matching resonant conditions. Capacitance  50  alone represents cell  12  when cell  12  is empty. When cell  12  generates a plasma, the power drawn by the generation of the plasma is represented by resistance  52 . 
     The main limitation on the performance of power supply  24  is the performance of switch  44 . Solid state IGBT switches work well up to frequencies of about 100 kilohertz at voltages up to one to two kilovolts. MOSFET switches can operate at frequencies up to several MHz at voltages between several hundred to several thousand volts, but as the frequency is increased, the power that can be supplied by power supply  24  with a MOSFET switch  44  decreases. In practice, the range of frequencies at which power supply  24  operates is from about 10 kilohertz to about 3 megahertz. 
     In operation, switch  44  is opened and closed at high frequency. A typical mode of operation is opening and closing switch  44  at a frequency of one megahertz at 50% duty. When the switching frequency is equal to the resonant frequency of load capacitance  50  with the parasitic inductance of transformer  48  combined with variable inductance  46 , a high AC voltage is developed across capacitance  50 . The maximum voltage attainable is limited by circuit losses in power supply  24  and by power absorbed by resistance  52 . The optimal voltage and interelectrode gap width is a function of the pressure of gas stream  30 . Device  10  may be operated at pressures of gas stream  30  from sub-Torr pressures to several Bars. Preferably, the pressure is on the order of tens of Torrs and the driving voltages are on the order of kilovolts. 
     Preferably, the high-frequency opening and closing of switch  44  is intermittent, a practice commonly known as “chopping”. This allows the plasma to relax and provides additional variation of the plasma chemistry. Preferably, this chopping is effected at a frequency between about 10 hertz and about 100 kilohertz. 
     FIG. 5 is a schematic illustration of a preferred version of device  10 , including mechanisms for adaptively controlling device  10  during operation. For clarity, DBD cells  12   a ,  12   b  and  12   c  are represented as boxes, with the serial arrangement of cells  12   a ,  12   b  and  12   c  represented by output end  28   a  being adjacent to input end  26   b  and output end  28   b  being adjacent to input end  26   c . Two kinds of sensors are illustrated, one for measuring the concentrations of atomic, ionic and molecular species in gas stream  30  as gas stream  30  transits from cell  12   a  to cell  12   b  and from cell  12   b  to cell  12   c , and the other for monitoring plasma parameters such as temperature, electrical conductivity and plasma density within cells  12   a ,  12   b  and  12   c.    
     Gas species concentrations are measured by laser induced fluorescence. To this end, a collimated beam  62  of monochromatic light from a laser  60  is directed by a beam splitter  64  into the region between output end  28   a  and input end  26   b  and by a mirror  66  into the region between output end  28   b  and input end  26   c . Fluorescence excited in gas stream  30  by beam  62  in the region between output end  28   a  and input end  26   b  and in the region between output end  28   b  and input end  26   c  is detected by spectrometers  68   a  and  68   b , respectively. This measurement arrangement is only illustrative. The scope of the present invention includes all suitable apparati and methods for measuring gas species concentrations, for example by laser interferometry, by infrared absorption spectrometry, or by simply diverting samples of gas stream  30  for on-line chemical analysis, for example using gas chromatography/mass spectrometer or residual gas analysis. Plasma parameters are measured using Langmuir probes  70   a ,  70   b  and  70   c , which protrude into gaps  22   a ,  22   b  and  22   c  respectively via output ends  28   a ,  28   b  and  28   c  respectively. Again, this method of measuring plasma parameters is only illustrative, the scope of the present invention including all suitable apparati and methods for measuring plasma parameters. Electrical signals representative of the readings obtained by spectrometers  68   a  and  68   b  and Langmuir probes  70   a ,  70   b  and  70   c  are conveyed by suitable input lines  74  to a microcomputer-based control system  72 . 
     Also shown in FIG. 5 is a source  80 , of a pressurized additive gas such as oxygen, nitrogen or hydrogen, connected to cell  12   c  by an electronically controlled valve  82  and a conduit  84 . Plasma electrons in the plasma of cell  12   c  ionize the molecules of the additive gas to create free radicals and ionic species that react with the undesired species of gas stream  30  and that interact with the original additive gas molecules. Conduit  84  is disposed to introduce the additive gas into gap  22   c  of cell  12   c  via input end  26   c . For clarity, only introduction of the additive gas into cell  12   c  is illustrated. In fact, the additive gas may be introduced to all of the DBD cells of device  10 . In addition, the additive gas may be introduced to gas stream  30  before gas stream  30  enters device  10  or after gas stream  30  leaves device  10 . 
     Control system  72  transmits control signals to power supplies  24   a ,  24   b  and  24   c  and to valve  82  via suitable control lines  76 . The output frequencies and voltages of power supplies  24   a ,  24   b  and  24   c  and the rate of flow of the additive gas into cell  12   c  thus are adjusted by control system  72  in accordance with the readings obtained from spectrometers  68   a  and  68   b  and from Langmuir probes  70   a ,  70   b  and  70   c  to maximize the destruction of undesired gaseous species in gas flow  30 . If one of the cells of device  10  is constructed in is the manner of cell  12   h  of FIG. 3, gap width  21  also can be adjusted, by appropriate signals sent from control system  72  to stepping motors  38 . For any given gaseous pollution abatement situation, it will be straightforward for one ordinarily skilled in the art to determine how to optimize the frequencies, voltages, gaps and gas flow parameters and to program control system  72  accordingly. For example, one optimal set of parameters for the abatement of the fluorine-containing gases listed above includes a pressure range for gas stream  30  is from about 0.1 Torr to about 200 Torr; a rate of flow on the order of a few hundred sccm for the impurities in gas stream  30  and also on the order of a few hundred sccm for additive gases such as oxygen and hydrogen; and widths of gaps  22   a ,  22   b  and  22   c  between about 1 mm and about 4 mm. 
     Device  10  also can be operated at atmospheric pressure. This ability to operate at atmospheric pressure greatly expands the range of situations to which device  10  is applicable 
     FIG. 7 is a schematic illustration of an expanded embodiment  10 ′ of device  10 . In addition to DBD cells  12   a ,  12   b  and  12   c , device  10 ′ includes three more DBD cells  12   j ,  12   k  and  12   l , also in series. Cells  12   j ,  12   k  and  12   l  are collectively in parallel with cells  12   a ,  12   b  and  12   c . “In parallel” means, not that cells  12   j ,  12   k  and  12   l  are electrically in parallel with cells  12   a ,  12   b  and  12   c , for, indeed, like cells  12   a ,  12   b  and  12   c , each of cells  12   j ,  12   k  and  12   l  has its own high frequency power supply  24   j ,  24   k  and  24   l , respectively; but rather that one portion of gas stream  30  traverses cells  12   a ,  12   b  and  12   c : entering cell  12   a  via input end  26   a , exiting cell  12   a  via output end  28   b  and immediately entering cell  12   b  via input end  26   b , exiting cell  12   b  via output end  28   b  and immediately entering cell  12   c  via input end  26   c , and finally exiting cell  12   c  via output end  28   c ; and another portion of gas stream  30  traverses cells  12   j ,  12   k  and  12   l : entering cell  12   j  via input end  26   j , exiting cell  12   j  via output end  28   j  and immediately entering cell  12   k  via input end  26   k , exiting cell  12   k  via output end  28   k  and immediately entering cell  12   l  via input end  26   l , and finally exiting cell  12   l  via output end  28   l . Embodiment  10 ′ has higher net throughput than embodiment 10, to handle high-volume gas streams  30 . 
     While the invention has been described with respect to a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of the invention may be made.