Modular dielectric barrier discharge device for pollution abatement

[0002] 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.

[0003] 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 CF₄, CHF₃, C₂HF₅, C₂H₂F₄and SF₆that are used in the fabrication of semiconductor devices. There are three general method to control emissions of these gases:

[0004] 1. Control the processes that generate or use such gases to minimize their production or use.

[0005] 2. In the case of gases deliberately introduced to industrial processes such as semiconductor device fabrication, collect and recycle the emitted gases.

[0006] 3. Convert the gases to environmentally safer compounds.

[0007] 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.

[0008] 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'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.

[0009] 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.

[0010] 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.

[0011] 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.

[0012] 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.

[0013] 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's law. Preferably, the power supplies are switching mode resonant power supplies.

[0014] The use of several small DBD cells instead of one large DBD cell has the following advantages:

[0015] 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.

[0016] 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.

[0017] 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.

[0018] 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.

[0019] The invention is herein described, by way of example only, with reference to the accompanying drawings, wherein:

[0020] FIG. 1 is a schematic illustration of a basic device of the present invention;

[0021] FIGS. 2A through 2F are cross sections of alternative constructions of a DBD cell;

[0022] FIG. 3 is a schematic axial cross section of another DBD cell;

[0023] FIG. 4 is a schematic diagram of a power supply;

[0024] FIG. 5 is a schematic illustration of a preferred device of the present invention;

[0025] FIG. 6 is an axial cross section of an improved embodiment of the DBD cell of FIG. 2E.

[0026] 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.

[0027] 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.

[0028] 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 12a, 12band 12c. Each DBD cell includes two electrodes: cell 12aincludes electrodes 14aand 16a, cell 12bincludes electrodes 14band 16b, and cell 12cincludes electrodes 14cand 16c. Electrode 14ais covered by a dielectric layer 18a. Electrode 14bis covered by a dielectric layer 18b. Electrode 14cis covered by a dielectric layer 18c. Electrode 16ais covered by a dielectric layer 20a. Electrode 16bis covered by a dielectric layer 20b. Electrode 16cis covered by a dielectric layer 20c. Dielectric layers 18aand 20adefine between them a gap 22a. Dielectric layers 18band 20bdefine between them a gap 22b. Dielectric layers 18cand 20cdefine between them a gap 22c. Electrodes 16a, 16band 16care grounded. Electrodes 14a, 14band 14care connected to high frequency power supplies 24a, 24band 24crespectively.

[0029] 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 12ahas an input end 26aand an output end 28a, cell 12bhas an input end 26band an output end 28b, and cell 12chas an input end 26cand an output end 28c. The cells are arranged in series, so that gas stream 30, after exiting cell 12avia output end 28a, immediately enters cell 12bvia input end 26b, and after exiting cell 12bvia output end 28b, immediately enters cell 12cvia input end 26c.

[0030] Electrodes 14a, 14b, 14c, 16a, 16b, and 16care made of an electrically conductive material, preferably a metal, most preferably copper, aluminum or stainless steel. Dielectric layers 18a, 18b, 18c, 20a, 20band 20care 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.

[0031] FIGS. 2A, 2B, 2C, 2D, 2E and 2F show alternative constructions of DBD cells. FIG. 2A is an axial cross section of a cell 12din which only one electrode 14dis covered by a dielectric layer 18d; electrode 16dis bare. Electrode 16dand dielectric layer 18ddefine between them a gap 22dthrough which gas stream 30 flows. FIGS. 2B, 2C, 2D, 2E and 2F are transverse cross sections of cylindrical DBD cells. FIG. 2B shows a cell 12ethat includes two electrodes 14eand 16e, in the form of cylindrical sections, on opposite sides of a dielectric tube 18e. In a device 10 including cells such as cell 12e, gas stream 30 flows through interior 22eof dielectric tube 18e. FIG. 2C shows a cell 12fin which one electrode is an electrically conductive cylinder 14fand the other electrode is an electrically conductive wire 16fconcentric with cylinder 14f. The inner surface of cylinder 14fis coated with a cylindrical dielectric layer 18f. In a device 10 including cells such as cell 12f, gas stream 30 flows through interior 22fof cylinder 18f. FIG. 2D shows a cell 12gin which the electrodes are concentric, electrically conductive cylinders 14gand 16g. The inner surface of cylinder 14gis coated with a cylindrical dielectric layer 18g. The outer surface of cylinder 16gis coated with a cylindrical dielectric layer 20g. In a device 10 including cells such as cell 12g, gas stream 30 flows through annulus 22gdefined by dielectric cylinders 18gand 20g. FIG. 2E shows a cell 12hin which the electrodes are concentric, electrically conductive cylinders 14hand 16h, cylinder 16hbeing solid rather than hollow. Cylinder 14his bare. The surface of cylinder 16his coated with a cylindrical dielectric layer 20h. In a device 10 including cells such as cell 12h, gas stream 30 flows through interior 22hof cylinder 14h. FIG. 2F shows a cell 12iin which there are three concentric electrodes: hollow, electrically conductive cylinders 14iand 15, and solid, electrically conductive cylinder 16i. The inner surface of cylinder 14iis coated with a cylindrical dielectric layer 18i. The surface of cylinder 16iis coated with a cylindrical dielectric layer 20i. In a device 10 including cells such as cell 12i, gas stream 30 flows through both an annulus 23 defined by cylinders 15 and 20iand an annulus 23’ defined by cylinders 18iand 15. In the operation of a cell such as cell 12i, cylinder 15 is connected to a power supply such as power supply 24a, 24bor 24c, and both cylinders 14iand 16iare grounded.

[0032] FIG. 6 is an axial cross section of an improved embodiment 112 of cell 12h. 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.

[0033] 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.

[0034] FIG. 3 is a schematic axial cross section of a DBD cell 12jthat is geometrically similar to cells 12a, 12band 12c, having two electrodes 14jand 16jwhose facing surfaces are coated with dielectric layers 18jand 20j, dielectric layers 18jand 20jdefining between them a gap 22j. Cell 12jis provided with a mechanism for changing width 21 of gap 22j. Specifically, cell 12jis mounted within an insulating housing that consists of an upper part 32 rigidly attached to electrode 14jand a lower part 34 rigidly attached to electrode 16j. 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 12j. 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.

[0035] Preferably, power supplies 24a, 24band 24care 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.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] 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 12a, 12band 12care represented as boxes, with the serial arrangement of cells 12a, 12band 12crepresented by output end 28abeing adjacent to input end 26band output end 28bbeing adjacent to input end 26c. 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 12ato cell 12band from cell 12bto cell 12c, and the other for monitoring plasma parameters such as temperature, electrical conductivity and plasma density within cells 12a, 12band 12c.

[0040] 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 28aand input end 26band by a mirror 66 into the region between output end 28band input end 26c. Fluorescence excited in gas stream 30 by beam 62 in the region between output end 28aand input end 26band in the region between output end 28band input end 26cis detected by spectrometers 68aand 68b, 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 70a, 70band 70c, which protrude into gaps 22a, 22band 22crespectively via output ends 28a, 28band 28crespectively. 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 68aand 68band Langmuir probes 70a, 70band 70care conveyed by suitable input lines 74 to a microcomputer-based control system 72.

[0041] Also shown in FIG. 5 is a source 80, of a pressurized additive gas such as oxygen, nitrogen or hydrogen, connected to cell 12cby an electronically controlled valve 82 and a conduit 84. Plasma electrons in the plasma of cell 12cionize 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 22cof cell 12cvia input end 26c. For clarity, only introduction of the additive gas into cell 12cis 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.

[0042] Control system 72 transmits control signals to power supplies 24a, 24band 24cand to valve 82 via suitable control lines 76. The output frequencies and voltages of power supplies 24a, 24band 24cand the rate of flow of the additive gas into cell 12cthus are adjusted by control system 72 in accordance with the readings obtained from spectrometers 68aand 68band from Langmuir probes 70a, 70band 70cto 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 12hof 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 22a, 22band 22cbetween about 1 mm and about 4 mm.

[0043] 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

[0044] FIG. 7 is a schematic illustration of an expanded embodiment 10′ of device 10. In addition to DBD cells 12a, 12band 12c, device 10′ includes three more DBD cells 12j, 12kand 12l, also in series. Cells 12j, 12kand 12lare collectively in parallel with cells 12a, 12band 12c. “In parallel” means, not that cells 12j, 12kand 12lare electrically in parallel with cells 12a, 12band 12c, for, indeed, like cells 12a, 12band 12c, each of cells 12j, 12kand 12lhas its own high frequency power supply 24j, 24kand 24l, respectively; but rather that one portion of gas stream 30 traverses cells 12a, 12band 12c: entering cell 12avia input end 26a, exiting cell 12avia output end 28band immediately entering cell 12bvia input end 26b, exiting cell 12bvia output end 28band immediately entering cell 12cvia input end 26c, and finally exiting cell 12cvia output end 28c; and another portion of gas stream 30 traverses cells 12j, 12kand 12l: entering cell 12jvia input end 26j, exiting cell 12jvia output end 28jand immediately entering cell 12kvia input end 26k, exiting cell 12kvia output end 28kand immediately entering cell 12lvia input end 26l, and finally exiting cell 12lvia output end 28l. Embodiment 10′ has higher net throughput than embodiment 10, to handle high-volume gas streams 30.

[0045] 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.