Patent Publication Number: US-6906280-B2

Title: Fast pulse nonthermal plasma reactor

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with Government support under Contract No. W-7405-ENG-36, awarded by the Department of Energy. The Government has certain rights in this invention. 
    
    
     CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not Applicable 
     INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention pertains generally to for processing pollutant containing gases, and more particularly to nonthermal plasma reactors. 
     2. Description of Related Art 
     The emission and discharge of volatile organic compounds (VOCs) are strictly regulated by the U.S. Conservation and Recovery Act (RCRA), the National Pollutant Discharge Elimination System (NPDES), and the National Emissions Standards for Hazardous Air Pollution regulations (NESHAPS). Technical and regulatory difficulties associated with current VOC treatment methods such as air-stripping (dilution), activated-carbon absorption, incineration, and thermal-catalytic treatment have prompted the search for alternatives. The drawbacks of present methods result in ineffective treatment, the generation of large secondary waste streams, and increased costs. 
     The present invention has recognized the prior art drawbacks, and has provided the below-disclosed solutions to one or more of the prior art deficiencies. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention can be used to effectively treat VOCs while meeting regulations in a timely and economical fashion. In addition to VOCs, the present invention can be used to treat other air pollutants and hazardous/toxic chemicals in gases (e.g., acid rain precursors NOx and SOx, odor causing chemicals, chemical/biological warfare agents, and industrial emissions). Furthermore, to operate fossil-fueled motor vehicles and other combustion-related engines or machinery under higher efficiency and reduced pollution output conditions in the future, it is desirable to have clean-burning, energy-efficient, hydrocarbon liquid fuels. Such higher-order hydrocarbons can be synthesized using a nonthermal plasma (NTP) device according to the present invention. 
     By way of example, and not of limitation, the present invention is a device that employs electrical discharges/nonthermal plasmas in a gaseous medium to destroy air pollutants or undesirable chemicals/chemical or biological agents, to process chemicals, or to synthesize chemical compounds. In nonthermal plasmas, the electrons are “hot”, while the ions and neutral species are “cold” which results in little waste enthalpy being deposited in a process gas stream. This is in contrast to thermal plasmas, where the electron, ion, and neutral-species energies are in thermal equilibrium (or “hot”) and considerable waste heat is deposited in the process gas. 
     In the present invention, the NTP reactor is applied to gas streams containing hazardous/toxic, or other undesirable pollutants or contaminants and to gas streams that are to be processed (i.e., changed in chemical form or transformed into other useful products). 
     In one aspect of the present invention, a nonthermal plasma reactor includes a discharge cell and a charging assembly. The charging assembly provides plural high voltage pulses to the discharge cell. Each high voltage pulse has a rise time between one and ten nanoseconds and a duration between three and twenty nanoseconds. 
     In another aspect of the present invention, a nonthermal plasma reactor includes a discharge cell and a first charging assembly and a second charging assembly that are electrically connected to the discharge cell. The charging assemblies alternatingly provide opposite polarity high voltage pulses to the reactor. 
     In yet another aspect of the present invention, a nonthermal plasma reactor includes a first capacitor plate and a second capacitor plate. A dielectric layer is disposed between the first capacitor plate and the second capacitor plate. Further, a spark gap switch is electrically connected to the first capacitor plate and a first electrode is electrically connected to the second capacitor plate. A second electrode is slightly spaced from the first electrode and a dielectric layer is disposed adjacent to the first electrode. Moreover, a gas discharge gap is established between the dielectric layer and the second electrode. In this aspect of the present invention, the first capacitor plate and the second capacitor plate provide plural high voltage pulses to the discharge cell. 
     In yet still another aspect of the present invention, a nonthermal plasma reactor includes a first capacitor plate, a second capacitor plate, and a first dielectric layer that is disposed therebetween. A first spark gap switch is electrically connected to the first capacitor plate. The reactor further includes a third capacitor plate, a fourth capacitor plate, and a second dielectric layer that is disposed therebetween. Further, a second spark gap switch is electrically connected to the third capacitor plate. In this aspect of the present invention, a first electrode is electrically connected to the second capacitor plate and the fourth capacitor plate and a second electrode is slightly spaced from the first electrode. A dielectric layer is disposed adjacent to the first electrode and a gas discharge gap is established between the dielectric layer and the second electrode. The first capacitor plate, the second capacitor plate, the third capacitor plate, and the fourth capacitor plate alternatingly provide opposite polarity high voltage pulses to the reactor. 
     In still yet another aspect of the present invention, a nonthermal plasma reactor includes a discharge cell and means for providing plural high voltage pulses to the discharge cell. Each high voltage pulse has a rise time of not more than ten nanoseconds. 
     In another aspect of the present invention, a method for treating pollutant containing gases includes providing a discharge cell. Plural high voltage pulses are provided to the discharge cell. Each high voltage pulse has a rise time of not more than ten nanoseconds. 
     In yet another aspect of the present invention, a method for treating pollutant containing gases comprises providing a discharge cell. Plural opposite polarity high voltage pulses are alternatingly provided to the discharge cell. Each opposite polarity high voltage pulse has a rise time of not more than ten nanoseconds. 
     An object of the invention is to provide a relatively high degree of contaminant removal. 
     Another object of the invention is to reduce contaminant removal costs. 
     Another object of the invention is to provide more efficient chemical processing/synthesis. 
     Another object of the invention is to provide for nonthermal treatment of pollutant containing gases. 
     Another object of the invention is to provide for simultaneous destruction and removal of multiple pollutants. 
     Another object of the invention is to eliminate the need for fuels or catalysts. 
     Another object of the invention is to provide a broad dynamic range for treatment of both rich and lean streams. 
     Another object of the invention is to provide for higher active species production efficiency with extremely short, high E/N pulses, where E/N is the reduced electric field strength when the process gas experiences electrical breakdown. 
     Further aspects of the invention will be brought out in the following portions of the specification, wherein the detailed description is for the purpose of fully disclosing preferred embodiments of the invention without placing limitations thereon. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) 
       The invention will be more fully understood by reference to the following drawings which are for illustrative purposes only: 
         FIG. 1  is a schematic diagram of electrical discharge streamers in a gas discharge gap between two electrodes. 
         FIG. 2  is a graph of average electron energy versus reduced electric field (E/N). 
         FIG. 3  is a side view of a first embodiment of a nonthermal plasma reactor with the housing cut away for clarity. 
         FIG. 4  is a schematic diagram of an electric circuit diagram representing the device shown in FIG.  3 . 
         FIG. 5  is a schematic diagram of a resonant-charging circuit for the nonthermal plasma reactor shown in FIG.  3 . 
         FIG. 6  is a graph of reduced electric field versus time. 
         FIG. 7  is a graph of reactor power versus time. 
         FIG. 8  is a side view of a second embodiment of a nonthermal plasma reactor with the housing cut away for clarity. 
         FIG. 9  is a schematic diagram of a circuit utilizing capacitive transfer circuits. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring more specifically to the drawings, for illustrative purposes the present invention is embodied in the apparatus generally shown in FIG.  1  through FIG.  9 . It will be appreciated that the apparatus may vary as to configuration and as to details of the parts, and that the method may vary as to the specific steps and sequence, without departing from the basic concepts as disclosed herein. 
     Referring initially to  FIG. 1 , a positive electrode and a negative electrode are shown and designated  10  and  12 , respectively. A gas discharge gap  14  is established between the electrodes  10 ,  12 . As shown, the gas discharge gap  14  has a width  16 . Moreover, three non-limiting, exemplary discharge streamers  18  are shown between the electrodes  10 ,  12  within the gas discharge gap  14 . Each discharge streamer  18  shown has a head  20  and a tail  22 . 
     It can be appreciated that a voltage pulse can be applied across the electrodes  10 ,  12 . If the applied voltage pulse rise time and pulse duration are comparable to the streamer transit time across the gap  14 , the drive circuit, described below, can influence the development of the discharge across the gap  14 . If the applied electric field rises fast enough, each discharge streamer head  20  can coalesce to create quasi-homogenous discharges. It is to be understood that quasi-homogenous discharges can have very favorable consequences. For example, the discharge operates for a larger fraction of its duration at a higher, and more favorable, reduced electric field, i.e., electric field divided by gas density (E/N). Further, the discharge operates at a higher average electron energy.  FIG. 2 , for example, shows that the average electron energy increases with increasing reduced electric field for oxygen gas. 
     Further results of the quasi-homogenous discharges are greater yields, i.e., number per unit energy, of free radicals and other active species because these yields generally increase with increasing electron temperature. Moreover, with more homogenous discharges, the radicals are spread over a larger volume and have lower peak concentrations. As such, there is less competition from radical-radical interactions which tend to reduce the concentrations of active species and therefore, more active species survive to react with entrained pollutants or feed gas species. 
       FIG. 3  shows a non-limiting, exemplary embodiment of a nonthermal plasma (NTP) reactor, generally designated  50 . As shown in  FIG. 3 , the reactor  50  includes a generally rectangular, box-shaped housing  52  in which a charging assembly  54  and a discharge cell  56  are disposed.  FIG. 3  shows that the charging assembly  54  includes a first capacitor plate  58  and a second capacitor plate  60 . The first capacitor plate  58  rests on a first dielectric layer  62  (e.g., Mylar) which insulates it from the housing  52 . Also, a second dielectric layer  64  is installed between the capacitor plates  58 ,  60 . A spark gap switch  66  is connected to the first capacitor plate  58 . 
     As shown in  FIG. 3 , the discharge cell  56  includes a first electrode  68  and a second electrode  70  that are separated by a dielectric layer  72 . Preferably, the dielectric layer  72  is made from a material such as glass. A gas discharge gap  74  is established between the electrodes  68 ,  70 . It is to be understood that after the capacitor plates  58 ,  60  are charged, the spark gap switch  66  can be used to control the electric pulse delivered across the electrodes  68 ,  70 . It is to be understood that pollutant containing gas can be supplied to the gas discharge gap  74  where it can be treated as described in detail below. 
     Referring now to  FIG. 4 , an electric circuit representing the device shown in  FIG. 3  is shown and is generally designated  100 .  FIG. 4  shows that the circuit  100  includes a first electrode  102  and a second electrode  104  that are separated by a discharge gap  106 .  FIG. 4  also shows that a first dielectric layer  108  and second dielectric layer  110  can be disposed between the electrodes  102 ,  104  and that the discharge gap  106  can be established between the dielectric layers  108 ,  110 . As shown in  FIG. 4 , a first inductor  112  is connected parallel to the electrodes  102 ,  104 . Moreover, a first capacitor  114  and a second capacitor  116  are also installed in the circuit  100  such that they are connected in series to each other and the combination thereof is connected parallel to the first inductor  112  and the electrodes  102 ,  104 . 
       FIG. 4  also shows a second inductor  118  that is connected to the circuit  100  adjacent to the second capacitor  116  the second inductor  118  represents the inherent inductance of the spark gap switch  126 . Further, the circuit  100  is connected to a power source  120  such as a direct current (DC) power source. A resistor  122  is connected between the power source  120  and the circuit  100 . As shown, the circuit  100  is also connected to ground  124 , e.g., at the second electrode  104 . The circuit  100  also includes a switch  126 , e.g. a spark gap switch as described above. It is to be understood that the above described circuit  100  can be used to create a fast-pulse nonthermal discharge between the electrodes  102 ,  104 . 
     Referring now to  FIG. 5 , a resonant-charging circuit is shown and is generally designated  150 . FIG.  5 . shows that the circuit  150  includes a discharge cell  152  having a first electrode  154  and a second electrode  156  separated by a discharge gap  158 . As shown in  FIG. 5 , a first inductor  160  is installed in the circuit so that it is parallel to the electrodes  154 ,  156 . Moreover, a first capacitor  162  and a second capacitor  164  are connected in series to each other and the combination thereof is connected parallel to the first inductor  160  and the electrodes  154 ,  156 . 
       FIG. 5  further shows a transformer  166  installed in the circuit  150 . The transformer  166  includes a low voltage (input) side  168  and a high voltage (output) side  170 . As shown the high voltage side  170  of the transformer  166  is installed in the circuit  150  so that it provides a high voltage signal to the capacitors  162 ,  164 .  FIG. 5  also shows a spark gap switch  172  is installed in the circuit parallel to the second capacitor  164 . The spark gap switch  172  is connected to ground  174  and can be used to control the electric pulses that are delivered to the discharge cell  152  between the electrodes  154 ,  156 . Preferably, in this embodiment, a first diode  176  is installed in the circuit  150  between the spark gap switch  172  and the transformer  166 . 
     As shown in  FIG. 5 , the low voltage side  168  of the transformer  166  is connected to a power source  178  such as an AC power source. A second diode  180  and a resistor  182  are connected parallel to each other and the combination thereof is installed in series within the circuit  150  between the low voltage side  168  of the transformer  166  and the power source  178 . A second inductor  184  is connected in series with the second diode  180  and resistor  182  combination between the power source  178  and the second diode  180  and resistor  182  combination. It is to be understood that the above described circuit  150  can be used to create a fast-pulse nonthermal discharge between the electrodes  154 ,  156  within the discharge cell  152 . 
       FIG. 6  shows a reduced electric field waveform generated, e.g., by the reactor  50  shown in  FIG. 3  with oxygen in the discharge cell  56 , i.e. within the gas discharge gap  74 . As shown in  FIG. 6 , the waveform peaks at approximately one and eight-tenths of a nanosecond (1.8 ns). This is a direct result of a high voltage pulse having an extremely fast rise time and short duration. 
       FIG. 7  shows an exemplary, non-limiting graph of the short-pulse electrical discharge power versus time, e.g., for the reactor  50  shown in FIG.  3 . As shown, the power peaks initially at approximately two and eight-tenths nanoseconds (2.8 ns) and as time elapses the amplitude of the power spikes decrease. Accordingly, very little power is wasted at times when electron temperature is low. 
     Referring now to  FIG. 8  an alternative embodiment of a nonthermal reactor, generally designated  200 . As shown in  FIG. 8 , the reactor  200  includes a generally rectangular, box-shaped housing  202  in which a first charging assembly  204  and a second charging assembly  206  are disposed. Each charging assembly  204 ,  206  is connected to a discharge cell  208  that is disposed in the housing  202  between the charging assemblies  204 ,  206 .  FIG. 8  shows that the first charging assembly  204  includes a first capacitor plate  210  and a second capacitor plate  212 . The first capacitor plate  210  rests on a first dielectric layer  214  which insulates it from the housing  202 . Also, a second dielectric layer  216  is installed between the capacitor plates  210 ,  212 . A spark gap switch  218  is connected to the first capacitor plate  210 . 
     Similar to the first charging assembly  204 , the second charging assembly  206  includes a first capacitor plate  220  and a second capacitor plate  222 . The first capacitor plate  220  rests on a first dielectric layer  224  which insulates it from the housing  202 . Also, a second dielectric layer  226  is installed between the capacitor plates  220 ,  222 . A spark gap switch  228  is connected to the first capacitor plate  220 . 
     As shown in  FIG. 8 , the discharge cell  208  includes a first electrode  230  and a second electrode  232  that are separated by a dielectric layer  234 . Preferably, in this embodiment, the dielectric layer  234  is made, e.g., from glass. A gas discharge gap  236  is established between the electrodes  230 ,  232 . It is to be understood that the capacitor plates  210 ,  212  of the first charging assembly  204  and the capacitor plates  220 ,  222  of the second charging assembly  206  can be oppositely charged. Moreover, the spark gap switches  218 ,  228  can be alternatingly fired in order to alternatingly deliver opposite polarity pulses to the discharge cell  208 . 
     Referring to  FIG. 9 , a circuit diagram utilizing capacitive transfer circuits is shown and is generally designated  250 .  FIG. 9  shows that the circuit  250  includes a first electrode  252  and a second electrode  254  that are separated by a discharge gap  256 .  FIG. 9  also shows that a first dielectric layer  258  and second dielectric layer  260  can be disposed between the electrodes  252 ,  254  and that the discharge gap  256  can be established between the dielectric layers  258 ,  260 . 
     As shown in  FIG. 9 , the circuit  250  includes a first capacitive-transfer circuit  262  and a second capacitive-transfer circuit  264  that provide pulses across the electrodes  252 ,  254  within the discharge gap  256 .  FIG. 9  shows that the first capacitive-transfer circuit  262  includes a storage capacitor  266  and a peaking capacitor  268  that are connected to the circuit  250  in series to each other and in parallel to the electrodes  252 ,  254 .  FIG. 9  also shows that the first capacitive-transfer circuit  262  is connected to a negative power source  270 . A first inductor  272  is installed between the power source  270  and the first capacitive-transfer circuit  262 . Moreover, a second inductor  274  and a switch  276  are shown between the first inductor  272  and the peaking capacitor  268 . It is to be understood that the second inductor  274  shown in the first capacitive-transfer circuit  262  represents the inherent inductance of the switch  276  and the connections associated therewith. 
     Similar to the first capacitive-transfer circuit  262 , the second capacitive-transfer circuit  264  includes a storage capacitor  278  and a peaking capacitor  280  that are connected to the circuit  250  in series to each other and parallel to the electrodes  252 ,  254 .  FIG. 9  also shows that the second capacitive-transfer circuit  264  is connected to a positive power source  282 . A first inductor  284  is installed between the power source  282  and the second capacitive-transfer circuit  264 . Moreover, a second inductor  286  and a switch  288  are shown between the first inductor  284  and the peaking capacitor  280 . It is to be understood that the second inductor  286  shown in the second capacitive-transfer circuit  264  represents the inherent inductance of the switch  288  and the connections associated therewith. 
     It is to be understood that the storage capacitors  266 ,  278  are rapidly switched into the closely coupled peaking capacitors  268 ,  280 . The capacitance of each peaking capacitor  268 ,  280  is less than its neighboring storage capacitor  266 ,  278 . Accordingly, the peaking capacitors  268 ,  280  “ring-up” to a higher voltage than the charge voltage on the storage capacitors  266 ,  278  and electrical discharges are created across the electrodes  252 ,  254 . It is to be understood that in order to deliver very fast time rise pulses to the electrodes  252 ,  254 , the inductances represented by the second inductors  274 ,  286  in each capacitive-transfer circuit  262 ,  264  must be kept very low. 
     It can be appreciated that in each circuit  100 ,  150 ,  250  described above additional circuit elements, e.g., resistors, inductors, etc., can be included in order to facilitate pulse shaping. 
     It is to be understood that each reactor  50 ,  200  is a fast-pulsed nonthermal plasma (NTP) reactor that can be used to generate highly reactive chemical species, such as free radicals. These reactive species, e.g., O-atoms, OH-radicals, N-radicals, excited N 2  and O 2  molecules, HO 2 -radicals, NH-radicals, CH-radicals, etc., readily decompose organic chemicals (e.g., VOCs), oxides of sulfur and nitrogen (SO 2  and NOx), and odor agents (e.g., aldehydes, H 2 S and many others) by breaking their chemical bonds. The result is the production of nonhazardous or easily-managed products. The free radicals, described above, can also play a key role in chemical synthesis, producing desirable products, e.g., creating higher-order hydrocarbon fuels from methane/natural gas. 
     Further, nonthermal plasmas can be created by the reactors  50 ,  200 . As described in detail above, each reactor  50 ,  200  makes use of an extremely fast-pulsed dielectric-barrier discharge arrangement. A high voltage pulse having an extremely fast rise time, approximately one to ten nanoseconds (1-10 ns), and duration, approximately three to twenty nanoseconds (3-20 ns), is applied to the electrodes thereby creating electrical-discharge streamers in the gas. With a short enough rise time, the development of the discharges can be influenced such that the discharge gap undergoes electrical breakdown at a reduced electric field, electric field divided by gas density (E/N), much higher than the static field (or the field with a slower rise time)—a condition sometimes called “overvolting”. This can create a quasi-homogeneous discharge condition in which most of the reactor active volume, i.e., the area between the electrodes, is filled with an electrical discharge having a high average electron energy. The discharges are the source of the active nonthermal plasma (NTP). 
     Each of the above-described NTP reactors  50 ,  200  are able to reduce hazardous compound concentrations in off-gases to very low levels by free-radical “cold combustion” or synthesize desirable chemical products using gaseous feedstocks. It is to be understood that although each NTP reactor  50 ,  200 , described above, has a generally rectangular box shape, each can be modified to have a generally cylindrical shape. 
     Although the description above contains many details, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of this invention. Therefore, it will be appreciated that the scope of the present invention fully encompasses other embodiments which may become obvious to those skilled in the art, and that the scope of the present invention is accordingly to be limited by nothing other than the appended claims, in which reference to an element in the singular is not intended to mean “one and only one” unless explicitly so stated, but rather “one or more.” All structural, chemical, and functional equivalents to the elements of the above-described preferred embodiment that are known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the present claims. Moreover, it is not necessary for a device or method to address each and every problem sought to be solved by the present invention, for it to be encompassed by the present claims. Furthermore, no element, component, or method step in the present disclosure is intended to be dedicated to the public regardless of whether the element, component, or method step is explicitly recited in the claims. No claim element herein is to be construed under the provisions of 35 U.S.C. 112, sixth paragraph, unless the element is expressly recited using the phrase “means for.”