Abstract:
A gas conversion system using microwave plasma is provided. The system includes: a microwave waveguide; a gas flow tube passing through a microwave waveguide and configured to transmit microwaves therethrough; a temperature controlling means for controlling a temperature of the microwave waveguide; a temperature sensor disposed near the gas flow tube and configured to measure a temperature of gas flow tube or microwave waveguide; an igniter located near the gas flow tube and configured to ignite a plasma inside the gas flow tube so that the plasma converts a gas flowing through the gas flow tube during operation; and a plasma detector located near the gas flow tube and configured to monitor the plasma.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Applications No. 61/501,767, entitled “Gas conversion system,” filed on Jun. 28, 2011, which is incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to gas conversion systems, and more particularly to systems converting gases using multiple gas conversion means with microwave plasma. 
     2. Discussion of the Related Art 
     In recent years, microwave technology has been applied to generate various types of plasma. In some applications, required capacity of gas conversion using plasma is very large, and it requires a high power microwave generator. The existing microwave techniques are not suitable, or at best, highly inefficient due to one or more of the following drawbacks. First, the existing systems lack proper scalability, where scalability refers to the ability of a system to handle varying amounts of gas conversion capacity in a graceful manner or its ability to be enlarged/reduced to accommodate the variation of the gas conversion capacity. For instance, the required gas conversion capacity may widely vary depending on the applications. Second, the economics of scale for a magnetron increases rapidly as the output power increases. For instance, the price of a 10 kW magnetron is much higher than the price of ten 1 kW magnetrons. Third, the system configured with a higher power magnetron would have a possibility that the whole system needs to be shutdown once either magnetron or plasma applicator has an issue. Thus, there is a need for a gas conversion system that has high scalability, less system down time, and is cheaper than currently available gas conversion system without compromising the gas conversion capacity. 
     SUMMARY OF THE INVENTION 
     In one embodiment of the present disclosure, a gas conversion system using a microwave plasma includes: a microwave waveguide for transmitting microwaves therethrough; a gas flow tube passing through the microwave waveguide and configured to transmit the microwaves through the gas flow tube; a first temperature controlling means for controlling a temperature of the microwave waveguide; a temperature sensor disposed near the gas flow tube and configured to measure a temperature of the microwave waveguide; an igniter located near the gas flow tube and configured to ignite a plasma inside the gas flow tube so that the plasma converts a gas flowing through the gas flow tube during operation; and a plasma detector located near the gas flow tube and configured to monitor the plasma. 
     In one embodiment of the present disclosure, a gas conversion system includes: an inlet gas manifold for supplying a gas; and a plurality of gas conversion units connected to the inlet gas manifold and configured to receive the gas therefrom. Each of the plurality of gas conversion units includes: a microwave waveguide for transmitting microwaves therethrough; a gas flow tube passing through the microwave waveguide and configured to transmit the microwaves through the gas flow tube; a first temperature controlling means for controlling a temperature of the microwave waveguide; a temperature sensor disposed near the gas flow tube and configured to measure a temperature of the microwave waveguide; an igniter located near the gas flow tube and configured to ignite a plasma inside the gas flow tube so that the plasma converts a gas flowing through the gas flow tube during operation; and a plasma detector located near the gas flow tube and configured to monitor the plasma. The gas conversion system also includes an outlet gas manifold connected to the plurality of gas conversion units and configured to receive therefrom. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a gas conversion system in accordance with one embodiment of the present invention. 
         FIGS. 2A-2C  are schematic cross sectional views of alternative embodiments of a portion of the gas conversion system in  FIG. 1 . 
         FIGS. 3A-3B  are schematic diagrams of various embodiments of an integrated gas conversion system according to the present invention. 
         FIG. 4  is a schematic diagram of an integrated gas conversion system in accordance with another embodiment of the present invention. 
         FIG. 5  is a schematic cross sectional view of an alternative embodiment of a portion of the gas conversion system in  FIG. 1  according to the present invention. 
         FIG. 6  is a schematic cross sectional view of an alternative embodiment of a portion of the gas conversion system in  FIG. 1  according to the present invention. 
         FIGS. 7A-7D  are top views of alternative embodiments of the gas flow tube in  FIG. 1  according to the present invention. 
         FIGS. 8A-8B  are perspective views of alternative embodiments of the integrated gas conversion system in  FIG. 4  according to the present invention. 
         FIGS. 9A-9B  are perspective views of alternative embodiments of the integrated gas conversion system in  FIG. 4  according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  is a schematic diagram of a gas conversion system  1  for generating microwave plasma and converting gas in accordance with one embodiment of the present invention. As illustrated, the gas conversion system  1  may include: a gas flow tube  26  that is transparent to microwave, such as glass, ceramic, or any other dielectric materials, preferably formed of quartz; a microwave supply unit  11  for providing microwave to the gas flow tube  26 ; and a waveguide  24  for transmitting microwave from the microwave supply unit  11  to the gas flow tube  26 , where the gas flow tube  26  receives a gas and/or gas mixture from a gas supply, such as flue gases. 
     The microwave supply unit  11  provides microwave to the gas flow tube  26  and may include: a microwave generator  12  for generating microwave; a power supply  13  for supplying power to the microwave generator  12 ; and an isolator  15  having a dummy load  16  for dissipating reflected microwave that propagates toward the microwave generator  12  and a circulator  18  for directing the reflected microwave to the dummy load  16 . 
     In one embodiment, the microwave supply unit  11  further includes a coupler  20  for measuring microwave powers; another coupler  17  located on the dummy load  16  to measure reflected microwave power to be dissipated at the dummy load  16 ; and a tuner  22  for reducing the microwave reflected from the gas flow tube  26 . The components of the microwave supply unit  11  shown in  FIG. 1  are well known and are listed herein for exemplary purposes only. Also, it is possible to replace the microwave supply unit  11  with a system having the capability to provide microwave to the gas flow tube  26  without deviating from the present invention. A phase shifter may be mounted between the isolator  15  and the tuner  22 . 
     The gas conversion system  1  may include a high voltage spark igniter  28  on the gas flow tube  26  for an easy ignition of plasma in the gas flow tube  26 ; a top cap  27  having a gas inlet  271  to receive gas and supply it into the gas flow tube  26 ; and a sliding short  35  to adjust a standing wave position for an efficient plasma. The top cap  27  is preferably made of a metal to avoid microwave leakage through the top of the gas flow tube  26 . Gas flow inside the gas flow tube  26  may have a swirling motion since the gas inlet  271  is configured as a side injection. The gas inlet  271  may be configured as a top injection to have a straight flow (not having a swirling motion) or may be configured as an angled injection. 
     The gas conversion system  1  may be used for a flue gas treatment. More particularly, it may be used for conversion of CO2 in the flue gas into CO and O2 by use of the plasma  101 . The gas conversion system  1  may include an inlet gas separator  41  for separating the flue gas into CO2 and other components. The inlet gas separator  41  may use an existing method, such as absorption, cryogenic, or membrane. The inlet gas separator  41  supplies CO2 to the gas flow tube  26  through the gas inlet  271 . A converted gas exhausted from the gas flow tube  26  is supplied to an outlet gas separator  42  for separating the converted gas into CO, O2, and CO2. The outlet gas separator  42  may use an existing method, such as absorption, pressure swing adsorption, or membrane. CO2 separated by the outlet gas separator  42  may be circulated to the gas inlet  271  for further conversion. Thus, the gas separator  42  and a gas line  421  form a gas circulation system. 
       FIG. 2A  is a schematic cross sectional view of an alternative embodiment of a portion of the gas conversion system  1  in  FIG. 1 . As depicted, temperature controlling means  241  and  261  are installed onto the waveguide  24  and the gas flow tube  26  respectively, to control the temperatures of the waveguide  24  and the gas flow tube  26 , respectively. Each of the temperature controlling means  241  and  261  may be a water-cooling system, a cooling system using other coolants, or a heater using a heating medium such as hot water, oil, or gas. The flows of the medium for the temperature controlling means  241  and  261  are shown as arrows  242  and  262 . The temperatures of the waveguide  24  and the gas flow tube  26  may be controlled by adjusting the medium flow rate and by sensing the temperature of waveguide or gas flow tube using a thermometer  29 . 
       FIG. 2B  is a schematic cross sectional view of an alternative embodiment of a portion of the gas conversion system  1  in  FIG. 1 . As depicted, air-cooling means, such as heat sink,  243  and  263  are installed onto the waveguide  24  and the gas flow tube  26  respectively, to control the temperatures of the waveguide  24  and the gas flow tube  26 , respectively. The air flow for cooling is illustrated as arrows  244 . The temperatures of the waveguide  24  and the gas flow tube  26  may be controlled by adjusting air flow rate and by sensing the temperature using a thermometer  29 . 
       FIG. 2C  is a schematic cross sectional view of an alternative embodiment of a portion of the gas conversion system  1  in  FIG. 1 . As depicted, a heat exchanger  264  is installed at downstream of the gas flow tube  26  so that the temperature of the gas exiting the reactor region is maintained at a predetermined level. The reactor region may be insulated with an insulation material  265  so that the gas temperature in the reactor region is maintained at a higher level to thereby increase the conversion efficiency of the reactor. The heat exchanger  264  may be a rapid gas cooling means using a coolant, such as water. 
       FIGS. 3A-3B  are schematic diagrams of various embodiments of an integrated gas conversion system according to the present invention.  FIG. 3A  illustrates an integrated gas conversion system having the four gas conversion systems  1   a - 1   d , where each of the four gas conversion systems  1   a - 1   b  is similar to the system  1  shown in  FIG. 1 . The flue gas is supplied to an inlet gas manifold  51  controlled by a controller  61 . The flue gas supplied to each of the four gas conversion systems  1   a - 1   d  is separated by a gas separator and converted by use of plasma, and subsequently sent to an outlet gas manifold  52 . Since each gas conversion system  1   a - 1   d  has similar mechanisms and functions of the system  1  in  FIG. 1 , gas separation and CO2 circulation are done inside of the gas conversion systems  1   a - 1   d . When the gas conversion system fails to operate, i.e., the plasma is extinguished inadvertently, the controller  61  controls gas distributions from the inlet gas manifold  51  so that the gas is not supplied to the failed gas conversion system. In addition, the controller  61  may control the total gas flow rate supplied to the gas conversion systems depending on the number of the gas conversion systems under operation. A detector for monitoring the plasma in each reactor region is described in conjunction with  FIG. 5 . 
       FIG. 3B  illustrates another integrated gas conversion system having the four gas conversion units  2   a - 2   d . Each gas conversion system  2   a - 2   d  has similar mechanisms and functions of the gas conversion unit  2  in  FIG. 1 . The gas conversion unit  2 , as depicted in  FIG. 1 , does not contain any inlet/outlet gas separator or gas circulation system. The flue gas is supplied to the inlet gas separator  41  and separated CO2 is supplied to the inlet gas manifold  51  controlled by the controller  61 . CO2 supplied to the four gas conversion systems  2   a - 2   d  are converted by plasma, and subsequently sent to the outlet gas manifold  52 . The converted gas collected at the outlet gas manifold  52  is supplied to the outlet gas separator  42 . Since each gas conversion system  2   a - 2   d  does not contain any gas separator or gas circulation system in  FIG. 1 , the gas separation and CO2 circulation are done outside of the gas conversion units  2   a - 2   d . When the gas conversion system fails to operate, i.e., the plasma is extinguished inadvertently, the controller  61  controls gas distributions from the inlet gas manifold  51  so that the gas is not supplied to the failed gas conversion system. In addition, the controller  61  may control the total gas flow rate supplied to the gas conversion systems depending on the number of the gas conversion systems under operation. A detector for monitoring the plasma in each reactor region is described in conjunction with  FIG. 5 . 
     Based on the embodiment shown in  FIG. 3B , one may configure another integrated gas conversion system by moving the outlet gas separator  42  and the CO2 circulation system into each gas conversion systems  2   a - 2   d . Or one may configure another integrated gas conversion system by moving only the outlet gas separator  42  into each gas conversion systems  2   a - 2   d.    
       FIG. 4  illustrates another integrated gas conversion system containing the four gas conversion systems  3   a - 3   d . Each of the gas conversion systems  3   a - 3   d  is similar to the gas conversion unit  2  in  FIG. 1 , with the difference that each of the gas conversion systems  3   a - 3   d  does not include the isolator  15 , the coupler  20 , the tuner  22 , and the sliding short  35 . Each of the gas conversion systems  3   a - 3   d  is fully optimized for efficient plasma generation, and thus these elements are not required for proper operation of the system. The flue gas is supplied to the inlet gas separator  41  and separated CO2 is supplied to the inlet gas manifold  51  controlled by a controller  61 . The separated CO2 is supplied to the four gas conversion systems  3   a - 3   d  having four gas flow tubes  26   a - 26   d , respectively, and subsequently converted by the plasma, and then sent to the outlet gas manifold  52 . The converted gas collected at the outlet gas manifold  52  is supplied to the outlet gas separator  42 . Since each gas conversion system does not have any gas separation or CO2 circulation system, gas separation and CO2 circulation are done outside the gas conversion systems  3   a - 3   d . When the gas conversion system fails to operate, i.e., the plasma is extinguished inadvertently, the controller  61  controls gas distributions from the inlet gas manifold  51  so that the gas is not supplied to the failed gas conversion system. In addition, the controller  61  may control the total gas flow rate supplied to the gas conversion systems depending on the number of the gas conversion systems under operation. A detector for monitoring the plasma in each reactor region is described in conjunction with  FIG. 5 . 
       FIG. 5  is a schematic cross sectional view of an alternative embodiment of a portion of the gas conversion system in  FIG. 1  according to the present invention. As depicted, a plasma detector  30  is installed onto the waveguide  24  to monitor the plasma, to thereby monitor the proper operation of the gas conversion system  1 . The plasma detector  30  may be an optical sensor to detect a light emission of plasma or a temperature sensor to detect a temperature increase due to plasma generation. The plasma detector  30  may be installed on the gas flow tube  26  instead. 
       FIG. 6  is a schematic cross sectional view of an alternative embodiment of a portion of the gas conversion system  1  in  FIG. 1  according to the present invention. A mesh plate  32 , preferably a grounded metal mesh plate, is installed at the bottom of the gas flow tube  26  to enhance the stability of gas flow and plasma, and to avoid a microwave leakage through the bottom of the gas flow tube  26 . The mesh size of the mesh plate  26  is much smaller than the wavelength of the microwave generated by the microwave supply unit  11 . It is preferred to install the mesh plate  32  at a location having a certain distance from the bottom surface of the waveguide  24  to have enough volume for plasma and avoid arcing inside the gas flow tube  26 . 
       FIGS. 7A-7D  are top views of alternative embodiments of the gas flow tube  26  in  FIG. 1  according to the present invention. As depicted, the cross sectional shape of the gas flow tubes  266 - 269  may be circle, oval, square, rectangle, or hexagon. It should be apparent to those of ordinary skill that other suitable geometrical shape can be used. 
       FIG. 8A  is a perspective view of an alternative embodiment of the integrated gas conversion system in  FIG. 4  according to the present invention. As depicted, the integrated gas conversion module  4  includes a plurality of, say fifty, gas conversion systems  3 . It contains an inlet gas manifold  51   a  controlled by a controller (not shown) and an outlet gas manifold  52   a . Each gas conversion system  3  is slidably mounted so that it can be easily accessed when maintenance is required. 
       FIG. 8B  is a perspective view of an alternative embodiment of the integrated gas conversion system in  FIG. 4  according to the present invention. As depicted, an integrated gas conversion system  5  includes a plurality of, say one hundred and ninety two, gas conversion modules  4 . It contains an inlet gas manifold  51   b  controlled by a controller (not shown) and an outlet gas manifold  52   b . Each gas conversion module  4  is slidably mounted so that it can be easily accessed when maintenance is required. The flue gas is supplied to the inlet gas separator (not shown) and separated CO2 is supplied to the inlet gas manifold  51   b  and then supplied to each gas conversion system  3  through the inlet gas manifold  51   a  on the gas conversion modules  4 . The gas converted by plasma is collected to the outlet gas manifold  52   b  through the outlet gas manifold  52   a  on the gas conversion modules  4 , and then delivered to the outlet gas separator (not shown). The operations before the inlet gas separator and after the outlet gas separator including CO2 circulation are the same as the system shown in  FIG. 4 , and the descriptions are not repeated for brevity. 
       FIG. 9A  is a perspective view of an alternative embodiment of the integrated gas conversion system in  FIG. 4  according to the present invention. As depicted, the integrated gas conversion module  400  includes a plurality of, say sixty, gas conversion systems  3 . It contains an inlet gas manifold  51   a  controlled by a controller (not shown) and an outlet gas manifold  52   a . Each gas conversion system  3  is radially arranged so that gas tubing is concentrated at the center for ease of plumbing and the human operator has enough space for maintenance. 
       FIG. 9B  is a perspective view of an alternative embodiment of the integrated gas conversion system in  FIG. 4  according to the present invention. As depicted, an integrated gas conversion system  500  includes a plurality of, say twenty, gas conversion modules  400 . It contains an inlet gas manifold  51   b  controlled by a controller (not shown) and an outlet gas manifold  52   b . The flue gas is supplied to the inlet gas separator (not shown) and separated CO2 is supplied to the inlet gas manifold  51   b  and then supplied to each gas conversion system  3  through the inlet gas manifold  51   a  on the gas conversion modules  400 . The gas converted by plasma is collected to the outlet gas manifold  52   b  through the outlet gas manifold  52   a  on the gas conversion modules  400 , and then delivered to the outlet gas separator (not shown). The operations before the inlet gas separator and after the outlet gas separator including CO2 circulation are the same as the system shown in  FIG. 4 , and the descriptions are not repeated for brevity. 
     It is noted that the integrated gas conversion systems shown in  FIGS. 3A ,  3 B, and  4  have only four gas conversion systems. It is also noted that the integrated gas conversion module shown in  FIG. 8A  and the integrated gas conversion system shown in  FIG. 8B  have fifty gas conversion systems and the one hundred and ninety two gas conversion modules, respectively. However, it should be apparent to those of ordinary skill in the art that the module or system may include any other suitable number of gas conversion modules or systems. Likewise, integrated gas conversion modules shown in  FIGS. 9A and 9B  may have other suitable number of gas conversion systems and modules. 
     The price of the microwave generator  12   a , especially the magnetron, increases rapidly as its power output increases. For instance, the price of ten magnetrons of the commercially available microwave oven is considerably lower than that of one high power magnetron that has an output power ten times that of the microwave oven. Thus, the multiple gas conversion systems in  FIGS. 3A-8B  allow the designer to build a low cost gas conversion system without compromising the total conversion capacity. Also, it allows for establishing a system having less system down time when a failure occurs by controlling the gas distribution. 
     It should be understood, of course, that the foregoing relates to exemplary embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.