Patent Publication Number: US-2019185770-A1

Title: Modular Hybrid Plasma Gasifier for Use in Converting Combustible Material to Synthesis Gas

Description:
RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 15/248,357, filed Aug. 26, 2016, which claims the benefit of Provisional Patent Application No. 62/210,979 filed Aug. 27, 2015. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to plasma arc reactors and systems. More particularly, the present invention relates to a modular plasma arc reactor and related methods of creating hybrid arc plasmas to gasify heterogeneous materials to produce synthesis gas. A synthesis gas consists mainly of carbon monoxide (CO) and hydrogen (H 2 ). 
     2. Prior Art Description 
     A plasma is commonly defined as a collection of charged particles containing equal numbers of positive ions and electrons, as well as excited neutrals. Although exhibiting some properties of a gas, a plasma is also a good conductor of electricity and can be affected by a magnetic field. One way to generate a plasma is to pass a gas through an electric arc. The arc heats the gas by resistive and radiative heating to very high temperatures within a fraction of a second. Essentially, any gas may be used to produce a plasma in such a manner. Thus, inert or neutral gases (e.g., argon, helium, neon, or nitrogen) may be used. Reductive gases (e.g., hydrogen, methane, ammonia, or carbon monoxide) may also be used, as may oxidative gases (e.g., oxygen or carbon dioxide) depending on how the plasma is to be utilized. 
     Plasma generators, including those used in conjunction with, for example, plasma torches, plasma jets and plasma arc reactors, generally create an electric discharge in a working gas to create the plasma. Plasma generators have been formed as direct current (DC) plasma generators, alternating current (AC) plasma generators, radio frequency (RF) plasma generators and microwave (MW) plasma generators. Plasmas generated with RF or MW sources are called inductively coupled plasmas. For example, an RF-type plasma generator includes an RF source and an induction coil surrounding a working gas. The RF signal sent from the source to the induction coil results in the ionization of the working gas by induction coupling to produce the plasma. DC and AC type generators may include two or more electrodes (e.g., an anode and cathode) with a voltage applied between them. An arc may be formed between the electrodes to heat and ionize the surrounding gas such that the gas obtains a plasma state. The resulting plasma may then be used for a specified process application. 
     Plasma reactors can be used for the high-temperature heating of material compounds to accommodate chemical or material processing. Such chemical and material processing may include the reduction and decomposition of hazardous materials. In other applications, plasma reactors have been utilized to assist in the extraction of a desired material, such as a metal or metal alloy, from a compound that contains the desired material. 
     Process applications utilizing plasma generators are often specialized. Consequently, the associated plasma reactors need to be designed and configured according to highly specific criteria. Such specialized designs often result in a device with limited usefulness. In other words, a plasma reactor which is configured to process a specific type of material using a specified working gas is not likely to be suitable for use in other processes wherein a different material is being processed using a different working gas. 
     In view of the shortcomings in the art, a need exists for a plasma reactor and associated system that has adjustable controls and provides improved flexibility regarding the plasma being generated. For example, it would be advantageous to provide a plasma reactor and system that enables the direct processing of solid materials into a gaseous state. It would further be advantageous to provide a plasma reactor and associated system which produces an improved arc and associated plasma column or volume wherein the arc and plasma volume may be easily adjusted and defined to optimize a plasma. These needs are met by the present invention as described and claimed below. 
     SUMMARY OF THE INVENTION 
     This invention describes a novel modular DC-DC hybrid plasma reactor system for industrial applications including gasification of biomass and non-biomass combustible materials to produce synthesis gas. Synthesis gas is mainly composed of CO and H 2 . The plasma reactor creates a large uniform high temperature (&gt;7000° K) plasma with tailored long residence time for materials processing. The plasma reactor has multiple sets of long electrodes that are placed longitudinally opposite each other within modular plasma units. The plasma units can be stacked to form an elongated plasma zone. Materials can continuously flow from one modular plasma unit into the next. In this configuration, an energy cascading effect is created. Due to the cascading energy from upstream plasma units, the bottom-most modular plasma unit produces the brightest plasma illumination. 
     Each plasma unit defines an internal plasma zone that is accessible through access ports. Electrode assemblies extend into the access ports. Each of the electrode assemblies has an electrode tip that is positioned within the internal plasma zone at a selected insertion depth. Each electrode tip is mounted in a tubular support jacket. A gas conduit for a supplied working gas surrounds at least a portion of the tubular support jacket. An arc is created at the electrode tip. A working gas flows through the gas conduit and is directed into the arc, therein creating plasma within the internal plasma zone. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the present invention, reference is made to the following description of exemplary embodiments thereof, considered in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a front view and schematic of an exemplary embodiment of a plasma gasifier apparatus containing multiple plasma units; 
         FIG. 2  is a selectively cross-sectioned top view of a plasma unit used in the exemplary embodiment of a plasma gasifier apparatus; 
         FIG. 3  is a fragmented perspective view of a plasma unit used in the exemplary embodiment of a plasma gasifier apparatus; 
         FIG. 4  shows a cross-sectional view of an electrode assembly used within the plasma gasifier apparatus; 
         FIG. 5  shows a first configuration for a tubular support jacket within the electrode assembly; and 
         FIG. 6  shows a second configuration for a tubular support jacket within the electrode assembly. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     Although the present invention plasma gasifier can be embodied in many ways, only one exemplary embodiment has been selected for the purposes of illustration and discussion. The exemplary embodiment represents one of the best modes contemplated for the invention. The illustrated embodiment, however, is merely exemplary and should not be considered a limitation when interpreting the scope of the claims. 
     Referring to  FIG. 1  in conjunction with  FIG. 2 , a plasma gasifier apparatus  10  is shown that internally generates a hybrid plasma. The plasma is used to process incoming material  12  and convert that incoming material into a synthesis gas  14 . The plasma gasifier apparatus  10  contains one or more plasma units  16  that are concentrically stacked. Although one plasma unit  16  can be used, performance is optimized through the use of a plurality of plasma units  16 . In this description, the term “hybrid” is used to refer to a “field free” (current and voltage free) plasma flame from one or more of the plasma units  16  to a “field active” (current and voltage active) arc state by superimposing an electric discharge within one or more plasma units  16 . 
     In  FIG. 1 , the plasma gasifier apparatus  10  is shown containing four stacked plasma units  16 . It should be understood that in actuality the plasma gasifier apparatus  10  can have any plurality of stacked plasma units  16 . By stacking the plasma units  16 , the plasma zone within the plasma gasifier apparatus  10  is lengthened. This produces an increase in the processing time for the incoming materials  12  introduced into the plasma gasifier apparatus  10 . The modular configuration created by stacking plasma units  16  enables an operator to manipulate the power settings in each of the plasma units  16  to achieve an overall temperature profile for the plasma gasifier apparatus  10 . An operator can also add or subtract modular plasma units  16  to achieve the desired residence time for complete gasification of a particular class of incoming material  12 . 
     Referring to  FIG. 2  in conjunction with  FIG. 1 , each plasma unit  16  has an annular body  18  with an inner wall  20  and an outer wall  22 . The inner wall  20  defines a central plasma zone  24 . The inner wall  20  is refractory and capable of containing the heat of the plasma without degradation. A preferred material for the inner wall  20  is graphite, however, certain refractory ceramics can also be used. A gap space  26  exists between the inner wall  20  and the outer wall  22 . The gap space  26  is packed with insulation  28 , such as high temperature ceramic fibers. In one embodiment of the insulation  28 , the ceramic fibers are, but not limited to, Zirconia fibers. Alternatively, granulated sand and/or granulated oxide materials can also be used as the insulation  28 . Ceramic fibers or granulated oxide materials have significant advantages over conventional solid high-density blocky oxide insulations. Granulated oxides and/or ceramic fiber blankets are very low-density packing materials. There are significant voids in these materials. The voids have very low thermal conductivity and have excellent thermal insulation properties. Very low-density thermal insulation materials also reduce the overall weight of the gasifier apparatus  10 . 
     The annular bodies  18  and central plasma zones  24  concentrically align when the plasma units  16  are stacked. The central plasma zone  24  of each plasma unit  16  is accessible through a plurality of access ports  30 . Preferably, each plasma unit  16  contains at least eight access ports  30 . Each of the access ports  30  is also lined with a sleeve of refractory material, such as graphite, that can maintain integrity in the heat field of plasma. 
     Most of the access ports  30  in each of the plasma units  16  receive electrode assemblies  32 . Each of the electrode assemblies  32  is surrounded by an insulator  34  that is sized to pass into the access ports  30  with tight tolerances. The tolerances prevent any significant gaps from existing between the insulator  34  and the interior of the access port  30  that can leak plasma out of the plasma gasifier apparatus  10 . As will later be explained in more detail, each of the electrode assemblies  32  contains an electrode tip  36  and a gas conduit  38  (shown in  FIG. 4 ). The electrode tip  36  extends into the central plasma zone  24  and creates an arc with another electrode tip during operation. The gas conduit  38  introduces a working gas  40  into the plasma zone  24  that is converted into plasma by the arc. Each of the electrode assemblies  32  is cooled by a coolant  42 . As such, it will be understood that each of the electrode assemblies  32  is coupled to a power supply  44  to receive electricity, a gas supply to receive the working gas  40 , and a coolant supply to receive coolant  42 . 
     Each plasma unit  16  receives the electrode assemblies  32  in sets of two. As such, each plasma unit  16  can receive two, four, six, eight or more of the electrode assemblies  32 , depending upon the number of access ports  30  present. A first set of electrode assemblies  32  are set at a first position P 1  and a second position P 2  on opposite sides of the central plasma zone  24 . Likewise, a second set of electrode assemblies  32  are set at positions P 3  and P 4 . A third set of electrode assemblies  32  are set at positions P 5  and P 6 . Accordingly, there are three sets of electrode assemblies  32  in the exemplary embodiment. Each set of electrode assemblies  32  contains one anode electrode and one cathode electrode. 
     The positions P 1 , P 2  of the first set of electrode assemblies  32  are disposed radially or circumferentially orthogonal to the positions P 3 , P 4  of the second set of electrode assemblies  32  and the positions P 5 , P 6  of the third set of electrode assemblies within each plasma unit  16 . The angle of separation between the electrode assemblies  32  of the second set and the electrode assemblies  32  of the third set is 90 degrees. The angle of separation between the electrode assemblies  32  of the first set and the electrode assemblies  32  of the second set is 45 degrees. The angle of separation between the electrode assemblies  32  of the first set and the third set is also 45 degrees. 
     Referring to  FIG. 3  in conjunction with  FIG. 2 , it will be understood that each of the electrode assemblies  32  can reciprocally move within the confines of the access ports  30 . The reciprocal movements are controlled by a corresponding linear actuator  46  that attaches to each of the electrode assemblies  32 . Each set of electrode assemblies  32  can be moved synchronously to or independent of each other. In a single plasma unit  16 , the separation (arc gap) between any set of electrode assemblies  32  can be adjusted by moving those electrode assemblies  32  into, or out of, the access ports  30 . 
     In the exemplary embodiment, three sets of electrode assemblies  32  are inserted into each plasma unit  16  through the access ports  30 . Preferably, at least two of the access ports  30  are used for observations of the plasma gasifier apparatus  10  in operation. Electrode assemblies  32  attach to the access ports  30  that are not being used for observation. Each of the electrode assemblies  32  has the linear actuator  46  that controls the movements of the electrode assemblies  32  into and out of the access ports  30 . 
     Referring to  FIG. 4 , it can be seen that each of the electrode assemblies  32  used in the plasma gasifier apparatus  10  includes an electrode tip  36 . The electrode tip  36  is made of a tungsten alloy or some other common plasma electrode material. The electrode tip  36  is mounted to the end of a tubular support jacket  48 , therein sealing one open end of the tubular support jacket  48 . The opposite open end of the tubular support jacket  48  is mounted to the top of a conductive tube  50 . The conductive tube  50  is coupled to an electrode base  52 , wherein the electrode base  52  receives current from the power supply  44 . Any current received at the electrode base  52 , travels through the electrode base  52  and into the conductive tube  50 . The current flows through the conductive tube  50  and into the tubular support jacket  48 . The current then flows through the material of the tubular support jacket  48  and into the electrode tip  36 . 
     The conductive tube  50  and the tubular support jacket  48  combine to define a common internal cooling compartment  54 . The coolant  42  is introduced into the cooling compartment  54  from an external supply. The coolant  42  flows through the conductive tube  50 , the tubular support jacket  48  and around much of the electrode tip  36  itself. In this manner, the coolant  42  directly cools the electrode tip  36 , the tubular support jacket  48  and the conductive tube  50  during operation. 
     The insulator  34  is positioned around the conductive tube  50  and most of the tubular support jacket  48 . The insulator  34  defines a central conduit  56 . The conductive tube  50  and the tubular support jacket  48  extend through the central conduit  56 . A gap space  58  exists between the interior of the central conduit  56  and the exterior surfaces of the conductive tube  50  and the tubular support jacket  48 . The gap space  58  is open proximate the electrode tip  36 . The working gas  40  used in creating the plasma is supplied into the gap space  58 . The working gas  40  travels through the gap space  58  and exits the electrode assembly  32  in a circle around the electrode tip  36 . 
     The insulator  34  can be homogenous. However, it is preferred that the insulator  34  be an assembly. In the shown embodiment, the insulator  34  includes a front-end ceramic section  62  and a back-end plastic section  60 . The two sections  60 ,  62  connect linearly to create the tubular body of the insulator  34 . 
     As previously stated, the working gas  40  exiting the gap space  58  is ejected in a circle around the electrode tip  36 . The generation of plasma is the most effective when the working gas  40  exiting around the electrode tip  36  does not disperse away from any arc that is emanating from the electrode tip  36 . The best way to ensure that the working gas  40  remains in a tight stream is to eject the working gas  40  in a directed laminar flow, rather than a random turbulent flow. 
     Referring to  FIG. 5  and  FIG. 6 , it will be understood that a laminar flow profile can be induced in the working gas  40  by providing flow channels in the exterior of the tubular support jacket  48 .  FIG. 5  shows straight flow channels  64 .  FIG. 5  shows spiral or swirl flow channels  66 . As the working gas  40  flows over the flow channels  64 ,  66 , the working gas  40  is provided with a directed flow, be it straight or spiral. This directed flow tends to be laminar or swirl for the flow rates being used. The directed flow of the working gas  40  through the flow channels  64 ,  66  has other unique performance characteristics. The tubular support jacket  48  is preferably made of a copper alloy that has a much better thermal conductivity than does the tungsten alloy of the electrode tip  36 . The tubular support jacket  48  is therefore a heat sink to the electrode tip  36 . As the working gas  40  flows through the flow channels  64 ,  66 , the working gas  40  cools the tubular support jacket  48 . This, in turn, cools the electrode tip  36 . This reduces over-heating of the electrode tip  36  and prevents excessive consumption and erosion of the electrode tip  36 . Furthermore, the high electrical conductivity of the tubular support jacket  48  reduces junction resistive heating within the electrode tip  36 . This allows high joule heating to occur at the electrode tip  36  for better thermionic emission of electrons that form and sustain an arc. 
     Referring back to  FIG. 1  and  FIG. 2 , it will be understood that in operation, each set of electrode assemblies  32  (one anode and one cathode) is coupled to the power supply  44 . The electrode assemblies  32  are preferably connected to the power supply  44  through water-cooled cables  70 . The water-cooled cables  70  provide both the cooling and current paths for the electrode assemblies  32 . 
     The coolant  42  for each electrode assembly  32  is supplied via a high-pressure pump. After cooling the electrode tip  36 , the coolant  42  exits the electrode assembly  32  and is stored in a reservoir. When fresh water is available, the coolant  42  in the reservoir  74  is kept cold by a cooling unit  76 , such as a portable water heat exchanger. In another embodiment, when fresh water is not available, the cooling unit  76  can be a chemical based chiller. 
     Arc Ignition and Plasma Formation 
     The arc can be ignited by (i) a high voltage discharge, (ii) a high frequency discharge, or by (iii) touching and withdrawing one electrode set from each other. 
     When high voltage or high frequency discharge is used to ignite an arc, a set of electrode tips  36  is brought in close proximity to each other. After the power supply  44  that supports the electrode assemblies  32  is energized, a high voltage or high frequency discharge is applied across the central plasma zone  24  between electrode tips  36 , which ignite an arc. 
     In a touch and withdrawal method of igniting an arc, the anode and cathode electrode tips  36  from a set of electrode assemblies  32  are brought into contact with each other momentarily after the power supply  44  is energized. As soon as a spark is generated, the electrode assemblies  32  are drawn apart quickly and an arc is ignited. 
     After a first arc is ignited, a second set of electrode assemblies  32  is moved into the first arc region for ignition. The second set of electrode assemblies  32  requires thermal conditioning for a few seconds in the arc before it is self-ignited. Thermal conditioning is required to heat the electrode tips  36  to a sufficient temperature for thermionic emission of electrons to occur. Different plasma units  16  in the same plasma gasifier apparatus  10  can be used to form a combined arc system. In this method, arc systems complement each other in heating the combined arc plasma to achieve a much higher energy state than is possible using a single plasma unit  16 . Additional plasma units  16  in the plasma gasifier apparatus  10  can be ignited in the same way. 
     The use of a plasma gasifier apparatus  10  with two or more plasma units  16  can generate very large and significantly high temperature arcs within the common plasma zone  24  using relatively low input power from each participating plasma unit  16 . 
     Plasma units  16  can be duplicated and stacked onto one other. In this way, when one or more plasma units  16  sustain arcs there is a field free (absence of current and voltage) high-energy plasma tail flame that can flow into other plasma units  16 . In this case the electrode assemblies  32  in other plasma units  16  superimpose discharges in the tail flame and reignite it back into an arc state. The stacked plasma units  16  produce a very large plasma column with very significant energy content. 
     The modular stacking configuration of plasma units  16  can therefore create a “hybrid plasma”. In this concept hybrid means that the “field free” (current and voltage free) plasma flame from the upstream unit is reheated to a “field active” (current and voltage active) arc state by superimposing an electric discharge in the downstream plasma units. The net plasma energy flow from one plasma unit  16  to another is called “energy cascading”. Energy cascading adds energy to the downstream plasma units and allows the downstream plasma units to operate with a lower energy requirement. 
     It will be understood that the embodiments of the present invention that are illustrated and described are merely exemplary and that a person skilled in the art can make many variations to those embodiments. All such embodiments are intended to be included within the scope of the present invention as defined by the claims.