WO2019221644A1

WO2019221644A1 - Plasma torch for thermal plasma jet generation

Info

Publication number: WO2019221644A1
Application number: PCT/SA2018/050028
Authority: WO
Inventors: Saud BIN JUWAIR; Abdullah ALABDULY; Ibrahim ALSHUNAIFI
Original assignee: King Abdulaziz City For Science And Technology
Priority date: 2018-05-14
Filing date: 2018-10-22
Publication date: 2019-11-21

Abstract

A non-transferred arc plasma torch with a method for the generation of plasma jet at high temperatures are disclosed. The main constituents of the torch assembly are a multiple-arc-producing cathode, a coolant distributing cell and an adjustable anode. The cathode comprises primary and secondary annular rods pointing towards the anode. The primary rod is pointing towards the center of the anode and is higher in length and diameter than the secondary ones. The secondary rods are similar in length and width. An internal coolant distribution cell was designed for internal cooling of the rods. The anode was configured to allow external adjustment of the gap between the electrodes. The electrodes were mounted into a jacketed cylindrical housing consisting an internal path within the jacket for efficient recycling of the coolant. The system is initiated by forming multiple arcs between the cathode and the anode leading to the ionization of the forming plasma gas that passed continuously through the torch. Thus, the power efficiency of the torch enhanced and the lifetime of the electrodes increased. Eventually, a thermal plasma jet exits the torch through the anode nozzle.

Description

PLASMA TORCH FOR THERMAL PLASMA JET GENERATION

Detailed Description Background of the Invention

Thermal plasma torches have found a wide range of industrial and environmental applications. These applications include, but not limited to, Nano-powder synthesis, metals purification, synthesis gas production via coal or organic waste gasification, decomposition of toxic chemicals (e.g. chemical weapons), etc. In general, these torches are employed for producing thermal plasma jet. These torches, comprises a cathode, hollow anode, a discharge gap, and are supplied with power and a forming plasma gas. The plasma torches is typically initiated by forming an electric field between the electrodes with a sufficient strength to ionize and break down the plasma forming gas (e.g. argon, helium, air, etc.) into cations and free electrons. As the concentration of the ionized gas increased, the gap between electrodes became electrically conductive and facilitated the formation of a plasma arc between the electrodes. Consequentially, the plasma gas that passes continuously between the arc and the internal wall of the anode undergoes heating and conversion to plasma stream before it exits the torch in the form of a hot jet.

Despite their wide and expanding range of applications, the conventional plasma torches suffer from the short lifetimes of the electrodes and low efficiency of power utilization. These shortcomings depress the economic feasibility of employing this type of plasma torches. Therefore, The main objectives of the current invention are three folds: (i) to develop a plasma torch with an enhanced power efficiency, (ii) to provide a mechanisms for extending the life of the electrodes and (iii) reduce the costs associated with the electrodes replacement. For achieving these objectives, a plasma torch was designed to simultaneously generate multiple arcs so that the volumetric ratio of the plasma arcs to the forming plasma gas is increased. Thus, the rates of heat and mass exchanges between the plasma arcs and the forming plasma gas

SUBSTITUTE SHEETS (RULE 26) are increased. In addition, the formation of multiple arcs are assumed to reduce the rates of electrodes erosion due to the distribution of current load and temperature on several rods instead of a single one. Therefore, the lifetimes of the cathode rods are expected to increase significantly. Another advantage of utilizing multiple rods instead of a single large cathode is to increase the surface area available for convective heat exchange with the forming plasma gas, which provides an additional mechanism of cathode cooling. Also, the invention disclosed herein includes an adjustable anode to facilitate (i) external adjustment of the discharge gap and (ii) the replacement of the anode without disassembling the torch components. The adjustability of the discharge gap can be employed to compensate the change in the length of cathode rods induced by gradual erosion so that the working hours of the electrodes can be increased. These design features are incorporated with efficient cooling mechanisms of the electrodes to extend the lifetime of electrodes further. Finally, and in accordance with the third objective of this invention, the simplicity of the structure of the electrodes are expected to play an essential role in reducing the costs and time required for their maintenance.

General Description of the Invention

The main objectives of the current invention are three folds: (i) to develop a plasma torch with enhanced power efficiency via increasing the rates of heat and mass exchanges between the plasma arc and the plasma forming gas, (ii) to provide a mechanisms for extending the life of the electrodes and (iii) reduce the costs associated with the electrodes replacement. For achieving the first objective, the plasma torch disclosed herein was designed to simultaneously generates multiple arcs to increase the volumetric ratio of plasma arcs to the plasma forming gas to enhance the rates of mass and heat transfer. Furthermore, the formation of multi arcs, reduces the erosion rates of the electrodes due to the distribution of current load and temperature on several rods instead of a single one. Therefore, the lifetimes of the cathode rods are expected to increase significantly. Another advantage of utilizing multiple rods instead of a single large cathode is to increase the surface area available for convective heat exchange with the plasma forming gas, which provides additional mechanism of cathode cooling. In addition, the invention disclosed herein include an adjustable anode to facilitate (i) adjusting the discharge gap and (ii) replacement of the anode without disassembling most of the torch components. The adjustability of the discharge gap can be

SUBSTITUTE SHEETS (RULE 26) employed to compensate the change in the length of cathode rods induced by erosion to increase the working hours of the electrodes. Theses design features were incorporated with effective cooling mechanisms of the electrodes to further prolong the lifetime of electrodes. Finally, the simplicity of the structure of the electrodes plays important role in reducing the costs and time required for their maintenance.

Brief Description of the Drawings

Figure 1 shows a schematic cross-sectional view of the non-transferred arc plasma torch.

Figure 2 shows schematics of the coolant distribution cylinder (a) external view, (b) cross-sectional view and (c) a bottom view.

Figure 3 shows a schematic of the tubes employed for passing the coolant from the distribution cell to the rods of the cathode.

Figure 4 shows a schematic of the thermal insulator disc employed for separation of the coolant distribution cell from the cathode cylinder.

Figure 5 shows schematics of the cathode cylinder.

Figure 6 shows schematics of the rods employed for arc generation.

Figure 7 shows schematics of the electric insulator separating the cathode set from the housing cylinder.

Figure 8 shows schematics of the jacketed cylinder employed for accommodating the electrodes and confinement of the discharge gap.

Figure 9 shows a 3D drawing of the adjustable anode employed for plasma jet generation.

Detailed Description of the Invention

The plasma torch disclosed herein comprises of a cathode cylinder 6 contains a set of a primary 13 and secondary annular rods 12 extending axially and pointing towards a hollow anode 22 with a nozzle 24. The primary (i.e. central) cathode rod 13 is pointing towards the center of the anode and is longer and wider in diameter than the secondary ones 12. The secondary cathode rods 12 are similar in length and width. The primary 13 and secondary 12 rods are separated from the anode 22 by a conical-shape gap in the anode

SUBSTITUTE SHEETS (RULE 26) 25. The cathode rods 12 and 13 are open and threaded at one end 36 and closed at the other 14. The closed ends of the rods 14 are pointing towards the anode 22 while the threaded ends 36 are connected to a perforated cathode cylinder 6.

As can be seen from fig. 5, the cathode cylinder 6 is flanged on both sides with a wide opening at one side 34 to provide a room for a thermal insulator dick 4 and an insert for the coolant-distributing cell 1. The other side of the cathode cylinder 31 is a flat base with threaded holes 35 to allow assembly of the primary 13 and secondary 12 rods. As can be seen from fig.5, the cathode cylinder contains a coolant outlet port 5 to pass the coolant returned from the cathode rods out of the torch. The flanges 32 and 33 are employed for attaching the cathode cylinder to the coolant-distributing cell 1 and a flanged insulating cylinder 8, respectively. The insulation cylinder 8 detailed in fig. 7 is preferably made from an electrically nonconductive material (e.g. polymer or ceramic) to prevent the jacketed housing 10 and the cathode cylinder 6 from contacting. It should be noted that the flanges could be replaced with threaded joints.

Figures 2 (a), (b) and (c) show the coolant distributing cell 1 employed for efficient cooling of the cathode rods. The cell 1 comprises a coolant inlet port 2, a flange 3 for attachment to the cathode cylinder 6 and a flat base 26 with threaded holes 27 and 28. The threaded holes 27 and 28 are used for attaching primary and secondary stainless steel tubes 15. These tubes facilitate the pass of the coolant to the cathode rods through the cathode cylinder for internal cooling. These tubes are threaded at one end 29 to allow attachment to the coolant distribution cylinder 1. In addition, the external diameters of the coolant tubes 27 and 28 are smaller than the internal diameters of the corresponding cathode rods 13 and 12. Thus gaps are formed to outlet the coolant to the cathode cylinder 6. The coolant distribution cylinder is separated from the cathode cylinder by a layer of a thermal insulator 4. The thermal insulator can be any material with low thermal conductivity such that the rate of heat exchange between the cathode cylinder 6 and the coolant distribution cylinder 1 is significantly minimized. For further extending the cathode rods lifetime, they may be capped with refractory materials.

SUBSTITUTE SHEETS (RULE 26) The rods of the cathode 12 and 13 and the anode 22 are mounted into a jacketed cylinder 10 shown in fig. 8. The jacketed cylinder 10 is preferably made of stainless steel and comprises a feed gas inlet port 11, coolant inlet 19 and outlet 20 ports and an internally threaded room for the anode 23. The jacket of the housing cylinder was configured in such way to include a coolant confining channel to direct the coolant liquid to pass from the inlet port 19 to the nearest point to the torch nozzle 21 in where the coolant is allowed to fill the jacket and pass out of the torch via an outlet port 20. This configuration was adapted to ensure sufficient cooling of the anode and to prevent the formation of stagnant zones of the coolant in the jacket 18. In addition, since the inlet 19 and the outlet 20 ports are positioned at the same level on the jacket 18, the direction of the coolant within the jacket can be switched depending on the orientation of the torch. For instance, when the torch is pointing upward, the inlet port 19 can be employed as a coolant outlet. The jacketed housing cylinder 18 is separated from the cathode set (1 and 6) by an electrically insulating material 8.

Figure 9 shows a 3D drawing of the anode 22. From the figure, it can be seen that the anode is a hollow cylinder consists of a conical gap 25 for electrical discharge and nozzle 39, with externally threaded end 38 and outer holes 40. The anode is preferably made of copper due to the high thermal conductivity to facilitate heat exchange with the jacketed cylinder 10. The outer holes of the anode 40 with the threaded end of the anode 38 facilitate the movement of the anode within the jacketed cylinder 10. This configuration was adapted to (i) facilitate the adjustment of the discharge gap without disassembling the torch, (ii) increase the lifecycles of the electrodes via compensating the loss on electrodes by erosion and (iii) reduce the time required for anode replacement.

The operational mechanism of the torch disclosed herein is expected to be similar to that generally employed in the conventional torches with some exceptions. Generally, the distance between the cathode rods and the anode defines the electric discharge zone, in which the formation of arcs takes place. The gap between the cathode rods and anode can be varied depending on the operational conditions such as the feed gas. Initially, cooling water should be

SUBSTITUTE SHEETS (RULE 26) passed through the cooling channels to maintain the electrodes temperatures. The forming plasma gas should be passed through the torch with a predetermined flow rate. The plasma torch disclosed herein can produce a plasma jet from any ionisable gas (e.g. argon, air, steam, helium, nitrogen or mixtures thereof). The torch is preferably powered using a direct current (DC) power supply. Plasma can be initiated by applying a high voltage between the rods of the cathode and the anode to begin the breakdown of the forming plasma gas into ions. With increasing the concentration of the ionized gas between the electrodes, the discharge gap becomes more conductive and facilitate the formation of arcs between the rods of the cathode and the anode. After initiation of the plasma arcs, the current can then be adjusted to the desired level of operation. Consequentially, the forming plasma gas that passes between the arc and the internal wall of the anode undergoes heating and conversion to plasma before it exits the anode nozzle in the form of a hot plasma jet.

SUBSTITUTE SHEETS (RULE 26)

Claims

1- The plasma torch assembly disclosed herein was designed to simultaneously generates multiple arcs to enhance the thermal efficiency of the torch and to reduce the erosion rates of the electrodes. The main constituents of the torch assembly are a multiple-arc-producing cathode, a coolant-distributing unit, a jacketed cylinder and an adjustable anode.

2- The multiple-arc -producing cathode according to claim 1 comprises a cathode cylinder (6) contains a set of a primary (13) and secondary annular rods (12) extending axially and pointing towards a hollow anode (22) with a nozzle.

3- The cathode cylinder (6) according to claim 2 is flanged on both sides with a wide opening at one side 34 to provide a room for a thermal insulator dick (4) and an insert for the coolant-distributing cell (1). The other side of the cathode cylinder (31) is a flat base with threaded holes (35) to allow assembly of the primary (13) and secondary (12) rods.

4- The cathode rods (12) and (13) according to claims 2 and 3 are open and threaded at one end (36) and closed at the other (14). The closed ends of the rods (14) are pointing towards the anode (22) while the threaded ends (36) are connected to a perforated cathode cylinder (6).

4- The primary rod (13) according to claim 2 is pointing towards the center of the anode and is longer and larger in diameter than the secondary rods (12).

5- The secondary rods (12) according to claim 2 are similar in length and diameter; the number and dimensions of the secondary rods may be varied depending on the torch dimensions and the input power characteristics.

6- The coolant distribution unit (1) according to claiml is designed for internal cooling of the primary (13) and secondary (12) rods of the cathode. The cell (1) comprises a cylinder with a coolant inlet port (2), a flange (3) for attachment to the cathode cylinder (6) and a flat base (26) with threaded

SUBSTITUTE SHEETS (RULE 26) holes (27) and (28). The threaded holes (27) and (28) are for attaching primary and secondary stainless steel tubes (15). These tubes (15) facilitate the pass of the coolant to the cathode rods through the cathode cylinder for internal cooling. These tubes (15) are threaded at one end (29) to allow attachment to the coolant distribution unit (1). In addition, the external diameters of the coolant tubes (27) and (28) are smaller than the internal diameters of the corresponding cathode rods (13) and (12).

7- The coolant distribution unit (1) according to the claims 1 and 6 is separated from the cathode cylinder (6) by a layer of a thermal insulator (4).

8 -The anode (22) according to claim 1 is a hollow cylinder with a threaded end (38). The anode cylinder contains a conical gap (25) for electrical discharge and a nozzle (39) and outer holes (40) for external adjustment.

9- The anode configuration according to the claim 8 was adapted to (i) facilitate the adjustment of the discharge gap without disassembling the torch, (ii) increase the lifecycles of the electrodes via compensating the loss on electrodes by erosion and (iii) reduce the time required for the replacement of the anode.

10- The rods of the cathode (12) and (13) and the anode (22) are mounted into a jacketed cylinder (10).

11- The jacket (18) of the housing cylinder (10) contains a coolant-confining channel to inforce the coolant liquid to pass from the inlet port (19) to the nearest point to the nozzle (21) in where the coolant is allowed to fill the jacket and pass out of the torch via an outlet port (20). This configuration was adapted to ensure sufficient cooling of the anode and to prevent the formation of stagnant zones of the coolant in the jacket (18). The jacketed housing cylinder (10) is preferably made of stainless steel, comprises a feed gas inlet port (11), coolant inlet (19) and outlet (20) ports, and is threaded at one end to facilitate assembly of the anode (23).

12- The multiple arc generation according to claim 1 provides a larger area available for heat and mass transfer between the plasma arcs and the working gas and hence enhances the thermal efficiency of the torch. In addition, the formation of multi arcs reduces the erosion rates of the

SUBSTITUTE SHEETS (RULE 26) electrodes due to the distribution of current load and temperature on several rods.

SUBSTITUTE SHEETS (RULE 26)