High-power, carbon dioxide plasma torch and uses thereof in plasma assisted waste conversion

Abstract

A high power, water-cooled plasma torch designed for operation with carbon dioxide as the working gas. The torch comprises a housing supporting coaxially aligned rear and front electrodes, both manufactured from copper or copper alloy. The rear electrode features a closed inner end and an open outer end while the front electrode features an inner cylindrical bore and terminates at its outer end into a cup-shaped face; between these electrodes is positioned an annular vortex generator, consisting of tangential apertures for the controlled introduction of carbon dioxide gas.

Description

[0001] HIGH-POWER, CARBON DIOXIDE PLASMA TORCH AND USES THEREOF IN PLASMA ASSISTED WASTE CONVERSION

[0002] FIELD OF THE INVENTION

[0003] This invention pertains to waste conversion systems and in particular, a high-power, carbon dioxide plasma torch and uses thereof in plasma assisted waste conversion.

[0004] BACKGROUND OF THE INVENTION

[0005] Plasma-assisted waste conversion processes such as those described in U.S. Patent No. 8,372,169 and International Publication No. WO 2011 / 106895 convert waste feedstock into a synthesis gas (syngas).

[0006] In plasma-assisted waste conversion processes, the plasma torches are used as sources of high-temperature heat (e.g., melting applications), and / or sources of ionised gas partially consisting of ions, electrons and radicals, which aid in speeding up chemical reactions such as decomposition of tars in the syngas.

[0007] Torches typically used in such systems are similar to those described in U.S. Patent No. 4,587,397 and U.S. Patent No. 5,451 ,740 incorporated by reference herein.

[0008] Generally, these torch types consist of two hollow, water-cooled copper electrodes separated by a gap insulator. A device receives compressed working gas and circulates it in a swirling motion between, into and out of the electrodes. Typically, the working gas used in the plasma torch is air because it is both economical and plentiful.

[0009] The syngas produced from such processes can be used in boilers, gas engines or gas turbines to produce electricity or to produce diesel fuel, ethanol, methanol, synthetic natural gas or hydrogen. The Fischer-Tropsch process is used to produce liquid fuels from syngas.

[0010] Components in the syngas including nitrogen, argon and nitrogen compounds, such as HCN, negatively impact downstream processes and / or may be pollutants. These undesirable components in the syngas can be difficult or expensive to remove.

[0011] The final composition of the syngas produced in the process is impacted by process conditions, feedstock, inputs into the process and the plasma working gas. The use of air as the working gas for the plasma torch contributes to the undesirable production of HCN and NH3 in the syngas. Alternative working gases have been contemplated in the art. Plasma torches using CO2 as a working gas were studied by Kaminska (2018), Engeling and Meier (2020), and Vadikkeettil et al (2021).

[0012] The torch described by Vadikkettil is a low power (50 kW) torch design with straight polarity having rod-type thoriated tungsten cathode and a copper anode. A combination of CO2 and air (20 Ipm) together with a small amount of argon is used as the working gas.

[0013] Similarly, the torch described by Kaminska was 50 kW torch having a rod-type thoriated tungsten cathode and a segmented copper anode. A complex mixture of gases including Ar or N2 at the cathode and Ar, N2, H2, or CO2 at the anode was used.

[0014] Engeling, and Meier (2020) also investigated and compared CO2 with air as the carrier gas for a waste-to-gas process, where the plasma is used as the heat source for direct waste conversion. Their study suggested that air plasma degrades and converts the solids to the gaseous product more completely than CO2 plasma due to its higher temperature and reactive oxygen.

[0015] Chan in US6810821 B2 describes a hazardous waste treatment method and apparatus that utilizes a combination of C02 / oxygen as the working gas to avoid nitrous oxide formation. In particular, Chan describes a system for treating the off-gas from a waste treatment system that reduces the carbon black present in the off-gas while avoiding the production of nitrogen oxides and other pollutants. The system includes an afterburner which a plasma torch having a nitrogen-free working gas, which, in one embodiment, is a carbon dioxide and oxygen mixture.

[0016] The plasma arc ionizes the working gas, thereby creating atomic oxygen, which assists in the removal of carbon black from the off gas.

[0017] This background information is provided for the purpose of making known information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention. SUMMARY OF THE INVENTION

[0018] An object of the present invention is to provide a high-power, carbon dioxide plasma torch and uses thereof in plasma assisted waste conversion.

[0019] In accordance with an aspect of the present invention, there is provided a high power, cooled plasma torch configured for carbon dioxide working gas comprising a torch housing, a rear electrode mounted within the housing, the rear electrode comprising a tubular copper or copper alloy member having a closed inner end and an open outer end, wherein the bore of the rear electrode has a diameter of between 23 / 32” and 7 / 8, a front electrode comprising a tubular copper or copper alloy member having a bore therethrough, wherein the bore of the front electrode has a diameter of between 11 / 16” and 27 / 32” at an inner cylindrical end portion, the front electrode extending at its outer end into an outward facing cup, the front electrode being mounted within the housing and spaced apart from and in coaxial alignment with the rear electrode, the front electrode having the inner cylindrical end adjacent the open outer end of the rear electrode, and a vortex generating means for generating a vortical flow of a working gas, wherein the vortex generating means is an annular ring having a plurality of tangentially directed apertures which is mounted between the rear electrode and the front electrode and in coaxial alignment therewith and wherein each aperture has a diameter of between 0.047” and 0.086”, wherein the high power, cooled plasma torch is at least a 500 KW plasma torch.

[0020] In accordance with some embodiments, the vortex generator comprises 8 apertures, wherein each aperture has a one-sixteenth of an inch (1 / 16”) diameter.

[0021] In accordance with some embodiments, the rear electrode bore diameter is twenty-five thirty seconds of an inch (25 / 32”).

[0022] In accordance with some embodiments, the front electrode bore diameter is three quarters of one inch (3 / 4”).

[0023] In accordance with one embodiment, the vortex generator comprises 8 apertures, wherein each aperture is with one-sixteenth of an inch (1 / 16”) diameter, the rear electrode bore diameter twenty-five thirty seconds of an inch (25 / 32”), and the front electrode bore diameter is three quarters of one inch (3 / 4”). In accordance with another aspect of the present invention, there is provided a system for providing a reduced nitrogen plasma, the system comprising the plasma torch of the invention, a working gas supply operatively conducted to the vortex generator, and a power supply comprising an arc starter, wherein the arc starter comprises a high voltage transformer, a voltage doubler circuit to increase the voltage to about 8850 volts and a high voltage capacitor and a high voltage discharging circuit.

[0024] In accordance with another aspect of the present invention, there is provided a method for refining a syngas, the method comprising the steps of (a) delivering the syngas to a refractory- lined chamber; (b) providing oxygen into said chamber; (c) torch heating said chamber with one or more of the plasma systems of the invention, and thereby producing refined syngas; and (d) removing the refined syngas from said chamber.

[0025] In accordance with another aspect of the present invention, there is provided an optimised high-power system for the partial or total replacement of compressed air with carbon dioxide gas (CO2) in hollow metal electrode water cooled torches and in particular to its use towards minimising the amount of nitrogen, nitrogenous compounds and argon in the synthetic gas produced in plasma-assisted waste conversion systems. Optimisation centres on the selection of the various component sizes (such as front electrode bore diameter, rear electrode bore diameter and vortex generator hole size along with operating parameters to achieve the objective of maximizing the combined electrode life. In preferred embodiments, the front electrode life and rear electrode life are substantially equal.

[0026] BRIEF DESCRIPTION OF THE DRAWINGS

[0027] These and other features of the invention will become more apparent in the following detailed description in which reference is made to the appended drawings.

[0028] FIG 1 (A) illustrates the plasma torch in accordance with one embodiment of the invention. FIG 1 (B) details the coolant inlet, coolant outlet and plasma gas inlet of the plasma torch of FIG 1 (A). FIG 1 (C) illustrates the added length to reach rear electrode area along with coolant flow around it and plasma gas being directed to the vortex generator of the plasma torch of FIG 1(A). FIG 1 (D) illustrates the electrode gap, gap insulator and front electrode detail of the plasma torch of FIG 1 (A). FIG 2(A) graphically represents the main components of the plasma torch of FIG 1 , including coolant, plasma gas, plasma plume and electrical connections. FIG 2(B) illustrates the expected arc attachment (AA) patterns on real-life electrode shapes for varying plasma gas flowrates and current scenarios. FIG 2(C) shows the desired arc attachment area on the front electrode face c / w valuation numbers indicating distance from the bore hole (range 0-5).

[0029] FIG 3 illustrates test results obtained by substituting a certain mass or volume of air with carbon dioxide (0% - 100%) at various power levels of operation.

[0030] FIG 4 shows a detailed graph comparing only the voltage as a function of percent substitution by mass and volume of CO2 respectively at 240 kW starting power (500A constant) operation.

[0031] FIG 5(A) shows a comparison of the maximum arc attachment as a function of the vortex generator size for the same electrode configuration at 360 kW operation. FIG 5(B) shows the substantiating calculations for optimal vortex generator size selection.

[0032] FIG 6 shows a comparison of the maximum arc attachment for three different electrode configurations with the same vortex generator size at the same power level.

[0033] FIG 7(A) shows a comparison of arc attachment of two plasma torches FE50RE52VG76 using air as a working gas and FE48RE50VG63 using CO2 as working gas. FE50RE52VG76 using CO2 as a working gas is also shown.

[0034] FIG 7(B) shows a comparison of arc attachment trends at low and high voltages for plasma torch FE50RE52VG76 using air as a working gas and FE48RE50VG63 using CO2 as a working gas.

[0035] FIG 8 shows the voltage vs flow for the “100% CO2” operation at the optimal configuration at various power levels and comparison of operation with “100% air”.

[0036] FIG 9 shows an illustration of plasma arc torch starter.

[0037] FIG 10 details reference feedstock composition, feed rate and other inputs FIG 11 details syngas production summary data from conversion of feedstock described in FIG 10.

[0038] FIG 12 details syngas production detailed data from conversion of feedstock described in FIG 10.

[0039] FIG 13 illustrates CFD results with no plasma torches in the system (only partial oxidation)

[0040] FIG 14 illustrates CFD results with plasma torches in the system using air as plasma gas.

[0041] FIG 15 illustrates CFD results with plasma torches in the system using carbon dioxide as plasma gas.

[0042] DETAILED DESCRIPTION OF THE INVENTION

[0043] The invention provides a high-power, carbon dioxide plasma torch and uses thereof in plasma assisted waste conversion. The geometry, operating conditions and electrodes of the plasma torch of the invention are configured for the use of carbon dioxide as a working gas thereby reducing nitrogen in the plasma. In some embodiments, the working gas is a combination of carbon dioxide and air. In other embodiments, the working gas is carbon dioxide. Optionally, the working gas is supplemented with oxygen.

[0044] In some embodiments, the working gas is at least 5 % carbon dioxide, at least 50% carbon dioxide, at least 75% carbon dioxide, at least 95% carbon dioxide, at least 99% carbon dioxide (by mass).

[0045] In embodiments, where the working gas is a combination of carbon dioxide, the remainder of the working gas is air and / or oxygen. The amount of air in the working gas is determined based on the intended use of the syngas and the impact the presence of nitrogen has on those downstream uses.

[0046] The invention further provides a method of reducing or minimizing the amount of nitrogen and nitrogen containing compounds in the syngas produced using plasma-assisted waste conversion. Minimizing nitrogen compounds including HCN, NH3 and nitrogen oxides in the syngas may also facilitates downstream gas clean-up by reducing known pollutants and compounds that are poisonous to downstream processes. The invention still further provides a method of producing a syngas from waste that has reduced or minimal nitrogen containing compounds or is nitrogen free. In such embodiments, the waste is converted to syngas using a waste conversion system that comprises an upstream horizontal moving grate gasifier configured to receive feedstock and a downstream carbon recovery vessel, optionally a vertical fixed bed updraft gasifier with associated solids material recovery vessel. Appropriate waste conversion systems are known in the art and include those described in U.S. Patent No. 8,372,169 and International Publication No. WO 2011 / 106895 both of which are incorporated by reference herein. Optionally, the waste conversion system is configured to minimize the amount of nitrogen containing gas inputted into conversion process, for example by limiting air in the feedstock and eliminating air input in the torches and other areas.

[0047] The high-power, carbon dioxide plasma torch of the invention was designed for operation at nominally 500 kW but said plasma torch can be made to operate higher if the power supply is designed appropriately. The invention described herein is limited to a maximum of 500kW. In some embodiments, the high power plasma torch is configured to have an operating voltage of about 525V to about 625V.

[0048] Referring to FIG. 1 , the plasma torch comprises an outer housing having water-cooled, hollow metal rear and front electrodes mounted therein.

[0049] The outer housing may be of a similar configuration to that described in U.S. Patent No. 4,587,397 incorporated by reference herein and includes one or more coolant conduits for connection to a coolant system. Appropriate coolants are known in the art and include but not limited to water.

[0050] The rear electrode (RE) is a hollow copper tube having a closed first end and open second end. The bore diameter of the rear electrode is between about 23 / 32” to about 28 / 32” and has a length of between 7” and 11”. In some embodiments, the rear electrode has a bore diameter of about 25 / 32” and a bore length of about 9”.

[0051] The front electrode (FE) is an open hollow copper alloy tube, optionally copper-chromium alloy, mounted spaced apart from and in coaxial alignment with the rear electrode’s open end. The outward facing potion of the front electrode is cupped shaped and includes a shoulder region to which the arc attaches. The diameter of the bore of the cylindrical portion of the front electrode is between about 11 / 16” to about 27 / 32” and has a length of between 4” and 8”. In some embodiments, the front electrode has a bore diameter of about 3 / 4” and a bore length of about 6”.

[0052] A gap is provided between the rear and front electrodes. A gap insulator is mounted therein. Appropriate gap insulators are known in the art and include hole sizes of 0.05 to 0.100”**. The gap is between 4mm and 6mm. In some embodiments, the gap is approximately 5 mm.

[0053] A vortex generator is mounted between the rear and front electrodes in closer proximity to the rear electrode. The gap insulator is located between the vortex generator / rear electrode and the front electrode to prohibit the contact between the rear electrode / vortex generator and the front electrode. The vortex generator is an annular ring having a plurality of tangentially directed apertures. In some embodiments, the diameter of each aperture is 0.0625”. The range is between 0.047” and 0.086”.

[0054] Working gas is delivered to the vortex generator similar to as described in U.S. Patent No. 4,587,397. The working gas passes through the tangential directed apertures in the vortex generator, so as to form a vortical flow of gas in the space between the rear and front electrodes.

[0055] Working gas is provided to the vortex generator from one or more gas sources. In some embodiments, the gas source is pressurized carbon dioxide. In other embodiments, working gas is provided from a gas mixer which receives gas from a source of carbon dioxide and a source of air.

[0056] The front and rear electrodes are operatively connected to a power supply which when operating generates an arc which extends from the rear electrode through the vortical flow of working gas to an appropriate attachment point on the front electrode. The power supply is configured to operate from 125kW to 500 kW with a representative operating voltage range between 400 and 640 Volts and an Open Circuit Voltage around 850 Volts. **.

[0057] The plasma torch system further includes an arc starter configured to provide an initial high voltage pulse between the positive (rear) electrode and the negative (front) electrode. The arc starter comprises a high voltage transformer followed by a voltage doubler circuit which increases the voltage to about 8850 Volts. This is used to charge a high voltage capacitor and a high voltage discharging circuit allows the energy in the capacitor to be discharged in less than one millisecond when requested by the torch operator. This provides the initial current flow path after which the normal DC power current can flow through and keep the arc between positive and negative electrodes alive.

[0058] The arc start system provides the required voltage, energy and arc discharging to break the gas medium between the DC plasma torch electrode gap and allow DC plasma torch power current to carry on with a live arc between the electrodes. Once the arc is successfully established, the arc starter system is cut off from the normal DC plasma torch arc operation.

[0059] Examples:

[0060] Various electrode and vortex generator configurations were tested during the development of a torch specifically configured for use of carbon dioxide and CO2 / Air mixtures as the working gas. The test torches were water-cooled, hollow metal electrode, plasma torches with aerodynamic arc control operating at power levels useful for commercial applications. All the data was obtained experimentally using a 500 kW rated, water-cooled, hollow metal electrode plasma torch.

[0061] Experience from sizing plasma torch components for air as the working gas in plasma torches was used as a baseline to design carbon dioxide plasma torches. Plasma torch components modified were front and rear electrode bore diameter and vortex generator hole diameter along with operating parameters such as desired voltage ranges to obtain a carbon dioxide plasma torch with maximized combined electrode life (preferably close-to-equal front and rear electrode life) and stable arc. The rear electrode length was left at about 9” for the rear bore length and about 6” for the front bore length respectively.

[0062] Extensive testing was performed to enable selection of the best electrode configuration and operating parameters. One of the electrode configurations used with air was used to gain knowledge of what size vortex generator that was optimal for 100% CO2 working gas. Also provided are supporting calculations and extrapolations.

[0063] The following nomenclature is used for each torch configuration tested:

[0064] FEnREnVGx wherein

[0065] FE - Front electrode

[0066] RE - Rear electrode

[0067] VG - Vortex Generator n indicates the bore diameter of the electrode (in sixty-fourths of one inch). x indicates the hole diameter of each of the eight holes used in the vortex generator (in thousands of one inch).

[0068] Electrode Size Optimization with Air (Used as Baseline)

[0069] The purpose of this series of tests was to understand what configuration of bore sizes gave the best arc attachment spread and how the differences in sizes affected reliability and arc attachment. Electrode sizing configurations were based around electrode size experience from testing in a commercial sized plant using 300kW torches. The front bore size was 48 / 64” (3 / 4”) and the rear bore size was 50 / 64” (25 / 32”). The configurations were organized and named as outlined in Table 1 below.

[0070] Table 1: Electrode Bore Size Configurations in 64ths of an inch

[0071] All electrode configurations were analyzed using the same test protocol. Each test was designed to achieve repeatable voltage points: 460V, and 600V, at a constant power of 400kW (as well as others) and held for an adequate time to achieve steady state. The arc attachment (AA) was marked on a scale of 0 - 5 following FIG 2(C).

[0072] If the torch was not able to maintain a consistent arc after a predetermined number of tries appropriate notes were taken and the operators moved on to the next configuration. This scenario explains the behavior of Q5, U5, Q6, and X8. These are marked in red in the table above and confirmed the upper functional bore size limit.

[0073] When testing the smallest extreme, the arc attachment was extremely close to the hole, hardly reaching an arc attachment of 1 . This was ruled out as a possible configuration after a full test with no real improvement or variation in arc attachment.

[0074] The best performing configurations using air as the working gas were the X6, Y5, Q4, and X4. The X6 configuration had the largest arc spread (0.5 - 4) but had many flame-out scenarios as the arc was spreading so far that it did not have a connection point, and the torch would go off. Z5 was acceptable but not as competitive. The X4 configuration was chosen as the best and most reliable configuration that simultaneously exhibited an adequate arc attachment spread (which is an indicator of electrode life) and the best steady-state operation capability.

[0075] It is worth noting that changes from X6 to Q6 or Q8 involve an increase in diameter of just 1 / 32”. This seemingly small change was adequate to result in a substantial operational change as described above. Adequate care was taken to confirm such findings through repetition of testing.

[0076] Electrode Size Optimization with Carbon Dioxide

[0077] In order to optimize the air plasma torch for use with carbon dioxide as the working gas. The following observation and criteria were considered:

[0078] The effect of substituting various amounts of air with CO2 at constant current or voltage was determined. FIGs. 3 and 4 show the effect of substituting the equivalent mass of CO2 for air and substituting the equivalent volume of CO2 for air at high power levels. Substituting air with an equal volume of CO2 increased the voltage. Substituting air with an equal mass of CO2 had a minor effect on the voltage beyond a concentration of ~5% - 10%. At higher CO2 concentrations (by mass) the voltage effectively plateaus or shows a minor increase. It was observed that required mass of 100% CO2 was ~87% the required mass of air.

[0079] FIG 5(A) shows the effect of vortex generator size on the arc attachment (AA). The curve was substantially flat and no trend was observed. All values were well in excess of desirable levels (much greater than 4 and attaching to the side of the cup which is not very well cooled) and more attributable to electrode sizing specifically for CO2 instead of vortex generator sizing. It was, therefore, decided to size and use vortex generator holes that would achieve approximately the same gas velocity through them as optimized experimentally with air. The volume of CO2 for equivalent operation is much lower than air, hence, the vortex generator hole size was calculated to be 0.0625” each (1 / 16”)). This was used throughout the remainder of the testing. Substantiating calculations are shown in FIG 5(B).

[0080] Three different electrode configurations (size combinations) were evaluated to select a configuration for detailed and extended testing using 100% CO2 as the working gas. The same vortex generator size (0.0625” each (1 / 16”)) was used. The experiments detailed in FIG 6 showed that during operation at 360kW a plasma torch having a front electrode with a bore diameter of %” (48 / 64” - FE48) and a rear electrode with a bore diameter of 25 / 32” (50 / 64” - RE50) with eight vortex generator holes of 0.0625” (VG63) had an adequately large arc attachment range, but not one that approached the outer perimeter or had excessive “variability”. Extrapolation to a size 1 / 64” smaller in both front and rear electrodes is expected to provide a preferable arc attachment (extrapolated to be ~3.8 in Figure 4) however operation to 500 kW is expected to shrink the arc attachment area somewhat as well. It is expected a torch having FE47RE49VG63 configuration would yield maximum arc attachments slightly lower than 4.0 at 360 kW operation. At 500 kWthe preferred combination is FE48RE50VG63.

[0081] The arc attachment of two plasma torches, the first configured for 100% air working gas and the second configured for 100% CO2 were compared. Graphical representation of the results comparing the arc attachment of F50R52VG76 for AIR vs F48R50VG63 for CO2 are shown in FIG 7(A). As shown, for a maximum arc attachment of 4.0” - 4.25” the lower voltage limits are about 500V for air and about 525V for CO2. In order to get a reasonable voltage range the upper voltage limit for CO2 operation has to be raised accordingly. In this case, arc attachment at the rear electrode must also be considered since arc attachment may stray into “undesirable” levels (back of rear electrode bore) if operation in excess of 640V is used frequently. Therefore, the preferred voltage range for 100% CO2 at 360 kW operation would, accordingly, be 525V - 625V vs 500V - 600V for corresponding operation with 100% air. Operation with partial CO2 is expected to be optimal with electrode sizes selected to be between that used with 100% CO2 working gas and that used with 100% air. Comparison with FE48RE50VG76 for AIR shows the much lower (and inferior) arc attachment trend. Comparison of arc attachment trends at low and high voltages at the same power level for FE48RE50VG70 for AIR and FE48RE50VG63 for CO2 are shown in Figure FIG 7(A). From the graph one can observe that the arc attachment at low voltage is much higher than the one at higher voltage in the case of CO2 for the same power level. The arc attachments for air at low voltage and high voltage tend to be very close to each other for the same power level. This is very likely due to the fact that the use of FE48RE50VG70 is not the optimal configuration for AIR but it is for CO2. It is provided herein just for comparison on the same FE48RE50 configuration albeit with a different vortex generator as that was not found to have a substantial effect on AA through testing of many different configurations. The fact that the arc attachment range is very small for air at that configuration is part of the reason that optimisation was performed which resulted in optimality at FE50RE52VG76.

[0082] The voltage as a function of volume flow for the two configurations shown above (Air Vs CO2 - both at optimal but different electrode configurations is shown in FIG 8.

[0083] Electrode Material Optimization

[0084] A comparison of pure copper and several copper alloys for the electrodes was conducted. Pure Oxygen-Free Copper (OFC) was found to be an excellent thermal and electrical conductor, but the copper-chromium alloy was found to be superior in that it had adequate conductivity but was also more resistant to electrode wear.

[0085] Copper alloy materials, in addition to chromium, include zirconium, tungsten, titanium, silver, etc and can be used with the copper on their own or with copper and another material (e.g., copper-chromium-zirconium alloy).

[0086] Arc Starter

[0087] The DC plasma torch of the invention runs at a voltage substantially lower than the voltage required to break the gas medium between the electrode gap (~5mm) and to provide the initial current flow through path to allow the normal DC current to flow between positive and negative electrodes. The arc starter of the invention was designed to provide an initial high voltage pulse between the positive electrode and the negative electrode. It uses a high voltage transformer followed by a voltage doubler circuit which increases the voltage to ~8850 Volts. This is used to charge a high voltage capacitor (item 2) and a high voltage discharging circuit allows the energy in the capacitor to be discharged in less than one millisecond when requested by the torch operator. This provides the initial current flow path after which the normal DC power current can flow through and keep the arc between positive and negative electrodes alive.

[0088] The arc starter of the invention includes the components shown in FIG 9(A). These components are: 1. Electrical high voltage charging circuit, 2. High voltage energy storing capacitor, 3. High voltage discharging contact and 4. Control elements to manage the capacitor charging and discharging sequence.

[0089] Once the arc is successfully established, the combined system is cut off from the normal DC plasma torch arc operation.

[0090] Two other designs were tried:

[0091] Alternative A: The initial discharging switch is as shown in FIG 9(B). This design included two high voltage IGBT switches (items labelled 1 , 2 on Figure 8b) linked in series, functioning as high voltage discharging contacts, and each IGBT switch included an optically isolated control element, items labelled 3, 4 in FIG 9(B).

[0092] There were over voltage issues with this configuration and as such an alternative configuration was designed.

[0093] The high voltage discharge switch was replaced with a single fast acting high voltage rated DC contact to achieve torch arc start purposes (item 3 in FIG 9(A)).

[0094] A simpler design was tried as well. This Arc Starter configuration is as shown in FIG 9(C). It involves 1. a high voltage AC ignition transformer, 2. a high voltage diode rectifier, 3. a high voltage contact, and 4. an ON / OFF control element.

[0095] This system configuration worked on a bench trial with no actual torch DC power supply system connections. However, when tried with the torch and power supply in place, it failed to generate an adequate arc at the torch electrodes tip and was, therefore, not used.

[0096] Heat and Material Balance (HMB) Comparison (Air Vs Carbon Dioxide in the torches)

[0097] A Heat and Material Balance (HMB) was projected using a modeling tool to compare the effect of using carbon dioxide instead of air in the torch. The results are summarised and detailed in FIG 11 and FIG 12 respectively using representative feedstock detailed in FIG 10. The results show very clearly that, as expected, the nitrogen content of the product gas drops significantly. The rest of the compounds change only marginally.

[0098] CFD Comparison (Air Vs Carbon Dioxide in the torches)

[0099] A computational fluid dynamics (CFD) model was developed to assess and compare the extent of NOx formation, as well as intermediate compounds such as HCN and NH3, for air and CO2 as plasma torch working fluids.

[0100] The CFD model consists of the geometry of a refining chamber, a syngas (fuel gas) stream flowing into the refining chamber, two plasma plumes and four oxygen nozzles. Syngas is mixed with plasma and undergoes partial oxidation and reforming reactions. The nitrogen content of syngas reacts with oxygen and plasma to form thermal and prompt NOx, defined as follows:

[0101] Thermal NOx: direct oxidation of nitrogen molecules at high temperatures

[0102] Prompt NOx: oxidation of nitrogen as a result of exposure to fuel rich conditions, lower temperatures and short residence times.

[0103] A first simulation is run in the absence of plasma torch gas to determine baseline NOx formation and distribution as a result of partial oxidation (Case A).

[0104] The simulation is repeated with a pair of plasma plumes with an estimated average plasma temperature using air (Case b) and pure CO2 (Case c) as working fluids with flowrates given in FIG 12.

[0105] The differences in nitrogen compounds concentration and distribution are determined for the three cases. It is shown that the plasma torch working with air contributes to additional NOx formation. FIG 13, FIG 14 and FIG 15 illustrate the differences in NO compound formation for these three cases, respectively. In these figures, iso-value contours of NO mass fraction formed by thermal and prompt formation are plotted on a horizontal plane in the immediate vicinity of the plasma torch outlets. Two distinct zones of NOx formation are observed: a zone shaped like a ring corresponding to the high oxygen concentration area formed by the 4 tangential oxygen nozzles. The second zone is associated with the plasma discharge zone and represents NOx formation due to very high plasma fluid temperatures. It can be seen that in the absence of air in plasma plume (Cases a and c), NOx formation occurs in the first zone only. In case of air plasma, NOx is formed in both zones, and the highest concentration of NOx is at the location of plasma plume discharge. Maximum local NOx concentrations in the plasma discharge region exceed 3% mass fraction for Case b, while the maxima corresponding to Cases a and c are about 0.1%.

[0106] The volume-averaged NO mass fraction formed by thermal and prompt formations for Cases a, b and c are 0.0045%, 0.0132 and 0.0084%, respectively.

[0107] In a nutshell, this means that there is reduction in NO formation between the use of air and CO2 as the plasma gas. When using CO2 NO is lower by approximately 36% my mass fraction as compared to when air is used.

[0108] CO2 Optimal Ratio

[0109] When testing the torch with CO2 as plasma gas, the ratio was tested based on two substitution comparisons: volume and mass. Under the mass substitution comparisons, the torch was tested at 240kW Starting (500A Constant) and 300kW Starting (625A Constant) respectively. The data from these tests are shown in FIG 3.

[0110] Substituting equal mass of CO2 instead of air has a minor effect on the voltage beyond a concentration of approximately 5% - 10%. At higher CO2 mass concentrations, the voltage plateaus or shows a very small increase. At 100% substitution of CO2, the mass of CO2 as compared to air generates the same voltage at approximately 87% of the mass of air.

[0111] As expected, substituting an equal volume of CO2 instead of air increases the voltage as shown in FIG 4.

[0112] REFERENCES

[0113] 1. The Reverse-Polarity Torch, S.L. Camacho, ISPC-B Tolyo, 1987, Paper number BV-07.

[0114] 2. Industrial-worthy plasma torches: State-of-the art, S.L. Camacho, Plasma Energy Corporation, Raleigh, North Carolina, USA; published in Pure & Applied Chemistry, volume 60, n0.5, pp. 619-632, 1988.

[0115] 3. A. Kaminska, The Effect of Gas Admixture on the Low Pressure Arc Properties, Romanian Journal of Physics, 63, 502, 2018. 4. K. W. Engeling, and A. J. Meier, Utilizing a C02 Carrier Gas in a Plasma Assisted Waste Conversion Test Cell for Space Applications, International Conference on Environmental Systems, 2020.

[0116] 5. Y. Vadikkeettil, P. Amarnath, S. Yugeswaran, and P. V. Ananthapadmanabhan, Comparative Study of Plasma Torch Characteristics Using Air and Carbon Dioxide, IEEE Transactions on Plasma Science, 07 December 2021.

[0117] 6. R. Snoeckx, and A. Bogaerts, Plasma technology - a novel solution for CO2 conversion, Chemical Society Reviews, 46 (5805-5863), 2017.

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[0120] 9. “Plasma Arc Torch”, S.L. Camacho et al, Patent no. 4,587,397.

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[0122] Although the invention has been described with reference to certain specific embodiments, various modifications thereof will be apparent to those skilled in the art without departing from the spirit and scope of the invention. All such modifications as would be apparent to one skilled in the art are intended to be included within the scope of the following claims.

Claims

WE CLAIM:1 . A high power, cooled plasma torch configured for carbon dioxide working gas comprising a torch housing, a rear electrode mounted within the housing, the rear electrode comprising a tubular copper or copper alloy member having a closed inner end and an open outer end, wherein the bore of the rear electrode has a diameter of between 23 / 32 and 7 / 8, a front electrode comprising a tubular copper or copper alloy member having a bore therethrough, wherein the bore of the front electrode has a diameter of between 11 / 16 and 27 / 32 at an inner cylindrical end portion, the front electrode extending at its outer end into an outward facing cup, the front electrode being mounted within the housing spaced apart from and in coaxial alignment with the rear electrode, the front electrode having the inner cylindrical end adjacent the open outer end of the rear electrode, and a vortex generating means for generating a vortical flow of a working gas, wherein the vortex generating means is an annular ring having a plurality of tangentially directed apertures which is mounted between the rear electrode and the front electrode and in coaxial alignment therewith and wherein each aperture has a diameter of between 0.047” and 0.086”, wherein the high power, cooled plasma torch is at least a 500 KW plasma torch.

2. The high power, cooled plasma torch according to claim 1 , wherein the vortex generator comprises 8 apertures.

3. The high power, cooled plasma torch according to claim 2, wherein each aperture is with one-sixteenth of an inch (1 / 16”) diameter.

4. The high power, cooled plasma torch according to any one of claims 1 to 3, wherein the rear electrode bore diameter is twenty-five thirty seconds of an inch (25 / 32”).

5. The high power, cooled plasma torch according to any one of claims 1 to 4, wherein the front electrode bore diameter is three quarters of one inch (3 / 4”).

6. The high power, cooled plasma torch according to any one of claims 1 to 5, configured to have an operating voltage of 525V - 625V.

7. The high power, cooled plasma torch according to any one of claims 1 to 6, wherein the front and rear electrodes are a chromium-copper alloy (C18200).

8. The high power, cooled plasma torch according to any one of claims 1 to 6, configured for a working gas having at 50% carbon dioxide, at least 75% carbon dioxide, at least 95% carbon dioxide, at least 99% carbon dioxide by mass.

9. The high power, cooled plasma torch according to any one of claims 1 to 6, configured for a working fluid being pure carbon dioxide.

10. A system for providing a reduced nitrogen plasma, the system comprising: a. the plasma torch of any one of claims 1 to 9, b. a working gas supply operatively conducted to the vortex generator, and a power supply comprising an arc starter, wherein the arc starter comprises a high voltage transformer, a voltage doubler circuit that increases the voltage to about 8850 volts and a high voltage capacitor and a high voltage discharging circuit.11 . A method for refining a syngas, the method comprising the steps of:(a) delivering the syngas to a refractory-lined chamber;(b) providing oxygen into said chamber;(c) torch heating said chamber with one or more of the systems of claim 10, and thereby producing refined syngas; and(d) removing the refined syngas from said chamber.

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