Method for co2 to co conversion

Abstract

Present application discloses a method for converting CO2 to CO, comprising: supplying CO2 to a gliding arc plasma reactor, said gliding arc plasma reactor comprising multiple parallel electrode pairings, and igniting a plasma in said gliding arc plasma reactor; operating said gliding arc plasma reactor at a pressure Po of at least 0.95 atm; and operating said gliding arc plasma reactor at a specific energy input SEI between 4.5 and 7.0 kJ / L CO2.

Description

[0001]Method for CO2 to CO conversion FIELD OF THE INVENTION The present invention relates to a method and apparatus for converting carbon dioxide (CO2) to carbon monoxide (CO) using a gliding arc plasma reactor. BACKGROUND The conversion of CO2 to CO is of significant interest due to the potential applications of CO in various chemical processes and the environmental benefits associated with CO2 reduction. Various methods exist for this conversion, and the present invention aims to provide an efficient and novel approach using a gliding arc plasma reactor. A variety of CO2 plasmolysis pathways exists, including the combination of various methods coupled with catalysis, or other electricity-based methods such as electrolysis. The reports describe a very wide range of process performance values. A nearly full conversion with a very high energy efficiency (EE) (up to 60%) were achieved with low pressure plasmas. It must be noted, however, that plasmas operated below atmospheric pressure require additional equipment with high energy costs (e.g., vacuum pumps), which limits their industrial potential. In contrast, atmospheric pressure plasmas (APPs) require no additional costs associated with low pressure equipment, although the energy efficiency values are generally lower. The research publications are unified by one notion: virtually every work is limited to very low flow rates and discusses the need of specific up-scaling methods in order to bring atmospheric plasma from the lab to industry. Microwave plasma currently shows the best results. Conversion of CO2 to CO with MW plasma shows high conversion and energy efficiency (EE), but typically at a very low pressure and feedstock rates. Increasing the pressure and / or feedstock rates results in reductions in both energy efficiency and conversion. Moreover, MW setups typically lead to high capital costs, as they require complex equipment, further limiting their ability to scale up. The present invention aims to resolve at least some of the problems and disadvantages mentioned above. The present invention targets at solving at least one of the aforementioned disadvantages. SUMMARY OF THE INVENTION The present invention and embodiments thereof serve to provide a solution to one or more of above-mentioned disadvantages. To this end, the present invention relates to a method for converting CO2 to CO according to claim 1. Preferred embodiments of the method are shown in any of the claims 2 to 15. A specific preferred embodiment relates to an invention according to claim 2. DETAILED DESCRIPTION OF THE INVENTION The present invention concerns a method for converting CO2 to CO. Unless otherwise defined, all terms used in disclosing the invention, including technical and scientific terms, have the meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. By means of further guidance, term definitions are included to better appreciate the teaching of the present invention. As used herein, the following terms have the following meanings: “A”, “an”, and “the” as used herein refers to both singular and plural referents unless the context clearly dictates otherwise. By way of example, “a compartment” refers to one or more than one compartment. “About” as used herein referring to a measurable value such as a parameter, an amount, a temporal duration, and the like, is meant to encompass variations of + / - 20% or less, preferably + / -10% or less, more preferably + / -5% or less, even more preferably + / -1% or less, and still more preferably + / -0.1% or less of and from the specified value, in so far such variations are appropriate to perform in the disclosed invention. However, it is to be understood that the value to which the modifier “about” refers is itself also specifically disclosed. “Comprise”, “comprising”, and “comprises” and “comprised of” as used herein are synonymous with “include”, “including”, “includes” or “contain”, “containing”, “contains” and are inclusive or open-ended terms that specifies the presence of what follows e.g. component and do not exclude or preclude the presence of additional, non-recited components, features, element, members, steps, known in the art or disclosed therein. Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order, unless specified. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein. The recitation of numerical ranges by endpoints includes all numbers and fractions subsumed within that range, as well as the recited endpoints. The expression “% by weight”, “weight percent”, “%wt” or “wt%”, here and throughout the description unless otherwise defined, refers to the relative weight of the respective component based on the overall weight of the formulation. Whereas the terms “one or more” or “at least one”, such as one or more or at least one member(s) of a group of members, is clear per se, by means of further exemplification, the term encompasses inter alia a reference to any one of said members, or to any two or more of said members, such as, e.g., any ≥3, ≥4, ≥5, ≥6 or ≥7 etc. of said members, and up to all said members. Unless otherwise defined, all terms used in disclosing the invention, including technical and scientific terms, have the meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. By means of further guidance, definitions for the terms used in the description are included to better appreciate the teaching of the present invention. The terms or definitions used herein are provided solely to aid in the understanding of the invention. Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment, but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to a person skilled in the art from this disclosure, in one or more embodiments. Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those in the art. For example, in the following claims, any of the claimed embodiments can be used in any combination. First aspect – method of CO2 to CO conversion In a first aspect, the invention relates to a method for converting CO2 to CO, comprising: - supplying CO2 to a gliding arc plasma reactor, said gliding arc plasma reactor comprising multiple parallel electrode pairings; - operating said gliding arc plasma reactor at a pressure Po of at least 0.2, preferably at least 0.95 atm; and - operating said gliding arc plasma reactor at a specific energy input SEI between 2 and 7 kJ / L CO2, preferably between 4.5 and 7.0 kJ / L CO2. Upscaling of plasma reactors is generally a difficult task. A particular issue with CO2 to CO conversion is the recombination of the reaction products, CO and O2, back to the reactant CO2. This recombination commonly occurs downstream of the plasma region and is highly temperature dependent. The recombination significantly decreases the overall conversion when it occurs, adding to the difficulties related to upscaling. Reducing the operating pressure alleviates this issue, which is readily apparent from Chatelier’s principle. However, operating at lower pressures decreases throughput, requires vacuum equipment and increases energy requirements to maintain such a vacuum. It was found that operating, in parallel, several plasma electrode pairings with an optimized power profile allows for higher conversion rates and throughput without sacrificing absolute conversion and energy efficiency. The applicants found that the optimal operating condition is defined by the specific energy input (SEI). Advantageously, present method uses gliding arc plasma reactors. Gliding arc plasma reactors are relatively simple, especially compared to microwave reactors. In a preferred embodiment, the multiple parallel electrode pairings comprise a series of multiple parallel reactor cathodes mounted on a common anode plate. The use of a multitude of modular, parallel reactor cathodes mounted on a single anode plate further results in a reactor design that is easy to produce, operate and adjust as necessary. This is particularly important as the reactor design, particularly the amount of parallel reactor cathodes in operation, must be adjusted to make significant changes to the reactor throughput or adjust for significant changes in the make-up of the reactor feedstock. In a particular preferred embodiment, the specific energy input is divided over multiple parallel electrode pairings operating at a limited power. In a preferred embodiment, each electrode pairing of the multiple parallel electrode pairings operating at an operating power between 0.5 and 2.0 kW, more preferably between 0.8 and 1.8 kW, more preferably between 0.9 and 1.5 kW, more preferably between 1.0 and 1.5 kW, preferably between 1.1 and 1.4 kW, more preferably between 1.1 and 1.3 kW, more preferably between 1.15 to 1.30 kW, most preferably around 1.25 kW. In another preferred embodiment, each electrode pairing of the multiple parallel electrode pairings operates at an operating power of at least 1.0 kW, more preferably at least 1.1 kW, more preferably at least 1.2 kW, more preferably at least 1.25 kW. The inventors found that operating a series of electrode pairings in parallel at these conditions shows significantly better results in terms of conversion, conversion rate and energy efficiency compared to working outside of this range. At lower operating power, the conversion of the CO2 to CO conversion in the plasma declines leading to lower conversion. At higher operating power, the temperature in the plasma afterglow is too high resulting in recombination of the products towards the reactant, also leading to lower conversion and a decrease in energy efficiency. In a preferred embodiment, the operating pressure Po is at least 0.2 atm, more preferably at least 0.5 atm, more preferably at least 0.8 atm, more preferably at least 0.9 atm, more preferably at least 0.95 atm, more preferably at least 0.97 atm, more preferably at least 0.99 atm, more preferably at least 1.00 atm, more preferably at least 1.01 atm, more preferably at least 1.03 atm, more preferably at least 1.05 atm. In another preferred embodiment, the operating pressure is preferably at most 5 atm, more preferably at most 4 atm, more preferably at most 3 atm, more preferably at most 2.5 atm, more preferably at most 1.5 atm. Present method obtains high energy efficiency and high conversion rate at atmospheric pressures. This is desirable as it allows higher throughput, no need for complex vacuum equipment as well as high energy requirements associated with such vacuums. In a further preferred embodiment, the specific energy input is at least 4.0 kJ / L CO2, more preferably at least 4.5 kJ / L CO2, more preferably at least 4.8 kJ / L CO2, more preferably at least 4.9 kJ / L CO2, more preferably at least 5.0 kJ / L CO2, more preferably at least 5.1 kJ / L CO2, more preferably at least 5.2 kJ / L CO2, more preferably at least 5.3 kJ / L CO2, more preferably at least 5.4 kJ / L CO2, more preferably at least 5.5 kJ / L CO2. In another further preferred embodiment, the specific energy input is at most 8.0 kJ / L CO2, more preferably at most 7.0 kJ / L CO2, more preferably at most 6.5 kJ / L CO2, more preferably at most 6.0 kJ / L CO2, more preferably at most 5.9 kJ / L CO2, more preferably at most 5.8 kJ / L CO2, more preferably at most 5.7 kJ / L CO2, more preferably at most 5.6 kJ / L CO2, more preferably at most 5.5 kJ / L CO2. In another preferred embodiment, the specific energy input is between 4 and 6 kJ / L CO2, more preferably between 4.5 and 6 kJ / L CO2, more preferably between 5.0 and 6.0 kJ / L CO2, more preferably between 5.0 and 5.7 kJ / L CO2, more preferably between 5.0 and 5.5 kJ / L CO2, most preferably about 5.3 kJ / L CO2. Lower specific energy input reduces the absolute conversion. In addition, lower specific energy input implies lower reactor throughput. The combined lower conversion and lower reactor throughput lead to a significant reduction in conversion rate or production rate of the product; even if relatively high energy efficiency can be obtained. Higher specific energy input also reduces the absolute conversion. Initially, near the upper range of the specific energy input, the conversion rate will drop but the energy efficiency remains relatively high. Without being bound to theory, the initial drop in conversion rate is partially offset by higher reactor throughput resulting in similar energy efficiency and higher conversion rate or production rate of product, despite lower absolute conversion. Further increases in SEI quickly lead to a drastic reduction in conversion due to recombination of products. This drastic reduction cannot be offset by higher throughput. In this regime both energy efficiency and conversion rate drop substantially. In a preferred embodiment, CO2 is supplied to the gliding arc plasma reactor at a flow rate of at least 20 L CO2 / min, more preferably at least 40 L CO2 / min, more preferably at least 50 L CO2 / min, more preferably at least 60 L CO2 / min, more preferably at least 70 L CO2 / min, more preferably at least 80 L CO2 / min, more preferably at least 100 L CO2 / min, more preferably at least 120 L CO2 / min, more preferably at least 150 L CO2 / min, more preferably at least 200 L CO2 / min, more preferably at least 300 L CO2 / min, more preferably at least 500 L CO2 / min, more preferably at least 1000 L CO2 / min. Present application allows significant upscaling of the reactor throughput, allowing large amounts of CO2 to be converted without significant decreases to the conversion and energy efficiency. In addition, present method achieves these results with relatively inexpensive equipment. In a preferred embodiment, the gliding arc plasma reactor is cooled downstream of the multiple electrode pairings. This is done to prevent recombination of the products CO and oxygen species (O or O2 or oxygen radicals) towards the reactant CO2. In a further preferred embodiment, the heat transfer capacity of the cooling unit is between 5% and 100% of the total operating power of the gliding arc plasma reactor. Preferably, the heat transfer capacity of the cooling unity is at least 5% relative to the total operating power of the gliding arc plasma reactor, more preferably at least 10%, more preferably at least 20%, more preferably at least 30%, more preferably at least 40%, more preferably at least 50%, more preferably at least 60%, more preferably at least 70%, more preferably at least 80%. A plasma reactor operating five electrodes at 1.2 kW per electrode; with a cooling unit subtracting 6 kW of heat from the downstream plasma zone, preferably the post- plasma chamber, this would correspond to a heat transfer capacity of the cooling unit of 100%. In a particular embodiment, the first aspect relates to a method for converting CO2 to CO, comprising: - supplying CO2 to a gliding arc plasma reactor, wherein CO2 is supplied to the gliding arc plasma reactor at a flow rate of at least 60 L CO2 / min; said gliding arc plasma reactor comprising multiple parallel electrode pairings, wherein said multiple parallel electrode pairings consist of multiple parallel cathodes mounted on a common anode plate, wherein said multiple parallel cathodes are electrically insulated from said common anode plate by an insulation ring, and igniting a plasma in said gliding arc plasma reactor; - operating said gliding arc plasma reactor at a pressure Po of at least 0.95 atm; and - operating said gliding arc plasma reactor at a specific energy input SEI between 4.5 and 7.0 kJ / L CO2. This combination of features allows operating the reactor in the specified SEI range at much higher flow rates compared to the prior art, i.e., achieving a significant increase in the scale of gliding arc reactors without deviating from the optimal operating conditions. In addition, this is achieved while still having as many common aspects as possible, and utilizing identical modular elements where not possible, to reduce material and production requirements. The combination of these features allows operating the reactor in the specified SEI range at much higher flow rates compared to the prior art, thereby achieving a significant increase in the scale of gliding arc reactors while maintaining optimal operating conditions. This improvement not only enhances the scalability of the process but also ensures that the energy efficiency and stability of the plasma reactor are preserved even under increased throughput. Additionally, this is accomplished with a focus on utilizing as many common aspects of the reactor design as possible. Where it is not feasible to maintain uniformity, identical modular elements are used to ensure compatibility, which in turn reduces material requirements and simplifies production processes. This modularity and commonality approach allows for streamlined manufacturing, minimizes costs, and promotes ease of maintenance and replacement, providing a robust and economical solution for large-scale plasma reactor applications. In a preferred embodiment, the gliding arc plasma reactor is powered by a switching-type high voltage power supply. In a preferred embodiment, the gliding arc plasma reactor is powered by a current-regulated power supply. In a preferred embodiment, the gliding arc plasma is powered by a DC power supply. More preferably, the gliding arc plasma reactor is powered by a current regulated, switching type high voltage power supply. Most preferably, the gliding arc plasma reactor is powered by a current regulated, switching type, high voltage DC power supply. In another or further preferred embodiment, the gliding arc plasma reactors comprise a series of multiple parallel reactor cathodes mounted on a common anode plate, wherein said multiple parallel cathodes are electrically insulated from said common anode plate by an insulation ring. Preferably said anode plate is grounded, and each cathode of said series of multiple parallel reactor cathodes is electrically connected to an independent power supply, preferably a direct current power supply. Preferably, each power supply is a switching-type DC power supply. In another or further preferred embodiment, each power supply is operated in power control mode (PCM), which is preferred to ensure the plasma reactor, and each electrode pairing therein, is utilized within the optimal operating conditions. These optimal operating conditions include the specific energy input as defined herein. As the specific energy input is on a molar basis, and thus variable with changes in flow rates or feedstock composition, the ability to control the power of each electrode independently allows optimal operation in a continuous process with variable feedstock compositions and flow rates, for example when converting flu or exhaust gasses comprising CO2. This independent power supply is independently controlled. In another embodiment, an independent power supply is provided and electrically connected to at most 5 cathodes, preferably at most 4 cathodes, more preferably at most 3 cathodes, more preferably at most 2 cathodes, most preferably a single cathode. When a single power supply is connected to multiple cathodes, these are connected in series. This reactor design strikes a balance between combining, such as the common anode, but also a shared reactor vessel for feedstock intake and product extraction; while also maintaining independent and fine grained control over a multitude of plasma electrode pairings so optimal plasma conditions can be maintained in a scalable manner. In order to ensure the desired operational window as discussed herein, the power supply is preferably controlled by its power output. During operation, electrode pairing failure, i.e. plasma reactor failure is preferably measured through deviations in its power supply. When multiple parallel electrodes are daisy chained to a common power supply; electrode pairing failure is more difficult to measure. Hence independent control and measurement of each cathode is preferred. In addition, if a reactor fails without detection while the power supply is controlled by its power output, then the remaining power will be distributed among the remaining reactors. This may lead to cascading failure due to overheating, as well as a deviation of the preferred plasma conditions. By providing independent control of each electrode pairing, optimal gliding arc conditions can be ensured throughout operation even when faced with variable feedstock conditions. In a further embodiment, each independent power supply is rated to provide at most 10 kW, more preferably at most 5 kW, more preferably at most 4 kW, more preferably at most 3 kW, more preferably at most 2 kW, most preferably at most 1 kW. By limiting the power supply rating to the abovementioned range, relatively available and standard components can be utilized. In a preferred embodiment, the plasma is a warm plasma. A warm plasma is a non- thermal, non-equilibrium plasma. The gas temperature is lower than the electron temperature therein. Preferably the plasma temperature is between 3000 and 5000K. Sufficiently high temperatures are required for plasmolysis of CO2. It is preferred to avoid plasma temperatures above 6000K. This may lead to the formation of carbon particles, requiring heterogeneous separation of particles from the gas stream downstream of the plasma reactor as well as appropriate piping and equipment. In a preferred embodiment, the gas temperature downstream of the plasma is at most 800°C, more preferably at most 700°C, more preferably at most 600°C, more preferably at most 500°C, more preferably at most 400°C, more preferably at most 300°C. Preferably, the gas temperature downstream of the plasma reactor is measured within 10 cm of said plasma reactor, more preferably at a distance 5 cm from the electrode pairing. By maintaining lower temperatures downstream of the electrode pairings, recombination of CO to CO2 is limited or avoided. In a preferred embodiment, the plasma reactor comprises a carbon source downstream of the plasma reactor. In a further preferred embodiment, the carbon source comprises carbon particles. In a further preferred embodiment, the carbon particles may be provided in a fixed-bed and / or fluidized bed. By providing a carbon source downstream of the plasma reactor, oxygen species formed by the dissociation of CO2 can be selectively fixated with carbon to form CO. Through the fixation of oxygen species, recombination of CO with oxygen species towards CO2 is limited or avoided. This drastically increases the conversion rate. In a preferred embodiment, the plasma reactor does not comprise carbon particles downstream of the plasma reactor. A carbon feedstock downstream of the plasma reactor is a suitable way to increase conversion by limiting recombination. However, it may also result in carbon particles in the product stream, which can complicate downstream operations. Present application provides a way to limit recombination of CO and oxygen species towards CO2 through optimization of the process, without the need to provide a carbon source. In a preferred embodiment, the multiple parallel electrode pairings consists of at least 2, more preferably at least 3, more preferably at least 4, Second aspect – Reactor design In a second aspect, present disclosure relates to a gliding arc plasma reactor comprising multiple electrode pairings. More preferably, said gliding arc plasma reactor comprises: ^ a cylindrical reactor vessel, wherein said cylindrical shape is characterized by a cross section in a radial plane and extends in an axial direction perpendicular to said radial plane; ^ an anode plate parallel to the radial plane, said anode plate separating the reactor vessel into a pre-plasma chamber and a post-plasma chamber, ^ a feedstock inlet allowing fluid communication to the pre-plasma chamber, ^ a product outlet allowing fluid communication to the post-plasma chamber, ^ multiple parallel reactor cathodes, wherein said multiple parallel reactor cathodes are mounted on said anode plate, wherein each parallel reactor cathode is electrically insulated from said anode plate by an insulation ring; wherein each parallel reactor cathode comprises a tangential gas inlet for fluid communication with the pre-plasma chamber and an axial plasma outlet for fluid communication with the post-plasma chamber; and ^ a power supply unit connected to said multiple parallel reactor cathodes. In a preferred embodiment, the gliding arc plasma reactor comprises at least one anode, preferably a single anode plate, more preferably a single grounded anode plate. Advantageously, multiple parallel reactor cathodes can be mounted on a single anode plate. This results in a modular, cheap and easily scalable setup of a multitude of reactor cathodes mounted on a combined anode plate. In addition, this setup allows the multitude of reactor cathodes to be conveniently housed in a single reactor housing; where the anode simultaneously acts as a separation between pre- plasma gas chamber and post-plasma gas chamber. In a further preferred embodiment, each parallel reactor cathode is electrically insulated from the anode plate with an insulation ring. In a preferred embodiment, the multiple parallel reactor cathodes consist of at least 2, more preferably at least 3, more preferably at least 4, more preferably at least 5, more preferably at least 7, more preferably at least 10, more preferably at least 12, more preferably at least 15, more preferably at least 20, more preferably at least 25 parallel reactor cathodes. Scaling with a series of modular, parallel reactor cathodes results in higher energy efficiency and conversion rate than a single, larger reactor. It was found that operating, in parallel, several plasma reactor electrode pairings with an optimized power profile allows for much larger throughput without significant decreases in energy efficiency or conversion or the need for complex equipment. In a preferred embodiment, the reactor housing comprises a cooling unit. Preferably said cooling unit is configured to cool the post-plasma chamber. More preferably, the reactor housing comprises a fluid jacket. More preferably, the reactor housing comprises a fluid jacket around the post-plasma chamber. The use of a cooling fluid within the fluid jacket allows cooling of the post-plasma chamber. In another preferred embodiment, the post-plasma chamber comprises a carbon bed. A carbon bed in the post-plasma chamber allows fixation of oxygen species with carbon, thereby limiting recombination of CO to CO2 and producing higher CO content. In a further preferred embodiment, a cooling unit and a carbon bed can both be utilized. EXAMPLES Experimental setup In a simplified chemical pathway, CO2is split into CO and atomic O (R2), with the latter recombining into O2(R3). We explicitly mention that we did not observe the formation of solid carbon under any experimental condition studied here. Hypothetically, C could be formed via further plasmolysis of CO (R4), but did not occur here. This clearly indicates that the temperature in our plasma setup was lower than that required for carbon formation (+-6000 K). ^ CO^⇌ CO + ^ O^(R1) CO^⇌ CO + O (R2) O + O ⇌ O^(R3) CO ⇌ C + O (R4) The measured CO2 concentration can be represented as the CO2 concentration in the reactor exhaust (^ ^^^ ^^^) (E1), where ^ is the molar flow rate into or out of the reactor, with the superscript indicating whether this is at the inlet or outlet. This is further used to obtain the absolute conversion of CO2(E2): ^(^^^)is the conversion of CO2, based on the measured concentration of CO2. A similar derivation can be carried out for the calculation of the other reaction products, in this case O2. ^(^^)is the conversion of CO2based on the measured concentration of O2(E3). We explicitly note that for every single experiment reported in this work, the shown conversion values are averages between the two conversion values obtained from the CO2 measurement and O2 measurement (E4). This was done to reduce the potential error due to the inherent uncertainty of the sensors. ^∙^^^^ ^(^^)=^^^^ ^^^^^^(E3) The energy efficiency (η) of the process, compared the standard reaction enthalpy (ΔH0 = +283 kJ / mol) for CO2 dissociation, is defined as shown in E5, where χ is the conversion, ^ ^^ ^^^^^is the total feed gas flow rate into the reactor, P is the power consumed during the process, and 23.6 L / mol is the molar volume of the gas at 17 oC (as calibrated by the manufacturer). In our case, the power is represented by the total power consumed from the plug as previously indicated. ^ (%) =^ (%) ∙ ^^^ (^^ / ^^^) ∙ ^^^^ ^^^^(^ / ^^^)^ (^^) ∙ ^^.^ (^ / ^^^)∙ ^^ (^ / ^^^) (E5) The ratio of power to total molar flow rate into the reactor is another useful metric, defined as the specific energy input (SEI), expressed in kJ / L, as shown in E6. From this ratio, the amount of energy consumed by the entire process can be defined as the energy cost (EC, MJ / mol) of the process (E7). The conversion rate (CR) is calculated as follows: ^^CR(^ / ℎ)=^^^^^^(^ / ^^^)∙ ^ (%) ∙ ^^(^^^ / ^)∙ ^^ (^ / ^^^)^^.^ (^ / ^^^) ∙^^^% (E8) Each experimental condition was repeated in triplicate, with the error bars shown representing the standard deviation between the gathered data. we should emphasise that these values are calculated based on the plug power. This is uncommon in publications found in literature, where the energy metrics are calculated based solely on plasma-deposited power. While the latter approach is more informative for fundamental studies, we infer that industry-oriented research should focus on the total plug power, such as the one reported in this work. To enable direct comparison between the values obtained here and those found in literature, we measured the energy efficiency of our PSUs, i.e., the efficiency of converting plug power into plasma-deposited power. The values were found to be ca. 77-83%. Examples Example 1 Figure 1 displays a schematic cutout of a gliding arc plasma reactor 110, with a common gas inlet 106 through which a feedstock gas comprising CO2 104 can be supplied into the pre-plasma chamber 105. From there it is divided over multiple parallel reactor cathodes 103, which are mounted on a single grounded anode 109. The gas inlet to each multiple parallel reactor cathode is tangential (not shown). In addition, the reactor cathodes are electrically insulated from the anode by an insulation ring (not shown). An axial gas outlet fluidly connects each reactor cathode to the post-plasma chamber 102. In the post-plasma chamber, a fixed carbon bed 101 provides a source of carbon for the fixation of oxygen species. The product gas 107 is collected from the post-plasma chamber from the axial common exhaust 108. Example 2 Figure 2 displays a schematic cutout of a gliding arc plasma reactor similar to figure 1. The fixed carbon bed 101 was removed; and a fluid jacket 111 was added. The removal of the fixed carbon bed lead to a product gas 107 which is entirely gaseous. No carbon particles were produced by the plasma reactor. This simplifies downstream operations. The addition of the fluid jacket 111 allowed cooling the post-plasma chamber by circulating cooling water through said fluid jacket. This had a beneficial effect on conversion; especially at higher operating power per electrode pairing. The setup of example 2 with five cathode plugs was tested over a range of conditions for the plasmalysis of CO2. The effect of increasing flow rate (30 – 80 L / min) for different configurations and numbers of active reactors was examined. It must be noted that in all experiments, the flow was divided between all reactors, i.e., even with one active reactor (Fig. 3) the total flow was going through all reactors, even the non-active ones. This was done as the specific aim of this work is to investigate the overall MRGAP performance rather than the performance of the individual GAP reactors within it. Likewise, the metrics of EC and SEI are reported for the overall setup, and are based on the overall flow rate and power. Nonetheless, the flow going through each individual reactor within the MRGAP increased when the total feed gas flow increased. The power was observed to increase near linearly across all cases as a function of increasing flow rate. The higher flow rate likely results in arc elongation, resulting in an increased voltage drop across the plasma. As power is the product of applied current and voltage, this increased voltage (at a constant current) results in higher power deposition into the reactor. In all of the following plots, the conversion and energy efficiency use the same axis scale for ease of comparison between figures, whereas the energy cost and power scales are fixed within a set of active reactors for ease of interpretation. Figures 3 to 6 Each of figures 3 to 6 displays the energy cost (1) , conversion (2), power (3) and energy efficiency (4) of one configuration of electrode pairings (shown inset) as a function of the feed gas flow rate. The standard deviation for each of these variables is also displayed. The energy cost (MJ / mol) is shown on the left most vertical axis. It is displayed in the center bar graph, with a top left to bottom right slanted, single line shading. The conversion (%) is shown on the left vertical axis, right of the energy cost. It is displayed in the left most bar graph, with a bottom left to top right slanted, single line shading. The power (kW) is shown on the right vertical axis, left of the energy efficiency. It is displayed in the line graph anchored to the centre bar graph. The energy efficiency (%) is shown on the rightmost vertical axis. It is displayed on the right bar graph, with a bottom left to top right slanted, double line shading. The horizontal axis for each of these graphs is the flow rate (L / min) of CO2. The active reactor nodes are displayed on each figure in gray. The inactive reactor nodes, through which there is fluid flow but to which no power is provided, are shown in white. Each of these figures and the data therein will now be described in more detail. Figure 3 Fig. 3 displays the energy cost (1) , conversion (2), power (3) and energy efficiency (4) of one active central reactor (a), and one outer reactor (b) (shown inset) as a function of the feed gas flow rate. In both configurations with one active reactor (Figs. 3a and 3b), increasing the flow rate from 30 to 80 L / min results in a gradual decrease in CO2conversion, from around 3.4% to 2% between 30 and 80 L / min. The energy efficiency of the process follows a contrary trend, increasing from around 17% (at 30 L / min) to a relatively constant value around 23% (for flow rates above and including 40 L / min). The power in both instances increased by a factor of 1.2 between 30 and 80 L / min in a near-linear manner. The energy cost of the process initially decreases from ca. 1.7 MJ / mol at 30 L / min as a function of increasing flow rate, reaching a steady minimal value around 1.2 MJ / mol for the flow rates in the range of 40 to 80 L / min. The highest conversion obtained with a single active reactor occurs at the lowest investigated flow rate, 30 L / min. Naturally, at higher flow rates, the fraction of gas treated by the plasma decreases, resulting in the observed downward trend in conversion. Despite this decreasing trend in conversion, the energy efficiency rises to a maximum value of 24% at 50 L / min. Above 40 L / min, in both configurations, this value remains relatively constant around 22-23%. This remains relatively constant as the decrease in conversion is balanced by the ratio of flow rate to power supplied to the process (i.e., the inverse of SEI); in other words, the conversion drops at the same rate as the drop in SEI. Figure 4 Figure 4 displays the energy cost (1), conversion (2), power (3) and energy efficiency (4) of three active reactors positioned linearly (a), and three active outer reactors (b) (shown inset) as a function of the feed gas flow rate. With three active reactors (Fig. 4a and 4b), the conversion profile as a function of increasing flow rate differs from that observed with one active reactor. The conversion initially rises from ca. 5.5% at 30 L / min, to a maximum value around 8% at 50 L / min. At higher flow rates (60-80 L / min), the conversion decreases again, returning to a value around 5.8% at 80 L / min in both configurations (similar to the values obtained at 30 L / min). Despite this peak-like behaviour in conversion, the energy efficiency of the process continually increases as a function of flow rate, rising from 9.5–10% at 30 L / min to a maximum value of 21% at 60 L / min. At higher flow rates, the energy efficiency remains relatively constant around this maximum value. As the flow rate increases, the power in both configurations continually rises, increasing from 3.6-3.7 kW at 30 L / min to a maximum value around 4.3–4.4 kW at 80 L / min. The energy cost of the entire process also shows a positive trend with the upscaled results, decreasing as a function of flow rate from 2.9 – 3 MJ / mol at 30 L / min to a minimum value around 1.3 MJ / mol at 50 L / min, remaining around this lowest value at higher flow rates in both configurations. At 30 L / min, the low conversion is likely due to the fact that the post-plasma chamber temperature is too high, facilitating more back-reactions, i.e., oxidation of CO to form CO218. At 80 L / min, the fraction of gas treated by the plasma is the lowest, similar to the single reactor configurations at high flow rates. This leads to low conversion values, but the highest energy efficiency, likely because the re- oxidation is minimal, and all product (CO) remains. The peak behaviour observed with three active reactors is also present in both configurations with four active reactors. In general, we observe a trend of the peak performance shifting towards higher flow rates with the use of more active reactors. This is especially important for the purpose of upscaling using this principle of parallelisation of plasmatrons. Another interesting finding is that for a given number of active reactors, the spatial configuration does not appear to impact the performance. Conversion, EE and EC values remain similar regardless of the location of the GAP reactors within the MRGAP reactor plate. This could be seen as counter-intuitive: to maintain the size of the overall setup within reasonable dimensions, the GAPs are positioned in a close range to each other, which suggests that the effluents of individual GAP reactors could interact with one another, leading to changes in the overall process performance. However, this isn’t the case, as is evident from the results. Figure 5 Figure 5 displays the energy cost (1), conversion (2), power (3) and energy efficiency (4) of four active reactors as a function of the feed gas flow rate, for configurations with four active reactors separated and equidistant from each other (a) and condensed (b). The conversion increases from 2.3% at 30 L / min to a peak value between 7.7 and 8.2% at 70 L / min. At an even higher flow rate (80 L / min), this value begins to decrease again. The energy efficiency follows the same trend as with fewer active reactors, rising from a low value around 3% at 30 L / min to its maximum of ca. 20% at 80 L / min. The power in both cases increases by a factor of 1.2: from around 5 kW at 30 L / min, to ca. 6 kW at 80 L / min. The energy cost for the process trends from a high value (around 11 MJ / mol at 30 L / min) to a low value as a function of flow rate, reaching a more viable value of 1.5 MJ / mol at 70 and 80 L / min. For flow rates to the left of the peak (≤60 L / min), the post-plasma chamber is not sufficiently cooled by either the cold gas stream or the active wall cooling (or a combination of both). This inefficient cooling leads to lower conversion, possibly by facilitating recombination reactions between CO and O / O2to reform the reactant CO2. The energy cost of the process is significantly influenced by the conversion at each step. At low flow rates, especially 30 L / min, this low conversion results in a high energy cost. At high flow rates, such as 70 and 80 L / min, the dramatic increase in conversion leads to an improvement in energy cost. Significant improvements to the energy cost can be obtained with a relatively small change in flow rate, due to its beneficial impact on conversion. This is evident between 30 and 40 L / min, where the flow rate increases by a factor of 1.3, which results in a conversion increase by a factor of 1.2, leading to major improvements to energy efficiency and energy cost. An opposite effect is noted for higher flow rates. From 70 to 80 L / min, the increase in flow rate is effectively offset by a minor decrease in conversion; resulting in a stable energy efficiency. However, the energy efficiency at these conditions is at its minimum value for these configurations. Operating at higher conversion and equal energy efficiency is preferred in particular to simplify downstream use and / or purification of the products. Figure 6 Figure 6 displays the energy cost (1), conversion (2), power (3) and energy efficiency (4) of five active reactors (shown inset) as a function of the feed gas flow rate. The most industrially relevant and interesting results were obtained for the flow rate variation analysis carried out with five active reactors (Figure 6). Once again, the peak-like behaviour for conversion can be observed, moving from a low value of 1.6% at 30 L / min to a maximum of 8.7% at 80 L / min. Promisingly, the energy efficiency for this configuration rises from 1.6% at 30 L / min to reach a maximum value of ca. 20% at 80 L / min, corresponding to the optimal conversion of 8.7%. The total power deposited in the system once again increases by a factor of 1.2 between the lowest and highest investigated flow rate, from 6 kW at 30 L / min to 7.1 kW at 80 L / min. As observed with four active reactors (see Fig. 5a and 5b above), the energy cost of the process follows an inverse trend to that observed for conversion under these conditions, decreasing from 17.8 MJ / mol at 30 L / min to a minimum value of 1.5 MJ / mol at 80 L / min. The maximal conversion obtained with all five active reactors is the highest amongst all investigated conditions and configurations, highlighting the synergistic effect between flow rate and number of active reactors (and hence plasma-deposited power). The conversion at lower flow rates is likely affected by the post-plasma chamber temperature being too high, and the produced CO and O2 recombining into CO2 (as also observed at low flow rates with three and four active reactors; see above). In summary, all data presented in Figs. 3-6 reveals that the trend of CO2conversion in all cases has a peak maximum, but this maximum is shifted from the lower flow rates with one active reactor toward higher flow rates with five active reactors. This is attributed to two factors: (i) a small number of active reactors at high flow rates means a lower fraction of plasma-treated gas, and (ii) thermal effects, where higher temperature (reached at lower flow rates with the increasing number of reactors) promotes recombination of CO with O / O2 into CO2. Figure 7 Figure 7 displays CO2 conversion rate as a function of the feed gas flow rate (a) for various reactor configurations (b). Furthermore, we compared the CR between configurations with different numbers of active reactors (Fig. 7). When plotted as a function of the feed gas flow rate, the CR increases for all configurations investigated. For the single active reactor condition, while the absolute conversion decreases as a function of flow rate, it does not initially do so proportionally (between 30 and 50 L / min). This results in a slight increase in CR, reaching a steady value around 185 g / h for flow rates above 50 L / min. A similar trend is observed with three active reactors, although the CO2 CR is larger, rising from ca. 200 g / h at 30 L / min to a steady maximal value around 500 g / h for flow rates ≥60 L / min. The trend of CR as a function of flow rate for four and five active reactors is quite similar, showing a steep rise between 50 and 70 L / min. For four active reactors, the CR increases by a factor of 3.7, while it increases by a factor of 6.8 for five active reactors. Importantly, the CR appears to plateau for three active reactors at around 60 L / min. Changing the flow rate from 70 to 80 L / min, with four reactors the relative increase in CR is lower than that with five reactors. This is another unambiguous indication that a larger number of active reactors yields best process metrics at higher flow rates. This is also clearly seen in Fig. 6, where the absolute conversion does not decrease when the flow rate increases from 70 to 80 L / min. In turn, this implies that increasing the flow rate even further could result in better process performance in the case of all five reactors. Although our current available infrastructure limited the investigation to flow rates not exceeding 80 L / min, a further study with higher flow rates is planned for future work. Under optimized conditions (five active reactors, 80 L / min CO2 feed), we achieved a process performance with ca. 9% CO2conversion, 20% energy efficiency, an energy cost of 1.5 MJ / mol CO2, and an exceptional CO2conversion rate of nearly 780 g / h. Figure 8 Figure 8 displays CO2conversion (a) and energy efficiency (b) as a function of the specific energy input. The configurations are identical to those shown in figure 7B. A useful method for comparing data with varying power inputs (such as the case with more or fewer active reactors) is plotting conversion as a function of SEI. The most ideal conditions should yield high conversion values for a low SEI value. As SEI is essentially the ratio of power input to flow rate, increasing SEI can be the result of either higher power input (at constant flow rate) or lower flow rate (at constant power input). In reality, as observed in Fig. 3 through Fig. 6, increasing flow rate results in increasing power deposition. In this instance, an increasing SEI value can be interpreted as the relative change in power being larger than the relatively change in flow rate between steps, and vice versa. The collected data shown in Fig. 8a reveals that an optimal SEI value exists for the peak conversion of CO2. This peak corresponds to approx.5.3 kJ / L regardless of the number of active reactors (3, 4, or 5). This peak indicates that to obtain optimum conversion, an ideal ratio between the power deposited into the system and the flow rate should be reached. Deviating from this optimal value results in a decrease in conversion and / or energy efficiency. However, while the same conversion can be reached at this SEI value for different configurations, the CR differs greatly. Around this peak SEI value of 5.3 kJ / L, the CR for three active reactors is 340 g / h, compared to 777 g / h with five active reactors. This relationship implies that parallelisation of reactors within a single unit is only limited by the amount of power and feed flow rate supplied. This data indicates high conversion, high conversion rate and high energy efficiency can be obtained by parallelization, while such results have not been obtained when increasing operating power and flow rates for single reactors regardless of their design. In terms of energy efficiency as a function of SEI, a different trend is observed in Figure 8b. With one active reactor, the best energy efficiency is obtained for the lowest SEI (23% at 1.4 kJ / L). However, the CR at these conditions (187 g / h) is too low to be of industrial interest. With more active reactors, a peak in energy efficiency emerges around 5.3 kJ / L. Further increasing the SEI only decreases the efficiency of the process, providing too much power into the system. While the best efficiency is obtained around this value with 3 active reactors (ca. 21% for an SEI of 4.2 kJ / L), the CR at this condition is still lower (510 g / h) than with more active reactors. For an energy efficiency of ca. 20%, five active reactors convert 777 g / h at an SEI of 5.4 kJ / L. In other words, a decrease in EE by a factor of 1.15, allows the CR to increase by a factor of 4.15. In terms of scaling up technology, the benefit of a largely increased CR evidently outweighs the minor losses made in energy efficiency.

Claims

CLAIMS 1. A method for converting CO2 to CO, comprising: - supplying CO2 to a gliding arc plasma reactor, said gliding arc plasma reactor comprising multiple parallel electrode pairings, and igniting a plasma in said gliding arc plasma reactor; - operating said gliding arc plasma reactor at a pressure Po of at least 0.95 atm; and - operating said gliding arc plasma reactor at a specific energy input SEI between 4.5 and 7.0 kJ / L CO2.

2. Method according to claim 1, wherein each electrode pairing of the multiple parallel electrode pairings operates at an operating power between 0.9 and 1.5 kW, preferably between 1.0 to 1.4 kW, more preferably between 1.1 to 1.3 kW.

3. Method according to any of claims 1-2, wherein said multiple parallel electrode pairings consist of multiple parallel cathodes mounted on a common anode plate, wherein said multiple parallel cathodes are electrically insulated from said common anode plate by an insulation ring.

4. Method according to any of claims 1-3, wherein the operating pressure Pois at least 0.99 atm, preferably at least 1.00 atm.

5. Method according to any of claims 1-4, wherein the specific energy input is between 5.0 and 6.0 kJ / L CO2, preferably between 5.0 and 5.5 kJ / L CO2.

6. Method according to any of claims 1-5, wherein said multiple parallel electrode pairings consist of at least five parallel electrode pairings.

7. Method according to any of claims 1-6, wherein said multiple parallel electrode pairings consists of at least five parallel cathodes mounted on a single common anode.

8. Method according to any of claims 1-7, wherein CO2 is supplied to the gliding arc plasma reactor at a flow rate of at least 60 L CO2 / min.

9. Method according to any of claims 1-8, wherein the gliding arc plasma reactor comprises a cooling unit downstream of said multiple parallel electrode pairings.

10. Method according to claim 9, wherein said cooling unit comprises a fluid jacket.

11. Method according to claim 9 or 10, wherein the cooling unit has a heat transfer capacity between 5% and 100% of the total operating power of the gliding arc plasma reactor.

12. Method according to any of claims 1-11, wherein the gliding arc plasma reactor further comprises a carbon bed downstream of said multiple parallel electrode pairings.

13. Method according to any of claims 1-12, wherein the gliding arc plasma reactor is powered by a switching-type high voltage power supply, preferably a current-regulated, switching-type high voltage power supply.

14. Method according to any of claims 1-13, wherein the method converts at least 500 g CO2 per hour, preferably at least 1000 g CO2 per hour.

15. Method according to any of claims 1-14, wherein the CO2 supplied to the gliding arc plasma reactor is at least 60 L CO2 / min and wherein each electrode pairing of the multiple parallel electrode pairings operates at an operating power between 0.9 and 1.5 kW.

Images