[0001] This invention relates to a method and apparatus for processing carbon dioxide.

BACKGROUND

[0002] Carbon dioxide is well recognised as an atmospheric pollutant that contributes to global warming. The removal of carbon dioxide from the atmosphere or the prevention of carbon dioxide from reaching the atmosphere is a desirable aim in improving the environment.

[0003] Current techniques for the sequestration of carbon dioxide involve burying the carbon dioxide or dissolving carbon dioxide in sea water, which has significant practical difficulties. The present invention, at least in its preferred embodiments, seeks to provide a potential alternative to existing methods of carbon dioxide sequestration.

BRIEF SUMMARY OF THE DISCLOSURE

[0004] In accordance with the present invention there is provided a method of processing carbon dioxide. The method involves creating a carbon dioxide plasma within a liquid medium and maintaining the plasma with carbon dioxide, whereby to generate at least carbon monoxide from the carbon dioxide.

[0005] Thus, in accordance with the invention, as carbon dioxide plasma is created within a liquid medium and can provide sufficient energy to convert carbon dioxide into carbon monoxide. The liquid medium prevents the process being affected by potential pollutants, such as nitrogen, and acts to dissipate heat from the recombinants including carbon monoxide.

[0006] In an embodiment of the invention creating the plasma comprises applying an electrical voltage between two spaced electrodes within the liquid medium. As an alternative, it would be possible to create the plasma by means of directed microwave or RF radiation at an appropriate wavelength..

[0007] During the process, the spacing between the electrodes may be maintained substantially constant. In this way, the energy density of the plasma can also be maintained constant. The spacing may be kept constant by monitoring the potential difference (voltage) between the electrodes for a given applied current and adjusting the spacing to maintain the potential difference at a constant level or by electro-optical, or electro-acoustic sensors providing feedback data to a monitoring system.

[0008] The carbon dioxide may be fed to the plasma through at least one of the electrodes. Thus a conduit for the carbon dioxide may be defined within at least one of the electrodes. The electrode may therefore be in the form of a hollow rod or tube. It is feasible for each of the electrodes to have such a conduit for supplying carbon dioxide to the plasma. Alternatively, the carbon dioxide may be supplied to the plasma through a separate nozzle or similar arrangement. An advantageof supplying the carbon dioxide through the electrode is that carbon dioxide is not lost outside of the plasma due to potentially disadvantageous kinetics.

[0009] The method may comprise feeding the plasma with carbon. The carbon may be provided as carbon black, graphite, coal or any other suitable form of solid carbon or carbonaceous material. The carbon may be in the form of an extrudable paste that can be supplied continuously to the plasma. The carbon may be in the form of a carbon rod that is fed to the plasma, for example between the electrodes. The addition of carbon aids the reduction of carbon dioxide to carbon monoxide.

[0010] Advantageously at least one of the electrodes may comprise solid carbon. Thus, the electrode may be consumed to feed the plasma with solid carbon. In this way, it is ensured in a simple manner that the carbon is directed as closely as possible to the plasma.

[0011] The liquid medium may simply provide a controlled environment for the plasma. However, in a presently preferred embodiment, the liquid medium is consumed by the plasma during processing. Thus, the liquid medium may comprise elements or compounds which can react with carbon dioxide to form other products.

[0012] In the presently preferred embodiment, the liquid medium comprises water.

Indeed in the presently preferred embodiment, the liquid medium is water. However, the water may be mixed with or have dissolved therein other compounds or elements. Thus, the liquid medium may be aqueous. In general, compounds of hydrogen, carbon and oxygen are desirably comprised in the liquid medium. For example, the liquid medium may comprise hydrocarbons or carbohydrates.

[0013] In embodiments of the invention, the plasma is sufficiently energetic that the method includes generating hydrogen by the consumption of the carbon dioxide and water by the plasma. In such embodiments, a product of the method is a mixture of carbon monoxide and hydrogen, also known as syngas.

[0014] Depending on the composition of the liquid medium and the plasma, the method may include generating oxygen.

[0015] The liquid medium may be maintained under pressure, for example contained within a pressure vessel. Alternatively, or in addition, the temperature of the liquid medium may be contolled as part of the method. The pressure of the liquid medium may be controlled as part of the method. The gas(es) generated by the method may be collected. The collected gases may be stored for future use. Alternatively, the gases may be processed immediately in other processes.

[0016] The invention extends to apparatus adapted to carry out the method described.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Embodiments of the invention are further described hereinafter with reference to the accompanying drawings, in which:

Figure 1 is a schematic diagram of a pressure vessel according to an embodiment of the invention;

Figure 2 is a schematic diagram of the carbon electrode feed mechanism of the embodiment of Figure 1; and

Figure 3 is a perspective view of the feed mechanism of Figure 2.

DETAILED DESCRIPTION

[0018] An embodiment of the present invention involves a new process to convert carbon dioxide in a cost effective manner utilising a plasma within a liquid medium to simultaneously reduce carbon dioxide and oxidise carbon such that the recombinants create a valuable commodity, syngas, a mixture of carbon monoxide and hydrogen gas.

[0019] According to an embodiment of the invention shown in Figure 1, this process takes place in a liquid, in this case water, contained in a pressurised reaction vessel 1. A carbon dioxide plasma is created by a transferred arc between consumable carbon electrodes 2, 3. Carbon dioxide is transmitted under pressure through a positively charged carbon electrode 2 where it is energised by the transfer of electrical energy to a negatively charged carbon electrode 3. When the carbon electrodes 2, 3 become incandescent they react with the water creating hydrogen gas (H ₂ ) and carbon monoxide. Plasma radicals also react with both the carbon and the water forming hydrogen and carbon monoxide.

Both gases bubble to the surface where they are contained in a gas space within the pressure vessel 1 prior to exiting into a storage system (not shown).

[0020] The advantage of using a plasma, which has potentially non-thermodynamic equilibria, lies in dealing with thermodynamically unfavourable reactions. Plasma is a state of matter consisting of a gas in which a significant proportion of the gas particles are ionised. A plasma is created when the atoms of the plasma gas become dissociated from their electrons due to being energised by electricity (or extreme heat). The present embodiment of the invention uses hot plasma (> 10 -15,000°C) for the process.

[0021] On a continuous basis carbon dioxide is energised by electrical discharge forming a dissociative plasma consisting of a range of carbon dioxide, carbon monoxide, carbon, and oxygen radicals. This plasma is extremely hot, highly conductive, and reactive. The carbon electrodes 2, 3 are heated to extremely high temperatures by the carbon dioxide plasma.

[0022] A liquid medium provides a controllable environment eliminating unwanted reactants such as nitrogen. Additionally the liquid medium provides immediate quenching to the products of the process, such as carbon monoxide, restricting carbon dioxide reformation. In this embodiment, water was used as the liquid medium. However, other mediums have been tested including biomass and waste sugar water solutions. These were tested in order to determine the efficacy of adding energy via the liquid medium. It was found that increased production of syngas was achieved with the addition of hydrocarbons to the water.

[0023] A carbon dioxide plasma overcomes one of the problems of electrical energy transfer within a liquid medium, i.e. resistivity of the liquid. Carbon dioxide plasma may be created by several electrical methods, included alternating current, direct current, radio frequency supply, high voltage and high current. Notwithstanding the energy utilised to create a carbon dioxide plasma it was found there was no loss of syngas production in practice.

[0024] Consumable carbon electrodes overcome one of the significant problems of creating hot plasma, i.e. electrode erosion by high temperatures, ion transfer and deposition. The present embodiment uses solid carbon rods 2 fed into the reaction site using a lead screw mechanism 4 operated by a stepper motor (not shown) controlled by a computer software system. For a continuous process an augur delivery system may be used for solid, and semi-solid carbonaceous materials.

[0025] A stainless steel pressure vessel 1 of approximately 240 litres rated at 3.5 bar operating pressure is shown in Figure 1. The pressure vessel 1 has a removable top cover section 5 held in place by a series of clamps and sealed with a solid nylon seal. The top cover 5 contains various ports using standard tri-clover fitting systems for the porting of temperature and pressure probes. The top cover also incorporates a burst disc in order to protect against any rapid increase in internal pressure associated with unexpected combustion of the syngas. A viewing window 9 for observing the plasma is provided in the side of the pressure vessel 1.

[0026] As shown in detail in Figures 2 and 3, mounted to the top cover is a carbon rod electrode carrier mechanism 4. The mechanism holds a hollow carbon rod 2 and its function is to move the carbon electrode 2 linearly relative to a stationary carbon block electrode 3 mounted at one end of the carrier 4. The mechanism 4 also serves to transmit carbon dioxide from a high pressure cylinder or other source located outside of the pressure vessel 1 through a regulator and hose to a gas line port 6 mounted to the flange 5 of the electrode carrier. A flexible tube (not shown) connected to the gas line port 6 carries the carbon dioxide to the electrode gas line port 10 where it is transmitted to the plasma reaction via a longitudinal passageway defined through the carbon rod electrode 2. This whole mechanism is immersed in water contained within the pressure vessel 1 such that the reaction site is underwater.

[0027] A lead nut 7 is moved by the rotary motion of a lead screw 8 turned by a stepper motor mounted to the mounting flange. In this reactor the lead screw 8 had a square threaded 2mm pitch screw. The carbon rod electrode 2 is mounted to the lead nut 7 for movement therewith. In order to establish optimum plasma densities tests have been carried out with various diameter carbon rods and varying current inputs. The lead nut 7 on the electrode carrier is a two part system. The electrode rod clamp is demountable and designed to clamp a specific diameter carbon rod 2. The clamp can be exchanged for a larger or smaller clamp once the desired rod diameter is selected.

[0028] The stepper motor is controlled by system software. Ports 6 for the carbon dioxide gas and power are provided on the mounting flange and are designed to transmit power and gas while maintaining pressure and isolating the power and carbon dioxide gas. The carbon block electrode 3 is electrically isolated from the pressure vessel 1 which itself carries part of the electrical load in order to complete a circuit.

[0029] The system is energised by a 10kW power supply with the capability to provide various waveform and frequency functions including AC/DC, sine, square, and triangle waveforms. Energy is carried to the electrodes 2, 3 via copper cables to the pressure vessel body and to the electrode 3. The electrical system is capable of carrying a total load of 10kW at currents of up to 350 amps. The arc length is controlled as this can have an effect on recombustion and gas production. Arc length is a function of resistivity as determined by the voltage necessary to overcome the given resistivity. Longer arcs require higher voltages which can be measured systemically at the power unit.

[0030] The described system allows a carbon dioxide plasma to be created which heats the carbon electrode until it is incandescent. An incandescent electrode allows simultaneous reactions to occur between the carbon and water, carbon dioxide plasma and water, and carbon dioxide plasma and carbon. Variables such as consumption of carbon, consumption of carbon dioxide, plasma energy density, pressure and temperature are controlled precisely in order to maintain a continuous process.

[0031] The product gas is a mixture of hydrogen (H ₂ ) and carbon monoxide (CO) (syngas) which is flammable and cannot be compressed as a mixed gas safely. Therefore a means of exhausting the syngas safely without significantly pressurising the system is provided. This is accomplished by having pressure regulators in the gas circuit before and after the pressure vessel with both a burst disc and a pressure safety valve. Additionally by minimising the gas volume less energy is contained within the system which can be released by unexpected combustion. The gas regulators are used to vary pressures within the vessel as pressure can have an effect on the stability of the process and the amount of gas generated by the process.

[0032] In the present embodiment, the automation software system uses “Labview 8.5” by National Instruments. The software receives as input the process parameters, including rod length, diameter and mass, electrical current to be used, required arc length, gas flow rate etc.

[0033] In operation of the reaction system, as a first step the reactor is purged with nitrogen to remove any residual gases inside. The purging step is repeated at the end of carbon dioxide processing to ensure that any residual gases are expunged from the reaction vessel.

[0034] To initiate the plasma arc, physical contact between the rod electrode 2 and the block electrode 3 is necessary due to the mode of operation of the electrical power supply used for testing. In this routine the control software sends a signal to the stepper motor resulting in linear movement of the rod electrode 2 until the rod electrode 2 and the block electrode 3 are sufficiently close, i.e. the voltage between the electrodes 2, 3 approaches zero. Once the arc is initiated, the arc remains stable if the gap between the rod electrode 2 and the block electrode 3 remains within very narrow limits of a few tens of a millimetre. At higher currents the electrode 2 tends to burn very quickly requiring quick action of the compensating system. Voltage, being directly linked to the distance between the electrodes, may be used as an indication that a compensating step is required. A range of acceptable voltages may be defined and the control algorithm may be based on making a compensating step down if the voltage is too high (the gap is too big) or up if the voltage is too low (the gap is too small). In this way the control algorithm can take care of both establishing the arc and then maintaining it.

[0035] A further embodiment of the invention is configured to convert carbon dioxide continuously. As the liquid medium is consumed in the reaction process it is replenished on a continuous basis. This embodiment of the invention utilises a pressurised reactor vessel in which liquid replacement is effected with pressurised pumps controlled automatically.

[0036] For a continuous process a carbonaceous electrode material may be used that is capable of being continuously fed to the reaction site, and is capable of transmitting carbon dioxide. The electrode material should be plastic enough that it can be delivered to the reaction site using an augur delivery system designed for semi-solid carbonaceous materials, yet stiff enough to maintain its structural integrity in a liquid medium so as to allow a plasma to exist at its face. The system is designed to inject the electrode material under high pressure to the reaction site maintaining gas and fluid seals such that the reactor vessel maintains its ability to convert carbon dioxide, combust carbon and deliver the product syngas safely.

[0037] According to this embodiment, carbon dioxide injection uses a ceramic injection head which allows carbon dioxide injection close to the reaction site without the injection head being adversely affected by the high temperature. The ceramic injection head heats the plastic carbonaceous material as it passes though the ceramic head causing the plastic material to harden sufficiently that it maintains its structural integrity in the liquid medium.

[0038] The hot plasma carbon dioxide reactor is able to convert carbon dioxide and generate a gas exhibiting combustibility. This shows that the reactor design is able to manage the recombinant process such that carbon dioxide, carbon monoxide, oxygen, and carbon ions are energised sufficiently to break water’s hydrogen-oxygen bond forming hydrogen and carbon monoxide.

[0039] The basic chemistry below shows that carbon dioxide plasma results in carbon dioxide, carbon monoxide, oxygen, and carbon ions which are then available to recombine to form carbon monoxide and oxygen gas. The basic reactions are as follows:

C0 ₂ + H ₂ - CO + H ₂ 0 AH ⁰ = 41.2 kJ/mol, 936.4 kJ/kg

2 C0 ₂ 2CO + 0 ₂ AH = 552 kJ/mol, 12545.5 kJ/kg C0 ₂ - c + 0 ₂ AH = 393.5 kJ/mol, 8943.2 kJ/kg

C + H ₂ 0 — CO + H ₂ AH ⁰ = 131.3 kJ/mol, 2984.1 kJ/kg

[0040] These may not be the only reactions occurring, and it is possible that there may be methane and other alkane reactions.

Examples of the outputs and inputs of the reaction vessel for nine separate test runs of the process according to an embodiment of the invention are shown in the table below, in which:

D is the diameter of the carbon rod electrode in mm; t is the run time of the process in minutes;

E is the energy input during the process in mJoules;

T is the peak temperature of the liquid medium in degrees C; P is the peak pressure within the pressure vessel in psia;

C, C0 ₂ , H ₂ 0, CO, H ₂ , and 0 ₂ are the volumes of the respective gases input or output from the reaction vessel in moles (an asterisk indicates a stoichiometrically calculated value, other values were measured with a mass flow meter and gas analyser).

[0041] In summary, a method of processing carbon dioxide involves creating a carbon dioxide plasma within a liquid medium, such as water. The plasma is fed with carbon dioxide, which generates at least carbon monoxide from the carbon dioxide. The liquid medium can be water, in which case hydrogen may also be generated by the plasma. A consumable carbon electrode 2 may be used. The electrode 2 may supply the carbon dioxide to the plasma via a passageway defined through the electrode 2.

[0042] Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of them mean “including but not limited to”, and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

[0043] Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments.

The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.