Patent Application
20120090985
The invention is in the field of the chemical conversion of materials using plasma.
Carbon dioxide decomposition may be a useful process in several circumstances. For example, in reaction processes in which Carbon Dioxide is a byproduct, CO2 decomposition may be a useful way in which to reduce the amount of CO2 discharged into the atmosphere. Additionally, in locations in which CO2 is in abundance, for example, the surface of Mars, CO2 decomposition may be used to produce fuel for mobility.
The endothermic plasma chemical process of carbon dioxide decomposition can be presented by the summarizing formula:
The enthalpy of the process is fairly high and close to that corresponding to hydrogen production from water. The total decomposition process (1.1) starts with and is limited by CO2 dissociation:
CO2→CO+O,ΔH=5.5eV/mol (1.2),
and then ends up with O conversion into O2 by means of either recombination or reaction with another CO2 molecule.
CO2 dissociation can be beneficial in a wide range of industrial applications, including treatment of power plant exhausts, synthesis of new transportation fuels, and even possible fuel production on Mars, where the atmosphere consists predominantly of CO2. Besides that, carbon monoxide generated in plasma (1.1) may be reacted with water to produce hydrogen without spending significant or appreciable additional energy in the thermo-catalytic shift reaction:
CO2+H2O→CO2,ΔH=−0.4eV/mol (1.3).
There is a need for an improved way to efficiently dissociate carbon dioxide.
In some embodiments, systems and methods for dissociation of carbon dioxide using a non-equilibrium gliding arc plasma reactor are disclosed. The non-equilibrium gliding arc is used in a reactor having a vortex flow pattern. With an input of CO2, the non-equilibrium, gliding arc plasma reactor at least partially decomposes CO2 using reactions 1.1 or 1.2. The vortex, or in some embodiments, reverse vortex, flow pattern is configured to increase the stay time of the CO2 reactant in the reactor, thus improving the efficiency of the reactor. In some embodiments, the input gas is not pure CO2, thus resulting in other gaseous reactions that may create other products or byproducts. The presently disclosed subject matter is not limited to having pure CO2 (including gas, liquid or solid) as the input nor is the subject matter limited to having only CO2, O2 and CO as the only possible outputs. For example, the reactor of the presently disclosed subject matter may have an additional carbon-based input stream, such as methane, that may produce CO and hydrogen gas (synthesis gas production).
In some embodiments, the reactor is primarily a cylindrical reactor with the body of the cylinder having two parts: an upper cylinder and a lower cylinder. The upper cylinder, in some embodiments, is configured to be at the high voltage potential and the lower cylinder, in some embodiments, is configured to be at the low (or ground) voltage potential. An insulator electrically isolates the upper and lower cylinders. It should be noted that the present subject matter should not be limited to a cylinder, as the other reactor configurations having similar properties will work as well.
Additionally, the use of the terms “upper” and “lower” merely describe the spatial relationship between the two parts and does not imply any limitation as to the orientation of the reactor with any other point of reference, such as the Earth. To help create the vortex flow, a diaphragm is situated on the output. The aperture of the diaphragm is adjusted based upon the flow rate of the input stream and may also be changed to adjust for possible efficiencies of the reactor. Additionally, the relative size of the upper and lower cylinders may be adjusted as well to, among other things, increase or decrease the recirculation flow of the reactants in the reactor.
These and other features of the subject matter are described below.
The foregoing summary, as well as the following detailed description of the subject matter is better understood when read in conjunction with the appended drawings. For the purposes of illustration, there is shown in the drawings exemplary embodiments; however, these embodiments are not limited to the specific methods and instrumentalities disclosed. In the drawings:
a is an exemplary illustration of a reactor showing a reverse vortex flow pattern;
Certain specific details are set forth in the following description and figures to provide a thorough understanding of various embodiments of the subject matter. Certain well-known details are not set forth in the following disclosure to avoid unnecessarily obscuring the various embodiments of the subject matter. Further, those of ordinary skill in the relevant art will understand that they can practice other embodiments of the subject matter without one or more of the details described below. Finally, while various methods are described with reference to steps and sequences in the following disclosure, the description as such is for providing a clear implementation of embodiments of the subject matter, and the steps and sequences of steps should not be taken as required to practice this subject matter.
The present invention may be understood more readily by reference to the following detailed description taken in connection with the accompanying figures and examples, which form a part of this disclosure. It is to be understood that this invention is not limited to the specific devices, methods, applications, conditions or parameters described and/or shown herein, and that the terminology used herein is for the purpose of describing particular embodiments by way of example only and is not intended to be limiting of the claimed invention.
Also, as used in the specification including the appended claims, the singular forms “a,” “an,” and “the” include the plural, and reference to a particular numerical value includes at least that particular value, unless the context clearly dictates otherwise. The term “plurality”, as used herein, means more than one. When a range of values is expressed, embodiments may include from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent “about,” it will be understood that the particular value forms another embodiment. All ranges are inclusive and combinable.
It is to be appreciated that certain features of the invention which are, for clarity, described herein in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention that are, for brevity, described in the context of a single embodiment, may also be provided separately or in any subcombination. Further, references to values stated in ranges include each and every value within that range.
Various embodiments of the present provide methods for dissociating carbon dioxide, each method comprising contacting a non-equilibrium, gliding arc discharge with carbon dioxide in a reactor, wherein some or all of the carbon dioxide is circulated within the reactor in a vortex flow pattern. In certain independent embodiments, at least 50 wt %, at least 60 wt %, at least 70 wt %, at least 80 wt %, at least 90 wt %, or at least 95 wt % of the carbon dioxide is circulated within the reactor in a vortex flow pattern. In other embodiments, the carbon dioxide is circulated in a reverse vortex flow pattern, while in still other embodiments, the carbon dioxide is circulated in a combination of vortex and reverse vortex flow patterns.
In other embodiments, the carbon dioxide is dissociated in the presence of at least one hydrocarbon, so as to produce a synthesis gas mixture, including those wherein the methods further comprise inputting a feedstock comprising carbon dioxide to a reactor chamber; and generating a gliding arc discharge plasma within said reactor.
In further embodiments of the methods and the reactors used in the methods, the reactors are conical or cylindrical in shape. In still further embodiments, each reactor comprises a reactor chamber having a high potential portion and a low potential portion, with the high potential portion being electrically isolated from the low potential portion; at least one orifice through which at least carbon dioxide can be input into the reactor, said at least one orifice directionally positioned for creating vortex flow within said reactor; a gap to facilitate a gliding arc discharge in conjunction with the inputting of the at least carbon dioxide; and an electrical power source having a high voltage potential in electrical communication with the high potential portion and a low voltage potential in electrical communication with the low potential portion; wherein the input, reactor chamber and electrical power source are configured to create a non-equilibrium gliding arc discharge within a vortex flow pattern. Other embodiments provide that the reactors further comprise an output orifice positioned at one end of the low potential portion of the chamber capable as acting as an output for the carbon dioxide and dissociated products therefrom and a diaphragm positioned at or proximate to the output orifice.
Other embodiments provide reactors and methods using same wherein the high potential portion of the reactor comprises a barrel-shaped electrode; and the low potential portion of the reactor comprises an electrically separate barrel-shaped electrode.
Still other embodiments provide reactors and methods wherein the high potential portion comprising barrel-shaped electrode has a volume at least about 10 vol %, at least about 25 vol % at least about 50 vol %, or at least about 10 vol %, larger than the low potential portion comprising the barrel-shaped electrode.
Whereas thus far, the chambers have been described in terms of a high and low potential portion, it should be appreciated that other embodiments provide reactors and methods wherein the potentials are reversed. That is, the portions of the reactors described above as high potential have electrical potentials that are lower than the potentials of the portions of the reactors described above as low potential, in each case, the terms high and low potential refer to the electrical potential with respect to one another.
Other embodiments provide methods wherein the at least one hydrocarbon is also input to the reactor, either in the feedstock comprising carbon dioxide or separately (for example, in separate feeds), or both. When so configured or used, the reactors or methods can be used to prepare synthesis gas. In certain embodiments, the reactors (and methods using same) comprise at least one input orifice at or proximate of the high potential portion of the chamber, said at least one input orifice capable of delivering at least one liquid hydrocarbon to the reaction chamber.
Plasmas, referred to as the “fourth state of matter,” are ionized gases having at least one electron that is not bound to an atom or molecule. In recent years, plasmas have become of significant interest to researchers in fields such as organic and polymer chemistry, fuel conversion, hydrogen production, environmental chemistry, biology, and medicine, among others. This is, in part, because plasmas offer several advantages over traditional chemical processes. For example, plasmas can generate much higher temperatures and energy densities than conventional chemical technologies; plasmas are able to produce very high concentrations of energetic and chemically active species; and plasma systems can operate far from thermodynamic equilibrium, providing extremely high concentrations of chemically active species while having a bulk temperature as low as room temperature.
Plasmas are generated by ionizing gases using any of a variety of ionization sources. Depending upon the ionization source and the extent of ionization, plasmas may be characterized as either thermal or non-thermal. Thermal and non-thermal plasmas can also be characterized by the temperature of their components. Thermal plasmas are in a state of thermal equilibrium, that is, the temperature of the free electrons, ions, and heavy neutral atoms are approximately the same. Non-thermal plasmas, or cold plasmas, are far from a state of thermal equilibrium; the temperature of the free electrons is much greater than the temperature of the ions and heavy neutral atoms within the plasma.
The initial generation of free electrons may vary depending upon the ionization source. With respect to both thermal and non-thermal ionization sources, electrons may be generated at the surface of the cathode due to a potential applied between the electrodes. In addition, thermal plasma ionization sources may also generate electrons at the surface of a cathode as a result of the high temperature of the cathode (thermionic emissions), or high electric fields near the surface of the cathode (field emissions), or as a result of ion and photon bombardment (secondary electron emission).
The energy from these free electrons may be transferred to additional plasma components, providing energy for additional ionization, excitation, dissociation, etc. With respect to non-thermal plasmas, the ionization process typically occurs by direct ionization through electron impact. Direct ionization occurs when an electron of high energy interacts with a valence electron of a neutral atom or molecule. If the energy of the electron is greater than the ionization potential of the valence electron, the valence electron escapes the electron cloud of the atom or molecule and becomes a free electron according to:
e
−+A→A++e−+e−.
As the charge of the ion increases, the energy required to remove an additional electron also increases. Thus, the energy required to remove an additional electron from A+ is greater than the energy required to remove the first electron from A to form A. A benefit of non-thermal plasmas is that because complete ionization does not occur, the power to the ionization source can be adjusted to increase or decrease ionization. This ability to adjust the ionization of the gas provides for a user to “tune” the plasma to their specific needs.
An exemplary thermal plasma ionization source is an arc discharge. Arc discharges have been otherwise used for applications such as metallurgy, metal welding and metal cutting and are known per se. Arc discharges are formed by the application of a potential to a cathode, and arc discharges are characterized by high current densities and low voltage drops. Factors relevant to these characteristics are the usually short distance between the electrodes (typically a few millimeters) and the mostly inert materials of the electrodes (typically, carbon, tungsten, zirconium, silver, etc). In the case of arc discharges the majority of electrons generated on the cathode surface are formed by intensive thermionic emission. Because of this intense generation of electrons at the cathode, current at the cathode is high, which leads to Joule heating and increased temperatures of the cathodes. Such high temperatures can result in evaporation and erosion of the cathode. The anode in arc discharges may be either an electrode having a composition identical or similar to the cathode or it may be another conductive material. For example, the anode in arc discharges used in metal welding or cutting is the actual metal to be welded or cut. Typical values for parameters of thermal arc discharges can be seen in Table 1:
Although thermal plasmas are capable of delivering extremely high powers, they have several drawbacks. In addition to the electrode erosion problems discussed above, thermal plasmas do not allow for adjusting the amount of ionization, they operate at extremely high temperatures, and they lack energy efficiency.
Non-thermal plasma ionization sources have alleviated some of the above-mentioned problems. Exemplary ionization sources for non-thermal plasmas include glow discharges, dielectric barrier discharges, and gliding arc discharges, among others. In contrast to thermal plasmas, non-thermal plasmas provide for high selectivity, high energy efficiencies, and low operating temperatures. In many non-thermal plasma systems, electron temperatures are about 10,000 K while the bulk gas temperature may be as cool as room temperature.
A glow discharge is a plasma source that generates a non-equilibrium plasma between two electrodes under a direct current. There are several types of glow discharges; a common one is in the “neon” lights. This glow discharge is established in a long tube with a potential difference applied between an anode at one end of the tube and a cathode at the other end. The tube is filled usually with an inert gas often under low pressure. Due to the potential difference between the electrodes, electrons are emitted from the cathode and accelerate toward the anode. The electrons collide with gas atoms in the tube and form excited species. These excited species decay to lower energy levels through the emission of light (i.e., glow). The ionized species generated by the collision of electrons with gas atoms travel toward the cathode and release secondary electrons, which are then accelerated toward the anode. This generation of electrons, referred to as secondary emission, is in contrast to the intensive formation of electrons at the surface of the cathode in arcs due to thermionic emission. Typical parameters of a glow discharge as described above are shown in Table 2:
Dielectric barrier discharge (DBD) may be generated using an alternating current at a frequency of from about 0.5 kHz to about 500 kHz between a high voltage electrode and a ground electrode. In addition, one or more dielectric barriers are placed between the electrodes. DBDs have been employed for over a century and have been used for the generation of ozone in the purification of water, polymer treatment (to promote wettability, printability, adhesion), and for pollution control. Dielectric barrier in DBDs prevents spark formation by limiting current between the electrodes.
Several materials can be utilized for the dielectric barrier. These include glass, quartz, and ceramics, among others. The clearance between the discharge gaps is typically between about 0.1 mm and several centimeters. The required voltage applied to the high voltage electrode varies depending upon the pressure and the clearance between the discharge gaps. For a DBD at atmospheric pressure and a few millimeters between the gaps, the voltage required to generate a plasma is typically about 10 kV. In certain embodiments, the ground electrode of the DBD may be an external conductive object, such as a human body. This is known as floating-electrode DBD (FE-DBD). FE-DBD has recently been utilized in medical applications.
All variety of plasma-chemical systems is traditionally divided into two major classes: thermal and non-thermal ones, characterized by their specific advantages and disadvantages [10]. Thermal plasma (usually arcs or Radio Frequency ICP-discharges) is associated with Joule heating, thermal ionization and enable to deliver high power (to over 50 Megawatts per unit) at high operating pressures. However, low excitation selectivity, very high gas temperature, serious quenching requirements and electrode problems result in limited energy efficiency and applicability of thermal plasma sources.
Non-thermal plasma (of low pressure glow, RF-, and microwave discharges) offers high selectivity and energy efficiency of plasma chemical reactions; it is able to operate effectively at low temperatures and without any special quenching. However, operating pressures and power levels of the non-thermal discharges are usually limited, which makes them not practical enough to render sufficient degrees of conversion and high production rates. Conventional thermal and non-thermal discharges cannot provide simultaneously a high level of non-equilibrium, high electron temperature, high electron density, and high power, whereas most prospective plasma chemical applications require both a high power for efficient reactor productivity and a high degree of non-equilibrium to support selectively chemical processes.
One of the critical challenges of modern plasma chemistry is to unite the advantages of thermal and non-thermal plasma systems by developing powerful and high-pressure discharges generating non-equilibrium cold plasma, which can be applied in particular for large scale exhaust gas cleaning, pollution control, fuel conversion, hydrogen production and surface treatment. One of the possible ways to create such hybrid plasma is to use the transient type of arc—the gliding arc (GA) discharge. This periodic discharge evolves during a cycle from arc to strongly non-equilibrium discharge with still relatively high level of electron density.
Next, the length continues increasing, but the supplied power is insufficient to balance energy losses of the thermal plasma to the surrounding gas. The arc cools down and either extinguishes or changes ionization mechanism to non-thermal one—if electric field is sufficient for that. The non-thermal plasma channel formed after the “equilibrium/non-equilibrium” transition keeps growing until extinguishing anyway closing a cycle. The next cycle starts immediately after the voltage reaches the breakdown value, usually just after the fading of the previous arc. A typical repetition rate of the arc is in the range of 10 Hz to 100 Hz and changes with the gas flow rate: the higher is the flow rate, the higher is the frequency. The gliding arcs can be arranged, depending on current and flow rate, as completely thermal (at high currents), completely non-thermal (lowest currents), and as transitional discharge (at intermediate currents). In some uses, the transitional regime may be the most practical one because it is mainly non-thermal though still powerful and remembering its “quasi-thermal past” (“the memory effect”).
The term “gliding arc” is used in the present subject matter as is understood by those skilled in the art. It should be understood that a plasma discharge in the present subject matter may be generated in various ways, for example, glow discharge. In a reactor implementing a glow discharge, a cathode current may be controlled mostly by the secondary electron emission, as occurs in glow discharge, instead of thermionic emission, as occurs in electrical arcs.
Vortex flow, as described herein, may be shown in reference to
For example,
It should also be understood that, although the reactor 10 of
Referring back to
Input fluid 2 is introduced to reaction chamber 12 via nozzles 14a, 14b, the outputs of which are preferably oriented tangential relative to wall 13 of reaction chamber 12, as shown by
By introducing input fluid 2 in this manner, as discussed above, a rotational force is imparted upon the fluids in reaction chamber 12, thus causing a rotation of the fluids in reaction chamber 12 in a clockwise direction in this embodiment. Thus, the velocity at which input fluid 2 enters reaction chamber 12 effects the rotational speed of the contents in reaction chamber 12. It should be noted that the input direction may be in a direction reverse to that shown in
Referring back to
Referring back to
The diameter of opening 32 may also be configured to establish, or prevent, a recirculation zone from forming. Reactor 10 may be configured to provide a way in which relatively hot fluids flowing from plasma region 40 may exchange a portion of their heat with fluids flowing to plasma region 40. For example, exemplary fluids 38a-c, which are flowing generally towards plasma region 40 receive heat from exemplary fluid 42a, which is flowing from plasma region 40. Exemplary fluid 42a, after exchanging heat with exemplary fluids 38a-c, may than flow back to plasma region 40, as shown by exemplary fluid 42b. Thus, a portion of the reaction heat generated in plasma region 40 and a portion of fluids in reaction chamber 12 recirculate within reactor 10. In one embodiment, if a recirculation zone is desired, the diameter of opening 32 in flange 30 may be approximately 10% up to 75% of the diameter of reaction chamber 12.
As discussed above, reverse vortex flow as used herein means that the vortex flow has axial motion initially caused by nozzles 14a and 14b along wall 13 of the chamber and then the flow turns back and moves along the axis to the “open” end of the chamber towards opening 32. An example in nature of this flow pattern may be similar to the flow inside a dust separation cyclone, or inside a natural tornado. Input fluid 2 travels in a circular motion, traveling in a downward and inward direction towards plasma region 40, as shown by exemplary fluids 38a-c.
A reverse vortex flow in reaction chamber 12 causes the contents of reactor 10 in reaction chamber 12 to rotate around plasma region 40, while output stream 22 travels in a direction upwards from the bottom of reactor 10 to opening 32. Along with other benefits that may not be explicitly disclosed herein, the rotation may provide necessary time for the heating of the contents flowing to and in the relatively hot plasma region 40 as the contents move downwardly around plasma region 40. Another benefit of the rotation may be that the reverse vortex flow may increase the residence time of reactants and products inside reaction chamber 12 while keeping the flow speed relatively high.
A vortex flow, such as the reverse-vortex flow described in
In an exemplary embodiment, incoming gas enters the cylindrical reactor through tangential inlet holes. Gliding arc discharge starts in the gap between 2 electrodes and stretches both ways (upward and downward) by incoming gas vortex. Incoming gas (CO2) splits into 2 flows (upward and downward). The ratio of these flows depends on diameters of both electrodes and diaphragm's diameter. Both high voltage and ground electrodes have extended length to maximize the residence time of CO2 contact with plasma arc and thus to increase CO2 dissociation efficiency. Restricted exit of ground electrode (diaphragm) serves the purpose of particular flow formation inside the reactor 600. The reaction products and byproducts exit reactor 600 through output orifice 610.
CH4+CO2→2CO+2H2ΔH=2.6eV/mol
Liquid or gaseous hydrocarbons could be introduced into plasma zone from the top of high potential electrode or tangentially in mixture with CO2. This so called dry reforming process is similar to steam reforming of methane, although it is more endothermic. At the same time in contrast to the steam reforming, the dry reforming can be more efficient in case of gaseous hydrocarbons due to complete gas phase nature of reaction.
This process consumes CO2 and produces synthesis gas, therefore it can be attractive for environmental control. Produced synthesis gas could be used for liquid fuel production or power generation.
While the embodiments have been described in connection with the preferred embodiments of the various figures, it is to be understood that other similar embodiments may be used or modifications and additions may be made to the described embodiment for performing the same function without deviating therefrom. Therefore, the disclosed embodiments should not be limited to any single embodiment but rather should be construed in breadth and scope in accordance with the appended claims.
This Application claims the benefit of U.S. Provisional Patent Application No. 61/388,861, filed Oct. 1, 2010, which is incorporated by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61388861 | Oct 2010 | US |
