Patent Grant
12273987
The present technology describes a system for controlling the electron energy distribution in an atmospheric and near-atmospheric low-temperature plasma by subjecting the plasma to a nanosecond pulsed voltage that is completely decoupled from the means that generated the plasma. The nanosecond-pulsed voltage generates a nanosecond pulsed electric field that affects the energy of the electrons in the plasma without affecting the energy of the ions. More specifically, the low-temperature plasma is generated by an electron-beam.
For the last 50 years, plasmas have been used in large industrial applications. Tailoring the type of plasma to a specific application is key to the commercial success of that application, as different types of plasma will have different efficiencies and different outcomes, depending on the application they are used for. For the last 20 years, there has been a large effort to use plasma for the dissociation of molecular species to produce chemicals such as ammonia, hydrogen, nitric acid, and others.
In the case of nitrogen fixation for example, Kevin H. R. Grootendorst et al1. give an extensive list of different types of plasma (
Referring to
O+N2→NO+N
N+O2→NO+O
NO is then further oxidized into NO2 at room temperature and then reacted with water to produce HNO3. Here again, the energy is not specifically provided to the chemical bonds of N2 and O2 molecules, but rather to heat the entire gas stream at high temperatures. Also, the arc plasma inherently carries high energy losses through ionization, electronic excitation, and radiation, which do not help the dissociation of the molecules.
Low-temperature plasmas have also been used to dissociate molecular gases such as methane and nitrogen. Kerscher et al2. have reviewed the use of an electron-beam generated plasma for the production of hydrogen through the pyrolysis of methane. Also, Richardson et al.3 have modeled the decomposition of nitrogen in an electron beam generated plasma. Other works have also used other types of on-thermal plasmas such as dielectric barrier discharge, hallo cathode, and gliding arc plasmas.
Wei Zhong Wang et al4. describe nitrogen fixation in a gliding arc plasma through chemical kinetics modelling and experimentation. In this case, a gliding arc plasma is used in a mixture of nitrogen and oxygen to produce NO.
For nitrogen molecule, the onset of the electronic excitation starts at a reduced electric field of 42 Td, which corresponds to about 2.3 eV. For N2 direct electron impact dissociation, the onset reduced field is about 120 Td, which corresponds to 4.5 eV.
The optimum energy for incident electrons onto a nitrogen molecule to maximize the vibrational excitation process and minimize other processes is around 1.8 cV. It is around 0.3 eV for the oxygen molecule. Any electron with an energy higher than 2 eV will increase energy losses in the rotational and electronic excitation channels as well as ionization for the oxygen molecule. In FIG. 3 by Walton et al6. we can see that the electron energy distribution for an electron beam generated plasma in nitrogen partly overlaps with the total excitation cross section and barely overlaps with dissociation cross section curve. This clearly shows that for these plasma conditions, no energy would be wasted in ionization and very little energy would be coupled to the dissociation process. It also shows that for a nitrogen molecule which has the optimum vibrational excitation for an electron energy between 0.5 eV and 4 eV, a good portion of the EEDF will not be put to work with optimum condition for vibrational excitation, as the energy of the electrons is too low.
In the case of carbon dioxide, all vibrational excitation occurs at reduced fields between 1 and 100 Td, which correspond to an electron energy range of 0.5 ev to 4 eV as shown in
None of the plasma systems described above have reached the efficiency of existing methods for the dissociation of any molecular species.
It is clear that there is a need for a plasma source, where the electron energy distribution function can be tailored to provide electrons with the optimum energy for vibrational excitation and minimize other processes such as electronic and rotational excitation.
Hadidi et al7. describe a system and method for selectively controlling the ion composition of an ion source. In that patent, a nanosecond pulse voltage generator is electrically connected to an electrode inside the low-pressure vessel where the plasma is generated. The plasma constitutes an impedance that is viewed by the plasma source power supply as a load. When a nanosecond pulsed voltage is applied to the plasma, it induces changes in the plasma that lead to a modification in the composition of the plasma. For example, certain ionic and neutral species may decline, whereas other ionic and neutral species may appear. This change in the chemistry of the plasma induces a change in the impedance, which in turn may force the power generation system to quickly adapt;
otherwise, the situation may lead to arcing and the destruction of the plasma source. The system described in the cited patent has its limitations as it does not decouple the means of creating the plasma from the means of controlling it. It also describes a low-pressure system, which is not suitable for continuous high-throughput processing of fluids.
Electron-beam generated plasma sources have been used in different applications. One of them is the destruction of NO2 emissions from thermal combustion processes8. They have also been used for the destruction of volatile organic compounds such as carbon tetrachloride and trichloroethylene9. An electron-beam generated plasma is ideal for generating secondary low-energy electrons. In these systems, a primary beam of electrons is accelerated from a vacuum chamber through a thin metallic foil window into the atmosphere. These high-energy electrons collide with the molecules that are at atmospheric pressure and produce secondary electrons that quickly thermalize due to the high collision rate. For an electron-beam with a primary energy of 100 keV, and given the energy required to produce an ion-pair in dry air, which is roughly 34 eV, each primary electron will produce almost 3000 secondary electrons before completely losing its energy.
Also, Matthew F. Wolford et al10. describe a pulsed electron beam system for the removal of NOx from flue gases. In this system, the extraction of the electrons from the cathode is pulsed and a pulsed plasma is created in a plasma channel. While this method lowers the overall energy input into the plasma, it does not decouple the means of creating the atmospheric plasma (the electron beam) from the means of controlling it. The same authors describe their method and apparatus in a published US patent application, US 2017/0087509 A1.
In addition, F. Kerschen et al2. describe a low-carbon hydrogen production via electron beam plasma methane pyrolysis. In this article, the authors demonstrate that hydrogen gas can be economically produced by decomposing methane in an electron beam generated plasma without direct CO2 emissions. The techno-economic analysis shows levelized costs of hydrogen for the plasma pyrolysis between 2.55 Euro/kg H2 and 5.00 Euro/kg H2. Also, the carbon footprint assessment indicates the high potential for a reduction of life cycle emissions by electron beam plasma methane pyrolysis (1.9 kg CO2 eq./kg H2 to 6.4 kg CO2 eq./kg H2, depending on the electricity source) compared to state-of-the-art hydrogen production technology (10.8 kg CO2 eq./kg H2).
We can clearly see that there is a great need for improving the efficiency of these plasma systems in order to lower the cost of the products they produce.
In one embodiment, the present technology describes a system to control the electron energy distribution function in an atmospheric pressure or near atmospheric pressure low-temperature plasma, where the means for generating the plasma is completely decoupled from the means for controlling it.
The electron energy distribution function control system relies on the application of a repetitive nanosecond pulsed voltage potential to the plasma to affect the energy of the electrons without affecting the energy of the ions.
The repetitive nanosecond pulsed voltage potential generator can generate voltage pulses that are between 0.1 volt and 50000 volts with pulse duration between 0.1 nanosecond and 100 nanoseconds at a repetitive rate between 500 Hz and 1 MHz.
In some embodiments, the pulse duration of the voltage potential is chosen to be less than the inverse of the plasma frequency, so as not to affect the ions in the plasma.
The repetitive nanosecond voltage potential pulse is applied to a set of electrodes that encompass the plasma, where one electrode is isolated from the ground and connected to the repetitive nanosecond voltage pulse generator and the second electrode is connected to the ground.
In another embodiment, the present technology intends to lower the energy consumption for dissociating molecular fluids in a low-temperature plasma by controlling the electron energy distribution function of the plasma and by incorporating a catalyst material near the controlled plasma to increase the dissociation rates of the molecular gas.
In yet another embodiment, the present technology intends to lower the energy consumption for dissociating molecular fluids in a low-temperature plasma by controlling the electron energy distribution function of the plasma and by incorporating a catalyst material near the controlled plasma to increase the dissociation rates of the molecular gas, and by incorporating a heating element into the catalyst material to increase its temperature in order to increase the dissociation rate of molecular gases.
The manufacturing of many chemical products relies on the use of feedstocks. In some instances, these feedstocks can be molecular fluids that need to be dissociated partially or entirely into their atomic state in order to produce radicals that react with other chemicals and produce the desired product. This requires the breaking of the chemical bonds that hold together the molecule. The dissociation can be achieved through high temperature (thermal dissociation), catalytic reactions (thermal catalysis), or direct electron impact (where an electron with an optimum energy collides with the molecule and transfers part of its energy into the molecule, which then causes it to dissociate). The limits of the cited processes reside in the fact that the energy required to dissociate the molecules far exceeds the bond energy of these molecules. The excess energy is wasted as heat in the process.
As an example of thermal catalysis, nitrogen gas is fed into a thermal catalytic reactor in a Haber-Bosch process to make it react with hydrogen and produce ammonia. This process operates at high temperature (300 to 500 C) and high pressure (100 to 250 bars).
In plasma processes, in order to minimize the energy required to break these molecular bonds and limit it as much as possible to the bond energy of the specific molecule, one needs to direct the energy specifically into the chemical bond, without wasting any energy in other secondary processes. One way that has been proposed in the literature is the so-called vibrational ladder climbing. In this process, an electron impact initiates a vibrational excitation of the lowest vibrational levels, followed by vibrational-vibrational collisions (ladder-climbing). The vibrational-vibrational collisions continue to gradually populate the higher vibrational levels, till the bond energy is reached, which eventually leads to the dissociation of the molecule.
The vibrational excitation can be achieved through either a photon-molecule interaction, or an electron-molecule interaction.
Low-temperature plasma: Low temperature plasma, or sometimes called non-thermal plasma is defined as a plasma where the energy of the electrons in the plasma is different than the energy of the ions in the plasma. Also, the energy of the electrons in the plasma (from 0.5 eV to 100 eV) is much larger than the energy of the ions in the plasma (from 0.01 eV to 0.5 eV). Also, a low temperature plasma gas temperature is between about 20 C and about 500 C.
Atmospheric pressure: It is defined as a pressure having a value of 1 atmosphere or 760 torr.
Slightly above atmospheric pressure is defined as a pressure having a value between 761 torr and 1300 torr (e.g., about 765 torr).
Slightly below atmospheric pressure is defined as a pressure having a value between 300 torr and 759 torr (e.g., about 750 torr).
Near atmospheric is defined as slightly below atmospheric pressure or slightly above atmospheric pressure.
A metallic foil is defined as a metal sheeting that can be made of aluminum, titanium, beryllium, or any metal or alloy with an atomic mass between 3 and 25 atomic mass units and having a thickness between about 10 μm and and 250 μm, and preferably under 100 μm.
Metallic is defined as consisting entirely or partly of metal or a metal alloy.
The present technology describes a means for producing electrons of an optimum energy that can achieve vibrational excitation of molecular gases. The present technology uses an electron beam generated in a vacuum and accelerated through a thin metallic window into a higher-pressure gas environment to create secondary electrons. Due to the absence of any applied electric field and high pressure in the electron beam channel, secondary electrons quickly thermalize and reach room temperature. Depending on the nature of the gas and pressure, the temperature of these electrons is between 0.1 and 2 eV.
Tailoring the energy of the secondary electrons to meet the optimum energy for electron impact vibrational excitation of a molecule is key to reducing the overall energy consumption of molecular dissociation. The present technology describes a means of controlling the secondary electron energy so as to optimize it to meet the optimum energy for electron vibrational excitation.
The means described above for providing electrons of optimum energy rely on the application of a pulsed nanosecond pulsed voltage that has a duration that is smaller than the inverse of the plasma frequency. This voltage will in turn generate a nanosecond pulsed electric field that can drive the electrons without affecting the ionic species in the plasma. This pulsed nanosecond voltage is described in U.S. Pat. No. 8,664,561 B2.
The main difference between the method and system described in that patent and the current disclosure is that in U.S. Pat. No. 8,664,561 B2, the pulsed voltage is applied to a low-pressure radio frequency plasma created by an inductively coupled plasma (ICP) source. When the nano pulse is applied, the plasma properties are affected and modified due to the creation of different species, which changes the impedance of the plasma, and which, in turn forces the ICP source to adapt. In the present technology, the plasma is created at atmospheric pressure or higher by an independent source that is the electron beam. When a nanosecond voltage pulse is applied, the plasma characteristics change as different species are created, but that has no effect on electron beam source that created the plasma in the first place. This effectively decouples the means for creating the plasma (electron beam) from the means of controlling it (the pulsed electric field or voltage).
At atmospheric pressure, it takes 33.9 eV11 to form an ion pair in nitrogen gas when the molecules are bombarded with high energy electrons. For each secondary electron created, one can consider that an energy of 34 eV was spent for the electron to form. However, while a secondary electron is formed, there are also other reactive species that form during the radiation of the molecules by the electron beam. Matzing et al.12 have produced a model in which they describe all the species produced in respectively nitrogen, oxygen, and water vapor when submitted to high energy electron impact. The equations below are per 100 eV of energy spent:
4.43N2→0.29N2*+0.885N(2D)+0.295N(2P)+1.87N(4S)+2.27N2++0.69N++2.96e− (1)
5.377O2→0.077O2*+2.25O(1D)+2.8O(3P)+0.18O*+2.07O2++1.23O++3:3e− (2)
7.33H2O→0.51H2+0.46O(3P)+4.25OH+4.15H+1.99H2O++0.01H2++0.57OH++0.67H++0.060++3.3e− (3)
For example, as we can see from equation (1), for 100 eV energy, 4.43 molecules of nitrogen get dissociated to form 2.96 secondary electrons in addition to 3.05 metastable nitrogen atoms (0.885N(2D)+0.295N(2P)+1.87N(4S)), 0.69 nitrogen ion, 0.29 excited nitrogen molecule, and 2.27 nitrogen molecule ion. Therefore, the 34 eV energy is spent in creating not only a free secondary electron, but also other reactive species such as nitrogen atoms and ions.
It is worth noting that the 3.05 nitrogen metastable atoms created are the sum of the different states that are N(2D), N(2P), and N(4S). It has been reported that the N(2D) state is more reactive than the N(2P) state13,14 and since these states production rates depend on the energy of the impacting electron on the nitrogen molecule, one can think of tuning the electron energy distribution in order to maximize the desired excited state.
For example, in the case of the production of NO as a precursor for the production of nitric acid, we have the following reaction of nitrogen with oxygen with reaction rates k:
k1
N(2D,2P)+O2→NO+O
k1(N2D)=6×10−12 cm3/s15
k1(N2P)<2×10−12 cm3/s16 (4)
We can see that the reaction rate of the N(2D) state with oxygen in reaction (4) is at least 3 times larger than the one for N(2P). The same reasoning goes for reaction of N(2D), N(2P) with hydrogen for the formation of NH in the case of ammonia production.
k2
N(2D,2P)+H2→NH+H
k2(N2D)=1.7×10−12 cm3/s17
k2(N2P)<8×10−16 cm3/s11 (5)
Here again we can see in reaction (5) that the N(2D) state has a reaction rate that is more than 4 orders of magnitude larger than the N(2P) state for the formation of the NH radical. Being able to control the excited states in order to produce more of the most reactive species is very important to lower the overall energy consumption for the production of ammonia.
We also can notice in equation (1) that secondary electrons are created. These electrons are very slow and have an energy between 0.1 and 1 eV, which is not enough to ionize any further. These low-energy electrons can still be useful if their energy is tailored to optimize specific reactions of interest. Molecular dissociation can occur not only through direct impact of high energy electrons, but also through the so-called vibrationally excited ladder climbing process. During this process, a lower vibrational level of the molecule is excited by a low-energy electron. Then, through vibrational-vibrational collision, the vibrationally excited molecule reaches higher vibrational levels of excitation, which eventually will lead to dissociation when the bond energy is reached. So finding a way to precisely control the electron energy distribution in the plasma can provide a means to increase the overall efficiency of the dissociation process by directing the energy of the electrons towards the vibrational excitation of the molecules and minimizing it for other processes (electronic excitation).
The production of NO and NH described above rely on the reaction of the excited nitrogen atom with other species. We can further improve the overall energy efficiency of the process by increasing the number of nitrogen atomic metastable (preferably the N(2D) state) by further decomposing the excited nitrogen molecule N2* in equation (1) and any remaining neutral nitrogen molecule. This can be done by tuning the energy of the secondary electrons in order to induce a vibrationally excited ladder climbing, which will eventually lead to the dissociation of the nitrogen molecule.
During the plasma processing of molecular gases, a cold plasma 16 is created in channel 18. When power supply 30 is energized to produce pulses of very fast voltage (electric field), the secondary electrons in the plasma will be subject to the electric field and will gain energy, while the ions in the plasma will not get affected due to their much larger mass and inertia. In order for this to happen, the nanosecond electric field pulse duration cannot be larger than the inverse of the electron plasma frequency. For this reason, choosing the right pulse generator is very important.
An embodiment of the present technology features further ionization and dissociation of the molecular species using a repetitive nanosecond voltage pulse by accelerating the low-energy secondary electrons into high energy levels that produce ionization and dissociation. Another parameter for modifying the electron energy distribution of the plasma is by modifying the gas pressure in the channel. Lower pressures will produce higher energy for the secondary electrons, as the collision frequency will get lower, and on the contrary, higher gas pressure in the channel will produce lower energy for the secondary electrons, as the collision frequency increases with pressure. Tailored gas pressure and tailored electron energy distribution can be combined to better control the dissociation process of molecular gases in the electron-beam generated plasma.
In another embodiment, the electron beam plasma can be combined with a catalyst to improve the reaction rates and increase dissociation efficiency.
In yet another embodiment, a catalyst is placed in the gas channel of the electron beam system that is modified with a fast voltage pulse generator as shown in
The so-called vibrational ladder climbing provides a less energy-intensive process, as the energy is directed towards making the molecule vibrationally excited to higher states, which leads to instability and eventually dissociation. This resonant dissociation process is well described by Laporta et al19. The authors cite on Page 2 of reference 19: “For incident electron energies, however, lower than 4 eV, the vibrational transition occurs via a resonant process involving the 2 Πg state of the N2− ion:
e−+N2(X1Σ+g;ϑ)→N−2(2Πg)→e−+N2(X1Σ+g;ω) (6)
where ϑ and ω are the initial and final vibrational level of N2. By means of the resonant process in reaction (6) the vibrational-excitation cross section is enhanced by several orders of magnitude. This process is very efficient and makes the vibrational temperature equilibrate very rapidly with the free electron temperature”.
In yet another embodiment, the catalysts material comprises a heating element 36 as shown in
In yet another embodiment, the gases that flow through the channel can be comprised of nitrogen, oxygen, methane, carbon dioxide, and water vapor and a mixture of any of these gases.
In yet another embodiment, additives fluids in a gaseous form can be added to the flowing gas or gases. Example of these additives can be hydrogen peroxide vapor, noble gases such as argon, neon and krypton, and other molecular compounds.
In yet another embodiment, the gas channel has multiple inlets and nanosecond pulsed voltage system is placed inside the gas outlet channel as shown in
Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, numerous equivalents to the specific procedures described herein. Such equivalents were considered to be within the scope of this technology and are covered by the following claims. The contents or all references, issued patents, and published patent applications cited throughout this application are hereby incorporated by reference.
This application claims priority to U.S. Provisional Application No. 63/322,878 filed Mar. 23, 2022. The entire contents of the foregoing applications are hereby incorporated by reference.
| Number | Name | Date | Kind |
|---|---|---|---|
| 8664561 | Hadidi et al. | Mar 2014 | B2 |
| Number | Date | Country |
|---|---|---|
| 2008135286 | Jun 2008 | JP |
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| Number | Date | Country | |
|---|---|---|---|
| 63322878 | Mar 2022 | US |
