Microwave Plasma Reactor Research Articles

Multiphysics simulation study in a microwave plasma reactor coupling a non-Maxwellian EEDF

Multiphysics simulation study in a microwave plasma reactor coupling a non-Maxwellian EEDF

Microwave plasma conversion of food waste using carbon foam: Production of heteroatom-doped graphene and combustible gas

The mass production of food waste (FW) has a terrible impact on the environment, but with proper treatment, FW can be transformed into a new resource. Currently, thermal conversion (especially incineration) has been the most common and efficient means of handling FW, but it also entails negative impacts, such as high carbon emissions and lower added value. In this study, a new high-efficiency thermal conversion process is proposed that uses carbon foams to induce the formation of microwave plasma (MP), which generates very high final temperatures to enable the direct conversion of FW into combustible gases and high-value-added heteroatom-doped microwave plasma graphene (HMPG) in 5–20 s. This work investigated the optimal carbonization temperature of the melamine sponge (carbon foam precursor) and the effect of microwave plasma reaction duration on the distribution of the gas products and the properties of HMPG, and HMPG was characterized in detail by Raman spectroscopy, X-ray diffraction, scanning electron microscope, etc. Ultimately, HMPG was tested in potassium ion battery anodes for evaluation of its energy storage potential, and the results showed that the capacities were able to reach ∼270 mAh g−1 at 50 mA g−1 after 60 cycles.

Analysis of a plasma reactor performance for direct nitrogen fixation by use of three-dimensional simulations and experiments

This study utilizes state-of-the-art in-situ measurements and advanced three-dimensional simulations to investigate the operation of a microwave plasma reactor for nitrogen fixation at a high subatmospheric pressure under various power inputs. It is shown that the system should be treated as a warm plasma due to negligible differences in vibrational and rotational temperatures and that it is required to account for chemical non-equilibrium. Complex flow field and significant temperature and concentration gradients are observed, emphasizing the need for three-dimensional simulations to accurately describe the interactions between mass, heat, and momentum transport and chemical reactions in non-equilibrium conditions. The innermost plasma regions reach temperatures close to 6000 K, while wall temperatures remain low due to presence of a low-temperature swirling flow field around the hot core. Analysis reveals that the swirling flow field around the hot core provides intrinsic partial quenching in regions with large temperature gradients which counteract the return to chemical equilibrium and enrich the flow surrounding the hot core. The inner flow exhibits a recirculation zone, stagnation point, and a transition into an increasingly parabolic flow profile closer to the reactor outlet. The agreement between the numerical modeling and the measurements is strong. In-situ Raman spectroscopy measurements confirm the calculated temperature field inside the reactor. Thermocouple measurements on the reactor wall are also in good agreement with the simulation. Additionally, Fourier-transform infrared spectroscopy measurements agree very well with the simulated amount of nitrogen oxide exiting the reactor at different microwave power inputs. The understanding and modeling capabilities are expected to support the development of future reactor designs that can facilitate higher yields.

Ortho C-H Bond Activations in an Atmospheric Microwave Plasma Ion Source.

C-H bond ortho-substitution reaction has always been a significant and challenging topic in organic chemistry. We proposed a synthesis method based on microwave plasma torches. High-resolution mass spectrometry was used to monitor rapid reaction products. 2-Alkylbenzimidazole can be formed through the reaction of phenylnitrenium ion and nitriles on a millisecond scale. This reaction can achieve the one-step formation of benzimidazoles from benzene ring single-substituted compounds without the addition of external oxidants or catalysts. A similar C-H bond activation reaction can be accomplished with ketones. Meanwhile, the microwave plasma reactor was modified, and the resulting 2-methylbenzimidazole was successfully collected, indicating the device has good application potential in organic reactions such as C-H bond activation reaction.

Metastable Controlled Plasma Profile in a Packed-Bed Microwave Plasma Reactor

Metastable Controlled Plasma Profile in a Packed-Bed Microwave Plasma Reactor

Computational study of a microwave plasma reactor based on the TM112 mode for diamond deposition

One of the main features of microwave plasma reactors is the electric field structure in the resonant cavity, which must be both intense and uniform in front of the substrate. For this reason, transverse magnetic modes are often used, especially axisymmetric modes because they produce an axisymmetric plasma. Microwave plasma reactors can be differentiated according to the chosen mode, because this has a direct influence on the diamond film growth process, among other features such as the coupling technique and the used quartz window. Another attractive characteristic of said reactors is obtaining large activation areas of the plasma. In this paper, we propose a microwave plasma reactor based on the TM112 cylindrical mode, which is subject to a computational study. Unlike axisymmetric modes, which activate the plasma on the cavity axis, the TM112 cylindrical mode presents two activation plasma areas. The reactor was designed following the methodology described by Silva et al., and using the Plasma, Radiofrequency (RF), and Heat transfer modules of the software COMSOL Multiphysics. The obtained results are presented in two stages. The first one is related to the initial electric field distribution of the TM112 mode. Next, the generation of the hydrogen plasma was simulated from the interaction of H2 gas with the TM112 microwave field. The plasma activation process is described in detail from graphics of the time evolution of the electron density, hydrogen density, and their respective temperatures until a steady state is reached. Additionally, the influence of the pressure on the concentration and the temperature of both electrons and gas in a steady state is analyzed. The presented results can be useful for the design of plasma reactors for diamond deposition.

Simulation of argon-excited microwave plasma reactor for green energy and CO2 conversion application

Microwave plasma as a potential tool to convert CO2 has been extensively studied in recent years. A simulated study on the plasma parameters via the variation of the operating pressure of a microwave plasma model has been performed in this study. The establishment of the model was based on the finite element method to analyse the spatial distribution of plasma parameters in the plasma torch over a period of time. Plasma parameters such as electron potential, density, and temperature were investigated at three different pressures, and the growth of electron potential and density were associated with time. The distribution of molecular ions was observed to be located more on the enter port of the microwave or waveguide near the location of the magnetron at the initial stage. The electron density was found to be constant after it reached maximum value for all the determined pressures. However, the electron temperature behaved differently as compared to the electron potential and density, the distribution of high electron temperature did not enhance during the processing time. The analysis of microwave plasma parameters is beneficial for plasma reactor designing, particularly for CO2 conversion.

Validation of the silicon nanoparticle production on the pilot plant scale via long-term gas-phase synthesis using a microwave plasma reactor

The formation of crystalline silicon nanoparticles by homogeneous gas-phase reactions as a direct way to produce high-purity raw material is applied. For this purpose, a microwave-assisted plasma reactor is used. Goal of this paper is to show the scalability of our process technology from laboratory scale to pilot plant scale while maintaining the particle characteristics. This is demonstrated by producing and analyzing silicon nanoparticles during long-term synthesis in a pilot-scale microwave plasma reactor over a period of six hours. The focus is on a high production rate in conjunction with consistent particle characteristics. A continuous production of the mostly spherical crystalline silicon particles with a count median diameter (CMD) of 23.4 nm and a geometric standard deviation of 1.5 is shown using TEM analysis. The stability of the synthesis process is monitored by means of regular sampling and analyzing batch samples extracted from the process every 30 min. Here it is shown that the CMD varies statistically between 21 and 26 nm. Moreover, the decomposition rate of the precursor was determined to be 99%, while the energy supply remained constant. A constant production rate of about 200 g∙h−1 is shown.

Computational modeling of CO2 conversion by a solar-enhanced microwave plasma reactor

The use of renewable energy to convert carbon dioxide (CO2) into higher-value products can help meet the demand for fuels and chemicals while reducing CO2 emissions. Solar-Enhanced Microwave Plasma (SEMP) CO2 conversion aims to combine the scalability and sustainability of solar thermochemical methods with the high efficiency and continuous operation of plasmachemical approaches. A computational study of a built SEMP reactor operating with up to 1250 W of microwave power together with up to 525 W of incident solar power at atmospheric pressure is presented. The study is based on a fully-coupled 2D computational model comprising the description of fluid flow, heat transfer, Ar-CO2 chemical kinetics, energy conservation for electrons and heavy-species, electrostatics, and radiative transport in participating media through the discharge tube, together with the description of the microwave electromagnetic field through the waveguide and the discharge tube. Numerical simulations reveal that the plasma is concentrated near the location of incident microwave energy, which is aligned with the radiation focal point, and that CO2 decomposition is highest in that region. The incident solar radiation flux leads to more uniform distributions of heavy-species temperature with moderately greater values throughout most of the discharge tube. Modeling results show that, at 700 W of electric power, conversion efficiency increases from 6.8% to 10.0% with increasing solar power from 0 to 525 W, in good agreement with the experimental findings of 6.4% to 9.2%. The enhanced process performance is a consequence of the greater power density of the microwave plasma due to the absorption of solar radiation.

Kinetic evaluation of heterocatalytic ozone-based activation of peroxymonosulfate using acid-treated graphene catalyst for the degradation of micropollutants

In this study, acid-treated graphene (Gr⎯COOH) catalyst was synthesized using a microwave plasma reactor, and its performance in different combinations of ozone and ozone activated peroxymonosulfate (O3, O3+Gr⎯COOH, O3+PMS, and O3+PMS+Gr⎯COOH) was kinetically evaluated. The yields of SO4•⎯ and HO• were quantified based on ozone consumption. Among the different operating parameters, the effect of pH (4−8) was particularly considered. The results showed that heterocatalytic ozone-based activation of peroxymonosulfate performed efficiently at neutral and alkaline conditions. The first-order rate constants for O3 decomposition in O3+PMS+Gr⎯COOH (1.2×10−2s−1) was higher than O3+PMS (8.5×10−3s−1) and O3 (2×10−4s−1) by a factor of 1 and 2 respectively. The radical yields were positively associated with pH. Higher pH values showed a notable increase in radical yields (η), with the maximum value of ηTotal (0.88) and ηHO• (0.53) obtained at pH 8 while ηSO4•⎯ showed maxima (0.46) at pH 7. This implies that at higher pH i.e., 8, HO• was the dominant reactive specie, which could be due to the presence of higher concentration of HO− and transformation of SO4•⎯ into HO•. Furthermore, the synergistic effects of the oxidative processes were tested considering atrazine (ATZ, 1 µM) and nitrobenzene (NB, 1 µM) as probe compounds. The best performance was obtained with O3+PMS+Gr⎯COOH process, reaching 96% and 81% of ATZ and NB removal respectively. The suppressive effects of inorganic ions and natural organic matter (NOM) on the degradation efficiency of ATZ and NB were also lower for O3+PMS+Gr⎯COOH process. These results confirmed that acid-treated graphene (Gr⎯COOH) catalyst in combination with ozone activated peroxymonosulfate could be an efficient and promising alternative oxidative process.

Spatially resolved measurement of the distribution of solid and liquid Si nanoparticles in plasma synthesis through line-of-sight extinction spectroscopy.

In many high-temperature gas-phase nanoparticle synthesis processes, freshly nucleated particles are liquid and solidify during growth and cooling. This study presents an approach to determine the location of the liquid-to-solid phase transition and the volume fraction and number density of particles of both phases within a gas phase reactor. Spectrally-resolved line-of-sight attenuation (LOSA) measurements are applied to a silicon nanoparticle aerosol generated from monosilane in a microwave plasma reactor. A phantom-based analysis using particle number density, particle size, and temperature distribution from direct numerical simulation (DNS) of the reacting flow indicates that the contributions from the two particle phases can be decoupled under practical conditions, even with noisy data. The approach was applied to analyze spatially and spectrally resolved LOSA measurements from the hot gas flow downstream of the plasma zone where both solid and liquid silicon particles coexist. Extinction spectra were recorded along a line perpendicular to the flow direction by a spectrometer with an electron-multiplying charge-coupled device (EMCCD) camera, and two-dimensional projections were deconvolved to obtain radial extinction coefficient distributions of solid and liquid particles across the cross-section of the flow. Particle number densities of both particle phases were retrieved simultaneously based on the size-dependent extinction cross-sections of the nanoparticles. The particle-size distribution was determined via thermophoretic sampling at the same location with subsequent transmission electron microscopy (TEM) analysis. The particle temperature distribution was determined from the particle's thermal radiation based on line-of-sight emission (LOSE) measurements. The approach for phase-selective data analysis can be transferred to other materials aerosol systems as long as significant differences exist in extinction spectra for the related different particle classes.

Tungsten doped diamond shells for record neutron yield inertial confinement fusion experiments at the National Ignition Facility

We report on fabrication and characterization of layered, tungsten doped, spherical about 2 mm diameter microcrystalline diamond ablator shells for inertial confinement fusion (ICF) experiments at the National Ignition Facility. As previously reported, diamond ICF ablator shells can be fabricated by chemical vapor deposition (CVD) on solid spherical silicon mandrels using an ellipsoidal microwave plasma reactor. In the present work, we further developed these ablator shells by embedding a W-doped diamond layer sandwiched between two undoped diamond regions. W incorporation in diamond was achieved by adding tungsten hexacarbonyl to the CH4/H2 CVD feed gas. We observe that the W doping concentration decreases with increasing deposition rate which, in turn, is controlled by adjusting the total gas pressure. Cross sectional microstructural analysis reveals sharp interfaces between doped and undoped regions of the diamond shell and uniform W distribution with concentrations up to about 0.3 at.%. At higher W concentrations (>0.3 at.%) formation of tungsten carbide precipitates is observed. Using a 3‐shock 1.6 MJ laser pulse, the targets described in this work produced the first laser driven implosion to break the 1 × 1016 neutron yield barrier, followed by experiments (described in future publications) with similar targets and slightly more laser energy producing yields as high as 4 × 1017.

Post-plasma quenching to improve conversion and energy efficiency in a CO2 microwave plasma

Transforming CO2 into value-added chemicals is crucial to realizing a carbon–neutral economy, and plasma-based conversion, a Power-2-X technology, offers a promising route to realizing an efficient and scalable process. This paper investigates the effects of post-plasma placement of a converging–diverging nozzle in a vortex-stabilized 2.45 GHz CO2 microwave plasma reactor to increase energy efficiency and conversion. The CDN leads to a 21 % relative increase in energy efficiency (31 %) and CO2 conversion (13 %) at high flow rates and near-atmospheric conditions. The most significant performance improvement was seen at low flow rates and sub-atmospheric pressure (300 mbar), where energy efficiency was 23 % and conversion was 28 %, a 71 % relative increase over conditions without the CDN. Using CFD simulations, we found that the CDN produces a change in the flow geometry, leading to a confined temperature profile at the height of the plasma, and forced extraction of CO to the post-CDN region.

Inertization of metals and hydrogen production as a byproduct from water hyacinth and water lettuce via plasma pyrolysis

Water hyacinth and water lettuce have been extensively used for phytoremediation of metals and metalloids. However, the reasonable disposal of phytoremediation plants is a difficult problem. This study aims to reduce metals and metalloids from water hyacinth and water lettuce, and produce hydrogen (H2) and methane (CH4) via an atmospheric-pressure microwave plasma reactor based on the circular economy concept. Inertization of metals and metalloids can be obtained by more than 60% for both water hyacinth and water lettuce. H2 and CH4 production of water hyacinth and water lettuce were 56.28%/57.30% and 3.75%/2% of volume fractions, respectively. Furthermore, total VOCs concentrations from the effluent gas were detected only at the values of 0.511% (water hyacinth) and 0.08% (water lettuce) of volume fractions, respectively. Overall, water hyacinth and water lettuce treated by atmospheric-pressure microwave plasma showed the potential of H2 and CH4 production as a by-product for alternative energy and inertization of metals/metalloids for the phytoremediation plants.

The role of phase transition by nucleation, condensation, and evaporation for the synthesis of silicon nanoparticles in a microwave plasma reactor — Simulation and experiment

A novel particle model is presented to simulate the synthesis of silicon nanoparticles from monosilane in a laboratory sized microwave plasma reactor. The simulations contribute essentially to the understanding of the particle formation process and the spatial and size distribution of particles observed in the experiment. The model approach features phase transition and explains the observed, tube-shaped particle formation zones by a competing nucleation, condensation, and evaporation process coupled with complex transport phenomena. The simulation results are in excellent agreement with experimental data from Rayleigh scattering and line-of-sight optical absorption with onion-peeling reconstruction (LOSA) measurements of the particle front, as well as with multiline SiO laser-induced fluorescence (LIF) temperature measurements. Particle size distributions determined from transmission electron microscopy (TEM) on thermophoretically sampled particles are in good agreement with the simulation results. Average diameters of 25.8 nm calculated in the simulation compare well to 27.6 nm measured in the experiment. It was found that thermophoresis has a crucial impact on particle trajectories, as it extends the particle residence time within the reactor by about 20% and provides the determining force for particles to escape zones of high temperature in which particles evaporate otherwise.The sectional model features two-way coupled phase transition formulations for the condensing matter, which is formed through the decomposition of monosilane diluted in argon/hydrogen mixtures. The process is investigated by the combination of two simulations with different grid resolutions, which show differences for the high Schmidt number particle phase only. The simulations feature a global monosilane decomposition reaction, while the microwave plasma source is simplified by a local heat source.

A Chemical Vapor Deposition Diamond Reactor for Controlled Thin‐Film Growth with Sharp Layer Interfaces

A microwave plasma reactor for diamond growth that allows for highly controllable process conditions is presented. The position of the diamond substrate within the reactor can be accurately controlled. Thus, equilibration of plasma conditions can be carried out after changes in process parameters. With this approach, sharp layer transitions among doped, undoped, and isotopically controlled diamond films can be obtained. In addition to the sample transfer, the growth temperature is maintained through a substrate heater, and a clean reactor environment is realized by a load‐lock sample exchange system. The plasma conditions are constantly monitored by optical emission spectroscopy. Using this system, the growth of nanoscopic sandwich structures is demonstrated with controlled isotopic ratios down to ≈10 nm thickness and N(V) layers below 50 nm are obtained on (001)‐oriented diamond. Growth rates and doping efficiencies depending on the used methane concentration are presented. Characterization with continuous‐wave optically detected magnetic resonance yields an average contrast of 4.1% per nitrogen vacancy (NV) orientation in layers with a thickness below 100 nm. Depending on the used methane concentration, surface morphology and NV doping homogeneity are influenced as observed by photoluminescence and atomic force microscopy measurements.

Application of Taguchi method and structural equation modeling on the treatment of e-waste

Resin, the e-waste from printed circuit board (PCB) scrap, has been used widely for the thermosetting of PCB. To obtain the optimum result of elements from e-waste in plasma technology, the understanding of parameters is urgently suggested. This study investigated the elimination of elements via an atmospheric-pressure microwave plasma reactor, considering experimental parameters: gas type, flux agent ratio, gas flow rate, pyrolysis duration, and microwave power. Moreover, the experimental data was quantitatively analyzed by structural equation modeling (SEM) via Partial Least Square (PLS). Based on the findings, the optimization parameters for removal efficiency were operated under nitrogen gas, a greater ratio of flux agents (2 g), 9 L/min of gas flow rate, pyrolysis in 10 min, and 1200 watt of microwave power. Furthermore, the type of gas was the most influential parameter in degrading the elements combined with other parameters and the removal efficiency ranging from 97.22% to 100%. Moreover, the crucible position inside the quartz tube has no significant impact on the degradation of elements. The model confirmed that crucible position and gas type exhibited significant influence in the degradation of elements. The findings of significant influence parameters give important information on the performance of an atmospheric-pressure microwave plasma reactor to treat e-waste. • Taguchi method finds the most influential parameters of plasma system. • Structural equation modeling gives the information of significant influence parameters. • Gas types and crucible positions have a significant impact on elements degradation. • Fuse of quartz sand creates the vitrification of resin from plasma treatment.

Applications of Plasma Produced with Electrical Discharges in Gases for Agriculture and Biomedicine

The use of thermal and non-thermal atmospheric pressure plasma to solve problems related to agriculture and biomedicine is the focus of this paper. Plasma in thermal equilibrium is used where heat is required. In agriculture, it is used to treat soil and land contaminated by the products of biomass, plastics, post-hospital and pharmaceutical waste combustion, and also by ecological phenomena that have recently been observed, such as droughts, floods and storms, leading to environmental pollution. In biomedical applications, thermal plasma is used in so-called indirect living tissue treatment. The sources of thermal plasma are arcs, plasma torches and microwave plasma reactors. In turn, atmospheric pressure cold (non-thermal) plasma is applied in agriculture and biomedicine where heat adversely affects technological processes. The thermodynamic imbalance of cold plasma makes it suitable for organic syntheses due its low power requirements and the possibility of conducting chemical reactions in gas at relatively low and close to ambient temperatures. It is also suitable in the treatment of living tissues and sterilisation of medical instruments made of materials that are non-resistant to high temperatures. Non-thermal and non-equilibrium discharges at atmospheric pressure that include dielectric barrier discharges (DBDs) and atmospheric pressure plasma jets (APPJs), as well as gliding arc (GAD), can be the source of cold plasma. This paper presents an overview of agriculture and soil protection problems and biomedical and health protection problems that can be solved with the aid of plasma produced with electrical discharges. In particular, agricultural processes related to water, sewage purification with ozone and with advanced oxidation processes, as well as those related to contaminated soil treatment and pest control, are presented. Among the biomedical applications of cold plasma, its antibacterial activity, wound healing, cancer treatment and dental problems are briefly discussed.

Volatile Organic Compound (VOC) Reduction from Face Mask Wastes via a Microwave Plasma Reactor

Since the outbreak of the COVID-19 pandemic, the driven of face masks as personal protective equipment has increased significantly. Thus, disposed face masks from users should be handled properly for preventing the contamination of medical waste and the potential spread of viruses to the environment. This study gives information for dealing with face masks and assessing the volatile organic compounds (VOCs) concentrations via an atmospheric-pressure microwave plasma reactor. Face mask samples were mixed with the flux agents, namely cullet/glass, Al2O3, SiO2 and CaCO3. Samples were compared with control (no addition of flux agents) and the addition of only cullet. Moreover, microwave power, gas flow rate and pyrolysis duration were controlled at 1000 W, 9 standard liter per minute (SLM) and 5 min, respectively. The total concentration of VOC with the absence of flux agents was 448.04 ppm. Furthermore, the fuse of cullet and SiO2-Al2O3-cullet in the mask reduced the concentration of VOCs by 314.77 ppm and 54.7 ppm, respectively. Furthermore, the combination of CaCO3-SiO2-Al2O3-cullet creates the vitrification of material with the presence of crystalline structure, where the compositions of Ca and Si were 13.55% and 19.12%, respectively. Moreover, the final composition of carbon from the flux agents was 17.92 ± 10.08%. This study confirms that the fuse of CaCO3-SiO2-Al2O3-glass/cullet reduced the VOC via plasma technology, which is a promising method to be implemented in order to reduce the concentration of VOC from the face mask waste.

Volatile Organic Compounds (VOCs) Distribution from PCB Waste and Vitrification by Reacting with Flux Agents

Resin, the part of printed circuit board (PCB) waste, consists of harmful elements and volatile organic compounds (VOCs) and can cause pollution, but it can also be potentially treated through vitrification. Hence, the purpose of this study is to investigate the distribution of VOCs and analyze the effects of different flux agent types, pyrolysis duration, flux agent ratio, and effect of different carrier gases to vitrify resin waste in an atmospheric-pressure microwave plasma reactor. The resins to the flux agents - shell powder, limestone and quartz sand ratio of 50:50 and 25:75 gave the best results for the vitrification compared to the ratio of 75:25 and control (absence of flux agent). Furthermore, the crystalline structures of CaCO3 and SiO2 dominated the final residue, and this confirmed the vitrification of the resin. Flux agents also can reduce the distribution of propylene and benzene from the plasma pyrolysis. A longer duration of the pyrolysis, a higher flux agent ratio and flux agent types can be recommended for plasma pyrolysis applications even though they have no significant impact on the inertization of the elements statistically. Shell powder is thus suggested as a cost-less flux agent which can create vitrification and inertization of the contaminants from resin in plasma discharges.