Search

A high rate fabrication of a thin film complex consisting of cubic-SiC nanocrystals, amorphous silicon and graphite was realized using an atmospheric pressure thermal plasma enhanced chemical vapor deposition (APTPECVD) process with SiCl4 and C2H2 as the silicon source and carbon source, respectively. The morphology, crystal structure and surface chemical composition of the products were characterized. The APTPECVD SiC nanocrystals have average diameters between 18 and 30 nm. A room temperature red region photoluminescence (PL) property originated from the quantum confinement effect of these SiC nanocrystals was observed under UV wavelength excitation. Moreover, pure SiC nanocrystals with a red region PL property can be obtained after a simple post-treatment, including calcining and etching processes. These red photoluminescent SiC nanocrystals can be utilized as biomarkers in bioimaging and drug delivery.

A generalized heat transfer and devolatilization model coupled with the thermal balance between the heating gas and particles was established to predict the complex coal pyrolysis behaviors in the practical plasma reactors. It was proved that this model could well describe the coal devolatilization behaviors in both the pilot-scale and lab-scale plasma reactors as the mechanisms of coal chemistry and particle-scale physics were incorporated. The achieved understanding on the reactor energy balance demonstrated that the heat recovery of the quenching process was crucial to the thermal efficiency and economic benefit of the overall project. The in-depth discussion of the influences of coal feed rate and particle size on the reactor performance revealed the dominant roles and presented the optimal values of these two factors. In particular, the simulation results of several coals could help to provide a simple, quick method of coal type selection for industrial plasma processes. © 2014 American Institute of Chemical Engineers AIChE J, 61: 913–921, 2015

BACKGROUND Decoloration of dyes is a top priority to enhance biodegradability in color-free wastewater post-treatment. A gas–liquid dielectric barrier discharge (DBD) falling film reactor has been developed as a promising approach to the decoloration of dyes. RESULTS Rhodamine B was employed as a model compound. The effects of residence time of the liquid film with different initial concentrations, oxygen flow rates and oxygen concentrations (using different diluent gases, e.g. argon and nitrogen) were investigated. Longer liquid film residence time and lower oxygen flow rate resulted in higher decoloration efficiency. The decoloration efficiency in oxygen and nitrogen discharge was slightly higher than that in oxygen and argon discharge at the same oxygen concentration. The mechanisms of decoloration and the production of oxidative species are clearly revealed. The highest decoloration efficiency (up to 97.4%) with initial concentration of 50 mg L−1 was obtained in pure oxygen discharge with liquid residence time of 1.9 s at an oxygen flow rate of 100 mL min−1. CONCLUSION The proposed gas–liquid DBD falling film reactor is a promising alternative to advanced oxidation processes (AOPs) for effective treatment of dyes. These results are helpful for further real plasma wastewater treatment. © 2014 Society of Chemical Industry

The inherent variability and insufficiencies in the co-production of propylene from steam crackers has raised concerns regarding the global propylene production gap and has directed industry to develop more on-purpose propylene technologies. The oxidative dehydrogenation of propane by CO2 (CO2-ODHP) can potentially fill this gap while consuming a greenhouse gas. Non-precious FeNi and precious NiPt catalysts supported on CeO2 have been identified as promising catalysts for CO2-ODHP and dry reforming, respectively, in flow reactor studies conducted at 823 K. In-situ X-ray absorption spectroscopy measurements revealed the oxidation states of metals under reaction conditions and density functional theory calculations were utilized to identify the most favorable reaction pathways over the two types of catalysts., The oxidative dehydrogenation of propane by CO2 (CO2-ODHP) can potentially fill the gap of propylene production while consuming a greenhouse gas. Here, the authors identify non-precious FeNi and precious NiPt catalysts supported on CeO2 as promising catalysts for CO2-ODHP and dry reforming, respectively, in flow reactor studies.

A novel approach, i.e., the reactive planar laser induced fluorescence (reactive-PLIF) technique, is employed to visualize the coupled mass transfer and reaction in a gas–liquid dielectric barrier discharge (DBD) reactor, by quantitatively recording the dynamic change of concentration field of fluorescence dye (e.g., Rhodamine B) in the liquid layer during its decoloration process. Firstly, the decoloration dynamics are revealed under the conditions of different discharge voltages in oxygen atmosphere. Increasing the discharge voltage can give rise to the increase of the density of DBD plasma filaments impacting on the surface of the liquid layer. Thus, more reactive species (O3, H2O2, OH and other radicals detected by optical emission spectroscopy) are generated, together with the intensification of the convective transport in the liquid layer confirmed by Particle Image Velocimetry (PIV) measurement. Consequently, it can be demonstrated that the simultaneous intensification of the coupled mass transfer and reaction is the reason for the significant enhancement of the decoloration efficiency when increasing the discharge voltage. Secondly, the decoloration dynamics in the discharge atmosphere of inert gas (i.e., argon) are revealed in comparison with the oxygen discharge at the same discharge voltage. It is found that O3 plays a more important role in the reactive species for Rhodamine B degradation. This study provides straightforward analysis on the mechanism of the interaction between discharge plasma and polluted water, which will be beneficial to develop highly efficient cold plasma techniques for advanced oxidation processes (AOPs).

Devolatilization is acknowledged as the first important step in coal conversion techniques. A comprehensive heat transfer and devolatilization model was established, with special consideration of the particle-scale physics and chemistry, to predict the internal heat transport and pyrolysis behavior of particles. The chemical percolation devolatilization model with corrected kinetic parameters and structure parameters was validated with a lot of experimental data and then adopted to describe the devolatilization behaviors under a broader range of temperatures, heating rates, and coal types. The newly achieved understanding of the integrated effect of heating rate and coal type on coal devolatilization could help to provide a preliminary coal rank selection method for industrial processes. In particular, in-depth discussion of the influences of heat conduction, volatiles diffusion, and endothermic heat of devolatilization inside particle indicated the dominant roles of these factors when the intensity of heat transfer was strong or the release of volatiles was rapid. © 2014 American Institute of Chemical Engineers AIChE J, 60: 2893–2906, 2014

Experiments were carried out in a vibrated fluidized bed to investigate the dynamic process of UV-enhanced synthesis of chlorinated polyvinyl chloride (PVC) using a gas–solid method, where a UV-Vis online analysis system was used to record the kinetics of chlorination. Meanwhile, a comprehensive mathematical model incorporating the distributed activation energy model was established to describe the hindrance effect of Cl atoms already inserted on the PVC polymer chain on the later insertion during the chlorination process. Each of the hypotheses in the model was validated by the well-designed experiments, and the model predictions matched well with the experimental data under various operating conditions. Sensitivity analyses of three primary operating parameters, that is, temperature, chlorine concentration, and UV intensity, were made for better understanding of the mechanism of PVC chlorination. It is anticipated that the modeling methodology in this work would be also suitable for similar gas–solid reaction systems. © 2014 American Institute of Chemical Engineers AIChE J, 60: 2235–2243, 2014

This work used thermal plasma to enhance the deposition process of SiC nanocrystals, with SiCl4 and CH4 as the Si source and C source, respectively. Thin films containing SiC nanocrystals, a-Si and graphite were deposited on the substrates. The morphology and crystalline structure of the samples were characterized by various techniques, including SEM, TEM, and XRD. SiC nanocrystals were observed being covered by carbon films and embedded in the network formed by graphite and a-Si. The effect of SiCl4 input rate on the deposition process and product properties was studied in detail, combining characterization techniques and optical emission spectroscopy (OES) diagnostic results. Based on the OES diagnostic of the plasma zone, the concentrations of atomic Si and C in the gas phase are concluded to be the main factors affecting the deposition process. Finally, a simple deposition mechanism is deduced based on the experimental results, which indicates the formation of SiC nanocrystals through the assembly of atomic species in the plasma.

Experiments were carried out in a drop tube furnace and a lab-scale plasma reactor to investigate the effects of coal properties, particle ultimate temperature and heating rate (i.e., 10−1 to 101 K/s for slow pyrolysis, 102 to 104 K/s for fast pyrolysis and 104 to 106 K/s for rapid plasma pyrolysis processes) on coal devolatilization performances. The results revealed that fast pyrolysis of coal would greatly increase the yield of light gases compared to slow pyrolysis process. The increased yield of light gases strongly depended on the coal rank and mainly came from the secondary tar cracking reactions. The comparisons of slow pyrolysis, fast pyrolysis and rapid plasma pyrolysis processes indicated that the major effect of the increased heating rate was to raise the carbon conversion to light gases and then greatly increase the yields of gaseous hydrocarbons correspondingly. In addition, fast pyrolysis behaved much closer to plasma pyrolysis for the coals with the volatile matter content higher than 37%. Therefore, understanding on the fast pyrolysis in a drop tube furnace could help to provide a simple, quick method for preliminary selection of coal rank for plasma pyrolysis process. Finally, a comprehensive single-particle heat transfer mechanism model was established to further discover the effect of heating rate on coal devolatilization and to improve the accuracy of the proposed coal rank selection method.

Nanocrystalline silicon is a promising alternative for the conventional crystalline silicon materials in the photovoltaic industry because of its better photostability and easy fabrication. However, the low deposition rates of conventional nanocrystalline silicon fabrication processes have hampered its application in industry. Thermal plasma has been used to successfully realize the high rate deposition of nanocrystalline silicon in this work. Optical emission spectroscopy (OES) diagnostic and thermodynamic equilibrium calculation are carried out to better understand the mechanism of deposition reactions and the effect of SiCl4 input rate on the nanocrystalline silicon deposition rate and product properties. Emission lines of atomic silicon, atomic hydrogen and atomic argon are observed. The results show that the amount of silicon related species in the gas phase is the main factor affecting the deposition process, which has a linear relationship with nanocrystalline silicon deposition rate, grain size and crystalline fraction at the high H2 dilution ratio of the deposition system.

Coal pyrolysis to acetylene in hydrogen plasma is carried out under ultrahigh temperature and milliseconds residence time. To better understand the complex gas-particle reaction behavior, a comprehensive computational fluid dynamics with discrete phase model has been established, with special consideration of the particle-scale physics such as the heat conduction inside particle. The improved chemical percolation devolatilization model that incorporates the tar cracking reactions is adopted. The model predictions are in good agreement with the performances of two pilot-plant reactors. The simulations reveal the detailed unmeasurable information of gas phase and particle-scale behaviors, then point out the facts that coal devolatilization almost finishes in the first 100 mm of the reaction chamber and the optimal particle diameter is suggested to be 20–50 μm. The different reactor performances during the scale-up from 2- to 5-MW unit are analyzed based on the detailed simulation results in combination with the operational experience. © 2012 American Institute of Chemical Engineers AIChE J, 59: 2119–2133, 2013

This work aims to improve the understanding on the principles of coal/hydrocarbons pyrolysis to acetylene (C2H2) in thermal plasma reactors by thermodynamic analysis in association with the practical operation experience at lab- and pilot-plant scale reactors. The thermodynamic predictions agree well with the experimental data in the literature and the representative data in megawatts plasma reactors. Accordingly, it is summarized that the effective mass ratio of C/H (RC/H) in the gas phase and the quench temperature are the two key factors influencing the reactor performance. The C2H2 concentration is very sensitive to the quench temperature varying around 1500–1800 K, which is therefore suggested to maintain above 1800 K for a better yield of C2H2. Higher effective mass ratio of C/H would have a more positive effect on the reactor performance in terms of C2H2 concentration. The additional input of hydrocarbons to the reaction system provides an effective means to flexibly adjust the performance of the plasma reactor for the raised RC/H. The additional water/steam helps to inhibit coking on the wall but reduce the RC/H, leading to the trend from thermal pyrolysis to gasification process.

Coal pyrolysis in thermal plasma provides a direct and cleaner route for producing acetylene from coal resources. Owing to the complex physical–chemical properties of coal and integrated multiple processes in milliseconds, the selection of coal feedstock becomes of great importance to the reactor performance. Experiments were carried out in a lab-scale plasma reactor to investigate the effects of coal properties and key operating conditions including the coal specific enthalpy, gas specific enthalpy, plasma atmosphere, and residence time. The results demonstrate that higher coal–gas mixture enthalpy leads to higher coal conversion and yield of light gases. The fixed carbon in coal can be partly converted to gaseous products in Ar/H 2 plasma, where the increased H 2 concentration enhances the carbon conversion, and correspondingly raises the yields of CO and C 2 H 2 . Since oxygen in coal is more easily released than carbon, the oxygen compounds in coal are disadvantageous to the acetylene yield. A non-coking coal with volatile matter content on dry ash-free basis higher than 37%, oxygen content lower than 13% and low ash content would lead to a better reactor performance. The relationship between rapid plasma pyrolysis and slow thermal pyrolysis processes is discussed by comprehensively comparing the gas products and solid residues in order to pursue an alternative, simple method of preliminary selection of coal rank for the plasma pyrolysis technology.

An online UV–Vis analysis system is established to reveal the dynamic characteristics of plasma-assisted synthesis of chlorinated polyvinyl chloride (CPVC). Dielectric barrier discharge (DBD) plasma is generated in a vibrated-bed reactor intermittently so that the decoupled processes, i.e., the plasma initiated chlorination step and the chlorine migration step, are mimicked and operated repeatedly at atmospheric pressure. The instant chlorine consumption rate shows corresponding cyclic curves, illustrating evident plasma initiation followed by chlorine migration inside particles. The mechanism of process decoupling concept is therefore demonstrated vividly. Under the same power density of plasma, temperature is ascertained as a key factor to influence the PVC chlorination process. PVC particles with smaller sizes tend to be chlorinated easily, while other particle properties such as specific surface area and microstructure also exert complicated effect on the chlorination process. SEM results show that plasma can destroy the film layer on the particle surface, which makes the secondary sub-particles inside a PVC particle expose to the environmental chlorination gas and plasma. A pyrolysis GC–MS analysis helps to identify the molecular structure of CPVC product in terms of the compositions of chlorobenzene and dichlorobenzene in the pyrolized products, which are formed by chlorine atoms bonded on polymer chains.

Cold atmospheric plasma is considered to be a promising approach for decontamination purposes, e.g. dyeing water decoloration. In order to better understand the complex mechanism of the plasma physics coupled with the plasma chemistry involved in the interaction of the polluted water with the discharge plasma, a novel approach was proposed to study the in situ oxidation process between the plasma and liquid phase in two dielectric barrier discharge (DBD) plasma reactors with different bottom shape (concave vs. plane), by using the planar laser induced fluorescence technique to visualize the process dynamics. Rhodamine B was employed as the tracer dye, which was gradually decomposed by the combined effect of the chemically active radicals (OH, O, H2O2, etc.) as well as the intense UV radiation in the DBD plasma process. The results showed that the DBD plasma filaments induced certain fluctuation on the Rhodamine B liquid layer, which accordingly intensified the mass transfer to a large extent thus accelerated the oxidation process. The comparison of the measured concentration fields in the two DBD plasma reactors illustrated that the DBD reactor #1 with concave bottom showed higher oxidation efficiency than the DBD reactor #2 with plane bottom. Additionally, the experiments demonstrated that the oxidation efficiency in the DBD plasma water treatment was much better than that in the reactor with pure oxidation by ozone gas, which can be further improved by injecting the additional oxygen gas bubbles into the liquid phase in the plasma reactor.

Using thermal plasma for coal pyrolysis to acetylene provides a direct route to make chemicals from coal resources, where the temperature field in the reactor plays a dominant role in the performance of coal devolatilization. A comprehensive computational fluid dynamics with discrete phase model (CFD-DPM) has been established to describe the rapid coal pyrolysis process in a reactor under ultra-high temperatures. The simulations based on this model helped to understand the complex gas–particle reaction behavior in the millisecond process of coal pyrolysis. The particle-scale physics such as the heat conduction inside solid materials, diffusion of released volatile gases, coal devolatilization, and tar cracking reactions were incorporated. The improved chemical percolation devolatilization (CPD) model was applied to describe the devolatilization behavior of rapidly heated coal based on the physical and chemical transformations of the coal structure. This model was proved to be qualified for describing the complex gas–particle reaction behavior with milliseconds residence time by the operation experience of a 5-MW plasma reactor. Then the simulations revealed the fact that the particle heating and devolatilization are strongly affected by the grade of the temperature and the residence time of coal particles in the high temperature zone(s). Highly concentrated energy input in the reactor may not intensify the reactor performance. As a potential solution, multi-stage heating design would provide more flexibilities to effectively adjust the devolatilization performances under the same energy input.

Liquid hydrocarbons including n-hexane, cyclohexane and toluene are pyrolyzed in H2/Ar plasma to investigate the effects of feedstock properties and key operating conditions (e.g., the feedstock specific input power and residence time) on the reaction performance. The experiments verify that the non-aromatic hydrocarbons show better chemical reactivity than partially aromatic substances. Meanwhile, the straight-chain alkanes and cycloalkanes have better yields of ethylene during the pyrolysis. The results also demonstrate that the pyrolysis reactions are almost completed within the first 0.8 ms in Ar/H2 plasma independent of the feed substances (i.e., liquid hydrocarbons), where the increased feedstock specific input power enhances the reactant conversions and correspondingly raises the yields of acetylene. At a feedstock specific input power of 4.7 × 104 kJ/kg, the n-hexane conversion is over 90 % and the yield of acetylene reaches 70 %. In addition, when using n-hexane as the feedstock, very little coke is formed during the course of reaction. Comprehensive comparisons of the current experiments with the data reported in the literature are made to point out the key influencing factors, i.e., the effective mass ratio of C/H (RC/H) in the gaseous phase and the quench temperature. Both two factors would need to be enhanced in order to get a better performance. Finally, the improvements on the specific energy requirement of this process are discussed.

The effort that China is making for cleaner production of chemicals especially based on technology innovations of green processing and synthesizing methods, is reviewed. An example is the rapidly expanding chlor-alkali industry in China, which manufactures a large volume of products of caustic soda, polyvinyl chloride (PVC), etc. With a distinct difference from conventional technologies, a series of novel processes are under investigation, both in academic research and in industrial demonstrations, such as clean coal conversion to acetylene using thermal plasma, mercury free catalysis to synthesize vinyl chloride monomer (VCM) and a plasma-assisted gas-solid contacting process to make chlorinated PVC (CPVC). All efforts are directed to improving the process effi ciency, saving the materials and energy cost, and accordingly to reducing the emission/disposal of waste gases, liquids and solid residues. We try to address the signifi cance of technology innovations to drive the industry reformation, in order to fully update the outdated processes with heavy pollution and energy consumption, to the state of the art green process and synthesis techniques.

Considering the inevitable high energy input to implement the CO 2 reforming of methane under high-temperature operation using conventional catalysis method, the low temperature conversion of CO 2 and methane in the coaxial dielectric barrier discharge (DBD) plasma reactor was investigated in this work. Steam was introduced to enhance the CO 2 reforming of methane with synergetic catalysis effect by cold plasma and catalyst. The experimental results showed that a certain percent of steam could promote the conversion of both CH 4 and CO 2 . Meanwhile, the carbon deposition was evidently reduced compared with the dry reforming of methane. With the increase of steam input, the steam reforming occurred predominantly. As a result, the hydrogen volume percentage in the product gases increased. In this way, the products with different H 2 /CO ratio could be achieved by changing the mole ratio of CH 4 /CO 2 /H 2 O at the reactor inlet. In particular, when the mole ratio of H 2 O/CH 4 increased to almost 3 corresponding to the pure steam reforming process, the conversion of CH 4 reached almost 0.95 and the selectivity to H 2 was almost 0.99 at 773K.

This paper gives an overview of the recent development of modeling and simulation of chemically reacting flows in gas–solid catalytic and non-catalytic processes. General methodology has been focused on the Eulerian–Lagrangian description of particulate flows, where the particles behave as the catalysts or the reactant materials. For the strong interaction between the transport phenomena (i.e., momentum, heat and mass transfer) and the chemical reactions at the particle scale, a cross-scale modeling approach, i.e., CFD–DEM or CFD–DPM, is established for describing a wide variety of complex reacting flows in multiphase reactors. Representative processes, including fluid catalytic cracking (FCC), catalytic conversion of syngas to methane, and coal pyrolysis to acetylene in thermal plasma, are chosen as case studies to demonstrate the unique advantages of the theoretical scheme based on the integrated particle-scale information with clear physical meanings. This type of modeling approach provides a solid basis for understanding the multiphase reacting flow problems in general.

Coal pyrolysis to acetylene in thermal plasma provides a direct route to make chemicals from coal resources, where the rapid heating and release of volatile matters in coal particles play the domin...

Dry reforming of CH4 with CO2 to produce syngas was investigated in a plasma reactor without catalysts at atmospheric pressure. The reactants passed through the plasma zone and reacted in milliseconds with high conversions and selectivity due to the localized high temperature. The results showed that both conversions and selectivity were higher when using a DC arc discharge than using a pulsed DC arc. Increasing the input energy density promoted the conversions of reactants. At an input power of 204 W, the conversions of CO2 and CH4 reached 99.3 and 99.6%, respectively, and the selectivity to products was almost 100%, where the molar ratio of CO2/CH4 was 1 with the reactants flow rate of 100 ml/min. Very little coke was formed during the course of reaction. Key parameters such as the pulse frequency, the input power and the total feed flow rate were studied to find the optimum operating condition.

The synergetic effect of commercial catalyst Ni/Al2O3 with the low-temperature plasma created by a dielectric barrier discharge (DBD) for the dry reforming of methane was investigated in a coaxial DBD reactor. Three different contact modes of prereduced catalyst with plasma were studied to determine the interaction of plasma and catalyst, especially their synergetic effect. When the catalyst was placed at the end of the discharge zone or absolutely separated with the discharge zone, there was no synergetic effect or much worse effect than only using plasma or only using catalyst. However, when the catalyst was fully filled in the annular discharge gap, the synergetic effect was achieved. In addition, with the increase of temperature, the synergetic effect appeared to be evident from 673 K. The experiments also demonstrated that the unreduced catalyst filled in the annular discharge gap can be in-situ reduced during the reaction at around 673 K by the assistance of the in-situ plasma. However, the synerget...

Low temperature conversion of CH4 and CO2 was investigated in a coaxial dielectric barrier discharge reactor at ambient pressure. Main parameters, including the input power, the residence time, the discharge gap, the molar ratio of the feed gases and the multi-stage ionization design were evaluated to understand the ways to improve the conversion of greenhouse gases and reduce the output of by-products. At certain input power, the conversion of CH4 and CO2 can reach 0.797 and 0.527, respectively, when the molar ratio of CH4/CO2 is one. When this ratio was low to 1:5, the conversion of CH4 was promoted to 0.843 and the selectivity to CO and H2 was almost 100%. The multi-stage ionization favored the conversion of CO2, which would also be an efficient design to promote the selectivity to the main products such as CO and H2 and suppress the selectivity to the by-products.

Hydrogenated nanocrystalline silicon (nc-Si:H) is a promising alternative for crystalline silicon (c-Si) in the photovoltaic industry. We proposed an atmospheric pressure thermal plasma enhanced CVD (APTPECVD) process for high rate deposition of nc-Si:H on silicon and glass substrates using SiCl4 as the deposition precursor. The deposition rate under typical operating conditions could reach 9.78 nm s−1, and the deposited nc-Si:H film thickness could reach 17.6 μm. The grain diameter and crystalline fraction of the deposition product were characterized using SEM, TEM, Raman spectroscopy and XRD. The photoluminescence performance at room temperature was discovered.