Main research areas of the laboratory:
– development and research of DC plasma sources for wide range of applications;
– research of processes and phenomena taking place in discharge channels, exhaust plasma jets and flows;
– diagnostics of plasma and high-temperature flow and development of diagnostic measures;
– research on interaction of plasma jets and substances in various plasma-technological processes;
– research and implementation of plasma neutralization process of hazardous substances;
– synthesis of catalytic and tribological coatings in plasma ambient and analysis of their properties;
– research of thermal and heterogeneous processes for reacting product flowing around catalytic surface;
– formation and modification of constructional material surfaces in plasma;
– synthesis of micro- and nano- dispersed granules and mineral fiber from hardly meltable materials and analysis of their properties;
– generation of water vapor plasma and its application for fuel conversion and neutralization of hazardous waste.
Researchers of the Laboratory of Plasma Processing have over 40 years of experience working in different fields of development, scientific research and application of atmospheric and reduced pressure plasma and are able to successfully simulate new plasma technologies, using plasma equipment, designed in the Laboratory. Different composition gas and its mixtures are used for plasma jets formation. The Laboratory contains pilot production technological equipment, which is used to change and modify mechanical, tribological, chemical and optical properties of layers of different material surfaces. Constant updating of technical base, development and disposal of available analytic equipment enables to perform research of plasma sources, diagnostics of plasma flows and jets, analysis of gas dynamic characteristics and heat-mass exchange.
On the basis of acquired knowledge, the Laboratory of Plasma Processing is carrying out the following researches:
Development of plasma sources and research of plasma jet
The Laboratory of Plasma Processing develops new plasma generators up to 200 kW of capacity and improves the existing ones. Recently, a new design water vapor plasma generator has been developed. Based on the knowledge of processes, occurring in the reactive discharge chambers, and by means of the theory of similarity of plasma processes, its volt-ampere and thermal characteristics were generalized and stable operation modes were determined.
By means of water vapor plasma generator, conversion of gaseous, solid and liquid organic materials into hydrogen enriched synthetic gas was carried out.
The Laboratory continues the investigations on heat transfer in plasmatron reactive arc zone, as well as variation of electric arc strength in laminar and turbulent flow regime, the impact of various factors on the characteristics of plasma flows and jets, specific features of arc radiation with different gas flows. Operating modes of linear electric gas arc heaters and reactors and their operating characteristics have been examined, conditions of increase of duration of operation have been determined, arc turbulence and new methods of energy application in plasma equipment have been analyzed.
Diagnostics of plasma and high temperature jets
Formation of high-temperature and plasma jet, its dynamics, heat exchange in the channels of different configuration, their cells, and in elements of heat exchangers are investigated in the Laboratory. Plasma diagnostics is performed in the Laboratory by applying numerical and experimental methods. A numerical research of heated gas jet in the channel was performed applying hydrodynamics software ANSYS Fluent. The calculation was performed to solve full Navier-Stokes and energy equations based on the dynamic k- model. However, the numerical research becomes especially difficult when multiphase jets are flowing, and the solid particles are injected into the jet. This occurs due to specific plasma properties; therefore, numerical research of two-phase plasma jets is performed applying software package Jets & Poudres, adjusted to modeling plasma jets.
Recently, non-contact methods have been widely applied for plasma diagnostics in the Laboratory. One of them is optical spectroscopy method; its main device is an optical spectrometer AOS-4. It is a very fast optical measurement system that may be used for the investigation of peaks of gas emission spectra in a wavelength range of 250–800 nm. The system is also used to examine composition of plasma elements and emission spectra.
A high-speed optical camera with CMOS sensor, 1280×800 pixel matrix, which enables high-speed recording and capturing of moving objects at a very high speed, is used for multiphase plasma flow visualization and determination of some dynamic characteristics. The Laboratory uses Phantom Miro M310 high-speed camera.
Formation of surface layers of construction materials by plasma technologies
Synthesis of coatings in plasma flows
Plasma spray technology for surface formation, developed in the Laboratory, was applied for catalytic, tribological and protective coatings formation as well as for hard ceramic coatings, which are employed for improving the operational characteristics of constructional material surface layers in mechanics, chemistry, energy and medicine.
These coatings improve the corrosion resistance up to 102–103 times, significantly decrease the friction coefficient and reduce the mechanical wear. The use of plasma technology decreases the demand for expensive constructional materials, since cheap materials covered with different thickness coatings replace large amounts of used expensive materials.
Having developed a non-equilibrium atmospheric pressure plasma jet with unbalanced temperature of individual components in the equipment presented in Fig., the activated and synthesized materials acquire different energies before reaching the treated surface. Necessary conditions for certain chemical reactions to combine into blocks in both plasma jet and the substratum surface are created. This enables the synthesis of phase Al2O3 coatings with highly active surface, which is especially relevant in the formation of catalytic coatings. The specific surface area of the coating was further enlarged by heating it at a certain temperature.
In the fields of science and production, a worldwide attention has recently been given to the renewable energy technologies, hydrogen energy, programs of fuel synthesis and saving, issues related to the reduction of environmental pollution and their solution. All these areas require special purpose and composition catalysts that are used in approximately 70% of chemical reactions carried out worldwide. The production of the up-to-date catalytic reactors is a time and finance consuming chemical process performed by precipitating platinum group metals. For this reason, the catalytic reactors are expensive; their ceramic substrates are non-durable, and the meshes often melt and block the reactors due to poor thermal conductivity. In the new generation of catalytic neutralizers, a metal substrate is substituted for ceramic one, and noble metals are replaced by cheaper metal oxides, zeolites and other materials that are successfully used as effective catalysts.
The mass and heat transfer processes taking place in the catalytic reactors made of coatings were examined using the equipment for studying catalyst coating characteristics developed in the Laboratory. Gas with CO concentrations, characteristic of internal-combustion engine, is emitted, and the temperature necessary for catalytic oxidation of the pollutant is reached when the combustion products mix with an oxidant in the air.
For the purpose of the work, the methodology for the research of dynamic and thermal characteristics of gas in the boundary layer zone was developed; the equipment and facilities for examining the jet structure were assembled. The distribution of velocity, temperature and substance concentration of the reactive gas next to the catalytic wall and the heat-mass exchange coefficients of the jet and the wall were established.
On the basis of metal oxide catalytic coatings, formed employing plasma method, catalytic reactors efficiently reducing the emission of CO, SO2, NOx, HC and other pollutants have been developed. By the catalytic combustion behavior, these reactors are very similar to the ones composed of noble metals. The work related to this issue continues in accordance with the project of the Baltic Sea Region Program 2007–2013. Presently, an innovative efficient catalyst for sulphur compounds oxidation is being developed on the basis of TiO2.
Carbon derivative coatings
Technological modification of surface layers of constructional materials by forming multifunctional coatings is widely applied in surface engineering. One of the possibilities of using plasma technology is the synthesis of plasma polymers. Plasma polymers are thin membranes precipitated by plasma method that may be applied in a wide range of fields: microelectronics, medicine, biotechnologies, manufacturing of semiconductors, etc. Plasma polymers are usually synthesized in a vacuum, but their structures have not been thoroughly studied yet. Due to the low price and good mechanical properties (resistance to corrosion, strength, small autonomous mass, small irrigation angle), hydro, halocarbon polymers and hydrogenated carbon films or their groups compete with the best up-to-date materials and alloys. After evaluation of the situation in the field of plasma polymer synthesis and research, it should be noted that plasma polymerization process requires more detailed knowledge, especially about the influence of coating parameters on the obtained plasma polymer properties and the stability of their time and temperature. One of the plasma polymer groups is innovative materials composed of plasma polymers mixed with metals or ceramics. Such composite materials form a new class of coatings, made of composites and non-composites, and are characterized by a variety of electric, optical and mechanical properties. The developed plasma polymers are mostly used as solid and protective coatings. The application of carbon derivatives for polymer synthesis is currently expanding.
Although the plasma coating formation process in the atmospheric pressure has been widely used in practice for a long time, yet, it has not been fully investigated in terms of physics. It is claimed that the chemical, physical and mechanical properties of the coating as well as its composition and structure are affected by about 50 factors. The prevailing ones are the following: composition of initial materials, dislocation of materials introduced in plasma jet, construction of plasmatron, working characteristics, distance from plasmatron to substrate, temperature, pressure and the type of working gas. Currently, a great deal of attention is directed towards developing solid carbon coatings of various composition and properties on different surfaces (steel, Al2O3, quartz glass, etc.) and investigating their properties by available methods.
To carry out the mentioned work, two plasma systems for synthesis of solid ceramic and diamond coatings were developed. They are equipped with modified plasma generators that supply non-equilibrium plasma jet. The devices operate in the ambient at the atmospheric and reduced pressure of gas, such as nitrogen, argon, hydrogen, acetylene, propane-butane and their mixtures. The coatings on the surfaces of stainless steel, quartz glass and silicon, obtained during the process of synthesis, are characterized by good properties of adhesion. The SEM, XRD, IR and Raman spectroscopy methods were applied for determining the following factors: the coatings surface structure, the size, shape and composition of their particles, their dependence on the composition of gas, constituting and transporting plasma, as well as the place and means of gas introduction into the plasmatron. It was noticed that all spectra of IR photoconductance and reflection have relations common to CHx, OH, CO, CO2 and C=C groups.
Following the performed research, the synthesis of supercondenser electrode coatings was realized, and carbon derivative coatings were obtained by developing them in the atmospheric-pressure plasma in argon/acetylene ambient. The electrical characteristics of the coatings enable increasing the capacity of supercondensers presently used in practice.
Melting of ceramic materials and synthesis of high-temperature metal oxide fiber
Traditional technology and equipment presently used to produce mineral fiber require continuous operation process, complex and expensive alloying furnaces and insulation materials. The quality and composition of fiber produced traditionally are also limited by the melting-point of raw materials: this method does not allow for production of high-temperature thermal insulation fiber, which is more and more often used in various fields.
Plasma technology is the only alternative to obtain a high quality high-temperature fiber. Plasma deposited fiber has unique properties such as resistance to high temperature, low thermal conductivity, and high chemical stability. By melting and stringing ceramic materials and forming mineral fiber, an experimental plasma device with 70–90 kW capacity plasma generator has been developed at the Laboratory of Plasma Processing. It enables to form a splint from dispersed particles, using air as plasma forming gas and auxiliary gas mixtures. Cheap and widely spread in nature ceramic materials (quartz sand, dolomite, clay, aluminum oxide, industrial ceramic waste, etc.) are used as raw materials for producing heat resistant ceramic fiber.
After conducting experimental and numerical research, it was determined that dynamic and energetic characteristics of plasma jet have major impact on the plasma process of pulping ceramic materials. Since melting temperature of ceramic materials reaches up to 2500 K, the temperature of plasma jet inflowing into the reactor should be 2500–3000 K, whereas velocity – 700–1000 m/s in order to completely melt and pulp dispersed ceramic particles. The average mass plasma jet temperature and velocity along the length of the reactor channel evenly reduces and changes at the end, respectively, by 14 and 10%, not taking into account plasma generator operating regimes. This enables to easily regulate plasma jet parameters in the reactor chamber. After getting physical explanation of the mechanism of ceramic fiber formation in plasma-chemical reactor, it was determined that melting of particles occurs in the reactor channel, whereas formation of fiber elements, which lasts 4–10 ms, occurs behind the reactor limits. After injection raw material and dispersed particles into the reactor, heat exchange occurs not only between plasma jet and reactor walls, but also among dispersed particles, which has impact on the reduction of plasma jet temperature. It was investigated that heat exchange of plasma jet and dispersed particles is more intensive the greater is the concentration of particles in the jet and the smaller their measurements are. With increase of concentration of mass cold dispersed particles in plasma jet from 6 to 24%, the heat flow into the reactor wall reduces from 6 to 31% due to intensive flow heat transfer to particles.
Plasma jet velocity is one the main factors conditioning the quality of ceramic fiber since with the increase of velocity of plasma jet discharge from the reactor by 60%, the developed fiber yield increases by 5%, whereas the fiber diameter comprising the splint and granular amount in it reduces.
The derived fiber is irreplaceable in the production of muffle furnaces, MHD generators and blast-furnaces, and due to splendid sound isolating properties – for sound isolation as well. The ceramic fiber can also be suitable in manufacturing of different filtrating materials, also as constructional, concrete solidifying material, whereas certain composition ceramic splint may serve as a catalyst.
Water vapor plasma technology
Rational exploitation of natural resources and environment protection from pollution by industries, household, medical waste are important tasks of the contemporary industry; it is likewise important to search for alternative energy resources. Decontamination of gaseous, liquid and solid waste is carried out by employing various methods. The hazardows waste can be disposed reducing the emission of pollutants into the environment, however the most rational way is to recycle them and get new useful products. Decomposition of different types waste with plasma method, due to unique plasma properties, it is characterized as an extremely environmentally friendly process. Plasma process uses water vapor, which is a coolant, and raw material, while in this plasma, practically all endothermic reactions can be carried out, the most persistent chemical compounds can be broken down to atoms; and by such water vapor plasma technology, synthetic gas (CO + H2) can be derived by decomposing organic waste.
The process and efficiency are determined by the device structure, technical characteristics and plasma jet parameters. Scientific literature lacks data about heat transfer in plasma-chemical reactors, mechanism of electric and thermal processes and interaction of plasma flow with treated materials. Therefore, the tasks of performing research are to model and construct a plasma-chemical reactor, designed for gaseous, liquid and solid waste decomposition and reveal mechanism for jet interaction with decomposable materials, investigate elemental composition of the resulting products, assess efficiency of the process. When the temperature is high (starting at 4000 K), water vapor mass enthalpy is about 6 times greater than air enthalpy. At high temperature, water vapor decomposes into oxygen, hydrogen and their compounds, which react in plasma-chemical reactions. Extremely rapid chemical processes occur in water vapor plasma, when reactive elements H and O are formed. Due to this flow property, hydrocarbons introduced into water vapor plasma are decomposed very efficiently. This technology may be applied for decomposition of waste and environmentally hazardous materials or turn them into synthetic gas during the conversion.
Various experiments of organic material decomposition were carried out. Chosen gaseous substances – hydrocarbons, liquid materials – toluene, glycerol, and solid materials – wood granules were introduced into the plasma-chemical reactor to perform the conversion.
The obtained results and conclusions
Linear direct current water vapor plasma generator with step formed electrode was designed and manufactured. Plasmatron was tested at a variety of modes by supplying air and overheated water vapor.
Thermal and electric properties of water vapor plasma generator were determined. It was established that the flow of discharged from plasmatron jet is turbulent. Heat transfer in water vapor plasmatron between electric arc, heated gas and electrode walls generally occurs by means of convection.
Parameters of plasma jet of gas heated in plasmatron were determined: flow of heated water vapor 2.63–4.48×10-3 kg/s, average temperature of plasma jet 2400–3300 K, average flow velocity 210–600 m/s, efficiency factor 0.7–0.78, Reynolds number 2750–6000.
After diagnostics of water vapor plasma jet performed by optical emission spectrometer, the results confirmed that water vapor in plasmatron discharge is dissociated and is comprised of OH, H2, O (I), Ar (I) elements. Detected atomic hydrogen peaks Ha (656.2 nm), Hb (486.1 nm) and Hg (434.1 nm) demonstrate that hydrogen atoms in water gas plasma are in excited state and are chemically very active. Spectral peaks of metal particles Cu and Fe are very intensive and demonstrate significant erosion of plasmatron electrodes, and care should be taken to reduce it.
After analyzing formation of active radicals in water vapor plasma by numeric methods, it was determined that with the increase of temperature up to 4100 K, dissociation of water vapor occurs. Concentration of water vapor decreases to 1%, while concentration of atomic elements H and O comprising it constantly increases.
The tests of gaseous, liquid and solid material conversion showed that in the ambient of water vapor plasma, synthetic gas H2+CO could be obtained. Its concentration in the general mass balance of the reaction products comprised over 55%.
After performing the tests on conversion of various organic waste, it was determined that the maximum efficiency (67% H2) is achieved by decomposing gaseous waste. Decomposing liquid and solid waste, 34–27% of H2 was obtained, since extra energy had to be provided for the gasification.
Projects implemented in the Laboratory
In 2014, researchers of the Laboratory participated in international projects and programs:
• 2012–2014 scientific group technological development project Ceramic fiber catalyst formed by plasma technologies for reducing pollution emission financed from State budget of Lithuania. Main project objective is to develop a catalytic ceramic fiber of desired properties by applying plasma technology, from which to produce metal oxide fiber catalyst of required properties designed for neutralizing environment pollution, to design and produce experimental research equipment of catalytic properties and realize research in real exhaust combustion product flows.
• National research program Energy for the Future project ATE02/2012 Research of local fuel thermal decomposition processes by developing efficient and ecological technologies.
• National research program Energy for the Future project ATE10/2012 Conversion of organic waste in water vapor plasma by reducing environmental pollution.
• EU support measure Promotion of high international level scientific research project Development of innovative thermal decomposition technology and its application for utilization of wastewater sewage (INODUMTECH). During the project, it is planned to develop a sample prototype of 100 kW power gasification process-technology designed to utilize the amount of sludge comprised in wastewater treatment enterprises of small Lithuanian towns. The project idea is implemented together with the Laboratory of Combustion Processes, Laboratory of Nuclear Engineering and Laboratory of Heat-Equipment Research and Testing.
• 2012–2014 state-funded work Synthesis of carbon coatings in argon-acetylene and in argon-hydrogen-acetylene plasma and investigation of their properties. In this work, measurements of hydrocarbon gas by plasma optical emission spectrometer were performed. Temperature of argon-acetylene and argon-hydrogen-acetylene plasma jet was evaluated. The influence of technological processes (power, amount of gas) on the temperature of discharged plasma jet was determined. Composition of argon-acetylene and argon-hydrogen-acetylene plasma, predominating radicals and types of particles and patterns of their change were analyzed.
Carbon structure coatings from argon-acetylene and argon-hydrogen-acetylene gas plasma were formed. Influence of acetylene gas flow and distance (temperature) on the type of formed carbon structure coatings, optical features and specific surface area was determined. Capacity of condensers with carbon electrodes was measured. Influence of hydrogen gas on the morphology and structure of the obtained coatings was assessed. Exposure of graphite-type carbon coatings to nanosecond and picosecond long impulses was performed. Influence of parameters of laser exposure process (number of impulses, energy of impulses, wave length) on elemental composition of graphite-type carbon coatings, types of connections was determined; optical features of coatings were evaluated.
• 2012–2016 long-term institutional scientific research and experimental (social, cultural) development program. The title of the work is: Experimental and numerical investigations of combustion and plasma processes for enhancement of energy generation technologies and renewable biofuel and for reduction of environment pollution; for implementation of the program, two work groups with separate goals, investigation of combustion and investigation of plasma processes, have been designated. In 2014, the following works were carried out in the Laboratory:
– Methodology for investigation of water vapor plasma jet was created, and analytical technology was selected;
– Dynamic and energetic characteristics of plasma jet were measured and analyzed;
– The spectral qualitative-quantitative analysis of water vapor plasma was performed;
– The assessment of the uncertainty of results was performed.
• 2013–2015 international activity COST TD1208 Plasma in Liquids. The researchers of the Laboratory implement an individual project in this activity Application of water vapor plasma for liquid waste processing, through implementation of which, new plasma-chemical reactor will be developed for decomposition of organic materials ofvarious composition and converting them into synthetic gas with increased amount of hydrogen. Researchers from 26 European countries participate in the activity.
The personnel of the Laboratory of Plasma Processing consists of 9 scientists with a doctoral degree, 1 junior researcher and highly experienced ancillary personnel: 3 engineers and 3 highly qualified foremen.
In 2014, the scientific and technological production of the Laboratory was presented at international (8 papers) and national (2 papers) conferences, 6 scientific articles were published in the journals listed in Thomson-Reuters database Web of Science Core Collection; PhD student Andrius Tamošiūnas defended his dissertation Investigation of Thermo-Hydro-Dynamic Processes in Water Steam Plasma and Its Application for Organic Waste Treatment and obtained a doctoral degree.
Air plasma jet discharged from direct current linear plasma generator
Distribution of temperatures (a) and velocities (b) in the air plasma jet with a speeding
50 m Al2O3 particle
Elemental composition of argon and water vapor plasma jets discharged from 47.6 kW power plasma generator, identified using optical spectroscopy method
Movement of alloy and granules and the process of mineral fibre formation in supersonic plasma jet, observed by high-speed video camera
Formation of various metal alloys in air plasma at atmospheric pressure
Catalytic Al2O3 coating (on the left) and its elemental composition (on the right)
Operating carbon coating synthesis facility generating argon/acetylene plasma
Morphology of amorphous graphite-type coating surfaces after nanosecond long radiation exposure
SEM images of zeolite fiber gained at different plasma flow velocities: 1 – 1600 m/s, 2 – 1500 m/s, 3 – 1200 m/s, 4 – 1000 m/s
Operating water vapor plasma facility designed for decomposition of organic materials
Dependence of change of reaction products on H2O/wood ratio. Continuous line – experimental results, dotted line – results calculated by digital software for temperature 2800 K