Pranas Valatkevicius

Preparation of catalytic coatings for heterogeneous catalysts employing atmospheric pressure non-equilibrium plasma

Abstract Well-adhered CuO and Cr 2 O 3 supported catalytic coatings with highly developed surface, excellent thermal and mechanical stability and high catalytic activity were prepared employing atmospheric pressure plasma spray technology. A system consisting of a linear DC plasma torch with a hot Hf cathode and cooling copper anode has been used. A temperature range of 2800–3500 K has been established as optimal. Coatings were analyzed by X-ray diffraction (XRD) structural patterns, scanning electron microscopy (SEM) and Brunauer–Emmet–Teller (BET) method analysis, as well as a catalytic combustion behavior study. The catalytic activity towards carbon monoxide (CO) oxidation was tested in the range 400–700 K.

Nitriding of an austenitic stainless steel in plasma torch at atmospheric pressure

Experiments with a nitrogen torch at atmospheric pressure have been performed in order to identify the role of surface processes in the mechanism of nitrogen transport during nitriding of stainless steel AISI 304. Unusually thick (∼175 μm) layers of supersaturated N solid-solution f.c.c. phase have been obtained for 10 min at 450 °C. Samples treated at 550 °C have a radically different structure. The scanning electron microscopy (SEM) surface and cross-sectional micrographs reveal that surface topography is an indicator of the degree of modification occurring in the nitrided layer. Surface vacancies generated by surface instabilities move deeply into the bulk at elevated temperatures and form a highly defective layer with pores and microcracks. The transport of nitrogen in austenitic stainless steel is driven by the fluxes of matrix atoms directed to stabilize surface instabilities. Nitrogen depth profiles simulated on the basis of a model with a surface-atom relocation process and activation energy of 1.15 eV, and including balanced fluxes of atoms in the bulk for relaxation of surface energy, are in qualitative agreement with experimental results.

Formation of carbon coatings employing plasma torch from argon-acetylene gas mixture

Amorphous carbon films were formed on Si (111) wafers from argon-acetylene gas mixture at atmospheric pressure by direct current (DC) plasma torch discharge. The Ar/C2H2 gas volume ratio varied from 12 to 100, and the distance between plasma torch nozzle exit and the samples was 0.005, 0.01 and 0.02 m. SEM revealed carbon coatings thickness in the range of 20-270 &mgr;m, and variation of the growth rate from 0.067 &mgr;m/s to 1.5 &mgr;m/s. Growth rate of the coatings increases decreasing Ar/C2H2 gas ratio and the distance. The Raman spectra of carbon films indicate the upward shift of the D (~1360 cm-1) and G (~1600 cm-1) peaks, compared to typical diamond-like carbon (DLC). a-C:H coatings deposited at higher Ar/C2H2 gas ratio (60 and 100) and distance d greater than or equal to 0.01 m contain high sp3 bond fraction and are attributed to DLC films. However Raman spectra shape and ID/IG ratio demonstrate existence of diamond phase mixed with glassy carbon phase. Films produced at lower Ar/C2H2 ratios are graphite-like carbon (GLC). The Fourier transform infrared (FTIR) spectroscopy has shown that film transparency increases decreasing acetylene gas content. Reflectance of the films depends on Ar/C2H2 gas ratio and distance, and varies from 60% up to 90%. The IR spectra showed clear evidence of C=C and C=O bonds in GLC films and presence of sp3 CH2 symmetric (2850 cm-1) and antisymmetric (2920 cm-1) modes in DLC coatings.

Water vapor plasma torch for synthesis gas production from organic waste

Plasma technologies have many fields of application, especially for treatment of materials and waste conversion. Water vapor plasma has special properties of high enthalpy and high activity flow. It can be a promising technology for gaseous, liquid and solid waste treatment and production of synthetic hydrogen-rich gas [1,2]. Atmospheric pressure water vapor plasma technology has been realized at Lithuanian Energy Institute, Plasma Processing Laboratory as an alternative to other reforming methods producing synthetic gas. An experimental linear, sectional, direct current arc plasma torch 55-70 kW of power with copper anode was developed and manufactured. The main plasma forming gas is overheated water vapor, produced by 5 bar of pressure steam generator. The operational characteristics of stable work of water vapor plasma torch were established. The flow was heated up to 2000-3000 K of temperature, the fuel conversion process from liquid organic waste, such as toluene and glycerol in plasma-chemical reactor was realized. Hydrogen and carbon monoxide gases (H2+CO) were produced as main products, additionally small amounts of CO2 and CxHy were obtained. The investigations confirmed that the conversion of toluene and glycerol into synthesis gas by means of atmospheric pressure water vapor plasma torch was successful. Liquid waste conversion rate exceed 100%. The main products of thermal plasma treatment are H2 (max. 56.9%) and CO (max. 20.7 %) with relatively high concentrations of CO2 (up to 12.6%). The results will be useful projecting new plasma equipment, designed for the decomposition of biomass and organic waste.

The Research on Atmospheric Pressure Water Vapour Plasma Generation and Application for the Destruction of Wastes

In the Lithuanian Energy Institute an experimental atmospheric pressure Ar/water vapour plasma torch has been designed and tested. The power of plasma torch was estimated 40 ÷ 69 kW, the mean temperature of plasma jet at the exhaust nozzle was 2300÷2900K. The chemical compositionof water vapour plasma was established from the emission spectrum lines at 300 ÷ 800nm range. The main species observed in Ar/water vapour plasma were: Ar, OH, H, O, Cu. The experiments on water vapour steam reforming were performed. The results confirmed that water vapour plasma has the unique properties – high enthalpy and environmentally friendly conditions. It could be employed for environmental purposes such as destruction of wastes into simple molecules or conversion to synthetic gas.