A. V. Shishkin

Synthesis of zinc oxide nanostructures during zinc oxidation by sub-and supercritical water

Solid zinc (Zn)S and liquid zinc (Zn)L are oxidized by water with the formation of zinc oxide (ZnO) nanostructures and the evolution of hydrogen. The maximum rate of this process, called chemical supercondensation by water (CSW), is realized on approaching the melting temperature of zinc from the left and right with increasing density of supercritical water. The CSW process begins with the formation of (ZnO)n clusters via the reaction (Zn)S,L + nH2O = [(Zn)S,L · (ZnO)n] + nH2, followed by their subsequent growth at n > 7 in the exothermal process of epitaxy on (Zn)S and coagulation of (ZnO)n in (Zn)L. The CSW of (Zn)S leads predominantly to the formation of nanowires and nanorods, while the CSW of (Zn)L practically always proceeds with the formation of nanoparticles. The rate of (Zn)S oxidation increases with the thickness of a layer converted into ZnO. This is related to the self-heating and local melting of (Zn)S in the course of CSW. The complete CSR of (Zn)S plates and cylinders results in the formation of highly porous nanostructural ceramics.

Synthesis of ZrO2 nanoparticles during zirconium oxidation by supercritical water

We have discovered that massive samples of solid zirconium (Zr)s are completely oxidized by supercritical water (SCW, T > 647 K, P > 22.1 MPa) with the formation of zirconium oxide nanoparticles (ZrO2)n. The particle size distribution, morphology, and features of the nanostructure formation depend on the process conditions. The kinetics of H2 production and zirconium oxidation has been determined using the method of SCW injection into a reactor with (Zr)s at various temperatures. The dependence of the oxidation induction time on the SCW parameters has been studied.

Synthesis of Fe x O y nanoparticles during iron oxidation by supercritical water

It is established that iron is oxidized by supercritical water (SCW) with the formation of H2 and nanoparticles of iron oxides (Fe3O4, FeO, and γ-Fe2O3). The kinetics of H2 production and iron oxidation has been studied by SCW injection at T = 673, 723, 773, 823, and 873 K into a reactor with iron particles. Data of X-ray diffraction and transmission electron microscopy show that the phase composition and morphology of synthesized oxide nanoparticles depend on the SCW temperature.

Conversion of brown coal in sub- and supercritical water at cyclic pressurization and depressurization

The conversion of brown coal in sub- and supercritical water at 310–460°C and pressures up to 30 MPa in the cyclic pressurization and depressurization modes is studied. The temperature dependences of coal organic matter (COM) conversion and the yield of volatile and condensed products are obtained. The temperature dependence of the yield of condensed substances has a maximum at 370°C. The fraction of high-molecular substances in condensed products is increased with increasing temperature. The cumulative conversion of COM into volatile and condensed products upon heating to 460°C was 31.4 and 8.6%, respectively. According to the data of mass spectrometry analysis of volatile products and the elemental analysis of the initial coal, carbonaceous residue after the conversion, and condensed products, 82.3% of oxygen and 74.7% of sulfur, are removed from COM. CO2 and H2S are the main products of the conversion of oxygen- and sulfur-containing groups.

Combustion of carbonized coal residue in a mixture of ammonium nitrate and supercritical water

The possibility of low-temperature oxidation of a solid carbonized coal residue in a mixture of NH4NO3 and supercritical water (723 K and 30 MPa) is shown for the first time and its mechanism is described. Conjugate processes of oxidation of the carbonized residue and formation of combustible gases H2 and CH4 caused by the participation of H2O in redox reactions was found. It was established that the ash residue has a high porosity and consists of agglomerated nanoparticles of silicon and metal oxides.

The effect of constant electric field on the oxidation rate of massive zinc in supercritical water and the formation of zinc oxide nanocrystals

It is established that the rate of oxidation of a massive zinc plate by supercritical water (SCW) at 673 K and 23 MPa increases and the morphology of obtained ZnO nanocrystals changes when strength E of the constant electric field applied perpendicular to the plate increases from 0 to 286 kV/m. A decrease in the SCW density at the same temperature results in the formation of a more compact nanostructured ZnO layer, while the increase in E leads to loosening of structure in the inner part of the ZnO layer.

Coupled processes of aluminum oxidation and asphaltite hydrogenation in supercritical water flow

The conversion of asphaltite (empirical formula CH1.23N0.017S0.037O0.01) in supercritical water (SCW) flow at 400°C and 30 MPa with and without addition of aluminum shavings is investigated. The composition and amount of the products and insoluble conversion residue are determined by means of liquid-adsorption chromatography, elemental analysis, IR and 1H NMR spectroscopy, mass spectrometry, and gas chromatography/mass spectrometry. It is found that SCW not only dissolves asphaltite components, but also participates in redox reactions. Hydrogen formation and heat evolution during aluminum oxidation by SCW promote the in situ hydrogenation of asphaltite, increase the fraction of aromatic and polyaromatic compounds in conversion products, decrease the yield of the insoluble conversion residue from 44.5 to 11.3%, and decrease the olefine fraction. When aluminum is added, the degree of asphaltite desulfurization that results from sulfur removal in the form of H2S increases by more than 3.5 times.

Obtaining of gas, liquid, and upgraded solid fuel from brown coals in supercritical water

Two new conversion methods of brown coals in water steam and supercritical water (SCW) are proposed and investigated. In the first method, water steam or SCW is supplied periodically into the array of coal particles and then is ejected from the reactor along with dissolved conversion products. The second method includes the continuous supply of water-coal suspension (WCS) into the vertically arranged reactor from above. When using the proposed methods, agglomeration of coal particles is excluded and a high degree of conversion of coal into liquid and gaseous products is provided. Due to the removal of the main mass of oxygen during conversion in the composition of CO2, the high heating value of fuels obtained from liquid substantially exceeds this characteristic of starting coal. More than half of the sulfur atoms transfer into H2S during the SCW conversion already at a temperature lower than 450°C.

Formation of combustible gases during interaction of tungsten and zirconium with supercritical fluid H2O/CO2

Oxidation of bulk samples of tungsten (923 K) and zirconium (773 and 873 K) by H2O/CO2 supercritical fluid (molar ratio [CO2]/[H2O] = 0.17–0.26) at a pressure of about 300 atm is investigated. Oxidation produces monoclinic WO3, monoclinic W19O55, monoclinic ZrO2, H2, CO, CH4, and carbon (on the surface of tungsten oxide). Differences in oxidation mechanisms for tungsten and zirconium are revealed. CO2 molecules take part in the oxidation of tungsten only after oxide formation in reaction with H2O. Zirconium is oxidized fully, and oxidation of tungsten terminates in the formation of the oxide layer at the metal surface.

Liquefaction of liptobiolith coal in supercritical water flow under nonisothermal conditions

Liquefaction of liptobiolith coal in steam and supercritical water flow at a constant increase in the temperature from 300 to 470°C is studied. Temperature dependences of the yield of liquid and volatile products and apparent kinetic parameters of the process are obtained. The yields of oil, resin, asphaltene, and volatile products are determined to be 23.2, 16.1, 5.1, and 14.1% of the coal organic matter (COM), respectively. The participation of water molecules in thermochemical transformations of COM results in a 13.2 wt % increase in the amount of oxygen in the conversion products and residue.