O. N. Fedyaeva

Disposal of hazardous organic substances in supercritical water

Studies devoted to the eco-friendly disposal of hazardous organic substances (OS) and waste through oxidation in supercritical water (SCW) are reviewed. One advantage of using SCW is the possibility of complete and rapid oxidation of OS in closed systems. The oxidation rate is determined by the temperature, proportions between the reagents, bond dissociation energies, and solubility of OS in SCW, decreasing in the series aliphatic > aromatic, heterocyclic > polyaromatic compounds. The main oxidation products are carbon dioxide, nitrogen, and water; sulfur, phosphorus, and halogens are converted into the respective mineral acids. There are a number of difficulties in the implementation of particular processes on the industrial scale, which impede the achievement of an acceptable performance of installations in terms of safety and stability. These difficulties are mainly associated with heterogeneous processes on the reactor walls, such as the corrosion of constructional materials and deposition of salts, which lead with time to changes in the kinetic characteristic of the main and coupled reactions.

Conversion of municipal sewage sludge in supercritical water

The conversion of the organic matter of municipal sewage sludge in water under supercritical conditions (T≤750°C; P≤30 MPa) was studied. According to mass-spectrometric data, CO2, H2, CH4, and NH3 were predominant among volatile conversion products. The kinetic parameters of conversion were determined. It was found that the rate of the process increased with temperature and mainly depended on the interaction of water molecules with sewage sludge carbon T > 600°C.

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.

Coal gasification with water under supercritical conditions

The conversion of an array of coal particles in supercritical water (SCW) was studied in a semibatch reactor at a pressure of 30 MPa, 500–750°C, and a reaction time of 1–12 min. The bulk conversion, surface conversion, and random pore models were used to describe the conversion. The quantitative composition of reaction products was determined, and the dependence of the rate of reaction on the degree of coal conversion, reaction time, and reaction temperature was obtained on the assumption of a first-order reaction and the Arrhenius function (E = 103 kJ/mol; A0 = 7.7 × 104 min−1). It was found that the gasification of coal under SCW conditions without the addition of oxidizing agents is a weakly endothermic process. The addition of CO2 to SCW decreased the rate of conversion and increased the yield of CO. It was found that, at a 90% conversion of the organic matter of coal (OMC) in a flow of SCW in a time of 2 min, the process power was 26 W/g per gram of OMC.

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.

Hydrogenation of bitumen in supercritical water flow and the effect of zinc addition

The composition of liquid and volatile products and residue of bitumen conversion in supercritical water (SCW) flow at 400°C and 30 MPa with and without addition of zinc savings to bitumen is investigated. The yield of liquid products upon bitumen SCW conversion reaches 47.3%. The data on oxygen and hydrogen balance in the initial bitumen, products, and conversion residue indicate the chemical participation of H2O molecules in the conversion. The yield of liquid products is increased up to 62.3% if zinc is added to the initial bitumen with a simultaneous increase of the H/C atomic ratio in the products. Hydrogenation is due to hydrogen evolution during zinc oxidation by supercritical water. Under these conditions, ZnO nanoparticles with a size of 50–100 nm are formed. The data of transmission electronic microscopy and X-ray diffraction analysis of oxidized zinc demonstrate the presence of about 0.5 wt % of ZnS and about 1.2 wt % of metal zinc clusters (up to 10-nm size) in the samples. According to the sulfur atoms’ balance accounting products and conversion residue, 12.6 wt % of bitumen sulfur participates in the formation of zinc sulfide.

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.

Brown coal gasification in combustion in supercritical water

The yield and composition of conversion products are investigated in a layout that provides countersupply of reagents (brown coal, supercritical water (SCW), and O2) into a vertical tubular reactor and drain of reactants into replaceable collectors under isobaric conditions (30 MPa). The coal (gross formula CH0.96N0.01S0.002O0.31) incorporated into coal-water slurry (CWS) stabilized by starch addition (1 wt.%), was supplied through the top end of the reactor, while SCW and SCW/O2 fluids were supplied through the bottom end. Based on the results of elemental analysis of liquid products and solid residue of conversion, and mass spectrometric analysis of volatile products, we obtained gross reactions of brown coal conversion in SCW and SCW/O2 fluids. It was found that addition of O2 to SCW leads to autothermal conversion conditions and an increase in the contribution from heterogeneous reactions between carbon and water, which provides additional yield of H2 and CH4.

The formation of Al2O3 nanoparticles in the oxidation of aluminum by water under sub- and supercritical conditions

Massive aluminum samples were oxidized by sub- and supercritical water with the formation of (AlOOH)n and (Al2O3)n nanoparticles. The release of H2 began at 523 K when the reagents were heated uniformly to 700 K. The time lag of the beginning of oxidation was 140 s when supercritical water was injected into a reactor with aluminum samples at 665 K and 23.1 MPa. Oxidized aluminum powders were analyzed using a transmission electron microscope. Predominantly large (300–400 nm) α-Al2O3 particles were formed when supercritical water was injected into a reactor with aluminum. Smaller (20–50 nm) γ-Al2O3 particles were also observed in samples oxidized by water vapor under temperature increase conditions. Kinetic equations describing the rate of H2 formation in the reaction of H2O with aluminum were obtained. Possible nanostructuring mechanisms are discussed.

Brown coal conversion under the action of supercritical water

A test bench was developed and the conversion of the organic matter of coal (OMC) in supercritical water (SCW) was studied under conditions of a continuous supply of a water-coal suspension to a vertical flow reactor at 390–760°C and a pressure of 30 MPa. From 44 to 63% OMC was released as liquid and gaseous products from coal particles (from the water-coal supension) during the time of fall to the reactor. This stage was referred to as the dynamic conversion of coal. The particles passed through the stage of the dynamic conversion of coal did not agglomerate in the reactor in the subsequent process of batch conversion in a coal layer at T = 550–760°C. The volatile products of the overall process of the dynamic and batch conversion of coal included saturated hydrocarbons (CH4 and C2H6), aromatic hydrocarbons (C6H6, C7H8, and C8H10), synthesis gas (H2 and CO), and CO2. At T < 600°C, CO2 and CO were the degradation products of oxygen-containing OMC fragments, whereas they also resulted from the decomposition of water molecules at higher temperatures in accordance with the reaction (C) + H2O = CO + H2. The mechanisms were considered, and the parameters responsible for the dynamic conversion of coal were calculated.