Jan W. Nowok

The cause of surface tension increase with temperature in multicomponent aluminosilicates derived from coal-ash slags

An explanation is proposed for the increase of surface tension with temperature in multicomponent aluminosilicate systems such as those derived from coal-ash slags. Two major factors are considered: (1) depolymerization of aluminosilicates caused by rearrangements of intermediate structures in the surface layers, and (2) the increase in surface entropy caused by evaporation of some ash slag components. Electron spectroscopy for chemical analysis spectra were recorded for oxygen 1s photoelectrons on quenched bulk slags and on sessile drops to gain insight into the depolymerization of coal-ash slags with temperature. The tests performed on quenched bulk slags indicated replacement of bridging oxygen [Si-O] with non-bridging oxygen atoms [Si-O−] as a function of increasing temperature. Mössbauer spectra showed an increase in ferrous iron from 4% to 12% of total iron as temperature rose from 1400 °C to 1500 °C. The increase in non-bridging oxygens resulted from the reduction of tetrahedrally coordinated Fe3+ to octahedrally coordinated Fe2+. Also, the intensity of the non-bridging oxygen 1s photoelectron peak was higher when detected on the surface of a sessile drop than when detected from the bulk of the drop.

The role of physical factors in mass transport during sintering of coal ashes and deposit deformation near the temperature of glass transformation

Abstract The role of physical properties of melts such as viscosity, diffusion, and surface/interfacial tensions in sintering and deformation mechanisms of ash deposits above glass-transformation temperature is discussed. The differential thermal analysis (DTA) technique was applied to measure glass transformation and crystallization temperatures. Sintering of selected coal ashes was performed as a function of temperature in air. The mechanical properties of sintered ashes were measured below and above the glass-transformation temperature, T g . It was found that sintering propensities of amorphous ashes and superplastic-like deformation of deposits above T g depend on mass transport phenomena in the intergranular liquid phase.

Strength development at low temperatures in coal ash deposits

At temperatures below approximately 1900°F, ash particles formed in coal-fired energy systems are relatively hard and not prone to sticking to system surfaces. However, if the ash collects on a surface not exposed to a shearing gas flow such as the downstream side of a heat exchanger or the surface of a hot-gas filter, the deposit can develop enough strength over a period of minutes to days so that it becomes difficult to remove, in some cases growing to sizes that impede the flow of gas. This paper presents data from ongoing measurements of the significance of ash and gas composition, deposit temperature, and time on the rates of strength development in simulated low-temperature ash deposits. Preliminary results of surface composition and particle-size distribution analyses of the ash, including submicron material, are also presented to explain the possible mechanisms of strength development.

Mass transport phenomena at the liquid metal/substrate (metal, carbide) interface

Abstract We have developed a model that describes surface mass transport in dissimilar materials such as liquid metal/substrate (metal, structural ceramic) and verified the model with experimental data taken from the literature and generated in our research on liquid aluminum and silicon carbide at 933 and 1253 K. Knowing the size of contact angles of drops on substrates as a function of time and temperature in a controlled atmosphere allows the calculation of apparent surface diffusion coefficients that are characteristic of the systems under consideration. Apparent surface diffusion appear to be two components related to adhesional wetting (physical effect), Ds, and reactive wetting (chemical effect), Dr, and can be expressed by the following equation: Ds +Dr = (γLV/ η) cos πeλLT= constant, where γLV and η are the surface tension and viscosity of the liquid, respectively; λ is a geometrical roughness of the solid surface; and πe is a contact angle at equilibrium.

A universal relation between diffusion, viscosity and surface tension in liquid metals in capillary-like media

There are two major driving forces that tend to produce the transport of matter in bulk materials with respect to the mean fluid motion: (a) the concentration gradient, and (b) velocity gradient. Thus, rate constants of such processes involve both diffusion (D) and viscosity ([eta]), respectively. Diffusion forces are related to the elementary displacement of atoms on an atomic scale. Viscous forces arises as a result of the rate of momentum interchange between atoms of adjacent layers. It can be assumed that both forces occur mostly between nearest-neighbor atoms. In capillary-like media, the mass transport is driven by the surface tension and the thermodynamic tendency of a liquid to minimize its surface area. Knowledge about the dynamic flow of liquids in capillary-like media is of the importance in many applications. The purpose of the work reported here is to apply the concept for simple systems such as liquid metals, and to see whether or not there is a relation between self-diffusion and viscosity in liquid metals, in capillary-like media, and to compare this relationship with that described by the Stokes-Einstein equation.

Transport properties of liquid phase in capillary-like media and its application to sintering of metallic and ceramic powders

Intergranular mass transport in materials plays an important role in successful bonding of particles, and controls the materials properties. This results from the processing conditions including the intergranular mass transport and interfacial reactions. The model of liquid mass transport of metals, molten salts, silicates, and molecular liquids, in capillary-like media is discussed. The model concentrates on the role of surface tension-to-viscosity ratio, γ/η, and volume diffusion on the liquid flow in fine pores with diameters comparable to the liquid phase above its critical thickness. We have found the following relation between two parameters: Dcap=(y/η)Lα, where α and L are a specific permeability and the mean diffusive jump length of atoms/ions/molecules, respectively. The specific permeability is related to the hydraulic permeability, taken from Darcys law, and depends on capillary radius and liquid/solid contact angle. It is demonstrated that the specific permeability depends on the interfacial reactions and heterogeneity of the system. The mass transport in liquid layers seems to be initiated by atoms with low interatomic distances (low atomic radii) in liquid metals or by the high non-bridging oxygen content in aluminosilicate melts.

Analysis of atomic diffusion in liquid metals at melting temperatures in capillary-like media

Abstract This is the second of two articles dealing with atomic transport in liquid metals in capillary-like media such as fine pores or grain boundaries with thicknesses above some critical value. Our previous studies analyzed the relationship between diffusivity and viscosity of liquid metals in capillary [J. W. Nowok, Scripta metall. mater.29, 931 (1993)]. A primary purpose of this article is to discuss an accuracy of calculating self-diffusion coefficients using known bulk properties of liquid metals such as viscosity, surface tension and fundamental physical properties of liquid behavior in capillary. The lack of accuracy of surface tension data causes errors in the calculation of diffusion coefficients. Also, the predominant influence on D appears to be the molar (molecular) volume of liquid metals. This results from changes in the attractive portion of the effective potential between atoms for liquids with low molecular volumes.

Rates and Mechanisms of Strength Development in Low-Temperature Ash Deposits

At temperatures below approximately 1900°F, ash particles formed in coal-fired energy systems are relatively hard and not prone to sticking to system surfaces. However, if the ash collects on a surface not exposed to a shearing gas flow such as the downstream side of a heat exchanger or the surface of a hot-gas filter, the deposit can develop enough strength over periods of minutes to days so that it becomes difficult to remove, in some cases growing to sizes that impede the flow of gas. This paper presents data from ongoing measurements of the significance of ash and gas composition, deposit temperature, and time on the rates of strength development in simulated low-temperature ash deposits. Preliminary results of surface composition and particle-size distribution analyses of the ash, including submicron material, are also presented to explain the possible mechanisms of strength development.

Sticking Mechanisms in Hot-Gas Filter Ashes

Large-scale hot-gas filter testing over the past 10 years has revealed numerous cases of cake buildup on filter elements that has been difficult, if not impossible, to remove. At times, the cake can bridge between candle filters, leading to filter failure. Physical factors, including particle-size distribution, particle shape, the aerodynamics of deposition, and system temperature, contribute to the difficulty in removing the cake, but chemical factors such as surface composition and gas–solid reactions also play roles in helping to bond the ash to the filter and to itself. In order to develop methods to predict the formation of sticky ash in hot-gas filtration systems, the University of North Dakota Energy & Environmental Research Center (EERC) worked with EPRI and a consortium of companies in partnership with the U.S. Department of Energy (DOE) to determine the factors causing hot-gas cleanup filters to develop deposits that can bridge the filters and cause them to fail. The primary deliverable was the Filter Bridging Index Code, a graphics-driven computer code to tie all of the knowledge together and make possible the prediction of rates of filter bridging based on coal, sorbent, filter, and system parameters. The objectives of this project were threefold: