40 research outputs found
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Support services for ceramic fiber-ceramic matrix composites. Annual technical progress report
Ceramic and advanced alloy corrosion in fossil energy systems is being investigated. During 1995-6, ash was collected for testing corrosion resistance of materials in air-blown fluidized-bed gasification systems. Descriptions of the activities are presented in this report, which is an extension of a technical paper on testing corrosion rates of ceramics in coal gasification systems. A section of this report covers factors affecting the composition of ash deposits
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Task 6.6 - Sialon Coatings for Alkali-Resistant Silicon Nitride: Semi-annual report, July 1-December 31, 1996
The efficiency of a gas turbine can be improved by increasing operating temperature. Construction materials should meet both high strength requirements and hot-alkali corrosion resistance. Structural ceramics based on silicon nitride are promising candidates for high temperature engineering applications because of their high strength and good resistance to corrosion. Their performance varies significantly with the mechanical properties of boundary phases which, in turn, depend on their chemical composition, thickness of the amorphous phase, and the deformation process. To make silicon nitride ceramics tough, SiAlON ceramics were developed with controlled crystallization of the amorphous grain boundary phase. Crystallization of the grain boundary glass improves the high temperature mechanical properties of silicon nitride ceramics
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Thermodynamic modeling of volatile hazardous metal behavior in the Vortec Vitrification System
The thermochemical equilibrium calculations indicate that at the temperature of a propane--air flame, some volatilization of uranium, plutonium, technetium, and cesium will occur. The expected concentrations of plutonium, technetium, and cesium in the flame will be very low because of the small maximum concentration of these elements in the projected feed materials for the first 30-day test. The quantities volatilized can generally be decreased by operating the flame in a fuel-rich mode, although this will lead to greater carbon monoxide production, which may be more objectionable. The concentrations of chlorine and fluorine, at least at the maximum levels in the projected Vortec feed, are not projected to greatly influence the vaporization rates. Therefore, blending to reduce the concentrations of those elements would most likely not be effective in reducing metal vaporization. Most of the elements vaporized condense by the time the gas cools to 2000 F. These elements would condense either on surfaces near the front of the heat recuperator or on entrained particulates or homogeneously as relatively pure submicron particles. Cesium would be expected to condense at the lower temperatures near the rear of the recuperator, although the expected maximum concentration in the Vortec feed material is extremely low so it should be greatly diluted by other particulates. The elements that condense on other entrained particles will form enriched surface coatings. Particles larger than 10{micro}m or so will be collected in the scrubber. Smaller particles, especially the submicron particles formed from homogeneous nucleation, should be largely collected in the HEPA filter. Deposits formed in the heat recuperator can normally be handled via sootblowing. To reduce handling problems, we suggest that the recuperator be oriented vertically so that the deposits blown off of the heat exchanger fall directly into the molten glass. The large size of the deposits should help to reduce the rate of revaporization, allowing the volatile elements to be removed with the glass. The volatile elements that do not deposit on system surfaces will be concentrated in the smaller particles. Therefore, the HEPA ash will be greatly enriched in these elements. If the HEPA filter is itself sent to a melter, the elements may revaporize and multiply the problems related to metal vaporization significantly. Therefore, the HEPA filters should be disposed of without high-temperature processing. Also, to reduce the formation of these very small particles, it is helpful to include in the feed larger particles to act as condensation nuclei that can then be collected in the scrubber. This can be accomplished by using feed materials with a fraction consisting of particles small enough that they will not be collected in the cyclone in the melter, but large enough that they will easily be collected by the scrubber. This is one advantage that firing bituminous coal has over gas firing; it provides a source of ash particles of the right size range to serve as nucleation sites, but large enough (depending on the coal) so that they can usually be collected efficiently in the scrubber system
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Support Services for Ceramic Fiber-Ceramic Matrix Composites
The Facility for the Analysis of Chemical Thermodynamics (FACT) computer code was used to calculate the vaporization and condensation behavior of germanium (Ge) and lead (Pb) in coal gasification systems. Since condensation occurs at specific temperatures, the elements can concentrate in deposits that foul or corrode structures within an integrated gasification combined-cycle system or form very small particles that may be sticky in particle filter systems or be difficult to collect in a particulate-control cyclone. The calculations were performed in two steps: (1) vaporization from ash constitutents at 1600C at a system pressure of 22.9 atm and (2) condensation of GeX and PbX components at lower temperatures. The calculations indicate that Ge vaporizes as GeS and GeO and condenses through chemical vapor deposition as solid GeO2, Pb vaporizes primarily as PbS, with some Pb metal, and condenses as PbS as high as 880C for concentrations in the feed of 100 ppm on a mass basis. Although these concentrations would never be expected in the raw fuel, such levels could be reached if by-product dusts are recirculated into the gasifier feed material. Therefore, the calculations are useful in determining the maximum amount of recirculated material that can be allowed in the feed material to prevent formation of condensates at specific temperatures. The calculations also indicate that chlorine in the fuel has little effect on the behavior of Ge, but increases the concentration of vapor phase Pb as PbCl4 at temperatures below 800F, most significantly near 400F, at which temperature approximately 1/10 of the lead may be in the vapor phase as PbCl4. It is expected that this vapor would be collected in the system's scrubber
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Energy and environmental research emphasizing low-rank coal: Task 6.2. Joining of advanced structural materials
Silicon carbide (SiC) is considered an attractive material for structural applications in fossil energy systems because of its corrosion and wear resistance, high thermoconductivity, and high temperature strength. These same properties make it difficult to sinter or join SiC. Conventional sintering techniques require applying pressure and heating to temperatures near 2000{degree}C, or the use of binders with lower melting temperatures, or pressureless sintering with the aid of carbon and boron to near full density about 2100{degree}C. The sintering temperature can be reduced to 1850{degree}--2000{degree}C if SiC is sintered with the addition of small quantities of Al{sub 2}O{sub 3} and Al{sub 2}O{sub 3} {plus} Y{sub 2}O{sub 3}. In addition, reaction sintering has been used by mixing Si and C with SiC powder and heating the mixture to 1400{degree}C to cause the Si and C to react and form SiC, which bonds the aggregate together. Work proposed for this year was to center on determining gas compositions that could be used to increase the sinterability of oxide binders and on using the binder and gas combinations to join bars of SiC, alumina, and mullite (3Al{sub 2}O{center_dot}2SiO{sub 2}). During the course of the year the focus was shifted to SiC joining alone, because it was felt that alumina and mullite are too prone to thermal shock for use in structural applications in fossil energy systems. Because of a thermal expansion mismatch between alumina and SiC, only SiC and mullite were investigated as joining aides for SiC. Therefore, the objectives of this work evolved into examining the sintering phenomena of SiC and mullite-derived binders at and below 1500{degree}C in various atmospheres and determining which conditions are suitable to form strong joints in monolithic SiC structures to be used at temperatures of 1000{degree}--1400{degree}C
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Task 6.3 -- Engineering performance of advanced structural materials. Semi-annual report, January 1--June 30, 1995
SiC sublimes without melting at temperatures over 2,000 C. This makes SiC difficult to use in the fabrication of large structures, because pieces made from SiC cannot be joined together in the same way that metals can be welded. Therefore, the size of the monolithic ceramic structures that can be manufactured are limited by the size of the sintering furnaces (approximately 10 feet for sintered alpha silicon carbide). In order to make larger objects such as heat exchangers, many small ceramic pieces must be fused or joined. In addition, repair of the objects will require the use of field joining techniques. At present, no joining techniques for high-temperature structural ceramics are routinely available. The objective of this work at the Energy and Environmental Research Center (EERC) is to develop a patentable technique for joining large silicon based advanced ceramics in the field. The key to developing a successful technique will be the use of reactive joining compounds to lower the joining temperature but without leaving continuous channels of unreacted compounds that can weaken the joint or be conduits for corrosion at temperatures over 1,400 C. Special efforts will be made in this project to transfer the developed technologies to the materials industry via licensing agreements through the EERC Foundation
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Thermal conductivity of coal ashes and slags
Generally, heat in solids is conducted by the free electrons in metals and alloys at low temperatures, by thermal vibrations of atoms that are observed in the stoichiometric dielectrics, by the free electrons and holes as well as lattice vibrations at the sufficiently high temperatures recorded in semiconductors, and also by ions in amorphous materials at high temperatures. In our case, the linear variations of both thermal and electrical conductivities suggest also that ionization of point defects related to nonstoichiometry, impurities, and dopants plays some role in the thermal conductivity at intermediate and high temperatures. They create free carriers, such as electrons and holes, with concentrations that increase with temperature. The magnitude of this electronic component of thermal conductivity is very low, since [sigma]/k is about 10[sup [minus]6]. Also, there is reason to expect the existence of electrically charged ceramic particles in a liquid-phase sintering medium that may introduce free charges. The ionic component in heat transfer, related to the diffusion of alkali ions, does not play any major role in this range of temperature and can be neglected. This component may take place above some critical temperature, across the surface, or through the volume of the material and is strongly dependent on the glass structure. Figure 7 shows the effect of porosity on the thermal conductivity of Beulah coal ash. Thermal conductivity decreases with the increase of porosity
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Energy and environmental research emphasizing low-rank coal: Task 6.1. Corrosion of advanced structural materials
In order to increase national energy self-sufficiency for the near future, energy systems will be required to fire low-grade fuels and use more efficient energy cycles than those available today. The steam cycle used at present is limited to a maximum steam temperature of 550{degrees}C and thus a conversion efficiency of 35%. To boost efficiency significantly, much higher working fluid temperatures are required, compelling subsystems to operate at much higher temperatures and, therefore, in much more corrosive environments than those currently used. Problems of special concern are corrosion and fatigue of direct-fired turbine blades, corrosion and blinding of hot-gas cleanup filters, catastrophic failure of high-temperature heat exchangers, and spalling and dissolution of refractory materials. The extreme conditions will require the use of advanced structural materials such as high-temperature ceramics for the construction of the subsystems. Unfortunately, little is known of the performance of these materials in actual coal combustion environments. Although some corrosion testing has been performed in the past, most has been done by groups experimenting with ash or slag stimulants composed of only one or two simple compounds. For this project performed at the Energy & Environmental Research Center (EERC), actual coal ash and slag will be used in simulated combustion conditions so that more realistic determinations of the mechanisms of corrosion can be made. The work includes three main research areas focusing on two fossil energy subsystems: high-temperature heat exchangers and hot-gas cleanup filters. The first area involves developing existing abilities in thermodynamic equilibrium calculations to determine the most appropriate corroding agents to include in the tests; the second area involves coal slag corrosion of high temperature heat exchangers; and the third, lower-temperature ash and gas corrosion hot-gas cleanup filters
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Stabilization of vitrified wastes: Task 4. Topical report, October 1994--September 1995
The goal of this task was to work with private industry to refine existing vitrification processes to produce a more stable vitrified product. The initial objectives were to (1) demonstrate a waste vitrification procedure for enhanced stabilization of waste materials and (2) develop a testing protocol to understand the long-term leaching behavior of the stabilized waste form. The testing protocol was expected to be based on a leaching procedure called the synthetic groundwater leaching procedure (SGLP). This task will contribute to the US DOE`s identified technical needs in waste characterization, low-level mixed-waste processing, disposition technology, and improved waste forms. The proposed work was to proceed over 4 years in the following steps: literature surveys to aid in the selection and characterization of test mixtures for vitrification, characterization of optimized vitrified test wastes using advanced leaching protocols, and refinement and demonstration of vitrification methods leading to commercialization. For this year, literature surveys were completed, and computer modeling was performed to determine the feasibility of removing heavy metals from a waste during vitrification, thereby reducing the hazardous nature of the vitrified material and possibly producing a commercial metal concentrate. This report describes the following four subtasks: survey of vitrification technologies; survey of cleanup sites; selection and characterization of test mixtures for vitrification and crystallization; and selection of crystallization methods based on thermochemistry modeling