ACERC
Abstracts - 1992 |
ACERC
Abstracts - 1992 |
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Thrust Area 2: Fuels Minerals, Fouling and Slagging |
Bartholomew, C.H.; Baker,
T.K.; Dayburjor, D.B. and Horsley, J.S.
Catalytica Studies Division, January 1992, Funded by Catalytica.
Catalysts comprising a metallic component on a refractory support are widely used in petroleum processing, chemical synthesis, and pollution control. Supported metal catalysts are subjected to high temperatures during use or regeneration. At these high temperatures, the activity of these catalysts declines because the surface areas of the metallic component and/or the support decrease. In general, the effects of surface area loss are more difficult to overcome than the effects of carbon deposition or poisoning. The sintering processes that lead to loss of surface area involve complex physicochemical phenomena. An understanding of the mechanism of sintering is important in developing new catalysts and in regenerating deactivated catalysts, and considerable research is being devoted to understanding the mechanisms of sintering and to reversing the effects of sintering. This study analyzes the causes and mechanisms of sintering, critically reviews the relevant scientific and patent literature, and recommends ways in which sintering can be minimized and deactivated catalysts can be regenerated.
Harb, J.N. and Yu, H.
Proceedings of the Ninth Annual International Pittsburgh Coal Conference,
235-240, Pittsburgh, PA, October 1992. Funded by ACERC.
Ash formed during combustion of pulverized coal produces a variety of operational and environmental problems. Recently, considerable effort has been expended toward the development of techniques and models to predict ash transformations and deposition during combustion. These models require a description of the coal mineral matter as input. It is well known that bulk ASTM analysis of the coal ash does not adequately represent observed deposition behavior. This fact has lead to the development of advanced analytical techniques to provide detailed information on coal mineral size composition, and association with the organic matrix.
The principal technique used for detailed analysis has been computer controlled scanning electron microscopy (CCSEM). Early development of this technique was performed by Huggins et al., who used a scanning electron microscope equipped with a Tracor Northern 2200 X-ray analysis system to examine coal minerals. Extensive development and refinement of the technique for both coal minerals and ash has been done at the Energy and Environmental Research Center at the University of North Dakota on several different Tracor Northern systems. Additional work on the association of the coal minerals with the organic matrix has been performed by Straszheim and co-workers at Iowa State University with use of a sophisticated image analysis system (LeMont Scientific DB-10). In spite of the significant advances that have been made, CCSEM analysis has not yet been standardized and questions persist as to the statistical significance of the data, the minimum number of particles required for the analysis, the quantitative nature of the composition data and the role of ZAF corrections, the influence of sample preparation on the results, etc.
The objectives of the present study were to: 1) implement the CCSEM technique on a JOEL 840A scanning electron microscope equipped with a Link eXL X-ray microanalysis and image analysis system, 2) verify the reproducibility and accuracy of the technique for quantitative analysis of the coal minerals, 3) develop the software routines necessary to determine the mineral content and composition of coal particles on a particle-by-particle basis, and 4) use the above techniques to examine two eastern coals recently tested in a utility boiler.
Bartholomew, C.H.; Neubauer,
L.R. and Smith, P.A.
Tenth International Catalysis Congress, Budapest, Hungary, January 1992.
Funded by US Department of Energy/Basic Energy Services and Brigham Young University.
Mössbauer absorption spectroscopy (MAS) and Mössbauer emissions spectroscopy (MES) studies of 1-3% 57Fe and 1% Co-57 on carbon and alumina supports were conducted as a function of a reduction temperature. Catalysts were prepared by nonaqueous evaporative deposition to maximize the reduction of cobalt and iron to the metal. Metal surface areas of the catalysts were determined by H2 adsorption, while extents of reduction to the metal were determined by both Mössbauer spectroscopy and by titration of reduced catalysts with oxygen at 673 K. MAS/MES data for 1 and 3% Fe-57/C and 1% Co-57/C catalysts reduced at 773 K indicate the presence of only one phase-superparamgnetic (SP) clusters of metal having diameters of about 1-2 nm. Room temperature isomer shifts for these carbon supported metal clusters of 0.10-0.14 mm/s indicate a decrease in electron density of the metal nuclei relative to the bulk metals. MES data for Co-57/Al2O3 suggest the existence of three phases: Cosp metal, Co(II) oxide, and Co(III) oxide, while MAS generally shows only the Fesp metal clusters and Fe(III) oxides to be present in 1-2% Fe-57/Al2O3, except for some ferromagnetic Fe metal in 2% 57Fe/Al2O3 reduced at 873 K. Isomer shifts for the metal clusters in the Al2O3-supported Co-57 and Fe-57 catalysts are -0.05 to -0.15 mm/s indicating an increase in the electron density at metal nuclei. The presence of small metals cluster of 1-5 nm in these catalysts is confirmed by H2 absorption. Moreover, Debye temperatures measured by Mössbauer are significantly lower than for bulk iron consistent with the lattice dynamics expected for small metal clusters having a large fraction of surface atoms. The very significant isomer shifts observed for SP metal phases by Mössbauer are consistent with electronic modification of small metal clusters in supported Co or Fe. That the isomer shift is positive for metal/C catalysts and negative for metal/Al2O3 catalysts indicates this effect must be due to metal-support interactions.
Benson, S.A.; Zygarlicke,
C.J. and McCollor, D.P.
Eighth Annual Coal Preparation, Utilization, and Environmental Control Contractors
Conference, Pittsburgh, PA, July 1992. Funded by US Department of Energy
and Morgantown Energy Technology Center.
The overall objective of this project is to develop a unified picture of the physical and chemical changes that occur in coal inorganic matter during combustion. Information obtained from studying the mechanisms of inorganic transformations will be used to predict the size and composition of ash particles based on coal composition and combustion conditions.
Benson, S.A.; Erickson,
T.A.; Hurley, J.P.; Zygarlicke, C.J. and Steadman, E.N.
Electric Power Research Institute Conference on Coal Quality, San Diego,
CA, August 1992, Funded by US Department of Energy, ABB-Combustion Engineering,
Electric Power Research Institute and ACERC.
The development of effective methods to predict ash behavior in utility boilers requires detailed information on ash-forming constituents in the coal, ash formation mechanisms, and behavior of the ash species in combustion systems. This paper described the application of advanced methods to characterize coal and provides examples of two methods to predict ash behavior. The advanced methods of coal analysis provide quantitative information on the size, association, and abundance of ash-forming species in the coal. The methodologies include computer-controlled scanning electron microscopy (CCSEM) and chemical fractionation. The CCSEM technique determines the size, abundance, and composition of 2000 to 3000 mineral grains in coal. The chemical fractionation technique is used to determine the abundance of organically associated inorganic components in lignite and subbituminous coals. The information obtained from the advanced methods of analyses is used as input into computer codes to predict ash behavior. The two methods to predict ash behavior include a fouling index and a phenomenological/mechanistic model. The fouling index provides the ability to rank coals based on their potential to produce convective pass deposits and was developed using data obtained from advanced methods of analyses, a knowledge base of ash behavior, and full-scale utility boiler operational data. The phenomenological or mechanistic models are used to predict the particle-size and composition distribution of ash and deposition potential of the ash as a function of boiler geometry and operational conditions. These predictive techniques have been developed through the use of full-scale utility boiler experience and have been verified for selected systems; however, these techniques are limited to certain types of coals and to certain regions of the boiler.
Zygarlicke, C.J.; Benson,
S.A.; Borio, R.W. and Mehta, A.K.
Electric Power Research Institute Conference on Coal Quality, San Diego,
CA, August 1992. Funded by US Department of Energy, Electric Power Research
Institute and ABB-Combustion Engineering.
Ash deposition during pulverized coal combustion was studied in full-, pilot-, and bench-scale systems. Ash deposits were produced from two high-volatile bituminous coals at 1) the Mississippi Power Company Watson Unit 4 utility boiler, 2) the ABB-Combustion Engineering Fireside Performance Test Facility (FPTF), and 3) The Energy & Environmental Research Center (EERC) optical access drop-tube furnace. The chemical and physical characteristics of the slagging and fouling deposits produced in the three systems were examined to determine the components that were responsible for deposit growth and strength development. Similar mineral and amorphous phases, elemental chemistries, and liquid-phase viscosities were observed for full-and bench-scale deposits generate under similar combustion conditions. Although the two test coals were very similar in composition, they performed differently, as evidenced in all three combustion regimes. The "Baseline" coal, a bituminous coal, which is also the baseline coal diet at Watson Unit 4, produced less severe slagging and fouling deposits than the alternate coal which was the "Alternate" a bituminous coal. All three combustion regimes produced data such as quantity and composition of low-viscosity silicate liquid phases, deposit adhesion or bonding strength, deposit crushing strength, and heat flux/heat transfer data which supported the conclusion that the Alternate deposits were more difficult to remove and caused greater impedance to heat transfer.
Zygarlicke, C.J.; McCollor,
D.P.; Benson, S.A. and Holm, P.L.
Twenty-Fourth Symposium (International) on Combustion, The Combustion
Institute, Sydney, Australia, July 1992. Funded by US Department of Energy,
Energy & Environmental Research Center and ACERC.
Synthetic coal model mixtures were used to determine the chemical and physical transformation mechanisms involved in the evolution of fly ash during combustion. Two calcium, silica, and sulfur synthetic coal systems were prepared: on system containing calcium as 10-µm calcite (Ca[min.]-Si-S), and the other containing calcium as ionically dispersed calcium acetate (Ca[org.]-Si-S). A third system consisted of sodium, silica, and sulfur with the sodium associated as sodium benzoate (Na[org.]-Si-S). Silica, in all three systems, consisted of a furfuyl alsohol/p-toluensulfonic polymer. The synthetic coal mixtures, each sized to 38-106 µm, were combusted in a bench-scale drop-tube furnace at gas temperatures of 900º, 1100º, 1300º, and 1500ºC and residence times of approximately two seconds. Fly ash produced from the Ca(min.)-Si-S mixture revealed significant interaction between the calcite and quartz at higher temperature, as evidenced by increases in particle size and in the levels of amorphous calcium silicate with increasing temperature. Temperatures were high enough to decompose calcite to calcium hydroxide and calcium oxide, which in turn reacted with sulfur o quartz. During the combustion of the Ca(org.)-Si-S mixture, the organically associated calcium reacted primarily with the surface of quartz grains at all temperatures. For both Ca-Si-S systems, calcium reacted with sulfur to form anhydrite at temperatures at or lower than 1300ºC. The Na(org.)-Si-S system revealed extensive melting, mineral particle coalescence, and formation of sodium sulfate at 900ºC; however, at 1500ºC, the fly ash contained only minuscule amounts of sodium or sodium-bearing phases and showed evidence for either char fragmentation or noncoalescence due to the absence of sodium sulfates and silicates.
Yu, H.
Characterization of Mineral Particles in Coal by Computer Controlled SEM-EDS,
M.S./BYU, December 1992. Advisor: Harb
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