ACERC
Abstracts - 1989 |
ACERC
Abstracts - 1989 |
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Thrust Area 2: Fuels Minerals, Fouling and Slagging |
Ramanathan, M.; Kalmanovitch,
D.P. and Ness, S.
To be published in Prog. Energy Comb. Sci., Special Issue on Ash Deposition,
Pergamon Press, 1989. Funded by ACERC (National Science Foundation and Associates
and Affiliates) and US Department of Energy.
PHOEBE is a new computer code developed at UND-EMRC as part of a long-term study of coal ash deposition phenomena in coal combustion systems. The task of developing a new code was undertaken to eliminate the various problems encountered with existing phase equilibria calculation packages and to apply better techniques in the minimization of the Gibbs free energy of the system. The results from PHOEBE for a couple of thermodynamic systems are presented and compared with their corresponding experimental values.
Kalmanovitch, D.P.; Zygarlicke,
C.J.; Steadman, E.N. and Benson, S.A.
To be published in Prog. Energy Comb. Sci., Special Issue on Ash Deposition,
Pergamon Press, 1989. Funded by ACERC (National Science Foundation and Associates
and Affiliates) and US Department of Energy.
Deposits formed during coal combustion in the UND EERC drop-tube furnace have been characterized. Pulverized lignite from Beulah, North Dakota, was combusted under carefully controlled conditions with gas temperatures ranging from 1100ºC to 1200ºC and deposit residence times ranging from 1 to 20 minutes. The deposits were characterized using X-ray diffraction and scanning electron microscopy techniques. The SEM techniques included morphological examination, back-scattered electron imaging, and a technique called scanning electron microscopy point count (SEMPC). SEMPC uses automated microprobe techniques to identify and quantify crystalline and amorphous phases present in a selected region of a deposit cross section. The SEMPC data was compared to the original mineralogical composition of the Beulah lignite, which was determined by a technique called computer controlled scanning electron microscopy (CCSEM). Comparisons of the deposit characteristics with coal mineralogy and fly ash characteristics led to insights into fundamental processes of ash deposit formation and growth.
Zygarlicke, C.J.; Steadman,
E.N.; Benson, S.A. and Kalmanovitch, D.P.
To be published in Prog. Energy Comb. Sci., Special Issue on Ash Deposition,
Pergamon Press, 1989. (Also ACS Div. Fuel Chem. Preprints of Papers,,
34 (2), 355-366). Funded by ACERC (National Science Foundation and Associates
and Affiliates) and US Department of Energy.
The mechanisms of coal ash formation were studied in relation to coal composition and combustion conditions. Monticello lignite, from Titus County, Texas, was analyzed to determine both mineralogical and organically bound components using computer controlled scanning electron microscopy (CCSEM) and chemical fractionation techniques, respectively. The coal was combusted in the EMRC drop-tube furnace at 1500ºC. Fly ash was collected and aerodynamically size-segregated into six stages. Scanning electron microscopy point count (SEMPC) was used to ascertain the bulk and surface chemistry of each fly ash size fraction. Fly ash composition and size distribution correlated well with the distribution of inorganics in the coal. Less than 3% of the coal mineral phases contained significant amount of Fe. Roughly 50% of the elemental iron was associated with acid-insoluble minerals. The remaining Fe was distributed in the finer fraction of the coal as organically bound species or soluble minerals. The smallest size fraction (<1.2 microns) had 26% Fe2O3 in bulk composition with less than 2% of the crystalline phases containing Fe.
Kalmanovitch, D.P. and Benson,
S.A.
Proceedings of the Sixth Annual International Pittsburgh Coal Conference,
2, 939-948, Pittsburgh, Pennsylvania, 1989. Funded by ACERC (National Science
Foundation and Associates and Affiliates) and US Department of Energy.
A drop-tube furnace system was used to evaluate the ash deposition propensity of Illinois #6 bituminous coal. The coal, size classified fly ash, and deposits were characterized using automated scanning electron microscope/microprobe techniques. The information derived from the SEM techniques consists of quantitative determination of the size, composition, and abundance of minerals in the coal and a distribution of phases in the fly ash and deposits. Based on the analysis of the ash using the SEM techniques, important parameters can be determined that are related to the ability of ash to form a deposit. These include viscosity distribution profiles that provide insight into sintering (strength development) behavior and base-to-acid ratios for each ash particle. The combustion conditions of the drop-tube furnace were adjusted to simulate the time-temperature profiles of combustors. Ash was collected using a multicyclone to provide size classified ashes for detailed analysis. Results indicate the majority of quartz and pyrite derived particles were greater than 11 micrometers. The smaller particles consisted mainly of amorphous materials. A deposit was formed under conditions that simulate fouling in the drop-tube furnace and analyzed using the SEM. Results indicate that the initial mechanism of deposition was by ash particles greater than 11 micrometers. These particles have liquid phases present that do not facilitate sintering processes either via low viscosity phases or via liquid reaction mechanisms. These particles have liquid phases present that do not facilitate sintering processes either via low viscosity phases or via liquid reaction mechanisms. The bulk of the deposit was shown to have been formed by preferential deposition of particles with liquid phases of low viscosity (between log 2 and log 4 poise) and high relative reactivity (based on base/acid distribution).
Bartholomew, C.H.; White,
W.E. ; Hecker, W.C.; Smith, D.M.
4th Annual Meeting of the Western States Catalysis Club, Denver, Colorado
, 1989. (Also presented at the Western States Section,The Combustion Institute,
Livermore, California, 1989). Funded by ACERC (National Science Foundation and
Associates and Affiliates).
Surface areas, pore volumes, and pore size distributions of five Argonne National Lab (ANL) coals (Pittsburgh No. 8, Illinois No. 6, Pocahontas No. 3, Beulah Zap lignite, and Utah Blind Canyon) and of two PETC coals (Lower Wilcox and Dietz) and chars derived from these coals area being measured in an ongoing study. Data obtained for several of these coals and chars will be reported. Surface areas, pore volumes and pore size distributions were measured by nitrogen and carbon dioxide adsorptions at 77 K and 195-300 K respectively; pore volumes and pore size distributions were also determined by NMR spin-relaxation measurements of samples saturated with water. Comparisons of accuracy and precision for static vacuum and flow desorption methods were made. Surface areas and pore volumes measured by adsorption in a static vacuum system and by desorption in a flow/TC detector system agree to better than 1-5% where adsorption equilibrium has obtained.
The results provide new insights into the surface and pore structure of coals and chars as functions of rank and charification. Surface areas of coals generally increase with decreasing rank. Chars have larger surface areas and pore volumes than the parent coals; indeed surface areas measured by nitrogen adsorption are up to two orders of magnitude larger, while those measured by carbon dioxide adsorption are 2-3 times larger. Pore volumes of chars measured by nitrogen adsorption are 10-20 time those of the parent coals. Large fractions of the internal surfaces of coals and pore diameters are microporous (pore diameters of 1 nm or smaller) and are not easily penetrated by nitrogen molecules at 77 K. In the case of some coals, while the pore volume increases during devolatilization, the shape of the pore size distribution stays the same. For other coals, the pore size distribution changes radically during devolatilization. This systematic study of surface areas and pore structures of coals and chars provides insights into physical changes that occur during coal devolatilization and char burnout. This information can be useful in characterizing the evolution of pore structure and its effect on diffusion of reactant in and products out during combustion of coal chars.
Zygarlicke, C.J.; Jones,
M.L.; Steadman, E.N. and Benson, S.A.
Advanced Combustion Engineering Research Center Report, 1989. Funded
by ACERC.
To assist with the overall goals of ACERC, especially Thrust Area 2, mineral behavior, a detailed characterization of the inorganic material in eight ACERC coals was completed. These analyses are essential to understanding the role inorganic components play in the behavior of coal in combustion systems. Of particular interest are the amount, size, and type of inorganic constituents found in coal. Without this information, it is not possible to predict the behavior of various coals in combustion systems. With this information, a coal's mineral behavior can be predicted (its fouling and slagging tendencies), as well as the impact of its inorganic components on the combustion process. The analysis of the ACERC coals included the following: 1) computer controlled SEM - to determine the size, quantity, juxtaposition, and association of mineral phases; 2) chemical fractionation - to determine organically bound inorganics (used with low-rank coals only); and 3) XRFA, XRD - used to verify inorganic constituents found with techniques noted above.
Benson, S.A.; Kalmanovitch,
D.P.; Zygarlicke, C.J. and Steadman, E.N.
Presented at the Fifteenth Biennial Low-Rank Fuels Symposium, Sponsored
by the University of North Dakota and the US Department of Energy, St. Paul,
Minnesota, 1989. Funded by ACERC (National Science Foundation and Associates
and Affiliates) and US Department of Energy.
The mechanisms of formation of troublesome fireside deposits in utility boilers as a result of combusting coals from the Powder River Basin and Fort Union Region are being examined in detail at the Energy and Environmental Research Center. Fireside ash fouling resulting from the combustion of Fort Union lignites has been correlated with the formation of sodium-rich liquid phases in deposits. As a result of combusting some Powder River Basin coals, calcium sulfate rich deposits form in the convective passes of boilers causing serious fouling problems. Detailed characterization of fouling deposits from utility boilers firing Powder River Basin and Fort Union coals has shown that complex processes. Scanning electron microscope/microprobe analyses have shown sodium- or calcium-rich phases are the materials that bind the ash particles together resulting in formation of deposits. In some cases the problem-causing phase is sulfate-based and in others the phase is aluminosilicate-based. The process of consolidation to form a strong-bonded deposit depends on the type of phase present. In the case of the sulfate-based deposits the mechanism appears to be low-temperature sintering in the absence of an extensive liquid phase. In the case of the aluminosilicate-based deposits the consolidation process is that of viscous flow sintering of a reactive liquid phase. In both cases, the abundance and association of alkali and alkaline earth elements in the coal, and their transformations during combustion and deposition, will control the extent of formation of troublesome deposits in a boiler.
Jones, M.L.; Kalmanovitch,
D.P.; Steadman, E.N.; Zygarlicke, C.J. and Benson, S.A.
Presented at the First Symposium on Advances in Coal Spectroscopy, Salt
Lake City, Utah, 1989. Funded by ACERC (National Science Foundation and Associates
and Affiliates) and US Department of Energy.
Techniques developed at Energy & Environmental Research Cetner (EERC) for the characterization of coal and coal ash by scanning electron microscopy (SEM) and electron microprobe analysis are detailed along with the application of these techniques. A description of the overall EERC approach to understanding coal ash formation and behavior in combustion systems and commentary on the significance of inorganic components in combustion systems are also presented.
The EERC approach to understanding inorganic components in combustion systems involves the detailed characterization of each component in the process by SEM techniques. Characterization of the minerals present in coal and in intermediate components (such as chars and fly ashed) is accomplished using a technique called CCSEM. The CCSEM technique uses a backscattered electron image to locate and determine the size, shape, and association of the minerals. A computer program then collects an energy dispersive spectra (EDS) from each particle in order to classify it according to mineral type. Since the CCSEM program is automated, data is collected from a large number of particles allowing for a statistical quantification of the minerals present.
The SEMPC technique is used to quantify the various phases in fly ashes and deposits. An automated program is used to raster the electron beam across the sample and an EDS analysis is collected from selected intervals until about 250 spectra are collected. These spectra are then subjected to the ZAF correction procedure that converts the data into chemical composition. The composition of each point is then classified into various crystalline and amorphous phases according to weight and molar ratios. In this manner, the overall chemistry of the sample is determined along with the quantities of the individual phases present. Thus, the SEM is used for the detailed characterization of mineral grains in the coal and materials produced from the various stages of coal combustion, intermediate formation, and finally, fly ash and deposit formation. This detailed characterization provides the data needed to elucidate the mechanisms of ash transformations during combustion and deposition.
CCSEM analysis of a North Dakota lignite revealed major amounts of quartz and aluminosilicate and minor amounts of pyrite, barite, gypsum and Ca-clay. CCSEM analysis of partially burned coal showed in increase in Ca-aluminosilicates indicating a reaction between the organically associated Ca and the clay minerals. SEMPC analysis of the fly ash corroborated this reaction, as it indicated the presence of mililite and anorthite in the larger sized fractions. Calcium silicate, anorthite, and melilite were also observed in the base of a deposit by the SEMPC technique. Predictions concerning the relative reactivity (base/acid ratio distribution) and physical properties (viscosity distribution) of the deposit were also made using the SEMPC technique.
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