ACERC
Abstracts - 1991 |
ACERC
Abstracts - 1991 |
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Thrust Area 2: Fuels Minerals, Fouling and Slagging |
Nowok, J.W.; Bieber, J.A.;
Benson, S.A. and Jones, M.L.
Fuel, 70:951-956, 1991. Funded by Morgantown Energy Technology Center.
The sessile drop technique has been used to determine the interfacial surface tension of Beulah, Illinois no. 6 and Pittsburgh no. 8 coal ash derived slags under various atmospheres. The physical and chemical effects that influence interfacial surface tension during heating cool slags are discussed. The results indicate that interfacial surface tension measurements obtained below the temperature of critical viscosity (Tcv) were lower than those determined above Tcv. This effect was attributed to a non-equilibrium state of surface tension due to the random distribution of surface active phases in the slag and higher polymerization of silicate structure. The interfacial surface tensions did not vary significantly with temperatures above Tcv.
Erickson, T.A.; Ludlow,
D.K. and Benson, S.A.
Energy & Fuels, 5:539-547, 1991. Funded by Pittsburgh Energy Technology
Center.
The interaction of sodium, sulfur, and silica at conditions typical in a pulverized coal furnace was investigated by using both model mixtures and a synthetic coal. The model mixtures consisted of selected inorganic constituents that were well mixed in proportions typically found in low-rank coal. The synthetic coal consisted of a furfuryl alcohol polymer with appropriate amounts of sodium, sulfur, and silica to duplicate the characteristics of low-rank coal. The model mixtures and synthetic coal were burned in a laminar flow (drop-tube) furnace at 900º, 1100º, 1300º, and 1500º C and residence times of 0.1, 0.5, 1.5, and 2.4s. The resulting char and fly ash particles were quickly quenched, collected, and analyzed with a scanning electron microscope (SEM) to determine size and composition. Results indicated that the formation of sodium silicates is favored by higher temperatures and longer residence times. Thermodynamic calculations and the model mixture studies indicated above 1100º C there is little interference in the formation of sodium silicates by sodium sulfates. In the synthetic coal studies, sodium sulfate particles were detected on the surface of the larger sodium silicate fly ash particles formed at lower temperatures. The size and prevalence of the sodium sulfate particles decreases as temperature was increased. Fly ash particle formation was characterized by fragmentation followed by coalescence. Fragmentation was more prevalent at higher temperatures and smaller fly ash particles were formed. Larger particles were formed at lower temperatures, indicating more complete coalescence with some cenosphere formation.
White, W.E.; Bartholomew,
C.H.; Hecker, W.C. and Smith, D.M.
Adsorption Science & Technology, 4:180-209, 1991. Funded by ACERC.
Multiple techniques (CO2 and N2 adsorptions, NMR spin relaxation of adsorbed water, He pycnometry, and Hg porosimetry) were combined in a comprehensive study to determine changes in surface area (CO2 and nitrogen), density (solid, particle, and bulk), and pore structure (pore size and volume distributions of micro-, meso-, and macropores) in high temperature char formation from rank-representative U.S. coals of the ANL and PETC Banks (i.e. Beulah Zap, Dietz, Utah Blind Canyon, Pittsburgh No. 8, and Pocahontas No. 3). Chars were formed at high heating rates in a flat flame burner (maximum temperature of 1473 K), a process representative of char formation in pulverized coal combustion. It was determined that most of the surface area of coals was found in micropores with radii less than 1.5 nm, while 95% or more of the pore volume in the coals (85% of that in chars) is contained in mesopores (radii > 20 nm). During high temperature formation of char in a flame: (1) CO2 surface areas (involving mainly micropores, rpore < 1.5 nm) increase 2-3 fold, while N2 surface areas, (involving mesopores, 1.5 nm < rpore < 20 nm) increase 20-200 fold, (2) solid densities increase about 25% due to graphitization, while particle densities decrease by about a factor of two due to large increases in particle porosity, (3) pore volumes are increased 5-10 fold, and (4) total porosities are increased 3-4 fold, most of this increase occurring in the macropore range. The larger surface areas and porosities of chars relative to coals may be explained by (i) the removal by pyrolysis of strongly adsorbed molecules or volatile hydrocarbons from micropores and small mesopores that would otherwise hinder access of CO2 and N2, (ii) creation of new pores during the restructuring process involved in charification, and (iii) opening up by gasification with oxygen of new pores previously blocked to gas adsorption. Preparation conditions (e.g. atmosphere, heating rate, and temperature) greatly affect the physical properties including surface area, porosity and density of the resulting chars. The degree of carbon burnout is an important correlating factor affecting these properties.
Zygarlicke, C.J.; Ramanathan,
M. and Erickson, T.A.
Engineering Foundation Conference on Inorganic Transformations and Ash Deposition
During Combustion, Palm Coast, FL, March 1991. Funded by US Department of
Energy and ACERC.
Two modeling approaches are being developed which will predict fly ash particle size and composition. Both approaches are phenomenological in that they require detailed coal input data and empirically derived knowledge of inorganic transformation phenomena that occur during coal combustion. The first approach is stochastic in construction and randomly combines initial coal inorganics depending on their distribution in the coal and outputs a predicted fly ash particle size and composition. The second approach is that of an expert system. The predicted fly ash results for Kentucky #9 bituminous coal compared fairly well with experimental fly ash using both modeling approaches.
Richards, G.H.; Harb, J.N.
and Zygarlicke, C.J.
Proc. of the Engineering Foundation Conference on Inorganic Transformations
and Ash Deposition During Combustion, (S.A. Benson, ed.), Engineering Foundation
Press, Palm Coast, FL, March 1991. Funded by ACERC.
A mathematical model of the deposition region of a laminar drop-tube furnace was used to examine the effect of variations in the size, composition, and physical properties of ash particles on deposition. Experimentally determined size and composition distributions for the fly ash of two western coals (Dietz and Utah Blind Canyon) were used as input to the model. Sticking coefficients were calculated using capture efficiencies based on particle viscosities and compared to experimental values.
Benson, S.A.; Zygarlicke,
C.J. and Jones, M.L.
Conference on the Effects of Coal Quality on Power Plants, St. Louis,
MO, 1991. Funded by ACERC and US Department of Energy.
Ash deposition in coal combustion systems is a result of the formation of low melting point phases that cause deposits to grow and develop strength. The ability of the inorganic components in coal to produce low-melting point phases depends upon the type and association of the inorganic components in the coal, and combustion conditions. The chemical and physical characteristics of western U.S. coals, typically subbituminous coal and lignite, are different from eastern U.S. bituminous coals. Specifically, many western coals contain high levels of organically associated alkali and alkaline earth elements, in addition to the major minerals such as various clays, quartz, carbonates, and sulfides. As a result, the ash species produced from lower rank coals varies dramatically from that of higher rank coals. At the present time, ash deposition resulting from lower ranked Western coals cannot be predicted effectively by using conventional ASTM methods of coal and coal ash analysis. This is due to the wide range of associations of inorganic components in coal. During combustion the ash produced varies in particle size and chemical composition. Therefore, relying on a single analysis of the bulk ash does not provide sufficient information to assess the ash behavior of the coal. The key information needed to understand and develop ways to predict and mitigate the formation of tenacious ash deposits produced from western coals is to define the pathways by which the inorganic components are transformed to produce low-melting point liquid phases. In many western coals the low-melting point liquid phases, consisting of alkali and alkaline earth sulfates and silicates, are in most instances responsible for the formation of deposits.
Bartholomew, C.H.; Gopalakrishnan,
R. and Fullwood, M.
ACS Fuel Division, 36(3):982-989, 1991 (4th Chemical Congress of North
America, New York, NY, August 1991). Funded by ACERC.
Catalysis by CaO and CaCO3 of the oxidation of a well-defined, high purity synthetic char, Spherocarb, was investigated at low reaction temperatures using thermal gravimetric analysis (TGA). Oxidation rates were likewise measured for fresh, demineralized, and Ca-impregnated samples of a high temperature char prepared in a flat-flame burner at about 1300 K from Beulah Zap coal. Spherocarb and demineralized Zap char were impregnated with Ca using aqueous impregnation and ion-exchange techniques. The resulting kinetic parameters for Spherocarb indicate significant catalytic effects--up to a 100 fold increase in reaction rate for CaCO3 and 3,000 in the case of CaO. The oxidation rates of CaO-catalyzed Spherocarb and Beulah Zap char are the same within experimental error, suggesting that the high reactivity of the Zap char is due in large part to catalysis by CaO.
Bartholomew, C.H.; Gopalakrishnan,
R. and Fullwood, M.
8th Annual International Pittsburgh Coal Conference, 1140, Pittsburgh,
PA, October 1991. Funded by ACERC.
Catalysis by CaO and CaCO3 of the oxidation of a well-defined, high purity synthetic char, Spherocarb, was investigated at low reaction temperatures using thermal gravimetric analysis (TGA). Oxidation rates were likewise measured for fresh, demineralized, and Ca-impregnated samples of a high temperature char prepared in a flat-flame burner at about 1300 K from Beulah Zap coal. Spherocarb and demineralized Zap char were impregnated with Ca using aqueous impregnation and ion-exchange techniques. The resulting kinetic parameters for Spherocarb indicate significant catalytic effects--up to a 100 fold increase in reaction rate for CaCO3 and 3,000 in the case of CaO. The oxidation rates of CaO-catalyzed Spherocarb and Beulah Zap char are the same within experimental error, suggesting that the high reactivity of the Zap char is due in large part to catalysis by CaO.
Eglinton, T.I.; McCaffrey,
M.A.; Huai, H. and Meuzelaar, H.L.C.
ACS Preprints, Division of Fuel Chemistry, 36(2):781-789 (201st ACS National
Meeting, Atlanta, GA, April 1991).
In offshore Peru high sedimentary organic carbon contents are a direct consequence of the extremely high primary productivity (ca. 1000g Carbon m²/yr) which, in turn, is supported by the upwelling of nutrient-rich waters near the coast. Diatoms represent the major phytoplankton type and give rise to sediments dominated by biogenic silica and planktonic organic matter. The remineralization of this large flux of organic matter to the bottom waters and sediments results in oxygen depletion over large areas of the shelf that, in turn, promotes organic carbon preservation in the underlying sediments. Sulfide from sulfate reduction is prevalent in the bottom waters and with a limited availability of iron (due to the dominant biogenic input coupled with a very low influx of detrital sediments) the excess sulfide is available for reaction with the organic matter. As a result high organic sulfur concentrations are found in the sediments.
The coastal Peru upwelling region is believed to be a modern analogue to the depositional environments of petroleum source rocks such as the Miocene Monterey Formation of the California Borderland. Because organic matter alteration pathways in surface sediments ultimately influence kerogen type and eventual petroleum yield, there has been interest in characterizing surface sediments such as those offshore Peru. Lipid, carotenoid and amino acid constituents as well as general biogeochemistry have been studied previously. However, studies of the macromolecular components of the sediments have been less extensive. This paper describes results from Py-MS analyses of sediment samples obtained from discrete intervals in a 1-meter core obtained from the upper continental shelf of the Peru Upwelling region. Factor and discriminant analysis of the Py-MS data revealed several distinct changes within this 1-meter section.
Weisbecker, T.; Zygarlicke,
C.J. and Jones, M.L.
Engineering Foundation Conference, Palm Coast, FL, March 1991. Funded
by NSP and Energy & Environmental Research Center.
Public and government concern for a clean environment has increased pressure on utilities to reduce emission levels from coal-powered generation. Government-instituted pollution compliance levels have been stiffened, and utilities are investigating new avenues to burn coal at or below compliance emission levels. One clear methods of meeting new emission restrictions is to burn compliance coal such as western subbituminous coals from the Powder River Basin (PRB). These coals are becoming increasingly popular to utilities because of their lower sulfur content. However, simple fuel switching is not without problems. Utility boilers, originally designed to burn higher-rank bituminous coals, can suffer significant derates from switching to lower-rank subbituminous coals or bituminous-subbituminous blends.
The Powder River Basin contains a vast amount of subbituminous coal reserves that display generally similar coal characteristics, such as ASTM ash chemistry and ash fusion temperatures. In comparison to eastern United States bituminous coals, the Powder River Basin coals have very low ash, sulfur, and iron contents and considerably higher calcium and sodium contents. Though somewhat similar in bulk as in composition, the PRB coals can have widely differing effects when burned in the same pc-fired or cyclone boiler. Differing combustion performance of PRB coals can be correlated with the distribution of inorganics in the coal. Low-rank coals are unique in that inorganics generally are found in two forms: 1) as discrete mineral phases such a quartz, kaolinite, and pyrite which are normally greater than on micron in size, and 2) as organically bound inorganics such as Na, Ca, and Mg which are in the submicron inorganic fraction of the coal because they are bonded as cations to carboxylic acid groups in the organic structure or are associated within the coal organic matrix as coordinated complexes.
The key to explain or predict the combustion performance of different PRB coals is to first gain a very complete inorganic characterization of the discrete minerals and organically bound inorganics. Methods have been developed which can now quantify both the discrete minerals and organically bound inorganics using computer-controlled scanning electron microscopy (CCSEM) and chemical fractionation, respectively. This paper correlates rigorous inorganic characterization of five Powder River Basin coals with combustion experience at the full scale. Combustion behavior noted included the general degree of fouling, the ability of the unit to maintain load, and stack opacity. An index was derived which correlates certain applicable parameters of the composition and distribution of organics in the five coals with general performance or experience at the full scale. Using a knowledge base of combustion inorganic transformations of PRB coals, an index was derived, based solely on the inorganic characterization data, which attempts to rank the coals according to their combustion performance. The index was then matched with full-scale observations of the coals' combustion performance. The information used in the knowledge base was derived from work conducted over several years at the EERC and elsewhere.
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