ACERC
Abstracts - 1990 |
ACERC
Abstracts - 1990 |
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Thrust Area 2: Fuels Minerals, Fouling and Slagging |
Harb, J.N. and Smith, E.E.
Progress in Energy and Combustion Science, 16, 169-190, (1990). Funded
by ACERC.
This review has examined fireside corrosion of pc-fired boilers in both the waterwall and superheater regions. The present understanding of corrosion phenomena has resulted in the development of strategies to control tube wastage. The corrosion problem persists, however, in spite of efforts to control it. The physical mechanisms that govern such corrosion are complex and not fully understood to date. The problem is complicated further by localized attack in the form of pits of cracks that may result in tube failure without a significant decrease in the average tube-wall thickness. Mechanistic models that allow quantitative prediction of local corrosion rates from first principles have not yet been developed. Hence, quantitative prediction of fireside corrosion rates is not feasible at the present time.
Mathematical models, however, may play an important role in the a priori prediction of boiler locations where corrosion problems are likely to develop, and the operating conditions under which corrosion is expected to occur. A combustion model could be used to simulate the environment inside a utility boiler for a variety of fuels and operating conditions. Once the conditions inside the boiler have been modeled, the corrosion behavior expected at a given location could be determined by comparing the local boiler environment with corrosion data obtained experimentally under similar conditions. Corrosion data would be accumulated from experience with industrial boilers and from well-defined laboratory experiments. Such a procedure could provide a valuable tool for use in boiler design and in the prediction of problems associated with a changing fuel supply.
Nowok, J.W.; Benson, S.A.;
Jones, M.L. and Kalmanovitch, D.P.
Fuel, 1990 (In press). Funded by US Department of Energy.
Sintering, crystallization, and mechanical properties of six coal ashes are being studied to obtain an understanding of ash deposit and agglomerate formation in combustion and gasification systems. Results show that viscous sintering is accomplished by neck growth between particles, shrinkage of closed pores, and/or by diffusion of slag constituents in a liquid film along the grain surfaces. These processes are dependent on temperature, atmosphere, and ash composition. In addition, studies show that the strength of sintered pellets produced from crystallized Beulah lignite ash are weaker than those produced from the amorphous form of the ash. Further, the amorphous form sintered in a CO/CO2 atmosphere is not as strong as the amorphous form sintered in air. Criteria are given for the relationship between compressive strength of sintered coal fly ashes and their activity in the sintering process measured by means of viscosity and surface tension.
White, W.E.; Bartholomew,
C.H.; Hecker, W.C. and Smith, D.M.
Adsorption Science & Technology, 1990 (In press). 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. and Steadman,
E.N.
Scanning Microse. Int., 4:579-590, 1990. Funded by US Department of Energy
and ACERC.
Research at the University of North Dakota Energy and Environmental Research Center (EER) has focused on methods to characterize the inorganic components in coals. Because the scanning electron microscope and electron probe microanalysis system (SEM/EPMA) provide both morphologic and chemical information, the SEM/EPMA system is well suited to the characterization of discrete minerals in coal. Computer-controlled scanning electron microscopy (CCSEM), along with simultaneous automated digital image collection, is one means of gaining more detailed insight into coal mineralogy. Computer-stored images of coal surfaces already analyzed for minerals using CCSEM can be reanalyzed to discern mineral morphologies and coal-to-mineral associations. Limitations may exist when using just CCSEM to characterize chemically and physically complex clay minerals without complimentary data on the association of the minerals to the coal organic matrix. Mineralogical investigations of San Miguel and Beulah lignites and Upper Freeport bituminous coal using CCSEM and automated digital image collection are given with a particular reference to the clay minerals present. Total mineral quantities generated for the three coals were in good agreement with total ash content, provided that organically bound constituents were taken into account for the lignites. Classification of the more complex aluminosilicate minerals was aided by the use of distribution plots of Si/A1 ratios and concentrations of ion exchangeable cations derived from the CCSEM analysis. Morphologic analysis of stored SEM images proved to be helpful in characterizing kaolinite group minerals.
Bae, I.; Maswadeh, W.; Yun,
Y.; Meuzelaar, H.L.C. and DuBow, J.
Accepted for publication in the ACS National Meeting Preprints, Boston,
Massechusetts,Spring, 1990. Funded by ACERC (National Science Foundation and
Associates and Affiliates).
Coal pyrolysis is a fundamental first step in combustion processes. Yet coals exhibit a wide variation in pyrolysis behaviors. The origins of these wide variations are, for a given set of experimental conditions, both structural and compositional in nature. Because of its thermochemical and catalytic properties, mineral matter plays an important role in both the thermodynamics (product mixes, activation energies) and kinetics of coal pyrolysis. The issue is further complicated by the manner in which mineral matter is distributed in various coals. While many classifications are possible, grouping into three classics is most common. These classes are: (1) discrete minerals such as clays, oxides (basic and acidic) and sulfides; (2) organometallic matter such as ion-exchangeable cations; and (3) dispersed trace elements and compounds. A considerable body of research exists for studying equilibrium and non-equilibrium effects of the various forms of coal minerals on coal combustion.
In the present paper the mineral matter effects on coal pyrolysis are being analyzed using an approach whereby observable spectroscopic (TG/MS) differences in the pyrolytic decomposition between fresh coal and demineralized coal are reconstructed from the sum of mineral matter effects on pyrolysis arising from adding back, singly and in pairs, individual minerals in various forms.
Harb, J.N.; Richards, G.H.
and Munson, C.L.
Proceedings of the ASME Ash Deposit and Corrosion Research Committee Seminar
on Fireside Fouling Problems, Brigham Young University, Provo, UT, 1990.
Funded by ACERC.
A mathematical model was developed to investigate particle deposition in a laminar drop-tube furnace. Specifically, simulations were performed to examine the effects of geometry, transport, plate temperature, and particle composition on deposition. Because of the geometry of the deposition region, the diameter of the inlet particle stream was narrowed and particle impaction rates were significantly enhanced near the center of the plate. The location of particle impact and the temperature of the particle upon impaction were both strongly dependent on particle size. The particle temperature at impaction was relatively insensitive to the plate temperature for particles greater than 15 to 20 µm in diameter at plate temperatures of 750K and 1400K. Calculation of composition effects indicated that particles of different sizes with similar compositions might exhibit significantly different sticking behavior owing to the formation of liquid phases.
Benson, S.A.; Zygarlicke,
C.J.; Toman, D.L. and Jones, M.L.
Proceedings of Seminar on Fireside Fouling Problems; ASME Research Committee
on Corrosion and Deposits from Combustion Gases, Washington, D.C., 1990.
Funded by US Department of Energy.
The transformations of inorganic constituents during pulverized coal combustion and ash deposition behavior have been examined in detail. A laboratory-scale laminar flow drop-tube furnace system equipped with intermediate ash collection and ash deposition devices was used to combustion pulverized Dietz subbituminous and Utah Blind Canyon bituminous coal. Intermediate ash and ash deposits were produced. The coal, intermediate ash, and ash deposits were analyzed using advanced methods including automated scanning electron microscopy and microprobe (SEM) analysis and chemical fractionation.
Chemical fractionation was used to determine the abundance of organically associated elements, and an automated SEM technique was used to determine the size, abundance, and composition of the discrete minerals in the test coals. High levels of organically associated alkali earth elements were observed in the Dietz coal. The Utah Blind Canyon had a lower level of organically associated elements and a higher level of minerals. The excluded minerals for the Dietz and Blind Canyon were determined to be 62.1 and 71.6 percent of the minerals, respectively.
The particle-size distribution of the minerals in the coal was compared to the size distribution of the fly ash produced. The results indicate that coalescence of the minerals occurred during combustion. The interaction of inorganic components during the combustion processes was significant as indicated by the types of phases that were identified in the various sized fractions of ash. The reaction of alkali and alkaline earth elements (likely organically associated) was significant for Dietz, as indicated by the formation of gehlenite (Ca2Al2SiO7).
The deposits produced from the Dietz and Blind Canyon coals exhibited quite different characteristics. Dietz produced the strongest deposit, but had a lower sticking coefficient when compared to the Blind Canyon deposit. The viscosity distribution profiles indicated that the liquid phase viscosity for the Dietz was much lower than that obtained for the Blind Canyon. The development of deposit strength is inversely proportional to the liquid phase viscosity. The low viscosity of the liquid phase present in the deposit is the reason for the high strength exhibited by the Dietz deposit.
Zygarlicke, C.J.; Toman,
D.L. and Benson, S.A.
Proceedings of the National Meeting of the American Chemical Society Division
of Fuel Chemistry, 35 (3), Washington, DC, 1990. Funded by US Department
of Energy.
Processes governing the evolution of the intermediate ash (inorganic gases, liquids, and solids) during pulverized coal combustion were examined in detail by combusting carefully sized fractions of Beulah lignite and Upper Freeport bituminous coals in a laminar flow drop-tube furnace. Char (partially combusted coal) and fly ash produced at various temperatures and residence times were analyzed using advanced scanning electron microprobe techniques. Fly ash was collected and sized in multicyclone and impactor devices. Work was focused on determining the relationship between the sizes of the original coal and coal minerals and the size of the resulting fly ash. Time-resolved size distributions of inorganic phases associated with chars show that Beulah and Upper Freeport phases exhibit some coalescence of inorganic phases with time. The Upper Freeport shows an initial increase in the amount of particles in the lower size ranges possibly due to fragmentation of minerals or the formation of smaller inorganic ash droplets from submicron minerals or organically associated inorganic constituents. The level of ash and coal minerals in size ranges greater than 3 microns is nearly equal for Upper Freeport, possibly indicating the influence of fragmentation. Size distributions of both the Upper Freeport coal minerals and resulting fly ash were larger than similar distributions for the Beulah. Both coals gave slightly smaller fly ash sizes for higher gas temperatures. In support of this observation, calculations revealed that both coals produced more fly ash particles per coal particle for higher combustion temperatures. The mechanism of fly ash formation for the Beulah was the result of partial coalescence of minerals and organically bound constituents. Upper Freeport ash revealed coalescence for the smaller (<3.0 µm) minerals. Using different coal sized fractions and the same gas temperature of 1500ºC, larger fly ash particle size distributions were observed for the smaller-sized coal fractions.
Bartholomew, C.H.; Gopalakrishnan,
R. and Fullwood, M.
Proc. ASME Seminar on Fouling of Western Coals, Brigham Young University,
Provo, UT, 1990. (Also presented at the National AlChE Meeting, Chicago,
1990). Funded by ACERC.
Catalysis by CaO of the oxidation of a well-defined, high purity synthetic char, Spherocarb, was investigated at low reaction temperatures using thermal gravimetric analysis (TGA). Spherocarb was impregnated with Ca using aqueous impregnation and ion-exchange techniques. The resulting kinetic parameters indicate a significant catalytic effect--10 to 100-fold increases in reaction rate. CO adsorption on CaO prepared by Ca(OH)2 decomposition was investigated using temperature-programmed desorption (TPD) of CO adsorbed at 298 K. Several high temperature peaks were observed consistent with heats of adsorption of 40-115 kJ/mal. These relatively large heats of adsorption are indicative of the presence of different, strongly adsorbed CO species on CaO and have significant implications for the catalysis of carbon oxidation and of CO oxidation to CO2 during char combustion. Experiments involving temperature-programmed reaction of hydrogen with adsorbed CO also indicate by the formation of methane that CO may adsorb dissociatively or at least dissociates in the presence of hydrogen to form methane.
Steadman, E.N.; Zygarlicke,
C.J.; Benson, S.A. and Jones, M.L.
Proceedings of Seminar on Fireside Fouling Problems, ASME, Washington,
DC, 1990. Funded by US Department of Energy and ACERC.
Techniques have been developed that allow for the detailed chemical characterization of coal inorganics, ash, and deposited ash material. This paper focuses on microanalytical techniques that utilize an automated scanning electron microscope and electron microprobe (SEM/EMPA). This combination provides both chemical as well as morphological information, making it a valuable tool in studies of inorganic components associated with coal combustion.
Two techniques are discussed. Computer-controlled scanning electron microscopy (CCSEM) is a technique used to determine size, shape, and semiquantitative composition of mineral grains in coal or char/ash intermediates. Scanning electron microscopy point count (SEMPC) is used to quantify the various phases in fly ashes and deposits. This combination of techniques provides a valuable set of tools to follow the fate of coal inorganic materials in coal conversion processes. The techniques discussed are still in the development stage. Plans for future activities to improve both techniques are discussed.
Bae, I.; Anani, M.; Maswadeh,
W.; Yun, Y.; Meuzelaar, H.L.C. and DuBow, J.
199th ACS National Meeting, 35 (2), 489-493, Boston, Massachusetts, 1990.
Funded by ACERC.
Coal pyrolysis is a fundamental first step in combustion processes. Yet coals exhibit a wide variation in pyrolysis behaviors. The origins of these wide variations are for a given set of experimental conditions, both structural and compositional in nature. Because of its thermochemical and catalytic properties, mineral matter plays an important role in both the thermodynamics (product mixes, activation energies) and kinetics of coal pyrolysis. The issue is further complicated by the manner in which mineral matter is distributed in various coals. While many classifications are possible, grouping into three classes is most common. These classes are: (1) discrete minerals such as clays, oxides (basic and acidic) and sulfides; (2) organometallic matter such as ion-exchangeable cations; and (3) dispersed trace elements and compounds. A considerable body of research exists for studying equilibrium and non-equilibrium effects of the various forms of coal minerals on coal combustion.
In the present paper the mineral matter effects on coal pyrolysis are being analyzed using an approach whereby observable spectroscopic (TG/MS) differences in the pyrolytic decomposition between fresh coal and demineralized coal are reconstructed from the sum of mineral matter effects on pyrolysis arising from adding back, singly, and in pairs, individual minerals in various forms.
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