Barthelson, SH
1994
New Analysis Techniques Help Control Boiler Fouling
Karner, F.R.; Zygarlicke, C.J.; Brekke, D.W.; Steadman, E.N. and Benson, S.A.
Power Engineering, 98:35-38, 1994. Funded by US Department of Energy, Brigham Young University and Electric Power Research Institute.
Severe boiler fouling can be controlled. But, it demands a thorough understanding of how minor changes in the chemical and physical properties of coal and ash and operating conditions can cause hardened deposits. Traditional analytical techniques have helped reveal some of these mechanisms in the past, but limitations inherent to the analysis left many questions unanswered.
Now, however, scanning electron microscopy and electron microprobe analysis (SEM/EMPA) provide researchers with the analytical tools necessary to truly understand deposit formation mechanisms. These new techniques are exposing the size, shape, and chemistry of the myriad individual ash particles formed when coal burns. This and other data can be used to predict combustion behavior and its impact on ash deposition, waterwall slagging, and other steam tube deposits.
Microanalysis is capable of helping operators to identify more efficient and cost effective corrective measures as well. Recognized control methods (i.e., soot-blowing or plant capacity reductions) are often expensive and ineffective. The same is true for trial and error operating changes. Logic dictates analyzing coal and ash deposits first and then modifying operating procedures accordingly; SEM/EMPA facilitates the process.
Transformation of Trace Metals in Coal Gasification
Zygarlicke, C.J.; McCollor, D.P. and Benson, S.A.
Proceedings of the Pacific Rim International Conference on Environmental Control of Combustion Processes, Maui, HI, August 1994. Funded by US Environmental Protection Agency, US Department of Energy/Morgantown Energy Technology Center and Brigham Young University.
Trace metals pose a potential problem to emerging coal gasification electric power-generating systems. Some of the trace metals in coal are considered air toxics when released into the atmosphere and can also cause the degradation of fuel cell efficiency due to contamination. The fate of trace metals during coal conversion in integrated gasification combined cycle and integrated gasification fuel cell systems is closely tied to how the trace metals are associated in the coal and gasification conditions. Bench-scale gasification experiments were performed using Illinois No. 6 coal to determine the partitioning of Hg, Se, As, Ni, Cd, Pb, and Cr into gas, liquid, and solid phases as a function of gasification conditions and coal composition. Entrained ash was generated in a pressurized drop-tube furnace and collected using a multicyclone and impinger sampling train. Coal analysis revealed arsenic, Hg, Ni, Pb, and Se to be primarily associated with pyrite. Cr was associated primarily with clay minerals, and Cd appeared to have either a sulfide or an organic association. Cr, Pb, and Ni are enriched in the ash particulate fraction (collected in multicyclones), while Cd, As, Se, and Hg are depleted in the particulate and are more enriched in the very fine particulate or vapor-phase fraction (collected in the final filter and impingers). Oxygen contents were varied to represent both combustion and gasification systems. Most of the work was conducted at lower oxygen/carbon (O/C) ratios. Lower O/C ratios resulted in more Hg being driven to the vapor phase. Under the constant O/C ratios, Hg, Se, and Cd showed increasing volatility with increasing temperature in the reaction zone.
Ash Formation, Deposition, Corrosion, and Erosion in Conventional Boilers
Benson, S.A.
Proceedings of the Second International Conference on Energy and Environment: Transitions in East Central Europe, Prague, Czech Republic, November 1994. Funded by US Department of Energy and Energy & Environmental Research Center.
The inorganic components (ash-forming species) associated with coals significantly affect boiler design, efficiency of operation, and lifetimes of boiler parts. During combustion in conventional pulverized fuel boilers, the inorganic components are transformed into inorganic gases, liquids, and solids. This partitioning depends upon the association of the inorganic components in the coal and combustion conditions. The inorganic components are associated as mineral grains and as organically associated elements, and these associations of inorganic components in the fuel directly influence their fate upon combustion. Combustion conditions, such as temperature and atmosphere, influence the volatility and the interaction of inorganic components during combustion and gas cooling, which influences the state and size composition distribution of the particulate and condensed ash species. The intermediate species are transported with the bulk gas flow through the combustion systems, during which time the gases and entrained ash are cooled. Deposition, corrosion, and erosion occur with the ash intermediate species are transported to the heat-transfer surface, react with the surface, accumulate, sinter, and develop strength. Research over the past decade has significantly advanced understanding of ash formation, deposition, corrosion, and erosion mechanisms. Many of the advances in understanding and predicting ash-related issues can be attributed to advanced analytical methods to determine the inorganic composition of fuels and the resulting ash materials. These new analytical techniques have been the key to elucidation of the mechanisms of ash formation and deposition. This information has been used to develop algorithms and computer models to predict the effects of ash on combustion system performance.
Pilot- and Bench-Scale Combustion Testing of a Wyoming Subbituminous/Oklahoma Bituminous Coal Blend
Zygarlicke, C.J.; Benson, S.A. and Borio, R.W.
Coal Blending and Switching of Low-Sulfur Western Fuels, 281-300, American Society of Mechanical Engineers, New York, NY, 1994. Funded by US Department of Energy and Electric Power Research Institute.
Combustion ash formation and deposition behavior of Wyoming subbituminous/Oklahoma bituminous coal blends were examined using small-scale testing equipment. Inorganic constituents in the parent coals affected ash behavior. Wyoming entrained ash was finer-sized and enriched in calcium aluminosilicates, while the Oklahoma fly ash was larger-sized and enriched in silica. The 70/30 Wyoming/Oklahoma blend produced worse slag deposits while the 90/10 Wyoming/Oklahoma blend produced worse fouling deposits.
Coal Ash Behavior and Management Tools
Erickson, T.A.; O'Leary, E.M.; Folkedahl, B.C.; Ramanathan, M.; Zygarlicke, C.J.; Steadman, E.N.; Hurley, J.P. and Benson, S.A.
The Impact of Ash Deposition on Coal-Fired Plants, Taylor & Francis, Inc., 1994. Funded by US Department of Energy, Electric Power Research Institute, Tow, Texaco, Shell, Union Electric, Kansas City Power and Light, Minnesota Power and Northern States Power.
Over the past five years, computer-based research tools have been increasingly applied in making important economic and operational decisions in the utility power industry. These tools-which include models, indices, databases, and data manipulation programs-are used by researchers, operators, and managers in the evaluation of coal utilization as an efficient and environmentally acceptable source of energy. Applicable tools that have been developed at, and are currently used by, the Energy & Environmental Research Center (EERC) include Partchar©, MINCLASS©, VISCAL©, MANAGER©, ATRAN, LEADER©, PHOEBE©, and PCQUEST©. These software applications range from databases for retrieving coal and coal product analysis, to computer codes to process coal and coal product analysis, to advanced models and indices to evaluate the operational impacts of specific systems.
1993
Ash Formation and Deposition
Benson, S.A.; Jones, M.L. and Harb, J.N.
Chapter 4, Fundamentals of Coal Combustion: For Clean and Efficient Use, (L.D. Smoot, ed.), Elsevier Science Publishers, The Netherlands, 1993. Funded by ACERC.
This chapter discusses the fundamental and applied aspects of coal ash formation and deposition. Ash deposition on heat-transfer surfaces has been examined for many years, resulting in voluminous literature on the subject. However, a precise and quantitative knowledge of the chemical and physical transformations of the inorganic components in coal during combustion has not been obtained due to the inability to quantitatively determine the inorganic composition of coal and to understand the complexity of the processes involved. The status of predictive methods for the fate of the inorganic constituents during combustion as a function of coal composition and combustion conditions is discussed herein. The composition of the coal ash produced under ASTM ashing conditions is used in most methods as an approximate guide to predict the behavior of inorganic constituents of a specific coal during combustion. This ashing technique can be used to predict average properties of the ash; however, examination of fly ash shows that many different types of particles are present, each having its own composition and probably its own melting behavior. Therefore, the behavior of individual fly ash particles may be very different from the predicted for the average ash composition. The extent of ash-related problems depends upon the quantity and association of inorganic constituents in the coal, the combustion conditions, and the system geometry. The inorganic constituents are distributed within the coal matrix in several forms, including organically associated inorganic elements; coal-bound, included minerals; and coal-free, excluded minerals. The primary mineral groups that are found in all coals consist of clay minerals, carbonates, sulfides, oxides, and quartz. However, the specific types of inorganic components present depend upon the rank of the coal and the environment in which the coal was formed.
Fuel Minerals, Fouling, and Slagging
Benson, S.A. and Harb, J.N.
Energy & Fuels, 7 (6):743-745, 1993. Funded by ACERC.
Thrust Area 2, Fuel Minerals, Fouling, and Slagging, is focused on obtaining a clear understanding of the role of fuel inorganic components in combustion processes. During the combustion process, the inorganic components are transformed to ash. The principal problems associated with ash are deposition on heat-transfer surfaces, erosion, corrosion, and formation of fine particulate that is difficult to collect. In addition, catalytic inorganic components impacts are noted in a number of coal combustion processes, e.g., burnout and devolatilization. The specific objectives of Thrust Area 2 are the following: (1) to characterize the chemical and physical transformations of inorganic components, (2) to characterize the catalytic role of inorganic components on oxidation and devolatilization, (3) to establish submodels for mineral behavior, (4) to perform key experiments to define model parameters, (5) to incorporate submodels into comprehensive code, and (6) to evaluate models.
Predicting Ash Behavior in Utility Boilers
Benson, S.A.; Hurley, J.P.; Zygarlicke, C.J.; Steadman, E.N. and Erickson, T.A.
Energy & Fuels, 7 (6):746, 1993. Funded by US Department of Energy and ACERC.
In recent years, significant advances have been made in the development of methods to predict ash behavior in utility boilers. This paper provides an overview of methods used to assess and predict ash formation and deposition. These prediction methods are based on a detailed knowledge of ash formation and deposition mechanisms that has been obtained through bench, pilot, and field-testing and detailed coal and ash characterization. The paper describes advanced methods of coal and ash analyses and the advantages of these methods over conventional methods. The advanced coal characterization methods provide sufficient data to predict size and composition distribution of fly ash. The composition and size data are used as inputs to mechanistic models that ultimately predict deposition propensities in various locations of utility boilers. Advanced indices based on advanced coal analysis data have also been developed and are being applied to predict convective pass fouling tendencies.
Ash Deposit Initiation in a Simulated Fouling Regime
McCollor, D.P.; Zygarlicke, C.J.; Allan, S.E. and Benson, S.A.
Energy & Fuels, 7 (6):761-767, 1993. (Also presented at the Seventh Annual Technical conference of the Advanced Combustion Engineering Center, Park City, UT, March 1993. Funded by US Department of Energy and ACERC.
Bench-scale pulverized coal combustion studies were performed to examine selected major factors that influence deposit initiation. Five coals of varying ranked and composition, including a Beulah-Zap lignite, a Dietz subbituminous coal, a Utah Blind Canyon western bituminous coal, and Illinois No. 6 and Pittsburgh No. 8 eastern bituminous coals, were fired in a laminar flow drop-tube furnace under simulated fouling conditions. Initially deposited particles as well as bulk fly ash were examined using scanning electron microscopy techniques. Deposited particle diameters ranged from 10 to 40 µm. Initial adhering particles were primarily iron-, iron-calcium-and iron-silica-aluminum-rich particles for the Pittsburgh No. 8, Illinois No. 6, Utah Blind Canyon, and Beulah coals. Utah Blind Canyon and Beulah deposits also included some calcium-silica-alumina-rich particles. Dietz deposits contained iron-and iron-calcium-rich particles, along with substantial barium- and barium-sulfate-rich species. These enriched particle species appeared common to all of the initial deposits, with their abundance being determined by the concentration in the original coals. The great majority of the initially deposited particles clustered in similar groupings above a certain "critical" mass and below a "critical" viscosity regardless of individual compositions. This indicates that the initial deposit particles are those with sufficient kinetic energy to impact the substrate inertially and with sufficiently low viscosity to adhere upon impaction. The propensity for initial ash deposition can be roughly related to the fraction of particles in the bulk fly ash possessing these mass and viscosity requirements.
Pilot- and Bench-Scale Combustion Testing of a Wyoming Subbituminous/Oklahoma Bituminous Coal Blend
Zygarlicke, C.J.; Benson, S.A. and Borio, R.W.
Proceedings of the Engineering Foundation Conference on Coal Blending and Switching of Western Low Sulfur Coals, Snowbird, UT, October, 1993 (in press). Funded by US Department of Energy, Electric Power Research Institute and ACERC.
Combustion ash formation and deposition behavior of Wyoming subbituminous/Oklahoma bituminous coal blends were examined using small-scale testing equipment. Inorganic constituents in the parent coals affected ash behavior. Wyoming entrained ash was finer-sized and enriched in calcium aluminosilicates, while the Oklahoma fly ash was larger-sized and enriched in silica. The 70/30 Wyoming/Oklahoma blend produced worse slag deposits while the 90/10 Wyoming/Oklahoma blend produced worse fouling deposits.
1992
Inorganic Transformation During Combustion
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.
Predicting Ash Behavior in Utility Boilers: Assessment of Current Status
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.
Examination of Ash Deposition in Full-, Pilot-, and Bench-Scale Testing
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.
Ash Particle Size and Composition Evolution During Combustion of Synthetic Coal and Inorganic Mixtures
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.
1991
Physicochemical Effects Influencing the Measurements of Interfacial Surface Tension of Coal Ashes
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.
Interaction of Sodium, Sulfur, and Silica During Coal Combustion
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.
An Overview of Ash Deposition During the Combustion of Western U.S. Coals
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.
1990
Sintering Behavior and Strength Development in Various Coal Ashes
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.
Inorganic Transformations and Ash Deposition During Pulverized Coal Combustion of Two Western U.S. Coals
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.
Trends in the Evolution of Fly Ash Size During Combustion
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.
A Microanalytical Approach to the Characterization of Coal, Ash, and Deposit
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.
1989
Deposition of Beulah Ash in a Drop-Tube Furnace Under Slagging Conditions
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.
Studies of Transformations of Inorganic Constituents in a Texas Lignite During Combustion
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.
Evaluation of Coals Using a Drop Tube Furnace
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).
Characterization of Mineral Matter in ACERC Coals
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.
Studies of Ash Deposit Formation From Powder River Basin and Fort Union Coals
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.
Application of SEM Techniques to the Characterization of Coal and Coal Ash Products
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.
1988-1987
An Overview of Fouling/Slagging with Western Coals
Jones, M.L. and Benson, S.A.
Proceedings of the Conference on the Effects of Coal Quality on Power Plants, 1987 Atlanta, Georgia. Funded by Coal Companies and Utility Companies.
Ash deposition is one of the greatest operational problems associated with the efficient utilization of low-rank coals in utility boilers. Also deposition can occur in two ways, slagging and fouling. For purposes of this discussion, fouling is defined as deposition in the convective section of the boiler and slagging as deposition in the convective pass, or fouling. The information required to better understand this process includes the mode of occurrence and abundance of the inorganic constituents, their reactions and transformations in the flame, mechanisms of ash transport and deposit growth, and interaction after deposition to form strong deposits. These issues are discussed in light of unique properties of the low-rank coals. Particular attention is paid to the mode of occurrence and abundance of the alkali and alkaline earth elements as well as their contribution to the liquid phase material critical to the development of strongly bonded ash deposits.
Promotion of Char Oxidation by Inorganic Constituents
McCollor, D.P.; Young, B.C.; Jones, M.L. and Benson, S.A.
Accept for publication in the Twenty-Second International Symposium on Combustion, 1988. Funded by US Department of Energy and Pittsburgh Energy Technology Center.
A North Dakota lignite has been demineralized and selectively reloaded with calcium, potassium, and sodium cations by an ion-exchange process. Chars produced from the treated samples were burned in a laminar-flow reactor and single-particle temperatures were determined by optical pyrometry. Results show that sodium and potassium cations present in the chars have little effect on the char particle temperatures at low concentration (<5000 ppm). The results are consistent with carbon dioxide being produced at the char surface by catalytic action of the char mineral matter.
Barthelson, SH
1994
New Analysis Techniques Help Control Boiler Fouling
Karner, F.R.; Zygarlicke, C.J.; Brekke, D.W.; Steadman, E.N. and Benson, S.A.
Power Engineering, 98:35-38, 1994. Funded by US Department of Energy, Brigham Young University and Electric Power Research Institute.
Severe boiler fouling can be controlled. But, it demands a thorough understanding of how minor changes in the chemical and physical properties of coal and ash and operating conditions can cause hardened deposits. Traditional analytical techniques have helped reveal some of these mechanisms in the past, but limitations inherent to the analysis left many questions unanswered.
Now, however, scanning electron microscopy and electron microprobe analysis (SEM/EMPA) provide researchers with the analytical tools necessary to truly understand deposit formation mechanisms. These new techniques are exposing the size, shape, and chemistry of the myriad individual ash particles formed when coal burns. This and other data can be used to predict combustion behavior and its impact on ash deposition, waterwall slagging, and other steam tube deposits.
Microanalysis is capable of helping operators to identify more efficient and cost effective corrective measures as well. Recognized control methods (i.e., soot-blowing or plant capacity reductions) are often expensive and ineffective. The same is true for trial and error operating changes. Logic dictates analyzing coal and ash deposits first and then modifying operating procedures accordingly; SEM/EMPA facilitates the process.
Transformation of Trace Metals in Coal Gasification
Zygarlicke, C.J.; McCollor, D.P. and Benson, S.A.
Proceedings of the Pacific Rim International Conference on Environmental Control of Combustion Processes, Maui, HI, August 1994. Funded by US Environmental Protection Agency, US Department of Energy/Morgantown Energy Technology Center and Brigham Young University.
Trace metals pose a potential problem to emerging coal gasification electric power-generating systems. Some of the trace metals in coal are considered air toxics when released into the atmosphere and can also cause the degradation of fuel cell efficiency due to contamination. The fate of trace metals during coal conversion in integrated gasification combined cycle and integrated gasification fuel cell systems is closely tied to how the trace metals are associated in the coal and gasification conditions. Bench-scale gasification experiments were performed using Illinois No. 6 coal to determine the partitioning of Hg, Se, As, Ni, Cd, Pb, and Cr into gas, liquid, and solid phases as a function of gasification conditions and coal composition. Entrained ash was generated in a pressurized drop-tube furnace and collected using a multicyclone and impinger sampling train. Coal analysis revealed arsenic, Hg, Ni, Pb, and Se to be primarily associated with pyrite. Cr was associated primarily with clay minerals, and Cd appeared to have either a sulfide or an organic association. Cr, Pb, and Ni are enriched in the ash particulate fraction (collected in multicyclones), while Cd, As, Se, and Hg are depleted in the particulate and are more enriched in the very fine particulate or vapor-phase fraction (collected in the final filter and impingers). Oxygen contents were varied to represent both combustion and gasification systems. Most of the work was conducted at lower oxygen/carbon (O/C) ratios. Lower O/C ratios resulted in more Hg being driven to the vapor phase. Under the constant O/C ratios, Hg, Se, and Cd showed increasing volatility with increasing temperature in the reaction zone.
Ash Formation, Deposition, Corrosion, and Erosion in Conventional Boilers
Benson, S.A.
Proceedings of the Second International Conference on Energy and Environment: Transitions in East Central Europe, Prague, Czech Republic, November 1994. Funded by US Department of Energy and Energy & Environmental Research Center.
The inorganic components (ash-forming species) associated with coals significantly affect boiler design, efficiency of operation, and lifetimes of boiler parts. During combustion in conventional pulverized fuel boilers, the inorganic components are transformed into inorganic gases, liquids, and solids. This partitioning depends upon the association of the inorganic components in the coal and combustion conditions. The inorganic components are associated as mineral grains and as organically associated elements, and these associations of inorganic components in the fuel directly influence their fate upon combustion. Combustion conditions, such as temperature and atmosphere, influence the volatility and the interaction of inorganic components during combustion and gas cooling, which influences the state and size composition distribution of the particulate and condensed ash species. The intermediate species are transported with the bulk gas flow through the combustion systems, during which time the gases and entrained ash are cooled. Deposition, corrosion, and erosion occur with the ash intermediate species are transported to the heat-transfer surface, react with the surface, accumulate, sinter, and develop strength. Research over the past decade has significantly advanced understanding of ash formation, deposition, corrosion, and erosion mechanisms. Many of the advances in understanding and predicting ash-related issues can be attributed to advanced analytical methods to determine the inorganic composition of fuels and the resulting ash materials. These new analytical techniques have been the key to elucidation of the mechanisms of ash formation and deposition. This information has been used to develop algorithms and computer models to predict the effects of ash on combustion system performance.
Pilot- and Bench-Scale Combustion Testing of a Wyoming Subbituminous/Oklahoma Bituminous Coal Blend
Zygarlicke, C.J.; Benson, S.A. and Borio, R.W.
Coal Blending and Switching of Low-Sulfur Western Fuels, 281-300, American Society of Mechanical Engineers, New York, NY, 1994. Funded by US Department of Energy and Electric Power Research Institute.
Combustion ash formation and deposition behavior of Wyoming subbituminous/Oklahoma bituminous coal blends were examined using small-scale testing equipment. Inorganic constituents in the parent coals affected ash behavior. Wyoming entrained ash was finer-sized and enriched in calcium aluminosilicates, while the Oklahoma fly ash was larger-sized and enriched in silica. The 70/30 Wyoming/Oklahoma blend produced worse slag deposits while the 90/10 Wyoming/Oklahoma blend produced worse fouling deposits.
Coal Ash Behavior and Management Tools
Erickson, T.A.; O'Leary, E.M.; Folkedahl, B.C.; Ramanathan, M.; Zygarlicke, C.J.; Steadman, E.N.; Hurley, J.P. and Benson, S.A.
The Impact of Ash Deposition on Coal-Fired Plants, Taylor & Francis, Inc., 1994. Funded by US Department of Energy, Electric Power Research Institute, Tow, Texaco, Shell, Union Electric, Kansas City Power and Light, Minnesota Power and Northern States Power.
Over the past five years, computer-based research tools have been increasingly applied in making important economic and operational decisions in the utility power industry. These tools-which include models, indices, databases, and data manipulation programs-are used by researchers, operators, and managers in the evaluation of coal utilization as an efficient and environmentally acceptable source of energy. Applicable tools that have been developed at, and are currently used by, the Energy & Environmental Research Center (EERC) include Partchar©, MINCLASS©, VISCAL©, MANAGER©, ATRAN, LEADER©, PHOEBE©, and PCQUEST©. These software applications range from databases for retrieving coal and coal product analysis, to computer codes to process coal and coal product analysis, to advanced models and indices to evaluate the operational impacts of specific systems.
1993
Ash Formation and Deposition
Benson, S.A.; Jones, M.L. and Harb, J.N.
Chapter 4, Fundamentals of Coal Combustion: For Clean and Efficient Use, (L.D. Smoot, ed.), Elsevier Science Publishers, The Netherlands, 1993. Funded by ACERC.
This chapter discusses the fundamental and applied aspects of coal ash formation and deposition. Ash deposition on heat-transfer surfaces has been examined for many years, resulting in voluminous literature on the subject. However, a precise and quantitative knowledge of the chemical and physical transformations of the inorganic components in coal during combustion has not been obtained due to the inability to quantitatively determine the inorganic composition of coal and to understand the complexity of the processes involved. The status of predictive methods for the fate of the inorganic constituents during combustion as a function of coal composition and combustion conditions is discussed herein. The composition of the coal ash produced under ASTM ashing conditions is used in most methods as an approximate guide to predict the behavior of inorganic constituents of a specific coal during combustion. This ashing technique can be used to predict average properties of the ash; however, examination of fly ash shows that many different types of particles are present, each having its own composition and probably its own melting behavior. Therefore, the behavior of individual fly ash particles may be very different from the predicted for the average ash composition. The extent of ash-related problems depends upon the quantity and association of inorganic constituents in the coal, the combustion conditions, and the system geometry. The inorganic constituents are distributed within the coal matrix in several forms, including organically associated inorganic elements; coal-bound, included minerals; and coal-free, excluded minerals. The primary mineral groups that are found in all coals consist of clay minerals, carbonates, sulfides, oxides, and quartz. However, the specific types of inorganic components present depend upon the rank of the coal and the environment in which the coal was formed.
Fuel Minerals, Fouling, and Slagging
Benson, S.A. and Harb, J.N.
Energy & Fuels, 7 (6):743-745, 1993. Funded by ACERC.
Thrust Area 2, Fuel Minerals, Fouling, and Slagging, is focused on obtaining a clear understanding of the role of fuel inorganic components in combustion processes. During the combustion process, the inorganic components are transformed to ash. The principal problems associated with ash are deposition on heat-transfer surfaces, erosion, corrosion, and formation of fine particulate that is difficult to collect. In addition, catalytic inorganic components impacts are noted in a number of coal combustion processes, e.g., burnout and devolatilization. The specific objectives of Thrust Area 2 are the following: (1) to characterize the chemical and physical transformations of inorganic components, (2) to characterize the catalytic role of inorganic components on oxidation and devolatilization, (3) to establish submodels for mineral behavior, (4) to perform key experiments to define model parameters, (5) to incorporate submodels into comprehensive code, and (6) to evaluate models.
Predicting Ash Behavior in Utility Boilers
Benson, S.A.; Hurley, J.P.; Zygarlicke, C.J.; Steadman, E.N. and Erickson, T.A.
Energy & Fuels, 7 (6):746, 1993. Funded by US Department of Energy and ACERC.
In recent years, significant advances have been made in the development of methods to predict ash behavior in utility boilers. This paper provides an overview of methods used to assess and predict ash formation and deposition. These prediction methods are based on a detailed knowledge of ash formation and deposition mechanisms that has been obtained through bench, pilot, and field-testing and detailed coal and ash characterization. The paper describes advanced methods of coal and ash analyses and the advantages of these methods over conventional methods. The advanced coal characterization methods provide sufficient data to predict size and composition distribution of fly ash. The composition and size data are used as inputs to mechanistic models that ultimately predict deposition propensities in various locations of utility boilers. Advanced indices based on advanced coal analysis data have also been developed and are being applied to predict convective pass fouling tendencies.
Ash Deposit Initiation in a Simulated Fouling Regime
McCollor, D.P.; Zygarlicke, C.J.; Allan, S.E. and Benson, S.A.
Energy & Fuels, 7 (6):761-767, 1993. (Also presented at the Seventh Annual Technical conference of the Advanced Combustion Engineering Center, Park City, UT, March 1993. Funded by US Department of Energy and ACERC.
Bench-scale pulverized coal combustion studies were performed to examine selected major factors that influence deposit initiation. Five coals of varying ranked and composition, including a Beulah-Zap lignite, a Dietz subbituminous coal, a Utah Blind Canyon western bituminous coal, and Illinois No. 6 and Pittsburgh No. 8 eastern bituminous coals, were fired in a laminar flow drop-tube furnace under simulated fouling conditions. Initially deposited particles as well as bulk fly ash were examined using scanning electron microscopy techniques. Deposited particle diameters ranged from 10 to 40 µm. Initial adhering particles were primarily iron-, iron-calcium-and iron-silica-aluminum-rich particles for the Pittsburgh No. 8, Illinois No. 6, Utah Blind Canyon, and Beulah coals. Utah Blind Canyon and Beulah deposits also included some calcium-silica-alumina-rich particles. Dietz deposits contained iron-and iron-calcium-rich particles, along with substantial barium- and barium-sulfate-rich species. These enriched particle species appeared common to all of the initial deposits, with their abundance being determined by the concentration in the original coals. The great majority of the initially deposited particles clustered in similar groupings above a certain "critical" mass and below a "critical" viscosity regardless of individual compositions. This indicates that the initial deposit particles are those with sufficient kinetic energy to impact the substrate inertially and with sufficiently low viscosity to adhere upon impaction. The propensity for initial ash deposition can be roughly related to the fraction of particles in the bulk fly ash possessing these mass and viscosity requirements.
Pilot- and Bench-Scale Combustion Testing of a Wyoming Subbituminous/Oklahoma Bituminous Coal Blend
Zygarlicke, C.J.; Benson, S.A. and Borio, R.W.
Proceedings of the Engineering Foundation Conference on Coal Blending and Switching of Western Low Sulfur Coals, Snowbird, UT, October, 1993 (in press). Funded by US Department of Energy, Electric Power Research Institute and ACERC.
Combustion ash formation and deposition behavior of Wyoming subbituminous/Oklahoma bituminous coal blends were examined using small-scale testing equipment. Inorganic constituents in the parent coals affected ash behavior. Wyoming entrained ash was finer-sized and enriched in calcium aluminosilicates, while the Oklahoma fly ash was larger-sized and enriched in silica. The 70/30 Wyoming/Oklahoma blend produced worse slag deposits while the 90/10 Wyoming/Oklahoma blend produced worse fouling deposits.
1992
Inorganic Transformation During Combustion
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.
Predicting Ash Behavior in Utility Boilers: Assessment of Current Status
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.
Examination of Ash Deposition in Full-, Pilot-, and Bench-Scale Testing
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.
Ash Particle Size and Composition Evolution During Combustion of Synthetic Coal and Inorganic Mixtures
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.
1991
Physicochemical Effects Influencing the Measurements of Interfacial Surface Tension of Coal Ashes
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.
Interaction of Sodium, Sulfur, and Silica During Coal Combustion
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.
An Overview of Ash Deposition During the Combustion of Western U.S. Coals
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.
1990
Sintering Behavior and Strength Development in Various Coal Ashes
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.
Inorganic Transformations and Ash Deposition During Pulverized Coal Combustion of Two Western U.S. Coals
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.
Trends in the Evolution of Fly Ash Size During Combustion
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.
A Microanalytical Approach to the Characterization of Coal, Ash, and Deposit
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.
1989
Deposition of Beulah Ash in a Drop-Tube Furnace Under Slagging Conditions
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.
Studies of Transformations of Inorganic Constituents in a Texas Lignite During Combustion
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.
Evaluation of Coals Using a Drop Tube Furnace
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).
Characterization of Mineral Matter in ACERC Coals
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.
Studies of Ash Deposit Formation From Powder River Basin and Fort Union Coals
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.
Application of SEM Techniques to the Characterization of Coal and Coal Ash Products
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.
1988-1987
An Overview of Fouling/Slagging with Western Coals
Jones, M.L. and Benson, S.A.
Proceedings of the Conference on the Effects of Coal Quality on Power Plants, 1987 Atlanta, Georgia. Funded by Coal Companies and Utility Companies.
Ash deposition is one of the greatest operational problems associated with the efficient utilization of low-rank coals in utility boilers. Also deposition can occur in two ways, slagging and fouling. For purposes of this discussion, fouling is defined as deposition in the convective section of the boiler and slagging as deposition in the convective pass, or fouling. The information required to better understand this process includes the mode of occurrence and abundance of the inorganic constituents, their reactions and transformations in the flame, mechanisms of ash transport and deposit growth, and interaction after deposition to form strong deposits. These issues are discussed in light of unique properties of the low-rank coals. Particular attention is paid to the mode of occurrence and abundance of the alkali and alkaline earth elements as well as their contribution to the liquid phase material critical to the development of strongly bonded ash deposits.
Promotion of Char Oxidation by Inorganic Constituents
McCollor, D.P.; Young, B.C.; Jones, M.L. and Benson, S.A.
Accept for publication in the Twenty-Second International Symposium on Combustion, 1988. Funded by US Department of Energy and Pittsburgh Energy Technology Center.
A North Dakota lignite has been demineralized and selectively reloaded with calcium, potassium, and sodium cations by an ion-exchange process. Chars produced from the treated samples were burned in a laminar-flow reactor and single-particle temperatures were determined by optical pyrometry. Results show that sodium and potassium cations present in the chars have little effect on the char particle temperatures at low concentration (<5000 ppm). The results are consistent with carbon dioxide being produced at the char surface by catalytic action of the char mineral matter.