ACERC
Abstracts - 1993 |
ACERC
Abstracts - 1993 |
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Thrust Area 2: Fuels Minerals, Fouling and Slagging |
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.
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.
Harb, J.N.; Zygarlicke,
C.J. and Richards, G.H.
J. Inst. Energy, 66:91- 98, 1993. Funded by ACERC.
A mathematical model of a laminar drop-tube furnace was used to examine the effect of variations in the size, composition and physical properties of ash particles on deposition. Experimentally determined size and composition distributions for the fly ash of two Western US coals (Dietz and Utah Blind Canyon) were used as input to the model. Particle trajectories in the furnace were simulated through the boundary layer to the cooled deposition surface. Particles of less than 25 µm cooled significantly before impacting the plate. Radiation had little effect on the particle temperature at impaction. Particle-capture efficiency was determined from the particle composition and temperature at impaction, based on the particle viscosity. Sticking coefficients were calculated from the particle impaction-capture efficiencies and compared with experimental values. Deposit size and shape were approximated from the deposition-rate data, and the temperature profile through the deposit was calculated.
Harb, J.N.; Richards, G.H.
and Munson, C.L.
Energy & Fuels, 7:20-214, 1993. Funded by ACERC.
This paper examines the use of computer calculations to estimate the phase and species composition of silica-based systems that are important in slagging and high-temperature fouling deposits that form in pc-fired utility boilers. Advanced numerical techniques were used to minimize the free energy of the system in order to determine the equilibrium composition and phase distribution while avoiding the numerical problems often associated with such calculations. The equilibrium model, which assumed ideal solutions of complex species, adequately approximated the behavior of a variety of systems for which experimental phase diagrams were available. The model, however, performed poorly for certain silica-rich systems due to an inadequate representation of the silica activity. Comparison of calculated results for actual coal ashes with the experimentally observed behavior showed good agreement for systems that did not have SiO2(1) in the calculated results. Calculations for ashes with high silica content predicted excessive amounts of liquid that were inconsistent with the experimental observations. The addition to the calculations of an empirical constraint on SiO2-(1), based on eutectic temperatures from ternary phase diagrams, yielded good agreement between the calculated results and the observed slagging behavior.
Richards, G.H.; Slater,
P.N. and Harb, J.N.
Energy & Fuels, 7, (6):774-781, 1993. (Also presented at the Annual
Advanced Combustions Engineering Research Center Conference, Park City,
UT, March 1993. Funded by ACERC.
A model has been developed to relate the deposition behavior of ash under slagging conditions to boiler operating conditions and coal composition data. This model has been incorporated into a comprehensive combustion code and used to investigate the effects of ash deposition rate, thermal conditions, and ash chemistry on slag growth in a pilot-scale combustor. Results for simulated deposits from a coal blend fired at 3.7 MBtu/h showed a relatively high liquid fraction corresponding to denser and presumably stronger deposits. The same coal blend fired at a lower rate produced deposits that were less dense because of the lower temperatures and heat flux levels in the combustor, as well as the lower ash deposition rates. Deposition from a cleaned version of the same blend was also simulated at 3.7 MBtu/h and showed less potential for liquid-phase formation than the uncleaned blend. These results are in qualitative agreement with experimental results and illustrate the importance of operating conditions on deposit formation.
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.
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.
Baxter, L.L.; Richards,
G.H.; Ottesen, D.K. and Harb, J.N.
Energy & Fuels, 7 (6):755-760, 1993. (Also presented at the Annual
Advanced Combustion Engineering Research Center Conference, Park City, UT,
March 1993). Funded by ACERC.
In situ Fourier transform infrared (FTIR) emission spectroscopy is used to identify the presence of silica, sulfates, and silicates as a function of time in coal ash deposits generated in Sandia's multifuel combustor, a pilot-scale reactor. Ash deposits are formed on a cylindrical tube in cross flow under experimental conditions that correspond to convection pass (fouling) regions of a commercial coal-fired boiler. The gas temperature, gas composition, particle loading, and extent of particle reaction in the combustor are typical of commercial boiler operation. The major classes of inorganic species deposited on the tube, including silicates and sulfates, are identified using the FTIR emission spectroscopy technique. Post mortem X-ray diffraction and conventional infrared absorption and reflectance analyses on the same deposits are used to corroborate the in situ FTIR emission data. The deposit composition from a western coal changes significantly as a function of both deposition time and combustion conditions. The observed changes include formation of sulfates and silicates. Such changes have implications for deposit properties such as tenacity and strength; the FTIR emission diagnostic shows promise as a method for monitoring such changes in practical systems.
Wall, T.F.; Baxter, L.L.
and Harb, J.N.
Proceedings of the Engineering Foundation Conference on Coal Blending and
Switching of Western Low-Sulfur Coals, Snowbird, UT, September 1993. Funded
by Australian Research Council, US Department of Energy, Pittsburgh Energy Technology
Center and ACERC.
The character of fireside ash deposits depends on the processes by which deposits are formed and subsequent reactions within the deposit and with furnace gases. The properties influencing furnace heat transfer, absorptivity for radiative transfer and thermal conductivity for conductive transfer are shown from many measurements to depend on this character. Illustrative trends in these properties as deposits mature and grow are presented together with their effect on furnace exit temperature and efficiency. The reflective character of initial deposits from particular coals is then considered with predictions and measurements of the spectral character of such deposits, during the first three hours of growth, using on-line FTIR spectroscopy.
Gopalakrishnan, R.; Fullwood,
M.; Moody, S.; Cope, R.F. and Bartholomew, C.H.
Proceedings of the 7th International Conference on Coal Science, Banff,
Canada, September 1993. Funded by ACERC.
This study is part of an ongoing program to investigate (a) rates and mechanisms of Ca-catalyzed oxidation of synthetic char and chars prepared from representative U.S. coals and (b) the chemical nature of active catalytic sites for oxidation on those inorganic mineral phases present in coal chars.
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.
McCollor, D.P.; Zygarlicke,
C.J.; Toman, D.L. and Evenstad, D.N.
Proceedings from the Impact of Ash Deposition on Coal Fired Plant Engineering
Foundation Conference, Solihull, UK, June 1993. Funded by US Department
of Energy and Electric Power Research Institute.
A bench-scale method has been developed to measure in situ the force required to remove coal ash deposits grown under simulated fouling conditions. A suite of eight coals, representing a range of rank and known fouling behavior, were examined using this method. Statistical analysis of the data resulted in a good correlation of adhesion strength with an independently derived index of high-temperature fouling propensity and with 1(coal feed time), number of soot blowings, and number of noneffective soot blowings. Using this method, a coal or coal blend can be rapidly screened for fouling behavior under conditions simulating those attained in a particular utility boiler.
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