Harb, JN
1997
Modeling of Ash Deposition in Large-Scale Combustion Facilities Buring Pulverized Coal
Wang, H. and Harb, J.N.
Progress in Energy and Combustion Science, 23:267-82(1997). Funded in part by ACERC.
Traditional approaches to the prediction of the deposition behavior of a coal usually involve the use of empirical indices and ASTM ash fusion temperatures. These approaches, however, can give misleading results and are often unreliable. In recent years, considerable effort has been made in the development of models that overcome some of the deficiencies of the traditional approaches, as reviewed in the first part of this paper. In spite of advances, these models still fail to describe the effect of deposition on boiler operation. The second part of this paper documents the efforts in the Advanced Combustion Engineering Research Center (ACERC) to integrate an ash deposition model with a comprehensive 3D coal combustion model. An ash deposition submodel, which includes the effects of both ash chemistry and operating conditions on slagging deposits, has been incorporated into the comprehensive combustion coed, PCGC-3. The submodel includes a statistically based particle cloud model for determination of impaction rates of fly ash on boiler walls. The fraction of impacting particles that stick to the surface is determined from the physical characteristics (viscosity) of both the particles and the deposit surface. The model includes a description of deposit growth that approximates both the physical properties and chemistry of the deposit as a function of combustion conditions (operating conditions). A key feature of the model is its ability to account for the effect of deposition on operating conditions in the boiler. Simulations of deposition in both pilot-scale and utility-scale combustion facilities are reported in the paper.
Modeling of Ash Deposit Growth and Sintering in PC-Fired Boilers
Wang, H. and Harb, J.N.
Presented at the Engineering Foundation Conference, Kona, Hawaii, November 2-7, 1997. Funded in part by ACERC.
A critical element of any boiler model is the ability to address the influence of inorganic matter of ash on boiler operation and performance. This paper describes a mathematical model that has been integrated into the comprehensive combustion code PCGC-3, and used to predict the effect of slagging on boiler operation and performance. The model includes a description of particle transport, impaction, and sticking. In addition, it features the ability to estimate the heat flux and heat transfer properties through a deposit during deposit growth. Viscous flow sintering is the principal mechanism responsible for changes in the local properties of the deposit. Consequently, a transient description of sintering has been included in the model in order to estimate the properties as a function of time, particle size and composition at different positions in the deposit. An energy balance is used to determine deposit temperatures and the heat flux through the deposit. This balance accounts for variable physical properties and is solved iteratively with a predictor corrector technique at each time step during deposit growth. Validation of the integrated model has been accomplished by performing three-dimensional simulations of deposition in pilot-scale combustor at different operating conditions. Simulations of ash deposition during operation of a utility boiler are presented.
1994
A Mathematical Model for the Build-Up of Furnace Wall Deposits
Harb, J.N.; Slater, P.N. and Richards, G.H.
The Impact of Ash Deposition on Coal Fired Plants, Taylor & Francis, Washington, DC, 1994. Funded by ACERC.
A theoretical study was performed to investigate the effect of ash chemistry and non-constant thermal properties on the calculated heat flux through a coal ash deposit. A mathematical model, previously developed to describe the build-up of furnace wall deposits, was used to predict the rate of deposit growth, thermal conductivity, and porosity of the deposit, as well as the heat transfer through the deposit. In this study, a method to predict deposit emittance and absorbance as a function of composition and particle size was added to the deposition model. Simulations showed that the ash chemistry had a significant effect on the thermal and physical properties of the deposit and, consequently, the heat flux through the deposit. Significant differences in the predicted heat flux through the deposit were observed when constant values (not varying with time and/or position) for the deposit emittance and thermal conductivity were assumed. The observed differences are largely due to the inability of the constant properties to adequately predict the resistance of the inner layer of the deposit that significantly affects the heat transfer through the deposit.
Characterization of Minerals and Coal/Mineral Associations in Pulverized Coal
Yu, H.; Marchek, J.E.; Adair, N.L. and Harb, J.N.
The Impact of Ash Deposition on Coal Fired Plants, Taylor & Francis, Washington DC, 1994. Funded by ACERC.
Computer Controlled Scanning Electron Microscopy was used to analyze the mineral composition of two eastern US bituminous coals. In addition, a procedure was developed to determine the association of the minerals with the coal matrix. The mineral content of each of the coals was also determined on a particle-by-particle basis and used to examine the association of various minerals in the coal. Examination of the mineral/mineral associations showed that the distribution of minerals in the coal was not random. In particular, results show lower levels of association for pyrite than for the other minerals examined.
Radiative Heat Transfer in Pulverized-Coal Fired Boilers - Development of the Absorptive/Reflective Character of Initial Ash Deposit
Richards, G.H.; Harb, J.N.; Baxter, L.L.; Bhattacharya, S.; Gupta, R.P. and Wall, T.F.
25th Symposium (International on Combustion, 1994 (in press). (Proceedings of the 25th Symposium (International) on Combustion, Irvine, CA, August 1994). Funded by Australian Research Council, US Department of Energy (Pittsburgh Energy Technology Center) and ACERC.
Emission Fourier transform infrared (FTIR) spectroscopy data provide in situ,. Time-resolved, spectral emissivity measurements for ash deposits generated from two U.S. Powder River Basin coals. The first 3 h of deposit growth on a tube in a cross flow in a pilot-scale furnace detail the development of surface emissivity with time. Measured emissivities vary significantly with wavelength, indicating the influence of the physical properties and chemical composition of the deposit. At long wavelengths (>7 µm), emissions features exhibit characteristics of silica, sulfates, and silicates. The spectral emissivity measured in this region approaches a steady value due to an increase in deposit thickness and the size of particles in the deposit. In contrast, deposits are not opaque at shorter wavelengths where the measured emissivity is influenced by the properties of the underlying metal surface. Theoretical predictions of the emissivity of a particulate layer were performed, and results are compared to the measured values. The theory adequately predicts the general features of spectral variation of the emissivity. The predicted trends in emissivity with particle size and deposit composition are also consistent with experimental observations. Total (Planck-weighted) emissivities are calculated from the measured spectral values for the deposits at the tube temperatures. They increase with time from the clean tube value (0.2-0.3) to values typical of deposits formed from western U.S. coals (0.45-0.55). Calculated total absorptivities are found to be lower than the corresponding emissivities.
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.
The Effect of Particle Composition and Temperature on the Deposition of Two Western U.S. Coals in a Laminar Drop-Tube Furnace
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.
Use of Equilibrium Calculations to Predict the Behavior of Coal Ash in Combustion Systems
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.
Simulation of Ash Deposit Growth in a Pulverized Coal-Fired Pilot Scale Reactor
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.
In Situ, Real-Time Characterization of Coal Ash Deposits Using Fourier Transform Infrared Emission Spectroscopy
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.
Ash Deposits, Coal Blends and the Thermal Performance of Furnaces
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.
1992
Characterization of Included and Excluded Minerals in Pulverized Coal
Harb, J.N. and Yu, H.
Proceedings of the Ninth Annual International Pittsburgh Coal Conference, 235-240, Pittsburgh, PA, October 1992. Funded by ACERC.
Ash formed during combustion of pulverized coal produces a variety of operational and environmental problems. Recently, considerable effort has been expended toward the development of techniques and models to predict ash transformations and deposition during combustion. These models require a description of the coal mineral matter as input. It is well known that bulk ASTM analysis of the coal ash does not adequately represent observed deposition behavior. This fact has lead to the development of advanced analytical techniques to provide detailed information on coal mineral size composition, and association with the organic matrix.
The principal technique used for detailed analysis has been computer controlled scanning electron microscopy (CCSEM). Early development of this technique was performed by Huggins et al., who used a scanning electron microscope equipped with a Tracor Northern 2200 X-ray analysis system to examine coal minerals. Extensive development and refinement of the technique for both coal minerals and ash has been done at the Energy and Environmental Research Center at the University of North Dakota on several different Tracor Northern systems. Additional work on the association of the coal minerals with the organic matrix has been performed by Straszheim and co-workers at Iowa State University with use of a sophisticated image analysis system (LeMont Scientific DB-10). In spite of the significant advances that have been made, CCSEM analysis has not yet been standardized and questions persist as to the statistical significance of the data, the minimum number of particles required for the analysis, the quantitative nature of the composition data and the role of ZAF corrections, the influence of sample preparation on the results, etc.
The objectives of the present study were to: 1) implement the CCSEM technique on a JOEL 840A scanning electron microscope equipped with a Link eXL X-ray microanalysis and image analysis system, 2) verify the reproducibility and accuracy of the technique for quantitative analysis of the coal minerals, 3) develop the software routines necessary to determine the mineral content and composition of coal particles on a particle-by-particle basis, and 4) use the above techniques to examine two eastern coals recently tested in a utility boiler.
1991
The Effect of Variations in Particle-to-Particle Composition on the Formation of Ash Deposits
Richards, G.H.; Harb, J.N. and Zygarlicke, C.J.
Proc. of the Engineering Foundation Conference on Inorganic Transformations and Ash Deposition During Combustion, (S.A. Benson, ed.), Engineering Foundation Press, Palm Coast, FL, March 1991. Funded by ACERC.
A mathematical model of the deposition region of a laminar drop-tube furnace was used to examine the effect of variations in the size, composition, and physical properties of ash particles on deposition. Experimentally determined size and composition distributions for the fly ash of two western coals (Dietz and Utah Blind Canyon) were used as input to the model. Sticking coefficients were calculated using capture efficiencies based on particle viscosities and compared to experimental values.
1990
Fireside Corrosion in PC-Fired Boilers
Harb, J.N. and Smith, E.E.
Progress in Energy and Combustion Science, 16, 169-190, (1990). Funded by ACERC.
This review has examined fireside corrosion of pc-fired boilers in both the waterwall and superheater regions. The present understanding of corrosion phenomena has resulted in the development of strategies to control tube wastage. The corrosion problem persists, however, in spite of efforts to control it. The physical mechanisms that govern such corrosion are complex and not fully understood to date. The problem is complicated further by localized attack in the form of pits of cracks that may result in tube failure without a significant decrease in the average tube-wall thickness. Mechanistic models that allow quantitative prediction of local corrosion rates from first principles have not yet been developed. Hence, quantitative prediction of fireside corrosion rates is not feasible at the present time.
Mathematical models, however, may play an important role in the a priori prediction of boiler locations where corrosion problems are likely to develop, and the operating conditions under which corrosion is expected to occur. A combustion model could be used to simulate the environment inside a utility boiler for a variety of fuels and operating conditions. Once the conditions inside the boiler have been modeled, the corrosion behavior expected at a given location could be determined by comparing the local boiler environment with corrosion data obtained experimentally under similar conditions. Corrosion data would be accumulated from experience with industrial boilers and from well-defined laboratory experiments. Such a procedure could provide a valuable tool for use in boiler design and in the prediction of problems associated with a changing fuel supply.
Theoretical Investigation of Ash Transport in a Laminar Drop-Tube Furnace Including Temperature and Compositional Effects
Harb, J.N.; Richards, G.H. and Munson, C.L.
Proceedings of the ASME Ash Deposit and Corrosion Research Committee Seminar on Fireside Fouling Problems, Brigham Young University, Provo, UT, 1990. Funded by ACERC.
A mathematical model was developed to investigate particle deposition in a laminar drop-tube furnace. Specifically, simulations were performed to examine the effects of geometry, transport, plate temperature, and particle composition on deposition. Because of the geometry of the deposition region, the diameter of the inlet particle stream was narrowed and particle impaction rates were significantly enhanced near the center of the plate. The location of particle impact and the temperature of the particle upon impaction were both strongly dependent on particle size. The particle temperature at impaction was relatively insensitive to the plate temperature for particles greater than 15 to 20 µm in diameter at plate temperatures of 750K and 1400K. Calculation of composition effects indicated that particles of different sizes with similar compositions might exhibit significantly different sticking behavior owing to the formation of liquid phases.
Harb, JN
1997
Modeling of Ash Deposition in Large-Scale Combustion Facilities Buring Pulverized Coal
Wang, H. and Harb, J.N.
Progress in Energy and Combustion Science, 23:267-82(1997). Funded in part by ACERC.
Traditional approaches to the prediction of the deposition behavior of a coal usually involve the use of empirical indices and ASTM ash fusion temperatures. These approaches, however, can give misleading results and are often unreliable. In recent years, considerable effort has been made in the development of models that overcome some of the deficiencies of the traditional approaches, as reviewed in the first part of this paper. In spite of advances, these models still fail to describe the effect of deposition on boiler operation. The second part of this paper documents the efforts in the Advanced Combustion Engineering Research Center (ACERC) to integrate an ash deposition model with a comprehensive 3D coal combustion model. An ash deposition submodel, which includes the effects of both ash chemistry and operating conditions on slagging deposits, has been incorporated into the comprehensive combustion coed, PCGC-3. The submodel includes a statistically based particle cloud model for determination of impaction rates of fly ash on boiler walls. The fraction of impacting particles that stick to the surface is determined from the physical characteristics (viscosity) of both the particles and the deposit surface. The model includes a description of deposit growth that approximates both the physical properties and chemistry of the deposit as a function of combustion conditions (operating conditions). A key feature of the model is its ability to account for the effect of deposition on operating conditions in the boiler. Simulations of deposition in both pilot-scale and utility-scale combustion facilities are reported in the paper.
Modeling of Ash Deposit Growth and Sintering in PC-Fired Boilers
Wang, H. and Harb, J.N.
Presented at the Engineering Foundation Conference, Kona, Hawaii, November 2-7, 1997. Funded in part by ACERC.
A critical element of any boiler model is the ability to address the influence of inorganic matter of ash on boiler operation and performance. This paper describes a mathematical model that has been integrated into the comprehensive combustion code PCGC-3, and used to predict the effect of slagging on boiler operation and performance. The model includes a description of particle transport, impaction, and sticking. In addition, it features the ability to estimate the heat flux and heat transfer properties through a deposit during deposit growth. Viscous flow sintering is the principal mechanism responsible for changes in the local properties of the deposit. Consequently, a transient description of sintering has been included in the model in order to estimate the properties as a function of time, particle size and composition at different positions in the deposit. An energy balance is used to determine deposit temperatures and the heat flux through the deposit. This balance accounts for variable physical properties and is solved iteratively with a predictor corrector technique at each time step during deposit growth. Validation of the integrated model has been accomplished by performing three-dimensional simulations of deposition in pilot-scale combustor at different operating conditions. Simulations of ash deposition during operation of a utility boiler are presented.
1994
A Mathematical Model for the Build-Up of Furnace Wall Deposits
Harb, J.N.; Slater, P.N. and Richards, G.H.
The Impact of Ash Deposition on Coal Fired Plants, Taylor & Francis, Washington, DC, 1994. Funded by ACERC.
A theoretical study was performed to investigate the effect of ash chemistry and non-constant thermal properties on the calculated heat flux through a coal ash deposit. A mathematical model, previously developed to describe the build-up of furnace wall deposits, was used to predict the rate of deposit growth, thermal conductivity, and porosity of the deposit, as well as the heat transfer through the deposit. In this study, a method to predict deposit emittance and absorbance as a function of composition and particle size was added to the deposition model. Simulations showed that the ash chemistry had a significant effect on the thermal and physical properties of the deposit and, consequently, the heat flux through the deposit. Significant differences in the predicted heat flux through the deposit were observed when constant values (not varying with time and/or position) for the deposit emittance and thermal conductivity were assumed. The observed differences are largely due to the inability of the constant properties to adequately predict the resistance of the inner layer of the deposit that significantly affects the heat transfer through the deposit.
Characterization of Minerals and Coal/Mineral Associations in Pulverized Coal
Yu, H.; Marchek, J.E.; Adair, N.L. and Harb, J.N.
The Impact of Ash Deposition on Coal Fired Plants, Taylor & Francis, Washington DC, 1994. Funded by ACERC.
Computer Controlled Scanning Electron Microscopy was used to analyze the mineral composition of two eastern US bituminous coals. In addition, a procedure was developed to determine the association of the minerals with the coal matrix. The mineral content of each of the coals was also determined on a particle-by-particle basis and used to examine the association of various minerals in the coal. Examination of the mineral/mineral associations showed that the distribution of minerals in the coal was not random. In particular, results show lower levels of association for pyrite than for the other minerals examined.
Radiative Heat Transfer in Pulverized-Coal Fired Boilers - Development of the Absorptive/Reflective Character of Initial Ash Deposit
Richards, G.H.; Harb, J.N.; Baxter, L.L.; Bhattacharya, S.; Gupta, R.P. and Wall, T.F.
25th Symposium (International on Combustion, 1994 (in press). (Proceedings of the 25th Symposium (International) on Combustion, Irvine, CA, August 1994). Funded by Australian Research Council, US Department of Energy (Pittsburgh Energy Technology Center) and ACERC.
Emission Fourier transform infrared (FTIR) spectroscopy data provide in situ,. Time-resolved, spectral emissivity measurements for ash deposits generated from two U.S. Powder River Basin coals. The first 3 h of deposit growth on a tube in a cross flow in a pilot-scale furnace detail the development of surface emissivity with time. Measured emissivities vary significantly with wavelength, indicating the influence of the physical properties and chemical composition of the deposit. At long wavelengths (>7 µm), emissions features exhibit characteristics of silica, sulfates, and silicates. The spectral emissivity measured in this region approaches a steady value due to an increase in deposit thickness and the size of particles in the deposit. In contrast, deposits are not opaque at shorter wavelengths where the measured emissivity is influenced by the properties of the underlying metal surface. Theoretical predictions of the emissivity of a particulate layer were performed, and results are compared to the measured values. The theory adequately predicts the general features of spectral variation of the emissivity. The predicted trends in emissivity with particle size and deposit composition are also consistent with experimental observations. Total (Planck-weighted) emissivities are calculated from the measured spectral values for the deposits at the tube temperatures. They increase with time from the clean tube value (0.2-0.3) to values typical of deposits formed from western U.S. coals (0.45-0.55). Calculated total absorptivities are found to be lower than the corresponding emissivities.
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.
The Effect of Particle Composition and Temperature on the Deposition of Two Western U.S. Coals in a Laminar Drop-Tube Furnace
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.
Use of Equilibrium Calculations to Predict the Behavior of Coal Ash in Combustion Systems
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.
Simulation of Ash Deposit Growth in a Pulverized Coal-Fired Pilot Scale Reactor
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.
In Situ, Real-Time Characterization of Coal Ash Deposits Using Fourier Transform Infrared Emission Spectroscopy
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.
Ash Deposits, Coal Blends and the Thermal Performance of Furnaces
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.
1992
Characterization of Included and Excluded Minerals in Pulverized Coal
Harb, J.N. and Yu, H.
Proceedings of the Ninth Annual International Pittsburgh Coal Conference, 235-240, Pittsburgh, PA, October 1992. Funded by ACERC.
Ash formed during combustion of pulverized coal produces a variety of operational and environmental problems. Recently, considerable effort has been expended toward the development of techniques and models to predict ash transformations and deposition during combustion. These models require a description of the coal mineral matter as input. It is well known that bulk ASTM analysis of the coal ash does not adequately represent observed deposition behavior. This fact has lead to the development of advanced analytical techniques to provide detailed information on coal mineral size composition, and association with the organic matrix.
The principal technique used for detailed analysis has been computer controlled scanning electron microscopy (CCSEM). Early development of this technique was performed by Huggins et al., who used a scanning electron microscope equipped with a Tracor Northern 2200 X-ray analysis system to examine coal minerals. Extensive development and refinement of the technique for both coal minerals and ash has been done at the Energy and Environmental Research Center at the University of North Dakota on several different Tracor Northern systems. Additional work on the association of the coal minerals with the organic matrix has been performed by Straszheim and co-workers at Iowa State University with use of a sophisticated image analysis system (LeMont Scientific DB-10). In spite of the significant advances that have been made, CCSEM analysis has not yet been standardized and questions persist as to the statistical significance of the data, the minimum number of particles required for the analysis, the quantitative nature of the composition data and the role of ZAF corrections, the influence of sample preparation on the results, etc.
The objectives of the present study were to: 1) implement the CCSEM technique on a JOEL 840A scanning electron microscope equipped with a Link eXL X-ray microanalysis and image analysis system, 2) verify the reproducibility and accuracy of the technique for quantitative analysis of the coal minerals, 3) develop the software routines necessary to determine the mineral content and composition of coal particles on a particle-by-particle basis, and 4) use the above techniques to examine two eastern coals recently tested in a utility boiler.
1991
The Effect of Variations in Particle-to-Particle Composition on the Formation of Ash Deposits
Richards, G.H.; Harb, J.N. and Zygarlicke, C.J.
Proc. of the Engineering Foundation Conference on Inorganic Transformations and Ash Deposition During Combustion, (S.A. Benson, ed.), Engineering Foundation Press, Palm Coast, FL, March 1991. Funded by ACERC.
A mathematical model of the deposition region of a laminar drop-tube furnace was used to examine the effect of variations in the size, composition, and physical properties of ash particles on deposition. Experimentally determined size and composition distributions for the fly ash of two western coals (Dietz and Utah Blind Canyon) were used as input to the model. Sticking coefficients were calculated using capture efficiencies based on particle viscosities and compared to experimental values.
1990
Fireside Corrosion in PC-Fired Boilers
Harb, J.N. and Smith, E.E.
Progress in Energy and Combustion Science, 16, 169-190, (1990). Funded by ACERC.
This review has examined fireside corrosion of pc-fired boilers in both the waterwall and superheater regions. The present understanding of corrosion phenomena has resulted in the development of strategies to control tube wastage. The corrosion problem persists, however, in spite of efforts to control it. The physical mechanisms that govern such corrosion are complex and not fully understood to date. The problem is complicated further by localized attack in the form of pits of cracks that may result in tube failure without a significant decrease in the average tube-wall thickness. Mechanistic models that allow quantitative prediction of local corrosion rates from first principles have not yet been developed. Hence, quantitative prediction of fireside corrosion rates is not feasible at the present time.
Mathematical models, however, may play an important role in the a priori prediction of boiler locations where corrosion problems are likely to develop, and the operating conditions under which corrosion is expected to occur. A combustion model could be used to simulate the environment inside a utility boiler for a variety of fuels and operating conditions. Once the conditions inside the boiler have been modeled, the corrosion behavior expected at a given location could be determined by comparing the local boiler environment with corrosion data obtained experimentally under similar conditions. Corrosion data would be accumulated from experience with industrial boilers and from well-defined laboratory experiments. Such a procedure could provide a valuable tool for use in boiler design and in the prediction of problems associated with a changing fuel supply.
Theoretical Investigation of Ash Transport in a Laminar Drop-Tube Furnace Including Temperature and Compositional Effects
Harb, J.N.; Richards, G.H. and Munson, C.L.
Proceedings of the ASME Ash Deposit and Corrosion Research Committee Seminar on Fireside Fouling Problems, Brigham Young University, Provo, UT, 1990. Funded by ACERC.
A mathematical model was developed to investigate particle deposition in a laminar drop-tube furnace. Specifically, simulations were performed to examine the effects of geometry, transport, plate temperature, and particle composition on deposition. Because of the geometry of the deposition region, the diameter of the inlet particle stream was narrowed and particle impaction rates were significantly enhanced near the center of the plate. The location of particle impact and the temperature of the particle upon impaction were both strongly dependent on particle size. The particle temperature at impaction was relatively insensitive to the plate temperature for particles greater than 15 to 20 µm in diameter at plate temperatures of 750K and 1400K. Calculation of composition effects indicated that particles of different sizes with similar compositions might exhibit significantly different sticking behavior owing to the formation of liquid phases.