Richards, GH
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.
Investigation of Mechanisms for the Formation of Fly Ash and Ash Deposits for Two Powder River Basin Coals
Richards, G.H.
Investigation of Mechanisms for the Formation of Fly Ash and Ash Deposits for Two Powder River Basin Coals, Ph.D./BYU, December 1994. Advisor: Harb
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
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.
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
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.
Richards, GH
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.
Investigation of Mechanisms for the Formation of Fly Ash and Ash Deposits for Two Powder River Basin Coals
Richards, G.H.
Investigation of Mechanisms for the Formation of Fly Ash and Ash Deposits for Two Powder River Basin Coals, Ph.D./BYU, December 1994. Advisor: Harb
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
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.
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
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.