Reade, WC
1996
Modeling High Temperature Char Oxidation
Reade, W.C.; Morris, K.; Ness, T. and Hecker, W.C.
Proceedings of the Tenth Annual Technical Review Meeting of the Advanced Combustion Engineering Research Center, Salt Lake City, Utah, March 6, 1996. Funded by ACERC.
A char oxidation model that predicts char burnout behavior over a broad range of temperature and oxygen concentration has been developed. The model accounts for variations in intrinsic reactivity and pore structure with char burnout. Intrinsic rates are obtained as a continuous function of char conversion from low-temperature isothermal TGA measurements. A port structure model based on three sizes of pores is included to calculate the effective diffusivity of oxidizer through the particle, which in turn is used to calculate the oxygen concentration profile as a function of particle burnout levels. Since the TGA data are base on reactivity per residual mass of the char sample, the effects of char heterogeneities and surface area evolution are implicit in the intrinsic kinetic, thus eliminating the need to predict surface area evolution which has been a problem in previous models.
Model predictions of high-temperature global rates and mass loss as a function of residence time, oxygen partial pressure, and parent coal rank have been compared with experimental data, both reported in the literature and obtained in our high temperature reactor. Chars tested to date include those from Pittsburgh #8, Pocahontas #3, Illinois #6, Deitz and North Dakota coals. To validate the model's ability to predict the effects of oxygen concentration, Pittsburgh #8 char was oxidized at two different temperatures and five different oxygen concentrations (ranging from 0-15%) in the high temperature reactor. The model predictions fit the experimental data very well for all 10 data pints, with variations ranging between 1 and 7%. Model predictions were also compared to the extensive high-temperature data of Sandi (Hurt and Mitchell, 1992) in order to test the model's ability to predict combustion behavior as a function of parent coal rank. The comparison was made for chars from 5 different coals and the agreement in burning rate and burning time for all five chars was again excellent.
1994
An Improved Method for Predicting High-Temperature Char Oxidation Rates
Reade, W.C.
An Improved Method for Predicting High-Temperature Char Oxidation Rates, M.S./BYU, December 1996. Advisor: Hecker
Modeling the Effects of Burnout on High-Temperature Char Oxidation
Reade, W.C. and Hecker, W.C.
Proceedings of the 17th Sympsium of the Rocky Mountain Fuels Society, Golden, CO, March 1994; Western States Section/The Combustion Institute, Davis, CA, March 1994; and AIChE Annual Meeting, San Francisco, CA, November 1994. Funded by ACERC.
In this work a char oxidation model that predicts changes in high-temperature reactivity with particle burnout is described. This model accounts for changes due to pore structure evolution and reactivity distributions, although there are many factors that can affect high-temperature reactivity (e.g., densification, mineral matter effects, etc.). Some factors, such as reactivity changes due to gasification-induced densification (Hurt, 1988), are implicitly included in the reactivity distribution effects. The only input parameters needed for this model are low-temperature kinetics (as measured by isothermal thermogravimetric analysis) and an approximate pore structure. This model is unique in that the pore structure evolution/reactivity distribution effects are lumped together in a preexponential that is a function of char conversion. The model predictions of high-temperature rates for two chars (Dietz and Pitt. #8) are compared with experimental data (Mitchell, et al., 1992) and show excellent agreement.
1992
Effects of Burnout on Char Oxidation Kinetics
Hecker, W.C.; McDonald, K.M.; Reade, W.; Jackson, C.D. and Cope R.F.
Twenty-Fourth Symposium (International) on Combustion, Sydney, Australia, July 1992. (Previously presented at the 1991 Annual Meeting of the American Institute of Chemical Engineers, Los Angeles, CA, November 1991). Funded by ACERC.
The development and validation of accurate coal combustion modeling requires separate characterization of the coal devolatilization and char oxidation processes. The characteristic time for heterogeneous oxidation of a char particle in a commercial combustor is 1-2 orders of magnitude greater than the time required for devolatilization and homogeneous oxidation of the volatiles. Thus the rate of combustion and hence the efficiency of the coal combustion process will be governed by the rate of oxidation of the nonvolatile coal char.
The effects of both extent of burnout and type of burnout on char oxidation rates and rate parameters have been investigated for chars prepared from Dietz (subbituminous B) coal. Intrinsic rates and rate parameters (reaction order, activation energy, and pre-exponential factor) were determined using isothermal thermogravimetric analysis (TGA). N2BET and CO2DP surface areas were measured, as was hydrogen to carbon ratio. Three types of burnout were studied and compared. Devolatilization mass loss (DML) was studied by devolatilizing the Dietz coal to various extents in a flat-flame methane burner (FFB) and then comparing the oxidation rates and other properties of the resulting chars. High-temperature oxidation burnout was studied by taking a given FFB Dietz char and oxidizing it to various conversion levels in a drop tube reactor at high temperatures characteristic of industrial combustors. The oxidation rates and kinetics of these partially burned out char samples were then determined using TGA. Low-temperature oxidation burnout was studied by oxidizing the given FFB Dietz char to a continuum of burnout levels in the TGA at low temperature (550 to 750 K) and obtaining the instantaneous oxidation rates, which are inherent in the TGA experiment.
The rate of oxidation was found to decrease with increasing devolatilization residence time, even after devolatilization mass loss has become constant. Increasing N2 and CO2 surface areas with devolatilization residence time are inverse to the decreasing rates, and a constant difference between N2 and CO2 surface areas indicates dramatic changes in the mesopore surface area during devolatilization, but not in the micropore surface area. Intrinsic rates of chars oxidized at high temperatures were found to decrease with burnout level, while those of chars oxidized at low temperatures were essentially constant with burnout level.
1991
Effects of Burnout on Intrinsic Char Oxidation Kinetics
McDonald, K.M.; Reade, W.C.; Cope, R.F. and Hecker, W.C.
Western States Section/The Combustion Institute, Los Angeles, CA, October 1991. Funded by ACERC.
The development and validation of accurate coal combustion modeling requires separate characterization of the coal devolatilization and char oxidation processes. The characteristic time for heterogeneous oxidation of a char particle in a commercial combustor is 1-2 orders of magnitude greater than the time required for devolatilization and homogeneous oxidation of the volatiles. Thus the rate of combustion and hence the efficiency of the coal combustion process will be governed by the rate of oxidation of the nonvolatile coal char.
The effects of both extent of burnout and type of burnout on char oxidation rates and rate parameters have been investigated for chars prepared from Dietz (subbituminous B) coal. Intrinsic rates and rate parameters (reaction order, activation energy, and pre-exponential factor) were determined using isothermal thermogravimetric analysis (TGA). N2BET and CO2DP surface areas were measured, as was hydrogen to carbon ratio. Three types of burnout were studied and compared. Devolatilization mass loss (DML) was studied by devolatilizing the Dietz coal to various extents in a flat-flame methane burner (FFB) and then comparing the oxidation rates and other properties of the resulting chars. High-temperature oxidation burnout was studied by taking a given FFB Dietz char and oxidizing it to various conversion levels in a drop tube reactor at high temperatures characteristic of industrial combustors. The oxidation rates and kinetics of these partially burned out char samples were then determined using TGA. Low-temperature oxidation burnout was studied by oxidizing the given FFB Dietz char to a continuum of burnout levels in the TGA at low temperature (550 to 750 K) and obtaining the instantaneous oxidation rates, which are inherent in the TGA experiment.
The rate of oxidation was found to decrease with increasing devolatilization residence time, even after devolatilization mass loss has become constant. Increasing N2 and CO2 surface areas with devolatilization residence time are inverse to the decreasing rates, and a constant difference between N2 and CO2 surface areas indicates dramatic changes in the mesopore surface area during devolatilization, but not in the micropore surface area. Intrinsic rates of chars oxidized at high temperatures were found to decrease with burnout level, while those of chars oxidized at low temperatures were essentially constant with burnout level.
Effects of Burnout on Char Oxidation Kinetics
Hecker, W.C.; McDonald, K.M.; Reade, W.C.; Jackson, C.D. and Cope R.F.
1991 Annual Meeting of the American Institute of Chemical Engineers, Los Angeles, CA, November 1991. Funded by ACERC.
The development and validation of accurate coal combustion modeling requires separate characterization of the coal devolatilization and char oxidation processes. The characteristic time for heterogeneous oxidation of a char particle in a commercial combustor is 1-2 orders of magnitude greater than the time required for devolatilization and homogeneous oxidation of the volatiles. Thus the rate of combustion and hence the efficiency of the coal combustion process will be governed by the rate of oxidation of the nonvolatile coal char.
The effects of both extent of burnout and type of burnout on char oxidation rates and rate parameters have been investigated for chars prepared from Dietz (subbituminous B) coal. Intrinsic rates and rate parameters (reaction order, activation energy, and pre-exponential factor) were determined using isothermal thermogravimetric analysis (TGA). N2BET and CO2DP surface areas were measured, as was hydrogen to carbon ratio. Three types of burnout were studied and compared. Devolatilization mass loss (DML) was studied by devolatilizing the Dietz coal to various extents in a flat-flame methane burner (FFB) and then comparing the oxidation rates and other properties of the resulting chars. High-temperature oxidation burnout was studied by taking a given FFB Dietz char and oxidizing it to various conversion levels in a drop tube reactor at high temperatures characteristic of industrial combustors. The oxidation rates and kinetics of these partially burned out char samples were then determined using TGA. Low-temperature oxidation burnout was studied by oxidizing the given FFB Dietz char to a continuum of burnout levels in the TGA at low temperature (550 to 750 K) and obtaining the instantaneous oxidation rates, which are inherent in the TGA experiment.
The rate of oxidation was found to decrease with increasing devolatilization residence time, even after devolatilization mass loss has become constant. Increasing N2 and CO2 surface areas with devolatilization residence time are inverse to the decreasing rates, and a constant difference between N2 and CO2 surface areas indicates dramatic changes in the mesopore surface area during devolatilization, but not in the micropore surface area. Intrinsic rates of chars oxidized at high temperatures were found to decrease with burnout level, while those of chars oxidized at low temperatures were essentially constant with burnout level.
Reade, WC
1996
Modeling High Temperature Char Oxidation
Reade, W.C.; Morris, K.; Ness, T. and Hecker, W.C.
Proceedings of the Tenth Annual Technical Review Meeting of the Advanced Combustion Engineering Research Center, Salt Lake City, Utah, March 6, 1996. Funded by ACERC.
A char oxidation model that predicts char burnout behavior over a broad range of temperature and oxygen concentration has been developed. The model accounts for variations in intrinsic reactivity and pore structure with char burnout. Intrinsic rates are obtained as a continuous function of char conversion from low-temperature isothermal TGA measurements. A port structure model based on three sizes of pores is included to calculate the effective diffusivity of oxidizer through the particle, which in turn is used to calculate the oxygen concentration profile as a function of particle burnout levels. Since the TGA data are base on reactivity per residual mass of the char sample, the effects of char heterogeneities and surface area evolution are implicit in the intrinsic kinetic, thus eliminating the need to predict surface area evolution which has been a problem in previous models.
Model predictions of high-temperature global rates and mass loss as a function of residence time, oxygen partial pressure, and parent coal rank have been compared with experimental data, both reported in the literature and obtained in our high temperature reactor. Chars tested to date include those from Pittsburgh #8, Pocahontas #3, Illinois #6, Deitz and North Dakota coals. To validate the model's ability to predict the effects of oxygen concentration, Pittsburgh #8 char was oxidized at two different temperatures and five different oxygen concentrations (ranging from 0-15%) in the high temperature reactor. The model predictions fit the experimental data very well for all 10 data pints, with variations ranging between 1 and 7%. Model predictions were also compared to the extensive high-temperature data of Sandi (Hurt and Mitchell, 1992) in order to test the model's ability to predict combustion behavior as a function of parent coal rank. The comparison was made for chars from 5 different coals and the agreement in burning rate and burning time for all five chars was again excellent.
1994
An Improved Method for Predicting High-Temperature Char Oxidation Rates
Reade, W.C.
An Improved Method for Predicting High-Temperature Char Oxidation Rates, M.S./BYU, December 1996. Advisor: Hecker
Modeling the Effects of Burnout on High-Temperature Char Oxidation
Reade, W.C. and Hecker, W.C.
Proceedings of the 17th Sympsium of the Rocky Mountain Fuels Society, Golden, CO, March 1994; Western States Section/The Combustion Institute, Davis, CA, March 1994; and AIChE Annual Meeting, San Francisco, CA, November 1994. Funded by ACERC.
In this work a char oxidation model that predicts changes in high-temperature reactivity with particle burnout is described. This model accounts for changes due to pore structure evolution and reactivity distributions, although there are many factors that can affect high-temperature reactivity (e.g., densification, mineral matter effects, etc.). Some factors, such as reactivity changes due to gasification-induced densification (Hurt, 1988), are implicitly included in the reactivity distribution effects. The only input parameters needed for this model are low-temperature kinetics (as measured by isothermal thermogravimetric analysis) and an approximate pore structure. This model is unique in that the pore structure evolution/reactivity distribution effects are lumped together in a preexponential that is a function of char conversion. The model predictions of high-temperature rates for two chars (Dietz and Pitt. #8) are compared with experimental data (Mitchell, et al., 1992) and show excellent agreement.
1992
Effects of Burnout on Char Oxidation Kinetics
Hecker, W.C.; McDonald, K.M.; Reade, W.; Jackson, C.D. and Cope R.F.
Twenty-Fourth Symposium (International) on Combustion, Sydney, Australia, July 1992. (Previously presented at the 1991 Annual Meeting of the American Institute of Chemical Engineers, Los Angeles, CA, November 1991). Funded by ACERC.
The development and validation of accurate coal combustion modeling requires separate characterization of the coal devolatilization and char oxidation processes. The characteristic time for heterogeneous oxidation of a char particle in a commercial combustor is 1-2 orders of magnitude greater than the time required for devolatilization and homogeneous oxidation of the volatiles. Thus the rate of combustion and hence the efficiency of the coal combustion process will be governed by the rate of oxidation of the nonvolatile coal char.
The effects of both extent of burnout and type of burnout on char oxidation rates and rate parameters have been investigated for chars prepared from Dietz (subbituminous B) coal. Intrinsic rates and rate parameters (reaction order, activation energy, and pre-exponential factor) were determined using isothermal thermogravimetric analysis (TGA). N2BET and CO2DP surface areas were measured, as was hydrogen to carbon ratio. Three types of burnout were studied and compared. Devolatilization mass loss (DML) was studied by devolatilizing the Dietz coal to various extents in a flat-flame methane burner (FFB) and then comparing the oxidation rates and other properties of the resulting chars. High-temperature oxidation burnout was studied by taking a given FFB Dietz char and oxidizing it to various conversion levels in a drop tube reactor at high temperatures characteristic of industrial combustors. The oxidation rates and kinetics of these partially burned out char samples were then determined using TGA. Low-temperature oxidation burnout was studied by oxidizing the given FFB Dietz char to a continuum of burnout levels in the TGA at low temperature (550 to 750 K) and obtaining the instantaneous oxidation rates, which are inherent in the TGA experiment.
The rate of oxidation was found to decrease with increasing devolatilization residence time, even after devolatilization mass loss has become constant. Increasing N2 and CO2 surface areas with devolatilization residence time are inverse to the decreasing rates, and a constant difference between N2 and CO2 surface areas indicates dramatic changes in the mesopore surface area during devolatilization, but not in the micropore surface area. Intrinsic rates of chars oxidized at high temperatures were found to decrease with burnout level, while those of chars oxidized at low temperatures were essentially constant with burnout level.
1991
Effects of Burnout on Intrinsic Char Oxidation Kinetics
McDonald, K.M.; Reade, W.C.; Cope, R.F. and Hecker, W.C.
Western States Section/The Combustion Institute, Los Angeles, CA, October 1991. Funded by ACERC.
The development and validation of accurate coal combustion modeling requires separate characterization of the coal devolatilization and char oxidation processes. The characteristic time for heterogeneous oxidation of a char particle in a commercial combustor is 1-2 orders of magnitude greater than the time required for devolatilization and homogeneous oxidation of the volatiles. Thus the rate of combustion and hence the efficiency of the coal combustion process will be governed by the rate of oxidation of the nonvolatile coal char.
The effects of both extent of burnout and type of burnout on char oxidation rates and rate parameters have been investigated for chars prepared from Dietz (subbituminous B) coal. Intrinsic rates and rate parameters (reaction order, activation energy, and pre-exponential factor) were determined using isothermal thermogravimetric analysis (TGA). N2BET and CO2DP surface areas were measured, as was hydrogen to carbon ratio. Three types of burnout were studied and compared. Devolatilization mass loss (DML) was studied by devolatilizing the Dietz coal to various extents in a flat-flame methane burner (FFB) and then comparing the oxidation rates and other properties of the resulting chars. High-temperature oxidation burnout was studied by taking a given FFB Dietz char and oxidizing it to various conversion levels in a drop tube reactor at high temperatures characteristic of industrial combustors. The oxidation rates and kinetics of these partially burned out char samples were then determined using TGA. Low-temperature oxidation burnout was studied by oxidizing the given FFB Dietz char to a continuum of burnout levels in the TGA at low temperature (550 to 750 K) and obtaining the instantaneous oxidation rates, which are inherent in the TGA experiment.
The rate of oxidation was found to decrease with increasing devolatilization residence time, even after devolatilization mass loss has become constant. Increasing N2 and CO2 surface areas with devolatilization residence time are inverse to the decreasing rates, and a constant difference between N2 and CO2 surface areas indicates dramatic changes in the mesopore surface area during devolatilization, but not in the micropore surface area. Intrinsic rates of chars oxidized at high temperatures were found to decrease with burnout level, while those of chars oxidized at low temperatures were essentially constant with burnout level.
Effects of Burnout on Char Oxidation Kinetics
Hecker, W.C.; McDonald, K.M.; Reade, W.C.; Jackson, C.D. and Cope R.F.
1991 Annual Meeting of the American Institute of Chemical Engineers, Los Angeles, CA, November 1991. Funded by ACERC.
The development and validation of accurate coal combustion modeling requires separate characterization of the coal devolatilization and char oxidation processes. The characteristic time for heterogeneous oxidation of a char particle in a commercial combustor is 1-2 orders of magnitude greater than the time required for devolatilization and homogeneous oxidation of the volatiles. Thus the rate of combustion and hence the efficiency of the coal combustion process will be governed by the rate of oxidation of the nonvolatile coal char.
The effects of both extent of burnout and type of burnout on char oxidation rates and rate parameters have been investigated for chars prepared from Dietz (subbituminous B) coal. Intrinsic rates and rate parameters (reaction order, activation energy, and pre-exponential factor) were determined using isothermal thermogravimetric analysis (TGA). N2BET and CO2DP surface areas were measured, as was hydrogen to carbon ratio. Three types of burnout were studied and compared. Devolatilization mass loss (DML) was studied by devolatilizing the Dietz coal to various extents in a flat-flame methane burner (FFB) and then comparing the oxidation rates and other properties of the resulting chars. High-temperature oxidation burnout was studied by taking a given FFB Dietz char and oxidizing it to various conversion levels in a drop tube reactor at high temperatures characteristic of industrial combustors. The oxidation rates and kinetics of these partially burned out char samples were then determined using TGA. Low-temperature oxidation burnout was studied by oxidizing the given FFB Dietz char to a continuum of burnout levels in the TGA at low temperature (550 to 750 K) and obtaining the instantaneous oxidation rates, which are inherent in the TGA experiment.
The rate of oxidation was found to decrease with increasing devolatilization residence time, even after devolatilization mass loss has become constant. Increasing N2 and CO2 surface areas with devolatilization residence time are inverse to the decreasing rates, and a constant difference between N2 and CO2 surface areas indicates dramatic changes in the mesopore surface area during devolatilization, but not in the micropore surface area. Intrinsic rates of chars oxidized at high temperatures were found to decrease with burnout level, while those of chars oxidized at low temperatures were essentially constant with burnout level.