McDonald, KM
1992
Low-Temperature Char Oxidation Kinetics: Effect of Preparation Method
McDonald, K.M.; Hyde, W.D. and Hecker, W.C.
Fuel, 71(3):319-323, 1992. Funded by ACERC.
Chars derived from Beulah-Zap (lignite A) and Dietz (sub-bituminous B) coals were prepared by three different methods utilizing three different reactor systems. These included a high heating rate method achieved in a methane flat flame burner, a moderate heating rate method achieved in a drop tube reactor, and a slow heating rate method achieved in a muffle furnace. The flat flame char was produced in a flame environment with excess oxygen, while the drop tube and muffle furnace chars were produced in inert environments. Low temperature oxidation rates and kinetic parameters were determined using isothermal thermogravimetric analysis (TGA) at temperatures between 550 and 950 K. Reactivities at different oxidation burnout levels (10%-75%) were compared on both an initial mass and an available mass basis. Using the available mass basis, rates in the intrinsic regime were found to be nearly identical for the different burnout levels. It was also found that the lower burnout levels are more highly influenced by diffusion effects. This was manifest by a decrease in the slope of the Arrhenius plot that began at a temperature of about 750 K for the char at 10% burnout compared to a temperature of nearly 900 K for the char at 75% burnout. In comparing the chars produced by the three different methods, reactivities in the reaction control regime showed that, for both coals, the drop tube char was more reactive than either the flat flame or muffle furnace char. Further tests indicated that the drop tube chars had a hydrogen to carbon ratio that was 2.5 to 5 times greater than the char from either of the other reactors and the devolatilization conversion was significantly less. The activation energies for all three Beulah-Zap chars and for the Dietz muffle furnace and flat flame chars were found to be 28.1±0.6 kcal/mol. A comparison of the reactivities for the flat flame burner chars of the lignite and the sub-bituminous showed that the lignite chars were more reactive by a factor of two. This was consistent over all burnout levels. Further work with the Dietz flat flame char showed the dependency on oxygen concentration, yielding an apparent reaction order of 0.67±0.03. This is in excellent agreement with data found in the literature.
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
Low-Temperature Char Oxidation Kinetics: Effect of Preparation Method
McDonald, K.M.; Hyde, W.D. and Hecker, W.C.
Fuel, 1991 (in press). Funded by ACERC.
Chars derived from Beulah-Zap (lignite A) and Dietz (sub-bituminous B) coals were prepared by three different methods utilizing three different reactor systems. These included a high heating rate method achieved in a methane flat flame burner, a moderate heating rate method achieved in a drop tube reactor, and a slow heating rate method achieved in a muffle furnace. The flat flame char was produced in a flame environment with excess oxygen, while the drop tube and muffle furnace chars were produced in inert environments. Low temperature oxidation rates and kinetic parameters were determined using isothermal thermogravimetric analysis (TGA) at temperatures between 550 and 950 K. Reactivities at different oxidation burnout levels (10%-75%) were compared on both an initial mass and an available mass basis. Using the available mass basis, rates in the intrinsic regime were found to be nearly identical for the different burnout levels. It was also found that the lower burnout levels are more highly influenced by diffusion effects. This was manifest by a decrease in the slope of the Arrhenius plot that began at a temperature of about 750 K for the char at 10% burnout compared to a temperature of nearly 900 K for the char at 75% burnout. In comparing the chars produced by the three different methods, reactivities in the reaction control regime showed that, for both coals, the drop tube char was more reactive than either the flat flame or muffle furnace char. Further tests indicated that the drop tube chars had a hydrogen to carbon ratio that was 2.5 to 5 times greater than the char from either of the other reactors and the devolatilization conversion was significantly less. The activation energies for all three Beulah-Zap chars and for the Dietz muffle furnace and flat flame chars were found to be 28.1±0.6 kcal/mol. A comparison of the reactivities for the flat flame burner chars of the lignite and the sub-bituminous showed that the lignite chars were more reactive by a factor of two. This was consistent over all burnout levels. Further work with the Dietz flat flame char showed the dependency on oxygen concentration, yielding an apparent reaction order of 0.67±0.03. This is in excellent agreement with data found in the literature.
Effects of Burnout on Char Oxidation Rates
Hecker, W.C.; McDonald, K.M.; Jackson, C.D. and Cope, R.F.
Proc. of the 1991 International Conference on Coal Science, 263-266, (IEA Coal Research, ed.), Butterworth-Heinemann, London, September 1991. Funded by ACERC.
To accurately model the combustion of coal, it is necessary to characterize both the coal devolatilization and char oxidation processes. To characterize char oxidation, the heterogeneous reaction of devolatilized coal with oxygen, it is important to understand how the kinetics of char oxidation vary with the burnout (or conversion) level of the char. For example, as a char particle burns, does its rate (normalized to the instantaneous amount of combustible material) vary or does it remain constant? It is also important to know how the degree of devolatilization achieved in preparing a char affects the oxidation kinetics of the resulting char. The objective of this study was to determine the effects of 1) degree of devolatilization, 2) extent of oxidation burnout, and 3) type of oxidation burnout on intrinsic char oxidation rates. Types of oxidation burnout include both those achieved at high temperatures (in a drop tube reactor) and those achieved at low temperatures (in a thermogravimetric analyzer, TGA). The intrinsic (i.e. free of transport limitations) rate and activation energy of each char was determined using isothermal TGA. Hydrogen/carbon ratio and N2 surface area were also measured, and attempts were made to relate these properties to differences in the observed char oxidation rates.
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.
1990
Low Temperature Kinetics of Coal Char Oxidation
McDonald, K.M.; Hyde, W.D. and Hecker, W.C.
Western States Section/The Combustion Institute, San Diego, CA, 1990. Funded by ACERC.
An accurate char oxidation submodel is an essential element of a realistic model of coal combustion. To aid in the development and validation of such a submodel, which will be used in comprehensive combustion codes being developed at ACERC, oxidation rates and rate parameters were measured for chars prepared from ACERC coals. In this study, chars derived from Beulah-Zap (lignite A) and Dietz (sub-bituminous B) coals were prepared in three different reactor systems: (1) a flat-flame methane burner, (2) an inert-atmosphere drop tube reactor, and (3) a muffle furnace. Low temperature oxidation rates and kinetic parameters were determined using isothermal thermogravimetric analysis (TGA) at temperatures between 550 and 950 K.
Reactivities at different oxidation burnout levels (10%-75%) were compared on both an initial mass and an available mass basis. The initial mass basis provided no clear distinction from which behavioral differences in the different burnout levels could be deduced. Basing rates on the available mass, however, elucidated the fact that in the intrinsic regime, rates are nearly identical for the different burnout levels. The available mass basis also made clear the fact that the lower burnout levels are more highly influenced by diffusion effects. This was manifest by a decrease in the slope of the Arrhenius plot beginning at a temperature of about 750 K for the char at 10% burnout compared to a temperature of nearly 900 K for the char at 75% burnout.
In comparing the chars produced in the three different reactors, reactivities in the reaction control regime showed that, for both coals, the drop tube char was more reactive than either the flat flame or muffle furnace char. Further tests indicated that the drop tube chars had a hydrogen to carbon ratio that was 2.5 to 5 times greater than the char from either of the other reactors and the percent conversion for devolatilization was significantly less. The activation energy for all three Beulah-Zap chars and for the Dietz muffle furnace and flat flame chars was found to be 28.10.6 kcal/mol. Data from the Dietz drop tube char is as yet insufficient to yield an accurate activation energy value.
A comparison of the reactivities for the flat flame burner chars of the lignite and the sub-bituminous showed that the lignite chars were more reactive by a factor of two. This was consistent over all burnout levels. Further work with the Dietz flat flame char showed an apparent reaction order of 0.66. This is in excellent agreement with data found in the literature.
1989
Effects of Preparation Variables on Reactivity and Surface Properties of ACERC Coal Chars
Hyde, W.D.; McDonald, K.M.; Cope, R.F.; Bartholomew, C.H. and Hecker, W.C.
Twelfth Symposium of the Rocky Mountain Fuels Society, Denver, 1989. (Also presented at the Rocky Mountain Regional AIChE Meeting, Salt Lake City, 1989). Funded by ACERC (National Science Foundation and Associates and Affiliates).
The combustion of coal consists of devolatilization and heterogenous char oxidation. The objective of this study is to understand the effect of devolatilization conditions on the oxidation rate (reactivity) and surface properties of the resultant char. Chars were produced by devolatilizing Pittsburgh No. 8 bituminous coal and North Dakota lignite at varying residence times and peak temperatures in a flat flame methane burner. Thermogravimetric analysis was used to determine the low-temperature reactivity of these chars. Trends of increased reactivity with decreased preparation temperature and residence time are observed. N2 and CO2 surface areas and pore size distributions were determined. Preliminary results indicate that surface areas increase with increased devolatilization. Char reactivity is also correlated with hydrogen content and aromatic cluster size.
Effects of Char Preparation Variables on Reactivity and Surface Properties of Chars Derived from ANL Coals
Hecker, W.C.; Hyde, W.D. and McDonald, K.M.
International Chemical Congress of Pacific Basin Societies, Honolulu, 1989. Funded by ACERC (National Science Foundation and Associates and Affiliates).
The objective of this study is to understand the effect of devolatilization conditions on the oxidation rate (reactivity) and surface properties of the resultant char. Chars were produced from 3 ANL coals (Pittsburgh No. 8, Beulah Zap, and Pocahontas No. 3) and Dietz subbituminous (PETC) by three different methods: (1) a flat flame methane burner, (2) a rapid heating-rate inert atmosphere reactor, and (3) a slow heating-rate muffle furnace. Thermogravimetric analysis (TGA) was used to determine the low-temperature reactivity of these chars as a function of residence time and peak temperature. CO2 surface areas pore size distributions were also determined for some samples. Trends of increased reactivity with decreasing rank and decreasing residence time were observed. Preliminary results indicate that surface area goes through a maximum with increasing devolatilization time for Pittsburgh No. 8 while it is essentially unaffected for the Zap lignite char. Char reactivity appears to be correlated with hydrogen content.
The Effects of Rank and Preparation Method on Coal Char Oxidation Rates
Hyde, W.D.; Hecker, W.C.; Cope, R.F.; Painter, M.M.; McDonald, K.M. and Bartholomew, C.H.
Western States Section/The Combustion Institute, Livermore, California, 1989. Funded by ACERC (National Science Foundation and Associates and Affiliates).
Coal char reactivity has been found to vary greatly depending on the rank and type of the parent coal. Also, the conditions under which a given coal is devolatilized to produce char can significantly effect the reactivity of the resulting char. Preparation conditions such as gas environment, heating rate, peak temperature, residence time, and particle size are very important in determining the resulting char reactivity in that they effect its chemical and physical structure.
The general objectives of this work are to (1) understand the effect of the rank of the parent coal on the oxidation rate (reactivity) of its derived char, (2) understand the effect of devolatilization conditions on the char oxidation rate, and (3) determine any correlations which may exist between char oxidation rates and the chemical and physical properties of the chars. Specifically, oxidation rates for char from 5 coals of various ranks were measured and compared. The differences in char reactivity of chars produced in three different char preparation apparatus: a muffle furnace, a flat-flame methane burner, and a high temperature inert-atmosphere reactor, were studied. The effects of peak temperature, residence time, and particle size were also studied. Finally, correlations of oxidation rate with hydrogen content, cluster size, and surface area were attempted.
Samples of chars from Beulah Zap (Lignite), Dietz (Subbituminous A), Utah Blind Canyon (hvC Bituminous), Pittsburgh #8 (hvA Bituminous), and Pocahontas #3 (lv Bituminous) were prepared at different residence times in the Flat-Flame Char Preparation Apparatus; samples of Pittsburgh #8, Beulah Zap, and Dietz were also prepared in the muffle furnace and the high temperature inert-atmosphere reactor. The low temperature reactivity of all the coal char samples was determined in a TGA using Tcrit as the reactivity indicator. Tcrit is defined at the temperature at which the mass loss of the sample reaches 11 percent per minute.
McDonald, KM
1992
Low-Temperature Char Oxidation Kinetics: Effect of Preparation Method
McDonald, K.M.; Hyde, W.D. and Hecker, W.C.
Fuel, 71(3):319-323, 1992. Funded by ACERC.
Chars derived from Beulah-Zap (lignite A) and Dietz (sub-bituminous B) coals were prepared by three different methods utilizing three different reactor systems. These included a high heating rate method achieved in a methane flat flame burner, a moderate heating rate method achieved in a drop tube reactor, and a slow heating rate method achieved in a muffle furnace. The flat flame char was produced in a flame environment with excess oxygen, while the drop tube and muffle furnace chars were produced in inert environments. Low temperature oxidation rates and kinetic parameters were determined using isothermal thermogravimetric analysis (TGA) at temperatures between 550 and 950 K. Reactivities at different oxidation burnout levels (10%-75%) were compared on both an initial mass and an available mass basis. Using the available mass basis, rates in the intrinsic regime were found to be nearly identical for the different burnout levels. It was also found that the lower burnout levels are more highly influenced by diffusion effects. This was manifest by a decrease in the slope of the Arrhenius plot that began at a temperature of about 750 K for the char at 10% burnout compared to a temperature of nearly 900 K for the char at 75% burnout. In comparing the chars produced by the three different methods, reactivities in the reaction control regime showed that, for both coals, the drop tube char was more reactive than either the flat flame or muffle furnace char. Further tests indicated that the drop tube chars had a hydrogen to carbon ratio that was 2.5 to 5 times greater than the char from either of the other reactors and the devolatilization conversion was significantly less. The activation energies for all three Beulah-Zap chars and for the Dietz muffle furnace and flat flame chars were found to be 28.1±0.6 kcal/mol. A comparison of the reactivities for the flat flame burner chars of the lignite and the sub-bituminous showed that the lignite chars were more reactive by a factor of two. This was consistent over all burnout levels. Further work with the Dietz flat flame char showed the dependency on oxygen concentration, yielding an apparent reaction order of 0.67±0.03. This is in excellent agreement with data found in the literature.
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
Low-Temperature Char Oxidation Kinetics: Effect of Preparation Method
McDonald, K.M.; Hyde, W.D. and Hecker, W.C.
Fuel, 1991 (in press). Funded by ACERC.
Chars derived from Beulah-Zap (lignite A) and Dietz (sub-bituminous B) coals were prepared by three different methods utilizing three different reactor systems. These included a high heating rate method achieved in a methane flat flame burner, a moderate heating rate method achieved in a drop tube reactor, and a slow heating rate method achieved in a muffle furnace. The flat flame char was produced in a flame environment with excess oxygen, while the drop tube and muffle furnace chars were produced in inert environments. Low temperature oxidation rates and kinetic parameters were determined using isothermal thermogravimetric analysis (TGA) at temperatures between 550 and 950 K. Reactivities at different oxidation burnout levels (10%-75%) were compared on both an initial mass and an available mass basis. Using the available mass basis, rates in the intrinsic regime were found to be nearly identical for the different burnout levels. It was also found that the lower burnout levels are more highly influenced by diffusion effects. This was manifest by a decrease in the slope of the Arrhenius plot that began at a temperature of about 750 K for the char at 10% burnout compared to a temperature of nearly 900 K for the char at 75% burnout. In comparing the chars produced by the three different methods, reactivities in the reaction control regime showed that, for both coals, the drop tube char was more reactive than either the flat flame or muffle furnace char. Further tests indicated that the drop tube chars had a hydrogen to carbon ratio that was 2.5 to 5 times greater than the char from either of the other reactors and the devolatilization conversion was significantly less. The activation energies for all three Beulah-Zap chars and for the Dietz muffle furnace and flat flame chars were found to be 28.1±0.6 kcal/mol. A comparison of the reactivities for the flat flame burner chars of the lignite and the sub-bituminous showed that the lignite chars were more reactive by a factor of two. This was consistent over all burnout levels. Further work with the Dietz flat flame char showed the dependency on oxygen concentration, yielding an apparent reaction order of 0.67±0.03. This is in excellent agreement with data found in the literature.
Effects of Burnout on Char Oxidation Rates
Hecker, W.C.; McDonald, K.M.; Jackson, C.D. and Cope, R.F.
Proc. of the 1991 International Conference on Coal Science, 263-266, (IEA Coal Research, ed.), Butterworth-Heinemann, London, September 1991. Funded by ACERC.
To accurately model the combustion of coal, it is necessary to characterize both the coal devolatilization and char oxidation processes. To characterize char oxidation, the heterogeneous reaction of devolatilized coal with oxygen, it is important to understand how the kinetics of char oxidation vary with the burnout (or conversion) level of the char. For example, as a char particle burns, does its rate (normalized to the instantaneous amount of combustible material) vary or does it remain constant? It is also important to know how the degree of devolatilization achieved in preparing a char affects the oxidation kinetics of the resulting char. The objective of this study was to determine the effects of 1) degree of devolatilization, 2) extent of oxidation burnout, and 3) type of oxidation burnout on intrinsic char oxidation rates. Types of oxidation burnout include both those achieved at high temperatures (in a drop tube reactor) and those achieved at low temperatures (in a thermogravimetric analyzer, TGA). The intrinsic (i.e. free of transport limitations) rate and activation energy of each char was determined using isothermal TGA. Hydrogen/carbon ratio and N2 surface area were also measured, and attempts were made to relate these properties to differences in the observed char oxidation rates.
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.
1990
Low Temperature Kinetics of Coal Char Oxidation
McDonald, K.M.; Hyde, W.D. and Hecker, W.C.
Western States Section/The Combustion Institute, San Diego, CA, 1990. Funded by ACERC.
An accurate char oxidation submodel is an essential element of a realistic model of coal combustion. To aid in the development and validation of such a submodel, which will be used in comprehensive combustion codes being developed at ACERC, oxidation rates and rate parameters were measured for chars prepared from ACERC coals. In this study, chars derived from Beulah-Zap (lignite A) and Dietz (sub-bituminous B) coals were prepared in three different reactor systems: (1) a flat-flame methane burner, (2) an inert-atmosphere drop tube reactor, and (3) a muffle furnace. Low temperature oxidation rates and kinetic parameters were determined using isothermal thermogravimetric analysis (TGA) at temperatures between 550 and 950 K.
Reactivities at different oxidation burnout levels (10%-75%) were compared on both an initial mass and an available mass basis. The initial mass basis provided no clear distinction from which behavioral differences in the different burnout levels could be deduced. Basing rates on the available mass, however, elucidated the fact that in the intrinsic regime, rates are nearly identical for the different burnout levels. The available mass basis also made clear the fact that the lower burnout levels are more highly influenced by diffusion effects. This was manifest by a decrease in the slope of the Arrhenius plot beginning at a temperature of about 750 K for the char at 10% burnout compared to a temperature of nearly 900 K for the char at 75% burnout.
In comparing the chars produced in the three different reactors, reactivities in the reaction control regime showed that, for both coals, the drop tube char was more reactive than either the flat flame or muffle furnace char. Further tests indicated that the drop tube chars had a hydrogen to carbon ratio that was 2.5 to 5 times greater than the char from either of the other reactors and the percent conversion for devolatilization was significantly less. The activation energy for all three Beulah-Zap chars and for the Dietz muffle furnace and flat flame chars was found to be 28.10.6 kcal/mol. Data from the Dietz drop tube char is as yet insufficient to yield an accurate activation energy value.
A comparison of the reactivities for the flat flame burner chars of the lignite and the sub-bituminous showed that the lignite chars were more reactive by a factor of two. This was consistent over all burnout levels. Further work with the Dietz flat flame char showed an apparent reaction order of 0.66. This is in excellent agreement with data found in the literature.
1989
Effects of Preparation Variables on Reactivity and Surface Properties of ACERC Coal Chars
Hyde, W.D.; McDonald, K.M.; Cope, R.F.; Bartholomew, C.H. and Hecker, W.C.
Twelfth Symposium of the Rocky Mountain Fuels Society, Denver, 1989. (Also presented at the Rocky Mountain Regional AIChE Meeting, Salt Lake City, 1989). Funded by ACERC (National Science Foundation and Associates and Affiliates).
The combustion of coal consists of devolatilization and heterogenous char oxidation. The objective of this study is to understand the effect of devolatilization conditions on the oxidation rate (reactivity) and surface properties of the resultant char. Chars were produced by devolatilizing Pittsburgh No. 8 bituminous coal and North Dakota lignite at varying residence times and peak temperatures in a flat flame methane burner. Thermogravimetric analysis was used to determine the low-temperature reactivity of these chars. Trends of increased reactivity with decreased preparation temperature and residence time are observed. N2 and CO2 surface areas and pore size distributions were determined. Preliminary results indicate that surface areas increase with increased devolatilization. Char reactivity is also correlated with hydrogen content and aromatic cluster size.
Effects of Char Preparation Variables on Reactivity and Surface Properties of Chars Derived from ANL Coals
Hecker, W.C.; Hyde, W.D. and McDonald, K.M.
International Chemical Congress of Pacific Basin Societies, Honolulu, 1989. Funded by ACERC (National Science Foundation and Associates and Affiliates).
The objective of this study is to understand the effect of devolatilization conditions on the oxidation rate (reactivity) and surface properties of the resultant char. Chars were produced from 3 ANL coals (Pittsburgh No. 8, Beulah Zap, and Pocahontas No. 3) and Dietz subbituminous (PETC) by three different methods: (1) a flat flame methane burner, (2) a rapid heating-rate inert atmosphere reactor, and (3) a slow heating-rate muffle furnace. Thermogravimetric analysis (TGA) was used to determine the low-temperature reactivity of these chars as a function of residence time and peak temperature. CO2 surface areas pore size distributions were also determined for some samples. Trends of increased reactivity with decreasing rank and decreasing residence time were observed. Preliminary results indicate that surface area goes through a maximum with increasing devolatilization time for Pittsburgh No. 8 while it is essentially unaffected for the Zap lignite char. Char reactivity appears to be correlated with hydrogen content.
The Effects of Rank and Preparation Method on Coal Char Oxidation Rates
Hyde, W.D.; Hecker, W.C.; Cope, R.F.; Painter, M.M.; McDonald, K.M. and Bartholomew, C.H.
Western States Section/The Combustion Institute, Livermore, California, 1989. Funded by ACERC (National Science Foundation and Associates and Affiliates).
Coal char reactivity has been found to vary greatly depending on the rank and type of the parent coal. Also, the conditions under which a given coal is devolatilized to produce char can significantly effect the reactivity of the resulting char. Preparation conditions such as gas environment, heating rate, peak temperature, residence time, and particle size are very important in determining the resulting char reactivity in that they effect its chemical and physical structure.
The general objectives of this work are to (1) understand the effect of the rank of the parent coal on the oxidation rate (reactivity) of its derived char, (2) understand the effect of devolatilization conditions on the char oxidation rate, and (3) determine any correlations which may exist between char oxidation rates and the chemical and physical properties of the chars. Specifically, oxidation rates for char from 5 coals of various ranks were measured and compared. The differences in char reactivity of chars produced in three different char preparation apparatus: a muffle furnace, a flat-flame methane burner, and a high temperature inert-atmosphere reactor, were studied. The effects of peak temperature, residence time, and particle size were also studied. Finally, correlations of oxidation rate with hydrogen content, cluster size, and surface area were attempted.
Samples of chars from Beulah Zap (Lignite), Dietz (Subbituminous A), Utah Blind Canyon (hvC Bituminous), Pittsburgh #8 (hvA Bituminous), and Pocahontas #3 (lv Bituminous) were prepared at different residence times in the Flat-Flame Char Preparation Apparatus; samples of Pittsburgh #8, Beulah Zap, and Dietz were also prepared in the muffle furnace and the high temperature inert-atmosphere reactor. The low temperature reactivity of all the coal char samples was determined in a TGA using Tcrit as the reactivity indicator. Tcrit is defined at the temperature at which the mass loss of the sample reaches 11 percent per minute.