Cope, RF
1994
Effects of CaO Surface Area on Intrinsic Char Oxidation Rates for Beulah Zap Chars
Cope, R.F.; Arrington, C.B. and Hecker, W.C.
Energy & Fuels, 8(5):1095, 1994. Funded by ACERC, National Science Foundation and US Department of Energy.
This work examines the effect of char burnout level and calcium content on the intrinsic char oxidation rates and physical properties of three series of chars. Three starting chars were prepared by devolatilizing a 63-74 mm fraction of North Dakota Zap lignite in a flat-flame burner; then, a portion of this char was washed in HCl to remove mineral matter; finally, a portion of the acid-washed char was reloaded with calcium. The three starting chars (ND (untreated), NDW (acid-washed), and NCa (Ca reloaded)) were then oxidized to various levels of conversion (6-92%) in a heated-wall drop-tube reactor (DTR) at high temperature (1523 K) in a 5-7% O2 environment. Low-temperature intrinsic oxidation rates were determined for each resulting sample using isothermal TGA (648-748 K, 10% O2). Other measured properties include burnout, N2 BET and CO2 DP surface areas, and CaO surface area. The latter was determined using a selective CO2 chemisorption technique. Intrinsic oxidation rate decreased as burnout increased for the calcium containing chars (ND and Nca). For the NDW char (69% Ca removed), the intrinsic rate was independent of burnout. Increased burnout produced general decreases in N2 BET and CO2 DP surface areas for all three chars, but they did not correlate well with rate. Increased burnout also produced decreases in C1) surface areas for the ND and Nca chars, but not for the NDW. The decreases in CaO surface area paralleled the decreases in intrinsic rates. This led to good correlation of CaO surface area with rate. Furthermore, the normalized values of rate per CaO surface area were essentially independent of burnout for the Ca-containing char series. This result suggests that catalysis by Ca is very significant during low-temperature oxidation. These results also indicate that the reason for decreasing rate with burnout is due to sintering of CaO with increased time of exposure to the high-temperature environment. Activation energies for the chars in the three series were found to be independent of burnout level and calcium content. Average values were 32.9 ± 1.4, 34.7 ± 3.3, and 33.6 ± 1.3 kcal/mol (uncertainties expressed as 95% confidence intervals) for ND, NDW, and Nca, respectively.
Improved Diameter, Velocity, and Temperature Measurements for Char Particles in Drop-Tube Reactors
Cope, R.F.; Monson, C.R.; Germane, G.J. and Hecker, W.C.
Energy & Fuels, 8(4):925-931, 1994. Funded by ACERC, Advanced Fuel Research and US Department of Energy.
Coal combustion researchers have typically used the average temperature and residence time of a burning particle cloud to determine the high-temperature reactivity of coals and chars. These average values, however, cannot account for particle-to-particle variations or their possible causes. Researchers at Sandia National Laboratories developed a pyrometry technique to simultaneously measure the temperature, velocity, and diameter of individual char particles burning in a transparent-wall flat-flame facility. This work reports two significant advances relative to the optical pyrometry technique. First, pyrometer modifications together with a new analysis technique now permit the particle properties to be measured for smaller/cooler particles. Second, the modified pyrometer has been implemented in two heated-wall drop-tube reactors, rather than transparent-wall, flat-flame burners. This is significant because drop-tube reactors allow greater flexibility/control of gas environments and operating pressures during char oxidation. Glowing reactor walls, however, present some unique challenges for these optical measurements. Means of overcoming these challenges are discussed, and reliable in situ measurement of particle temperatures, velocities, and diameters is verified. The results of measurements made in these drop-tube reactors, both for calibration tests and actual oxidation tests with Spherocarb and a Utah bituminous coal char, are also presented.
1993
Mineral Matter Catalysis During High- and Low-Temperature Char Oxidation of Lignite Chars
Cope, R.F.; Swensen, M.R. and Hecker, W.C.
Proceedings of 7th Annual International Conference on Coal Science, Banff, Alberta, Canada, September 1993. Funded by US Department of Energy and ACERC.
Many researchers have studied the impact of mineral catalysis, particularly by CaO, on the low-temperature (<=750 K) oxidation rates of synthetic and low rank coal-derived chars. Fewer have studied Ca effects during high-temperature (>=1300 K) char oxidation. The objectives of this ongoing study are to determine (1) the significant of CaO catalysis during the oxidation of a lignite char at both high and low particle temperatures, and (2) the effect of high-temperature burnout level on oxidation rates and CaO catalysis. Three char series, with and without potentially catalytic metal ions, were prepared. High-temperature rates and particle temperatures were measured for each in a drop-tube reactor. Low-temperature (intrinsic) rates and activation energies were determined using isothermal TGA. Changes in char densities and surface areas (N2, CO2, and CaO) were also measured and related to changes in the observed oxidation rates.
Effects of CaO, CaSO4 and Coal Rank on Low- and High-Temperature Char Oxidation Rates
Gopalakrishnan, R.; Fullwood, M.; Moody, S.; Cope, R.F. and Bartholomew, C.H.
Proceedings of the 7th International Conference on Coal Science, Banff, Canada, September 1993. Funded by ACERC.
This study is part of an ongoing program to investigate (a) rates and mechanisms of Ca-catalyzed oxidation of synthetic char and chars prepared from representative U.S. coals and (b) the chemical nature of active catalytic sites for oxidation on those inorganic mineral phases present in coal chars.
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 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.
Improved Temperature, Velocity and Diameter Measurements for Char Particles in Drop-Tube Reactors
Cope, R.F.; Hecker, W.C.; Monson, C.R. and Germane, G.J.
Western States Section/The Combustion Institute, Los Angeles, CA, October 1991. Funded by Advanced Fuel Research, Morgantown Energy Research Center and ACERC.
Early attempts to determine the high-temperature reactivity of coals and chars were hampered by the inability to measure a burning particle's temperature and residence time. Researchers have typically approximated these values with the average temperature and residence time of a cloud of burning particles. Average values, however, do not account for particle-to-particle variations or their possible causes. In 1984 researchers at Sandia National Laboratories developed an optical technique to simultaneously measure the temperature, velocity and diameter of individual particles burning in a flat flame facility. This work reports modifications to the Sandia technique that allow measurement of smaller particles (ca. 30-200 µm) and its application to particles burning in atmospheric and elevated pressure drop-tube reactors.
The modified pyrometer is applied to drop-tube reactors, rather than flat flame burners, to allow a broad range of well-controlled gas environments and operating pressures for char oxidation experiments. Electrical wall heating allows good control of particle temperature histories, however glowing reactor walls present some unique challenges. Diffuse radiation emitted from the reactor walls enters the pyrometer's optical path, producing excess noise in the particle signal. This noise has been minimized by optimizing the design of the reactors' optical ports and the alignment of the pyrometer. The possibility of temperature measurement error caused by wall emissions reflecting off of the particle is shown to be minimal by Maxwell's electromagnetic equations.
Particle properties are obtained by focusing the burning particle's image onto a coded aperture that, as re-designed at Brigham Young University, contains a series of carefully sized blackouts and windows. Temperature is measured by conventional 2-color pyrometry, while velocity is obtained from the particle's transit time. Particle size is extracted from the signal by means of a unique geometric/statistical fitting technique developed during this work. Proper operation of the modified pyrometer has been verified during oxidation of both Spherocarb and coal char in the drop-tube reactors.
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.
1989
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.
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.
Oxidation Rate Measurements of Coals and Derived Chars
Wells, W.F.; Hyde, W.D.; Cope, R.F.; Smoot, L.D.; Hecker, W.C. and Bartholomew, C.H.
Twelfth Symposium of the Rocky Mountain Fuels Society, Denver, 1989. Funded by ACERC (National Science Foundation and Associates and Affiliates).
To aid in the development and provide validation data to the char oxidation submodel, measurements of oxidation rates at high and low temperatures are being collected for chars prepared from five select ACERC coals. Chars are prepared using a flat flame burner and a recently constructed inert atmosphere drop-tube reactor heated using an inductively coupled plasma. The drop-tube reactor is also used to obtain reaction rates at high temperatures; low temperature rates were measured in a TGA. Chemical and physical properties of the fuel were measured. Multivariate statistics are used to correlate fuel properties to reaction rates.
Effects of Pressure and Coal Rank on Carbon Conversion in an Entrained-Coal Gasifier
Cope, R.F.; Smoot, L.D. and Hedman, P.O.
Fuel, 68, 806-808, 1989. Funded by US Department of Energy (Morgantown Energy Technology Center).
Elevated-pressure gasification tests were completed with North Dakota lignite, Wyoming subbituminous and Illinois No. 6 bituminous coals. Carbon conversion values obtained in theses tests were compared with those obtained with the same three coals at atmospheric pressure. Increased pressure produced greater increases in carbon conversion as coal rank decreased.
1988
Effects of Pressure and Coal Rank on Carbon Conversion in an Entrained-Coal Gasifier
Cope, R.F.; Smoot, L.D. and Hedman, P.O.
Accepted for publication Fuel, 1988. 9 pgs. Funded by Morgantown Energy Technology Center.
Between 1946 and 1962, the U.S. Bureau of Mines developed and operated five different atmospheric pressure and elevated pressure entrained-coal gasifiers. From 1963 to 1982, Bituminous Coal Research, Inc. and others developed and operated the Bi-Gas pressurized two-stage entrained-coal gasifier. These programs demonstrated the feasibility of gasifying US coals in entrained gasifiers, and investigated the effects of such operating variables as pressure and coal rank. A number of other entrained coal gasification processes have demonstrated the ability to operate at elevated pressure with several coals of different rank (e.g. Texaco, Shell and Mountain Fuel Resources), but little research has been performed with these processes to determine how carbon conversion is affected by coal rank at elevated pressures.
Sebastion, Strimbeck et al., and Brown et al. reported that carbon conversion increased as coal rank decreased in atmospheric pressure entrained-coal gasification. Preliminary tests in the BI-Gas study show a similar coal rank effect at elevated pressures, but simultaneous changes in other operating variables (i.e. coal, oxygen and steam feed rates) obscure the combined effects of pressure and coal rank on carbon conversion. In a limited investigation of pressure effects in a laboratory gasifier (using bituminous coal only), McIntosh and Coates observed a 10 to 20 percent increase in exit carbon conversion as operating pressure was increased from 1050 to 2320 kPa. Studies conducted by Azuhata et al., showed that for a diffusion flame and an O2/coal ratio greater than 0.8, increasing the operating pressure form 100 to 500 kPa produced a 10-15 percent increase in the carbon conversion of Utah bituminous coal. A similar pressure effect was not observed in the Azuhata et al. premixed flame data. The objective of this study was to determine the combined effects of coal rank and elevated operating pressure on carbon conversion, in entrained-coal gasification.
Cope, RF
1994
Effects of CaO Surface Area on Intrinsic Char Oxidation Rates for Beulah Zap Chars
Cope, R.F.; Arrington, C.B. and Hecker, W.C.
Energy & Fuels, 8(5):1095, 1994. Funded by ACERC, National Science Foundation and US Department of Energy.
This work examines the effect of char burnout level and calcium content on the intrinsic char oxidation rates and physical properties of three series of chars. Three starting chars were prepared by devolatilizing a 63-74 mm fraction of North Dakota Zap lignite in a flat-flame burner; then, a portion of this char was washed in HCl to remove mineral matter; finally, a portion of the acid-washed char was reloaded with calcium. The three starting chars (ND (untreated), NDW (acid-washed), and NCa (Ca reloaded)) were then oxidized to various levels of conversion (6-92%) in a heated-wall drop-tube reactor (DTR) at high temperature (1523 K) in a 5-7% O2 environment. Low-temperature intrinsic oxidation rates were determined for each resulting sample using isothermal TGA (648-748 K, 10% O2). Other measured properties include burnout, N2 BET and CO2 DP surface areas, and CaO surface area. The latter was determined using a selective CO2 chemisorption technique. Intrinsic oxidation rate decreased as burnout increased for the calcium containing chars (ND and Nca). For the NDW char (69% Ca removed), the intrinsic rate was independent of burnout. Increased burnout produced general decreases in N2 BET and CO2 DP surface areas for all three chars, but they did not correlate well with rate. Increased burnout also produced decreases in C1) surface areas for the ND and Nca chars, but not for the NDW. The decreases in CaO surface area paralleled the decreases in intrinsic rates. This led to good correlation of CaO surface area with rate. Furthermore, the normalized values of rate per CaO surface area were essentially independent of burnout for the Ca-containing char series. This result suggests that catalysis by Ca is very significant during low-temperature oxidation. These results also indicate that the reason for decreasing rate with burnout is due to sintering of CaO with increased time of exposure to the high-temperature environment. Activation energies for the chars in the three series were found to be independent of burnout level and calcium content. Average values were 32.9 ± 1.4, 34.7 ± 3.3, and 33.6 ± 1.3 kcal/mol (uncertainties expressed as 95% confidence intervals) for ND, NDW, and Nca, respectively.
Improved Diameter, Velocity, and Temperature Measurements for Char Particles in Drop-Tube Reactors
Cope, R.F.; Monson, C.R.; Germane, G.J. and Hecker, W.C.
Energy & Fuels, 8(4):925-931, 1994. Funded by ACERC, Advanced Fuel Research and US Department of Energy.
Coal combustion researchers have typically used the average temperature and residence time of a burning particle cloud to determine the high-temperature reactivity of coals and chars. These average values, however, cannot account for particle-to-particle variations or their possible causes. Researchers at Sandia National Laboratories developed a pyrometry technique to simultaneously measure the temperature, velocity, and diameter of individual char particles burning in a transparent-wall flat-flame facility. This work reports two significant advances relative to the optical pyrometry technique. First, pyrometer modifications together with a new analysis technique now permit the particle properties to be measured for smaller/cooler particles. Second, the modified pyrometer has been implemented in two heated-wall drop-tube reactors, rather than transparent-wall, flat-flame burners. This is significant because drop-tube reactors allow greater flexibility/control of gas environments and operating pressures during char oxidation. Glowing reactor walls, however, present some unique challenges for these optical measurements. Means of overcoming these challenges are discussed, and reliable in situ measurement of particle temperatures, velocities, and diameters is verified. The results of measurements made in these drop-tube reactors, both for calibration tests and actual oxidation tests with Spherocarb and a Utah bituminous coal char, are also presented.
1993
Mineral Matter Catalysis During High- and Low-Temperature Char Oxidation of Lignite Chars
Cope, R.F.; Swensen, M.R. and Hecker, W.C.
Proceedings of 7th Annual International Conference on Coal Science, Banff, Alberta, Canada, September 1993. Funded by US Department of Energy and ACERC.
Many researchers have studied the impact of mineral catalysis, particularly by CaO, on the low-temperature (<=750 K) oxidation rates of synthetic and low rank coal-derived chars. Fewer have studied Ca effects during high-temperature (>=1300 K) char oxidation. The objectives of this ongoing study are to determine (1) the significant of CaO catalysis during the oxidation of a lignite char at both high and low particle temperatures, and (2) the effect of high-temperature burnout level on oxidation rates and CaO catalysis. Three char series, with and without potentially catalytic metal ions, were prepared. High-temperature rates and particle temperatures were measured for each in a drop-tube reactor. Low-temperature (intrinsic) rates and activation energies were determined using isothermal TGA. Changes in char densities and surface areas (N2, CO2, and CaO) were also measured and related to changes in the observed oxidation rates.
Effects of CaO, CaSO4 and Coal Rank on Low- and High-Temperature Char Oxidation Rates
Gopalakrishnan, R.; Fullwood, M.; Moody, S.; Cope, R.F. and Bartholomew, C.H.
Proceedings of the 7th International Conference on Coal Science, Banff, Canada, September 1993. Funded by ACERC.
This study is part of an ongoing program to investigate (a) rates and mechanisms of Ca-catalyzed oxidation of synthetic char and chars prepared from representative U.S. coals and (b) the chemical nature of active catalytic sites for oxidation on those inorganic mineral phases present in coal chars.
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 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.
Improved Temperature, Velocity and Diameter Measurements for Char Particles in Drop-Tube Reactors
Cope, R.F.; Hecker, W.C.; Monson, C.R. and Germane, G.J.
Western States Section/The Combustion Institute, Los Angeles, CA, October 1991. Funded by Advanced Fuel Research, Morgantown Energy Research Center and ACERC.
Early attempts to determine the high-temperature reactivity of coals and chars were hampered by the inability to measure a burning particle's temperature and residence time. Researchers have typically approximated these values with the average temperature and residence time of a cloud of burning particles. Average values, however, do not account for particle-to-particle variations or their possible causes. In 1984 researchers at Sandia National Laboratories developed an optical technique to simultaneously measure the temperature, velocity and diameter of individual particles burning in a flat flame facility. This work reports modifications to the Sandia technique that allow measurement of smaller particles (ca. 30-200 µm) and its application to particles burning in atmospheric and elevated pressure drop-tube reactors.
The modified pyrometer is applied to drop-tube reactors, rather than flat flame burners, to allow a broad range of well-controlled gas environments and operating pressures for char oxidation experiments. Electrical wall heating allows good control of particle temperature histories, however glowing reactor walls present some unique challenges. Diffuse radiation emitted from the reactor walls enters the pyrometer's optical path, producing excess noise in the particle signal. This noise has been minimized by optimizing the design of the reactors' optical ports and the alignment of the pyrometer. The possibility of temperature measurement error caused by wall emissions reflecting off of the particle is shown to be minimal by Maxwell's electromagnetic equations.
Particle properties are obtained by focusing the burning particle's image onto a coded aperture that, as re-designed at Brigham Young University, contains a series of carefully sized blackouts and windows. Temperature is measured by conventional 2-color pyrometry, while velocity is obtained from the particle's transit time. Particle size is extracted from the signal by means of a unique geometric/statistical fitting technique developed during this work. Proper operation of the modified pyrometer has been verified during oxidation of both Spherocarb and coal char in the drop-tube reactors.
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.
1989
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.
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.
Oxidation Rate Measurements of Coals and Derived Chars
Wells, W.F.; Hyde, W.D.; Cope, R.F.; Smoot, L.D.; Hecker, W.C. and Bartholomew, C.H.
Twelfth Symposium of the Rocky Mountain Fuels Society, Denver, 1989. Funded by ACERC (National Science Foundation and Associates and Affiliates).
To aid in the development and provide validation data to the char oxidation submodel, measurements of oxidation rates at high and low temperatures are being collected for chars prepared from five select ACERC coals. Chars are prepared using a flat flame burner and a recently constructed inert atmosphere drop-tube reactor heated using an inductively coupled plasma. The drop-tube reactor is also used to obtain reaction rates at high temperatures; low temperature rates were measured in a TGA. Chemical and physical properties of the fuel were measured. Multivariate statistics are used to correlate fuel properties to reaction rates.
Effects of Pressure and Coal Rank on Carbon Conversion in an Entrained-Coal Gasifier
Cope, R.F.; Smoot, L.D. and Hedman, P.O.
Fuel, 68, 806-808, 1989. Funded by US Department of Energy (Morgantown Energy Technology Center).
Elevated-pressure gasification tests were completed with North Dakota lignite, Wyoming subbituminous and Illinois No. 6 bituminous coals. Carbon conversion values obtained in theses tests were compared with those obtained with the same three coals at atmospheric pressure. Increased pressure produced greater increases in carbon conversion as coal rank decreased.
1988
Effects of Pressure and Coal Rank on Carbon Conversion in an Entrained-Coal Gasifier
Cope, R.F.; Smoot, L.D. and Hedman, P.O.
Accepted for publication Fuel, 1988. 9 pgs. Funded by Morgantown Energy Technology Center.
Between 1946 and 1962, the U.S. Bureau of Mines developed and operated five different atmospheric pressure and elevated pressure entrained-coal gasifiers. From 1963 to 1982, Bituminous Coal Research, Inc. and others developed and operated the Bi-Gas pressurized two-stage entrained-coal gasifier. These programs demonstrated the feasibility of gasifying US coals in entrained gasifiers, and investigated the effects of such operating variables as pressure and coal rank. A number of other entrained coal gasification processes have demonstrated the ability to operate at elevated pressure with several coals of different rank (e.g. Texaco, Shell and Mountain Fuel Resources), but little research has been performed with these processes to determine how carbon conversion is affected by coal rank at elevated pressures.
Sebastion, Strimbeck et al., and Brown et al. reported that carbon conversion increased as coal rank decreased in atmospheric pressure entrained-coal gasification. Preliminary tests in the BI-Gas study show a similar coal rank effect at elevated pressures, but simultaneous changes in other operating variables (i.e. coal, oxygen and steam feed rates) obscure the combined effects of pressure and coal rank on carbon conversion. In a limited investigation of pressure effects in a laboratory gasifier (using bituminous coal only), McIntosh and Coates observed a 10 to 20 percent increase in exit carbon conversion as operating pressure was increased from 1050 to 2320 kPa. Studies conducted by Azuhata et al., showed that for a diffusion flame and an O2/coal ratio greater than 0.8, increasing the operating pressure form 100 to 500 kPa produced a 10-15 percent increase in the carbon conversion of Utah bituminous coal. A similar pressure effect was not observed in the Azuhata et al. premixed flame data. The objective of this study was to determine the combined effects of coal rank and elevated operating pressure on carbon conversion, in entrained-coal gasification.