Gale, TK
1996
Effects of Pyrolysis Heating Rate on Intrinsic Reactivities of Coal Chars
Gale, T.K.; Bartholomew, C.H. and Fletcher, T.H.
Energy & Fuels, 10(3):766-755, 1996. Funded by ACERC.
The main objective of this work was to determine the effects of pyrolysis heating rate on intrinsic O2 reactivity of coal chars. Relationships of intrinsic reactivity to other pyrolysis conditions and char physical and chemical structure were also investigated, and empirical correlations were obtained. Two different entrained flow reactors (a flat flame methane-air burner and a drop tube reactor) were used to prepare chars under a variety of different pyrolysis conditions at maximum particle temperatures and heating rates of 840-1627 K and 104 to 2 s 105 K/s, respectively. Intrinsic reactivities of a lignite and two bituminous coal chars decrease with increasing preparation heating rate. Maximum particle temperature and heating rate are difficult preparation parameters to separate and were closely coupled in this work, as in most entrained flow coal research. Indeed, much of the work described in the literature defining the effects of pyrolysis heating rate on coal char reactivity; has utilized vast residence time differences, comparing data from fixed bed (residence time of ~ 1 h) and entrained flow reactors (residence time of ~100 ms). It is concluded from this work that observations made on the basis of such experimentation are a function more of residence time and reactor variations (packed or fixed bed, as opposed to entrained flow) than particle heating rate. This work also provides evidence that intrinsic reactions of O2 with coal char (for the three coals observed in this study) are not significantly influenced by large differences in char meso- or micropore surface area obtained by varying pyrolysis conditions.
1994
Decreases in the Swelling and Porosity of Bituminous Coals During Devolatilization
Gale, T.K.; Bartholomew, C.H. and Fletcher, T.H.
Combustion and Flame, 1994 (in press). (Also presented at the 25th International Symposium on Combustion, Irvine, CA, August 1994). Funded by ACERC.
Concern about comparability and validity of different methods for producing coal chars for reactivity experiments has led to research on the effect of devolatilization conditions on the char physical and chemical structure. Particle diameter and porosity changes during devolatilization significantly affect char oxidation rates. In particular, physical properties of chars prepared in drop tube reactors differ greatly from chars prepared in flat flame burner experiments. Recent data indicate that the presence of oxygen in the gas atmosphere has no effect on swelling until char oxidation has begun. The present research concentrates on the effects of heating rate, particle temperature and residence time on the swelling and porosity of a plastic coal, and compares these results with a non-plastic coal. The heating rate at which the transition from increasing swelling to decreasing swelling occurs in approximately 5 x 10³ K/s for swelling coals. Swelling coals also reach a maximum porosity near this heating rate. At low particle heating rates swelling gradually increases versus heating rate in contrast to a decline in the swelling at high heating rates in a narrow heating rate region of 2 x 10^4 to 7 x 10^4 K/s. Non swelling bituminous and lignite coals continue to increase in porosity beyond the heating rate of 2 x 10^4 K/s.
Effects of Pyrolysis Conditions on Internal Surface Areas and Densities of Coal Chars Prepared at High Heating Rates in Reactive and Non-Reactive Atmospheres
Gale, T.K.; Fletcher, T.H. and Bartholomew, C.H.
Energy & Fuels, 1994 (in press). Funded by ACERC.
Concern about comparability and validity of different methods for producing coal chars for research has motivated this investigation of the effects of devolatilization conditions on the physical properties of coal chars. It is evident from the findings of this study that care must be taken to prepare chars under conditions similar to those of full-scale coal combustion boilers prior to performing char oxidation studies. Two different entrained flow reactors were used to prepare chars under a variety of different pyrolysis conditions at maximum particle temperatures and heating rates between 840 to 1627 K and 10^4 to 2 x 10^5 K/s respectively. Under these conditions micro-pore (CO2) surface area generally increases with residence time and mass release for lignite and bituminous coals, as does true density. Micro-pore surface area also increases somewhat with increasing maximum particle temperature and heating rate. Meso-pore (N2) surface area is most affected by reactive gas atmospheres (carbon activation). The presence of steam in the post flame gases of methane/air flat flame burners is a significant factor in increasing meso-pore surface are of chars prepared in such burners, even though the mass conversion by steam gasification is small. Partial char oxidation with O2 significantly affects char N2 and CO2 surface area at these heating rates and residence times (50 to 100 ms), sometimes increasing and sometimes decreasing internal surface area. Low rank lignite and sub-bituminous coals have higher potentials for forming chars with increased N2 surface are than bituminous coals. The moisture content of low rank coals may be more important than rank. Lignite with high moisture content yields char with a significantly higher N2 surface area than char prepared from lower moisture content lignite. However, initial coal moisture has less effect on CO2 surface area.
Effects of Pyrolysis on Coal Char Properties
Gale, T.K.
Effects of Pyrolysis on Coal Char Properties, M.S./BYU, August 1994, Advisor: Bartholomew
1993
Comparison of Reactivity and Physical Structure of Chars Prepared Under Different Pyrolysis Conditions, i.e. Temperature, Gas Atmosphere, and Heating Rate
Gale, T.K.; Bartholomew, C.H. and Fletcher, T.H.
Proceedings of the International Conference on Coal Science, Banff, Canada, September 1993. Funded by ACERC.
Coal combustion consists of basically two main steps: 1) pyrolysis and oxidation of the liquid and volatile matter, and 2) subsequent oxidation of the residual porous char matrix. Char oxidation is the slower of these two steps and is difficult to bring to completion. Pyrolysis significantly affects the resulting char structure, porosity, internal surface area and chemical composition (e.g. H/C ratios) and hence the char oxidation rate. A highly porous char particle is more accessible to reactant molecules and will, therefore, have a higher reactivity in the reaction zone influenced by pore diffusion. Under surface reaction controlled conditions, reactivity increases with increasing internal surface area and H/C ratio.
A number of different experimental methods and reactor types are currently used to produce chars for laboratory study. These different reactors typically operate under conditions that are quite reproducible from one run to another. However, variations in pyrolysis conditions from one method to another and from one reactor type to another may be large. Comparisons of data obtained in different laboratories are often rationalized by matching experimental conditions thought to be most critical such as temperature and residence time, or temperature and total volatiles yield. However, comparing chars at the same residence time or the same mass loss may not be valid, because at different heating rates and/or gas-phase oxygen concentrations, the chemical and physical nature of the pyrolysis will vary.
The objective of this research was to determine effects of variations in pyrolysis conditions on char structure and reactivity for a group of chars prepared from coals of low to high rank. Heating rate, temperature, residence time, and gas atmosphere during pyrolysis were the main variables in the study.
Gale, TK
1996
Effects of Pyrolysis Heating Rate on Intrinsic Reactivities of Coal Chars
Gale, T.K.; Bartholomew, C.H. and Fletcher, T.H.
Energy & Fuels, 10(3):766-755, 1996. Funded by ACERC.
The main objective of this work was to determine the effects of pyrolysis heating rate on intrinsic O2 reactivity of coal chars. Relationships of intrinsic reactivity to other pyrolysis conditions and char physical and chemical structure were also investigated, and empirical correlations were obtained. Two different entrained flow reactors (a flat flame methane-air burner and a drop tube reactor) were used to prepare chars under a variety of different pyrolysis conditions at maximum particle temperatures and heating rates of 840-1627 K and 104 to 2 s 105 K/s, respectively. Intrinsic reactivities of a lignite and two bituminous coal chars decrease with increasing preparation heating rate. Maximum particle temperature and heating rate are difficult preparation parameters to separate and were closely coupled in this work, as in most entrained flow coal research. Indeed, much of the work described in the literature defining the effects of pyrolysis heating rate on coal char reactivity; has utilized vast residence time differences, comparing data from fixed bed (residence time of ~ 1 h) and entrained flow reactors (residence time of ~100 ms). It is concluded from this work that observations made on the basis of such experimentation are a function more of residence time and reactor variations (packed or fixed bed, as opposed to entrained flow) than particle heating rate. This work also provides evidence that intrinsic reactions of O2 with coal char (for the three coals observed in this study) are not significantly influenced by large differences in char meso- or micropore surface area obtained by varying pyrolysis conditions.
1994
Decreases in the Swelling and Porosity of Bituminous Coals During Devolatilization
Gale, T.K.; Bartholomew, C.H. and Fletcher, T.H.
Combustion and Flame, 1994 (in press). (Also presented at the 25th International Symposium on Combustion, Irvine, CA, August 1994). Funded by ACERC.
Concern about comparability and validity of different methods for producing coal chars for reactivity experiments has led to research on the effect of devolatilization conditions on the char physical and chemical structure. Particle diameter and porosity changes during devolatilization significantly affect char oxidation rates. In particular, physical properties of chars prepared in drop tube reactors differ greatly from chars prepared in flat flame burner experiments. Recent data indicate that the presence of oxygen in the gas atmosphere has no effect on swelling until char oxidation has begun. The present research concentrates on the effects of heating rate, particle temperature and residence time on the swelling and porosity of a plastic coal, and compares these results with a non-plastic coal. The heating rate at which the transition from increasing swelling to decreasing swelling occurs in approximately 5 x 10³ K/s for swelling coals. Swelling coals also reach a maximum porosity near this heating rate. At low particle heating rates swelling gradually increases versus heating rate in contrast to a decline in the swelling at high heating rates in a narrow heating rate region of 2 x 10^4 to 7 x 10^4 K/s. Non swelling bituminous and lignite coals continue to increase in porosity beyond the heating rate of 2 x 10^4 K/s.
Effects of Pyrolysis Conditions on Internal Surface Areas and Densities of Coal Chars Prepared at High Heating Rates in Reactive and Non-Reactive Atmospheres
Gale, T.K.; Fletcher, T.H. and Bartholomew, C.H.
Energy & Fuels, 1994 (in press). Funded by ACERC.
Concern about comparability and validity of different methods for producing coal chars for research has motivated this investigation of the effects of devolatilization conditions on the physical properties of coal chars. It is evident from the findings of this study that care must be taken to prepare chars under conditions similar to those of full-scale coal combustion boilers prior to performing char oxidation studies. Two different entrained flow reactors were used to prepare chars under a variety of different pyrolysis conditions at maximum particle temperatures and heating rates between 840 to 1627 K and 10^4 to 2 x 10^5 K/s respectively. Under these conditions micro-pore (CO2) surface area generally increases with residence time and mass release for lignite and bituminous coals, as does true density. Micro-pore surface area also increases somewhat with increasing maximum particle temperature and heating rate. Meso-pore (N2) surface area is most affected by reactive gas atmospheres (carbon activation). The presence of steam in the post flame gases of methane/air flat flame burners is a significant factor in increasing meso-pore surface are of chars prepared in such burners, even though the mass conversion by steam gasification is small. Partial char oxidation with O2 significantly affects char N2 and CO2 surface area at these heating rates and residence times (50 to 100 ms), sometimes increasing and sometimes decreasing internal surface area. Low rank lignite and sub-bituminous coals have higher potentials for forming chars with increased N2 surface are than bituminous coals. The moisture content of low rank coals may be more important than rank. Lignite with high moisture content yields char with a significantly higher N2 surface area than char prepared from lower moisture content lignite. However, initial coal moisture has less effect on CO2 surface area.
Effects of Pyrolysis on Coal Char Properties
Gale, T.K.
Effects of Pyrolysis on Coal Char Properties, M.S./BYU, August 1994, Advisor: Bartholomew
1993
Comparison of Reactivity and Physical Structure of Chars Prepared Under Different Pyrolysis Conditions, i.e. Temperature, Gas Atmosphere, and Heating Rate
Gale, T.K.; Bartholomew, C.H. and Fletcher, T.H.
Proceedings of the International Conference on Coal Science, Banff, Canada, September 1993. Funded by ACERC.
Coal combustion consists of basically two main steps: 1) pyrolysis and oxidation of the liquid and volatile matter, and 2) subsequent oxidation of the residual porous char matrix. Char oxidation is the slower of these two steps and is difficult to bring to completion. Pyrolysis significantly affects the resulting char structure, porosity, internal surface area and chemical composition (e.g. H/C ratios) and hence the char oxidation rate. A highly porous char particle is more accessible to reactant molecules and will, therefore, have a higher reactivity in the reaction zone influenced by pore diffusion. Under surface reaction controlled conditions, reactivity increases with increasing internal surface area and H/C ratio.
A number of different experimental methods and reactor types are currently used to produce chars for laboratory study. These different reactors typically operate under conditions that are quite reproducible from one run to another. However, variations in pyrolysis conditions from one method to another and from one reactor type to another may be large. Comparisons of data obtained in different laboratories are often rationalized by matching experimental conditions thought to be most critical such as temperature and residence time, or temperature and total volatiles yield. However, comparing chars at the same residence time or the same mass loss may not be valid, because at different heating rates and/or gas-phase oxygen concentrations, the chemical and physical nature of the pyrolysis will vary.
The objective of this research was to determine effects of variations in pyrolysis conditions on char structure and reactivity for a group of chars prepared from coals of low to high rank. Heating rate, temperature, residence time, and gas atmosphere during pyrolysis were the main variables in the study.