The lighter density (<1.5 g/cm 3) fractions produced from two waste coals sampled from the waste ... more The lighter density (<1.5 g/cm 3) fractions produced from two waste coals sampled from the waste coal disposal sites at thermochemical plants situated in South Africa were used as feed materials for liquefaction with tetralin. The liquefaction residues from the lighter density and untreated lighter density fractions were used in pyrolysis experiments. Pyrolysis of the lighter density fractions was carried out in a Fischer assay oven at 750 o C and 920 o C under an argon atmosphere. The advanced analytical techniques (gas chromatography-mass spectrometry (GC-MS) and nuclear magnetic resonance (NMR) spectroscopy) were employed to characterise the pyrolysis products. Also, the lighter density fractions, liquefaction residues and their chars were examined using conventional and advanced analytical techniques. The pyrolysis char yields of the liquefaction residues ranged between 74% and 76%, and those of the coal float fractions ranged between 67.0-71.5%. Gas pyrolysis yields ranged between 16.0% and 20.0% for the residues and between 14.5% and 18.4% for the lighter density fractions, while the pyrolytic water and the tar products of the lighter density fractions were slightly higher than those of the liquefaction derived residues. The proton NMR analysis of the tars from the residues shows marginally higher amounts of aromatic protons than those of the lighter density fractions. Chars which were generated after pyrolysis of the liquefaction derived residues show higher porosity values than those from the pyrolysis of the lighter density fractions. The differences in the porosities are attributed to the opening of pores and extraction of some lower molecular mass aliphatic species from the coal matrix during liquefaction. The pyrolysis products distribution and characterisation of the products showed that the residues (waste material) generated after tetralin liquefaction of the float fractions from the float-sink experiments of waste coals may be utilised for thermochemical processes (pyrolysis).
The possible catalytic effect of a 5% loading by mass of crystallized salt from spent Benfield so... more The possible catalytic effect of a 5% loading by mass of crystallized salt from spent Benfield solution (SBFS) was investigated and compared with the catalytic influence of a 5 mass% loading of K 2 CO 3 on the char-CO 2 gasification reactivity of a bituminous medium rank C South African Highveld coal. The effect was investigated using known thermogravimetric investigation techniques. Coal char samples were prepared using the coal and catalyst-loaded samples by heating the samples in nitrogen to 1000°C. The prepared char samples were further subjected to thermogravimetric analyses by heating them at isothermal temperatures between 900 and 990°C in a CO 2 atmosphere to simulate gasification reactions of the coal char. Results from isothermal char-CO 2 gasification experiments showed that the catalyst-loaded coal samples exhibited higher gasification reactivities than the coal sample and the reactivities increased in the order: coal \ coal ? 5% SBF \ coal ? 5% K 2 CO 3. It was observed that the relative initial reactivity of the catalyst-loaded samples increased in order of magnitude [ 3.5 in comparison with the coal char sample. The addition of catalysts to the coal was found to reduce the estimated activation energies of the catalyst-loaded samples during the gasification process. The estimated activation energies (calculated from the initial reaction rate of the gasification reaction) of the samples were 215 kJ mol-1 for the coal char, 146 kJ mol-1 for the char from the coal ? 5% SBF and 118 kJ mol-1 for the char from the coal ? 5% K 2 CO 3. The crystallized spent Benfield salt may be utilized as a CO 2 gasification catalyst for coal. Keywords CO 2 gasification reactivity Á Catalytic effect Á Spent Benfield salt Á K 2 CO 3 Á Activation energy & C. A.
This is a repository copy of Density functional theory molecular modelling and experimental parti... more This is a repository copy of Density functional theory molecular modelling and experimental particle kinetics for CO2-char gasification.
The carbon dioxide gasification characteristics of three biomass char samples and bituminous coal... more The carbon dioxide gasification characteristics of three biomass char samples and bituminous coal char were investigated in a thermogravimetric analyser in the temperature range of 850-950 °C. Char SB exhibited higher reactivities (R, R, R) than chars SW and HW. Coal char gasification reactivities were observed to be lower than those of the three biomass chars. Correlations between the char reactivities and char characteristics were highlighted. The addition of 10% biomass had no significant impact on the coal char gasification reactivity. However, 20 and 30% biomass additions resulted in increased coal char gasification rate. During co-gasification, chars HW and SW caused increased coal char gasification reactivity at lower conversions, while char SB resulted in increased gasification rates throughout the entire conversion range. Experimental data from biomass char gasification and biomass-coal char co-gasification were well described by the MRPM, while coal char gasification was b...
ABSTRACT The chemical–structural properties of four South African bituminous coals were investiga... more ABSTRACT The chemical–structural properties of four South African bituminous coals were investigated using wide angle X-ray diffraction–carbon fraction analysis (WAXRD–CFA), attenuated total reflection Fourier transform infra-red spectroscopy (ATR–FTIR), solid state 13C nuclear magnetic resonance spectroscopy (ss 13C NMR), and high resolution transmission electron microscopy (HRTEM). The aromaticity of the samples determined by solid state 13C NMR ranged from 0.74 to 0.87 and compared well with the WAXRD–CFA results (0.73–0.86). WAXRD–CFA, ATR–FTIR and 13C NMR data showed that the lower iso-rank coal samples contained more aliphatic moieties; while the higher rank sample contained higher fractions of polyaromatic moieties and saturated long chain hydrocarbons. The lattice parameters determined from WAXRD–CFA show that lower rank coals investigated are structurally less well-ordered than the higher rank coal. Also, its fringes were quantified as having the greatest preferential alignment of the coals examined. HRTEM aromatic fringe image analysis revealed that the carbon lattice of the samples consist of aromatic fringes of varying lengths, L (3 Å ⩽ L ⩽ 95 Å), which corresponded to a molecular weight distribution ranging from 75 to 1925 amu, assuming circular catenation. The coal with the highest volatile matter yield was found to exhibit a higher frequency of lower molecular weight fringes; while the higher rank coal possessed the most higher molecular weight fringes. The average molecular weights determined for the samples from 13C NMR varied between 504 and 544 amu; and compared well with the values from HRTEM aromatic fringe image analysis data. Thus, there was good agreement and consistency between the different techniques. It was demonstrated that, for the characterisation methods used in this study; that vitrinite reflectance impacts more on the chemical–structural properties of these coals than maceral composition does.
ABSTRACT Thermal effect causes greater transformation of the vitrinite-rich coal. The intertinite... more ABSTRACT Thermal effect causes greater transformation of the vitrinite-rich coal. The intertinite-rich coal forms denser chars. The vitrinite-rich coal yields isotropic cokes. Chars are chemically similar but physically distinct. a b s t r a c t The development of coal char structures with pyrolysis has been extensively investigated but has traditionally been focused on the carboniferous coals in the northern Hemisphere. In this investigation, the properties of pyrolysis chars generated from inertinite-and vitrinite-rich coals (81% and 91% mmb by volume, respectively) collected from selected South African coalfields were determined. Chars were generated at 450, 700 and 1000 °C. The properties of coals and chars were examined using the chemical, physical, petrographic, solid-state 13 C NMR and X-ray diffraction analytical techniques. The objective was to generate results for further studies including molecular modelling and atomistic reaction kinetics. A good correlation was found between the total maceral scan (rank) and aromaticity as the pyrolysis temperature increased, as well as between the aromaticity measurements with XRD and NMR techniques. The chemical structure of the intertinite-rich and vitrinite-rich chars at 700–1000 °C was remarkably similar in terms of the proximate, ultimate, total maceral scan and aromaticity values. Greater transition occurred in the vitrinite-rich coal, implying a thermally more activated coal. Differences in the physical structure of the chars at these temperatures were observed in terms of the surface area using the Dubinin–Radushkevich (D–R), the Brunauer–Emmet–Teller (BET) and Langmuir methods as well the microporosity from the CO 2 adsorption method. The macerals were not distinguishable at 700–1000 °C. However, the differences in maceral composition of the coals resulted in substantially different char forms during thermal conversion. The intertinite-rich coal formed more denser chars and higher proportions of thicker-walled networks (60–65% by volume). The vitrinite-rich coal showed higher proportions of isotropic &amp;#39;&amp;#39;coke&amp;#39;&amp;#39; (91–95% by volume), which contributes to a high distribution of surface area and micropores. Therefore, on a chemical level, the high temperature chars were similar. Differences existed in the physical structure at high temperatures. The physical structures, char forms and crystallite diameter (L a) significantly distinguished the chars at high temperatures, where L a for inertinite-rich chars was 37.6 Å compared with 30.7 Å for the vitrinite-rich chars. The L a property, in particular, played a significant role in investigations for the molecular structural properties of the inertinite-and vitrinite-rich chars, including their reactivity behaviour with carbon dioxide gas.
ABSTRACT In this paper, we compare the surface area, porosity and other physical-structural prope... more ABSTRACT In this paper, we compare the surface area, porosity and other physical-structural properties of four bituminous coals from three different coal fields of South Africa determined using carbon dioxide (CO2) and nitrogen (N-2) low pressure gas adsorption (LPGA), mercury intrusion porosimetry (MIP), and advanced Small Angle X-ray Scattering (SAXS) analytical techniques. Consistent with previous findings, N-2 adsorption underestimated the surface area and porosity of the samples compared to results obtained using the other techniques, but it provided a good insight into the pore size distribution of mesopores. The surface areas and porosities of the samples determined from SAXS were found to be larger than any of the values obtained using the other techniques. This is attributed to SAXS probing a wider range of pores, including pores that are closed to, or restricted in access by, gas adsorption or mercury intrusion and also capturing the properties of pores of diameters between 5 and 17 angstrom, which are not readily measured by any of the other techniques used in this investigation. However, we show here that because each technique probes different pore size ranges in coal, a combination of SAXS with other techniques (gas adsorption, MIP, etc.) provides a richer picture of the nature of the porosity in coals.
ABSTRACT An investigation was undertaken to determine the effect of carbon structural properties ... more ABSTRACT An investigation was undertaken to determine the effect of carbon structural properties on the reactivity of chars derived from inertinite-rich coal. The structures of chars were characterized by X-ray diffraction and the reactivity evaluation with respect to gasification and combustion involving experimentation and reaction rate modeling was carried out. The effect of the aromaticity on the intrinsic reaction rates and associated activation energy for gasification and combustion of chars were determined based on chemical reaction rate controlled kinetics. Two sets of results were examined consisting of gasification with four char samples derived from different inertinite-rich coals prepared with a devolitalization temperature of 900 °C (Set 1) and three chars from the same parent coal also rich in inertinites prepared at different devolitalization temperatures, namely 550, 700 and 850 °C (Set 2). It was found that the aromaticity and fraction of crystalline carbon of the four chars prepared at the same temperature (Set 1) varied in the ranges 0.85–0.95 and 0.52–0.72 respectively and the chars prepared at different temperatures (Set 2) in the ranges 0.82–0.92 and 0.44–0.74 respectively. The structural properties of the chars within each set of results were also very different which required reaction rate modeling for the determination of intrinsic reactivities. The random pore model with chemical reaction controlling described the reaction kinetics accurately and it was found that the intrinsic reactivity of the char decreased with increasing aromaticity while the activation energies decreased (aromaticity &lt; 0.92) which indicated the effect of the variation of the distribution of aromatic/aliphatic structures on the intrinsic reaction rate. It is proposed that the aromaticity of chars be used as a predictive index for char reactivity under chemical reaction controlled conditions.
ABSTRACT The chemical–structural properties of four South African bituminous coals were investiga... more ABSTRACT The chemical–structural properties of four South African bituminous coals were investigated using wide angle X-ray diffraction–carbon fraction analysis (WAXRD–CFA), attenuated total reflection Fourier transform infra-red spectroscopy (ATR–FTIR), solid state 13C nuclear magnetic resonance spectroscopy (ss 13C NMR), and high resolution transmission electron microscopy (HRTEM). The aromaticity of the samples determined by solid state 13C NMR ranged from 0.74 to 0.87 and compared well with the WAXRD–CFA results (0.73–0.86). WAXRD–CFA, ATR–FTIR and 13C NMR data showed that the lower iso-rank coal samples contained more aliphatic moieties; while the higher rank sample contained higher fractions of polyaromatic moieties and saturated long chain hydrocarbons. The lattice parameters determined from WAXRD–CFA show that lower rank coals investigated are structurally less well-ordered than the higher rank coal. Also, its fringes were quantified as having the greatest preferential alignment of the coals examined. HRTEM aromatic fringe image analysis revealed that the carbon lattice of the samples consist of aromatic fringes of varying lengths, L (3 Å ⩽ L ⩽ 95 Å), which corresponded to a molecular weight distribution ranging from 75 to 1925 amu, assuming circular catenation. The coal with the highest volatile matter yield was found to exhibit a higher frequency of lower molecular weight fringes; while the higher rank coal possessed the most higher molecular weight fringes. The average molecular weights determined for the samples from 13C NMR varied between 504 and 544 amu; and compared well with the values from HRTEM aromatic fringe image analysis data. Thus, there was good agreement and consistency between the different techniques. It was demonstrated that, for the characterisation methods used in this study; that vitrinite reflectance impacts more on the chemical–structural properties of these coals than maceral composition does.
The development of coal char structures with pyrolysis has been extensively investigated but has ... more The development of coal char structures with pyrolysis has been extensively investigated but has traditionally been focused on the carboniferous coals in the northern Hemisphere. In this investigation, the properties of pyrolysis chars generated from inertinite- and vitrinite-rich coals (81% and 91% mmb by volume, respectively) collected from selected South African coalfields were determined. Chars were generated at 450, 700 and 1000 C. The properties of coals and chars were examined using the chemical, physical, petrographic, solid-state 13C NMR and X-ray diffraction analytical techniques. The objective was to generate results for further studies including molecular modelling and atomistic reaction kinetics. A good correlation was found between the total maceral scan (rank) and aromaticity as the pyrolysis temperature increased, as well as between the aromaticity measurements with XRD and NMR techniques. The chemical structure of the intertinite-rich and vitrinite-rich chars at 700–1000 C was remarkably similar in terms of the proximate, ultimate, total maceral scan and aromaticity values. Greater transition occurred in the vitrinite-rich coal, implying a thermally more activated coal. Differences in the physical structure of the chars at these temperatures were observed in terms of the surface area using the Dubinin–Radushkevich (D–R), the Brunauer–Emmet–Teller (BET) and Langmuir methods as well the microporosity from the CO2 adsorption method. The macerals were not distinguishable at 700–1000 °C. However, the differences in maceral composition of the coals resulted in substantially different char forms during thermal conversion. The intertinite-rich coal formed more denser chars and higher proportions of thicker-walled networks (60–65% by volume). The vitrinite- rich coal showed higher proportions of isotropic ‘‘coke’’ (91–95% by volume), which contributes to a high distribution of surface area and micropores. Therefore, on a chemical level, the high temperature chars were similar. Differences existed in the physical structure at high temperatures. The physical structures, char forms and crystallite diameter (La) significantly distinguished the chars at high temperatures, where La for inertinite-rich chars was 37.6 Å compared with 30.7 Å for the vitrinite-rich chars. The La property, in particular, played a significant role in investigations for the molecular structural properties of the inertinite- and vitrinite-rich chars, including their reactivity behaviour with carbon dioxide gas.
Experimental measurements and DFT atomistic modelling were conducted to elucidate the mechanisms ... more Experimental measurements and DFT atomistic modelling were conducted to elucidate the mechanisms for gasification chemistry of char with CO2 gas. The molecular models used were based on experimental representations of coal chars derived from the vitrinite- and inertinite-rich South African coals at 1000 C. The HRTEM and XRD techniques were used to construct parallelogram-shaped PAH stacks of highest frequency in the vitrinite-rich (7 · 7) and intertinite-rich (11 · 11) char structures. Computations were executed to get the nucleophilic Fukui functions, at DFT–DNP level, to elucidate the nature and proportions of carbon active sites and quantify their reactivity. The DFT–DNP-computed reaction pathways and transition states, to obtain the energy of reaction and activation energies for the gasification reactions of CO2 with active carbon sites were examined. These results were compared with TGA experimental results at 900–980 C. The mean nucleophilic Fukui function of the H-terminated char models and active sites located at similar edge positions decreased with increasing size of char molecules and followed the sequence: zigzag > armchair > tip active sites. The mean DFT–DNP values for the activation energy of 233 kJ mol1 at the reactive carbon edge was in agreement with the experimental 191 ± 25 kJ mol1 and 210 ± 8 kJ mol1 for the respective chars.
Abstract
The chemical-structural properties of four South African bituminous coals were investiga... more Abstract The chemical-structural properties of four South African bituminous coals were investigated using wide angle X-ray diffraction-carbon fraction analysis (WAXRD-CFA), attenuated total reflection Fourier transform infra-red spectroscopy (ATR-FTIR), solid state 13C nuclear magnetic resonance spectroscopy (ss 13C NMR), and high resolution transmission electron microscopy (HRTEM). The aromaticity of the samples determined by solid state 13C NMR ranged from 0.74 to 0.87 and compared well with the WAXRD-CFA results (0.73-0.86). WAXRD-CFA, ATR-FTIR and 13C NMR data showed that the lower iso-rank coal samples contained more aliphatic moieties; while the higher rank sample contained higher fractions of polyaromatic moieties and saturated long chain hydrocarbons. The lattice parameters determined from WAXRD-CFA show that lower rank coals investigated are structurally less well-ordered than the higher rank coal. Also, its fringes were quantified as having the greatest preferential alignment of the coals examined. HRTEM aromatic fringe image analysis revealed that the carbon lattice of the samples consist of aromatic fringes of varying lengths, L (3Å ≤ L ≤ 95Å), which corresponded to a molecular weight distribution ranging from 75 to 1925 amu, assuming circular catenation. The coal with the highest volatile matter yield was found to exhibit a higher frequency of lower molecular weight fringes; while the higher rank coal possessed the most higher molecular weight fringes. The average molecular weights determined for the samples from 13C NMR varied between 504 to 544 amu; and compared well with the values from HRTEM aromatic fringe image analysis data. Thus, there was good agreement and consistency between the different techniques. It was demonstrated that, for the characterisation methods used in this study; that vitrinite reflectance impacts more on the chemical-structural properties of these coals than maceral composition does.
In this paper, we compare the surface area, porosity and other physical–structural properties of ... more In this paper, we compare the surface area, porosity and other physical–structural properties of four bituminous coals from three different coal fields of South Africa determined using carbon dioxide (CO2) and nitrogen (N2) low pressure gas adsorption (LPGA), mercury intrusion porosimetry (MIP), and advanced Small Angle X-ray Scattering (SAXS) analytical techniques. Consistent with previous findings, N2 adsorption
underestimated the surface area and porosity of the samples compared to results obtained using the other techniques, but it provided a good insight into the pore size distribution of mesopores. The surface areas and porosities of the samples determined from SAXS were found to be larger than any of the values obtained using the other techniques. This is attributed to SAXS probing a wider range of pores, including pores that are closed to, or restricted in access by, gas adsorption or mercury intrusion and also capturing the properties of pores of diameters between 5 and 17 Å, which are not readily measured by any of the other techniques used in this investigation. However, we show here that because each technique probes
different pore size ranges in coal, a combination of SAXS with other techniques (gas adsorption, MIP, etc.) provides a richer picture of the nature of the porosity in coals.
h i g h l i g h t s " The standard and XRD properties occurring during devolitalization was deter... more h i g h l i g h t s " The standard and XRD properties occurring during devolitalization was determined. " Significant graphitization occurs at 900°C with aromaticity equal to 0.95. " The random pore model showed that the intrinsic reactivity correlates with the aromaticity. " The activation energy increased with increasing aromaticity.
The lighter density (<1.5 g/cm 3) fractions produced from two waste coals sampled from the waste ... more The lighter density (<1.5 g/cm 3) fractions produced from two waste coals sampled from the waste coal disposal sites at thermochemical plants situated in South Africa were used as feed materials for liquefaction with tetralin. The liquefaction residues from the lighter density and untreated lighter density fractions were used in pyrolysis experiments. Pyrolysis of the lighter density fractions was carried out in a Fischer assay oven at 750 o C and 920 o C under an argon atmosphere. The advanced analytical techniques (gas chromatography-mass spectrometry (GC-MS) and nuclear magnetic resonance (NMR) spectroscopy) were employed to characterise the pyrolysis products. Also, the lighter density fractions, liquefaction residues and their chars were examined using conventional and advanced analytical techniques. The pyrolysis char yields of the liquefaction residues ranged between 74% and 76%, and those of the coal float fractions ranged between 67.0-71.5%. Gas pyrolysis yields ranged between 16.0% and 20.0% for the residues and between 14.5% and 18.4% for the lighter density fractions, while the pyrolytic water and the tar products of the lighter density fractions were slightly higher than those of the liquefaction derived residues. The proton NMR analysis of the tars from the residues shows marginally higher amounts of aromatic protons than those of the lighter density fractions. Chars which were generated after pyrolysis of the liquefaction derived residues show higher porosity values than those from the pyrolysis of the lighter density fractions. The differences in the porosities are attributed to the opening of pores and extraction of some lower molecular mass aliphatic species from the coal matrix during liquefaction. The pyrolysis products distribution and characterisation of the products showed that the residues (waste material) generated after tetralin liquefaction of the float fractions from the float-sink experiments of waste coals may be utilised for thermochemical processes (pyrolysis).
The possible catalytic effect of a 5% loading by mass of crystallized salt from spent Benfield so... more The possible catalytic effect of a 5% loading by mass of crystallized salt from spent Benfield solution (SBFS) was investigated and compared with the catalytic influence of a 5 mass% loading of K 2 CO 3 on the char-CO 2 gasification reactivity of a bituminous medium rank C South African Highveld coal. The effect was investigated using known thermogravimetric investigation techniques. Coal char samples were prepared using the coal and catalyst-loaded samples by heating the samples in nitrogen to 1000°C. The prepared char samples were further subjected to thermogravimetric analyses by heating them at isothermal temperatures between 900 and 990°C in a CO 2 atmosphere to simulate gasification reactions of the coal char. Results from isothermal char-CO 2 gasification experiments showed that the catalyst-loaded coal samples exhibited higher gasification reactivities than the coal sample and the reactivities increased in the order: coal \ coal ? 5% SBF \ coal ? 5% K 2 CO 3. It was observed that the relative initial reactivity of the catalyst-loaded samples increased in order of magnitude [ 3.5 in comparison with the coal char sample. The addition of catalysts to the coal was found to reduce the estimated activation energies of the catalyst-loaded samples during the gasification process. The estimated activation energies (calculated from the initial reaction rate of the gasification reaction) of the samples were 215 kJ mol-1 for the coal char, 146 kJ mol-1 for the char from the coal ? 5% SBF and 118 kJ mol-1 for the char from the coal ? 5% K 2 CO 3. The crystallized spent Benfield salt may be utilized as a CO 2 gasification catalyst for coal. Keywords CO 2 gasification reactivity Á Catalytic effect Á Spent Benfield salt Á K 2 CO 3 Á Activation energy & C. A.
This is a repository copy of Density functional theory molecular modelling and experimental parti... more This is a repository copy of Density functional theory molecular modelling and experimental particle kinetics for CO2-char gasification.
The carbon dioxide gasification characteristics of three biomass char samples and bituminous coal... more The carbon dioxide gasification characteristics of three biomass char samples and bituminous coal char were investigated in a thermogravimetric analyser in the temperature range of 850-950 °C. Char SB exhibited higher reactivities (R, R, R) than chars SW and HW. Coal char gasification reactivities were observed to be lower than those of the three biomass chars. Correlations between the char reactivities and char characteristics were highlighted. The addition of 10% biomass had no significant impact on the coal char gasification reactivity. However, 20 and 30% biomass additions resulted in increased coal char gasification rate. During co-gasification, chars HW and SW caused increased coal char gasification reactivity at lower conversions, while char SB resulted in increased gasification rates throughout the entire conversion range. Experimental data from biomass char gasification and biomass-coal char co-gasification were well described by the MRPM, while coal char gasification was b...
ABSTRACT The chemical–structural properties of four South African bituminous coals were investiga... more ABSTRACT The chemical–structural properties of four South African bituminous coals were investigated using wide angle X-ray diffraction–carbon fraction analysis (WAXRD–CFA), attenuated total reflection Fourier transform infra-red spectroscopy (ATR–FTIR), solid state 13C nuclear magnetic resonance spectroscopy (ss 13C NMR), and high resolution transmission electron microscopy (HRTEM). The aromaticity of the samples determined by solid state 13C NMR ranged from 0.74 to 0.87 and compared well with the WAXRD–CFA results (0.73–0.86). WAXRD–CFA, ATR–FTIR and 13C NMR data showed that the lower iso-rank coal samples contained more aliphatic moieties; while the higher rank sample contained higher fractions of polyaromatic moieties and saturated long chain hydrocarbons. The lattice parameters determined from WAXRD–CFA show that lower rank coals investigated are structurally less well-ordered than the higher rank coal. Also, its fringes were quantified as having the greatest preferential alignment of the coals examined. HRTEM aromatic fringe image analysis revealed that the carbon lattice of the samples consist of aromatic fringes of varying lengths, L (3 Å ⩽ L ⩽ 95 Å), which corresponded to a molecular weight distribution ranging from 75 to 1925 amu, assuming circular catenation. The coal with the highest volatile matter yield was found to exhibit a higher frequency of lower molecular weight fringes; while the higher rank coal possessed the most higher molecular weight fringes. The average molecular weights determined for the samples from 13C NMR varied between 504 and 544 amu; and compared well with the values from HRTEM aromatic fringe image analysis data. Thus, there was good agreement and consistency between the different techniques. It was demonstrated that, for the characterisation methods used in this study; that vitrinite reflectance impacts more on the chemical–structural properties of these coals than maceral composition does.
ABSTRACT Thermal effect causes greater transformation of the vitrinite-rich coal. The intertinite... more ABSTRACT Thermal effect causes greater transformation of the vitrinite-rich coal. The intertinite-rich coal forms denser chars. The vitrinite-rich coal yields isotropic cokes. Chars are chemically similar but physically distinct. a b s t r a c t The development of coal char structures with pyrolysis has been extensively investigated but has traditionally been focused on the carboniferous coals in the northern Hemisphere. In this investigation, the properties of pyrolysis chars generated from inertinite-and vitrinite-rich coals (81% and 91% mmb by volume, respectively) collected from selected South African coalfields were determined. Chars were generated at 450, 700 and 1000 °C. The properties of coals and chars were examined using the chemical, physical, petrographic, solid-state 13 C NMR and X-ray diffraction analytical techniques. The objective was to generate results for further studies including molecular modelling and atomistic reaction kinetics. A good correlation was found between the total maceral scan (rank) and aromaticity as the pyrolysis temperature increased, as well as between the aromaticity measurements with XRD and NMR techniques. The chemical structure of the intertinite-rich and vitrinite-rich chars at 700–1000 °C was remarkably similar in terms of the proximate, ultimate, total maceral scan and aromaticity values. Greater transition occurred in the vitrinite-rich coal, implying a thermally more activated coal. Differences in the physical structure of the chars at these temperatures were observed in terms of the surface area using the Dubinin–Radushkevich (D–R), the Brunauer–Emmet–Teller (BET) and Langmuir methods as well the microporosity from the CO 2 adsorption method. The macerals were not distinguishable at 700–1000 °C. However, the differences in maceral composition of the coals resulted in substantially different char forms during thermal conversion. The intertinite-rich coal formed more denser chars and higher proportions of thicker-walled networks (60–65% by volume). The vitrinite-rich coal showed higher proportions of isotropic &amp;#39;&amp;#39;coke&amp;#39;&amp;#39; (91–95% by volume), which contributes to a high distribution of surface area and micropores. Therefore, on a chemical level, the high temperature chars were similar. Differences existed in the physical structure at high temperatures. The physical structures, char forms and crystallite diameter (L a) significantly distinguished the chars at high temperatures, where L a for inertinite-rich chars was 37.6 Å compared with 30.7 Å for the vitrinite-rich chars. The L a property, in particular, played a significant role in investigations for the molecular structural properties of the inertinite-and vitrinite-rich chars, including their reactivity behaviour with carbon dioxide gas.
ABSTRACT In this paper, we compare the surface area, porosity and other physical-structural prope... more ABSTRACT In this paper, we compare the surface area, porosity and other physical-structural properties of four bituminous coals from three different coal fields of South Africa determined using carbon dioxide (CO2) and nitrogen (N-2) low pressure gas adsorption (LPGA), mercury intrusion porosimetry (MIP), and advanced Small Angle X-ray Scattering (SAXS) analytical techniques. Consistent with previous findings, N-2 adsorption underestimated the surface area and porosity of the samples compared to results obtained using the other techniques, but it provided a good insight into the pore size distribution of mesopores. The surface areas and porosities of the samples determined from SAXS were found to be larger than any of the values obtained using the other techniques. This is attributed to SAXS probing a wider range of pores, including pores that are closed to, or restricted in access by, gas adsorption or mercury intrusion and also capturing the properties of pores of diameters between 5 and 17 angstrom, which are not readily measured by any of the other techniques used in this investigation. However, we show here that because each technique probes different pore size ranges in coal, a combination of SAXS with other techniques (gas adsorption, MIP, etc.) provides a richer picture of the nature of the porosity in coals.
ABSTRACT An investigation was undertaken to determine the effect of carbon structural properties ... more ABSTRACT An investigation was undertaken to determine the effect of carbon structural properties on the reactivity of chars derived from inertinite-rich coal. The structures of chars were characterized by X-ray diffraction and the reactivity evaluation with respect to gasification and combustion involving experimentation and reaction rate modeling was carried out. The effect of the aromaticity on the intrinsic reaction rates and associated activation energy for gasification and combustion of chars were determined based on chemical reaction rate controlled kinetics. Two sets of results were examined consisting of gasification with four char samples derived from different inertinite-rich coals prepared with a devolitalization temperature of 900 °C (Set 1) and three chars from the same parent coal also rich in inertinites prepared at different devolitalization temperatures, namely 550, 700 and 850 °C (Set 2). It was found that the aromaticity and fraction of crystalline carbon of the four chars prepared at the same temperature (Set 1) varied in the ranges 0.85–0.95 and 0.52–0.72 respectively and the chars prepared at different temperatures (Set 2) in the ranges 0.82–0.92 and 0.44–0.74 respectively. The structural properties of the chars within each set of results were also very different which required reaction rate modeling for the determination of intrinsic reactivities. The random pore model with chemical reaction controlling described the reaction kinetics accurately and it was found that the intrinsic reactivity of the char decreased with increasing aromaticity while the activation energies decreased (aromaticity &lt; 0.92) which indicated the effect of the variation of the distribution of aromatic/aliphatic structures on the intrinsic reaction rate. It is proposed that the aromaticity of chars be used as a predictive index for char reactivity under chemical reaction controlled conditions.
ABSTRACT The chemical–structural properties of four South African bituminous coals were investiga... more ABSTRACT The chemical–structural properties of four South African bituminous coals were investigated using wide angle X-ray diffraction–carbon fraction analysis (WAXRD–CFA), attenuated total reflection Fourier transform infra-red spectroscopy (ATR–FTIR), solid state 13C nuclear magnetic resonance spectroscopy (ss 13C NMR), and high resolution transmission electron microscopy (HRTEM). The aromaticity of the samples determined by solid state 13C NMR ranged from 0.74 to 0.87 and compared well with the WAXRD–CFA results (0.73–0.86). WAXRD–CFA, ATR–FTIR and 13C NMR data showed that the lower iso-rank coal samples contained more aliphatic moieties; while the higher rank sample contained higher fractions of polyaromatic moieties and saturated long chain hydrocarbons. The lattice parameters determined from WAXRD–CFA show that lower rank coals investigated are structurally less well-ordered than the higher rank coal. Also, its fringes were quantified as having the greatest preferential alignment of the coals examined. HRTEM aromatic fringe image analysis revealed that the carbon lattice of the samples consist of aromatic fringes of varying lengths, L (3 Å ⩽ L ⩽ 95 Å), which corresponded to a molecular weight distribution ranging from 75 to 1925 amu, assuming circular catenation. The coal with the highest volatile matter yield was found to exhibit a higher frequency of lower molecular weight fringes; while the higher rank coal possessed the most higher molecular weight fringes. The average molecular weights determined for the samples from 13C NMR varied between 504 and 544 amu; and compared well with the values from HRTEM aromatic fringe image analysis data. Thus, there was good agreement and consistency between the different techniques. It was demonstrated that, for the characterisation methods used in this study; that vitrinite reflectance impacts more on the chemical–structural properties of these coals than maceral composition does.
The development of coal char structures with pyrolysis has been extensively investigated but has ... more The development of coal char structures with pyrolysis has been extensively investigated but has traditionally been focused on the carboniferous coals in the northern Hemisphere. In this investigation, the properties of pyrolysis chars generated from inertinite- and vitrinite-rich coals (81% and 91% mmb by volume, respectively) collected from selected South African coalfields were determined. Chars were generated at 450, 700 and 1000 C. The properties of coals and chars were examined using the chemical, physical, petrographic, solid-state 13C NMR and X-ray diffraction analytical techniques. The objective was to generate results for further studies including molecular modelling and atomistic reaction kinetics. A good correlation was found between the total maceral scan (rank) and aromaticity as the pyrolysis temperature increased, as well as between the aromaticity measurements with XRD and NMR techniques. The chemical structure of the intertinite-rich and vitrinite-rich chars at 700–1000 C was remarkably similar in terms of the proximate, ultimate, total maceral scan and aromaticity values. Greater transition occurred in the vitrinite-rich coal, implying a thermally more activated coal. Differences in the physical structure of the chars at these temperatures were observed in terms of the surface area using the Dubinin–Radushkevich (D–R), the Brunauer–Emmet–Teller (BET) and Langmuir methods as well the microporosity from the CO2 adsorption method. The macerals were not distinguishable at 700–1000 °C. However, the differences in maceral composition of the coals resulted in substantially different char forms during thermal conversion. The intertinite-rich coal formed more denser chars and higher proportions of thicker-walled networks (60–65% by volume). The vitrinite- rich coal showed higher proportions of isotropic ‘‘coke’’ (91–95% by volume), which contributes to a high distribution of surface area and micropores. Therefore, on a chemical level, the high temperature chars were similar. Differences existed in the physical structure at high temperatures. The physical structures, char forms and crystallite diameter (La) significantly distinguished the chars at high temperatures, where La for inertinite-rich chars was 37.6 Å compared with 30.7 Å for the vitrinite-rich chars. The La property, in particular, played a significant role in investigations for the molecular structural properties of the inertinite- and vitrinite-rich chars, including their reactivity behaviour with carbon dioxide gas.
Experimental measurements and DFT atomistic modelling were conducted to elucidate the mechanisms ... more Experimental measurements and DFT atomistic modelling were conducted to elucidate the mechanisms for gasification chemistry of char with CO2 gas. The molecular models used were based on experimental representations of coal chars derived from the vitrinite- and inertinite-rich South African coals at 1000 C. The HRTEM and XRD techniques were used to construct parallelogram-shaped PAH stacks of highest frequency in the vitrinite-rich (7 · 7) and intertinite-rich (11 · 11) char structures. Computations were executed to get the nucleophilic Fukui functions, at DFT–DNP level, to elucidate the nature and proportions of carbon active sites and quantify their reactivity. The DFT–DNP-computed reaction pathways and transition states, to obtain the energy of reaction and activation energies for the gasification reactions of CO2 with active carbon sites were examined. These results were compared with TGA experimental results at 900–980 C. The mean nucleophilic Fukui function of the H-terminated char models and active sites located at similar edge positions decreased with increasing size of char molecules and followed the sequence: zigzag > armchair > tip active sites. The mean DFT–DNP values for the activation energy of 233 kJ mol1 at the reactive carbon edge was in agreement with the experimental 191 ± 25 kJ mol1 and 210 ± 8 kJ mol1 for the respective chars.
Abstract
The chemical-structural properties of four South African bituminous coals were investiga... more Abstract The chemical-structural properties of four South African bituminous coals were investigated using wide angle X-ray diffraction-carbon fraction analysis (WAXRD-CFA), attenuated total reflection Fourier transform infra-red spectroscopy (ATR-FTIR), solid state 13C nuclear magnetic resonance spectroscopy (ss 13C NMR), and high resolution transmission electron microscopy (HRTEM). The aromaticity of the samples determined by solid state 13C NMR ranged from 0.74 to 0.87 and compared well with the WAXRD-CFA results (0.73-0.86). WAXRD-CFA, ATR-FTIR and 13C NMR data showed that the lower iso-rank coal samples contained more aliphatic moieties; while the higher rank sample contained higher fractions of polyaromatic moieties and saturated long chain hydrocarbons. The lattice parameters determined from WAXRD-CFA show that lower rank coals investigated are structurally less well-ordered than the higher rank coal. Also, its fringes were quantified as having the greatest preferential alignment of the coals examined. HRTEM aromatic fringe image analysis revealed that the carbon lattice of the samples consist of aromatic fringes of varying lengths, L (3Å ≤ L ≤ 95Å), which corresponded to a molecular weight distribution ranging from 75 to 1925 amu, assuming circular catenation. The coal with the highest volatile matter yield was found to exhibit a higher frequency of lower molecular weight fringes; while the higher rank coal possessed the most higher molecular weight fringes. The average molecular weights determined for the samples from 13C NMR varied between 504 to 544 amu; and compared well with the values from HRTEM aromatic fringe image analysis data. Thus, there was good agreement and consistency between the different techniques. It was demonstrated that, for the characterisation methods used in this study; that vitrinite reflectance impacts more on the chemical-structural properties of these coals than maceral composition does.
In this paper, we compare the surface area, porosity and other physical–structural properties of ... more In this paper, we compare the surface area, porosity and other physical–structural properties of four bituminous coals from three different coal fields of South Africa determined using carbon dioxide (CO2) and nitrogen (N2) low pressure gas adsorption (LPGA), mercury intrusion porosimetry (MIP), and advanced Small Angle X-ray Scattering (SAXS) analytical techniques. Consistent with previous findings, N2 adsorption
underestimated the surface area and porosity of the samples compared to results obtained using the other techniques, but it provided a good insight into the pore size distribution of mesopores. The surface areas and porosities of the samples determined from SAXS were found to be larger than any of the values obtained using the other techniques. This is attributed to SAXS probing a wider range of pores, including pores that are closed to, or restricted in access by, gas adsorption or mercury intrusion and also capturing the properties of pores of diameters between 5 and 17 Å, which are not readily measured by any of the other techniques used in this investigation. However, we show here that because each technique probes
different pore size ranges in coal, a combination of SAXS with other techniques (gas adsorption, MIP, etc.) provides a richer picture of the nature of the porosity in coals.
h i g h l i g h t s " The standard and XRD properties occurring during devolitalization was deter... more h i g h l i g h t s " The standard and XRD properties occurring during devolitalization was determined. " Significant graphitization occurs at 900°C with aromaticity equal to 0.95. " The random pore model showed that the intrinsic reactivity correlates with the aromaticity. " The activation energy increased with increasing aromaticity.
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Papers by Gregory Okolo
> tip active sites. The mean DFT–DNP values for the activation energy of 233 kJ mol1 at the reactive carbon edge was in agreement with the experimental 191 ± 25 kJ mol1 and 210 ± 8 kJ mol1 for the respective chars.
The chemical-structural properties of four South African bituminous coals were investigated using wide angle X-ray diffraction-carbon fraction analysis (WAXRD-CFA), attenuated total reflection Fourier transform infra-red spectroscopy (ATR-FTIR), solid state 13C nuclear magnetic resonance spectroscopy (ss 13C NMR), and high resolution transmission electron microscopy (HRTEM). The aromaticity of the samples determined by solid state 13C NMR ranged from 0.74 to 0.87 and compared well with the WAXRD-CFA results (0.73-0.86). WAXRD-CFA, ATR-FTIR and 13C NMR data showed that the lower iso-rank coal samples contained more aliphatic moieties; while the higher rank sample contained higher fractions of polyaromatic moieties and saturated long chain hydrocarbons. The lattice parameters determined from WAXRD-CFA show that lower rank coals investigated are structurally less well-ordered than the higher rank coal. Also, its fringes were quantified as having the greatest preferential alignment of the coals examined. HRTEM aromatic fringe image analysis revealed that the carbon lattice of the samples consist of aromatic fringes of varying lengths, L (3Å ≤ L ≤ 95Å), which corresponded to a molecular weight distribution ranging from 75 to 1925 amu, assuming circular catenation. The coal with the highest volatile matter yield was found to exhibit a higher frequency of lower molecular weight fringes; while the higher rank coal possessed the most higher molecular weight fringes. The average molecular weights determined for the samples from 13C NMR varied between 504 to 544 amu; and compared well with the values from HRTEM aromatic fringe image analysis data. Thus, there was good agreement and consistency between the different techniques. It was demonstrated that, for the characterisation methods used in this study; that vitrinite reflectance impacts more on the chemical-structural properties of these coals than maceral composition does.
underestimated the surface area and porosity of the samples compared to results obtained using the other techniques, but it provided a good insight into the pore size distribution of mesopores. The surface areas and porosities of the samples determined from SAXS were found to be larger than any of the values obtained using the other techniques. This is attributed to SAXS probing a wider range of pores, including pores that are closed to, or restricted in access by, gas adsorption or mercury intrusion and also capturing the properties of pores of diameters between 5 and 17 Å, which are not readily measured by any of the other techniques used in this investigation. However, we show here that because each technique probes
different pore size ranges in coal, a combination of SAXS with other techniques (gas adsorption, MIP, etc.) provides a richer picture of the nature of the porosity in coals.
> tip active sites. The mean DFT–DNP values for the activation energy of 233 kJ mol1 at the reactive carbon edge was in agreement with the experimental 191 ± 25 kJ mol1 and 210 ± 8 kJ mol1 for the respective chars.
The chemical-structural properties of four South African bituminous coals were investigated using wide angle X-ray diffraction-carbon fraction analysis (WAXRD-CFA), attenuated total reflection Fourier transform infra-red spectroscopy (ATR-FTIR), solid state 13C nuclear magnetic resonance spectroscopy (ss 13C NMR), and high resolution transmission electron microscopy (HRTEM). The aromaticity of the samples determined by solid state 13C NMR ranged from 0.74 to 0.87 and compared well with the WAXRD-CFA results (0.73-0.86). WAXRD-CFA, ATR-FTIR and 13C NMR data showed that the lower iso-rank coal samples contained more aliphatic moieties; while the higher rank sample contained higher fractions of polyaromatic moieties and saturated long chain hydrocarbons. The lattice parameters determined from WAXRD-CFA show that lower rank coals investigated are structurally less well-ordered than the higher rank coal. Also, its fringes were quantified as having the greatest preferential alignment of the coals examined. HRTEM aromatic fringe image analysis revealed that the carbon lattice of the samples consist of aromatic fringes of varying lengths, L (3Å ≤ L ≤ 95Å), which corresponded to a molecular weight distribution ranging from 75 to 1925 amu, assuming circular catenation. The coal with the highest volatile matter yield was found to exhibit a higher frequency of lower molecular weight fringes; while the higher rank coal possessed the most higher molecular weight fringes. The average molecular weights determined for the samples from 13C NMR varied between 504 to 544 amu; and compared well with the values from HRTEM aromatic fringe image analysis data. Thus, there was good agreement and consistency between the different techniques. It was demonstrated that, for the characterisation methods used in this study; that vitrinite reflectance impacts more on the chemical-structural properties of these coals than maceral composition does.
underestimated the surface area and porosity of the samples compared to results obtained using the other techniques, but it provided a good insight into the pore size distribution of mesopores. The surface areas and porosities of the samples determined from SAXS were found to be larger than any of the values obtained using the other techniques. This is attributed to SAXS probing a wider range of pores, including pores that are closed to, or restricted in access by, gas adsorption or mercury intrusion and also capturing the properties of pores of diameters between 5 and 17 Å, which are not readily measured by any of the other techniques used in this investigation. However, we show here that because each technique probes
different pore size ranges in coal, a combination of SAXS with other techniques (gas adsorption, MIP, etc.) provides a richer picture of the nature of the porosity in coals.