The effects of chemical and physical properties of chars derived from inertinite–rich, high ash coals on gasification reaction kinetics
Okolo, Gregori Nworah
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With the increasing global energy demand and the decreasing availability of good quality coals, a better understanding of the important properties that control the behaviour of low–grade coals and the subsequent chars in various utilisation processes, becomes pertinent. An investigation was therefore undertaken, to study the effects of chemical and physical properties imparted on chars during pyrolysis on the subsequent gasification reaction kinetics of typical South African inertinite–rich, high ash Highveld coals. An attempt was made at following these changes in the transition from coals to chars by a detailed characterisation of both the parent coals and the respective chars. These changes were determined using various conventional and advanced techniques, which included among others, carbon crystallite analysis using XRD and char carbon forms analysis using petrography. Three of the four original coals were characterised as Bituminous Medium rank C (coals B, C and C2), while coal D2 was found to be slightly lower in rank (Bituminous Medium rank D). The coals were rich in inertinites (> 54 vol. %, mmb with coal C2 having as high as 79 vol. %, mmb) and high in ash content (> 26.7 wt. %, db) and cabominerite and minerite contents (26 – 39 vol. %, mmb). The inertinitevitrinite ratios of the coals were found to range from 1.93 to 26.3. Characterization results show that both volatile matter and inherent moisture content decreased, while ash, fixed carbon and elemental carbon contents increased from coals to chars, indicating that the pyrolysis process was efficient. Elemental hydrogen, oxygen and nitrogen contents decreased, whereas total sulphur contents increased from coals to chars. This reveals that the total sulphur contained in the char samples was associated with the char carbon matrix and the minerals. Hydrogen–carbon and oxygen–carbon ratios decreased considerably from coals to chars showing that the chars are more aromatic and denser products than the original coals. Despite the fact that mineral matter increased from coals to chars, the relative abundance of the different mineral phases and ash components did not exhibit significant variation amongst the samples. The alkali index was, however, found to vary considerably among the subsequent chars. Petrographic analysis of the coals and char carbon forms analysis of the chars reveal that total reactive components (TRC) decrease while the total inert components (TIC) increase from coals to chars. The 0% gain in TIC observed in char C2 was attributed to its relatively high partially reacted maceral char carbon forms content. Total maceral reflectance shifted to higher values in the chars (4.43 – 5.28 Rsc%) relative to the coals (1.15 – 1.63 Rsc%) suggesting a higher structural ordering in the chars. Carbon crystallite analyses revealed that the chars were condensed (smaller in size) relative to the parent coals. Lattice parameters: interlayer spacing, d002, increased, while the average crystallite height, Lc, crystallite diameter, La, and number of aromatic layers per crystallite, Nave, decreased from coals to chars. Carbon aromaticity generally increased whereas the fraction of amorphous carbon and the degree of disorder index decreased from parent coals to the respective chars. Both micropore surface area and microporosity were observed to increase while the average micropore diameter decreased from coals to chars. This shows that blind and closed micropores were “opened up” during the charring process. Despite the original coal samples not showing much variation in their properties (except for their maceral content), it was generally observed that the subsequent chars exhibited substantial differences, both amongst themselves and from the parent coals. The increasing orders of magnitude of micropore surface area, microporosity, fraction of amorphous carbon and structural disorderliness were found to change in the transition, a good indication that the chars’ properties varied from that of the respective parent coals. Isothermal CO2 gasification experiments were conducted on the chars in a Thermax 500 thermogravimetric analyser in the temperature range of 900 – 950 °C with varying concentrations of CO2 (25 – 100 mol. %) in the CO2–N2 reaction gas mixture at ambient pressure (0.875 bar in Potchefstroom). The effects of temperature and CO2 concentration were observed to be in conformity with established trends. The initial reactivity of the chars was found to increase in the order: chars C2 < C < B < D2, with char D2 reactivity greater than the reactivity of the other chars by a factor > 4. Gasification reactivity results were correlated with properties of the parent coals and chars. Except for the rank parameter (the vitrinite reflectance), no significant trend was observed with any other coal petrographic property. Correlations with char properties gave more significant and systematic trends. Major factors affecting the gasification reactivity of the chars as it pertains to this investigation are: parent coal vitrinite reflectance, and: aromaticity, fraction of amorphous carbon, degree of disorder and alkali indices, micropore surface area, microporosity and average micropore diameter of the chars. The random pore model (chemical reaction controlling) was found to adequately describe the gasification reaction experimental data (both conversions and conversion rates). The determined activation energy ranged from 163.3 kJ·mol–1 for char D2 to 235.7 kJ·mol–1 for char B; while the order of reaction with respect to CO2 concentration ranged between 0.52 to 0.67 for the four chars. The lower activation energy of char D2 was possibly due to its lower rank, lower coal vitrinite reflectance and higher alkali index. The estimated kinetic parameters of the chars in this study correspond very well with published results in open literature. It was possible to express the intrinsic reactivity, rs, of the chars (rate of carbon conversion per unit total surface area) using kinetic results, in empirical Arrhenius forms.
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