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. / Thesis (M.Ing. (Chemical Engineering))--North-West University, Potchefstroom Campus, 2011.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:nwu/oai:dspace.nwu.ac.za:10394/4890 |
| Date | January 2010 |
| Creators | Okolo, Gregori Nworah |
| Publisher | North-West University |
| Source Sets | South African National ETD Portal |
| Detected Language | English |
| Type | Thesis |
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