An investigation was undertaken to compare the kinetics of combustion and gasification
reactions of chars prepared from two South African coals in different reaction atmospheres:
air, steam, and carbon dioxide. The two original coals were characterised as vitrinite-rich
(Greenside) and inertinite-rich (Inyanda) coals with relatively low ash content (12.5-16.7 wt.
%, adb). Chars were prepared from the parent coals under nitrogen atmosphere at 900 °C.
Characterisation results show that the volatiles and moisture were almost completely driven
off from the parent coals, indicating that the pyrolysis process was efficient. Physicalstructural
properties such as porosity and surface area generally increased from the parent
coals to the subsequent chars. The heterogeneous char-gas reactions were conducted
isothermally in a TGA on ~1 mm size particles. To ensure that the reactions are under
chemical reaction kinetic control regime, different temperatures zones were selected for the
three different reaction atmospheres. Combustion reactivity experiments were carried out
with air in the temperature range of 387 °C to 425 °C; gasification reactivity with pure steam
were conducted at higher temperatures (775 °C - 850 °C) and within 825 °C to 900 °C with
carbon dioxide. Experimental results show differences in the specific reaction rate with
carbon conversion in different reaction atmospheres and char types. Reaction rates in all three
reaction atmospheres were strongly dependent on temperature, and follow the Arrhenius type
kinetics. All the investigated reactions (combustion with air and gasification with CO2 and
steam) were found to be under chemical reaction control regime (Regime I) for both chars.
The inertinite-rich coals exhibit longer burn-out time than chars produced from vitrinite-rich
coals, as higher specific reaction rate were observed for the vitrinite-rich coals in the three
different reaction atmospheres. The determined random pore model (RPM) structural
parameters did not show any significant difference during steam gasification of Greenside
and Inyanda chars, whereas higher structural parameter values were observed for Greenside
chars during air combustion and CO2 gasification (ψ > 2). However a negative ψ value was
determined during CO2 gasification and air combustion of Inyanda chars. The RPM
predictions was validated with the experimental data and exhibited adequate fitting to the
specific rate of reaction versus carbon conversion plots of the char samples at the different
reaction conditions chosen for this study. The activation energy determined was minimal for
air and maximum for CO2 for both coals; and ranged from 127-175 kJ·mol-1 for combustion,
214-228 kJ·mol-1 and 210-240 kJ·mol-1 for steam and CO2 gasification respectively. / MIng (Chemical Engineering), North-West University, Potchefstroom Campus, 2015
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:nwu/oai:dspace.nwu.ac.za:10394/15519 |
| Date | January 2015 |
| Creators | Rambuda, Mpho |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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