| Summary: | Catalytic gasification has been studied extensively in order to develop more efficient and
economic coal conversion processes. Fundamental studies regarding catalytic gasification
have thus far focused on experimentation with small coal particles and powders. The lack of
knowledge regarding the application of large coal particles in steam gasification studies, in
particular catalytic steam gasification, is the motivation behind this investigation.
A washed bituminous, medium rank-C Highveld coal (seam 4) was selected for this study,
and a general characterisation of the coal was conducted. It was found that the ash content
of the washed coal is 12.6 wt.% (air-dried basis). Based on the gross calorific value of 26.6
MJ/kg (air-dried basis), the coal sample was graded as a grade B coal. XRF analysis of the
ash indicated that the coal is rich in SiO2 and Al2O3, with a low potassium oxide content
(0.53 wt.%) which is typical for South African coal.
Potassium carbonate (K2CO3) was selected as catalyst, and the excess solution
impregnation method was used to impregnate large coal particles (5 mm, 10 mm, 20 mm
and 30 mm). The pH of the impregnation solution stabilised after three weeks, which led to
the assumption that impregnation is complete. Two methods were used to determine the
catalyst loading obtained after impregnation: XRF was used to determine the wt.% K in the
ash, while ion specific electrode (ISE) was used to measure the [K+] decrease in the
impregnation solution. XRF results indicated the maximum catalyst loading obtainable for
large coal particles, with the specific impregnation method, to be between 0.68 – 0.83 wt.%
K (coal basis). XRF can be used to determine the catalyst loading by measuring the K
content in the ash, while ISE can be used to semi-quantitatively predict the catalyst loadings
of large coal particles. The catalyst distribution was studied using SEM and tomography
analyses. SEM scans showed that the formation of cracks occurred as a result of
impregnation, and EDS analysis indicated that the majority of the catalyst is concentrated
around the outer surface of the particles. Tomography scans, and mineral volume analysis,
indicated that the mineral matter of the coal particles increased after impregnation.
The effect of catalyst addition on reactivity was investigated by conducting steam gasification
experiments with 5 mm and 10 mm particles, in a large particle TGA. The 20 mm and 30
mm particles did not remain intact after impregnation and were therefore not used for the
reactivity experiments. Reactivity experiments were performed at temperatures ranging from
800 °C to 875 °C, with a steam concentration of 80 mol%. Graphs illustrating conversion as a function of time indicated that the addition of K2CO3 to the coal samples increased the
reaction rate. This was quantified by determining the reactivities of the raw and catalysed
samples using linearised homogeneous model plots. The reaction rate was found to be
temperature sensitive, and independent of particle size, which indicated that experiments
were conducted in the chemical reaction control regime. A slight decrease in activation
energy was observed with the addition of K2CO3, from 191 kJ/mol (raw coal) to 179 kJ/mol
(catalysed coal). Microscope images of raw and catalysed chars indicated that the addition
of a catalyst may reduce agglomeration. === Thesis (M.Ing. (Chemical Engineering))--North-West University, Potchefstroom Campus, 2012
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