Catalytic steam gasification of large coal particles
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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.
- Engineering