The influence of particle size and density on the combustion of Highveld coal
Van der Merwe, George William
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Coal from the Highveld seam 4 deposit was studied to determine and understand the influence of density and particle size on the high temperature combustion characteristics. All of the charring, as well as the combustion experiments were conducted in a high temperature horizontal tube furnace where charring was done at 1100 °C and combustion at 1000 °C. The furnace was equipped with carbon monoxide and carbon dioxide analysers to monitor carbon conversion during experimentation. The parent coal was characterised in terms of proximate analysis, ultimate analysis, calorific value analysis and FTIR analysis. The chars were investigated with FTIR analysis. The coals were categorized according to density from +1.4 g.cm-3 to -2.0 g.cm-3 using 0.2 g.cm-3 intervals and the particle size effect was studied using particles of 20, 30 and 40 mm diameter. The characterization results showed that the moisture content of the coal samples tended to decrease as the particle density increased and that the smaller particles tended to have a higher moisture content. The volatile matter and fixed carbon content decreased as the coal density increased, while both had a relatively even distribution across the size ranges. The ash content was found to significantly increase as the density increased. Elemental analysis revealed that that carbon-, hydrogen- and nitrogen- content decreased as density increased with a relatively even distribution of the different size ranges. High sulphur values were observed in the high density particles. This was attributed to a higher pyrite content in this fraction. Smaller particles tended to have a higher oxygen content. Calorific value analysis showed that the high density fraction had a very low heating value and that the overall sample value would be improved by removing the high density fraction. FTIR analysis showed that the properties of density separated fractions varied significantly; this was most prominent in the substitution structures occurring on the aromatic structures. It was found that the chemical composition of the size separated particles remained relatively constant. The properties of char particles with different densities tended to become more similar, especially with regard to their aromatic substitution structures. The characterization of the different coal samples showed that the parameters were generally significantly different for the different density fractions but did not vary significantly across the particle size ranges. It was also found that significant shattering occurred at heating rates of 50 °C/min, while very little shattering was observed at 15 °C/min. The low density particles formed porous ash residues, while the high density particles formed very hard and solid ash residues that could contain unreacted carbon in the core. Combustion studies showed that particle density and size had a significant influence on the time required for complete conversion of the chars. The low density particles required less time for full conversion than the high density particles and the smaller particles reacted faster than the larger particles. Modelling of the experimental data showed that the shrinking unreacted core model can be used to describe the combustion characteristics of both the size and density separated particles. The controlling mechanism was found to be a combination of internal and external diffusion. From the modelling results it was found that effective ash layer diffusion became more prominent as the density increased and that the obtained mass transfer coefficients correlated well with values found in literature.
- Engineering