The influence of particle size on the steam gasification kinetics of coal
Abstract
Steam gasification has been extensively researched in order to optimise and efficiently utilise coal. Reactivity on powdered coal has received considerable attention, however, due to equipment limitation large coal particle research has not progressed to the same extent. The lack of knowledge regarding the steam gasification reactivity of large coal particles is the main motivation of this study. A South African Highveld seam 4 coal was used in this investigation. Conventional coal characterisation was conducted on a representative sample of the run-of-mine coal sample. The results obtained for the conventional analysis are typical for what is found in literature for a South African Highveld seam 4 coal. The run-of-mine coal was sieved into particle size fractions for easy hand selection of large coal particles. The single coal particles were hand selected on size and shape and afterwards a density cut (1400 – 1500 kg/m3) was used as the final selection criterium. The particles, selected according to size, shape and density, were used for the petrographic analysis, char pore structure analysis and reactivity experiments. The petrographic analysis of the raw coal particles was conducted on 5 and 30 mm particles. Both samples are clasified as inertinite rich bitiminous, medium rank C coal. The maceral
concentartion varied with particle size. The char pore structure of the 5, 10, 20 and 30 mm coal particles were also studied. It was observed that an increase in the particle size
decreased the char porosity, reduced pore diameter and increased surface area (BET
surface area for gas adsorption and pore area for mercury porosimetry). Steam gasification reactivity experiments using 5, 10, 20 and 30 mm coal particles at
gasification temperature ranging from 775 to 900 °C were conducted. The ash produced
after gasification was studied to determine the degree of fragmentation. A large degree of fragmentation was observed for the 30 mm coal particles when compared to the other (smaller) coal particles. To quantitatively determine the influence of particle size on the reactivity of coal, the validity of powdered reactivity models were tested on the reactivity results of large coal particles. Fundamental models, like the homogenous, shrinking core and random pore models, were found to fit most of the experiments, but the fitted constants lacked a chemical / physical meaning. The semi-empirical Wen model accurately predicts the experimental carbon loss and was used for modelling. The initial reactivities obtained from the Wen model were used to quantatively determine the influence of temperature and particle size on the steam gasification kinetics. The activation energy obtained from the Arrhenius plots for the 5, 10, 20 and 30 mm particles are 165, 145,
150 and 143 kJ/mol, respectively. In order to determine the influence of particle size on the reactivity of coal the initial reactivity
obtained from the Wen model was normalised using the 30 mm coal particle reactivity. This showed that a six fold decrease in particle size resulted in a twofold increase in steam gasification reactivity. Also, no significant difference in reactivity is observed for the 20 and 30 mm coal particles and it is proposed that the large degree of fragmentation of the 30 mm particle is responsible for this phenomenon. The increase in reactivity observed with a
decrease in particle size is proposed to be a combination of different conversion
mechanisms as well as a combination of several different factors (fragmentation,
petrographic composition and char pore structure) which are dependent on coal particle size.
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