The influence of particle size and devolatilisation conditions on the CO2 gasification of Highveld coal

Abstract

The influence of particle size and devolatilisation conditions on the CO2 reactivity of Highveld seam 4 run of mine coal were investigated in a large particle thermo–gravimetric system. Particle sizes of 5 mm, 10 mm, 20 mm and 40 mm were chosen for both the devolatilisation and the gasification experiments. The devolatilisation experiments were done by placing the coal particles in an electrically preheated tube furnace at isothermal temperatures of 450°C, 700°C and 850°C. The reactivity of the resulting chars was subsequently determined at 900°C using 100% CO2. Coal samples were characterised in terms of proximate analysis, ultimate analysis, calorific value as well as SSNMR. The characterisation results indicated that there is no significant difference across the particle size range in terms of the chemical composition and the structural parameters obtained from SSNMR. Devolatilisation results showed that an increase in temperature resulted in an increase in both devolatilisation rate and volatile yield, with the volatile yield at 850°C being similar to that obtained from proximate analysis for all particle sizes. An increase in particle size resulted in an increase in devolatilisation time and a decrease in devolatilisation rate. For devolatilisation of all particle sizes at 450°C, a distinct plateau in the weight loss curves was observed. The plateau is indicative that the devolatilisation is controlled by heat transfer through the coal particle. This phenomenon is less pronounced at higher temperatures. Due to significant particle fragmentation of large particles at high temperatures, the gasification kinetics of 40 mm chars obtained at 700°C and 850°C could not be determined. The rate of gasification increased with a decrease in particle size while the devolatilisation temperature showed no significant effect for 5 mm and 10 mm particles. Some scatter in the gasification results was observed for 20 mm particles. This might be due to a higher degree of particle fragmentation occurring at the onset of gasification. Different conversion times were found to have a linear relation to the particle size. The gasification results were modelled using a shrinking unreacted core model in the kinetic controlled regime. The reaction kinetic constant showed some particle size dependence that could be due to particle fragmentation and the temperature profile inside the oven. The model showed a good fit for all results.