The influence of minerals content and petrographic composition on the gasification of inertinite rich high ash coal

Abstract

Coal particles with different densities have different mineral and maceral compositions and this affects the gasification reaction rates, especially in coals with high ash contents. This study involved the characterization of six Highveld coals (coals A – F) as well as a coal blend (coal G) consisting of several of these single- source coals in terms of chemical, maceral, mineral and structural properties. This initial characterization was supported by the evaluation of the pyrolysis gas compositions (at 600ºC) that can be obtained from the seven parent coals. The second part of this study included the density separation of two coals (coals B and G) into five density fractions by means of dense medium separation. the coal fractions as a function of particle density. Finally these coal fractions (1 mm) were gasified under a pure CO2 atmosphere at atmospheric pressure (87.5 kPa) and temperatures ranging from 1000 ºC to 1070 ºC to study the effect of particle density on gasification behaviour of coals. These gasification experiments were modelled with the random pore model and the activation energy of each fraction was calculated. From the initial characterization it was found that the parent coals have ash contents in excess of 18.6 % and are inertinite rich (> 68 %); properties which are typical of coals derived from the Highveld coal field. It was observed that coal B has a very low calorific value (17.5 MJ.kg-1) considering the amount of fixed carbon contained in this coal (48.1 %). This motivated further research into the properties and gasification kinetics of density separated fractions prepared from coals B and G. Density separation showed that five coal fractions with uniquely varying properties can be prepared by this procedure. Several trends were observed relating coal properties to particle density, the most prominent of which are: • Ash increases from 8.9 % to 73 % with increasing particle density. • Fixed carbon decreases from 60 % to 10 % with increasing density. • Qualitative maceral segregation occurs, as inertinite tends to concentrate in the dense fractions (1.6 g.cm-3 – 2.0 g.cm-3), while vitrinite and liptinite remain in the lighter fractions (-1.4 g.cm-3 – 1.6 g.cm-3). • The calorific value increases from 3.3 MJ.kg-1 to 28.3 MJ.kg-1 with decreasing particle density. • The coal surface area increases from 38.0 m2.g-1 to 142.6 m2.g-1 with decreasing particle density. It was found at low densities (< 1.8 g.cm-3) that particle density does not have a significant influence on the CO2 gasification reactivity. Further increase in particle density in excess of 1.8 g.cm-3 results in increasing the gasification reactivity. The random pore model was found to give an adequate fit to the experimental gasification data in the temperature range of 1000 ºC to 1070 ºC. The activation energy for the CO2 gasification reaction varies from 163 kJ.mole-1 to 225 kJ.mole-1 for the various coal fractions. It was observed that Ca, Mg and K act as catalysts in the CO2 gasification reaction.