ABSTRACT
In this investigation, coal fines enriched with inertinite were used for direct liquefaction experiments. For comparison, a vitrinite-rich coal typically utilized in coal-to-liquid processes was also employed. To assess the impact of mineral matter content, demineralization was used to remove most of the inorganic constituents. The findings revealed that the inertinite-rich coal exhibited lower liquefaction conversions due to a reduced proportion of reactive macerals and elevated levels of inorganic mineral matter. These conversion values exhibited a strong correlation with the quantity of reactive macerals present in the parent coals. For the inertinite-rich coal, the presence of inorganic mineral matter impeded the liquefaction process but facilitated the CO2 gasification reactions of the derived chars. To evaluate their potential in gasification processes, CO2 gasification experiments were conducted and the reactivities and apparent gasification activation energies of both coal chars, liquefaction residue chars, and preasphaltene and asphaltene (PAA) chars were calculated. These calculations were carried out using the random pore model (RPM) and volumetric reaction model (VRM). The chemistry, reactivity, and kinetics of residue gasification conversion are not thoroughly understood, yet they hold significant importance in optimizing syngas production within gasification processes. The findings from this work highlight significant differences in liquefaction conversion values, product distribution, and composition. These differences are influenced by factors such as maceral composition, inorganic mineral matter content, hydrogen-donor capabilities of the solvent, and liquefaction reaction temperatures. Additionally, these variables affect the CO2 gasification reactivity of liquefaction solid residue chars.
ABSTRACT
The objective of this study, the first of its kind on these specific South African low-sulfur coals, was to capture H2S and SO2 produced under inert and oxidizing conditions from sulfur compounds present in the coals. The capturing agents were calcium and magnesium oxides formed during the transformation of calcite and dolomite. The effectiveness of two different scrubbing solutions (0.15 M cadmium acetate and 1.1 M potassium hydroxide) for absorption of volatilized H2S and SO2 was also investigated. The bituminous coal (coal A) contained dolomite, calcite, pyrite, and organic sulfur. Lignite (coal B) had a high organic sulfur content and contained gypsum, no or low dolomite and pyrite contents, and no calcite. A third sample (coal C) was prepared by adding 5 wt % potassium carbonate to coal A. Under oxidizing conditions and at elevated temperatures, FeS2 produced Fe2O3, FeO, and SO2. It transformed to FeS and released H2S under inert conditions. Organic sulfur interacted with organically bound calcium and magnesium at 400 °C in an inert atmosphere to form calcium sulfate and oldhamite ((Ca,Mg)S). CaO, produced from calcite or dolomite, reacted with SO2 and O2 at 950 °C to form calcium sulfate. Treatment of lignite at 400-950 °C resulted in 96-98% evolution of sulfur as gases. Hydrogen sulfide formation increased with the increasing thermal treatment temperature under inert conditions for the three coals. Under oxidizing conditions, sulfur dioxide formation decreased with the increasing temperature when heating coals B and C. The lowest ratio (0.01) of H2S to SO2 was achieved during thermal treatment of the blend of coal and potassium carbonate (coal C), implying that almost all of sulfur was retained in the coal C ash/char samples. In situ capturing of sulfur gases by CaO and MgO and by the added K2CO3 in coal C to form calcium/magnesium/potassium sulfates and potassium/calcium/magnesium aluminosilicate glasses during utilization of these and similar coals could reduce the percentage of sulfur volatilized from the coals by 54-100%, thereby potentially decreasing their impact on the environment.