Desulfurization of Organic Sulfur from a Subbituminous Coal by Electron-Transfer Process with K4[Fe(CN)6

Abstract

The desulfurization reaction involving direct electron transfer from potassium ferrocyanide, K4[Fe(CN)6], successfully removed organic sulfur from a subbituminous coal. The temperature variation of desulfurization revealed that increase of temperature enhanced the level of sulfur removal. Moreover, the desulfurization reaction was found to be dependent on the concentration of K4[Fe(CN)6]. Gradual increase in the concentration of K4[Fe(CN)6] raised the magnitude of desulfurization, but at higher concentration the variation was not significant. The removal of organic sulfur from unoxidized coal slightly increased with reduced particle size. Desulfurization from oxidized coals (prepared by aerial oxidation) revealed a higher level of sulfur removal in comparison to unoxidized coal. Highest desulfurization of 36.4 wt % was obtained at 90 °C and 0.1 M concentration of K4[Fe(CN)6] in the 100-mesh size oxidized coal prepared at 200 °C. Model sulfur compound study revealed that aliphatic types of sulfur compounds are primarily responsible for desulfurization. Because of higher stability, thiophene and condensed thiophene-type of compounds perhaps remained unaffected by the electron-transfer agent. Infrared study revealed the formation of oxidized sulfur compounds (sulfoxide, sulfone, sulfonic acid, etc.) in the oxidized coals. Band intensities in these oxidized compounds due to common −SO and −SO2 units increased in their respective regions. The desulfurization reaction in different systems is well-represented by the pseudo-first-order kinetic model. The intrinsic rate constants were found to be in the range of (1.8−3.5) × 10-5 s-1, which implies that the reaction proceeds at a slow and steady rate. The activation energy of the reaction in different systems falls in the range of 5.2−8.8 kJ mol-1. Lower values of frequency factor (range: (1.9−5.5) × 10-4 s-1) revealed that during the course of the reaction an associated type of compound (activated complex) was formed. Application of the transition state theory indicated that the desulfurization reaction proceeds with the absorption of heat (endothermic reaction) and is nonspontaneous in nature.