Role of FeS Catalyst in the Hydromodification of Lignite in a Subcritical Water–CO System

[Image: see text] The impacts of FeS catalysts on the hydromodification and structural evolution of lignite were investigated using Fourier transform infrared spectroscopy, nuclear magnetic resonance, and X-ray photoelectron spectroscopy. The results indicate that the caking property of lignite can be significantly improved in the presence of the FeS catalyst. When 6.0 wt % FeS was added, the maximum caking index (G(RI)) of modified coal reached 95. During the hydromodification, FeS has little effect on the intrinsic water gas shift reaction, but it can increase the CO conversion by promoting the decomposition and hydrogenation of coal so that more active hydrogen is generated and introduced into modified coal. FeS is conducive to the rupture of distal aliphatic groups in the extractible solutes, which promotes the entrance of hydrogen into the aromatic nucleus (H(ar)) and α positions (H(α)) of asphaltenes and β positions (H(β)) of preasphaltenes. After the catalytic hydromodification, the longer side chains or bridge bonds break and are hydrogenated to form the aliphatic structures with a shorter chain or a higher branched degree. Meanwhile, more oxygen-containing functional groups were removed along with the reduction of volatiles in the modified coal. The synergistic effect of FeS on these factors is favorable for the generation of plastic materials, which contributes to the development of the caking property of lignite.