Comparative Study of the Catalytic Performances of Na2CO3 and γ-FeOOH in Hydroliquefaction To Propose a Two-Stage Lignite Catalytic Liquefaction Process

High oxygen content in lignite increases hydrogen consumption during hydroliquefaction, and simultaneously the produced water decreases hydrogen partial pressure, which negatively affects lignite liquefaction behaviors. Therefore, the liquefaction performances and oxygen-removal behaviors of Xilinhaote (XL) lignite during liquefaction at different temperatures in the presence of Na2CO3 (NC) and FeOOH (FO) were investigated. Results showed that the oxygen contents of liquefaction residues (LRs) obtained at 330-380 degrees C were 9.37-13.84% without catalyst and decreased to 7.90-13.02% and 6.38-11.73% with the addition of FO and NC, respectively. The addition of NC could promote the polarized breakdown of aromatic C-aryl(-) C-alkyl ether bonds via ionic pathways in the presence of water, which facilitated the depolymerization of lignite macromolecular structures, resulting in significant increase in coal conversions at 330-380 degrees C. To enhance liquefaction performance, an optimized two-stage catalytic liquefaction process for lignite with high oxygen content was proposed, which integrated the deoxygenation treatment of lignite over Na2CO3 and the hydrogenation of Na2CO3-treated lignite over iron catalyst. In the two stage catalytic liquefaction processes, coal conversions and oil yields significantly increased to 90.66-92.73% and 52.73-57.56%, respectively, compared to 84.61% and 43.52% in single-stage liquefaction of XL over FO at 450 degrees C. Furthermore, in the two-stage catalytic liquefaction process, the treatment of XL over NC in stage I presented higher coal conversion (92.73%) and oil yield (57.56%) than those over FO in stage I (90.66% and 52.73%). Surprisingly, H-2 consumption of the former (2.17%) was lower than that of the latter (2.91%).