Comparison of two-stage and single-stage liquefaction schemes for their effect on conversion and liquid yield of two low rank coals

Balancing the rates of coal radical fragments’ generation and hydrogen donation is crucial to inhibit the radical fragments’ condensation for a high oil yield and low hydrogen consumption in direct coal liquefaction (DCL). Two-stage liquefaction (TSL) has been proposed to be effective for balancing these rates under mild conditions but little work can be found in the literature on the comparison of TSL with single-stage liquefaction (SSL). This work studies direct liquefaction of two low-rank coals (NMH and SW) of very different volatile content under various TSL and SSL schemes in systems with different hydrogen donation ability. The coal conversion and products yield in different liquefaction schemes are correlated with their organic structure characterized by 13 C NMR and predicted by a structure model. It is found that for the total 40 min liquefaction, the TSL scheme with 10 min at 420 ℃ followed by 30 min at 455 ℃ results in the highest liquid yield and lowest hydrogen consumption per liquid yield when the coal is very active but the hydrogen donation ability is insufficient. Otherwise, SSL at 455 ℃ is preferred. The coal conversion, liquid yield and hydrogen consumption per liquid yield of NMH under above TSL scheme at coal:THN= 1:2 are 85.5 wt%, 55.9% and 48.4 mmol-H/g-Liquid, respectively.