The liquefaction conditions of brown coal with bottom recycling were investigated in the presence of an iron-sulfur catalyst using a continuous reactor system for the complete conversion of the coal into distillate and the long-term stable operation of a plant. Two kinds of heavy fractions were used as recycled bottoms. CLB (coal liquid bottom, b.p. > 320 degrees C) and HDAO (hydrogenated-deashed oil, b.p. > 250 degrees C), where they were produced in primary hydrogenation and secondary hydrogenation, respectively. HDAO was hydrogenated over Ni-Mo/Al2O3 catalyst and included HDB (hydrogenated bottom, b.p. > 420 degrees C). The distillate yield increased in proportion to an increase in the amount of the recycled bottom (CLB and/or HDB) in the feed solvent. Fe1-xS contained in the CLB had almost the same catalytic activity as that of Fe2O3-S catalyst, and HDAO had a higher ability of hydrogen donation. They played important roles in the enhanced conversion of the coal and recycled bottom into distillate. The increased gas flow rate (GFR) through the reactors by gas circulation from the gas-liquid separator to the first reactor markedly increased the distillate yield at conditions with bottom recycling. The prolonged nominal residence time (theta(NT): defined by the ratio of slurry-feed rate to reactor volume) also markedly enhanced the conversion of heavy fraction at higher GFR condition with bottom recycling. Accordingly, the complete conversion of the coal, which provides CLB yield of zero, is confirmed as being achieved at conditions of 450 degrees C and 14.7 MPa by cooperative effects of bottom recycling and increases in GFR and theta(NT). At such conditions, a liquefaction plant is confirmed to be ratably operated for a long time because the reactor liquid still maintains its fluidity. The yields of all products at the conditions for the complete conversion of the coal (CLB yield of zero) were determined by using their correlation to the CLB yield. The distillate yield correlated very well to CLB yield regardless of factors affecting the conversion such as bottom recycling, GFR and theta(NT), and attained 66.3 wt% on daf coal at the conditions of CLB yield of zero. (C) 2000 Elsevier Science Ltd. All rights reserved.