Pyrolysis of lignite with internal recycling and conversion of oil

The present authors studied pyrolysis of lignite with internal recycling of a heavier portion of the liquid product (oil), employing a reactor that experimentally simulated continuous pyrolysis in a gas-solid countercurrent moving-bed reactor. The lignite particles traveled through the isothermal section at the temperature (T_iso) of 140 or 180 °C and then the non-isothermal section from T_iso to the peak pyrolysis temperature (T_p) of 600 °C. Among the volatile products, a lighter portion of the oil (LO) with a normal boiling point of <360 °C was allowed to escape from the reactor together with steam and non-condensable gas, while the heavier portion of the oil was sorbed by the moving particles in both sections, recycled to the pyrolysis and then converted into char, water, and non-condensable gases inside the reactor. The oil recycling increased the char yield by up to 6 wt % dry lignite. LO consisted mainly of alkanes/alkenes (37-49 wt %), lower oxygenates (acids, ketones, esters, and ethers; 17 wt %), phenols (6-26 wt %), and monoaromatic hydrocarbons (12-21 wt %), but it contained no triaromatics. LO was evaporated completely upon heating to 200 °C, leaving no residue. T_iso influenced the composition of LO. The heaviest compounds varied from C₁₇ to C₂₁ paraffins for T_iso of 140-180 °C, respectively.