Effect of volatiles’ reaction on composition of tars derived from pyrolysis of a lignite and a bituminous coal

Abstract

In coal pyrolysis, the reaction of volatiles plays an important role in determination of tar yield and composition but has not been studied in detail in the literature. This work studies the volatiles’ reaction in pyrolysis of a bituminous coal (SM) and a lignite (HLBE) using a two-stage fixed-bed laboratory reactor with the second-stage temperature of 440–700 °C and the residence time of 1.5–6.9 s. The tars were characterized by solvent extraction with tetrahydrofuran (THF) and hexane, and by simulated distillation (SIMDIST) of the THF soluble matter and high performance liquid chromatography (HPLC) analysis of the hexane soluble matter. It was found that the main component of the tars was oil, which was more in the HLBE tar than in the SM tar. As much as >50 wt% oil and tar retained in the chromatograph columns. The volatiles reacted at temperatures as low as 440 °C and the rate was more pronounced at 600 °C in 4.2 s. The volatiles’ reaction reduced the oil yield while increased the pitch yield. The aromatics in the hexane soluble matter were mainly 2-ring compounds and their content increased by the volatiles’ reaction. The major tar fractions have boiling points of 200–538 °C and their content varied systematically in the volatiles reaction. The findings are important for design of efficient coal pyrolysis reactors.