Influence of thermal pretreatment on pyrolysis of lignite

The effects of thermal pretreatment on pyrolysis behavior of Shengli lignite were investigated in a fixed bed reactor. In comparison with raw coal, the thermally pretreated coal has lower content of hydroxyl group and lower ratio of aromatic-H to aliphatic-H. The yield of pyrolysis water is lowered after thermal pretreatment in N₂, N₂+O₂ and CO₂ atmospheres. The pretreatment also causes a higher CO₂ content in pyrolysis gas, which thus lowers the heating value of the gas. Pretreating the coal in superheated steam raises the tar yield by 3~4 weight percentage points, and the composition of tar varies with the atmosphere and temperature of the pretreatment. The light fraction (with boiling points below 360℃) increased by 27 weight percentage points by pretreatment in 200℃-steam. The pretreatments at 200 and 250℃ in the mixture of steam and simulated flue gas elevate the light oil and phenol oil fractions in tar by 60 and 42 weight percentage points, respectively.