Effect of volatiles' reaction on coking of tar during pyrolysis of Naomaohu coal in a downer-bed reactor

The volatiles' reaction has a significant influence on the yield and quality of tar during pyrolysis of low-rank coals. To clarify the effect of volatiles' reaction on the coking behavior of tar, pyrolysis experiments of Naomaohu coal were conducted in fixed-bed and downer-bed reactors at various heating rates (10, 58 and >103 °C/min) and temperatures (550–800 °C). The tar composition was analyzed by gas chromatography with the simulated distillation method and a comprehensive two-dimensional gas chromatography/mass spectrometry. The structure of coke suspended in tar (Coke-S) and char was characterized by electron spin resonance, Raman and thermogravimetry. The reaction of volatiles during coal pyrolysis is more significant in the downer-bed reactor, due to the increased interaction between different volatile radical fragments or with char, leading to increased coke yield and lower tar quality. In addition, the volatile radical fragments are dominated by the core structure of aromatic carbon at higher than 650 °C in the downer-bed reactor. In the second thermal field in the downer-bed reactor, Coke-S produced by these fragments has more macromolecular structures and greater stability than char obtained at the same temperature. •Volatiles' reaction is more obvious in downer-bed reactor with the continuous feeding.•The volatile fragments are dominated by aromatic structures at 650 °C or higher.•The I(Gr+Vr+Vl)/ID ratio of coke suspended in tar is smaller than that of char, except at 550 °C.•The coke suspended in tar has more radicals and greater stability than char at same temperature.