Selective Production of Phenolic Monomers and Biochar by Pyrolysis of Lignin with Internal Recycling of Heavy Oil

Pyrolysis is the simplest way to convert lignin into phenolic compounds as monomers but inevitably producing oligomers as heavy oil (HO). We propose a particular type of pyrolysis, which recycles such oligomers entirely to pyrolysis, employing the parent lignin for their capture/recycling without any chemicals and catalysts. Continuous pyrolysis of a Japanese cedar lignin was simulated by repeating fixed-bed pyrolysis of HO-loaded lignin and demonstrated steady state of yields of gas, water, char, and light oil (LO) that consisted solely of monomers and furans and lower acids, with neither discharge of HO from the system nor accumulation therein. HO was converted to char, water, and LO with once-through conversion of around 47%, which was contributed by not only self-pyrolysis of HO but also its copyrolysis/carbonization with the parent lignin. The HO recycling increases the LO yield by a factor of 1.4. Catechols (catechol and methylcatechols) and guaiacols (guaiacol, 4-ethylguaiacol, creosol, vanillin, and apocynin) constitute 42 and 22 wt % of LO, respectively. The char shows an increased production in the sequential runs with a near-unchanged elemental composition, volatile matter content, and calorific value.