Pyrolysis of Lignin in Gas-Phase Isothermal and cw-CO ₂ Laser Powered Non-Isothermal Reactors

Pyrolysis of lignin in the gas-phase using "wall-less" and cw-IR CO ₂ laser powered homogeneous pyrolysis (LPHP) non-isothermal and continuous droplet evaporation (CDE) isothermal reactors has been performed. Hydrolytic lignin was dissolved in an acetone/water (9:1) mixture and dispersed into LPHP and CDE reactors using a constant output atomizing device and a syringe pump, respectively. Large differences in gas phase depolymerization of lignin have been seen in both reactors. The temperature distribution in the LPHP reactor was evaluated by thermocouple measurements and validated by the method of "chemical thermometer" and COMSOL Multiphysics simulations. The gas phase delivery of lignin into the hot zone of the LPHP reactor under "wall-less" conditions led to the breakdown of lignin into paramagnetic fragments that deposited onto the cell walls, which were studied by electron paramagnetic resonance spectroscopy. Phenolics were not detected at all in the LPHP reactor. However, major phenolics (phenol(s), guaiacol(s), syringol(s)) were detected in the CDE reactor. At laser powers higher than 20 W and a high initial concentration of dispersed lignin (5 g/L), an intensification of demethoxylation reactions of phenolics by concomitant alkylation of aromatic rings along with the expulsion of CO from phenoxy rings (to account for formation of indenes and naphtalenes) was achieved in the LPHP reactor when a prepyrolyzed mixture from the CDE reactor entered the LPHP reactor. The yields of qualified biofuel products such as styrene, different alkyl benzenes, and surrogate fuels of diesel such as indene, 1,2-dihydro naphthalene, naphthalene, and other derivatives were detected in the LPHP reactor with decreasing amounts of phenolics. These results represent valuable observations about the primary mechanism of lignin depolymerization.